Fundamental studies on the mechanisms and kinetics of reduction of dense synthetic nickel oxide have been carried out in H2-N2 and H2-H2O mixtures. The influences of temperature, hydrogen partial pressure, and hydrogen-steam ratio on the reduction process were systematically investigated. The kinetics of the reduction process were followed metallographically by measuring the advance of the nickel product layer. The microstructures of the reduction products and their changes during heating were characterized using a high resolution scanning electron microscopy. In H2-N2 mixtures and H2-H2O mixtures with low steam content, it was found that the initial reduction rates were first order with respect to hydrogen partial pressure. In both sets of mixtures, it was found that the progress of NiO reduction was not a monotonic function of temperature. A minimum rate of advancement of NiO reduction was observed in the temperature range 700oC – 800oC depending on the hydrogen partial pressures and reduction time. A number of distinctly different nickel product microstructures, originating at the Ni-NiO interface were observed under various reduction conditions, namely coarse fibrous nickel with fissures, fine porous nickel-planar interface, large porous nickel-irregular interface and dense nickel product layer. The microstructures of reduction product were found to change with temperature and time. It was found that heating the coarse fibrous nickel structure resulted in a re-crystallization, grain growth and densification of nickel product. When the heat treatments were carried out on the porous nickel structures, the number of pores decreases with increasing temperature and time, which was accompanied by the increase in the pore sizes. The microstructures and kinetics of the reduction of nickel oxide were found to be a function of temperature, gas composition and reaction time. The study provides strong evidence for a link between the reduction kinetics and the changes in the reduction product microstructures. Mechanisms and kinetics of the reduction of nickel oxide have been discussed by considering reduction conditions, information on the structures and elementary processes involving in the reduction process.

The cleanliness of steel impacts the success of steel production and the physical and chemical properties of the final product. Improving the cleanliness of steel, therefore, becomes a necessity in the present time, with an ever-increasing demand for high-quality steel products. The cleanliness can be improved by removing the harmful inclusions through flotation or by modifying their composition and morphology to less detrimental forms. The present study focuses on better understanding the second approach, a specific modification method commonly known as the calcium (Ca) treatment in advanced high strength steel (AHSS) production. The chemical and morphological evolutions of Al2O3 inclusions under experimental and industrial conditions, as well as the formation of CaS and MnS inclusions, were studied in this work. Six laboratory experiments with different combinations of calcium and sulfur contents of liquid steel were conducted. Samples were taken at different time durations after calcium addition. The inclusions on the sample cross-sections were analyzed using an automated SEM-EDS system to obtain their chemical, size distribution, population, and morphological information. Similar steps were taken in the analysis of industrial samples. The findings obtained based on the automated SEM-EDS analyses were further supported and validated against other analysis results such as manual SEM analysis, thermodynamics, and kinetics calculations. The modification mechanism for Al2O3 inclusions was established in the first part of the study. After adding 10 ppm, 20 ppm, and 35 ppm Ca, small-sized calcium aluminates CAx (C and A denote CaO and Al2O3, respectively) inclusions become the primary oxygen bearer instead of Al2O3 inclusions. The modification extent of the CAx inclusions depends on the Ca content. CaS inclusions also form at the early stage of calcium treatment. In the later stage, CaS inclusions act as the Ca source to modify the remaining Al2O3 inclusions to CAx inclusions and simultaneously modify the existing CAx inclusions until equilibrium is reached. CaO inclusions only form in steel containing 20 ppm S and 35 ppm Ca; the primary oxygen bearer will change from Al2O3 to CaO, followed by a transformation from CaO to CaS. In other S and Ca contents, CaO inclusions do not form. This finding clarified that modification of Al2O3 inclusions is mainly driven by dissolved Ca and CaS inclusions, with CaO showing a minor direct impact. Moreover, the experimental studies showed that total area fractions of liquid and semi-liquid inclusions, which are inclusions that are partially liquid and partially solid, are correlated with the thermodynamic stability of CaS inclusions. The fraction of liquid inclusions decreases after the area fraction of CaS inclusions drastically increases when steel chemistries allow stable CaS inclusions to precipitate. The correlation between the steel chemistries and inclusions was improved by iv incorporating more data from industrial heats. Three modification indexes were proposed to estimate the control of CAx, CaS, and MnS inclusions. The fraction of CAx inclusions with more than 50 pct liquid, and the area fraction of CaS and MnS inclusions in tundish samples were correlated with the Ca, Al, Mn, and S contents of liquid steel. Later, these modification indexes were incorporated to evaluate the effectiveness of calcium treatment quantitatively. This makes the present study the first to discuss the correlations between Ca, Al, Mn, and S contents and the number of inclusions in the open literature. The correlations were validated against industrial data, they may be used in industry to determine the optimum Ca content for inclusion control and modification. Based on the experimental and industrial data, the coarsening of CaS inclusions was initially governed by mass transport, then shifted to collision-related mechanisms. When agitation is absent, Brownian motion shows the most significant impact on the growth of CaS inclusions, while turbulent flow is the critical cause of collision and coagulation when the melt is stirred, such as in industrial conditions. It has been found that CAx inclusion growth mainly occurs in the early stage after Ca addition. The potential reason is that the lack of attraction prevents coagulation after CAx inclusions are modified to liquid and semi-liquid. / Thesis / Doctor of Philosophy (PhD)

