Effect of added cobalt ion on electro-deposition of copper from sulfate bath using graphite and Pb–Sb anodes

Effect of added Co²⁺(aq) on copper electro-deposition was studied using Pb–Sb and graphite anodes and a stainless steel cathode. The presence of added Co²⁺(aq) in the electrolyte solution was found to decrease the anode and the cathode potentials. The optimum level of Co²⁺(aq) concentration in the electrolyte, with respect to the maximum saving of power consumption was established. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were used to study the influence of added Co²⁺(aq) on the anodic and the cathodic processes in a copper sulfate-sulfuric acid electrolyte. The oxygen-evolution potential is depolarised at lower current densities (≤ 150 A/m²) and attains saturation at [Co²⁺]_o ? 600 ppm; whilst at higher current densities (≥ 300 A/m²) it is depolarised with [Co²⁺]_o ≥ 300 ppm. The presence of Co²⁺ promoted the deposit of a smoother and brighter copper cathode as measured by surface reflectivity. X-ray diffraction (XRD) showed that added Co²⁺ changed the preferred crystal orientations of the copper deposits. Scanning electron microscopy (SEM) indicated that the surface morphology of the copper deposited in the presence of added Co²⁺ has well-defined grains. Analysis of cathode copper deposits found negligible cobalt.     

Hydrothermal purification and enrichment of Chilean copper concentrates: Part 1: The behavior of bornite,covellite and pyrite

The nature and basic kinetics of the hydrothermal reactions of bornite, covellite and pyrite with copper sulfate solutions were investigated, as a previous study on the behavior of the bulk Chilean copper concentrates. The reaction of bornite produced digenite at < 160 °C and djurleite above this temperature. The reaction products formed a continuous, non porous layer. The rates were of zero order with respect [Cu²⁺]_(aq), and the activation energy was ～ 100 kJ/mol. Covellite was transformed to digenite at < 200 °C and to chalcocite (Q and M) at > 200 °C. The reaction was characterized for an irregular nucleation and growth of the products, which were non-protective. The order with respect the [Cu²⁺]_(aq) was 0.6 and the activation energy ～ 110 kJ/mol. The pyrite reaction was significant at > 200 °C and extensive at 240 °C. It produced a mixture of digenite/chalcocite-Q, with small amounts of djurleite. The transformation has a high Philling–Betworth ratio (～ 1.6), causing the break of the product layers. The reaction order and the activation energy were also 0.6 and ～ 110 kJ/mol, respectively. The rate of the bornite reaction should be controlled by solid-state ionic diffusion, whereas for covellite and pyrite the rates should be limited by electrochemical processes. Towards the hydrothermal transformation, the reactivity of the main phases present in Chuquicamata-type concentrates was: bornite > chalcopyrite > covellite > sphalerite > pyrite.     

Stripping of copper from CYANEX® 301 extract with thiourea–hydrazine–sodium hydroxide solution

The stripping of copper from the organic extract of bis(2,4,4-trimethylpentyl) phosphinodithioic acid, CYANEX® 301, using an aqueous mixture of thiourea, hydrazine and sodium hydroxide has been investigated. The optimal concentrations of the aqueous solution were found to be 1.0 M, 5 × 10^− 2 M and 5.0 M, respectively which led to 95% stripping of copper from CYANEX 301 with concomitant regeneration of the extractant. The characterization of the stripped copper product was done using a combination of microanalyses, cyclic voltammetry and X-ray diffractometry (XRD). The product was made up of two non-stoichiometric copper sulfides, Cu_xS with x = 1.60 and 1.77. Cu_1.60S was the major product. Hydrazine appears to play the role of reducing the disulfide species of CYANEX 301, R₂P(S)S–S(S)PR₂, formed during the extraction step, back into CYANEX 301, whereas thiourea provides a source of sulfur in the formation of the stripping products.     

