The production of ultra clean coal by sequential leaching with HF followed by HNO 3

Abstract

A UK bituminous coal with particle size <62 μm and containing 5.0 wt% ash and 2.4 wt% S was treated with a two-stage leaching sequence of aqueous HF followed by aqueous HNO 3. The ash and S contents reduced to 0.2 wt% and 1.3 wt%, respectively. In addition, the calorific value (CV) dropped from 31.5 to 29.5 MJ/kg, and the N content increased from 2.0 to 2.8 wt%, due to attack on the carbonaceous matrix during the HNO 3 leach. Interestingly, this attack only occurs when HNO 3 reacts with and dissolves pyrite, suggesting that it is the products of reaction between HNO 3 and pyrite which react with the coal. It is proposed here that localised concentrations of sulphuric and nitric acids at the pyrite site may react to form the powerful nitrating agent NO 2 + which then reacts with the carbonaceous coal matrix. The fluoride content of the coal increases from 80 to 3240 ppm after the HF leach, yet drops to 130 ppm following the HNO 3 leach. The mineral matter remaining in the coal after the leaching sequence consists largely of Fe, which is most likely present as finely disseminated unreacted pyrite.