Zinc recovery from spent ZnO catalyst by carbon in the presence of calcium carbonate

Zinc recovery from the spent zinc oxide catalyst by carbon in the presence of calcium carbonate was studied using an X-ray diffractometer (XRD), an atomic absorption spectrometer (AAS), and a scanning electron microscope (SEM). The spent zinc oxide catalyst was determined to be composed of 87.5 wt pct zinc oxide and 3.1 wt pct zinc sulfide. The results of X-ray diffractometry revealed that calcium carbonate decomposed to calcium oxide and carbon dioxide; zinc oxide and zinc sulfide were reduced to zinc vapor and carbon monoxide evolving from solid sample; and sulfur content was scavenged as calcium sulfide remained in the solid. Steps involved in this reaction system were summarized to explain the overall reaction. The experimental results of atomic absorption spectrometry showed that the initial rate of zinc recovery and final zinc recovery can be increased by increasing either the sample height, the reaction temperature or the initial bulk density. Furthermore, they were found to increase with decrease in either the argon flow rate, the molar ratio of Zn_total/C, the molar ratio of Zn_total/CaCO₃, the grain size of the spent catalyst, the agglomerate size of carbon, or the agglomerate size of calcium carbonate. Empirical expressions of the initial rate of zinc recovery and final zinc recovery have been determined.     

Hydrochloric acid leaching of nickel sulfide precipitates

Nickel sulfide precipitates produced by the AMAX Acid Leach Process for oxide nickel ores were leached in hydrochloric acid. The effects of process variables such as temperature, acid concentration, stoichiometric excess of HCl, gas sparging and heat treatment of feed were investigated. The nickel leachability was found to be in the 60–80% range. Chemical and mineralogical examination of the leach residues indicated the presence of NiS₂. This higher nickel sulfide is insoluble in hydrochloric acid, and its presence hinders the leaching of NiS. Several methods are suggested to reduce the sulfur content in order to attain complete dissolution. The thermodynamics and kinetics of nickel sulfide leaching are briefly discussed.     

Oxidation of nickel sulfide

The oxidation of nickel sulfide whose atomic fraction of sulfur,x _s, is 0.40 to 0.44 was studied in a mixed O₂-N₂ gas stream at 923, 973, and 1023 K. The oxygen partial pressure was maintained at 2.0 x 10⁴ Pa. In the oxidation of nickel sulfide ofx _s = 0.40 and 0.41, a dense NiO layer was formed on the sulfide surface without the evolution of SO₂ gas, because of the low sulfur activity. Diffusion of nickel within the inner sulfide core toward the surface controlled the oxidation rate during the first one minute of oxidation. Subsequently, the oxidation rate was controlled by the diffusion of nickel through the formed NiO layer. In the oxidation of nickel sulfide ofx _s = 0.44 at 973 and 1023 K, the reaction proceeded irregularly to the interior of the sulfide core with the evolution of SO₂ gas, and a porous oxide layer was formed, due to the high sulfur activity of nickel sulfide. For the same reason, this oxidation was also accompanied by the dissociation of nickel sulfide. Under the experimental conditions ofx _s = 0.42, 1023 K and x_s = 0.44,923 K, the oxidation started with weight increase and without the evolution of SO₂ gas, and in the subsequent stage the weight decreased and SO₂ gas was evolved. K. HAJIKA, former Graduate Student     

Na2SO4-induced accelerated oxidation (hot corrosion) of nickel

The Na₂SO₄-induced accelerated oxidation of nickel has been studied at 1000°C. It has been found that low oxygen activities in the Na₂SO₄, which are produced by the formation of NiO, cause the sulfur activity of the Na₂SO₄ to be increased. Nickel and sulfur from the Na₂SO₄ combine to form nickel sulfide and the oxide ion activity of the Na₂SO₄ is increased. The accelerated oxidation of nickel occurs because oxide ions react with NiO to form a nonprotective oxide scale. The accelerated oxidation of nickel is not self-sustaining since oxide ions are not produced when conditions in the Na₂SO₄ are no longer favorable for the formation of nickel sulfide.     

Kinetics of cobalt sulfide reduction in the presence of calcium oxide

Kinetic measurements have been made on the hydrogen reduction of solid cobalt sulfide in the presence of calcium oxide. The cobalt metal yield was compared with that of the direct reduction reaction over the temperature range 600 to 800°C at various hydrogen flow rates, and calcium oxde to cobalt sulfide mixing ratios. It was found that the presence of calcium oxide caused a sharp increase in the reaction rate—for example a 15 fold increase in conversion was achieved at 700°C after 24 min of reaction. Low hydrogen flow rates were found to be desirable, and an optimum mixing ratio of 3.0 established.     