Identification of arsenato antimonates in copper anode slimes

It was found that, in copper electrolyte, the combination of As(V) and Sb(V) can form arsenato antimonic acid (AAAc) and, the reactions of AAAc with As(III), Sb(III), and Bi(III) can produce the precipitates of arsenato antimonates. During copper electrorefining, the As, Sb, and Bi deposited into the anode slime from the electrolyte are dominant in the forms of arsenato antimonates. It is extremely difficult to separate pure arsenato antimonates from copper anode slimes, while it is easy to synthesize arsenato antimonates using H₂O₂ to oxidize As(III) and Sb(III) in copper refining electrolyte. The composition and structure of the arsenato antimonates were determined with chemical analysis, IR and XRD techniques. The characteristic bands in the IR spectra of arsenato antimonates are δ of As–OH and Sb–OH at 1126.8 cm^− 1, ν_as of As–OH at 1029.7 cm^− 1, ν_as of As–OX(X = As, Sb) at 819.5 cm^− 1, ν_as of Sb–OH at 618.4 cm^− 1, ν_as of Sb–OY(Y = As, Sb) at 507.2 cm^− 1, and ν_as of Sb–OBi at 470 cm^− 1.The arsenato antimonates form irregular masses of amorphous structure because there are many OH groups in AAAc, the OH groups bond with As(III), Sb(III), and Bi(III) at random, which makes the arsenato antimonates formed in copper refining electrolyte have no fixed ratios for As/Sb/Bi. The formation of the arsenato antimonates can be expressed as follows:aH₃AsO₄ + bH[Sb(OH)₆] + cMeO ⁺ →Me_cAs_aSb_bO_{(3a+5b+c/2+1)}H_{(a+5b−2c+2)}·xH₂O + cH⁺ + (a + b + c / 2 − 1 − x)H₂O, where Me = As(III), Sb(III) and Bi(III); a ≥ 1, b ≥ 1, c ≤ (3a + b)     

Kinetics and mechanism of stripping of Mn(II)–D2EHPA complex by sulphuric acid solution

The kinetics of stripping of Mn(II)–D2EHPA chelate (MnA₂.HA.H₂A₂) existing in kerosene phase by aqueous sulphuric acid solutions at constant sulphate ion concentration of 0.50 M have been investigated by the constant interfacial area stirred (Lewis) and non-stirred (Hahn) cells. Both pseudo-rate constant (q) or (rate / area) and flux (F) methods of the rate data treatment have been applied. The empirical rate equations have been derived. Results have been compared among themselves and other published works on Mn(II)–D2EHPA chelate stripping kinetics. Rate constants obtained from (q) or (rate / area) and (F) methods differ in magnitude and units and an explanation of this has been given.An analysis of rate equations obtained from the Lewis cell experimentation (F_b = 10^− 4.1[MnA₂.HA.H₂A₂]_(o)(1 + 0.005[H⁺]_(i)^− 1)^− 1 (1 + 7.94[H₂A₂]_(o)^0.5)^− 1) suggests that the process is chemically controlled at lower aqueous acidity and higher free D2EHPA concentration regions; whereas, it is diffusion-controlled at higher aqueous acidity and lower free D2EHPA concentration regions. In the investigated D2EHPA concentration and aqueous acidity regions, majority of data fall in the intermediate controlled region. The suggested mechanisms are supported by the values of activation energy (E_a). In the kinetic regime, the reaction step: MnA⁺ → Mn²⁺ + A⁻ occurring in the bulk aqueous phase is the slowest step which proceeds through S_N2 mechanism as indicated by the comparable higher negative value of ΔS^± in the stated condition.In Hahn cell technique, the empirical rate equation is: F_b = 10^− 5.9[MnA₂.HA.H₂A₂]_(o)(1 + 0.0025[H⁺]_(i)^− 1)^− 1(1 + 2.8[H₂A₂]_(o)^0.5)^− 1. Analysis of this equation together with the E_a value of 12–42 kJ/mol indicate that the stripping process is completely diffusion controlled in the low pH and extractant concentration region and in other parametric condition, that is intermediate controlled. For both cells, the ratio of (k_f) to (k_b) equals almost to K_ex of 10^− 2.44 obtained from the distribution study.     

