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Tenova Bateman SSA

Tenova Mining & Minerals

NORTH RIVER NAMIB LEAD / ZINC

PROJECT

FEASIBILITY STUDY

Volume 1

Section: 3 Process Development

NORTH RIVER NAMIB LEAD ZINC PROJECT

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TABLE OF CONTENTS

1 PROCESS ENGINEERING ................................................................................ 4

1.1 Process Design Criteria............................................................................... 4

1.2 Mintek Testwork .......................................................................................... 5

1.3 Head Analyses ............................................................................................ 6

1.4 Mineralogy .................................................................................................. 6

1.5 Crushing Work Index and Abrasion Index ................................................... 7

1.6 Bond Ball and Rod Work Indices ................................................................. 7

1.7 Grindmill Testwork ...................................................................................... 8

1.8 Flotation Testwork ....................................................................................... 8

1.9 Flotation of Tailings ..................................................................................... 8

1.10 Flotation of Ore ......................................................................................... 10

1.11 Sedimentation and Filtration Testwork ...................................................... 12

1.12 Mass Flow Testwork ................................................................................. 13

2 PROCESS DESCRIPTION .............................................................................. 13

2.1 ROM Receipt and Crushing ...................................................................... 13

2.2 Tailings Reprocessing ............................................................................... 17

2.3 Milling ........................................................................................................ 19

2.4 Floatation of Ore ....................................................................................... 21

2.5 Flotation of Tailings ................................................................................... 23

2.6 Reagents .................................................................................................. 23

2.7 Sampling ................................................................................................... 24

2.8 Concentrate Dewatering............................................................................ 25

2.9 Tailings Disposal ....................................................................................... 26


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2.10 Process Water .......................................................................................... 27

2.11 Compressed Air ........................................................................................ 28

2.12 Emergency Power ..................................................................................... 28

2.13 Operating and Control Philosophy ............................................................. 28

3 CAPEX AND OPEX ......................................................................................... 28

3.1 Capex ....................................................................................................... 28

3.2 Opex ......................................................................................................... 30

4 CONCLUSIONS AND RECOMMENDATIONS ................................................ 30


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1 PROCESS ENGINEERING

The purpose of the process section of the final report is to:

Compile all process deliverables and process related information,

including process design criteria and mass balance

Describe how the process design and testwork programme were

developed

Describe the process flow and utilities requirements

Explain the mechanical equipment selection for tailings reprocessing

and fresh ore

Explain the operational and control philosophy for tailings reprocessing

and fresh ore

Detail the material and resource requirements for input into capex and

opex calculations

Review the capex and opex estimates

Make recommendations for the implementation of the project.

1.1 Process Design Criteria

The Process Design Criteria were baselined from various past studies and

from similar operations. A conservative view has been taken on which

values to use for Bond Work Indices, flotation residence times and other

parameters.

In order to improve the accuracy of the final cost estimate, Mintek was

contracted to undertake a testwork programme which has run in parallel with

the project. A copy of the Mintek testwork proposal and scope of work is in

the report appendices.

The Mintek testwork outputs were fed back into the process design criteria,

project capex and opex where possible. The latest testwork informed the

design of the crushing and milling circuit. However the flotation testwork was


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not completed in time to update the design of the flotation circuit and

downstream equipment, for which layouts, civil and structural costs are

based on the Rev A design criteria e.g. lead and zinc rougher flotation

residence times, concentrate filtration rates and tailings settling rates.

The process design allows for two distinct plant feeds, namely zinc-rich

tailings from the existing tailings dam and fresh ore from the underground

mine. North River Resources (NRR) requested the option of a tailings

retreatment facility, comprised of a closed circuit ball mill, followed by zinc

flotation. Reprocessing of tails could proceed in parallel with mine

development and the construction of a front-end crushing circuit as well as a

lead flotation circuit upstream of the zinc flotation circuit. Alternatively

tailings could be retreated at a later date, such as prior to mine closure. The

process design allows for easy switching between tailings and fresh ore,

although the optimum ball charge is different.

The processing of tailings is important both economically and strategically

for NRR. However, the economics of fresh ore processing are more

attractive than for tailings reprocessing. Therefore the process design is

based on fresh ore, and tailings will be reprocessed at whatever rate the

capacity of the overall circuit will economically allow. The mill will have the

capacity to mill 250 000 tpa of crusher product to a target grind of 80%

passing 75 microns. The capacity of the mill for tailings will be higher due to

their finer size distribution. However, the retention time of the flotation plant

will be reduced at higher tonnages, with consequent losses of recovery.

The design criteria value of 300 000 tpa of tailings was chosen to represent

a reasonable trade-off between milling throughput and flotation recovery.

