KWAME NKRUMAH UNIVERSITY OF

SCIENCE AND TECHNOLOGY

COLLEGE OF ENGINEERING

GEOLOGICAL ENGINEERING DEPARTMENT

LECTURE NOTES FOR

GED 152: BASIC MINERAL SCIENCE

Prepared by:

E. K. Appiah-Adjei / K. Aboraa

January, 2018

i

COURSE OUTLINE

WEEK ACTIVITY

1 – 2 REGISTRATION AND COURSE INTRODUCTION

• Overview of the course

3 – 4 METALS, MINERALS AND THEIR ORES

• Definitions

• Metals: Types and Importance

• Minerals: Types and Occurrence

5 – 6 OVERVIEW OF MINERAL PROCESSING

• Professional Disciplines in Metal Production

• Mineral Processing Techniques

7 – 10 COMMUNITION

• Crushing

• Grinding

• Particle Size Separation

11 CYANIDATION

12 REVISION

13 – 16 EXAMINATIONS

REFERENCE MATERIALS:

1. Lecture Notes

2. Kesler, S.E. (1994). Mineral Resources, Economics and the Environment. Macmillan

College Publishing Company, New York, p164 – 234.

3. Wills, B.A. and Napier-Munn, T. (2006). Mineral Processing Technology: An Introduction

to the Practical Aspects of Ore Treatment and Mineral Recovery. 7th Edition, Elsevier

Technology and Books, 444pp.

4. Relevant materials online

NOTES:

1. Grading [Final Exam (70%) and Continuous Assessment (30%)]

2. Continuous Assessment [Mid-Semester Exam, Class Test and Class Exercise]

3. Mid-Semester Examination on 16th March 2018?

4. Visit to a Quarry

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TABLE OF CONTENTS

COURSE OUTLINE ........................................................................................................................... i

TABLE OF CONTENTS ................................................................................................................... ii

1. METALS, MINERALS AND THEIR ORES ............................................................................. 1

1.1 INTRODUCTION ................................................................................................................ 1

1.2 METALS .............................................................................................................................. 2

1.2.1 FERROUS AND FERROALLOY METALS (STEEL INDUSTRY METALS) ......... 3

1.2.2 NON-FERROUS METALS .......................................................................................... 4

1.2.3 GEMSTONES ............................................................................................................... 5

1.2.4 NUCLEAR METALS ................................................................................................... 5

1.2.5 THE ELECTRONICS METALS AND MINERALS ................................................... 5

1.2.6 INSULANTS AND REFRACTORIES ........................................................................ 6

1.2.7 NATURAL ABRASIVES ............................................................................................. 6

1.2.8 FERTILIZER AND CHEMICAL INDUSTRIAL MINERALS................................... 6

1.2.9 MINERAL MATERIALS FOR CONSTRUCTION AND MANUFACTURING ...... 6

1.3 ORE MINERALS ................................................................................................................. 7

1.3.1 IRON (Fe) ORES .......................................................................................................... 7

1.3.2 BASE METAL ORES ................................................................................................... 8

1.3.3 LIGHT METAL ORES ................................................................................................. 9

1.3.4 PRECIOUS METAL ORES ........................................................................................ 11

1.3.5 THE GRADE OF ORES ............................................................................................. 12

2. OVERVIEW OF MINERAL PROCESSING ........................................................................... 13

2.1 INTRODUCTION .............................................................................................................. 13

2.2 MINERAL PROCESSING TECHNIQUES ...................................................................... 14

2.3 COMMINUTION ............................................................................................................... 16

2.3.1 CRUSHING................................................................................................................. 16

2.3.2 FINE GRINDING ....................................................................................................... 23

2.4 PARTICLE SIZE SEPARATION ...................................................................................... 27

2.4.1 SCREENING............................................................................................................... 27

2.4.2 COMMERCIAL SCREENING .................................................................................. 29

2.4.3 CLASSIFICATION..................................................................................................... 31

2.5 CYANIDATION ................................................................................................................ 32

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1. METALS, MINERALS AND THEIR ORES

1.1 INTRODUCTION

The earth’s crust is the original source of all metals known to man. The forms in which the metals

occur in the crust of the earth depend on their reactivity with the environment. Generally, 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, while the less reactive metals are found

principally in native or metallic form like gold and platinum in the earth. However, other metals

like silver, copper, and mercury are found in native as well as in the form of sulphides, carbonates,

and chlorides.

The earth’s crust is made up of rocks of different characteristics and properties depending upon

their composition. This composition is a combination of different proportions of minerals and/or

native metals(s). Thus, every piece of rock material that you pick up is mainly made up of two or

more minerals and/or native metal(s). The relative quantities of different minerals in the rocks

determine their peculiar properties.

A mineral may be defined as:

i. A naturally occurring inorganic substance, which has a definite chemical composition,

distinctive physical properties and a specific crystalline structure (Technical Definition).

ii. In a broad non-technical sense, the term embraces all inorganic and organic substances of

economic value that are extracted from the earth for use by man e.g. coal, chalk, clay, granite,

mineral fuels (oil and natural gas), etc.

An Ore is defined as a naturally occurring aggregate of minerals containing precious or useful

metals or metalloids that may be extracted at a profit. When a particular mineral (or group of

minerals) occurs in such significantly high proportions and in such quantities that the rock mass in

which it occurs could be exploited with relative ease for social or economic use, the rock mass is

referred to as an ore deposit. An ore deposit must contain materials that are:

• valuable,

• in concentrations, which can profitably be mined, transported, milled and processed, and

• able to be extracted from a rock by mineral processing techniques.

Certain geological processes occur in the earth’s crust, which lead to the accumulation

(concentration) of minerals in ore deposits in the form of mineral-bearing beds, veins, placers, or

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solutions. The ultimate goal for exploiting an ore is to obtain one or two important metallic or non-

metallic materials. Thus, earth resources are exploited in order to recover metals and non-metals for

specific purposes beneficial to man.

Most ores are mixtures of extractable minerals and extraneous rocky material described as gangue.

They are frequently categorized according to the nature of the valuable mineral as:

a) Native ores where the metal/mineral is present in elemental form,

b) Sulphide ores where the metal is contained a as sulphide compound,

c) Oxidized ores where the valuable mineral is present as an oxide, sulphate, silicate, carbonate

or some hydrated form of these compounds, and

d) Complex ores where more than one valuable mineral is present in profitable quantities.

Ores are, also, classified by the nature of their gangues as calcareous or basic (lime rich) and

siliceous or acidic (silica rich). Usually, 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 e.g. galena and sphalerite associate themselves

commonly, as do copper sulphide minerals and sphalerite to a lesser extent.

The minimum metal content (grade) required for a deposit to qualify as an ore varies from metal to

metal. For example, 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. A deposit can be classified as an ore deposit (or of economic value) if the contained value

per tonne > the extraction cost (processing cost + losses + other costs) per tonne.

1.2 METALS

A metal (for the purposes of Minerals and Metallurgical Engineering) may be defined as any

category of electropositive elements that: (1) usually have a shiny surface, (2) are generally good

conductors of heat and electricity, (3) can be melted or fused, and (3) can be hammered into thin

sheets or drawn into wires. Commonly, any object made of metal including an alloy of two or more

metallic elements may also be referred to as metal. Due to the abundance of iron as an industrial

metal, all metals are broadly classified as ferrous and non-ferrous.

Aside the above classification, metals may also be classified as primary (virgin) (i.e. metals

extracted from ores, natural brines or ocean water) or secondary (i.e., metals derived from scrap

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e.g. steel produced from scrap). This classification notwithstanding, the quality and properties of a

particular metal, whether primary or secondary, are essentially the same.

1.2.1 FERROUS AND FERROALLOY METALS (STEEL INDUSTRY METALS)

i. Ferrous Metals

The term "ferrous" is derived from the Latin word meaning "containing iron". Ferrous metal is a

metal with iron as its major constituent. Iron is the most important, widely used and about the

cheapest metal in terms of price per unit weight. Iron has a specific gravity of 7.57. There are

several iron based alloys; almost all of which are susceptible to rust or corrosion when exposed to

moist air.

