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Mineral and coal processing.

Mineral and Coal Processing

THE 14th Congress of the Council of Mining and Metallurgical Institutions (CMMI) was held in Edinburgh, Scotland, from July 2-6, 1990. The proceedings,[1] Minerals, Materials and Industry, have been published by The Institution of Mining & Metallurgy and contain 53 papers covering a wide range of mining-related topics, including education and finance. The third CMMI Distinguished Lecture, |The Future for Metals', was delivered by Professor Kelly, University of Surrey, England, on the opening day of the CMMI congress.[2] The use of metals in the future is questioned because of the decreased profitability of the metal industries in the last decades and the growing use of commonplace substitutes.

The third International Mineral Processing Symposium was held in Istanbul, Turkey, in September 1990. A total of 42 papers was presented. The symposium proceedings volume reflects the current developments in mineral and coal processing technology.[3]

The 1990 Society for Mining, Metallurgy & Exploration (SME) Annual Conference was held in Salt Lake City, Utah, U.S., at the end of February 1990. This meeting, which incorporated the Gold '90 and Control '90 symposia, contained a total of 420 papers. However, many of those papers outside the symposia were presentations or appeared as conference preprints.

The 22nd Annual Meeting of the Canadian Mineral Processors took place in Ottawa, Canada, in January. The proceedings volume contains 29 technical papers, including several dealing with environmental issues.[4]

Many other meetings were held during 1990, often dealing with single subject areas or technical topics, and these are reviewed in the relevant section.

Environmental Issues

The growing environmental concerns that will continue into the 1990s are having an increasing impact upon the minerals industry, resulting in a number of publications and conferences devoted to these issues. Mining and Mineral Processing Wastes,[5] the proceedings of a regional symposium held in Berkeley, California, at the end of May 1990, covers a wide range of subjects from regulations and permitting to case studies of waste management practices. An international conference on Industrial Minerals and the Environment was held at the University of Leeds, England, in September 1990.

The treatment of wastes and limitation of discharges into the environment, while creating many problems for the minerals industry, also offers opportunities for innovation in waste treatment processes and for equipment supply companies. The mining industry, however, has a high profile in the eyes of the public and no matter what efforts and improvements it makes many opinions are already fixed. As one mining company executive has stated, the industry stands in danger of being declared "guilty before being proven innocent"! One example is the public concept of the dangers of cyanide. Cyanide is a naturally occurring compound for which there are ample geochemical and biogeochemical degradation mechanisms, as well as engineered solutions, to prevent its unwarranted spread throughout the environment.[6]

Computers

The Control '90 symposium was held as part of the 1990 AIME Annual Meeting. The symposium, dealing with computer control of mineral and extractive processes, provides an update to the Control '84 symposium. The proceedings, which provide a most up-to-date volume of information on computer control, have been published by the SME.[7]

The 22nd International Symposium on the Application of Computers & Operations Research in the Minerals Industry (APCOM) was held in Berlin, Germany, in September 1990.[8]

Model-based control has been developed during the 1980s and is now being applied at a number of operating plants. Sastry[9] has discussed the principles and methodology of mineral process modelling.

The field of artificial intelligence has contributed immensely to process control, and expert systems and fuzzy logic are just some areas of artificial intelligence. Ynchausti and Hales[10] have discussed the application of expert systems in the minerals industry. Meech[11] has described a series of rule-based Expert Systems that have been developed for the teaching of a number of topics in mineral-related disciplines. In the near future we may see another facet of artificial intelligence, namely neural networks, applied in the minerals industry.

Dowling Jr, Klimpel and Aplan[12] have presented a method of distributing error to predict mass balances for the instantaneous sampling of copper flotation circuits. The least absolute sum approach advocated is simple to use and repetitive calculations are easily handled, with no user intervention.

Hodouin and Flament[13] report on methods to retrieve the hidden information contained in data using on-line analytical techniques on mineral processing plants.

Dynamic optimization has been shown to be a cost-effective method to increase production or to improve product quality on a plant. Herbst, Pate and Oblad[14] have discussed the alternatives for the dynamic optimization of mineral processing operations.

One method to evaluate the technical efficiency of flotation is the release analysis. In this method the separability of the minerals is pushed, by repeated cleaning of the concentrate, to that given by liberation of the minerals. The separation usually achieved may then be compared with this practical limit. The method is elegant due to the presentation of all the technical information (recovery, concentrate grade and yield) on one curve: the grade-gradient curve. The elegance of the method is diminished for complex separations. Labonte et al[15] have constructed a computer program that can be used to determine the economic and technical separation optima for such complex ore separations without drawing the curve.

Comminution and Liberation

The proceedings of the International Symposium on Comminution, held at the Camborne School of Mines in September 1989, appeared as a special issue of the Journal of Minerals Engineering.[16] This symposium, the first in an annual series, featured 20 papers, many concerned with developments in comminution methods.

