Mineral Processing Technology PDF (Elsevier, 2006)

Mineral Processing Technology An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery, by Barry A. Wills, Tim Napier-Munn ISBN: 0750644508 Publisher: Elsevier Science & Technology Books Pub. Date: October 2006 Preface to 7th Edition Although mining is a con...

Mineral Processing Technology An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery, by Barry A. Wills, Tim Napier-Munn ISBN: 0750644508 Publisher: Elsevier Science & Technology Books Pub. Date: October 2006 Preface to 7th Edition Although mining is a conservative industry, economic drivers continue to encourage innovation and technological change. In mineral processing, equipment vendors, researchers and the operations them- selves work to develop technologies that are more efficient, of lower cost and more sustainable than their predecessors. The results are apparent in new equipment and new operating practice. Any textbook needs to reflect these changes, and Barry Wills' classic is no exception. It is nearly 30 years since Mineral Processing Technology was first published, and it has become the most widely used English-language textbook of its kind. The sixth edition appeared in 1997 and Barry and his publishers felt that it was again time to bring the text up to date. They approached the Julius Kruttschnitt Mineral Research Centre at the University of Queensland to take on the challenging task. My colleagues and I agreed to do so with some trepidation. The book's well-deserved reputation and utility were at stake, and the magnitude of the task was clear. Revising someone else's text is not an easy thing to do successfully, and there was a real danger of throwing the baby out with the bath water. The value of Mineral Processing Technology lies in its clear exposition of the principles and practice of mineral processing, with examples taken from practice. It has found favour with students of mineral processing, those trained in other disciplines who have converted to mineral processing, and as a reference to current equipment and practice. It was important that its appeal to these different communities be preserved and if possible enhanced. We therefore adopted the following guidelines in revising the book. The 7th edition is indeed a revision, not a complete re-write. This decision was based on the view that "if it ain't broke, don't fix it". Each diagram, flowsheet, reference or passage of text was considered as follows. If it reflected current knowledge and practice, it was left unchanged (or modestly updated where necessary). If it had been entirely superseded, it was removed unless some useful principle or piece of history was being illustrated. Where the introduction of new knowledge or practice was thought to be important to preserve the book's currency, this was done. As a consequence, some chapters remain relatively unscathed whereas others have experienced substantial changes. A particular problem arose with the extensive references to particular machines, concentrators and flow- sheets. Where the point being illustrated remained valid, these were generally retained in the interest of minimising changes to the structure of the book. Where they were clearly out of date in a misleading sense and/or where alternative developments had attained the status of current practice, new material was added. It is perhaps a measure of Barry Wills' original achievement that it has taken more than a dozen people to prepare this latest edition. I would like to acknowledge my gratitude to my colleagues at the JKMRC and elsewhere, listed below, for subscribing their knowledge, experience and valuable time to this good cause; doing so has not been easy. Each chapter was handled by a particular individual with expertise in the topic (several individuals in the case of the larger chapters). I must also thank the editorial staff at Elsevier, especially Miranda Turner and Helen Eaton, for their support and patience, and Barry Wills for his encouragement of the enterprise. My job was to contribute some of the chapters, to restrain some of the more idiosyncratic stylistic extravagancies, and to help make the whole thing happen. To misquote the great comic genius Spike Milligan: the last time I edited a book I swore I would never do another one. This is it. Tim Napier-Munn December 2005 Contributors Chapters 1, 4, 9, 11, 14 Prof. Tim Napier-Munn (JKMRC) Chapters 2 and 16 Dr Glen Corder (JKTech) Chapter 3 Dr Rob Morrison (JKMRC) and Dr Michael Dunglison (JKTech) Chapters 5 and 6 Dr Toni Kojovic (JKMRC) Chapter 7 Dr Frank Shi (JKMRC) Chapter 8 Marko Hilden (JKMRC) and Dean David (GRD Minproc, formerly with JKTech) Chapters 10, 13 and 15 Dr Peter Holtham (JKMRC) Chapter 12 Dr Dan Alexander (JKTech), Dr Emmy Manlapig (JKMRC), Dr Dee Bradshaw (Dept. Chemical Engineering, University of Cape Town) and Dr Greg Harbort (JKTech) Appendix III Dr Michael Dunglison (JKTech) Acknowledgements Secretarial assistance Vynette Holliday and Libby Hill (JKMRC) Other acknowledgements Prof. J-P Franzidis (JKMRC) and Evie Franzidis for their work on an earlier incarnation of this project. Dr Andrew Thornton and Bob Yench of Mipac for help with aspects of process control. The Julius Kruttschnitt Mineral Research Centre, The University of Queensland, for administrative support. The logos of the University and the JKMRC are published by permission of The University of Queensland, the Director, JKMRC.  Table of Contents 1 Introduction 1 2 Ore handling 30 3 Metallurgical accounting, control and simulation 39 4 Particle size analysis 90 5 Comminution 108 6 Crushers 118 7 Grinding mills 146 8 Industrial screening 186 9 Classification 203 10 Gravity concentration 225 11 Dense medium separation (DMS) 246 12 Froth flotation 267 13 Magnetic and electrical separation 353 14 Ore sorting 373 15 Dewatering 378 16 Tailings disposal 400 App. 1 Metallic ore minerals 409 App. 2 Common non-metallic ores 421 App. 3 Excel spreadsheets for formulae in chapter 3 Introduction Minerals and ores lattice. The term "mineral" is often used in a much more extended sense to include anything Minerals of economic value which is extracted from the earth. Thus, coal, chalk, clay, and granite do not The forms in which metals are found in the crust come within the definition of a mineral, although of the earth and as sea-bed deposits depend on details of their production are usually included in their reactivity with their environment, particularly national figures for mineral production. Such mate- with oxygen, sulphur, and carbon dioxide. Gold and rials are, in fact, rocks, which are not homoge- platinum metals are found principally in the native neous in chemical and physical composition, as or metallic form. Silver, copper, and mercury are are minerals, but generally consist of a variety found native as well as in the form of sulphides, of minerals and form large parts of the earth's carbonates, and chlorides. The more reactive metals are always in compound form, such as the oxides crust. For instance, granite, which is one of the and sulphides of iron and the oxides and silicates of most abundant igneous rocks, i.e. a rock formed by aluminium and beryllium. The naturally occurring cooling of molten material, or magma, within the compounds are known as minerals, most of which earth's crust, is composed of three main mineral have been given names according to their composi- constituents, feldspar, quartz, and mica. These tion (e.g. g a l e n a - lead sulphide, PbS; sphalerite - three homogeneous mineral components occur in zinc sulphide, ZnS; cassiterite- tin oxide, SnO2). varying proportions in different parts of the same Minerals by definition are natural inorganic granite mass. substances possessing definite chemical compo- Coals are not minerals in the geological sense, sitions and atomic structures. Some flexibility, but a group of bedded rocks formed by the accu- however, is allowed in this definition. Many mulation of vegetable matter. Most coal-seams minerals exhibit isomorphism, where substitution were formed over 300 million years ago by of atoms within the crystal structure by similar the decomposition of vegetable matter from the atoms takes place without affecting the atomic dense tropical forests which covered certain areas structure. The mineral olivine, for example, has of the earth. During the early formation of the the chemical composition (Mg, Fe)2 SiO4, but the coal-seams, the rotting vegetation formed thick ratio of Mg atoms to Fe atoms varies in different beds of peat, an unconsolidated product of the olivines. The total number of Mg and Fe atoms decomposition of vegetation, found in marshes in all olivines, however, has the same ratio to and bogs. This later became overlain with shales, that of the Si and O atoms. Minerals can also sandstones, mud, and silt, and under the action exhibit polymorphism, different minerals having of the increasing pressure and temperature and the same chemical composition, but markedly time, the peat-beds became altered, or metamor- different physical properties due to a difference in phosed, to produce the sedimentary rock known crystal structure. Thus, the two minerals graphite as coal. The degree of alteration is known as and diamond have exactly the same composi- the rank of the coal, the lowest ranks (lignite or tion, being composed entirely of carbon atoms, brown coal) showing little alteration, while the but have widely different properties due to the highest rank (anthracite) is almost pure graphite arrangement of the carbon atoms within the crystal (carbon). 