KONKOLA COPPER MINES plc

TAILINGS LEACH PLANT

REPORT ON THE OPERATIONS OF LEACHING, WASHING AND SOLVENT

EXTRACTION SECTIONS

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Contents

Contents ……………………………………………………………………………………………………………………………..…1

Nomenclature…………………………………………………………………………………………………………………....…..2

List of figures……………………………………………………………………………………………………………………..……2

1.0 Introduction……………………………………………………………………………………………………………………...3

2.0 Leaching section plant operations………………………………………………………………………………..……3

2.1 Pre-Leach Thickeners………………………………………………………………………………………….…4

2.2 Kamana Water Reticulation Pump Station……………………………………………………….……5

2.3 Pre-Leach Agitators…………………………………………………………………………………………….…6

2.4 Leach Pachucas……………………………………………………………………………………………..………6

3.0 Washing section plant operations………………………………………………….……………………………..……9

3.1 CCD Thickeners………………………………………………………………………………………………..……9

3.2 Old Reagent Plant…………………………………………………………………………………………………10

4.0 Solvent Extraction plant operations…………………………………………………………………..………………12

4.1 Solvent Extraction Streams……………………………………………………………………………………13

4.2 Column Flotation Cells and Natco Filters……………………………………………….………………14

5.0 Observations and Recommendations…………………………………………………………………………………16

6.0 Conclusion……………………………………………………………………………………………………………………….…16

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Nomenclature

CCD: Counter Current Decantation

CPLT: Current Pre-Leach Thickener

gpl: Grams per liter

HPLS: High Pregnant Liquor Solution

LPLS: Low Pregnant Liquor Solution

MV: Mixing Vessel

O/A: Organic to Aqueous ratio

O/F: Overflow

PD: Phase Disengagement

RPLT: Reclaim Pre-Leach Thickener

SX: Solvent Extraction

TD: Tailings Dam

U/F: Underflow

List of Figures

Figure 1: Leaching section process flow sheet……………………………………….8

Figure 2: Washing section process flow sheet……………………………………….11

Figure 3: Solvent extraction section process flow sheet……………………………..15

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1.0 INTRODUCTION

The ore mined at Nchanga underground and open pits contain copper in form of sulphides

and oxides. The sulphides are recovered by the flotation concentration process in form of

concentrates which are treated via pyro metallurgy route to produce copper cathodes. The

copper in oxide form report to the tailings which are supposed to be discarded but since they

contain the oxide copper minerals in recoverable quantities, a different route to recover this

copper in oxide form becomes necessary. The Tailings Leach Plant was established for this

purpose to treat oxide copper minerals present in the tailings via a hydrometallurgy route.

This is achieved by dissolving the oxide copper in the tailings using concentrated sulphuric

acid, separating the dissolved copper in solution from the barren solids and then purifying

and concentrating the solution by using the solvent extraction process and finally recovering

the copper in solution by electro winning to produce copper cathodes as the final product.

The Tailings Leach Plant treat tailings from the Nchanga concentrators (Old East Mill, New

West Mill and New East Mill) and old tailings damps (Currently TD3). The old tailings

damps contain recoverable acid soluble copper which were damped at the time when the

processing of low grade tailings was not feasible at Nchanga.

The plant is comprised of seven sections; Leaching, Washing, Muntimpa tailings pump

station /Post leach agitator area, Lime plant, Dams, Solvent Extraction and Tank Houses.

This report details the operations in the leaching, washing and solvent extraction sections.

2.0 LEACHING SECTION

This is the section which receives feed for treatment at the Tailings Leach plant and the

whole copper recovery process starts here. The leaching section is comprised of the Current

Pre-Leach Thickener (CPLT), Reclaim Pre-Leach Thickener (RPLT), Kamana Water

Reticulation Pump Station, Pre-Leach Agitators and Leach Pachucas. The main purpose of

this section is to dissolve out the acid soluble copper present in the feed (Tailings) by using

concentrated sulphuric acid.

