EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

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I would like to thank the Renco mine plant personnel for their support and encouragement during the
course of my learning course at the mine. Furthermore I wish to express my earnest gratitude to the Plant
Superintendant Mr B. Patel and the plant foreman Mr J. Sigogo for the learning experience and the . I
would like to applaud the plant metallurgist Mr T. M. Chisango and Mr N. Midzi of the E.N.R for the
brotherly support and pledged commitment to the student’s mentoring. lastly I would like to glorify the
almighty for the knowledge and protection he hath granted me, for it is not by anyone else’s power or
might that I have accomplished what I have and lived to this very day.

MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

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MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

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Renco gold mine is a subsidiary of Rio Zim private limited, formally Rio TINTO Zimbabwe limited.
The mine was officially commissioned in June 1982. Renco is situated about 95 kilometers south east of
Masvingo, and approximately 5 kilometers north of Bangala Dam in Chiredzi District level. Renco
mine’s mineral processing plant comprises several unit operations aimed at producing the targeted gold.
The main operations are size reduction, thickening, Carbon in Leaching, flotation, Concentrate Leach
Plant (CLP), elution, electrowinning and smelting.The mineralogy is characterized with the following
which disturb in gold processing.

The mineralization includes gold, maldonite, elemental bismuth and sulphides. The main sulphide is
pyrrhotite with minor chalcopyrite and very minor pyrite and variable amounts of bismuth sulphides,
bismuthinite and galenobismutite.
The gold occurs in various forms but mainly extremely fine-grained, about 15-20µm in diameter and with
intergrowths of bismuth. For this reason very fine grinding is required (about 90% passing 45 microns).
The gold is also associated with the sulphides, mainly chalcopyrite and to a lesser extent, pyrrhotite, and
rarely pyrite. The gold is also sometimes locked to bismuth or bismuth bearing minerals. Bismuth occurs
mainly as free grains as well as in association with the gold.

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Renco mine’s mineral processing plant is principally partitioned in two, i.e. the particle size reduction and
the extraction section. The size reduction section is of utmost importance as it deals with the liberation
(exposure) of the mineral of interest (gold). Size reduction involves crushing and milling the ore to a size
at which economic extraction can be achieved1. The extraction section is then responsible for the recovery
of the gold which entails leaching, flotation, elution and gold smelting.

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MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

This is an open circuit in which -250mm (10’’) run of ore mine is drawn from the 800t shaft Ore Bin by
two belt feeders (C2 & C3) onto a short intermediate conveyor belt (C4) then on to a primary crusher
feed conveyor belt (C5).A 3200kg suspended electromagnet with a working gap of 350mm is sited at
the head pulley of the crusher feed conveyor belt (C5) to protect the crushing units from tramp metal
and hammer heads.

The primary 1800x120mm single decked vibrating scalping screen, with a circular 65mm aperture size,
bypasses the fine undersized (-65mm) material onto the stockpile feed conveyor (C6). The oversize
particles proceed into a choke-fed, Osborn Telsmith Single Toggle jaw crusher (with 54t/hr working
capacity). The stockpile feed conveyor conveys the crushed product together with the primary scalping
screen undersize onto a 2600tonne capacitated stockpile.

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The secondary crushing section is the second stage of the communition process and is advancement
towards the achievement of the above highlighted functions. The circuit consists of a 1300mm (51”)
Allis Chalmer hydrocone crusher in a closed circuit with a 50mm by 28mm double deck vibrating
screen. A 1300 mm 4ft 3inch Symons gyratory crusher serves as a standby crushing unit.

The approximately 80% -65 mm material is drawn from the stockpile by a vibrating feeder (VF2) or two
stand-by conveyors feeders onto a secondary screen feed conveyor (C7). The secondary screen is a
double decked square aperture vibrating screen with a 40mm top deck and 28mm bottom deck (covered
with polyurethane). The screen oversize is collected into a 20t surge chute, whilst the undersize
(nominally 100 % - 28mm) passes onto a mill ore bin feed conveyor belt (C9). The material in the chute
gravitates into the secondary crusher (a hydrocone crusher or the standby Symons crusher unit) with
about 28mm close sized gap setting.

