Characterization and alkaline decomposition/cyanidation of beudantite-jarosite materials from Rio Tinto gossan ores

Jarosite-type minerals are the major silver carriers in the gossan ores fro m Rio Tinto (Spain). Two types of minerals were found. one corresponding to beudantite variable enriched in sulfate; the other is potassium jarosite c ontaining various amounts of arsenate acid lead. They are isostructural wit h cell parameters intermediate between those reported for end members. Silv er is present in both jarosites as dilute solid solution (230 ppm Ag in ave rage). The cyanidation of potassium jarosite in saturated Ca(OH)(2) at 70-1 00 degrees C consists of two step in series: a slow step of alkaline decomp osition followed by a fast step of Ag complexation from the decomposition s olids. The alkaline decomposition is characterized by the simultaneous remo val of sulfate and K ions and the formation of an amorphous hydroxy-arsenat e of Fe, Pb and Ca. The kinetics are chemically controlled, with an activat ion energy of 86.5 kJ mol(-1). The nature of the alkaline decomposition of beudantite was similar but extremely slow at less than or equal to 100 degr ees C. (C) 1999 Canadian institute of Mining and Metallurgy. Published by E lsevier Science Ltd. All rights reserved.