Thesis (PhD (Process Engineering))--University of Stellenbosch, 2007. / Nickel-Copper sulphide ores are the most important Platinum Group Metal bearing ores. The South African deposits are exceptionally rich in the platinum group metals (PGMs) and production of the PGMs is the primary purpose of treating these ores. The methods used in the recovery of the PGMs from the nickel-copper ores generally consists of ore concentration by physical techniques, pyrometallurgical concentration and hydrometallurgical extraction of the base metals followed by the PGMs. Pyrometallurgical concentration produces Ni-Cu matte, which is treated by hydrometallurgical processes to recover the nickel, copper, cobalt and the precious metals. In this study, the leaching behaviour of a Ni–Cu matte in CuSO4–H2SO4 solution during the repulping (pre-leach) stage at Impala Platinum Refineries was studied. The repulping stage is basically a non–oxidative atmospheric leach stage, in which nickel, iron and cobalt are partially dissolved, while the copper is precipitated. To understand the nature of the leaching process during this stage of the base metal refining operation, the effects of variations in the key process variables such as temperature, stirring rate, particle size, pulp density, residence time, initial copper and acid concentrations were investigated. The pre-leached matte was then pressure leached to ascertain the effect of process conditions in the pre-leach stage on the subsequent pressure leach stage. It was found that the leaching mechanism entails dissolution of metal alloys out of sulphide minerals with transformation of Ni3S2 to NiS. Aqueous copper precipitates as metallic copper and as chalcocite. The matte is leached by both acid and the cementation process, especially in the early stage when the Cu2+ ions are present. Galvanic interaction of the sulphide minerals and/or the Ni alloy also enhances the leaching process. The leaching kinetics of Ni was characterized with the shrinking core model and was found to be controlled by diffusion through surface layer. An activation energy of 31 kJ/mol was obtained, which also suggested a diffusion controlled leaching reaction. Atmospheric leaching tests indicated that Ni extraction increased slightly in the temperature range 50 – 60 oC, however no significant increase was observed from 60 to 80 oC, probably because the leaching process was found to be diffusion controlled. The slight increase in nickel dissolution at higher temperatures (>60 oC) may be attributed to the transformation of Ni3S2 to NiS, which is easier to leach. Co extraction appeared to be insensitive to temperature changes, while Fe extraction was low at 50 oC but increased significantly at 60 – 80 oC. The Ni extraction increased gradually with increase in the stirring rate from 145 to 400 rpm while Co and Fe extractions were insensitive at 145 and 205 rpm, but increased substantially at 400 rpm. This was probably due to increased mass transfer rate and transformation of Ni3S2 to NiS. With pulp density, Ni and Co extractions appeared to be insensitive to changes in the pulp density as only a slight increase in extractions was observed when the density was reduced from 1.7 kg/L to 1.6 kg/L. Similar iron extractions were achieved at 1.7 and 1.75 kg/L but increased significantly at 1.6 kg/L. It was found that Ni and Co extractions were not significantly affected by changes in the particle size, probably because metal alloys were liberated and hence exposed to the leaching solution. Iron extraction could not be determined accurately because of iron precipitation at pH above 3. Generally the leaching of metals did not depend on the initial copper concentration in the investigated range of 25 - 48 g/L Cu. It was also observed that initial acid concentration did not have an effect on Ni extraction, probably due to the fact that most of the nickel was leached by the process of cementation. However, Co and Fe extractions increased when the acid increased from 90 g/L to 110 g/L, but no further increase was noted at 125 g/L. A residence time of 5 hours was found to be adequate as there was no significant increase in metal extractions when the residence time was increased beyond 5 hours. As much as 20% Ni, 40% Co and 80% Fe can be extracted from the Ni-Cu matte during the repulping stage of the leaching process studied, provided the investigated conditions prevail. Generally the rate of Cu cementation increased with increasing temperature and pulp density, but decreased with an increase in particle size, acid and copper concentrations. The rate of stirring did not affected Cu cementation. It was found that aqueous copper precipitated from the solution within 90 minutes when the temperature was