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.     

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.     

Some aspects of the electrochemistry of the flotation of pyrrhotite

The iron sulfide mineral, pyrrhotite (Fe_(1–x)S), has long been known to be more difficult to recover by flotation from alkaline slurries than many other base metal sulfide minerals. This paper summarizes the results of an electrochemical study of the surface reactions that occur during the flotation of nickeliferous pyrrhotite in the recovery of nickel and the platinum group metals. Mixed potential measurements conducted with natural pyrrhotite electrodes in various stages of an operating flotation plant showed that the mineral potential is positive to the equilibrium potential of the xanthate/dixanthogen couple. Similar results were obtained during batch flotation experiments and in synthetic solutions in the laboratory. Cyclic voltammetric and potentiostatic current/time transient experiments were used to investigate the oxidation of pyrrhotite under various conditions. In addition, the reduction of oxygen, the reaction of copper ions and the oxidation of xanthate ions at the mineral surface were investigated. The formation of dixanthogen on pyrrhotite surfaces is thermodynamically favourable in plant flotation slurries. However the interaction with xanthate at pH values above 7 is inhibited by a surface species formed during the conditioning prior to xanthate addition. In acidic solutions copper ions react readily with pyrrhotite to form a species, possibly CuS that can be oxidized at potentials above 0.4 V. At pH 9 this species does not form and there is no electrochemical reaction between pyrrhotite and copper ions. The beneficial effects of copper ions to flotation performance appear to be related to an enhancement of the oxidation of xanthate.     

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.     

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.     

Galvanic interaction of pyrite with Cu activated sphalerite and its effect on xanthate adsorption

In the presence of Cu ions, a packed bed electrochemical reactor (PBER) was employed to deliberately avoid or induce galvanic coupling between pyrite and Cu‐activated sphalerite. The effect of galvanic interaction on Cu ions uptake and xanthate adsorption were investigated. Solution chemistry and surface chemistry studies (ethylenediaminetetraacetic acid extraction and time of flight secondary ion mass spectrometry) have observed that when sphalerite and pyrite were galvanically coupled, Cu ions migrated from the pyrite surface to the surface of the sphalerite. Along with the marked decrease in the adsorption of Cu ions on pyrite, xanthate adsorption on the minerals also dramatically dropped. The pseudo‐adsorption rate constant for the minerals in the mixed mode is only 0.0583 s^?1, much less than that in the decoupled mode, which is 0.1368 s ^?1. This testing program shows that the galvanic coupling of minerals contributes to more copper transfer and Cu ions preferentially adsorbed by sphalerite rather than pyrite. This affects the pyrite surface and causes it to become xanthate unflavoured.     

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.     

Correlation between surface free energy and floatability of galena with potassium ethyl xanthate

From the maximal extrapolated film pressure π_max values for n-octane and n-propanol, the dispersion and non-dispersion components of the surface free energy of galena precoated with potassium ethyl xanthate (collector) were estimated and they were then correlated with the floatability of galena. The collector layer deposited by methanol evaporation did not produce any significant changes in the surface free energy of galena. Polymolecular adsorption of the collector occurred from aqueous solution. The adsorbed collector layer of ~8 statistical monolayers in thickness produced a considerable decrease in the dispersion component of the surface free energy and complete disappearance of non-dispersion interactions. At this surface coverage, a certain change in the properties of the adsorbed layer appeared which was probably connected with the formation of polymolecular layers containing dixanthogen. The work of the water adhesion being significantly lower than the work of water cohesion appeared insufficient to achieve a high floatability of galena.     

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.     