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The tertiary crushing section draws the ideally 28mm material from the 1 200t mill ore bin (MOB) with
an approximate dead ore of 300t. The tertiary crushing section comprises a Barmac autogeneous crusher
(BMC1 and a standby BMC2 unit) in a closed circuit with a Vibramac double decked, watter-washed
vibrating screen. An online weghtometer sited beneath the mill ore bin (above the bin’s discharge

MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

conveyor; C10), measures the tonnage to the wet screen. The weghtometer also helps control the rate of
ore withdrawal from the bin. It also gives the tonnage of the day which is significant to the metallurgical
accounting. The weigtometer is calibrated on every planned maintaince to determine the error of either
overload or underload of the belt. The vibrating wet screen has a 12 mm top deck and 3mm aperture sized
bottom deck.

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This section is sub-divided into two operations, namely primary and secondary milling.

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The primary milling circuit consists of a 2.886meter by 5.782meter (9ft 6inch 17ft) Allis Chalmer ball
mill (with a steel lined grate discharge), in a closed circuit with a 500mm (20inch) rubber lined hydro-
cyclone. Make-up water is added in the sump and is regulated by an overhead auto-sump level controller
to maintain the sump level at about 75% volume and it also prevents air entrainment which is detrimental
to cyclone operation. The wet screened undersized material (i.e. -3mm) together with primary mill
discharge gravitate into the primary mill feed sump, from which the material is pumped (by either P1/ P2)
to the operating 500mm primary cyclone (either Cy1/2 respectively). The primary hydro-cyclones have a
spigot diameter of 145mm for classification.

The ideally 75% -45µm cyclone overflow (O/F) with a pulp density of 15% solids by weight is laundered
to the secondary mill feed sump, whilst the coarse underflow (U/F) with about 78% density gravitates
back into the primary mill feed chute.

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The secondary milling section consists of an 8ft by 10ft rubber lined Overflow Crusters ball mill (with an
open discharge regrind mill), in a closed circuit with eight overhead cluster cylones . The slurry in the
secondary mill feed sump is pumped by either P123/124 sump to the mill’s on-line operating cluster
cyclones. Normally five of the eight cluster cyclones are in use at any given time. The cluster cyclones’
overflow (ideally 85% -45µm) with a density of about 12% solids by weight proceeds to the thickener,
while the oversize underflow gravitates into the secondary mill feed chute for further grinding.
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MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

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Main purpose of the 23 meter Dorr thickener is to increase the solids concentration from the about 12%
solids (cyclone overflow) to about 55% solids which is required for efficient leaching operation. The
overflow cyclone density is very low (i.e. 12% solids) indicating that it contains a lot of water which can
consume a large amount of chemicals (NaCN and NaOH). This dewatering is usually achieved by use of a
substance called a flocculant. Flocculants are organic substances with activated sites on which small
particles bind together thus making them heavy and they settle down.

The flocculant solution is prepared in a mixing tank and is a mixture of 1kg Yangfloc and water. The
flocculant solution is then pumped into the thickener (at an intended rate of 44g/hr i.e. 0.059g/L
concentration) to enhance agglomeration and thickening. In the leach circuit, pulp dilutions result in leach
densities of around 50% solids (with spillages).

The overflowing ‘clean’ water from the thickener is laundered to the mill process water storage tank and
from which it is then pumped to the following points:

a. To the mill sump, where it is used for diluting the mill product and controlling the mill sump level.
b. The 2 000m3 water storage tank at the tailings dam which is also used for dust suppression.

The under-flowing thickener pulp (about 55% solids density) is then pumped by either P6 / 7 to a bank of
four 17m3 Outokumpu pre-conditioning cells.

