Electrochemical interactions of industrially important platinum-containing minerals

Pt–Te₂ and Pt–Bi–Te are two important platinum-carrying minerals typically found in altered platinum reserves. Mineralogical studies indicate the Pt–Pd–Bi–Te class to react poorly on the flotation stimuli and indications are that these minerals are slow floaters. This paper investigates the fundamental interactions of potassium ethyl xanthate with these minerals by employing electrochemically controlled contact angle measurements, FT-IR spectroscopy and voltammetry. The electrochemical investigations revealed that Pt–Te₂ is more noble than Pt–Bi–Te at potentials lower than 300 mV (SHE) in sodium tetraborate buffer solution and the reduction peak observed during the return sweep of Pt–Bi–Te is mainly associated with the reduction of bismuth oxidation products. Both the minerals show increased anodic currents when polarised at 0.3 V (SHE) in a 0.05 M Na₂B₄O₇ solution containing 1 × 10⁻³ M potassium ethyl xanthate. According to the results of the rest potential measurements, formation of dixanthogen should be thermodynamically favourable and the very high rest potentials are attributed to the catalytic effect of platinum on the oxygen reduction reaction. The formation of dixanthogen on both the minerals was confirmed by ex situ FT-IR spectroscopy. Contact angle measurements revealed the surface of Pt–Bi–Te to be hydrophobic in nature whereas small contact angles were realized on Pt–Te₂. Surface roughness of the mineral could be responsible for these small contact angles.     

巯基乙酸抑制硫化矿物的电化学机理研究

研究了巯基乙酸（TGA）对硫化矿物电极电位和动电位的影响。试验表明巯基乙酸对黄铁矿、方铅矿和黄铜矿有抑制作用,对毒砂和亲锌矿没有抑制作用。电化学测试表明巯基乙酸对表面覆盖有捕收剂膜的硫化矿物动电位影响较小,能够显降低硫化矿物的静电位,它对硫化矿物抑制的电化学机理为当硫化矿物的静电位EMS低于黄药氧化为双黄药的可逆电位EX^-/X2时,硫化矿物表面双黄药不稳定被还原,从而降低了可浮性;反之,双黄药保持稳定,不被抑制。用巯基乙酸实现两种硫化矿物浮选分离的电化学条件为：EMS1＜EX^-X2(EX^-/GPbX2）＜EMS2。     

An investigation into electrochemical interactions between platinum group minerals and xanthate: Voltammetric study

In the flotation of platinum group minerals (PGMs) containing significant amounts of tellurides and arsenides it is generally assumed that these minerals will respond in the same way as sulphides to collectors such as xanthates which are typically used in sulphide flotation. This paper presents the results of a fundamental study which has been conducted to provide a greater insight into the comparative interactions between synthetic moncheite (PtTe₂) and cooperite (PtS) with sodium ethyl xanthate (SEX). Cyclic voltammetry has been used to investigate these interactions in the absence and presence of sodium ethyl xanthate (SEX) in aerated and de-aerated solutions. Reduction potentials of the SEX/dixanthogen couple have been measured and compared to published data. Reduction potentials of possible reactions of the minerals have been predicted from thermodynamic calculations and used to attempt to interpret the potentials measured in the cyclic voltammetry investigations. Results have shown differences in the oxidation behaviour of the minerals as well as in the SEX interaction with the minerals. The effect of pH was investigated in the case of the Pt minerals and fractional surface coverages were calculated with a view to ultimately correlate these results with flotation behaviour. Proposals are made with respect to the various reactions occurring under the conditions studied.     

双黄药在磁黄铁矿电极表面的电化学形成及吸附研究

本文研究了丁基黄药在磁黄铁矿表面氧化的电化学过程。热力学和循环伏安研究表明丁黄药离子在磁黄铁矿表面氧化形成双黄药 ;恒电流阶跃法导出了氧化过程的电化学动力学方程 η =0 0 98-0 0 74lg〔1-(t/τ) 1/ 2 〕 (V) ,利用动力学参数可以定量地描述氧化速率 ;计时电量法的研究确定了磁黄铁电矿电极表面双黄药吸附层厚度随着 pH值升高而变薄 ,在 pH =6 82、 9 18和 11 0的条件下 ,吸附层厚度分别为 3 0 9、 2 14和 0 98个单分子层     

