Physical Chemistry Research On Silica In Manufacture Process Of Alumina By Sub-molten Salt

The Alumina and Silica separate question was investigated and optimized, which was especially developed to solve the problems of high concentration of Silica, low crystal effecinency of Sodium Aluminate and the liquid-solid separation after digestion. The main innovations are as:The digestion mechanicsm of silicate in sub-molten salt was studied by the diasporic bauxite with the low alumina to silica mass ratio (1.34). The research indicates that the reaction temperature is the most significant factor in the digestion process by sub-molten salt; with the rise of temperature, the diaspore, the quartz, and the illite, et al. could be effectively dissolved. The four factors(temperature, caustic modulus, concentration of sodium oxide, concentration of silica) could speed the digestion.The alumina and silica reaction dynamics were built; The research indicates that the digestion process accounts to the retract model; The chemical reaction is the rate controlling process of sub-molten salt digestion; The silica digestion dynamics isThe alumina reaction dynamics isIn order to utilize the diasporic bauxite that the low alumina to silica mass ratio, reduce leachinging rate of silica, three methods were used to reduce the concentration of silica: digestion the bauxite by concentrating silica solution, secondy sweetening digestion by flotation slag, desilication solution by temperature-drop program. The study shows that three methods could effectivly reduce silica concentration and the leaching rate of silica.Through the three methods, the most significant method is the desilication solution by temperature-drop program. The optimum parameters are: After digestion at 453K for two hours, decrease temperature by 4K/min the temperature speed to 413K and constant temperature 3 hours.A simple method was introduced to estimate the Gibbs energy of complicate ferrite desilication additives. The thermodynamic calculation indicates that calcium hydroferrite, calcium hydroferrocarbanate, calcium hydroferrosulfate could be systhesised at low temperature. The rise of theperature in conductive to the improvement of the stability and the synthesis speed of ferrite desilication aaditives.The desilication products were identified by experiment, it is calcium hydroferrosilicate. The thermodynamic study indicates that the Gibbs energy of desilication reaction are negative, the desilication reaction could spontaneous carry on in sub-molten salt. The desilication additives reaction tendency is : 3CaO·Fe₂O₃ CaCO₃·12H₂O > 3CaO·Fe₂O₃·CaSO₄·12H₂O > 3CaO·Fe₂O₃·6H₂O ; the optimal desilication additive is calcium hydroferrocarbanate.Through factor and orthogonal experiments, the systhesis of HCFC was studied. The research indicates that HCFC is metastable compound in solution, reaction temperature and reaction time have the interation to the systhesis; the optimal factors are: temperature 303K, reaction time 12hours, the liquid to solid mass ratio is 20, rpm 500r/min, the calcium oxide grain size 120-150 order.With regard to deep desilication from sub-molten salt solution, the HCFC desilication mechanism was studied; the desilication dynamics was researched by the dynamic balance method; the HCFC dynamics is-dS/dt=64.385e^25768.355/RT×S(1.27878)the reaction activation energy is 25.768KJ/mol, the reaction orderis 1.279, the diffusion is the rate controlling process of desilication reaction; In desilication process, HCFC decomposes into calcium hydroferrite and calcium hydroxide, the silica compound calcium hydroferrite into calcium hydroferrosilicate.As the reaction time pass, the desilication efficiency enhance, and the desilication product grain grows up; the optimal reaction time is 180 minute; Increasing temperature could enhance desilication efficiency and the desilication product grain, reduce the content of sodium oxide and alumina in desilication product, th optimal temperature is 403-413K. The significant factor in desilication is concentration of sodium oxide, for the rise of free concetration of sodium oxide can cause the decomposition of desilication product and the alumina loss; the optimal total concentration of sodium oxide is 550g/L.