GED 152: BASIC MINERAL SCIENCE

Instructor: Dr. Emmanuel K. Appiah-Adjei

Geological Engineering Department KNUST, Kumasi

GED 152: BASIC MINERAL SCIENCE

Lecture: 10:00 – 11:55 @ NBSF2 (Friday)

10:00 – 11:55 @ 304 (Monday)

Credit: 2

Contact: ekappiah-adjei.soe@knust.edu.gh

Room A107 near the Computer Room

TAs: Barnor and Ntori

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COURSE OBJECTIVES

• Introduce metals, minerals and their ores

• Present the processes and professional disciplines

related to obtaining metals/economic materials from

the earth

• Discuss various mineral processing techniques

• Discuss the various particle size separation

techniques and the equipment used

• Present gold cyanidation process

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COURSE OUTLINE WEEK ACTIVITY

1 – 2 REGISTRATION AND COURSE INTRODUCTION

Overview of the course

3 – 5 METALS, MINERALS AND THEIR ORES

Definitions, Metals, Minerals, etc.

6 MINERAL PROCESSING

Overview of processes involved

7 – 10 COMMUNITION

Crushing, Grinding and Sizing (Screening)

11 CYANIDATION

12 REVISION

13 – 16 EXAMINATIONS

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LITERATURE

• Lecture Notes

• Lecture Slides

• Mineral Resources, Economics and the Environment (Kessler, 1994)

• Mineral Processing Technology by (Wills and Napier-Munn, 2006)

• Relevant materials online?

NOTES

Attendance, Punctuality, Cheating, etc.

Field visit to a quarry?

Grading {Exam (70%) + CA (30%) = 100%}

CA (Mid-Semester Exam, Assignments, and Tests)

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METALS

• Metal is any category of electropositive elements that usually have a shiny surface, generally good conductors of heat and electricity, and can be melted or fused, hammered into thin sheets, or drawn into wires

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METALS – cont’d

• All known metals originates from the earth crust

• The crust is made up of rocks of different characteristics and properties depending upon their mineralogical composition

• Any piece of rock is an aggregate of minerals

Mineral is a naturally occurring, inorganic solid with a definite chemical composition and a specific crystalline structure

(non-technically, it may include organic or non-solid substances extracted from the earth, e.g. crude oil)

Ore is a naturally occurring aggregate of minerals containing precious (useful) metals (metalloids) that may be extracted at profit

An ore deposit must contain metal(s) that: - are valuable, - are high in concentration to be mined profitably, and -can be extracted from a rock by mineral processing techniques

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Metals may, broadly, be classified as ferrous and non-ferrous

o Ferrous metal is a metal with iron as its major constituent

- Fe is the most widely used and about the cheapest metal available

- Fe has SG=7.57 and forms several alloys that rust easily

- Common alloys of Fe are Steel (Fe + < 2%C + other elements);

Pig Iron (Fe + >2%C + other elements); Wrought Iron; Mild Steel; etc.

- Metals (e.g. Mn, Si, Ni, V, etc. ) that combine with Fe or steel to produce alloy steels or ferrous alloys are known as ferroalloy metals

METALS – cont’d

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o Non-Ferrous Metals are metals that do not contain Fe

- They may be, broadly, subdivided as:

1. Precious metal

• They are metals of high economic value

• Less reactive but have high luster and electric conductivity

• May be grouped as:

-Traditional Precious Metals (e.g. Au and Ag) are better known for their use in art, jewelry, and coinage as well as industrial usage

- Platinum Group Metals (e.g. Pt, Pd, Rh, etc.) are mainly used in refining, manufacturing and jewellery industries

2. Base Metals

• Common and inexpensive metals that oxidize/corrode relatively easily. Examples are Cu, Pb, Zn, Sn, etc.

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3. Light Metals

• They have lower densities than most metals e.g. Al (SG = 2.7), Ti (SG = 4.51), Mg (SG = 1.74), Be (SG = 1.85), etc.

4. Chemical and Industrial Metals

• Used largely in chemical or industrial applications e.g. As, Hg, Bi, Cd, Sb, Se, Ta, etc.

5. Soft Metals

• They are soft and very active chemically; used as chemical reagents but have no engineering application. Examples are K, Ca, Na, Li, etc.

6. Rare Earth Metals

• Are a group of closely related metals, from lanthanum to lutetium in the periodic table

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GEMSTONE - Is an attractive mineral, which when cut and polished, is used to make

jewelry or other adornments e.g., Diamond, Emerald, Ruby, Sapphire, etc.

