Refine Of Ammonium Aluminum Sulfate Dodecahydrate And Preparation Of Its Subsequent Products

Sulphuric acid leaching is one of the most commonly used methods for extracting aluminum from fly ash by acid method.In traditional sulfuric acid process,aluminum ions in acid leaching solution are precipitated in the form of aluminum sulfate crystals,which are converted into other process products by further reaction.The recovery rate of aluminum ions in the process is low.Because the solubility of ammonium aluminum sulfate hydrate is much lower than that of aluminum sulfate at the same temperature,the recovery of aluminum can be greatly improved if aluminum ions in acid leaching solution are precipitated in the form of ammonium aluminum sulfate hydrate crystal.Therefore,this paper carries out an attempt in this regard.However,the precipitated crystals of ammonium aluminum sulfate dodecahydrate contain a large number of calcium and iron impurities,which seriously affect the purity of the subsequent products,and constitute one of the technical bottlenecks in the process of extracting aluminum by ammonium aluminum sulfate method.In view of the above problems,this paper carried out the research on impurity removal and purification of ammonium aluminum sulfate dodecahydrate.The effect of solution state and material composition ratio on the purity of crystalline products during evaporation crystallization process was mainly investigated.The technological conditions for impurity removal and purification of ammonium aluminum sulfate dodecahydrate were optimized.The purified ammonium aluminum sulfate dodecahydrate was used to prepare industrial-grade crystalline aluminum chloride and aluminum hydroxide.The main research contents and conclusions are as follows:?1?Ammonium aluminum sulfate dodecahydrate was purified by evaporative crystallization.The effect of solution state and material composition ratio on impurity removal of ammonium aluminum sulfate dodecahydrate in evaporative crystallization process was investigated,and the process route of purification of ammonium aluminum sulfate dodecahydrate was optimized.The results show that the crystallization rate of aluminum ion is higher in metastable evaporative crystallization solution,but a small amount of calcium sulfate impurities are mixed in precipitated ammonium aluminum sulfate dodecahydrate crystals,while the crystallization rate of aluminum ion in stable evaporative crystallization solution decreases greatly,but almost all calcium ions are precipitated.The optimized process shows that the purity of ammonium aluminum sulfate dodecahydrate can reach 99.1% by first removing calcium impurities from the solution by evaporation crystallization in stirring state and then evaporating the remaining solution in stationary state.?2?The effects of solution state,concentration of hydrochloric acid and material composition ratio on ion concentration in the process of preparing crystalline aluminum chloride by reaction of crude ammonium aluminum sulfate dodecahydrate with hydrochloric acid were investigated.It was found that in metastable state,the lower the impurity content of calcium and iron in reaction crystallization,the lower the acidity corresponding to the beginning of precipitation of crystalline aluminum chloride.When the content of ammonium aluminum sulfate was 98%,the critical point of precipitation of crystalline aluminum chloride was the lowest,and the corresponding concentration of hydrochloric acid was 6.0 mol/L.In stable state,the impurity content of calcium and iron did not affect the precipitation of crystalline aluminum chloride.When the concentration of hydrochloric acid reaches 8.0 mol/L,the crystalline aluminum chloride begins to precipitate.Crystalline aluminum chloride was prepared from purified ammonium aluminum sulfate dodecahydrate.The purity of aluminum chloride was over 93% and the yield was about 58%.?3?The effects of reaction temperature and ammonia concentration on the yield and purity of aluminum hydroxide prepared by the reaction of ammonium aluminum sulfate dodecahydrate with ammonia aqueous solution were investigated.Under the conditions of 20% concentration of ammonium aluminum sulfate dodecahydrate solution,1 m L/min entrance rate of peristaltic pump and 2 h entrance time,the optimum conditions for preparing aluminum hydroxide were obtained: reaction temperature 70 ?,ammonia concentration 5%.Under these conditions,the crystalline form of aluminum hydroxide is beta-Al?OH?₃ with good morphology.The highest yield of the product is 91.8% and the purity is as high as 99%.This study can provide reference data for the improvement of aluminum recovery rate and product purity in sulfuric acid leaching-crystallization process,and also can provide optimization ideas for the purification of other valuable elements and the preparation of process products.