The behavior of thiourea and flotation reagents in zinc electrowinning circuits

The effect of thiourea and flotation reagents on the electrowinning of zinc from industrial electrolytes was studied, and all the compounds were found to reduce the zinc deposition current efficiency and to change the properties of the zinc deposits. The effectiveness of activated carbon, two-stage cementation, and hot acid leaching on the destruction/removal of the organic compounds also was addressed. Activated carbon pretreatment of thiourea-containing electrolytes restored the current efficiency for 1-hour zinc deposits to values comparable to those obtained for thiourea-free electrolytes. The activated carbon pretreatment, however, altered the deposit morphology and orientation, but produced a cyclic voltammogram similar to that of the thiourea-free solution. Two-stage cementation did not counteract the harmful effects of thiourea. Hot acid leaching destroyed the thiourea but generated large concentrations of ferrous ion that reduced the current efficiency. The ferrous concentrations, however, were readily controlled by KMnO₄ or MnO₂ oxidation. None of the treatment options (activated carbon, two-stage cementation, or hot acid leaching) was effective in controlling the flotation reagents, and their moderately harmful effect on zinc electrowinning persisted. Even low concentrations of these reagents polarized zinc deposition, and this resulted in a “glue-type” zinc deposit.     

The behavior of thiourea and flotation reagents in zinc electrowinning circuits

The effect of thiourea and flotation reagents on the electrowinning of zinc from industrial electrolytes was studied, and all the compounds were found to reduce the zinc deposition current efficiency and to change the properties of the zinc deposits. The effectiveness of activated carbon, two-stage cementation, and hot acid leaching on the destruction/removal of the organic compounds also was addressed. Activated carbon pretreatment of thiourea-containing electrolytes restored the current efficiency for 1-hour zinc deposits to values comparable to those obtained for thiourea-free electrolytes. The activated carbon pretreatment, however, altered the deposit morphology and orientation, but produced a cyclic voltammogram similar to that of the thiourea-free solution. Two-stage cementation did not counteract the harmful effects of thiourea. Hot acid leaching destroyed the thiourea but generated large concentrations of ferrous ion that reduced the current efficiency. The ferrous concentrations, however, were readily controlled by KMnO₄ or MnO₂ oxidation. None of the treatment options (activated carbon, two-stage cementation, or hot acid leaching) was effective in controlling the flotation reagents, and their moderately harmful effect on zinc electrowinning persisted. Even low concentrations of these reagents polarized zinc deposition, and this resulted in a “glue-type” zinc deposit.     

MnO2 reduction by calcium lignosulfonate: the kinetics and mechanism studies

ABSTRACT

The kinetics and mechanism of the reduction of manganese dioxide ore by calcium lignosulfonate (CLS) in acid solution were investigated. The results showed that the reducing process mainly occurred after the adsorption and then decomposition of CLS by ore and the concentration of sulphuric acid exerted an enhancing effect on both CLS adsorption and manganese extraction. The effects of leaching temperature, the mass ratio of CLS to MnO₂ in ore, concentration of sulphuric acid as well as leaching time on per cent leached of Mn were also discussed. The experimental data were well interpreted with a shrinking core model of internal diffusion, and an overall kinetic model was established. The reaction rate constant was found to be proportional to the mass ratio of CLS to MnO₂ in ore and concentration of sulphuric acid, and the apparent activation energy was determined to be 69.4?kJ?mol^?1 by using Arrhenius expression.     

A Visual Insight into the Oxidation of Sulfide Minerals During Bioleaching and Chemical Leaching of a Complex Ore

A scanning electron microscope (SEM) study was performed to provide a visual insight into the oxidation patterns of sulfide minerals during chemical and bacterial leaching of a complex ore for 3 days. The mineral grains were studied under SEM before and after bacterial and chemical leaching with or without the addition of ferrous iron to generate ferric iron in situ by bacteria or chemical oxidant (MnO₂). Both mesophilic and moderately thermophilic cultures of bacteria were used in bioleaching tests. A limited oxidation of sphalerite and pyrite, similar to those in acid leaching (control), was observed to occur when no ferrous iron was added. However, the initial addition of ferrous iron into bioleaching media was shown to significantly improve the oxidation of sphalerite and pyrite. Galena was readily oxidized in the presence or absence of bacteria. Sphalerite was oxidized more extensively/selectively than chalcopyrite and pyrite, consistent with their respective nobility/electrochemical activity. Provided that chemical/biological oxidation of sphalerite was intensive, a sulfur-rich layer appeared to form on mineral surface. But, no such layer on pyrite surfaces was discernable. Supplementary bioleaching data were also provided to support SEM observations and to further elucidate the bioleaching characteristics of these sulfide phases. It can be inferred from this study that the oxidation of sulfides proceeds most discernibly via “indirect mechanism” and the generation of ferric iron by bacteria in sufficient quantity is essential for the effective oxidation of sulfide minerals.     

