Reactions of lime with carbonate-containing solutions

Abstract

In the Bayer process for refining alumina, lime (either quicklime or slaked lime) is added to the process liquor to precipitate calcium carbonate and restore alkalinity to the solution. The work described in this paper was undertaken to elucidate the mechanism of this precipitation reaction about which surprisingly little is known. In the first part of the study, the composition of calcium oxide disks after reacting with sodium carbonate solutions was examined by XRD and TGA. TGA results show that the amount of calcium carbonate formed on the disks increases with sodium carbonate concentration but the amount of calcium hydroxide reaches a minimum at a solution concentration of about 0.05 M carbonate and then increases at higher carbonate concentrations. The kinetics of the formation of calcium carbonate were also followed using HPLC to monitor the change in bulk carbonate concentration due to the reaction of powdered lime in sodium carbonate solutions. The effect of the agitation speed and temperature on the reaction rate indicates that the formation of calcium carbonate is a diffusion-controlled process. To account for these findings it is suggested that at concentrations above 0.05 M carbonate a more porous heterogeneous CaCO 3–Ca(OH) 2 layer appears owing to direct attack on the surface by carbonate before a layer of calcium hydroxide can form. Scanning electron micrographs of disks after reaction in carbonate solutions support this suggestion. The second part of this study deals with the dissolution of lime in the presence of sodium carbonate. The concentration of Ca 2+ from dissolution of calcium hydroxide disks was determined by AAS. It was found that the dissolution process is slowed by the formation of calcium carbonate, and is diffusion controlled. The reaction of powdered calcium oxide in the sodium carbonate solution was followed by calorimetry. It was found that the relationship between the extent of dissolution and the carbonate concentration is similar to that obtained by TGA for the formation of calcium carbonate, suggesting the same mechanism.