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Please use this identifier to cite or link to this item: http://hdl.handle.net/1807/24672

Title: Evaluating the Potential of Scaling due to Calcium Compounds in Hydrometallurgical Processes
Authors: Azimi, Ghazal
Advisor: Papangelakis, Vladimiros G.
Department: Chemical Engineering and Applied Chemistry
Keywords: Calcium sulphate
Gypsum
Anhydrite
Hemihydrate
Chemical Modelling
Solubility measurements
Transformation mechanism
Transformation kinetics
Hydrometallurgy
Scaling
Laterite Pressure Acid Leaching
Solid phase transformation
Issue Date: 4-Aug-2010
Abstract: A fundamental theoretical and experimental study on calcium sulphate scale formation in hydrometallurgical solutions containing various minerals was conducted. A new database for the Mixed Solvent Electrolyte (MSE) model of the OLI Systems® software was developed through fitting of existing literature data such as mean activity, heat capacity and solubility data in simple binary and ternary systems. Moreover, a number of experiments were conducted to investigate the chemistry of calcium sulphate hydrates in laterite pressure acid leach (PAL) solutions, containing Al2(SO4)3, MgSO4, NiSO4, H2SO4, and NaCl at 25–250ºC. The database developed, utilized by the MSE model, was shown to accurately predict the solubilities of all calcium sulphate hydrates (and hence, predict scaling potential) in various multicomponent hydrometallurgical solutions including neutralized zinc sulphate leach solutions, nickel sulphate–chloride solutions of the Voisey’s Bay plant, and laterite PAL solutions over a wide temperature range (25–250°C). The stability regions of CaSO4 hydrates (gypsum, hemihydrate and anhydrite) depend on solution conditions, i.e., temperature, pH and concentration of ions present. The transformation between CaSO4 hydrates is one of the common causes of scale formation. A systematic study was carried out to investigate the effect of various parameters including temperature, acidity, seeding, and presence of sulphate/chloride salts on the transformation kinetics. Based on the results obtained, a mechanism for the gypsum–anhydrite transformation below 100°C was proposed. A number of solutions for mitigating calcium sulphate scaling problems throughout the processing circuits were recommended: (1) operating autoclaves under slightly more acidic conditions (~0.3–0.5 M acid); (2) mixing recycled process solutions with seawater; and (3) mixing the recycling stream with carbonate compounds to reject calcium as calcium carbonate. Furthermore, aging process solutions, saturated with gypsum, with anhydrite seeds at moderate temperatures (~80°C) would decrease the calcium content, provided that the solution is slightly acidic.
URI: http://hdl.handle.net/1807/24672
Appears in Collections:Doctoral
Department of Chemical Engineering and Applied Chemistry - Doctoral theses

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