THE BEHAVIOR OF THALLIUM DURING JAROSITE PRECIPITATION

Jarosite precipitation provides an effective means of eliminating thal lium from zinc processing circuits, and a systematic study of the exte nt and mechanism of thallium removal during the precipitation of ammon ium, sodium, and potassium jarosites was carried out. Thallium (as Tl) substitutes for the ''alkali'' ion in the jarosite structure. Nearly ideal jarosite solid solutions are formed with potassium, but thalliu m is preferentially precipitated relative to either ammonium or sodium . Approximately 80 pet of the dissolved thallium precipitates during t he formation of ammonium jarosite; the extent of thallium removal is v irtually independent of thallium concentrations in the 0 to 3000 mg/L Tl range and of the presence of 75 g/L of dissolved Zn. Although the d eportment of thallium is nearly independent of (NH4)(2)SO4 or Na2SO4 c oncentrations >0.1 RI, the precipitates made from more dilute media ar e relatively enriched in thallium. Likewise, the precipitates made fro m dilute ferric ion media are also Tl-rich. Low solution pH values or low temperatures both significantly reduce the amount of jarosite form ed, but the precipitates made under these conditions are enriched in t hallium. Furthermore, because thallium jarosite is more stable than th e ammonium or sodium analogues, the initially formed precipitates are consistently Tl rich. The presence of jarosite seed accelerates the pr ecipitation reaction, but dilutes the thallium content of the product. The results suggest that most of the thallium in a hydrometallurgical zinc circuit could be selectively precipitated in a small amount of j arosite, by carrying out the precipitation reaction for a short time i n the absence of seed and from solutions having low alkali concentrati ons.