Tanzania Gold Mines

7/29/2019 Tanzania Gold Mines

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FREQUENTLY ASKED QUESTIONS

ABOUT GOLD MINING OPERATIONS IN TANZANIA

Gilay Shamika

2010


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Forewords

Foreign Direct Investment is still new in Tanzaniacompared to our neighboring countries. This is due

to economic ideology of Tanzania adopted during

independence. The Citizens of Tanzania have a right

to be educated with this new ideology of

privatization and globalization to mitigate thediscontents and rumors associated with all projects

under foreign direct investment.


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1.0 INTRODUCTION

Mining operations in Tanzania is at its initial stages compared to other countries which

have more than hundred years practicing mining operations. The good thing of beingthe follower is to have advantage of not repeating the same mistakes done by the

leaders/beginners.

By having references from those who have experience, it is better to be more vigilant to

read between the lines of those who have experience to get rid of being manipulated by

them.


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2.0 FREQUENTLY ASKED QUESTIONS

Among other questions, these are commonly asked;

Question 1: How many major gold mining companies currently operating in

Tanzania?

Answer: There are six (6) major gold mines. These are Bulyanhulu Gold Mine(BGM),

Geita Gold Mine (GGM), North Mara Gold Mine(NMGM),Tulawaka Gold Mine(TGM),

Golden Pride Mine (Resolute Nzega) and Buzwagi Gold Mine.

Question 2: What are the Ores and types of Ores processed at each major gold mine?

Answer: Ores are the Rocks containing valuable minerals. Two types of Ores available,

Oxide Ore and Sulphide Ore. Oxide ore at Geita Gold Mine, North Mara Gold Mine,

Tulawaka Gold Mine ,Golden Pride Mine and Buzwagi Gold Mine which has a reserve

of Oxide and Sulphide Ore. Buzwagi is currently mine and process Oxide ores.

Bulyanhulu Gold Mine is only mine currently process sulphide ore by froth flotation

method.

Question 3: What is the difference between Oxide ore and Sulphide ore?

Answer: In Oxide ore, the gold is not locked in a complex manner. It is embedded in a

gangue and sometime appears free while in Sulphide ore, the gold is locked within the

Sulphide minerals (pyrite and chalcopyrite) in a complex manner as a compound.

Question 4: What type of method used to mine the Ores?

Answer: Two methods are used, open pit/cut and underground mining. Bulyanhulu

use underground mining. Tulawaka uses both methods. The rest of mines use open pit

mining. The decision of which method to be used, depends on the Ores localization.

Question 5: What type of gold processing method used by major gold mines?

Answer: Carbon-in-Leach/pulp (CIL/CIP) and Froth Flotation process. The type of ore

suggests the process to be used. Sulphide ore is processed by flotation and Oxide ore by

CIL/CIP for economic recovery. The Sulphide Ore at BGM requires froth floatation

technology for economic recovery of gold and silver. Froth floatation technology results

in the production of Copper Concentrate as part of its gold processing. Other Mines

have Oxide Ore which requires CIL/CIP process in gold recovery.

On June 2007 CIL plant was commissioned at BGM to treat Cleaner tails from flotation.

Cleaner tails behave like Oxide Ore; gold is no longer locked in complex compounds.


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Question 6:What is the difference between CIL/CIP and Froth Flotation?

Answer:Carbon-In-Leach (CIL), leaching and adsorption takes place simultaneously (in

one tank) while Carbon-In-Pulp (CIP), leaching and adsorption takes place into

different time and different tanks (then mixed in one tank).CIL/CIP process uses carbon

porous for adsorption of gold particles.

Froth flotation uses density differences to separate valuable and non-valuable minerals

by the use of air injections in Roughers and Cleanears. (Roughers and Cleaners are

flotation machines)

Question 7: Why production of Copper Concentrate at Bulyanhulu Gold Mine?

Answer: Froth floatation technology results in the production of Copper Concentrate as

part of its gold processing while CIL/CIP results in production of calcines and then gold

bars after smelt. As explained above, Sulphide ore at BGM requires Flotation processwhich produce Copper Concentrtes.Other mines has Oxide Ore which requires CIL/CIP

and produce gold bars.

Questions 8:What is a content of Copper Concentrate at Bulyanhulu Gold mine?

Answer:Copper Concentrate contains 250g/t gold, 200g/t silver and 16% Copper

Questions 9: How much gold is present in a container of Copper Concentrate

exported?

Answer: The copper concentrate contains approximately 250 g/t gold, 200 g/t silver and

16 % copper. The Copper Concentrate is exported into containers with approximatelyweight of 21tons.

Therefore each container of Copper Concentrate exported contains approximately 5kg

of gold;

i.e. (250g/t gold *21t)=5250g (5.2kg) gold

Questions 10: Do other countries export copper concentrate to China other than

Tanzania through Bulyanhulu Gold Mine?

Answer: Many countries not only Bulyanhulu Gold Mine which produce and exportCopper Concentrate to China or Japan.USA and Australia are among other countries

producing Copper Concentrate and export to China.(Seewww.tmaa.go.tz,)
http://www.tmaa.go.tz/http://www.tmaa.go.tz/http://www.tmaa.go.tz/http://www.tmaa.go.tz/


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Questions 11: Why Zambia has a Copper smelter and Bulyanhulu Gold Mine (BGM)

still say not viable to construct a copper smelter in Tanzania?

