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Coal Tech 2020

Coal Tech 2020 is a collaborative research programme launched by the South African coal industry in 1998. The aim of the programme is to carry out research that will assist in the optimal use of South Africa’s coal reserves.

CoalTech 2020 funds a number of research projects in the areas of coal geology, underground and surface mining, coal preparation, and the impact of coal mining on the environment and on water resources, as well as into the human and social issues related to coal mining and the coal industry.

A study aimed at re-evaluating dense-medium cyclone processing of fine coal was motivated to the CoalTech 2020 Steering Committee and they agreed to sponsor the project.


The Beneficiation Methods Of Fine Coal

Froth flotation is probably the best-established method of cleaning minus 0,5 mm fines and is in extensive use abroad. In South Africa, however, froth flotation was confined until recently to the upgrading of coking-coal fines. Although these fines are classified, according to South African standards, as being amenable to froth flotation, the process is far from straightforward. Studies directed towards a search for more selective reagents included various frothers4, like Aerofroth 65, Aerofroth 73, Aerofroth 77, pentanol, tri-ethoxy-butane (TEB), crude tar acids, and eucalyptus oil, in conjunction with paraffin as collector. Eucalyptus oil appeared a very promising reagent, even when used on its own. It was also found effective for certain weakly coking coals5, 6. Other collectors5,6, such as kerosene, dieselene, and turpentine, were investigated, but the outcome was not entirely satisfactory. It is possible that more careful studies would reveal more effective reagents, and this is being pursued.

In contrast, the flotation of non-coking coals proved disappointing in the extreme. Their poor floatability is attributed to the small amount of ‘bright’ coal they contain.

As a means of cleaning fine coal, water-only cyclones appear attractive for two reasons: they offer likely capital-cost advantages, and they can easily be integrated in existing dense-medium washing plants. However, it has been found7,8 that the accuracy of separation is not as a rule sufficiently high to enable this type of separator to rank as an alternative to any of the established dense-medium processes for small coal. A degree of cleaning is undoubtedly achieved, but the ecart probable is about 0,25.

Tests have indicated that the water-only cyclone operates both as a density separator and as a classifier. This results in the cutpoints of various size fractions of the feed being distributed over a wide range of relative densities, and the overall cutpoint is noticeably higher if a considerable amount of fine material is present. The separating effect in the minus 150 f1-mfraction is almost negligible, and that on the coarser fractions is merely an ash reduction; for example, the ash content can be reduced from 17,6 to 15,4 percent at a yield of 75,2 per cent, while the theoretical yield according to the washability characteristics is as high as 94,0 per cent at the same ash content.

Tests on the compound water cyclone9 or tricone produced promising results. For example, a raw coal smaller than 0,5 mm with an ash content of 18,2 per cent was washed to give a product with an ash content of 12,8 per cent. The ash content of the plus 150 f1-msize fraction was found to be 9,4 per cent, and that of the minus 150 f1-m fraction was still as high as 16,7 per cent. Appropriate desliming can be applied, but that leaves the fraction between 150 and 75f1-m almost untreated. It is therefore considered unlikely that the hydrocyclone will find wide application in the beneficiation of minus 0,5 mm coal.

Concentrating tables are widely used in the U.S.A. but they are not yet in commercial use in South Africa. Researchlo, however, has indicated that the low capacity of these units can be a major drawback: a quarter-size table cannot handle much more than 0,5 tlh if coal of 7 per cent ash is to be produced from coal smaller than 1 mm. Ecart probable values were about 0,08 with serious ‘tails’ increasing the ash content, and the organic efficiencies were as low as 72 per cent.

Various combinations of flotation, water-only cyclones, and tables were investigated5,6, but a product of about 7 per cent ash could be obtained only when the table was used as the second-stage unit. However, this configuration has the disadvantage of low capacity.


Dense-Medium Beneficiation Of Fine Coal Revisited

Dense-medium beneficiation of fine (minus 0.5 mm) coal is not a new concept and has been used in South Africa previously. The dense-medium fine coal plant operated for almost 18 years and, despite proving a difficult plant to operate, did succeed in producing a fine coal product with a low ash content of 7%. No other beneficiation process available could achieve this.

Dense-medium beneficiation of fine coal was also used inother countries, of which the plants at Homer City in the USA and Curragh in Australia were the best known. The first application of dense-medium cleaning of fine coal was.

however, at the Tertre plant in Belgium1. This plant was built in 1957. About nine years later, a second fine coal densemedium plant was built at the Winterslag Mine, also in Belgium. According to reports, this plant operated satisfactorily for a period of some 17 years.