1.2 Mintek Testwork

The aim of the testwork was to confirm or update the ore characteristics and

key process design criteria:


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Mineralogical analyses and head assays for tailings and fresh ore

samples

Crushing work index and abrasion index for ore and waste

Bond rod and ball mill indices for ore

Grindmill testwork to establish the specific energy for ball milling

tailings

Flotation circuit configuration for ore and tailings

Grades and recoveries for ore and tailings

Mass pulls for ore and tailings

Flotation kinetics for ore and tailings

Reagent consumptions for ore and tailings

Sedimentation rate and flocculent consumption for final tailings

Filtration rates for lead and zinc concentrates.

1.3 Head Analyses

The tailings sample assays 2.98% zinc, slightly higher than the value used

for the PDC. However the ore sample assays 3.91% zinc and 1.94% lead,

which are somewhat lower than the PDC values. There is 23.4% iron in the

ore sample, rising to 25.8% in the tailings sample. Calcium measures 12%

in the tailings and 15.3% in the ore. Total sulphur is 15.4% in the tailings and

13.4% in the ore.

1.4 Mineralogy

The mineralogical reports show that, apart from being depleted in galena,

the tailings have other noticeable differences compared to fresh ore, i.e.

Slightly lower in sphalerite (4.89% vs. 6.13%)

Much higher in Fe-Oxides (11.96% vs. 2.85%)

Lower in calcite (18.82% vs 30.57%)

Much higher in sulphates (5.43% vs. 1.03%).


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The high sulphates combined with the low calcite means that the tailings

slurry samples have a low natural pH of around 6 and there appears to be

some buffering in solution. This means a lot of alkali is required to reach

target pH’s of 10-11, making some commonly-used flotation recipes

uneconomical.

Oxidation of sphalerite is not apparent from microscopic analysis, but

surface oxidation may explain why the sphalerite in the tailings sample is

apparently not activated by copper sulphate.

Electron microprobe analysis shows that the galena is composed of lead

and sulphur only. However, the sphalerite contains on average 10.48% iron

in the tailings and 11.03% iron in the ore. These represent minimum values

that are achievable in the concentrate if the sphalerite were quantitatively

recovered.

1.5 Crushing Work Index and Abrasion Index

The crushing work index of 10.87 ± 1.89 kWh/t for the ore sample is

indicative of soft ore, verging on very soft. The waste rock was very soft with

a CWI of 8.69 ± 1.63 kWh/t. The Bond Abrasion Indices for ore and waste

were both less than 0.1, where less than 0.2 is indicative of low abrasivity.

These results indicate that single stage impact crushing of ROM ore with a

maximum size of 400 mm to a ball mill feed size of 15 mm is achievable

without causing excessive equipment wear.

1.6 Bond Ball and Rod Work Indices

The Bond Ball Work Index of 10.9 kWh/t at a closed screen size of 106 µm

and Rod Work Index of 10.7 kWh/t are also indicative of soft ore. Despite

the low nominal tonnage of 32 tph a motor power of 450 kW is still required

to achieve the fine target grind of 80% -75 µm.


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1.7 Grindmill Testwork

The grinding of tailings is not accurately predicted by Bond’s Law. Batch

milling tests are therefore used to derive breakage and selection functions,

which are then used to simulate the continuous circuit and output the

specific energy requirements.

The Mintek Grindmill testwork gives a specific energy of 3.72 kWh/t for

grinding of tailings to 80% -75µm using a ball top size of 30 mm. This rises

to 6.46 kWh/t with a ball top size of 70 mm. This means that there is the

potential to mill tailings at a much higher rate than fresh ore. In practice the

processing of tailings is likely be limited by the recovery versus retention

time relationship in the zinc flotation circuit, which has been designed for 20

minutes rougher retention time for a fresh ore feed of slightly less than 30

tph and 30% solids by mass.

The higher energy requirement for milling tails with large balls means that

switching the plant feed from ore to tailings would result in high electrical

consumption if the ball charge was not switched to smaller balls. Switching

from tailings to fresh ore would be easier as it would only require larger balls

to be added to the mill.

1.8 Flotation Testwork

The majority of the Mintek testwork was to confirm flotation process design

criteria. These include the effect of grind, rougher flotation rates, number of

cleaning stages, reagent addition points and reagent dosages, and finally

locked cycle cleaner tests. The flotation of tailings and fresh ore are

discussed separately.

1.9 Flotation of Tailings

The tailings dam sample contained over 32% fines in the -38 micron

fraction. The first scouting test used a polypropylene glycol methyl ether as

frother, copper sulphate as activator and PAX, a strong collector, at the


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natural pH of the slurry –made with fresh water from site. Sodium silicate

was used to prevent coating of ore particles with slimes and increase

selectivity, but a low recovery (51.7%) was obtained at a high rougher mass

pull (22.9%).

The second scouting test used lime instead of sodium silicate with the aim of

depressing pyrite. This approach was recommended by the plant foreman of

a tailings retreatment plant which ran between 1990 and 1992. The recovery

dropped to only 34%.

