Fundamental Applied Research On Process Of Alumina Recovery From High-alumina Fly Ash By Sub-molten Salt Method

High alumina fly ash(HAFA)is a new type of Al-rich resource discovered in North Western China in recent years.The annual output of HAFA is higher than 50 million tons now and it needs to be urgently exploited and utilized.As the only whole-wet alkali process,the technology of alumina extraction from HAFA by sub-molten salt method has showed a good industrialization prospect and its' industrial large-scale test is being carried out at this moment.However,part of the alumina extraction mechanism and the scale-up law for industrial application are still unclear.In order to achieve high efficiency and low cost extraction of alumina from HAFA,this dissertation systematically addresses the following fundamental research topics:rule of phase transformation in the digestion process,microcosmic reaction mechanism and high temperature dissolution kinetics of intermediate product,formation and organization of by-product,industrial application research of new process.In this dissertation,the following results and progress were obtaineed:(1)The leaching rules of HAFA reacting in different kinds and sizes of reactors were investigated.Through the optimization of the reactor,the reaction temperature for alumina extraction decreased from 280 to 240?.The main reason for this optimization was thought to be the differences of the mixture effect between two types of reactor,which was found by the simulation calculation results.The optimized leaching parameters of two kinds of HAFA and some theoretic support for scale-up design of reactor were obtained at last;(2)A two-stage digestion mechanism for alumina extraction from HAFA by Sub,molten Salt method was proposed and Na8Al6Si6O24(OH)2(H2O)2 was determined as an important intermediate product in this process.With the aid of a specially designed rapid real-time sampling device,the kinetics of the decomposition of Na8Al6Si6O24(OH)2(H2O)2 at high temperatures has also been studied and fitted by the Avrami-Erofeev equation,the apparent activation energy of the reaction of Na8Al6Si6O24(OH)2(H2O)2 decomposition was calculated as 255.55kJ/mol;(3)The formation and regulation rules of the sodium aluminosilicate hydrate calcium by-product were investigated.The results show that the initial caustic ratio had the biggest impact on its generation.Under the condition of the final caustic ratio higher than 14,it can completely avoid its emergence.At the same time,the effect of the wraparound of NaCaHSiO4 during the high temperature digestion process was determined as the main reason for its formation.The way of improving the stirring speed to enhance mass transfer was proposed as the method to inhibit the formation of by-product;(4)The formation mechanism of the pipe scaling in the industrial amplification was deeply investigated.The method of controlling material retentional time to suppress the pipe scaling was proposed and the feasibility of this method had been verified by a 18 days of continuous industrial operation test.By this regulation way,the scarring speed decreased 76.65%finally;The above laboratory work had already been verified by a 100 tons level experimental pilot and a 10000 tons level middle trial.This industrial application of this process has yielded several good results.The success of the industrial middle trial research gives a solid foundation for the future commercialization as well.