Separation of Nickel from Technetium
Louie El-Azzami, Ph.D.
Research Assistant
Dr. Eric Grulke, Professor
Table of Contents
C. Criteria Release of the US
and Other Countries
D. Other Markets for This Technology
E. Prior and Present Processes
F. Need For An Efficient Process
II. Metal Distillation and Vaporization
A. Separation of Metallic Mixtures
B. Proposal for Separation of Nickel and Technetium by
Physical Vapor Deposition
1. Theoretical Feasibility of Separation
IV. Appendix: Prior & Present Arts of Electrorefining
& Electrowining
B. US patent 5,156,722: Decontamination of radioactive
metals (5)
C. US patent
5,183,541: Decontamination of radioactive metals (6)
D. US patent 5,217,585: Transition metal decontamination
process (6)
E. US 5,262,019:
Decontamination of radioactive metals (7)
F. US 5,439,562:
Electrochemical decontamination of radioactive metals by alkaline processing
(9)
G. US 5,487,748: Method for removal of technetium from
radio-contaminated metal (10)
List of Figures
Figure
1. The phase diagram of nickel-technetium (19)
Figure
2. The phase diagram of nickel-rhenium (20)
Figure
3. The schematic diagram of a Knudsen effusion cell mass spectrometer
Figure
4. Vaporization apparatus for the recovery of nickel from Tc-contaminated nickel ingots.
Figure
IV.D.1. Process flow diagram of Ni Recovery from Tc-contaminated Ni.
Figure
IV.E.1. Process flow diagram to
decontaminate transition metals.
Figure
IV.G.1. The process flow diagram for the decontamination of Ni.
List of Tables
Table
1. Physical and thermal properties of nickel and technetium
Table
2. The vapor pressures of nickel and technetium
Table
3. Physical properties of nickel and technetium
Table
4. The Vapor Pressures of Nickel and Technetium
Table
5. Cost of decontamination of 9700 tons of contaminated nickel
Table
7. The radioactivity of technetium vs. nickel recovery.
The
recovery of nickel (Ni) from Department of Energy (DOE) gaseous diffusion plant
barriers contaminated with radionuclides and specifically, the separation of Ni
from technetium-99 (99Tc) has proven to be difficult. Manufacturing Sciences Corporation (MSC), utilizing
electro-refining approaches, could not remove 99Tc from
volumetrically contaminated Ni to levels that would allow the free release of
the Ni for commercial and industrial uses. The various methods applied by Manufacturing Sciences Corporation (MSC)
are reported in the attached Appendices. The electro-refining methods employed by MSC resulted in Ni containing
residual 99Tc. The residual 99Tc
in Ni purified by MSC’s electro-refining methods resulted in congressional
opposition to the release of decontaminated Ni from the K-25 plant at Oak Ridge
and further resulted in a moratorium being issued by the Secretary of DOE for
the release of decontaminated scrap metal. The present proposal employs an approach that does not rely on
electro-refining for the separation of 99Tc from volumetrically
contaminated Ni.
A
major obstacle to the free release of surface or volumetrically contaminated
scrap metal originating in radiation impacted areas within the DOE Complex is
the scrap metal industry’s position of “zero tolerance.” The scrap metal industry produces metals
that are used in industrial and consumer products. The industry position is that the release of scrap metal from
radiation impacted areas at DOE facilities into the industry’s recycled scrap
metal flow path would have both short and long-term negative impacts. The industry fears the rejection of its
products by customers and the contamination of its processing systems if
decontaminated material containing residual radioactivity is released. The scrap metal industry supported the Secretary
of Energy’s decision to impose a moratorium on the release of volumetrically
contaminated metals. The DOE moratorium halted the release of MSC’s
electro-refined nickel from the Oak Ridge, TN K-25 facility Ni that was
volumetrically contaminated with residual 99Tc.
The Nuclear Regulatory
Commission (NRC) continues to evaluate national standards for the potential
release of materials from radiation-impacted facilities. The release of DOE scrap metal from
radiation-impacted areas, including radiation impacted areas at DOE facilities,
would be required to meet standards established by the NRC and Agreement
States. DOE’s moratorium on the release
of volumetrically contaminated metals remains in place until the NRC and
Agreement States establish national standards for the release of
radiation-impacted materials. Furthermore, the DOE’s suspension of the unrestricted release of
recycled scrap metal from radiological areas within DOE facilities will remain
in effect until the DOE develops release criteria and establishes the release
criteria through DOE Order 5400.5.
DOE indicated in its April 2002 draft Guide “DOE G
441.1-XX, CONTROL AND RELEASE OF PROPERTY WITH RESIDUAL RADIOACTIVE MATERIAL
for use with DOE 5400.5, Radiation Protection of the Public and the
Environment,” that the DOE’s principle requirements for the release of
scrap metal from radiation impacted areas are intended to meet the following
goals:
• Property is evaluated,
radiologically characterized, and, where appropriate, decontaminated before
release.
• The level of residual radioactive material in property to be released
is as near background levels as is reasonably
practicable, as determined through the DOE ALARA process requirements, and
meets DOE authorized limits.
• All property releases are appropriately certified, verified,
documented, and reported; public involvement and notification needs are
addressed; and processes are in place to appropriately maintain records.
Prior
to the release of both surface and volumetrically contaminated scrap metal from
DOE and other licensed radiation impacted facilities, a number of important
concerns must be overcome to ensure the released material will not have a
negative impact on both public health and the public perception of the scrap
metal industry’s use of the material in commercial and industrial products.
The
upgrades, maintenance, decontamination and radiation decommissioning of the
gaseous diffusion process at the Paducah Gaseous Diffusion Plant (PGDP) in
Paducah, Kentucky have generated and will generate substantial quantities of
nickel and other metals volumetrically contaminated with radioactive
materials. The estimated amount of
contaminated nickel could reach 44,794 tons (1). The most frequently identified contaminant in the nickel is
technetium-99 (99Tc). However, traces of neptunium (Np), plutonium (Pu), protactinium (Pa),
thorium (Th), and uranium (U) have also been identified in the nickel.
There
is interest in recovering the PGDP nickel and recycling it to the industrial
sector. However, in additon to the many regulatory, industry, and public
acceptance issues associated with any use of decontaminated material outside of
the nuclear industry, there remains a significant technical challenge. The most challenging technical problem
relative to the decontamination and release of the PGDP nickel is the present
lack of an ultra-high efficiency method necessary to separate technetium-99 from
nickel. The other radioactive materials
can be separated via electrolysis processes.
At present, application of the best available
electrolysis process technologies leave approximately one (1) Becquerel (Bq) of
technetium-99 activity per gram of nickel when applied to material containing an
initial technetium-99 activity of 320 Becquerels per gram. The presence of residual radioactivity in
decontaminated Ni from the current separation processes is not acceptable to
government, industry, and the public. This project will explore a new alternative separation method based on
the large differences between the vapor pressures of nickel and
technetium.
This
project will develop and demonstrate a technically effective, cost-efficient,
and cost-effective process using physical vapor deposition to recover pure
nickel with no detectable traces of technetium. The slag left behind will be composed of technetium with small
levels of nickel. The proposed physical
vapor deposition process consists of two steps: 1) Nickel is preferentially
evaporated from solid or liquid solutions of Ni/99Tc, and 2) The nickel vapor is deposited on cold
surfaces to produce 99Tc-free nickel plates.
A physical vapor deposition
process can be designed only after the fundamental data on the
vapor-liquid-solid-solid phase equilibria of the metal mixtures in question are
collected. A unique GC/MS system
designed for metal vapor evaluation will be constructed to obtain the needed
data. Similar systems were constructed
at Lawrence Livermore Laboratory (1969) and Los Alamos (1983). Mr. Bert Lynn of the University of Kentucky
(UK) is a GC/MS expert and a skilled instrument designer. Mr. Lynn will collaborate on design,
construction and commissioning of the new GC/MS.
New data obtained with the
proposed GC/MS will redefine the phase diagrams for Ni-99Tc and
other metallic mixtures and will permit thermodynamic phase equilibria models
to be developed and applied to the process design. Data to be obtained includes vapor pressures, heats of
vaporization, heats of sublimation, activity coefficients, and separation
factors for the Ni-99Tc pair at different temperatures. The data will be incorporated into phase
diagrams that will account for the vapor phases of the constituents. This will revolutionize the ability of
material and chemical engineers to determine the feasibility of separation.
The proposed research will
investigate the physio-chemical system of Ni-99Tc. Presently, there are no phase diagrams that
relate metal vapor compositions to the metallic liquid-solid system. The
proposed research approach will provide data for a relatively unexplored area
of metal separation. The proposed
research has applications for many non-radioactive systems such as scrap metal
recycling and alloy purification. This
project will deal with the recovery of nickel from nickel ingots volumetrically
contaminated with technetium-99. Proposals are being prepared for other funding sources to study other
metallic systems.
The nickel-rhenium system has
been chosen to be a surrogate model system for this method of separation to
verify the analytical and physical vapor separation technologies. This system was chosen because rhenium and
technetium are in the same chemical group. Rhenium is not radioactive, so the research will be able to generate significant
amounts of data to validate the feasibility of the separation without the need
for a costly health physics program.
Technetium-99 is a b-emitting radionuclide with maximum beta energy of 297 keV and a
half-life of 2.1x105 years. Volumetrically contaminated Ni may have a 99Tc activity of up
to 5000 Bq/g or more, which is more than one order of magnitude above the
maximum international release criteria of 74 Bq/g metal total activity. Certain
countries have specified lower release criteria of 1.0 Bq/g or less total
activity. In the United States, the
federal government and regulatory agencies handle release of material
containing residual radioactive material on a case-by-case basis. Presently, release of scrap metal or
recycled metal containing detectable radioactivity is not acceptable to the
public.
The scrap metal industry is a
secondary metal industry. The
dismantling and the decommissioning of nuclear facilities along with scrap
metal inventories at DOE sites would
supply the scrap metal industry with millions of tons of metals. This requires the decontamination of the
scrap metals before release for public use. The principal administrative authorities responsible for controlling the
release of scrap metal from nuclear facilities are the DOE, the Nuclear
Commission Regulatory (NCR), Department of Defense (DOD), and Agreement States (1). These authorities have a jurisdictional authority over 30,000 structures, of which
8,000 are contaminated (1). The DOE
has an estimate of 149,665 tons of contaminated metals in its existing
inventory (1). This inventory consists
of 119,232 tons carbon steel, 10,699 tons Ni, 7,462 tons stainless steel, 2,353
tons aluminum (Al), 1,975 tons copper (Cu)/brass, and 7,943 tons of other
metals (1).