The tailings dam is now at least 22 years old and there is evidence of

oxidation of iron minerals such as pyrrhotite to sulphate. For this reason, 5

kg/t of lime was required to raise the pH to 10.5 and 8.5 kg/t was required to

raise the pH to 10.8. Therefore some alternative to the use of lime as a

pyrite depressant was indicated. Addition of Cytec 7261 iron depressant

improved the recovery but did not improve the selectivity.

The use of sodium silicate was revisited, this time conditioning at 5 kg/t and

70% solids. A zinc recovery of 74% was achieved without milling, increasing

to nearly 90% when milling to 80% passing 75µ. However, mass pulls were

high, zinc grades were low and iron grades were high.

By reducing the reagent dosages and mass pull, better selectivity could be

obtained, but at the expense of recovery. The best quality concentrate

contained 46% zinc and 19% iron at a recovery of 35%. Striking a better

compromise between grade and recovery has proved to be a difficult

challenge, and the extra effort put into this testwork contributed to delays in

the overall Mintek testwork program.

Aeration of the pulp was tried at a late stage of the testwork with the aim of

preferentially oxidising the iron sulphides and depressing them. The results


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were promising, but the detailed assays are not available to date. Therefore

an economical process for tailings cannot be ruled out at this stage.

The municipal water supplied for the testwork was originally at pH 8, but

over the prolonged testwork period, the pH climbed to 10.7 and there was

some precipitation of solids. This effect may prove beneficial in treating

tailings material which has a low natural pH of around 6.

1.10 Flotation of Ore

Initial results with ore showed faster than expected lead flotation kinetics,

with a recovery of over 90% lead achievable at less than 5% mass pull.

Initially some zinc reported to the lead concentrate, but this was reduced to

an acceptable level by reduction of the collector dosage in the lead float.

The flotation of ore follows a more conventional route than for tailings. A

small amount of hydrated lime (200 g/t) is used to raise the pH in the mill

above 8.5 to prevent formation of hydrogen cyanide. 300 g/t of zinc sulphate

and 80 g/t of sodium cyanide are then added to the mill to depress zinc in

the lead float. 365 g/t more lime, 30 g/t of XP200 frother and 15 g/t of

sodium ethyl xanthate are then added to the conditioner. The lead

concentrate is then subjected to cleaning and re-cleaning stages.

A flotation grade of 62.5% lead, 1.0% zinc and 4.1% iron was achieved at a

recovery of 83%, which is in line with the process design criteria. The

laboratory rougher flotation retention time was 3 minutes.

Zinc flotation kinetics were also fast, but mass pulls were initially too high,

indicating that reagent dosages for the zinc float were also high.

Flotation of zinc starts with the addition of 1.9 kg/t of lime to conditioner 1 to

raise the pH to 11.5 and the zinc is then activated with 500 g/t of copper


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sulphate. 15 g/t more XP200 frother is added to conditioner 2, with 60 g/t of

SEX and 125 g/t of Aero 7261 iron depressant.

A flotation grade of 53.0% zinc and 13.4% iron was achieved at a recovery

of 83%, which is in line with the process design criteria. A recovery of 73%

silver at a grade of 170 g/t was also achieved (telephonically reported). The

laboratory rougher flotation retention time was only I minute, however, and

this process would be very difficult to control in practice. For this reason

Mintek recommends the use of the Cytec iron depressant, which slows the

kinetics. Therefore the baseline conditions for locked cycle tests are still to

be determined, based on the results with depressant. Aeration was a

variable which was incorporated at a late stage in the testwork, and is also

showing promising results. However, Mintek remains confident of the grades

and recoveries achievable for lead, zinc and silver.

There was some evidence of oxidation of the ore sample reported by

Mintek. Fresh ore should require less lime to raise the pH. Because of the

recycling of reagents, the locked-cycle tests should give reduced reagent

consumptions that will be a better prediction of continuous plant operation.

The full-scale retention times must be adjusted to compensate for the faster

kinetics associated with a small laboratory batch cell. However this requires

specialised testwork and modelling, which are beyond the scope of this

study. Instead the following scale-up factors can be used as a basis for

comparison of the PDC with the testwork:

Flotation Stage Scale-Up Factor

Conditioner 100% (i.e. none)

Rougher 200%

Cleaner 250%

Recleaner 300%


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The practical implications are that the conditioner and rougher cell volumes

are considerably larger than required, the cleaner cells are slightly larger

and the recleaner cells are slightly smaller, based on the PDC. However,

additional recleaner capacity was included in the design to allow for higher

than expected froth carrying and lip loading capacities associated with

pockets of high grade ore.

Concentrate mass pulls obtained from testwork are considerably lower than

allowed for in the PDC. The rougher mass pull for lead is a quarter of the

design value, while the rougher mass pull for zinc is half the value in the

PDC. This means that higher than expected head grades and recoveries

should not overload the concentrate sumps. Again, locked-cycle tests will

give the best prediction of the plant retention times and mass balance.

