Pretoria, South Africa
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Naude G.,Exxaro Resources Ltd. | Hoffman J.,South African Nuclear Energy Corporation | Theron S.J.,Exxaro Resources Ltd. | Coetzer G.,Exxaro Resources Ltd.
Minerals Engineering | Year: 2013

High quality char, produced during the initial coal gasification process, is commonly utilised as a reductant during the smelting of chromite ores. The rate of the gasification reaction and gasification performance will depend on the relevant coal properties. These properties include organic and inorganic composition, coal porosity and pore size distribution, permeability, swelling index and intrinsic reactivity. Quantitative evaluation of these factors affecting the behaviour of coal during pyrolysis and the subsequent influence of those properties on the reactivity of the char product is of paramount significance to further our understanding of the natural differences inherent in coal. X-ray computed tomography allowed three-dimensional characterisation of pores, organic and inorganic constituents. Pyrolysis of coal resulted in increased pore volumes, and by extension, increased porosity. However, the percentage increase in porosity differs from sample to sample, which may be related to the volatile matter content of the coal. The widening and cleanup of pores during gasification result in a decrease in organic content. The organic phases will influence the behaviour of mineral matter during pyrolysis, since the former will fluidise and become devolatilised, while the mineral matter will not become fluid or volatilise, therefore the minerals will migrate and settle and, ultimately, form agglomerates. The lack of significant swelling in the TB1 sample may suggest that higher levels of inertinite were present in the sample, while increased amounts of swelling in the TB2 and TB3 samples may result from increased levels of vitrinite, with the TB3 sample possibly containing the largest percentage vitrinite. The proportion of porous chars will increase with increasing vitrinite content in the parent coal, and therefore, since the TB3 char is highly porous, the possibility that the TB3 sample contains a high proportion of vitrinite becomes highly probable. However, these results need to be verified by quantifying the maceral phases through petrographic analysis. Nevertheless, inherent differences in the nature of coal will result in differences in behaviour when subjected to gasification. Although X-ray computed tomography could be used to successfully identify and quantify various properties within a sample, some limitations still remain. Further work utilising complementary techniques is required to positively identify the remaining unknown phases as well as quantify the organic components. © 2013 Elsevier Ltd. All rights reserved.

Theron J.A.,Exxaro Resources Ltd | Le Roux E.,Exxaro Resources Ltd
Journal of the Southern African Institute of Mining and Metallurgy | Year: 2015

This paper provides guidelines on performing mass and energy balance modelling involving coal and coal derivatives. Usually, the inputs to a pyrometallurgical process would be specified in terms of elements and compounds. Reliable thermochemical data is more widely available for species involving uniquely defined, relatively smaller molecules. However, in the case of coal, the molecules are extremely large and not uniquely defined. Consequently, modelling processes involving coal and its derivatives involve several potential pitfalls. These are outlined in the present paper. It was found that coal proximate analysis should not be regarded as absolute; it could vary with several parameters, including heating rate. For modelling, the use of ultimate analyses should be considered a preferable option to proximate analyses, where 'fixed carbon' and 'volatiles' are not defined in terms of chemical composition. Significant errors could be incurred if the larger molecules are neglected during calculation of the calorific value (CV) of coal gas (the gas liberated when coal is heated in the absence of oxygen). For elemental analysis determination, the oxygen content (which is calculated by balance) should be checked to ensure it is within the expected range. For representation of sulphur in coal, one should avoid doublecounting due to SO3 in the ash analysis. Potentially, oxygen in coal could be represented as O2, H2O, CO, or CO2. However, use of some of these species without considering the experimentally determined gross CV leads to significant errors in the energy balance. If coal enthalpy is calculated from elemental analyses without correction, representation of coal oxygen as H2O(1) gives reasonable accuracy. Coal volatiles could be represented by a complex mixture of compounds, even using different oxygen-containing species than these four, provided the enthalpy is corrected. It is recommended that an 'enthalpy correction value' be incorporated in energy balances involving combustion, devolatilization, or conversion of coal and coal derivatives, e.g. coke, char, or tar. That would imply that proximate analysis, elemental analysis, as well as the gross CV would be required for all solid or liquid coal-derived substances being modelled. No other correction due to carbon being present in a form other than graphite should be used, as that would imply double-counting some effects. © The Southern African Institute of Mining and Metallurgy, 2015.

