The Current Focus for R&D within the South African Environment G.J. de Korte CSIR, Pretoria, South Africa Abstract Coal plays a major role in the South African economy and the country is reliant on coal for almost all the electricity generated. Coal is also important to the chemical and metallurgical industries. The quality of coal mined in South Africa is gradually decreasing and coal fields that have until now been largely unexploited will be mined in the future. The coal from these coal fields is very different from that currently mined and new techniques will be required to mine and beneficiate the coal. Research is necessary to drive the search for better beneficiation processes and currently virtually all coal-related research in South Africa is conducted through the Coaltech Research Association. A number of coal processing projects have been completed by Coaltech since its formation in 1999 and some of these projects are discussed in the paper.
Introduction Coal is very important to the South African economy and almost all the electricity generated in the country is from coal-fired power stations. Coal is also presently the number one mineral in terms of sales value and has surpassed that of gold in recent years. Approximately 253 million tonnes of coal is mined in South Africa annually and the main consumer of coal locally is Eskom, the government owned power utility. The other large consumer of coal is the petrochemical company Sasol. South Africa also exports approximately 68 million tonnes of coal annually through the port of Richards Bay which is situated on the east coast of the country in the Kwa-Zulu Natal province. Table 1 below summarises the approximate distribution of coal use in South Africa. Table 1: Coal use in South Africa (Hall, 2013) User / application Million tonnes per annum % of total Eskom 126 50 Sasol 40 16 Export 68 27 Other local 19 8 Total 253 100 The main source of coal in South Africa has been the Witbank/Highveld coal fields of the central basin situated in the Mpumalanga and Kwa-Zulu Natal provinces. These coalfields have been extensively mined and the coal is expected to start becoming depleted by approximately 2040. The country still has some 60 billion tonnes of coal reserves left but much of this coal is in the Waterberg and Limpopo coal fields in the northern part of the country. The coal from the Waterberg and Limpopo coal fields differs significantly from the coal in the Mpumalanga coal fields and consists of thin layers of coal interspersed with layers of shale. Geologists use the term barcode which gives a good description of the appearance of the coal. The coal contains some bright coal and a semi-soft coking coal can be produced. The yield of the semi-soft coking fraction is however low (in the region of 10 to 15%) and unless a middling fraction can also be produced, mining of the coal is not economically viable. The production of a middling product increases the overall product yield to about 50% and this requires double-stage coal processing plants. The coal is furthermore difficult to process since it contains relatively high amounts of near-density material. The quality of the remaining coal in the Witbank / Highveld coal field is gradually decreasing as mining companies start to exploit coal reserves which were previously considered unattractive. Much of the coal has been mined using bord-and-pillar mining leaving as much as 60% of the coal behind as pillars. A number of current mining operations now extract the remnant pillars using opencast mining methods and this presents a number of problems which include spontaneous combustion, high proportions of contamination and scrap steel in the run-of-mine coal as well as increased fines content all of which has to be managed in the coal processing plants. Coal processing in South Africa is thus becoming more difficult due to the nature of the raw coal being mined and research is required to find improved methods to mine and beneficiate the coal. At present, almost all coal-related research in South Africa is conducted by the Coaltech Research Association.
