Theoretical Study On The Carbon Dioxide Neutralization Treatment Of Hydrated Calcium Aluminates

Hydrated calcium aluminates are important compounds in the alumina refining process by alkali method, which can be generated easily in various stages of production, resulting in the loss of alumina.3CaO·Al2O3·6H2O (C3AH6) is the foremost one among the numerous hydrated calcium aluminates, whose chemical property is similar with hydrogarnet. So to study the recovery of alumina from the hydrated calcium aluminates not only can raise alumina recovery rate, but also can be an important guideline for recovering alumina from dealkalization residue (mainly containing hydrogarnet); in addition, it can make reference to deal with the solid calcium aluminate produced by other industries.In this paper a novel method by which alumina is recovered from hydrated calcium aluminates at normal pressure and low temperature has been studied, and the related reaction mechanism has been discussed. The research consists of two steps, firstly hydrated calcium aluminates were decomposed into Al (OH) 3 and CaCO3 by using CO2 as the reagent in water medium, then alumina was dissolved from the residue by using alkali liquor. Main contents and conclusions are as follows:(1) Using C3AH6 as research object. Thermodynamic analysis of the process of decomposition and dissolving alumina were done. Results show that the decomposition by using CO2 as the reagent and the dissolving alumina by using alkali liquor are feasible, which provided a theoretical guidance for experimental research in this paper.(2) The influencing factors such as temperature, the concentration of CO2 in the mixture of CO2 and air, and reaction medium, the mechanism of decomposition were studied. Results show that the reaction rate of C3AH6 with CO2 is extremely slow both in the air and in the ethanol medium. However, in the medium of water, the reaction occurs quickly with the products of Al (OH) 3, CaCO3 and H2O, because the reaction of molecules between C3AH6 and CO2 has been changed into the reaction among the ions in aqueous solution as a result of the solvation of C3AH6 and the formation of HCO3. The reaction rate increases with the increase of the reaction temperature, CO2 concentration and reaction time. The mechanism of decomposition, that the reaction rate is determined mainly by the solvation of C3AH6 in the initial stage of the reaction and by the formation of HCO3 in the late reaction stage, is proposed by combining with the change of pH value in the reaction process.(3) The influencing factors such as initial concentration of Na2OK, L/S radio, reaction temperature, reaction time, CaCO3 and composition of aluminate solution and the mechanism of dissolving alumina were studied. Results show that the dissolution rate of alumina increases with temperature increment; it also increases with the elevation of initial concentration of Na2OK firstly, but decreases later. During the progress of dissolving alumina, the dissolution reaction of Al(OH)3 and the anti-causticization reaction of CaCO3 occur at the same time. Obviously the promotion of Al(OH)4- to the reaction of anti-causticization is confirmed. Increasing the initial concentration of Na2Oc and the reaction temperature can effectively inhibit the anti-causticization reaction, but the addition of sodium oxalate on the inhibition of anti-causticization is not obvious.