The precipitation of hematite from ferric chloride media at atmospheric pressure
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I. INTRODUCTION
IRON occurs abundantly in the earth’s crust and is a common impurity in most base-metal ores and concentrates.[1] Depending on the combination of technologies used to produce the base metal, most of the iron is rejected as mill tailings, smelter slag, or a hydrometallurgical residue. In acid-based hydrometallurgical circuits, however, a significant fraction of the iron dissolves, and, at some point in the process, the dissolved iron must be eliminated to prevent its interference with the subsequent purification or metal recovery operations. In this regard, Table I indicates the estimated amounts of soluble iron generated during the production of four metals as a consequence of acid leaching or, in the case of steel, acid pickling operations.[2] The amount of soluble iron generated in the metallurgical industry greatly exceeds the current demand for soluble iron chloride or iron sulphate salts. In this regard, Rosato and Agnew[3] have estimated the North American demand for iron chlorides to be 222,000 tonne/yr and that of the iron sulphates to be 55,000 tonne/yr. As a consequence, it is common practice in the industry to precipitate the dissolved iron and to dispose of the resulting precipitate. Simple neutralization is sometimes used, but this produces Fe(III) hydroxide-type precipitates which are difficult to filter and adsorb large amounts of the associated base metals. To circumvent these problems, most metal producers precipitate readily filterable jarosite or goethite (a-FeO?OH) compounds, which do not incorporate significant amounts of the associated base metals. Such processes, however, generate large volumes of residues, which commonly contain unacceptably high levels of hazardous impurities like As, Pb, and Cd. The steady accumulation of large tonnages of such impurity-containing residues is a growing environmental J.E. DUTRIZAC and P.A. RIVEROS, Research Scientists, are with the Mining and Mineral Sciences Laboratories, CANMET, Natural Resources Canada, Ottawa, ON, Canada, K1A OG1. Manuscript submitted April 13, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
concern, despite the fact that the residues are presently stored in secured impoundment basins. Paradoxically, as the base-metals industry continues to impound large tonnages of iron-rich residues, vastly greater amounts of iron are mined from primary sources to satisfy the demand for a diversity of iron products. Environmental considerations dictate that useful iron products should be recovered from the metallurgical processing streams and effluents, both to reduce the volume of residues impounded and to minimize the mining of primary iron ores. Examples of useful, or potentially useful, iron products include iron salts for water treatment, electrolytic iron, and hematite. Hematite is especially interesting, as it is the starting material for the production of pigments, ferrites, and, of course, iron and steel. Furthermore, hematite is the most acceptable iron product for intermediate storage or disposal, because of its high ther
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