Float-Sink (Washability) Test
Float-sink tests are done on samples of coal from beds that will likely need washing (beneficiation, preparation) to remove sulfur or ash (rock and mineral matter) from the coal to meet the desired specifications of the end user. The float-sink analysis determines how much coal can be separated from rock and minerals in fluids of different densities.
Methods for float-sink analysis are outlined in ASTM method D4371-06 (American Society for Testing and Materials, 2013, p. 566–573). In a standard float-sink analysis, set amounts of the representative coal sample are crushed to a specified particle size and placed in liquids of known density or across a range of densities. Coal has relatively low density or specific gravity (1.2 to 1.5) compared to other rocks such as shale (2.4 to 2.8) or minerals such as pyrite (4.9 to 5.2). Hence, float-sink analyses are usually run in liquids with densities between 1.3 and 2.0. Because coal is less dense than most rocks and minerals (such as pyrite), it floats in liquids of equal or greater density than the coal; rock and pyrite sink. The amount of material floating is skimmed from the top of the test container, dried, and weighed as the float fraction at that liquid’s density. The density and ash content of the float fraction are also measured. The material that sinks is also dried and weighed, and its density and ash content is measured. This is the sink fraction.
If a coal sample will be sent to a preparation plant that uses a fixed density for separating coal from rock and minerals, then a single float-sink test may be done on a sample at the fluid density used in the plant. The single float-sink test defines the relative amount of float-product and sink-reject that should result from the coal being washed in that preparation plant.
The second type of float-sink analysis tests a coal sample in a succession of density baths to determine the optimal density for separation of the coal from rocks and minerals. This type of analysis is used when the density of separating fluids can be adjusted in a preparation plant or when different plants (with different density requirements) could be used to wash a coal. The crushed coal sample is placed in the lowest-density liquid bath. Then the sink fraction is immersed in a slightly higher-density liquid. The analysis is repeated in a series of increasingly dense liquids. At each step, the float and sink fractions are dried and weighed, and the density and ash content are measured.
The results of a succession of float-sink tests at different fluid densities are generally shown graphically on a chart in which sample run density and ash content (percent) is on the X axis and float yield (percent) and sink yield (percent) is on the Y axis. Results are plotted as washability curves. Usually, a series of curves, based on different equations, is plotted:
Cumulative float curve = (ash % of cumulative float) / (weight % of cumulative float)
Cumulative sink curve = (ash % of cumulative sink) / (weight % of cumulative sink)
Density (densimeteric, specific gravity) curve = (Weight % of cumulative float / (midpoint relative density)
Elemental ash curve = (weight % of cumulative float) / (ash % of fractional sample)
Changes in the slope of the different curves show the relative amount of change in results for each density fraction. The results can be used to calculate the optimal density of fluid needed for economic separation of coal from rock and minerals and the amount of mined coal that could be washed to specific parameters at that density in a preparation plant. The curves can also be used to calculate the amount of rejects that would have to be disposed of at specific densities.
In some circumstances, washability data need to be obtained for different particle sizes of coal to determine if more pyrite or ash can be economically separated from coal in preparation plants that have multiple particle-size circuits. In these cases, washability data may be gathered for coarse, intermediate, and fine coal particles. In general, separating coal from intergrown minerals is increasingly difficult with decreasing particle size and concentration (Stach and others, 1982).
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