6.   METALLURGICAL PROCESSING

6.3.1  

Grootegeluk Mine

Grootegeluk Mine commenced operation in 1980 to produce a blend coking coal for use in the steelworks coke ovens. Thermal coal was produced in the washing plant (“GG1”) as a co-product and stored for use at Matimba Power Station.

A second washing plant (“GG2”) was constructed and commissioned in 1986 to produce power station thermal coal to augment the throughput of the first plant so as to produce the full tonnage of thermal coal for the Matimba Power Station.

The design of the large capacity plants (GG1 and GG2) is conventional with a primary wash of 150 +0.5mm coal to remove a discard fraction followed by a secondary wash on GG1 plant of –25mm coal, the large coal (–150 +25mm) being crushed to –25mm before the secondary wash. The newer washing plant GG2 is scheduled for a rebuild to increase the RoM feed and to include the secondary wash of destoned –150mm coal which is crushed to –25mm before rewashing. The fine coal is recovered on de-watering screens for the –0.5 +0.1mm fraction and on horizontal vacuum filters (0.1 – 0mm). The coal reports to the power station thermal coal fraction.

Grootegeluk plans to enlarge the second wash plant (GG2), to include two stage washing to produce increased tonnages of blend coking coal. Increased feed tonnage to the plant (GG2 and GG6) will maintain the tonnage of thermal coal, together with increased tonnage of blend coking coal and the associated discard material.

Small plants were constructed later including a crushing plant to produce raw coal for the Power Station (“GG3”) and washing plants which produce low ash metallurgical and steam coal (“GG4 and GG5”) for sale to local and export markets. The small washing plants (GG4 and GG5) produce 10% and 15% ash coal which has low phosphorous content making it suitable for either steam raising of specialised metallurgical applications such as pulverized coal injection of coal in blast furnaces or in the iron reduction plants. The plants produce a +0.5mm co-product of power station smalls and fine coal that reports to the power station smalls.

The plant operations are very largely influenced by the demand for power station thermal coal. The fraction of this coal within the RoM coal is set by the coal characteristics and the production of higher grade co products. This will be maximised by the construction of the GG6 project which extends the second large coal washing plant (GG2) to produce blend coking coal and power station coal, giving higher overall revenue/t RoM produced.

Further expansion is under investigation together with Eskom on the extension of the Power Station by approximately 60%, which will increase the production of the associated high-grade co-products by a similar amount. The operational costs of R3.20/t RoM for coal treatment are very low showing an efficient coal washing and handling operation.

The fundamental requirements of a coal washing plant are to recover the products in the raw coal and produce a discard that may be safely dumped with no risk of long-term pollution or combustion.

Ideally the product yields should correspond with the geological yields but this is approached rather than attained. The geological yield is measured by crushing a 102mm core from a drill rig to –25mm and then separating the different products by heavy liquids to give the geological yield of products and a discard having minimum energy and minimal tendency to combust after dumping.

The geological factor shows a loss of blend coking coal to power station thermal coal, resulting in lower mine revenue. This result gives a target for the plant designer and operator rather than a measure of operating performance. It can be expected that this factor may be improved over time from incremental plant improvement. Any changes that are proposed will consider improving this factor as part of any design work.

A large plant such as Grootegeluk is subject to continuous review and update in order to minimise operating cost and to maximise product revenue from the mine as a whole. New technologies improve recovery of different size fractions and developments within the mine change requirements for each part of the plant operation.

Continuous review is required on the plant feed top size so as to maximise revenue. The present top size of 150mm is large in the context of typical plant design and the coal characteristics where the coal in the RoM adheres well with shales present in narrow plates within the coal Seams. Crushing to a smaller size may increase yield but at the cost of producing fines which results in the loss of high quality coal to the unwashed fines which report to the power station feed product. Part of this review should encompass examination of new washing techniques for the smaller sizes of coal of less than 2mm and the primary wash recrush using a ring roller crusher, known to produce excessive fines.

The plant discard contains no products <1mm as these are included unwashed in the power station feed. The discard is of low calorific value of 5MJ/kg, much lower than most mine discards, but it has a tendency for spontaneous combustion in the dump. This results in a requirement for soil or other fine inert material to seal the discard in the dump.

The discard CV is a very good measure of plant performance. The above value shows little loss of energy in the discard.

A basic audit of the mechanical condition of the coal handling infrastructure and coal washing plants was undertaken in order to determine whether the maintenance carried out by the plant is sufficient such that no unexpected costs arise due to neglect or under funding of plant maintenance and upkeep.

The plant maintenance system was examined so as to understand the present and predicted maintenance costs for surface equipment. The audit was validated by inspecting plant mechanical availability records to establish that the plants are in a fully ready condition to run to the operating schedules.

The plant inspection further established the general condition of surface equipment including washing plant, mechanical platework, steel and control relative to a well maintained plant with a scheduled life of at least 10 years. The rotating machinery on the plant is well-guarded. Refurbishment of grating and sheeting around the plant includes a total major refurbishment of the original plant (GG1) this year.