Cadmium extraction from chloride solutions with model N-alkyl- and N,N-dialkyl-pyridine-carboxamides

N-alkyl- and N,N-dialkyl-pyridine-carboxamides with the amide group at the 2nd, 3rd or 4th position were used to recover cadmium(II) from acid chloride solutions. It was found that N,N-dialkyl-pyridine-carboxamides in toluene diluent are effective extractants for the recovery of cadmium from chloride solutions at pH < 2. Cadmium extraction ability rises as the distance of the amide group from the pyridine nitrogen increases. In strongly acid chloride media, extractants (L) with N,N-dialkyl-amide group at position 3 or 4 in the pyridine ring form cadmium ion pairs: (LH⁺)₂(CdCl₄²⁻) but the dialkyl derivative of picolinamide probably forms the complex (LH⁺)₂(CdCl₄²⁻)(LHCl). Monoalkyl-pyridine-carboxamides are not suitable for cadmium extraction from chloride solutions because in weak-acid systems (pH > 3, [Cl⁻] = 0.02–4 M) the N-alkyl-pyridine-3-carboxamides form very slightly soluble cadmium complexes and the N-alkyl-pyridine-2-carboxamides do not extract cadmium(II) from chloride solutions under these conditions. In strongly acid systems ([HCl] = 0.01–4 M) N-alkyl-pyridine-2-carboxamide with a branched carbon chain is ineffective, but the hydrochlorides and cadmium complexes of the rest of the N-alkyl-amides are slightly soluble in hydrocarbon diluents. In ethanol solutions hydrophobic pyridine-3-carboxamides form (CdCl₂)_nL₂ complexes (n = 2 or 3) with cadmium chloride.     

The oxidation of thiosulfates with copper sulfate. Application to an industrial fixing bath

This paper presents the transformation of thiosulfate using Cu(II) salts, such as copper sulfate, at pH between 4 and 5. The nature and kinetics of this process were determined. In the experimental conditions employed, the reaction between thiosulfates and Cu(II) ions produces a precipitate of CuS and the remaining sulfur is oxidized to sulfate, according to the following stoichiometry: 1 mol thiosulfate reacts with 1 mol Cu²⁺ and 3 mol H₂O, generating 1 mol copper(II), 1 mol sulfate and 2 mol H₃O⁺. In the kinetic study, the apparent reaction order was ≈ 0 with respect to H₃O⁺ concentration, in the interval 1.0 · 10^? 4–1.0 · 10^? 5M H₃O⁺; of order 0.4 with respect to Cu²⁺ in the interval 0.21–0.85 g L^? 1 Cu²⁺; and of order 0 with respect to S₂O₃^2? in the interval 0.88–2 g L^?1 S₂O₃^2?. The apparent activation energy was 98 kJ mol^? 1 in the interval 15–40 °C. On the basis of this behavior an empirical mathematical model was established, that fits well with the experimental results. The thiosulfate transformation process using copper(II) sulfate was applied to an industrial fixing bath that proceeded from the photographic industry; after this, the resulting effluent contained less than 10 mg L^? 1 of thiosulfates.     

Formation of complex compounds of univalent copper at the Cu<Superscript>0</Superscript>-Cu<Superscript>2+</Superscript> interphase boundary in chloride solutions

Kinetics and a mechanism of formation of compounds of univalent copper at the interphase boundary Cu⁰-Cu²⁺ in the solution containing excess chlorine anions are investigated by the spectrophotometry method. It is shown that the [CuCl₂]^? and [CuCl₃]^2? complex anions are formed during the contact of metal copper with the Cu²⁺ ions. The former ones are characterized by the appearance of the band in the region of 233 nm in the spectra of electron absorption, and the latters ones—in the region of 273 nm. The dependence of intensity of absorption bands on the contact time of the Cu²⁺ ions with copper is used to estimate the kinetic parameters of formation and oxidation of the [CuCl₂]^? and [CuCl₃]^2? ions. A thickness of the reaction layer, in which they are formed, is determined. The formation of the Cu(I) compounds during the contact interaction of copper with the Cu²⁺ ions is confirmed by the results of electrochemical investigations. The mechanism of cathode reduction of copper-containing compounds on a copper electrode is suggested.     

