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Processing Red Mud with Sulfuric Acid and Organic Substitutes for selective leaching of Scandium and Yttrium

Baltzakis Emmanouil

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Summary

The Red Mud is a byproduct of the aluminum production industry, derived from the processing of bauxite. The collection and processing of the enormous quantity of red mud produced pose both an issue and a challenge, as it contains metals with technological and economic value known as rare earth elements. This present thesis focuses on the selective extraction of rare earth elements, specifically scandium and yttrium, using sulfuric acid aqueous solutions and organic acids (citric and oxalic acid). Laboratory solutions, as well as natural materials such as lemon and vinegar, were employed as sources of citric and acetic acid, respectively. Experiments were conducted significant using a thermal processing method in a water bath at 70 °C. All samples had a solid/liquid ratio of 1 g of red mud to 40 mL of aqueous solution. Initially, experiments were carried out with sulfuric acid at molecular concentrations of 1M and 0.5 M for 1, 2, 4, 24, and 48 hours, respectively. Subsequently, a thermal processing time of 2 hours in a water bath was chosen, and experiments involved mixing different ratios of sulfuric acid with organic acids, as well as mixing organic acids of both laboratory and natural origin. The results showed the highest recovery percentages for scandium (91%) and yttrium (94%) using a 1 M sulfuric acid aqueous solution with a processing time of 1 hour for scandium and 48 hours for yttrium. When citric and acetic acids were used, the selectivity regarding yttrium and scandium was significantly better compared to sulfuric acid, although they exhibited a lower extraction rate of rare earth elements, with a higher extraction rate of 40% in the citric acid aqueous solution. In the aqueous solutions where organic acids and sulfuric acid were mixed, satisfactory results were obtained, especially for mixing ratios of 50%-50% and 75%-25% (sulfuric acid-organic acid) in terms of rare earth element leaching efficiency. Laboratory organic acids showed a slightly higher recovery percentage of scandium and yttrium compared to organic acids of natural origin. These results suggest a potential reduction in environmental impact by using environmentally friendly and low-cost products such as vinegar and lemon, thereby decreasing the use of hazardous and harmful chemical substances.

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