2 Wills' Mineral Processing Technology Metallic ore processing These large fluctuations in oil prices have had a significant impact on metalliferous ore mining, Metals due to their, influence both on the world economy The enormous growth of industrialisation from the and thus the demand for metals, and directly on the eighteenth century onward led to dramatic increases energy costs of mining and processing. It has been in the annual output of most mineral commodi- estimated that the energy cost in copper produc- ties, particularly metals. Copper output grew by a tion is about 35% of the selling price of the metal factor of 27 in the twentieth century alone, and (Dahlstrom, 1986). aluminium by an astonishing factor of 3800 in the The price of metals is governed mainly by same period. Figure 1.1 shows the world produc- supply and demand. Supply includes both newly tion of aluminium, copper and zinc for the period mined and recycled metal, and recycling is now 1900-2002 (data from USGS, 2005). a significant component of the lifecycle of some All these metals suffered to a greater or m e t a l s - about 60% of lead supply comes from lesser extent when the Organisation of Petroleum recycled sources. There have been many prophets Exporting Countries (OPEC) quadrupled the price of doom over the years pessimistically predicting of oil in 1973-74, ending the great postwar indus- the imminent exhaustion of mineral supplies, the trial boom. The situation worsened in 1979-81, most extreme perhaps being the notorious "Limits when the Iranian revolution and then the Iran-Iraq to Growth" report to the Club of Rome in 1972, war forced the price of oil up from $13 to nearly which forecast that gold would run out in 1981, zinc $40 a barrel, plunging the world into another and in 1990, and oil by 1992 (Meadows et al., 1972). deeper recession, while early in 1986 a glut in the In fact major advances in productivity and tech- world's oil supply cut the price from $26 a barrel nology throughout the twentieth century greatly in December 1985 to below $15 in 1986. Iraq's increased both the resource and the supply of invasion of Kuwait in 1990 pushed the price up newly mined metals, through geological discovery again, from $16 in July to a peak of $42 in October, and reductions in the cost of production. This although by then 20% of the world's energy was actually drove down metal prices in real terms, being provided by natural gas. which reduced the profitability of mining compa- In 1999, overproduction and the Asian economic nies and had a damaging effect on economies crisis depressed oil prices to as low as $10 a barrel heavily dependent on mining, particularly those from where it has climbed steadily to a record in Africa and South America. This in turn drove figure of over $60 a barrel in 2005, driven largely further improvements in productivity and tech- by demand especially from the emerging Asian nology. Clearly mineral resources are finite, but economies, particularly China. supply and demand will generally balance in such Figure 1.1 World production of aluminium, copper and zinc for the period 1900-2002  Introduction 3 a way that if supplies decline or demand increases, Table 1.2 Abundance of metal in the oceans the price will increase, which will motivate the Element Abundance Element Abundance search for new deposits, or technology to render in sea-water in sea-water marginal deposits economic, or even substitution (tonnes) (tonnes) by other materials. Interestingly gold is an exception, its price Magnesium 1015-1016 Vanadium } 109_1010 Silicon 1012-1013 Titanium having not changed much in real terms since Aluminium Cobalt } the sixteenth century, due mainly to its use as Iron 101~ Silver 1012_1013 a monetary instrument and a store of wealth Molybdenum Tungsten (Humphreys, 1999). Zinc Chromium / Estimates of the crustal abundances of metals are Tin Gold (total processing costs deposit. + losses + other costs) per tonne Due to the action of these many natural agencies, mineral deposits are frequently found in sufficient A major cost is mining, and this can vary enor- concentrations to enable the metals to be profitably mously, from only a few pence per tonne of recovered. It is these concentrating agencies and the ore to well over s High-tonnage operations development of demand as a result of research and are cheaper in terms of operating costs but have discovery which convert a mineral deposit into an higher initial capital costs. These capital costs are ore. Most ores are mixtures of extractable minerals paid off over a number of years, so that high- and extraneous rocky material described as gangue. tonnage operations can only be justified for the They are frequently classed according to the nature treatment of deposits large enough to allow this. of the valuable mineral. Thus, in native ores the Small ore bodies are worked on a smaller scale, to metal is present in the elementary form; sulphide reduce overall capital costs, but capital and oper- ores contain the metal as sulphides, and in oxidised ating costs per tonne are correspondingly higher ores the valuable mineral may be present as oxide, (Ottley, 1991). sulphate, silicate, carbonate, or some hydrated form Alluvial mining is the cheapest method and, if of these. Complex ores are those containing prof- on a large scale, can be used to mine ores of very itable amounts of more than one valuable mineral. low contained value due to low grade or low metal Metallic minerals are often found in certain associ- price, or both. For instance, in S.E. Asia, tin ores ations within which they may occur as mixtures of a containing as little as 0.01% Sn are mined by allu- wide range of particle sizes or as single-phase solid vial methods. These ores had a contained value solutions or compounds. Galena and sphalerite, for of less than s but very low processing costs example, associate themselves commonly, as do allowed them to be economically worked. copper sulphide minerals and sphalerite to a lesser High-tonnage open-pit and underground block- extent. Pyrite (FeS2) is very often associated with caving methods are also used to treat ores of low these minerals. contained value, such as low-grade copper ores. Ores are also classified by the nature of their Where the ore must be mined selectively, however, gangues, such as calcareous or basic (lime rich) as is the case with underground vein-type deposits, and siliceous or acidic (silica rich). An ore can be mining methods become very expensive, and can described as an accumulation of mineral in suffi- only be justified on ores of high contained value. cient quantity so as to be capable of economic An underground selective mining cost of s extraction. The minimum metal content (grade) would obviously be hopelessly uneconomic on a required for a deposit to qualify as an ore varies tin ore of alluvial grade, but may be economic on a from metal to metal. Many non-ferrous ores hard-rock ore containing 1.5% tin, with a contained contain, as mined, as little as 1% metal, and often value of around s much less. In order to produce metals, the ore minerals must Gold may be recovered profitably in ores be broken down by the action of heat (pyromet- containing only 1 part per million (ppm) of the allurgy), solvents (hydrometallurgy) or electricity metal, whereas iron ores containing less than (electrometallurgy), either alone or in combination, about 45% metal are regarded as of low grade. the most common method being the pyrometallur- Every tonne of material in the deposit has a gical process of smelting. These chemical methods  Introduction 5 consume vast quantities of energy. Treatment of 1 t their level. For instance, it is necessary to remove of copper ore, for instance, consumes in the region arsenopyrite from tin concentrates, as it is difficult of 1500-2000 k W h of electrical energy, which at to remove the contained arsenic in smelting and the a cost of say 5 p/kW h is around s well above process produces a low-quality tin metal. the contained value of all current copper ores. Against the economic advantages of mineral Smelters are often remote from the mine site, processing, the losses occurred during milling and being centred in areas where energy is relatively the cost of milling operations must be charged. The cheap, and where access to roads, rail or sea-links is latter can vary over a wide range, depending on available for shipment of fuel and supplies to, and the method of treatment used, and especially on products from, the smelter. The cost of transporta- the scale of the operation. As with mining, large- tion of mined ore to remote smelters could in many scale operations have higher capital but lower oper- cases be greater than the contained value of the ore. ating costs (particularly labour and energy) than Mineral processing is usually carried out at the small-scale operations. As labour costs per tonne mine site, the plant being referred to as a mill or are most affected by the size of the operation, so, concentrator. The essential purpose is to reduce the as capacity increases, the energy costs per tonne bulk of the ore which must be transported to and become proportionally more significant, and these processed by the smelter, by using relatively cheap, can be more than 25% of the total milling costs in low-energy physical methods to separate the valu- a 10,000 t d-1 concentrator. able minerals from the waste (gangue) minerals. Losses to tailings are one of the most impor- This enrichment process considerably increases the tant factors in deciding whether a deposit is viable contained value of the ore to allow economic trans- or not. Losses will depend very much on the ore portation and smelting. mineralogy and dissemination, and on the tech- Compared with chemical methods, the phys- nology available to achieve efficient concentration. ical methods used in mineral processing consume Thus, the development of froth flotation allowed relatively small amounts of energy. For instance, the exploitation of vast low-grade copper deposits to upgrade a copper ore from 1 to 