Study On Extraction Of Alumina And Silica From Activated Fly Ash And Red Mud By Gypsum

With the development of the power industry,the emission of fly ash from coal-fired power plants in China is increasing day by day.It is one of the industrial solid wastes with a large amount of emissions in China.At present,the massive accumulation of fly ash has caused serious environmental and social problems.The extraction of Al₂O₃ from fly ash is of great significance for solving the problem of shortage of bauxite resources and high dependence on foreign countries,reducing mining and protecting environment.In this paper,the effect of adding a certain amount of red mud and desulfurization gypsum on the extraction of alumina during the process of extracting alumina from fly ash was studied.In order to reduce the complexity of the experimental process,pure gypsum was used instead of desulfurization gypsum.Alumina was extracted by the calcination activation-sulfuric acid leaching process in the raw material,and at the same time,the method for recovering silica from the de-alumina residue was preliminarily discussed.The main research contents and results are as follows:?1?The effect of adding red mud on the extraction rate of alumina was studied in the process of extracting alumina from calcined fly ash with gypsum as activator.The experimental results show that the extraction ratio of alumina can be increased by adding red mud to adjust the ratio of aluminum to silicon.When the ratio of aluminum to silicon in the raw material is 1:1,the calcination temperature is 1200?and the calcination time is 1 h,the extraction rate of aluminum can achieve about 89%,the extraction rate of alumina was increased by about 3 percentage points compared to the single fly ash.?2?After mixing fly ash and red mud in proportion,the effect of gypsum dosage,roasting temperature and roasting time on the activation effect of raw materials was studied with gypsum as the activator.The optimum activation conditions were found as follows:molar ratio of Ca/Al of 1.4:1,roasting temperature of 1200?,roasting time of 30 min;XRD characterization of roasted clinker showed that mullite phase disappeared in fly ash,indicating that calcium sulfate could destroy mullite structure in the fly ash,which was accompanied by the formation of calcareous feldspar phase;the hydrated garnet phase in red mud disappeared,and the main unit body of calcareous nepheline phase remained unchanged.?3?The roasted clinker was leached by dilute sulfuric acid,and the optimum technological conditions were discussed by single factor test and optimized by orthogonal test.The optimum acid leaching conditions were obtained as follows:acid leaching temperature is 90?,sulfuric acid concentration is 3 mol/L,liquid-solid ratio is 20 mL/g,acid leaching time is 30 min.At this time,the dissolution rate of alumina was stable at about 95%;XRD characterization of de-alumina residue showed that the main phase was CaSO₄ and SiO₂,indicating that alumina was leached completely.The?-Al₂O₃ product obtained by calcination of aluminum sulfate crude solution after removing impurities at 820?for 1 h had a yield of 87.67%,and the purity of?-Al₂O₃ is 97.46%.In addition,the purity of CaSO₄ obtained during the calcium removal process is high and it can be used directly as a product.?4?The silicon in raw materials is mainly concentrated in the de-alumina residue.Na₂SiO₃ solution can be leached by sodium hydroxide solution to achieve the purpose of separating silica.The optimum technological conditions of alkali leaching were studied experimentally.The results show that when sodium hydroxide solution concentration is 40%,the alkali leaching temperature is90?,the alkali immersion time is 90min,and liquid-solid ratio is 7:1,the leaching rate of SiO₂ can reach 86%.Silica was prepared from Na₂SiO₃ solution by two steps carbonation process,then precipitate was filtered,washed and dried to obtain silica product.