Simultaneous leaching of a deep-sea manganese nodule and chalcopyrite in hydrochloric acid

Hydrochloric acid leaching of chalcopyrite and a manganese nodule, in combination, was studied using powder samples. Chalcopyrite, which does not dissolve well in HCl, was effectively leached in the presence of a manganese nodule at 3 to 4 M HCl. The rates of dissolution of metal values from the nodule were also enhanced in the presence of chalcopyrite. Dissolution was found to occur through three routes: (1) the galvanic interaction between CuFeS₂ and MnO₂, (2) the action of the Fe³⁺/Fe²⁺ redox couple, and (3) the action of Cl₂ gas generated from the MnO₂-HCl reaction on CuFeS₂. The last route appeared to make the major contribution to the dissolution. The combined leaching of a nodule and chalcopyrite appears to be promising from both technical and economic points of view.     

Active manganese oxide on MnOx–CeO2 catalysts for low-temperature NO oxidation: Characterization and kinetics study

MnO_x–CeO₂ catalysts were synthesized to investigate the active sites for NO oxidation by varying the calcination temperature. XRD and TEM results showed that cubic CeO₂ and amorphous MnO_x existed in MnO_x–CeO₂ catalysts. High temperature calcination caused the sintering of amorphous MnO_x and transforming to bulk crystalline Mn₂O₃. H₂-TPR and XPS results suggested the valence of Mn in MnO_x–CeO₂ was higher than pure MnO_x, and decreased with the increasing calcination temperature. The turnover frequency (TOF) was calculated based on the initial reducibility according to H₂-TPR quantitation and kinetic study. The TOF results indicated that the initial reducibility of amorphous MnO_x with high valence manganese ions was equivalent to the active sites for NO oxidation. It can be inferred that the amorphous MnO_x plays a key role in low-temperature NO oxidation.     

High-efficiency simultaneous extraction of rare earth elements and iron from NdFeB waste by oxalic acid leaching

Iron can not be recovered at high value because only rare earth elements are effectively recovered from NdFeB waste via oxidation roasting-hydrochloric acid leaching process.In this study,a new method for leaching NdFeB waste with oxalic acid was developed.The high-efficiency,simultaneous and high-value recovery of rare earth elements and iron was realized to simplify the process and improve the economic benefit.Results of the oxalic acid leaching experiments show that under the optimum leaching conditions at 90℃ for 6 h in the aqueous solution of oxalic acid(2 mol/L) with a liquid-solid ratio of60 mL/g,the iron leaching efficiency and precipitation rate of rare earth oxalate reach 93.89% and 93.17%,respectively.Rare earth oxalate and Fe(C₂O₄)3^3- were left in the residue and the leaching solution,respectively.The leaching mechanism was further analyzed by characterising the leach residues obtained through X-ray powder diffraction(XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy(SEM-EDS).Results of the leaching kinetics study indicate that the process of oxalic acid leaching follows the shrinking nucleus model,and the leaching kinetics model is controlled by the mixed factors of diffusion and chemical reaction.The leaching residue was calcined at 850℃ for 3 h and then decomposed into rare earth oxide,which can be directly used to prepare rare earth alloy via molten salt electrolysis.For the leaching solution,ferric oxalate solution was reduced using Fe powder to prepare the ferrous oxalate(FeC₂O₄-2H₂O).     