raised to 80 oC. However, under the present pre-leach temperature of about 60 oC complete Cu cementation can only be achieved after about 5 hours. The cementation reaction was found to follow a mixed control mechanism, with two distinct activation energies namely 18.2 kJ/mol at 70 – 80 oC and 74.6 kJ/mol at 50 – 70 oC. This suggested that the rate of cementation reaction is probably controlled by a boundary layer diffusion mechanism at higher temperatures. At low temperatures the rate is probably controlled by a surface reaction mechanism. The pressure leaching experiments, which were aimed at investigating the response of pre-leached matte to the subsequent pressure leaching process, showed that Ni extractions were similar for the investigated pre-leach temperature of 50 – 80 oC and stirring rate of 145 – 400 rpm. For the pre-leach stage conditions of 60 – 80 oC and 205 – 400 rpm Ni3S2 was transformed into NiS, which is easier to leach in the pressure leaching stage. However, because of the aggressive conditions prevailing in the pressure leaching stage, all the nickel minerals were leached at about the same rate. In the case of pulp density, Ni extraction was comparable for all the investigated pulp densities (1.6 – 1.75 kg/L). This was probably due to the fact that Ni3S2 transformed to NiS in the pre-leach stage. It was found that Ni extraction increased with increasing residence time for the investigated time of 1 hour to 9 hours, probably due to the changes in the mineral phases of the matte as indicated above. The copper minerals (Cu2S and Cu1.96S) transformed into Cu2S and Cu1.8S with aqueous copper being precipitated, and were not leached under the applied conditions. All the cobalt and iron dissolved in the pressure leaching stage. A semi-empirical kinetic model was developed for the pre-leaching stage. A comparison of the model predictions and the experimental data for the dissolved species during the batch leaching process showed that the model can satisfactorily fit the trends in the leaching of the metals.

The innovation of crucible steel, a high-carbon, homogeneous, slag-free steel, is regarded as a milestone in the history of the development of ferrous metallurgy. Associated in popular literature with the making of swords, particularly in the Early Islamic period, crucible steel, also known as wootz, possesses exceptional properties of hardness and strength. While much is now understood about its metallurgical composition and structure, little is known of its origins and spread. Few archaeological sites have been uncovered and to date pre-industrial production of this alloy is only known from Central Asia and South Asia. Previous studies have largely focused on individual sites in isolation from wider regional patterns of ferrous metallurgy. As a refining process of iron, it is argued here that crucible steel has a symbiotic relationship with the smelting technologies that produced the raw material for refining. This thesis explores the value of assessing crucible steel production within its wider landscape, cultural and technological context by presenting the evidence from Northern Telangana, India. Historical sources and recent archaeological field surveys have shown that Telangana has a rich metallurgical past, including the manufacture of crucible steel. Despite this, little archaeological work has been conducted in the region to elucidate the nature, scale and diversity of the metallurgical technologies that underpinned its production. Following a major reconnaissance survey in 2010 by the Pioneering Metallurgy Project, the present study tackled the assessment of the large body of field data and the recording of the technological waste assemblage collected. By combining detailed morphological analyses of the collected materials and contextual information recorded during field survey, a better understanding of the techno-cultural role of crucible steel was gained. Technological variations were identified across the survey area and the inter-relationship between iron smelting and crucible steel was assessed. The study reveals that crucible steel was embedded within a long-established local and regional tradition of iron smelting and concludes that it represented the intensification of a pre-existing iron processing industry. The evidence points to a widespread crucible steel production industry with varying degrees of site specialisation, indicating that it was perhaps more common than the few isolated sites commonly referred to in the literature suggests. The comparison of the material evidence with other production sites in Central and South Asia also revealed close parallels to the latter suggesting that they belonged to the same regional manufacturing tradition.