Hydrothermal purification and enrichment of Chilean copper concentrates. Part 2: The behavior of the bulk concentrates

The hydrothermal treatment of Chilean Codelco-type copper concentrates with copper sulfate solutions was investigated as a mean of removal of impurities and subsequent increase of the copper assay. The behavior of the mineral phases (digenite, chalcopyrite, covellite, bornite, pyrite and sphalerite) was similar to those obtained in previous works from pure mineral samples. An almost complete transformation of bornite, chalcopirite, covellite and sphalerite into Cu_2 ? xS phases was obtained at 225 °C–240 °C. The highest degree of elimination (around 80%) of impurities was in Zn, Cd, Tl and Bi. An intermediate elimination (40–70%) was achieved for Pb and Te, with only moderate elimination (20–40%) of Mo, Hg, Sb and As. Temperature was the variable having the greatest influence on the elimination of the impurities. A concentrate containing 33% Cu, 33% S, 22% Fe and 2% Zn was converted to a highly enriched concentrate containing 70% Cu, 19% S and 3% Fe. The advantages of a concentrate of this type would include: (1) raising by more than twice the smelting capacity due to the high copper content, (2) generation of a minimum amount of slag, (3) reduction by almost 50% in sulfur emissions, (4) substantial reduction of wastes containing hazardous metals and, finally (5), retention of the option to hydrometallurgical copper recovery since the neo-formed Cu_2 ? xS phases are more reactive than chalcopyrite to the chemical or biochemical leaching.     

Chemistry of the Ca–Se(IV)–H2O and Ca–Se(VI)–H2O systems at 25 °C

The Ca–Se(IV)–H₂O and Ca–Se(VI)–H₂O systems were studied by contacting either selenious acid or selenic acid solution with calcium oxide to attain equilibrium at 25 °C for one month. Analysis of the final solid phases and the associated solution, together with X-ray diffraction analysis and a study into the graphed relationships, showed the existence of three calcium selenites in the Ca–Se(IV)–H₂O system — Ca₂SeO₃(OH)₂·2H₂O (Se(IV) = 4.8 × 10^− 5–2.8 × 10^− 4 M); CaSeO₃·H₂O (Se(IV) = 2.8 × 10^− 4–0.86 M) and Ca(HSeO₃)₂·H₂O (Se(IV) > 0.86 M). It also showed four calcium selenates in the Ca–Se(VI)–H₂O system — Ca₂SeO₄(OH)₂ (Se(VI) = 0.21–0.39 M); CaSeO₄·2H₂O (Se(VI) = 0.40–9.1 M); CaSeO₄ (Se(VI) = 10.2 M) and CaSe₂O₇ (Se(VI) > 10.8 M). The X-ray diffraction analyses reported and SEM analyses indicate a high degree of crystallinity of all seven compounds. The stability and solubility regions for these compounds were defined versus pH, and the conventional solubility constants and conditional free energies of formation for the less soluble CaSeO₃·H₂O, Ca₂SeO₃(OH)₂·2H₂O, CaSeO₄·2H₂O and Ca₂SeO₄(OH)₂ were calculated from solubility data obtained.     

Chloride leaching of chalcopyrite

Two new process flowsheets have been developed which combine chloride leaching of copper from chalcopyrite with solvent extraction, to selectively transfer copper to a conventional sulfate electrowinning circuit. Chloride leaching with copper(II) as oxidant offers significant advantages for copper including increased solubility and increased rates of leaching. Both process flowsheets were similarly designed with a two stage counter-current leach but differ with respect to iron deportment. The goethite model flowsheet includes sparging of air or oxygen to the second leach stage to aid precipitation of iron as goethite (FeOOH). The hematite model flowsheet precipitates iron as hematite (Fe₂O₃) downstream from the leach in a dedicated autoclave. A mass balance has been completed for both process flowsheets and this determined the concentrations of copper and iron species in feed liquor returning to the leach following copper solvent extraction.The optimum leach extraction conditions were determined by varying grind size, temperature and residence time for both leach model scenarios. Leach tests were conducted using a chalcopyrite concentrate from Antamina in northern Peru, which contains a low to moderate amount of gangue material. The hematite model was also examined using a Rosario concentrate from Chile which contained chalcocite in addition to chalcopyrite and significant pyrite. Leach tests based on the hematite model were successful in achieving copper extractions > 95% in 4–6 h at 95 °C after fine grinding the concentrate (P₉₀ = 41 μm). However, copper extraction exceeded 99% from the finely ground Rosario concentrate (P₉₀ = 37 μm). In the goethite model leach tests, 89% copper extraction was achieved under optimum conditions in the atmospheric conditions tested.     