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2006. / An understanding of the behaviour of chromium in mattes equilibrated with chrome-saturated slags is essential for the prediction and control of chromium deportment in these melts. The main ore reserves of South Africa’s platinum group metals (PGM) are associated with the Merensky and UG2 reefs of the Bushveld Complex. The gradual depletion of the pyroxenitic Merensky reef over the years has necessitated the PGM industry to exploit the underlying chromiterich UG2 reef. The problem with UG2 reef is that it contains significant amounts of chromium, this being typically 5% (reported as Cr2O3) against <1% for the Merensky reef (in concentrates). Chromium has a number of deleterious effects on base-metal smelting processes. Under given conditions, it forms chromite spinels, which can accumulate and, over a period of time, form undesirable build-ups resulting in reduced furnace operational volume. The chromite spinels also tend to increase slag viscosity, thereby impacting negatively on the slag/matte separation, which leads to matte entrainment in slag. Moreover, high viscosities lead to problems with tapping of the furnace melts. Finally it can be said that chromium that deports to the matte during smelting will normally tend to precipitate as spinels during subsequent converting, and lead to the formation of very stable and unwanted slag foams in the converter. In literature, there exists very little published work on the behaviour of chromium in sulphur-saturated systems such as matte-smelting furnaces. The few publications that exist in this area do not cover the effect of controlling all the pertinent variables simultaneously on the behaviour of chromium in mattes in equilibrium with chromium-containing slags. Hence the main focus of the oxygen fugacities on the mineralogical and deportment behaviour of chromium in a matte-slag system. All the three variables (temperature, oxygen fugacity (pO2), and sulphur fugacity (pS2)) were investigated at three levels. Temperatures studied were 1300°C, 1400°C, and 1500°C. Oxygen and sulphur fugacities were established by controlling the mix-ratios of purified carbon dioxide, carbon monoxide, sulphur dioxide, and argon. Oxygen fugacities were maintained at 10-10atm, 10-8atm, and 10-6atm; whereas sulphur fugacities were maintained at 10-6atm, 10-4atm, and 10- 2atm. In order to investigate the effect of each of these three variables (temperature, pS2, and pO2) at different levels of the other variables, a completely randomised 33 full factorial experimental design was adopted. The study revealed that chromium is generally present in matte as both dissolved CrS and as precipitated oxidic and sulphidic chromium spinel phases. It was shown that as the conditions become more oxidising (pO2 = 10-6atm), CrS (which is soluble in matte) becomes a predominant phase, and as the conditions become more reducing (pO2 = 10-10atm), the sulphospinel, daubreelite (FeCr2S4), becomes a more predominant phase. Oxidic chromium spinels were found to be present in matte under the more oxidising conditions (pO2 of 10-6atm) of this investigation. The presence of the above-mentioned phases was confirmed using X-ray diffraction. Subject to the experimental conditions employed in this research, chromium was found to partition the least to matte under the conditions of; low temperature, high pO2, and low pS2. Conversely, chromium was found to partition the most to matte under the conditions of; high temperature, low pO2, and high pS2.

Copper losses to slag are generally between 0.7-2.3% during the copper matte smelting stage. In this study, the aim was to reduce these losses in the slag phase. For this purpose, usage of some additives (especially calcined colemanite labeled as CC, boric oxide-B2O3 and calcium oxide-CaO as well) as flux material was investigated. The flash furnace matte-slag (FFM-FFS) obtained from Eti Copper Inc. and a master matte-slag (MM-MS) produced synthetically were used as starting materials. Additives were tested in various amounts under two different atmospheres (N2 and low Po2 obtained by mixture of CO2/CO gases). Temperature and duration were also used as experimental variables. Experimental results have indicated that 2 hours was sufficient to obtain a low copper content in slag about 0.3% and 0.4% for FFS and MS, respectively. It was also seen that the copper content in slag decreased with increasing CC addition at all oxygen partial pressures and at all temperatures. Furthermore, the addition of all additives up to 4% had great influence in lowering the copper content in the final slags (~0.3%Cu). From FactSage calculations, it could be concluded that the colemanite addition decreased the liquidus temperature which led to early melting of slag and allowed enough duration for settling of matte particles within the slag without substantial changing its viscosity, which resulted in less mechanical copper losses to the slag. By using colemanite in copper production, it was possible that a new application area for boron compounds which are produced in Turkey could be created.