FUEL MINERALS -Examples are Coal, Oil, and Natural Gas

NUCLEAR METALS - Examples are Uranium, Polonium, Thorium, Caesium, etc.

ELECTRONICS METALS AND MINERALS - Examples are Mercury, Silica, Mica, Cadmium, Germanium, Gallium, etc.

INSULANTS AND REFRACTORIES - Examples are Asbestos, Magnesite, Perlite, Graphite, Andalusite, etc.

NATURAL ABRASIVES

- Examples are Industrial Diamonds, Silica sand, Garnet, Emery, Feldspar, etc. 12

FERTILIZER AND CHEMICAL INDUSTRIAL MINERALS

-Examples are Limestone, Dolomite, Lime, Phosphate, Rock salt, Potash, Sulphur, Fluorite, Iodine, Sodium Sulphate, Bromine, etc.

MINERAL MATERIALS FOR CONSTRUCTION/MANUFACURING

• Construction Minerals

- e.g. Cement, Rock Aggregates, Common Clay, Gypsum, etc.

• Fillers, Extenders, Pigments, and Filters

- e.g. Clays, bentonite, talc/pyrophyllite, asbestos, mica, zeolites, etc.

• Glass and Ceramics Raw Materials

- e.g. Soda Ash, Limestone, Feldspar, Sodium Sulphate, etc.

Metals may also be classified as Primary (Virgin) –i.e. metal extracted from ores, brines or ocean water– or Secondary metals –i.e. metals derived from scrap e.g. steel produced from scraps

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ORE MINERALS • Ore minerals are those minerals that contain the valuable metals in

an ore; metals are extracted from them

• Gangue Minerals are the valueless minerals associated with an ore mineral

• The wall rock broken with the ore is called waste

• The unbroken (in-situ) rock adjacent to the ore body is called a country rock

• Ores may be classed by the nature of the valuable mineral as:

1. Native Ore: the metal is present in elemental form

2. Sulphide Ore: the metal is present as sulphide

3. Oxidized Ore: metal may occur as oxide, sulphate, or carbonate

4. Complex Ore: contains more than one valuable mineral

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Common ore types include:

1. Fe ores e.g., Haematite, Native Iron, Magnetite, Limonite, etc.

2. Pb ores e.g., Native Pb, Galena, Anglesite, Cerussite, Crocoisite, etc

3. Zn ores e.g., Native Zn, Sphalerite, Zincite, Willemite, etc.

4. Cu ores e.g., Native Cu, Chalcopyrite, Chalcocite, Covellite, etc.

5. Sn ores e.g., Cassiterite and Stannite

6. Al ores e.g., Bauxite(Gibbsite, Diaspore & Boehmite), Corundum etc

7. Au ores e.g. Native Au, Sylvanite, Calaverite, Petzite, etc.

8. Ag ores e.g. Native Ag, Argentite, Pyrargyrite, Proustite, etc

9. Mn ores e.g., Pyrolusite (MnO2), Manganite (MnO(OH)), Rhodocrosite (MnCO3), Rhodonite (MnSiO3), etc.

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Ore Grade

- It is a measure of the valuable material (metal or mineral) in an ore

- It is used to assess the feasibility of mining operation

- It is expressed as % or ratio of a specific quantity per unit weight or volume of the ore depending on valuable component concerned

- Precious metals (Au, Pt) and gems are usually expressed as amount of material per ton of ore ( e.g. g/t, carat/t, oz/t, ppm, etc.)

- Base metals (Cu, Zn) and industrial minerals (Mn) are usually expressed as %

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Assignment 1

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Processes involved in obtaining metals from the earth are:

1. Mineral Exploration (i.e., Exploration Geology)

• Search for ore deposits and identify their mode of occurrence

• Establish the type and importance of mineral(s) or metal(s) in the ore deposit

• Estimate the quantity of both the ore and mineral (or metal)

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

2. Mining (i.e., Mining Engineering)

• Establish technical and economic feasibility of exploiting the metal

• Identify and design the method for mining the ore deposit

• Organise men and materials to carry out the exploitation of the ore

• Supervise the actual extraction of the ore material

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Bogoso Mines 19

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3. Mineral Dressing (i.e., Mineral Processing)

• Is the process of separating the mineral grains from the gangue

• Prepare the mined ore and render it amenable for treatment

• Extract the highest possible proportion of the mineral/metal from the gangue and discard the less useful associated materials.