Steel, in general, is an alloy of iron and 0.02 to 2% carbon and, sometimes, with an admixture of

other elements. Ferrous metals that consist of iron and more than 2% carbon are called cast irons or

pig irons1. Steel is tougher, stronger and more expensive than cast iron. Another form of iron is

wrought iron; this has low carbon content (generally less than 0.3%, but mostly less than 0.1%

carbon) and contains up to 2% slag (particularly iron silicate), which gives it its ductility and

toughness. Another low carbon ferrous metal is mild steel, which contains 0.12 to 0.25% carbon

(i.e. low carbon), and practically no slag.

In steels, the carbon is not present in elemental form but is combined with the iron as the compound

iron carbide, called cementite, with a small amount dissolved in the iron. Cast iron and pig iron

contain amounts of carbon varying from 2 to 4%. The carbon, here, is present both as iron carbide

and elemental carbon in the form of graphite.

ii. Ferroalloy Metals

Iron forms a series of useful alloys with carbon and some non-ferrous metals that are collectively

referred to as ferroalloys or “ferrous alloying metals”. The ferroalloy metals are those that, in

combination with iron or steel, produce alloy steels or ferrous alloys. The available ferroalloy

metals include: Manganese (Mn), Chromium (Cr), Vanadiun (V), Nickel (Ni), Cobalt (Co),

Tungsten (W), Molybdenum (Mo), Silicon (Si), Columbium (Nb), Tantalum (Ta) and Tellurium

(Te).

1 Pig Iron contains about 94.3% Fe, 4.5% C, 0.6% Mn, 0.5% Si, and trace amounts of other impurities such as Sulphur

and phosphorus

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1.2.2 NON-FERROUS METALS

All pure metals other than iron may be, broadly, classified as non-ferrous metals. This may be

subdivided into precious metals, base metals, light metals, and rare earth metals as follows:

i. Precious Metals

Precious metal is a rare metal of high economic value. Chemically, precious metals are less reactive

than most elements, have high luster and high electrical conductivity. They were very important as

currency in the past but are now regarded, mainly, as investment and industrial commodities. The

precious metals may be grouped into:

a. Traditional Precious Metals: e.g. Gold (Au) and Silver (Ag)

b. The Platinum Group Metals, PGM (sometimes called Noble Metals): They are used

primarily as catalysts in the automotive, chemical, and petroleum refining industries.

Examples of the PGM are Platinum (Pt), Palladium (Pd), Rhodium (Rh), Ruthenium (Ru),

Iridium (Ir) and Osmium (Os). Due to their corrosion resistant properties, they are used in

the chemical, electrical, glass, dental and medical industries. They are also used in the

jewellery industry because of their beauty.

ii. Base Metals

Base metals were so named by ancient alchemists and have no clear definition. However, they

might have been so named on being the first group of metals they discovered before the precious

and ferrous metals. The base metals are common and inexpensive metals that oxidize or corrode

relatively easily. They commonly form sulphide and oxide minerals. Examples of base metals are

Copper (Cu), Lead (Pb), Zinc (Zn) and Tin (Sn). Some publications include mercury, antimony,

and even iron to the list of base metals, but for the purpose of classification in this course, these

metals fall under other categories of metal.

iii. Light Metals

As they are collectively called, these metals have lower densities than most metals. These are

Aluminium (Al), Titanium (Ti), Magnesium (Mg) and Beryllium (Be) with Specific Gravities (SG)

of 2.7, 4.51, 1.74 and 1.85 respectively.

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iv. Chemical and Industrial Metals

These comprise metals that are used, largely, in chemical or industrial applications and are not as

popular as the more abundant metals. Examples of this class of metals include Antimony (Sb),

Arsenic (As), Bismuth (Bi), Cadmium (Cd), Germanium (Ge), Indium (In), Mercury (Hg),

Rhenium (Re), Selenium (Se), Tantalum (Ta), Thallium (Tl), Zirconium (Zr), Rare Earths and Soft

Metals.

v. Soft Metals

These are soft and very active, chemically, and are used as chemical reagents. They, however, have

no direct engineering applications. They include Potassium (K), Calcium (Ca), Lithium (Li), and

Sodium (Na).

vi. Rare Earth Metals

The rare earths are a group of closely related metals, from lanthanum to lutetium in the periodic

table. Yttrium and Scandium are commonly included in the rare earths because of their chemical

similarities and tendency to occur in the same deposits. The list of rare earth metals are as follows:

Cerium (Ce), Dysprosium (Dy), Erbium (Er), Europium (Eu), Holmium (Ho), Lanthanum (La),

Lutetium (Lu), Neodymium (Nd), Praseodymium (Pr), Samarium (Sm), Scandium (Sc), Terbium

(Tb), Yttrium (Y), Ytterbium (Yb), Thulium (Tm) and Gadolinium (Gd).

1.2.3 GEMSTONES

Gemstone is a ‘mineral’, which is used to make jewellery (or adornments) in a cut and polished

form. Examples of gemstones are Diamonds, Emerald, Ruby, Sapphire, Alexandrite (Chrysoberyl),

Amber, Aquamarine, Jade, Opal, Pink Topaz, Spinel, Tourmaline, Agate, Amethyst, Zircon, etc.

1.2.4 NUCLEAR METALS

These are kinds of metals used in the nuclear industry for generation of energy and other purposes.

Examples include Uranium (U), Caesium (Cs), Rubidium (Rb), Hafnium (Hf), Beryllium (Be),

Actinium (Ac) Polonium (Po) and Thorium (Th).

1.2.5 THE ELECTRONICS METALS AND MINERALS

These are used in electronic industry for production of different tools. Examples include Cadmium,

Mercury, Mica, Silica, Germanium, Tellurium, Selenium, Rhenium, Indium, Gallium, etc.

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1.2.6 INSULANTS AND REFRACTORIES

Examples are Asbestos, Magnesite, Perlite, Graphite, Vermiculite, Kyanite, Silimanite, and

Andalusite. The latter three minerals are different crystal structures of aluminium silicate (Al2SiO5).

1.2.7 NATURAL ABRASIVES

Examples are Industrial Diamonds (diamonds that cannot be used as gems), Silica sand, Garnet

(mostly found in metamorphic rocks), Emery (impure form of corundum), Feldspar and Diatomite.

1.2.8 FERTILIZER AND CHEMICAL INDUSTRIAL MINERALS

Examples of this group are Limestone (CaCO3), Dolomite (CaMg(CO3)2,) Lime (CaO), Phosphate

(has no definite chemical composition and two of its major natural varieties are fluorapatite and

chlorapatite), Potash (K2O), Sulphur, Nitrogen Compounds and Nitrate, and Other Agricultural and

Chemical Minerals like Fluorite, Iodine, Sodium Sulphate and Bromine.

1.2.9 MINERAL MATERIALS FOR CONSTRUCTION AND MANUFACTURING

i. Construction Minerals

Examples are Cement, Aggregate (Sand and Stone; Crushed Rock), Dimension Stone, Common

Clay, and Gypsum.

ii. Fillers, Extenders, Pigments, and Filters

Examples are Clays (kaolinite and montmorillonite), diatomite, fuller’s earth, barite, bentonite,

talc/pyrophyllite, asbestos, mica, zeolites, and mineral pigments (e.g., ochre –limonite rich, sienna –

haematite, and umber –purplish and rich in manganese oxide).

iii. Glass and Ceramics Raw Materials

Examples are Soda Ash, Limestone, Nepheline Syenite, Boron, Sodium Sulphate, Lithium,

Industrial Sand (mainly quartz), Flourspar, Strontium, Feldspar, Arsenic, Aplite, Selenium, etc.

iv. Abrasive and Refractory Minerals

Examples include industrial and synthetic diamonds, other natural abrasives (chert, flint,

novaculite, garnet, wollastonite, and emery), graphite, kyanite and related minerals (sillimanite,

andalusite).

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1.3 ORE MINERALS

Ore Minerals: Ore minerals refer to minerals that contain the valuable metals in an ore. Ore

minerals may be native metals or chemical compounds of the metals. They are the source of metals

and earth resources.

Gangue Minerals, on the other hand, are the valueless minerals that are associated with the ore

mineral. For example, a typical lead-zinc ore may contain the minerals galena (PbS), sphalerite

(ZnS), pyrite (FeS2), siderite (FeCO3), and quartz (SiO2). Thus, with respect to Lead and Zinc,

galena and sphalerite may be the ore minerals whereas pyrite, siderite, and quartz might constitute

the gangue minerals. Note that in another ore deposit, galena can become a gangue mineral whereas

siderite could become the ore mineral.

