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

Chapter One Introduction 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, SnO 2 ). 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 SiO 4 , 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). 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). 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 seabed 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). 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 FeS 2 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: Contained value per tonne > total processing costs + losses + other costs) per tonne (Continues...) Excerpted from Wills' Mineral Processing Technology by Barry A. Wills Copyright © 2006 by Julius Kruttschnitt Mineral Research Centre. Excerpted by permission of Butterworth-Heinemann. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher. Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site. 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. 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 Cover 1 Preface to 7th Edition 2 Contributors 3 Acknowledgements 4 Contents 5 Chapter1 7 Chapter2 36 Chapter3 45 Chapter4 96 Chapter5 114 Chapter6 124 Chapter7 152 Chapter8 192 Chapter9 209 Chapter10 231 Chapter11 252 Chapter12 273 Chapter13 359 Chapter14 379 Chapter15 384 Chapter16 406 Appendix I - Metallic ore minerals 415 Appendix II - Common non-metallic ores 427 Appendix III - Excel Spreadsheets for formulae in chapter 3 434 Index 443