Coaltech The Coaltech Research Association was established in 1999 as the Coaltech 2020 Research Program. It is a collaborative initiative which has as its aim the development of technology and the application of research findings that will enable the South African coal industry to remain competitive, sustainable and safe, well into the 21st century. The Coaltech Research Association is an association incorporated under Section 21 of the Companies Act of 1973. The Coaltech Shareholders are Anglo Coal, Xstrata Coal, Eskom, Exxaro Coal, Sasol Mining, BHP Billiton Energy Coal South Africa, Total Coal, CSIR and the Chamber of Mines. Bon Terra Mining, Kuyasa Mining, Kangra Mining, Leeuw Mining, University of the Witwatersrand, University of Pretoria, National Research Foundation, National Union of Mineworkers and the Department of Minerals and Energy are partners in the Coaltech work program. Coaltech is funded by voluntary contributions from the shareholders. In exchange for their contribution, every shareholder obtains representation on the Coaltech board as well as on the steering committees. Seven research areas are supported: (i) underground mining (ii) surface mining (iii) geology and geophysics (iv) coal preparation (v) surface environment (vi) engineering (vii) human and social. Each research area has a steering committee consisting of representatives of the shareholders and the research providers. The research providers are employees of science councils, universities and private consulting firms. The chairpersons of the committees are senior representatives from industry. Since industry is represented on the committees, research is kept practical and relevant. The committees meet at least every other month to monitor project progress. The initial focus of the research was on extending the useful life of coal mining in the Witbank/Highveld coalfields while sustaining job opportunities and utilising the available infrastructure to the year 2020 and beyond. Although much of this research is still continuing, the anticipated depletion of the reserves in the Witbank/Highveld coalfields has necessitated the research focus to also now include the Waterberg coalfield, situated in the northern Limpopo Province. It is expected that most of South Africa s coal will be mined from this coalfield in the future. The geology of the coalfield is very different from that of the Witbank/Highveld area and presents some formidable challenges to mining and processing engineers. Current coal preparation research projects are focusing on dry processing and screening of low-grade raw coals. The supply of coal to the local power generation utility, Eskom, has drastically changed in the last few years. In the past, Eskom was supplied with coal from a few large, dedicated mines. The coal was mined from high-grade reserves, crushed and sent to the power stations via conveyor belts. Presently, a large number of small suppliers truck coal by road to Eskom stations. Much of this coal is recovered from low-grade reserves, discard dumps and slurry ponds. In order to meet Eskom s quality specifications, most of the coals require some beneficiation, or at least de-stoning, but for many small operators,
building and operating a dense-medium plant for this purpose is too expensive. Coaltech therefore decided to investigate the viability of inexpensive, dry beneficiation techniques. Work conducted thus far included the evaluation of dry screening of coal at small apertures (3 to 6 mm) using the Bivi-tec screen, and dry processing of coal using the FGX (Fuhe Gan fa Xuan mei = compound dry type coal washer) as well as dual-energy x-ray transmission sorting. Water is scarce in South Africa and dry processing is very attractive for this reason. The facts that coal remains dry (which maintains the heat value) and that no slurry is produced make dry processing that much more attractive. Coaltech has now been in existence for fourteen years and is considered to be a successful research program due to the following factors: it is a collaborative partnership between mineral and energy industries, labour unions, the government, universities and other research organizations it is "industry needs" driven it assists in the career development of post graduate students there is formal involvement and partnership with universities it is jointly funded by the coal industry, the state and the CSIR one of Coaltech s most important successes is the good communication between the different mining companies which came about as a result of the companies conducting joint research projects. This has benefited all the shareholders and the coal industry at large. Three of the projects researched as part of the Coaltech program, namely fine coal processing with dense medium cyclones, the 3-product cyclone and dry beneficiation of coal using the FGX separator and x-ray sorting, have been implemented in the South African coal industry and are discussed in more detail in this paper. Fine coal dense medium separation Spirals are still used extensively in South Africa to process fine (minus 1 mm) coal. Changes in the export market brought about a change in the specification of thermal export coal and most contracts now specify the required coal quality on an as-received basis. This implies that the moisture content of coal has to be brought into consideration when assessing the quality of the coal. Fine coal, due to its high moisture content, negatively influences the quality of the coal railed to Richards Bay. Unless the fine coal can be processed to a high calorific value (approximately 28 MJ/kg ad) and dewatered to low moisture levels (below 15% ar), it is not economically viable to include fine coal in the final export product and as a result, many of the spiral plants in South Africa were de-commissioned. A better method for processing fine coal was sought and this led to a research project aimed at evaluating dense-medium cyclone processing of fine coal. A 25 t/h pilot dense medium cyclone plant was designed by the Coaltech coal processing steering committee and tested at four different collieries. It proved that dense medium processing was more efficient than spirals for many of the coals in the Witbank/Highveld coalfield and that fine coal could be upgraded to a calorific value of 28 MJ/kg (ad). The use of dense medium cyclones to beneficiate fine coal is not new, and dates back to 1957 when the first such plant was built in Belgium. Since then dense medium fine coal plants have been constructed in the United States, Australia (Kempnich, van Barneveld and Lusan, 1993) and South Africa. These plants met with mixed success and currently there are only two fine coal dense medium cyclone plants in operation both in South Africa.