Answer:There are two types of Smelters, Copper Concentrate Smelter and Copper Ore

Smelter. The one at Zambia is Copper Ore Smelter which treats Copper Ore and not

Copper concentrate.

Most of Copper Concentrate smelters installed worldwide has feed capacities greater

than 150,000 tonnes per year and BGM produces only 50,000tonnes per year;

The total Copper Concentrate construction cost exceeds US$ 500 Million. (See

www.tmaa.go.tz,)

Question 12: Why Planes are landing and taking the minerals without the

government knowing what is going on?

Answer: The Government through Tanzania Minerals Audit Agency (TMAA) knowsproduction and export of gold and Copper Concentrate at each mine on daily basis. The

TMAA Auditors are present in all major gold mines full time .TMAA Auditors are

present at each mine all the time and nothing is done without their presence. TMAA

field Auditors do what is called insitu audit.They witness and involved on every

production and export of gold.

Question 13: How does the Government know if the assays (purity) provided by

mines are correct?

Answer: For every gold bars produced, the samples are taken between two parties,TMAA auditor and mine staff. TMAA sent samples to TMAA laboratory and the mine

to the SGS analab. The assay reconciliation is done at an appropriate time. Hence

TMAA laboratory act like TBS for mines assays. E.g. The gold bar of 25kg has an assay

of 80%.This means the amount of gold out of those 25kg is (25*80%) = 20kg of gold.

Questions 14: Who will be responsible to rehabilitate disturbed land after mines

closure?

Answer: TMAA under environmental audit, assess whether mining companies have

given to the Government adequate financial assurance to guarantee the costs ofreclamation (including shut-down, closure and post-closure). This will ensure the costs

of reclamation are covered by the mine operator and not the Government.
http://www.tmaa.go.tz/http://www.tmaa.go.tz/http://www.tmaa.go.tz/


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Question 15: Major gold mining companies have declared losses, how does the

government approve the same?

Answer: The government through TMAA Financial analyst has the responsibility to

confirm the said. They also review financial projections and budgets on revenues,

capital and operational expenditures of the major gold mining companies for the

purpose of forecasting potential Government revenues (win-win situation) from major

gold mines in form of taxes and royalties.

Question 16: Does TMAA also involved in knowing production and sales of gold

mined by ARTISANAL and SMALL SCALE MINERS?

Answer: YES, TMAA is responsible for major gold mining companies and ARTISANAL

and SMALL SCALE MINERS.TMAA under the directorate of Minerals Production and

Export has introduced two divisions one dealing with major mines and the other with

small and medium mines. The same effort and activities enforced to major mines are

also conveyed to small and medium mines.


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3.0 GOLD PROCESSING TECHNOLOGIES

Within extractive industry, gold extraction is one of a complicated and the longestprocess. For someone who is not familiar with the gold process, he/she imagine gold is

easily found because is seen as a readymade. The way it look when already processed,

you cant imagine when you compared when it was still in ore.!

Gold processing is a laborious job and gambling in nature a lot of uncertainties. When

you tell somebody a ton of ore gives out 5g of gold and even less, it seems as a

ridiculous or a joke but that is a way it is. On the side of value, there is a common

saying in streets that if you get gold which can be full within a match box, no need of

work. you will spend the money for your life time. It is not true.

Amalgamation was the earliest method of gold processing. Mercury was the chemical

used to agglomerate the gold slimes and then smelt it to get ingot. For the mercury to be

more effective to recover substantial amount of gold, the ore should have high grade

and particle sizing need to be in micro.

Later came the invention of Carbon- In -Leach (CIL) and Carbon-In- Pulp (CIP).These

technologies are believed to be invented in South Africa. These technologies are able to

recover the Ore with grade less than 1g/t. Based on this efficiency; a lot of plants were

installed to process gold and increased the gold trade worldwide. The problem of these

methods was the gravity gold that is the gold with large particles which cannotinter/absorbed into carbon porous when passed through leaching without dissolving.

The problem was solved by introduction of gravity circuit such as knelson concentrator,

Gemini table and grizzly/shaking screen on materials from grinding circuit. All these

operation units deal with gold particles which are large and believed wont dissolve

efficiently into CIL therefore they need special care to recover them.

Currently the Gemini table is not used and other technology known as ICR or ILR is

used to treat those gravity gold with trade name of ACACIA. The Optical Sorting

technology is on pilot a test which is aimed to reduce waste materials and hence

minimize the grinding cost of waste Ore. The Optical sorting will remove all waste

materials from the feed materials and hence have relevant throughput to be grinded.

It is anticipated with the advancement of technology the treatment of ore will go

beyond the ore with grade less than o.5g/t.


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4.0 CIL AND FLOTATION GOLD PROCESSING

TECHNOLOGIES

Typical example of CIL and Flotation technologies at Bulyanhulu Gold Mine.

4.1 CIL GOLD PROCESSING

In Summary: The CIL (Carbon In- Leach) plant process is a vague technology after

amalgamation. Its invention has brought alarming recovery and commissioning of gold

process plants. The main concept of CIL is the capture of gold by using Carbon porous.

