Solubility of Chromium in DON Smelting

Chromia-bearing raw materials in nickel and copper matte smelting are difficult to process due to their tendency of forming solid chromite spinel precipitates, leading to formation of mushy slag and buildups in the smelting vessel. The solubility of chromia in smelting slags, and especially in mattes, are not known accurately and new data for iron-silicate slags in equilibrium with low-iron nickel mattes have been measured at 1350–1450°C. Typical copper-bearing nickel mattes with Ni:Cu≈2 (w/w) in the DON (Direct Outotec Nickel) process with 2 to 10 wt% [Fe]_matte have been equilibrated in carefully controlled S₂-O₂-SO₂-Ar gas atmosphere experiments with the corresponding silica saturated iron-silicate slags. The phase assays post quenching were measured by electron probe X-ray microanalysis, including the molten slag and matte as well as the solid phase of chromite spinel. Laser ablation ICP mass spectrometry was used to measure the trace elements in the matte. An additional variable in the slag composition was magnesia concentration, varying from zero to 10 wt% (MgO)_slag. The solubility of chromium in the slag at 1400°C was ≈0.7 wt% (Cr) and in the nickel matte 30–100 ppm [Cr], depending on the iron concentration of the sulfide matte. The impact of MgO on the chromium concentration in slag was small and within the experimental error of the measurements.