Solid phase changes in chemically and biologically leached copper smelter slag

- A sample of copper smelter slag was tested for chemical and bacterial leaching.
- The major phases of the Cu slag were fayalite, glassy silicates, and magnetite.
- The slag sample contained minor amounts of oxides, metallic Cu, and sulfides.
- The most reactive mineral in the leaching was fayalite.
- Cu-sulfides in the slag were particularly refractory.

Pyrometallurgical processes for sulfide concentrates produce slag waste materials that contain multiple metals as carry-over impurities. Varied quantities of the metal content can be solubilized from the slag through chemical acid leaching or bioleaching. In this study, copper smelter slags were leached with and without iron- and sulfur-oxidizing bacteria in acid, sulfate-rich solutions and solid phase changes were determined. The samples (10% pulp density) were leached at 28 °C in stirred tanks at pH 2.5 and 3.0. The duration of the experiment was 37 d and the adjusted pH decreased to 2.3 in both sets of experiments. The primary metals of interest in the slag were Cu and Zn, with minor amounts of Ni and Co, and As, Sb, and Pb at trace levels. The metals in the slag were distributed in fayalite, glassy silicates and magnetite phases as well as minor amounts in oxides, metallic Cu and sulfides. Based on the solution analyses, the dissolution yields were 44% and 34% for Cu, 14% and 13% for Zn and Co, and 4.5% and 4.0% for Ni at initial pH values of 2.5 and 3.0, respectively. Fayalite was the most reactive mineral and Cu-sulfides were particularly refractory. The elemental composition of leached residues was matched with solid phase changes and the distribution of metals in the mineral phases.