Carbons are soaked into the tank containing gold and gold particles are

attracted/absorbed by carbon porous. Then the carbons are taken out and soaked into

Cyanide which dissolves the gold contained into carbon. The carbons are removed

leaving behind dissolved gold which is collected through electrolysis by Electro

winning cells.

4.1.1 Pre-Leach Thickener

The slurry from a grinding circuit advance to the pre-leach thickener. The feed slurry to

the pre-leach thickener is 8% solid. The purpose of pre-leach thickener is to increase %

solid to 55-62% solid. At BGM, the feed to CIL plant is the Cleaner flotation tails which

behave like Oxide Ore.

4.1.2 Carbon- In- Leach

After that the slurry with almost 55% solid is pumped to CIL tank 1 up to CIL tank

6.The concentration of 1200ppm of NaCN is added to CIL 1,conc of 800ppm to CIL

2,conc of 500ppm to CIL 4 and 300ppm to CIL 6.The air is introduced in all tanks and

agitation to ensure leaching reaction. The leaching and adsorption takes place

simultaneously in CIL plant.

4.1.3 Leaching

Cyanide dissolves exposed gold and silver particles contained in slurry producing

sodium gold cyanide complex which is consequently adsorbedonto activated carbon.

Gold dissolution is performed in alkaline media. Lime is added to boost alkalinity and

to suppress generation of HCN gas which is poisonous gas. The operational pH in CIL

tank 1 is 10.5 11, CIL tank 2-6 is 10.5.During dissolution the gold particles dissolve as

follows


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Most of the gold is dissolved by the reaction

OHOHCNAuHOCNAu 42024222

222

(1)

Small but significant portion of gold is dissolved according to reactionOHCNAuHOCNAu 440284 2

22. (2)

Since gold and silver are inert, the complexes formed are very stable in pregnant leach

solution and hence overcome this inertness.

4.1.4 Adsorption

The gold and silver complexes formed during leaching are adsorbed onto surface of

activated carbon. The adsorption process can be hindered, if the amount of excess

cyanide in the slurry is too high or if the pH is too high.

The adsorption process starts at the instant when fresh or regenerated carbons are

introduced into CIL tank 6. The carbons are advanced countercurrent to the slurry

flow, by periodically pumping some of the content of each tank until the concentration

of the loaded carbon in CIL 1 reaches (600 1000) g/tone.

4.1.5 Acid washing circuit

The loaded carbon from the screen is poured into the acid wash column, while the

slurry is getting back to the CIL 1. At the bottom of the column, HCl acid is introduced

where it will wash by dissolve and remove inorganic contaminants such as calcium

salts, magnesium and sodium salts, fine minerals trapped in the large and medium size

pores of the carbon like silica, silicates, and aluminates.The residual acid is eliminated using water washing. The solubility of the deposited salt

and the concentration of the acid wash utilized will greatly determine the efficient of

the acid wash circuit.

4.1.6 Elution

This is a stripping process, a process of getting back the precious metals from the

carbon. Stripping process is based on the mass transfer of a soluble gold or silver

compound generated by a favorable concentration gradient and high temperature. This

process takes place in an elution column.

Elution sequence

1. Fill the elution column with loaded carbon and prepare strip solution(NaOH and NaCN)

2. Preheat


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The preheat step is the first stage of the stripping process. This stage heats

the pre-soak to the desired temperature to enable the gold to be desorbed.

This satisfies the necessary requirement for successfully stripping. In this

step the pre-soak solution (2% NaOH and 2% NaOH) is circulated

through the column using the strip solution pump. The solution is

pumped through the heat exchanger, back to strip solution tank. . The

solution is preheated to a temperature of 120Cand pressure of 130kPa

3. ElutionWhen the strip temperature reaches 120C, the timed elution cycle( 17hrs)

starts and the strip solution is advanced to the electrowining cell

4. Cool-DownAutomatically, the thermal heater stopped when the elution time of 17 hrs has elapsed.

4.1.7 Carbon Regeneration Kiln.The purpose of the above is to dry and regenerate carbon and made it active again,

about 80% of the total energy in regeneration kiln is used to evaporate the water.

Temperature of up to 750C which is enough to remove organic containments. The

regenerated carbons are called barren carbon and are recycled back in CIL 6 tank.

4.1.8 Eletrowinning.

The purpose of electrowinning is to recover the gold and silver from the solution by

electrolysis method. Pregnant eluate from the carbon stripping circuit is passed throughelectowinning cells. The pregnant solution circulates through the cells until the solution

become barren where by the cells are shut down. At the cathode, the gold and silver is

washed from it, and then it flows to a vacuum filter for dewatering. The sludge is dried,

mixed with fluxes, and smelted in a furnace and finally the Dore bars are produced.

4.1.9 Detoxification Process

This is the process of reducing the concentration of cyanide from the slurry to an

environmental level before sent to tailings. The slurry from CIL 6 passes through safety

carbon screen to recover any carbon which has passed via the enter tank screen. The

slurry passed through the screen, flows to detoxification tank

In detox tank Sodium Meta-Bi-Sulphite, Air to supply oxygen and Slaked lime are

added.

