Effect Of Sodium Containing Additives On The Activation Of Coal Fly Ash For Alumina Extraction

Coal fly ash,produced from coal-fired power plants,is one of the largest industrial solid wastes in China.The large number of coal fly ash production and emission would inevitably result in serious environmental pollution.Hence,it is very necessary to develop an efficient and environmental method of disposal of fly ash.With a rich content of alumina（usually around 15～40%）,coal fly ash is considered as a potential alternative to bauxite for alumina production.Nevertheless,the aluminum in coal fly ash is mainly exist in mullite,which is generally considered a non-reactive matter.It is difficult to extract alumina from coal fly ash by direct acid leaching.Thus,it is necessary to activate the fly ash before its utilization.In this paper,the effect of Na-containing additives on the activation of coal fly ash for alumina extraction was investigated.Furthermore,The action mechanisms were also explored by the means of XRD and TG-DTG.In additions,the effect of Fe,Ca impurity on the activation coal fly ash with Na₂CO₃ additive was also studied.The main conclusions are as follows:（1）The activation behavior and mechanism of coal fly ash in the attendence of Na₂B₄O₇·10H₂O,Na₂CO₃,NaNO₃,NaOH,NaF,Na₂SiO₃,Na₃PO₄·12H₂0,Na₂SO₄,NaCl were discussed.The results showed that the addition of Na₂B₄O₇·10H₂O,Na₂CO₃,NaNO₃,NaOH,NaF,Na₂SiO₃ promoted the activity of the coal fly ash and the extraction of aluminum all significantly reached～80%when the calcination temperature increased to 800～900℃.In addition,the alumina extraction reached 49.5%,46.9%,25.3%with the additives of NaOH,NaF,Na₂SiO₃ at 400℃,significantly higher than that of other additives.However,the additives of Na₃PO₄·12H₂O,NaCl,Na₂SO₄ slightly improved the extraction of aluminum,the maximum aluminum extraction only reached 43.3%,32.1%and 32.1%,respectively.（2）The effect of Fe₂O₃,CaO impurity on the activation of fly ash with Na₂CO₃ additives was discussed.With the addition of iron oxide,some iron-containing material such as Na₂Fe₂O₄ formed,and the alumina extraction was slightly declined but remained above～82%.As for the CaO impurity,a small amount of calcium-containing material such as Na₄CaSi₃O₉,Ca₃Al₂O₆ generated with the change of CaO content,but the extraction of aluminum remained above 84%.Overall,the existence of Fe₂O₃,CaO could result in the emergence of some new substances during the activation but had little influence on the extraction of aluminum.