The wall rock broken with the ore is called waste. Note that the unbroken (in-situ) rock adjacent to

the ore body is called a country rock. Thus, all metals originate from natural deposits in the earth’s

crust and these deposits are called ores. The important ore minerals from which the most popular

metals can be extracted are:

1.3.1 IRON (Fe) ORES

Iron, as a metal, is next to Aluminium in abundance within the earth’s crust (4.6%). The common

available iron ores include:

Native Iron Fe Usually alloyed with Nickel etc. Strongly magnetic. Iron-

grey. SG = 7.3-7.8

Hematite Fe2O3 Reddish, nonmagnetic (but becomes magnetic on heating),

soft to hard. Contains 70% iron. This is the most important

source of iron. SG = 4.9-5.3

Magnetite Fe3O4 A heavy, black, magnetic mineral that contains about

72.4% Fe. SG = 5.18

Limonite Fe2O3.xH2O Yellowish to yellowish-brown. Contains 59-63% Fe.

Limonite is a rock rather than a specific mineral

Goethite Fe2O3.H2O Brownish black, sometimes yellowish or reddish. Becomes

magnetic after heating. Main component of Limonite.

62.9% Fe. SG = 4-4.4

Siderite,

Chalybite

FeCO3 Pale Yellowish brown through brownish-black to brownish-

red. 48.3% Fe. SG = 3.7-3.9

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1.3.2 BASE METAL ORES

i. Lead

Native Lead Pb Bluish-grey metal. Quickly oxidises on exposure to air.

Native lead is rare. SG = 11.34

Galena PbS

Lead Sulphide

Lead grey in colour. This is the most important source of

Lead. SG = 7.15 - 7.6. Almost always contains Silver

Sulphide. If silver occurs in economic quantities, the ore is

called argentiferous galena

Anglesite (Lead

Vitriol)

PbSO4

Lead Sulphate

White, sometimes with a blue, green, grey, or yellow tint.

SG = 6.3 – 6.4

Cerussite PbCO3

Lead Carbonate

White or greyish, sometimes tinged with blue or green with

copper salts. Occurs in the oxidising zone of lead veins. SG

= 6.55

Crocoisite

Crocoite

PbCrO4 SG = 5.9-6.1. Different shades of red. Occurs where lead

lodes traverse rocks containing chromium.

ii. Zinc

Native Zinc Zn Bluish-white brittle metal. Its surface becomes tarnished on

exposure to moist air. SG = 7.15. Native Zinc is rare.

Sphalerite

ZnS

Zinc Sulphide

Black or brown, sometimes yellow or white and rarely,

colourless. Most important source of zinc. SG = 3.9 – 4.2.

Almost always found in association with galena.

Zincite

Spartalite

ZnO

Zinc oxide

Deep red, but when in very thin scales deep yellow by

transmitted light. SG = 5.4 – 5.7

Smithsonite ZnCO3 SG = 4-4.5. White, greyish, greenish, brownish-white. The

zinc is often partly replaced by iron. A little lime, magnesia

or cadmium oxide is often present

Willemite,

Wilhelmite

Zn2SiO4 SG = 4-4.1. Green, yellow, or brown

Goslarite,

White Vitriol

ZnSO4.7H2O SG = 2.1. White.

iii. Tin

Cassiterite

Tinstone

SnO2

Tin Oxide

SG = 6.8 – 7.1. 78.6% Tin. Usually black or brown in

colour.

Stannine, Stannite,

Tin Pyrites Bell Metal Ore.

Cu2SnFeS4 Sulphide of tin,

copper and iron

SG = 4.4. 27.5% Tin. Steel-grey when pure; iron-

black, sometimes bronze or bell-metal colour.

iv. Copper

Native Copper Cu Copper-red in colour. SG = 8.9. Relatively easy to

find. MP = 1,100°C. Next to Ag in electrical

conductivity.

Chalcopyrite CuFeS2 SG = 4.1 – 4.3. 34.5% Cu. Brass yellow colour.

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Copper Pyrites Principal source of copper.

Chalcocite Cu2S SG = 5.5 – 5.8. 79.8% Cu. Brass yellow colour.

Valuable source of copper. Formed by the alteration of

primary copper sulphides in the zone of secondary

enrichment.

Bornite, Erubescite Peacock Ore

Cu3FeS3 SG = 4.9 – 5.4. 63% Cu. Copperly red colour.

Valuable ore of copper. Occurs as a primary deposit.

Covellite, Covelline CuS SG = 4.6. 66.4% Cu. Indigo-blue colour. .

Enargite Cu3AsS4 SG = 4.44. 48% Cu. Greyish-black to iron-black

colour. Important ore of copper.

Tennantite Sulphide of Copper &

Antimony

Cu3AsS3 SG = 4.37 – 4.49. Blackish lead-grey to iron black.

Occurs in association with other copper ores

Famatinite Sulphide of Copper &

Antimony

Cu3SbS4 Greyish to copper-red

Tetrahedrite Cu3SbS3 SG = 4.5 – 5.1. Colour is between steel grey and iron-

black

Cuprite Cu2O SG = 5.8 – 6.15. 88.8% Copper. Occurs in the upper

oxidised zones of copper lodes.

Tenorite, Melaconite CuO SG = 6.25. 79.85% Copper. Dull black masses or

shining and flexible scales.

Chalcanthite CuSO4.5H2O SG = 2.12 – 2.3. 25.4% Copper. Sky blue and

sometimes greenish colour.

Malachite Cu2CO3(OH)2 SG = 3.5 - 4. 57.3% Copper. Occurs with other ores of

copper.

Azurite, Chessylite, Blue Carbonate of

copper.

Cu3(CO3)2(OH)2 SG = 3.7 – 3.8. 55.1% Copper. Deep Azure blue.

Chrysocolla CuSiO3.2H2O SG = 2 – 2.2. Bluish-green, sky-blue, or turquoise-blue.

Atacamite Cu2(OH)3Cl SG = 3.76. 59.4% Copper. Bright deep green to

blackish green.

1.3.3 LIGHT METAL ORES

i. Aluminium (Al) Ores

Aluminium (Al), the most abundant metal, does not occur in a free state in nature but in

combination with other elements and constitutes about 8% of the earth’s crust. The only

commercial ore of aluminium is bauxite, which is a rock that contains hydrated oxides of

aluminium; namely, gibbsite, diaspore, and boehmite.

Bauxite This is a mixture of gibbsite, diaspore and boehmite

Gibbsite Al(OH)3 SG = 2.35. 65.4% Al2O3.

Diaspore HAlO2 SG = 3.5. 85.4% Al2O3

Boehmite AlO(OH) SG = 3

Corundum Al2O3 100% Al2O3. Varieties are: (i) the gemstones ruby

(red), sapphire (blue), oriental amethyst (purplish),

oriental emerald (green), oriental topaz (yellow),

and (ii) emery – a greyish-black variety containing

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admixed magnetite and haematite that is crushed,

powdered and sifted, and the powder used for

polishing hard surfaces.

Cryolite Na3AlF6 SG = 2.97. Colourless, snow-white, reddish,

brownish, brick-red, and even black.

Cyanite Al2O3.SiO2 63.2% Al2O3

Leucite KAlSi2O6 23.5% Al2O3

Alunite Na3AlF6 37.0% Al2O3

Alum KAl(SO4)2 SG = 1.75. Readily soluble in water.

ii. Magnesium (Mg) Ores

Magnesium is a silver white metal that tarnishes easily to the oxide, MgO. The metal is used in the

manufacturing of light alloys and castings, especially aircraft, and in various metallurgical

processes. It is obtained from magnesium chloride recovered from sea-water (contains 0.13% Mg)

or saline residues. The chief magnesium mineral of economic importance is Magnesite (MgCO3),

which is used for furnace-linings and other purposes.

Periclase, Native

Magnesia

MgO Dark green grains.

Brucite Mg(OH)2 SG = 2.39. 41% Mg. White, often bluish, greyish

and greenish

Magnesite MgCO3 SG = 2.8 - 3. 28% Al2O3. White, greyish-white,

yellowish or brown

Dolomite MgCO3.CaCO3 SG = 2.8 – 2.9. 13% Al2O3. White and often tinged

with yellow and brown, and sometimes with red,

green or black.