Main purpose of this study was to investigate the production of high carbon ferromanganese by smelting a manganese ore located in Erzincan. Time, charge basicity, fluorspar addition and coke to ore weight ratio were used as experimental variables. After characterization of the ore, a mixture of 100 grams of uncalcined manganese ore and carefully weighed amounts of coke powder, lime, fluorspar and scrap iron were put into a conical graphite crucible. The graphite crucible with its contents was covered with a graphite lid and placed in an electronically controlled muffle furnace. The furnace was heated to 1600 oC and held at this temperature for a predetermined time and cooled to room temperature. The crucible was taken out of the furnace and the metal and slag phases formed were removed from the crucible by breaking the crucible and carefully weighed. Finally, the metal and slag phase were subjected to chemical analysis. The experimental results show that Erzincan manganese ore can be used in the production of high carbon ferromanganese.

Thesis (PhD.(Metallurgy)--University of Pretoria, 2008. / Includes bibliographical references.

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A large number of Li-ion batteries used today will reach their End-Of-Life (EOL) in a few years. After their EOL, the recovery of their precious elements is required. By applying physical separation, a fraction with fine particle size is left behind which is known as Black Mass (BM). BM is rich in LIB precious materials, including Li metal oxides and graphite. In this study, pyrometallurgical recycling of BM is investigated. In the first step, the BM high-temperature transformations are being studied, focusing on reducing Li metal oxides, Li evaporation, and F removal. In the second step, Fe and Cu oxides are added to the BM to investigate how the graphite remaining in the BM can be used as a reducing agent and form alloys with Co and Ni. The use of mechanical activation as a mean to improve the kinetics of the reactions and the efficiency of the reduction reaction was also studied. To model the experiments in this study, thermodynamic softwares (FactSage and HSC) were also employed.