The
decommissioning of DOE gaseous diffusion plant facilities commenced with the
K-25 plant (1998-2006). The proposed
schedule for decommissioning of the following DOE facilities are the Portsmouth
plant (2007-2015), then the Paducah plant (2015-2023), and finally the rest of
the plants (2023-2058) (1). The
existing and future contaminated scrap metal at DOE facilities may total up to
1,068,022 tons (1). That projected inventory
consists of 903,897 tons carbon steel, 53,990 tons Cu/brass, 44,818 tons Ni, 26,960
tons stainless steel, 36,070 tons Al, 291 tons lead (Pb), 83 tons monel, and 1,913
tons of other metals (1).
Prior
and present processes for removal of 99Tc from volumetrically
contaminated Ni have not been effective for removal of all the residual 99Tc. The
presence of residual 99Tc in processed Ni metal
from K-25 gaseous diffusion plant Ni barriers in Oak Ridge prevented the
release of the reprocessed Ni. Existing
processes that have been utilized to decontaminate volumetrically contaminated
material include ion exchange, solvent extraction, melt refining, inductoslag
refining, and electrolysis.
There have been several different processes applied
for the recovery of Ni. Ni is selectively stripped by an organic oxime from an
acidic solution of Ni & Cu and then Ni electrowinning (2). Another process is the removal of Ni by
liquid-liquid extraction (3,4). Muller,
et al., extracted Ni from aqueous solutions that contained large amounts of
alkali metal ions by contacting the solution with an organic solution of
di-2-ethylhexyl phosphoric acid and naphthenic acid (3). Fujimoto et al. used
organic mixtures of 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester and/or
3,5,5-timethylhexyl phosphoric acid mono-3,5,5-timethylhexyl ester and/or
isodecyl phosphoric acid monoisodecyl ester to separate cobalt (Co) from Ni (4).
Previous work demonstrated that
metallic Ni contaminated with fission products can be decontaminated to remove
actinides by direct electrorefining which is based on the differences in
reduction potential along the electromotive force (emf) series. Actinide
removal is favored by two phenomena during electrorefining. Actinides have a
significantly higher reduction potential relative to nickel and they are
normally won from molten salt electrolyte rather than from aqueous electrolyte
(see U.S. Pat.
Nos. 3,928,153 and 3,891,741)
(5).
Electrorefining
and electrowinning of Ni decontaminated streams have been utilized but these
processes encountered problems with the co-deposition of the 99Tc and Ni on the cathodic cell. This problem was solved by
converting 99Tc(VII) to 99Tc(IV) which prevents it from co-depositing with Ni
(6-10). Other problems arise with the
electrowinning and electrorefining processes because of the generation of large
volumes of radioactively contaminated acid wastes especially when large amounts
of Ni have to be recovered.
There remains a need for an
economical and efficient method to refine and decontaminate metals from nuclear
facilities. More specifically, there remains a
need to separate technetium-99 from contaminated metals in a simple manner.
The
radiochemical decontamination art is presented with unique practical problems
not shared with traditional extraction technologies. Radiochemical extraction
technologies are generally concerned with the economic recovery of
radiochemicals. Routine process inefficiencies that result in residual amounts
of radiochemicals in process streams, by-products, or waste streams raise only
normal economic issues of process yield and acceptable process costs relative
to the radiochemical recovery. The various process streams and the
radiochemicals will continue to be held by the regulated nuclear community in
order to minimize the potential for release to the general public.
In contrast to radiochemical extraction
technologies are technologies necessary to remove radiochemicals from
contaminated materials such as nickel and precious metals. The presence of only residual parts per
million concentrations of fission daughter products such as technetium-99 in
remediated nickel and other recycled products will make the products
unacceptable to the public. The release of material containing residual
radioactivity to unregulated non-nuclear markets has been prevented by industry
opposition, public perception, politically and by lack of a sound regulatory
approach. Residually contaminated material and product must then be employed in
less valuable regulated nuclear markets or it must be further decontaminated at great financial
cost. (6).
Two of the more
important practical considerations in the commercial feasibility of waste
processing recycling, and
recovery operations include the effectiveness of so-called
"secondary" processing steps and the volume of wastes which are
generated by these secondary processing steps. wo of the more important practical considerations
in the commercial feasibility of waste processing operations
include the effectiveness of so-called "secondary" processing steps
and the volume of wastes which are generated by these secondary processing steps, such as ion exchange
processing and like sorption processes for removing unwanted ionsFor example, when.
Where, e.g., strong acid ion exchange resins of the type generally
employed in nuclear waste processing operations are employed,
adsorbed cations may leak from the resin beads whenre the pH
of the solution is from about 2 to about 5. In addition, t The resin beads
may be degraded by process reagents such as, e.g., oxidants (such as peroxides) and ozone that may ,
which may be added to maintain the oxidation potential of the a process
streams. Also, iIt is normally
desirable to incinerate contaminated resin beads because they tend to resist
compaction and, therefore, volume
reduction. Resin
beads,H however, resin beads cannot
easily be incinerated because the resin polymers tend to foul incineration
equipment such as incineration grating and also may release noxious offgases.
Furthermore, residence time distributions of process streams in ion exchange,
carbon beds and the like may result in undesirable side reactions or subsequent
reactions which produce other (and perhaps non-extractable) species, such as
the reduction of a pertechnetate cation to nonionic technetium oxide. Also,
bBy-passing
is anothera practical concern which that may substantially reduce the
effectiveness of ion exchange and carbon beds aand columns.
The Ni reduction process
requires high metal concentration in solution during purification. This
combined with similarities in properties between Ni and 99Tc make
separation extremely difficult. Accordingly, there remains a need for an
economical and efficient method to decontaminate metals and more specifically,
to separate technetium
from these metals in a simple
manner.
The physical vapor deposition
of nickel will be more efficient than all the prior and present processes. The advantages of the physical vapor deposition of Ni-99Tc are as follows: 1) Obtain Hhighly pure nickel can be produced, 2) Technetium-99c is
also purified in this process and could become awhich makes it a secondary product, 3) Tthe process cost is lower than that ofIt is cheaper than electrolysis, and 4) nNo additional waste streams are generatedDoes not generate additional waste.
Metal distillation and vaporization has been
utilized as a means to purify metals from contaminants. The
efficiency of metal distillation depends on the difference of the boiling
points and the vapor pressures of the metallic components of the molten
mixture. Ginder, et. al. purified zinc
(Zn) from cadmium (Cd) by metal distillation (13, 14, and 15). The influent Zn-Cd mixture contained 0.47% Cd
(B.P = 767oC) and the remaining Zn (B.P
= 905oC). The vapor pressure of Cd
is higher than that of Zn. Distilling
the Zn-Cd mixture
at 975oC, Ginder was able to purify Zn and
reduce the content of Cd to 0.0013%.
U.S. patent Patent 5,582,630
demonstrates how to purify magnesium (Mg) from metallic and nonmetallic
elements by vacuum distillation (16). The final Mg product contained 5.87 ppm Zn and 0.73 ppm of other metals
(25 ppb Al, <10 ppb As, <5 ppb B, <10 ppb Bi, <20 ppb Ca, 18 ppb
Cd, <10 ppb Co, <10 ppb Cr, <20 ppb Cu, <10 ppb Fe, <10 ppb Ga,
<10 ppb In, <10 ppb K, < 5 ppb Li, 41 ppb Mn, <10 ppb Mo, 36 ppb
Na, <10 ppb Ni, 36 ppb Pb, 22 ppb Sb, 226 ppb Si, < 10 ppb Sn, < 1 ppb
Th, 10 ppb Ti, <1 ppb U, <30 ppb V, and 23 ppb Zr).
Tayama and Hodozuka used vacuum distillation to
purify iIndium
(In) from silicon (Si), iron (Fe), Ni, Cu, gallium (Ga), antimony (Sb), and
lead (Pb) (1). The In feed had the following contaminants: 0.14
ppm Si, 0.15 ppm Fe, 2.3 ppm Ni, 0.28 ppm Cu, 0.03 ppm Ga, 0.02 ppm Sb, and 0.2
ppm Pb. The purified In had the following impurities:
<0.03 ppm Si, <0.01 ppm Fe, <0.01 ppm Ni, <0.01 ppm Cu, <0.01
ppm Ga, <0.01 ppm Sb, and 0.01 ppm Pb.
Weil distilled a mixture of metals to recover mMagnesium. The mixture was
composed of 85.654% Mg, 10.7% Al, 0.67% Cu, 0.006% Fe, 0.24% Mn, 0.01% Ni,
0.16% Si, and 2.56% Zn. The distillate
was composed of >97.805% Mg, <0.01% Al, <0.01% Cu, <0.003% Fe,
<0.01% Mn, 0.002% Ni, 0.02% Si, and 2.86% Zn. The residue was composed of 90.16% Al, 0.3% Mg, 5.9% Cu, 0.04%
Fe, 2.1% Mn, 0.08% Ni, 1.4% Si, and 0.02% Zn (18).
Historically, metal
distillation and vaporization has not been applied to separate
radiocontaminated metals from the radioactive contaminants.
To
apply metal distillation and vaporization techniques to the recovery of metal from Tc-99 contaminated metal stock, tThe physical
properties and phase diagram of the Ni-Tc system must be investigated to provide the fundamental
data needed for the design of the separation process. and defined. Once the physical properties
and phase diagram of the Ni-Tc system have been defined, a non-radioactive surrogate system with
a similar phase diagram can be used to investigate the feasibility
of separation.
Figure 9 1 representsdemonstrates the liquid-solid phase diagram of Ni-Tc. The
phase diagram of Ni–Tc system is a simple pertectic phase diagram with a
peritectic temperature of 1495oC and composition of 29.4
atomic % Tc. The phase diagram illustrates that, in the region of
interest (<10 ppm Tc), the phase of the Tc for the Ni-Tc composition
above the melting point of Ni, is liquid. This is critical because the phase diagram shows no formation of
alloys in that region, which implies that separation of these metals is
possible.
 |
Figure 102 represents.
demonstrates the liquid-solid phase diagram of
nickel-rhenium. The phase diagram behaves
in a similar way to that of nickel-technetium. This is one of the main reasons to choose nickel-rhenium system
as a surrogate
system to model the Ni-Tc system. Another reason is that the nickel-rhenium
system is a non-radioactive system which makes it less cumbersome and less expensive to
work with. This will permit collection
of which
provides a large amount of experimental data to ensure the
validity of the work. Tables 1
and 2 show the physical properties and pure component vapor pressures of nickel and technetium.