The seventh European Symposium on Comminution was held in Ljubljana, Yugoslavia, organized by the Yugoslav Committee for Mineral Dressing. This meeting featured 64 papers in three poster sessions and eight plenary lectures.[17] This was followed by a two-day international workshop on worldwide comminution research needs. This was organized by the International Comminution Research Association to explore the boundaries of knowledge and highlight important areas for intensive research.

Crushing plants in mines have usually been conservatively designed. Aggregate producers, however, processing a low-value material and forced to meet more stringent product requirements, have adopted more advanced technology in the hunt for cost savings. Svensson and Steer[18] report that the mining industry is now following suit. They focus on three areas of technical development -- machine design, plant flowsheet and control figuration -- and indicate how a mine can benefit from the interaction of these three areas in a modern comminution plant.

Sawant[19] also discusses advances in cone crusher productivity and ease of operation. The effect of varying the head throw and head speed on the productivity of the crusher are significant, with setting held constant. The larger the throw, the higher the capacity. As the throw increases, the energy consumption/tonne decreases. Also, as the speed of head gyrations is increased the product becomes more cubical.

Recent innovative crushing technology has made the division between crushing and grinding less distinct. The TIDCO Barmac autogenous crushing mill offers the production of a remarkably uniform cubical particle shape throughout a wide size range.[20] The product from the autogenous mill also contains a disproportionate amount of fines, which means that the crusher has potential as a substitute for a conventional crusher-rod mill circuit.

In a paper dealing with the status of comminution simulation in Australia, McKee and NapierMunn[21] demonstrate how simulation has come of age. It is now being used routinely by practising engineers in producing cost-effective plant designs and in modifying existing circuits to increase performance or to adapt to changing feed conditions.

Gao and Forssberg[22] have investigated the behaviour of the selection and breakage functions of an iron ore with a broad particle-size distribution. They have demonstrated that the breakage functions are non-normalizable. It has also been shown that, once a reliable selection function is obtained, it is possible to back-calculate a reduced set of averaged breakage functions. Kelly and Spottiswood[23] have discussed the breakage function (better called the breakage distribution function) and the authors have concluded that it can be considered to result from two different breakage processes: shatter and cleavage. In size reduction equipment in practice, an individual fracture event usually involves both processes, and may also involve re-breakage of the progeny products.

An alternative to the traditional matrix models for crushing simulation has been recently suggested.[24] A kinetic-type model for jaw and cone crushers, in which the residence time is directly related to the crusher's closed or open side setting, is proposed by the author. For closed circuit simulation a kinetic-type model of screening is intended under the novel technique for parameter condensation.

The application of an expert system for the selection of crushing circuits offers advantages over traditional selection procedures.[25] Fracture toughness is used to predict the required power. This, together with other circuit information, permits the expert system to select possible circuit designs, releasing experienced personnel to analyse the choice of circuit configurations. A new Bond grindability test has been developed which not only significantly reduces the experimental testwork involved but also implicitly provides the necessary data for as many sizes as are required.[26] The new method is based upon a computer simulation which closely parallels the traditional method.

Thermally assisted liberation (TAL) has been suggested as a possible method for improving recovery of minerals from appropriate ores. A review by Fitzgibbon and Veasey[27] surveys research in this area. An example of the successful application of TAL in the field of secondary metal recovery is discussed. Wonnacott and Wills[28] describe the application of a gravity circuit regrind model in an attempt to optimize TAL in the treatment of a tin ore.

The U.S. Bureau of Mines has reported on its recent testwork to use microwaves to improve the grindability of ores.[29] The data indicate that up to 20% reduction in conventional Bond Work Index measurements may be achieved, with the added potential of improved liberation characteristics. The high cost of microwave energy still remains a significant negative factor for this technology.

The study of the breakage kinetics of solids reveals that one of the methods used for modifying the grinding rate is to use some chemical compounds as additives. Reported results using two different non-metallic substances, calcite and quartz, and a large variety of surfactants with different polarities, have shown grindability modifications and a new characteristic parameter was established.[30]

Image analysis techniques are finding increasing application in the minerals industry. Texture distribution can be ascertained by image analysis. The results of textural analysis and the degree of liberation of comminution products of a chromite ore have been reported.[31]

Grinding

Mill sizes can be expected to continue to grow in the near future since larger plants are more favourable owing to the specific investment costs, as well as operating and maintenance costs. This trend is further enhanced by the depletion of higher-grade ores, since the throughput of the grinding plants has to be increased to produce the same amount of finished mineral product. The design concepts for modern tube mills, including the use of finite element methods for the design of bearing components, are discussed in a two-part article by Krupp Polysius AG, F.R.G.[32] New developments of components in the girth-gear drive are presented. These have led to a new drive system for tube mills known as Combiflex. This design combines the slide shoe bearing, the girth gear and the reducer in a compact unit. As a result of the compact design, the amount of money to be invested in the civil construction part has been reduced considerably.

