The proposed DBM plant flowsheet is based on run-of-mine milling followed by carbon-in-leach, with gold recovery by elution, electrowinning and smelting.
Report context
This section of the July 2012 technical report for the DBM Project describes the proposed recovery methods for gold processing, based on a Pre-Feasibility Study. The report examines the selected processing route, evaluates the viability of uranium recovery, details the proposed plant flowsheet and equipment selection, and provides estimates for energy, water and consumable requirements. The processing design assumes ore mineralogy similar to that currently being mined in the area.
Processing route
Process route selection
For the purposes of process and plant design for the Pre-Feasibility Study, it was assumed that the ore to be processed will be similar in mineralogy and ore processing characteristics to the ores currently being mined in the area. The closest metallurgical plants to the DBM Project area are the old Harmony Merriespruit plant, the Joel plant and the Beatrix plant. The Harmony plant was designed many years ago and its processing route was not considered. Joel plant uses run-of-mine milling followed by cyanide leaching and carbon-in-pulp. Beatrix also uses run-of-mine milling followed by carbon-in-leach. The Beatrix ore contains smectite type clays, which are preg-robbing, so carbon-in-leach is well suited to this ore. Neither Joel nor Beatrix makes use of gravity concentration.
Based on this review, the process route selected was run-of-mine milling, followed by carbon-in-leach, with gold being recovered by elution, carbon reactivation, electrowinning and smelting. Should testwork show that gravity concentration could make a significant contribution to gold recovery, it could be included in the flowsheet. Similarly, if the ores do not contain preg-robbing minerals, then carbon-in-pulp could be considered. The report notes that a number of gold plants around the world that have not identified preg-robbing minerals in their ores still elect to use carbon-in-leach over carbon-in-pulp as it has lower capital cost and a simpler flowsheet (no carbon-in-pulp tanks). Carbon-in-leach results in lower gold loading on activated carbon than carbon-in-pulp plants, requiring a larger elution plant. Overall, a carbon-in-leach plant is lower capital cost than a carbon-in-pulp plant and ensures that no gold will be lost to preg-robbing minerals. The process route utilises technology and equipment that is well proven on metallurgical plants on the gold mines of the Witwatersrand and Free State.
Evaluation of uranium recovery viability
A study was carried out to determine whether it would be viable to recover uranium from any or all of the DBM reefs. During the DBM Concept Study, the various reefs were analysed for uranium as U₃O₈. The High Carbon Leader reef showed the highest uranium grade at 280 ppm. The highest uranium content of the other reefs was 140 ppm. Based on planned mining rates from the various reefs, a uranium plant feed rate of 15,000 tonnes per month of High Carbon Leader reef was assumed. The capital and operating costs for a plant of this capacity were determined and revenue calculated, all in current money terms.
The study showed that treating ore from the High Carbon Leader reef, the highest grade reef in terms of uranium content, would require a uranium price of $85 per pound of uranium (in current money terms) to make the process viable. This compares to the current uranium price of $55 per pound. The report concludes it is therefore unlikely that uranium recovery will be viable in the near future from any of the DBM reefs.
Proposed plant flowsheet
Run-of-mine milling and classification
ROM ore is withdrawn from the shaft headgear bin with vibrating feeders onto a conveyor that transfers the ore to the mill silos. No crushers are included in the circuit as the ore from underground will have a top size of 400 mm, which is an ideal feed size for ROM milling. In ROM milling, the large rock particles are used for grinding in the mill. An absence of these sized particles will result in increased steel ball consumption. Ore is withdrawn from the mill silos with vibrating feeders and fed to the ROM mills.
The mill discharge is pumped to cyclones where classification by particle size takes place. The cyclone underflow containing the coarse particles is returned to the mill for regrinding, while the cyclone overflow containing the fine particles passes to the thickener. A linear screen on the thickener feed removes woodchips and any tramp material such as plastic particles. These particles, if not removed, will blind the carbon screens in the carbon-in-leach circuit.
