A New Method For High-level Upgrading Of High-ash Lignite And Its Mechanism

Based on the conventional hydrothermal treatment(HT)and alkali digestion/acid leaching demineralization method(DD method),this paper has proposed a new method for stepwise upgrading of high-ash lignite.The method is first to deoxygenate lignite by HT and then to demineralize lignite by successive Ca(OH)2 digestion/acid leaching(abbreviated as HT-D method).The effects and mechanism of this method on lignite upgrading were analysed in detail.The main contents and results of this research are outlined as follows.The Yuxi high-ash lignite mined from Yunnan province was upgraded by HT,DD method,and HT-D method,respectively.The effects of three methods on the lignite upgrading were evaluated from the proximate and ultimate analyses,and the changes in the high heat value and contents of acidic functional groups.Experimental results showed that,three methods could effectively remove oxygen and acid functional groups from lignite.However,only the HT-D method could,to a larger extent,remove mineral matter from lignite.After an optimal HT-D treatment at both HT and digestion temperatures of 330 ℃,the O/C molar ratio,the total acid content and the ash content were reduced from 0.30 to 0.09,from 3.98 mmol/g to 1.01 mmol/g,and from 24.1%to 3.5%(all dry basis),respectively.The higher heat value(HHV)increased from 17.4 MJ/kg to 29.7 MJ/kg.The energy recovery rate reached 75～80%via this treatment.The mechanism study revealed that,Yuxi lignite is enriched with quartz and clays.The lignite was also abuandant with acidic functional groups,which had a negative effect on the interactions of Ca(OH)2 with the mineral matter in the lignite.This led to a low removal of mineral matter by the DD method.In contrast,the HT-D method could significantly enhance the removal of mineral matter,because most acidic functional groups were removed during the HT and this allowed clays and quartz to be alkalized sufficiently by Ca(OH)2 forming acid-soluble aluminoslicate salts.The contents of 12 trace elements in the raw lignite and treated lignites were determined,and the results were explained according to the modes of occurrence of different trace elements.Experimental results showed that,the HT could noticeably remove As,probably because most As in the lignite was soluble in a weaker acid.The DD method could remove part or most of Be,V,Mn,Sr,Cd and Pb.This was because these elements were partly or mainly present in the lignite in the forms of sulfide,carbonate or carboxylate,which were leached by the acid washing.The HT-D method exhibited higher removals of all trace elements than the DD method,especially for Ga and Li.This was attributed to the predominant presence of these two elements in the lignite as aluminosilicates.