Technically, the most elegant and sophisticated drive concept for tube mills is the ring motor, although it also entails the highest investment cost. There are virtually no performance limits to this drive system. Owing to the power electronics required, the drive can easily be built in a variable-speed version, thereby allowing optimization of the grinding process. Economic advantages arise from potential savings in terms of specific energy consumption, wear and optimum capacity utilization. Reducing the rotational speed makes it possible to avoid the damaging effect of the grinding media striking the lining directly where there is a reduced level of material in the mill due to changing grindability of the ore, especially useful in semi-autogenous (SAG) milling.

Svalbonas[33] has examined several recent mill structural failures and state-of-the-art mill designs. The worrying conclusions offered are that mill buyers still need to exercise caution, and that mill fabricators are still not immune from cost reduction pressures, which keep the spectre of failure hovering not far from some of today's designs. On the positive side, most mill failures are reparable. Mill reparability, however, is small consolation for the downtime lost due to failure and continually required repair maintenance.

Grinding Process Development (GPD) Co. Ltd of Montreal, Canada, has developed a new method of performance analysis which can contribute significantly towards the understanding, evaluation and subsequent improvement of ball mill circuits.[34],[35] The method has adapted "Functional Performance Analysis" from the value engineering techniques and applied them to ball milling.

Functional performance analysis subdivides overall ball mill circuit performance into a number of key elements, of which total performance is comprised. Examination of these key elements can give important insights into ways of improving circuit performance.

Autogenous and Semi-autogenous Grinding. The attractions of fully autogenous (FAG) or semi-autogenous grinding (SAG), namely reduced capital and operating costs plus simplified circuits, have been reported for many years. The key saving of a FAG system over SAG milling relates to ball consumption. These attractions must be balanced against the fact that the systems are not suitable for all ore types, need extensive testing prior to design and are relatively difficult to control and optimize.

The proceedings of two major conferences held at the end of 1989 have now been published by the host universities. The first was an International Conference, held in Vancouver, Canada, in September 1989, where 51 papers were presented during six technical sessions.[36] The latter meeting was a more national affair held at Murdoch University, Perth, Australia, in December 1989.[37] Both proceedings contain excellent review papers by G. B. Siddal[38] dealing with the growth of SAG milling in Australia over recent years; from fewer than six installations in 1984 to over 60 by 1989. By comparison, the Caribou Concentrator in New Brunswick, Canada, is one of the few North American base metal mines attempting fully autogenous milling.[39]

A further claimed benefit of fully autogenous and semi-autogenous grinding is its impact on metallurgical performance. It is often claimed to give improved metallurgical performance, but rarely can these systems be compared with conventional grinding on the same ore under plant conditions. Results have been presented for the fully autogenous milling system operated at the Aitik Mine, Sweden.[40] Mineral recovery is directly compared for conventional and fully autogenous grinding.

Many difficulties have been experienced in the commissioning, operating and optimization of FAG and SAG circuits and the basic control strategies available have been reviewed.[41],[42] The adoption of superior mill control strategies demands further instrumentation such as load cells or bearing pressure gauges and programmable logic controllers together with the more common variable indicators.

The A-2 expansion of Codelco's concentrator at Chuquicamata[43] incorporates new 9.75 m-dia. x 4.57 m-long SAG mills. The SAG mills are run by a dedicated computer, using Expert Control software supported by a system for Control and Automization of Industrial Processes (SCAIP), together called SUPERSAG. The SUPERSAG system examines the principal variables and takes whatever steps are necessary to maximize tonnage.

Today, liner designs and materials have evolved to a point where maintenance-free liner life of two to three years is possible in FAG mills and one year in the more severe operating conditions encountered in SAG mills. Parks and Kjos[44] provide an overview of FAG and SAG mill liner functions, criteria for evaluating performance, designs, wear-resistant materials and maintenance problems.

Screen and Classification

Efficient screening is dependent upon good design, installation and operation. Suttill[45] has reviewed the factors to be considered in screen selection, screen installation and maintenance, screening machines, probability screens, screen shaking, sieve bends, and screening media.

Quarrying, along with many other industries, has long sought a means of determining the oversize within screened products on a continuous basis. A system based on computer analysis of video images of representative samples taken from the main product streams has been developed.[46] The system is not intended to replace laboratory analysis but rather to provide on-going information about plant performance.

Restarick[47] has discussed the development of a two-stage cylinder-cyclone system (Figure 1), incorporating a variable concentric aperture valve between the cylinder and the cyclone units. By adjusting this interstage flow-control valve, the classification size of the cyclone can be varied instantly as required. Applications of this system discussed include split conditioning of flotation feeds, desliming and classification prior to spirals and other gravity devices, and densification of feed to gold cyanidation plants.

A useful method for performing mass balance computations for a network of interconnecting hydrocyclones treating mineral suspensions has been presented.[48] Any configuration of hydrocyclones, recycle loops and particle size distributions can be considered.

Gravity Concentration

Gravity separation techniques are experiencing a revival of interest: they are cheap to operate and potentially more environmentally acceptable. An International Symposium on Gravity Separation Technology, containing 22 papers, was held at the Camborne School of Mines, Cornwall, England, in September 1990.[49]

The past, present and future of gravity concentration is reviewed by Turner.[50] The future will depend on the development of methods to recover even finer particles perhaps using electrochemical or centrifugal forces combined with improved methods of comminution.

