MAINTAINING AN OXALATE FREE PRECIPITATION CIRCUIT AT AUGHINISH ALUMINA
O’Connell, A., Vaughan, J., Beaulieu, S., Ryan, T., Mooney, A. and Baxter, H.
Maintaining an oxalate free precipitation circuit is key to product quality at Aughinish Alumina. This is achievable by removing oxalate from a side stream of spent liquor by deep evaporation. A small sidestream of spent liquor is evaporated to approximately double the caustic strength so that the oxalate and other salts become supersaturated. A crystallization process follows whereby with no seeding, oxalate and other salts co precipitate to an end concentration. This concentration is still supersaturated and therefore further atmospheric crystallization follows.
The atmospheric crystallization is a seeded process where time, temperature, and surface area are important to allow further oxalate precipitation. There is evidence that seed poisoning occurs in this part of the process. Testwork has shown that theoretical equilibrium is well below that achieved in the plant. How to affect the equilibrium and maximize oxalate production is presented.