Factors Affecting the Precipitation of Potassium Jarosite in Sulfate and Chloride Media
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JAROSITE precipitation is widely used in the metallurgical industry to precipitate iron in a readily filterable form from hydrometallurgical processing solutions;[1,2] jarosite precipitation effectively controls a number of impurity species, as well.[3,4] The general formula for jarosite-type compounds is KFe3(SO4)2 (OH)6, but it is well established that a number of species, + such as Na+, Ag+, NH+ 4 , H3O , etc., can replace the monovalent potassium ion in the jarosite structure.[2,4] Potassium jarosite is the most common naturally occurring jarosite species[5,6] and is also the thermodynamically most stable species.[7–10] Under comparable synthesis conditions, the rate of precipitation of potassium jarosite is faster than that of either ammonium jarosite, sodium jarosite, or hydronium jarosite.[11,12] Thus, the precipitation of potassium jarosite in the metallurgical industry could offer several advantages J.E. DUTRIZAC, Research Scientist, is with the Mining and Mineral Sciences Laboratories, CANMET, Ottawa, ON, Canada. Contact e-mail: [email protected] Manuscript submitted May 29, 2008. Article published online November 15, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
related to both the rate and extent of iron removal in the process and the stability of the jarosite residues in disposal impoundments. Despite the potential advantages of potassium jarosite precipitation, almost all commercial processes are based on the use of sodium jarosite or ammonium jarosite; the reasons are entirely related to the much lower costs of the sodium or ammonium reagents relative to the corresponding potassium salts. Nevertheless, the precipitation of potassium jarosite remains an interesting option in some metallurgical operations, in which the nearly complete iron removal from acidic sulfate or chloride media is desired or the control of potassium is an important process consideration. From the early 1970s, numerous studies have been carried out to demonstrate that potassium jarosite is readily synthesized from acidic ferric sulfate solutions containing soluble potassium ions.[13–15] Although these and subsequent[2,16] studies have shown that temperature, ferric-ion concentration, and solution acidity are important parameters for the precipitation of potassium jarosite, none of the studies provided a systematic examination of the precipitation parameters over the range of temperatures and solution compositions likely to be encountered in the metallurgical industry. VOLUME 39B, DECEMBER 2008—771
In contrast, such systematic investigations have been reported for the precipitation of sodium and ammonium jarosites,[17] silver jarosite,[18] and thallium jarosite.[19] Given the widespread natural occurrence of potassium jarosite and its potential importance in metallurgical processing, a systematic series of experiments was carried out to define the parameters affecting the precipitation of potassium jarosite (KFe3(SO4)2(OH)6) over a wide range of solution concentrations and at temperatures up to 220 °C. The results of these
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