Table 3 demonstrates the
physical properties of nickel and technetium. Nickel has a much lower boiling point than technetium. Table 4 illustrates that the relative
volatility of nickel to technetium is in the order of 106. In conclusion, it is obvious that the
separation of nickel and technetium by physical vapor deposition is realistic.
To measure the
concentrations of the metal vapors at high temperatures, a quadrupole mass
spectrometer coupled to a Knudsen effusion cell must be assembled. Figure 3 shows the schematic diagram of a
Knudsen effusion cell mass spectrometer (21). Figure 4 illustrates a physical vapor
deposition apparatus to purify Ni.
The proposed concepts are:
a)
Separation of metals from metal mixtures due to the
difference of vapor pressures.
b)
Constructing vapor-liquid-solid phase diagrams of
metallic mixtures.
c)
Developing correlations for vapor pressures,
activities, and enthalpies as a function of temperature for the metallic
systems.
d)
Design of metal-metal separation processes such as
physical vapor deposition, batch distillation, or continuous distillation.
Presently, there are three
ways to deal with metals contaminated with radioactive material which are:, dumping, storing, and the BNFL electrolysis method. The cost of the physical vapor deposition method vs. the cost
of the above methods is tabulated in Table 54. The cost is based on the decontamination of
9700 tons of contaminated nickel. The London Exchange Metal quoted
the price of nickel to be $6.6452/lb ($14.6192/kg). The price of Tc was quoted by the
National Lab. at Oak Ridge to be $50/g. The costs
of the electrolysis process, Nevada and Envirocare disposal sites were quoted from DOE/EM-0567 Report (22).
 |
To measure the
concentrations of the metal vapors at high temperature, a quadrupole mass
spectrometer coupled to a Knudsen effusion cell mustwhich
is enclosed by a high temperature furnace has to be assembled. Figure 3 shows the schematic diagram of a
Knudsen effusion cell mass spectrometer (21).
a)
Developing correlations for vapor pressures,
activities, and enthalpies as a function of temperature for the metallic systems.
Tables 6 5 and 7 6 illustrates that the
process of physical vapor deposition can reduce Tc levels in Ni much lower than
the electrolysis method. In addition,
Tc is purified with only 4 ppm Ni. The
electrolysis method produces large amounts of radioactive acidic and basic
wastes, which have to be treated by the Purex method or dumped or stored.
1.
S. Cohen & Associates (SCA). 2001. “Potential Recycling from Nuclear Facilities.” Volume 1. Prepared for U.S. Environmental Protection
Agency, Office of Radiation and Indoor Air, Washington, DC.
2.
R. Skarbo, US Patent 3,853,725, Selective stripping
process, 1974.
3.
M. Muller, B. Witzke, and A. Gottfried, US Patent
4,162,296, Liquid-liquid extraction of nickel, 1979..
4.
H. Fujimoto, N. Miura, and T. Noguchi, US Patent
4,196,076, Separation of cobalt and nickel by solvent extraction, 1980.
5.
T. Snyder, W. Gass, S. Worcester, and L. Ayers, US
Patent 5,156,722, Decontamination of radioactive metals, 1992.
6.
T. Snyder, W. Gass, S. Worcester, and L. Ayers, US
Patent 5,183,541, Decontamination of radioactive metals, 1993.
7.
T. Snyder, W. Gass, G. Boris, and L. Ayers, US Patent
5,217,585, Transition metal decontamination process, 1993.
8.
Snyder; Thomas S., Ayers; Laura
J., Cooney; Chuck A., Boris;
Gregory F., Goad; Dwight F., Robbins; Kevin D., Watkins; Darrell B., US Patent 5,262,019, Decontamination of radioactive
metals, 1993.
9.
T. Snyder and D. Goad,
US Patent 5,439,562, Electrochemical decontamination of radioactive metals by
alkaline processing, 1995.
10.
G. Hradil, US Patent 5,458,745, Method for removal of
technetium from radio-contaminated metal, 1995.
11.
T. Snyder and A. Murray, US Patent 4,792,385,
Electrolytic decontamination apparatus and encapsulation process, 1988.
12.
Remi Rynard, Roger Tindy, and Edmond Daniel, US Patent
3,711,000, Surface apparatus for handling an elastic column, 1973.
13.
P. Ginder, W. Pierce, and R. Waring, P. Ginder, W. Pierce, and
R. Waring, US Patent 1,994, Purifying zinc metal, 1932.
14.
P. Ginder, W. Pierce, and R. Waring, US Patent
1,994,351, Purifying zinc
metal, 1933.
15.
P. Ginder, W. Pierce, and R. Waring, US Patent
1,994,357, Purifying zinc metal, 1933.
16.
R. Lam and D. Marx, US Patent 5,582,630, Ultra high
purity magnesium vacuum distillation purification method, 1996.
17.
K. Tayama and T. Hodozuka, US Patent Application
2003/0145683, Method and apparatus for enhanced purification of high purity
metals, 2003.
18.
W. Weil, British patent 638,904, Improvements in or
relating to the purification of aluminum and magnesium, 1945.
19.
P. Nash,
Ni-Tc phase diagram, Bulletin of Alloy
Phase Diagram, Vol. 6, No. 2, 1985.
20.
A. Nash & P. Nash, Ni-Re phase diagram System, Bulletin of Alloy Phase Diagram, Vol. 6, No. 4, 1985.
21.
A. Banerjee, R. Prasad, V. Venugopal, Gibbs free
energy of formation of calcium rhodite, Thermochimica Acta, 417 (2004) 59-65.
22. Manufacturing
Sciences, DOE/EM-0567, Decontamination and conversion of nickel radioactive
scrap metal, 2001.
Electro-refining using
aqueous acid electrolytes is known to be effective for the removal of actinides
from contaminated Ni; in such a technique the Ni is deposited selectively on a
cathode, with the actinide ions remaining in solution due to their lower
electrochemical reduction potential. Conventional electro-refining is however
ineffective for reducing Tc concentrations in Ni; Tc is found to co-deposit
with Ni at the cathode in a ratio that is the same as, or higher than, that in
which it is found in the electrolyte. To reduce the concentrations of Tc in Ni ingots, new techniques were
implemented. As listed below. (There is
some in the patents in the experiment sections)
1) A technique in which solvent extraction is
combined with electrorefining is described in Snyder et. al.. U.S. Pat. No.
5,156,722. Solvent extraction is used to separate heptavalent Tc from the
electrolyte in which radio-contaminated Ni is dissolved, followed by
electrowinning to recover Ni.
2) The process described in Snyder et. al. U.S. Pat. No.
5,183,541 and U.S. Pat. No. 5,156,722 employs an electro-refining cell that
utilizes a semi-permeable membrane. Tc is chemically precipitated in the anodic
compartment, using a variety of agents to reduce it to its tetravalent state,
and is removed by filtration. A hydrochloric acid-based electrolyte is used
because it is more amenable than sulfuric acid to the chemical
precipitation of Tc.
3) U.S. Pat. No. 5,217,585,
also to Snyder et. al., describes an
electrorefining process in which the Tc-containing Ni is again electrolytically
dissolved in an acid electrolyte. The electrolyte is contacted with activated
carbon to absorb pertechnetate ions, after which the solution is filtered and
transferred to an electrowinning cell, where the Ni is recovered at the cathode.
The contaminated carbon is subsequently incinerated to produce technetium containing
ash, which can be encapsulated for disposal.
4) In U.S. Pat. No. 5,262,019, Snyder et al
address the foregoing(contaminated ash?) problem by providing an
electro-refining process with separate electrolytic dissolution and
electrowinning steps. Contaminated nickel is first electrolytically dissolved
in a sulfuric acid electrolyte, followed by treatment of the filtered
nickel-laden electrolyte with an ion exchange resin to remove pertechnetate and
other ions; the treated electrolyte is then processed in an electrowinning cell
to deposit purified Ni at the cathode.
5) In U.S. Pat. No. 5,439,562, Snyder et al
developed a novel method for decontaminating radio-contaminated nickel
comprising, in an electrorefining cell having a semi-permeable membrane,
cathodically depositing Ni from an alkaline solution containing electrolyte, Ni
ions, and radioactive ions. Preferably, the electrolyte solution is ammonium
sulfate maintained at a pH of least about 10.
6) In U.S. Pat. No.
5,458,745, Hardil et al employs a three-step process to separate Tc from
radio-contaminated metal. The contaminated metal is dissolved in an acid
solution; the Tc, present in the resultant solution as pertechnetate ions, is
quantitatively reduced to its metallic state through a metal displacement
(cementation) reaction with a base metal of lower reduction potential; and the
desired metal is electrolytically recovered from the solution.
All tThese
techniques were not capable of meeting the release criteria for radioactive
materials.
Figure 1 shows the process of
the decontamination of radio(actinide?)-contaminated Ni. The contaminated Ni ingots are fabricated
into electrodes. The electrode is dissolved in sulfuric acid (H2SO4)
to produce a solution containing actinide ions and at least 30 g/L of Ni to
oxidize Tc to produce pertechnetate anions. The pertechnetate anions and actinides
are removed by countercurrent solvent extraction with a barren solution
containing tri-n-octylphosphine oxide (TOPO), and di-2-ethyl hexyl phosphoric
acid (D2EHPA) mixtures dissolved in a long-chained aliphatic
hydrocarbon such as kerosene. The organic-to-aqueous
phase contact ratios for the extraction are between 0.25 and 20 to produce a
decontaminated Ni containing raffinate and a contaminated, loaded solvent
stream. The Tc-rich solvent stream
is diluted by 2-6N HCl and incinerated. The organic-to-aqueous phase contact
ratios for the stripping circuit are 0.10-10. The decontaminated, Ni is passed
through an absorbent (carbon column) to remove residual organic solvents. The
raffinate is electrowon in an electrolysis cell with acidic electrolyte to remove
residual actinides present, and Ni is recovered at the cathode.