Spillage from plant conveyors is well controlled and any accommodation of spillage collected and returned to the product stockpiles. Conveyor walkways are in good condition.

HDPE piping has been installed in the older as well as the new sections of the plant and spare key sections are available on site as replacements should these pipes fail.

The steel piping around the plant is in good condition indicating that the clarified water is not corrosive, as is sometimes the case on other plants. The product stacker/reclaimers were operating during the visit. Scheduled overhauls are included in the preventative maintenance plan. The plant maintenance system in use is based on preventative maintenance to ensure maximum availability.

Each unit selected as a significant piece of equipment is individually identified and a scheduled preventative maintenance job card generated to ensure maintenance is carried out to manufacturers recommendation, or based on operating experience. The maintenance is scheduled during maintenance periods and a condition report generated on the condition of the equipment and wear resistant liners that are inspected as part of the maintenance procedure.

The cost of plant maintenance, part of the plant operating cost, is favourable because of the large size of the equipment in the modules, and the module throughput.

The Grootegeluk Plant is well maintained operating at high availabilities and at rated feed tonnages to the plant. The availability, measured as time on coal, is 90% of the available time after taking off the Sundays and scheduled maintenance periods.

The plant yields are satisfactory, as shown by the small energy losses to discard. There are losses of coking coal to lower value power station coal, as shown by the geological factors for the plant. These however are complex, but should be subject to continuous design review so as to improve the mine revenue. The first design opportunity occurs during the extension to the GG3 plant (GG6 Project) to produce coking coal. There are no potential increases in maintenance costs during the life of the plant on a cost/t RoM basis.

The requirement for covering of soil or inert sand on the dump to control the oxidation reactions of the discard, which result in spontaneous combustion is an ongoing matter. It is matter which requires careful control, as has occurred in the past. The costs associated with this, should not be significantly more than those incurred at present. Crushing of discard, using a crusher, which generates fines, may be an alternative to reduce air flows in the discard dump. This crusher may be at some stage be followed by a jig to remove liberated coal and further reduce the tendency for the dumps to burn. Other alternatives include different techniques in dumping waste, as is happening at present in dumping waste in the pit. The occasion where it is possible to use these technologies may occur for reasons outside the plant itself, such as improvement in waste dumping when the mine overall size is increased to meet future plans, which raises the coal handling throughput.

6.3.2  

Leeuwpan Mine

The plant consists of a conventional dense medium drum and dense medium cyclone plant with spirals to produce saleable products. A new jig has been installed to destone the coal from the upper part of the Seam to produce a power station feed coal or a feed to the existing plant.

The plant produces a large number of products in different size ranges and grades resulting in a large medium section relative to the plant throughput.

The plant is extensively lined with ceramics and utilises HDPE medium piping throughout in order to minimise maintenance and the RoM coal throughput demonstrates a high availability.

The jig plant was not fully commissioned or integrated into the operation at the time of the site visit.

The coal Seam consists of an upper and lower section. The upper Seam is low grade with 60% discard when producing the 15% ash products sold at present. A power station coal with 25% ash may be produced directly in the jig, which is used to remove waste carbonaceous shales. The lower part of the Seam is washed to produce a 15% ash product with little discard.

The coal is crushed to –80mm in open circuit before washing and to –50mm as product coal to the power station or to low ash products, there being no demand for +50mm coal from the present market. Roll crushers are used to minimise fines production. The geological yield factor of 86.5% product to feed coal ratio indicates that the losses to discard is significant even allowing for the slimes that are filtered and dumped in the pit. The new supply facility for power station coal includes the crushing of feed coal to 50mm from the jig product coal that may be rewashed in the drum/cyclone plant, or delivered directly to stockpile for railage to the power station. This results in the drum product crusher having no feed when the jig is part of the coal treatment circuit because the plant feed coal is –50mm.

A new crusher or relocating the drum product crusher to crush the main plant feed coal to –50mm coal may improve the geological factor.

The RoM coal is trucked to a loading point and dumped over a fixed grizzley into a receiving bin. The coal is discharged on an apron feeder and crushed in a double roll feeder to –180mm and conveyed to a double deck screen, the oversize coal is crushed to –80mm nominal size (–100mm maximum). The sized coal is conveyed to the plant feed silo. A magnet removes any tramp material.

The coal is discharged from the silo onto each of two plant feed belts and the tonnage controlled by a belt weigher on the plant feed conveyors.

The silo is emptied between batches of coal as the design is not mass flow with no cut-off possible between batches of different grade.

The feed coal is discharged into a head chute where it is pulped with water and screened on a 2.4m wide double deck inclined screen. The top deck has a 25mm to 30mm cut point and the oversize reports by belt conveyor to the drum washer. A single drum washes oversize coal from the two modules.

The coal is mixed with magnetite medium in the launder feeding the drum and the coal is separated into product (floats) and discard (sinks). The product and discard are drained and rinsed on dedicated screens and report to specific conveyors. The product is crushed to –50mm as there is no market for larger coal and then screened at 25mm with the –25mm reporting to the peas product from the cyclone module. The large coal medium circuit and dilute medium circuit are conventional with headboxes to set the flows and a single magnetic separator recovers magnetite from the dilute medium slurry. The separating density of 1.5 – 1.6 g/ml results in low loading of the magnetic separator.