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The purpose of the pre-conditioning section as the name suggests is to set optimum conditions for
improved efficiency of cyanide leaching. Pre-conditioning thus entails increasing the pulp pH to 10.5
and raising the dissolved oxygen content of the slurry from 2ppm to about 5ppm. The pulp that comes
from the mills is a neutral mixture of three different types of ore i.e. pyrhotite (Fe 1-x S), chalcopyrite
(Fe2S) and quartz.

MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

Caustic soda (sodium hydroxide; NaOH) solution is added upstream of the “Preconditioning” cells to
raise the pH. The caustic soda solution (prepared in a 3.53m3 tank by mixing and agitating a solution of
700Kg of caustic soda with water) is introduced into the precondition cell at a regulated rate. The
desired basic pH of 10.5 also offers alkaline protection from CN- that forms a very toxic hydrogen
cyanide (HCN) gas under acidic pH <7.

Gold dissolves under alkaline conditions and oxidizing conditions in the presence of a suitable
complexing ion. In this case the complexing ion is a result of the dissolution of NaCN in solution.
Discharge from the pre-conditioning cells gravitates through a 140cm x 720cm Delko or Derek linear
screen (with a 0.65mm aperture) into the pre-conditioning sump from which it is pumped to the leach
section. The screen removes foreign substances such as wood chips, rubber, plastics and etc, which would
hinder mineral dissolution. In the sump 160g/L (1.25Kg/t) solution of aqueous sodium cyanide is added as
a 10% solution (i.e. about 400ppm free CN in the 1st leach tank). 200 g/t of mercuric chloride also used to
be added to this “Precon” sump as a 4 % solution. This practice was however abandoned due to its
negative environmental impact and as the bismuth content of the Renco ore had since decreased from
originally about 250ppm to 10-15ppm. Moreover the leach efficiency and mineral recovery have since
been seen to be relatively just as good without addition of the costly mercuric chloride. The slurry is then
pumped from the pre-conditioning section by either P10/11 to the leach and adsorption section.

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This section is divided into two units totalling six tanks (390m3 each) i.e. one ‘Leach tank’ and five
‘Carbon in leach (CIL) tanks’; all fitted with 11Kw agitators.

Each tank has a retention time of approximately 10 hours at 750tpd (≈37.5m 3/hr at 55% solids), hence
total retention time in leach tanks is 20.8 hrs. According to Elsner, cyanide selectively dissolves gold in
the presence of oxygen as follows;

4Au + 8NaCN + O 2 +2H 2 O 4 NaAu(CN)2 + 4NaOH i

Apart from the oxygen picked up by the slurry en-route the hydro-metallurgical circuit, commercial
oxygen is added from beneath the tanks, to boost the dissolved oxygen level to above 8.2ppm (i.e. the
maximum solubility of oxygen in water under standard conditions). The dissolved oxygen in the leach
MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012
EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

tanks should be maintained above 18.5g/l. On average dissolution of the leach section is approximately
64%. The CIL tanks have Airlifts to enable the counter current movement of carbon (against the flow of
pulp). Roughly the carbon concentration is maintained at 23 g/t in the 5th CIL tank; at 8 g/t in the other
three CIL tanks and at 5 g/t in the measuring vessel CIL tank 2. The tanks have bypass routes which allow
planned maintenances without disrupting production. The five CIL tanks are fitted with cylindrical carbon
retention screens.
Cyanide leaching should always occur under alkaline conditions, as low pH result in the formation of
hydrogen cyanide gas. This not only wastes cyanide but is an extremely toxic gas i.e.
H+ + CN- HCN

Hydrogen cyanide gas is very stable under acidic conditions and it dissociates to give free cyanide in
alkaline conditions.
Renco ore formerly had high Bismuth contents and mercuric chloride would be added to improve
recoveries during cyanidation. Albeit this desired effect, the practice has since been aborted due to the
chemical’s exorbitant costs and negative environmental impact. The ore’s bismuth concentrations have
also dropped to about 15% hence recoveries are Just as fine without the HgCl2.

