Ammonium jarosite formation in ferric chloride leaching processes

Synopsis

Ammonium jarosite is readily formed from FeCl₃−NH₄Cl leaching solutions containing an independent source of sulphate. The principal factors affecting NH₄-jarosite formation are pH, temperature and the concentrations of sulphate and FeCl₃. At 98° C in 4 M NH₄C1 media, no precipitate forms when the pH is <0.4; pH values >1.8 result in difficult-to-filter products. Only traces of jarosite are precipitated at 50°C, but the amount of product increases systematically with increasing temperatures above this value. A minimum sulphate concentration is required for NH₄-jarosite precipitation, and the critical sulphate concentration is a function of the ferric ion concentration. When excess sulphate is present, the amount of jarosite increases directly with the FeCl₃ concentration. The addition of jarosite seed accelerates the precipitation reaction. The jarosite precipitation reaction is relatively independent of the concentrations of NH₄Cl (2.0–5.0 M) and the FeCl₂ reaction product (0.0–1.0 M FeCl₂). Regardless of the PbCl₂ concentration of the solution, the jarosite products precipitated from 4 M NH₄C1 solutions have Pb contents <0.2%. The Pb content of the ammonium jarosite exceeds 1% only when the NH₄C1 concentration is <2 M. Regardless of the AgCl concentration of the solution, the silver content of the ammonium jarosite is always <0.01% Ag. The low levels of Pb and Ag incorporation in the ammonium jarosite are attributed to the strong anionic complexation of these elements in concentrated chloride media.