Wills' Mineral Processing Technology: An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery

By Barry A. Wills

Wills' Mineral Processing Technology provides practising engineers and students of mineral processing, metallurgy and mining with a review of all of the common ore-processing techniques utilized in modern processing installations. Now in its Seventh Edition, this renowned book is a standard reference for the mineral processing industry. Chapters deal with each of the major processing techniques, and coverage includes the latest technical developments in the processing of increasingly complex refractory ores, new equipment and process routes. This new edition has been prepared by the prestigious J K Minerals Research Centre of Australia, which contributes its world-class expertise and ensures that this will continue to be the book of choice for professionals and students in this field.This latest edition highlights the developments and the challenges facing the mineral processor, particularly with regard to the environmental problems posed in improving the efficiency of the existing processes and also in dealing with the waste created. The work is fully indexed and referenced.

• The classic mineral processing text, revised and updated by a prestigious new team
• Provides a clear exposition of the principles and practice of mineral processing, with examples taken from practice
• Covers the latest technological developments and highlights the challenges facing the mineral processor
• New sections on environmental problems, improving the efficiency of existing processes and dealing with waste.

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 18, 2011
• ISBN: 9780080479477

Book preview

Wills’ Mineral Processing Technology

An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery

Seventh Edition

Barry A. Wills

Staff of the Julius Kruttschnitt Mineral Research Centre The University of Queensland, Australia

Editor

Tim Napier-Munn

Butterworth-Heinemann

Table of Contents

Cover image

Title page

Preface to 7th Edition

Contributors

Acknowledgements

Chapter 1: Introduction

Publisher Summary

Minerals and ores

Mineral processing methods

Efficiency of mineral processing operations

Concentration

Economics of tin processing

Economics of copper processing

Economic efficiency

Chapter 2: Ore handling

Publisher Summary

Introduction

The removal of harmful materials

Ore transportation

Ore storage

Feeding

Chapter 3: Metallurgical accounting, control and simulation

Publisher Summary

Introduction

Sampling and weighing the ore

Slurry streams

Automatic control in mineral processing

Neural networks

Circuit design and optimisation by computer simulation

Mass balancing methods

Introduction to mass balances on complex circuits

Design of experiments and plant trials

Chapter 4: Particle size analysis

Publisher Summary

Introduction

Particle size and shape

Sieve analysis

Sub-sieve techniques

Chapter 5: Comminution

Publisher Summary

Introduction

Principles of comminution

Comminution theory

Grindability

Simulation of comminution processes and circuits

Chapter 6: Crushers

Publisher Summary

Introduction

Primary crushers

Secondary crushers

Crushing circuits and control

Chapter 7: Grinding mills

Publisher Summary

Introduction

The motion of the charge in a tumbling mill

Tumbling mills

Grinding circuits

Chapter 8: Industrial screening

Publisher Summary

Introduction

Performance of screens

Factors affecting screen performance

Screen types

Chapter 9: Classification

Publisher Summary

Introduction

Principles of classification

Types of classifier

Chapter 10: Gravity concentration

Publisher Summary

Introduction

Principles of gravity concentration

Gravity separators

Jigs

Pinched sluices and cones

Spirals

Shaking tables

Pneumatic tables

Duplex concentrator

Mozley Laboratory Separator

Centrifugal concentrators

Gold ore concentrators

Chapter 11: Dense medium separation (DMS)

Publisher Summary

Introduction

The dense medium

Centrifugal separators

DMS circuits

Typical dense medium separations

Laboratory heavy liquid tests

Efficiency of dense medium separation

Construction of partition curves

Organic efficiency

Chapter 12: Froth flotation

Publisher Summary

Introduction

Principles of flotation

Classification of minerals

Collectors

Frothers

Regulators

The importance of pH

The importance of pulp potential

The role of bubble generation and froth performance

Entrainment

The engineering of flotation

Electroflotation

Agglomeration-skin flotation

Flotation plant practice

Reagents and conditioning

Control of flotation plants

Typical flotation separations

Flotation of copper ores

Oxidised copper ores

Chapter 13: Magnetic and electrical separation

Publisher Summary

Introduction

Magnetic separation

Chapter 14: Ore sorting

Publisher Summary

Introduction

Electronic sorting principles

Examples

Chapter 15: Dewatering

Publisher Summary

Introduction

Sedimentation

Chemical formula

Filtration

Drying

Chapter 16: Tailings disposal

Publisher Summary

Introduction

Methods of disposal of tailings

Appendix I: Metallic ore minerals

Appendix II: Common non-metallic ores

Appendix III: Excel Spreadsheets for formulae in chapter 3

Index

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 themselves 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 flowsheets. 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 , DrMichael 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), , DrEmmy Manlapig (JKMRC) and , DrDee Bradshaw,     (Dept. Chemical Engineering, University of Cape Town)

Dr Greg Harbort (JKTech)

Appendix III

Dr Michael Dunglison (JKTech)

Acknowledgements

1

Introduction

Publisher Summary

This chapter discusses the minerals and ores, metallic ore processing, mineral processing methods, and efficiency and economy of mineral processing operations. Minerals by definition are natural inorganic substances possessing definite chemical compositions and atomic structures. Gold and platinum metals are found principally in the native or metallic form. Silver, copper, and mercury are found in the native form as well as in the form of sulphides, carbonates, and chlorides. The more reactive metals are always in compound form such as the oxides and sulphides of iron and the oxides and silicates of aluminium and beryllium. In the economics of ore processing, a major cost is mining, and this can vary enormously. High-tonnage operations are cheaper in terms of operating costs but have higher initial capital costs. Alluvial mining is the cheapest method and, if on a large scale, can be used to mine ores of very low contained value because of low grade or low metal price or both. Mineral processing—sometimes called ore dressing, mineral dressing, or milling—follows mining, and it prepares the ore for extraction of the valuable metal in the case of metallic ores and produces a commercial end product from products such as iron ore and coal. It is also a process of physically separating the grains of valuable minerals from the gangue minerals, to produce an enriched portion, or concentrate, containing most of the valuable minerals, and a discard, or tailing, containing predominantly the gangue minerals.

