The mechanism of the adsorption of alkyl xanthate species on galena

Oxidation of galena produced thiosulphate and sulphate ions at the mineral surface but flotation activity, using isopropyl xanthate as collector, was found to depend on the concentration of thiosulphate ions at the surface. It is proposed that the thiosulphate ion reacts with di-isopropyl dixanthogen formed by oxidation of xanthate ion in solution. It was demonstrated that these species react in homogeneous solution to give xanthate and xanthogen-thiosulphate ions. The proposed surface reaction may account for the formation of a layer of adsorbed xanthate ions and for the surface redox potential being raised to a value where more xanthate ions can be chemisorbed and where dixanthogen is stable.     

Electrochemical Aspects of Pyrrhotite and Pentlandite in Relation to their Flotation with Xanthate. Part-I: Cyclic Voltammetry and Rest Potential Measurements

Separation of pyrrhotite from pentlandite is highly important in flotation practice of massive nickel sulphide ores. Flotation reactivity of these minerals was studied using cyclic voltammetry and rest potential measurements, which included dissolved oxygen concentration and butyl xanthate addition as the main parameters at a solution pH of 9.2. Under the experimental conditions used, the voltammograms, obtained in the absence and presence of xanthate (at 0.1 mol m^–3) and dissolved oxygen, provided no direct evidence for any anodic peaks that can be associated with xanthate electroadsorption or its possible transformation to dixanthogen, as an agent of superior hydrophobicity. However, the current densities corresponding to the most prominent anodic potential for pyrrhotite were noticeably lower in the presence of xanthate, suggesting its involvement in the formation of a partially passivating film. According to results of rest potential measurements, formation of dixanthogen will be common to both of these minerals in oxygen-saturated solutions to promote their bulk flotation. However, under oxygen-deficient conditions in the presence of xanthate, pyrrhotite develops rest potentials that are much lower than the equilibrium potential for dixanthogen formation. Since dixanthogen formation is essential for pyrrhotite floatability, controlling the dissolved oxygen level and potential in the pentlandite–pyrrhotite flotation environment at low levels is expected to promote selective flotation of pentlandite, which has been demonstrated independently.     

The mechanism of the adsorption of ethyl xanthate ion on galena (II)

Studies of the adsorption of ³⁵S labelled ethyl xanthate ion and of ³⁵S labelled diethyl dixanthogen show that both species are competitively chemisorbed to an approximately equal extent on a galena surface, but that ethyl xanthate ion is oxidised to a species which is physically adsorbed, while diethyl dixanthogen is reduced to ethyl xanthate ion which is partially desorbed. It is suggested that galena surfaces are sufficiently heterogenous that oxidative chemisorption and oxidation of ethyl xanthate ion can take place on the anodic parts of the surface while diethyl dixanthogen can be reductively chemisorbed or completely reduced on the cathodic areas. The relationship of these processes to the normal flotation of galena is discussed.     

Etude electrochimique de l'oxydation simultanee de la galene (PbS) et de l'ethylxanthate—condition de formation du xanthate de plomb en surface de la galene

The electrochemical oxidation of galena (PbS) has been investigated in the pH range 9–10, by controlled potential electrolysis using a special bed electrode. In the absence of xanthate, the oxidation of PbS leads to the formation of elemental sulphur, thiosulphate and lead species (Pb(OH)₂). Simultaneous oxidation of galena and xanthate has also been studied and the products characterized by their uv-visible spectrum. The oxidation produces lead xanthate and dixanthogen. It has been established that lead xanthate comes from an ion exchange reaction between excess xanthate in solution and lead species resulting from the lead sulphide oxidation.     

Mössbauer Spectroscopy of Fe Xanthates

Abstract

Mössbauer spectroscopic studies indicate that, depending on pH, two different precipitates are formed when appropriate solutions of iron ions are mixed with ethyl xanthate solutions. Below pH 3.5, ferric ethyl xanthate is obtained, regardless of whether ferric or ferrous reactants are used; ferrous xanthate can be obtained only with highly concentrated solutions and in strictly neutral or even reducing conditions. Above pH 3.5 a ferric hydroxy-xanthate, with one or more OH groups replacing the xanthate groups, is formed. No analogy with the Cu²⁺ → Cu⁺ reduction and the accompanying stoichiometric oxidation of xanthate to dixanthogen could be observed in the Fe³⁺ → Fe²⁺ systems, in which dixanthogen is detected only as a product of ferric xanthate decomposition in alkaline pH's.     