The purpose of the hydrate lime is to control pH. The sodium metabisulphite breaks

down to produce sulphur dioxide, which is required along with the oxygen for cyanide

destruction. Cyanide reduction is complete into two stages. The first stage of cyanide


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reduction starts from detoxification tank 1 where cyanide is expected to be reduced to

below 100ppm, after that slurry flows by gravity to detoxification tank 2 where it is

reduced further to below 20ppm.

4.2.0 CIL tailings

Tails from detoxification tank is pumped to the tailings dam. At BGM, tails are pumped

to thickener where by about 65% solids of the underflow are obtained; the slurry is

pumped to the paste conditioner and mixed with rougher tails for back fill and other to

tailings.


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5.0 GRAVITY PLANT

The gravity circuit is the primary recovery of gold. The coarse gold from grinding

circuit is recovered at gravity circuit. The rest of gold is then recovered by CIL or

flotation process.

Gravity recovery techniques capture liberated gold and electrum particles from the

slurry that circulates around the Ball Mill grinding circuit.The slurry that feeds the

Gravity Recovery Plant is screened to remove oversize particles, and then fed to the

Knelson Concentrators.

The Knelson Concentrator employs a 30-inch bowl that rotates. Water is pumped

through small holes in the bowl, which fluidizes a rotating bed of slurry and forces the

lighter materials out of the bowl. Most of the slurry is not captured in the Knelson

Concentrator bowl. Heavier particles are captured in the bowl and are periodically

dumped into a concentrate holding tank.

The gold bearing gravity concentrate from the holding tank is directed to the ACACIA

(formerly were passed over Gemini Tables) for intensive leaching and then to the

electrowining cell where is collected and dried.

The ACACIA and ILR technology is the replacement of Gemini table. ACACIA is a

trade marker but principally is a reactor like ILR but this is more advanced (intensive

leaching)

The gravity concentrate is dried for over 12 hours in a calcine oven. It is then mixed

with fluxes (silica, borax, soda ash, nitre) and smelted in a furnace. The dore bars are

removed from the moulds, then cleaned, sampled, weighed and stamped.


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6.0 FROTH FLOTATION THEORY

Flotation process is only practised at BGM due to the reasons discussed above. It is

mainly used in extraction of copper, nickel, and platinum. Buzwagi is expected to use

flotation process. This study is aimed to provide detailed knowledge of flotationprocess starting from its theory and practise

What is flotation?

Key note:

Flotation use density differences to separate valuable minerals and wastes by injected air.


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Figure 1. Flotation equipment (rougher/cleaner)


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Figure 2.Flotation layout-Rougher and Cleaners


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7.0 PRINCIPLES OF FLOTATION

Valuable minerals in the ore are present as small particles embedded in the host rock.

Before flotation can take place, the primary and secondary grinding circuits must

reduce the ore into fragments small enough to break most of the valuable mineralparticles free from the host rock.

Mineral separation by the process of flotation is achieved in agitated slurry into which a

multitude of air bubbles has been introduced. Bubbles of air are made to rise through

the mineralised slurry and form froth on the surface of the flotation cell.

There are two sets of conditions that can describe the flotation environment.

7.1 Chemical conditions:

The interaction of chemical reagents with the mineral particles to make thevaluable minerals selectively attaches to the air bubbles, making minerals

hydrophobic.

7.2 Physical-mechanical conditions:

Which are determined by the flotation, machine.

Chemicals are added to the pulp to assist in the formation of bubbles, froth. Other

chemicals cause some minerals to stick to the bubbles and rise to the surface. Usually,

the desired minerals are made to stick to the bubbles, while the waste mineral particles,

gangue, and remain in the pulp. The froth is then skimmed off to complete the

separation.

The froth is called the concentrate, and the slurry remaining behind is called the tailings

or tails.

7.3 Surface Chemistry

Flotation relies on the differences in surface properties of different minerals to achieveseparation. Surfaces can be either hydrophilic (water loving), hydrophobic (water hating) or

mineral surface can also be electrically charged and can be oxidised. A particle may

have both hydrophilic and hydrophobic patches.

If the particle is hydrophilic, the contact angle with water is large.

If the particle is hydrophobic, the contact angle with water will be smaller preferring the air and

rejecting the water.


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Figure 3.Froths and Entrainment

7.4 Particles can enter flotation froth by either of two ways.

The hydrophobic particles attach to air bubbles or are entrained in the water carried

along by the rising bubbles. In this way gangue and other unwanted particles enter the

froth, particularly at finer particle sizes. Particles entrained in this manner may be

washed out of the froth as the froth drains.

Hydrophobic particles attach to bubbles help to slow drainage, which in turn helps to

stabilise the froth.

Hydrophilic particles which are drawn into the froth due to entrainment carry a

stabilised water film which can increase the water content and further stabilise the

froth.

Figure 4. Bubbles carrying valuable particles


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7.5 Liberation and Separation

The whole flotation process relies on achieving a specific degree of liberation so that the

valuable particles can be recovered into the froth and

non-valuables left behind as tails.

The range of particles that exist after grinding can have any one

of the forms listed in the following diagram: Each of these particles behaves differently

in the flotation circuit.