The electrolysis cell
operating range preferably includes a current density of 10-300 A/ft 2 with an efficiency of 80-98%, pH = 1-6, and a cell-operating voltage of 2-4
V/cell. The electrolysis cell is preferably operated at a temperature in the
range of 25oC-60oC. The electrolyte additives can include
0-30 g/L free H2SO4, 60 g/L H3BO3 and 20-40 g/L Cl- to improve both the plating rate and the character
of the plated deposit. Suitable examples of Cl- sources which may be
used include NaCl, CaCl2 and NiCl2.
The Ni and the Tc half-cell reactions are given by reactions 1, 2. 3, and 4:
1) Anode Ni - 2e- -> Ni(ii) E = +0.23 V
2) Cathode Ni(II) + 2e- -> Ni(s) E = -0.23 V
3) Tc + 4H2O - 7e- -> TcO4- + 8H+ Eo =
-0.472 V
4) TcO4- + 7e-+
8H+ -> Tc + 4H2O Eo = +0.472 V
Controlling pH, temperature and anolyte oxidation
potential, metallic Ni is won at the cathode. The problem arises due to the fact that Tc will track Ni and co-deposit
at the cathode. Employing a reducing
acid (preferably HCl (aq)) reduces Tc in the feedstock
solution at
the anode.
Equations (5) and (6) potentially describe the
half-cell reactions that allow TcO2 precipitation without influencing
Ni recovery at the cathode. In a highly concentrated Ni solution (particularly
in a chloride electrolyte in which nickel forms no chloride complexes but
remains as bare Ni (II)), at least one possible pertechnate-nickel complex
can be formed with which is positive: [(TcO4)-.XNi+2 ]2x-1.
Not only does this complex provide a positive charge which would be attracted
to the cathode but, if x equals 1 or 2, then it would explain why Tc
concentrates in the cathodic Ni product relative to the Tc contaminated level
in the Ni feedstock. Note also that Tc
cationic complexes can form
as well.
In a strong oxidizing acid, Tc, present either as a pertechnate-Ni ion
complex or a lower valence, positive Tc complex, migrates from anode to cathode
during nickel electrorefining where it is reduced chemically with the cathodic
Ni product, and equations 5 and 6 do not occur anodically.
______________________________________________________________
Anodic Reactions in Cathodic
Reaction in
Reducing
Electrolyte Reducing Electrolyte
______________________________________________________________
(5) Tc - 7e- + 4H2O
+ TcO4- + 8H+ 4e- + 4H+ -> 2H2
(6) TcO4- + 4H+ + 3e- -> TcO2 + 2H2O
______________________________________________________________
The
complete electrochemical formation of TcO2 in solution would force insoluble
TcO2 to the precipitate in the slimes at the anode by equations 5 and 6, but
complete precipitation is unlikely using oxidizing electrolyte conditions
because reactions 5 and 6 are difficult to drive to completion in oxidizing
media. Further, both the heptavalent Tc state and its pertechnetnate
ion are quite stable in oxidizing electrolytes. Therefore, a chemical reduction of Tc must boost the strictly electrochemical behavior to drive
reactions 5 and 6 to
completion.
A reducing acid such as aqueous HCl is substituted by the present invention for
the oxidizing acid to promote the formation of TcO2 by anodic reaction shown in
equations 5 and 6. Moreover, the oxidation potential of the electrolyte must be
controlled to maintain conditions favoring TcO2 formation. Further, increasing
anodic half cell voltages to > 0.8 V provides an overall cell voltage of >1.2
V to enhance this reaction.
Ni
contaminated with radioactive Tc is dissolved in HCl(aq) to produce a solution
that contains pertechnetate ions and Ni ions. Fe+2 and Sn+2 are added continuously to the
aqueous solution (0.05-5N) externally of an electrochemical cell to reduce the
pertechnetate ions to a TcO2 precipitate. The TcO2 precipitate is continuously separated from
the aqueous solution externally of the cell and then the separated aqueous
solution is introduced into the cell for cathodically depositing metal from the
aqueous solution.
The TcO2 precipitate has a residence time of <1 hr in the aqueous
solution. The multivalent metal ion is
added to the aqueous solution at a low valence by applying a voltage between an anode comprised of the multivalent metal and a cathode in an
electrochemical cell. The aqueous solution has a pH < 2.
removed in the course of the electrorefining step.
The complete electrochemical formation of TcO2 in solution would force insoluble TcO2 to the precipitate in the
slimes at the anode but complete precipitation is unlikely using oxidizing
electrolyte conditions because reactions 5 and 6 are difficult to drive to completion
in oxidizing media. Further, both the heptavalent Tc state and its
pertechnetate ion are quite stable in oxidizing the electrolytes. Therefore, a
chemical reduction of Tc must boost the strictly electrochemical behavior to
drive reactions 5 and 6 to completion. Further, increasing anodic half cell voltages to > 0.8 V
provides an overall cell voltage of > 1.2 V to enhance this reaction.
Chemical reductants are added to the anodic chamber to enhance Tc valence
reduction from VII to IV.
Figure 21. shows the process of
decontamination of Ni by the use of HCl as a reducing agent. The
electrochemical cell 10 has an anode 12 in an anode chamber 14 and a cathode 16
in a cathode chamber 18 which are electrically connected by a voltage source
20. The anode 12 is normally comprised of the metal to be recovered at the
cathode 16. The anode chamber 14 and the cathode chamber 18 are separated by a
semipermeable membrane 22 which permits the transfer of the electrolytic
solution from one chamber to the other chamber. Preferably, the solution is
circulated through an external circuit from the anode chamber 14 to the cathode
chamber 18 and then back to the anode chamber 14 through the membrane 22.
Alternatively, the solution may circulate within the cell 10 between the
chambers (not shown). The cell 10 may have a drain line 24 for removing anode
slimes, including TcO2 in some practices, which form in the anode
chamber 14. The cell 10 typically operates at a temperature range of 25-60oC
and at a current density of 10-300 A/ft2 with an efficiency of about
80% or more at a cell voltage of 2-4 V/cell.
The electrochemical cell 10 employs HCl having a pH=1-4.5 where Ni is to be
recovered. The solution may contain 40-105 g/L metal. Boric acid or other plating
agents are added in an amount of 60 g/L to improve the plating rate and the
character of the plating deposit.
Preferably, a reductant is added to an aqueous hydrochloric acid solution in
the case where the contaminated metal is Ni or its alloy. Reductants such as Fe+2,
Cu+2, Sn+2, Ti+2, V+2 or other
multivalent ions may be advantageously added to the solution in the form of
soluble salts such as chlorides, as is indicated by addition arrow 26. Gaseous
reductants alternatively may be added by sparging the gases into the HCl
solution in the anode
chamber 14.
The process can be altered to substantially reduce the codeposition of the
reductant at the cathode, titanium or vanadium ions are added as reductants for
nickel. Advantageously, these multivalent metal ions will form cations having a
low valence state of +2 which reduce the pertechnetate ions and concomitantly
are themselves oxidized to a higher valence state of +3 or +4 in the anode
chamber 14. The precipitated TcO2 generally reports to the anodic
slimes. The cations in the higher valence state are reduced from the high
valence state to the low valence state in the cathode chamber 18 without
cathodically depositing on the cathode 16. Then the reductant may be
recirculated to the anode chamber 14 to repeat the cycle. Also, the reductant
concentration may be closely maintained within a controlled range with little
loss of reductant to the slimes and low volumes of waste may be generated. In
addition, a dimensionably stable electrode may be deposited. In practice,
deposited cathodes may be subject to scaling or flaking where the reductant is
a transition metal which codeposits with the metal to be recovered. Thus the
selection of the candidate reductants (such as ferrous, stannous or cuperous
ions in the
case of nickel) include this
consideration.
Preferably the aqueous solution in the anode chamber 14 is pumped from the
electrochemical cell 10 via a pump 28 in an external line 30 through a strong
base anion exchanger 32 for capturing pertechnetate ions which may not have
been reduced or may have been generated. The polished aqueous solution from the
anion exchanger 32 flows into a holding tank 34 where the activity of the solution
may be continuously analyzed. The solution may then be introduced into the cell
cathode chamber 18 via a pump 36 in a
line 38.
Another adaptation of the process can be made to remove substantially all of
the Tc-containing species from the metal-containing solution in the cathode
chamber 18. The aqueous
solution in the anode chamber 14 containing pertechnetate ions and metal ions
is pumped via a pump 40 in an external line 42 into a pipeline reactor 44 or
other substantially plug flow reactor for closely controlling the concentration
of added technetium reductants and the residence time of the TcO2 precipitate in the metal-containing solution. A reductant such as Fe+2,
Sn+2 or Cu+2 ions in an aqueous solution may be pumped by
a pump 46 from a make-up tank 48 or other suitable source into the reactor 44.
In addition, an aqueous suspension of filter aid may be conveniently added from
a make-up tank 52 by a pump 54 to the precipitate-containing solution in the
reactor 44. The filter aid preferably contains graphite or activated carbon and
also a powdered anion exchange resin so that Tc which re-oxidizes to
the pertechnetate species and goes back into solution may be adsorbed. The
suspension flows from the pipeline reactor 44 into a rotary drum filter 56 or
other suitable (and preferably continuous) separating device for separating the
precipitate and the filter aid from the aqueous solution. The precipitate and
filter aid are discharged as a sludge, as is shown by discharge arrow 58.
Preferably the residence time of the precipitate in the reactor 44 and in the
filter 56 is > 1 hr, and more preferably < 0.5 hr. The metal-containing
solution is then pumped through the anion exchanger 32 to the cathode chamber
18. Data indicates that the activity of the solution of the metal-containing
solution after the anion exchanger 32 will be from about 1% to about 10% of the
activity of the solution before the anion exchanger 32.
Beaker tests have shown that the precipitate begins to
redissolve as complex ions into the aqueous solution shortly after the
precipitate forms. Thus, the anode slimes may be a significant source of Tc
contamination in the case where TcO2 precipitates from the solution
inside the cell anode chamber 14. The beaker tests were conducted on HCl
solutions at a pH=2 and at a temperature of 25oC. The solutions
generally contained 90 g/L Ni and 3000-4000 ppm Tc.
In one series of tests, ten samples of the contaminated solution were each
charged with up to 50 g FeCl2/50 mL solution or 50 g SnCl2/50
mL solution to precipitate TcO2. The samples were not filtered
immediately after precipitation. Several weeks were permitted to lapse between
precipitation and analysis of the activity and of the technetium concentration
of the solutions. The analyses of the samples with initial activities over 4000
Bq/g charged with FeCl2 indicated the following concentrations with
week long residence times in the filtrates:
__________________________________________________________
g FeCl2 g-mole FeCl2 Tc Activity Conc.