黄原酸盐介质中黄铁矿的电化学行为：（1）双黄药在其表面形成的电极过程

采用循环伏安扫描法，恒电流阶跃法研究了黄铁矿电极在丁基黄原酸钾溶液体系中的电化学行为，根据电化学试验结果可以判定，黄药在黄铁矿电极表面形成双黄药；双黄药的形成是分步进行的，首先会发生黄原酸根离子的电化学吸附，然后再氧化成双黄药。     

Classification of the Major Copper Sulphides into semiconductor types,and associated flotation characteristics

Recoverable economic copper sulphide minerals such as chalcopyrite, bornite, chalcocite and covellite often occur together in varying proportions in the major copper-bearing ores, and have individual flotation requirements and characteristics. Pyrite also occurs in these ores to varying extents as the sulphide gangue, and is problematic because of its natural tendency to float quickly and easily. In a bulk sulphide float, selectivity against pyrite is desirable, particularly if it does not host other paymetals such as gold or silver. At the same time it is a requirement to float all of the copper sulphides despite their electrochemical differences. The electrochemistry and semiconductor properties of these minerals are reviewed, together with implications for flotation with and without collector addition. Mixed collector systems for the improved flotation of these sulphides are proposed as a solution. The use of xanthate and dithiophosphate in the collector suite allows the co-existence of dixanthogen and free dithiophosphate radical because the latter has a higher redox potential requirement than xanthate to oxidize to the dithiolate. Because some of these minerals require dixanthogen, and others, free thiolate, to generate surface hydrophobicity, a bulk flotation of all the species becomes possible in the overlapping area of Eh and pH between the two dithiolate equilibrium lines on the Pourbaix Diagram. The arsenic-signature copper minerals are added to the study, since many copper operations encounter arsenic as a penalty element in the saleable concentrate. It is shown that the addition of arsenic to the copper and iron sulphides alters the semiconductor and electrochemistry properties, and in turn, its flotation characteristics. The degree of mineral association and liberation between these minerals can be a complicating factor due to textural associations, and should also be considered in the process as a next step.     

Surface potential and wettability of enargite in potassium amyl xanthate solution

The voltammetric, ultraviolet techniques and contact angle measurements were used to investigate the oxidation of xanthate and the reduction of dixanthogen on synthetic and natural enargite electrodes which were kept under a fixed potential. The effect of applied potential on the enargite electrode surface in potassium amyl xanthate (PAX) solutions at pH 10 and 7 was investigated by contact angle measurement after the electrodes were conditioned for 10 min. The effect of PAX concentration on enargite floatable potential range was also studied. The hydrophobic film on the enargite surface was extracted using hexane and examined by ultraviolet spectroscopy. The results indicate that the hydrophobic film on the enargite surface in PAX solution was dixanthogen, and it began to form on the enargite electrode surface in 7×10⁻⁴ M PAX solution from −0.2 V vs. Standard Calomel Electrode at pH 10 and from −0.275 V vs. Standard Calomel Electrode at pH 7. The floatable potential range of enargite was 60–70 mV narrower in 7×10⁻⁵ M PAX solution than in 7×10⁻⁴ M PAX solution. The potential needed to reduce dixanthogen on enargite surface was as low as −0.6 V vs. Standard Calomel Electrode.     

Analysis of alkylxanthate collectors on sulphide minerals and flotation products by headspace analysis gas-phase infrared spectroscopy (HAGIS)

Direct surface analysis by infrared spectroscopy is a useful technique for detection of xanthate species on sulphide minerals. However, infrared detection limits require xanthate dosages that are often one or two orders of magnitude higher than are used in practice. This paper presents an alternative approach to the analysis of collectors on mineral surfaces. A novel gas-phase infrared technique (HAGIS) has been applied to the indirect analysis of ethylxanthate on lead, zinc and copper sulphide mineral surfaces and also on flotation products from copperzinc and leadzinc ores at xanthate concentrations which are comparable to those used in the mineral processing industry. HAGIS relies on the detection of the gas-phase species generated by the thermal decomposition of xanthates adsorbed on mineral surfaces. In this study, the primary decomposition products generated under an air atmosphere include carbonyl sulphide (COS), carbon dioxide (CO₂), carbon disulphide (CS₂) and ethanol. The amounts of these species were compared with product distributions observed for the decomposition of known ethylxanthate complexes. Correlations between the decomposition species provide evidence for the speciation of the collector in the flotation product. The decomposition species can be readily detected from xanthate dosages that are much lower than are the detection limits of traditional, direct surface analysis techniques.     

