Studies on oxychlorination of MoS2 in a fluid bed reactor

Studies on the chlorination kinetics of molybdenum sulfide in the presence of a mixture of oxygen, chlorine, and nitrogen gases have been carried out in a fluidized bed reactor at temperatures of 250 to 350 ° and particle size range of 75 to 200 μm. The reaction rate with respect to surface area of MoS₂ particles as well as the composition of reactant gases has been determined and the specific rate constants evaluated. The oxychlorination of MoS₂ has been determined to be of first order with respect to surface area of particles and the overall reaction is found to be controlled by chemical reaction.     

Kinetics and mechanism of the desulfurization of liquid blast furnace slags by Ar-H2O gas mixtures

The rate of evolution of sulfur-bearing gases from liquid silicate slags at 1400 ‡C when exposed to Ar-H₂O gas mixtures has been studied. The rate was first order with respect both to the concentration of sulfur ions adsorbed on the surface of the slag and to the partial pressure of water vapor when P_{H 2}O was greater than 0.15 atm. When the partial pressure of water vapor was less than 0.15 atm, the rate was second order with respect to the concentration of sulfur ions adsorbed on the surface early in the reaction. At longer times in the latter case, the rate was independent of the partial pressure of water vapor, but first order with respect to the concentration of sulfur ions adsorbed on the surface. It was concluded, based on the kinetics of desulfurization, that the sulfur-bearing species evolved from the surface of the slag was S₂ when the rate was second order with respect to the concentration of adsorbed sulfur ions, but SO₂ was evolved when the rate was first order with respect to the concentration of adsorbed sulfur ions. Under the conditions employed in the experiments, H₂S never evolved from the surface of the slag, although it did form, in some instances, in the gas phase. B. AGRAWAL, formerly a Graduate Student at Massachusetts Institute of Technology     

Chlorination of commercial molybdenite concentrate in a fluidized bed reactor

Studies on recovery of molybdenum from commercial grade molybdenite using the technique of fluidized bed chlorination in the presence of oxygen are presented. Molybdenum recovery above 99 pct at a chlorine utilization efficiency of 84 pct has been achieved for a fluidizing gas flow-rate of 3 L/min of the gases Cl₂, O₂, and N₂ mixed in the proportion of 2∶5∶23, respectively, at 300 °C. The investigations on kinetics showed that the overall oxychlorination reaction is controlled by chemical reaction and is of first order with respect to particle surface area.     

Kinetics of the direct synthesis of molycarbide by reduction-carburization of molybdenite in the presence of lime

Kinetic studies were conducted on the carbon monoxide reduction of molybdenite in the presence of lime. Contrary to the expectation that the MoS₂ (s)+CaO (s)+CO (g) reaction will result in metal formation, molycarbide was found to form and no Mo was detected in the product. This is explained on the basis of thermochemical considerations, which indicate that the Mo₂C formation is more feasible by eight orders of magnitude. The effects of quantity of lime in the charge, CO flow rate, temperature (1123 to 1298 K), and time of reduction have been studied. Kinetic analysis reveals that the results on the MoS₂ (s) conversion to Mo₂C (s) fit into a modified parabolic rate law. Based on the thermochemical calculations and experimental findings, the probable reaction scheme has been identified. Molycarbide appears to form by a three-successive solid-gas reaction path consisting of (1) metal formation by the MoS₂ (s)+CO (g) reaction followed by (2) in-situ carburization of Mo metal by CO (g), and finally (3) the scavenging of the COS (g) by lime, resulting in CaS (s). The latter two reactions drive the overall reaction forward. Further, out of these three consecutive reactions, progress of the overall MoS₂+CaO+CO reaction seems to be governed by the intrinsic kinetics of the first one. Calcium molybdate, which forms as a transitory phase, plays a significant role by modifying the linear kinetics of the MoS₂ (s)+CO (g) to one of parabolic nature.     