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The elution section is concerned with the stripping of gold from the carbon, yielding a concentrated gold
bearing solution apt for the subsequent process (i.e. electrowinning). Airlifting is employed to transfer the
loaded carbon from CIL tank two to the Elution section. The trommel screen above tank two separates the
loaded carbon from the slurry. A 2.52m3 capacitated measuring vessel, sited above the elution column,
receives the gravitating loaded carbon. The carbon is drained into the elution column below the vessel.

The elution process is based on the Anglo American Research Laboratory (AARL) system and it involves
copper desorption. The acid washing sequence starts with a 3% hydrochloric acid solution to remove
calcium, magnesium and base metals which reduce the efficiency of the adsorption process. Two alpha-
Laval plate heat exchangers heat the feed solutions using steam from a 1.5MW electrode boiler. The acid
wash goes on for 20 minutes (at 98˚C) after which the spent solution is pumped to the tailings. All traces
MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012
EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

of acid must be removed from the elution column prior to treatment with caustic/cyanide solution. This is
accomplished by a one hour water-wash also at 98˚C using demineralised water. The fresh loaded carbon
is only water washed for an hour at 120ºC. This time and temperature is sufficient to maintain calcium
levels on the carbon at about 0.25% i.e. harmless to adsorption efficiencies.
The pregnant solution is pumped to one of the two catholyte holding tanks in the electrowinning building.
The efficiency of the elution section is approximately 99.5% hence the eluted carbon rests in the order of
17.610 grams per tonne. The eluted carbon is then dewatered over a sieve bend and in a vibrating holding
tank. The barren carbon is then thermally regeneration in a Welman kiln at 650-700ºC. It is then water
quenched on a Sweco screen where any carbon fines are removed before being reintroduced to the CIL
(through the last tank). Loss of carbon is in the order of 40-50 grams per tonne milled, mainly emanating
from the regeneration process.

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The eluate from the process is circulated through four Anglo American Research Laboratory (AAL) type
cells. The cathode consists of 700grams of wire wool wrapped around the cathode support tube. The
anode section is separated from the cathode by a semi permeable membrane. The anolyte (20% caustic
soda solution) circulates through the anode compartment while the pregnant catholyte solution circulates
through the cathode. At 110 amps and 4.5 volts the gold deposit on the wire wool. The efficiency of the
electrowinning circuit is about 99.5% and the spent catholyte is re-circulated through the tanks to utilize
the residual cyanide. The remaining gold in solution is, of course, reabsorbed on carbon.

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At Renco smelting occurs weekly. The catholyte cells are pulled out and the gold-rich cathodes are
stripped and replaced with fresh wire wool and put back on-line. The gold-bearing wire wool is then dried
in a retort at about 350ºC to do away with any water or volatile liquids on the wool. The dry wire wool is
then smelted in a 15KW induction furnace. The flux comprises a 5: 1 ratio mixture of borax Na2Ba4O7 and
silica SiO2. Oxygen is blown-in through a glass stick inlet used for stirring the molten mixture. The
oxygen oxidizes all the adsorbed iron into the slag. Typically bullion has a fineness of about 960ppt and
20ppt for silver (>96% Au).

FFLLLOOOTTTAAATTTIIIOOONNN SSSEEECCCTTTIIIOOONNN
MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012
EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

This section recovers chalcopyrite (CuFeS2) usually found in association with some gold and silver. The
pulp’s pH has a major role and as such, its control is of utmost importance. At a pH range of 7.5 - 8.5 the
sulphide minerals are floatable. Acidic conditions also corrode the pipe-work and flotation cells moreover
the reagents used favour alkalinity hence the need to lower the pH (from about 10 to around 8) prior to
flotation. This reduction in pH is achieved in the cyanide destruction section. Some reagents also need to
be added to the pulp prior to flotation, to allow sufficient time for them to attach to the minerals.

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This section comprises three 55m3 tanks and preconditioning tank 3. Cyanide is destroyed by the
introduction of SO2 to the pulp in sufficient quantities as to form a weak sulphuric acid. It is this acid that
sees to the reduction of the pH to 7.5 by converting the alkaline cyanide to a non-toxic cyanide oxide.