Fluidized Bed Selective Oxidation-Sulfation Roasting of Nickel Sulfide Concentrate: Part II. Sulfation Roasting

The fluidized bed sulfation roasting process followed by water leaching was investigated as an alternative process to treat nickel sulfide concentrate for nickel production. The effects of several roasting parameters, such as the sulfation gas flow rate, roasting temperature, the addition of Na₂SO₄, and the roasting time, were studied. 79 pct Ni, 91 pct Cu, and 95 pct Co could be recovered with minimal dissolution of Fe of 4 pct by water leaching after two-stage oxidation-sulfation roasting under optimized conditions. The sulfation roasting mechanism was investigated, showing that the outermost layer of sulfate melt and the porous iron oxide layer create a favorable sulfation environment with high partial pressure of SO₃. Sulfation of the sulfide core was accompanied by the conversion of the sulfide from Ni_1?x S to Ni₇S₆ as well as inward diffusion of the sulfation gas.     

Phase equilibria in the metal-sulfur-oxygen system and selective reduction of metal oxides and sulfides: Part I. The carbothermic reduction and calcination of complex mineral sulfides

The difference in the standard Gibbs free energy for the formation of any two oxides or sulfides is the chemical potential for selective reduction of metals from complex minerals. The magnitude of the Gibbs free energy difference is shown by plotting the univariant relationships for relevant sulfides and oxides. In this investigation, three examples of mineral sulfides are considered, and the experimental results are compared with the predicted thermodynamic calculations. These examples include the reduction conditions for nickel and iron sulfides and pentlandite (Fe,Ni)₉S₈ and chalcopyrite (CuFeS₂) minerals. The reduction behavior of mineral sulfides, such as those of nickel, cobalt, iron, and copper, is illustrated by referring to both the sulfide and alloy phase equilibria. In particular, the solution thermodynamic properties of the metallic phase equilibria are featured for determining the physical chemistry of preferential or selective reduction of the metal oxides and sulfides. The mechanism for the reduction of the aforementioned sulfide minerals is explained with the aid of the governing phase equilibria for the calcination process. The results from the carbothermic reduction of sulfide minerals are also compared. The important roles of lime and calcium sulfate in controlling the emission of sulfurous gases during the reduction reaction are explained. A qualitative analysis of reduction reactions of nickel and iron sulfides is reviewed to provide a comparison of the mechanism for complex nickel-bearing minerals. The importance of these results in producing alloy and pure metallic phases is also examined.     

Absorption of gaseous oxygen by liquid cobalt,copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts’ method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600°C, and for copper was 1250°C. For initial oxygen pressures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm³ (STP) O₂/cm² of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm³ (STP) O₂/cm² under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and proportional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissociative adsorption of oxygen molecules at the gas/oxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative adsorption of oxygen molecules at the gas/oxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen absorption rate compared to that of liquid cobalt. R. H. RADZILOWSKI, formerly a Graduate Studient at The University of Michigan     

Absorption of gaseous oxygen by liquid cobalt, copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts' method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600‡C, and for copper was 1250‡C. For initial oxygen pres-sures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm³ (STP) O₂/cm² of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm³ (STP) 0₂/cm² under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and pro-portional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissocia-tive adsorption of oxygen molecules at the gasJoxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative ad-sorption of oxygen molecules at the gasJoxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen ab-sorption rate compared to that of liquid cobalt. Formerly a Graduate Student at The University of Michigan     

Oxidation of cobalt sulfide

Dense plate of cobalt sulfide was oxidized in a mixed O₂-N₂ gas stream at 873 to 1123 K. Atomic fraction of sulfur of the sulfide was between 0.505 and 0.525, and the partial pressure of oxygen in the gas stream was varied between 5.05 x 10₃ and 2.02 x 10₄ Pa. At lower temperature, cobalt diffused from the interior of sulfide to the surface due to the lower sulfur activity, and a dense oxide layer was formed without the evolution of SO₂ gas. The oxidation rate was controlled by the diffusion of cobalt in the sulfide in the initial few minutes, and it was controlled by the diffusion of cobalt through the oxide layer in the subsequent oxidation. At higher temperature, the oxidation of cobalt sulfide proceeded accompanying the evolution of SO₂ gas due to the higher sulfur activity, and a porous oxide was formed. The oxidation rate was determined by the mass transfer of oxygen through the gas boundary film and the oxide layer.     

