Factors affecting alkali jarosite precipitation
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I.
INTRODUCTION
THE formation
of jarosite-type compounds (MFe3" where M = Na, K, gb, NH4, Ag, H30, etc.) is of great interest to mineralogists and metallurgists alike. 1 Mineralogically, jarosite-type compounds are encountered wherever sulfides, especially pyrite, undergo rapid oxidation under acidic conditions; such environments exist in pyritic coal seams, sulfide ore deposits, 2's or in soils developed from pyrite-bearing marine sediments. 4 Metallurgically, the interest originates in the ability of jarositetype compounds to precipitate iron from acidic leach solutions in a readily filterable form and to give relatively low losses of the common divalent base metals such as Zn "+, Cd 2+, Cu 2+, or Co 2+ in the washed residues. Processes have been developed to remove iron from zinc sulfate processing solutions5-8 and, more recently, to precipitate iron from copper chloride leach solutions9 or from cobalt sulfate processing liquors. ~~ For either mineralogical or metallurgical applications it is important to have a firm understanding of the factors which affect both the extent of jarosite precipitation and the composition of the jarosite. For example, mineralogists are concerned with the partitioning of various alkali ions during jarosite formation in supergene deposits and with the extent of hydronium (H30 +) substitution for the alkalis. Metallurgists are keenly concerned with the effect of temperature, seeding, pH, etc. on the yield and composition of the product as well as with the practical aspects of carrying out the precipitation reaction on a commercial scale. Given the widespread importance and usage of jarositetype compounds, it might be assumed that the factors affecting alkali jarosite precipitation, as well as the (SO4)2(OH)6
J. E. DUTRIZAC is a Research Scientist wi~hCANMET, Energy, Mines and Resources Canada, 555 Booth Street, Ollawa, Ontario, Canada K1A 0GI, Manuscript submitted February 1, 1983. METALLURGICALTRANSACTIONS B
thermodynamics and kinetics of reaction, were all well known and defined. Recent reviews of the jarosite literature, H,12however, indicate that this is not the real situation. Although a number of studies have reported on the precipitation of alkali jarosites, none has been fully systematic for all the important metallurgical parameters, and significant discrepancies exist among the published works. ~2 In the present investigation, the physical and chemical parameters affecting alkali jarosite formation have been defined systematically using sodium jarosite as the model. Some work was also done on potassium, rubidium, and ammonium jarosites to ascertain their relative jarosite forming potential, and partitioning coefficients among several pairs of the alkali jarosites have been presented.
II.
EXPERIMENTAL
Reagent grade chemicals were used for all synthesis experiments, Most of the jarosites were formed simply by dissolving the appropriate sulfate reagents in water, adjusting the pH with either Li2CO3 or H2SO4, and then heating to 97 ~ in a 2-L reaction kettle. Solid lithi
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