| Desilication product (Na2O·Al2O3·xSiO2-·yH2O, DSP), which is produced from the process of alumina production by the alkali treatment of bauxite. Large amount of DSP containing considerable alumina and sodium oxide is required to be dealt with. In Bayer process of alumina production, most DSP is discharged with red mud, which not only takes up the land and pollutes the environment but also wastes resources. In sintering process of alumina production, the method of allowing DSP back into the sintering system is generally used to recover the alumina and sodium oxide, which results in the decrease of the alumina content in the sinter and increase of production cost.Based on the previous research work, the new hydro-treatment technology of DSP was studied in this thesis by combining thermodynamic calculation with experiments. Three steps of the technology, namely de-alkalization, decomposition of hydrogarnet (3CaO·Al2O3·xSiO2·(6-2x)H2O) and dissolution of alumina, were studied. Furthermore, reaction mechanisms of decomposition and dissolution were discussed. Main contents and conclusions are as follows.(1) By the reaction of DSP and lime in the aqueous system, hydrogarnet and sodium hydroxide are obtained, and the sodium oxide is recovered through liquid-solid separation. The influencing factors, such as the molecular ratio of CaO to Na2O (CaO/Na2O), reaction temperature, reaction time and liquid-solid mass ratio (L/S), were studied, and the effect of CaO/Na2O is most significant. The optimal conditions to recover sodium oxide are as follows, CaO/Na2O4, temperature90-100℃, time3hours, L/S10, and the highest sodium oxide recovery reaches95%.(2) For the decomposition of hydrogarnet, the influencing factors such as reaction temperature, reaction time, L/S, CO2concentration and the decomposition mechanism were investigated. The results demonstrate that calcium carbonate, hydrated calcium silicate and aluminum hydroxide with imperfect crystalline are produced when hydrogarnet react with CO2in aqueous system. The temperature is most significant and increasing temperature favors the decomposition of hydrogarnet, and the temperature of90-100℃are chosen. There are two routes for the reaction of hydrogamet and CO2, the reaction occurred mainly through the solvation of hydrogarnet in the initial stage of the reaction, and calcium carbonate, hydrated calcium silicate and aluminum hydroxide are formed by the reaction of CO2with Ca2+, Al(OH)4-,SiO2(OH)22-and OH-, which are produced by the ionizing of hydrogarnet. As the reaction proceeded, the solution changed from weak alkline to weak acidic, and the final products were produced directly by the reaction of hydrogarnet and HCO3-.(3) The influencing factors of Na2Ok concentration, reaction temperature, reaction time, L/S and the mechanism of CaO-Na2O-CO2-SiO2-Al2O3systerm in the dissolution of alumina were conducted. The results show that the optimal conditions are Na2Ok=75g/L, temperature75-80℃, time45minutes, L/S10, and the highest recovery of alumina reaches53%. The results of thermodynamic calculation and experiments indicate that the secondary reaction occurring during the dissolution readily causes the loss of alumina, and the anti-causticization of CaCO3in sodium aluminate solution is the main reason for the secondary reaction. The anti-causticization rate of CaCO3in pure NaOH solution is low, while it is significant when CaCO3in sodium aluminate solution, indicating that Al(OH)4-anion can significantly promote the anti-causticization of CaCO3in sodium aluminate solution. The reaction rate of anti-causticization improves with the increase of temperature, increases first and then decreases with the increase of the mole ratio of Na2O to Al2O3(ak), and reaches the maximum with ak of approximately3. The Na2CO3concentration increases because of the anti-causticization of CaCO3, which intensifies the secondary reaction. Therefore, the anti-causticization must be inhibited in order to improve the leaching rate of alumina. |
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