The sizing screen coal undersize coal reports to the lower deck of the plant feed sizing screen.

Sized small coal of –25+6mm is rinsed and dewatered on the lower deck of the sizing screen and discharges into a launder where it is mixed with magnetite medium. The slurry is pumped to an 800mm diameter cyclone where the floats and sinks are produced, and the medium removed on a 3mm wide split drain panel and screen to produce a peas product and discard. The dilute medium circuit is similar to the drum dilute medium circuit.

The –6mm undersized coal from the plant feed sizing screen is collected as slurry and the –1mm coal and water removed through a fixed drain panel and a 2.4m wide screen. The coal is rinsed on the screen and discharged into a launder where it is mixed with magnetite medium. The screen was overloaded with water with no discernable de-watering section of the screen before discharge.

The medium/coal slurry is pumped to a 710mm diameter cyclone and a product/discard panel and 3m wide split screen drains medium from the coal product and discard streams. The product reports to a specific conveyor and sampled while discard reports to the general discard conveyor, together with discard from the drum plant and the peas plant.

The fine coal slurry collected from the cyclone feed screen and panel is pumped to a desliming cyclone and the –1mm +100 micron material spiraled in two stages to produce a product and discard.

These are cycloned to produce a de-watering screen feed and the product de-watered and conveyed to a product stockpile. The de-watered discard reports to the discard conveyor.

Discard is conveyed to the discard bin and then trucked to the pit where it is dumped as part of the fill.

Water from the fine coal plant reports to three 22m diameter thickeners where it is clarified and water recirculated. The thickener underflow is pumped out at high solids content and filtered on plate and frame filters.

The water is recovered and the filters periodically discharged onto a stockpile from where it is dumped into the pit as fill so that there is no slimes lagoon with the associated water collection problems.

The filter cake could be mixed with the power station feed coal as at Grootegeluk when the jig is commissioned and the coal grade allows this addition.

The coal yields are 40% for top coal and 90% for bottom coal. The plant produces 145ktpm from 220ktpm RoM coal giving a nominal yield of 66%. The product coal specification is 15% but there is a penalty in producing a 14% ash small coal (–25mm), especially from the lower Seam. Any discard reports predominantly to the drum as large discard.

The coal is washed as –100mm coal but all coal is sold as –50mm coal. Any coal produced in the new jig is crushed to –50mm in the jig product crusher.

Quality control is achieved through accurate sampling of the coal products using belt samplers. The different sections of the coal Seam are washed separately to optimise yield. The laboratory is contracted out to produce sample results to schedule. There have been no recorded customer complaints on the coal product specification.

The plant operating costs and throughput will change with the addition of the new jig. The existing plant and infrastructure is in good condition with no build-up of maintenance tasks so no change is expected in this area of the plant. The jig maintenance and operating costs are recovered from revenue generated from sales of power station coal and increased throughput of coal to the drum/cyclone plant when operating on the low yield upper Seam because of the removal of excessive discard in the jig.

A fine coal separation plant is under consideration due to the high value of low ash coal from this mine. The mining, laboratory and the product loading and plant cleaning is outsourced. The supervisory staff and control room operators are directly employed.

The laboratory is contracted out to CMT. The equipment was adequate for analysis and sales quality control. There is some dust issuing from the laboratory mill when the coal was crushed before splitting down and sampling.

The coal washing plant was running at the time of the visit through from the coal receiving bunker to outloading of products. The rotating machinery on the plant is well-guarded. Walkways, grating and handrailing were in acceptable condition and all areas were accessible with no buildup of spillage. The plant staff use adequate personal protective equipment.

A basic audit of the operation and physical condition of the coal washing plant and associated conveyors was undertaken in order to determine whether the condition and maintenance carried out is sufficient that no unexpected costs arise due to neglect or under funding of plant maintenance and upkeep.

A visual inspection of plant and equipment was conducted to:

• Examine operational efficiencies;
• Audit production;
• Determine quality standards.

The steelwork is in good condition with no visible corrosion on steel and no evidence of changes to steelwork since the plant was built.

No undue vibration in the structure was noticed. The screens are well supported and there is no evidence of stress fractures of steel members. There is no evidence of corrosion caused by the clarified water in the plant and to wash the floors.

The small coal platework is ceramic lined and in good condition. There is no evidence of patching of chutes and underpans. All conveyors are in good condition with no visible damage to belts or damaged conveyor idlers. The belt scrapers operate with no tell-tale spillage at return idlers. There is no damage to conveyor trestles from mobile equipment indicating good plant design and layout.

HDPE piping is used extensively throughout the plant. This is good practice, carried low maintenance risk and maintenance friendly. There was one leak on the plant which was under repair during the visit.

Electrical equipment appears to be in good condition and no electrical motors were seen under spillage which reduces cooling efficiency.