Mechanical Properties and Dry Sliding Wear Behaviour of Molybdenum Disulphide Reinforced Zinc–Aluminium Alloy Composites

ZA-27 alloy is a lightest alloy which offers excellent bearing and mechanical properties in automobile and industrial applications. In this study, the MoS₂ particles with 0.5, 1 and 1.5 (wt%) weight percentages were reinforced in ZA-27 alloy to form composites, which were fabricated by using ultrasonic assisted stir casting method. The ZA-27/MoS₂ composite specimens were examined for chemical composition with the aid of XRD technique and EDS. Microstructure analysis of the ZA-27/MoS₂ composites was studied using SEM. Tests were conducted for mechanical properties such as tensile strength and hardness on ZA-27/MoS₂ composites samples as per ASTM standards. Dry sliding wear behavior of the composites was tested at various operating conditions by using pin-on-disc apparatus. Microstructural images of the ZA-27 composites reveal that there is a uniform dispersion of the MoS₂ particles in the base material. From the results it is observed that the mechanical properties increases with ZA-27 reinforced with 0.5 wt% MoS₂ composite and further decreases with increase in the filler content. The enhanced wear resistance is observed in ZA-27 reinforced MoS₂ composites as compared to the unreinforced alloy. The wear rate of the ZA-27 composites decreases with the increase in filler content, further the worn surfaces as examined using SEM reveals the wear mechanism explaining the improved wear resistance of the particulate composites.     

Re-oxidation Kinetics of Flash-Reduced Iron Particles in H2-H2O(g) Atmosphere Relevant to a Novel Flash Ironmaking Process

A novel flash ironmaking process based on hydrogen-containing reduction gases is under development at the University of Utah. The goal of this work was to study the possibility of the re-oxidation of iron particles in a H₂-H₂O gas mixture in the lower part of the flash reactor from the kinetic point of view. The last stage of hydrogen reduction of iron oxide, i.e., the reduction of wustite, is limited by equilibrium. As the reaction mixture cools down, the re-oxidation of iron could take place because of the decreasing equilibrium constant and the high reactivity of the freshly reduced fine iron particles. The effects of temperature and H₂O partial pressure on the re-oxidation rate were examined in the temperature range of 823 K to 973 K (550 °C to 700 °C) and H₂O contents of 40 to 100 pct. The nucleation and growth kinetics model was shown to best describe the re-oxidation kinetics. The partial pressure dependence with respect to water vapor was determined to be of first order, and the activation energy of re-oxidation reaction was 146 kJ/mol. A complete rate equation that adequately represents the experimental data was developed.     

THE IMPORTANCE OF SURFACE PROPERTIES AND PARTICLE SHAPE IN THE COMPACTION OF GRAPHITE AND MoS2

Abstract

Adsorptive studies of the surfaces of graphite and MOS₂have shown that these consist of two distinct types of site. The sites on the basal-plane surface differ from those on the edge surface with respect to their relative affinities for different organic compounds. These findings led to the development of grinding techniques to produce graphite and MoS₂ powders possessing different ratios of basal-plane:edge-surface area.

Grinding graphite and MoS₂ in the presence of low-viscosity, volatile hydrocarbons produced very thin flake-like powders, consisting predominantly of basal-plane surface. These fine flakes showed a high affinity for long-chain n-paraffins and were therefore termed oleophilic solids. Grinding under reduced pressure also produced very fine powders, having, however, a more granular structure exhibiting a far lower ratio of basal-plane: edge-surface area. These were termed polar solids to distinguish them from the solids ground in liquid hydrocarbons.

The cold-forming properties of the various powders have been compared under uniaxial compaction. The conversion of synthetic and natural graphite powders to the oleophilic form resulted in marked improvements in both compact strength and modulus. Synthetic graphite converted to the polar form would not form a compact at cold-forming pressures up to 800 MN/m².

The cohesive properties of the oleophilic graphite powders were improved by heating to 900°C in hydrogen. Electrical-resistivity measurements showed that cold-formed oleophilic graphite compacts exhibited a marked anisotropy. The improved cold-forming properties of the powders are ascribed directly to improved cohesion via basal-plane site interactions, coupled with the facility of the flake powders to take up a preferential orientation during compaction in order fully to utilize the extensive basal-plane sites available for cohesion.

The differences between the oleophilic and polar forms of MoS₂ were less marked. It is believed that interparticle cohesive junctions are more readily formed via edge/edge interactions, and basal-plane junctions do not play as important a role in the cohesion of MoS₂ as in that of graphite.

The corrosion and abrasion of metal surfaces by graphite and MoS₂ have been examined. In all cases the powders converted to the oleophilic form showed reduced abrasive and corrosive characteristics when compared to similar powders converted to the polar form. These improvements are believed to result from the reduction of the possibilities of edge interactions with the metal surfaces.     