The testwork results therefore confirm the conservative process design

basis used for the flotation plant, and indicate that capex can be reduced

here.

1.11 Sedimentation and Filtration Testwork

Sedimentation tests were planned for the flotation tailings and sedimentation

and filtration tests were planned for the concentrates. This testwork included

optimisation of flocculent addition for the tailings thickener.

The testwork on filtration rates was scheduled to follow the locked-cycle

testwork, which will produce the most representative samples of

concentrate. Flotation testwork delays means that suitable samples have not

been tested to date. There is a concern that, at the fine grind employed, it

may not be possible to produce a filter cake with the target moisture content

of 9% at the specified tonnage. This problem could be exacerbated by poor

blending of ROM ore, resulting in higher than expected concentrate


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production. However, given the low rainfall and high evaporation in the area,

NRR is confident that producing dry concentrates will not be a problem.

Sedimentation testwork on tailings will also follow the locked-cycle testwork.

1.12 Mass Flow Testwork

Mintek was responsible for all the testwork apart from the mass flow

testwork, which was undertaken by Green Technical, an independent

consultant, using the Tenova Bateman laboratory facility.

The 0-4 mm size fraction is critical in determining the bulk flow properties of

solids. The fine ore bin and feeder design for crusher product / ball mill feed

was therefore based on bulk solids flow testwork undertaken by

Greentechnical (Pty) Ltd. The large opening at the bottom of the bin is

dictated by the need to avoid bridging and rat-holing from occurring. This in

turn requires an oversized belt feeder to feed the mill. A cheaper vibrating

feeder of the required size for the silo opening would not be able to control

the low feed rate.

2 PROCESS DESCRIPTION

2.1 ROM Receipt and Crushing

OVERSIZE

SCREENIMPACT

CRUSHERUNDERSIZE TO MILLING

ROM STOCKPILE

ROM BINFRESH ORE ONLY


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MOBILEIMPACT

CRUSHER

ROM STOCKPILE

ROM BINFRESH ORE & TAILS UNDERSIZE TO MILLING

The ore body is not homogeneous, but consists of marble host rock

interspersed with zones of massive galena and sphalerite. For this reason,

some blending should be undertaken to ensure a relatively constant feed

grade to the concentrator plant, even though double handling of the ore will

have a cost implication. Multiple stockpiles will increase the amount of

handling involved and will require extra space, and so a single blending

stockpile will be used.

ROM ore with a top size of 400 mm will be brought to the tip in 15 t trucks.

The ROM tip will have a capacity of 2000 t of ROM ore. Trucks will be able

to drive onto the tip and spread their loads laterally to facilitate blending. A

front end loader will profile the ramps on the tip and will transfer ore from the

tip into the ROM feed bin, further blending the ore so as to reduce

fluctuations in the lead and zinc head grade. The ROM feed bin will be able

to accommodate 50 t of ore. The bin will be equipped with a static grizzly

with a 400 x 400 mm aperture size. The small scale mining method is not

expected to generate much oversize material. Any oversize material will be

pushed off the grizzly and broken with a hammer in a demarcated area next

to the bin where no pedestrian traffic will be allowed. Broken material will be

retrieved by front end loader as required.

Several options were considered for the crushing circuit. The most

conservative option included a primary jaw crusher and secondary cone

crusher in closed circuit with a vibrating screen. However, there was early

evidence that the ore was relatively soft and of low abrasiveness, which was

later confirmed by Mintek testwork. This allowed consideration of impact

crushers which could take the ROM ore from a top size of 400 mm to a top

size of 12-15 mm in a single crushing stage.


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The use of impact crushers means that extra attention should be paid to the

accompanying high noise levels and dust generation.

Consideration was given to a mobile impact crusher, which has the

advantage of minimal installation cost. This option is the most flexible as it

lends itself to the treatment of both tailings and fresh ore, while minimising

redundant equipment once the tailings resource has been exhausted.

A modular skid mounted impact crushing circuit was also considered since

the ground conditions are particularly favourable. However, the extra

simplicity and reduced cost of operation and maintenance of a modular skid-

mounted plant means that this option is recommended if only fresh ore is

treated.

The modular skid-mounted concept also allows for reduced construction

time and reduced civil costs when compared with more permanent crushing

installations. This approach was used wherever it was practical to do so.

The modular crushing circuit is supplied as a complete skid-mounted

package by Metso. It comprises ROM bin (26 m3) with fixed grizzly (400 x

400 mm aperture), vibrating grizzly feeder for pre-screening of -38 mm fines

from the crusher feed , impact crusher (110 kW), single deck closed circuit

vibrating screen (4 m L x 1.5 m W), plus all conveyors and transfer chutes. A

belt magnet and metal detector are included. In order to cut costs, Metso

have used narrow conveyors (500 mm) and steep conveyor lifts of 16-18

degrees. When these lifts were queried, Metso supplied the diagram below,

which shows that the lifts are safely within Metso’s design envelope.

