Decomposition of alkyl dixanthogens in aqueous solutions

Alkyl dixanthogens, (ROCSS)₂, decompose in aqueous solution in the presence of nucleophiles in many ways.It is proposed here that in alkaline solution the principal methods of decomposition of ethyl dixanthogen are by simultaneous attack of OH^? ions on the sulphur-sulphur bond to give products which include xanthate ion (ROCSS^?) and peroxide (H₂O₂) and on the carbon-sulphur bond to give products which include monothiocarbonate ion (ROSCO^?), sulphide ion (S^2?), and sulphur (S⁰). Above pH 12 reaction is complete in a few minutes, and more monothiocarbonate than xanthate is formed. At pH 9 the reaction takes over 20 h and more xanthate than monothiocarbonate is formed.The primary products react further to give various ions which depend in part on the pH of the system. In alkaline solution some of the xanthate and peroxide react to give perxanthate (ROCSSO^?). In acid solution both xanthate and monothiocarbonate decompose rapidly; CS₂ is formed from xanthate and OCS from monothiocarbonate.In the presence of other nucleophiles at pH 9.2, dissolved dixanthogen decomposes much more quickly than with OH^? alone, and other reactions occur. With thiosulphate a higher proportion of xanthate is formed together with some xanthyl thiosulphate and monothiocarbonate but no perxanthate. With sulphite (in the absence of oxygen) or cyanide the products include xanthate and monothiocarbonate but no perxanthate. With sulphite in the presence of oxygen, perxanthate is also formed.Suspensions of dixanthogens react slowly but in a similar fashion to dissolved dixanthogens.Longer-chain dixanthogens are much less soluble than ethyl dixanthogen but, in general, react in a similar way. Higher temperatures increase the rate of decomposition by OH^?.This work has various implications in operating plants.     

An electrochemical investigation of pyrite flotation and depression

The oxidation of ethyl xanthate on pyrite electrodes, and the influence of the flotation depressants hydroxide, cyanide, and sulphide, have been investigated using cyclic voltammetry. A layer of a hydrated iron oxide has been identified on pyrite surfaces. Xanthate does not interact with this layer but is oxidized to dixanthogen at positive potentials. An increase in pH results in an increase in the background current due to oxidation of the mineral, and at pH=11 this reaction becomes faster than xanthate oxidation. Cyanide interacts with the electrode to form a surface species which inhibits xanthate oxidation. Sulphide gives rise to an anodic wave preceding the wave due to xanthate oxidation. The flotation and depression of pyrite are interpreted in terms of mixed-potential mechanisms.     

Flotation of galena. Influence of grain size,of pulp deoxygenation,and of cleaning with ammonium acetate

The flotation of < 10, 10–20, and 20–40 μm galena fractions was studied. For uncleaned galena a given collector coverage produced better floatability with increasing grain size. Nitrogen had a detrimental effect only for the < 10 μm fraction, producing at a given collector coverage a recovery smaller than that obtained with air.Galena cleaned with 400 g/l ammonium acetate had very poor floatability, although xanthate abstraction was fairly high; this confirms that strong xanthate adsorption is necessary for flotation. Formation of monothiocarbonate was small in all cases, which points to a very minor influence, if any, of this compound in the flotation process.In blank flotation tests, or for very low residual xanthate concentrations, a peak at 208 nm and a shoulder at 255 nm were observed. The former was assigned to the uncomplexed Pb²⁺ ion, and the latter was tentatively attributed to the PbOH⁺ ion.Lead in solution results from dissolution of the oxidation products of galena, as galena itself has an exceedingly low solubility. The curve for total lead in solution vs. initial xanthate concentration, had a minimum for an initial xanthate concentration of 10^?5M, the further increase in dissolved lead is attributed to formation of complexes such as PbX⁺ (X = xanthate). Dissolved lead concentrations were nearly as high for cleaned as for uncleaned galena, which indicates a high oxidation rate of the mineral.     