Minerals and ores

Minerals

The forms in which metals are found in the crust of the earth and as sea-bed deposits depend on their reactivity with their environment, particularly with oxygen, sulphur, and carbon dioxide. Gold and platinum metals are found principally in the native or metallic form. Silver, copper, and mercury are found native as well as in the form of sulphides, carbonates, and chlorides. The more reactive metals are always in compound form, such as the oxides and sulphides of iron and the oxides and silicates of aluminium and beryllium. The naturally occurring compounds are known as minerals, most of which have been given names according to their composition (e.g. galena – lead sulphide, PbS; sphalerite – zinc sulphide, ZnS; cassiterite – tin oxide, SnO2).

Minerals by definition are natural inorganic substances possessing definite chemical compositions and atomic structures. Some flexibility, however, is allowed in this definition. Many minerals exhibit isomorphism, where substitution of atoms within the crystal structure by similar atoms takes place without affecting the atomic structure. The mineral olivine, for example, has the chemical composition (Mg, Fe)2 SiO4, but the ratio of Mg atoms to Fe atoms varies in different olivines. The total number of Mg and Fe atoms in all olivines, however, has the same ratio to that of the Si and O atoms. Minerals can also exhibit polymorphism, different minerals having the same chemical composition, but markedly different physical properties due to a difference in crystal structure. Thus, the two minerals graphite and diamond have exactly the same composition, being composed entirely of carbon atoms, but have widely different properties due to the arrangement of the carbon atoms within the crystal lattice. The term mineral is often used in a much more extended sense to include anything of economic value which is extracted from the earth. Thus, coal, chalk, clay, and granite do not come within the definition of a mineral, although details of their production are usually included in national figures for mineral production. Such materials are, in fact, rocks, which are not homogeneous in chemical and physical composition, as are minerals, but generally consist of a variety of minerals and form large parts of the earth’s crust. For instance, granite, which is one of the most abundant igneous rocks, i.e. a rock formed by cooling of molten material, or magma, within the earth’s crust, is composed of three main mineral constituents, feldspar, quartz, and mica. These three homogeneous mineral components occur in varying proportions in different parts of the same granite mass.

Coals are not minerals in the geological sense, but a group of bedded rocks formed by the accumulation of vegetable matter. Most coal-seams were formed over 300 million years ago by the decomposition of vegetable matter from the dense tropical forests which covered certain areas of the earth. During the early formation of the coal-seams, the rotting vegetation formed thick beds of peat, an unconsolidated product of the decomposition of vegetation, found in marshes and bogs. This later became overlain with shales, sandstones, mud, and silt, and under the action of the increasing pressure and temperature and time, the peat-beds became altered, or metamorphosed, to produce the sedimentary rock known as coal. The degree of alteration is known as the rank of the coal, the lowest ranks (lignite or brown coal) showing little alteration, while the highest rank (anthracite) is almost pure graphite (carbon).

Metallic ore processing

Metals

The enormous growth of industrialisation from the eighteenth century onward led to dramatic increases in the annual output of most mineral commodities, particularly metals. Copper output grew by a factor of 27 in the twentieth century alone, and aluminium by an astonishing factor of 3800 in the same period. Figure 1.1 shows the world production of aluminium, copper and zinc for the period 1900–2002 (data from USGS, 2005).

Figure 1.1 World production of aluminium, copper and zinc for the period 1900–2002

All these metals suffered to a greater or lesser extent when the Organisation of Petroleum Exporting Countries (OPEC) quadrupled the price of oil in 1973–74, ending the great postwar industrial boom. The situation worsened in 1979–81, when the Iranian revolution and then the Iran–Iraq war forced the price of oil up from $13 to nearly $40 a barrel, plunging the world into another and deeper recession, while early in 1986 a glut in the world’s oil supply cut the price from $26 a barrel in December 1985 to below $15 in 1986. Iraq’s invasion of Kuwait in 1990 pushed the price up again, from $16 in July to a peak of $42 in October, although by then 20% of the world’s energy was being provided by natural gas.

In 1999, overproduction and the Asian economic crisis depressed oil prices to as low as $10 a barrel from where it has climbed steadily to a record figure of over $60 a barrel in 2005, driven largely by demand especially from the emerging Asian economies, particularly China.

These large fluctuations in oil prices have had a significant impact on metalliferous ore mining, due to their influence both on the world economy and thus the demand for metals, and directly on the energy costs of mining and processing. It has been estimated that the energy cost in copper production is about 35% of the selling price of the metal (Dahlstrom, 1986).

The price of metals is governed mainly by supply and demand. Supply includes both newly mined and recycled metal, and recycling is now a significant component of the lifecycle of some metals – about 60% of lead supply comes from recycled sources. There have been many prophets of doom over the years pessimistically predicting the imminent exhaustion of mineral supplies, the most extreme perhaps being the notorious Limits to Growth report to the Club of Rome in 1972, which forecast that gold would run out in 1981, zinc in 1990, and oil by 1992 (Meadows et al., 1972).