The sulphur dioxide is bubbled into two of three 55m3 i.e. in ‘Cyanide destruction tank 1, 2’. The sulphur
dioxide (SO2) is produced by burning elemental sulphur in small pressure vessels with a controlled flow of
compressed air i.e.
S + O2 → SO2

Potassium amly xanthate (PAX) or sodium isobutyl xanthate (SIBX) is also introduced in the last 55m 3
tank which serves as a flotation pre-conditioning tank’. The collector renders the sulphide mineral water-
repellent hence it adheres to the rising air bubbles.

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Senfroth is added in the feedbox to stabilise the rising air bubbles in the flotation cells. The flotation Plant
consists of three roughers, three Scavengers, three Cleaners and two Re-Cleaner cells. The Re-Cleaners
concentrates gravitate into a sump from which they are pumped to a miniature concentrates leach plant.
The Scavenger tails are pumped to the tailings dam.
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The CLP is a small Carbon in Leach section of the main plant. It comprises one holding tank, six carbons
in leach tanks and a tailings tank. The six carbon in leach tanks are fed with carbon which absorbs the gold
and the loaded carbon is finally airlifted into sacks. The rich copper tails are then pumped to the Oliver
filter. Generally, typical analysis of the concentrate is 11% copper, 65 grams per tonne gold and 45 grams
MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012
EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

per tonne silver. Recovery of copper is about 65% and for gold about 24% which improves the total gold
recovery of the whole plant by roughly 3.5%.

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The purpose of the carbon regeneration section is to restore carbon that has passed through the Carbon in
Leach (CIL) and elution sections to the same activity as virgin carbon. The stripped carbon is transferred
from the elution column to the DSM screen by water pumped from the carbon transfer pump. The DSM
screen separates the water from the carbon, which then falls into the kiln feed hopper. The water gravitates
to the fine carbon recovery screen where any carbon broken down by the elution process is recovered.
Dewatered carbon is withdrawn from the kiln feed hopper at a controlled rate by a screw feeder which
discharges into the carbon regeneration kiln. A vibrating bottom section prevents hang up in the hopper.

The regeneration kiln heats and maintains the carbon at around 700°C for 30minutes allows the carbon to
cool to around 200°C and then discharges onto a gyratory Sweco screen which removes any carbon which
is broken down during this regeneration process. The kiln is sized to operate for 16 hours, per day. Spray
water on the screen washes the attrited carbon to the fine carbon recovery screen and .the coarse reusable
carbon discharges into the transfer vessel. The transfer vessel is sized to contain the contents of the elution
column but is discharged every 12 hours to the last adsorption tank as required by the carbon movement
schedule of 630 kg of carbon every 12 hours. Broken down carbon which is finer than 850 micron and
also any carbon spillage in the elution area is recovered on the fine carbon recovery screen. It was
intended that it be burnt in order to recover any contained gold.

The fine carbon which is lost from the circuit is thus replaced by adding carbon and water to the carbon
pre-conditioning vessel where any fine carbon or sharp edges are removed by strong agitation. This
carbon slurry is then drained from the carbon preconditioning vessel, screened on the kiln discharge
Sweco screen and then added to the carbon entering the transfer vessel. A carbon loss due to breakdown is
expected to be less than 30 kg per day.

R
REEN
NCCO
OMMIIN
NEE TTA
AIILLIIN
NGGSS D
DAAM
M

The tailings dam for Renco is located 0.5 Km south west of the mine and 0.4 Km from the processing
plant. Renco mine effluents and tailings system consists of an active tailings dam, new tailings dam and a

MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

return water catchment dam. The old tailings dam was constructed using spigot discharge techniques. The
new tailings dam is to the immediate west of the old tailings dam and was constructed by hydrocone
classification techniques. Below the new tailings dam lies the return water dam which is a rock-clad

MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012

EDWELL VIMBAI SANYAMANDWE ZIMBABWE SCHOOL OF MINES

MINERAL PROCESS AND EXTRACTIVE METALLURGY FIRST YEAR 2012