Tailings from the Nchanga concentrators (current tails) and those from reclamation dams

(reclaim tails) are fed in the two thickeners i.e CPLT and RPLT with the addition of

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flocculants which help in the settling of fine solid particles. The design was that reclaim tails

be fed to RPLT and current tails to CPLT but currently the two types of tails are blended for

better settling rates (reclaim tailings are relatively courser than the current tailings). Current

tails are delivered via four lines i.e south, center, north and new east mill tails lines while

reclaim tails come through three lines i.e line 1, 2 and 3. The three reclaim tails lines are run

two at a time while reserving one line on standby.

Before leaching, the water in the tailings is reduced by use of conventional thickeners to

produce underflow slurry with less water and overflow water recycled for plant use.

2.1 Pre-Leach Thickeners

There are two pre-leach thickeners, RPLT and CPLT. RPLT was designed to treat reclaim

tails from old tailings dams while CPLT was designed to treat current tails from the Nchanga

concentrators but currently the tailings are blended i.e. reclaim and current tails are treated

together both at RPLT and CPLT. And in cases where CPLT is down some tails are diverted

to RPLT and vice versa while others are shut down so that the operating thickener is not over

burdened. The two thickeners are each equipped with three underflow pumps. Two pumps

are run at any one time at RPLT while one is run at any one time at CPLT. The rest of the

pumps are on standby.

The pre-leach thickeners serve the purpose of reducing the water content in feed before

leaching takes place to minimize the consumption of acid and also minimize the amount of

raffinate generated and reduce the amount of raffinate for bleeding.

The tails fed to the two thickeners come at densities in the range 1100 – 1300 kg/m3 (14 –

36% solids) and after removing water the underflow densities are increased to the range 1550

– 1650 Kg/M3 (56 – 62% solids).

During the thickening process two products are obtained overflow water and underflow

slurry. Overflow water is recycled for plant use. RPLT overflow water is mainly used for

tailings reclamation at TD3 and CPLT used mainly for hosing, gland water for slurry pumps

and muntimpa line flushing.

Important Parameters to Monitor and Control on the Thickeners

Torque: the thickener rake torque measure the resistance of the rakes to

rotation. A high torque means high resistance to rotation and a torque in

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excess of 45 percent stops the rakes. When the rakes stop it means going

down on the thickener, hosing out all the solids and then start over which is

costly, time consuming and a loss on production. Therefore, the rake torque is

always monitored so that it does not exceed 10 percent.

Underflow density: underflow densities must be maintained between 1550

and 1650 Kg/M3 which is the desired normal operating density required for

cost optimum water removal. This is monitored by way of controlling the

underflow pulling rates. If the density falls below 1550 Kg/M3 the pulling rate

is reduced to build up the densities and vice versa if the densities go beyond

1650 Kg/M3.

Flocculent dosage: flocculation affects the settling rate of finer particles in

the thickener and hence the overflow clarity. It is desired that a clear overflow

is obtained at all times. Dosing the right amount is very important. Under

flocculation for example is very dangerous as it can cause a shooting torque

(sudden rise in torque) and stop the thickener rakes from running at once.

MV Density: the mixing vessel densities must be maintained in the range

1150 - 1250 Kg/M3 to prevent hindered settling at the feed well and obtain

good settling rates and hence clear overflow water and normal rake torque.

This is achieved by adding dilution water in the mixing vessel.

2.2 Kamana Water Reticulation Pump Station

Kamana water reticulation pump station is there to supply water for use in the plant for

hosing, gland water, muntimpa line flushing and dilutions where required. The source of

water is either CPLT overflow launder or reservoir 150’ (Note: Water is drawn from reservoir

150’ only if the launder level at CPLT is very low or CPLT is down).

