For the treatment of refractorygold ores: Increased gold recovery rates Robust technology Environmentally friendly Commercially viable and cost

effective

BIOX

Ashanti Goldfields, Ghana, the largest BIOX plant in the world.

The BIOX process, which pre-treats refractory sulphide

gold ores such as pyrite, arsenopyrite and pyrrhotite,

was developed to increase gold recovery rates during

the metallurgical extraction process. The gold in these

sulphide ores is encapsulated in sulphide minerals

which prevent the gold from being leached by cyanide.

The BIOX process destroys the sulphide minerals and

exposes the gold for subsequent cyanidation, increasing

recovery rates.

The BIOX process has many real advantages over

conventional refractory processes such as roasting,

pressure oxidation and nitric acid leaching. These

include:

Improved rates of gold recovery

Significantly lower capital costs

Low running costs

Robust technology that is suited to remote areas

Low level of skills required for operation

Environmentally friendly

Ongoing process development and improvement

Rights to the process, which has been available

commercially for more than 15 years, are currently

held by Biomin Technologies SA, a subsidiary of Gold

Fields Limited. However, the initial research and

development into the process was conducted by Gencor

Process Research (now Billiton Process Research).

The BIOX processThe process itself uses a combination of three bacteria

that occur naturally, thiobacillus ferrooxidans,

thiobacillus thiooxidans and leptospirillum ferroxidans,

to break down the sulphide mineral matrix in the ore

being treated, thus freeing the occluded gold for

subsequent cyanidation. The bacteria attach themselves

to the metal sulphide surfaces in the ore, resulting in

the accelerated oxidation of the sulphides.

The BIOX process involves the continuous feeding

of the flotation concentrate slurry to a series of stirred

reactors.

Low pH levels and a high slurry temperature enhance

the efficiency of the process and it is important that

these parameters are controlled within narrow ranges

so as to maintain the right balance of bacteria in order

to achieve the optimum rate of oxidation.

The reactors are aerated and the slurry temperature

is maintained at the optimum level of 40-45C. As the

oxidation reactions of sulphide minerals are exothermic,

it is necessary to cool the tanks so as to maintain the

slurry temperature within the optimum range. This is

done by circulating cooling water and removing the

excess heat via a cooling tower.

The pH level is controlled by adding limestone or

sulphuric acid to the slurry. Since direct sulphide

oxidation requires high levels of oxygen, large volumes

of air have to be injected and dispersed in the slurry.

This is one of the main engineering challenges in the

design of a full-scale bio-reactor.

Furthermore, sufficient carbon dioxide is required for

the bacteria to maintain cellular growth. This is obtained

from the injected air as well as carbonate minerals.

Should the latter be absent, limestone is added.

The bacteria also require nutrients to sustain growth.

Nitrogen, phosphorous and potassium are added to

the primary reactors in various forms and quantities,

depending on the composition of the concentrate being

treated.

2

A photomicrograph of the thiobacillus ferrooxidans bacteriawhich is a component of the bacterial culture used in the

BIOX process.

3Flow diagram of the BIOX process

Concentrate

Nutrients

Stock tank

Air

Coolingwater

BIOX

reactors

CCD washthickeners

Cyanidationand CIP

Limestone

Residualdeposit

Neutralisationdeposit

Wash water

The overall residence time in the bio-oxidation reactors,

which is mainly a function of the mineralogy, typically

varies between four and six days. For an ore where the

gold is locked mainly in arsenopyrite, a shorter residence

time is expected to achieve optimum gold liberation

than with an ore where most of the gold is occluded

in pyrite. This is because the oxidation rate of

arsenopyrite is faster than that of pyrite.

Some ores require only partial sulphide oxidation to

liberate the gold. The circuit can be simplified for such

ores and the residence time reduced to two days or

less.

During the bacterial oxidation process, elements like

iron, sulphur and arsenic are dissolved. After oxidation,

the BIOX product is washed in a counter-current

decantation circuit and the solution is neutralised in

a controlled two-stage process with limestone and/or

lime. The precipitates formed meet environmental

standards set in the United States and can be safely

deposited onto tailings dams. The BIOX process is

thus a non-polluting, environmentally clean means of

treating refractory ore.

To save water, the neutralised effluent can be mixed

with flotation tailings and thickened. The overflow

solution can be recycled as dilution water in the milling,

flotation and BIOX sections of the plant. This makes

the process ideally suited for arid regions.

The washed BIOX product is treated in a conventional

cyanidation plant from which the gold is finally

recovered.

Operating parameters of the BIOX processTemperature: 40-45C

pH: 1.2-1.6

Percentage solids in feed: 20%

Dissolved oxygen: >2ppm

Retention time: 4-6 days

Nutrients fertiliser type ammonium, potassium and phosphorus salts

4Plants in operationFour BIOX plants are currently in operation in South

Africa, Ghana, Brazil and Australia with the most

successful of these being Ashantis Sansu plant near

Obuasi in Ghana. The Tamboraque plant in Peru is

currently being recommissioned.

