Utilization of CO2 as a precipitant to prepare aluminum hydroxide from NaAl(OH)4 solutions: Study on reaction kinetics and mechanism

Abstract

The carbonation reaction of sodium aluminate solution is the most important, green and economical process for preparing aluminum hydroxide and alumina. Controlling carbonation reaction rate and hydroxyl ion concentration (pH) at the reaction endpoint of NaAlO₂ solution is the most critical technical requirement. However, carbonation reaction mechanism is still controversial, and there is no relevant study on the reaction kinetics of hydroxide ion, which affects the control of carbonation reaction and reactor design. This paper reports the study of the change of pH and hydroxide ion concentration in situ, to propose a two-stage carbonation reaction mechanism of NaAlO₂ by comparing with carbonation reaction of NaOH solution. The first stage of reaction mechanism is the neutralization reaction between CO₂ and uncombined NaOH of NaAlO₂ solution. The second stage of reaction mechanism is a sequence of three chemical reactions: (i) NaAlO₂ decomposition generating NaOH, (ii) neutralization between generated NaOH and CO₂, and (iii) hydrolysis of carbonate ions. Secondly, a new experimental method of spontaneous-decomposition reaction of NaAlO₂ solution was innovatively proposed, revealing two first-order decomposition reactions. The reaction rate constant was about 3 × 10⁻³ s⁻¹. Furthermore, the rate equation for hydroxyl ion describing a two-stage carbonation reaction, controlled by mass transfer rate and chemical reaction rate was derived. The correlation coefficients of fitted experimental data were all greater than 0.99. Research found that the decomposition rate constant of carbonation reaction is greater than the spontaneous-decomposition rate constant of NaAlO₂ solution. The decomposition rate of carbonation reaction was affected by the decreasing rate of hydroxide ion concentration. Finally, CO₂ consumption measured by experiment and calculated by kinetics was completely consistent, which verifies the correctness of reaction mechanism and proposed rate equations.