However, there is still a concern about the belt angle at the loading point,

which we would recommend not being more than five degrees, and this

should be revisited at the implementation stage.


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If an economical recipe for tailings reprocessing is forthcoming from

testwork, then a ball mill feed must be produced from the hard aggregated

tailings. These tailings are likely to contain some tramp metal and rocks. A

mobile impact crusher would be ideal for this application, designed for

building rubble containing structural steel and concrete. This would make a


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skid-mounted crushing plant redundant, since the mobile crusher could

alternate between tailings and fresh ore.

The crushed fresh ore will be stored in a 300 ton fine ore bin. This will allow

just less than 10 hours of milling time in the event of crushing circuit

downtime. It should be noted that the availability of the mobile crusher is

likely to be less than the fixed crusher circuit. Because there is a 300 ton

fine ore bin, the mill availability can be maintained for fresh ore. However,

for tailings reprocessing the fine ore bin will be bypassed and the mill

availability will be dependent on the mobile crusher availability.

2.2 Tailings Reprocessing

CYCLONEMECHANICAL

RECLAIMOVERFLOW TO

ZINC FLOTATIONBALL MILL

MOBILEIMPACT CRUSHER

UNDERFLOW

The initial project strategy involves initial reprocessing of approximately

700kt of existing tailings through the ball mill and zinc flotation circuit while

the underground mine is developed. The crushing circuit and lead flotation

circuit would be constructed in time to receive fresh ore from underground.

However the strategy is flexible, and it may be economically preferable to

defer tailings reprocessing to the end of end of the project.

The tailings dam is comprised of a solid crust of agglomerated material at

the surface which gradually transforms into a slimy liquid at depth.

Mechanical reclamation of the surface tails to ball mill feed size could be

achieved by impact crushing, but the major unknown is the location and

extent of the wet material which would require specialised hydraulic

monitoring.


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The uncertainty in the depth and location of the slimes means that even

mechanical reclamation may require experience in tailings dam

management. The risk increases for hydraulic reclamation, where tailings

dam management experience is essential, and where the investment in

equipment cannot be justified without knowing the amount of slimes to be

treated.

Furthermore the companies which specialise in hydraulic mining of tails

(Fraser Alexander & Steffanuti Stocks) do not design systems but prefer the

build/own/operate/maintain business model.

For these reasons, and to guarantee no equipment redundancy, only the

mechanical reprocessing of tails will be allowed for initially. It is likely that

the residual moisture in the dump has dropped and that, once wet areas are

exposed, this moisture will quickly evaporate. Therefore the need for

hydraulic reclamation can be re-evaluated at a later date.

The mechanically reprocessed tails will be discharged by front end loader

into the feed hopper of a mobile impact crusher (capacity 2.6 m3). The

mobile crusher has a maximum capacity of 160 tph (depending on the ore)

and is equipped with a feeder, pre-screen for fines removal, impact crusher,

closed circuit screen and recirculating belt to return oversize to the crusher

feeder. The screen and pre-screen undersize will combine and be

discharged onto a transfer conveyor. The transfer conveyor will discharge

via a transfer chute onto the mill feed conveyor. The reprocessed tails will

bypass the fine ore bin in order to avoid re-compaction.


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2.3 Milling

BALL MILL CYCLONE

UNDERFLOW

Lime, NaCN & ZnSO4

FINE ORE BINCYCLONE OVERFLOW TO LEAD FLOTATION

CRUSHER PRODUCT

A conventional overflow ball mill was selected because it is a simple and

robust design in comparison to a grate discharge mill. It is also preferred for

fine grinding because an overflow mill has a longer retention time than the

same size grate discharge mill. A rubber lining was preferred, as it is quicker

and easier to install, giving a lower installed cost. Other advantages are

reduced maintenance and reduced noise levels. Reagent additions are for

fresh ore only.

The nominal throughput for fresh ore is 250 000 tpa, which at 90%

availability equates to 32 tph. The target grind is 80% -75 microns although

the flotation testwork has revisited coarser and finer grinds for both tailings

and fresh ore. In order to generate a narrow particle size distribution for the

flotation feed, the ball mill is in closed circuit with a hydrocyclone and a

circulating load of 250% was chosen as a suitable compromise between

efficient milling and the size of the mill discharge pumps.

The mill discharge pumps and other main slurry pumps in the milling and

flotation areas will employ expeller seals in order to avoid having a separate

gland service pumping system. The mill discharge, lead tailings, zinc tailings

and tailings underflow pumps will be variable speed and will have standby

capacity. Tech-Taylor valves are specified on the discharge lines to allow

rapid changeover or starting of a standby pump.


