Abstract
Experimental work has been performed to investigate the precipitation mechanism of aluminum hydroxide phases from sodium aluminate/sodium carbonate pregnant solutions by carbon dioxide gas purging. Such solutions result from leaching calcium aluminate slags with sodium carbonate solutions, in accordance with the Pedersen process, which is an alternative process for alumina production. The concentration of carbonate ions in the pregnant solution is revealed as a key factor in controlling the nature of the precipitating phase. Synthetic aluminate solutions of varying sodium carbonate concentrations, ranging from 20 to 160 g/L, were carbonated, and the resulting precipitating phases were characterized by X-ray diffraction analysis. Based on the results of the previous carbonation tests, a series of experiments were performed in which the duration of carbonation and the aging period of the precipitates varied. For this work, a synthetic aluminate solution containing 20 g/L free Na2CO3 was used. The precipitates were characterized with X-ray diffraction analysis and Fourier-transform infrared spectroscopy.
Highlights
The resulting aluminate solution is supersaturated in aluminum trihydroxide, which is precipitated through seeding precipitation, which takes advantage of the temperature dependence of aluminum trihydroxide solubility in alkaline solutions
A benefit of the Pedersen process is that the hydrometallurgical conditions employed are moderate and, most importantly, the issue of bauxite residue is eliminated as, in its place, a calcium-based residue is produced in the leaching stage with potential applications
The results show slow alumina hydrate formation kinetics during carbonation, indicating that the system under investigation requires a sufficiently long aging period in order to precipitate as much of the aluminum as possible
Summary
A major drawback of the process is the excessive production of the solid residue of the leaching stage, the so-called bauxite residue, which contains all the unreacted components of the original ore, as well as phases that have precipitated during the caustic digestion step. The Pedersen process was abandoned in 1969, studying the process from the perspective of sustainability and circular economy, several advantages stand out [4]. It promises a complete utilization of bauxite ores, even some rejected by the Bayer process [5,6,7], since it aims at the production of iron and alumina. A benefit of the Pedersen process is that the hydrometallurgical conditions employed are moderate and, most importantly, the issue of bauxite residue is eliminated as, in its place, a calcium-based residue is produced in the leaching stage with potential applications
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