Environmental protection has become the focus of

world-wide researches in the gold mining industry and favors the development of environmentally sound processes, such as the Coal-Gold Agglomeration (CGA) process, which is an alternative to existing gold processing methods, such as cyanidation and mercury amalgamation.

INTRODUCTION

This process of gold recovery was developed and

patented by the British Petroleum (BP) research team in the early eighties and was to replace the old hazardous forms of mining; and has an advantage over the other methods of gold mining, in that gold is recovered by a procedure which has little or no negative impact on the environment, low operation as well as low cost involvement.

The gold mining industry especially the small-scale

mining industry has relied mainly upon the use of highly polluting chemicals, such as mercury and cyanide to recover gold from its ores. These methods of gold mining pose considerable hazards to the environment due to the effects these chemicals have.

The process is based on the recovery of hydrophobic

gold particles from ore slurries into agglomerates formed from coal and oil. The oils often used in this process include diesel oil, kerosene and vegetable oils. The oil acts as the bridging liquid between the coal and gold particles where the coal is the carrier of the mineral and enables effective separation of the oil phase.

The coal-oil agglomerate is prepared by mixing coal

powders of the size of 0.074 to 0.149 millimeters and diesel oil of dosage 15% - 30% in mass.

COA PREPARATION AND

GOLD RECOVERY

CGA process can be applied for extracting gold from

hard (rock) ore, alluvial ore and other gold bearing ore which are difficult for cyanidation process because of high copper or arsenic content, and difficult for gravity process because of small gold particle size and certain refractory gold ores as well.

In this process of gold recovery, the slurry is burned

causing the gold to form larger particles that can be extracted using gravity methods using water (i.e. flotation)

This can be done in the lab by putting a weighted milled

ore into an adhesion tank. Water is then added to form the pulp. The pH of the resulting pulp is adjusted by the addition

of sodium carbonate or sodium silicate. Collectors are added successively and respectively and

pulp is stirred. COA is then put into the tank and mixed. Separation of COA from pulp is performed by filtration

and froth floatation. After adding flocculants , the residual pulp is filtered . The filtered cake is dried and sample is taken for analysis.

EXPERIMENTAL PROCEDURE

After the crushed gold-bearing ores have been passed

through a ball mill, many gold particles can be liberated or exposed from the gangue mineral. When the gold particle and the other minerals associated with them become hydrophobic (i.e. by means of PH modification, addition of collectors, depressants and activators), coal-oil agglomerate (COA) can easily capture the oleophilic gold particles and the other minerals associated with them. The COA then is burnt and the resultant residue is smelt to produce gold bullion or extracted by hydrometallurgical method to produce purer gold product.

INDUSTRIAL PROCEDURE

The coal-oil agglomerates are recycled to increase

their gold loading, after which they are separated and further treated to produce gold bullion or bars of gold.

A flow diagram of the whole experimental setup is found on slide 13.

When the chief gold bearing minerals are sulphides

such as pyrite , yellow arsenic etc. , (although the gold recovery is high) the consumption of coal-oil agglomerate would be too high for the process to be economical.

The works carried out in the CGA process can recover gold effectively from many kinds of gold bearing minerals and can be operated successfully in commercial scale.

Figure 1—A Simplified flow diagram of the Experimental Setup of the Coal Gold Agglomeration (CGA) process

Laboratory scale batch tests were performed on

artificial mixture gold slurry and gold recoveries of up to 85% were found under optimized conditions. By recycling the coal/oil phase, it was found that the gold loading onto the agglomerates was increased.

EXPERIMENTAL EVIDENCE

• The ground size of the ore: gold recovery increase

with ground size becoming small (high surface area).

• The pH value of the pulp: pH of the medium should be basic. The favorable pH is about 9 and this is obtained by adjusting the pH of pulp using sodium carbonate.

• The time of adhesion: The speed of gold recovering in CGA process is extremely rapid and keep constant with time.

• The stirring strength : increasing stirring increases rate of precipitation in a saturated solution.

EFFECTS OF TECHNOLOGICAL

FACTORS ON GOLD RECOVERY.

The coal gold agglomeration (CGA) process is a novel gold recovering technique, which has following advantages.

Studies on gold recovery from amalgamation tailings by CGA process shows that about 88% gold can be captured by coal-oil agglomeration and the gross gold recovery is more than 85%.

ADVANTAGES OF USING CGA

The gold particle from the fine (minus 5

micrometers) to the coarse (plus 300micrometers) can be recovered effectively.

The short residence time and low reagent consumption lead to a low capital and operation cost comparing with tradition cyanidation and mercury amalgamation processes.

ADVANTAGES OF USING CGA

The high selectivity can be achieved against gangue

sulphide, minerals by depressants and pH modification, as in froth floatation.

No cyanide or mercury is needed; therefore the process has good environmental protection.

Silver and platinum group metals can also be recovered.

ADVANTAGES OF USING CGA

Tests performed on an industrial ore yielded slightly

lower gold recoveries, and X-ray Diffraction (XRD) analysis on the coal/oil phase showed that minerals other than gold was recovered into this phase.

CHALLENGES OF CGA

A comparative study conducted whereby the CGA

process was compared to mercury amalgamation showed that, Gold recoveries obtained through amalgamation were 15% lower than by the agglomeration process, which indicates that this process is relatively better.

CONCLUSION

1. HUGHES-NARBOROUGH, C., WALL, N.C., and WILLEY, G. Coal Gold Agglomeration: An innovative approach to the recovery of gold in environmentally sensitive areas, IV International Mineral Processing, vol. 2, 20–22 October 1992. 2. HOUSE, C.I., TOWNSEND, I.G., and VEAL, C.J. Separation Process. Pat. Int. Cl (4) C22B1/16, /GB87/00480, w088/00248 (1988). HOUSE, C.I., TOWNSEND, I.G., and VEAL, C.J. Separation Process. Pat. Int. Cl (4) C22B1/16, /GB87/00480, w088/00248 (1988). 3. BONNEY, C.F. Coal Gold Agglomeration—A novel Approach to Gold Recovery; Randol Gold Forum 88, Scottsdale, Arizona, 23–24 January 1988. 4. HOUSE, C.I., TOWNSEND, I.G., and VEAL, C.J. Coal Gold Agglomeration, International Mining, September 1988. 5. BUCKLEY, S.A., HOUSE, C.I., and TOWNSEND, I.G. A Techno-Economic Assessment of the Coal Gold Agglomeration Gold Recovery Process, The Canadian Mineral Processors Conference, Ottawa, January 1989.

REFERENCE

MESSRS ;

KOPGLI ROLAND

KWAKYE SAMUEL

ANANE BOATENG JOHN

DOMFEH AUGUSTINE

ANDERSON RICHARD

WILSON EBENEZER

AGLUBI YAW MANASEH

MEMBERS