Converter slag leaching in ammonia medium/column system with subsequent crystallisation with NaSH

Converter slags are by-products of pyrometallurgical processing of copper concentrates in Chile, which contain significant amounts of copper (close to 40.0% in the material considered here), converting them from a passive material (material that is not feasible to process) to an active material (it can be processed). This study analyses column leaching of converter slag with the use of ammonium hydroxide. A preliminary analysis was carried out on acid leaching (in an agitated system). Later, in ammonia medium, several variables were assessed, including particle size and NH₄OH concentration, as well as different grades of acid purity to adjust the pH of the leaching solution. The experiments were carried out in 1.2 m high columns with a cross-section diameter of 7.5 × 10⁻² m, using 2.0 kg of slag. The results show that using an acid system (i.e. ordinary leaching), reagent consumption reached 473.9 kg H₂SO₄/ton of slag, with copper recovery of only 50.8% and Fe recovery of over 67.0%. On the other hand, using an ammonia system, the recovery values reached 87.7% for Cu, with almost no impurities, with reagent consumption of 3.8 kg H₂SO₄/ton of slag. The working pH was 10.5. These recovery levels are due to the leaching of Cu₂O and Cu° from the converter slag. When using an acid contaminated with impurities to adjust the pH, the copper extraction rate was increased. The impurities of antimony, bismuth and arsenic are not significant. The copper in pregnant solution (PLS) obtained was put into contact with NaSH to generate hydrated chalcanthite (CuSO₄·5H₂O) with a high grade of purity, while the remaining solution can be returned to the leaching stage due to its high level of ammonia content.