Carnallite MgCl2.KCl.6H2O SG = 1.6. 8% Mg.

iii. Titanium (Ti) Ores

Rutile TiO2 SG = 4.2. Reddish-brown, red, yellowish, or black

Anatase,

Octahedrite

TiO2 SG = 3.82-3.95. Brown, indigo-blue or black

Brookite TiO2 SG = 4. Hair-brown, reddish, iron-black.

Ilmenite FeTiO3 SG = 4.5-5. Iron-black.

iv. Beryllium (Be) Ores

Native beryllium does not occur in nature. The two minerals of beryllium used as gemstones are:

Beryl Be3Al2(Si6O18) SG = 2.7. Emerald-green and deep green

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Beryllium

aluminium silicate

(emerald), pale blue (aquamarine). Available as a

by-product in the mining of mica and feldspar

deposits in pegmatites.

Chrysoberyl,

Alexandrite

BeAl2O4 SG = 3.6-3.8. Shades of green. Occurs in alluvial

deposits, and in place in granite, pegmatite, gneiss

and mica-schist. Alexandria greenish but is reddish

by artificial light, and is used as a gem.

1.3.4 PRECIOUS METAL ORES

Gold and the platinum group metals occur mostly in native metallic state or as alloys with

themselves rather than being extracted from ore minerals. For example, gold is generally alloyed

with some silver whilst platinum might be alloyed with one or more of the other platinum group

metals. Copper, iron, palladium, rhodium, and even bismuth, have been found in native gold.

Silver, unlike the other precious metals, has several ore minerals of its own. However, in few cases,

gold and the platinum group metals may occur in ore minerals as shown below.

i. Gold (Au)

Minerals known as gold tellurides might occur in veins and replacement-deposits. Some of the gold

tellurides are:

Sylvanite (Au, Ag)Te2 SG = 8-8.2. Pale Yellow. 24.5% Au and 13.4% Ag

Calaverite (Au, Ag)Te2 SG = 9. Pale Yellow. Gold is predominant. Occurs

mostly as AuTe2

Petzite (Au, Ag)2Te SG = 8.7-9. Steel-grey to iron-black colour.

ii. Silver (Ag)

Ore minerals of silver are:

Native Silver Ag SG = 10.1-11.1. White. Could be up to 99% pure.

Often contains gold, copper, platinum, mercury,

bismuth, etc.

Argentite Ag2S SG: 7.19-7.36. 87.1% Ag. Blackish lead-grey.

Cerargyrite (horn

silver)

AgCl SG: 5.8. 75.3% Ag. Pale shades of grey, sometimes

greenish or bluish. Colourless when pure.

Polybasite (Ag, Cu)16Sb2S11

Sulphide of silver,

antimony, copper

and Arsenic

SG: 6-6.2. 70% Ag. Iron-black.

Pyrargyrite (ruby

silver)

Ag3SbS3 SG: 5.7-5.9. 59.9% Ag. Black to reddish.

Proustite Ag3AsS3 SG: 5.55-5.64. 65.4% Ag. Reddish

Freibergite (Pb, As)8Sb5S12 SG: 6-6.4. 22.23% Ag. Light steel-grey to dark

lead-grey.

Hessite Ag2Te SG: 8.4. Lead-grey

NB: Silver is also associated with ores of lead, zinc, copper and other metals

12

iii. Platinum Group

This is a greyish white lustrous metal. Its resistance to acids and chemical influence, generally,

renders it of particular use in the laboratory, dentistry and in the electrical and other industries

including jewellery. Platinum occurs as:

Native Pt Pt SG = 21.46. Melting point of 1760°C. Native

platinum could be alloyed with Fe, Ir, Os, Au, Rh,

Pa, and Cu, with Pt ranging from 45 to 86%.

Native Pt is the most important source of the metal.

Sperrylite PtAs2

Platinum Arsenide

SG: 10.6. Tin-white

Almost all the other Platinum Group metals occur in the crude native platinum state. Amongst its

group of metals, palladium, rhodium, and iridium have some industrial applications whereas

ruthenium and osmium have very limited application.

Palladium (Pd) is a silver-white metal with SG-11.3-12, oxidises more readily than platinum, has a

melting point of 1546°C, is hard, and used in dental alloys and as a catalytic agent in the chemical

industry. Osmium (Os) is a bluish-grey metal with SG 22.48 (the heaviest of metals) and fuses at

2200°C. It is, however, of little commercial importance.

Iridium (Ir) is a steel-white metal, SG-22.4, and melting point of 2290°C. Its chief application is in

the dental, electrical and jewellery trades. Rhodium (Rh) is a white metal with SG of 12.1 and melts

at 2000°C. It is used in the manufacturing of thermal couples for crucibles, and rhodium plating.

Ruthenium (Ru) is a white, hard and brittle metal of SG 12.2. It has few or no industrial application.

1.3.5 THE GRADE OF ORES

The grade of an ore refers to the quantity of valuable component in the ore expressed as a

percentage or ratio of a specified quantity per unit weight or volume depending upon the metal or

valuable component concerned. The various expressions of ore grade are:

% (e.g., Lead, Zinc, Copper, Tin) carat/cu. yd (e.g., Diamond)

gm/t (e.g., Gold, Silver, Platinum) dwt/t (e.g., Gold, Silver, Platinum)

oz/t (e.g., Gold, Silver) carat/m3 (e.g., Diamond)

carat/t (e.g., Diamond) gm/ m3 (e.g., Gold, Silver)

13

2. OVERVIEW OF MINERAL PROCESSING

2.1 INTRODUCTION

In order for minerals and, for that matter, metals to be made available for use by man, they must be

found in the earth, mined and extracted by specific established processes. The processes by which

the minerals are found to the point where the useful metals (or other valuables) they contain are

extracted in a useful form are covered under various professional disciplines indicated in Table 1.

Table 1: Processes and professional disciplines related to the production of metals

Process

Industrial

Activity

Professional

Discipline

State of metal at the

end of the process

• Search for and identify ore deposits and

their mode of occurrence.

• Declare the type and importance of

mineral(s) or metal(s) in the ore deposit.

• Estimate the size and quantity of both ore

and metal.

• Establish the quality of the mineral (i.e.

grade).

Min

eral

Explo

rati

on

Explo

rati

on G

eolo

gy

Min

ing E

ngin

eeri

ng

Metal is combined with

large amount of gangue

Gangue

Mineral/Metal Ore

• Establish the technical and economic

feasibility of exploiting the metal.

• Identify and design the method by which

the ore material is extracted from within

the earth’s crust.

• Organise men and materials to carry out

the exploitation of the ore.

• Supervise the actual extraction of the ore

material. M

inin

g

Min

ing E

ngin

eeri

ng

Metal is highly

combined with gangue.

Gangue

Mineral/MetalBroken ore

• Prepare the extracted ore and render it

amenable for further treatment.

• Extract the highest possible proportion of

the mineral/metal from the gangue and

discard the less useful material associated

with the mineral by physical and

chemical means. Min

eral

Dre

ssin

g

Min

eral

Pro

cess

ing

Metal is partially

combined with gangue.

RemainingGangueMineral/Metal

• Separate metals from their ores or, in

most cases, from an enriched

(concentrated) mineral to obtain the final

metal product (approx. 99.99% purity).

The objective is to separate a metal from

elements and chemical components with

which it is combined.

Sm

elti

ng a

nd

Ref

inin

g

Extr

acti

ve

Met

allu

rgy

Metal is in its pure state

Mineral /Metal

14

Process

Industrial

Activity

Professional

Discipline

State of metal at the

end of the process

• Study the detailed characteristics of the

metal/material in its final form and the

various ways in which it could be

utilised.

Mat

eria

l F

abri

cati

on,

conver

sion, al

loyin

g,

etc.

Physi

cal

Met

allu

rgy

Pure metal. Might or

might not be combined

with other metals and

converted to useful

materials.

Fabricated Metal

2.2 MINERAL PROCESSING TECHNIQUES

As indicated, a mined ore mostly consists of valuable minerals and gangue. Mineral processing,

sometimes called ore dressing, mineral dressing, ore preparation or milling, follows after actual

mining of most ores and prepares the ore for extraction of the valuable metal (in the case of metallic

ores) and produces a commercial end product. Apart from regulating the size of the ore, it is a

process of mechanically 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 bulk of gangue minerals. Basically, it is a cleaning

operation that is carried out by utilising differences in physical properties between valuable and

gangue minerals such as those of appearance, density, magnetic and surface nature to separate

them. If the ore contains more than one valuable mineral, mineral dressing techniques are employed

to separate them.