Dissolution of zinc ferrite samples in acids

The dissolution of rotating discs of synthetic zinc ferrite — the principal constituent of the ‘Moore Cake’ residue in zinc extraction plants — was studied in mineral acids, particularly in 1–5 N H₂SO₄ at 70–99°C. This dissolution was found to be directly proportional to the surface area, and the order of the zinc ferrite-sulphuric acid reaction with respect to proton activity, [H⁺], to be 0.6. The apparent energy of activation was established as 15 kcal/mole, and the chemical reaction on the solid surface as the rate-controlling step.What appeared to be ‘non-stoichiometric’ or preferential dissolution of zinc (over iron) from zinc ferrite was observed during the initial stages of reaction. This was attributed to the existence of trace amounts (undetectable by X-ray methods) of unreacted zinc oxide grains in the zinc ferrite matrix. This is, to our knowledge, the first time that electron microprobe analysis has been used to identify and analyse these grains. Prolonged sintering at 1200°C for 48 hours eliminated the ZnO phase.Dissolution of zinc ferrite in acid is stoichiometric. A typical dissolution rate is ～ 10^?8 mol cm^?2 sec^?1, which corresponds to almost complete extraction of zinc from ‘Moore Cake’ particles in 2–5 N H₂SO₄ solution at 95°C in 1–2 hours.     

Sulfuric acid oxygen-pressure leaching of Ni3S2 prepared by a wet process

Ni₃S₂ prepared by a wet process was easily leached as nickel sulfate at 383 K, p_o2 1 MPa, and sulfuric acid concentration of 0.1–0.15 mol L⁻¹. The leaching reaction proceeds through the intermediate formation of NiS prior to complete dissolution. A constant leaching rate was observed for most of the duration of the reaction, and this has been attributed to an increase in the specific surface area of the sulfide particles. A thin sulfur layer was formed on the sulfide; the diffusion of oxygen through the sulfur layer was found to be rate-determining.     

Kinetics of sulfation of chalcopyrite with steam and oxygen in the presence of ferric oxide

The kinetics of suifation of chalcopyrite with/without ferric oxide addition has been studied in the fixed bed for the temperature range 673 to 773 K in the absence of external mass transfer effects such as particle size of ore and flow rate of oxidizing gases such as steam and oxygen. The suifation reaction was observed to be topochemical. The activation energy value of 30.5 kJ/mol was found when no catalytic addition was made. The rate of suifation increases with the addition of ferric oxide. The rate constant values obtained without and with 10 pct Fe₂O₃ were 5.5 × 10³ min^?1 and 7.00 × 10³ min^?1, respectively. The activation energy value for the roasting in the presence of the catalyst was 29.2 kJ/mol under these conditions. Examination of the kinetic data indicates that the reaction occurred on the surface of the mineral particles and proceeded through the reactant and product phase boundary. The sulfated products were also characterized by metallography, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and X-ray diffractometry (XRD) studies.     

Sliding Wear Behavior Solid Lubricant Based Ni–Al–Bronze Surface Composite Developed by Friction Stir Processing

In this study, a nickel aluminium bronze (NAB) metal matrix composite reinforced with solid lubricants i.e. graphite and molybdenum disulphide (MoS₂) was prepared by friction stir processing. Friction stir processing (FSP) refined the grain structure as compared to the as-cast NAB. The micrographs of graphite reinforced matrix revealed fine globular α phase with some elongated morphology α phases, whereas MoS₂ reinforced surface composite mainly exhibited fine α phase particles. FSP also resulted in the distribution of solid lubricant particles in the NAB matrix. The hardness of the composites decreased with the addition of the solid lubricants in NAB matrix. SEM–EDS analysis of the reinforced NAB matrix confirmed the presence of solid lubricants. The influence of solid lubricants on the sliding wear behavior of NAB metal matrix was investigated by using the design of experimental approach. The experimental results revealed better wear resistance of the NAB–MoS₂ surface composite as compared to graphite reinforced and FSPed NAB surface. SEM–EDS analysis of worn out surfaces and wear debris were carried out for understanding the wear mechanism.     

