Arsenopyrite oxidation

For arsenopyrite, oxidation reactions include the reactions producing elemental sulphur ... 

The oxidation order of iron, sulfur, and arsenic in arsenopyrite has been controversial. Nesbitt and Muir (1998, 140) concluded that iron and arsenic oxidize faster than sulfur, at least when arsenopyrite is exposed to air. In contrast, Craw, Falconer and Youngson (2003, 80) proposed the following reaction to explain arsenopyrite oxidation in water at pH 4-9 ... 

Both Reactions 3.50 and 3.51 indicate that sulfur oxidizes before iron and that As(I-) and As(0) in arsenopyrite oxidize to As(III). Nevertheless, (Craw, Falconer and Youngson (2003), 81) warn that higher than expected pH readings in their experimental data suggest that at least Reaction 3.50 may be a too simplistic description of arsenopyrite oxidation. Some of the arsenic from arsenopyrite may fully oxidize to As(V) rather than existing as H3As03° as predicted by Reaction 3.50. Using X-ray photoelectron spectroscopy (XPS), Nesbitt and Muir (1998) confirmed that As(III) is not the only arsenic species in surface oxidation products on arsenopyrite. As(V) and even traces of As(I) are also present. 

Walker, F.P., Schreiber, M.E. and Rimstidt, J.D. (2006) Kinetics of arsenopyrite oxidative dissolution by oxygen. Geochimica et Cosmochimica Acta, 70(7), 1668-76. 

Rimstidt et al. (1994) measured the rate of reaction of arsenopyrite under conditions typical of acid mine-drainage environments. Arsenopyrite was observed to be more reactive than pyrite, chalcopyrite, galena, and sphalerite. Oxidation of arsenopyrite led to the formation of scorodite on the surface. The activation energies for arsenopyrite oxidation varied from 18 kJ mol at 0-25 °C, to a slightly negative of 6 kJ moF at 25-60 °C. Rimstidt et al. (1994) attributed the negative activation energies to competition... 

Collinet M-N, Morin D (1990) Characterization of arsenopyrite oxidizing Thiobacillus. Tolerance to arsenite, arsenate and ferric iron. Antonie van Eeeuwenhoek 57 237-244 Collins MJ, Arciero DM, Hooper AB (1993) Optical spectropotentiometric resolution of the hemes of hydroxylamine oxidoreductase. Heme quantitation and pH dependence of Em. J Biol Chem 268 14655-14662... 

M-N Collinet, D Morin. Characterization of arsenopyrite oxidizing Thiobacillus. Tolerance to arsenite, arsenate, ferrous and ferric iron. Antonie van Leeuwenhoek 57 237-244, 1990. 

MG Monroy-Fernandez, C Mustin, P de Donato, J Berthelin, P Barion. Bacterial behavior and evolution of surface oxidized phases during arsenopyrite oxidation by Thiobacillus ferrooxidans. In T Vargas, CA Jerez, JV Wiertz, H Toledo, eds. Biohydrometallurgical Processing. Vol. 1. Santiago University of Chile, 1995, pp 57-66. 

III. Arsenopyrite—538 °C exotherm, arsenopyrite oxidizes and is transformed to y-Fe20v it releases AS2O3 and SO2 789 °C exotherm, y-FejO changes to o-Fe203. 

Metafile arsenic can be obtained by the direct smelting of the minerals arsenopyrite or loeUingite. The arsenic vapor is sublimed when these minerals are heated to about 650—700°C in the absence of air. The metal can also be prepared commercially by the reduction of arsenic trioxide with charcoal. The oxide and charcoal are mixed and placed into a horizontal steel retort jacketed with fire-brick which is then gas-fired. The reduced arsenic vapor is collected in a water-cooled condenser (5). In a process used by Bofiden Aktiebolag (6), the steel retort, heated to 700—800°C in an electric furnace, is equipped with a demountable air-cooled condenser. The off-gases are cleaned in a sembber system. The yield of metallic arsenic from the reduction of arsenic trioxide with carbon and carbon monoxide has been studied (7) and a process has been patented describing the gaseous reduction of arsenic trioxide to metal (8). 