Mineral processing involves four general types of operations; namely (i) comminution or particle

size reduction, (ii) sizing or separation of particle sizes by screening or classification, (iii)

concentration by taking advantage of physical and surface chemical properties, and (iv)

dewatering or solid/liquid separation. Aside these, a number of auxiliary materials handling

operations are also considered as a branch of mineral processing such as storage (as in bin design),

conveying, sampling, weighing, slurry transport, and pneumatic transport. Normally, a plant is

purposely constructed in the mining vicinity within which various types of equipment are installed

to embark on the necessary mineral processing operations. This plant may be referred to as a mill,

mineral processing plant, or simply, a plant. The excavated ore that is transported to the mill for

processing is referred to as the run-of-mine (ROM) ore, and might be fed directly to the plant or

initially deposited on stockpiles (which some mines refer to as the “ROM pad”) from where they

are later fed into the plant.

15

Some metals occur as chemical compounds; hence their ore minerals must be subjected to a kind of

metallurgical process, i.e. pyrometallurgy (heat) or hydrometallurgy (chemical), to break up the

chemical bond and liberate the metallic elements. This is the realm of Extractive Metallurgy. The

processes involved in most extractive metallurgy operations are extremely expensive and not

feasible to be applied on the entire run-of-mine ore. Thus, mineral processing methods are applied

to the ore to get rid of as much gangue as possible before applying the extractive metallurgical

process.

Also, most ore minerals are usually finely disseminated and intimately associated with gangue

minerals such that the various minerals must be dissociated from each other before the valuable

ones can be liberated from the valueless components (gangue). At the end of the mineral dressing

operation, the gangue would have been reduced to a finely ground powder or sand (depending upon

the type of ore) in the form of slurry, called tailings, that is disposed in a specially prepared area.

Thus, the two primary operations of mineral processing are comminution and concentration.

However, within these primary operations are several other important unit operations such as

screening or sizing, classification, thickening, dewatering and filtration.

Extractive metallurgy is the science, technique, and practice of separating metals from their ores or,

in most cases, from an enriched (concentrated) ore mineral to obtain the final metal product

(approx. 99.99% purity). The objective is to separate a metal from elements and chemical

components with which a particular metal of interest is combined. There are three main

subdivisions of extractive metallurgy, viz.:

1. Pyrometallurgy (fire metallurgy): This is the application of high temperatures to bring about

decomposition of the feed material to separate and consolidate the metal. In other words,

pyrometallurgy makes use of chemical reactions at high temperatures to separate metals from

their ores. It is the oldest division of extractive metallurgy. The pyrometallurgical process may

be represented in general terms as follows:

Mineral + gangue + reducing agent + flux + heat = metal + slag + gas

2. Hydrometallurgy: This involves the separation of metals in aqueous solution from the rest of

their ores, followed by precipitation in metallic form. “Many hydrometallurgical processes are

related to ore dressing than any other extraction method, and, particularly in the leaching of

gold and silver ores, the ore-dressing and hydrometallurgical operations are so closely

interrelated as to defy separate classification” (Joseph Newton, Extractive Metallurgy, John

Wiley & Sons, 1959, page 413).

16

3. Electrometallurgy – the use of electrical energy to decompose the pure mineral in the molten

state or dissolved in a mixture of molten salts. It deals mainly with the extraction and refining

of metals by the use of electric currents –known as electrolytic process.

2.3 COMMINUTION

Comminution refers to the various size reducing processes, i.e. crushing and grinding, undertaken

during processing of the mined ores.

2.3.1 CRUSHING

Crushing is the first stage in the comminution operation whereby the hard run-of-mine ore is

reduced to the desired size before entering the next stage. Currently, the largest crushing plant

might accept a feed with a maximum particle size of 1.524 m (60”) and reduce it to a product size

as small as 5 mm. To achieve the final reduction ratio, the crushing operation may be carried out in

one, two, or three stages, and are designated as primary, secondary, and tertiary crushing,

respectively. There is a working relationship between the size a machine can accept (its gape) and

the maximum size which will pass through the discharge end (its set). This relationship is called the

reduction ratio. The factors that determine the selection of crushing equipment and a crushing

circuit are:

• Tonnage rate (throughput)

• The size distribution of feed (run-of-mine ore)

• Desired product size

• Method of feeding

• Ore Characteristics (e.g. compressive strength, abrasiveness, clay content, etc.)

i. Primary Crushing

Primary crushing machines are of two main types; viz. Jaw and Gyratory crushers. The largest

crusher can accept a feed size of up to 2.1336m x 1.524m (84”x60”) and reduce it to a product size

of 101.6mm to 152.4mm (4” to 6”). The product size is determined by the set of the primary

crusher.

17

a) Jaw Crushers

The jaw crusher was invented by E.W. Blake in 1858 and is, mainly, used for primary and

secondary crushing. Jaw crushers in use today are variations of the original Blake jaw crusher. A

typical present-day jaw crusher is illustrated in Fig 1.

Figure 1: A typical Jaw Crusher with mouth openings of up to 2m x 3m. (Source: A.R.

Bailey, A textbook of Metallurgy)

This crusher consists of a heavy main frame carrying a fixed jaw and a swing (or moving) jaw. The

swing jaw is pivoted at the top and goes through an oscillating motion by means of toggle plates

and a pitman driven by the eccentric on the main driving shaft. As the back toggle rises, it presses

the lower end of the pitman forward in a movement that is transmitted via the front toggle to the

swing jaw to effect a to and fro motion of the jaw (see Fig. 1). In some crushers, the movement is

transmitted directly from an eccentric to the jaw instead of through pitman and toggles. Rock is fed

down between the jaws, and is nipped and crushed by them as the swing jaw closes. The jaw plates

and cheek or side plates are subjected to severe abrasion in crushing the ore. These plates are made

of chilled cast iron, manganese steel, or some other wear-resistant alloy, and they are replaceable

when worn out. Some jaw crushers have vertically corrugated jaw plates while others might use

smooth plates. The wear on the plates is expressed in terms of pounds of steel consumed per ton of

ore crushed.

The point of entry of the feed is known as the mouth of the crusher. The dimensions of the mouth

are given by the width of the crushing jaws and the gape or opening of the mouth measured at right

angles to the fixed jaw. The size of a crusher is measured by the dimensions at the mouth in inches

or millimetres as the case might be. The gape is always given first and the width of the jaw second.

For example, a 1219 mm (48”) x 1829 mm (72”) means a crusher with a 1219 mm gape and a jaw

width of 1829 mm. The gape is, almost, always the smaller of the two dimensions, and it

determines the size of the largest single piece of rock that will enter the crusher. Thus, a 1219 mm x

18

1829 mm crusher will not accept any rock larger than 1219 mm in diameter. In practice, the largest

size of material to be fed into the crusher must be 80 % of the gape.

The throat of a crusher is the point at which the rock is discharged. The throat dimension varies

depending upon whether the swing jaw is in the open or closed position. The difference in

millimetres (or inches) between the open and closed throat dimensions (moving-jaw displacement)

is known as the stroke or throw of the crusher. The throw varies from a minimum of 9.53mm (3/8”)

in small crushers to a minimum of 25.4 mm (1-inch) in big ones with the maximum possible throw

being about three times the minimum in any case. With brittle rock (e.g. acid rock such as

quartzite), short throw may be best but for a tough rock (basic rock), a long throw is necessary.

The rock is discharged at all times at the throat, but most of the discharge takes place while the

swing jaw is receding. Thus, many pieces will be discharged that are larger than the minimum

throat opening and some pieces will be discharged that are almost as large as the maximum throat

opening. For jaw crushers, the maximum opening is given as the “set”. The ratio of the “gape to

set” is the reduction ratio (e.g. 8 to 1), which is the same as the ratio of the largest particle in the

feed to the largest particle in the discharge.

Commonly, crushers are adjusted to produce a selected product size. There are three main

interrelated parameters for setting crushers; these are:

1. Closed-side setting (CSS) – This is the minimum distance between the surfaces at the

closed position.