In fact major advances in productivity and technology throughout the twentieth century greatly increased both the resource and the supply of newly mined metals, through geological discovery and reductions in the cost of production. This actually drove down metal prices in real terms, which reduced the profitability of mining companies and had a damaging effect on economies heavily dependent on mining, particularly those in Africa and South America. This in turn drove further improvements in productivity and technology. Clearly mineral resources are finite, but supply and demand will generally balance in such a way that if supplies decline or demand increases, the price will increase, which will motivate the search for new deposits, or technology to render marginal deposits economic, or even substitution by other materials.

Interestingly gold is an exception, its price having not changed much in real terms since the sixteenth century, due mainly to its use as a monetary instrument and a store of wealth (Humphreys, 1999).

Estimates of the crustal abundances of metals are given in Table 1.1 (Taylor, 1964), together with the actual amounts of some of the most useful metals, to a depth of 3.5 km (Chi-Lung, 1970).

Table 1.1

Abundance of metal in the oceans

The abundance of metals in the oceans is related to some extent to the crustal abundances, since they have come from the weathering of the crustal rocks, but superimposed upon this are the effects of acid rain-waters on mineral leaching processes; thus the metal availability from sea-water shown in Table 1.2 (Chi-Lung, 1970) does not follow precisely that of the crustal abundance. The sea-bed may become a viable source of minerals in the future. Manganese nodules have been known since the beginning of the nineteenth century (Mukherjee et al., 2004), and recently mineral-rich hydrothermal vents have been discovered and plans are being made to mine them (Scott, 2001).

Table 1.2

Abundance of metal in the oceans

It can be seen from Table 1.1 that eight elements account for over 99% of the earth’s crust; 74.6% is silicon and oxygen, and only three of the industrially important metals (aluminium, iron, and magnesium) are present in amounts above 2%. All the other useful metals occur in amounts below 0.1%; copper, for example, which is the most important non-ferrous metal, occurring only to the extent of 0.0055%. It is interesting to note that the so-called common metals, zinc and lead, are less plentiful than the rare-earth metals (cerium, thorium, etc.).

It is immediately apparent that if the minerals containing the important metals were uniformly distributed throughout the earth, they would be so thinly dispersed that their economic extraction would be impossible. However, the occurrence of minerals in nature is regulated by the geological conditions throughout the life of the mineral. A particular mineral may be found mainly in association with one rock type, e.g. cassiterite mainly associates with granite rocks, or may be found associated with both igneous and sedimentary rocks (i.e. those produced by the deposition of material arising from the mechanical and chemical weathering of earlier rocks by water, ice, and chemical decay). Thus, when granite is weathered, cassiterite may be transported and re-deposited as an alluvial deposit.

Due to the action of these many natural agencies, mineral deposits are frequently found in sufficient concentrations to enable the metals to be profitably recovered. It is these concentrating agencies and the development of demand as a result of research and discovery which convert a mineral deposit into an ore. Most ores are mixtures of extractable minerals and extraneous rocky material described as gangue. They are frequently classed according to the nature of the valuable mineral. Thus, in native ores the metal is present in the elementary form; sulphide ores contain the metal as sulphides, and in oxidised ores the valuable mineral may be present as oxide, sulphate, silicate, carbonate, or some hydrated form of these. Complex ores are those containing profitable amounts of more than one valuable mineral. Metallic minerals are often found in certain associations within which they may occur as mixtures of a wide range of particle sizes or as single-phase solid solutions or compounds. Galena and sphalerite, for example, associate themselves commonly, as do copper sulphide minerals and sphalerite to a lesser extent. Pyrite (FeS2) is very often associated with these minerals.

Ores are also classified by the nature of their gangues, such as calcareous or basic (lime rich) and siliceous or acidic (silica rich). An ore can be described as an accumulation of mineral in sufficient quantity so as to be capable of economic extraction. The minimum metal content (grade) required for a deposit to qualify as an ore varies from metal to metal. Many non-ferrous ores contain, as mined, as little as 1% metal, and often much less.

Gold may be recovered profitably in ores containing only 1 part per million (ppm) of the metal, whereas iron ores containing less than about 45% metal are regarded as of low grade. Every tonne of material in the deposit has a certain contained value which is dependent on the metal content and current price of the contained metal. For instance, at a copper price of £2000/t and a molybdenum price of £18/kg, a deposit containing 1% copper and 0.015% molybdenum has a contained value of more than £22/t. The deposit will be economic to work, and can be classified as an ore deposit if:

A major cost is mining, and this can vary enormously, from only a few pence per tonne of ore to well over £50/t. High-tonnage operations are cheaper in terms of operating costs but have higher initial capital costs. These capital costs are paid off over a number of years, so that high-tonnage operations can only be justified for the treatment of deposits large enough to allow this. Small ore bodies are worked on a smaller scale, to reduce overall capital costs, but capital and operating costs per tonne are correspondingly higher (Ottley, 1991).

Alluvial mining is the cheapest method and, if on a large scale, can be used to mine ores of very low contained value due to low grade or low metal price, or both. For instance, in S.E. Asia, tin ores containing as little as 0.01% Sn are mined by alluvial methods. These ores had a contained value of less than £1/t, but very low processing costs allowed them to be economically worked.