Today, not one of these dense-medium plants remains operational.Spirals were introduced into the South African coal industry in the early 1980s and soon became accepted as the fine coal beneficiation process of choice. Currently, almost every coal processing plant in South Africa employs spirals to beneficiate fine coal.

Although spirals are normally capable of yielding products of acceptable quality, there are some instances,especially in the case of the No. 4 Seam coal, where spiralscannot produce the required product quality. It then becomes more economical to discard the raw fine coal than to process it with spirals. Obviously, this practice does not make the best use of the available coal reserves.

There is a definite need to produce fine coal at a higher product quality than that which is possible with spirals. For this reason, dense-medium beneficiation of fine coal was reconsidered.


The Primary Cyclone Circuit To Controll Feed Density

The primary cyclone circuit incorporates acluster of eight Minerals cyclones. The feed density to the cluster is controlled to ensure the correct cut point isobtained while the number of cyclones operatingcan be altered to match the feed flow volume.The cyclone overflow product is 20% solidswith a P80 of 53 µm containing mostly liberatedmagnetite . The underflow is mostlyunliberated magnetite and low-SG gangue andreports to the existing flotation tailings sump. Inthe size classification circuit, 47% of themagnetite is recovered from the cyclone feed.“The effective classification of the magnetiteis paramount to the success of the separationprocess.

This is complicated by the bimodaldensity properties of the high-SG magnetite andthe low-SG gangue. This was evident throughoutthe metallurgical test work campaigns and was approached to model the suitabilityof cyclones as a classification .”A regrind circuit treats the cyclone underflowand recovers the unliberated magnetite.

The additional magnetite captured passesthrough a set of regrind magnetic separators.The concentrate produced is directed onto thecleaner magnetic separator circuit, mixing oncemore with the liberated magnetite from theprimary cyclone cluster, while the tailings arediscarded as final tails.

The cleaner magnetic separator circuit worksin the same way as the rougher and regrindmagnetic separator circuits, using drums andmagnets to further separate the magnetite fromthe non-magnetic material but at a differentmagnetic strength. The slurry is introduced intothe cleaner separator first, which is a permanentlow intensity magnet and operates with amagnetic strength of 750 Gauss. The cleanerseparator produces a concentrate which flowsinto the feed launder of the following finisherdrum separator which also operates at 750Gauss. The last finisher drum separator isoperated at 550+ Gauss and serves as apolishing separator for magnetite and nonmagneticgangue materials. At the end of thisstage, 99.8% of the magnetite is recovered fromthe feed and a clean concentrate containing upto 98% iron oxides is produced, which issuitable for sale.

The concentrate is directed to the dewateringcircuit while the tailings are recycledback to the rougher and cyclone feedsumps to be re-used as dilution water inthese earlier stages of processing,minimising raw water usage in theprocess.

The concentrate is dewatered using two ceramic disc filters to create a product ofsuitable density for transportation. Asthe discs rotate through the slurry,capillary action causes the liquid to drawthrough the discs while the solids buildup on the external surface of the discs toform magnetite cake. The cake (finalmagnetite concentrate) contains around8-10% moisture. It is removed from thediscs with a scraper and discharged intothe concentrate stockpile for storageuntil it is time for transportation. Thefinal product is a premium grade iron oreconcentrate containing around 90-98%magnetite.


Fine Dry Milling Andair Classification Systems

This equipment manufacturer specialises in fine dry milling andair classification systems. Its systems are inoperation on all five continents, processingindustrial minerals, cement, lime, fly ash, claysand numerous other fine and ultrafine dryproducts. RSG maintains a state of the art pilotplant for air classification, dry ball milling,crushing and ultrafine grinding.

As part of a program of continuous productimprovement, International InnovativeTechnologies (IIT) has further developed itspatented m series of high efficiency verticalmilling equipment with the introduction of anew second generation system for high yieldminerals and powder milling applications. Thistechnology is suitable for the milling of a widerange of natural raw materials and industrialproducts, such as aluminium oxide, siliconcarbide, zirconia, calcium carbonate andlimestone products‚ coal‚ fly ash and differenttypes of slag.

The upgraded system uses heavy dutycastings in a range of modular grinding optionsto meet the needs of different materials, particlesize requirements and throughput volumes.These include a special twin mill systemconfiguration, using two grinding modulesoperating in parallel from a common feed, forcomplete flexibility of material throughput andincreased production outputs.