If gravity concentration is included in the flowsheet, a portion of the cyclone underflow will be fed into the gravity concentrator. The tailings from the gravity concentrator will be returned to the mill, while the gravity concentrate will pass to the smelthouse for further upgrading and smelting.
Thickening
Lime and flocculant are added to the thickener feed to aid settling of the finer particles. The lime addition is controlled to provide the optimum pH in the carbon-in-leach circuit for gold leaching. Thickener underflow is pumped to the carbon-in-leach circuit. Thickener overflow water is returned to the mill process water tank. The plant feed sample for gold accounting purposes will be taken from the feed to the carbon-in-leach tanks using an automatic cross cut sampler.
Carbon-in-leach circuit
Sodium cyanide is added to the carbon-in-leach tanks to dissolve the gold. Granular activated carbon made from coconut shells is added into the last carbon-in-leach tank to adsorb the dissolved gold. The carbon is pumped up the carbon-in-leach circuit counter current to the pulp flow using recessed impeller pumps that minimise abrasion of the carbon. Carbon from the first (head) carbon-in-leach tank is pumped to the loaded carbon screen. The screen underflow (pulp) flows back into the carbon-in-leach tank and the loaded carbon is washed on the loaded carbon screen.
Elution and carbon regeneration
The loaded carbon then passes to the loaded carbon tank, from where it is fed into the elution column. The loaded carbon tank is also used as an elutriator to wash any remaining woodchip and plastic particles out of the loaded carbon.
Two elution processes commonly used in the gold industry are described: the Zadra process and the Anglo American Research Laboratories process. In the Zadra process, eluting solution (eluate) containing sodium cyanide and sodium hydroxide (caustic soda) at 120°C is passed through the elution column to strip the gold off the carbon. The solution then passes to the electrowinning cells where the gold is electrolytically plated from the solution. From the electrowinning cells, the solution returns to the elution column to strip more gold off the carbon. This circulation of eluate through the elution column, to the electrowinning cell and back to the elution column typically takes approximately 16 hours, until the gold has been virtually completely eluted off the carbon. In the Anglo American Research Laboratories process, the eluate does not pass directly to the electrowinning cell but is stored in the eluate tank. Fresh eluate is passed through the elution column until the elution process is complete. The eluate is then passed through the electrowinning cell to electroplate the gold. The Zadra process is considered to be simpler to operate than the Anglo American Research Laboratories process, so the Zadra process has been selected for the DBM plant.
Once the elution process is complete, the eluted carbon is washed and transferred to the regeneration kiln feed tank. The carbon is fed into the regeneration kiln. At a temperature of 750°C, any volatile organic matter is distilled from the carbon. This process reactivates the carbon. The carbon exits the kiln into a quench tank. From the quench tank, the carbon is screened to remove fines and is then acid treated with dilute hydrochloric acid to dissolve any calcium and base metals which have adsorbed onto the carbon during the gold adsorption process. The acid washed regenerated carbon is then washed to remove residual traces of acid and returned to the last carbon-in-leach tank for the adsorption process to be repeated.
A quantity of fresh activated carbon needs to be added to the plant on a regular basis to make up for carbon losses caused by abrasion of the carbon in the carbon-in-leach agitators and pumps. Fresh carbon, received in bulk bags, is poured into an agitated tank to which water has been added. The carbon is agitated in the tank to remove the rough edges on the carbon particles. If this is not carried out, these rough edges will be abraded off shortly after the carbon has been added to the carbon-in-leach tanks and will leave the carbon-in-leach tanks in the tailings, but having adsorbed some gold, resulting in gold losses.
Electrowinning and smelting
In the electrowinning cells, the gold is plated onto steel wool cathodes. Once electroplating is complete, the cathodes are removed from the cells, washed and calcined in a furnace. The product from calcining is then mixed with fluxes and smelted, to produce gold bullion bars containing approximately 90 per cent gold. The slag resulting from smelting is crushed, milled and tabled on a gravity table to recover any gold prills from the slag. The gold concentrate is added to the smelt, while the slag is returned to the plant ROM mill.