Interaction of thionocarbamate and thiourea collectors with sulphide minerals: a flotation and adsorption study

The ability of O-isopropyl-N-ethyl thionocarbamate (IPETC), O-isobutyl-N-ethoxycarbonyl thionocarbamate (IBECTC) and butyl ethoxycarbonyl thiourea (BECTU) collectors to increase the flotation of the sulphide minerals, chalcopyrite, galena and pyrite, has been studied. For each collector, the flotation characteristics of these minerals, flotation rate constant and flotation recovery maximum, have been calculated from the flotation data and compared as a function of pH and collector concentration. Overall, the flotation performance of these collectors is stronger for chalcopyrite than for galena and pyrite. Flotation increases with collector concentration and decreasing pH values. For chalcopyrite, the collector performances of BECTU are slightly better than those of IPETC but far superior to those of IBECTC, especially at high pH values or at low collector concentrations. The flotation performance of these collectors has been shown to be in good agreement with the amount of collector adsorbed at the mineral surface. The affinity of BECTU for the various minerals has been calculated using a multilayer adsorption model.     

Electrochemical study of the pyrite-oxygen-xanthate system

Polarograms have been determined with pyrite electrode in a buffer solution at pH 9.1, with and without potassium ethyl xanthate and dissolved air, and the resulting oxidation-reduction processes interpreted.It has been observed that the addition of dixanthogen in a nitrogenated solution does not produce any hydrophobicity at the surface of pyrite electrode and does not alter the rest potential and cathodic polarogram.The experimental observations and thermodynamic interpretations provide some support for the production of dixanthogen, developing the first hydrophobic layer, directly at the surface of the pyrite by the action of oxygen of air.     

Thermodynamic calculations on sulfide flotation systems, II. Comparisons with electrochemical experiments on the galena-ethyl xanthate system

Extensive thermodynamic calculations have been carried out to study the oxidation of galena and its flotation by potassium ethyl xanthate (KEX). In order to include some kinetic effects, three cases have been considered by assuming that sulfur oxidation can proceed as far as the formation of sulfate, thiosulfate and elemental sulfur.The results of these calculations have been compared with those of linear sweep voltammetry and intermittent galvanostatic polarization experiments conducted on galena at pH 9.2. Analysis of the experimental data indicates that in xanthate-free solutions, galena is oxidized primarily to PbO and S⁰ and, to a lesser extent, S₂O₃²⁻, rather than to the thermodynamically most favored PbOH⁺ and SO₄²⁻. Tests carried out in the presence of collector confirm the previous finding (Woods, 1971) that the interaction of xanthate with the mineral begins with chemisorption by a one-electron reaction. This cannot, of course, be predicted by calculations based on bulk thermodynamics.     

Electrochemical studies of sulphides,I. The electrocatalytic activity of galena,pyrite and cobalt sulphide for oxygen reduction in relation to xanthate adsorption and flotation

The electrocatalytic activity of galena, pyrite and Co₃S₄ for oxygen reduction has been studied by potentiostatic methods. Open circuit potentials of the sulphide electrodes have also been measured as a function of pH in nitrogen, air and oxygen atmospheres and also in the presence of H₂O₂ and ethyl xanthate. The adsorption of xanthate on sulphides was followed by observing bubble attachment to the electrodes.The catalytic activity for oxygen (or H₂O₂) reduction (the cathodic currents), the electrode potentials and the xanthate adsorption as shown by bubble attachment within certain pH limits, all varied as $\text{Co}{}_3\text{S}{}_4\text{> pyrite (≈ PbS in H}{}_2\text{O}{}_2\text{) ? PbS}$ indicating considerable dependence of the redox processes in flotation on the d - electron character of the sulphides.In the absence of oxygen, xanthate is probably bonded to the water structure of the surface through hydrogen-bonding, thus keeping the surface hydrophilic. Such adsorption reduces the electrode potential and inhibits oxygen reduction.     

