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By M. Reuter, C. Schuffeneger, T. Seppälä, L. Holappa

Reaction kinetics of solid-state reduction of chromite pellets (laboratory made and industrial) and lumpy ore was investigated by the thermogravimetric analysis (TGA) technique. Carbon monoxide was used as the reducing agent, with the presence of graphite in proximity to chromite. The experiments were carried out at both rising temperature and constant temperature in the range of 1450 - 1550°C. Scanning electron microscopy was employed to observe and analyse the structure and composition changes of the chromite samples. For the lab-made chromite pellets, the effects of particle size and bentonite addition on the reduction behaviour were studied. The pellet with smaller particle size gave a higher reduction rate. Bentonite addition had slightly negative effect on the reduction rate. When reducing industrial pellets, temperature and CO flow rate had positive influence on the reduction rate. Reduction of lumpy ore took place slower than reduction of a pellet. In all samples, chromite reduction proceeded longer in the outer part of the sample than in the centre part. The porosity of the chromite samples was determined before and after the experiments. The porosity of lumpy chromite increased significantly during the reduction. However, the porosity of lab-made chromite pellet decreases due to sintering effect. The reaction mechanism is discussed on the basis of experimental results and earlier investigations.

Jan 1, 2004

By A. Moss

The Palabora Mining Company has successfully extended the life of its operation by developing a 30 000 tpd block cave underground mine below the now closed open pit. The process of transitioning from surface to underground mining has been technically, operationally and managerially challenging. This has been due to a number of factors, in particular, the development of a block cave in a very competent rock mass. The competence of the rock mass is clearly demonstrated in the open pit that preceded the block cave, one of the steepest deep open pit excavations in the world. With cave breakthrough, however, the pit was ?instantaneously? deepened by 400 m causing a major failure of the north-west wall involving close to 100 Mt of waste material.

Jan 1, 2006

By Li-ming Qi

During mining, the stress in the coal face varies, and, at the same time, the gas flows from the coal body into the excavation, which will cause the gas pressure to change continuously. Assuming the coal gas content of the coal to be constant, and applying the law of energy conservation, the influence of stress concentration on the gas pressure was analysed. It was found that the stress is proportional to the gas pressure. Finally, as a result of analysis and the law of gas flow, the gas pressure dynamic distribution ahead of tunnelling was modelled, and the mechanism by which blasting induces outburst was explained by making use of the model. Keywords: gas pressure, stress concentration, excavation, tunnel.

Jan 1, 2007

By G. Bacon

The past decade has witnessed unprecedented growth in the development and implementation of solvent extraction technology in the extractive metallurgy of nickel and cobalt. Solvent extraction is proving to be a powerful tool, opening new opportunities for simpler, more cost efficient and environmentally sound metal refining processes. These advances, however, could not have been possible without the cooperative efforts between the nickel industry and the people and organizations involved with reagents, materials and equipment development and supply as well as solvent extraction research, aided by the accumulated knowledge and practical experience from the copper and uranium solvent extraction operations. This paper provides an overview of these developments and some of the challenges ahead.

Jan 1, 2002

By J. H. Potgieter

The method used to assess the reactivity of lime was evaluated to gauge it usefulness as a quality control tool. The effect of particle sizes, different lime and water masses for a fixed s:l ratio and the same for various s:l ratios on the reactivity values obtained, were investigated. An attempt to introduce the amount of grit produced during the slaking reaction as an additional measure of limeís quality and reactivity, proved to be unsuccessful. It was also found that the relative amount of lime slaked during the different tests was independent of the reactivity value of the lime. The investigation confirmed the usefulness of the accepted test procedure as are liable method to measure the reactivity of lime, despite various reservations expressed from a theoretical point of view.