High-tonnage open-pit and underground block-caving methods are also used to treat ores of low contained value, such as low-grade copper ores. Where the ore must be mined selectively, however, as is the case with underground vein-type deposits, mining methods become very expensive, and can only be justified on ores of high contained value. An underground selective mining cost of £30/t would obviously be hopelessly uneconomic on a tin ore of alluvial grade, but may be economic on a hard-rock ore containing 1.5% tin, with a contained value of around £50/t.

In order to produce metals, the ore minerals must be broken down by the action of heat (pyrometallurgy), solvents (hydrometallurgy) or electricity (electrometallurgy), either alone or in combination, the most common method being the pyrometallurgical process of smelting. These chemical methods consume vast quantities of energy. Treatment of 1 t of copper ore, for instance, consumes in the region of 1500–2000 kWh of electrical energy, which at a cost of say 5 p/kW h is around £85/t, well above the contained value of all current copper ores.

Smelters are often remote from the mine site, being centred in areas where energy is relatively cheap, and where access to roads, rail or sea-links is available for shipment of fuel and supplies to, and products from, the smelter. The cost of transportation of mined ore to remote smelters could in many cases be greater than the contained value of the ore.

Mineral processing is usually carried out at the mine site, the plant being referred to as a mill or concentrator. The essential purpose is to reduce the bulk of the ore which must be transported to and processed by the smelter, by using relatively cheap, low-energy physical methods to separate the valuable minerals from the waste (gangue) minerals. This enrichment process considerably increases the contained value of the ore to allow economic transportation and smelting.

Compared with chemical methods, the physical methods used in mineral processing consume relatively small amounts of energy. For instance, to upgrade a copper ore from 1 to 25% metal would use in the region of 20–50 kW h t−1. The corresponding reduction in weight of around 25:1 proportionally lowers transport costs and reduces smelter energy consumption to around 60–80 kW h in relation to the weight of mined ore. It is important to realise that, although the physical methods are relatively low energy users, the reduction in bulk lowers smelter energy consumption to the order of that used in mineral processing, and it is significant that as ore grades decline, the energy used in mineral processing becomes an important factor in deciding whether the deposit is viable to work or not.

Mineral processing reduces not only smelter energy costs but also smelter metal losses, due to the production of less metal-bearing slag. Although technically possible, the smelting of extremely low-grade ores, apart from being economically unjustifiable, would be very difficult due to the need to produce high-grade metal products free from deleterious impurities. These impurities are found in the gangue minerals and it is the purpose of mineral processing to reject them into the discard (tailings), as smelters often impose penalties according to their level. For instance, it is necessary to remove arsenopyrite from tin concentrates, as it is difficult to remove the contained arsenic in smelting and the process produces a low-quality tin metal.

Against the economic advantages of mineral processing, the losses occurred during milling and the cost of milling operations must be charged. The latter can vary over a wide range, depending on the method of treatment used, and especially on the scale of the operation. As with mining, large-scale operations have higher capital but lower operating costs (particularly labour and energy) than small-scale operations. As labour costs per tonne are most affected by the size of the operation, so, as capacity increases, the energy costs per tonne become proportionally more significant, and these can be more than 25% of the total milling costs in a 10,000 t d−1 concentrator.

Losses to tailings are one of the most important factors in deciding whether a deposit is viable or not. Losses will depend very much on the ore mineralogy and dissemination, and on the technology available to achieve efficient concentration. Thus, the development of froth flotation allowed the exploitation of vast low-grade copper deposits which were previously uneconomic to treat. Similarly, the introduction of solvent extraction enabled Nchanga Consolidated Copper Mines in Zambia to treat 9 Mt/ yr of flotation tailings, to produce 80,000 t of finished copper from what was previously regarded as waste (Anon., 1979).

In many cases not only is it necessary to separate valuable from gangue minerals, but it is also required to separate valuable minerals from each other. For instance, porphyry copper ores are an important source of molybdenum and the minerals of these metals must be separated for separate smelting. Similarly, complex sulphide ores containing economic amounts of copper, lead and zinc usually require separate concentrates of the minerals of each of these metals. The provision of clean concentrates, with little or no contamination with associated metals, is not always economically feasible, and this leads to another source of loss other than direct tailing loss. A metal which reports to the wrong concentrate may be difficult, or economically impossible, to recover, and never achieves its potential valuation. Lead, for example, is essentially irrecoverable in copper concentrates and is often penalized as an impurity by the copper smelter. The treatment of such polymetallic base metal ores, therefore, presents one of the great challenges to the mineral processor.

Mineral processing operations are often a compromise between improvements in metallurgical efficiency and milling costs. This is particularly true with ores of low contained value, where low milling costs are essential and cheap unit processes are necessary, particularly in the early stages, where the volume of material treated is relatively high. With such low-value ores, improvements in metallurgical efficiency by the use of more expensive methods or reagents cannot always be justified. Conversely high metallurgical efficiency is usually of most importance with ores of high contained value and expensive high-efficiency processes can often be justified on these ores.

Apart from processing costs and losses, other costs which must be taken into account are indirect costs such as ancillary services – power supply, water, roads, tailings disposal – which will depend much on the size and location of the deposit, as well as taxes, royalty payments, investment requirements, research and development, medical and safety costs, etc.

Non-metallic ores

Ores of economic value can be classed as metallic or non-metallic, according to the use of the mineral. Certain minerals may be mined and processed for more than one purpose. In one category the mineral may be a metal ore, i.e. when it is used to prepare the metal, as when bauxite (hydrated aluminium oxide) is used to make aluminium. The alternative is for the compound to be classified as a non-metallic ore, i.e. when bauxite or natural aluminium oxide is used to make material for refractory bricks or abrasives.