25% metal which were previously uneconomic to treat. Simi- would use in the region of 2 0 - 5 0 k W h t -1. The larly, the introduction of solvent extraction enabled corresponding reduction in weight of around 25:1 Nchanga Consolidated Copper Mines in Zambia proportionally lowers transport costs and reduces to treat 9 Mt/yr of flotation tailings, to produce smelter energy consumption to around 60-80 kW h 80,000 t of finished copper from what was previ- in relation to the weight of mined ore. It is impor- ously regarded as waste (Anon., 1979). tant to realise that, although the physical methods In many cases not only is it necessary to sepa- are relatively low energy users, the reduction in rate valuable from gangue minerals, but it is bulk lowers smelter energy consumption to the also required to separate valuable minerals from order of that used in mineral processing, and it is each other. For instance, porphyry copper ores significant that as ore grades decline, the energy are an important source of molybdenum and the used in mineral processing becomes an important minerals of these metals must be separated for factor in deciding whether the deposit is viable to separate smelting. Similarly, complex sulphide ores work or not. containing economic amounts of copper, lead and Mineral processing reduces not only smelter zinc usually require separate concentrates of the energy costs but also smelter metal losses, due to minerals of each of these metals. The provision the production of less metal-bearing slag. Although of clean concentrates, with little or no contamina- technically possible, the smelting of extremely low- tion with associated metals, is not always econom- grade ores, apart from being economically unjusti- ically feasible, and this leads to another source of fiable, would be very difficult due to the need to loss other than direct tailing loss. A metal which produce high-grade metal products free from dele- reports to the "wrong" concentrate may be difficult, terious impurities. These impurities are found in the or economically impossible, to recover, and never gangue minerals and it is the purpose of mineral achieves its potential valuation. Lead, for example, processing to reject them into the discard (tailings), is essentially irrecoverable in copper concentrates as smelters often impose penalties according to and is often penalized as an impurity by the copper 6 Wills' Mineral Processing Technology smelter. The treatment of such polymetallic base 0.01 ppm. Diamond deposits are mined mainly for metal ores, therefore, presents one of the great chal- their gem quality stones which have the highest lenges to the mineral processor. value, with the low-value industrial quality stones Mineral processing operations are often a being essentially a by-product; most industrial compromise between improvements in metallur- diamond is now produced synthetically. gical efficiency and milling costs. This is particu- larly true with ores of low contained value, where Tailings retreatment low milling costs are essential and cheap unit processes are necessary, particularly in the early Mill tailings which still contain valuable compo- stages, where the volume of material treated is rela- nents constitute a potential future resource. New or improved technologies can allow the value tively high. With such low-value ores, improve- contained in tailings, which was lost in earlier ments in metallurgical efficiency by the use of processing, to be recovered, or commodities more expensive methods or reagents cannot always considered waste in the past can become valuable be justified. Conversely high metallurgical effi- in a new economic order. Reducing or eliminating ciency is usually of most importance with ores of tailings dumps or dams by retreating them also high contained value and expensive high-efficiency reduces the environmental impact of the waste. processes can often be justified on these ores. The cost of tailings retreatment is sometimes Apart from processing costs and losses, other lower than that of processing the original ore, costs which must be taken into account are indirect because much of the expense has already been met, costs such as ancillary s e r v i c e s - power supply, particularly in mining and comminution. There are water, roads, tailings disposal- which will depend many tailings retreatment plants in a variety of much on the size and location of the deposit, applications around the world. The East Rand Gold as well as taxes, royalty payments, investment and Uranium Company (ERGO) closed its oper- requirements, research and development, medical ations in 2005 after 28 years of retreating over and safety costs, etc. 870 Mt of the iconic gold dumps of Johannesburg, significantly modifying the skyline of the Golden Non-metallic ores City and producing 250t of gold in the process. Also in 2005 underground mining in Kimberley Ores of economic value can be classed as metallic closed, leaving a tailings dump retreatment opera- or non-metallic, according to the use of the mineral. tion as the only source of diamond production in the Certain minerals may be mined and processed for Diamond City. Some platinum producers in South more than one purpose. In one category the mineral Africa now operate tailings retreatment plants for may be a metal ore, i.e. when it is used to prepare the recovery of platinum group metals (PGMs), and the metal, as when bauxite (hydrated aluminium also chromite as a by-product from the chrome-rich oxide) is used to make aluminium. The alternative UG2 Reef. is for the compound to be classified as a non- Although these products, particularly gold, tend metallic ore, i.e. when bauxite or natural aluminium to dominate the list of tailings retreatment oper- oxide is used to make material for refractory bricks ations because of the value of the product, there or abrasives. are others, both operating and being considered as Many non-metallic ore minerals associate with potential major sources of particular commodities. metallic ore minerals (Appendix II) and are mined For example coal has been recovered from tailings and processed together, e.g. galena, the main source in Australia (Clark, 1997), uranium is recovered of lead, sometimes associates with fluorite (CaF2) from copper tailings by the Uranium Corporation and barytes (BaSO4), both important non-metallic of India, and copper has been recovered from minerals. the Bwana Mkubwa tailings in Zambia, using Diamond ores have the lowest grade of all mined solvent extraction and electrowinning. The Kolwezi ores. The richest mine in terms of diamond content Tailings project in the DRC which proposes to (Argyle in Western Australia) enjoyed grades as recover oxide copper and cobalt from the tailings high as 2 ppm in its early life. The lowest grade of 50 years of copper mining is expected to be the deposits mined in Africa have been as low as largest source of cobalt in the world.  Introduction 7 The re-processing of industrial scrap and some ores. However, in the majority of cases the domestic waste is also a growing economic activity, energy consumed in direct smelting or leaching especially in Europe. It is essentially a branch of low-grade ores would be so enormous as to of mineral processing with a different feedstock, make the cost prohibitive. Compared with these though operation is generally dry rather than wet processes, mineral processing methods are inexpen- (Hoberg, 1993; Furuyama et al., 2003). sive, and their use is readily justified on economic grounds. If the ore contains worthwhile amounts of more Mineral processing methods than one valuable mineral, it is usually the object "As-mined" or "run-of-mine" ore consists of valu- of mineral processing to separate them; similarly able minerals and gangue. Mineral processing, if undesirable minerals, which may interfere with sometimes called ore dressing, mineral dressing or subsequent refining processes, are present, it may milling, follows mining and prepares the ore for be necessary to remove these minerals at the sepa- extraction of the valuable metal in the case of ration stage. metallic ores, and produces a commercial end There are two fundamental operations in mineral product of products such as iron ore and coal. Apart processing: namely the release, or liberation, of from regulating the size of the ore, it is a process the valuable minerals from their waste gangue of physically separating the grains of valuable minerals, and separation of these values from minerals from the gangue minerals, to produce an the gangue, this latter process being known as enriched portion, or concentrate, containing most concentration. of the valuable minerals, and a discard, or tailing, Liberation of the valuable minerals from the containing predominantly the gangue minerals. The gangue is accomplished by comminution, which importance of mineral processing is today taken involves crushing, and, if necessary, grinding, to for granted, but it is interesting to reflect that less such a particle size that the product is a mixture than a century ago, ore concentration was often a of relatively clean particles of mineral and gangue. fairly crude operation, involving relatively simple Grinding is often the greatest energy consumer, gravity and hand-sorting techniques performed by accounting for up to 50% of a concentrator' s energy the mining engineers. The twentieth century saw consumption. As it is this process which achieves the development of mineral processing as a serious liberation of values from gangue, it is also the and important professional discipline in its own process which is essential for efficient separation right, and without physical separation, the concen- of the minerals, and it is often said to be the key tration of many ores, and particularly the metallif- to good mineral