Witwatersrand Consolidated Gold Resources' 2012 prefeasibility study for the DBM project in South Africa rejects uranium recovery as uneconomic at the uranium prices of the time.
The Pre-Feasibility Study of the De Bron-Merriespruit Project (DBM Project), South Africa, was prepared for Witwatersrand Consolidated Gold Resources Limited (Wits Gold), with contributions from Turgis Consulting and Snowden, and filed in 2012. For the plant design, the study assumes DBM ore will be similar in mineralogy and processing behaviour to the ores mined nearby. It looked at the closest plants. Harmony's old Merriespruit plant was too dated to consider, Joel uses run-of-mine (ROM) milling with leaching and carbon-in-pulp (CIP), and Beatrix uses ROM milling with carbon-in-leach (CIL) because its smectite clays are preg-robbing. Neither uses gravity concentration.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Report | Pre-Feasibility Study of the DBM Project | Not applicable | For Wits Gold, filed 2012 |
| Process route | ROM milling and CIL | Not applicable | Gravity and CIP to be added or considered if testwork supports them |
| ROM top size | 400 | mm | No crushers in the circuit |
| Elution | Zadra, about 16 h | Not applicable | Eluate of sodium cyanide and caustic at 120°C |
| Bullion | About 90% gold | Not applicable | After calcining and smelting |
| Carbon regeneration | 750 | °C | Kiln, then dilute hydrochloric acid wash |
| Uranium in High Carbon Leader reef | 280 | ppm U3O8 | Other reefs up to 140 ppm |
| Uranium plant case | 15,000 t/month | Not applicable | High Carbon Leader reef |
| Breakeven uranium price | 85 | US$/lb | Compared with US$55/lb at the time |
Overview
The study selected ROM milling followed by CIL, with gold recovered by elution, carbon reactivation, electrowinning and smelting. It notes that many gold plants without preg-robbing minerals still choose CIL over CIP, because CIL has a lower capital cost and a simpler flowsheet with no separate adsorption tanks. CIL gives lower carbon loadings and so needs a larger elution plant, but it is cheaper overall and ensures no gold is lost to preg-robbing minerals. Gravity concentration would be added only if testwork showed a significant contribution, and CIP considered only if the ore proves free of preg-robbers.
The study also examined uranium recovery. The High Carbon Leader reef has the highest uranium grade, at 280 ppm U3O8. At an assumed plant feed of 15,000 t a month of this reef, uranium recovery would need about US$85 per pound to be viable, against a price of about US$55 at the time, so it is unlikely to be viable in the near future.
Key Process Stages
ROM milling. Vibrating feeders draw ore from the shaft headgear bin onto a conveyor to the mill silos. There are no crushers, because underground ore with a top size of 400 mm is ideal ROM mill feed. The large rocks act as grinding media, and the report notes that without them steel ball consumption would rise.
Classification and thickening. Mill discharge is cycloned, with the underflow returned to the mill. Cyclone overflow passes a linear screen that removes woodchips and plastic, which would otherwise blind the CIL screens, and then goes to a thickener dosed with lime and flocculant. Gravity concentration on part of the cyclone underflow is allowed for if testwork justifies it.
CIL. Thickener underflow is leached with sodium cyanide. Coconut-shell activated carbon, added to the last tank, is pumped counter-current by recessed impeller pumps to limit abrasion. Loaded carbon is washed and elutriated to remove remaining woodchips and plastic.
Zadra elution and electrowinning. The study chose Zadra over AARL elution as simpler to operate. Eluate of sodium cyanide and caustic at 120°C circulates between the column and the electrowinning cells for about 16 hours, plating gold onto steel wool cathodes.
Smelting. The cathodes are washed and calcined, then smelted with fluxes into bullion of about 90% gold. Smelting slag is crushed, milled and tabled to recover gold prills, and the remaining slag returns to the ROM mill.
Carbon regeneration. Eluted carbon is reactivated in a kiln at 750°C, quenched, screened, acid washed with dilute hydrochloric acid and returned to the last CIL tank. New carbon is pre-attritioned to knock off rough edges that would otherwise wear off, carrying adsorbed gold to the tailings.
Tailings. After the carbon safety screen and cyanide detoxification, tailings go to the tailings dam. Water is recovered through a penstock to the return water dam.
Additional Interesting Data and Summary
The report cites trade-off studies, including ROM milling against crushing and ball milling. It notes that Witwatersrand plants moved from multi-stage crushing and ball milling to ROM milling from the mid-1970s, for lower capital and operating costs.
In summary, the 2012 DBM PFS takes a proven Witwatersrand and Free State route: crusherless ROM milling, CIL to guard against preg-robbing and Zadra elution, and it leaves gravity concentration and uranium recovery out unless future testwork or prices justify them.