This thesis studied the iron smelting technology of Southwest China. It introduces the natural and human environment of Southwest China and gives a systematic review on the important archaeological sites of Southwest China. A total of 75 iron smelting related sites of Southwest China were surveyed. Five of these sites were excavated and studied in detail including the furnace structures, smelting related materials and their smelting process. A statistical study of over 5,100 iron objects (and bronze and iron bi-metallic objects) in published excavation reports was carried out to understand the pattern and statistical distribution of iron objects excavated in Southwest China. This was followed by metallographic analysis of 66 samples taken from 42 iron objects and slags (mostly from the Lijiaba site, and from the Qiaogoutou site). The slag samples (from the Xuxiebian site) helped to identify the bowl-shaped furnaces, that discovered at the iron smelting sites of Southwest China, as refining furnaces. The results of the metallographic studies helped to characterise the range of technologies that developed in Southwest China primarily during the Han dynasty (202BC-220AD). According to these studies, some issues such as the origin and development of technology, the labourers’ identities, the origin of the blacksmiths, and the management and policy of iron production in Southwest China are discussed.

The copper smelting industry has under gone extreme change over the past three decades. These changes have reordered dramatically the demand for inputs and the way in which those inputs have been utilized. The stimulus for change has come from multiple sources, and chief among these stimuli has been the mandate to sharply curtail the atmospheric release of sulfur dioxide. Even though the total emissions were lower than those from steam generation of electricity and from the refinery and petro-chemical industry, the perceived local and regional impact of sulfur dioxide forced extreme changes in the utilization of fundamental inputs of capital, labor, energy and materials.This study attempts to analyze these input use changes by modeling the industry as a translog cost function and by generating a number of associated elasticities. In addition to the four basic inputs, the model includes as control variables output, and other variables that represent pollution abatement and technical change.The challenge of estimating a large model on a limited number of observations has delivered information that is more limited in scope than was originally desired. The proxy for technical change did not produce significant parameters and the pollution abatement proxy is limited in its participation in the results. The range of elasticities computed reveal a picture of an industry characterized by inelasticity, in general, labor and energy being part of the exceptions. The industry is found to be sensitive to output level in its degree of elasticity among inputs.The translog model is found to be an effective tool for industry analysis. The promise of detailed analytical information may be even greater at the firm level where data are more accurate and the number of observations far greater.

The oxidation and ignition behaviour of the four major sulfide minerals present in the nickel concentrates smelted at the Kalgoorlie Nickel Smelter (KNS) has been established. These minerals are pyrite (FeS2), pyrrhotite (Fe1-nS, where n = 0 to 0.125), pentlandite ((FeNi)9S8) and violarite ( Ni2FeS4 ).The characteristic behaviour of these sulfides has been examined using Thermogravimetry-Differential Thermal Analysis (TG-DTA) under normal oxidation conditions ( l0ºC/min, air atmosphere). By increasing the heating rate to 40ºC/min and using an oxygen atmosphere, the tendency of the sulfides to ignite was established. Ignition was characterised by a highly exothermic reaction which occurred in association with a rapid mass loss over a short time span. Significant overheating of the samples beyond the temperature of the surroundings was observed. Pyrite, pyrrhotite and violarite all exhibited ignition behaviour while pentlandite did not.Using Isothermal Thermogravimetry (TG) the sulfides were subjected to shock heating conditions (heating rate = 3000-5000ºC/min, oxygen atmosphere) analogous to those which exist in an industrial flash smelter. The order of reactivity of the sulfides agreed with that observed during TG-DTA ignition trials. Even under these more intensely oxidising conditions pentlandite did not ignite. The effect of particle size on the ignition temperature was determined, larger particles igniting at a higher temperature. The magnitude of this effect varied according to the characteristics of the minerals.Products collected during Isothermal TG were examined by optical microscopy, Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA). Using these techniques it was possible to establish the morphology of the products and hence, to propose mechanisms for the reaction of the four sulfide minerals under ignition conditions.In order to ++ / simulate the thermal environment which exists in the KNS, a pilot scale model of the reaction shaft was used. Nickel sulfide concentrates of varying mineralogy and particle size distribution were smelted under various conditions. The effect of larger particle size and increasing oxygen partial pressure on the reactivity of these concentrates was established.The products were quenched at the base of the shaft and collected for examination by optical microscopy, SEM and EPMA. Products ranged from unreacted to completely oxidised particles. The morphology and composition of these species were identified. Approximately 30 particles in each of 26 samples were examined with a view to establishing the frequency of occurrence of the particular product types in concentrates of differing mineralogy and particle size. This allowed proposals to be made regarding the fate of the individual sulfide minerals during ignition in the pilot scale flash reactor.