Tailings disposal
The pulp passes from tank to tank down the carbon-in-leach train of tanks, counter currently to the carbon. When the pulp exits the last carbon-in-leach tank, it passes to the carbon safety screen. Here the pulp passes through the screen to the cyanide detoxification tanks, and any carbon particles that have passed through the last interstage screen due to a hole in the screen will be recovered on the safety screen. This recovered carbon will either be smelted or sent to a by-product smelter to recover contained gold.
The carbon-in-leach tailings then pass to the tailings tank prior to being pumped to the tailings dam. On the tailings dam, water is recovered through a penstock system and flows to the return water dam, from where it is pumped back to the plant for re-use.
Plant equipment selection trade-off studies
ROM milling versus crushing and ball milling
In gold plants built prior to the 1970s, the typical flowsheet consisted of two or three stages of crushing followed by ball milling. From the mid-1970s, these circuits were replaced by ROM milling plants without crushing. The ROM milling plants had lower capital requirements (fewer items of equipment) and lower operating costs (lower labour and maintenance costs). In 2009, the Modder East plant was constructed using a crushing/ball milling flowsheet. The plant capital expenditure was reported to be lower than other plants constructed at that time, but there were a number of reasons for the lower capital expenditure. Notwithstanding the Modder East situation, ROM milling remains a lower capital and operating cost route.
Use of an existing plant
Consideration was given to the possible use of an existing plant to treat the DBM ore, with the objective of reducing capital expenditure. Harmony Gold offered to sell the Joel plant to Wits Gold, after which a due diligence on the plant was undertaken by Turgis personnel.
The Joel metallurgical plant was built in the mid-1980s by Johannesburg Consolidated Investments with a design throughput capacity of 120,000 tons per month. The plant was put on care and maintenance approximately 10 years ago. The Joel operation was later taken over by Harmony and the plant was re-commissioned in 2009. As the required plant throughput was 80,000 tons per month, only two of the three mills were re-commissioned. The thickeners, leach, carbon-in-pulp and tailings disposal were also re-commissioned. To increase the capacity of the plant to 120,000 tons per month would require the purchase and installation of a third mill and an elution, carbon treatment plant and smelting equipment. Refurbishment of existing equipment would also be required. A new tailings dam would be required in 7 years’ time as the existing dam will have reached its maximum capacity.
It was estimated that the cost of refurbishing the existing plant, including the cost of a new tailings dam, would be R296 million, as compared to the estimated cost of a new plant of R700 million. In addition, ore would have to be transported from DBM to Joel at an annual cost of R25 million. The plant operating costs of the Joel plant would be higher than those of a new plant, due to higher maintenance costs of an older plant, and higher labour costs due to a higher number of operating units (3 mills instead of 2). The Joel plant uses a carbon-in-pulp process, whereas DBM will require carbon-in-leach, so the Joel plant would need to be converted to carbon-in-leach. It was therefore recommended by Turgis that purchasing the Joel plant would not be a viable option, which was accepted by Wits Gold.
One ROM mill versus two ROM mills
It is theoretically possible to mill 120,000 tons per month to the required product size in one mill utilising ROM milling. The report provides comparative mill dimensions and power draws: for 120,000 tons per month with one mill, the required mill diameter is 6.09 m (20 ft), length 10.36 m (34 ft), with motor power drawn of 4,376 kW. For 120,000 tons per month with two mills, each mill would have diameter 4.88 m (16 ft), length 9.14 m (30 ft), with motor power drawn of 2,175 kW each.
The largest ROM mills currently installed on South African gold mines are the 4.88 m diameter by 10.06 m long (16 ft by 33 ft) mills at the Harmony One gold plant in the Free State. The Beatrix ROM mills are 4.88 m diameter by 9.14 m long (16 ft by 30 ft) and were designed to mill 60,
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Monthly milling capacity | 120,000 t/month | Design comparison |
| Single-mill motor power | 4,376 kW | Design comparison |
| Two-mill motor power | 2 × 2,175 kW | Design comparison |
Technical qualifications
The report compares conceptual mill configurations; the selected final configuration is not inferred.
