Study Of Theoretical Basis And Innovative Technology For Efficient Separation And Utilization Of Aluminum And Silicon From Coal Fly Ash Based On Alkali Activation Via Hydrothermal Process

Coal fly ash is a secondary resource with aluminum and silicon as main components.However,the Al₂O₃/Si O₂ mass ratio of coal fly ash is low,and Al-Si separation and utilization technology via traditional processes mainly aims at the extraction of alumina from coal fly ash,which would produce silicate slag that much more difficult to use,and would cause secondary pollution to the environment.This study aims at the synchronous separation and utilization of aluminum and silicon from coal fly ash.The aluminum-and silicon-components were separated under hydrothermal conditions with coexistence of Ca（OH）₂ and Na OH,achieving the synchronous extraction of alumina and synthesis of tobermorite fibers.The alumina lixivium was used to prepare pseudo-boehmite,and tobermorite fibers were used to prepare calcium silicate board and adsorption materials,which could achieve the value-added utilization of aluminum and silicon from coal fly ash.The conclusions and innovations are as follows.The reaction rules of aluminum-and silicon-components under the action of Ca（OH）₂-Na OH were elucidated.It can be obtained from thermodynamic and pure-mineral experimental results,the aluminum-and silicon-components were mainly dissolved as Al（OH）₄^-,H₂Si O₄^2-and HSi O₄^3-,etc.under alkali activation.H₂Si O₄^2-and HSi O₄^3-were combined with Ca²⁺to form calcium silicate hydrate,Al（OH）₄^-was enriched in the solution.When the concentration of Al（OH）₄^-attained the supersaturated concentration for zeolites formation,it would promote the formation of zeolites.In the reaction system with high concentration of silicate ions,calcium silicate hydrates like riversideite,tobermorite,foshagite and xonotlite,etc.were formed.In the reaction system with low concentration of silicate ions,the reaction of silicate and calcium ions mainly produced tobermorite.In the reaction system with coexistence of silicate and aluminate ions,in addition to tobermorite,zeolites like hydogarnet and analcime were also formed.Crystallization behaviors of tobermorite in Ca（OH）₂-Na OH solution were elucidated.Under hydrothermal conditions at temperatures above200 ^oC,silicon-components were efficiently dissolved and transformed into flake-like tobermorite of low crystallinity.The flake-like tobermorite was transformed into tobermorite fibers of higher crystallinity as the continuous dissolution of silicon-components.When the temperature exceeds 210 ^oC or the concentration of Na OH exceeds 30 g/L,tobermorite is structurally unstable and tends to transform into xonotlite.When the silicon-components were completely dissolved,the crystallinity of tobermorite was declined as reaction time was extended.If aluminate ions were existed in the hydrothermal reaction system,aluminum would enter into tobermorite crystals via Al-Si substitution and promote tobermorite crystallization,and the amount of Al-Si substitution could up to about 20%.Mechanisms on dissolution and transformation of aluminum-and silicon-components of coal fly ash under the action of Ca（OH）₂ and Na OH were revealed.The amorphous substance was preferentially dissolved and transformed into amorphous calcium silicate hydrate under hydrothermal consitions at 100 ^oC～160 ^oC.When the temperature attained about 200 ^oC,quartz and mullite can be efficiently dissolved,and calcium silicate hydrate was gradually transformed into tobermorite fibers.The aluminate ions were enriched in the solution,thus achieving the separation of aluminum-and silicon-components.When the concentration of aluminate ions attained the supersaturated concentrations for zeolites formation,Na-type zeolites like analcime were formed.When the aluminum-and silicon-components were completely dissolved,Ca²⁺would substitute Na⁺in analcime structure and promote the formation of Ca-type zeolites like hydrogarnet.The efficiency of Al-Si separation was lowered by zeolites formation.A novel process was proposed to separate aluminum-and silicon-components from coal fly ash based on alkali activation via hydrothermal method,achieving the synthesis of tobermorite fibers and extraction of alumina synchronously.The alumina leaching ratio attained60.14%at 200 ^oC～220 ^oC for 1 h～3 h with 20 g/L～30 g/L Na OH and Ca O/Si O₂ molar ratio around 1.0.The alumina lixivium was used to prepare pseudo-boehmite product via carbonation process.The solid products of hydrothermal reactions were mainly tobermorite fibers with 5μm～10μm in length and 0.01μm～0.50μm in diameter,aggregating into porous spherical constructions,which can be used to prepare calcium silicate boards and adsorption materials.It can be obtained from 30 L pilot-scale test and 2 m³ semi-industrial scale test that the new process was achieved the separation of aluminum-and silicon-components via“one-step hydrothermal process”,which was characterized of simple technical flow,short process,value-added products,etc.The new process was technically and economically feasible,which provides a viable approach for resource recovery and value-added utilization of coal fly ash.