Based on the outcome of the test work conducted on the Coaltech test plant, the spiral plant at Leeuwpan Mine was replaced with a fine coal dense medium plant. This plant is still in operational condition but is not presently utilised since the mine s product specifications have changed allowing them to by-pass the fine coal directly to product. Coaltoll (Pty) Ltd. recently constructed a new fine coal dense medium cyclone plant at Muhanga Mines which is situated near Middelburg in the Mpumalanga province. The design of the plant is largely based on the experimental Coaltech plant and has a design capacity of 50 t/h. The plant was commissioned during late October 2013 and is presently employed to process all the fine coal arising from the main coal processing plant at Muhanga. Fine coal is also recovered from stockpiles and processed via the new fine coal processing plant. A view of the plant is shown in Figure 1. Figure 1 View of the Coaltoll dense medium fine coal plant at Muhanga The plant has now been in operation for some five months and so far is proving to be effective. The required product quality is being produced, operation of the plant is reportedly trouble-free and magnetite consumption is low. The magnetite consumption, measured from the amount of magnetite in the magnetic separator effluent, is approximately 1.4 kg/t but Coaltoll claims that it is even lower than this. With the permission of the owners, Coaltech recently carried out an efficiency evaluation of the plant and obtained the results shown in Table 2.
Table 2 Performance data for dense medium cyclone plant (feed size - 1.0 + 0.1 mm) Parameter Circulating medium RD 1.24 Circulating medium RD 1.27 Circulating medium RD 1.30 Feed % (ad) ash 31.5 28.5 35.1 Product % (ad) ash 13.8 13.7 16.8 Discard % (ad) ash 47.9 49.7 56.2 Product yield % (ad) 48 59 54 D 50 cut-point density 1.563 1.604 1.670 Ep 0.078 0.053 0.063 Organic efficiency % 83.6 93.6 92.0 Sink in float % 8 6 6 Float in sink % 6 4 4 Total misplaced % 14 10 11 Near-density material (+/-0.1 RD) 21.8 23.3 21.0 The partition curve obtained for the RD 1.24 test is shown in Figure 2. Figure 2: Partition curve for RD 1.24 test The separation efficiency obtained in the cyclone is influenced by the particle size distribution of the feed, the feed rate to the cyclone, the medium-to-coal ratio and the cyclone feed pressure. The 3-product cyclone Many of the coal mines in the Witbank area currently produce thermal coal for the export market. The quality of this coal is typically 6000 kcal/kg NAR and is exported via the Richards Bay export harbor. To produce coal with this heating value, the raw coal has to be processed at a low relative density and, as a result, low product yields are often obtained. The reject coal obtained from the plants can, however, be re-processed at a higher relative
density to yield a thermal coal, typically with a heat value of around 21 MJ/kg (ad), which is sold to Eskom. To enable this two-stage processing of the coal, most plants consist of two separate washing stages, normally a low relative density primary plant and a secondary, high-density section. Some plants are only equipped to produce a single product and in some of these plants, an export coal is produced whilst the rejects is stockpiled and reprocessed through the plant at a later stage to yield a middling coal for use by Eskom. This implies that additional running time is required since the plant cannot process run-of-mine coal while it is re-washing rejects. The 3-product cyclone, which is used extensively in China (Zhao et al, 2010; Zhao and Yu, 2012), can affect a two-stage separation using a single washing unit and a single medium circuit. The 3-product cyclone was developed in Russia during the late 1960 s and a large number of these cyclones have since been installed in Russia and China. Coaltech investigated the application of the 3-product cyclone and found that it would be well suited to South African conditions. A 3-product cyclone was installed at Anglo American s Umlalazi coal preparation plant to facilitate testing of the concept and the cyclone has now been in commercial operation for about two years. It has proven to be very successful in this specific application. Anglo American has since installed four more 3-product cyclones at their Navigation Colliery and is planning another installation at Greenside Colliery. One of the 3- product cyclones installed at Navigation is shown in Figure 3. Figure 3 Three product cyclone in Navigation plant