Wells[51] has looked at the fundamental differences in approach to the design of gravity concentration and flotation circuits. The translation of good design principles into practice is described with examples. Current practice in Canadian gold mills that use some form of gravity concentration has been reviewed.[52]

Modern dynamic dense medium circuits offer many opportunities for simplifying processes, for savings of costs and energy, and extending the reserves of ores which can be mined economically. The development of such systems and the medium control circuits are described by Burton et al.[53] Logical designs, good materials, effective circuits and the elimination of high-wear items have made modern plants simple to operate and convenient for integration with continuously operating circuits with minimal supervision.

Water-only cyclones are gaining wide popularity due to their simple design and few maintenance problems. The cone angle in water-only cyclones varies from 80 [degrees] to 140 [degrees] and the vortex finder extends down through the length of the cylindrical body. Considerable scope exists for the extension of their use from coal preparation to the beneficiation of lead-zinc ores, cassiterite and placer deposits of gold.[79]

The Kelsey Centrifugal Jig is an Australian development using the principles of the conventional mineral jig but with the additional feature of being able to vary the apparent gravitational field by operating within a centrifuge.

The design consists of a rotating bowl surrounded by a concentrate and tailings launder assembly. The bowl contains an impeller to distribute the feed, a parabolic wedge wire screen to retain the ragging, and concentrate hutches to capture and discharge the concentrate. On a conventional jig the retention screen and ragging are on a horizontal plane, where in a centrifugal jig the screen and ragging are vertical. Pulsing is provided by means of a series of diaphragms, which operate sequentially as the jig rotates. The results from pilot plant trials, using a 5 t/h unit, of this technology on various process streams at Rio Kemptville Tin, Nova Scotia, Canada, have been reported.[84] The grade/recovery relationships were consistently better than results obtained from conventional shaking tables.

Modern generation spiral concentrators have a place in the flowsheet of a wide range of minerals. Davies, Goodman and Deschamps[85] have given examples of recent installations, including environmental applications in the area of contaminated soil cleaning and coal dump reclamation. Advances in polymer technology have had significant effects on spiral design, construction and application.

The efforts to treat finer particles by utilizing centrifugal forces in gravity concentration devices continue. The multi-gravity separator (MGS)[80] has now successfully been scaled up to a mine-size unit, treating up to 5 t/h, and is being evaluated for a wide variety of processing applications.[81],[82] A potential application is the non-toxic recovery of fine gold.[83]

Some methodologies for a rational financial consideration of dense medium separation applied for preconcentration have been proposed.[78] The computer software developed is described and the possible use of the financial model in a plant control system is mentioned. The dense media separation process is not economic in all situations, however, but recent developments in the progress towards increasing efficiency -- especially sharper separations of finer sizes -- will make it competitive by comparison with most traditional systems.

Magnetic and Electrostatic Methods

An important area where magnetic and electrostatic separation is almost exclusively used is in the beneficiation of the heavy minerals produced by the beach sands industry. The formulation of flowsheets for the dry mill treatment of rutile-zircon-ilmenite concentrates has developed rapidly over the past 20 years with the opening up of deposits containing difficult mineral suites.

The use of rare earth (RE) high-intensity magnetic separators for the separation of ilmenite from several heavy mineral deposit concentrates has been tested and compared with the conventional technology involving the use of electromagnetic induced magnetic roll (IMR) separators and cross belt separators.[86] Performance is shown to be comparable and it is claimed serious consideration should be given to the RE separators due to their simplicity, high capacity, compactness and lower operating costs, for applications where IMR and cross-belt electromagnetic separators have been traditionally selected.

Superconducting magnetic separators for industrial use have been manufactured by three different companies and are presently operational in three continents (Figure 2). Stadtmuller[87] has presented details of the features unique to superconducting magnets that, when properly exploited, can result in magnetic separators that can yield clear economic benefit through increased capacity and low processing costs.

A recent review of magnetic separation[88] reports that while magnetic separation has the advantage that it can be environmentally friendly, problems of selectivity must be overcome before it finds wider application in mineral separation. One method of improving selectivity is to alter the magnetic properties of mineral phases. Rowson and Rice report the use of microwave irradiation of a caustic coal slurry to improve the magnetic desulphurization of coal.[89],[90] The surfaces of pyrite particles are converted to strongly magnetic phases, probably pyrrhotite or magnetite, allowing improved magnetic separation.