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A 400 mm diameter cyclone was chosen because it allows for flexible

operation i.e. higher volumetric throughputs to allow tailings to be treated at

a higher tonnage than fresh ore. Fresh ore will be fed using the conventional

configuration, while tailings can be treated using a reverse configuration, i.e.

feeding the mill discharge sump rather than the feed hopper.

It will be possible to mill tailings at a substantially increased rate compared

with fresh ore. Based on a specific energy of 3.72kWh/t, it would be possible

to mill at over 100 tph. If the standby slurry pumps are started on the main

process lines the increased pressure drop means increasing the flow rate to

match the mill’s capacity for tailings will not be possible. However a

substantial increase in throughput would be achievable, particularly if the

zinc flotation were run at higher feed % solids and/or lower retention time. A

nominal capacity of 300 000 tpa has been chosen as a reasonable

compromise between throughput and recovery.

Running the plant at higher throughputs of tailings without sacrificing

retention time would be possible if the lead and zinc circuits were run in

parallel for zinc recovery, but this would require minor modifications to the

existing plant design and early construction of the lead flotation circuit. The

optimum ball filling and ball size distribution will be different for fresh ore and

tailings, with a lower ball filling and smaller balls required for tailings.

Therefore it is preferable not to switch from tailings feed to ore and back

again.

The top-up costs for forged steel balls were quoted at N$12589/t delivered.

The equivalent high-chrome balls are more expensive at N$16785/t, but

have a longer wear life. If the high chrome balls wear at 75% of the rate of

the steel balls, then the effective cost is the same. Claims have been made

that reductions in media consumption of 60% are possible, with no adverse

effect on metallurgy, i.e. 40% of the wear rate of steel balls (Ref. “The

Application of High-Chrome Grinding Media at MMG Century Mine for


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Improved Grinding Media Consumption and Metallurgy Performance”,

Greet, Obeng, Kinal and de Bosscher, Metallurgical Plant Design and

Operating Strategies (MetPlant 2013), 15 - 17 July 2013, Perth WA). This

translates to potential savings of nearly 50% in opex. However, steel balls

were chosen for the opex calculations, as these were used by Mintek for the

testwork.

2.4 Floatation of Ore

LEAD ROUGHERS

LEAD CLEANERS

LEAD RECLEANERS

LEAD CONDITIONER

CYCLONE OVERFLOW

ZINC CONDITIONER

1

ZINC CONDITIONER

2

ZINC ROUGHERS

ZINC CLEANERS

ZINC RECLEANERS

LIME & CuSO4

XP200 & SEX & LIME

XP200 & SEX & AERO 7261

TAILINGS THICKENER

ZINC CONC FILTER

LEAD CONC FILTER

The skid-mounted modular philosophy was also discussed with flotation cell

vendors. However, none of the proposals included this as an option. For in-

house design, the traditional concrete “mushrooms” are considered more

cost-effective than designing steel skid-mounted structures from scratch.

Based on historical testwork, which indicates the lead and zinc sulphides are

fast-floating and that iron is slower floating, a rougher/cleaner/recleaner

configuration (no scavenger cells) was chosen for both lead and zinc

flotation, subject to confirmation by the current Mintek testwork. The

reagents chosen have been in common use for many years. The exception

is Aero 7261, a newer product which has proven effective in depressing


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pyrrhotite. XP200 is a polypropylene glycol methyl ether frother. A cheaper

frother could be tested at a later stage.

Flotation cells will be individually stepped tank cells with forced air

introduced via low pressure blowers (part of vendor package). The air flow

and the cell levels will be automatically controlled. This level of automation is

considered essential for concentrate product quality control.

The retention times chosen for the flotation circuit were conservative. This

means that the cell volumes, froth carrying capacity and cell lip loading will

also be conservative. However, the nature of the ore body is such that high

grade pockets of sphalerite and galena are likely to come through to the

float even though there is some blending of ore at the ROM tip. A safety

factor is therefore prudent, although too much retention time can result in

the unwanted recovery of slower-floating iron sulphides.

Lip loading and froth carrying constraints will be most tested in the final

cleaning stage. For this reason the circuit design has a 200% safety factor

for zinc and a 250% safety factor for lead on the recleaner cell residence

times. The different values are dictated by the standard flotation cell sizes,

and not because a higher safety factor was required for lead.

Combined sump pumps were specified for concentrate sumps. These have

the disadvantage of lower mechanical efficiency, but the advantage of

increased simplicity and flexibility over the traditional centrifugal pump /

sump arrangement.


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2.5 Flotation of Tailings

ZINC CONDITIONER 2

ZINC ROUGHERS

ZINC CLEANERS

ZINC RECLEANERS

TAILINGS THICKENER

UNDERFLOW TO TSF

OVERFLOW TO PROCESS WATER TANK

ZINC CONC FILTER

ZINC FILTER CAKE

O/F TO PROCESS WATER TANK

Na2SiO3 & CuSO4 & SEX

LIME & CuSO4 & SEX

CYCLONE OVERFLOW

ZINC CONDITIONER 1

The reprocessing of tailings should ideally be accommodated within the

existing flowsheet. If attritioning and aeration are required, as seems likely,

then this could be accomplished by earlier construction of the lead flotation

circuit, provided that a lead removal step is also not required because of

smelter penalty charges.