Ashanti, GhanaAshanti Goldfields Company investigated several

refractory treatment options including roasting, pressure

oxidation and nitric acid leaching. The BIOX

technology was selected for, among other reasons, its

lower capital and operating costs, reduced technical

risk, relatively benign environmental impact and its

ease of operation.

Designed with an initial capacity of treating 720 tonnes

of concentrate per day, the Sansu plant has since been

expanded and currently has four modules processing

960 tonnes per day in all. It is by far the largest bio-

oxidation plant in the world and its modular design

makes it possible to apply the technology to large

refractory deposits. The simplicity of the process also

makes it ideal for remote areas.

Furthermore, the metallurgical performance of the

Sansu BIOX plant has been highly satisfactory in

coping with the local ore of which there are two types,

which differ widely regarding their mineralogical and

bio-oxidation characteristics. This demanded a plant

design with sufficient flexibility to treat both concentrates,

either individually or as a blend.

The capital cost of the plant totalled US$25 million

(1994 terms) and the operating cost is currently

US$17/tonne milled.

Fairview, South AfricaThe BIOX plant at the Fairview mine in Barberton,

which was the initial pilot plant, has been fully operational

for 15 years. It was originally designed to treat 10

tonnes a day but with the success of the project this

has been increased over time to 55 tonnes of concentrate

per day.

Much of the innovative research work on the bio-

oxidation of refractory gold ores conducted in the late

1970s and early 1980s was driven by the need to

replace the outdated Edwards roasters at Fairview,

which at the time were seriously contributing to

atmospheric and water pollution in the environmentally

sensitive Barberton area of South Africa.

This plant has played a vital role in the ongoing

development of the BIOX process as the scale of the

The Tamboraque plant in Peru is currentlybeing recommissioned.

The Fairview BIOX Plant in Mpumalanga province, South Africa,which currently treats 55 tonnes of concentrate a day.

5The BIOX plant at Wiluna Gold Mine, Western Australia, whose reserves are mostly refractory sulphides.

Sao Bento, BrazilA BIOX plant is also in operation in Brazil at the

Sao Bento Mine where a pressure oxidation circuit

is used to treat the refractory flotation concentrate.

This plant has confirmed the viability of combining

bio-oxidation with pressure oxidation as a cost-effective

method of increasing capacity at an existing

oxidation plant.

Wiluna, AustraliaThe fourth plant, in operation at Wiluna Gold Mine,

Western Australia, was commissioned in 1993. Although

originally designed to treat 115 tonnes per day of

concentrate this has subsequently been increased to

operation has lent itself to the testing of new equipment,

design modifications and process optimisation.

With the purchase of Fairview by Avgold Ltds Eastern

Transvaal Consolidated, Avgold concluded a licensing

agreement with Gold Fields entitling it to use the BIOX

technology to treat its concentrate. Gold Fields has

retained access to the plant and site to enable it to

continue its development work and train operators for

new BIOX plants.

The plant has been both a technical and commercial

success, confirming the simplicity of the process and

the robustness of the technology.

For further information on the BIOX process, contact:

Pieter van Aswegen

Gold Fields Limited/Biomin Technologies SA

24 St Andrews Road

Parktown

2193

South Africa

6

158 tonnes per day. Since the cost of power initially

amounted to 50% of the operating costs, a natural

gas-fired power station was built to replace the diesel

generation and limit costs.

Future developmentsTo date more than 250 concentrate and whole ore

samples have been tested for compatibility with the

BIOX process and more than 15 integrated pilot

plants to monitor and refine the design of commercial

plants have been completed. One such pilot plant,

together with a detailed commercial process design,

was completed for Golden Star Resources Bogoso

Mine in Ghana.

Currently, several projects are at the feasibility stage

or awaiting financial backing. In addition, technology

licence agreements have been signed with gold mining

companies in Uzbekistan, Greece and Australia. Of

these, the largest project is that in Uzbekistan where

the agreement with the Navoi Mining and Metallurgical

Combinat is for a BIOX plant with an ultimate capacity

of 2,055 tonnes of concentrate a day.

Project development framework

Requirements Duration Results

BIOX amenability 10 kg concentrate 2.5 months Detailed report with:

testing 10 litre process water Rate of oxidation

Gold recovery vs oxidation

Pre-feasibility study Treatment rate 1 month Conceptual flow diagram

(optional) Concentrate grade Preliminary equipment list

Unit cost data Order-of-magnitude cost estimates

Pilot plant run Testwork agreement 4 months Detailed report with:

1,000 kg sample Design parameters

Reagent consumptions

Operating strategy

Process design Licence agreement 1 month Process design specifications

package Key design criteria Mass balances

Process flow diagrams

Basic engineering Approval of contractor 9 - 12 months Bacterial inoculum

design, construction Operating manual

and commissioning Operator training

Plant commissioning

On-going technical support

Tel: +27 11 644 2574

Fax: +27 11 484 0631

E-mail:

Or visit the website:

www.goldfields.co.za or

www.gold-fields.com