The behavior of thallium during jarosite precipitation
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TRODUCTION
Thallium is a trace constituent of most zinc ores and concentrates. On a global basis, the average Tl content of zinc concentrates is 20 ppm, but Tl contents as high as 100 ppm have been reported in individual concentrates.[1] In the conventional roast-leach-electrolysis process for zinc concentrates, the thallium remains in the calcine and eventually dissolves in the leaching circuit. The presence of dissolved Tl in zinc processing solutions is undesirable, as thallium codeposits with zinc and contaminates the product.[2] For this reason, thallium must be continuously removed from the zinc circuit to maintain its concentration at ,5 mg/L, and ideally at ,1 mg/L, in the purified solution sent to electrolysis.[3] The market for thallium is extremely limited; for example, consumption in the United States is less than 2000 kg/y.[4] The element is also very toxic, even at low concentrations; for these reasons, the common practice in the zinc industry is to stabilize the thallium in a residue or precipitate which can be impounded indefinitely in a suitably controlled site. The jarosite process is widely used in the zinc industry to eliminate iron, sulfate, and alkalis as an easily filterable jarosite-type compound (MFe3(SO4)2(OH)6, where M 5 K, Na, NH4, H3O, Ag, Tl, etc.). Jarosite precipitates are relatively stable and they incorporate thallium in their structure.[5] For these reasons, jarosite precipitation is commonly used to eliminate and stabilize thallium in zinc operations. Despite the general importance of jarosite precipitation for thallium control in the zinc industry, relatively little is known about thallium jarosite or Tl-bearing alkali jarosites. J.E. DUTRIZAC, Research Scientist, is with CANMET, Ottawa, ON, Canada K1A 0G1. Manuscript submitted December 10, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
Dutrizac and Kaiman[5,6] synthesized end-member thallium jarosite and determined its structure. The thallium compound was shown to be isostructural with jarosite; it is the most dense of all jarosite-type compounds. Yaroslavtsev et al.[7] reported on the relative partitioning of thallium, as well as a number of other elements, between the solution and potassium jarosite precipitated after 8 h at 90 7C from solutions containing 28 g/L Fe (as ferric sulfate) and 98 g/L Zn (as zinc sulfate) at an initial room-temperature pH of 1.5. Their data were discussed in terms of a capture coefficient, K, which was defined as K5
impurity in jarosite (wt pct) impurity in initial solution (g/100 cc)
[1]
The greater the tendency for the impurity to concentrate in the jarosite, the higher is the value of K. For solutions initially containing 0.3 to 0.72 g/L Tl+, the value of K ranged between 6.7 and 5.0. Virtually identical results were obtained when Tl3+ was used, and the implication is that Tl3+ is rapidly reduced to the Tl+ state. In any event, the conclusion is that thallium is extensively incorporated in potassium jarosite, even when the thallium is present at relatively low concentrations in the synthes
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