Kinetic Studies of the Electrochemical Treatment of Nitrate and Nitrite Ions on Iridium‐Modified Carbon Fiber Electrodes

Electrochemical reductions of nitrate and nitrite ions in neutral solution were studied using an Ir‐deposited carbon fiber electrode. The objective of the study was to set up a process to remove nitrate and nitrite ions from industrial and municipal wastes as well as from the nitrate and nitrite ion enriched water supplies. Since nitrate and nitrite ion reduction kinetics is very slow on plain carbon, the iridium‐modified fibers showed a significant reduction of nitrate and nitrite ions. Nitrogen and ammonia were identified as the two major gaseous products of the reduction process. A first‐order inhibition kinetics model is proposed and was used to analyze the nitrate and nitrite ion reduction kinetics. The reduction rates of nitrate and nitrite ions were favored at higher cathode potential (cf. –950 mV and –850 mV) in a solution of pH = 7.0 with hydrogen evolution efficiency increasing with cathodic potential. A simple model for the batch re‐circulation process was developed for the reduction of nitrate ion at surface‐modified fiber electrode. There was a good agreement between the model predictions and experimental results.     

Effect of Sample-Related Sources of Error on the Analysis of Pulp and Paper Mill Process Liquids by Ion Chromatography: Part I—Determination of Sulphur Species

Several factors which could affect the accuracy of measurements of sulphur species by ion chromatography have been identified. Analyses of sulphide, sulphite, sulphate and thiosulphate were conducted on samples of kraft liquors, spent sulphite liquor and newsprint white water. Erroneously high results were observed in the determination of sulphide in kraft white and green liquors, presumably due to shifts in equilibrium between elemental sulphur and polysulphide ions during sample dilution. Another problem was encountered during the analysis of sulphite in kraft liquors. Sulphite concentrations in diluted liquors rose rapidly with increase in time between dilution and injection into the ion chromatograph. Evidence presented in this paper suggests that, in diluted kraft liquors containing polysulphide and/or elemental sulphur, the sulphite concentration increases rapidly, possibly due to hydrolysis of elemental sulphur. Determination of thiosulphate was also subject to positive errors, due to oxidation of sulphide to thiosulphate. The effect of storage conditions on the stability of sulphur species was also determined. Room- temperature storage was the most deleterious for white water samples containing low levels of sulphur ions. Long-term stability in all samples was much improved by refrigeration or freezing. Kraft liquors were very stable even when stored at room temperature. Various recommendations are made to maintain sample integrity and minimize analysis errors.     

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.     

Cation exchange of surface protons on silica gel with cupric ions

The exchange of surface protons on silica gel with cupric ions was investigated under two different conditions. The equilibrium constant was determined and the mechanism of the exchange was suggested. In ammonia water, two protons exchange with one tetrammine cupric ion and formed two ammonium ions. In acidic solution, where one proton exchanges with one cupric ion, the apparent equilibrium constant increases with an increase in the amount of copper loading. The structure of surface species on silica gel was suggested from the mechanism of the exchange and from ESR study. The surface species prepared in acidic solution is changed into the one prepared in ammonia water by exposure to ammonia. The change in the structure of surface species by the adsorption of foreign molecules was also studied.     

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.     

离子交换玻璃表面性质研究

针对离子交换增强玻璃稳定性问题,详细研究了由钾离子作为增强离子的玻璃表面性质.利用光学显微镜观察玻璃表面在加速风化条件下表面被侵蚀的情况,并用原子吸收光谱(AAS)研究玻璃在该加速风化条件下,表面K+偏析的情况.通过场发射扫描电子显微镜(FEGSEM)测试K+离子交换深度,用X射线光电子能谱(XPS)研究玻璃在离子交换前后表面元素含量的变化.研究表明,在加速风化条件下,离子交换增强玻璃表面较普通玻璃更容易受到侵蚀,K+离子向表面偏析明显.研究发现造成此种现象的原因是:在交换过程中,处于KNO3电解质中的玻璃表面吸附一层K+离子,形成Sterm紧密层,由于Stern层的距离只有几纳米,玻璃表面对K+的吸附力非常大,离开溶液后,表面层的活性大量K+离子依旧存在玻璃表面,致使离子交换玻璃表面稳定性降低.     