Grinding modules can also be arrangedvertically in series for multi-stage millingrequired for the production of certain grades ofmaterial.

The second generation m series mills alsoincorporate pressurised oil cooling andlubrication for extended bearing life, enhancedsealing integrity and the cooling of criticalequipment assemblies. In addition, systems canalso be provided with forced air circulationthrough the mill for product cooling andmoisture removal where necessary.

In addition a number of special features havebeen introduced to both reduce wear in the millcaused by hard materials and eliminate thedanger of contamination of product.Compact and powerful‚ the centrifugalgrinding mechanism of the m-series is extremelyefficient with the vertical material flow path andspecial roller assembly ensuring that the forceproduced is translated into maximum particlegrinding power. To meet the specific requirements of particularly demandingapplications and hard minerals, this feature hasbeen further enhanced in the upgraded systemwith a range of material options for the grindingrollers and rings, including abrasion resistanttool steels, high chrome iron and ceramictooling.


The Integration Of Underground Pre-Concentration

The majority of Canadian hard-rock narrow-vein mines are mature, and face the need to mine deeper deposits in the face of increasing costs and declining grades. Obstacles to the successful mining of such deposits include ground control, material handling and ventilation. In addition to these issues, the mining of narrow-vein deposits faces additional economic challenges in the low productivity of the typical narrow-vein mining method, the management of dilution and therefore grade, and the optimal integration of fill into the mining cycle.

UBC has been involved with INCO in a strategic research initiative into mine-mill integration, and more specifically underground pre-concentration, since 2000. Several enabling technologies such as coarse-particle mineral processes, hydraulic transport and modular cemented backfill systems have been identified, and integrated using systems engineering techniques into a conceptual underground mining and processing system for the rejection and disposal of waste. The grade of ore delivered to surface is substantially increased through the rejection of barren waste underground. Previous research has indicated that substantial operating and capital cost savings can be achieved, as well as a reduction in the surface footprint of the operation

A case study has been undertaken for Falconbridge’s Onaping Depth Deposit in Sudbury, Ontario to assess the potential impacts of underground pre-concentration. Geological research and mineralogical evaluation of typical Sudbury Igneous Complex ores indicate that between 20 – 70% of the ore mined can be rejected with a good metallurgical recovery. Several coarse-particle separation technologies have been identified and tested for use in the underground process plant. Waste thus rejected is highly competent, and is suitable for use as a source of aggregate for cemented backfill of superior mechanical properties. A conceptual process plant has been developed and integrated into the underground mining and material handling system. Several impacts of the implementation of underground pre-concentration on the operation have been identified, including:

• Lowering of the cost of metal production through rejection of up to 60% dilution underground at greater than 95% metal recovery

• Savings in underground haulage, hoisting, surface transport, milling and tailings disposal

• Lowering the cutoff grade through the reduction in operating costs

• Increasing the mineral reserve though lowering of the effective cutoff grade

• Decrease in selectivity of mining, thus increasing productivity

• A possible increase in minimum stoping height (ground conditions permitting), facilitating the use of more productive mining equipment and the introduction of automated roof-bolters

• Potential change in mining method to bulk-mining techniques due to the inclusion of a continuous, efficient and effective waste-rejection step within the mining cycle

• Improvement in ground control and a potential reduction in rockbursting due to the introduction of superior backfill into the mining void.


Dewatering Of Fine And Ultrafine Coal

The dewatering of fine and ultrafine coal is one of the most important area of coalpreparation. Not only does it enable maximisation of revenue from existing reserves andplant products by reducing moisture, but it also enables coal currently discarded to berecovered. The techniques of recovering slimes arising from the plant as well as recoveringexisting slimes dams have been developed, which raises yields from mined coal as well asreducing costs of disposal and potentially costly environmental constraints.

Each individual operation has its own economics associated with it in terms of physical constraints as well as marketing, so the smorgasbord of equipment presented can be fitted into each individual case as required.

A number of points have arisen out of this work which needs to be addressed in order to give a better picture of what equipment should be preferred in the South African context. It is hoped that a more systems approach is adopted, as there is often a one piece of equipment fits all approach.

Sample at a number of mines, possibly 12, do standard filter test, permeability, quality and particle size distributions. Also pass these to various equipment manufacturers for their own testing followed by their equipment sizing and costing. This needs to be done to eliminate the “average” used in this report and get a better picture of the variability of dewatering coals.

Do true benchmarking, investigate unrelated industries that dry materials, e.g. laundries!?, sugar to broaden the knowledge base.