processing. In order to produce erous ores, would be hopelessly uneconomic (Wills clean concentrates with little contamination with and Atkinson, 1991). gangue minerals, it is necessary to grind the ore It has been predicted, however, that the impor- finely enough to liberate the associated metals. tance of mineral processing of metallic ores Fine grinding, however, increases energy costs, and may decline as the physical processes utilised can lead to the production of very fine untreat- are replaced by the hydro and pyrometallurgical able "slime" particles which may be lost into the routes used by the extractive metallurgist (Gilchrist, tailings. Grinding therefore becomes a compromise 1989), because higher recoveries are obtained by between clean (high-grade) concentrates, operating some chemical methods. This may certainly apply costs and losses of fine minerals. If the ore is when the useful mineral is very finely dissemi- low grade, and the minerals have very small grain nated in the ore and adequate liberation from the size and are disseminated through the rock, then gangue is not possible, in which case a combina- grinding energy costs and fines losses can be high, tion of chemical and mineral processing techniques unless the nature of the minerals is such that a may be advantageous, as is the case with some pronounced difference in some property between highly complex ores containing economic amounts the minerals and the gangue is available. of copper, lead, zinc and precious metals (Gray, An intimate knowledge of the mineralogical 1984; Barbery, 1986). Also new technologies such assembly of the ore is essential if efficient as direct reduction may allow direct smelting of processing is to be carried out. A knowledge not 8 Wills' Mineral Processing Technology only of the nature of the valuable and gangue liberation. Conventional optical microscopes can minerals but also of the ore "texture" is required. be used for the examination of thin and polished The texture refers to the size, dissemination, sections of mineral samples, and in mineral sands association and shape of the minerals within the applications the simple binocular microscope is a ore. The processing of minerals should always be practical tool. However, it is becoming increas- considered in the context of the mineralogy of ingly common to utilise the new technologies of the ore in order to predict grinding and concen- automated mineral analysis using scanning elec- tration requirements, feasible concentrate grades tron microscopy, such as the Mineral Liberation and potential difficulties of separation (Hausen, Analyser (MLA) (Gu, 2003) and the QEMSCAN 1991; Guerney et al., 2003; Baum et al., 2004). (Gottlieb et al., 2000). Microscopic analysis of concentrate and tailings The most important physical methods which are products can also yield much valuable informa- used to concentrate ores are: tion regarding the efficiency of the liberation and concentration processes (see Figures 1.2a-i for examples). It is particularly useful in trou- (1) Separation based on optical and other proper- bleshooting problems which arise from inadequate ties. This is often called sorting, which used Figure 1.2a Chromite ore. Relatively coarse grain size, and compact morphology of chromite (C) grains makes liberation from olivine (O) gangue fairly straightforward Figure 1.2b North American porphyry copper ore. Chalcopyrite (C) precipitated along fractures in quartz. Liberation of chalcopyrite is fairly difficult due to "chain-like" distribution. Fracture is, however, likely to occur preferentially along the sealed fractures, producing particles with a surface coating of chalcopyrite, which can be effectively recovered into a low-grade concentrate by froth flotation  Introduction 9 Figure 1.2c Mixed sulphide ore, Wheal Jane, Cornwall. Chalcopyrite (C) and sphalerite (S), much of which is extremely finely disseminated in tourmaline (T), making a high degree of liberation impracticable Figure 1.2d Hilton lead-zinc ore body, Australia. Galena (G) and sphalerite (S) intergrown. Separate "clean" concentrates of lead and zinc will be difficult to produce, and contamination of concentrates with other metal is likely Figure 1.2e Copper-zinc ore. Grain of sphalerite with many minute inclusions of chalcopyrite (C) along cleavage planes. Fracturing during comminution takes place preferentially along the low coherence cleavage planes, producing a veneer of chalcopyrite on the sphalerite surface, making depression of the latter difficult in flotation 10 Wills' Mineral Processing Technology Figure 1.2f Lead-zinc ore. Fine grained native silver in vein networks and inclusions in carbonate host rock. Rejection of this material by heavy medium separation could lead to high silver loss Figure 1.2g Flotation tailings, Palabora Copper Mine, South Africa. Finely disseminated grains of chalcopyrite enclosed in a grain of gangue, and irrecoverable by flotation. Maximum grain size of chalcopyrite is about 20 microns, so attempts to liberate by further grinding would be impracticable Figure 1.2h Gravity circuit tailings, tin concentrator. Cassiterite (light grey) locked with gangue (darker grey), mainly quartz. The composite particle is very fine (less than 20 I~m), and has reported to tailings, rather than middlings, due to the inefficiency of gravity separation at this size. Loss of such particles to tailings is a major cause of poor recovery in gravity concentration. In this case, the composite tailings particles could be recovered by froth flotation into a low-grade concentrate  Introduction 11 Figure 1.2i Tin concentrate, assaying about 60% tin. Although there is some limited locking of the cassiterite (light grey) with gangue (darker grey), the main contaminant is arsenopyrite (white), which, being a heavy mineral (S.G. 6), has partitioned with the cassiterite (S.G. 7) into the gravity concentrate. The arsenopyrite particles are essentially liberated, and can easily be removed by froth flotation, thereby increasing the tin grade of the concentrate and avoiding smelter penalties due to high arsenic levels to be done by hand but is now mostly accom- as magnetite (Fe304), while high-intensity plished by machine (see Chapter 14). separators are used to separate paramagnetic (2) Separation based on differences in density minerals from their gangue. Magnetic sepa- between the minerals. Gravity concentra- ration is an important process in the bene- tion, a technology with its roots in antiq- ficiation of iron ores, and finds application uity, is based on the differential movement of in the treatment of paramagnetic non-ferrous mineral particles in water due to their different minerals. It is used to remove paramagnetic hydraulic properties. The method has recently wolframite ((Fe, Mn) WO4) and hematite enjoyed a new lease of life with the develop- (Fe203) from tin ores, and has found consid- ment of a range of enhanced gravity concen- erable application in the processing of non- trating devices. In dense medium separation metallic minerals, such as those found in particles sink or float in a dense liquid or mineral sand deposits. (more usually) an artificial dense suspension; (5) Separation dependent on electrical conduc- it is widely used in coal beneficiation, iron tivity properties. High-tension separation can ore and diamond processing, and in the pre- be used to separate conducting minerals concentration of metalliferous ores. from non-conducting minerals. This method (3) Separation utilising the different surface is interesting, since theoretically it represents properties of the minerals. Froth flotation, the "universal" concentrating method; almost which is one of the most important methods all minerals show some difference in conduc- of concentration, is effected by the attach- tivity and it should be possible to separate ment of the mineral particles to air bubbles almost any two by this process. However, within the agitated pulp. By adjusting the the method has fairly limited application, and "climate" of the pulp by various reagents, its greatest use is in separating some of the it is possible to make the valuable minerals minerals found in heavy sands from beach or air-avid (aerophilic) and the gangue minerals stream placers. Minerals must be completely water-avid (aerophobic). This results in sepa- dry and the humidity of the surrounding air ration by transfer of the valuable minerals to must be regulated, since most of the electron the air bubbles which form the froth floating movement in dielectrics takes place on the on the surface of the pulp. surface and a film of moisture can change (4) Separation dependent on magnetic properties. the behaviour completely. The biggest disad- Low intensity magnetic separators can be used vantage of the method is that the capacity of to concentrate ferromagnetic minerals such economically sized units is low. 12 Wills' Mineral Processing Technology In many cases, a combination of two or more of fine magnetite particles onto the surfaces of non- of the above techniques is necessary to concen- magnetic minerals in the slurry. trate an ore economically. Gravity separation, for Some refractory copper ores containing instance, is often used to reject a major portion sulphide and oxidised minerals have been pre- of the gangue, as it is a relatively cheap process, treated hydrometallurgically to enhance flotation It may not, however, have the selectivity or effi- performance. In the Leach-Precipitation-Flotation ciency to produce the final clean concentrate, process, developed in the years 1929-34 by the Gravity concentrates therefore often need further Miami Copper Co., USA, the oxidised minerals upgrading by more expensive techniques, such as are dissolved in sulphuric acid, after which the froth flotation, copper in solution is precipitated