Richards J.M.,Exxaro Resources Ltd. | Naude G.,Exxaro Resources Ltd. | Theron S.J.,Exxaro Resources Ltd. | McCullum M.,Exxaro Resources Ltd.
Journal of the Southern African Institute of Mining and Metallurgy | Year: 2013

For most of the 20th century optical petrography has been the primary petrological and mineralogical tool used to characterize coal. The development of quantitative SEM-based techniques, e.g. QEMSCAN®, for coal began only about a decade ago. The application of these techniques for coal lagged behind other commodities, but they are currently being developed with the aim to provide 'one complete analysis' for coal. Quantitative SEM-based techniques are supplemented by quantitative X-ray diffraction (XRD). Recent indications are that these more modern techniques cannot replace the 'standard' petrographic and chemical evaluations, but rather complement them where and when required. The great advantage of quantitative SEM-based techniques is that they are very rapid, with the result that large volumes of samples can be processed on a routine basis. This is ideal for coal type identification, since the results can be used in the creation of 'intelligent' composites. This can lead to the more speedy evaluation of coal deposits by reducing the number of samples on which detailed metallurgical and characterization test work is required, without an increase in the overall statistical error of the resource model. Coal petrography, however, remains important for the prediction of the coking characteristics of certain coals and coal products. As a consequence it is therefore important that any coal laboratory be able to produce data with confidence. This requires strict quality control and assurance protocols that adhere to international standards.© The southern african institute of mining and metallurgy, 2013.

Van Der Merwe G.W.,Exxaro Resources Ltd.
AACE International Transactions | Year: 2012

Quite often project planning is developed during the execution phase for the next part of the project activities which deprives the team of informing the up-stream activities of the information that would be required by the down-stream activities. This behavior results in information not collected during execution to inform next step activities. To this end, the formulation and definition of proper project execution plans throughout the project lifecycle are vital to ensure that all requirements throughout the lifecycle are collected, considered and accommodated in the program, processes, procedures and the schedules. Regardless of the implementation model, the strategic planning forms a critical part of successful project execution. For purpose of this discussion, a process plant project is taken as an example.

Ledgerwood J.,Exxaro Ltd Resources | Van Zyl W.,Exxaro Ltd Resources | Van Der Westhuyzen P.V.A.,Exxaro Ltd Resources
8th International Heavy Minerals Conference 2011 | Year: 2011

Namakwa Sands is a mineral processing company based on the West Coast of South Africa and is solely owned by Exxaro Resources Limited. Mineral ore is mined and processed, concentrated and both chemical and mechanically upgraded before being sent for electrostatic and magnetic separation units. The final high value mineral products report as Zircon, Rutile and Ilmenite. The recovery of non-magnetic material is largely dependent upon a number of factors, including but not limited to dew point, ambient temperature, feed rate, mineral size distribution, specific gravity and mineral content in head feed. One important limiting factor to recovery in typical dry mill operations is the particle size distribution of the feed. Variance in the particle size has catastrophic consequences for recovery. This is the reason for this study. Until recently large variances in nonmagnetic production were viewed to be a function of machine operation. This view was later rejected with the feed size distribution determined as the major driver of low recoveries, for Namakwa Sands orebody 50 per cent variance in particle size distribution can be noticed within one day. This is because of the successive mining blending techniques employed. The predicament is that the two modes were so closely placed together that recovery of a single mode is almost impossible without the consequence of greatly reducing recovery. The second mode was found at 125 μm while the first was at 90 μm. Interestingly, for the mentioned feed type a cumulative size distribution of a unimodal feed type is almost exactly the same as for a bimodal size distribution. Thus a plot of cumulative size distribution will show no difference due to the modes being so close together. This is the reason why this phenomenon was not noticed before. The study also found that, using an high tension roll (HTR) or rare earth roll, a fine stream with a single mode at 90 μm versus a coarse stream with a single mode at 150 μm would yield completely different results interms of yields. Larger conductive particles are thrown from the roll while small particles loose charge faster (entrainment). Results in terms of the bimodal size distribution show that even though 25 per cent of the feed type is greater than 150 μm the yields followed a similar pattern to that of the coarse particles rather than that of the majority (75 per cent) fine particles. This confirms what was published by M Ziemski and P N Holtham in 2005 on charge decay rates; they mentioned that particle bed effects played a major role in recovery.

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