The effect of zinc cyanide on the flotation of gold from pyritic ore

In gold processing plants, the process water contains a considerable amount of weak acid dissociable (WAD) cyanide species. The cyanide-containing water recycled to flotation circuits has a deleterious effect on mineral flotation. Zinc cyanide can be a major constituent of the process water due to the cyanide leaching of zinc minerals and the zinc cementation applied to precipitate gold and silver. In the present study, the effect of zinc cyanide on the flotation of gold from a pyritic ore was evaluated and the competitive adsorption between zinc cyanide species and the collector, potassium amyl xanthate (PAX) or Aerophine promoter (3418A), on gold was studied by surface enhanced Raman spectroscopy (SERS). It was found that free cyanide did not depress the flotation of liberated gold, but zinc cyanide with a CN/Zn ratio of 2 did. The depression of gold flotation by zinc cyanide was due to the preferential adsorption of Zn(CN)₂ on gold surface over either 3418A or amyl xanthate in the solution, rendering gold surface hydrophilic.     

The role of calcium in xanthate adsorption onto sphalerite

This paper presents results of a laboratory study designed to look into the effect of calcium on sphalerite hydrophobization with xanthate making use of zeta potential, contact angle, microflotation and UV/vis spectrometry techniques. The experimental results show that xanthate adsorbs onto the mineral surface at neutral and slightly acid pH as reflected by contact angle, floatability (by microflotation) and xanthate adsorption (by UV/vis spectrometry) measurements. Apparently, xanthate adsorption is due to both the less negative surface charge of the mineral and to the presence of Zn²⁺ in equilibrium with the mineral, rather than Zn(OH)₂, which is the product of the mineral oxidation with dissolved oxygen at pH above 8. When calcium is present in the aqueous solution at concentrations similar to that of saturation for gypsum precipitation (e.g., about 0.017 mol/L), calcium apparently adsorbs on the sphalerite surface decreasing its negative charge (e.g., zeta potential) and promoting xanthate adsorption and hydrophobization, as reflected by contact angle and floatability measurements (ca. 30° and 33% recovery, respectively). Calcium adsorption appears to be favoured by the increase in pH, as reflected by the increased xanthate adsorption, thus suggesting that the calcium hydroxo-complex (CaOH⁺) is the adsorbing species. At pH 10 and 11, precipitation of calcium hydroxide (observed by SEM-EDS measurements at pH 11) appears to counteract xanthate adsorption, substantially decreasing mineral hydrophobicity. Calcium and xanthate appear to display some type of chemical interaction while calcium and dithiophosphinate do not.     

黄铁矿氧化抑制行为及机理研究

利用纯矿物浮选试验研究了次氯酸钠、过硫酸铵及含钙药剂 CK等氧化剂对黄铁矿、黄铜矿可浮性的影响 ,并采用实际矿石进行了验证。根据黄铁矿的电化学性质、接触角测定和黄药在矿物表面的吸附量测定 ,分析了黄铁矿在氧化状态下受抑时的表面性质。结果表明 ,氧化剂能减小黄铁矿的接触角 ,增大亲水性 ,阻止或减少黄药在矿物表面的吸附从而使黄铁矿受到抑制。     

Flotation of colemanite from realgar

Pure colemanite and realgar crystals, collected from Emet Colemanite deposit in Kutahya, Turkey, were used to determine the adsorption mechanisms of flotation reagents on mineral surfaces. The floatability of pure minerals and separation conditions by flotation were also investigated.Infrared spectrophotometer studies revealed that sodium oleate was adsorbed chemically on colemanite surfaces, and floated pure colemanite minerals with over 98% recovery. Potassium amyl xanthate was determined to be the best collector for realgar, and floated pure realgar minerals with over 88% recovery. The optimum separation conditions were also determined by means of flotation by using artificial mineral mixtures. The separation was obtained by floating realgar with potassium amyl xanthate, and colemanite was left in the suspension. Finally, optimum conditions were applied to a natural sample. Realgar was floated off and a colemanite concentrate was obtained in the suspension having 96.99% B₂O₃ recovery and 33.93% B₂O₃ grade with 860 ppm As content.     