The pump station is equipped with six pumps, three to supply high pressure and the other

three to supply low pressure water. Pumps 277/231, 235 and 234 supply high pressure and

pumps 277/232, 229 and 228 low pressure water. High pressure pumps supply water in the

plant for hosing and dilutions while low pressure pumps supply water to gland water pump

station for use with slurry pumps at CPLT, RPLT, CCDs and Muntipa pumps. It is important

to keep overflow water as clear as possible to prevent the supply of dirty water which has

negative effects on plant operations. If the overflow clarity at CPLT becomes very bad, a

bloke valve is closed to stop water in the launder from going to kamana. And in that case a

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bloke valve is opened in kamana to start drawing water from reservoir 150’. During this time

transfer of water to RPLT using pumps 273/198 and 199 are also stopped until the conditions

are normalized.

2.3 Pre-Leach Agitators

There are two pre-leach agitators north and south which act as feed buffer before RPLT and

CPLT underflow are fed to the leach pachucas. At the agitators are seven slurry pumps 102,

103, 105, 135, 136, 137 and 138 which feed material into leach Pachucas. Pumps 103, 105

and 135feed A-bank, 102, 136 and 138feed B-bank and C-bank is fed by pumps102 and 137.

The pumps run one at a time to each bank allowing at least one pump on standby in case the

running pump fails.

2.4 Leach Pachucas

A Pachuca is an air agitated cylindrical leaching vessel with a conical bottom. Air is injected

at the bottom of the vessel for agitation and then slurry, acid and raffinate are fed on top.

Raffinate is added to break the densities coming from the pre-leach thickeners for easy

operations of pachucas and CCDs (see discussion on lowering densities. There are four banks

of pachucas A, B, C and D. These banks have four pachucas each except for D bank which

has three. Banks A, B and C has two receiving and two discharging pachucas labeled 1 – 4 i.e

1A – 4A, 1B – 4B and 1C – 4C. 1 and 2 are receiving pachucas while 3 and 4 are discharging

pachucas. Currently only A, B and C banks are operational D bank is under rehabilitation.

For A and B banks, 3A discharges to CCD2 while 4A discharges to CCD3. The design was

that all the two can discharge to either CCD2 or CCD3 but some lines have been spaded. So

when discharging to CCD2 (normal operation) and running on 4 pachucas the configuration

is 1-2-4-3 out. The configuration will depend on the pachucas available and the receiving

thickener at the CCDs. Bank C operates normally both 3C and 4C can discharge to either

CCD2 or CCD3.

Important Parameters to Control

The parameters controlled at leach pachucas are pH, agitation air pressure, residence

time and Pachuca density.

pH: This affects the leaching efficiency. High pH above the set point will lower

the leach efficiency and low pH below the set point will increase free acid tenor in

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the PLS at CCDs and hence lower extraction efficiency at solvent extraction plant

and also a waste of acid and an increase in the cost but with no significant change

on the leach efficiency. The pH set point is 1.8 – 2.2.

Agitation air pressure: This will lower the leach efficiency if lower than is

required as there will be poor mixing and contact between acid and copper

minerals in the feed. It is therefore important that there is good agitation in the

pachucas at all times. Poor agitation also cause blockage of interconnection lines.

The minimum agitation pressure is 3 bars.

Pachuca density: When Pachuca densities are higher than 1250 Kg/M3settling is

affected at the receiving CCD thickener causing increased total suspended solids

in PLS which is undesirable and result in more crud generation in solvent

extraction streams. Therefore the Pachuca densities are maintained in the ranges

1150 – 1250 Kg/M3 and this is achieved through the addition of raffinate at the

pachucas.

Residence Time: A low residence time will result in low leach efficiency and

vice versa. Residence time is controlled by adjusting the flows to a receiving

Pachuca i.e raffinate and slurry flow. The residence time is set at 30 minutes for

each Pachuca making two hours per bank when running on four pachucas. Also

the number of pachucas available per bank will affect the residence time. It is

always desired to run on four pachucas to have good residence time and hence

favorable leach efficiency.