The separation efficiency of the three product cyclone at Umlalazi has been evaluated on a number of occasions and found to be consistently very good. The separation achieved in the primary stage is at least as good as that of any conventional dense medium cyclone whilst the efficiency of the second stage is also quite acceptable. The performance of the unit at Umlalazi is summarized in Tables 3 and 4 below. Table 3 Summary of efficiency parameters for primary three product cyclone Parameter - 50 + 20 mm - 20 + 10 mm - 10 + 1 mm Combined - 50 + 1 mm Feed % (ad) ash 33.6 26.3 26.6 29.0 Product % (ad) ash 11.5 11.1 10.7 11.1 Discard % (ad) ash 57.1 45.3 44.0 48.9 Product yield % (ad) 51 55 52 52 D 50 cut-point density 1.545 1.540 1.536 1.538 Ep 0.018 0.014 0.025 0.020 Organic efficiency % 99.5 99.2 96.3 98.0 Sink in float % 1 2 4 2 Float in sink % 1 1 4 2 Total misplaced % 2 3 8 5 % Near-density (+/-0.1RD) 17.8 30.5 36.1 28.7 Table 4: Summary of efficiency parameters for secondary cyclone Parameter - 50 + 20 mm - 20 + 10 mm - 10 + 1 mm Combined - 50 + 1 mm Feed % (ad) ash 57.1 45.3 44.0 48.9 Product % (ad) ash 33.1 29.0 26.3 29.0 Discard % (ad) ash 74.5 60.4 60.8 65.9 Product yield % (ad) 42 48 49 46 D 50 cut-point density 1.816 1.726 1.701 1.747 Ep 0.032 0.020 0.042 0.042 Organic efficiency % 99.5 96.6 91.6 91.8 Sink in float % 1 1 3 3 Float in sink % 1 3 6 5 Total misplaced % 2 4 10 8 % Near-density (+/-0.1RD) 17.8 24.7 28.4 20.7 The three product cyclone unfortunately has some limitations. The main issue is the cutpoint density of the second stage which depends on the density of the circulating medium and the resulting cut-point density of the first stage. When the density of the medium fed to the unit is adjusted, both the primary and secondary cut-point densities are affected. Whilst the first stage cut-point density can be directly controlled, that of the second stage is indirect and not easily controlled. A second issue with the three product cyclone is that the lowdensity separation is carried out first, hence all the shale and stone contained in the feed has to pass through both stages. This causes increased wear in the unit but also requires that the capacity of the unit be large enough to ensure that the capacity of the second stage cyclone is not exceeded. The unit therefore works best with coals with high primary stage yields. The Waterberg coal in South Africa typically has primary (low-density) yields of less than 20% and overall yields of less than 50%. The three product cyclone is therefore not ideal for processing this coal. It does, however, work very well for coal from the No. 4 Seam in the central basin.
In China the three product cyclones are fed with raw coal (no desliming) but in South Africa the feed is typically de-slimed at 1 mm prior to processing. Dry beneficiation of coal One of the strategic actions identified by the coal preparation sub-committee of Coaltech was to investigate methods that may be employed to beneficiate low-grade raw coals. The objective is to find low-cost methodologies to either de-stone raw coal and/or discard coal for subsequent use in power generation and to pre-beneficiate raw coals which are not presently considered economically viable to exploit. In addition, dry beneficiation technologies were considered as South Africa is a water-scarce area and the availability of water in the Waterberg coal field is expected to be a problem. Two dry beneficiation technologies were thus far evaluated namely the FGX which was developed and employed in China and dual-energy transmission x-ray sorting. FGX The FGX unit provides a very simple means of removing stone from raw coal and discard. It does not require the use of water and the risk of pollution is reduced since no slurry is produced. The FGX unit also offers the potential of being able to remove stone and shale from low-grade raw coals at a low capital and operating cost (Honaker, 2007) Exxaro Coal purchased a 10 t/h pilot FGX unit for their own use and kindly made the unit available to Coaltech for evaluation. A photograph of the FGX pilot unit which was used for tests at Exxaro s NBC Mine is shown in Figure 4.
Figure 4 FGX Pilot dry processing machine at Exxaro s NBC Mine The FGX machine is basically a modification of the FMC shaking table and has a perforated table surface. The raw coal feed to the unit consists of coal crushed to a top-size of about 50 mm. The coal is fed onto the table surface with a vibrating feeder to control the feed rate. The surface of the table has a series of riffles fitted to the surface and when vibration is applied to the table coal stratifies on the table in such a fashion that the lighter coal moves towards the top and front of the table whilst heavy particles move to the bottom and back of the table. There are at present two full-scale FGX plants in operation in South Africa at Middelkraal and Onverdacht Mines. The plant at Onverdacht is shown in Figure 5.
