A gold processing plant is a linked set of circuits that turns run-of-mine ore into doré bullion. Ore is crushed and milled until the gold is liberated, classified by particle size, then run through a gravity circuit to catch coarse free gold before cyanide leaching dissolves what remains. Activated carbon adsorbs the dissolved gold. Loaded carbon is then stripped in an elution column and the metal is plated onto cathodes by electrowinning ahead of smelting.
At Gold Fields’ South Deep mine on the Far West Rand, that journey takes up to five days from the moment ore enters the plant. South Deep’s gold plant has the capacity to treat around 330,000 tonnes of ore a month. Every design decision behind those five days traces back to a single input: what the ore is actually made of.
Ore Mineralogy Decides the Flowsheet Before You Order Equipment
Mineralogy sets the grind size, the recovery route and the reagent load. Free-milling ore gives up its gold once grinding liberates it from the host rock, so a conventional mill and cyanide leach will do the work. Refractory ore locks gold inside sulphides or carbonaceous material, which means an oxidation step has to sit ahead of the leach before cyanide can reach anything.
The split between gravity recoverable gold and leachable gold changes the circuit as well. About 15% of South Deep’s gold is recovered through its gravity concentrator circuit, which shortens processing time for that portion of the metal. An ore body with very little coarse free gold makes that circuit hard to justify.
Tailings retreatment adds a further SA-specific variable. South Deep runs a dedicated tailings re-treatment section consisting of a thickener followed by a CIL circuit, and two of its dormant tailings facilities are being re-mined using a water cannon. Feed from a tailings dam arrives fine and already oxidised, often still in slurry form. The comminution requirement drops away almost entirely while thickening and reagent handling become the constraint.
MechProTech (MPT) sizes equipment against the metallurgical requirement of the specific commodity rather than fitting a flowsheet around a fixed catalogue. That order of operations matters most on marginal ore bodies where a wrong mill selection is expensive to correct once concrete is poured in Benoni or on site.
The Circuits Inside a Gold Processing Plant
What is a gold processing plant?
A gold processing plant is the surface facility that separates gold from the waste rock it was mined with, using a sequence of mechanical and chemical stages. It sits between the mine and the refinery. The output is doré, an unrefined alloy bar that goes on to a refiner for final purification.
Most South African gold plants are built from six functional circuits. Each one hands a defined product to the next, and a bottleneck anywhere upstream caps the whole plant.
| Circuit | What it does | Typical equipment |
| Comminution | Reduces run-of-mine rock so gold particles are liberated | Jaw and cone crushers, SAG or AG mills, ball mills, autogenous scrubbers |
| Classification | Sorts particles by size and returns coarse material for regrinding | Hydrocyclones, barrel screens and trommels, incline and horizontal screens |
| Gravity recovery | Captures coarse free gold before it reaches the leach | Centrifugal concentrators, shaking tables, intensive leach reactors |
| Leach and adsorption | Dissolves gold in cyanide and adsorbs it onto carbon | Agitated leach tanks, interstage screens, air injection systems |
| Carbon handling | Strips gold from loaded carbon and plates it out | Acid wash columns, elution columns, electrowinning cells, regeneration kilns |
| Dewatering and reagents | Recovers process water and doses the chemicals the leach depends on | Thickeners, filters, flocculant and reagent make-up plants |
MPT’s manufactured range covers four of those six circuits: comminution, classification, hydrometallurgy and materials handling. Mapping which supplier covers which circuit early keeps the interfaces between packages from becoming someone else’s problem at commissioning.
Comminution and Classification: Where Recovery Is Won or Lost
Comminution is the first stage and the most power-hungry one. Grind too coarse and gold stays locked in the host rock, so the leach never sees it. Grind too fine and you burn power and steel for recovery you were going to get anyway.
South Deep’s milling circuit shows one workable answer for hard Witwatersrand material: a single-stage semi-autogenous grinding mill with a pebble crusher for primary milling, followed by secondary ball milling. Classification is handled by cyclones, which sort fine particles in suspension for downstream processing and send coarse material back to the ball mill.
MPT manufactures its own ball mills and autogenous scrubbers at its Benoni facility, handling up to 1,000 tonnes per hour. The TITAN mill runs on a girth gear and dual pinion drivetrain with a high pressure lubrication system, reaching up to 80% of critical speed at a 40% ball charge. The drive is bi-directional, which the company reports prolongs liner life by up to 70% without losing grinding performance.Units are delivered as modular, platform-mounted assemblies built and tested at Benoni before dispatch.
A single modular skid base carries up to 1.3 MW, with the inlet chute and discharge sump held in the frame so the unit can be transported and relocated as one piece.
Grinding media feed is its own recovery variable. The TITAN Ball Charger is an automated media feeder with a PLC programmed to monitor mill power draw, so charge is topped up against actual load rather than a fixed schedule. On the classification side, MPT produces large barrel screens and trommels for mill discharge alongside horizontal and incline screens. MultotecMechProTech
Gravity Recovery and the Leach Circuit
Gravity recovery earns its place by pulling coarse free gold out before it enters the leach. Centrifugal concentrators separate by density without any reagent addition. The concentrate they produce goes to an intensive leach reactor or a table for cleanup, and gold recovered on this route spends far less time in the plant than gold that travels the full cyanide circuit.