as cement copper Ores which are very difficult to treat (refractory), by the addition of metallic iron. The cement due to fine dissemination of the minerals, complex copper and acid-insoluble sulphide minerals are mineralogy, or both, respond very poorly to the then recovered by froth flotation. This process, with above methods, several variations, has been used at a number of A classic example is the huge zinc-lead-silver American copper concentrators, but a more widely deposit at McArthur River, in Australia. Discov- used method of enhancing the flotation perfor- ered in 1955, it is one of the world's largest mance of oxidised ores is to allow the surface zinc-lead deposits comprising measured, indicated to react with sodium sulphide. This "sulphidisa- and inferred resources totalling 124 Mt with up tion" process modifies the flotation response of to 13% Zn, 6% Pb and 60g/t Ag (in 2003). For the mineral causing it to behave, in effect, as a 35 years it resisted attempts to find an economic pseudo-sulphide. Such chemical conditioning of processing route due to the very fine grained mineral surfaces is widely used in froth flotation texture of the ore. However, the development of (see Chapter 12); sphalerite, for example, can be the proprietary IsaMill fine grinding technology made to respond in a similar way to chalcopy- (Pease, 2005) by the mine's owners Mount Isa rite, by allowing the surface to react with copper Mines (now Xstrata), together with an appro- sulphate. pilate flotation circuit, allowed the ore to be Recent developments in biotechnology are successfully processed and the mine was finally currently being exploited in hydrometallurgical opened in 1995. The concentrator makes a bulk operations, particularly in the bacterial oxidation lead-zinc concentrate with a very fine product of sulphide gold ores and concentrates (Brierley size of 80% smaller than 7 p~m. There are many and Brierley, 2001; Hansford and Vargas, 2001). stages of flotation cleaning to achieve the neces- The bacterium Acidithiobacillus ferrooxidans is sary product grades with sufficient rejection of mainly used to enhance the rate of oxida- silica. McArthur River is a good example of how tion, by breaking down the sulphide lattice and developments in technology can render previously thus liberating the occluded gold for subse- uneconomic ore deposits viable. Process evolution quent removal by cyanide leaching (Lazer et al., for McArthur River continues, with the propri- 1986). There is good evidence to suggest etary Albion atmospheric leaching process being that certain microorganisms could be used to considered for the direct treatment of concentrates enhance the performance of conventional mineral (Anon., 2002). processing techniques (Smith et al., 1991). It has Chemical methods, such as pyrometallurgy or been established that some bacteria will act as hydrometallurgy, can be used to alter mineralogy, pyrite depressants in coal flotation, and prelim- allowing the low cost mineral processing methods inary work has shown that certain organisms to be applied to refractory ores (Iwasaki and can aid flotation in other ways, with potential Prasad, 1989). For instance, non-magnetic iron profound changes to future industrial froth flotation oxides can be roasted in a weakly reducing atmo- practice. sphere to produce ferromagnetic magnetite. It has Extremely fine mineral dissemination leads to also been suggested (Parsonage, 1988) that the high energy costs in comminution and high losses magnetic response could be increased without to tailings due to the generation of difficult- chemically altering the minerals, by the adsorption to-treat fine particles. Much research effort has  Introduction 13 been directed at minimizing fines losses in recent Run-of-mineore years, either by developing methods of enhancing mineral liberation, thus minimizing the amount of comminution needed, or by increasing the efficiency of conventional physical separation i' Comminution I processes, by the use of innovative machines or t , by optimising the performance of existing ones. Several methods have been researched and devel- oped to attempt to increase the apparent size of , t fine particles, by causing them to come together Product and agglomerate. Selective flocculation of certain handling minerals in suspension, followed by separation , , , of the aggregates from the dispersion, has been Figure 1.3 Simple block flowsheet successfully achieved on a variety of ore-types at laboratory scale, but plant application is limited (see Chapter 15). rejection. The next block, "separation", groups Ultra-fine particles in a suspension can be the various treatments incident to production agglomerated under high shear conditions if the of concentrate and tailing. The third, "product particle surfaces are hydrophobic (water-repellent). handling", covers the disposal of the products. A shear field, caused by vigorous agitation, of suffi- The simple line flowsheet (Figure 1.4) is for cient magnitude to overcome the energy barrier most purposes sufficient, and can include details of separating the particles is necessary to bring them machines, settings, rates, etc. together for hydrophobic association. Although the phenomenon of shear flocculation is well known Ore it has not, as yet, been exploited commercially I (Bilgen and Wills, 1991). Selective agglomeration of fine particles by oil (+) l Crushers is a promising method, and has been devel- oped to a commercial scale for the treatment Screens of fine coal (Capes, 1989; Huettenhain, 1991). (-) In the oil agglomeration process an immiscible (+) Gdr ding liquid (e.g. a hydrocarbon) is added to the suspension. On agitation, the oil is distributed Classification over oleophilic/hydrophobic surfaces and particle (-) impact allows inter-particle liquid bridges to j Separation 1 form, causing agglomeration. The oleophilicity of specific minerals can be controlled, for example, Concentrate Tailing by adding froth flotation reagents. As yet, the oil agglomeration process has not been used to treat Figure 1.4 Line flowsheet. (+)indicates oversized material returned for further treatment and (-) ultra-fine minerals outside the laboratory (House undersized material, which is allowed to proceed to and Veal, 1989). the next stage The flowsheet Milling costs The flowsheet shows diagrammatically the sequence of operations in the plant. In its simplest It has been shown that the balance between milling form it can be presented as a block diagram costs and metal losses is crucial, particularly with in which all operations of similar character are low-grade ores, and because of this, most mills keep grouped (Figure 1.3). In this case comminu- detailed accounts of operating and maintenance tion deals with all crushing, grinding and initial costs, broken down into various sub-divisions, such 14 Wills' Mineral Processing Technology as labour, supplies, energy, etc. for the various areas subsequent hydrometallurgical processes, such as of the plant. This type of analysis is very useful in leaching, it may only be necessary to e x p o s e the identifying high-cost areas where improvements in required mineral. performances would be most beneficial. It is impos- In practice, complete liberation is seldom sible to give typical operating costs for milling achieved, even if the ore is ground down to the operations, as these vary enormously from mine grain size of the desired mineral particles. This to mine, and particularly from country to country, is illustrated by Figure 1.5, which shows a lump depending on local costs of energy, labour, water, of ore which has been reduced to a number of supplies, etc., but Table 1.3 is a simplified example cubes of identical volume and of a size below of such a breakdown of costs for a 100,000t/d that of the grains of mineral observed in the copper concentrator. Note the dominance of original ore sample. It can be seen that each grinding, due mainly to power requirements. particle produced containing mineral also contains a portion of gangue; complete liberation has not been attained; the bulk of the major m i n e r a l - the Table 1.3 Costs per metric tonne milled for a gangue - has, however, been liberated from the 100,000 t/d copper concentrator minor m i n e r a l - the value. Item C o s t - US$ Percent cost per tonne Crushing 0.088 2.8 Grinding 1.482 47.0 Flotation 0.510 16.2 Thickening 0.111 3.5 Filtration 0.089 2.8 Tailings 0.161 5.1 Reagents 0.016 0.5 Pipeline 0.045 1.4 Water 0.252 8.0 Figure 1.5 "Locking" of mineral and gangue Laboratory 0.048 1.5 Maintenance support 0.026 0.8 The particles of "locked" mineral and gangue Management support 0.052 1.6 are known as middlings, and further liberation Administration 0.020 0.6 Other expenses 0.254 8.1 from this fraction can only be achieved by further comminution. Total 3.154 100 The "degree of liberation" refers to the percentage of the mineral occurring as free parti- cles in the ore in relation to the total content. This can be high if there are weak boundaries Efficiency of mineral processing between mineral and gangue particles, which is often the case with ores composed mainly of operations rock-forming minerals, particularly sedimentary minerals. Usually, however, the adhesion between Liberation mineral and gangue is strong and, during comminu- One of the major objectives of comminution is tion, the various constituents are cleft across. This the liberation, or release, of the valuable minerals produces much middlings and a low degree of liber- from the associated gangue minerals at the coarsest ation. New approaches to increasing the degree of possible particle size. If such an aim is achieved, liberation involve directing the breaking stresses then not only is energy saved by the reduction of at the mineral crystal boundaries, so that the rock