The control is by PLC and Scada with all required information on well-designed screens.

The planned preventative maintenance is similar to the other Kumba operations and based on examining major equipment. Weekly maintenance schedule allows 24h/wk for maintenance using mine employed tradesman (helpers are contractors).

The equipment selected such as screens, ceramic lined cyclones and Warman pumps assists maintenance through proved reliability.

6.3.3  

Tshikondeni Mine

A technical audit of the coal preparation plant was carried out on 11 July 2005 and 12 July 2005 to determine if the plant was operating at the required tonnage and efficiencies.

During the visits discussions were held with the Plant Manager, Plant Controller and operating personnel. Inspections were made of the primary crushing screening and stockpile areas, the secondary crushing and screening plant, the dense medium cyclone plants, the froth flotation plants spiral plants and thickener, product stockpiles out-loading, waste dump and slimes dams and the laboratory. Operating procedures, reporting systems and accounting systems were discussed.

A previous audit of the plant was carried out by DMP Consulting in November 2001 and the information from that audit was used for comparison purposes.

For the period April, May, June 2005 the plant treated an average of 65,352tpm and produced an average of 34,273tpm of sales versus a budget of 32,500tpm at a 49.7% yield.

The plant is capable of producing the budgeted sales per month at the current plant yield which varies from 48% – 54%.

RoM coal is delivered by lorry to the RoM tip bin. Coal is conveyed from the tipping bin to a raw coal scalping screen where the 200mm oversize is discharged to ground for disposal by front end loader and lorry. The –200mm raw coal is delivered to a 3,000t capacity raw coal bin then conveyed to a screening and crushing plant where the coal is reduced to a –13mm nominal top size, then conveyed to the plant feed bins.

Raw coal from the plant feed bins is conveyed by two plant feed conveyors to the coal washing plant. The coal preparation plant at Tshikondeni was extended in 1997 and comprises of a twin module dense medium cyclone plant, a twin module froth flotation plant, a single module thickening plant and a product storage and out-loading plant.

Desliming screens in the plant fitted with 1.6mm slot aperture decks size the coal at 1.4mm. The +1.4mm raw coal is separated in 2 x 710mm diameter dense medium cyclones and the –1.4mm raw coal in 2 banks of 6 x 6m³ froth flotation cells. Double stage spiral concentrators were added later to treat the flotation tailings and recover any misplaced fine coal. In order to meet the require product specification the individual plant products are controlled as shown in Table 6.8.

The cyclone product quality is normally adjusted to control final product quality.

The plant is currently fed at 150t/h which is well below its design capacity. The operating philosophy is to maximise yield at the expense of tonnage through-put. Batch washing and operating on extended hours at a lower feed rate is reported to have increased the yield by 4% – 6%.

For the Tshikondeni plant configuration, a plant discount factor of 0.925 can be applied to the theoretical yields for predicting expected plant yields. In practice a plant does not always operate at maximum efficiency due to breakdowns, stops and starts, control problems, etc. and a further factor of 0.975 should be considered giving an overall plant discount factor on yield of 0.902. These factors vary with washing yield and plant configuration but are a suitable guide for the current operation.

Last year the plant yield averaged 54.0% compared to the typical average of 56.1%. The difference can be attributed to the fact that the typical washabilty data is based on a specific area.

Typical washability data from each of the three shafts compare very favourably in terms of relative density and yield when producing a 12% ash product. However the variability in the Seams has led the mine to operate on a batch wash system as this has proven over time to give a higher yield. A homogenising stockpile could be installed to control the variability of the feed but this is deemed impractical on cost with respect to the short life of mine.

Run of mine coal from the incline shaft is delivered by conveyor to a bi-furcated chute. From the bifurcated chute it is conveyed to either the “A” frame RoM ground stockpile where it is recovered by vibrating feeders and front end loader and returned to the circuit or conveyed to the RoM tip bin.

The RoM tip bin receives coal by bottom discharge lorry from the operating shafts. Coal is withdrawn from the tip bin by belt feeder and conveyed to a raw coal inclined scalping screen fitted with a 200mm spacing bar deck. A belt magnet removes tramp iron and discharges to ground for later removal. The scalping screen feed conveyor is fitted with a single idler weightometer and belt speed indicator.

Oversize from the scalping screen, which is mainly waste, is discharged by chute to ground level where it is later removed by front end loader into lorries. The oversize currently averages 2.0%. Undersize from the scalping screen is conveyed to a twin outlet concrete raw coal silo of 3,000t capacity.

6.3.4  

Arnot Colliery

The plant supplies Arnot Power Station at a nominal rate of 5,000ktpa. There is a large stockpile between the plant and power station such that there is no production link between the plant and power station. There is little storage between the mining operation and the stockpile resulting in a close link between the mining operation and the plant. This link is carefully monitored to ensure that the mining operation is never affected by the plant. The overall conveyor capacity is over 1,200t/h for all overland conveyors to handle the product from a continuous mining machine.