FROTH FLOTATION

Flotation Theory and Reagents

The investigation into improvements and understanding of the chemical environment of froth flotation is fraught with difficulties. However, advances continue to be made at the research and application levels. New analytical techniques have allowed reactions to be followed in situ.[49],[50],[51]

Fuerstenau and co-workers[52],[53] have studied the adsorption of xanthate on selected sulphide minerals in the virtual absence and presence of oxygen, whilst Houot and Duhame[55] report the importance of oxygenation of pulps in the flotation of sulphide ores. The wetting behaviour of chalcopyrite in the absence and presence of xanthates was investigated by Pang and Chander.[55] Electrochemical studies of the flotation process continue to yield valuable insights into the fundamentals of flotation chemistry and a study of the influence of the electrochemical behaviour of the flotation of Mt Isa copper and lead-zinc ores has been reported.[56]

The steps from experimental curiosity through to commerical product can be slow and tortuous in spite of the technological advances offered by the new development. The development of a patented novel new collector for the flotation of zinc is described by Plaumann et al.[57] The collector F1 (2-hexylthioethylamine hydrochloride), based upon chelation chemistry, has several unique structural features that suggest atypical mechanisms for zinc collection. Laboratory and plant data demonstrated significant flexibility with respect to grade and recovery at dosages lower than those typically used for xanthates. The advantages of a sulphide mineral collector, AEROPHINE 3418A, an aqueous solution of di-isobutyldithiophosphinate, have been presented by Mingione.[58] It has proven effective as a collector for the sulphide minerals of copper and particularly lead, as well as for secondary gold and silver values.

Often the separation of arsenopyrite and pyrite, both containing disseminated gold, is desired so that they can be fed to separate extractive processes. O'Connor, Bradshaw and Upton[59] have investigated the flotation of arsenopyrite from an arsenopyrite/pyrite ore and report that arsenopyrite recovery can be optimized by using a two-stage flotation process in which a dithiophosphate is added at pH11 in the first stage and copper sulphate and a dithiocarbamate in the second stage.

While several research groups have established that collectorless flotation is significant for sulphide minerals, the work has been mostly at the laboratory scale rather than at the plant scale. Labonte et al[60] have shown that collectorless flotation does translate into plant operation.

Chalcopyrite is the sulphide mineral most susceptible to collectorless flotation. Detailed studies are required to exploit fully the practicality of collectorless flotation. Even with the partial results obtained so far, it seems evident that some ores might be amenable to full collectorless (selective) flotation while other ores may use it as part of an overall process to improve selectivity. The behaviour of precious metals (which depends on their mineralogy) will be the determing factor.

Recent studies on the use of nitrogen in the processing of complex sulphide ores by flotation have been described.[61] The findings suggest the use of nitrogen in developing alternative routes for the rejection of pyrite in processing complex sulphide ores. This approach could reduce dependence on chemical reagents with associated reduced costs and environmental impact.

The results of continuous testwork, with a novel arrangement of Noranda-designed mini-cells, using nitrogen gas for pyrite flotation, has also been reported.[62] Pyrite could be selectively floated ahead of sphalerite recovery. Furthermore, low xanthate additions were required for pyrite flotation, but sphalerite could be floated without copper activation.

Andrews[63] has provided a comprehensive review of the main developments in cassiterite flotation by each of the main collector systems. These systems include alkyl carboxylates, alkyl sulphates, sulphonates and sulphosuccinamate derivatives, alkyl hydroxamic acid, aryl arsonic acid, alkyl phosphonic acid derivatives and cationic collectors.

In the U.S. mining industry, about 40% of the tonnage of the ore floated is industrial minerals, but, because of the non-selective nature of the flotation reagents employed, reagent consumption is ten times as great as the volume of reagents used in selective sulphide flotation. Crozier[64] has reviewed the flotation of non-metallic minerals, dealing with reagent technology, process design and industrial practice.

The use of differential flotation for the separate treatment of fine and coarse size fractions has long been advocated. Huang and Sivamoharn[65] show significant improvements in selectivity and reduced collector consumption in the flotation of calcite using differential flotation compared with conventional flotation.

A new method for the flotation of petalite, used as a source of lithium in the glass and ceramics industry, has been presented by Lyyra et al.[66] A quaternary amine was used as a collector, following activation with hydrofluoric acid, the flotation of microcline and albite being depressed by the addition of [K.sup.+] and [Na.sup.+] ions. Quartz was depressed by keeping the pH of the pulp below 3.

Macromolecular organic depressants are assuming importance, due to the ever increasing need for cost-effective flotation reagents to process even lower-grade ores containing problematic gangue minerals. Polymers such as guar gum and carboxymethylcellulose (CMC) are used as depressants for layered silicate gangue minerals, e.g. talc and pyrophyllite. Recently, it was found that hemicelluloses are easy to apply and unexpectedly act as highly selective depressants for such systems. Hemicelluloses have been evaluated as gangue depressants for the beneficiation of layered silicate ores which are difficult to process.[67]

Flotation Practice and Machines

Froth washing in mechanical cells offers a number of metallurgical and economical benefits resulting from significant reductions in gangue entrainment. Froth washing was first successfully practised more than 25 years ago in some Soviet plants. Recently, trials of froth washing have been conducted in many parts of the world and the method has been gaining more acceptance, especially in North America. Kaya et al[68] report on four recent froth washing trials in North America and conclude that froth washing can be tested easily and implemented for existing flotation banks at very nominal investment and should be a prerequisite for column flotation.