Effect of preparation method on MnOx-CeO2 catalysts for NO oxidation

The NO oxidation reaction was studied over MnO_x-CeO₂ catalysts prepared by co-precipitation, impregnation and mechanical mixing method, respectively. It was found that the co-precipitation was the most active and a 60% NO conversion was achieved at 250 °C. X-ray diffraction (XRD), Brumauer-Emmett (BET), H₂-temperature programmed reduction (H₂-TPR) and oxygen storage capacity (OSC) techniques were employed to characterize the physical and chemical properties of the catalysts. XRD results showed that amorphous MnO_x or Mn-O-Ce solid solution existed in co-precipitation and impregnation prepared sample, while crystalline MnO_x was found in mechanical mixing catalyst. A larger surface area was observed on co-precipitation prepared catalyst compared to those prepared by impregnation and mechanical mixing. The strong interaction between MnO_x and CeO₂ enhanced the reducibility of the oxides and increased the amount of Mn⁴⁺ and activated oxygen, which are favorable for NO oxidation to NO₂.     

Aluminum Leaching from Calcined Coal Waste Using Hydrochloric Acid Solution

Coal waste is the largest industrial solid waste generated from coal preparation plants during the processing and cleaning of coal for electric power generation. Aluminum oxide (Al₂O₃, 16–36%), silicon dioxide (SiO₂, 45–58%), and carbon (4–25%) account for more than 90% of the coal waste. In this paper the results of aluminum leaching from calcined coal waste are reported. Techniques including XRD, SEM, and FTIR were used to characterize the coal waste before and after calcination. Three additives were evaluated during the calcination process to improve the extraction of Al₂O₃ by hydrochloric acid dissolution. The results from the leaching experiments show that coal waste is activated after calcination at a temperature between 650°C and 750°C. With addition of sodium fluoride effective aluminum dissolution of 90% was achieved under certain conditions. The extent of leaching was found to be 20% higher than that achieved without sodium fluoride addition.     

Kinetic study on carbothermic reduction of MnO2 with graphite

The carbothermic reduction of MnO₂ with graphite has been investigated for the reduction mechanism in the temperature range of 1000–1300°C. Although several researchers studied the reduction step from MnO₂ to MnO, it was attempted to employ the in situ testing of reduction behaviour with the analysis of product gas composition simultaneously from the different points of view. The overall reduction rate increased with temperature, which was controlled by the carbon gasification. It is because the reduction rate of MnO₂ to MnO was much faster than the carbon gasification at the experimental temperatures. This was confirmed by the generation of negligible amount of CO compared with CO₂ in the analysis of product gases. Furthermore, the results could explain the difficult formation of manganese carbide from MnO₂ in contrast with the carbide formation from pure MnO.     

The leaching of chalcopyrite with cupric chloride

A comparative study of electrochemical leaching and chemical leaching of chalcopyrite was performed mainly at 343 K to elucidate the leaching mechanism of chalcopyrite with CuCl₂. Also, the morphology of the leached chalcopyrite surface was studied by using a single chalcopyrite crystal. The leaching with CuCl₂ produced a porous elemental sulfur layer on the chalcopyrite surface, showing a similar morphology to that produced during leaching with FeCl₃. The leaching kinetics were found to be linear over an extended period, followed by an acceleration stage, as a result of an increase in the reaction surface area. The leaching rate of chalcopyrite was proportional to C(CuCl₂)^0.5, whereas it was inversely proportional to C(CuCl)^0.5. The mixed potential of chalcopyrite exhibited a 66 mV decade⁻¹ dependency upon C(CuCl₂), and—69 mV decade⁻¹ upon C(CuCl). Based on these observations together with other findings, an electrochemical mechanism involving the oxidation of chalcopyrite and CuCl ₋ ² and the reduction of CuCl⁺ was proposed. The Tafel plot between the mixed potential and the current density obtained by converting the rate of chemical leaching gave a straight line whose slope was in good agreement with that of the electrochemical leaching. These findings strongly support the electrochemical mechanism of chalcopyrite leaching with cupric chloride.     

Oxidation Studies of SiAlON/MgAlON Ceramics with Fe2O3 and CaO Impurities, Part I: Kinetics

The oxidation behaviors of composites SiAlON/MgAlON phases (β-SiAlON, 15R-SiAlON and MgAlON) synthesized from the residue during the leaching treatment of salt cake and corresponding synthetic samples were investigated in air by thermogravimetric measurements. Combined kinetics, viz. linear law + arctan law + parabolic law, are used to describe the kinetics of oxidation in isothermal mode. The oxidation studies reveal the effects of impurities, namely, Fe₂O₃ and CaO, present in the salt cake residue. The addition of Fe₂O₃ results in a lower activation energy and more aggressive oxidation. The addition of CaO caused the shrinkage during the synthesis and liquid formation during the oxidation above 1673 K (1400 °C). The impurities of CaO and Fe₂O₃ in the leaching residue can result in an aggressive oxidation at low temperature and a protective oxidation at temperatures above the eutectic point.     

