Factors affecting the precipitation of chromium(III) in jarosite-type compounds
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INTRODUCTION
CHROMIUM salts are widely used for metal finishing, steel plating, and leather tanning.[1] Also, chromium is a minor constituent of many ores and metallurgical concentrates; for example, zinc concentrates contain up to 125 ppm Cr and have a reported average Cr content of 40 ppm.[2] In all of the preceding applications, Cr-bearing processing solutions and Crbearing effluent streams are generated. In most instances, the chromium is present either in the hexavalent (chromate) or trivalent oxidation states.[3] Because chromium salts are toxic and can create processing problems, the dissolved chromium must be precipitated, or otherwise eliminated, at some point in the processing flow sheet. Jarosite precipitation is widely used in the metallurgical industry to precipitate iron in a readily filterable form from processing solutions, and jarosite precipitation effectively controls a number of impurity species as well.[4,5] The type formula for jarosite-type compounds is KFe3(SO4)2(OH)6, but it is known that a number of species can replace the monovalent K cation, the trivalent Fe cation, and the divalent SO4 anion in the jarosite structure.[5] In fact, it is well established that the divalent chromate anion (CrO4) substitutes completely for sulfate in the jarosite structure,[6] and the chromate analogue of potassium jarosite has been identified in contaminated soil from a metal plating facility.[7] More recently, studies have detailed the solubility of the chromate analogue and have demonstrated that an ideal solid solution series exists between KFe3(SO4)2(OH)6 and KFe3(CrO4)2(OH)6.[8,9] The behavior of Cr(III) during jarosite precipitation is less well defined, although the Cr(III) analogues of both hydronium jarosite J.E. DUTRIZAC and T.T. CHEN, Research Scientists, are with CANMET, Mining and Mineral Sciences Laboratories, Ottawa, ON, Canada K1A-OG1. Contact e-mail: [email protected] Manuscript submitted April 21, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
(H3OCr3(SO4)2(OH)6) and potassium jarosite (KCr3(SO4)2(OH)6) have been synthesized from chromium sulfate solutions at 300 °C.[10,11] Furthermore, Kolitsch and Pring[12] have reported extensive Cr(III) substitutions in some naturally occurring phosphate and arsenate analogues of jarosite-type compounds. Although the preceding discussion indicates the existence of Cr(III) analogues of jarosite-type compounds, the factors affecting the precipitation of these species have not been thoroughly defined. Such information is important, however, not only for the control of chromium in processing circuits but also for the synthesis of the Cr(III) analogues themselves, which have been shown to have unusual magnetic properties.[13,14] Accordingly, a systematic series of experiments was carried out to define the behavior of Cr(III) during the precipitation of jarosite-type compounds at temperatures up to 220 °C. The results of those studies are presented in this article. II. EXPERIMENTAL A. Synthesis Procedures To try to precipitate the Cr(III) analogue of p
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