the amount of fines produced, but any subsequent can be broken without breaking the mineral grains separation stages become easier and cheaper to (Wills and Atkinson, 1993). operate. If high-grade solid products are required, Many researchers have tried to quantify degree then good liberation is essential; however, for of liberation with a view to predicting the behaviour  Introduction 15 of particles in a separation process (Barbery, 1991). of valuable mineral, with an accepted degree of The first attempt at the development of a model for locking with the gangue minerals, and a middlings the calculation of liberation was made by Gaudin fraction, which may require further grinding to (1939); King (1982) developed an exact expres- promote optimum release of the minerals. The sion for the fraction of particles of a certain size tailings should be mainly composed of gangue that contained less than a prescribed fraction of minerals. any particular mineral. These models, however, Figure 1.6 is a cross-section through a typical suffered from many unrealistic assumptions that ore particle, and illustrates effectively the libera- must be made with respect to the grain structure tion dilemma often facing the mineral processor. of the minerals in the ore, in particular that libera- Regions A represent valuable mineral, and region tion is preferential, and in 1988 Austin and Luckie AA is rich in valuable mineral but is highly concluded that "there is no adequate model of intergrown with the gangue mineral. Comminu- liberation of binary systems suitable for incorpora- tion produces a range of fragments, ranging from tion into a mill model". For this reason liberation fully liberated mineral and gangue particles, to models have not found much practical application. those illustrated. Particles of type 1 are rich in However, some fresh approaches by Gay, allowing mineral, and are classed as concentrate as they multi-mineral systems to be modelled (not just have an acceptable degree of locking with the binary systems) free of the assumptions of pref- gangue, which limits the concentrate grade. Parti- erential breakage, have recently demonstrated that cles of type 4 would likewise be classed as tail- there may yet be a useful role for such models ings, the small amount of mineral present reducing (Gay, 2004a,b). The quantification of liberation is the recovery of mineral into the concentrate. Parti- now routinely possible using the dedicated scan- cles of types 2 and 3, however, would probably ning electron microscope MLA and QEMSCAN be classed as middlings, although the degree of systems mentioned earlier, and concentrators are regrinding needed to promote economic liberation increasingly using such systems to monitor the of mineral from particle 3 would be greater than in degree of liberation in their processes. particle 2. It should also be noted that a high degree of liberation is not necessary in certain processes, and, indeed, may be undesirable. For instance, it is possible to achieve a high recovery of values by gravity and magnetic separation even though the valuable minerals are completely enclosed by gangue, and hence the degree of liberation of the values is zero. As long as a pronounced density or magnetic susceptibility difference is apparent between the locked particles and the free gangue Figure 1.6 Cross-sections of ore particles particles, the separation is possible. A high degree of liberation may only be possible by intensive fine grinding, which may reduce the particles to such a During the grinding of a low-grade ore the bulk fine size that separation becomes very inefficient. of the gangue minerals is often liberated at a On the other hand, froth flotation requires as much relatively coarse size (see Figure 1.5). In certain of the valuable mineral surface as possible to be circumstances it may be economic to grind to a exposed, whereas in a chemical leaching process, a size much coarser than the optimum in order to portion of the surface must be exposed to provide produce in the subsequent concentration process a a channel to the bulk of the mineral. large middlings fraction and a tailings which can In practice, ores are ground to an optimum grind be discarded at a coarse grain size. The middlings size, determined by laboratory and pilot scale test- fraction can then be reground to produce a feed to work, to produce an economic degree of libera- the final concentration process (Figure 1.7). tion. The concentration process is then designed This method discards most of the coarse gangue to produce a concentrate consisting predominantly early in the process, thus considerably reducing 16 Wills' Mineral ProcessingTechnology Feed Such separations are, of course, never perfect, 1 Primarygrind so that much of the middlings produced are, in fact, misplaced particles, i.e. those particles which ideally should have reported to the concentrate or the tailings. This is often particularly serious when Pre-con!entration I treating ultra-fine particles, where the efficiency of I 1 separation is usually low. In such cases, fine liber- Middlings Tailings ated valuable mineral particles often report in the middlings and tailings. The technology for treating fine-sized minerals is, as yet, poorly developed, Se~ila~dti~nd and, in some cases, very large amounts of fines are 9' discarded. For instance, it is common practice to Concentrate Middlings Taili~ngs remove material less than 10txm in size from tin concentrator feeds and direct this material to the tailings, and, in the early 1970s, 50% of the tin mined in Bolivia, 30% of the phosphate mined in Figure 1.7 Flowsheetfor process utilising two-stage Florida, and 20% of the world's tungsten were lost separation as fines. Significant amounts of copper, uranium, grinding costs, as needless comminution of liber- fluorspar, bauxite, zinc, and iron were also simi- ated gangue is avoided. It is often used on minerals larly lost (Somasundaran, 1986). which can easily be separated from the free gangue, Figure 1.8 shows the general size range applica- even though they are themselves locked to some bility of unit concentration processes (Mills, 1978). extent with gangue. It is the basis of the dense It is evident that most mineral processing techniques medium process of preconcentration (Chapter 11). fail in the ultra-fine size range. Gravity concentra- tion techniques, especially, become unacceptably inefficient. Flotation, one of the most important of the Concentration concentrating techniques, is now practised success- The object of mineral processing, regardless of the fully below 10 txm but not below 1 p~m methods used, is always the same, i.e. to separate It should be pointed out that the process is also the minerals into two or more products with the limited by the mineralogical nature of the ore. values in the concentrates, the gangue in the tail- For example, in an ore containing native copper it ings, and the "locked" particles in the middlings. is theoretically possible to produce a concentrate Figure 1.8 Effectiverange of application of conventional mineral processingtechniques  Introduction 17 containing 100% Cu, but, if the ore mineral was a lead ore, but the recovery would be very low. On chalcopyrite (CuFeS2), the best concentrate would the other hand, a concentrating process might show contain only 34.5% Cu. a recovery of 99% of the metal, but it might also The recovery, in the case of the concentration put 60% of the gangue minerals in the concentrate. of a metallic ore, is the percentage of the total It is, of course, possible to obtain 100% recovery metal contained in the ore that is recovered from by not concentrating the ore at all. the concentrate; a recovery of 90% means that 90% There is an approximately inverse relationship of the metal in the ore is recovered in the concen- between the recovery and grade of concentrate in trate and 10% is lost in the tailings. The recovery, all concentrating processes. If an attempt is made when dealing with non-metallic ores, refers to the to attain a very high-grade concentrate, the tailings percentage of the total mineral contained in the ore assays are higher and the recovery is low. If high that is recovered into the concentrate, i.e. recovery recovery of metal is aimed for, there will be more is usually expressed in terms of the valuable end gangue in the concentrate and the grade of concen- product. trate and ratio of concentration will both decrease. The ratio of concentration is the ratio of the It is impossible to give figures for representative weight of the feed (or heads) to the weight of the values of recoveries and ratios of concentration. A concentrates. It is a measure of the efficiency of concentration ratio of 2 to 1 might be satisfactory the concentration process, and it is closely related for certain high-grade non-metallic ores, but a ratio to the grade or assay of the concentrate; the value of 50 to 1 might be considered too low for a low- of the ratio of concentration will generally increase grade copper ore; ratios of concentration of several with the grade of concentrate. million to one are common with diamond ores. The The grade, or assay, usually refers to the content aim of milling operations is to maintain the values of the marketable end product in the material. Thus, of ratio of concentration and recovery as high as in metallic ores, the per cent metal is often quoted, possible, all factors being considered. although in the case of very low-grade ores, such Since concentrate grade and recovery are metal- as gold, metal content may be expressed as parts lurgical factors, the metallurgical efficiency of any per million (ppm), or its equivalent grams per tonne concentration operation can be expressed by a curve (gt-1). Some metals are sold in oxide form, and showing the recovery attainable