Oxidation of acetate and octanoate and its relation to glucose metabolism in contracting porcine carotid artery

The oxidation of octanoate and acetate was measured in segments of porcine carotid arteries to ascertain whether the oxidation of exogenous fatty acid substrates (acetate and octanoate) is augmented during contraction induced by K(+)-depolarization. The oxidation of acetate increased from 7 +/- 1 to 14 +/- 2 nmol/min/g (P < 0.01) during sustained isometric contraction. Octanoate oxidation increased from 11 +/- 1 to 14 +/- 1 nmol/min/g (P < 0.05). The rate of oxidation of neither acetate nor octanoate was affected by the presence or absence of glucose either in resting or contracting arteries Acetate or octanoate oxidation could account for the majority of O2 consumption during contraction. Octanoate but not acetate inhibited glucose uptake and glycolysis in resting muscles. In contrast to augmented acetate and octanoate metabolism during contraction, there was a "down-regulation" of glucose metabolism in contracting muscles as evidenced by a decrease in the rate of glucose uptake, glycolysis and lactic acid production during sustained isometric contraction. Thus, contractile activation of vascular smooth muscle is associated with a shifting pattern of substrate utilization. Exogenous acetate or octanoate can serve as the primary oxidative substrate during sustained isometric contraction.     

Enhancement of chalcopyrite leaching by ferrous ions in acidic ferric sulfate solutions

The effects of ferrous ions on chalcopyrite oxidation with ferric ions in 0.1 mol dm⁻³ sulfuric acid solutions were investigated by leaching experiments at 303 K in nitrogen. With high cupric ion concentrations, the chalcopyrite oxidation was enhanced by high concentrations of ferrous ions and copper extraction was mainly controlled by the concentration ratio of ferrous to ferric ions or the redox potential of solutions. Ferrous ions, however, suppressed the chalcopyrite oxidation when cupric ion concentrations were low. A reaction model, which involves chalcopyrite reduction to intermediate Cu₂S by ferrous ions and oxidation of the Cu₂S by ferric ions, was proposed to interpret the results.     

Effects of calcium substitute in LaMnO3 perovskites for NO catalytic oxidation

La1-x Cax MnO3 (x=0-0.3) perovskite-type oxides were synthesized by citrate sol-gel method. The physical and chemical properties were characterized by X-ray diffraction (XRD), Brumauer-Emmett-Teller method (BET), X-ray photoelectron spectroscopy (XPS), NO+O2 -TPD (temperature-programmed desorption), activated oxygen evaluation and H2 -TPR (temperature-programmed reduction) technologies. The results showed that NO catalytic oxidation activity was significantly improved by Ca substitution, especially for lower temperature activity. The La0.9 Ca0.1 MnO 3 sample showed the maximum conversion of 82% at 300 oC. The monodentate nitrates played a crucial role for the formation of NO2 . The reducibility of Mn 4+ ions and reactivity of activated oxygen were favorable for the catalytic performances of NO oxidation.     

Microbial Dissolution of Zn-Pb Sulfide Minerals Using Mesophilic Iron and Sulfur-Oxidizing Acidophiles

The purpose of this study is to test the feasibility of using mixed culture of iron and sulfur-oxidizing bacteria for the dissolution of metals from high-grade zinc and lead sulfide ore. Considering that the roll crusher could reduce the ore size to less than 2 mm, this size fraction was selected in order to study the possibility of removing mill circuit. Effects of parameters such as pulp density, initial pH, Fe²⁺, oxidation–reduction potential (ORP), and pH fluctuations were investigated, as well. The maximum Zn dissolution was achieved under the conditions of initial pH 2, initial 75 g/L FeSO₄ · 7H₂O, and pulp density of 50 g/L. The results indicated that under the optimum conditions, about 68.8% of zinc was leached during 24 days of bacterial leaching treatment. The lead recoveries were low (about 1%), because of precipitation of Pb as lead arsenate chloride. Furthermore, the surface studies by using SEM images showed that during chemical leaching the ore dissolution starts from surface discontinuities, but in bacterial leaching all surface becomes involved. In addition, in another process the ore was leached separately with sulfuric acid and sodium hydroxide, and then final results were compared to the bacterial leaching tests in order to find the optimum hydrometallurgical method to extract zinc and lead from these ores.     