Electrochemistry of the chalcocite-xanthate system

The interactions between chalcocite and ethylxanthate have been examined in an electrochemical-microflotation cell containing a particle-bed working electrode. Cyclic voltammetry has shown the formation of a chemisorbed product commencing at –0.5 V versus SCE with a bulk oxidation product, possibly cuprous xanthate, forming above –0.23 V. Relating the charge passed on a potentiostated mineral bed after addition of xanthate to spectrophotometric changes in xanthate concentration suggested the presence of a fast chemical reaction and a charge transfer reaction. The strong correlation between flotation response and potential indicates that chemisorption of ethylxanthate produces a layer sufficiently hydrophobic for chalcocite flotation.     

Electrochemical behaviour of copper in aqueous solution containing potassium ethylxanthate

The electrochemical behaviour of copper in aqueous potassium ethylxanthate (KEX) is studied by using potentiodynamic techniques at different sweep rates, complemented with SEM and EDAX. In NaCl solutions a cuprous xanthate film is formed at low potentials, the initial stage of this reaction being the electroadsorption of KEX on copper competing with the adsorption of chloride ions and water. At low surface coverages for electroadsorbed KEX the electrodissolution of copper is partially inhibited as compared to plain NaCl solutions. As the KEX monolayer is completed, a tri-dimensional cuprous xanthate film grows in the electrode surface. On subsequent increase of the applied potential a complex anodic layer is formed leading to copper passivity. Passivity breakdown promoted by either chloride ions or electro-oxidation of the organic film can be observed when the potential exceeds a certain critical value.Facultad de Ciencias Exactas, Universidad Nacional de La Plata.     

Relaxation of the lead-deficient sulfide surface layer on oxidized galena

The surface oxidation of galena in air and in aqueous media has been investigated using electron spectroscopy and linear potential sweep voltammetry to elucidate the incongruent oxidation indicated by previous X-ray photoelectron spectroscopic studies. It has been confirmed that the initial oxidation reaction involves the removal of lead atoms from the sulfide lattice to generate a metal-deficient surface layer. It has been shown that, when the mineral is removed from the oxidizing environment, the composition of this layer relaxes slowly back towards that of the bulk mineral as a result of lead atoms diffusing from the bulk to the surface to fill metal vacancies in the sulfide lattice. The relaxation accounts for the absence of a shifted component in sulfur electron spectra from galena exposed to air or basic solutions in equilibrium with air. Correlation of the potential dependence of self-induced flotation and the extent of surface sulfur excess determined electrochemically indicates that a metal-deficiency equivalent to about half a monolayer of excess sulfur is necessary for significant flotation to be effected.     

A SERS spectroelectrochemical investigation of the interaction of 2-mercaptobenzothiazole with copper, silver and gold surfaces

The interaction of the sulfide mineral flotation collector, 2-mercaptobenzothiazole, with silver, copper and gold surfaces has been investigated by surface enhanced Raman scattering (SERS) spectroscopy. 2-mercaptobenzothiazole, the copper, silver and gold compounds of this species, and the dithiolate, 2,2-dithiobis(benzothiazole) were characterised by ¹³C NMR and Raman spectroscopy to provide a basis for identifying surface species. SERS investigations showed that, at pH 4.6 where the solution species is in the protonated form, and at 9.2, where it is present as the ion, adsorption on each metal occurs over a wide potential range. Attachment of the organic compound occurs through bonding between the exocyclic sulfur atom and metal atoms in the surface. X-ray photoelectron spectroscopy confirmed that the adsorbed layer was of monolayer thickness. Adsorption of the protonated 2-mercaptobenzothiazole occurs on copper at pH 4.6 at potentials below that at which charge transfer adsorption commences.     

Selective Separation of Cu,Zn, and As from Solution by Flotation Techniques

Abstract

The selective precipitation and flotation of copper, zinc, and arsenic ions from dilute aqueous solutions were investigated. Phase separation was accomplished effectively by the dissolved-air technique for the production of fine gas bubbles, and a short-chain xanthate was applied as the collector for copper ions, dialkyldithiocarbamate for zinc, and ferric sulfate for the pentavalent arsenic. The procedures followed were ion flotation for copper and zinc, and adsorbing colloid flotation for arsenic (without a surfactant).     

Cathodic processes in the leaching and electrochemistry of covellite in mixed sulfate–chloride media

The cathodic processes that occur on a covellite (CuS) surface in mixed sulfate–chloride solutions in the absence and presence of copper(II) ions have been studied using potentiostatic transients and cyclic voltammetry at rotating disk electrodes in the potential range 0.3–0.7 V (versus SHE). This range is relevant to the oxidative leaching of this copper mineral in sulfate and chloride lixiviants. Variations in the concentrations of sulfate and chloride ions had a small effect on the cathodic reduction of covellite in the potential range of 0.5–0.3 V, although the presence of chloride ion resulted in a significant increase in the anodic current on the reverse sweep. On the other hand, addition of copper(II) ions resulted in enhanced cathodic currents and subsequent anodic currents in both sulfate and chloride solutions due to reduction of covellite to an undefined reduced copper sulfide species. Reduction of copper(II) to copper(I) ions becomes the preferred cathodic reaction as the concentration of chloride ions increases, becoming mass transport controlled at a rotating disc electrode at potentials below about 0.4 V. Potentiostatic measurements at potentials negative to the mixed potential in acidic chloride solutions have shown that reduction of copper(II) ions is reversible and have been used to estimate the rate of oxidative dissolution of the mineral which value agrees reasonably well with previously reported leaching rates under similar conditions. Reduction of dissolved oxygen has been found to be very much slower that that of copper(II) ions under ambient conditions.     