Monitor actual equipment in terms of costs, feed and performance.

Initiate testing of simple techniques which can be done quickly and at reasonable cost, e.g. air purging centrifuging

Calculate properly the cost of environmental issues such as slimes dams, pumping underground, dumps, loss of fine coal forever etc, so that these costs may be included in the justification for recovery of extra coal. This must be done in terms of likely future legislation as well as present cost.


Efficient Cyclone Systems For Fine Particle Collection

The optimised design of gas cyclones is of particular relevance for the recovery of highly valuable products, such as in the production of active pharmaceutical ingredients (API) through drying systems such as spray or fluid bed drying. Traditionally, these have been captured with reverse flow gas cyclones, but fine and low density particles remain difficult to capture. Bagfilters, although highly efficient, are to be avoided due to possible contamination problems and filter product hold-up.

In addition, small electrostatic precipitators are difficult to clean, and consequently have not been used for this purpose. This article addresses the use of optimised designs of reverse-flow gas cyclones that can be coupled with mechanical/electrostatic recirculation systems . The Hurricane designs are the solution of numerical global optimisation problems, with the objective of maximising collection efficiency, while obeying several imposed operating and geometrical constraints . The simulation model used by the optimiser is based on the predictive properties of a finite diffusivity model. Recently, this model was extended to deal with recirculation , allowing the possibility of optimising the already highly efficient systems.

Cyclones are gas-solid separation devices characterised by low investment and operating costs, which have been used in the API industries for valuable product recovery . The simulation of reverse-flow cyclones has been the subject of several different approaches, but none of the proposed theories is capable of consistently giving good predictions when applied to predicting experimental grade-collections obtained with different geometries, operating conditions and particlesize distributions. Of all these, the model predictions are dependent on the knowledge of the particles turbulent dispersion coefficient, which depends on operating conditions, cyclone geometry and particle size. These difficulties have led cyclone designers to base their geometries on empirical testing, such that a widely accepted basic rule to succeed in cyclone design is to use only geometries that have been experimentally tested.

Since each cyclone manufacturer has its own high efficiency (HE) design, the question of which design is indeed better for a particular application is relevant. Also, it is highly unlikely that the optimum design can be found by empirical testing, as there are too many design parameters involved. This has been experimentally confirmed, who tried to improve spray-dryer recovery using re-designed cyclones. Despite various attempts at re-designing the cyclones, no improvement could be observed for capturing particles below about 2 μm, and severe losses (up to 20%) were observed.


Industrial Minerals of Mozambique

Mozambican resources of industrial minerals and rocks are large enough to cover most of the requirements of the national industry and to contribute also to the export. The most recent data on more than forty industrial raw materials of Mozambique will help the geologist, the mining engineer, the planners and decision-makers in the government to find the best solution for the present and future industrial development.

This era substitutes expensive copper by common silica, special metallic alloys by ceramic masses in the space industry, metallic engines by the construction of ceramic engines that are better than the metallic ones, microelectronics use structural properties of cheep silicate materials, miniaturization is based on the use of rare earth elements and new composite materials display extra ordinary properties by combining metallic and nonmetallic materials. But even the common products of the modern silicate industry, when using nontraditional processes, can attain special properties – special cements can be used for the production of hulls of ships, ultra fine ground limestone can substitute kaolin in paper and save about 30% of synthetic material when used as a filler, besides an improvement of physical and mechanical properties, smectites (bentonites) and other absorbing materials can save up to 70% of fertilizers in sandy soils which otherwise could be washed away etc.

Abundant industrial mineral resources are kaolin, materials for refractories, high quality graphites, metallurgical grade fluorite, large reserves of nepheline syenites, rare-earth minerals, zirconium and lithium minerals, large reserves of gypsum and anhydrite (also for sulphuric acid production), glass sand, limestone and diatomite. Also building materials are ubiquitous.

Missing or in small quantity or of a low quality are white ceramic clays (ball and bonding clays), industrial salts, asbestos, magnesite and dolomite, vermiculite, of the chemical materials these are sodium carbonate and sulphate, borates, bromine, iodine and nitrates; phosphates (apatite) are abundant but of low quality.

The volume is introduced by a brief review of the Mozambican geology and mining.While the knowledge of the Archean and Precambrian regions in the W a NW is good and adequate to the needs of a mineral exploration, the situation in the NE comprising the provinces Niassa, Nampula and Cabo Delgado is somehow obscure in spite of the fact, that a new geological map, scale 1 :1,000 000, has just been published. This region needs a detailed geological mapping and further exploration work for graphite, marble, nepheline syenite, ultrabasic rocks, kimberlites and pegmatites must be supported.