2.6 Reagents

Reagents will be stored in a dedicated storage area. They will be made up

on a daily basis and stored in tanks using a skid mounted concept. Most of

the reagents will be delivered in solid form. Each solid reagent will require

two agitated tanks: one for mixing and one for dosing. When the dosing tank

is empty, it will become the mixing tank for the next batch of reagent.

Relatively high reagent concentrations were chosen so as to reduce the

consumption of fresh water for dissolution, i.e. 10% for xanthate and

sulphate, 20% for cyanide and 30% slurry for hydrated lime. The liquid


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reagents that do not require mixing will be pumped directly from the

containers that they are delivered in.

The reagents selected and the dosage rates were based on historical

testwork, particularly the work undertaken by Kupfermelt Metal Processing.

The reagent selection, dosage points and dosage rates will be updated for

opex purposes, and the capex implications will be discussed.

Each reagent will be delivered by a dedicated peristaltic pump to a particular

point in the process using flexible stainless steel tubing. Dosage rates can

be varied using variable speed drives. Flow rates will be measured and

controlled manually.

Reagent spillage will be pumped to the final tailings sump, except for lime

which is contained within a dedicated sump and can be pumped back into

the mixing or dosing tank.

2.7 Sampling

Sampling of lead flotation feed (or zinc flotation feed if tailings are

processed) will be by means of a two-stage cross-cut sampler. Sampling of

lead flotation tails and zinc flotation tails are by single stage cross cut

samplers of the same size as the flotation feed primary sampler. Therefore

there may be the opportunity to save costs and reduce the height of the

cyclone structure to accommodate a single stage sampler, if a slightly larger

sample is collected.

Sampling of lead and zinc flotation concentrates will also be carried out by

single stage cross cut samplers


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2.8 Concentrate Dewatering

LEAD CONC FILTER

LEAD FILTER CAKE

FILTRATE TO PROCESS WATER

TANK

LEAD FILTER FEED TANK

LEAD RECLEANER

CONC

ZINC CONC FILTER

ZINC FILTER CAKE

FILTRATE TO ZINC PROCESS WATER TANK

ZINC FILTER FEED TANK

ZINC RECLEANER

CONC

The process design was initially based on thickening followed by filtration.

Filter presses were chosen because of their simplicity and cheapness. Filter

presses with manual discharge of cake and cloth washing were considered

the most appropriate design for the duty. However, the filter press sizes for

thickener underflow feed were only slightly smaller than if they were fed with

thickener feed. Therefore concentrates will be filtered only.

The lowest capex option was chosen. (Delkor FAST Filter Presses). This is

a new design with a smaller footprint, but this option should only be chosen

for the construction phase if there are at least a few suitable local reference

installations, as there are some reservations about the closing mechanism,

which doesn’t use the typical hydraulic ram.

Confirmatory testwork will be required to confirm throughput and moisture

content. The final moisture specified may necessitate the use of more

expensive filter options, such as membrane squeezing, or the use of more

expensive Larox-type vertical filter presses, which are employed in many

concentrator plants. However, air-drying is an obvious option in a desert

environment to mitigate process risk.


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There is a concern that pockets of high-grade ore may reach the plant and

overload the concentrate filters. This could be overcome by increasing the

capacity of the filter feed tanks. A cheaper alternative would be to build a

bund wall around the filter feed tanks, and allow them to overflow during

periods of high concentrate production. The products could then be

manually returned to process when capacity allows.

Lead concentrate filtrate will be recycled as process water. Zinc concentrate

filtrate will only be recycled within the zinc section, to avoid activation of zinc

in the lead float.

2.9 Tailings Disposal

TAILINGS THICKENER

U/F TO TSF

O/F TO PROCESS WATER TANK

CATCHMENT DAM RETURN

ZINC ROUGHER TAILINGS

Flotation tailings thickening will be carried out in a high-rate thickener. A

high-rate thickener will have a lower capex than a larger conventional

thickener, although there is an opex penalty because of increased flocculent

consumption. A skid-mounted manually operated flocculent plant has been

included. A hydrocyclone is often specified upstream of the thickener, with

the cyclone overflow being thickened and the thickener and cyclone

underflow being combined and pumped to tails. This level of complexity is

not justified for the scale of operation, given the relatively small potential

saving in thickener size.


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Where centrifugal pumps are specified for thickener underflow, vendors

usually prefer a gland seal pump or expensive mechanical seals. For this

reason, seal water will be taken from the high pressure potable water line.

The thickener underflow will be pumped to a final tailings sump, from where

it will be pumped to the tailings dam.