毒砂和黄铁矿吸附黄药的表面活性

采用液相组分的微量分析方法研究黄药在毒砂与黄铁矿表面的吸附与矿物表面发生的变化之间的联系。证明药剂的吸附与矿物的表面活性之间存在增函数关系,矿物表面活性越大,吸附量越大。发现介质pH、捕收剂初始浓度等均会影响矿物的表面活性,从而影响黄药的吸附。因此,与传统的看法不同,从表面活性的角度认为黄药的吸附与矿物表面金属阳离子的过量密切相关。     

丁黄药体系铁闪锌矿的浮选行为与电化学研究

研究了以丁黄药为捕收剂时铁闪锌矿的浮选性质, 在酸性条件(pH <6)下, 其回收率约为60%, 在pH>8 的碱性条件下, 其回收率急剧下降;pH=6.0 时, 铁闪锌矿在0.2 ～ 0.6 V 的电位区间, 回收率大于50%;pH=9.18 和pH =11.0 时, 无论怎样调节矿浆电位, 铁闪锌矿浮选回收率低于50 %。从热力学的角度而言铁闪锌矿与黄药的作用有可能产生双黄药X₂ 和黄原酸锌ZnX₂, 但ZnX₂ 稳定存在的区域不大。采用循环伏安电位扫描研究方法, 研究了丁黄药在铁闪锌矿表面的电化学作用机理, 丁黄药在铁闪锌矿表面的吸附和氧化形成疏水性物质的反应不明显;在高碱条件下, 铁闪锌矿自身的氧化严重阻滞了丁黄药在其表面的吸附和氧化形成疏水性物质。电化学试验结果与浮选试验结果能较好地对应起来。     

Gold enhanced spectroelectrochemical investigation of 2-mercaptobenzothiazole,isopropyl xanthate and butylethoxycarbonylthiourea adsorption on minerals

The interaction of the flotation collectors 2-mercaptobenzothiazole, potassium isopropyl xanthate and butylethoxycarbonylthiourea with chalcopyrite, chalcocite, pyrite and galena has been investigated using surface enhanced Raman scattering (SERS) spectroscopy. For metals and minerals, adsorption occurs via charge transfer to form a metal–sulfur bond and, in situations for which the reversible potential of the formation of the bulk phase is known, at underpotentials to the reversible compound formation.     

Collectability of physically adsorbed xanthate ion–dixanthogen associates

Under discussion is collectability of ethyl and butyl xanthate species resulted from nonstoichiometric interaction with oxidizer. It is visually proved that solution contains fine micro-drops stabilized by negative charge. The size and ζ–potential of micro–drops are determined together with the spreading velocity of emulsion over water surface. The mentioned velocity is higher than the spreading velocity of products of non-stoichiometric interaction between xanthate and heavy metal salt. The products of interaction between xanthates and oxidizers are known as desorbable species (DS), as at the moment of rupture of water film between mineral particle and air bubble they can detach from particle surface and attach to air–water interface. Spreading of DS over the interface forces water out of the film. The forces applied to liquid in the film from the side of DS of ethyl and butyl xanthates are evaluated. The volume–flow rate of water from the film is related with the surface pressure of reagent species active at the air–water interface. The surface pressure of dixanthogen–xanthate emulsion is evaluated as a function on initial concentration of xanthate. Collectability of the reagent depends on the surface tension of DS solution and is governed by the structure of hydrocarbon fragment of the agent.     

Surface characteristics and flotation behaviour of platinum and palladium tellurides

The telluride minerals, moncheite ((Pt,Pd)(Bi,Te)₂ and PtTe₂) and merenskyite ((Pd,Pt)(Bi,Te)₂ and PdTe₂), contribute between 20% and 45% of the PGMs present in the Platreef ore which is located in the northern limb of the Bushveld Complex of South Africa. There is evidence of these minerals reporting to the tailings and the present investigation was aimed at determining their flotation behaviour and to relate this to their surface characteristics.Microflotation, zeta potential determinations, ToF-SIMS analyses (time of flight-secondary ion mass spectrometry) and X-ray photoelectron spectroscopy (XPS) were used to characterise the flotation and surface behaviour of the synthesised samples. Copper activation of both moncheite and merenskyite samples reduced the flotation response compared with xanthate on its own. When xanthate is added on its own, it is adsorbed on the mineral surfaces at a higher concentration compared to the copper activated minerals.Oxidation of the PGE telluride minerals negatively affects the flotation performance of the Pt and Pd bismuth telluride samples respectively but not the Pt and Pd telluride samples. Grinding finer reduced recoveries and increasing the calcium ion concentration from 80 ppm to 500 ppm in the synthetic water did not negatively affect the flotation response of the PdTe₂ mineral.     