Arsenic trioxide may be made by burning arsenic in air or by the hydrolysis of an arsenic trihaUde. Commercially, it is obtained by roasting arsenopyrite [1303-18-0] FeAsS. It dissolves in water to a slight extent (1.7 g/100 g water at 25°C) to form a weaMy acidic solution which probably contains the species H AsO, orthoarsenous acid [36465-76-6]. The oxide is amphoteric and hence soluble in acids and bases. It is frequendy used as a primary analytical standard in oxidimetry because it is readily attainable in a high state of purity and is quantitatively oxidized by many reagents commonly used in volumetric analysis, eg, dichromate, nitric acid, hypochlorite, and inon(III). 

An alternative route increasingly investigated is bio-oxidation using bacteria to oxidize pyrite or arsenopyrites at 45°C. 

The adsorption of collectors on sulfide mineral occurs by two separate mechanisms chemical and electrochemical. The former results in the presence of chemisorbed metal xanthate (or other thiol collector ion) onto the mineral surface. The latter yields an oxidation product (dixanthogen if collector added is xanthate) that is the hydrophobic species adsorbed onto the mineral surface. The chemisorption mechanism is reported to occur with galena, chalcocite and sphalerite minerals, whereas electrochemical oxidation is reportedly the primary mechanism for pyrite, arsenopyrite, and pyrrhotite minerals. The mineral, chalcopyrite, is an example where both the mechanisms are known to be operative. Besides these mechanisms, the adsorption of collectors can be explained from the point of interfacial energies involved between air, mineral, and solution. 

The modern trend is to employ processes based on aqueous oxidation of pyrite and arsenopyrite, and the chemical reactions involved can simplified as ... 

Rusanen, L., Aromaa, J., Forsen, O. (2013). Pressure oxidation of pyrite-arsenopyrite refractory gold concentrate. 

Six sulphide species were observed in the non-ferromagnetic heavy mineral concentrates (NFM-HMCs) of bedrock samples arsenopyrite pyrite > chalcopyrite > bismuthinite = molybdenite = cobaltite. Chalcopyrite, pyrite and bismuthinite do survive in near-surface till but only in minor amounts (<8 grains/sample). Although the Co-rich composition of arsenopyrite is possibly the strongest vector to Au-rich polymetallic mineralization in the study area, sandsized arsenopyrite is absent in C-horizon tills, suggesting that arsenopyrite more readily oxidizes than chalcopyrite and pyrite in till, and therefore is an impractical indicator mineral to detect mineralization using surficial sediments at NICO. 

Prior to gold extraction by cyanidation, refractory gold ores are either roasted or pressure oxidized to liberate the gold contained as submicroscopic particles or in solid solution in arsenopyrite and arsenic-rich pyrite. Gold extraction from such ores require roasting or pressure oxidation or bacterial oxidation prior to cyanidation to destroy the sulfide structure. 

The source of As in the ARS and NATA is arsenopyrite, a phase that is stable in low Eh, high pH conditions of unoxidized residue. Oxidation would have been initiated during deposition of the residue and would have progressed in the ARS during the 50 years of exposure. During this time, As rich runoff would have flowed into the runoff area (RA) where As was concentrated by plant material. 

All tailings samples except two were visibly oxidized and were selected based on distinct visual characteristics thought to be indicative of different mineralogy. Of the two unoxidized samples, one was distinctly arsenopyrite rich (CAR02) and another was from saturated tailings beneath a thin organic layer (MG04). 

Native gold and its alloys, which are free from surface contaminants, are readily floatable with xanthate collectors. Very often however, gold surfaces are contaminated or covered with varieties of impurities [4], The impurities present on gold surfaces may be argentite, iron oxides, galena, arsenopyrite or copper oxides. The thickness of the layer may be of the order of 1-5 pm. Because of this, the flotation properties of native gold and its alloys vary widely. Gold covered with iron oxides or oxide copper is very difficult to float and requires special treatment to remove the contaminants. 

Separation of arsenopyrite and pyrite is important from the point of view of reducing downstream processing costs. Normally, roasting or pressure oxidation followed by cya-nidation is used to recover gold. 