Tailings thickener overflow will be recycled as process water.

2.10 Process Water

RAW WATER MAKE-UP

PROCESS WATER TANK

THICKENER OVERFLOW

BALL MILL, LEAD FLOTATION, ETC.

The water requirement for the project will be potable quality and therefore

expensive. The tailings dam design indicates a recovery of 49-59% of the

water that is pumped to tailings. Concentrate products will also be

dewatered in filter presses and the water recycled.

The recycling of copper sulphate from the zinc concentrate filtrate to the

lead circuit must be avoided, since otherwise zinc will be activated and will

report to the lead concentrate. The most conservative option is to have

separate tailings thickeners after lead flotation and zinc flotation, and to

keep the water streams separate, but this adds substantially to capex.

What has proven to work on a large scale operation is to recycle the water in

the zinc concentrate stream to the zinc circuit only, since the copper

sulphate is mainly contained in the zinc concentrate stream, and this is the

preferred design.


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2.11 Compressed Air

PROCESS AIR COMPRESSOR

FILTER PRESSESMILL GREASECONTROL VALVES

BACK-UP COMPRESSED AIR FROM MINING

AMBIENT AIR

The major users of compressed air will be the filter presses. Additional

compressed air is required for application of grease to the mill girth gear, as

well as for the control valves in the flotation circuit.

A single compressor has been sized for this duty, with three air receivers -

one air receiver for each of the two filter presses and an additional air

receiver for the remaining plant. The plant air will be dried by a refrigerator

drier.

A back-up compressed air supply line at 7 bar will be taken from the mining

compressor station.

2.12 Emergency Power

Electricity will be supplied by NamPower. An emergency generator will

supply power to the thickener rake and filter feed tank agitators in the event

of a power outage. Emergency lighting will be battery-powered.

2.13 Operating and Control Philosophy

Refer to Volume 2 – Process in the report appendicies.

3 CAPEX AND OPEX

3.1 Capex

The limited mine life means that the project is highly capex-sensitive. Capital

cost cutting measures include the following:


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Design factor of 10% for transfer equipment (pumps & conveyors) only

Standby equipment limited to the main process flow and process water

pumps

Common discharge line for running / standby pumps

HDPE piping more economical than rubber-lined steel (although

increased support required).

Single stage impact crushing to reduce ROM ore to ball mill feed size

Skid-mounted crushing circuit to reduce construction time and civil

costs

High-rate thickener instead of conventional thickener for tailings

Filtration of tailings without intermediate thickening

Flotation cells limited to a minimum of three in series

Reduction in fine ore bin size by increasing crushing plant operating

hours

Reduction in reagent tank sizes by daily make-up.

Skid-mounting of reagent tanks.

Reduction in capex inevitably implies increased opex, and a fit-for-purpose

philosophy should ensure that opex penalties are not excessive. For

instance, the opex of a single stage crushing plant would be prohibitive if the

ore had a high crushing work index or was abrasive, but the ore is soft and

non-abrasive. The relative capital costs obtained from a single supplier are

summarised below.

Crushing Plant

Description

Equipment Cost

(Uninstalled)

2-Stage Jaw / Cone N$ 10.192 m

2-Stage Jaw / Impact N$ 7.343 m

Single-Stage Impact N$ 5.849


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For instance the capex for the German-built Rubble Master RM80 Go mobile

crusher of N$ 4.371 m is less than half the cost of a Metso mobile crusher

with similar specs (closed-circuit screen for proper ball mill feed size control)

at N$ 9.456 m. The Metso machine features a bigger crusher which is also

bigger than the fixed impact crusher specified by Metso for the same duty.

Although the Metso mobile crusher was not favourably priced, the Metso

fixed crushing plant was the cheapest offering that was technically

acceptable (incorporating feed to the fine ore bin). It should be noted that

Metso mobile equipment is sold through a third party (Barloworld

Equipment).

Although capex reduction was a priority, there are opportunities to make

further reductions e.g. by using a contractor or purchasing second-hand

equipment, provided the risk is acceptable.

3.2 Opex

Refer to Section 7 of this report.

4 CONCLUSIONS AND RECOMMENDATIONS

The Mintek testwork on tailings was exhaustive, and delayed completion of

the testwork on fresh ore. There were indications at the time of writing that

the zinc can be extracted profitably, but the margin would always be low at

best, especially when compared with the economics for fresh ore.

The fresh ore has excellent milling and flotation characteristics, and high

recoveries and grades are achievable. Controlling the flotation process has

proven to be a challenge because of the faster than expected flotation

kinetics, so determining the optimal flotation cell configuration, optimum

residence times and the right reagent dosages will be critical to the


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profitability of the project. Locked cycle testwork will be the best method

(after pilot plant testwork) of finalising the flotation plant design, and will

produce the most representative samples to complete the flitration and

sedimentation tests, and to update the process process design if required,

hopefully with cost savings.

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