Column flotation is the most significant innovation in the mineral processing industry in recent years and has become an important alternative to mechanical flotation. Industrial acceptance of flotation columns now seems firmly established, with new plants being built, incorporating them into initial flowsheets. The technology has been most widely accepted and applied in cleaning service where it has frequently proven to be superior to mechanically agitated flotation. Roughing in columns has not been ignored, but early testing indicated that columns may not be capable of achieving adequate recovery, the main performance objective in rougher flotation, and, to date, columns have not been seen as serious competitors for mechanical roughers. Also, the required capacity of flotation columns for this duty must be considered. Furey[69] reports that roughing by column flotation should not be arbitrarily ruled out, and that an initial evaluation of column roughing should not be restricted to the standard control strategies.

One of the operational variables that needs to be controlled for stable operation of a flotation column is the froth depth, or "interface level" as it is commonly called. Small froth depths result in a reduction of concentrate grade if particle entrainment is only partially eliminated, while excessive froth depths reduce recoveries as a consequence of the smaller volume of collection zone. The development of a conductivity probe for the detection of the interface level within flotation columns has been described.[70] A new column flotation measurement system, the C-probe, has recently been put on the market using this conductivity technique.[71] The 4 m probe, with a 2.1 m detection length, can generate a conductivity profile within a minute and provide a level detection accuracy of [+ or -]5 cm. Other potential applications for the C-probe are controlling thickeners and waste-water treatment processes. Flotation columns do have disadvantages, with the major problem being their height requirement.

The Jameson cell[72] removes the central portion of the Canadian column and the air and slurry are contacted in a central |downcomer' (Figure 3). The feed slurry is pumped into the Jameson cell and is forced through a nozzle, forming a jet of slurry which draws air into the central downcomer. This forms a |froth', comprising 50-60% by volume of air. The aerated slurry discharges into a tank and the rising air bubbles are again washed by the application of wash water if desired. Residence time in the downcomer is around 10 seconds, with about a further minute in the body of the cell. Again control is through a simple level controller and the cell is self-aspirating.

TREATMENT OF SPECIFIC ORES

Gold Ores. As progressively more gold ores are brought into production that are not directly amenable to cyanidation the need for process mineralogy to assist in process design and optimization increases. Chryssoulis and Cabri[91] have examined the significance of gold mineralogical balances in planning detailed metallurgical testing and process optimization of difficult gold ores.

Sulphide Ores. The Red Dog zinc-lead mine was brought into production during 1990 by Cominco.[92] Situated in north-west Alaska, it is one of the largest, lowest-cost zinc producers in the world. The process flowsheet (Figure 4) contains many features that are new to zinc/lead recovery. Among these are the modular construction aspect, and the use of innovative equipment such as tower mills, flotation columns and pressure filters. A grinding sequence consisting of SAG mill/ball mill/tower mill was chosen as it offered low capital and operating costs coupled with a high level of reliability.

This circuit design eliminates fine crushing and achieves high energy efficiency through the use of tower mills, which can reportedly result in a reduction in power needs of up to 40%. The ball mill was included to protect the tower mills from the possible adverse effects of SAG mill oversize and surging.

Rougher flotation is carried out in a series of Maxwell tank flotation cells. Both lead and zinc cleaning flotation circuits utilize Maxwell cells and flotation columns. Elemental sulphur is present in the ore. It is removed in a pre-flotation step before the ore enters the lead circuit. Dewatering of flotation concentrates is by four Ingersoll Rand Lasta filter presses, said to be the world's largest pressure filters. Each consists of eighty four 4.9 [m.sup.2] leaves.

Wood and Duval have described the development and operation of the milling circuits treating the different Selbaie orebodies.[93] The current flowsheets are described, with discussion of some unusual features. Les Mines Selbaie currently exploits two copper-zinc orebodies in northern Quebec, Canada which exhibit very different metallurgical behaviour. The A-1 zone yields reasonable separation of copper and zinc while the B zone gives a bulk sulphide float.

Oxide Ores. The chief mineral raw material for the production of antimony and antimony (III) oxide, used in alloying with lead and as a fire-retardant filler in plastics, is the oxide mineral stibnite ([Sb.sub.2][O.sub.3]). The ore is concentrated by flotation, occasionally supplemented by gravity concentration. Flotation is conducted at natural pH; commonly used collectors are amyl or butyl xanthate. A model has been presented describing the reactions in stibnite flotation.[94]

An investigation of the behaviour of minerals in the Mt Wright spiral circuit to investigate methods of improving iron oxide liberation and recovery was undertaken by Wilson, Petruk and Cote.[95] Sized fractions of samples within the circuit were analysed using MP-SEM-IPS image analysis and recommendations made as to how the objectives might be achieved.