Performance Evaluation Of Hydrocyclone Filter

The success of microirrigation depends on the ability of the system to prevent emitter clogging. Emitters with small orifices are used to deliver the required low flow rates and these small orifices can easily be clogged by particulate matter, biological growths, chemical precipitates or combination of these present in irrigation water.

The hydrocyclone filter was tested by studying the variation of clean pressure drop with flow rate and variation of pressure drop, discharge, influent concentration and filtration efficiency with elapsed time. Clean pressure drop is the pressure drop obtained across the inlet and outlet of the hydrocyclone filter when clean water is fed through it.

Experiments were carried out with four known concentrations of solid suspension, viz. 300; 600; 900 and 1,200 mg L-1, at the Hydraulics Laboratory of Kelappaji College of Agricultural Engineering and Technology, Tavanur, Kerala, India. The filter was tested for ten hours using recirculated irrigation water that was prepared with known amount of solid. The hydrocyclone filter was installed on a metallic frame to facilitate less vibration caused due to swirling of wate A 3 hp centrifugal pump having 20 m hydraulic head was used to pump the known concentration of muddy water prepared in a sump (length, 5.85 m; width, 1.4 m; and depth, 0.86 m) through the hydrocyclone filter. The delivery point of the 3 hp centrifugal pump was connected to the inlet flange of the hydrocyclone filter with a PVC pipe of 0.0508 m diameter. Near the pump delivery, an outlet pipe was provided for collecting the influent samples of initial concentration at regular time intervals .

The sampling process was controlled by fitting a gate valve to the outlet pipe. Near the inlet point of the hydrocyclone filter, a gate valve and a pressure gauge were provided to control the inflow rate and to monitor the inlet water pressure, respectively. The outlet of the hydrocyclone filter was directed to the sump using a PVC pipe of 0.0508 m diameter. The outlet pipe consisted of a pressure gauge, a gate valve and a water meter to measure the water pressure, to control the flow and outflow discharge, respectively. The two gate valves on either side of the hydrocyclone filter were used to provide sufficient pressure difference inside the cyclone chamber. Outflow samples were collected from an outlet provided near the outlet pipe. The sampling process was controlled by fitting a gate valve to the outlet. A centrifugal pump of 1.5 hp was used to stir the solid suspension in the sump. Both suction and delivery of the 1.5 hp pump were immersed in the sump. Tap water passing through a 200 micron sieve was used as source of clean water to the sump .

The capacity of the sump for each concentration was fixed to 5 m 3 . The sump was cleaned well and tap water through 200 micron sieve was used to fill the sump up to 0.61 m height to make the sump capacity, 5 m 3 . Then, the sump was covered with a plastic sheet to prevent entry of foreign particles into it. Before conducting the experiment, the hydrocyclone filter was properly cleaned by pumping the clean water using 3 hp pump. During the cleaning process, the gate valves at the sample collection points were closed and the gate valves at inlet and outlet of the hydrocyclone filter were opened fully.

The filter was tested first with clean water to determine the clean water pressure drop at inlet and outlet of the hydrocyclone filter. Known concentration of solid suspension, i.e., 300; 600; 900 Performance evaluation of hydrocyclone filter for microirrigation Eng. Agríc., Jaboticabal, v.27, n.2, p.373-382, maio/ago. 2007 377 and 1,200 mg L -1 were prepared by mixing 1.5; 2.0; 3.5 and 4 kg of field solid in to 5 m 3 of clean water present in the sump. The field solid was dried and sieved through 2 mm-size sieve.

The solid particles of size larger than 2 mm were not taken in the present study to simulate irrigation water and to avoid the wearing to the impeller of the 3 hp pump. A 1.5 hp centrifugal pump was used to mix the suspension continuously for ten hours of the experiment. After one hour of thorough mixing of the solid suspension with the small pump (1.5 hp), the experiment was started with an inlet flow rate of 20 m 3 h -1 . The gate valve at the outlet flange of hydrocyclone filter was partially closed to achieve the maximum inlet pressure at the inlet pressure gauge. Observations, such as inlet pressure, outlet pressure, and water meter readings were noted at every five-minute interval.

Similarly, samples from the influent and effluent were collected in plastic containers of capacity 500 mL each, from the inlet and outlet points. After completion of one batch of concentration, the sump was cleaned and filled with clean water and another concentration was prepared to test.