Tc
Sample 50ml soln L (Bq/g Fe) (ppb)
__________________________________________________________
1 0.5 0.08 566 908
2 1.25 0.2 591 947
3 2.5 0.4 386 947
4 5.0 0.8 370 620
5 50 8.0 1910 3086
__________________________________________________________
The analyses of similar feed samples charged with SnCl2 indicated
the following concentrations at long residence times in the filtrates:
__________________________________________________________
g SnCl2 g-mole SnCl2 Tc Activity Conc. Tc
Sample 50ml soln L (Bq/g Sn) (ppb)
__________________________________________________________
6 0.5 .053 257 413
7 1.25 0.13 333 535
8 2.5 0.263 434 697
9 5.0 0.525 528 848
10 50. 5.25 837 1347
__________________________________________________________
This series of tests indicates that as the concentration of the reductant is
increased, the Tc
activity
and concentration increases.
In
another series of tests, five samples of contaminated solution were each
charged with 5 grams of ferrous chloride per 50 mL of contaminated solution
(such as Sample 4 above). These samples were held for 0.5-6 hr and then
filtered. The analyses of the samples indicated the following activity and Tc
concentration of the filtrates:
__________________________________________________
Residence Time Activity
Tc Conc. Tc
Sample (hours) (Bq/g Sn) (ppb)
__________________________________________________
11 0.5 10.2 16
12 1 9.2 15
13 2 26.9 43
14 4 20.9 33
15 6 30.3 49
__________________________________________________
A comparison of Samples 11 and 12 with Samples 13-15 indicates that the Tc
concentration of the filtrate was substantially less when the residence time
was less than about one hour. Thus, the TcO2 should be precipitated
and separated from the aqueous solution within a residence time of about one
hour if the redissolution of Tc from the oxide is to be minimized. Preferably,
the addition and separation steps are performed continuously to closely control
the reductant concentration and to minimize the re-dissol-
ution of the Tc.
In another practice of the
process to efficiently reduce the pertechnetate ions as they are anodically
dissolved, multivalent metal ions in a low valence state are added to the
solution in the anodic chamber by applying a voltage between a secondary anode
comprised of the multivalent reductant metal and a cell cathode. The reductant
anode may be located near the contaminated anode so the pertechnetate anions
are reduced before they have a substantial opportunity to form more stable
complex ions, which are not repelled by the cathode and disperse throughout the solution. In
addition, the voltage supplied to the reductant anode may be controlled to minimize the addition of excessive
amounts of reductant to the
solution.
In the demonstration test, Ni
ions and pertechnetate ions were anodically dissolved into an electrolytic
solution 80 provided as a 2 N HCl solution containing 30-60 g/L boric acid. The
Ni feed activity was over 4000 Bq/g. The anodic slimes which formed were
filtered from the solution and their activities (disintegrations/minute) were
analyzed as follows:
______________________________________
Filtrate Filtercake
pH oC DPM DPM
______________________________________
0 25 -- 2200
0 60
-- 2500
2 25 1000 180000
2 60
800 320000
4 25 1000 280000
4 60 500 310000
______________________________________
Thus this practice may be employed to efficiently reduce the pertechnetate ions
to a TcO2 which may be separated to provide a relatively clean
metal-containing filtrate. It is noted that a commercial-type cell having an
anode in an anode chamber and a cathode in a cathode chamber would provide an
even cleaner filtrate.
A beaker test was conducted without the use of added reductants such as
multivalent metal ions, reducing gases and the like to demonstrate the net
behavior difference between a hydrochloric acid solution (a reducing
environment) and a sulfuric acid solution (a mildly oxidizing environment) in
the anodic dissolution of contaminated nickel. Nickel anodes contaminated with
about 0.7 ppm Tc were dissolved in 2 N acid solutions at about room
temperature. The solutions were permitted to sit prior to filtration of the
slimes from the solution and analysis of their activities. The analysis
indicated the following activities:
______________________________________
Filtrate Sludge
Acid DPM DPM
______________________________________
H2SO4 1200 1500
HCl 0 400
______________________________________
Thus, although H2SO4 may be
employed in the decontamination of metals containing Tc, this test demonstrates
that a reducing acid such as HCl (and/or another reductant) will more effectively separate the Tc from the solution and thereby permit the
cathodically
recovered metal to be more
completely decontaminated
Figure
IV.D.31 shows the removal of Tc
from contaminated Ni. A transition
metal such as Ni which may be contaminated with up to about 10 ppm or more of
Tc, U, Th and other transuranic elements is introduced (as indicated by
addition arrow 10) into an anodic dissolution tank 12.
The transition metal in the
dissolution tank 12 is dissolved in an aqueous sulfuric acid solution. The acid solution may be fresh and/or
recycled acid. The diagram shows two recycle streams introduced into the
dissolution tank 12 via a recycle line 14 and a bleed
line 16.
The powder size is in the range of from about 0.1-100 micron and the slurry in
a process line 19 from the dissolution tank 12 comprises from about 0.1-50 g/L
of powder. Assuming a surface area of about 500 m2/g, it will be
seen that the dissolving Tc will be exposed to a large total surface area. Such powders do
not tend to clog and do not tend to interfere with the electrical properties of
the electrolyte solution in the dissolution tank 12.
The powder-containing slurry may be fed to a premix tank 20 where it is mixed
with known filter aids such as diatomaceous earth, powdered anion and/or cation
exchange resins, from a filter aid make-up tank 22 and flocculants from a
flocculent make-up tank 24 (which flocculants may consist of polyelectrolytes,
Al or Zr complexes). The slurry is then fed to a filter 26 or other suitable
device for separating the Tc-containing solids from the aqueous acid solution.
The Tc-containing filter cake may then be washed with water to remove residual acid
and ions. About two bed volumes of water or more will sufficiently wash the cake
in any instances. Preferably, the filter cake is then incinerated in a high
temperature incinerator 28 to produce a technetium-containing ash, which most
preferably is encapsulated in a glass structure by known means for permanent
disposal. U.S. Pat. No. 4,792,385 provides the method of incineration and
encapsulation systems (10). Similarly, the wash water may be neutralized,
evaporated and the evaporator bottoms encapsulated. The total volume
of radioactive wastes produced by this practice is substantially reduced.
The
metal-containing acid filtrate from the filter 26 may optionally be fed to an
ion exchange polishing column 30 (as shown) and then to an electrochemical cell
36 where the dissolved metal may be electrowon. The filtrate from the filter 26 may be
analyzed on-line to verify the decontamination efficiency of the previous
operation free of self-shielding due to the presence of the dissolved metal. If
the activity of the filtrate is too high, then the process can be easily taken
off-stream and corrected before off-specification nickel is plated.
The electrochemical cell 36 may operate at 25-60 oC and at a current
density of 10-300 A/ft2 with an efficiency of about 80% or more at a
cell voltage of 2-4 V/cell. The aqueous acid solution in the electrochemical
cell preferably has a pH of between 1 and 6 and, in the case where nickel is
recovered from a hydrochloric acid solution, preferably between 1 and 4.5. The
solution in the electrochemical cell contains from about 40-105 g/L of
contaminated metal. The solution may also contain up to about 60 g/l of boric
acid or other suitable plating agent to improve the plating rate and the
character of the plating deposit. It is these operating conditions that depart
from traditional radiochemical decontamination conditions and necessitate the
intimate contact of the electrolyte with the adsorption/exchange media.
The
metal-depleted aqueous acid solution from the electrochemical cell 36 may be
recycled to the dissolution tank 12 via process line 14. Preferably, the
oxidation potential of the aqueous solution in the dissolution tank 12 is
controlled by injecting a peroxide into the solution by a suitable means.
Thus, e.g., a water soluble peroxide may be dissolved in metal-depleted acid
and then added via an addition line 40 to the process line 14.
The metal-depleted recycle stream may be employed to introduce graphite or
activated carbon powder into the dissolution tank 12. Thus, e.g., a
metal-depleted solution bleed stream may be diverted via process line 42 from
the process line 14 to an in-line mixer 44, which may be a turbine impeller of
an injection pump, for intimately mixing the solution with the carbon powder.
The powder may be fed from a feed bin 46 by a solids feeder 48 or by other
suitable means to the mixer 44, which may then pump the slurry into the
dissolution tank 12. In a preferred practice, graphite or activated carbon
powder is added in amounts sufficient to produce a slurry in the aqueous
solution from the dissolution tank 12 containing 0.1-50 g/L powder. Preferably, the
powder size is between about 0.1 microns and 100 microns.
The
carbon powder may be introduced into the aqueous solution by other suitable
means. In an alternative practice, low grade graphite anodes may be employed as
an inert anode in either the dissolution tank 12 (in processes where the
contaminated metal is anodically dissolved) or in the electrochemical cell 36.
These anodes will disintegrate over time to provide the required adsorbent. In
another practice, a graphite frame may be employed to hold the contaminated
metal anodes.
In
a full scale pilot plant test of a prototype process, a technetium-contaminated
nickel sample from the Oak Ridge uranium diffusion cascade was dissolved in a
sulfuric acid solution and then slurried with between 0.1 g/L and 50 g/L of a 0.1
micron to 100 micron graphite powder. The slurry was mixed with conventional
filter aids and flocculants in a premix tank and then filtered. The filtrate
was then fed to an electrochemical cell where decontaminated nickel was
electrowon from the sulfate solution. The plated nickel was analyzed at less
than 1 Bq/g (total activity).
In
addition, bench scale tests were performed in which Tc-contaminated
nickel was dissolved in a sulfuric acid solution that was then slurred with less than about 1 g/100 cc
solution of graphite powder. The graphite was filtered from the solution and
the activity of the filtrate and the adsorbent determined to be:
______________________________________
FILTRATE ADSORBENT
SAMPLE NO. ACTIVITY ACTIVITY KD
______________________________________
1 400 76 0.19
2 1600 753 0.47
3 1100 179 0.16
______________________________________
Thus the, practice of the present procedures/method will effectively
decontaminate metals such as nickel so that they may be released to commercial
markets.
Figure IV.D.313. Process flow diagram of Ni Recovery
from Tc-contaminated Ni.