Fluidized bed roasting of molybdenite-effect of operating variables

The results of an investigation on the fluidized bed roasting of molybdenite are reported. Molybdenite mixed with quartz was subjected to an oxidizing roast in a 22 mm diam stainless steel batch fluidized bed reactor. Enriched air (with O₂) or diluted air (with N₂) was used as the fluidizing and oxidizing gas. In addition to the MoS₂ content of the solids and the O₂ content of the gas, the effect of temperature and flow rate was also examined. For the range of variables investigated, it was found that the temperature influences the rate of the roasting reaction greatly. The gas flow rate affects the conversion favorably up to a certain fluidizing flow rate. An increase in the O₂ content of the gas and the MoS₂ of the solids results in higher conversion levels. The unreacted core kinetic model was applied to the results; and the energy of activation for the reaction was obtained from the Arrhenius plot as 31,100 cal/gmol of MoS₂. The data obtained should be useful in the design and operation of larger scale roasting reactors.     

Kinetics and Reaction Mechanisms of High-Temperature Flash Oxidation of Molybdenite

The kinetics and reaction mechanism of the flash oxidation of +35/–53 μm molybdenite particles in air, as well as in 25, 50, and 100 pct oxygen higher than 800 K, has been investigated using a stagnant gas reactor and a laminar flow reactor coupled to a fast-response, two-wavelength pyrometer. The changes in the morphology and in the chemical composition of partially reacted particles were also investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and electron microprobe. High-speed photography was also used to characterize the particle combustion phenomena. The effects of oxygen concentration and gas temperature on ignition and peak combustion temperatures were studied. The experimental results indicate that MoS₂ goes through a process of ignition/combustion with the formation of gaseous MoO₃ and SO₂ with no evidence of formation of a molten phase, although the reacting molybdenite particles reach temperatures much higher than their melting temperature. This effect may be a result of the combustion of gaseous sulfur from partial decomposition of molybdenite to Mo₂S₃ under a high gas temperature and 100 pct oxygen. In some cases, the partial fragmentation and distortion of particles also takes place. The transformation can be approximated to the unreacted core model with chemical control and with activation energy of 104.0 ± 4 kJ/mol at the actual temperature of the reacting particles. The reaction was found to be first order with respect to the oxygen concentration. The rate constant calculated at the actual temperatures of the reacting particles shows a good agreement with kinetic data obtained at lower temperatures. The ignition temperature of molybdenite shows an inverse relationship with the gas temperature and oxygen content, with the lowest ignition temperature of 1120 K for 100 pct oxygen. Increasing the oxygen content from 21 to 100 pct increases the particle combustion temperature from 1600 K to more than 2600 K. A high oxygen content also resulted in a change of the reaction mechanism from relatively constant combustion temperatures in air to much faster transient combustion pulses in pure oxygen.     

Reduction of molybdenite with carbon in the presence of lime

The thermodynamics of the MoS₂-C-CaO system has been studied in order to understand the carbothermic reduction of molybdenite in the presence of CaO. Kinetic studies were also conducted with mixtures of MoS₂+C+CaO in the temperature range of 900 °C 1200 °C. The reduction of MoS₂ with carbon in the presence of lime proceeds through the direct oxidation of MoS₂ by CaO to form intermediate molybdenum oxidized species, MoO₂ and CaMoO₄, which subsequently undergo reduction by CO to yield mixtures of Mo, Mo₂C, and CaS. Complete conversion of MoS₂ can be obtained at 1200 °C in less than 20 minutes for molar concentrations of MoS₂:C:CaO=1:2:2. The kinetic model ln (1−X)=kt was used to determine the rate constants. The activation energy found for the temperature range studied was 218.8 kJ/mol.     

Intrinsic Kinetics of Chlorination of WO<Subscript>3</Subscript> Particles With Cl<Subscript>2</Subscript> Gas Between 973 K and 1223 K (700 °C and 950 °C)

Chlorination is one of the methods applied in extractive metallurgy for the treatment of minerals to obtain valuable metals, such as titanium and zirconium. The possibility of applying chlorination metallurgy to other metals such as tungsten was the major aim of this study. The kinetics of the chlorination of tungsten oxide (WO₃) particles has been investigated by thermogravimetry between 973 K and 1223 K (700 °C and 950 °C) and for partial pressures of chlorine ranging from 15 to 70 kPa. The starting temperature for the reaction of WO₃ with chlorine is determined to be about 920 K (647 °C). The influence of chlorine diffusion through the bulk gas phase and through the particle interstices in the overall rate was analyzed. In the absence of these two mass-transfer steps, a reaction order of 0.5 with respect to chlorine partial pressure, and an activation energy of 183 kJ/mol were determined. For tungsten oxide particles of less than 50-μm size, a complete rate expression has been obtained.     