Jan 1, 2002

By G. D. Buson

Driefontein Consolidated Limited has a proud history in the South African gold mining industry dating back to March 1945 with the formation of the West Driefontein Gold Mining Company. The reclamation gold plant at West Driefontein was originally built as a uranium recovery plant in 1970. With the collapse of the uranium oxide yellow cake market, the uranium plant was closed in 1988and successfully converted into a gold plant, recovering gold from repulped tailings dams1. The West Driefontein Reclaim plant utilized milling, leaching, conventional rotary vacuum filtration and cementation to recover the gold at a rate of 200,000 dry tons per month. The No. 1 tailings dam (at a head grade of 0.8 Au g/t) began to near the end of its life during 1996, and the next slimes dams to be treated, the No. 2tailings dam (at a lower head grade of 0.56 Au g/t) was found too marginal for reprocessing using the filtration technology. Alternate technologies were investigated, particularly the carbon-in-pulp (CIP) plant based on the Anglo-American Corporation (AAC) pump-cell technology at the Vaal Reefs Exploration and Mining Company?s East gold plant which was operating profitably at lower head grades2. The AAC pump-cell technology is based on the carousel mode of operation2. The suitability of CIP technology for the West Driefontein Reclaim plant was investigated using a pilot plant pump-cell supplied by Kemix The data gathered were utilized in sizing a full scale AAC pump-cell plant. This plant was motivated, built and commissioned on the 22nd of November 1996. Incorporating 6 eighty cubic metre AAC pump-cells. The West Driefontein Reclaim plant initially treated repulped slimes dam material. A subsequent change of feed material to current arisings (from the main gold plant) occurred in1997 in order to convert mills for waste rock treatment. Currently the reclaim plant treats a combination of current arisings and waste rock (milled at the reclaim site). The new CIP plant replaced twenty-seven rotary vacuum filters. A comparison between the filters and CIP in terms of operating costs was undertaken, as well as a performance analysis of the CIP plant to determine its flexibility in terms of variation of the throughput, pulp density, gold input, leaching efficiency prior to CIP, and the rate of carbon movement.

Jan 1, 1999

By D. J. Greeff

By making use of genetic programming, empirical models for metallurgical processes can be evolved that are more cost-effective than models determined by means of classical statistical techniques. These methods explore populations of candidate models assembled from sets of variables, parameters and simple mathematical operators, and do not require explicit specification of model structures. The application of the proposed strategies is illustrated by means of a case study pertaining to the leaching of nickeliferous chromite ores. Modelling of the nickel and cobalt extraction from the ores yielded models of similar accuracy compared to those obtained by non-linear regression and artificial neural networks. However, in the case of the iron extraction from the ores, the genetic programming model was significantly more accurate than both the regression and neutral network models.

Jan 1, 1998

By L. Holappa

Ferroalloys like FeCr and FeMn are mostly produced in submerged electric arc furnaces. Typical for these processes is that the amount of slag is quite high. The role of slag for the process is thus very essential and specific chemical, physical and electrical properties are required. Slags in FeCr process are Al2O3-MgO-SiO2based with minor contents of CaO and CrOx. Slags in FeMn process are principally similar originating from silicate minerals but can have quite a large composition range depending on the manganese ore and its gangue minerals. Characteristic for FeMn process is that the slag is rich in manganese oxide up to above 30%. A common feature is that the ?ore oxides?, CrOx and MnOxin slag are not only divalent oxides but also containing some higher valent of Cr3+ and Mn3+, respectively. In FeMn production conditions the slag consists mainlyMn2+. This is a clear difference from CrOx containing slags in which both two and three valent coexists in slags in FeCr process conditions. Oxidation state of the slag depends on the ambient oxygen partial pressure and temperature in the examined process. Thermodynamics of slags in ferroalloys production are discussed in terms of phase diagrams, activities and oxidation state of chromium and manganese oxides. Further, slag properties influencing reduction kinetics and furnace operation i.e. viscosity and electrical resistivity are discussed. Keywords: review, slags, ferroalloys, ferrochromium, ferromanganese, thermodynamics, physical properties.

Jan 1, 2004

By R. A. Williams

Electrical impedance tomographic methods have been applied to a variety of mineral and chemical processes in the laboratory and on plant. This review paper surveys technical developments in measurement methods to enable quantitative extraction of key process information in the form of mapping of concentration profiles, mixture homogeneity and suspension velocity. The methods for determining velocity are summarized, since this is of especial interest when applied to opaque systems where most conventional optically based measurements fail. A recent significant development is the use of ultra-high speed measurement that now allows velocity vectors to be extracted for fast moving mixtures and fluids (up to several metres per second). New data are reported pertinent to low and high solids concentration flow in pipelines. Use of the methods offer improvements to design of such systems but also points to the use of tomographic electrical sensor as online flow measurement and fault detection. This offers new opportunities for applications and benefits in the design and monitoring of mineral slurries and sludges.

