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:: Gravitaur - Gold Recovery from Gravity Concentrates
A new hydrometallurgical technology for recovering gold from gravity concentrates is being evaluated in a joint R&D venture between Mintek and AngloGold. The commercial-scale demonstration plant, designed to treat 250 kg of concentrate per batch, is situated at AngloGold's Vaal Reefs no. 9 gold plant in Orkney.
The Gravitaur ('Gravity Au Recovery') process shares a number of features with Mintek's successful Minataur(tm) hydrometallurgical gold-refining technology. As in the Minataur process, the gold is solubilised by chloride leaching and recovered by reduction-precipitation. However, with gravity concentrates as the feed, an intermediate solvent-extraction step to upgrade the gold tenor of the solution is unnecessary, and the gold is precipitated directly from the leachate as a 'three nines' (99.9 per cent) pure fine powder.
Feed materials containing as little as 1 per cent gold can be successfully treated (processing of concentrates containing less than 1 per cent is technically feasible, but would not be economic). Recoveries are targeted at higher than 99 per cent, resulting in a discardable tail which is not achievable with other hydrometallurgical processes.
Another feature of the Gravitaur process is the ability to utilise cheap chlorine generated on-site at ambient pressure, using a simplified version of the chlor-alkali membrane cell. This method has benefits in terms of safety and cost.
The process can also easily be fully automated, and the batch operation makes for good gold accounting.
With the introduction of centrifugal-type concentrators, gravity concentration is being increasingly used in gold recovery operations, either as the sole process or as the primary step to recover 'free' gold, with the tailings being treated by cyanidation. A variety of hydrometallurgical and pyrometallurgical options can be used to process the concentrator products, but these typically recover less than 80 per cent of the contained gold. A major drawback associated with these routes is that they generate a solid residue or middlings fraction that requires recycling to recover the remaining gold.
The Gravitaur process achieves maximum gold recovery in the form of a high-purity final product, eliminating the need for recycle of gold-bearing solid residues, and avoiding environmental problems such as gas emissions or chemical contamination.
Gravitaur process description
The concentrate is leached in hydrochloric acid and chlorine to dissolve gold, some silver, and base metals such as iron, copper, and manganese. The slurry is filtered to produce a filtrate containing a few grams per litre gold.
The leach residue, which consists mainly of silica, together with insoluble silver chloride and small amounts of occluded gold, is repulped and returned to the milling circuit.
After air sparging to remove most of the residual chlorine, the leach filtrate is treated with ferrous sulphate solution, which reduces the aqueous gold to metallic powder. Very little silver, and none of the base metals, are precipitated. The gold powder has a purity of approximately 99.9 per cent, and can be further processed directly.
All gases from tanks and filter are scrubbed before being vented. The scrubber effluent, brine and caustic bleed streams, and wash waters are de-chlorinated. These effluents, together with the gold-barren leach filtrate, which contains hydrochloric and sulphuric acid, are adjusted to pH 11 before being returned to the milling circuit. The caustic soda by-product from the chlorine cells is used for scrubbing and neutralisation.
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