The washing plant capacity is large due to the size of the plant and the large coal size –300 + 25mm coal handled. The screen capacity allows operation at up to 500t/h, equivalent to 1,200t/h RoM coal. The washing plant is operated only to control the abrasion index of the coal, due to rock reporting to the coal underground. The abrasion index is a progressive index over a month so the feedback from coal samples allows the tonnage that is washed to be pre-planned and scheduled. The typical operating time is 10% – 20% of the conveyor operating time. The raw coal crusher sizing screens may be checked and maintained when the washing plant is operating.

RoM coal is crushed underground at each shaft to –300mm the feed size to the ring roller crusher and washer. The coal is collected underground in a surge bin and is conveyed to the shaft bin of total storage capacity of 400t. The coal is conveyed to the primary screening plant where it is fed to three screens which screen out the oversized coal which is conveyed to a ring roller crusher with a 40mm aperture screen. The crushed coal and the screened fines are collected in the fines bin and then conveyed to the power station stockpile. The primary screening plant has three further screens which screen out oversize coal which is conveyed to the secondary bin. The coal is screened at 25mm. The fine coal is conveyed to the power station stockpile feed conveyor or to a variable angle screen where the –25 +15mm coal is screened out and may be conveyed to the washing plant feed silo together with the –300 +25mm coal.

The crushed power station coal from either the screening plant or the DMS plant is conveyed to the power station.

The dense medium separation plant is operated intermittently to remove large discard so that the abrasive index of the coal is to specification. There is a large inertia in the supply chain due to the power station stockpile and the sample collection specification so that the cumulative index for each month is the key factor in determining the operational time for the plant. The large coal either +25mm or +15mm is fed to the wet feed sizing screen where –12mm coal is removed to a de-watering screen. The screen underflow slurry is pumped to a cyclone and the cyclone spigot solids recycled to the de-watering screen and to the power station coal.

The large coal is fed to the Drewboy washer where the product floats in a magnetic medium of 1,7RD. It is removed from the bath by paddles and reports to the product drain and rinse screen where medium is drained from the coal and the coal rinsed with water to produce a product coal and a dilute medium. The coal is returned to the product conveyor and reports to the power station colliery stockpile.

The discard sinks in the Drewboy where it is removed and drained in the inclined discard wheel and discharged onto the discard conveyor and trucked from the discard bin to the Arnot Colliery opencast operation as fill. The fines from the washing plant are collected on the dewatering screen with the –12mm undersize coal from the feed screen and a –12mm +100 micron de-watered coal is discharged onto the plant product conveyor. The slimes from the plant are collected in the thickener a 10m, diameter high rate unit and periodically filtered on a horizontal belt filter to produce a filter cake that is added to the product coal.

The operating costs are low for conveying coal at R2/t coal handled on surface and R6/t of RoM coal for washing because of the low operating time for the plant.

6.3.5  

Matla Colliery

The Matla Colliery surface coal handling operation is a conveying, screening and crushing operation to produce a –40mm crushed coal which is the feed coal to Matla Power Station. The only beneficiation and control is achieved by a waste picking operation at the crushing plant and at the shafts before the overland conveyor. The No. 5 Seam coal, if mined, will be washed in an off-site plant. No plans were presented.

The sizing and crushing plant consists of two streams each operated at up to 1,800t/h RoM coal. The coal is sized on 40mm mesh screens and the undersized coal bypasses the crushers. The oversized coal passes over a picking belt with two stations where large sandstone waste is removed at 0,2 – 0,4% of the feed rate. The rock is removed in order to control the abrasive index of the coal (approximately 150 – 200t/day removed).

The coal is crushed in one ring roller crusher/stream with a standby crusher and the crushed coal reports to the small screened coal. The coal is conveyed to the stockpile where it is stacked by trucks. The capacity of the conveyors that feed the power station is 1,800t/h per stream. The power station capacity is rated at 15Mtpa requiring an operating time of 347h/month for each crushing plant at 1800t/h. This is 63% of the time on coal out of a total available time of 580h/month. There is a 500kt capacity stockpile between the crushing plant and the power station making the operation of the crushing plant independent of the power station. Raw coal silos at the feed to the crushing plant from the shaft overhead conveyors ensure that the availability of the crushing plant is easily achieved.

The RoM coal is delivered to a shaft silo at each shaft. It is crushed to –300mm and waste picked at the shafts before the coal is conveyed by overland conveyors to the crushing plant. The coal is dumped into silos to provide storage between the shafts and the crushing plant.

The coal is discharged from the silo by four vibrating feeders feeding a single conveyor belt of 1200mm width. Some –40mm coal is screened out on selected feeders and reports to a fines belt below the screen feed belt.

The coal reports to two sets of two 2,2m x 6m Dabmar double deck sizing screens in series where the –40mm coal is removed by the lower deck. The oversize coal reports to the picking belt where 150 – 200t/day stone is removed from 35,000 t/day of coal on a typical day.

The oversize coal is then charged into a ring roller crusher to reduce it to –40mm nominal size and the crusher product is discharged onto the product belt. Dust from the crusher is collected in a scrubber and the dust is dumped as a dense slurry on the coal conveyors.