Leach efficiency is very important in this section as it the overall objective at the end

of the day to achieve. The projected leach efficiency is currently at 87%. Figure 1

below shows the process flow sheet of the leaching section.

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Figure 1: Leaching Section Process Flow Sheet

CPLT

RPLT

Reclaim Tails

Current Tails

To Tailings Reclaimation

O/F

U/F

U/F

Other Plant Use

O/F

To Kamana

N/ S Pre-Leach

Agitators

TO CCD2/CCD3

Agitation Air From Compressor

Acid + Raffinate +Effluent

B -Bank

A -Bank

C -Bank

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3.0 WASHING SECTION

The washing section receives feed from the three banks of pachucas (A, B and C banks) to

separate the leached copper in solution from the solids. There are five thickeners in this

section named CCD1-CCD5.The thickeners are arranged in a way that overflow and

underflow flow in opposite direction or counter current hence the name Counter-Current

Decantation (CCD). CCD2 is the receiving thickener while CCD1 is a clarifier to CCD2

overflow and CCD3-CCD5 are wash thickeners. CCD3 overflow is clarified using the

clarifier at solvent extraction section.

The objective of the washing section is to recover solution copper in the thickener overflow

and minimal solution copper loss in CCD5 underflow so that the target wash efficiency of

92% is achieved. Also PLS free acid has to be controlled between 2-3 grams per liter in order

not to affect extraction efficiency at solvent extraction plant.

This section is only comprised of the CCD thickeners and old reagent preparation plant.

3.1 CCD Thickeners

As earlier mentioned, there are five CCD thickeners in operation and each is equipped with

three underflow pumps except for CCD1 which has two. Underflow pumps run two at a time

and keeping one on standby. The underflow pumps discharge into two independent lines.

CCD2 is the receiving thickener during normal operations and CCD1 is the clarifier to CCD2

overflow. The three discharge lines from the leach pachucas are discharged into CCD2 MV

by gravity. CCD2 overflow is pumped to CCD1 using pumps 147, 148 and 149 located at

blue tank behind CCD3 for clarification before sent for solvent extraction as HPLS.CCD2

underflow is pumped as feed to CCD3. Overflow from CCD3 gravitates to clarifier at SX as

LPLS and the underflow is pumped to CCD4 for washing out of the solution copper in

underflow. CCD4 overflow gravitates to CCD3 and the underflow is pumped to CCD5 for a

final wash. Washing is achieved by the addition raffinate to CCD5.Overflow from CCD5

gravitates to CCD4 and the underflow is pumped to Muntimpa for neutralization and

disposal.

The wash liquor (raffinate) addition to CCD5 has to be continuous and is very important such

that if it is not available you can shut down the plant as no washing can take place resulting in

high copper losses.

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3.2 Old Reagent Plant

This is where the reagents used for flocculation in all the thickeners at TLP are prepared.

There are two types of reagents prepared, acid duty for use with CCD thickeners and non-

acid duty for use with CPLT and RPLT. The reagents being used currently are Superfloc

N100 for the acid duty thickeners and Flopam AN 923 for the non-acid duty thickeners. The

acid duty reagents are tailored to operate in an acidic environment while the non-acid duty

reagents operate in a non-acidic environment. When preparing the reagents raffinate is used

for dilution on the acid duty side and water on non-acid duty but on the acid duty if there is

no raffinate water is also used. Reagent mixing is done according to the instructions given to

the operator by the plant metallurgist who make necessary calculations depending on dosage

required in the plant.

There are six pumps at reagent plant, three for acid duty and three for non-acid duty side. On

the acid duty side pumps 163, 165 and 167 are used to transfer flocculent the CCD

thickeners. Only one pump is run at a time. On the non-acid duty side, pumps 172, 173 and

174 are used to transfer flocculent to RPLT and CPLT. During normal operations 173 and

174 are used to transfer reagent to CPLT and RPLT. 172 is used to transfer reagent from east

and west preparation tanks to northeast and northwest storage tanks but it can also be used to

transfer reagent direct to RPLT and CPLT in cases where both pumps 173 and 174 are

defective.