Everything the gravity circuit misses goes to the leach. Cyanide dissolves gold from the milled slurry in a series of agitated tanks, with lime holding pH in the region of 10.5 and air or oxygen injected to sustain the reaction. A 2021 review in Minerals Engineering put cyanide behind roughly 80% of global gold extraction, citing Aylmore.
What is the difference between CIP and CIL gold processing?
Carbon-in-pulp leaches first, then moves the pregnant slurry into separate tanks where activated carbon adsorbs the dissolved gold. Carbon-in-leach puts the carbon into the leach tanks themselves, so dissolution and adsorption happen in the same vessel.
CIL suits ores with preg-robbing carbonaceous material, because carbon competes for gold the moment it dissolves. CIP gives more control over adsorption conditions and is often the cheaper build where the ore behaves predictably. Interstage screens hold carbon inside each tank while slurry passes forward, and screen condition is a live recovery issue on both circuits.
Reagent dosing decides operating cost here more than equipment does. Cyanide and lime both have to arrive at the right concentration and the right moment, with flocculant dosing the same story downstream. Reagent preparation gets treated as a circuit rather than an afterthought for exactly that reason.
Stripping the Carbon: Elution and Electrowinning
Loaded carbon leaves the adsorption circuit carrying the gold, and the back end of the plant exists to get it off again. At South Deep, carbon withdrawn from the circuit is first processed in an acid column that removes impurities, then transferred to an elution column where it is soaked in heated caustic cyanide solution. Hot water is pumped through the column to strip the gold into solution. Ftmmachinery
The pregnant eluate goes to electrowinning, where gold plates onto steel wool cathodes under current. Cathode sludge is dried and smelted into doré bars. Stripped carbon is regenerated in a kiln and returned to the adsorption circuit, since carbon replacement is a standing consumable cost.
How long does it take to process gold ore into doré?
South Deep reports up to five days between ore entering the processing plant and the start of smelting. That figure is specific to its circuit configuration. Plants with a larger gravity contribution move part of their gold through much faster, while refractory circuits carrying an oxidation stage run considerably longer.
MPT supplies elution plants alongside its tanks with mixers, and its hydrometallurgy range extends from leaching tanks through to electrowinning cells. For an operator building out the back end, that keeps the tank farm and the elution package inside a single locally manufactured scope rather than split across import lead times.
Thickeners and Reagent Plants: The Circuits Operators Underspecify
The thickener is the quietest circuit in the plant and one of the most consequential. Classified mill product is very dilute, so a thickener de-waters it into a dense slurry before the leach. It recovers process water for reuse, raises slurry density to the concentration the leach wants and buffers the leach against upsets in the grinding circuit.
Water recovery carries extra weight for South African operations, where process water is a permitting question as much as an operating one. Every cubic metre returned from the thickener overflow is a cubic metre that does not have to be drawn or paid for.
MPT’s Hi-Rate Thickener uses a bolt-together tank designed for transport and on-site assembly, with a maximum width of 3.9 m and a height of 4.7 m. Full and half-radius rake arms use constant velocity rakes to keep sedimentation consistent and prevent blockages. Tank floor and side walls are 6 mm mild steel plate finished with a two-coat epoxy system for corrosion resistance. Mechprotech
Reagent preparation sits directly alongside it. The HYDRAMIX reagent plant uses a cyclic tank design where tanks rotate from make-up to hydration to dosing, which removes the need for transfer pumps. The Hydramix tank farm mixes and hydrates powdered and liquid reagents at up to 500 kg per hour, with the reagent loading system and access platform supplied as one integrated plant. ScienceDirect
What a Gold Processing Plant Costs and How Long Setup Takes
How much does a gold processing plant cost?
There is no single figure, and any supplier quoting one before seeing your ore is guessing. Capital cost tracks throughput first, then ore hardness, then the recovery route the mineralogy forces on you. A free-milling ore at modest tonnage running gravity and CIL sits at one end of the range. A refractory ore needing pressure oxidation ahead of the leach sits far above it, because the oxidation circuit alone can rival the cost of everything around it.
Build method moves the number as much as circuit selection does. Conventional stick-built plants carry long on-site construction programmes, with the cost and schedule risk that comes with them. Modular units are assembled and tested in the workshop before they ship, which compresses site work into installation and connection.
MPT builds to that second model. Equipment is manufactured and tested at Benoni before dispatch, and the company reports that material can be flowing through a unit within ten days of it leaving the workshop. Modular skid mounting also makes relocation realistic, which matters for operations working a defined tailings resource or a short reserve life.
Pilot plants are the cheapest way to reduce the risk on all of it. Running representative ore through a small circuit gives you real grind and recovery figures, along with actual reagent consumption, before you commit capital to full-scale equipment.
Sizing a Circuit for Your Ore Body
A sizing conversation moves faster when you arrive with a representative ore sample that has mineralogical work behind it. Add the target throughput and the recovery figure your project economics depend on. Those inputs let an OEM size a mill and scope a reagent plant against something real rather than an assumption.
MechProTech has designed and supplied mineral processing equipment since 1998, manufacturing in-house at its Benoni workshop. The entire range is built locally and in use on plants internationally. Get in touch to talk through your flowsheet and where locally manufactured equipment fits into it