A study of solid-aqueous equilibria by the speciation approach in the hydronium alunite-sulfuric acid-water system at hi
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INTRODUCTION
THE sulfuric acid pressure leaching process for the production of nickel and cobalt from equatorially located nickeliferous limonites is conducted at high temperatures (230 ~ to 280 ~ and acidities (PH25*c < 1).tt-41Under these conditions, the target metals are leached into the acid sulfate liquor, with recoveries exceeding 95 pct.m In contrast, iron and aluminum, which are the most abundant elements in the ore, are controlled in the leach liquor by hydrolysis and precipitation as hematite, FezO3, and hydronium alunite, (H30)A13(SOa)2(OH)6 , respectively. Unfortunately, these solid products are known to be the principal components of autoclave scale. In fact, in industrial practice (i.e., at Moa Bay, Cuba) the scaling of the internal surfaces of the autoclaves and their connecting pipe-work is so severe that frequent forced shutdowns are required in order to remove the material.t3.5] In addition, scaling of leaching ore particles by these solids may contribute to secondary nickel and cobalt losses by retarding diffusional processes, as suggested by Sobol.tS] While hematite chemistry has been previously studied by several authors to some extent, very little information is available for hydronium alunite systems. Thus, a study of hydronium alunite formation chemistry and solubility is presently warranted prior to investigating its role in autoclave scaling. A reaction pathway for aluminum from the ore source (gibbsite) to the precipitation product (hydronium alunite) during pressure acid leaching has been suggested to occur as follows, tr,7'sl
B.C. BLAKEY, formerly Graduate Student, Department of Chemical Engineering and Applied Chemistry, University of Toronto, is currently with INCO Ltd, JRGR Laboratory, Mississauga, ON, Canada L5K IZ9. V.G. pAPANGELAKIS, Assistant Professor, is with the Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada M5S 1A4. Manuscript submitted January 25, 1995. METALLURGICALAND MATERIALSTRANSACTIONS B
(1) Gibbsite, which is the principal soluble aluminum phase in limonites, first transforms to boehmite during ore preheating. This transformation[61 takes place between 135 ~ and 155 ~ 3Al(OH)3~.,--~ 3AIOOH(~.~ + 3H20
[1]
(2) Boehmite is then acid leached at autoclave temperatures (230 ~ to 270 ~ 3AIOOH{s) + 9H + ~ 3A13+ + 6H20
[2]
(3) Hydronium alunite finally precipitates in situ at the same temperature (i.e., between 230 ~ and 270 ~ 3AP + + 2SO4- + 7H20 (H30)A13(SO4)2(OH)6/+, + 5H§
[3]
The lack of information available on the equilibrium behavior of aqueous systems is usually attributed to the difficulties in assessing the equilibrium picture (i.e., solution speciation-complexation) directly at high temperatures (i.e., T > > 100 ~ This is primarily due to the inability of conventional analytical techniques to quantify high-temperature solution compositions exactly. To make such a determination by conventional means, samples from high-temperature systems must be withdrawn and cooled and analyzed at room temperature. T
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