Effect of annealing treatment on the crystallisation and leaching of dumped base metal smelter slags

Leaching tests of base metals contained in two smelter slags were undertaken in ammonia and nitric acid solutions aiming to recover Co, Cu and Zn. Leaching tests were conducted at 25 and 60 °C at pH = 0 and 3 in HNO3 and pH = 12 in NH4OH media. XRD analysis revealed that the dumped slags were amorphous. Annealing these slags at 1180 °C produced crystalline phases comprising diopside, magnetite and fayalite. SEM and EDS analysis revealed that Cu and Pb compounds have concentrated in the magnetite phase, whereas another phase rich in Zn and Cu was located in the diopside matrix. ICP-OES analysis of the pregnant leaching solutions (PLS) showed that 30-60% of Co, Cu and Zn were released from the amorphous slags treated in HNO3 at pH = 0, and lesser in ammonia. However, the contamination by Fe and Pb was higher at pH = 0. The contamination of the PLS obtained by leaching of the crystallised slags remained low. The low Fe and Pb contamination was attributed in this case to the chemical stability of the crystalline phases formed upon annealing treatment. The higher solubilisation of metals contained in amorphous slags was attributed to the collapse of silicate structures during nitric acid leaching at pH ∼ 0.