Many non-metallic ore minerals associate with metallic ore minerals (Appendix II) and are mined and processed together, e.g. galena, the main source of lead, sometimes associates with fluorite (CaF2) and barytes (BaSO4), both important non-metallic minerals.

Diamond ores have the lowest grade of all mined ores. The richest mine in terms of diamond content (Argyle in Western Australia) enjoyed grades as high as 2 ppm in its early life. The lowest grade deposits mined in Africa have been as low as 0.01 ppm. Diamond deposits are mined mainly for their gem quality stones which have the highest value, with the low-value industrial quality stones being essentially a by-product; most industrial diamond is now produced synthetically.

Tailings retreatment

Mill tailings which still contain valuable components constitute a potential future resource. New or improved technologies can allow the value contained in tailings, which was lost in earlier processing, to be recovered, or commodities considered waste in the past can become valuable in a new economic order. Reducing or eliminating tailings dumps or dams by retreating them also reduces the environmental impact of the waste.

The cost of tailings retreatment is sometimes lower than that of processing the original ore, because much of the expense has already been met, particularly in mining and comminution. There are many tailings retreatment plants in a variety of applications around the world. The East Rand Gold and Uranium Company (ERGO) closed its operations in 2005 after 28 years of retreating over 870 Mt of the iconic gold dumps of Johannesburg, significantly modifying the skyline of the Golden City and producing 250 t of gold in the process. Also in 2005 underground mining in Kimberley closed, leaving a tailings dump retreatment operation as the only source of diamond production in the Diamond City. Some platinum producers in South Africa now operate tailings retreatment plants for the recovery of platinum group metals (PGMs), and also chromite as a by-product from the chrome-rich UG2 Reef.

Although these products, particularly gold, tend to dominate the list of tailings retreatment operations because of the value of the product, there are others, both operating and being considered as potential major sources of particular commodities. For example coal has been recovered from tailings in Australia (Clark, 1997), uranium is recovered from copper tailings by the Uranium Corporation of India, and copper has been recovered from the Bwana Mkubwa tailings in Zambia, using solvent extraction and electrowinning. The Kolwezi Tailings project in the DRC which proposes to recover oxide copper and cobalt from the tailings of 50 years of copper mining is expected to be the largest source of cobalt in the world.

The re-processing of industrial scrap and domestic waste is also a growing economic activity, especially in Europe. It is essentially a branch of mineral processing with a different feedstock, though operation is generally dry rather than wet (Hoberg, 1993; Furuyama et al., 2003).

Mineral processing methods

As-mined or run-of-mine ore consists of valuable minerals and gangue. Mineral processing, sometimes called ore dressing, mineral dressing or milling, follows mining and prepares the ore for extraction of the valuable metal in the case of metallic ores, and produces a commercial end product of products such as iron ore and coal. Apart from regulating the size of the ore, it is a process of physically separating the grains of valuable minerals from the gangue minerals, to produce an enriched portion, or concentrate, containing most of the valuable minerals, and a discard, or tailing, containing predominantly the gangue minerals. The importance of mineral processing is today taken for granted, but it is interesting to reflect that less than a century ago, ore concentration was often a fairly crude operation, involving relatively simple gravity and hand-sorting techniques performed by the mining engineers. The twentieth century saw the development of mineral processing as a serious and important professional discipline in its own right, and without physical separation, the concentration of many ores, and particularly the metalliferous ores, would be hopelessly uneconomic (Wills and Atkinson, 1991).

It has been predicted, however, that the importance of mineral processing of metallic ores may decline as the physical processes utilised are replaced by the hydro and pyrometallurgical routes used by the extractive metallurgist (Gilchrist, 1989), because higher recoveries are obtained by some chemical methods. This may certainly apply when the useful mineral is very finely disseminated in the ore and adequate liberation from the gangue is not possible, in which case a combination of chemical and mineral processing techniques may be advantageous, as is the case with some highly complex ores containing economic amounts of copper, lead, zinc and precious metals (Gray, 1984; Barbery, 1986). Also new technologies such as direct reduction may allow direct smelting of some ores. However, in the majority of cases the energy consumed in direct smelting or leaching of low-grade ores would be so enormous as to make the cost prohibitive. Compared with these processes, mineral processing methods are inexpensive, and their use is readily justified on economic grounds.

If the ore contains worthwhile amounts of more than one valuable mineral, it is usually the object of mineral processing to separate them; similarly if undesirable minerals, which may interfere with subsequent refining processes, are present, it may be necessary to remove these minerals at the separation stage.

There are two fundamental operations in mineral processing: namely the release, or liberation, of the valuable minerals from their waste gangue minerals, and separation of these values from the gangue, this latter process being known as concentration.

Liberation of the valuable minerals from the gangue is accomplished by comminution, which involves crushing, and, if necessary, grinding, to such a particle size that the product is a mixture of relatively clean particles of mineral and gangue. Grinding is often the greatest energy consumer, accounting for up to 50% of a concentrator’s energy consumption. As it is this process which achieves liberation of values from gangue, it is also the process which is essential for efficient separation of the minerals, and it is often said to be the key to good mineral processing. In order to produce clean concentrates with little contamination with gangue minerals, it is necessary to grind the ore finely enough to liberate the associated metals. Fine grinding, however, increases energy costs, and can lead to the production of very fine untreatable slime particles which may be lost into the tailings. Grinding therefore becomes a compromise between clean (high-grade) concentrates, operating costs and losses of fine minerals. If the ore is low grade, and the minerals have very small grain size and are disseminated through the rock, then grinding energy costs and fines losses can be high, unless the nature of the minerals is such that a pronounced difference in some property between the minerals and the gangue is available.

