The Basal Reef is the predominant gold-bearing reef at Bambanani. The Steyn facies of the Basal Reef cover approximately 90% of Bambanani’s mining lease area and overlie, with a very slight angular sub-conformity, the UF1 quartzite of the Welkom Formation. It is in turn overlain by the khaki shale unit of the Harmony Formation in the north.

To the south, it is overlain by the younger waxy brown leader quartzite, which erodes the khaki shale. The presence and thickness of the khaki shale may influence decisions to undercut the Basal Reef. While the reef’s thickness may vary from a few centimetres to more than 10m, it is typically between 1m and 3m thick.

The Stuurmanspan Fault in the west and the De Bron-Vermeulenskraal Fault system in the east are bound to the Basal Reef at Bambanani. Both are northward-striking dextral extensional faults with significant westerly downthrows. The reef dips easterly, varying from 25 degrees in the west to 45 degrees in the east but, in places, local deformation against a fault leads to vertical reefs. Smaller faults break up the reef but are generally sub-parallel to the main structures.

As Bambanani is in the final stages of its life of mine, mining is limited to extraction of the shaft pillar. Mining of the shaft pillar is focused on mini longwalls on the north side and, at the centre of the pillar, it is separated by safety pillars that have been left along designated geological structures.

Most of the panels are mined on full width, leaving a reef beam of approximately 80cm in the hanging wall in order to build a beam to support the shale. The challenge remains to control the stoping width and the stability of the beam in a highly fractured and faulted environment with sill intrusions, and a weak waxy brown quartzite hangingwall above the shale that is complicated by ball and pillow formations.

Backfill has been successfully introduced in all panels. The quality of installation has improved drastically as the crew has acquired knowledge and understanding of its underground application. The focus is currently on improving the volume of backfill placed versus the square metres mined, as well as quality control, which includes regular testing of the backfill product.

The seismic system is operational and the data gathered is used and applied in the design of the mining sequence. Seismic responses are also monitored and correlated with monthly production data to establish the relationship between volumes mined and the seismic response.

Work on the shaft pillar continues at levels 66, 69, 71 and 73. Ore is transported via a decline system – from 58 to 75 levels – on the northern side of the shaft pillar, to Bambanani West, from where it is hoisted to surface. The shafts are linked by cross-tramming at 60 level.

Bambanani does not have its own mineral processing plant and the mine’s ore is transported by rail for 7km to the Harmony 1 plant for processing. This is a centrally located plant that is used by other Harmony mines in the Free State.

Harmony One Gold Plant is located near Bambanani shaft, on the southern edge of the City of Welkom in the Free State Province of South Africa. The plant was built in 1986, and the milling, leaching and carbon-in-pulp technology reflects the technology which was current at the time. The operations at the Harmony One Plant consist of the following.

An ore receiving bay, where ore is brought to the plant by rail from 5 shafts. The railway hoppers discharge individually into one of the two concrete, rail-lined inverted cones. The ore is rapidly withdrawn from the apex of the cone via a shuttle belt feed conveyor feeding onto a main silo feed conveyor. Each silo has a live storage of approximately 3 000 tons. Ore is discharged onto the mill feed conveyor via a pneumatically operated Langlaagte chute.

The ore is taken up the slow-moving conveyers from underneath the silos and discharged directly into the feed hopper for the six fully autogenous mills. For control purposes the mill feed belts and the mill discharge pumps both have variable speed drives. Each mill is a closed circuit with a 1200mm primary cyclone with mass flow measurement on the feed. The primary cyclone overflow is screened on a 600µm linear screen for the removal of coarse woodchips and tramp steel.

Calcium Oxide (lime) is added to the 6 thickeners (operating in 3 parallel modules) as slaked lime with levels of CaO being controlled at between 0.014 and 0.016% CaO. The lime maintains a protective level of alkalinity in the leach section to prevent the generation of poisonous HCN (cyanide) gas in the process. Thickening is carried out in six 60m diameter, cable torque thickeners. Flocculent is used to assist the settling rate and is added at the rate of approximately 1 to 3 g/t. Each thickener is equipped with a fixed and variable speed underflow pump. The variable speed pump is used for transferring the thickened slime (± 53% solids) to the leach circuit.

The leach circuit consists of three modules, each with nine 800m3 mechanically agitated draught tube circular tanks. The feed to leach is screened for woodchips, using three Mintek circulating tanks fitted with 800µm aperture mesh screens. Air is injected under the draught tube impeller for oxygen distribution to the pulp. Liquid cyanide is automatically added to the leach reception tank, with the initial level of the cyanide being controlled by a TAC1000 online automatic sampler. The concept is that should a reduction in output be required then one module can be shut down whilst the other two are running at full capacity.

The slurry then passes to one of the three adsorption modules, each of which has seven 450m3 tanks where it passes through the tanks in counter-flow to the carbon movement, which adsorbs the gold that is in solution. The gold depleted slurry from the seventh tank flows over a vibrating carbon safety screens and is pumped to the residue tank. The slurry from the residue tank is sampled by an automatic online WAD cyanide analyser. The majority of residue is pump to the slimes dams, but a portion is also pumped to a backfill plant for the generation of backfill material. The backfill plant is located at the decommissioned Bambanani West shaft site. The carbon that has been pumped upstream is recovered at the first adsorption tank by pumping the slurry over vibrating carbon screen.

The elution section has three separate elution modules that process the carbon from the three CIP modules. One 1.25m diameter elution column is used in each of the elution trains and the cycle of water/acid washing, first and second strips are all done in the one column by passing the various solutions through the column. The AARL process is used at 130°C and 450 kpa. After the carbon has been stripped it is taken through three 9 m length regeneration.

The gold rich eluate from the columns is pumped across to the eluate tanks located inside of the smelt house where the gold in solution is recovered by zinc precipitation. The zinc precipitate is filtered out by rotary vacuum drum filters and then calcined in one of the 9 large ovens at 800°C to oxidise as much of the base metals as possible. Finally, the concentrate is mixed with silica-borax-Mn flux and smelted into gold bullion in one of three electric arc furnaces.