High-efficiency Control Of Calcium And Iron Removal From High-alumina Fly Ash And Its Recycling

It is estimated that by 2020,China’s annual emissions of fly ash will reach 900 million tons,and its accumulated stockpiles will reach 3 billion tons,which will occupy a large amount of land resources and pose a huge threat to the ecological environment,and cause waste of secondary resources.China has abundant high-aluminum fly ash resources,which are mainly distributed in central and western Inner Mongolia and northwestern Shanxi.The sum of its alumina and silica content exceeds 70 to 80%,however the large amount of impurity elements such as calcium and iron contained in it have a huge adverse effect on the subsequent high-value utilization.This article takes high-aluminum fly ash as the research object,using XRF,XRD,SEM-EDS,FT-IR and particle size analysis,starting from the ore phase characteristics of high-aluminum fly ash,the removal of calcium and iron by acid leaching high-aluminum fly ash,the preparation of polyaluminum ferric chloride with acid pickling solution and product characterization are studied.The main conclusions obtained in this article are as following:Based on the determination of the chemical composition and mineral phase characteristics of high-alumina fly ash,the effects of different acid species and reaction parameters on the calcium and iron removal performance of high-alumina fly ash were systematically studied,and the optimal Calcium removal conditions.The content of CaO and Fe2O3 in high-aluminum fly ash in fluidized bed is high,reaching 11.03%and 3.43%,respectively;the main mineral phases are mullite and quartz,the calcium-containing minerals are calcium sulfate and calcareous feldspar,and the iron-containing minerals are red Iron ore;micro-morphology shows mainly irregular molten particles with fluffy structure,uneven particle size,flakes stick together to form floc.When the acid concentration is in the range of 2一 10 mol/L,the dust removal effect of fly ash first increases and then decreases;when the reaction temperature is in the range of 60～100℃,the dust removal effect of fly ash first increases and then decreases;the reaction time is in the range of 1.0～3.0 h,the impurity removal effect increased at first and then remained unchanged;the liquid-solid ratio had less influence on the impurity removal effect.The optimal process conditions for removing calcium and iron from high-aluminum fly ash are:selecting hydrochloric acid as an impurity removing agent,an acid concentration of 6 mol/L,a reaction temperature of 80℃,a reaction time of 2.0 h,and a liquid-solid ratio of 4.Under this condition,after removing calcium and iron the content of CaO in the fly ash was 0.69%and the content of Fe2O3 was 0.65%.The calcium and iron removal rates reached 95%and 85%respectively.The kinetics of the removal of calcium and iron from high-aluminum fly ash was calculated,and the evolution behavior of the main ore phases and the mechanism of removing calcium and iron were studied by means of particle size,XRD and SEM.The removal of calcium and iron in high-alumina fly ash is controlled by internal diffusion.The activation energy of calcium removal reaction is 25.61 kJ·mol-1,and the rate equation of calcium removal reaction is:1-2α/3-(1-α)2/3=32.46×exp(-25610/RT)×t;the activation energy of the iron removal reaction is 31.84 kJ·mol-1,and the rate equation for the iron removal reaction is:1-2α/3-(1-α)2/3=79.84×exp(-31840/RT)×t;After the removal of calcium and iron,only the mullite phase and the quartz phase are included in the high-aluminum fly ash.Because there are more glass phases in fly ash,a small amount of calcium and iron impurities still remain.The effects of different alkalizing agents,solution components and reaction conditions on the performance of calcium aluminum and ferric acid leaching solutions for preparing polyaluminum ferric chloride were studied,and the optimal process parameters were determined.The best process conditions are:using sodium hydroxide as alkalizing agent,Al/Fe molar ratio is 15,pH is 3.0,polymerization temperature is 85℃,and polymerization time is 3.0 h.Under these conditions,the removal rate of turbidity of simulated water samples of kaolin can reach more than 90%.The prepared polyaluminum ferric chloride product has a application in treating wastewater with wide pH range,and the turbidity removal rate can reach more than 90%in the range of 6 to 8;The flocculation performance is good in the range of 100 to 150 mg/L.Characterization and analysis of the structure of the prepared polyaluminum ferric chloride product.Solid product is yellow,alumina content of 29.7%,an iron oxide content of 3.6%,and a salt base degree of 64.3%,all in accordance with national standards;XRD and infrared spectroscopy show that aluminum and iron in polyaluminum ferric chloride products have been effectively polymerized.There are both polymers complexed with hydroxyl and polymers complexed with hydroxy;the products of polyaluminum ferric chloride are The chain-like three-dimensional structure having more pores.This structure increases the specific surface area,and the porosity is relatively large,which enhances the adsorption bridging effect.The product has a good net capture and sweeping ability.