Industrial Minerals. The ninth Industrial Minerals International Congress was held in Sydney, Australia, during March 1990. The proceedings,[96] containing 27 papers, feature commodity and marketing issues, particularly in relation to the |Pacific Rim' countries. The processing papers have been published separately.[97]

The treatment of beach sands is covered in a number of papers in both volumes. Of particular interest is the commercial development of the WIM-150 deposit in Victoria, Australia.[98] The fine grain size of the deposit, typically 50 microns (about a quarter the size of conventional beach sand minerals), presents challenges in achieving efficient mineral separation. Flotation technology has been developed both to extract a heavy mineral concentrate and to separate the individual minerals. Additional work has focused on novel gravity techniques suited to fine size materials.

Walker[99] has reviewed the processing options for low-grade phosphate ores. Deposits at three locations within Australia and its territories are considered. The large distances of these deposits from present processing plants pose special problems such that some form of beneficiation will be a key requirement for any future development. Meanwhile, Hall[100] has reviewed the application of column flotation in the treatment of industrial minerals. Results from the processing of fluorspar, graphite, chromite, talc and phosphates are presented.

Increasingly stringent dust standards are becoming harder to meet in the dry milling processes for asbestos. It has been reported that the future of the asbestos industry could lie in wet processing.[101] Following a thorough research and development programme a wet process facility has been developed for Baie Verte Mines, Newfoundland, Canada, for recovery of chrysotile asbestos.

COAL PREPARATION

The 11th International Coal Preparation Congress was held in Tokyo, Japan, with the proceedings being published by the Mining and Materials Processing Institute of Japan (MMIJ).[102] The organizing committee achieved its objective of widening the scope of the papers presented with coverage of chemical, as well as physical, cleaning and coal utilization. With increased competition amongst energy resources and environmental pressures, the importance of coal preparation is being more and more seriously recognized.

Comminution

Despite the gaps in coal-particle breakage knowledge, modern coal breaking and crushing equipment represents a mature technology, with proven design principles and familiar production sequences. Developments within the field tend to be incremental, rather than revolutionary. A broad range of coal breakers and crushers allows producers to tailor the machine to specific process requirements; these are reviewed by Carter.[103]

Gravity Processing

Sulphur removal continues to be of major concern as permissible emissions are reduced. The problem is that some of the sulphur content is organic sulphur combined in the coal structure and much of the remaining sulphur, present as pyrite, is finely disseminated throughout the coal, which makes physical separation difficult. Recent tests with the Multi-Gravity Separator have been reported to be encouraging.[104]

The Parnaby process, originally developed to recover coal from waste dumps, is gradually finding applications to run-of-mine coal.[104]

The Parnaby process incorporates two major independent features: the Parnaby barrel and the Parnaby cyclone, a horizontal-body, autogenous-medium cyclone. The barrel separates using a combination of autogenous dense-medim and dynamic effects, while the cyclones and associated sieve bends extract fine solids from the exiting barrel pulp and return the autogenous medium to the barrel for continuing use.

Improvements to dense media cyclone performance that arise from the use of micronized magnetite (over 70% less than 5 microns) are reported by Klima, Killmeyer and Hucko.[105] The various stages that make up the Micro-Mag Process are described, with the focus on cyclone performance, though also included are preliminary results for media recovery. The results indicate very good separation of ash and pyrite down to 38 microns. The process is scheduled to be scaled up to 50-250 kg/h in 1992.

The trend for producing high-quality coals for coal-slurry fuels is discussed and the available systems for obtaining such premium products are examined by Ferrara, Rombini and Ruff.[106] To this end, a multistage dense-medium dynamic separation, such as a Tri-Flo system, is particularly suitable. By using a three-stage separator it is possible to perform the high-density cut for rejecting the refuse in the first stage and then to perform the low-density cut, which requires a sharper separation, in the second and third stages (Figure 5).

Froth Flotation

Froth flotation is generally recognized as the best method for processing minus 600 micron coal. Unfortunately, conventional flotation cells are unable to upgrade effectively coal streams containing a high proportion of fine coal or clay slimes. Vickers and Morris[107] have discussed aspects of research into fine coal treatment. An important outcome was the development of empirically derived performance prediction techniques requiring minimal analytical information. It has been shown that the importance of |superfine' (minus 63 microns) particles in fine coal treatment is paramount and that their behaviour can be modelled in coal flotation and filtration.

All coal preparation plants are different, if not in design then in the rank of the coal processed by them or in the final product specification. Therefore, it is important that flotation reagent design be flexible to accommodate these changes in operating conditions. Briscoe and Vander Veen[108] have outlined some of the current trends and logic in the design of state-of-the-art flotation reagents.

Franzidis and Anderson[109] have reported the results of an investigation into the events occurring on a microscale during coal flotation, i.e. the individual steps within the flotation process which are not well understood but whose quantification would greatly assist in optimizing flotation performance. Collector oil dispersion, adsorption (including selectivity) and influence of air bubble creation and attachment were studied. It was found that coal recovery was largely dependent on froth stability since oil dispersion and adsorption were adequate.