Complexes Formed by Quaternary Ammonium Salts with Xanthates,and Synergistic Effects in Adsorption of Xanthate Ions on Reduced Copper Powders

Abstract

Precipitates formed in equimolar solutions of long chain xanthates and alkyl trimethyl ammonium bromides (C _n TAB) have been analyzed by chemical, X-ray diffraction, and infrared spectroscopic methods and were found to consist of a 1:1 complex of long-chain anions and cations. It is inferred that analogous ion-pair complexes, but in a solubilized form, exist in mixed solutions of shorter chain compounds, e.g., potassium ethyl xanthate (KEtX) and C₁₂TAB. Tests showed that a reduced copper surface adsorbs xanthate ions from such mixed solutions under anaerobic conditions when no adsorption of uncomplexed xanthate ion is possible. In the temperature range 4 to 20°C, an Arrhenius activation energy of 22.6 kcal/mole was evaluated for adsorption from equimolar solutions KEtX and C₁₂TAB. Characteristic changes in the behavior of xanthate due to its complexing with C _n TAB may have important applications in separation of solids by flotation, particularly under oxygen-deficient or reducing conditions, i.e., when normally flotation with xanthate ions is impossible, owing to their inability to adsorb under anaerobic condition.     

Electrochemical studies of gold ore processing wastewater containing cyanide, copper, and sulfur compounds

The electrochemistry of real gold ore processing wastewater solutions from copper sulfide containing gold ore has been investigated. Analysis shows that the wastewater contains a range of sulfur compounds in various oxidation states from sulfide to sulfate. The electrochemical characteristics of the gold ore processing wastewater were evaluated using rotating disk, cyclic voltammetric, polarization and preparative electrolysis studies. The solutions show clear differences versus synthetic alkaline copper cyanide solutions. The copper cyanide/copper oxide catalysis normally seen in synthetic alkaline copper cyanide solutions is strongly inhibited. Two components of the wastewater solution identified as inhibiting the copper cyanide/copper oxide catalysis are copper sulfide complexes and thiocyanate. The inhibition of the copper cyanide/copper oxide catalysis appears to have an initiation time possibly related to the accumulation of copper-sulfur compounds at the electrode surface. The passivated surface is still able to oxidize cyanide, though at a maximum rate that corresponds to the limiting current for free cyanide assuming 1 electron per cyanide. The lack of the copper oxide coating that typically forms during oxidation of synthetic alkaline copper cyanide solutions, plus possibly the presence of various sulfur compounds, results in corrosion at higher anodic potentials when stainless steel is used as an electrode. However, stainless steel can be successfully used as an electrode material to treat the solutions if the potential is carefully controlled.     

Inferring the distribution of iron oxidation species on mineral surfaces during grinding of base metal sulphides

Electrochemistry plays an important role in the flotation of base metal sulphide minerals. During grinding a galvanic interaction occurs between minerals and grinding media and controls the iron contamination on mineral surfaces, which depresses mineral flotation significantly. In this study, the galvanic interaction was quantified by measuring the iron oxidation species originated from grinding media by ethylene diamine-tetra acid (EDTA) extraction in single mineral and mixed mineral systems. It was found that the extent of galvanic interaction between minerals and grinding media was intimately associated with the electrochemical reactivity of minerals. The nobler the mineral, the stronger the galvanic interaction with grinding media, and the higher the amount of iron oxidation species from grinding media. For both galena and chalcopyrite a linear relationship was observed between the amount of iron oxidation species and flotation recovery in single mineral systems. This relationship was able to predict the iron oxidation species on galena and chalcopyrite surfaces when they were mixed with pyrite separately. The distribution of iron oxidation species onto the two minerals in the mixture changed with the ratio of the mineral surface areas and was correlated with mineral flotation recovery. The major cathodic mineral in the mixture was dictated by the combination of mineral surface area and reactivity and drew iron oxidation species from the grinding media.     

Soluble Copper Xanthate Complexes

Abstract

When solutions of cupric perchlorate (10^?6 to 10^?2 M) are mixed with solutions containing nonequivalent quantities (concentrations) of xanthate (O-alkyl-dithiocarbonate) ions, clear solutions containing soluble nonionic and ionic complexes are obtained, despite the fact that the solubility product for a given metal xanthate salt may be exceeded by several orders of magnitude. Ultraviolet and visible (UV and VIS) spectroscopy, and dc and ac differential polarography have been employed to study these complex species.