An intimate knowledge of the mineralogical assembly of the ore is essential if efficient processing is to be carried out. A knowledge not only of the nature of the valuable and gangue minerals but also of the ore texture is required.

The texture refers to the size, dissemination, association and shape of the minerals within the ore. The processing of minerals should always be considered in the context of the mineralogy of the ore in order to predict grinding and concentration requirements, feasible concentrate grades and potential difficulties of separation (Hausen, 1991; Guerney et al., 2003; Baum et al., 2004). Microscopic analysis of concentrate and tailings products can also yield much valuable information regarding the efficiency of the liberation and concentration processes (see Figures 1.2a–i for examples). It is particularly useful in troubleshooting problems which arise from inadequate liberation. Conventional optical microscopes can be used for the examination of thin and polished sections of mineral samples, and in mineral sands applications the simple binocular microscope is a practical tool. However, it is becoming increasingly common to utilise the new technologies of automated mineral analysis using scanning electron microscopy, such as the Mineral Liberation Analyser (MLA) (Gu, 2003) and the QEMSCAN (Gottlieb et al., 2000).

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

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

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 μm), 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

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

The most important physical methods which are used to concentrate ores are:

(1) Separation based on optical and other properties. This is often called sorting, which used to be done by hand but is now mostly accomplished by machine (see Chapter 14).

(2) Separation based on differences in density between the minerals. Gravity concentration, a technology with its roots in antiquity, is based on the differential movement of mineral particles in water due to their different hydraulic properties. The method has recently enjoyed a new lease of life with the development of a range of enhanced gravity concentrating devices. In dense medium separation particles sink or float in a dense liquid or (more usually) an artificial dense suspension; it is widely used in coal beneficiation, iron ore and diamond processing, and in the preconcentration of metalliferous ores.

(3) Separation utilising the different surface properties of the minerals. Froth flotation, which is one of the most important methods of concentration, is effected by the attachment of the mineral particles to air bubbles within the agitated pulp. By adjusting the climate of the pulp by various reagents, it is possible to make the valuable minerals air-avid (aerophilic) and the gangue minerals water-avid (aerophobic). This results in separation by transfer of the valuable minerals to the air bubbles which form the froth floating on the surface of the pulp.

(4) Separation dependent on magnetic properties. Low intensity magnetic separators can be used to concentrate ferromagnetic minerals such as magnetite (Fe3O4), while high-intensity separators are used to separate paramagnetic minerals from their gangue. Magnetic separation is an important process in the beneficiation of iron ores, and finds application in the treatment of paramagnetic non-ferrous minerals. It is used to remove paramagnetic wolframite ((Fe, Mn) WO4) and hematite (Fe2O3) from tin ores, and has found considerable application in the processing of non-metallic minerals, such as those found in mineral sand deposits.

(5) Separation dependent on electrical conductivity properties. High-tension separation can be used to separate conducting minerals from non-conducting minerals. This method is interesting, since theoretically it represents the universal concentrating method; almost all minerals show some difference in conductivity and it should be possible to separate almost any two by this process. However, the method has fairly limited application, and its greatest use is in separating some of the minerals found in heavy sands from beach or stream placers. Minerals must be completely dry and the humidity of the surrounding air must be regulated, since most of the electron movement in dielectrics takes place on the surface and a film of moisture can change the behaviour completely. The biggest disadvantage of the method is that the capacity of economically sized units is low.

In many cases, a combination of two or more of the above techniques is necessary to concentrate an ore economically. Gravity separation, for instance, is often used to reject a major portion of the gangue, as it is a relatively cheap process. It may not, however, have the selectivity or efficiency to produce the final clean concentrate. Gravity concentrates therefore often need further upgrading by more expensive techniques, such as froth flotation.

Ores which are very difficult to treat (refractory), due to fine dissemination of the minerals, complex mineralogy, or both, respond very poorly to the above methods.

A classic example is the huge zinc–lead–silver deposit at McArthur River, in Australia. Discovered in 1955, it is one of the world’s largest zinc–lead deposits comprising measured, indicated and inferred resources totalling 124 Mt with up to 13% Zn, 6% Pb and 60 g/t Ag (in 2003). For 35 years it resisted attempts to find an economic processing route due to the very fine grained texture of the ore. However, the development of the proprietary IsaMill fine grinding technology (Pease, 2005) by the mine’s owners Mount Isa Mines (now Xstrata), together with an appropriate flotation circuit, allowed the ore to be successfully processed and the mine was finally opened in 1995. The concentrator makes a bulk lead–zinc concentrate with a very fine product size of 80% smaller than 7 μm. There are many stages of flotation cleaning to achieve the necessary product grades with sufficient rejection of silica. McArthur River is a good example of how developments in technology can render previously uneconomic ore deposits viable. Process evolution for McArthur River continues, with the proprietary Albion atmospheric leaching process being considered for the direct treatment of concentrates (Anon., 2002).

Chemical methods, such as pyrometallurgy or hydrometallurgy, can be used to alter mineralogy, allowing the low cost mineral processing methods to be applied to refractory ores (Iwasaki and Prasad, 1989). For instance, non-magnetic iron oxides can be roasted in a weakly reducing atmosphere to produce ferromagnetic magnetite. It has also been suggested (Parsonage, 1988) that the magnetic response could be increased without chemically altering the minerals, by the adsorption of fine magnetite particles onto the surfaces of non-magnetic minerals in the slurry.

