For the last few weeks I have been processing an electrowin sludge from a gold/copper mine carbon strip plant.
The sludge is formed from the cathodic reduction of metals solubilised from the stainless steel anodes in the electrowin cell.
Major metal components of the sludge are manganese, nickel and chrome.
The sludge contains around 1% gold with some silver, no PGMs.
The sludge also contains high levels of chloride from the strip circuit water.
As an electrowin product, the gold is encapsulated in the manganese and is not in the main accessible for cyanide leaching.
The dry product is a fluffy brown solid which becomes a brown slime, virtually unfilterable, when wet.
As there are a few hundred kilograms of this material the operator was quite interested in recovering the gold values.
If the sludge is placed in acid solution and the mangaqnese dissolved around 70% of the gold also instantly dissolves as a gold chloride.
If placed in a pressure filter I could only filter 5 litres of liquor in 24 hours from this pulp.
The first requirement was to improve the filtration of the pulp.
I did this by furnacing the dry sludge for 8 hours at 500C.
The operating temperature range was 450 to 550C, furnacing outside of this range caused further problems.
The furnaced material was then digested in HCl and enough HNO3 was added to drive the gold dissolution to maximum values.
An attempt was made to put the gold values from this pulp onto carbon however the manganese swamped the carbon and allowed only low gold adsorption.
I then took the pulp and added NaOH to a pH around 10.
All of the metal values including the gold precipitated out.
I then added cyanide to the pulp at 3 times stochiometric for the gold values and solubilised just the gold and silver.
As the gold had precipitated rather than been electrowon it was accessible to a cyanide leach.
The pulp was then pressure filtered and the liquor was contacted with carbon and the gold loaded thereon.
The carbon was later ashed with the ash digested in aqua regia and the gold recovered with metabisulfite.
Note that the gold could not be recovered from the filtered liquor by electrowinning as there was a high chloride level in the liquor which would lead to high chlorine evolution levels.
The gold levels at all stages were monitored by AAS and residues were assayed by aqua regia digest.
It appeared that the initial sludge was an artefact of attack on the stainless steel anodes by the copper rich strip solution.
The treatment process I developed is not the only one possible but is what I felt was the most cost effective.
If the above can help anyone with a similar problem feel free to use it.
Deano