• It involves 4 operations: communition, sizing, concentration and dewatering

• A plant is usually constructed in a mine for the above operations

• Run-of-Mine (ROM) ore refers to excavated ore that is transported to the mill for processing

• Tailings is gangue material that has been reduced to a finely ground powder or sand in the form of slurry after the processing

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4. Smelting and Refining (Extractive Metallurgy)

• Separate metals from their ores or, in most cases, from an enriched (concentrated) ore mineral to obtain the final metal product

• Objective is to purify the metal to the desired quality (99.9% purity)

• 3 main subdivisions of the extractive process are:

i. Pyrometallurgy – involves applying high temp and some chemicals

ii. Hydrometallurgy – involves separation of metals in aqueous solution from the rest of their ores, followed by precipitation in metallic form

iii. Electrometallurgy – involves the use of electrical energy

5. Material Fabrication, Alloying, etc. (Physical Metallurgy)

• Study the detailed characteristics of the metal/material in its final form and the various ways in which it could be utilised

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COMMINUTION

• It refers to the various size reducing processes undertaken during processing of mined ores

• It involves crushing and grinding of the ores (or rock material)

CRUSHING

• It is the first stage in the comminution operation whereby hard ROM ore is reduced to the desired size

• The largest crushing plant may accept a feed with maximum particle size of 1.524m and reduce it to about 5mm product size

• It may be carried out in 1, 2, or 3 stages (viz. primary, secondary, and tertiary crushing respectively) before acquiring the desired size

• Reduction ratio is the working relationship between the gape and set of a crusher

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• Factors that determine crushing equipment and circuit selection are:

1. Tonnage rate (throughput)

2. The size distribution of feed (i.e. run-of-mine ore)

3. Desired product size

4. Method of feeding

5. Ore characteristics (e.g. compressive strength, abrasiveness, clay content, etc)

Primary Crushing

• Two main types of primary crushers available are Jaw and Gyratory

• The largest crusher can accept a feed size of up to 2.1336m x 1.524m and reduce it to product size of 101.6mm to 152.4mm

• The product size is determined by the set of the primary crusher.

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Jaw Crusher • To and fro movement of the moving jaw

crushes the ore to desired sizes

• It is mainly used for primary and secondary crushing operations

• Mouth – is the point of entry of the feed into the crusher; its dimensions are given by the width of the crushing jaws and the gape

• The size of the crusher is measured by the dimensions at the mouth in mm or inches; e.g., 48’’ X 72’’ (i.e., gape by jaw width)

• Throat - is the point at which the rock is discharged from the crusher

• Stroke (or Throw) is the difference in mm(or in.) between the open and closed throat dimensions of a crusher

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• Crushers are usually adjusted to produce a selected product size

-3 main, interrelated, parameters used in crusher settings are:

1. Closed-Side Setting (CSS) – is the minimum distance between the surfaces at the closed position

2. Open-Side Setting (OSS) – is the maximum distance between the surfaces at the open position

3. Throw – is the distance in the direction of compression that the moving wear surface travels between the open-side setting and the closed-side setting.

The average of the CSS and OSS is the best measure of the size of the discharged product from the jaw crusher

Capacity of a crusher is defined as the number of tons of finished product made per hr; It decreases as the reduction ratio increases

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Gyratory Crusher • It consists of a fixed crushing surface within

which is a moving crushing surface that crushes the ore as the surfaces converge

• The size of a gyratory crusher is expressed by the maximum opening at the mouth in mm(or inches); 1000 mm gyratory crusher has a gape of 1000 mm

• It can be used for primary and secondary crushing operations

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Comparing Jaw and Gyratory Crushers

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

• Gyratory crushers can be choke-fed by direct dumping from trucks while Jaw crushers require a scalping grizzly and a feeder

• Jaw crushers require less maintenance than the gyratory crusher

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

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Secondary Crushing

• The essence is to further break down primary crushing products to a size suitable for feeding a grinding mill

• The maximum feed size into a secondary crusher is about 152 mm in diameter and discharges a product size between 6.5 to 65 mm

• There is often a closed circuit arrangement between the primary and secondary crushing operations

Feed ≤80 cm

Grizzly 15 cm spacing

+15cm Primary Crusher

Screen 15mm

-15cm

Final product, -15mm

Secondary Crusher

+15mm -15cm

Conveyor Belt

A typical closed circuit crushing flow sheet

Fine-Ore Bin

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Cone Crusher

• It is exclusively used for primary and secondary crushing operations

• It operates in a similar way to the gyratory crusher but at faster rate

• It consists of a conical crushing head that gyrates within an inverted truncated cone called the bowl