Influence of frothers on floatability. I. Flotation of single minerals (quartz and synthetic chalcocite)

The flotation and sorption properties of chalcocite and quartz in potassium ethyl xanthate (EtXK), frothers (α-terpineol and n-amyl alcohol), and (xanthate + frother) mixed solutions were investigated. Surface tension and frothing properties of the solutions were also measured.Floatability and sorption properties of the minerals investigated strongly depended on any controlled or uncontrolled change occurring at the mineral surface. Surface oxidation of chalcocite was shown to be an especially important parameter.The frothers studied showed some collecting properties in respect to chalcocite and quartz and influenced the xanthate sorption on chalcocite.Comparison of the flotation recoveries in frother solutions respectively with and without xanthate indicated an occurrence of noticeable collector-frother interactions during formation of the bubble/chalcocite grain aggregate. No such interactions were found for quartz.Influence of the frothers on floatability of the minerals was observed at different frother concentrations but at close values of surface pressure (π) and of retention time (rt).     

On the cathodic reaction coupled with the oxidation of xanthates at the pyrite/aqueous solution interface

The adsorption of ethyl, propyl and butyl xanthates on pyrite has been studied through electrokinetics, batch adsorption tests, and quantification of Fe²⁺ ions in solution. Adsorption isotherms for the three alkyl xanthates indicate that their adsorption to dixanthogen produces Fe²⁺ ions in solution and decreases the pyrite zeta potential negatively. It seems that the oxidation reaction of xanthates to dixanthogen on pyrite is coupled with the reduction reaction of surface-ferric hydroxide to ferrous ions, leading to the dissolution of hydrophilic ferric hydroxide and growth of hydrophobic dixanthogen on the surface of pyrite. Flotation of pyrite is presented as a function of pH using various ethyl xanthate concentrations. The floatability results are explained in terms of the surface coverage relationship between ferric hydroxide and dixanthogen, which is pH dependent.     

Removal of lead minerals from copper industrial flotation concentrates by xanthate flotation in the presence of dextrin

Potato starch and dextrins resulting from thermolysis of potato starch in the absence of reagents and presence of -amino acids are promising depressants for separation of lead and copper minerals present in the Polish industrial copper concentrates. The polysaccharides were used for differential xanthate flotation of the final industrial concentrates produced by flotation with sulfhydryl collectors in the absence of depressants. The polysaccharides depressed galena and provided froth concentrate rich in chalcocite and other copper minerals as well as cell product containing lead minerals. The best results of separation were obtained in the presence of plain dextrin prepared by a thermal degradation of potato starch. The industrial concentrate containing 18.5% Cu and 5.5% Pb was divided into a froth product containing 38.1% Cu with 77% recovery of copper and a cell product assaying 7.3% Pb with 83% recovery of lead. It was accomplished using 2500 g/t of dextrin, 50g/t of potassium ethyl xanthate, and 50 g/t of frother (α-terpineol). The pH of flotation was 8.0–8.2.     

Oxidation-Reduction effects in the flotation of chalcocite and cuprite

A laboratory study of the batch flotation of chalcocite from chalcocite-quartz mixtures and of cuprite from cuprite-quartz mixtures with potassium ethyl xanthate as collector has shown that the oxidation-reduction state of the flotation pulp can have a pronounced influence on mineral floatabilities. At pH 11 chalcocite floated over a relatively narrow Eh range of about 300 mV; pH had no influence on the potential of the lower flotation boundary in reducing conditions but had a significant effect on the potential of the upper boundary in oxidizing conditions. Below this upper limit, the floatability was reversible with respect to Eh. Provided the Eh was in correct region chalcocite could be floated in the absence of measurable concentrations of dissolved oxygen.Cuprite displayed a high level of floatability with ethyl xanthate for which, by contrast with chalcocite, no flotation limit in reducing conditions was found; over a small range of potentials close to zero, its behaviour was strongly pH dependent.An attempt to account for the floatabilities of chalcocite and cuprite in terms of the formation of cuprous ethyl xanthate on their surfaces did not lead to correlations with the observed behaviour in reducing conditions but provided a rough correlation with the upper flotation potential limit. It is believed that more detailed and properly controlled comparative flotation studies of the chalcocite-xanthate and cuprite-xanthate systems could help to resolve some of the uncertainties associated with the effects of Eh, pH and oxygen concentration in sulphide mineral flotation.     