Key Processes
- Run-of-mine milling without crushing
- Cyclone classification and trash screening
- Pre-leach thickening with lime and flocculant
- Carbon-in-leach with coconut-shell carbon
- Zadra elution and electrowinning onto steel wool
- Calcining and smelting to bullion with slag prill recovery
- Kiln regeneration and acid washing of carbon
- Cyanide detoxification and tailings dam with water return
Source: Pre-Feasibility Study of the De Bron-Merriespruit Project (DBM Project), South Africa, prepared for Witwatersrand Consolidated Gold Resources Limited, 2012.
Earlier report summaries
Earlier versions of this profile, based on older technical reports, are retained below for source traceability. Dates and figures reflect each report’s own effective date and are superseded by the current information above.
De Bron Merriespruit — 2011 Technical Report
This preliminary assessment describes a proposed gold processing route incorporating milling, pre-leach thickening, and carbon-in-leach recovery for the De Bron-Merriespruit Project in South Africa.
Report context
The Witwatersrand Consolidated Gold Resources Ltd Preliminary Assessment of the De Bron-Merriespruit Project (DBM Project), South Africa, bears an effective date of 5 August 2011 and report number 30836-01. The document was compiled by Turgis Consulting with contributions from Snowden Mining Industry Consultants. The processing section of this technical report (Section 17, Recovery Methods) presents a proposed mineral processing design; no historical operating data from a processing plant are included because the project is at a preliminary assessment stage. Metallurgical testwork results are referenced in Section 1.13 but the report does not include detailed testwork data in the processing sections.
Processing route
Mineral Processing Overview
The report presents a processing design for gold recovery only. Uranium recovery is mentioned but no process description is provided for uranium; Section 17.1 states "Uranium Recovery" as a heading but contains no further text in the extracted content. The gold processing route comprises milling, pre-leach thickening, and a carbon-in-leach (CIL) plant. The flowsheet is shown in a Process Flow Diagram (Section 17.3) but the diagram itself is not reproduced in the extracted text.
Milling Plant
The milling plant is described as a component of the processing circuit. No specific mill type, dimensions, power requirements, or throughput capacity are stated in the extracted sections. The milling plant feeds the pre-leach thickener.
Pre-leach Thickener
A pre-leach thickener is included in the circuit between milling and the CIL plant. No thickener diameter, underflow density, or other design specifications are provided in the extracted text.
CIL Plant
The carbon-in-leach plant is the primary gold recovery step. No number of tanks, tank volumes, retention times, or carbon concentrations are specified in the extracted sections. The CIL plant receives feed from the pre-leach thickener.
Elution, Electrowinning, Carbon Reactivation and Smelt House
The report lists elution, electrowinning, carbon reactivation, and a smelt house as downstream processes following the CIL plant. No equipment specifications, elution temperatures, electrowinning cell details, or smelting furnace descriptions are provided.
Reagents and Consumables
Section 17.8 is titled "Reagents and Consumables" but contains no specific reagent types, consumption rates, or storage requirements in the extracted text.
Tailings Disposal
Section 17.9 is titled "Tailings Disposal" but contains no description of the tailings disposal method, dam design, or deposition plan in the extracted text.
Key reported parameters
No specific design parameters, historical operating data, or metallurgical testwork results are reported in the extracted processing sections. The table below records what is stated and what is absent.
| Parameter | Unit | Proposed Design | Historical Operating Data | Testwork Basis |
|---|---|---|---|---|
| Gold recovery method | , | Carbon-in-leach | Not reported | Referenced in Section 1.13 but not detailed in extracted sections |
| Uranium recovery method | , | Not described | Not reported | Not reported |
| Milling configuration | , | Not specified | Not applicable | Not reported |
| Pre-leach thickener size | , | Not specified | Not applicable | Not reported |
| CIL tank configuration | , | Not specified | Not applicable | Not reported |
| Elution method | , | Not specified | Not applicable | Not reported |
| Electrowinning configuration | , | Not specified | Not applicable | Not reported |
| Carbon reactivation method | , | Not specified | Not applicable | Not reported |
| Reagent types | , | Not specified | Not applicable | Not reported |
| Tailings disposal method | , | Not specified | Not applicable | Not reported |
Project website: https://miningdataonline.com/property/4605/De-Bron-Merriespruit-Project.aspx
Technical qualifications
The report is a preliminary assessment. No processing plant has been constructed or operated for this project. The processing sections present proposed design only, with no testwork data, mass balances, equipment specifications, or performance estimates included in the extracted text. The report states that metallurgical testwork is referenced in Section 1.13 but that section is not included in the extracted processing content. All processing parameters are unverified at the preliminary assessment stage.
Source: Witwatersrand Consolidated Gold Resources Ltd, Preliminary Assessment of the De Bron-Merriespruit Project (DBM Project), South Africa, effective 5 August 2011, Report Number 30836-01, Section 17 Recovery Methods (Mineral Processing).