Thesis (MScEng) –Stellenbosch University, 2010 / ENGLISH ABSTRACT: More mining houses are turning their attention to the processing of ore mined from the UG2 reef. This is mainly due to the depletion of the Merensky reef or the low availability of ore from the Platreef. With the higher UG2 ore concentration in the process feed, companies experience more problems with the processing of the ore due to its high chrome content. Although there are various possible solutions to the problems experienced in the processing of UG2 ore, very little of thise solutions are actually implemented in the industry. When smelting chrome-bearing ore, the chrome can go to any of three phases: matte phase, spinel phase or glass phase. If it reports to the furnace matte phase it can lead to problems in the down stream processing. When chrome forms part of the spinel phase it forms a solid, refractory-like material which, to an extent, is an unwanted material inside the furnace. Lastly the chrome can report to the glass phase (liquid phase) which is the more desireable phase to which chrome should report to since it will have little or no impact on downstream processing. There were four main objectives for this research project namely to conduct a literature review to understand the problem of chrome in the smelting process, to do an experimental simulation of a matte and slag phase in one crucible, to interpret the experimental results and compare the experimental results to thermodynamic predictions obtained using FactSageTM. From literature the following aspects were found to be important: Silica structures Research has shown that silica incorporates different metal ions into its structures. When the metal ions are incorporated into the silica structures they are seen as part of the glass phase. By incorporating cations such as chromium into the silica structure the structure is able to maintain its neutral electrical charge. Basicity of slag Oxides once melted will either donate or accept oxide ions. The group of oxides that are in excess will determine whether that specific slag is acidic or non acidic. This is important to our study since the basicity can determine the stability of the spinel phase. Partial pressures The partial pressure of the system is important since it forms part of the equilibrium constant calculations - meaning that partial pressures determine the stability of certain species. Currently different arguments exist as to how the sulphur pressure inside a molten bath is maintained. What is important, however, is that for the system in this research project a log oxygen partial pressure of -8 and a log sulphur partial pressure of -3.5 was chosen. These values were based on previous research done on PGM and copper smelting processes. Chrome deportment Previous research on chrome deportment shows a relationship between temperature and the amount of chromium dissolving into the glass phase as well as an increased chromium(II)oxide solubility (when compared to chromium(III)oxide) in a silicate melt. Regarding slag chemistry and chrome deportment very little work has been published for the system found in the PGM industry but it is mentioned that by adjusting certain slag additives (alumina, lime and silica) the chrome deportment can be manipulated. The scope of this project was to investigate the effect of chromium, lime and silica on chrome deportment. More specifically, the effect of lime. The reason being that lime was originally added as 10%wt of the feed stream in the processing of Merensky ore to act as a fluxing agent. With the new furnace design and higher power densities the slag are maintained at 200oC to 350oC higher than when Merensky ore was smelted. With these higher slag temperatures fluxing agents will play a smaller role meaning that lime additions become less important. Literature studies also showed that additional lime in a slag system can stabilize the spinel structure (which is an unwanted phase). Removing lime would be advantageous out of a chrome presepective as well as an economic point of view. The research was conducted in three sections namely the determination of the time required for this particular system to reach equilibrium, the investigation of different additives on chrome deportment using a controlled atmosphere and synthetic slags and, lastly, a comparison of the experimental results obtained to thermodynamic predictions. For the equilibration studies reaction time periods of 4,7,11 and 16 hours were used. From the results it was found that the alumina crucible dissolved into the glass phase continually. This indicated that equilibrium was not reached. However, a reaction time, rather than an equilibrium time, was chosen where the species and phases had enough time to react. This was based on literature, on observations of species diffusing between the matte and slag phase as well as on two-point analysis (diffusion gradients). A reaction time of nine hours was chosen With the reaction time fixed, the effect of different slag additives on chrome deportment was investigated. For the addition of chrome it was seen that an 1.5%wt increase in the starting material increased the chrome content of the slag phase by 0.025%wt. For the same increase in chromium in the starting materials the chrome content of the spinel phase increased by 2.1% indicating that chromium has a tendency to report to the spinel phase. The increase in chromium had a minor effect on the chrome content of the matte phase, however, since the chrome content only increased from 0.025%wt to 0.028%wt. Plotting the results showed that increasing the lime in the starting material decreases the chromium content in the glass phase. For the 39%wt silica system the chromium content in the glass phase decreased from 0.75%wt to 0.46%wt for an increase in the lime content from 1.7%wt to 7.3%wt. The same trend was seen for the 33%wt silica system. For the spinel phase an increase of 1.27%wt was seen when the lime content of the starting materials was increased from 1.7%wt to 10.1%wt for a 33%wt silica system. The same increase in lime increased the chrome content of the matte phase from 0.03%wt to 0.06%wt for a 33%wt silica system. Silica also proved to affect chrome deportment. Increasing the silica content of the starting materials from 25%wt to 39%wt increased the amount of chromium in the glass phase from 0%wt to 0.46%wt for a 10%wt lime system. The same effect is seen for a lower lime content except that more chromium were incorporated into the silica structure. A silica increase from 32.4%wt to 39%wt resulted in a chromium decrease from 5.2%wt to 0% in the spinel phase for a 10%wt lime system. The same trend was seen for the 1.5%wt lime system. An increase in the silica levels lead to an increase in the chrome level of the matte phase. When silica is increased from 32%wt to 39%wt the chrome content of the matte phase increased from 0.06%wt to 0.07%wt. The last part of the research project entailed the comparison of the trends observed with the experimental results to trends obtained from thermodynamic predictions. FactSageTM is a program that uses model equations to predict the Gibbs free energies for different phases. The program is therefore also able to predict the amount of different phases present at equilibrium. This is called thermodynamic ”optimization”. In section 6 trends that were observed from FactSageTM results are compared to the trends found in the experimental results. It is important to note that it is only trends that are evaluated and not actual values since FactSageTM calculations are for a system that is at equilibrium and (as explained above) this system is not at complete equilibrium. The comparison however was good. Several trends found in the experimental results were confirmed by the results from FactSageTM. These included the relationships of chromium fed versus chromium spinel, lime fed versus chromium spinel, lime fed versus chromium in glass, silica fed versus chromium in glass, silica fed versus chromium in spinel and silica fed versus chromium in matte. To conclude, slag additions can be used to manipulate chrome deportment to an extent. Secondly, FactSageTM can be used for thermodynamic predictions but a proper understanding as well as some form of validation of the specific system investigated is still needed. Due to time constraints and the difficulty of experimentally simulating this multi-phase system in the lab, only a few parameters were investigated. In order to obtain a more complete understanding of the system the effect of partial pressures and temperature should also be investigated.