Some refractory copper ores containing sulphide and oxidised minerals have been pretreated hydrometallurgically to enhance flotation performance. In the Leach-Precipitation-Flotation process, developed in the years 1929–34 by the Miami Copper Co., USA, the oxidised minerals are dissolved in sulphuric acid, after which the copper in solution is precipitated as cement copper by the addition of metallic iron. The cement copper and acid-insoluble sulphide minerals are then recovered by froth flotation. This process, with several variations, has been used at a number of American copper concentrators, but a more widely used method of enhancing the flotation performance of oxidised ores is to allow the surface to react with sodium sulphide. This sulphidisation process modifies the flotation response of the mineral causing it to behave, in effect, as a pseudo-sulphide. Such chemical conditioning of mineral surfaces is widely used in froth flotation (see Chapter 12); sphalerite, for example, can be made to respond in a similar way to chalcopyrite, by allowing the surface to react with copper sulphate.

Recent developments in biotechnology are currently being exploited in hydrometallurgical operations, particularly in the bacterial oxidation of sulphide gold ores and concentrates (Brierley and Brierley, 2001; Hansford and Vargas, 2001). The bacterium Acidithiobacillus ferrooxidans is mainly used to enhance the rate of oxidation, by breaking down the sulphide lattice and thus liberating the occluded gold for subsequent removal by cyanide leaching (Lazer et al., 1986). There is good evidence to suggest that certain microorganisms could be used to enhance the performance of conventional mineral processing techniques (Smith et al., 1991). It has been established that some bacteria will act as pyrite depressants in coal flotation, and preliminary work has shown that certain organisms can aid flotation in other ways, with potential profound changes to future industrial froth flotation practice.

Extremely fine mineral dissemination leads to high energy costs in comminution and high losses to tailings due to the generation of difficult-to-treat fine particles. Much research effort has been directed at minimizing fines losses in recent 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 processes, by the use of innovative machines or by optimising the performance of existing ones. Several methods have been researched and developed to attempt to increase the apparent size of fine particles, by causing them to come together and agglomerate. Selective flocculation of certain minerals in suspension, followed by separation of the aggregates from the dispersion, has been successfully achieved on a variety of ore-types at laboratory scale, but plant application is limited (see Chapter 15).

Ultra-fine particles in a suspension can be agglomerated under high shear conditions if the particle surfaces are hydrophobic (water-repellent). A shear field, caused by vigorous agitation, of sufficient magnitude to overcome the energy barrier separating the particles is necessary to bring them together for hydrophobic association. Although the phenomenon of shear flocculation is well known it has not, as yet, been exploited commercially (Bilgen and Wills, 1991).

Selective agglomeration of fine particles by oil is a promising method, and has been developed to a commercial scale for the treatment of fine coal (Capes, 1989; Huettenhain, 1991). In the oil agglomeration process an immiscible liquid (e.g. a hydrocarbon) is added to the suspension. On agitation, the oil is distributed over oleophilic/hydrophobic surfaces and particle impact allows inter-particle liquid bridges to form, causing agglomeration. The oleophilicity of specific minerals can be controlled, for example, by adding froth flotation reagents. As yet, the oil agglomeration process has not been used to treat ultra-fine minerals outside the laboratory (House and Veal, 1989).

The flowsheet

The flowsheet shows diagrammatically the sequence of operations in the plant. In its simplest form it can be presented as a block diagram in which all operations of similar character are grouped (Figure 1.3). In this case comminution deals with all crushing, grinding and initial rejection. The next block, separation, groups the various treatments incident to production of concentrate and tailing. The third, product handling, covers the disposal of the products.

Figure 1.3 Simple block flowsheet

The simple line flowsheet (Figure 1.4) is for most purposes sufficient, and can include details of machines, settings, rates, etc.

Figure 1.4 Line flowsheet. (+) indicates oversized material returned for further treatment and (–) undersized material, which is allowed to proceed to the next stage

Milling costs

It has been shown that the balance between milling costs and metal losses is crucial, particularly with low-grade ores, and because of this, most mills keep detailed accounts of operating and maintenance costs, broken down into various sub-divisions, such as labour, supplies, energy, etc. for the various areas of the plant. This type of analysis is very useful in identifying high-cost areas where improvements in performances would be most beneficial. It is impossible to give typical operating costs for milling operations, as these vary enormously from mine to mine, and particularly from country to country, depending on local costs of energy, labour, water, supplies, etc., but Table 1.3 is a simplified example of such a breakdown of costs for a 100,000 t/d copper concentrator. Note the dominance of grinding, due mainly to power requirements.

Table 1.3

Costs per metric tonne milled for a 100,000 t/d copper concentrator

Efficiency of mineral processing operations

Liberation

One of the major objectives of comminution is the liberation, or release, of the valuable minerals from the associated gangue minerals at the coarsest possible particle size. If such an aim is achieved, then not only is energy saved by the reduction of the amount of fines produced, but any subsequent separation stages become easier and cheaper to operate. If high-grade solid products are required, then good liberation is essential; however, for subsequent hydrometallurgical processes, such as leaching, it may only be necessary to expose the required mineral.

In practice, complete liberation is seldom achieved, even if the ore is ground down to the grain size of the desired mineral particles. This is illustrated by Figure 1.5, which shows a lump of ore which has been reduced to a number of cubes of identical volume and of a size below that of the grains of mineral observed in the original ore sample. It can be seen that each particle produced containing mineral also contains a portion of gangue; complete liberation has not been attained; the bulk of the major mineral – the gangue – has, however, been liberated from the minor mineral – the value.

