Study On Digestion Characteristics Of Desilication Concentrate From Roasted Low-grade Bauxite And Extraction Alkali From Red Mud

In recent years,Chinese high-quality bauxite resources are increasingly exhausted.Efficient utilization of low-grade bauxite is conductive to the sustainable development of Chinese alumina industry.In this paper,the low-grade bauxite and Bayer red mud were used as raw materials for experiments.The digestion experiment was carried out on the desilication concentrate obtained after roasting and alkali-soluble desilication to simulate the crystal change in the digestion process.The effects of digestion temperature,digestion time,caustic concentration and lime addition on digestion performance were systematically investigated.The effects of roasting temperature,roasting time,leaching time,liquid-solid ratio and Ca O addition on the alkali extraction from red mud were studied.The phase and morphology changes of roasted ore,desilication concentrate and roasted red mud were systematically studied by XRF,XRD,SEM-EDS and BET methods.After roasting,the effects of roasting temperature,roasting time,leaching time,liquid-solid ratio and Ca O content on the alkali extraction from red mud were investigated.The digestion mechanism of Al₂O₃and Na⁺is studied by digestion kinetics and Materials Studio computer simulation.In the digestion process of roasted ore,the optimum condition was a digestion temperature of 280 ^oC,a digestion time of 70 min,a lime addition of 10%,and a caustic soda concentration of 245 g/L,the digestion rate of Al₂O₃was 90.37%.Under the same conditions of other digestion conditions,the best addition of lime for desilication concentrate was 8%,and the digestion rate of Al₂O₃was 94.85%.The digestion effect of desilication concentrate was obviously better.Compared with the desilication concentrate,the roasted ore was porous,but the red mud particles of desilication concentrate were more dense.The BET specific surface area of roasted ore and desilication concentrate was 29.817 m²/g and 21.252 m²/g respectively,the cumulative pore volume was 0.077 cm³/g and 0.078 cm³/g respectively,and the average pore size was 10.4126 nm and 14.7620 nm respectively.The effect of roasting on the red mud phase was studied.Through roasting,the phase of iron oxide and ferric silicate in the red mud disappeared,and a new phase of calcium ferrosilicon sodium aluminate appeared.The lattice parameters of sodium aluminosilicate hydrate in red mud were the largest at 700 ^oC,which were a=b=12.6638 nm,c=5.1615 nm.With the roasting temperature increasing,the specific surface area and pore volume of red mud gradually decreased,pore diameter firstly increased and then decreased,particles gradually refine,but the aggregation degree of red mud particles gradually increases.The best technology of extracting alkali from red mud by doping Ca O after roasting was as follows:the content of Ca O was 8%,the ratio of liquid to solid was 6:1,the leaching time was 90 minutes,the leaching temperature was 95 ^oC,and the leaching rate of Na⁺was 72.6%.The results of kinetic analysis show that Al₂O₃from roasted ore and desilication concentrate was controlled by internal diffusion,and their apparent activation energies were 63.99 k J/mol and 49.06 k J/mol,respectively.And the relationship between the relative digestion ratio of Al₂O₃and temperature of desilication concentrate and roastedoreunderspecificdigestionconditionswere y=u?T?=-899.87+6.75223T-0.01143T²and y=u?T?=-188.49+1.49077T-0.00179T².The computer simulation study of Materials Studio shows that the reaction of the?113?surface of Al₂O₃in the process of alkali leaching results to decrease of crystal size.The binding energy of Al₂O₃structure of desilication concentrate with caustic soda was less than that of roasted ore.The interplanar spacing of desilicated concentrate decreases and the length Al—O increases,with two bond lengths of1.8540 nm and 1.9710 nm,respectively.The free energy of the reaction between sodium aluminosilicate hydrate and calcium hydroxide increased first and then decreased,the binding energy decreased first and then increased,and the crystal surface area of sodium aluminosilicate hydrate was directly proportional to the surface energy,which was conducive to the chemical reaction and improved the efficiency of replacing Na⁺by Ca²⁺.