The spectral changes taking place on the addition of potassium ethyl xanthate to cupric perchlorate solutions, and also turbidity measurements, indicate that copper xanthate complexes are formed before the precipitation of cuprous ethyl xanthate, Cu(S₂ COC ₂ H ₅). In very dilute solutions containing near equivalent concentrations of the reactants, a spectrum containing three broad maxima (235, 285, and 375 nm) is obtained and is interpreted as due to a nonionic cupric complex, Cu(S₂ COC ₂ H ₅)₂, (I). In the presence of excess cupric ions a second complex, an ionized cupric monoxanthate, Cu(S₂COC₂H₅)+, (II), is formed with bands at 256, 283, 325, and 382 nm. The relative concentrations of the reactants determine not only which complex is formed in these dilute solutions, but also whether or not the cuprous xanthate precipitates.

The species in solutions giving the spectrum of complex (I) is not stable. It decomposes quickly to cuprous xanthate and dixanthogen. Solutions of the ionic cupric complex (II) are stable for many for many hours. It is suggested that the ionic complex (II) may play a role in activating sphalerite in alkaline solutions.     

Modification of Pyrite and Sphalerite Flotation by Dextrin

Abstract

The role of dextrin in the xanthate flotation of pyrite and sphalerite was examined by means of flotation tests and microelectrophoretic measurements. Floatability and ζ-potential of both minerals were found to be depressed in a suitable pH range due to the formation of their respective superficial metal hydroxide layers. The influences of surface oxidation and flocculation of mineral particles were also discussed. The selective flotation of sphalerite from pyrite in an acidic medium in the presence of copper sulfate was found to be possible, as shown by artificial mixture separation studies. In this way, the possible exclusion of cyanides (for environmental reasons) may be advanced.     

Continuous Precipitate Flotation of CuS/ZnS

Abstract

The precipitate flotation of copper and zinc as sulfides in dilute aqueous solutions (50-250 ppm metal ion concentration) was investigated in the laboratory in continuous flow. The dispersed-air flotation technique was followed, leading to a selective recovery of copper sulfide of the order of 95% in a high acidic pH region (of 1.7) by a laurylamine ethanolic solution as collector and with the addition of cetyl-pyridinium chloride as frother. The precipitate flotation of zinc sulfide was then accomplished with the same method at pH 5.0 as a second separation stage (in the presence of minor amounts of copper).     

Comportement electrochimique de l'ethylxanthate sur differents types d'electrodes: platine,or, galene,mercure

The electrochemical behaviour of ethylxanthate (C₂H₅OCS^?₂) has been studied by cyclic voltammetry, polarography, and controlled-potential electrolysis, in the pH range 9–10.The oxidation product, at platinum, gold and galena electrodes was found to be dixanthogen (C₂H₅OCS₂)₂; the mechanism of the reaction is complicated by the adsorption of xanthate and dixanthogen.With the dropping mercury electrode, an anodic wave was found at ?0,3 V vs sce; the electrochemical reaction involves the oxidation of mercury in the presence of xanthate, and the formation of a mercuric compound: Hg(C₂H₅OCS₂)₂. Strong adsorption of C₂H₅OCS^?₂ and Hg(C₂H₅OCS₂)₂ occurs.     

Continuous monitoring of EDTA extractable iron from mineral slurries using a microfluidic chip

In mineral processing, iron oxidation species formed on the mineral surface play a dominant role in depressing mineral flotation and must be monitored. Current analytical methods rely on off-line sample preparation followed by conventional analytical techniques, which are labour intensive and time delayed. This paper reports the successful application of an open microfluidic chip for real-time continuous monitoring of ethylenediaminetetraacetic acid (EDTA) extractable iron (derived from iron oxidation species) in mineral slurries. A detailed mineralogical study of the mineral ore before and after leaching by EDTA has been undertaken to understand the leaching behaviour of the EDTA-slurry extraction system, showing good agreement with solution analysis.     

Oxidation Sequence of Sulfide Minerals in Operating and Out-of-Service Mine Waste Storage

To determine the oxidation sequence for sulfides in stored mine waste and to assess the effect of sulfide oxidation on the environment and the processing characteristics of anthropogenic deposits, the electrode potentials of sulfide minerals were measured in alkali solutions under conditions similar to storage conditions. The surface oxidation of sulfides was studied in relation to the nature and concentration of the dissolved gas. The influence of sulfide surface inhomogeneities and mineral concretion on the electrochemical oxidation was investigated. The oxidation rate of minerals was found to be increased by impurities with higher electropositive potentials.