for any value of hence the grade may be quoted in terms of the concentrate grade. Figure 1.9 is a typical recovery- marketable oxide content, e.g. %WO 3, %U308, etc. grade curve showing the characteristic inverse In non-metallic operations, grade usually refers to relationship between recovery and concentrate the mineral content, e.g. %CaF 2 in fluorite ores; grade. Mineral processes generally move along diamond ores are usually graded in carats per 100 a recovery-grade curve, with a trade-off between tonnes (t), where 1 carat is 0.2 g. Coal is graded grade and recovery. The mineral processor's chal- according to its ash content, i.e. the amount of lenge is to move the whole curve to a higher point incombustible mineral present within the coal. Most so that both grade and recovery are maximised. coal burned in power stations ("steaming coal") has an ash content between 15 and 20%, whereas "coking coal" used in steel making generally has an ash content of less than 10% together with appro- priate coking properties. The enrichment ratio is the ratio of the grade of the concentrate to the grade of the feed, and again 0 is related to the efficiency of the process. r Ratio of concentration and recovery are essen- n" tially independent of each other, and in order to evaluate a given operation it is necessary to know both. For example, it is possible to obtain a very Grade of concentrate high grade of concentrate and ratio of concentration by simply picking a few lumps of pure galena from Figure 1.9 Typical recovery-grade curve 18 Wills' Mineral Processing Technology Concentrate grade and recovery, used simulta- Example 1.1 neously, are the most widely accepted measures A tin concentrator treats a feed containing 1% of assessing metallurgical (not economic) perfor- tin, and three possible combinations of concentrate mance. However, there is a problem in quanti- grade and recovery are: tatively assessing the technical performance of a concentration process whenever the results of two High grade 63% tin at 62% recovery similar test runs are compared. If both the grade Medium grade 42% tin at 72% recovery and recovery are greater for one case than the Low grade 21% tin at 78% recovery other, then the choice of process is simple, but if the results of one test show a higher grade but a Determine which of these combinations of grade lower recovery than the other, then the choice is and recovery produce the highest separation effi- no longer obvious. There have been many attempts ciency. to combine recovery and concentrate grade into a single index defining the metallurgical efficiency of the separation. These have been reviewed by Schulz Solution (1970), who proposed the following definition: Assuming that the tin is totally contained in Separation efficiency (S.E.) - Rm - Rg (1.1) the mineral cassiterite (SnO2), which, when pure, contains 78.6% tin, and since mineral recovery (Equation 1.2) is 100 C x concentrate grade/feed where R m - % recovery of the valuable mineral, grade, for the high-grade concentrate: R g - % recovery of the gangue into the concentrate. 100 62-Cx63x--, and s o C = 9. 8 4 1 x 1 0 -3 1 Suppose the feed material, assaying f % metal, Therefore, separates into a concentrate assaying c% metal, and a tailing assaying t% metal, and that C is the 0.984 x 78.6 x ( 6 3 - 1) (S.E.) (Equation 1. 3 ) - fraction of the total feed weight that reports to the ( 7 8. 6 - 1) 1 concentrate, then: =61.8% 100Cc Rm -- ~ (1.2) Similarly, for the medium-grade concentrate, from f Equation 1.2: i.e. recovery of valuable mineral to the concentrate is equal to metal recovery, assuming that all the 7 2 = 100 C x 42/1 valuable metal is contained in the same mineral. Therefore, The gangue content of the concentrate- 1 0 0 - (lOOc/m)%, where m is the percentage metal C = 1.714 10 -2, content of the valuable mineral, and S.E. (Equation 1.3) -- 71.2%. 1 0 0 ( m - c) i.e. gangue content -- For the low-grade concentrate, from Equa- m tion 1.2: Therefore, Rg = C x gangue content of concen- trate/gangue content of feed: 7 8 - 100 x C x 21/1 100C(m - c) Therefore, C - 3.714 x 10 -2, and S.E. (Equa- (m-f) tion 1.3) - 75.2%. Therefore, Rm - Rg = 100Ccf { l O O C ( mf -)c ) Therefore, the highest separation efficiency is achieved by the production of a low-grade (21% tin) concentrate at high (78%) recovery. lOOCm(c- f) Although the value of separation efficiency can (1.3) (m--f)f be useful in comparing the performance of different  Introduction 19 operating conditions on selectivity, it takes no as possible to this target grade. Although the effect account of economic factors, and, as will become of moving slightly away from the optimum may apparent, a high value of separation efficiency does only be of the order of a few pence per tonnes not necessarily lead to the most economic return. treated, this can amount to very large financial Since the purpose of mineral processing is to losses, particularly on high-capacity plants treating increase the economic value of the ore, the impor- thousands of tonnes per day. Changes in metal tance of the recovery-grade relationship is in deter- price, smelter terms, etc. obviously affect the NSR- mining the most e c o n o m i c combination of recovery concentrate grade curve, and the value of the and grade which will produce the greatest financial optimum concentrate grade. For instance, if the return per tonne of ore treated in the plant. This will metal price increases, then the optimum grade will depend primarily on the current price of the valu- be lower, allowing higher recoveries to be attained able product, transportation costs to the smelter, (Figure 1.11). refinery, or other further treatment plant, and the cost of such further treatment, the latter being very dependent on the grade of concentrate supplied. A High high grade concentrate will incur lower smelting P ~ s ~ ~ ~ Low costs, but the lower recovery means lower returns of final product. A low grade concentrate may achieve greater recovery of the values, but incur greater smelting and transportation costs due to the included gangue minerals. Also of importance are impurities in the concentrate which may be penal- ized by the smelter, although precious metals may Concentrategrade produce a bonus. The net return from the smelter (NSR) can Figure 1.11 Effect of metal price on NSR-grade be calculated for any recovery-grade combina- relationship tion from: NSR = Payment for contained metal It is evident that the terms agreed between the - (Smelter charges + Transport costs) concentrator and smelter are of paramount impor- tance in the economics of mining and milling oper- This is summarised in Figure 1.10, which shows ations. Such smelter contracts are usually fairly that the highest value of NSR is produced at an complex. Concentrates are sold under contract optimum concentrate grade. It is essential that the to "custom smelters" at prices based on quota- mill achieves a concentrate grade which is as close tions on metal markets such as the London Metal Exchange (LME). The smelter, having processed the concentrates, disposes of the finished metal to the consumers. The proportion of the "free market" s " Valuation price of the metal received by the mine is deter- mined by the terms of the contract negotiated between mine and smelter, and these terms can ~NSR vary considerably. Table 1.4 summarises a typical Target low-grade smelter contract for the purchase of tin concentrates. As is usual in many contracts, one assay unit is deducted from the concentrate assay in ~._ ~ m e n t assessing the value of the concentrates, and arsenic ' Transport present in the concentrate is penalised. The concen- trate assay is of prime importance in determining Concentrategrade the valuation, and the value of the assay is usually Figure 1.10 Variation of payments and charges with agreed on the result of independent sampling and concentrate grade assaying performed by the mine and smelter. The 20 Wills' Mineral Processing Technology Table 1.4 Simplified tin smelter contract Material Tin concentrates, assaying no less than 15% Sn, to be free from deleterious impurities not stated, and to contain sufficient moisture as to evolve no dust when unloaded at our works. Quantity Total production of concentrates. Valuation Tin, less 1 unit per dry tonne of concentrates, at the lowest of the official London Metal Exchange prices. Pricing On the 7th market day after completion of arrival of each sampling lot into our works. Treatment charge s per dry tonne of concentrates. Moisture s per tonne of moisture. Penalties Arsenic s per unit per tonne. Lot charge s per lot sampled of less than 17 tonnes. Delivery Free to our works in regular quantities, loose on a tipping lorry or in any other manner acceptable to both parties. assays are compared, and if the difference is no of the century, only nine mines of any conse- more than an agreed value, the mean of the two quence remained in Britain, where 300 had flour- results may be taken as the agreed assay. In the ished 30 years earlier. From alluvial or secondary case of a greater difference, an "umpire" sample is deposits, principally from South-East Asia, comes assayed at an independent laboratory. This umpire 80% of mined tin. assay may be used as the agreed assay, or the mean Unlike copper, zinc and lead, production of tin of this assay and that of the party which is nearer has not risen dramatically over the years and has to the umpire assay may be chosen. rarely exceeded 250,000 t/yr. The use of smelter contracts, and the impor- The real price of tin spent most of the first tance of the by-products and changing metal prices, half of the twentieth century in a relatively narrow can be seen by briefly examining the economics band between US$10 and US$15/t (19985), with of processing two base metals - tin and c o p p e r - some excursions (Figure 1.12; USGS, 2005). From whose fortunes have fluctuated over the years for 1956 its price was regulated by a series of markedly different reasons. international agreements between producers and consumers under the auspices of the International Tin Council (ITC), which mirrored the highly Economics of tin processing successful policy of De Beers in controlling the gem diamond trade. Price stability was sought Tin constitutes an interesting case study in the through selling from the ITC's huge stockpiles vagaries of commodity prices and how they impact when the price rose and buying into the stockpile on the mineral industry and its technologies. when the price fell. Almost a half of the world's supply of tin in the From the mid-1970s, however, the price of mid-nineteenth century was mined in south-west tin was driven artificially higher at a time of England, but by the end of the 1870s Britain's world recession, expanding production and falling premium position was lost, with the emergence consumption, the latter due mainly to the increasing of Malaysia as the leading producer and the use of aluminium, rather than tin-plated steel, discovery of rich deposits in Australia. By the end cans. Although the ITC imposed restrictions on  Introduction 21 Figure 1.12 Tin prices 1900-2002 the amount of tin that could be produced by its tively low. Production of high-grade concentrates member countries, the reason for the inflating tin also incurs relatively low freight charges, which is price was that the price of tin was fixed by the important if the smelter is remote. Malaysian dollar, while the buffer stock manager's For these reasons it has been traditional in the dealings on the LME were financed in sterling. The past for hard-rock, lode tin concentrators to produce Malaysian dollar was tied to the American dollar, high-grade concentrates, but high tin prices and which strengthened markedly between 1982 and the development of profitable low-grade smelting 1984, having the effect of increasing the price of processes changed the policy of many mines tin in London simply because of the exchange rate. towards the production of lower-grade concen- However, the American dollar began to weaken trates. The advantage of this is that the recovery of in early 1985, taking the Malaysian dollar with it, and effectively reducing the LME tin price from tin into the concentrate is increased, thus increasing its historic peak. In October 1985, the buffer stock smelter payments. However, the treatment of low- manager announced that the ITC could no longer grade concentrates produces much greater prob- finance the purchase of tin to prop up the price, as lems for the smelter, and hence the treatment it had run out of funds, owing millions of pounds to charges at "low-grade smelters" are normally much the LME traders. This announcement caused near higher than those at the high-grade smelters. Freight panic, the tin price fell to s and the LME charges are also correspondingly higher. halted all further dealings. In 1986 many of the Suppose that a tin concentrator treats a feed world's tin mines were forced to close down due containing 1% tin, and that three possible combi- to the depressed tin price, and prices continued nations of concentrate grade and recovery are (as to fall in subsequent years. The following discus- in Example 1.1): sion therefore relates to tin processing prior to the collapse, including prices and costs. The same prin- High grade 63% tin at 62% recovery ciples can be applied to the prices and costs of any Medium grade 42% tin at 72% recovery particular period including the present day. Low grade 21% tin at 78% recovery It is fairly easy to produce concentrates containing over 70% tin (i.e. over 90% cassiterite) Assuming that the concentrates are free of from alluvial ores, such as those worked in South- arsenic, and that the cost of transportation to East Asia. Such concentrates present little problem the smelter is s of dry concentrate, then the in smelting and hence treatment charges are rela- return on each tonne of ore treated can be simply 22 Wills' Mineral Processing Technology calculated, using the low-grade smelter terms set does a smelter share the risks of changing metal out in the contract in Table 1.4. price, as it performs a service role, changes in For instance, at a grade of 42% tin and 72% smelter terms being made more on the basis of recovery, the weight of concentrate produced from changing smelter costs rather than metal price. The 1 t of ore is 17.14 kg (from Example 1.1). mine does, however, reap the benefits of increasing The smelter payment for tin in this concentrate is metal price. At a tin price of s the NSR per tonne P x 17.14 x ( 4 2 - 1)/100,000 of ore from the low-grade smelter treating the where P = tin price in s 42% tin concentrate is s while the return Assuming a tin price of s then the net from the high-grade smelter, treating a 63% Sn smelter payment is s concentrate, is s Although this is a differ- The smelter treatment charge is s x concen- ence of only 21/t of ore, to a small 500td -~ trate weight = s and the transportation cost is tin concentrator this change in policy from rela- s tively low- to high-grade concentrate, together The net smelter return for the processing of 1 t of with the subsequent change in concentrate market, concentrator feed is thus s - (6.59 + 0.34) = would expect to increase the revenue by s x s Therefore, although the ore contains, at free 500 x 365 = s per annum. The concentrator market price, s worth of tin per tonne, the mine management must always be prepared to change its realises only 62% of the ore value in payments policies, both metallurgical and marketing, in this received. way if maximum returns are to be made, although Production of a lower-grade concentrate incurs production of a reliable grade-recovery relationship higher smelter and freight charges, but increases is often difficult due to the complexity of opera- the payment for contained metal, due to the higher tion of lode tin concentrators and variations in feed recovery. Similar calculations show that at a grade characteristics. of 21% tin and 78% recovery, the payment for tin It is, of course, necessary to deduct the costs is increased to s but the total deductions also of mining and processing from the NSR in order increase to s producing a net smelter return to deduce the profit achieved by the mine. Some of s of ore treated. of these costs will be indirect, such as salaries, Clearly, lowering the concentrate grade to 21% administration, research and development, medical tin, in order to increase recovery, has increased and safety, as well as direct costs, such as oper- the separating efficiency (Example 1.1), but has ating and maintenance, supplies and energy. The adversely affected the economic return from the breakdown of milling costs varies enormously from smelter, the increased charges being more impor- mine to mine, depending very much on the size and tant than the increase in revenue from the metal. complexity of the operations. Mines with very large Increasing the grade to 63% tin can obviously reduce charges even further, particularly if the ore reserves tend to have very high throughputs, concentrate can be sent to a higher-grade smelter and so although the capital outlay is higher, the with lower treatment charges. operating and labour costs tend to be much lower Assuming a treatment charge of s of concen- than those on smaller plants, such as those treating trate, and identical payments and freight charges, lode tin ores. Mining costs also vary enormously, the payment for metal in such a concentrate would and are very much higher for underground than for be only s but the charges are reduced to open-pit operations. s The NSR per tonne of ore treated is thus If mining and milling costs of s and s respec- s In this case, therefore, the return is highest tively per tonne of ore are typical of underground from the low-grade smelter treating a medium- tin operations, then it can be seen that at a tin grade concentrate. This situation may change, price of s the mine, producing a concentrate however, if the metal price changes markedly. If of 42% tin, which is sold to a low-grade smelter, the tin price falls and the terms of the smelter makes a profit of s 48---s of ore, contracts remain the same, then the mine profits which at a throughput of 500t d -1 corresponds to will suffer due to the reduction in payments. Rarely a gross annual profit of s It is also clear  Introduction 23 It of mined ore (1% Sn) Contained value = s (fmp) I I Cost s I I r oss,~ I Cost s Concentrate (72 % recoven]) Contained value s (fmp) I Tailing Contained value s (fmp) !, Transport & smelting I I i I I Figure 1.13 Breakdown of costs and revenues for treatment of lode tin ore (fmp = free market price) that if the tin price falls to s the mine loses Bolivia, had the highest production costs, being s 38.96 = s for every tonne of ore treated. above s in 1985. Alluvial operations, such The breakdown of revenue and costs at a tin as those in Malaysia, Thailand and Indonesia, have price of s is summarised in Figure 1.13. lower production costs (around s in 1985). The mine profit per tonne of ore treated can be Although these ores have much lower contained summarised as: values (only about s - 2/t), mining and processing Contained value of o r e - (costs + losses) costs, particularly on the large dredging operations, are extremely low, as are smelting costs and losses, = s - (40 + 8 + 23.80 + 8.40)) - 4.80/t due to the high concentrate grades and recoveries Since 1 t of ore produces 0.0072t of tin in produced. In 1985 the alluvial mines in Brazil concentrates, and the free market value of this produced the world's cheapest tin, having produc- contained metal is s the total effecti