• Its size is denoted by the bottom diameter of the crushing head or cone (size ranges from 0.6 to 3 m) or by the feed opening and mantle diameter (e.g., 200 – 2050)

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Other available crushers include:

1. Roller crushers - mainly used for crushing easily fractured materials like soft limestone, chalk, clay, etc

2. Impact crushers - can crush hard rock material (e.g., granite, basalt, bluestone, etc) with diameters from 100 to 500mm. Its final product is desirable for use as aggregate in highway and railway surfaces, etc.

http://www.fam.de/english/Products (then go to mineral processing)

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R.O.M. Ore

60t rock Box

Apron Plate Feeder (40kW) No.1 Conveyor

(42’’, 5kW)

Primary Crusher

(110kW) (Toggle jaw)

+8’’

-8’’

Primary vibrating

screen (20kW)

No.2 Conveyor

(42’’,20kW)

Secondary Vibrating Screen

(15kW) -4’’

No.3 Conveyor

(42’’,15kW)

No.4 Main Conveyor

(14600’’,30,450kW)

Secondary Crusher

[(2*75kW) (Twin Roll)]

-8’’+ 4’’

100t Surge Bin

Gate feeder

(40 kW)

To Washing Plant

Crushing Flowsheet @ GBC, Awaso

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Power Calculations for Crushers

• Bond’s theory is widely accepted for determining the power (i.e. motor size) of crushers

• The theory is based on the “Work Index” for the material being crushed

- 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 200#.

• Specific Power,

• Total Power (in kW) = (crusher capacity) x (W) x (factor)

- factor for 1o Crusher = 0.75; for 2o and 3o Crushers = 1

• To convert kW to Hp, multiply by 1.341

1001,

P

Rr

RrWWiIndexWork

(Established by Allis Chalmers)

W = Wi x 11.0 x ( F80 - P80 ) / ( F80 x P80 ) (by SME Vol. 2)

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Example 1

Assume a crusher has a capacity of 2,000 short tons of average material per day and a Work Index of 13.0. If the crusher feed has 80% passing 3-inch and a final product of 80% passing ¾-inch is desired and the crusher works continuously for 14 hours per day, calculate the input power required.

Example 2

What is the power required to crush 100 ton/h of limestone if 80% of the feed pass a 2-in screen and 80% of the product a 1/8 in screen? The work index for limestone is 12.74.

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Example 1

An open circuit cone crusher at a quarry in Buoho is fed with 2100 tons of granitic material per day. The crusher works 16 hours in a day and the material has a work index of 16 kWhr/ton. If the crusher has the capability to reduce the material from 80% passing through 160 mm screen to 80% passing through 20mm, what is the power needed to carry out this operation?.

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FINE GRINDING

• It involves breaking the crushed material into fine particles through attrition and compression at the grain size level

• There are two basic grinding processes; wet and dry grinding

• Dry grinding is employed where a dry end-product is required

- Examples of materials that require dry grinding are coal for powdered fuel,

cement clinker, talc, metal powders, drugs, etc.

• Wet-grinding is done in a mill that rotates on a horizontal axis and contain wear resistant grinding media -i.e. balls, rods, or pebbles- which grind the material in the mill under water

- The mixture of grinding media, ore, and water are collectively referred to as crop load

• Grinding is, usually, performed with a grinding mill; each grinding mill is identified by the type of grinding media utilized

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Types of Grinding Mills

1. Ball Mill

– Is a slightly inclined or horizontal rotating cylinder partially filled, usually, with steel balls, which grinds material to the necessary fineness by friction and impact of the tumbling balls

- The ball size usually exceeds 3 times the maximum feed size and the feed size should not be coarser than 10 to 20 mm

- The longer the grinding action, the smaller the average particle size of the product; length of time the ore remains in the mill depends on the feed rate

- Its capacity is often expressed as the number of tons of material less than some specified size (e.g., -150 microns) per hour

- Size of ball mill is ordinarily given as its diameter (e.g., 2-meter mill); If the dimension is given as 2 x 3-metre mill, then 2m is diameter and 3m is length

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Factors that affect the capacity of ball mills include:

• Size and shape of the mill

• Nature of mill lining and feed

• Rate of feed

• Speed of rotation of the mill

• Pulp density (amount of water used with the ore) and Pulp level (amount of pulp in mill)

• Size and weight of balls in the mill

• Size of product desired

• Whether grinding circuit is open or closed

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Types of Grinding Mills Cont’d