The critical particles for balancing the flotation circuit in terms of grades and recoveries

are particles B and C.

If no further grinding can be justified, recovery of the particle to concentrate represents

additional valuable mineral recovery but a reduction in grade due to its gangue content.


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7.6 Grade and Recovery

Grade provides an indicator of the extent to which the valuable minerals of interest has

been concentrated from the other minerals, has been achieved.Recovery indicates the

extent to which the minerals of interest have been recovered and not lost to tailings. A90% overall recovery means that of the percentage of copper entering the flotation

circuit 10% is lost to tailings

The objectives of achieving a maximum recovery and maximum grade are invariably in

conflict. An improvement in the recovery usually means that the grade of the

concentrate decreases and visa versa. The ideal rougher concentrate grade would be 10

to 15 times the rougher feed. Example 1% rougher feed increase to a concentrate of 10%

to 15% copper concentrate.

Figure 5. Grade and Recovery relation


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7.7 Common Reagents Used

7.7.1 Xanthate

Xanthate makes minerals hydrophobic. Most minerals are naturally hydrophilic. Toachieve a separation by flotation, the surfaces of one mineral must be selectively

rendered hydrophobic by the addition of a collector that selectively adsorbs and

provides the required hydrophobic surface. A simple description of the surface

adsorption would show the collector having a charge opposite to the mineral surface

charge, and being electrostatically drawn to the surface.

Figure 6: Chemistry of Xanthane

7.7.2 LimeAct as a modifier to improve collector absorption and selectivity. These charges can be

controlled by manipulation of the chemical environment of the pulp, particularly its

pH. The electrical charge on the surface influences whether the collector will be

adsorbed onto it or not. The pH improves copper flotation, betters recoveries and also

acts as a depressant. The Cleaner pH should not be too high as it starts to depress

copper.

7.7.3 Frother

Frother is used to produce a stable froth above the pulp. Rother is active at the air,

water interface. They are essential to the production of small bubbles and enable a

stable froth to form.


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A frother is required to provide froth above the pulp that is stable enough to preventundue froth breakage and subsequent return of particles to the pulp before the froth is

removed. It is important, however that the froth break down rapidly once removed;

otherwise problems occur in slurry pumping and in subsequent processing steps.

Figure 7. Nature of hydrophilic and hydrophobic


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8.0 FLOTATION UNIT OPERATIONS

The main flotation machines are rougher and cleaner.

Flotation machines introduce a mixture of air, water, particles and chemicals to producean overflow product and an underflow of non floated particles. Mechanical agitation

allows valuable minerals to contact and adhere to air bubbles.

8.1 Roughers

The primary objective of the rougher is to float all free copper liberated in the grinding

circuit and to achieve both a reasonable recovery and a reasonable grade of concentrate.

Final adjustment of air ingestion will depend on the ore characteristics and operational

experience. Non-valuable minerals, which are not floated, remain in the pulp andthereby discharged with the pulp.

Figure 8.Rougher flotation machine


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8.2 Cleaners

The primary objective of the Cleaners is to upgrade the rougher concentrate and reject

the remaining gangue and to produce a saleable product at a high recovery. Due to the

position of the impellor the agitator motor is prone to drawing high amps at low air

additions and high pulp densities. The reduction of motor amps can be achieved by

increasing aeration or decreasing pulp densities.

Figure 9. Cleaner flotation machine

Figure 10: Cleaners and Roughers in series

Residence Time: The longer particles spend in a flotation cell, the more likely they are

to collide with a bubble yielding a higher recovery. The residence time depends on both

the volumetric flow rates. Higher the flow, shorter the residence time, Lower the flow,

longer the residence time.


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9.0 FLOTATION PROCESS PLANT

The Flotation Plant recovers the gold and electrum locked within the sulphide minerals

(pyrite and chalcopyrite). Rougher flotation is the first stage, in which sulphide minerals

are collected. Special reagents are mixed with the slurry in a conditioning tank, whichthen flows to the rougher cells. Air is injected and bubbles rise to the surface in a froth,

carrying the valuable sulphide minerals on their surface. The froth overflows the cells

and is collected. At this stage it is called rougher concentrate. The slurry that is not

collected as froth flows through the cells and becomes waste material called tailings.

The rougher concentrate is sent to a Regrind Mill for further grinding. Cyclone

classifiers separate the ground concentrate particles to ensure the correct sized particles

are sent to the next stage of flotation. The re-grinding produces particles of 17 microns

(0.017 mm) and liberates chalcopyrite and gold particles from the pyrite.

Figure 11. BGM flotation circuit


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The next stage of flotation is called cleaner flotation, because low grade material is

cleaned, to produce a higher grade concentrate. Lime is added to the cleaner circuit

to depress the low-grade pyrite and prevent it from floating, while CMC is added to

depress the slimes, which consist mainly of non-sulphide gangue. The copper mineral

(chalcopyrite) attaches to the froth that overflows the cells, and the pyrite is not

collected and becomes tailings. Four stages of cleaning are used to continuously

upgrade the copper concentrate to produce a final concentrate at an acceptable grade

for sale.