Formation of metallic phase on reducing-gas injection in multicomponent oxide melt. Part 2. Density and surface properties

The density and surface tension of melts of ferronickel (0–100% Ni) and oxidized nickel ore are measured by the sessile-drop method, as well as the interface tension at their boundary in the temperature range 1550–1750°C. The composition of the nickel ore is as follows: 14.8 wt % Fetot, 7.1 wt % FeO, 13.2 wt % Fe₂O₃, 1.4 wt % CaO, 16.2 wt % MgO, 54.5 wt % SiO₂, 4.8 wt % Al₂O₃, 1.5 wt % NiO, and 1.2 wt % Cr₂O₃. In the given temperature range, the density of the alloys varies from 7700 to 6900 kg/m³; the surface tension from 1770 to 1570 mJ/m²; the interface tension from 1650 to 1450 mJ/m², the density of the oxide melt from 2250 to 1750 kg/m³; and its surface tension from 310 to 290 mJ/m². The results are in good agreement with literature data. Functional relationships of the density, surface tension, and interphase tension with the melt temperature and composition are derived. The dependence of the alloy density on the temperature and nickel content corresponds to a first-order equation. The temperature dependence of the surface tension and interphase tension is similar, whereas the dependence on the nickel content corresponds to a second-order equation. The density and surface tension of the oxide melt depend linearly on the temperature. The results may be used to describe the formation of metallic phase when carbon monoxide is bubbled into oxide melt.     

Sulfide capacities of fayalite-base slags

The sulfide capacities of fayalite-base slags were measured by a gas-slag equilibration technique under controlled oxygen and sulfur potentials similar to those encountered in the pyrometallurgical processing of nonferrous metals. The oxygen pressure range was from 10^−9.5 to 10⁻¹¹ MPa and the sulfur pressure range from 10⁻³ to 10^−4.5 MPa, over a temperature range of 1473 to 1623 K. The slags studied were FeO-SiO₂ at silica saturation and those with addition of CaO, MgO, and Al₂O₃ to determine their effect on sulfide capacities. For these slags, the sulfide capacities were found to vary from 10^−3.3 to 10⁻⁵. The sulfide capacities increased with increasing temperature from 1473 to 1623 K. A comparison of the reported plant data on sulfur content of industrial slags shows good agreement with the present experimental results. The present data will be useful in estimating metal losses in slag due to metal sulfide entrainment in nonferrous smelters.     

Oxidation kinetic studies of zinc sulfide in a fluidized bed reactor

Kinetic studies of the oxidation of zinc sulfide were carried out in a fluidized bed reactor over a temperature range of 740° to 1000°C with O-N gas mixtures of 20 to 40 pct O₂. A mathematical model was developed to describe the overall conversion of the solids. Application of the model to the experimental data indicated that the chemical reaction at the outer boundary of the unreacted sulfide core was the rate-limiting step for the process. The temperature dependence of the kinetic constant corresponded to an activation energy of 40,250 cal per mole. Oxygen starvation in the bed was not limiting in any of the experimental runs, but an increase in the inlet-oxygen mole fraction resulted in a substantial increase in reaction rate.     

Production of calcium metal by aluminothermic reduction of Egyptian limestone ore

Calcium metal was successfully produced from Egyptian limestone ore under vacuum using aluminium as a reducing agent. The aluminothermic reduction process of CaO by aluminium metal was followed up by collected amounts of produced calcium metal. The progress of the reduction process was investigated by the phase analysis of produced reaction residues through X-ray diffraction. The technical parameters affecting the reduction progress were investigated such as reductant stoichiometry, reduction temperature and reduction time. Maximum metallic yield (30%) was obtained by using charge containing 19.4% aluminium powder reduced at 1300°C for 90?min under vacuum (10^?3?mbar). The reduction residues composed mainly of unreacted calcium oxide, tri-calcium aluminate (3CaO·Al₂O₃), mayenite (12CaO·7Al₂O₃) and traces of calcium aluminide (CaAl₄). The probable reaction sequences of the aluminothermic reduction of calcium oxide were also discussed.     

Solubilities of carbon dioxide in sodium silicate melts

Carbonate solubilities in Na₂O-SiO₂ melts were measured over the composition range X_{Na 2}O/ (X_{Na 2}O + X_{SiO 2}) = 0.5 to 1.0 and the temperature range 1100 to 1300 ‡C. The solubility increased with increasing X_Na2_O and decreased with increasing temperature. Carbonate capacities calculated from the experimental results compared favorably with values for sulfide and phosphate capacities obtained from the literature. In addition, an excellent correlation was obtained between carbonate capacity and the activity of sodium oxide. The carbonate capacity, which is an easier parameter to obtain, is a good measure of the basicity of sodium silicate melts. It would appear that carbonate capacity could be an excellent basicity index for iron and steelmaking slags as well as for fluxes used in other high temperature technologies. Formerly with University of Toronto. Formerly with the Department of Metallurgy, University of Tokyo.     