2. Pebble Mill – It consists of a rotating drum with rock pebbles as the grinding media

- The rotating drum causes friction and attrition between rock pebbles and ore

particles; may be used where product contamination from the steel balls or rods is to be avoided

3. Rod Mill – It is similar to the pebble mill but has steel rods as grinding media.

- the rotating drum causes friction and attrition between steel rods and ore

particles to effect grinding

4. Grinding Rolls - It consists of a pair of vertical cylindrical rollers rotating in opposite directions

- ore is fed between two rollers that are pushed firmly together while their rotating motion pushes the ore through a small gap between them to grind the ore

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Closed Circuit Grinding

• It helps to avoid both over-grinding and under-grinding

• Over-grinding leads to energy wastage and inefficiency in concentration process

• Classifier is a device that separate ball-mill discharge into 2 portions: oversize (o/s) and undersize (u/s; i.e., product)

• Circulating load is the o/s material that is returned from the classifier to the mill

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Particle Size Separation

• Sizing in mineral processing is done by screening or classification

Screening is the mechanical separation of particles on the basis of size alone

- It takes place on a screening surface that consists of several equal sized apertures or openings

- Materials that pass thru the apertures are referred to as undersize (-) and those that remain on the screen are known as oversize (+)

- Available screening equipment include stationary screens and grizzlies; gyrating screens; vibrating screens; wire mesh screens, etc.

- These screens may be made of welded bars, steel plate punched with round, square, rectangular, or octagonal holes; or wire cloth

- Screen size may be described in terms of “space” or “mesh”

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Laboratory Screening

- Screening in the laboratory is done with sieves (i.e. sieve analysis)

- Sieve analysis in the lab is done with standard series of screens (e.g. ASTM, Tyler, BS, etc.)

- The screens are designated by their opening and mesh numbers

Commercial Screening

- is done to separate crushed ore or rocks into 2 or more fractions

- mainly categorized as stationary and dynamic screens

1. Grizzly – is a screen with coarse opening used to separate broken rock or ore

- It is primarily used for sizing feed of ore into crushers

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Revolving Screens or Trommels

- Used for sizing coarse material in gravel washing, coal washing, and quarry plants

- Consist of a cylindrical (or conical) screening or perforated surface mounted on a revolving frame.

Punched Plate or Woven Wire Screens

- Mostly consist of steel woven-wire vibratory screens stretched tightly on a metal frame

- Range in size from about 36 x 36 inches to 72 x 60 inches; Rapid

vibration of the screen surface tends to prevent “blinding” or clogging of the screen meshes.

- Opening or aperture of punched-plate and coarse woven-wire screens is given in inches or mm while finer woven screens are given by a mesh number

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Capacity of a screen is a measure of the amount of material that can be screened in a given time;

-To obtain maximum effectiveness, the capacity must be small and vice versa

Classification

- Refers to sizing operations that exploits the differences in settling velocities exhibited by particles of different sizes and densities.

- The equipment used 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

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WASHING FLOWSHEET

No.4 Main Conveyor

Blade Mill

Recycled water

from Tailings Dam

Double Deck vibrating Screen

60’’ Spiral classifier

Static Screen

-4’’

Water Spray

-4’’+1’’

Water Spray

30’’ Conveyor

-1’’+1/8’’

Lump Ore

-4’’+1/8’’

24’’ Conveyor with

Travelling feeder

-1/8’’+60 mesh

Classifier Recovery

Clean water spray from

Clear Water Dam

No.1 Sherwin Screen

Ghana Railways 43 &25t

wagon to Takoradi

-60 mesh

Classifier Overflow

Pumped to Tailings Dam

No.2 Sherwin

Screen

+ 60 mesh

-60 mesh

Pumped to Tailings Dam

+60 mesh Tipped on

Fines Stockpile

Lump

Stockpile

1600t Rail

Bin

-1/8’’

Underflow

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CYANIDATION

- Is a metallurgical process of extracting Au and Ag from their ores

- The process (alias MacArthur-Forrest Process) was invented in 1887

- Is based on the fact that gold dissolves in dilute solutions (0.05%) of Na or K cyanide in the presence of oxygen; NaCN is commonly used

• The basic procedure followed for the extraction of gold are:

1. Contacting the finely ground ore with a dil. NaCN (presence of 02)

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

- 2 ways of doing the contacting: Vat leaching or Heap leaching

2. The solution carrying freely swimming gold ions is isolated (e.g. by carbon), and subsequently separated by electrolysis process

3. The solution with Au ions is deoxygenated, goes through a filter-press and, using zinc dust, precipitates are recovered

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