Extraction of rare earths and yttrium with high molecular weight carboxylic acids

The extraction behaviour of trivalent rare earths namely La, Ce, Pr, Nd, Sm, Gd, Dy and Ho including Y (M(III), where M represents rare earths and yttrium ) from chloride medium has been studied with the solutions of high molecular weight carboxylic acids such as cekanoic, naphthenic, neo-heptanoic and Versatic 10 in dodecane. The effects of equilibrium pH, extractant concentration, metal ion concentration etc have been investigated. Using slope analysis technique it has been inferred that the metal ions form monomeric complex of the type [M(HA₂)₃] with carboxylic acids (H₂A₂, the dimer form). The stoichiometry of the species has also been confirmed using non-linear least square regression method. The carboxylic acids show different behaviour for Y extraction, it resembles to that of heavy rare earth (Ho), for sterically hindered acids (neo-heptanoic and Versatic 10) and to that with lighter rare earths (Ce, Pr) for the less sterically hindered acids (cekanoic and napthenic). The extraction order for the rare earths has been found to be the same with the four acids, i.e., La < Ce < Pr < Nd < Sm < Gd < Dy < Ho. The extraction constant (K_ex) of the systems and the separation factor amongst the rare earth pairs have been evaluated.     

Sorption behaviour and mechanism of ytterbium(III) on imino-diacetic acid resin

The sorption behaviour and mechanism of a novel chelate resin, imino-diacetic acid resin (IDAAR), for Yb(III) has been investigated in HAc–NaAc medium. The sorption of Yb(III) obeys the Freundlich isotherm. Optimum sorption for Yb(III) on IDAAR is at pH 5.13 and the statically saturated sorption capacity is 187 mg/g resin at 298 K. Yb(III) can be eluted using 1~2 mol L^− 1 HCl and the resin can be regenerated and reused without apparent decrease of sorption capacity. The apparent sorption rate constant is k₂₉₈ = 1.57 × 10^− 5 s^− 1; the apparent activation energy is 13.8 kJ mol^− 1 and the enthalpy change ΔH of IDAAR for Yb(III) is 29.8 kJ mol^− 1. The sorption mechanism of IDAAR for Yb(III) was examined by using chemical methods and IR spectrometry. The molar coordination ratio of the functional group of IDAAR to Yb(III) is 3:1 with the coordination compound formed between oxygen atoms in the functional group of IDAAR and Yb(III).     

Product inhibition by sulphide species on biological sulphate reduction for the treatment of acid mine drainage

It is well recognised that the product of sulphate reduction, i.e. the sulphide species formed, may inhibit the biological process. In this paper, we further the kinetic study of biological sulphate reduction using the mixed population of complete oxidisers growing on acetate for which kinetic data has been reported previously as a function of sulphate concentration, temperature, dilution rate and volumetric sulphate loading using chemostat culture by Moosa et al. to provide kinetic insight into this inhibition.The effect of a feed sulphide concentration in the range 0.50 to 1.25 kg m^− 3 on the biological sulphate reduction process is established using chemostat culture at pH 7.0 ± 0.2. Further, the chemical speciation of sulphide as undissociated H₂S or dissociated HS⁻ on process inhibition is reported through the variation of operating pH across the range pH 6.0 to pH 7.5 at a sulphate feed concentration of 2.5 kg m^− 3. It is clearly shown that inhibition is chiefly mediated by the undissociated H₂S sulphide species, rather than the total sulphide concentration. This inhibition was shown to affect the maximum specific growth rate constant and the death rate constant in the Contois rate equation presented previously while having negligible effect on KS describing substrate affinity.     