Simulation of the sulfide phase formation in a KhN60VT alloy

The conditions of the existence of sulfide phases in Fe–Ni–S alloys and four-component Fe–50 wt % Ni–0.001 wt % S–R (R is an alloying or impurity element from the TCFE7 database) systems are studied using the Thermo-Calc software package and the TCFE7 database. The modification of nickel superalloys by calcium or magnesium is shown to increase their ductility due to partial desulfurization, the suppression of the formation of harmful sulfide phases, and the uniform formation of strong sulfides in the entire temperature range of metal solidification. The manufacturability of superalloys can decrease at a too high calcium or magnesium content because of the formation of intermetallics with a low melting temperature along grain boundaries.     

Surface Effects in Sulfidation Reactions

Abstract

Pure iron and an Fe–41 wt% Ni alloy are reacted at temperatures in the range 793–1073 K with H₂–H₂S–N₂ atmospheres corresponding to equilibrium P _S2 levels from 6.5 × 10^?5 to 0.65 Pat The kinetics of iron sulfidation are intermediate in form to linear and parabolic rate laws. The instantaneous parabolic rate constant is found to increase with extent of reaction until a constant value is reached. For fixed equilibrium sulfur pressures, the instantaneous rate increases with hydrogen sulfide partial pressure; for fixed hydrogen sulfide partial pressure, the instantaneous rate decreases as the equilibrium sulfur pressure is increased. It is demonstrated that hydrogen sulfide is the reactant species. A Langmuir-Hinshelwood kinetic model based on the slow dissociation of adsorbed hydrogen sulfide accounts satisfactorily for the unusual gas-phase compositional effects, and also for the rate at which the reacting system approaches steady state. Similar effects are found for the Fe–41%Ni alloy, where nickel sulfide whisker formation results from localized catalysis of the hydrogen sulfide dissociation reaction.     

Sulfide capacity of high alumina blast furnace slags

Sulfide capacities of high alumina blast furnace slags were experimentally determined using the gas-slag equilibration technique. Two different slag systems were considered for the current study, namely, CaO-SiO₂-MgO-Al₂O₃ quaternary and CaO-SiO₂-MgO-Al₂O₃-TiO₂ quinary system. The liquid slag was equilibrated with the Ar-CO-CO₂-SO₂ gas mixture. Experiments were conducted in the temperature range of 1773 to 1873 K. The effects of temperature, basicity, and the MgO and TiO₂ contents of slags on sulfide capacity were studied. As expected, sulfide capacity was found to increase with the increase in temperature and basicity. At the higher experimental temperature, titania decreases the sulfide capacity of slag. However, at the lower temperature, there was no significant effect of titania on the sulfide capacity of slag. Sulfide capacity increases with the increase in MgO content of slag if the MgO content is more than 5 pct.     

Study of the reduction of UO2 by magnesium or calcium dissolved in molten chlorides

The reduction of solid UO₂ to uranium by magnesium or calcium dissolved in their molten chlorides has been studied. The rate of reduction per unit area of UO₂ surface, at constant temperature and concentration of reductant in the molten chloride, was found to increase with time to a constant value. The rate of reduction per unit area was observed to be proportional to the concentration of reductant in the molten salt. The small increase observed in the reaction rate over the temperature range 750° to 850°C, suggests that the reduction is controlled by transport of the reductant to the reaction site. Solidified salt, containing UO₂ pellets which had been partially reduced, was sectioned, polished, and examined microscopically. The products of the reduction reaction form concentric layers around the UO₂ pellets. Layers of metallic uranium and oxide containing small amounts of dispersed salt alternated with layers of salt containing small amounts of metallic uranium and oxide. The layers ruptured, presumably because the volume of the products, uranium and oxide, is greater than the volume of the UO₂. Therefore, an impervious layer did not form on the oxide surface to inhibit the reduction reaction.     

Reoxidation on the Surface of Molten Low‐Carbon Aluminum‐Killed Steel

The unwanted formation of oxide on the surface of molten steel is a process that plagues virtually all grades of steel and often results in unacceptable defect distributions in the finished product. The kinetic rates of the reaction remain largely unknown due to the difficulties in experimental measurements at high temperature involving reactive molten melts. This paper presents recent experimental work and analysis of oxide evolution on the surface of Al‐killed steel melts at oxygen partial pressures of Po₂= 1~5×10^?5atm, by using a Confocal Scanning Laser Microscope (CSLM) equipped with a gold image furnace. The effects of gas flow rate (170~300 cm³/min) and temperature (1580~1630°C) were investigated. The oxide phase formed on the melt surface was in all cases Al₂O₃ but not the thermodynamically stable FeAl₂O₄. It was found, under the range of experimental conditions in this study, that the rate controlling mechanism for oxide nucleation and growth was gas phase mass transfer of oxygen to the melt surface. The morphology of the oxide changed gradually from distinctly dendritic at low gas flow rates to aggregates as the flow rate was increased.