Kinetic study on arsenopyrite dissolution in nitric acid media by the rotating disk method

Abstract

The kinetics of arsenopyrite compact samples dissolution in nitric acid media was investigated using the full factorial experiment model (FFEM). The effects of nitric acid concentration (2–5 M), temperature (303–348 K), disk rotation frequency (3.33–10 rps) on the arsenopyrite dissolution rate were studied. Nitric acid concentration and temperature have a significant effect, and disk rotation frequency does not. With an increase in nitric acid concentration from 4 M to 5 M and temperature from 318 K to 333 K, the dissolution rates of iron and arsenic increase by 30.1–30.4 times, from 0.26–0.27 × 10–6 mol dm−2 s−1 to 7.83–8.21 × 10–6 mol dm−2 s−1. Increasing temperature to 348 K does not lead to a significant increase in the dissolution rate. The data obtained were processed using the FFEM to obtain kinetic equations (R2 = 0.95): WFe=0.04CHNO31.8e-4419Tω0.1, WAs=0.04CHNO31.8e-4420Tω0.1. Using SEM–EDS and XPS analyses, the mechanism of arsenopyrite passivation was determined. Under mild conditions (nitric acid concentration of 3 M and temperature of 303 K), local conglomerates containing iron polysulphides (Fe2Sx) are formed on the arsenopyrite surface. They do not create diffusion difficulties when dissolving arsenopyrite. Under harsh conditions (nitric acid concentration of 5 M and temperature of 333 K), polysulphides begin to aggregate. It leads to the formation of the passivating film containing elemental sulphur. The film is thin (20–30 μm) and porous. While the arsenopyrite dissolution rate is high. Further increase of parameters leads to thickening and compaction of the passivating film and decrease in the arsenopyrite dissolution rate.