Figure 11.Larox Concentrate filter press

The water in the final concentrate slurry must be removed to permit cost effective

transport and treatment. The dewatering process consists of thickening and filtration.

The thickening process decants the water from the slurry, to increase the slurry density

from 15 % solids to 55 % solids. Pressure filtration, utilizing the Larox filter press,further reduces the water content in the slurry, to produce a cake of 8 % moisture

content. The concentrate is then loaded into containers for transport to overseas

smelters.BGM Process Plant produces approximately 75 tonnes of concentrate per day.

The copper concentrate contains approximately 250 g/t gold, 200 g/t silver and 16 %

copper.

Filter plate packFilter cloth


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10.0 Some of Glossary of Terms Used in Mineral Processing

Activity: A measurement of the ability of carbon to adsorb gold and silver in aCIL/CIP plant.

Adsorption: The process whereby the gold and silver become attached to thesurface of carbon particles in a CIL/CIP plant.

Absorption : Anode: The positive terminal of a battery or electrowinning cell. Apron feeder: A mechanical device for feeding material on to a conveyor belt or

another piece of equipment.

Attrition: A process involving the removal of the rough edges on fresh carbon.The carbon is added to a tank filled with water and stirred. The collisions

between the carbon particles wear away the rough edges.

Ball mill: A device in which grinding of the ore occurs due to the action ofgrinding balls.

Barren solution : A solution from which the valuable metals have been removedin a CIL, CIP plant

Cataract: The action of the balls in a SAG, Ball mill operating at high speed.

Cascade: The action of the balls in SAG, Ball mill operating at low speed. Cathode: The negative terminal of a battery or electro winning cell. Caustic soda : An alkaline chemical used in the gold recovery process. Also

known as sodium hydroxide (Na OH).

Circulating load: The portion of a mill discharge that is returned to the mill feedin a closed circuit.

Classifier: A device for separating the particles in a stream into fine and coarsefractions.

C.I.P. : The Carbon In Pulp process- leaching and adsorption takes placesimultaneously (in one tank) while


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CIL: The Carbon In Leach process leaching and adsorption takes place intodifferent time and different tanks (then mixed in one tank)

Closed circuit:A system is said to be in closed circuit when a portion of theproduct is returned to the feed for further processing.

Counter current decantation: A method of washing the solids in a pulp byfeeding the pulp through a number of thickeners operating in series. The

washing solution travels in the opposite direction to the solids.

Critical speed :The speed of a mill at which the grinding media is held againstthe mill liners by centrifugal force so that grinding ceases.

Cyanide: Sodium cyanide Na CN. Mainly used in a CIP or CIL circuit to dissolvethe gold and place it into solution form.

Cyclone: A cyclone (also known as a hydro cyclone) is a device for classifyingthe coarse and fine particles in a pulp. Centrifugal forces acting on the feed

particles cause the coarse fraction to be discharged from the bottom of the

cyclone at the apex or spigot. The fine particles overflow at the top. The device

contains no moving parts.

Desorption: The process involving the stripping of gold and silver from theloaded carbon in a CIL/CIP plant.

Educator: A device for lifting carbon using air or water in a CIL/CIP plant. Eluant: The solution used to strip the gold and silver from the loaded carbon in a

CIL/CIP plant.

Eluate : The solution containing the gold and silver after removal from thecarbon In a CIL/CIP plant.

Elution: The process of stripping the gold and silver from the loaded carbon.(Same as desorption) in a CIL/CIP plant.

Feeder: A device for transferring solids at a controlled rate to a process. Filtrate: The liquid fraction recovered from the filtration process. Flocculants: A chemical used to increase the settling rate of solids in a pulp. Flux: A chemical added during the smelting of gold and silver for the purpose of

scouring impurities from the molten mass.


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Gangue: The minerals forming the worthless component of the ore. Grizzly: A type of heavy duty screen having a screen deck of parallel bars. Heat Exchanger: A device for raising or lowering the temperature of a fluid by

heat transfer to or from another fluid.

Impellor: A component in a pump which causes the fluid to be pumped. Labyrinth: A lubrication sealing arrangement around a bearing that allows

grease to be discharged but prevents foreign matter from entering.

Launder: A drain or channel. Lifter bars: Liners in a ball or S.A.G. mill designed to pick up some of the ball

charge to improve the grinding characteristics.

Liner: A rubber or steel replaceable wearing surface in a mill or pump. Micron: One millionth of a metre or one thousandth of a millimetre. Open circuit: A system is said to be in open circuit when the feed stream passes

straight through the system and onto the next section of the process without the

presence of any circulating load.

Ore: Rock containing a valuable mineral. Ounce (Oz): A unit measure of gold, 1oz = 31g for American std and 1oz = 28g

for British std.

Overflow: The product stream from a classifier containing the fine material.Abbreviated as O/F.

Oversize: The product stream from a screen containing the coarse material.Abbreviated as O/S.

pH: A measure of the acidity or alkalinity of solution. Solutions having a pH ofless than 7 are acidic. Solutions having a pH of more than 7 are alkaline. Neutral

solutions have pH of 7.

P80: The measurement of the size of the solids in a product stream. A P80 valueof 500 microns means that 80% by weight of the solids will pass through a screen

with a 500 micron aperture. For a feed stream, the corresponding symbol is F80.