The crushed coal from each plant is conveyed to the stockpile area and stacked using mobile equipment. The coal can be conveyed directly to the power station or recovered from the stockpile. The coal is then screened over a guard screen, one for each of two streams and then conveyed to the power station. The oversize coal is recycled to the feed to the sizing and screening plant. A new crusher is under construction to crush oversized coal.

The coal reports without any treatment to the power station. Some rock from the roof reports to the product during changes of the long and short wall operations. This is removed by waste picking at the shafts and at the screening and crushing plant. This is done to control the abrasive index of the coal as delivered to the power station. The coal is at present within specification.

The coal is crushed to –40mm and a guard screen ensures that no oversize coal reports to the power station.

The coal is sold on contained energy so is weighed on calibrated conveyor scales and automatically sampled. Because of the large stockpile there is a large momentum on quality change so that short-term quality control is not required.

The change from short wall mining to the use of continuous miners is expected to reduce contamination at the coalface.

The plant throughput is planned to increase from 11 – 14Mtpa. This will require upgraded control to increase plant availability to the rated capacity of 63% on line, based on a three-shift operation.

The operating costs will not change from the present levels and the enhanced control system can readily be financed by the increased coal tonnage.

6.3.6  

New Clydesdale Colliery

The total Coal Preparation plant configuration is capable of treating 600tph of RoM and is rated at a maximum capacity of 240kt RoM per month or 2,880kt RoM per year.

The washing plant can be seen as comprising four distinct modules or sections.

The first of these is the screening plant, which crushes and screens RoM coal down to –40mm in size from the received 150mm raw product. At present No. 4 Seam lower seam coal is received from the Diesel Power opencast operation (65ktpm). The coal is reduced at the Diesel Power site to –150mm before being trucked approx 6,5km to the plant tip area.

Size reduction in the screening plant is done via 2 Jeffrey double roll crushers of which the primary crusher set at 75mm is in “open circuit” and the secondary crusher set at 40mm is in “closed circuit.” This screening plant can handle 300tph and is fed via a vibrating feeder under the stockpile, which is fed by the trucks bringing across this No. 4 Seam coal. The second of the modules is the HMS section or inland plant, which treats 100tph of the screened and crushed product. The washing is done in a 600mm diameter Multotec mild steel DMS cyclone at a density of 1,680 producing a product and a discard. Currently this product is sized for domestic customers; three separate products are made:

	Small nuts	40mm x 28mm	A grade (27,85MJ/kg)	4,000tpm.
	Peas	28mm x 10mm	A grade (27,85MJ/kg)	11,500tpm.
	Duff	10mm x 0,5mm	A grade (27,85MJ/kg)	on demand.

Currently all the duff is reporting to the Export product line.

The budget for this plant is 180kt of sales (15.5ktpm). Yield in this plant is currently at 63% of the coal fed into it. Sufficient area is available to stockpile large quantities of washed product.

The third of the modules is the PCI or export plant (Module A), which treats the other 200tph of the screened and crushed product. This plant was erected by Bateman in 1997/1998 and was part of the double stage LAC plant. The washing is done in a 810mm diameter Multotec ceramic lined DMS cyclone at a density of 1,680 producing a product and a discard. About 60% of this product is returned to the HMS product for screening into peas, small nuts and duff. The remaining 40% (sizing 40mm x 40mm) is placed on the Export coal stockpile. Yield in this plant is currently at 63% of the coal fed into it. There are two export stockpiles, each capable of holding 8kt each. Should FEL’s be utilised stockpiling capacity can be increased to at least 150kt.

The final module is a stand alone Module B component which produces both domestic and inland washed coal. At present feed for this plant comprises 75ktpm of No. 2 Seam coal mined underground. This coal is sent to the module B plant via a system of surface conveyor belts. Some spare Mafube coal that is trucked across is also being fed into this plant Expected tonnages of this RoM until the end of 2005 is expected to be around 20ktpm. This plant was commissioned in early 2003 after it was extended and modified from the stagnant LAC portion of the existing Bateman plant. A crusher, thickener and a feed in section were added to make it operational. This plant has a feed capacity of 300tph of RoM feed. Feed is as mentioned from the overland conveyor system. The crushing is done in a Hazemag impact crusher (single stage from 150mm to 40mm – open circuit). The washing is done in two 810mm diameter Multotec ceramic lined DMS cyclone at a density of 1,680m³ producing a product and a discard. The washed product is fed over a sizing Dabmar screen to produce:

	Small nuts	40mm x 28mm	A grade (27,85MJ/kg)	4,000tpm.
	Peas	28mm x 10mm	A grade (27,85MJ/kg)	11,500tpm.

This tonnage assists the budget of Inland sales to be made. The remaining tonnage reports to the Export stockpiles. Yield in this plant is currently at 62% of the coal fed into it.