Magnesia mixture as a regulator in the separation of pyrite from chalcopyrite and arsenopyrite

The aim of this paper is to find an effective method for the separation of the undesirable constituents, namely, chalcopyrite and arsenopyrite from pyrite used for the production of H₂SO₄. A new effective method is developed for co-depressing chalcopyrite with arsenopyrite by AsI₃, followed by the addition of magnesia mixture. This method has been shown to be based on the fact that iron sites exist in the three minerals, whereas copper and arsenic sites exist only in chalcopyrite and arsenopyrite, respectively. This is coupled with the ability of both Cu(I) and Cu(II) to precipitate As(III) in the form of insoluble copper arsenides, namely Cu₃As, Cu₃As₂. In contrast, neither Fe(II) nor Fe(III) form stable arsenides. Consequently, As³⁺ ions are selectively adsorbed onto the surface of chalcopyrite. The facility for oxidizability of As(III) is well known and hence it adsorbs oxygen from the pulp and changes to As(V) of higher valency and smaller size, with ionic potential over 10. Accordingly, it yields a stable complex anion with covalent bonding, namely, [AsO₄]^3?. These newly created arsenate sites on the surface of chalcopyrite, as well as the corresponding original arsenate sites on the surface of arsenopyrite combine with magnesia mixture to form cations leading to the formation of tightly abutting strongly hydrophilic layers of … AsO₄NH₄Mg.6H₂O. The spread of this hydrophilic film on arsenopyrite and chalcopyrite surfaces leads to the screening of their surfaces, making them difficult of access for the collector, ethyl xanthate. Since the pK_a of xanthic acid occurs at pH below 3, xanthate species predominate at pH above 8 and are adsorbed selectively on the pyrite surface in sufficient quantity for its selective flotation and hence for its separation to take place in the pH range 8–9.     

Depression of galena by slime gangue in an agitated pulp

The effect of gangue on mineral behaviour in a miniature batch version of an industrial flotation machine was examined. The flotation systems consisted of a sulphide, galena, with a specific collector, potassium ethyl xanthate, and a particular non-sulphide gangue in each case. Three gangue minerals were selected: corundum, fluorite and quartz. Flotation experiments were augmented with spectrophotometric adsorption and zeta potential measurements. Based on this data, the validity and significance of the slime-coating hypothesis in the context of the environment characteristic of industrial flotation processes were assessed.     

Influence of frothers on floatability. II. Flotation of chalcocite and quartz mixtures

Influence of frothers (α-terpineol or n-amyl alcohol) on the floatability of chalcocite and quartz mixtures, with potassium ethyl xanthate as a collector at pH 5.5–6, was investigated with a Hallimond tube.Activation of quartz by the presence of chalcocite was observed and interpreted as due to copper ionic species originating from chalcocite oxidation products. Best selectivity of separation between chalcocite and quartz was obtained when the frother concentration was kept sufficiently low. Collector—frother interactions during formation of three-phase mineral grain/solution/gas bubble contacts were observed.     

Interaction of finely ground galena and potassium amylxanthate in relation to flotation, 3. Influence of acid and neutral grinding

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Hallimond tube flotation and microelectrophoresis have been utilized to investigate the reactions in the adsorption-abstraction of K-amylxanthate on finely ground galena. The mineral was ground in a laboratory stainless steel rod mill under controlled conditions (pH 4.0 and 7.0) using HCl as a pH regulator. X-ray photoelectron spectroscopic (XPS) studies have been carried out in order to characterize the surface oxidation products after grinding (weak amounts of S_n and PbS₂O₃). The two-stage adsorption process discovered in previous studies was confirmed. For low concentrations or submonolayer capacity, the layer is formed with 1:1 monocoordinated lead xanthate and dixanthogen. For higher values of surface coverage, it is composed of lead xanthate (stoichiometric at pH 7 and non-stoichiometric at pH 4), amyldixanthogen and amylcarbonate disulphide. In the second stage mainly dixanthogen is formed. This stage corresponds to complete flotation and to a sharp decrease in zeta potentials.     

