Integrated process for coal chemical demineralization and spent caustic regeneration- A pilot scale study

Indian coal washing plants face tremendous pressure in maintaining clean coal yield and handling the rejects. Conventional coal washing plants generate clean coal with 15-18% ash content and 35-50% yield by several physical beneficiation techniques. Handling and disposal of the rest of 50% reject is an environmental concern and hence, alternate processes for utilization of high-ash washery rejects are of high importance. In this work, the chemical demineralization process is explored at a pilot scale to produce the low ash clean coal from different washery rejects. 44.6%-67.8% demineralization is achieved with the single-stage alkali-acid leaching process and 68.4%-78.3% by multi-stage leaching at optimum operating conditions. However, the economics of the process depends on the regeneration of the spent alkali solution. Therefore, a two-step methodology for spent-alkali regeneration is developed. In the first step, carbonation of spent-alkali with CO2 separates almost >95% of the silica and alumina impurities. In the second step, 89.2% caustic soda is recovered by causticization using quick lime.