Refer to figure 2 for the washing section process flow sheet.

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Figure 2: Washing Section Process Flow Sheet

CCD1

CCD5

CCD4

CCD2

CCD3

PACHUCA DISCHARGE

136A U/F

136 U/F

U/F

U/F

U/F

U/FTo Muntipa

O/FO/F

To SX Clarifier

Blue Tank

O/F

O/FTo HPLS Tank

Raffinate

O/F

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4.0 SOLVENT EXTRACTION SECTION

The leached solution is full of dissolved impurities and its copper tenor is very low for electro

winning therefore it has to be purified and concentrated. The solvent extraction plant serves

this purpose. The process involves contacting PLS with an organic solvent into which copper

ions from the PLS are loaded (extraction) and their after contacting the loaded organic with

spent electrolyte to strip off the copper ions from the loaded organic (stripping). Extraction

involves acid generation while stripping involves acid consumption. Therefore, extraction

requires minimal amount of acid in PLS compared to stripping stage which requires high acid

tenor in spent electrolyte. The acid tenor in spent should be between 170 – 175 grams per liter

and 2-3 grams per liter in PLS to achieve targeted extraction and stripping efficiency of 98

and 65% respectively.

Solvent extraction plant is a very critical plant as the organic used is very flammable and

toxic. Organic spills as to be prevented from going into the main drain so that no pollution

results due to organic spills.

Organic spills are washed into the SX sumps where is it recovered by use of tricanter

centrifuge machines and pumped back into the streams. Not only pollution that can be caused

by organic spills but also increased costs as it is the most expensive consumable in the SX

plant.

The plant comprises the solvent extraction streams, organic recovery columns and natco

filters, spillage ponds, new PLS ponds and the clarifier.

4.1 Solvent Extraction Streams

Concentration and purification takes place in the streams with mixer-settler arrangement. A

split circuit is used at Nchanga solvent extraction plant where two streams of PLS are fed on

each end of extraction and producing two different streams of raffinate as compared to the

conventional circuit where only one stream of PLS is fed to the extraction and producing only

one stream of raffinate. The split circuit is preferred at Nchanga SX as it increases the PLS

flows resulting in increased production.

The solvent extraction circuit at Nchanga is 3 Extract and 2 Strip meaning there are three

extraction stages and two striping stages per train and there are a total of four trains (three

trains are run at any one time while keeping the fouth on standby). The streams are Alpha,

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Bravo, Charlie and Delta. Each stream is equipped with two organic pumps running one at a

time keeping the other on standby in case the other pump running becomes defective. The

products of extraction are loaded organic and raffinate. The raffinate produced is used for

leaching and washing, the excess is neutralized and disposed off. The loaded organic is feed

to the striping circuit where the products are advance electrolyte and stripped organic.

Advance electrolyte is pumped to the column flotation cells and natco filters for recovery of

entrained organic and removal of suspended solids before pumping to the tank house for

electro winning while the stripped organic is recycled to the extraction circuit.

The mixer-settlers are arranged in the order E3, E2, E1, S1 and S2. E is for extraction and S for

stripping. Stripped organic from the organic pump is pumped to E3 together with LPLS from

clarifier. The organic flows from E3 via E2, E1 and S1 to S2 and into the organic tank and

then recycled back to E3.

HPLS is pumped to E1 and comes out from E2 as high raffinate (two stage extraction). The

LPLS pumped to E3 comes out from E3 low raffinate (single stage extraction). The organic

from E1 organic weir is called loaded organic and is feed to the stripping side where it is

contacted with spent electrolyte which is high in free acid content and loses its copper into

the electrolyte. Spent electrolyte at 35-40 gpl copper and 170-175 gpl free acid is fed to S2

mixer and come out from S1 advance weir as advance electrolyte with a copper tenor of 45 –

60 gpl and 130-150 gpl free acid tenor.