Diffusion-Controlled Attack of Glass Surfaces by Aqueous Solutions

Simple binary alkali oxide silica glasses and some ternary glasses derived from the binary compositions by substituting alkaline earth oxides for silica were subjected to the action of water and various aqueous solutions (pH 1.4 to 10.9) and the kinetics of the reactions were studied. In every case a root-time relation of extraction, suggesting an ion-exchange controlled diffusion mechanism, was observed, followed by a time relation of extraction. The rate and duration of diffusion depended on the pH of the solution and the reaction temperature. Above a critical pH the hydrogen ion concentration of the attacking solution was so low that the surface sites began to be occupied preferentially by alkali ions, increasing their concentration at the leached layer-solution boundary and slowing the diffusion process. For potassium glasses only, where alkali ion distribution in the leached layer was exponential, the diffusion coefficients calculated from the concentration profiles agreed well with those calculated from the extraction of potassium ions by a diffusion mechanism.     

Effect of surface preparation prior to cerium pre-treatment on the corrosion protection performance of aluminum composites

The effect of surface preparation prior to cerium treatment on the corrosion protection of AA6061 T6–10% Al₂O₃ in NaCl solution was investigated using electrochemical impedance spectroscopy and DC polarization techniques. A new surface preparation method consisted of pre-etching followed by oxide-thickening is proposed. Optical microscope and scanning electron microscopy showed a marked decrease in the number and depth of pits for the pre-etched as compared to the as-polished specimens. X-ray photoelectron spectroscopy revealed that pre-etching eliminated the active sites where the chloride ions can adsorb. Simultaneously, the adsorption of cerium ions is uniformly on the surface. The oxide-thickening step has a vital role in the protection mechanism. It was found that presence of a thick oxide layer completely changed the mechanism of protection.     

Adsorption behavior of fluoride ions using a titanium hydroxide-derived adsorbent

The removal behavior of fluoride ions was examined in aqueous sodium fluoride solutions using a titanium hydroxide-derived adsorbent. The adsorbent was prepared from titanium oxysulfate (TiOSO₄·xH₂O) solution, and was characterized by X-ray diffraction, scanning electron microscopy, thermogravimetry-differential thermal analysis, Fourier transform infrared spectrum and specific surface area. Batchwise adsorption test of prepared adsorbent was carried out in aqueous sodium fluoride solutions and real wastewater containing fluoride ion. The absorbent was the amorphous material, which had different morphology to the raw material, titanium oxysulfate, and the specific surface area of the adsorbent (96.8 m²/g) was 200 times higher than that of raw material (0.5 m²/g). Adsorption of fluoride on the adsorbent was saturated within 30 min in the solution with 200 mg/L of fluoride ions, together with increasing pH of the solution, due to ion exchange between fluoride ions in the solution and hydroxide ions in the adsorbent. Fluoride ions were adsorbed even in at a low fluoride concentration of 5 mg/L; and were selectively adsorbed in the solution containing a high concentration of chloride, nitrate and sulfate ions. The adsorbent can remove fluoride below permitted level (< 0.8 mg/L) from real wastewaters containing various substances. The maximum adsorption of fluoride on the adsorbent could be obtained in the solution at about pH 3. After fluoride adsorption, fluoride ions were easily desorbed using a high pH solution, completely regenerating for further removal process at acidic pH. The capacity for fluoride ion adsorption was almost unchanged three times after repeat adsorption and desorption. The equilibrium adsorption capacity of the adsorbent used for fluoride ion at pH 3 was measured, extrapolated using Langmuir and Freundlich isotherm models, and experimental data are found to fit Freundlich than Langmuir. The prepared adsorbent is expected to be a new inorganic ion exchanger for the removal and recovery of fluoride ions from wastewater.     

用淀粉黄原酸盐处理含镉废水的研究

主要研究用淀粉黄原酸盐(ISX)处理含镉废水,探讨了淀粉黄原酸盐用量、反应液pH值、反应时间、废水浓度等因素对镉离子去除率的影响。结果表明:当处理20 mL含Cd2+浓度为40 mg/L废水时,pH值为8,ISX用量为0.3 g,处理时间为15 m in,废水中镉的去除率可达97%。