Chemical demineralization of coal involves the selective removal of inorganic ash-bearing minerals from around 40% to about 10%. This is carried out by alkali leaching of coal using NaOH at temperatures of 150 – 200 oC. The existing process flow is explained in Figure 1 below. During this process, NaOH reacts with alumina and silica in coal (of size less than 0.5mm) and forms soluble sodium silicate (Na2SiO3) & sodium aluminate (NaAlO2). At high temperature and concentration, sodium silicate and sodium aluminate combine to form an insoluble precipitate of sodium alumino-silicates (Sodalite), which is deposited on the surface of coal. Thus, the ash reduction is less in the first stage of leaching.
Since alkali leaching on its own is unable to remove the ash from the coal, we need to go for a subsequent acid leaching stage, to dissolve the sodalite. In this stage, the alkali-leached coal is treated with 7.5% (w/w) hydrochloric acid under ambient conditions, which results in the formation of sodium chloride, silicon tetrachloride, and aluminum chloride. Thus, the insoluble sodalite dissociates into the acid and, subsequently, reduces ash content in the coal. The typical quantities needed for these substances is shown in table 1 below.
The cost of recovering sodium in the form of NaOH (from NaCl present in the spent acid solution) is very expensive. Owing to the cost associated with the higher requirements of chemicals, and the energy-intensive process of regenerating the reagents from the spent solution by dewatering, chemical beneficiation methods have not been employed widely on a commercial scale. Economic regeneration of spent alkali and acids can reduce the cost of chemicals in the cleaning method.
Thus, for economic reasons, a two-stage leaching process is not feasible. Hence, we are looking for solutions that will enable us to achieve desired chemical leaching of the coal in the alkali leaching stage itself and facilitate the recovery of spent alkali solution, without the need of subsequent acid leaching. This can be attained if we can achieve either of the following:
We are not interested in recovering sodium after acid leaching stage.
Figure 1: Simplified process flow
Table 1: Chemical consumption for the treatment of 1 ton of raw coal with 40 % ash content
|NaOH||80 Kg||20 % (w/w)|
|HCl||132 Kg||7.5 % (w/w)|