Close monitoring and control of PLS and organic flows in the streams is very important as

they affect other parameters in the plant like O/A ratios and mixer phase continuity. The

desired O/A ratios are 1.0 – 1.2 on extraction and 2.0 – 2.2 on strip. The ratios are measured

every after two hours so that if there are intolerable deviations to the set parameters necessary

changes to PLS and organic flows can be made. Mixer phase continuity is used to control

entrainments. Aqueous continuous has less aqueous entrainment in organic but more organic

entrainments in aqueous. It is therefore important to run a mixer in the correct phase

continuity to avoid these entrainments. Aqueous entrainment into stripped organic on the

stripping circuit is undesirable as it increases the acid tenor in stripped organic and lower the

extraction efficiency on extraction circuit. Therefore at Nchanga SX, the strip mixers are run

in organic continuous to avoid aqueous entrainments. E3 and E2 on the extraction circuit are

also run in organic continuous to prevent organic entrainments in raffinate. E1 is run in

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aqueous continuous in order to avoid aqueous entrainments in loaded organic and hence

impurities from extraction to strip.

The other monitored parameters are phase disengagement time (PD) which is the time taken

for the phases to separate. A high PD than required will cause entrainments and also shows

deterioration of organic due to impurities. The required PD is required 120 seconds.

4.2 Column Flotation Cells and Natco Filters

The advance electrolyte from the streams has entrained organic and suspended solids and

requires removal before electro winning has they have detrimental effects on the quality of

the cathodes. This organic and TSS removal is achieved through the use of column flotation

cells and natco filters.

There are two column flotation cells, primary and secondary operating in series. Advance

electrolyte from S2 advance weir is pumped to the primary column using column feed pumps

066 and 067. The columns are packed with polyethene bags locally known as onion bags and

coalescers which trap the entrained organic from the advance. After passing the advance

electrolyte in the columns it gravitated into the raw feed tank for storage before pumping to

the filters for removal of TSS and remaining entrained organic.

The advance electrolyte in the raw feed tank is fed to the natco filters. The filters are filled

with silica sand, garnet and anthracite which trap the organic and suspended solids. The

filters are fed with pumps 063 and 064 and after filtration the final advance electrolyte goes

to the advance electrolyte storage tank from where it is pumped to the tank house using

pumps 007 and 009for electro winning.

Figure 3 below shows the process flows in detail in the solvent extraction section.

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E3

E2

E1

S1

S2

Figure 3: Solvent Extraction Section Process Flow Sheet

Stripped Organic Tank

Lo Raffinate

Low PLS

High PLS

South PLS Tank

North PLS Tank

Stripped Organic

Loaded Organic

Advance Electrolyte

Clarifier /CCD3

O/F

CCD1/CCD2O/F

Raw Feed Tank

Spent electrolyteSpent Electrolyte

Tank

Spent electrolyte from T/H

Advance electrolyte to T/H

Advance Electrolyte Tank

NATCO Filters

X6

Hi Raffinate Reclaim Tails

Current Tails

To Tailings Reclaimation

Organic Recovery Columns

X2

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5.0 OBSERVATIONS AND RECOMMENDATIONS

For the period I have been on the plant I have interacted with almost every employee in the

covered sections and have learnt and observed a lot of which some have a negative impact on

plant performance. Most of the things mainly are as a result of employee demotivation.

People have no motivation to work and I strongly recommend management to work out

things and see how employee motivation can be improved for better plant performance. Have

also observed the unwillingness of some employees to teach and cooperate with new

employees coming on the plant either full or part time for various programs saying it is not

their responsibility and that it is the responsibility of the training department. This is serious

as it can bring about demotivation and loss of interest to new employees and hence a failure

in learning and development.

6.0 CONCLUSION

In conclusion I would say the familiarization or rather the acquaintance program me is good

and necessary to be undertaken but should be revised and closely monitored for better

performance and utilization of new employees so that both employees especially graduates

and the company have a mutual benefit.

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