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Pinion: The small drive gear on the ball mill and the S.A.G. mill. This mesheswith the ring gear.

Precipitate: Fine solid material settling out as the result of a reaction betweentwo liquids.

Pregnant solution: The solution containing the gold and silver after desorptionin a CIL/CIP plant.

Pulp: A mixture of solids and water. Pulp density: A measure of the solids concentration in the slurry. Ppm : Particles per million (1g/t = 1ppm) Reactivation: A process wherein carbon is heated to 700 Degrees Celsius in the

absence of air to increase its activity.

Reagent: A chemical used in a process. Reduction ratio: The ratio of the size of the mill feed to the size of the mill

product i.e. F80/P80.

Refractory: A material that resists heat or chemical attack. Residue: The solid product recovered during filtration. Reverberatory: Refers to a furnace that is indirectly heated. S.A.G.(Semi-autogenous grinding). : Autogenous grinding is self grinding.

Semi autogenous grinding is grinding using a limited ball charge.

Scrubber: A device for removing the dust from a stream of air. S.G.: Specific Gravity or Relative Density which is measure of the density of a

solid, liquid or gas compared with the density of water.

Sizing analysis: A method of determining the size range of solids in a stream byweighing the amount retained on a series of standard test screens.

Slag: The product obtained when the molten fluxes solidify. Stripping: A process of circulating a pregnant solution to electrowinning cell for

electrolysis. After 16hrs the solution becomes barren. All gold is attached to the

Cathodes.


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Stripping ratio: The amount extracted vs. waste e.g. stripping ratio of 7:2 means7tonnes mined produce 2g of gold.

Thickener: A thickener is a device in which the density of a pulp is increased.The products are (i) a pulp of higher density than the feed and (ii) clarified water.

Trommel: A cylindrical revolving screen. Trunnion: The surface on each end of a mill which turns on a lubricated slipper

bearing.

Underflow: The product stream from a classifier containing the coarse material.Abbreviated as U/F.

Undersize: The product stream from a screen containing the coarse material.Abbreviated as U/S.

Under speed : A safety system where a slipping conveyor belt trips out its feedsupply system or when a piece of equipment is overloaded.

Weightometer: An electronic device to continuously weigh material on a movingconveyor belt.

Lot: A lot refers to a quantity of material of unknown grade and composition.Some examples of a lot are: A truckload of ore or an exploration drill core

sample.

Increment: An increment is a group of particles which has been randomlyextracted from the lot in a single operation of a sampling device.

Sample: A sample is defined as a single increment or group of increments whichhave been extracted from the lot. A sample is not just any part of a lot. The

extraction of a sample must respect the sampling rules. If these rules are not

followed, then you will have collected a specimen, not a sample.

Specimen: A specimen is a part of the original lot, but it has been obtainedwithout respecting the sampling rules. A specimen should never be used to

represent the lot.


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Homogeneity refers to the degree of unity of all the elements of a lot. If thecondition occurs where all elements of the lot are exactly identical, the lot is said

to be homogenous.

Heterogeneity refers to the degree of difference of all the elements in a lot. If theelements of a lot have a degree of difference it would be described as being eitherheterogeneous or as being non homogenous.


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11.0 Taxation and Fiscal Terms Used in Mining Contracts

The following are the common terms used in the mining contract to clarify fiscal and tax

clauses;

Capital allowance:

The tax relief given on the expenditure a company makes on capital goods.

Under Tanzanian tax laws the cost of all capital equipment incurred in a mining

operation can be offset against the income from the mine in the year in which it is

spent. If the profits of the mine are smaller than the capital allowances that could

be claimed the capital allowances are then inflated in the following year (see

unredeemed capital expenditure, below) and capital allowances are then given in

the following year on the inflated sum.

Capital expenditure:

A companys expenditure on capital goods.

Capital expenditure ring fencing:

A company is only allowed to deduct the cost of capital expenditure at a

particular mine against the income of that mine when calculating its tax bills. As

a result it cannot deduct the cost of capital expenditure at another mine that is

not yet making money from the income of the mine now generating profits. Thisbrings forward the date on which tax is paid on profits but it may also have the

effect of discouraging local reinvestment of profits.

Capital gains tax:

Tax on the surplus obtained from the sale of an asset, such as a mine, land or the

company as a whole. It is a tax charged on the difference between the amount

received on the sale of the asset and the amount it cost.


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Capital goods:

Durable goods such as machines, tools, furnaces and other equipment used by

mining companies to extract gold. These goods are not imported into Tanzania

by trading companies to be sold on to consumers. Instead they are used by the

company that buys them for use in its own gold extraction operations. This

equipment is expected to be used by the company over a number of years. It is

the fact that it has a life expectancy of several years that identifies it as being a

capital item. The cost of the capital expenditure is claimed as an expense to

reduce profits in different ways for accounting and tax purposes. For accounting

purposes it is charged as depreciation. For tax it is claimed as a capital

allowance.

