Experimental Study On The Desilication Of Coal Fly Ash By Alkali Leaching And Sintering Process

Coal fly ash,the main industrial by-product,is derived from coal combustion in thermal power plant.The annual production of coal fly ash in China is great and still continuous growing.Therefore,there is an urgent need for the rational utilization ways of coal fly ash.Using high-alumina coal fly ash in the extraction of alumina industry not only can solve the environmental population problems that are caused by the accumulation of coal fly ash,but also be able to ease the situation of bauxite resources’ shortage in China.The coal fly ash used in this dissertation was taken from the Shizuishan thermal power plant in Ningxia province.This dissertation focused on to study the influence of the alkali leaching conditions to the desilication rate and the ratio of Al2O3 to SiO2 and the phase transition in the alkali leaching process.Comparative studies in the optimization conditions of coal fly ash’s desilication were carried out to investigate the solubility and phase transition of mullite and quartz,which are the main crystal phases in the coal fly ash.At last,the phase transitions in the sintering process of zeolite,mullite and quartz reacting with Na2CO3 and CaCO3 were discussed.The main research contents and conclusions are as follow:(1)The coal fly ash,ball-milled and dried,was mixed with NaOH solution to disilication.The effects of the reaction temperature,the reaction time,liquid/solid mass ratio and the mass concentration of NaOH solution to the desilication rate and the ratio of Al2O3 to SiO2 were investigated.The obtained optimum conditions were the reaction temperature of 90℃,the reaction time of 3 hours,liquid/solid mass ratio of 9 and the mass concentration of NaOH solution of 35%.After leaching the coal fly ash in this condition,the desilication rate is 47%and the ratio of Al2O3 to SiO2 raises from 0.77 in the raw material to 1.45.(2)The Phase transition of coal fly ash in the alkali leaching process were investigated.The formation of the zeolite phases was mainly affected by the mass concentration of NaOH solution.The ratio of Al2O3 to SiO2 of the zeolite increased with the increase in the mass concentration of NaOH solution.The largest value of the ratio of Al2O3 to SiO2 is 1:2.At the fixed mass concentration of NaOH solution in 10%and 35%,the influence of the reaction temperature,the reaction time,liquid/solid mass ratio on the ratio of Al2O3 to SiO2 of the zeolite that generated in the alkali leaching process were studied.Between the experimental ranges,all of the reaction conditions show minor influences on the formation of zeolite.(3)The dissolution rate and phase transition of mullite and quartz,which are the two kinds of major crystal phases of the coal fly ash,in the optimum conditions of desilication were discussed.The results indicated that the dissolution rate of mullite and quartz are lower than 5%,and there were not zeolites formed.The conclusion indicated that the glass phase of the coal fly ash is the major parts which reacted in the desilication process.(4)Mullite,quartz and zeolite were separately mixed with Na2CO3 and CaCO3 then sintered.The influence of the sintering temperature and the dosage of Na2CO3 and CaCO3 to the sintering products were discussed.The experimental results showed that,when the sintering temperature was lower(750℃,850 ℃),some intermediate phases,such as sodium silicate and calcium silicate,would form.But when the reaction temperature is higher than 850℃,all of the phase would transform into NaAlO2,Na2CaSiO4.