磨矿过程硫化矿物表面电化学性质及其对浮选的影响

本文根据浮选电化学理论分析讨论了丁黄药和乙硫氮在方铅矿、闪锌矿及黄铁矿三种硫化矿物表面的电化学作用过程,分析了磨矿环境具有低电位还原气氛产生的原因。腐蚀电偶测定结果表明,磨球介质与硫化矿物之间、方铅矿与黄铁矿之间的原电池相互作用减弱了黄铁矿表面的捕收剂作用过程,有利于方铅矿与黄铁矿的浮选分离。浮选试验结果表明利用磨矿过程进行浮选的工艺有较大的优越性     

Effects of salinity on xanthate adsorption on sphalerite and bubble–sphalerite interactions

The adsorption of amyl xanthate on sphalerite in NaCl solutions of different concentrations and in saline water (viz. simulated sea water) and the surface properties of treated minerals were investigated. The interactions between xanthate treated sphalerite particles and air bubbles were examined using induction time measurement, which revealed that the salt ions could inhibit the adsorption of xanthate on sphalerite due to the competitive adsorption, but also could compress the electrical double layer at mineral/water interface. The induction time of sphalerite treated in potassium amyl xanthate (PAX) with saline water was shorter than that of sphalerite treated in PAX with NaCl of the same ion concentration, which implies that the water composition (e.g. Ca²⁺, Mg²⁺) could play an important role in the bubble–particle interaction. The hydrophobicity, chemical composition, and charge property of PAX treated sphalerite surfaces were characterized using contact angle measurement, Cryo-X-ray photoelectron spectroscopy (Cryo-XPS) and zeta potential determination, respectively, which support the inhibition effect of salt ions on the xanthate adsorption on sphalerite. In the case of saline water, the xanthate decomposition products were confirmed by XPS, which could further lower the hydrophobicity of the treated sphalerite. Our results provide insights into the basic understanding of the salinity effects on the xanthate adsorption on sphalerite and the bubble–mineral interactions in flotation.     

Adsorption of copper sulphate on PGM-bearing ores and its influence on froth stability and flotation kinetics

Copper sulphate is used as an activator in the flotation of base metal sulphides as it promotes the interaction of collector molecules with mineral surfaces. It has been used as an activator in certain platinum group mineral (PGM) flotation operations in South Africa although the mechanisms by which improvements in flotation performance are achieved are not well understood. Some investigations have suggested these changes in flotation performance are due to changes in the froth phase rather than activation of minerals by true flotation in the pulp zone. In the present study, the effect of copper sulphate on froth stability was investigated on two PGM containing ores, namely Merensky and UG2 (Upper Group 2) ores from the Bushveld Complex of South Africa. Froth stability tests were conducted using a non-overflowing froth stability column. Zeta potential tests and ethylenediaminetetraacetic acid (EDTA) tests were used to confirm the adsorption of reagents onto pure minerals commonly found in the two ores. The results of full-scale UG2 concentrator on/off copper sulphate tests are also presented. The UG2 ore showed a substantial decrease in froth stability in the order of reagent addition: no reagents > copper > xanthate > copper + xanthate, while Merensky ore showed a slight decrease. It was shown through zeta potential measurements that copper species were to be found on plagioclase, chromite, talc and pyrrhotite surfaces and through EDTA extraction that this copper was in the form of almost equal amounts of Cu(OH)₂ and chemically reacted copper ions on the Merensky and UG2 ore surfaces. In certain cases, the presence of copper sulphate and xanthate substantially increased the recovery, and therefore the implied hydrophobicity, of pure minerals in a frothless microflotation device. It was, therefore, proposed that increases in hydrophobicity beyond an optimum contact angle for froth stability, were the cause of instabilities in the froth phase and these were found to impact grade and recovery in a full-scale concentrator. Differences in the extent of froth phase effects between the different ores can be attributed to differences in mineralogy.