Corporate tax:

The tax paid by companies on their taxable profits. Taxable profits are those

declared in their accounts (see net profit before tax) but some adjustments are

usually made for tax purposes. The most important by far is to add back to that

figure for net profit before tax the depreciation charge and to then deduct from

the resulting sum the capital allowance claim made for expenditure made on

capital goods. Since the expenditure on capital goods often exceeds the

depreciation charge it is common for taxable profits to be lower than accounting

profits and for the actual tax due to be less than that which is apparently

appropriate when multiplying the declared net profit before tax by the publishedcorporation tax rate.

Depreciation:

The accounting charge made to reflect the cost of a companys capital goods used

to produce its gold during a period. This is also sometimes called amortisation.

The capital goods a company uses are gradually worn out in use. The

depreciation charge reflects this fact and a charge is made for this for accounting

purposes. Depreciation charges do not involve any cash expenditure; the cash

was spent when the equipment was purchased. There are several methods for

working out this depreciation cost, for example dividing the original cost of themachine by the number of years it is expected to last, or by working out how

money tonnes of ore it should be able to process and dividing the cost of the

machine by this total to calculate an expected cost of using the equipment per

tonne processed which is then used to calculate a charge in the accounts based on

the amount of ore actually processed.


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By definition these depreciation charges will last for a number of years after the

time the capital goods were purchased until it either is, or is for accounting

purposes, considered to be worn out. This accounting treatment is very different

from the equivalent charge made for tax that is called a capital allowance.

Gross profits:

Calculated by deducting all the direct costs of extracting ore from the value of

ore sold in a period, but without taking overhead costs into account. Direct costs

are expenses such as the cost of employing miners, buying materials used in the

mining process, paying for the power consumed when extracting ore, paying

royalties due on the value of ore sold and the cost of shipping the ore to ports for

export. In a mining operation depreciation is usually a direct cost and as such is

deducted from sales income in the process of calculating gross profit. Overhead

costs are not directly related to the production process and might include the costof management, accounting, marketing, running offices, finance costs such as

loan interest, advertising (if necessary), and training. Withholding taxes on the

supply of services from overseas will probably be an overhead cost. These

overhead costs are not charged against sales when calculating the gross profit,

but are deducted from gross profit to calculate net profit before tax.

Net profits:

Sales income less direct costs and overheads (see definition of gross profit abovefor more information).

Royalty:

Effectively a sales tax charged on the market value of the gold sold. How this

market value is calculated will determine the actual royalty a company pays.

Companies might claim to sell gold to their buyers at a lower price (called the

reference price) than the price of gold on international commodity exchanges,

which means they would pay a lower royalty. It is therefore important for

mining agreements to have proper market pricing arrangements in place. In thecase of gold these will usually be fixed on an internationally recognized

exchange.


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Tax allowance:

Expenses that can be offset against income when calculating profit and that are

also allowed as an offset against income for taxation purposes. Some of these,

such as capital allowances, are not accounting entries at all and are only

calculated for tax purposes.

Taxable income:

The net profit before tax when adjusted for depreciation charges that are not

allowed for tax, the capital allowances that are claimed for tax instead of

depreciation and any other adjustments to profit required by taxation law.

Because capital allowance arrangements tend to be so much more generous than

their accounting equivalent called depreciation no Tanzanian mining company

has declared a taxable income to the Tanzanian government in the last ten years.

Unredeemed capital expenditure:

If a mining company has incurred capital expenditure that is greater than the

amount needed to cancel all of its taxable income for the year on a particular

mine, then the balance of capital expenditure not offset for tax in that year is

carried forward for offset against the income of the next year. The peculiarity is

that the balance of unredeemed capital expenditure is increased at the start of the

next year by 15% as if this sum had been spent on additional capital goods eventhough this has not actually occurred. As a result the date on which the first tax

is due from a mining operation can be deferred for a considerable period of time.

This additional unredeemed capital expenditure has been cancelled in some

recent mining contract renegotiations as the deductions have no economic

substance. It seems likely that it was introduced at a time of high inflation to

make sure that the real value of the amount expended was offset against income

apparently worth more in a later period, but this has no relevance now and the

allowance has instead been used as a way to defer tax payments for considerable

periods.


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Value-added tax:

An indirect tax charged on the sale value of goods or services supplied. Most

businesses can reclaim the VAT charged to them for the purposes of running

their business. As a result they only pay over to the government the difference

between the tax they charge to their customers and the VAT they are charged on

their purchases. This, very approximately, equates to a tax on their value

added, hence the name of the tax. VAT is also usually charged on the import of

goods into Tanzania so that a retail company selling imported goods would need

to pay VAT on importing goods into the company and then reclaim this cost

when accounting for the VAT it has charged to its customers on their subsequent

sale. Mining companies, however, are exempted from this charge on imports.

This is largely because most of the gold they produce is exported and there is noVAT on the value of goods exported. As a result to make the mining companies

pay VAT on their imports would simply create a situation where they had to

make continual claims for it to be refunded by the government. Not charging

VAT on exports is a characteristic common to all VAT systems all over the world.

Windfall tax:

An additional tax levied by the government on extractive companies when there

are above predicted price increases of commodities on international markets (aboom). This tax is levied on windfall profits which arise not because of any

action on the part of the company but because the price of the commodity they

are dealing in has risen for reasons beyond their control e.g. there being a

worldwide shortage.

Tanzania Gold Mines

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