All export trains are loaded on the Colliery via FEL’s onto a conveyor belt transecting the two export stockpiles. Currently 3 Liebherr FEL machines are used for these trains and the average time taken to load one of these trains is 6.8 hours. This conveyor feeds a silo situated directly over the rail line and weighbridge. Wagons are loaded (58t each) in 100 truck blocktrains (5.8kt). The Colliery locomotives take these wagons to the Bezuidenhoutsrus Siding approx 5,5km away, which is linked to the main RBCT rail line.

Current export budget is 830ktpa of entitlement coal plus an additional 240kt BEE component totalling 1,070kt. This equates to some 16 blocktrains per calendar month.

RBCT is 480 kilometres from the Colliery. The weighbridge is a Klerkscale accredited (SABS) module that is frequently checked. Current costs in the coal preparation plant are ZAR13.78/t of RoM washed coal, of which ZAR2.20/t is for water and electricity.

Plant budget for 2005 staff wise is 74 persons; 3 x 9-hour shifts are worked during the week (45hr week) and each of the shifts comprise 15 employees. The rest of the number is made up of drivers, weighbridge clerks and laboratory assistants.

Magnetite used is medium grade type from Martin and Robson and is received in bulk from their depot at Broodsynersplaas, which is close to the Colliery (about 15km away). Magnetite consumption is fairly high at 2kg/t although a figure of 1kg/t is being worked towards. This has been affected by dirty water. Flocculant consumption is 18g to 20g per RoM feed ton. Flocculant is the granular type received in 25kg bags from Sudchemie. Water usage is 1,5m³ per RoM feed ton.

6.3.7  

North Block Complex

Glisa Plants: The Eskom and export coal crushing plants were operating well, producing crushed coal in the correct size grades. The Eskom coal and high grade coal screening and crushing plants each have a capacity of over 300tph. The condition of the equipment is good through regular scheduled maintenance and it has a life which is much longer than the scheduled operation at Glisa. The crushers are suitable for the proposed new plant at Belfast and the screens are in good condition and will operate for the life of plant with normal maintenance and replacement of components.

• Eskom Plant: The plant consists of a three stage crushing circuit in open circuit to produce an Eskom Spec Coal. The primary crusher is a feeder breaker set at 250mm followed by a primary double roll crusher which reduces the coal to 80mm nominal (100mm max). The coal is then screened at 50mm and the +50mm is crushed in a MMD double roll crusher to –50mm in open circuit. The plant produced a coal with significant oversize resulting in a rejection of coal at the power station. An examination of other plants shows that the crusher, usually a rolling ring crusher has a discharge aperture between 40 and 45mm diameter so as to meet specification.

  Models of a crusher circuit showed that a double roll crusher must be set with the gap between rolls at least 5mm less than required. The capacity of the crusher was 836t/h for a 900mm wide crusher set at 50mm between rolls so there is no capacity restraint in closing the crusher gap between the rolls. The required capacity of the plant of around 300t/h is much less than the maximum operating capacity.

• High Grade Coal Plant: This plant capacity is 150ktpm making the required availability less than the plant capacity. At present only a day shift operation is required to screen and load the coal. Coal of D grade (24MJ/kg) is selectively mined from No. 2 Lower Seam. It is sized and sold to both the export and inland market. The feed capacity of the equipment is larger than that required for the plant feed by trucks. The expected feed capacity determined by the crushers and screens is in excess of the usual trucking capacity and is expected to be 300 – 400t/h RoM. The coal is dumped through a fixed grizzley into a bin and discharged by a vibrating feeder into 2 double roll crushers mounted one above the other to crush the coal to –80mm. The coal is conveyed to an inclined double deck screen 1.8m x 4.8m where cobbles and large nuts are screened out. The cobbles are stockpiled and loaded onto trucks or recycled to a tertiary double roll crusher and conveyor. The undersized coal is conveyed to a second double deck inclined screen of 1,8 x 4,8m where small nuts, peas and duff is screened out.

  Based on the screen and conveyor sizes the capacity of the screening plant of approximately 300t/h is in excess of the mine requirements. At the time of the visit the recycle coal crushing stream was not used due to the demand for large coal. The condition of the plant was acceptable for the operation with access and safety well addressed and all major equipment and conveyors serviced to schedule, based on the preventative maintenance plan.

• Strathrae Plant: The Strathrae Plant is a complete washing plant with a capacity of 130ktpm. The present operating cost of R20/t will not be reduced immediately as the coal throughput is increased from 50ktpm to the scheduled 130ktpm because further refurbishment of the plant screens is required to improve plant availability.

  This is temporary as the mechanical work is limited and the steelwork, platework, crushers and conveyors are in adequate condition requiring a continuation of the present scheduled maintenance.

  The dump reclamation at Strathrae is a temporary operation to be completed in 12 months. It is operated with mobile equipment which builds a layered stockpile of crushed RoM coal and screen waste from the existing dump. This is then loaded onto trucks and transported to Eskom power stations.

  The Strathrae coal washing plant is located on the Carolina road some 23km from Carolina itself. It is utilised to wash coal from local mini pit mines and other deposits including possibly washing Matla Colliery No. 5 Seam coal.