Column flotation, with the use of counter-current wash water, can give improved performance on such streams. Interest in column flotation continues with co-operative efforts between research establishments and industry at a plant-scale being reported. Four commercial scale flotation columns, 2.4 m dia. by 6.7 m high, are in operation at a preparation plant in Virginia, U.S. The four columns currently treat about 18.14 t/h of fine refuse (55.6% ash), producing about 5.4 to 7.2 t/h of clean coal containing approximately 6% ash.[110]

Hydrodynamic analyses indicate that the use of microbubbles (10 to 40 microns dia.), i.e. bubbles substantially smaller than those used in conventional flotation, can greatly improve the flotation rate and, hence, coal recovery. This is the basis for the Microbubble Column Flotation (MCF) process developed at Virginia Polytechnic Institute and State University.[111] The process has been developed to pilot plant scale (0.25 t/h) and is designed to produce premium coal products (less than 1% or 2% ash) using a variety of micronized U.S. coals as feed stock. A novel microbubble generator is incorporated into the MCF cell (Figure 6).

Ultrasonic energy can be used to sharpen flotation separation by liberating surface and porebound clay, ash and pyrite particles from larger coal particles and by surface polishing the minus 74 micron clay and pyrite particles to increase their hydrophobicity.[112]

Dewatering

In coal preparation plants, there are two solid-liquid separation steps that are critical to the efficiency and economy of an operation: dewatering minus 850 micron clean coal, and the thickening of fine tailings. Dahlstrom[113] has reviewed some basic fundamentals of dewatering, which can avoid many of the potential problems. By the very nature of deep-mine coal production and the consequential treatment through a coal preparation plant to achieve the necessary qualities, two constituents, moisture and fines, cause considerable difficulties in the handling characteristics of the final product to the customer. Jones[114] describes some of the solutions developed, used and evaluated by British Coal to ensure that coal fines are utilized in the final product blend rather than discarded as waste.

Cyclones. Childs[115] has described a novel approach to process water recovery. Instead of a thickener, high-performance Mozley hydrocyclones and horizontal belt filters do the same job. Advantages included lower capital cost, easier operation and maintenance and considerably reduced space requirement. A review of cyclone operation and applications in coal preparation has been published.[116]

Filtration. The most important economical research target for solid/liquid separation is, and remains, to push mechanical dewatering as far as possible in order to reduce or completely avoid thermal drying. A paper by Kubitza et al[117] describes the basis for pressure filtration and results obtained at Ruhrkohle AG, F.R.G.

Brown[118] describes the merits and disadvantages of the air blow filter press, the Autojet filter, membrane filter presses, tube presses and high-pressure conventional plate filter presses as applied to the dewatering of clean coal flotation concentrate and uncleaned raw slurry.

Centrifuges. A number of innovations in screen bowl centrifuge design and operation are presented by Salomon and Orphanos.[119] These developed from unexpected performance results from commercially installed screen bowl centrifuges when tested at throughputs from 100% to 200% of the normally rated design capacity.

Process Monitoring, Control and Optimization

Systems for rapid determination of ash contents using the backscatter principle for on-line measuring on a partial stream have been operational for approximately 20 years. To avoid complicated mechanical systems requiring comprehensive maintenance, and for widening the measuring range, transmission units have been developed. The measuring is done using separate high- and low-energy gamma radiation to determine the belt load height and the ash content. For continuously monitoring the moisture content, transmission units were developed, based on the principle of determining the attenuation and the phase shift of microwaves (Figure 7). Bachmann et al described the operation of these units at Ruhrkohle AG, F.R.G.[120] The previous system is limited to coal sizes and moisture contents where the product stream is or can be conveyed. Laurila[121] discusses the real-time control of fine coal circuits, where coal slurries occur, by on-line measurement of ash and mass balances. The On-Line Slurry Ash Monitor, developed by Process Technology Inc. (PTI), is capable of measuring ash content on a slurry stream containing particles up to 9.5 mm (3/8") dia. The device is used as the basis for a process control strategy which computes on-line ash and mass balances. These balances provide the base data for a process control strategy which adjusts reagent addition to the froth flotation circuit, concentrating table end elevation and tilt in the intermediate coal circuit and media gravity in the coarse coal circuit.

PHOTO : The new SAG mills at Chuquicamata (see p. 249).

PHOTO : Wemco flotation cells.

PHOTO : Fig. 1: Pre-densifier cyclone.

PHOTO : Fig. 2: The Cryofilter -- HGMS high gradient magnetic separator provides a high beneficiation rate for kaolin slurries.

PHOTO : Fig. 3: Schematic of the Jameson cell.

PHOTO : Fig. 4: Simplified flowsheet of Red Dog lead-zinc operation.

PHOTO : Fig. 5: Three-stage two-density Tri-Flo dense medium separator.

PHOTO : Fig. 6: Schematic of the MCF cell bubble generator.

PHOTO : Fig. 7: Arrangement of measuring paths for a combined ash and moisture monitor.

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S. T. Hall, Ph.D., C.Eng., Lecturer in Minerals Engineering, Department of Mining Engineering, University of Nottingham, Nottingham, U.K.
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Publication:Mining Magazine
Date:Jan 1, 1991
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