Silver catalyzed bioleaching of low-grade copper ores. Part III: Column reactors

A study of the effect of different variables (inoculation, [Ag], silver addition mode, reactivation of the ore (i.e. delayed silver addition), composition of the pregnant liquid solution, ore particle size, pH and aeration) on the silver-catalyzed bioleaching of two different low-grade copper (> 600 μm) has been investigated in column reactors.The two low-grade copper ores tested, lower K-ore and Pinto Valley Dump ore, behaved similarly during the silver catalyzed process. After more than 300 days of leaching, the addition of both silver and bacteria improved the copper extraction from both low-grade chalcopyritic ores. Copper extraction in the presence of silver displays paralinear kinetics with two stages very well defined, with the first stage being much faster than the second one.Copper extraction from the K-ore surpassed 70% after 350 days of bioleaching using low concentrations of silver (1.4 g Ag/kg Cu). The addition of silver was effective even after the bioleaching process was started. Conversely, silver addition mode and aeration were not significant variables in the bioleaching process. The effect of a pregnant liquid solution collected from a first bioleaching experiment demonstrated that the build up of the solution can negatively affect the copper extraction during the silver-catalyzed chalcopyrite process.Copper extraction from PVD ore surpassed 60% after 300 days of bioleaching using 1.24 g Ag/kg Cu. Copper extraction was substantially affected by both [Ag] and ore particle size. For the ranges between 0.25 and 1.00 g Ag/kg Cu and between < 5.08 × 10⁴ and < 1.27 × 10⁴ μm, the dissolution of copper was favoured with the highest amount of silver and the smallest particle size. On the other hand, silver concentration, pH and silver addition mode were not influential variables in the process.Chemical analyses, XRD and SEM/EDX studies were carried out on different residues after 100, 200 and 325 days of experimentation. Chemical analysis showed that copper is selectively bioleached in the presence of silver and bacteria, while silver remains in the solid phase. The SEM study showed that pyrite remains mainly unattacked during the bioleaching of chalcopyrite from the low-grade copper ore. Jarosite, gypsum and elemental sulphur precipitates have been identified on the residues.     

Extraction of cadmium (II) from phosphoric acid media using the di(2-ethylhexyl)dithiophosphoric acid (D2EHDTPA): Feasibility of a continuous extraction-stripping process

The extraction of cadmium from phosphoric media has been studied. The D₂EHDTPA was used as extractant and dodecane as diluent. No third phase was observed in the investigated conditions.A continuous micro-pilot scale mixer-settler was successfully tested for both extraction and stripping. More than 99% extraction rate was obtained in steady-state conditions with a flow rate ratio Aqueous/Organic equal to 1.1. Continuous stripping was performed using HCl 4 M. More than 96% of the cadmium was stripped in one continuous mixer-settler stage for flow rate ratio equal to 0.7. Results were in good agreement with the predicted values based on the McCabe–Thiele method. Experimental mixer-settler stages behave as ideal ones (Murphree efficiency > 98%).An optimal flow sheet is proposed to purify the Wet Phosphoric Acid (WPA) and to recover a relatively concentrated cadmium solution (1 g L^? 1). Two ideal stages operating at phase ratio A/S equal to 5/1 are required for the extraction step leading to a very depleted raffinate (< 0.2 µg L^? 1). For the stripping step, six stages are required (S/A = 5/1). The recovered organic phase contains less than 2 µg L^? 1 and could be recycled in the extraction step.     

The electrochemical oxidation of chalcopyrite in ammoniacal solutions

The anodic dissolution of chalcopyrite in ammoniacal solutions was investigated using electrochemical methods. At low overvoltages, the formation of a copper deficient sulfide layer, Cu_1-xFeS₂ through a charge transfer reaction is proposed based upon the dependence of the rest or open circuit potential on solution composition and the presence of a Tafel region of appropriate slope. In addition, a current peak that occurs at 10⁻⁴ A/cm² is a function of the square root of the voltage scanning speed and is explained in terms of a charge transfer reaction. At larger overvoltages, constant potential experiments and mass balances performed at various anodic potentials indicate that the dissolution is consistent with the overall reaction, CuFeS₂ + 4NH₃ + 9OH^- = Cu(NH₃) ₄ ⁺² + Fe(OH)₃ + S₂O ₃ ⁼ + 3H₂O +9e ^-, although some copper may be released to solution in the cuprous state and some ferrous iron has been identified in the product film. Currentvs time data taken during constant potential experiments were found to obey a linear rate relationship. This was interpreted in terms of the formation of a layer of constant thickness which is corroded at the outer interface at the same rate it is formed at the inner interface.. The model proposed is typical of the corrosion of some metals. An examination of the polarization curves shows the dissolution reaction to be first order with respect to [OH^-]. The lack of dependence on [Cu²⁺] indicates that the catalytic effect of cupric ion during oxygen pressure leaching is related only to the cathodic reduction of O₂ in agreement with the results of previous investigations.     