Figure IV.E.1 lays out the
process flow diagram designed for the decontamination of ORNL Ni. The ORNL Ni is contaminated with about 0.85
ppm Tc, < 0.5 ppb Np 237, < 0.005 ppb Pu 239, 0.93 ppm U 235 and 1.74 ppm
total U and has a gross beta activity of about 4000 Bq/gm.
The ORNL nickel is generally available in the form of 24 inch diameter by 18
inch high 2,200 pound ingots, which must be converted to electrodes having a
much higher surface area per unit weight geometry for effective electrolytic
dissolution. Desirably, the electrode preparation involves a minimum amount of
reprocessing, product losses and waste generation. Accordingly, contaminated
feed electrodes may
be prepared by "shot" production or thin sheet rolling from the
contaminated ingots.
Shot production generally
involves the steps of induction melting, screening, free-fall through a shot
tower and collection. Induction melting is preferred over arc melting due to health
physics constraints to control the spread and volatilization of radioactive contaminants.
The particle size should be sufficiently small that the product
"flowers" can be loaded into titanium baskets for electrolytic
dissolution. The chief advantages of a shot process are the generation of high
surface area particles, ease of reloading and minimal rework. In addition,
various known "slags" can be introduced during the melting operation
to extract radiochemicals such as uranium and transuranics.
Planar electrodes may be
produced by rolling thin sheets of about one-quarter inch to about 2 inch thicknesses to
generate high surface areas. Planar electrodes may be preferred to
"flowers" because anodic slimes which tend to form during the
dissolution step can be readily removed from planar anodes in most cases. In
addition, a rolling process is inherently simpler than shot production and
substantially eliminates dust control problems of shot production.
Once the electrode geometry
is established, a suitable number of contaminated nickel electrodes 8 are
loaded into a dissolution tank 10 in a "side-by-side" configuration. Up
to 60 or more electrodes may be employed as shown in the tank 10. Half of the electrodes
may be connected together as anodes and the other electrodes may be connected
together as cathodes. Thus, inert cathodes need not (but could) be employed to
complete an electrical circuit. The side-by-side contaminated electrodes are preferably connected
to the dissolution process power supply through a reversing switch, which alternates
polarity on the electrodes so each electrode spends half of its time as an
anode and half as a cathode. The polarity may be reversed at 5-30 s intervals.
Preferably, the electrode polarity is reversed when the initial current flux
drops by about 10% due to the rise in electrode polarization. Concentration
polarization is eliminated and anodic slimes are removed by the frequent
polarity reversal. A voltage of 2-5 V is applied per cell, and preferably,
about 3 V, to dissolve the transition metal and the radioactive contaminants
with the reduction of protons to H2(g) at the cathodes. Gases and
vapors from the tank 10 may be vented via an overhead hood 12 and piping 14 to
an off gas scrubber (not shown). Also, the acid solution in the dissolution
tank 10 may be re-circulated
by a pump through a carbon filter for removing slimes and particulates from the
acid solution (not shown).
Preferably, nickel is dissolved into an acid solution having a pH=0.5-2. Because the acid solution
is comprised of recycled acid solution from the electrowinning step, which
generally does not economically plate nickel at substantially lower
concentrations, the acid solution will typically contain at least about
60 g/L Ni. Anodic
dissolution and hydrogen reduction are efficiently affected under these
conditions. The
acid solution from the tank 10 contains 70-100 g/L Ni. The acid solution may
have a gross beta activity > 100 Bq/gm. [Sulfuric acid and contains 145-245 g/L
sulfate??].
The acid solution may also contain up to about 30 g/L boric acid, which
functions as a plating agent in the electrowinning step. The temperature of the
acid solution may vary between 35-75oC.
The redox potential of the acid solution in the dissolution tank 10 is maintained at
about 0.3 V so that the Tc (+4) species is oxidized to the Tc (+7)
species. The contaminated acid solution
from the dissolution step is transferred by a pump 20 via a pipe 22 to an
oxidation tank 24 where it is contacted with an oxidizing agent. Ozone is employed as the oxidizing agent,
the acid solution from the oxidization tank 24 contains from 2 ppm to 10 ppm
ozone. In the process shown, some of the acid solution in the oxidization tank
24 is recycled by pump 26 in pipe 28 and a second pipe 30 back to the
dissolution tank 10 for maintaining the acid solution in the dissolution tank
10 at the proper redox potential.
The contaminated acid solution in the oxidization tank 24 may be transferred
downstream via the pump 26 in the pipe 28 and another pipe 32 to a stripping
tank 34 for removing unreacted oxidizing agents such as ozone from the acid
solution by sparging regenerative air or steam supplied by a pipe 36 into the
stripping tank 34. Alternatively, where hydrogen peroxide or the like is employed
as the oxidizing agent, ion exchange "blocking" (not shown) may be
employed in place of gas stripping to remove the unreacted oxidizing agent. Removal of such
unreacted oxidizing agents protects downstream equipment such as ion exchange
resins and the like from oxidation.
The contaminated acid solution in the stripping tank 32 may be transferred via
a pump 40 in a pipe 42 to a pH adjustment tank 44 where boric acid or other
buffer may be added via a pipe 46 to the acid solution for adjusting the
solution pH=2.5-4.5. Analysis of pilot plant operating data (for
decontaminating ORNL nickel) indicates that the gross beta activity of the acid
solution may be as high as about 150 Bq/g at this point in the process.
The contaminated acid solution in the pH adjustment tank 44 may be pumped
downstream by pump 50 in pipe 52 through particulate filter 54 for removing
radioactive particulates from the acid solution thereby reducing the gross beta
activity of the acid solution to < 20 Bq/g. Preferably, the filter 54 is
a hybrid sand-powdered ion exchange resin filter. In a preferred practice,
filter 54 is a downflow filter with a layer of sand over a layer of powdered
ion exchange resins having a fabric mesh extending horizontally through the
resin.
The contaminated acid
solution may be transferred downstream via pump 56 in pipe 58 through one or
more anion exchangers 60, cation exchangers 62 and mixed ion exchangers 64
(which are shown as one exchanger each) to one of two hold tanks 66.
Preferably, sulfate resins are employed at an operating temperature of less than about
60oC. Mixed ion exchangers 64 may be employed to eliminate the effects of any
localized pH variations that may occur in a pure anion or cation column. Both radioactive
anions and cations may be sorbed from the acid solution. Analysis of pilot
plant operating data (for decontaminating ORNL nickel) indicates the gross beta
activity of the solution after the anion exchangers 60 may be as low as 10 Bq/g
and as low as 8 Bq/g after the cation exchangers 62 where the gross beta
activity of the acid solution before the filtration step is as high as 150 Bq/g.
Preferably, at least two anion exchangers 60, cation exchangers 62 and mixed
ion exchangers 64 are employed on-line in series. Each on-line ion exchanger
60, 62, and 64 is monitored to determine radiochemical activity breakthrough.
When the first (i.e., the upstream) ion exchanger bed in tandem breaks through,
it is taken off-line and regenerated while the second bed in tandem is
"moved-up" to the first position and a fresh bed is installed in the
second position.
The radioactivity of the acid
solution in the hold tanks 66 is verified before charging the solution to the
downstream electrowinning step. If the gross beta activity exceeds approximately 20 Bg/L the acid
solution may be recycled via a pump 70 in a pipe 72 and a pipe 74 to upstream
of the anion exchanger 60. Preferably, the recycled acid solution is returned
to the oxidation tank 24 for adjusting its redox potential to assure complete
oxidization of Tc to the Tc (+7) species. If the gross beta activity is less
than about 20 Bq/g, the acid solution may be transferred via pump 70 and pipe
76 to an electrowinning tank 80. Preferably, the acid solution is not charged
to the electrowinning tank 80 unless its gross beta activity is less than about
10 Bq/g. In a preferred practice of the present procedure/method, and after the gross beta
activity of the acid solution in a hold tank 66 has been verified to be below
about 20 Bq/g, the pH of the solution may be buffered with boric acid or other
suitable buffer to between about 4.0 and 4.5 before charging the solution to
the electrowinning step.
The electrowinning tank 80 is operated at ~2-6 V/cell, and preferably at about
3 V/cell to plate nickel product at the cathode while generating oxygen gas at
the anode. The electrowinning tank 80 as shown has three cells with 20
anodes/cell, although other configurations may be employed. Inert anodes must
be employed to maintain clean electrolytes in the electrowinning step. Thus,
commercially available titanium anodes coated with a platinized layer or
iridium oxide layers are preferably employed. Conventional graphite electrodes
are not acceptable as they tend to spill submicron particles into the solution, which may adsorb
technetium complexes and pass through ion exchangers then be adsorbed on the cathode product.
Also, "inert" stainless steel anodes are not employed because they
contaminate the plating bath with chromium, which tends to inhibit plating by
increasing the surface stresses on the cathodic deposits. Analysis of pilot
plant data (for decontaminating ORNL nickel) indicates that Ni plated in
accordance with this practice has beta activities due to Tc from 1.6 Bq/g to
17.0 Bq/g or less.
The off gases from the electrowinning tank 80 may be vented via a hood 82 and
piping 84 to a gas scrubber or other waste treatment facility (not shown). In
addition, the acid solution in the electrowinning tank 80 may be recirculated
by centrifugal pumps through external carbon filters for removing particulates
in the solution (not shown).
The barren acid solution from the electrowinning tank 80, now containing about
60 g/L-75 g/L Ni and up to about 30 g/L boric acid is transferred via pump 86 in pipe 88 to recycle tank 90 for recycling the barren acid solution to
dissolve additional contaminated nickel. The barren acid solution will
inherently have a lower pH than does the acid solution charged to the
electrowinning tank 80 because of the electrowinning step. Preferably, sulfuric
acid or other suitable acid is added via a pipe 92 to the barren acid solution
in the recycle tank 90 in order to further reduce the pH of the barren acid
solution to the pH of the solution in the dissolution step, which is about 0.5
to about 2. In addition, and as shown, the barren acid solution is
transferred via pump 94 in pipe 96 back to the oxidation tank 24 (for adjusting
the redox potential of the recycled acid solution to about 0.3 V) and then to
the dissolution tank 10 via pipe 26.
Accordingly, the technical risks of product contamination are managed by an
effective process screen (comprising a particulate filter for trapping TcO2
colloidal sized and larger particles out of aqueous acid solutions, and an
anion exchanger and a cation exchanger for trapping both positive and negative
pertechnetate complexes) between the dissolution step and the electrowinning
step and recycling of all excessively contaminated solutions to upstream of the
screen. Importantly, the decontamination is verified at the process screen
before the electrowinning step.