A novel recovery process of metal values from the cathode active materials of the lithium-ion secondary batteries

A novel process was conducted with experiments which separated and recovered metal values such as Co, Mn, Ni and Li from the cathode active materials of the lithium-ion secondary batteries. A leaching efficiency of more than 99% of Co, Mn, Ni and Li could be achieved with a 4 M hydrochloric acid solution, 80 °C leaching temperature, 1 hour leaching time and 0.02 gml^− 1 solid-to-liquid ratio. For the recovery process of the mixture, firstly the Mn in the leaching liquor was selectively reacted and nearly completed with a KMnO₄ reagent, the Mn was recovered as MnO₂ and manganese hydroxide. Secondly, the Ni in the leaching liquor was selectively extracted and nearly completed with dimethylglyoxime. Thirdly, the aqueous solution in addition to the 1 M sodium hydroxide solution to reach pH = 11 allowed the selective precipitation of the cobalt hydroxide. The remaining Li in the aqueous solution was readily recovered as Li₂CO₃ precipitated by the addition of a saturated Na₂CO₃ solution. The purity of the recovery powder of lithium, manganese, cobalt and nickel was 96.97, 98.23, 96.94 and 97.43 wt.%, respectively.     

Effect of pH,iron concentration,and pulp density on the solubilization of uranium from ore material in chemical and microbiological acid leach solutions: Regression equation and confidence band analysis

The solubilization of uranium from a finely ground ore material was investigated in leaching tests each lasting for about 24 h. Ferric iron, added as Fe₂(SO₄)₃ or produced by prior microbiological oxidation of FeSO₄, accelerated the rates of uranium leaching, as compared with those obtained with sulfuric acid or acidic ferrous sulfate. Pulp density and initial pH were also test variables. Yields of up to 100% uranium extraction were obtained within 24 h. Quadratic response surfaces were fitted to the experimental leach curves and confidence bands were calculated to assess the significance of the effects of pH, iron concentration, and pulp density on uranium solubilization. The general separation of the confidence bands indicated that each factor had a significant effect.     

Surface reactive species on MnOx（0.4）-CeO2 catalysts towards soot oxidation assisted with pulse dielectric barrier discharge

MnOx(0.4)-CeO2 was investigated for soot oxidation assisted with a pulse dielectric barrier discharge(DBD).The catalysts were evaluated and characterized with TPO(temperature programmed oxidation),X-ray diffraction(XRD),Raman and X-ray photoelectron spectroscopy(XPS).The ignition temperature Ti for soot oxidation decreased from 240.8 to 216.4 oC with the increase of the pulse DBD frequencies from 50 to 400 Hz,lower than that of the case without pulse DBD present(253.4 oC).The results of XRD,Raman and XPS agreed well with the TPO activities of MnOx(0.4)-CeO2 towards soot oxidation.More solid solution of ceria and manganese,and surface reactive species including O2–,O– and Mn4+ were responsible for the enhancement of soot oxidation due to pulse DBD injection in the present study.For solid solution favors to the activation and transformation of those species,which are believed to be involved in the soot oxidation in a hybrid catalysis-plasma.     

Kinetics of nitric acid leaching of cerium from oxidation roasted Baotou mixed rare earth concentrate

The kinetics of nitric acid leaching of cerium was investigated for the oxidation roasted Baotou mixed rare earth concentrate. The effects of leaching temperature, HNO₃ concentration, liquid–solid ratio (L/S) and stirring rate on rare earth extraction were studied. The XRD and SEM mapping analysis of the samples before and after acid leaching shows that the roasted bastnaesite is completely leached. Besides, the decomposition process of oxidizing roasting was also obtained by TG–MS and XRD. Different kinetics models were applied in this leaching process. The results of dynamic fitting show that the leaching process can be described by a new variant of the shrinking-core model. And the leaching rate is controlled by both the interfacial transfer and diffusion through the product layer. The apparent activation energy is calculated as 76.78 kJ/mol and the reaction orders with respect to HNO₃ concentrations and liquid–solid ratio are determined to be 7.609 and 2.516, respectively. Besides, an empirical rate equation is obtained to describe the process.