Metals (May 2021)
Deposition of Arsenic from Nitric Acid Leaching Solutions of Gold–Arsenic Sulphide Concentrates
Abstract
At present, the processing of refractory gold–arsenic sulphide concentrates is becoming more relevant due to the depletion of rich crude ore reserves. In the process of the nitric acid leaching of arsenic sulphide minerals, solutions are formed containing 20–30 g/L of arsenic (III). Since market demand for arsenic compounds is limited, such solutions are traditionally converted into poorly soluble compounds. This paper describes the investigation of precipitating arsenic sulphide from nitric acid leaching solutions of refractory sulphide raw materials of nonferrous metals containing iron (III) ions using sodium hydrosulphide with a molar ratio of NaHS/As = 2.4–2.6, which is typical for pure model solutions without oxidants. The work studied the effect of temperature, the pH of the solution and the consumption of NaHS and seed crystal on this process. The highest degree of precipitation of arsenic (III) sulphide (95–99%) from nitric acid leaching solutions containing iron (III) ions without seed occurs with a pH from 1.8 to 2.0 and a NaHS/As molar ratio of 2.8. The introduction of seed crystal significantly improves the precipitation of arsenic (III) sulphide. An increase in seed crystal consumption from 0 to 34 g/L in solution promotes an increase in the degree of transition of arsenic to sediment from 36.2 to 98.1% at pH = 1. According to SEM/EDS and XRF sediment data, from the results of experiments on the effect of As2S3 seed crystal consumption, acidity and molar ratio of NaHS/As on the precipitation of arsenic (III) sulphide and the Fetotal/Fe2+ ratio in the final solution, it can be concluded that the addition of a seed accelerates the crystallisation of arsenic (III) sulphide by increasing the number of crystallisation centres; as a result, the deposition rate of As2S3 becomes higher. Since the oxidation rate of sulphide ions to elemental sulphur by iron (III) ions does not change significantly, the molar ratio of NaHS/As can be reduced to 2.25 to obtain a precipitate having a lower amount of elemental sulphur and a high arsenic content similar to that precipitated from pure model solutions.
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