2. Open-side setting (OSS) – This is the maximum distance between the surfaces at the open

position.

3. Throw – This is the distance in the direction of compression that the moving wear surface

travels between the open-side setting and the closed-side setting.

Both the open-side and closed-side settings might be used in tabulations of crusher performance in

publications and manufacturers manuals. It is usually important to check the type of setting used in

any particular case but, in most cases, the closed-side setting is used in sizing the products of jaw

crushers. For the purpose of size determination only, the average of the CSS and OSS is the best

measure of the size of the discharged product. The capacity of a crusher is defined as the number of

tons of finished product made per hour. Capacity decreases as the reduction ratio increases.

19

b) Gyratory Crushers

Generally, the gyratory crusher (Fig. 2) consists of a fixed crushing surface in the form of the

frustum of an inverted cone within which is a moving crushing surface in the form of the frustum of

an erect cone. The ore is crushed in the downward-converging annular space between the two

crushing surfaces. The action of the gyrating head is such that the head is alternately approaching

and receding from the outer shell at any point, and, as the particles of rock fall into this space, they

are nipped and crushed. The size of a gyratory crusher is expressed by the maximum opening at the

mouth in millimetres (or inches). Thus a 1000 mm gyratory crusher has a gape of 1000 mm. Like

the jaw crusher, gyratory crushers can be used for primary and secondary crushing.

Fig 2a: Section through a typical Gyratory

Crusher

Fig 2b. Top view of a Nordberg Gyratory

Crusher

A comparison between Jaw and Gyratory Crushers is as follows:

• Given the same size of feed, the capacity of the gyratory crusher exceeds that of a jaw

crusher.

Crushing head

attached to the

spindle Spider cap

enclosing

low friction

bearings

Spider

Arm

Shield

Concaves

20

• Gyratory crushers can be choke-fed by direct dumping from trucks. Jaw crushers require a

scalping grizzly and a feeder.

• Jaw crushers require less maintenance.

• For the same tonnage capacity, the gape of a jaw crusher allows it to handle more awkward

oversize material than the gyratory.

ii. Secondary Crushing

A secondary crusher, generally, takes the discharge of a primary crusher and breaks it down further

to a size suitable for feeding a grinding mill. The maximum feed size of a secondary crusher is not

likely to exceed 152 mm (6”) in diameter and discharges a product size between 6.5 mm to 65 mm.

Invariably, the product of secondary crushing is fed into a fine-ore bin, which must have sufficient

capacity to receive all the ore accepted for treatment and to keep the plant running continuously,

though the mine delivers its ore periodically.

Often two or more secondary crushers would be needed if the primary crusher discharge is fairly

coarse. A closed circuit often exists between primary and secondary crushing operations whereby

the oversize products of the secondary crusher are returned to the primary crusher for re-crushing.

For instance, if the feed has a maximum size of, say, 80 cm and the final product required is, say,

≤15 mm, the crushing circuit could be designed as shown in Fig. 3.

a) The Cone Crusher

Cone crushers are exclusively used for secondary and tertiary crushing. The crusher operates in

similar fashion to the gyratory crusher in that the crushing head is attached to a gyrating spindle.

However, unlike the gyratory crusher in which the spindle is hung from the upper end, the spindle

of a cone crusher is supported in a universal bearing below a gyrating head or cone. The crusher

(Fig. 4) consists of a conical crushing head that gyrates within an inverted truncated cone called the

Feed

≤80 cm

Grizzly

15 cm

spacing +15cm Primary

Crusher

Screen

15mm

-15cm

Final product, -15mm

Secondary

Crusher

+15mm -15cm

Conveyor Belt

Figure 3: A typical closed circuit crushing flow sheet

Fine-Ore

Bin

21

bowl. The crushing head (cone) and the bowl are lined by manganese steel or other wear-resisting

alloy. The main shaft is gyrated by means of a long eccentric like those of the standard gyratory

crusher though the crusher operates at a higher speed than a primary gyratory. Thus the cone

crusher is, sometimes, referred to as a type of secondary gyratory.

A feature of the cone crusher is the feed-distributing plate mounted on top of the main shaft. The

size of a cone crusher is denoted by the bottom diameter of the crushing head or cone and range in

size from 0.6 to 3 m. However, some manufacturers may designate their crusher sizes by both the

feed opening and the mantle diameter –e.g. a model 200-2050 may mean a crusher with a feed

opening of 200 mm and a mantle diameter of 2050 mm. Unlike that of primary crushers, the set is

minimum discharge opening, since during its last two or three nips, the particle of ore must pass

through a parallel-sided crushing zone.

Fig 4a: Cone Crusher Fig 4b: Cone Crusher with material being crushed

Apart from the above crushers, other crushers that could be used for primary, secondary and tertiary

crushing to a limited degree are roller crusher, impact crusher, hammer mills, gravity stamps, etc.

iii. Power Calculations for Crushers

Several attempts have been made to establish an acceptable principle for estimating the energy

required to reduce a unit weight of rock (such as 1 tonne) from a given feed size to the required

product size. In the process, Kick’s law, the Rittinger law, and Bond’s theory have evolved.

However, of all the theories, it is Bond’s theory that has gained the widest acceptance. Thus,

Bond’s theory is used to determine the power (and hence the motor size) of jaw, gyratory, and cone

crushers. Bond’s theory is based on the “Work Index” for the material being crushed.

22

The Work Index is the total work input in KWhr/ton required to reduce a rock particle from a

theoretically infinite particle size to 80 % passing 100 microns or to approximately 67 % passing 75

microns (Sieve 200#). Given that:

W = work input in KWhr/ton,

F = feed size (size in microns of the sq. hole which 80% of feed passes),

P = product size (size in microns through which 80% passes), and

Rr = reduction ratio or F/P,

Work Index from commercial operations is calculated by the relation established by Allis Chalmers

as:

An alternative equation from the SME Mining Engineers Handbook Vol. 2 for calculating the

approximate unit power consumption is given as:

W = Wi x 11.0 x ( F80 - P80 ) / ( F80 x P80 ) ……………(2)

where W is specific power required in kWhr/ton, P80 is 80% passing size of the product in microns

(µ), and F80 is 80% passing size of the feed in (µ), and Wi is the Work Index of material being

crushed. The total power in kW is found by multiplying the specific power by the crusher

throughput and a factor of 0.75 for primary crushing and 1.0 for secondary and tertiary crushing,

i.e.: Total kW = (crusher capacity) x (W) x (factor). To convert kW to Hp, multiply by 1.341.

Example:

Assume a crusher has a capacity of 2,000 short tons of average material per day for materials with a

Work Index of 13.0. If 80% the crusher feed passes a 3-inch grizzly and a final product of 80%

passing ¾-inch sieve is desired from the crusher for working continuously for 14 hours per day,

calculate the input power required.

Solution (using Eqn. (1) above):

Hours per day = 14

Tons per hour = 143

Feed size (80%) = 3” = 76.2 mm = 76200 µm

Product size (80%) = ¾’ = 19.05 mm = 19050 µm

)1......(..............................1001

)(P

Rr

RrWWiIndexWork

23

Reduction Ratio Rr = F

P =

76200

19050 = 4

Wi = (W x 2) x 190.5½ =W x 2 x 13.8 = W x 27.6

W = Wi/27.6 = 13/27.6 = 0.471 KWhr/ton

Hence, input power = W x TPH = 0.471 x 143 x 1 = 67.4 kW = 67.4 x 1.341 Hp = 90.3 Hp.

NB: Try to use the Eqn. (2) to calculate the power input and compare with Eqn. (1).

The work indexes of selected materials are as follows:

Material Sp. Gr Wi

Bauxite 2.38 9.45

Galena 5.39 10.19

Glass 2.58 3.08

Gold Ore 2.86 14.83

Granite 2.68 14.39

Granite 2.68 14.39

Manganese Ore 3.74 12.46

Quartz 2.64 12.77

Sandstone 2.68 11.53

Granite 2.68 14.39

2.3.2 FINE GRINDING

i. Wet and Dry Grinding

The two basic grinding processes are wet grinding and dry grinding. Though most ores are reduced

by wet grinding (in the presence of water) before being processed, some raw materials require dry

grinding (without the use of water).