Jan 1, 2008

By R. C. W. Webber

Mining up to depths of 5 000 m would be a world first and, accordingly, no previous experience in the determination of acceptable heat stress limits, criteria or indices is wholly applicable. However, some South African gold mines are already operating at depths beyond 3000 m and much of the knowledge gained in reaching and working at such depths will be helpful in making adequate provision for acceptable environmental control at the greater depths being contemplated. Accordingly, it is necessary to take cognisance of the industry?s experience in deep-level mining and of standards and regulations already established in South Africa and elsewhere in order to ensure acceptable working conditions and control standards, that compare favourably and defensibly with those in other mining industries. The local and international use of heat stress limits, criteria and indices were investigated as it was necessary to determine to what extent any other indices, limits or criteria would be applicable to South African deep mine conditions. In addition, it was necessary to establish whether there was a single heat stress index that could be used for South African ultra deep mining conditions. It was found that an appropriate combination of heat stress indices would be required in planning for and ultimately controlling thermal conditions in ultra-deep mining. The depths being contemplated and the concomitant potential heat hazard present too great a risk for reliance on a single environmental component of heat stress, such as wet-bulb temperature at present in common use locally. The study recommends that a heat stress index, preferably Air Cooling Power (ACP), be used to design an ultra deep mine?s ventilation system and that wet-bulb temperature be used to monitor and control the system once it is implemented.

Jan 1, 2003

By J A. Rabe, PAP Fouche, K O?Neill

Two of the most important issues mines have to deal with are tracking the ore mined from pit to plant and to test the efficiency of beneficiation equipment. Kumba Resources and Saco Systems are working together at moment to develop a RFID tracer system to help address these issues. This paper will focus on the tracking of ore from pit to plant. The main advantages for ore tracking is to ensure plant feed blending and dilution according to plan. The RFID tracer system works on the basis of a tracer that physically simulates the ore properties and radio frequency technology that facilitates communication with the tracer and allowing information to be stored on the tracer. This paper tends to indicate the progress on the application of RFID tracer technology in the minerals processing environment as tested at Kumba Resources so far. It is concluded that the system has great potential, but some important problems remain

Jan 1, 2005

By J. H. Schubert

Mintek has developed a level-control system, FloatStar, for use in flotation circuits. Instead of controlling levels separately in each unit, the controller monitors levels throughout the circuit. It can thus take account of the global effects of any control actions. A collaborative project between Mintek and Karee Platinum Mine has begun, looking at aspects of flotation control. Karee plant was chosen as the site for this work because of its good existing control infrastructure. As a first stage of the joint project FloatStar was installed. Analysis during installation and running of the FloatStar has illustrated how circuit design and measurement technology can affect process control. Severe interactions have been observed in the levels of adjacent banks. The effect of FloatStar on flotation performance was tested and an improvement in level control was observed. Similarly, a faster settling of the plant after start-ups was observed. Both the improved level control and faster start-ups lead to higher platinum recovery.

Jan 1, 1999

By Ö. Uysal

This study deals with the presentation of the mechanized longwall mining project introduced some years ago to increase productivity and output of Aegean Lignite Establishment (ELI) owned by Turkish Coal Enterprises (TKI.) and the comparison of this method with a planned alternative new mining method called ?shortwall mining? where continuous miners will be introduced as primary coal excavation machines. Following the planning phase, production costs have been calculated for both mining methods on the basis of a panel having a height of 7.5 m. In addition to cost analysis, both methods have been evaluated using ?the multi-dimensional objective system?. In this evaluation the value synthesis has been carried out according to scoring models. Lastly, the advantages and disadvantages of both production methods have been stated by analysing them according to related criteria. In the light of outputs gathered from the analyses, it has been determined that the shortwall mining method will out perform the longwall method. Keywords: continuous miner, shortwall mining, longwall mining, multi-dimensional objective system.

Jan 1, 2006

The processes by which particles and bubbles interact capture many of the central concepts of colloid science and hydrodynamics. This area of research embraces hydrodynamics, interfacial(including capillary) forces, particle and bubble behaviour and solution chemistry. In this review, we discuss collision, attachment and stability efficiencies. We deal with the identification of a flotation ?domain?, the deformation of a bubble surface upon interaction with a solid surface, the kinetics of three phase contact line expansion and the determination of attachment efficiencies through to the direct measurement of bubble-particle interaction forces. We have recently carried out a test of collision theory where the latter embraces both inertial and centrifugal forces. The real challenge is now to quantitatively describe the behaviour of real systems and to incorporate a reliable description of the behaviour of particles in flotation froths. The approach described is very robust when applied to flotation, both from qualitative and quantitative points of view, providing an intellectual foundation for understanding this complex process in separation science and technology.