Galvanic interaction between galena and pyrite in an open system

1IntroductionMost sulfide minerals in nature have perfect con-ductivity and possess semiconducting properties.Gal-vanic corrosion may occur when two sulfide mineralswith different potentials are connected together in thesolution.The mineral with the highe…     

A vibrational spectroscopic study of the oxidation of pyrite by molecular oxygen

In situ flow-through attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy has been used to investigate the formation, and step-wise oxidation, of sulfoxyanions at the pyrite surface during oxidation by molecular oxygen. The surface was studied under two different pH regimes (pH 2.5 and pH 5.6) and under two light conditions (dark and ultraviolet light). It was experimentally observed that multiple sulfoxyanions were present at the pyrite surface during oxidation by molecular oxygen, spectroscopically illustrating the concept of sulfur step-wise oxidation. The results from photochemical experiments were complex and suggest a dependence on both pH and surface speciation.     

The effect of slime coatings of the serpentine minerals,chrysotile and lizardite,on pentlandite flotation

Serpentine minerals in the gangue often interfere with the concentration of nickel sulfide ores. The influence of slime coatings of the serpentine minerals chrysotile and lizardite on the flotation of unoxidized pentlandite has been studied. In addition to microflotation testing at pH 9 with potassium amyl xanthate collector, electrophoresis and scanning electron microscopy have been used. The formation of a slime coating was found to be directly related to the surface charges on the slime and the sulfide mineral. Chemical additives which modified the slime surface charge, such as carboxy methyl cellulose, dextrin, sodium pyrophosphate and sodium silicate, were shown to be effective in reducing the adverse influence of the slime on the flotation of pentlandite.     

Adsorption of xanthate and starch on synthetic graphite

In the processing of sulphide ores containing graphite, starch is occasionally employed for depressing graphite while xanthate is used as a sulphide collector. In such a system the adsorption of starch by graphite is also influenced by xanthate. In the present investigation the adsorption of xanthate and starch at various values of pH and adsorbate concentrations have been carried out. The results show that xanthate is hardly adsorbed in the xanthate-N₂ system, but in the presence of oxygen it is catalytically oxidised to dixanthogen which adsorbs on graphite. Xanthate and starch are partly adsorbed at the same or adjoining sites of graphite with starch being adsorbed faster than xanthate. The ability of starch to depress graphite is reduced in the xanthate-starch system.     

Proximity effects on semiconducting mineral surfaces II:: Distance dependence of indirect interactions

In a previous study, we described proximity effects on surfaces of the semiconducting minerals galena and pyrite, whereby a chemical reaction at one surface site modifies the reactivity of a remote surface site several Ångstroms or even nanometers away (Becker et al., 2001). The modification of interest does not arise because of a direct “through space” interaction between the two sites, but rather an indirect interaction via the electronic structure of the substrate. Here we investigate the distance and direction dependence of proximity effects using quantum mechanical modeling. The direct and indirect interactions between co-adsorbed oxygen atoms and between adsorbed oxygen atoms and point defects on vacuum-terminated galena (100) surfaces were modeled. Density functional theory cluster and plane wave pseudopotential calculations were used to calculate the modifications to the adsorption energy as a function of separation. Energy-distance plots indicate that the proximity effect energy can become very strong at separations decreasing below about 5 to 6 Å, and persist at increasing separations up to 12 Å in a slowly decaying form. A strong attractive indirect interaction out-competes direct electrostatic repulsion for O-vacancy interactions. An oscillatory asymptotic behavior is found for co-adsorbed O-O indirect interactions, which indicates that the proximity effect energy can vary with surface crystallographic direction. It implies the presence of a strong organizing force on like adatoms that may explain the progressive oxidation of certain sulfide minerals by patchwork growth. These findings begin to pave the way for improved adsorption isotherms and extended surface complexation models that will include the specific influence of semiconductor-type proximity effects.