Dry grinding is employed where a dry end-product is called for and can be processed up to the

required state without the use of water. Among the raw materials that require dry grinding are coal

for powdered fuel, cement clinker, talc, metal powders, drugs, and chemical salts. In this method,

the grinding media and feed are loaded into the grinding mill and worked dry until the desired state

of attrition has been attained.

24

In the processing of ores, some methods are such that either the finely comminuted mineral surfaces

have to be protected by adding protecting chemicals or the mineral surface be cleaned; these are

best achieved by grinding under water. Hence, wet grinding methods shall be discussed in this

course.

ii. Wet-Grinding Mills

Fine wet-grinding is done in a mill that rotates on a horizontal axis and contains wear resistant

balls, rods, or pebbles (collectively known as grinding media), which grind the material in the mill

under water. The Hardinge mill has a short cylindrical section terminated by two conical sections

whereas most other mills are cylindrical in shape. The mixture of grinding media, ore, and water is

collectively referred to as the crop load.

A ball mill contains steel balls; a rod mill contains steel rods; and a pebble mill contains flint

pebbles. Exceptionally long mills with relatively small diameters are called tube mills. These mills

are often collectively designated by the term “tumbling mill”. They are all nearly half filled with

crushing bodies, collectively referred to as grinding media. Under this course, the ball mill (Fig 5)

will be used to gain general insight into the principles of milling.

Fig 5: Section through a ball mill Fig 6. Closed Circuit grinding

The grinding media of a ball mill consist of cast iron or steel balls. When a ball mill is rotated about

its horizontal axis, the balls are carried up the side by centrifugal force, and when they reach a

certain height they break or fall away from the mill shell and drop back towards the centre of the

mill. This action results in a cascading action of the ball charge, and the resulting impact, together

Scoop

Trunnion

25

with the abrasive action caused by the balls rolling upon one another grinds up the ore particles

mixed with the ball charge. The faster the mill is rotated, the higher the balls are lifted. When the

speed of rotation reaches a certain point, the balls are said to have been carried through a complete

circle. When this happens, the charge is said to centrifuge and all grinding stops. The minimum

speed at which a mill is said to centrifuge is known as the critical speed of the mill, and is given by

the relation:

Nc = 42.29

D …………………………………. (3)

where Nc is the critical speed in revolutions per minute and D is the internal diameter of mill in

metres.

Commercial ball mills operate within the range 50 – 80% (usually 75%) of the critical speed to

ensure that the ball charge undergoes satisfactory cascading action. The feed to a ball mill should,

usually, not be coarser than 10 to 20 mm and the ball size usually exceeds 3 times the maximum

feed size. The ball mill consists of a steel shell in which the inside is completely lined. The purpose

of the lining is: (1) to provide a means of lifting the balls and ore to improve grinding, and (2) to

protect the mill. The lining material may consist of replaceable wear-resistant steel plates (e.g.

manganese steel) or rubber. Metal liners may be smooth or corrugated.

Most ball mills are mounted on trunnions such that feed enters the mill through one trunnion and is

discharged through the other on the opposite end. Feeding is done by means of rotating spiral

scoop, which picks up the feed and delivers it to the mill when the scoop reaches the upper half of

its revolution. Ball mills are usually run with between 45% and 50% of the volume occupied by the

crop load whereas the pulp in the mill is made up of 20 to 50% by weight.

iii. Product Size

The product size from a grinding operation could be as small as required for effective beneficiation.

However, the average size of the discharged ground ore depends primarily upon how long the

material remains in the mill. The longer the grinding action the smaller the average particle size of

the product. The length of time the ore remains in the mill depends in turn upon the feed rate since

the product must discharge at the same rate as the feed enters the mill. Consequently, it is

impossible to say that a ball mill of a given size has a certain capacity since the mill will discharge

material as rapidly or as slowly as it is fed in. Capacity is often expressed as the number of tons of

material less than some specified size (say -150 microns) per hour. However, the most important

factors that affect the capacity of ball mills are:

26

• Size and shape of mill

• Nature of mill lining

• Rate of feed

• Speed of rotation of the mill

• Nature of feed

• Pulp density (amount of water used with the ore)

• Pulp level (amount of pulp in the mill)

• Size of balls in the mill

• Size of product desired

• Weight of ball load

• Whether grinding circuit is open or closed

The size of ball mill is ordinarily given as its diameter. A 2-meter mill is 2 metres in diameter.

However, if two dimensions are stated, such as 2 x 3-metre mill, then the reference is to a mill that

is 2 metres in diameter and 3 metres long; i.e., the first number refers to the diameter. At times 2 or

3 ball mills are run in series (i.e. primary, secondary, and tertiary grinding) when particularly fine

grinding is required.

iv. Closed Circuit Grinding

If one is operating a single mill and desires to grind from, say, 15 mm feed to 100% passing 150

microns (-150µ), then one would charge the mill and run it until all the material passes through a

150 micron screen. By the time this is achieved, a very large proportion of the material would have

been ground to a size much smaller than 150 microns as the balls would continue to grind both

large and small particles to the extent that particles would undergo multiple grinding. This batch

grinding would have two harmful effects: (1) large amount of over ground material in the discharge

that would make concentration processes highly difficult and inefficient, and (2) waste of energy

due to unnecessary over-grinding. On the other hand, to avoid over-grinding, one can run the mill

by continuously feeding and discharging from the mill, in which case the residence time of material

in the mill will be relatively short (open-circuit grinding) and result in excessive oversize product.

To avoid both over-grinding and under-grinding, commercial ball mills are operated in closed

circuit with a classifier. The classifier is a device that takes the ball-mill discharge and separates it

into two portions; i.e. the finished product, which is ground as fine as desired and oversize material.

The oversize is returned to the mill for further grinding (see Fig. 6). The material that is returned

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from the classifier to the mill is called the circulating load of the system. In this way, the ore can be

passed through the ball mill rapidly enough to prevent serious over-grinding (sliming), but the

classifier prevents the accompanying oversize material from escaping from the circuit. The

circulating load is commonly 200 to 1000 percent of the amount of new feed entering the system.

When stage ball milling is in use, the primary ball mill is often used in open circuit while the

secondary mill is used in closed circuit.

2.4 PARTICLE SIZE SEPARATION

Particle size separation, or sizing, in mineral dressing may be done by either screening or

classification.

2.4.1 SCREENING

Screening is a term that applies to the mechanical separation of particles on the basis of size alone.

The operation takes place on a screening surface that consists of several equally sized apertures or

openings. Particles smaller in size than the screen openings pass through and those that are larger

are rejected and remain over the surface. Sizing and sieving are terms that are also used to describe

the screening operation.

During the screening process, material is made to flow over the surface of the screen by gravity

along an inclined screen, by rotating, shaking, or vibrating the screen or, less commonly, by raking

devices. A common approach is to combine gravity and one or two of the other methods. Screens

may be made of welded bars, steel plate punched with round, square, rectangular or octagonal

holes, or wire cloth. Cloth is the most popular material in laboratory screening.

Sieves are used to determine particle size in screen analysis in the laboratory. Materials that pass

through the apertures in sieves (or screens) are referred to as undersize (indicated by the minus

sign, e.g. -3mm fraction). The materials that remain on the screen are known as the oversize

(indicated by the plus sign, e.g. +3mm fraction). The total weight of material to be analysed is

found before screening. The amount remaining on the screen is weighed and calculated as a

percentage of the total material.

Screen size may be described in terms of “space” or “mesh”. Space (opening or aperture) is the

actual dimension of the clear opening such as between adjacent wires in square or octagonal holes,

the diameter of circular holes, or the inner short dimension of a rectangular hole. Mesh is the

number of openings per inch measured from centre to centre of parallel wires. Space is, however,

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measured in inches, millimetres or microns. One micron (µ) is a millionth of a metre –i.e., 1 x 10-

6m or a thousandth of a millimetre (1 x 10-3 mm). It is worthy of note that a square opening allows a

larger throughput than a circular aperture of the same diameter.

Testing sieves are made in sizes as fine as 400 mesh; a material finer than this, known as subsieve

material, might be sized by rising currents of water, air or other form of classification. A popular

series of screens is the Tyler Series in which the aperture of any screen is approximately equal to

1.414 or 2 times the aperture of the next finer screen as shown in Table 2.