Ni metal contaminated with Tc
is dissolved in ammonium sulfate in an electrolytic cell containing a
semi-permeable membrane. The
electrolyte has a pH in the range of 10-13. The solution contains nickel ions and radioactive ions. Ni is selectively recovered from the
solution by cathodically depositing substantially radioactive-free Ni
from the solution in the electrofining cell.
The electrodes of the cell
are driven by a common current and controlled by a single rectifier. The
oxidation potential of the radioactive contaminants in the solution is adjusted
by the addition of ozone.
The electrolyte is maintained
at a temperature of 40-60oC., and the electrochemical cell is
operated at a current density of about 10-300 A/ft2 with an
efficiency of about 80% or more at a cell voltage of about 1-4 V/cell.
Figures 51 & 62 depict a conventional
single electrowinning dissolution system and an entire plant layout,
respectively. In figure 5 the contaminated metal is first dissolved in
anodic dissolution tank 10. The contaminant-containing solution is then
transferred out of the dissolving tank to oxidation tank 12, which is equipped
with a dispersion system and optionally a blower 26, where the oxidation
potential of the radioactive contaminants is adjusted. Next, the
solution is transferred from the oxidation tank to tank 14, which is equipped
with a gas dispersion system and optionally a blower, where oxidants are
removed from solution. Then, the solution is transferred through filter 16
which removes solids, and then through a series of ion exchangers 18, 20 which
remove the radioactive contaminants from solution. Next, the solution is transferred to a series
of holding tanks 22, 24, and subsequently to an electrowinning plating cell
(not shown) where nickel is cathodically plated.
Figure 62 is a schematic
representation of a full scale plant layout utilizing the dissolution system of
Figure 51. A series of dissolving
tanks 100 (as described above) are serially connected to each other
electrically (heavy lines) and via electrolyte piping (thin lines). The
dissolving tanks are also connected to holding tanks 120,122 and electrolyte
return tanks 130. The dissolving tanks are driven by power supplies 124. Upon
dissolution, the solution is transferred to a series of electrowinning plating
tanks 126, which are serially connected to power supplies 128, for cathodic
deposition of nickel. Thus, while such electrowinning methods are generally
effective, but they
require the use of separate, multiple tanks and power supplies, with a
concomitant high cost in capital and operating expense.
The methods of recovering nickel in accordance with the procedures/method are illustrated in the accompanying drawings. Referring to figure 73, the electrorefining
electrochemical cell 202 has an anode 200 in an anode chamber 201 and a cathode
204 in a cathode chamber 205 which may be electrically connected by a voltage
source 208 and controlled by a single rectifier (not shown). The anode chamber
201 and the cathode chamber 205 are separated by a semi-permeable membrane 206
which permits the transfer of the electrolytic solution from one chamber to the
other, and completes the electrical circuit. The semi-permeable membrane may be
comprised of any material which permits ionic flow, but prohibits bulk or
osmotic flow, between anode chamber 201 and cathode chamber 205. Preferred
materials include polymers such as polysulfone, sintered glass frit, and finely
powdered ceramic particles compressed between porous membranes. The cell 202
may have a drain line 214 for removing anodic slimes, including technetium
oxide, which form in the anode chamber. The cell may also be equipped with a
blower 212 for agitation and/or aeration of the solution.
The anode 200 is normally comprised of the metal to be recovered at the cathode
204. The nickel to be decontaminated is generally available in the form of 24
inch diameter by 18 inch high 2,200 pound ingots, which must be converted to
anodes having a much higher surface area per unit weight geometry for effective
electrolytic dissolution. Preferably, the electrode preparation involves a
minimum amount of reprocessing, product losses and waste generation.
Accordingly, contaminated anodes are preferably prepared by "shot"
production or thin sheet rolling from contaminated ingots. Methods for
preparing such electrode are described in U.S. Pat. No. 5,262,019.
Alternatively, the contaminated anode may be in powder or bar form.
The
cathode employed to evaluate the methods may be comprised of one or more materials selected from the group
consisting of titanium, platinum, stainless steel, graphite and nickel. Ni is
preferred.
The
electrochemical cell typically operates at 40-60oC, and at a current
density of about 10-300 A/ft2 with an efficiency of about 80% or
more at a cell voltage of 1-3 V/cell.
The electrochemical cell 202 may employ any suitable aqueous solution having a
pH above 7 as an electrolytic solution, including for example, ammonium
sulfate, ammonium chloride, ammonium hydroxide, ammonium carbonate, with
ammonium sulfate being preferred. Initially, the contaminated anode may be
dissolved in an acidic solution of sulfuric or hydrochloric acid, having a
pH=1-3, and a temperature of 30-75oC. The amount of anion in
solution is not critical, as long as there is a sufficient amount to support
nickel dissolution. Preferably, the electrolyte contains from 100-250 g/L SO42-,
and preferably 145-245 g/L sulfate, added as H2SO4. Next,
a balancing cation may be added to the solution in an amount sufficient to
increase the pH of the electrolyte to the desired alkaline range. Preferably,
the pH of the electrolyte is adjusted by adding at least one cation selected
from the group consisting of ammonium, sodium, calcium with ammonium being
preferred. For example, the pH of the electrolyte may be adjusted by adding
sodium hydroxide, calcium hydroxide, and/or by sparging with NH3 Alternatively, the contaminated anode may be placed directly in an alkaline
solution having the appropriate pH.
The
alkaline electrolyte may contain 30-280 g/L Ni. Preferably, the alkaline
electrolyte contains 60-130 g/L, and preferably 70-100 g/L Ni. The alkaline
electrolyte may also contain up to ~30 g/L boric acid, which functions to
improve the plating rate and the character of the plating deposition.
Other
electrolytes, such as fluoroboric acid, fluorosilicic acid, hydrochloric acid,
nitric acid and the like, and other suitable process conditions may be employed
in other applications of the this
method, as long as such materials and process conditions support nickel
plating, are compatible with the cellular construction materials, and do not
substantially increase the amount of secondary waste generated. The alkaline
electrolyte may have a gross beta activity of 2000-6000 Bq/gm prior to treatment.
In the preferred application of the method depicted in figure 37, the solution is
circulated within cell 202 between chambers 201 and 205. In evaluating this
particular method, the alkaline solution in the cathode chamber 205 may be
maintained at a slighter higher pressure (1-1.2 atm), to promote flow/transfer
of the alkaline solution from the cathode chamber to the anode chamber to
dissolve additional contaminated nickel. The alkaline solution in
electrochemical cell 202 also may be recirculated by pumps 210 which may be
equipped with carbon filters (not shown) for removing organics and other
particulates in the solution. In addition, a blower device 212 may be utilized
for cell agitation and aeration.
Unprecipitated
contaminants remain in solution, accumulating in the anolyte, which is periodically removed and treated.
Any precipitated contaminant, such as TcO2, generally reports to the
anodic slimes, which may be periodically removed from the cell, treated and buried. For
example, re-circulating carbon beds may be placed in both the anolyte and
catholyte chambers to receive hydrocarbon and radioactive contaminants. Thus, there is no
requirement for any separate processing step using ion exchange means or
solvent extraction to remove the radioactive contaminants, and any process step
to separate the radioactive contaminants from the Ni-containing solution
external to the electrochemical cell is optional. As a result, low volumes of
waste are generated.
In
another modification of this
application substantially radioactive-free nickel is electrorefined at the
cathode by controlling pH and adjusting the oxidation potential of the
radioactive contaminants in a separate step, which occurs externally of the
electrochemical cell. This approach is particularly effective in the removal
of trace amounts of radioactive-contaminants from the nickel-containing
solution prior to the deposition of nickel in the cathode chamber. In this alternative
of the method which is shown in figure 7, the solution may be circulated
through an external circuit from the anode chamber 201 to the cathode chamber
205 and then back to the anode chamber 201 through the membrane 206. The
composition of the electrolyte solution and operating conditions of the
electrolytic cell generally are as described above.
The
contaminate-containing solution in the anode chamber 201 is pumped from the
electrochemical cell 202 (via pump 216) in an external line 218 into a vessel
220 where the oxidation/reduction potential of the radioactive contaminants in
solution may be adjusted. Preferably, the contaminate-containing solution is
contacted with an oxidizing agent that oxidizes the contaminants to a valence
state where the contaminants may be readily precipitated and removed from
solution in later processing steps. Preferably, the radiocontaminant-containing
solution is contacted with an oxidizing agent selected from the group
consisting of hydrogen peroxide, ozone, oxygen gas, nitric acid-nitrous acid
mixtures, and permanganate. Preferably, oxidizing agents such as peroxide and
ozone are used to oxidize any Tc species in the alkaline solution to Tc (+7).
Depending upon the oxidizing agent, any unreacted oxidizing agent remaining in
solution after the oxidation step, may be removed to protect downstream filters
and ion exchangers from possible oxidation. In those practices where ozone is
employed as an oxidizing agent, air or steam is preferably employed to strip
the ozone from the alkaline solution following the oxidation step. In practices
where hydrogen peroxide is employed as the oxidizing agent, ion exchange
"blocking" may be used to remove residual impurities.
Following
the oxidation adjustment (or redox) step, the nickel-containing solution may be
pumped (via pump 221) through a contaminant polishing means 222, which may
consist of, for example, a filtering means and/or an ion exchange means, for
removing contaminants from the solution. For example, the nickel-containing
alkaline solution may be filtered through a particulate filter to separate particulates
from the alkaline solution. The particles may be colloidal sized particles
stemming from the contaminant metal itself (such as TcO2) or
processing contaminants such as resin fragments or the like. Preferably, the
filter is a hybrid sand-powdered ion exchange resin filter. Alternatively, or
in addition to the filtration step, the alkaline solution may flow through one
or more ion exchangers having an anionic or mixed resin bed, for sorbing
cationic technetium complexes and other radioactive/transition/actinide metal
ions. Preferably, resins are employed at an operating temperature > 60oC.
Mixed ion exchangers may be employed to eliminate the effects of any localized
pH variations that may occur in a pure anion column and which may be
deleterious to the reaction.