Leaching kinetics of synthetic CaWO4 in HCl solutions containing H3PO4 as chelating agent

In this study, the dissolution kinetics of synthetically prepared CaWO₄ in HCl solutions containing H₃PO₄ was studied. The effects of process parameters such as stirring speed, temperature and acid concentrations on the dissolution rate of CaWO₄ were investigated. The reaction rate was found to be of 1 / 3 and 2 / 3 order with respect to HCl and H₃PO₄ concentrations, respectively, and the activation energy for the dissolution reaction to be 60 kJ mol^− 1. The rate equation for the dissolution reaction was derived using the Avrami equation and the rate determining step was the chemical reaction on the surface of solid particles.     

The effect of chloride ion on the ferric chloride leaching of galena concentrate

Previous investigations of the ferric chloride brine leaching of galena concentrate have shown that additions of chloride ion result in accelerated dissolution rates. The current study has provided the necessary information to extend and modify these previous results by incorporating the important effect of chloride ion on the dissolution kinetics. As part of this study the solubility of lead chloride in ferric chloride-brine solutions has been determined and results indicate that additions of either FeCl₃ or NaCl increase the PbCl₂ solubility. This is attributed to the effect of complexing on the level of free chloride ion. In addition, the dissolution kinetics of elemental lead and lead chloride were also determined and compared with the kinetics of PbS dissolution. It is significant that the rate of dissolution of PbCl₂ decreases as the concentration of Cl⁻ is decreased and as the concentration of dissolved lead increases. These results along with SEM examination of partially reacted Pb shot show that solid PbCl₂ forms on the surface long before the bulk solution is saturated with lead. The PbCl₂ is proposed to form by a direct electrochemical reaction between Cl⁻ and PbS prior to the formation of dissolved lead. The reaction was determined to be first order with respect to Cl⁻ and closely obeys the following kinetic model based on a rate limiting charge transfer reaction at the surface: The model is in excellent agreement with experimental results up to about 95 pct reaction as long as the solubility of PbCl₂ is greater than about 0.051 M. Where these conditions are not met, deviation from the surface reaction model occurs due to the extremely slow dissolution rate of PbCl₂. Therefore the effect of Cl⁻ on the brine leaching of PbS is attributed to two factors, the direct reaction of Cl⁻ with the pbS surface and the effect of Cl⁻ on the dissolution rate of PbCl₂. The overall dissolution process is viewed as occurring in three stages; in the first stage the reaction is controlled by the surface reaction and described by the model above, then as solid PbCl₂ is produced the diffusion of Cl⁻ would be equal in importance with the surface reaction,i.e, the second stage. As the reaction proceeds further, a shift in the rate-limiting step from surface reaction to product layer or pore diffusion occurs, the third stage. Thus the rate-determining step would no longer be just the surface reaction as observed experimentally at longer reaction times. The practical implications of these results for the processing of a complex sulfide concentrate using sequential, selective, or total leach approaches are also discussed.     

The effects of molybdenum additions to nickel-chromium alloys on their sulfidation properties

The effect of molybdenum additions to γ-phase Ni-Cr on the reaction of the alloys with H₂/H₂S atmospheres at 700°C has been investigated. The effect is to reduce the rate of internal sulfidation of the alloys, diminish greatly the extent of liquid sulfide formation and slow the growth rate of external solid sulfide scale. The first two effects are thermodynamic in origin whilst the last effect is due to favorable doping of Cr₂S₃ and to the formation within a Cr₃S₄ scale of MoS₂ precipitates which partially block outward metal diffusion.     

The effects of molybdenum additions to nickel-chromium alloys on their sulfidation properties

The effect of molybdenum additions to γ-phase Ni-Cr on the reaction of the alloys with H₂/H₂S atmospheres at 700°C has been investigated. The effect is to reduce the rate of internal sulfidation of the alloys, diminish greatly the extent of liquid sulfide formation and slow the growth rate of external solid sulfide scale. The first two effects are thermodynamic in origin whilst the last effect is due to favorable doping of Cr₂S₃ and to the formation within a Cr₃S₄ scale of MoS₂ precipitates which partially block outward metal diffusion.