In China more than two-thirds of available coal reserves are in arid areas, where, to beneficiate the run-of-mine coal, there is not enough water resource required by conventional processing. Developing efficient dry beneficiation technology is of great significance for efficient coal utilization in China, notably the clean coal technology (CCT). The dry coal benefi-ciation technology with air-dense medium fluidized bed utilizes air-solid suspension as beneficiating medium whose density is consistent for beneficiation, similar in principle to the wet dense medium beneficiation using liquid-solid suspension as separating medium. The heavy portion in feedstock whose density is higher than the density of the fluidized bed will sink, whereas the lighter portion will float, thus stratifying the feed materials according to their density.
In order to obtain efficient dry separation in air-dense medium fluidized bed, stable fluidization with well dispersed micro-bubbles must be achieved to ensure low viscosity and high fluidity. The pure buoyancy of beneficiation materials plays a main role in fluidized bed, and the displaced distribution effect should be restrained. The displaced distribution effects include viscosity displaced distribution effect and movement displaced distribution effect. The former is caused by viscosity of the fluidized bed. It decreases with increasing air flow velocity. Movement displaced distribution effect will be large when air flow rate is too low or too high. If medium particle size distribution and air flow are well controlled, both displaced distribution effects could be controlled effectively. A beneficiation displaced distribution model may be used to optimize beneficiation of feedstock with a wide particle size distribution and multiple components in the fluidized bed. The rheological characteristics of fluidized beds were studied using the falling sphere method. Experimental results indicated that the fluidized bed behaves as a Bingham fluid. The plastic viscosity and yield stress can be obtained by measurement of the terminal settling velocity of spheres and linear regression of the experimental data. Both plastic viscosity and yield stress increase with increasing size of the fluidized particles. The drag coefficient can be calculated with favorable agreement with experimental data.
The first dry coal beneficiation plant with air-dense medium fluidized bed was established by CUMT with an output of 320000 t·a-1 and a probable error Ep value up to 0.05 was achieved. The plant was accepted by the Chinese government in June, 1994. Since then, new applications have been found including a 700000 t·a-1 dry coal beneficiation plant put up for commercial testing. To realize coal dry beneficiation of full size range of 300~0 mm, further research on dry coal ben-eficiation of different size fractions has been under way at the Mineral Processing Research Center of CUMT, leading to the following results:
• Dry beneficiation technology with a vibrated air-dense medium fluidized bed for fine coal of size fraction 6~0.5 mm
Ash content was reduced from 16.57% to 8.35%, with yield up to 80.20% and Ep value up to 0.065.
• Coal dry beneficiation technology with a deep air-dense medium fluidized bed for > 50 mm coal
An Ep value up to 0.02 was achieved. This technology is of great value for waste removal from 300~50 mm large feedstock, especially for big surface mines in China.
• Coal triboelectric cleaning technology for < 1 mm pulverized Coal
Coal is comminuted down to 320 mesh (0.043 mm) to fully liberate the embedded minerals, yielding an ultra-low ash coal (less than 2%). Currently a pilot system with triboelectric cleaning has successfully passed technical appraisal.
• Three-product beneficiation technology with dual-density fluidized bed
This technology yields three products: clean coal, middling and tailing, with the following typical results: Ep value of 0.06~0.09 for the upper layer with a density of 1.5~1.54 g·cm-3 and Ep value of 0.09~0.11 for the lower layer with density of 1.84~1.9 g·cm-3.