Jan 1, 1999

By R. G. Tarr

During an intense and accurate study of initiation systems used on an AngloGold mine, Mine A, poor drilling practices were revealed. Graphic results of all actual measurements taken are shown below. [ ] Accurate drilling of shot holes will determine whether a mining operation is payable or not and at what pay limit mining can take place. Losing 30 cm of a potential 120 cm-hole length will result in a number of consequences, i.e.: ?Higher stoping costs ?Drilling of one shot hole twice in every five shifts ?Increased explosive cost as a result ?Excess stress on hangingwall and footwall due to bad face shape ?Excessive blast damage before the area is worked out. No wonder we have such a high-risk operation. Drilling accuracy is crucial to the profitability on any mine as profits are directly related to the quality and utilization of the drilled shot holes.

Jan 1, 2000

Like it or not, CPD is upon us. As from 1 January 2006, all registered engineering professionals, i.e. professional engineers, professional engineering technologists, professional certificated engineers and professional engineering technicians, were required to undertake CPD in order to keep their professional registration current. The rules regarding CPD have been gazetted under Government Gazette No. 28328, Board Notice 131 on 23 December 2005, and they also appear on the ECSA website?www.ecsa.co.za. For the convenience of our members they are reproduced in full in this issue of the SAIMM Journal.

Jan 1, 2006

By C. Kasatuka

Globalization creates investment opportunities for enterprises around the world. Attracting foreign investment has been a key challenge for economic growth in developing countries. Corruption, political instability, armed conflict, and other internal problems affect foreign direct investment (FDI) inflows negatively. A framework of criteria provides a basis for comparing non-commercial risk (NCR) among twenty-three mineral economies. Countries competing for foreign direct investment inflows are ranked from least to most risky. This study compares Inward FDI Performance Index (IND) with NCR ratings in developing countries and shows that risk is increasingly mitigated by means of appropriately structured insurance. Given the option for countries with similar mineral potential, investors will generally invest in the country that offers lower non-commercial risk. Keywords: foreign direct investment, non-commercial risk, developing countries.

Jan 1, 2006

Milling circuits are known to be difficult to control. In order to address these difficulties, it was decided to develop and test a model predictive controller (MPC) for a milling circuit. A milling simulator was written using Microsoft Visual C++ and used to design and test the MPC controller. Due to the large number of tuning parameters in a controller of this type, an optimizing algorithm was written that automatically tunes the controller. Good, robust control was obtained when the controller was tuned using this algorithm. Control was further improved by combining feedback and feed forward control and by using quadratic programming in the MPC algorithm. The movement of the manipulated variables was reduced by using a novel technique that allows the controlled variable to vary within a specified region around the set point. An optimiser controller was also successfully implemented to control the circuit at its maximum throughput.

Jan 1, 2000

By M. J. Nicol

Mr Vegter has analysed, in some depth, the approximations made in the treatment of the kinetic data presented in the original by ourselves (ref.1) and has come to the conclusion that ?under the experimental conditions used by Adams and Nicol (1986) the steady-state assumption was inaccurate?. We are acutely aware of the fact that the experimental method adopted does not permit the attainment of a true steady-state in the elution reactor. However, this does not eliminate the attainment of a pseudo steady-state by a suitable choice of the experimental parameters with the accompanying simplification of the analysis of the data with the aim of drawing the reader?s attention to the most important objective of the paper viz. the effect of various operating parameters on the rate of elution of gold.

Jan 1, 1997

By S. Luyckx

This paper reports hardness and abrasive wear resistance test results from a wide range of WC-Co alloys, varying in cobalt content as well as WC grain size. The results show that the abrasive wear resistance increases with increasing hardness and that the rate of increase is higher for larger WC grain sizes. The increase in abrasive wear resistance was found to be parabolic up to a critical hardness value and exponential above that value. The transition from parabolic to exponential behaviour was found to occur at the same contiguity value for all grain sizes, which suggests that at that contiguity value the WC skeleton becomes continuous in all WC-Co alloys.

Jan 1, 2004