Figure 5 FGX plant at Onverdacht Mine The efficiency of the plant at Middelkraal has been determined and the results are summarised in Table 5. Table 5 Efficiency of separation FGX (feed size - 50 + 0 mm) Parameter Middling added to product Middling added to discard Feed % (ad) ash 40.4 40.4 Product % (ad) ash 31.9 31.6 Discard % (ad) ash 60.2 57.5 Product yield % (ad) 70 66 D 50 2.007 1.973 Ep 0.217 0.239 Organic Efficiency 86.8 82.6 Sink in Float 7 7 Float in Sink 11 14 Total Misplaced 18 20 Near-density Material (+/- 0.1 RD) 8.4 7.7 The FGX is a very simple unit and therefore easy to operate. Although it can be effectively applied for removing shale and stone from the coal, the separation of the minus 6 mm size fraction is very poor. The bulk of the minus 6 mm coal reports to the product stream with
almost no upgrade. When the coal fed to the FGX is dry, dust tends to be a problem while damp or wet coal results in blinding of the perforated table. In South Africa, it was found more productive to remove the minus 6 mm size fraction from the feed to the FGX and bypass it directly to the product conveyor. This not only increases the capacity of the plant but also reduces the dust problem and furthermore eliminates blinding of the perforated table by damp coal. X-ray sorting of coal In conventional coal processing techniques, the density difference between coal and shale or stone is used to separate the coal from the unwanted contaminants. X-ray sorters are able to distinguish lighter coal from heavier minerals based on the differences in atomic density. Development of the technology was pioneered in the Netherlands (de Jong, van Houwelingen and Kuilman, 2004) and in Germany. Extensive test work has been conducted on x-ray sorters in South Africa and the results obtained have shown that these units are well suited to de-shale coarse run-of-mine coal. A typical application for an x-ray sorter would be where coal is mined together with in-seam partings and, as in many instances, transported over relatively long distances to processing plants. By employing x-ray sorting, the bulk of the stone and shale can be removed from the raw coal at the source, leaving a partly beneficiated coal to be transported with a subsequent saving in transport and processing costs. Figure 6 shows a view of an x-ray sorter in this type of application. Figure 6 X-ray sorter in operation
Table 6 below shows a summary of the results obtained from test work conducted on a sorter employed to remove contamination from run-of-mine coal. In this specific case, it was found that a significant amount of contamination could be removed from the raw coal with very little loss of coal. Table 6 Summary of efficiency - x-ray sorter (feed size - 100 + 30 mm) Parameter Value Feed % (ad) ash 71.0 Product % (ad) ash 59.5 Discard % (ad) ash 81.4 Product yield % (ad) 48 D 50 cut-point density 2.062 Ep 0.288 Organic efficiency % 79.4 Sink in float % 28 Float in sink % 3 Total misplaced % 31 % Near-density material 1.9 X-ray sorters effectively operate on a per-particle basis and the size range of the feed to the unit is therefore important. A size range of about 3:1 is ideal and a suitable feed would therefore be coal sized between 150 mm and 50 mm. Smaller coal, down to 12 mm, can be sorted but at a reduced feed rate. Moisture does not affect the operation of the sorter. Conclusion The quality of raw coal mined in South Africa is declining and processing of the coal is becoming increasingly more difficult. At the same time, economic conditions are becoming ever more demanding. It is therefore of utmost importance that research be conducted to identify equipment and techniques that can be employed to beneficiate coal in the most costeffective way. References Coaltech Research Association 2014, website. http://www.coaltech.co.za. Accessed on 25 March 2014. de Jong, T.P.R., van Houwelingen, J.A. and Kuilman, W. 2004 Automatic sorting and control in solid fuel processing: opportunities in European perspective, Geologica Belgica (2004) 7/3-4: 325-333. Hall, I. 2013, South African Coal Roadmap Presentation to FFF Council Meeting, 27 February 2013, SRK Offices, Johannesburg. Honaker, R.Q. 2007, Dry Coal Cleaning Using the FGX Separator, Paper presented at the SA Coal preparation Conference and exhibition, Sandton, 10-14 September 2007. Kempnich, R.J., van Barneveld, S. and Lusan, A. 1993, Dense medium cyclones on fine coal the Australian experience, 6th Australian Coal Preparation Congress, Proceedings, Paper E1.
Zhao, S., Zhang, C., Xu, X., Yao, W., Chen, J., Yuan, Z. and Zhang, H. 2010 Super-Large Gravity-Fed Three-Product Heavy Medium Cyclone Proceedings of the XVI ICPC, Lexington, KY, USA, May 2010, Pp 296 305. Zhao, S. and Yu, J. 2012 Novel efficient and simplified coal preparation process International Coal Prep 2012, Lexington, KY, April 30 - May 3, 2012, Paper 9.