  The RoM coal is dumped into a receiving bin and discharged using a feeder breaker set to give –300mm coal. It is conveyed to a secondary crusher plant consisting of two double roll crushers and conveyed to a tertiary plant consisting of a 1,8m inclined double deck screen with the oversize fed to a Osborne double roll crusher to produce –100mm feed coal to the plant.

  The coal is conveyed to the plant feed stockpile. The coal is discharged from the stockpile and fed to the plant at a controlled rate where it is washed in large coal (–100 +25mm) and small coal (–25 +1mm) plants. The fines (–1 mm) are deslimed and the –1mm +0,1mm coal is washed on spirals. The –0.1mm slimes from the thickener is pumped underground.

  The plant capacity is determined by the product drain and rinse screens, together with the separation equipment and the feed screens. A typical size distribution is taken, based on opencast and underground coal to size the equipment so that any coal feed may be washed in the plant.

  The coal yields are set for approximately 70% based on the product screen split. This will become important if larger cyclones are installed. The coal is washed as –100mm coal (–80mm nominal) and there are sales for all size fractions. The –100 micron coal is pumped to underground voids in worked out sections of the mine.

6.3.8  

Sintel Char Plant

The plant was designed by B Morgan and Associates and is based on a well-tried and tested concept of a continuous vertical retort used extensively in the past in the UK and elsewhere (Johannesburg Gasworks) to produce towns gas and non-metallurgical coke for domestic use. The process mass flow diagram given in the report shows a conventional flow diagram based on well-established technology. No calculations were available to check on the mass flow figures given.

The heart of the process is the retort itself. B Morgan and Associates designed and built a plant based on their technology for United Carbon Producers, known as the Prolon Plant situated near Ogies on the Witbank Coalfield. This plant produced char, but was subsequently closed for business reasons. The engineering and operating experience gained from this plant together with technical input from Kumba was used as a base to design a new char retort for the Sintel Project, designated a Modern Char Retort. The concept of this new retort is well-described in the feasibility study report and the reasons for the change in design configuration are discussed in principle. In effect the new retort is based on engineering out the perceived deficiencies in the Proton Plant to produce a modular design with two smaller retorts to give a similar throughput. The changes in design are well-reasoned and there is every reason to conclude that the Modern Char Retort will have an improved performance when compared to the original design. However there are some aspects of the plant that require further consideration particularly as no detailed drawings were available, only basic schematics.

It is most important that the mass flow through both the feed bunker, the body of retort itself and the char discharge section is as even as possible. Channels, “rat holes” or blockages in any part of the system with adversely affect plant performance. Kumba are well aware of this and stated that the well-known principles of mass flow developed by Jenike and Johanson will be applied to the design. This can only be checked once detailed drawings are available. These questions will need to be answered once design drawings are available.

Char strength is an important parameter, particularly important if briquettes are to be manufactured. No tumbler or drop shatter test results on char or briquettes were available.

Char and formed coke plants constructed in the past incorporated a “soakng pit” where the product was allowed to cool and as a result char strength increased. In the Sintel Process the char is cooled by gas before quenching in water. How this affects char strength, if at all, is not clear. It would be prudent for Kumba to consider an alternative to the quenching system proposed. This should result in a decrease in the free moisture content of the final product and could improve char strength. It would be advisable to incorporate sufficient space in the plant layout to install such a system should the final product be adversely affected by water quenching.

Tramp iron in the coal feed is a serious problem for a char plant as it could result in jamming the coal feeder mechanism. Tramp iron should not be present in beneficiated coal as it should be removed in the beneficiating process, but this is not always the case. In addition to the tramp iron magnet shown in the schematic it would seem advisable to include a metal detector which stops the feed belt when tramp iron is detected.

Correct coal sizing is stated to be an important factor in the efficient operation of the retort. No details of the screening plant were given in the schematic drawing. With no stockpile or mixing (“blending”) facility for the plant feedstock, feedstock quality control is in the hands of the Grootegeluk beneficiation plant. The screening plant therefore requires careful design and should be sited as far as possible in such a way that degradation after screening is kept to a minimum.

The plant, although forming part of the Grootegeluk area, will be operated and run as a separate company.

SRK’s understanding is that it will have its own management, operation and maintenance teams. We believe that this approach is correct as a char plant is substantially different to a standard coal washing plant operation and requires different skills, operating and maintenance strategies. Special attention needs to be taken during the staff selection process.

The decision for major maintenance to be co-ordinated with Grootegeluk and to use Grootegeluk’s facilities and major equipment is correct. Otherwise, the new plant will need to incur additional, and in this case avoidable, costs. The manning of the site operation, with the eight operators and four multi-skill millwrights, appears correct. This is in addition to the management, lab and office personnel.

Plant availability is planned at 85% average and is acceptable being within industry norms. Maintenance on this type of plants is heavy and time consuming. Ideally an allowance of 10% for planned maintenance and a further 5% to 10% for unscheduled maintenance breakdown/maintenance work should be made, therefore the 85% availability will comply with this requirement.