The production of ultra clean coal by chemical demineralisation

A high-volatile UK coal, with a particle size of <500 μm, an ash content of approximately 7.9% by weight and a sulphur content of 2.6% by weight, was treated with aqueous HF followed by aqueous HNO3. The reaction residence time and temperature for both treatments were 3 h and 65°C, respectively. HF reduces the ash content to approximately 2.6% by weight. The remaining ash largely consists of fluoride compounds such as AlF3, NaAlF4, CaF2 and MgF2, which form during leaching, and pyrite (FeS2), which does not react with HF. HNO3 then further reduces the ash content to approximately 0.6% by weight, by dissolving fluoride compounds and the Fe present as FeS2. The remaining ash consists largely of unreacted FeS2, which is encapsulated in the coal structure. This investigation also showed that HNO3 only reacts with FeS2 above a particular HNO3 concentration, which suggests that it is consumed preferentially, and to a certain extent, with the organic coal structure. The final sulphur content following treatment with HF and HNO3 was 1.4% by weight.