Preferably,
at least two ion exchangers are employed on-line in series. Each on-line ion
exchanger is monitored to determine radiochemical activity breakthrough. When
the first (i.e., the upstream) ion exchanger bed in tandem breaks through, it
is taken off-line and regenerated while the second bed in tandem is
"moved-up" to the first position and a fresh bed is installed in the
second position.
If
desired, after the contaminant polishing step and before the alkaline solution
is reintroduced into the cathode chamber, the solution may be pumped to one or
more holding tanks 224, where the contamination level/activity of the alkaline
solution may be monitored so that an excessively contaminated alkaline solution
is not fed into the cathode chamber. If the contaminant level or radioactivity
of the alkaline solution is too high, say > 40 ppb (75 Bq/g), the alkaline
solution may be recycled via a pump 228 in an external line 226 to upstream of
the redox vessel. The recycled solution is returned to the vessel 220 for
adjusting its oxidation potential to assure, for example, complete oxidation of
Tc to the Tc +7 species. The oxidation/reduction and contaminant polishing
steps may be repeated until the desired level of contaminant has been removed.
When the contaminant/activity level of the alkaline solution is considered
acceptable, the nickel-containing alkaline solution may be reintroduced to the
cathode chamber of the electrorefining electrochemical cell.
A solution containing about 90 g/L of Ni and 5.4 mg/L of U was plated under the
operating conditions set forth in Table 1. Results are set forth in Table 2.
During Experiments 1 and 2, which were conducted in an acidic electrolyte, the
seed cathode surfaces were passivated by oxidation when the cathodes were
removed from the bath to establish the weight gain, rendering the cathodes
substantially inoperable for additional runs. Electrode passivation did not
occur during Experiments 3 and 4 which were conducted in an alkaline
electrolyte. As the data for Experiments 3 and 4 indicates, over 99% of the Ni
was recovered by cathodic deposition in an alkaline electrolyte. The purity of
the recovered Ni was not analyzed.
TABLE 1
______________________________________
TEST CONDITIONS
NUMBER ANODE CATHODE pH
______________________________________
1 GRAPHITE STAINLESS ACIDIC
2 INERT NICKEL ACIDIC
3 GRAPHITE STAINLESS BASIC
4 INERT NICKEL BASIC
______________________________________
TABLE 2
______________________________________
RESULTS
BASIC TESTING - EXPERIMENTS 3 AND 4
U Ni EFF. NITRATE
mG/L mG/L % G/L
______________________________________
TEST 3 5.4 24 99.5 157
TEST 4 5.4 12.5 99.8 194
______________________________________
To
best understand the electrolytic separation of Ni and Tc, the
oxidation/reduction potentials of those elements should be considered. In an
acidic solution, Tc exists predominantly in a heptavalent form, as
pertechnetate ions (TcO4-), and Ni exists as Ni2+.
The reduction of these ions to the metallic state is governed by the following
half reactions:
TcO4- + 8H+ +7e- ó Tc + 4H2O Eo= 0.477V
Ni2+ +2e- ó Ni Eo.=
-0.250V
The
magnitude of the standard potential indicates the driving force of the reaction
to proceed from left to right. Therefore, the pertechnetate ions are more
readily reduced to the metallic state than are the nickel ions. Additionally,
the standard potentials indicate that metallic Ni will act as a strong reducing
agent with respect to pertechnetate ions. Combining the two foregoing
half-reaction equations yields the following reaction:
TcO4- + 16H+ + 7Ni ó 7Ni2++ 2Tc + 8H2O D Eo =0.727 V
The equilibrium constant for the foregoing reaction can be determined from the
Nernst equation:
D E = D Eo -(0.059/n) log
[Ni2+ ]7 /[ H+]16 [TcO4-]2
wherein n is the number of electrons (14 in this case ) transferred. At
equilibrium, de=0, and the equilibrium constant can be expressed as:
k = [Ni2+ ]7 /[ H+]16 [TcO4-]2
D Eo = (0.059/n) log k
log k = n D Eo/0.059
k = 10(n D Eo/0.059)
k = 10(14*0.727/0.059) =3.22*10172
[TcO4- ]=([Ni2+ ]7 /3.22 x 10172 [H+ ]16)1/2
In a typical sulfate-based electrolyte, the Ni concentration would be about 1 M
and the pH would be about 4 ([H+ ] = 0.0001 M). This yields a
pertechnetate concentration of 5.5x10-55 M. Thus, for all practical
purposes the displacement reaction can be considered to go to completion, such
that the removal of technetium is quantitative.
The level of Tc contamination in feedstock nickel is typically 0.3 ppm, which
is approximately 1 g Tc for every 3300 kg of feedstock Ni. In the displacement
reaction, 2 moles Tc are reduced for every 7 moles Ni oxidized; to reduce 1 g
Tc, therefore, 2 g of Ni would be dissolved.
Since
the displacement reaction tends to encapsulate the reducing metal, it is
beneficial to use a powder, or other high surface area medium, to maximize the
surface area and, in turn, Tc loading on the metal. In metal-displacement
reactions, the metal ion subjected to reduction forms a metallic layer
approximately 0.25 micron thick before the reaction ceases, due to
encapsulation of the base metal. Powdered nickel is widely available in a range
of particle sizes, with 5 microns being typical. Assuming a spherical geometry,
this provides, as a conservative estimate, a surface area of 1348 cm2 /g. A 0.25 micron coating of Tc deposited over the calculated surface area
translates to approximately 0.4 g of Tc reduced per gram of powdered Ni. Since
approximately 2 g Ni are oxidized to reduce 1 g Tc, this indicates that the
nickel will be almost completely displaced by Tc.
Distilled water, with a pH of 3 and having an initial activity of 3.9x103 Bq/ml due to the presence of Tc-99 as ammonium pertechnetate, is contacted with
5 g/l of activated nickel powder. The resultant solution, at 25oC,
is agitated for 20 minutes to allow sufficient solid-liquid contact for the
heterogeneous displacement reaction to proceed. After an additional period of
20 minutes, the solution is settled and the clear solution is decanted. It is
found to have an activity of 16 Bq/ml, representing a technetium removal of 99.2%.
Allowing the reaction to proceed for a full hour produces an activity level of
7 Bq/ml, representing 99.8% removal.
An
acid solution (pH = 2), containing 5.25 g/l of Ni, as NiSO4, and
having an initial activity of 0.935x103 Bq/ml due to the presence of
Tc-99 as ammonium pertechnetate ions, is contacted with 5 g/l of activated Ni
powder. The resultant solution, at 25oC, is agitated for 20 minutes
to allow sufficient solid-liquid contacting for the heterogeneous displacement
reaction to proceed. After an additional period of 20 minutes, the solution is
settled and the clear solution is decanted. It is found to have an activity of
5.1 Bq/ml,
Figure
IV.G.81 shows a single cell,
generally designated by the numeral 1, suitable to use in carrying out an
electrorefining process embodying the present invention. Although the
decontamination of radio-contaminated nickel is specifically discussed, it will
be appreciated that the system illustrated is suitable for carrying out a wide
range of decontamination reactions, within the scope of the instant invention.
The
depicted cell 1 is divided into cathodic and anodic compartments 3 and 2,
respectively, by a semi-permeable membrane 6, which may consist of a chemically
impervious cloth. The radio-contaminated metal (e.g., Ni) is employed as the
anode 4, which is electrolytically dissolved in a sulfuric acid-based
electrolyte contained in the anodic compartment 2. The electrolyte for nickel
decontamination will typically comprise 50-100 g/L of Ni ion, 65-120 g/L of
sulfate radical, an effective amount (generally up to 40 g/L) of boric acid as
a plating agent, and optionally up to 50 g/L of chloride ion. The pH of the
electrolyte will normally be maintained between 1 and 4; a pH value of about
1.5 will generally be optimal in the absence of chloride in the electrolyte,
and a pH of 3.0 will generally be optimal if chloride ion is present in
significant concentrations. The cell will normally be operated at a solution
temperature maintained between 20oC, and 80oC, with 60oC
often producing the best results.
Anolyte
is transferred from the anodic compartment 2 by way of line 8 and pump 9,
through a filter 11 to remove particulates, and then through a bed 13 of Ni
powder, where the pertechnetate ions are reduced to the metallic state. The
solution then passes through a second filter 16 to remove any suspended matter,
which may include nickel powder carried over from the bed 13. A fraction of the
treated solution is returned to the anodic compartment 2 through line 17, with
the balance flowing through line 14 to the cathodic compartment 3. In this
manner Tc is removed from the anolyte solution on a continuous basis.
The portion of the anolyte solution diverted to the cathodic compartment 3
through line 14 serves to maintain the desired nickel concentration therein,
while also maintaining the solution level above the level in the anodic
compartment 2. This forces the electrolyte to flow from the cathodic compartment
3 to the anodic compartment 2 through the semi-permeable membrane 6, due to the
resultant hydrostatic pressure differential. Because the anolyte diverted to
the cathodic compartment has been subjected to the metal displacement reaction
in bed 13 and because hydrostatic pressure prevents flow from the anodic
chamber 2 to the cathodic chamber 3, the technetium concentration in the
catolyte will be maintained at a very low level (e.g., below 10 Bq/ml). The
flow of treated anolyte is so proportioned as to maintain the Ni concentration
in the cathodic compartment 3 sufficiently high for effective nickel deposition
on the cathode 5, which will desirably be of seed nickel or stainless steel
construction. Ni deposited from the catholyte will normally have an activity
below 17 Bq/g, and U and other actinides will not codeposit due to their low
reduction potentials; rather they will accumulate in the electrolyte. Drainage
for maintenance and cleaning of the cell may be affected through line 15.
The
cell is operated under steady or pulsating direct current, delivered to the
electrodes 4 and 5 from the power supply 7, usually at a level of 2 to 6, and
preferably 3, volts. Current density will normally be maintained at 50-250 A/ft2.
The
system will usually be so designed that the liquid will be subjected to intimate
contact with the treating metal for a period of about 10-30 minutes, so as to
allow the displacement reaction to approach equilibrium concentrations.
Initially, it may be necessary or desirable to activate the metal surface by
acid flushing, such as with concentrated sulfuric acid or sulfurous acid, as
taught in U.S. Pat. No. 3,117,000 (12). Particles of any powder employed will
generally have a diameter of 2 microns or larger; it is believed however that 5
micron particle will to afford almost complete utilization of the base metal
for the displacement reaction, while at the same time minimizing the
difficulties that would be encountered in the handling of ultra-fine powders.
