Arsenic transformation during the pressure oxidation and curing process: a thermodynamic and kinetic study
Creators: name: Liu, Yanhua
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401103
subject: Global and planetary environmental engineering
classificationCode: 401903
subject: Hydrometallurgy

description: In pre-treatment of sulfide ores such as pyrite and arsenopyrite, pressure oxidation (POX) is an efficient technique for releasing the refractory gold prior to cyanidation. However, there is ongoing debate surrounding the mechanisms of arsenic-bearing sulfide mineral dissolution, and phase transformation during POX and curing process, in part due to limited thermodynamic data and in-situ studies. The scope of this thesis is to further our understanding of the behavior and transformation of arsenic and ferric species during POX and curing via thermodynamic and kinetic modelling along with experimental approaches. Towards this aim, the first step of this thesis was to fill in the gaps in our knowledge of the thermodynamic properties of arsenate minerals and partial sulfate minerals at elevated temperatures (25 – 300oC). In this work, the thermodynamic properties for arsenate and sulfate minerals were estimated at temperatures between 25 to 300oC using a polyhedral method, which demonstrated good agreement with the available experimental data from literature. Subsequently, based on the calculated data, a model of the species distribution for ferric and arsenic species was constructed to investigate the solubility of scorodite at room temperature as a function of pH. The work highlighted the importance of the equilibrium with ferric oxide and hydroxide phases, and proposed this to be the reason for the variation in literature data.

Subsequently, Eh-pH diagrams for the Fe-S-H2O and Fe-As-S-H2O systems were developed at typical POX and curing temperature on the basis of the calculated data for arsenate and sulfate minerals. Based on the analysis of thermodynamic diagrams for Fe-S-H2O system at 225oC, the formation of basic ferric sulfate (BFS: Fe(OH)SO4) was found to be favored under stronger acidic conditions and higher solution redox potentials, while hematite (Fe2O3) and szomolnokite (FeSO4∙H2O) are formed at low solution redox potentials. This is consistent with results that were observed from in situ powder X-ray diffraction (PXRD) measurements using a capillary setup for the in-situ pressure leaching of pyrite at 225oC. Eh-pH diagrams developed for the Fe-As-S-H2O systems at 100oC and 225oC predicted the predominant solid phases precipitating during POX of arsenopyrite at a pH range of 0 to 0.5, which included BFS, FeSO4∙H2O, hydronium jarosite ((H3O)Fe3(SO4)2(OH)6) and angelellite (Fe4O3(AsO4)2). The Eh-pH diagrams predicted the FeSO4∙H2O precipitate to be the stable phase during the annealing stages. The in-situ PXRD experimental results confirmed the formation of FeSO4∙H2O, BFS and hydronium jarosite during the annealing stages, while BFS and/or hydronium jarosite dissolved during cooling. 

To understand the stability of the arsenic-bearing phases during pressure leaching and curing process, a series of POX experiments were conducted in a laboratory 300 mL Parr autoclave with a synthetic mixture of pyrite and arsenopyrite. For a feed of pure arsenopyrite, the arsenic in the POX product is present as basic ferric arsenate sulfate (BFAS) prior to curing. This is followed by a transformation to scorodite after 18 h of curing at 90 – 120oC, which also improves arsenic removal from solution by nearly 60%. The transformation does not occur with curing at 60oC, and the arsenic remains as BFAS. For a feed of Fe/As = 2 to 4, the arsenic is present as BFAS in the POX product before curing. Toxicity characteristic leaching procedure (TCLP) results for the arsenic in the leach extract range from 10 – 15 ppm for the BFAS product without curing to 5 ppm for the scorodite and BFAS after curing. Above Fe/As = 4, the arsenic is present as surface-adsorbed arsenate and sulfate-substituted arsenate, both of which are less stable. As a concurrent work, optimization using response surface methodology (RSM) was performed to maximize arsenic removal from solution. Good agreement was demonstrated between the model and the experimental results, and the experimentally validated optimized model was found to provide a 10% increase in arsenic removal over the best model input. The maximum arsenic removal for the investigated system was achieved at 107oC, a critical threshold temperature, over which scorodite formation decreases with temperature. Thermodynamic modelling revealed the formation of soluble FeHAsO4+ complexes over scorodite above this threshold temperature, decreasing arsenic fixation at higher temperatures.

Furthermore, the POX of arsenopyrite and arsenopyrite-pyrite mixtures in a H2SO4–O2 system was investigated with the aim of using the oxygen consumption kinetics to probe the oxidation mechanisms. Kinetic analysis of arsenopyrite dissolution at 180 to 220oC indicated an activation energy (Ea) of 48.46 kJ/mol, suggesting a mixed control system. The order of the reaction to acidity was 0.16 at 200oC, suggesting that [H+] is likely not the rate-limiting factor. The addition of Fe(II) ions to the feed enhanced arsenopyrite oxidation, due to the formation of Fe(III) as a surrogate oxidant to drive the reaction. The Ea for Py/Aspy of 1:1 exhibited two behaviors, calculated to be 77.32 kJ/mol below 200oC and decreasing to 34.59 kJ/mol above 200oC. The former was attributed to surface control due to the oxidation of elemental sulfur, followed by a switch to mixed control at elevated temperatures. As the Py/Aspy in the feed was increased to 2:1 and 3:1, a single behavior was observed, with Ea values of 56.01 kJ/mol and 43.35 kJ/mol, respectively. These values imply a mixed reaction control, and the values begin to approximate literature values for the oxidation of pure pyrite.