Recovering gold

Materials World magazine
,
1 Mar 2017

Colin Low explains how a new technique is providing a more efficient and environmentally friendly way of reclaiming gold.

Gold captures our attention, thanks to its preciousness, beauty and worth. It is extremely hard to produce with only 25kt mined in 2010 and, in 2014, it is believed that only 183,600 tonnes in total were extracted worldwide.

At the Atomic Weapons Establishment (AWE), UK, gold is being widely used to support and understand the safety and performance of the UK’s nuclear deterrent. This is due to gold’s unique properties, for example its ductility and resistance to chemical reaction. 

Commercially, gold is regularly electroplated from a cyanide-based solution to form corrosion-resistant and highly conductive coatings for use in a range of applications from electronics through to heat shielding and aerospace applications. However, gold has other properties of significant interest, such as being able to exploit its infrared and X-ray resistance in the manufacture of tiny sub-millimetre targets, typically the size of a grain of sugar, for plasma physics research at the Orion laser facility, UK, where research into high energy density physics phenomena takes place. The targets support experimental trials to underwrite the safety and performance of the UK’s weapon systems. Since the Comprehensive Nuclear-Test-Ban Treaty was signed in 1996, simulation testing has been undertaken above-ground in controlled test chambers.

Components are electroformed, a process whereby a master mandrel is electroplated and subsequently withdrawn during the plating process to form a finished component. Electroforming components with gold requires strict control for the accounting of this material.  

Over time, the plating solution becomes expired or impure and needs either replacing or reclaiming with the remaining gold. Traditionally, spent gold cyanide plating solutions were sent away for gold reclamation via a third party. A sample of solution would be taken and analysed to determine the amount of gold content. A contract would then be placed with the refiner, and the solution shipped for reclamation. This presents challenges due to the cyanide content of the plating solution and higher cost associated with the legislative obligations and safeguards that had to be put in place before the shipment happens.

Recovering gold

In a drive to improve efficiency, process and cost, AWE has investigated the potential to introduce other methods with high recovery rates. This resulted in the Gold Bug method, which uses an extremely high surface area cathode, of a known weight, enclosed within a gold bug cell to provide the anode and form a circuit.

The complete cell is then placed within the plating solution, and the outlet of the process filtration is directed into the cell to produce a new plating solution to be seen by the cathode. By using a 25-30amp DC power supply, through conventional electrolysis, the gold is deposited directly onto the high surface area cathode. Recovery rates can reduce strengths to less than five parts per million – the traditional operating gold content is in the region of 25g/l or 2,500 parts per million.

After a period of time, the cathode is removed from the cell, washed, rinsed, dried and then re-weighed. The increase in weight is the result of physical gold recovered. The plated cathode can then be sent directly for reclaim as a non-hazardous material. Additionally, while the process is in operation, through disassociation, it destroys the free cyanide within the plating solution and the remaining cyanide solution can then be neutralised within the plating tank. This is then disposed of as a non-cyanide containing solution, at a significantly reduced cost. 

Each cathode is capable of a recovering rate of up to 3kg of gold, at which point the cathode is simply replaced and the process is repeated. As well as the cost savings, the technique is a more efficient and environmentally friendly way of reclaiming gold. It has also been identified as a suitable means for the recovery of silver and other precious metals used at AWE from spent plating solutions. Additionally, by placing the gold bug within the rinse water effluent system, any gold or metal ions passed to these rinses through and any drag out can also be recovered in the same way, further reducing waste and cost. 

AWE provides and maintains warheads for Trident, the UK’s nuclear deterrent. The company has been at the forefront of the UK nuclear deterrence programme for more than 60 years, delivering to UK Government innovative solutions to national nuclear security. For more information, visit www.awe.co.uk

Colin Low began his career in 1979 as an apprentice with the Ministry of Defence and completed his apprenticeship in 1983 as an instrument maker/toolmaker. He has spent most of his career in engineering, specialising in materials finishing. He has also spent time working as a classic aircraft restorer, and successfully raced F2 sidecars in the UK and abroad.