The performance,kinetics and microbiology of sulfidogenic fluidized-bed treatment of acidic metal- and sulfate-containing wastewater

A sulfidogenic fluidized-bed reactor (FBR) process was developed for treating acidic metal- and sulfate-containing wastewater. The process operating parameters were determined and the bacterial diversity of the FBR was described. The process was based on sulfate reduction by sulfate-reducing bacteria (SRB), precipitation of metals as sulfides with the biogenic H₂S and neutralization of the water with biologically produced bicarbonate alkalinity. The lactate- and ethanol-utilizing FBRs precipitated over 600 mg Zn l^− 1 day^− 1 and 300 mg Fe l^− 1 day^− 1 at a hydraulic retention time of 6.5 h. Metal precipitation was over 99.8% and effluent soluble Zn and Fe concentrations were below 0.1 mg l^− 1. Zinc and iron precipitated predominantly as ZnS, FeS₂ and FeS. The wastewater pH was increased from 2.5–3 to 7.5–8.5 during the treatment. Acetate oxidation was the rate-limiting step in ethanol oxidation. Ethanol oxidation was more affected by sulfide toxicity than was acetate oxidation. The FBR microbial communities contained SRB related to members of the genera Desulfovibrio, Desulfobulbus, Desulfotomaculum, Desulfobacca and Desulforhabdus, and also many species that do not reduce sulfate. The FBR communities contained many previously undescribed bacteria. This study demonstrated the feasibility of the sulfidogenic FBR for the concomitant removal of acidity, metals and sulfate from wastewaters.     

Electrochemical modeling and study of copper deposition from concentrated ammoniacal sulfate solutions

A fundamental investigation of the electrolytic deposition of copper from concentrated aqueous ammoniacal solutions has been carried out based on the thermodynamic analysis of the system Cu–NH₃–H₂O. The speciation of copper vs. pH and redox potential was modeled in high ionic strength solutions, in which the activity coefficients of the system species were estimated according to the Modified Bromley's Methodology. The electrochemical behavior of the redox system Cu(0)/Cu(I)/Cu(II) in concentrated aqueous ammoniacal solutions was studied at pH = 9.5 and the cathodic reactions in these solutions were determined. It was found that metallic copper was formed under strongly reductive redox conditions, while under mildly reductive to mildly oxidative conditions the cuprous di-ammine complex species dominate. Under highly oxidative conditions the cupric tetra-ammine complex species predominated. According to the theory and results, the cathodic deposition of copper from concentrated aqueous ammoniacal solutions proceeds in a two-step reduction mechanism. The cupric ammine species are first reduced to cuprous di-ammine, which in turn is reduced to metallic copper. The electrochemical experiments revealed that copper deposition over time follows a sigmoid-type curve, verifying the two-step mechanism. The main feature of these sigmoid curves was the presence of an induction period with negligible copper deposition, followed by an acceleration period where the copper deposition rate gradually increased. By increasing the applied cell voltage, the induction period was significantly reduced or disappeared.     

Tungsten recovery from alkaline leach solutions as synthetic scheelite

The recovery of tungsten from alkaline leach solutions has been studied examining the effect of temperature, pH, Ca/WO₃ molar ratio and nature of the precipitated calcium tungstate. The precipitation kinetics of calcium tungstate, upon the addition of aqueous sodium tungstate to calcium solutions, was followed by potentiometric measurements using a calcium ion-selective electrode. Two models, a crystal growth model and a second-order reaction opposed by zero-order reaction, have been used to test the experimental data. Both models show that the apparent activation energy of CaWO₄ precipitation falls in the range 58 to 67 kJ mol^− 1.The kinetic data shows that the maximum recovery of precipitated calcium tungstate occurs at pH ≥ 8.5 with a 10% excess of CaCl₂ at 50 °C over a period of 20 min using sodium tungstate solutions of 100 g L^− 1 and 150 g L^− 1 WO₃.