Table 2: Tyler Standard Screen scale

Mesh No Opening (ins) Opening (mm) Opening (micron) Diameter of wire (mm)

… 1.050 26.67 26.67x106 3.7592

… 0.742 18.8468 18.85 x106 3.429

… 0.525 13.335 13.34 x106 2.667

… 0.371 9.4234 9.42 x106 2.3368

3 0.263 6.6802 6.68 x106 1.778

4 0.185 4.699 4.70 x106 1.651

6 0.131 3.327 3.33 x106

8 0.093 2.362 2.36 x106

…. …

150 0.0041 0.10414 0.104 x106 0.06604

200 0.0029 0.07366 0.074 x106 0.05334

Another standard for the gradation of aggregates is that prescribed by the ASTM (American Society

for Testing Materials) in which the series of screens and sieves are designated in terms of opening

and mesh numbers as shown in the Table 3.

Table 3: ASTM Series of Testing Sieves

Screen or

Sieve

Designation

Nominal Opening Equivalents

mm inches microns

4 in 101.6

3 in 76.2

2 in 50.8

1.5 in 38.1

1 in 25.4

0.75 in 19.1

0.5 in 12.7

0.375 in 9.53

0.25 in 6.35

No.4 4.76 0.187 4760

6 3.36 0.132 3360

8 2.38 0.0937 2380

12 1.68 0.0661 1680

16 1.19 0.0469 1190

20 0.84 0.0331 840

30 0.59 0.0232 590

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40 0.42 0.0165 420

50 0.297 0.0117 297

70 0.21 0.0083 210

100 0.149 0.0059 149

140 0.105 0.0041 105

150 0.1 0.0039 100

200 0.074 0.0029 74

270 0.053 0.0021 53

400 0.037 0.0015 37

2.4.2 COMMERCIAL SCREENING

Commercial screening operations are used to separate crushed ore or stones into two or more

fractions. The purpose of commercial screening might be to remove fines so that they may bypass

crushing or to classify crushed material into different size ranges.

To facilitate movement of material over the surface of screens, some are shaken or vibrated

mechanically whereas others are stationary with a sloping surface. Thus, the two main types of

commercial screens are stationary and dynamic screens.

i. Grizzly

The only commercial stationary screen is the grizzly (Fig. 6). In a way, grizzlies are large industrial

screens with relatively coarse openings used to separate broken rock or ore into two or more

fractions and may reject boulders from the feed material. They are made up of parallel bars or

chains. Old mine rail tracks are sometimes used to construct grizzlies locally.

Figure 6: A vibrating bar grizzly

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The primary use of grizzly is for sizing the feed for ore crushers. Thus if a crusher produces a -100

mm (-3.94-inch) product, then the feed is usually passed over a 100 mm grizzly. Thus only the

+100 mm material is fed to the crusher with the undersize from the grizzly joining the crusher

discharge. Grizzlies can be shaken or vibrated mechanically, electrically, or by the impact of falling

rock (spring loaded ones). Inclined grizzlies are usually set at an angle of 25–50o to facilitate the

flow of oversize material into the crusher. However, flat grizzlies are often used to retain large

oversize, which might cause trouble if allowed to flow into the crusher. The grizzly is sturdy

enough to act as an anvil if such oversize is to be sledged down by hand or by a hydraulic breaker.

They are used to screen dry material.

ii. Revolving Screens or Trommels

Used for sizing coarse material in gravel washing, coal washing, and quarry (stone treating) plants.

They consist of a cylindrical (sometimes conical) screening or perforated surface mounted on a

revolving frame.

Revolving screens rotate on rollers and have nothing on the internal of the screens. Trommels are

supported from a shaft through the axis of the cylinder carrying two or more spiders to which the

screen is attached. They are 24-84 inches in diameter and 6-30ft in length. They are used for

screening material between ¼ and 21/2 inches in diameter. They may be used for dry or wet

screening.

iii. Punched Plate or Woven Wire Screens

Several types exist, but mostly consist of woven-wire vibratory screens. They are woven of steel

wire and stretched tightly on a metal frame. A high speed vibrator produces a vibratory motion at

right angles to the plane of the screen.

They vibrate at a frequency of about 1000 to 3600 cycles per minute and the screens range in size

from about 36 x 36 inches to 72 x 60 inches and might be set at an angle of 30 to 40o to the

horizontal. The rapid vibration of the screen surface tends to prevent “blinding” or clogging of the

screen meshes. Blinding occurs when some of the holes in the screen are blocked (or jammed).

Blinding might be caused by near particle size material that sits on the hole instead of going

through it.

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The opening or aperture of punched-plate and coarse woven-wire screens is commonly given in

inches or millimetres. The aperture size of finer woven screens is given by a mesh number as

shown in Tables 2 and 3. Screening may be done either wet or dry but damp material cannot be

screened effectively.

iv. Capacity and Efficiency of Screens

The capacity of a screen is the measure of the amount of material that can be screened in a given

time. It is measured in tons per square foot per hour per millimetre of aperture. Capacity and

effectiveness are opposing factors. To obtain maximum effectiveness, the capacity must be small,

and large capacity is obtainable only at the expense of a reduction in effectiveness.

2.4.3 CLASSIFICATION

This refers to sizing operations that exploit the differences in settling velocities exhibited by

grounded particles of different sizes and densities. The equipment used in this separation technique

includes ore sorters, gas cyclones, hydrocyclones, mechanical classifiers, rotating trammels, etc.

The equipment employs one of the two principal separation methods: viz. sink-and-float and

differential settling.

The sink-and-float method uses a liquid sorting medium, the density of which is intermediate

between that of the light material and that of the heavy material. Then the heavy particles settle

through the medium, and the lighter ones float, for a separation to be obtained. This method has the

advantage that, in principle, the separation depends only on the difference in the densities of the

two substances and is independent of the particle size. This method is also known as the heavy-

fluid separation and is used to treat relatively coarse particles, usually greater than 10-mesh.

Differential settling methods utilize the difference in terminal velocities that exist between

substances of different density. The density of the medium is less than that of either substance.

Consider two particles X and Y settling through a medium of density with X heavier than Y. If

the smallest particle of X settles faster than the largest particle of Y, then complete separation of the

two particles X and Y can be achieved.

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2.5 CYANIDATION

Cyanidation, also known as MacArthur-Forrest Process, is a leaching method of extracting gold,

copper and silver from their ores. The process was invented in 1887 at Scotland by John Stewart

MacArthur and was funded by the brothers Dr Robert Forrest and Dr William Forrest. It is based on

the fact that gold dissolves in dilute solutions (0.05%) of sodium or potassium cyanide in the

presence of oxygen. However, sodium cyanide is what is commonly used in the industry because of

its strong property for dissolving metals. Usually, about 300-500 tons of sodium cyanide is used for

the extraction of a ton of gold. The process is an all-sliming process whereby the ore is finely

ground (to about 200 mesh size) and agitated with a cyanide solution. The basic procedure followed

in using the process for extraction of gold is as follows:

1. Contacting the finely ground ore with a dilute solution of sodium (or potassium) cyanide in

the presence of atmospheric oxygen. This causes the dissolution of gold, along with the

production of sodium cyanoaurite and sodium hydroxide, and is known as the Elsner Reaction:

4Au + 8NaCN + O2 +2H2O 4NaAu(CN)2 + 4NaOH

[Similarly for Silver: 4Ag + 8NaCN + O2 +2H2O 4NaAg(CN)2 + 4NaOH]

There are two main ways of contacting the ore with the cyanide solution; these are:

i. Vat leaching: where the ore and solvent are put in large tanks for hours to dissolve the gold.

This process is very suitable when the grade of the ore is more than 20 g/ton.

ii. Heap leaching, which is very suitable for low-grade ores. Under this, a solution of sodium

cyanide is sprayed onto a heaped ore to dissolve the gold. The gold enriched solution drains

off at the bottom of the heap where it is pumped to a recovery plant.

2. Separating the solids from clear solution: The freely swimming gold ions is separated from the

gold solution using activated carbon, which tends to adsorb the gold ions onto itself. Again, strong

solutions of sodium cyanide (and sodium hydroxide) are used to leach the gold from the carbon.

Afterwards, the gold is recovered by the Merrill-Crowe process or electrowinned onto steel wool by

electrolytic process.

3. In the final process, the solution with gold ions is deoxygenated and passed through a filter-press

where the gold is displaced from solution by reduction with zinc metal powder and the precipitate

of gold are collected.