Figure 1.5 Locking of mineral and gangue

The particles of locked mineral and gangue are known as middlings, and further liberation from this fraction can only be achieved by further comminution.

The degree of liberation refers to the percentage of the mineral occurring as free particles in the ore in relation to the total content. This can be high if there are weak boundaries between mineral and gangue particles, which is often the case with ores composed mainly of rock-forming minerals, particularly sedimentary minerals. Usually, however, the adhesion between mineral and gangue is strong and, during comminution, the various constituents are cleft across. This produces much middlings and a low degree of liberation. New approaches to increasing the degree of liberation involve directing the breaking stresses at the mineral crystal boundaries, so that the rock can be broken without breaking the mineral grains (Wills and Atkinson, 1993).

Many researchers have tried to quantify degree of liberation with a view to predicting the behaviour of particles in a separation process (Barbery, 1991). The first attempt at the development of a model for the calculation of liberation was made by Gaudin (1939); King (1982) developed an exact expression for the fraction of particles of a certain size that contained less than a prescribed fraction of any particular mineral. These models, however, suffered from many unrealistic assumptions that must be made with respect to the grain structure of the minerals in the ore, in particular that liberation is preferential, and in 1988 Austin and Luckie concluded that there is no adequate model of liberation of binary systems suitable for incorporation into a mill model. For this reason liberation models have not found much practical application. However, some fresh approaches by Gay, allowing multi-mineral systems to be modelled (not just binary systems) free of the assumptions of preferential breakage, have recently demonstrated that there may yet be a useful role for such models (Gay, 2004a,b). The quantification of liberation is now routinely possible using the dedicated scanning electron microscope MLA and QEMSCAN systems mentioned earlier, and concentrators are increasingly using such systems to monitor the degree of liberation in their processes.

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 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 fine size that separation becomes very inefficient. On the other hand, froth flotation requires as much of the valuable mineral surface as possible to be exposed, whereas in a chemical leaching process, a portion of the surface must be exposed to provide a channel to the bulk of the mineral.

In practice, ores are ground to an optimum grind size, determined by laboratory and pilot scale testwork, to produce an economic degree of liberation. The concentration process is then designed to produce a concentrate consisting predominantly of valuable mineral, with an accepted degree of locking with the gangue minerals, and a middlings fraction, which may require further grinding to promote optimum release of the minerals. The tailings should be mainly composed of gangue minerals.

Figure 1.6 is a cross-section through a typical ore particle, and illustrates effectively the liberation dilemma often facing the mineral processor. Regions A represent valuable mineral, and region AA is rich in valuable mineral but is highly intergrown with the gangue mineral. Comminution produces a range of fragments, ranging from fully liberated mineral and gangue particles, to those illustrated. Particles of type 1 are rich in mineral, and are classed as concentrate as they have an acceptable degree of locking with the gangue, which limits the concentrate grade. Particles of type 4 would likewise be classed as tailings, the small amount of mineral present reducing the recovery of mineral into the concentrate. Particles of types 2 and 3, however, would probably be classed as middlings, although the degree of regrinding needed to promote economic liberation of mineral from particle 3 would be greater than in particle 2.

Figure 1.6 Cross-sections of ore particles

During the grinding of a low-grade ore the bulk of the gangue minerals is often liberated at a relatively coarse size (see Figure 1.5). In certain circumstances it may be economic to grind to a size much coarser than the optimum in order to produce in the subsequent concentration process a large middlings fraction and a tailings which can be discarded at a coarse grain size. The middlings fraction can then be reground to produce a feed to the final concentration process (Figure 1.7).

Figure 1.7 Flowsheet for process utilising two-stage separation

This method discards most of the coarse gangue early in the process, thus considerably reducing grinding costs, as needless comminution of liberated gangue is avoided. It is often used on minerals which can easily be separated from the free gangue, even though they are themselves locked to some extent with gangue. It is the basis of the dense medium process of preconcentration (Chapter 11).

Concentration

The object of mineral processing, regardless of the methods used, is always the same, i.e. to separate the minerals into two or more products with the values in the concentrates, the gangue in the tailings, and the locked particles in the middlings. Such separations are, of course, never perfect, 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 treating ultra-fine particles, where the efficiency of separation is usually low. In such cases, fine liberated valuable mineral particles often report in the middlings and tailings. The technology for treating fine-sized minerals is, as yet, poorly developed, and, in some cases, very large amounts of fines are discarded. For instance, it is common practice to remove material less than 10 μm 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 Florida, and 20% of the world’s tungsten were lost as fines. Significant amounts of copper, uranium, fluorspar, bauxite, zinc, and iron were also similarly lost (Somasundaran, 1986).

Figure 1.8 shows the general size range applicability of unit concentration processes (Mills, 1978). It is evident that most mineral processing techniques fail in the ultra-fine size range. Gravity concentration techniques, especially, become unacceptably inefficient. Flotation, one of the most important of the concentrating techniques, is now practised successfully below 10 μm but not below 1 μm

Figure 1.8 Effective range of application of conventional mineral processing techniques

It should be pointed out that the process is also limited by the mineralogical nature of the ore. For example, in an ore containing native copper it is theoretically possible to produce a concentrate containing 100% Cu, but, if the ore mineral was chalcopyrite (CuFeS2), the best concentrate would contain only 34.5% Cu.

The recovery, in the case of the concentration of a metallic ore, is the percentage of the total metal contained in