The h-pH diagrams of surface oxidation of arsenopyrite and pyrite are shown in Fig. 2.16 and Fig. 2.17, respectively. Figure 2.16 shows that jBh-pH area of self-induced collectorless flotation of arsenopyrite is close to the area forming sulphur. The reactions producing elemental sulphur determine the lower limit potential of flotation. The reactions producing thiosulphate and other hydrophilic species define the upper limit of potential. In acid solutions arsenopyrite demonstrates wider potential region for collectorless flotation, but almost non-floatable in alkaline environment. It suggests that the hydrophobic entity is metastable elemental sulphur. However, in alkaline solutions, the presence of... 

Figure 2.17 shows that although the metastable elemental sulfur maybe present at pyrite surface, pyrite does not exhibit self-induced collectorless floatability except in very strong acidic media and a narrow oxidized Eh range. Such behavior may be similar to that of arsenopyrite in alkaline solutions due to the formation of hydroxides, thiosulphate. 

Pyrite and arsenopyrite have similar oxidation and self-induced collectorless flotation behavior. It is generally suggested that anodic oxidation of pyrite occurs according to reactions (2-24) in acidic solutions (Lowson, 1982 Heyes and Trahar, 1984 Trahar, 1984 Stm et al., 1991 Chander et al., 1993). The oxidation of pyrite in basic solutions takes place according to reactions (2-25). Since pyrite is flotable only in strong acidic solutions, it seems reasonable to assume that reaction (2-24) is the dominant oxidation at acidic solutions. Whereas pyrite oxidizes to oxy-sulfur species with minor sulphur in basic solutions. 

The oxidation of arsenopyrite has some more arguments. Kostina and Chernyak (1979) concluded that the reaction was significant only in caustic soda solution and that the adsorption of hydroxide ion was a key initiating step in the oxidation reaction. Beattie and Poling (1987), Sanchez and Hiskey (1988) and Feng (1989) considered that the oxidation of arsenopyrite was observed only imder basic conditions by a reaction of the type ... 

Durm et al. (1989) atttibuted the oxidation of arsenopyrite electrode in acid to the reaction of type ... 

The reaction product was identified as a-sulphur using XRD and SEM analysis. Sanchez and Hiskey (1991) reinvestigated the oxidation reaction of arsenopyrite and a two-step reaction sequence was suggested by the electrochemical measurements. The initial step was described by... 

Because arsenopyrite is floatable in acidic conditions and non-floatable in basic conditions (see Fig. 2.16), it seems reasonable to assume that reactions (2-63) or (2-28) and (2-29) are dominant oxidation in acidic solutions. Elemental sulphur is responsible for the hydrophobicity of arsenopyrite in acidic media. In alkaline solutions, reactions (2-64) and (2-65) may be dominant resulting in the formation of oxy-sulfur species and arsenate species with minor sulphur. 

From the Eqs. (3-1) to (3-13), the h-pH diagram of sodium sulphide solution is constructed with element sulphxir as metastable phase considering the presence of barrier (about 300kJ/mol) or overpotential (about 3.114 mV) of sulphide oxidation to sulphate and shown in Fig. 3.7. It is obvious that the lower limit of potential of sodium sulphide-induced collectorless flotation of pyrite, pyrrhotite and arsenopyrite at various pH agree well with the potential defined respectively by reactions of Eq. (3-9) producing elemental sulphur. The initial potential... 

Sun et al. (1993a) reported the effects HS ion concentration on the adsorption of HS , the amount of extracted sulphur and sulphur-induced flotation of pyrite as shown in Fig. 3.11. The results show that dining sodium sulphide-induced collectorless flotation, it involves the adsorption of HS ion on the mineral and the HS" adsorbed can be oxidized into sulphur to render pyrite and arsenopyrite surface hydrophobic due to the fact that the adsorption density of HS" ion increases with the HS" ion concentration and the amount of extracted sulphur and hence the flotation rate increases with the increase of adsorption density. It suggests that the mechanism of sodium sulphide-induced collectorless flotation of pyrite takes place hy reactions ... 

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