Microwave-enhanced Roasting Of Diaspore Ore From The Basic Research Of Alumina Extraction

Even though China has rich bauxite resources,most of them are diasporic-characterized as tough processing demands,good crystalline state,compact structure,and aluminum silicon ratio of 4?7.Thus,the economic effect of alumina production in treating such refractory ores by Bayer process is greatly diminished by a number of problems,such as strict digestion condition,large amount of red mud,low alumina recovery rate,etc.In addition,the application of the sintering processes,including combined sintering-Bayer method and intensified sintering method are limited by high sintering temperature,complex technical flowsheet,and pollution problems.In order to compete with the international alumina industry,it is necessary to introduce new methods and technologies into the domestic alumina industry to improve aluminum recovery,reduce energy consumption,and enhance efficiency.Microwave heating technology is attracting more and more research attention for its cleanness,rapid and efficiency.And MH technology is applied to extracting alumina from low grade diasporic bauxite ores in our research.Specifically,a slurry mixture of diasporic bauxite and alkaline(Ca(OH)2 + supersaturated NaOH solution)is roasted in microwave.This research aims to take the selective heating effect of microwave energy to intensify the decomposition process of refractory diasporic bauxite.A1 bearing minerals in the raw bauxite reacts rapidly with NaOH to form the water-soluble phase(NaAlO2),while Si bearing minerals reacts with Ca(OH)2 to form the water-unsoluble phase(Ca2SiO4).Then the roasted products are leached by low-concentration alkali solution at the atmospheric pressure.The thermodynamics and kinetics of the reaction are studied in this work.Particularly,the heating performance,phase composition,microstructure,and leaching efficiency of the roasted product are analyzed by DTA-TG,XRD,and SEM-EDS techniques.In addition,by the comparison experiments of microwave-conventional roasting,wet mixing-dry mixing,NaOH system-Na2CO3 system,the mechanisms of intensifying action of microwave roasting-wet mixing method are discussed.The main contents of this work are as follows:1.The thermodynamics of main reactions of binary system,ternary system,and quaternary system are calculated for the reactions of the main mineral phases in diasporic bauxite with NaOH and Ca(OH)2.And the order of these reactions are determined.2.The effects of operation parameters,such as the amount of NaOH,the amount of Ca(OH)2,microwave roasting temperature,and holding time on the phase transformation of diasporic bauxite-NaOH system and diasporic bauxite-NaOH-Ca(OH)2 system,are investigated.The optimal conditions of microwave roasting process are obtained as follows:the amount of NaOH is accrording to the nNa/(nAl+nFe)mole ratio of 1;the amount of Ca(OH)2 is accrording to the nCa/nSi mole ratio of 2;microwave roasting temperature is 800 ?,and holding time is 20 min.3.The effects of different microwave roasting conditions(the amount of NaOH and Ca(OH)2,microwave roasting temperature,and holding time)and leaching conditions(leaching temperature,leaching time,and liquid-solid ratio)on the leaching performance of the roasted products are studied.The optimal conditions of the leaching process are obtained as:leaching temperature 80 ?,leaching time 20 min,and liquid-solid ratio 15:1.At these conditions,the extraction rates of Al,Na and Si can reach 96.68%,98.2%and 7.05%,respectively.4.In order to explore the mechanism of microwave roasting process,the phase structure and leaching performance of roasted products by conventional heating and microwave heating are compared.The results show that microwave heating method can effectively promote the reaction of NaOH with Al bearing minerals in raw bauxite.Compared with conventional heating,microwave heating can either reduce the roasting temperature by nearly 400 ? or shorten reaction time by 3 times.The apparent activation energy of 31.31 kJ/mol by microwave hearing is lower than that of 40.84 kJ/mol by conventional heating.5.In order to reveal the mechanism of intensifying action of water on the high temperature reactions in the microwave field,the phase structure and leaching performance of roasted products of wet mixing and of dry mixing in microwave roasting process are compared.The results show that compared with dry mixing,wet mixing method can effectively promote reactions of bauxite with NaOH and Ca(OH)2,and inhibit the dehydration reaction of AlOOH,and so can affect beneficially to the formation of NaAlO2 and Ca2SiO4.6.Through the comparisons of dielectric properties,viscosity and saturated vapor pressure,it is found that compared with Na2CO3 solution,NaOH solution is more conducive to heat transfer and water locking in raw slurry,and has a high pressure effect in part micro-area in the microwave field.The extraction rate of A1 of the product roasted at 800 ? for 20 min prepared by Na2CO3 is only 70%,and it is lower than that of A1 of the product prepared by NaOH for about 27%under the same roasting conditions.7.The mechanisms of intensifying action of wet mixing-microwave heating method are mainly the following:(a)creation of a hot core at the peak location of the electric field;(b)preheating of the outer layer facilitated by heat conduction and vapor from the hot core,and the existence of temperature gradients between different minerals with different microwave absorbing properties and the polar molecules in the microwave field to increase the reaction area;(c)the reaction layer expands gradually from inside to outside;and(d)by prolonging roasting time,the dielectric loss increases,the penetration depth decreases,and the temperature gradient in the sample gradually equalizes,leading to a homogeneous product.In conclusion,in order to solve the problems in the existing treatment processes of diasporic bauxite,a novel process,wet mixing-microwave heating,is proposed for extracting alumina from diasporic bauxite ores.This method is found to be efficient of low temperature treatment of diaspore ore.In addition,the mechanisms of intensifying action of wet mixing-microwave heating method are discussed.