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Deano

Well-known member
Joined
Feb 23, 2014
Messages
468
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
 
I don't think I'll ever have an opportunity to test this procedure, but it was an interesting read.

Good work!

Göran
 
I was a bit rushed when I posted the last effort.

What I was wanting to say but missed doing so was that the post was really about the effect in a leach of having manganese in solution.

For most people this involves an acid leach, having manganese in solution in alkaline solution is not an everyday procedure.

Having manganese in solution means that any gold present will not precipitate out until the manganese has been itself reduced to Mn2+.

You can be adding precipitating agents in industrial quantities to achieve this depending on the manganese levels.

The idea of adding a whole lot of reductant such as metabisulfite to the solution usually results in the dropout of brown manganese complexes as well as the gold, there is a small window where only gold is precipitated.

If someone has a solution which has known gold dissolved but which will not drop this gold when the correct amount of precipitant has been added then the presence of manganese in solution should be investigated.

The presence of manganese in solution also interferes with solvent extraction both in the loading cycle and the phase separation stages.

Manganese in solution will usually be sourced from having stainless steel present in the leach step.


Deano
 
I process considerable gold obtained from stripping of manganese containing materials and have never had an issue in selective gold precipitation or solvent extraction or resin with Mn(II/IV) in acidic/oxidizing milieu. Usually my big demon is Fe(III).
 
Could the manganese carry over to gold recovered by the copper(II) chloride leach? I know that the nickel substrate is mostly left intact from AP solution. I believe that manganese is used in the Kovar alloy.

Fe Ni Co C Si Mn
balance 29% 17% < 0.01% 0.2% 0.3%

Would this be enough to cause problems with the precipitation with SMB?
 
The material I was working with was the sludge from a carbon strip electrowin cell from a gold circuit.

A quick analysis of the sludge showed around 80% Mn, 10% Ni and 10% Cr with 1% Au.

I have had considerable experience with gold recovery from acid liquors containing low levels of these metals and have had few problems.

It was only when I worked with high tenor leaches of these metals that I had problems, filtration being the worst.

Multi cycle aqua regia leaches still left a problem residue.

I don't know if it was the manganese per se that was the problem or if it was the combination of Mn and the other metals.

I did run a diluted leach and had little problems in recovery from such.

Running more dilute liquors on the total was not an option due to the volume of liquor involved. I really don't want to spend my retirement on this one project.

The main message is to avoid dissolving stainless steel in your gold leaches, low levels are probably OK for small scale processing.

Deano
 
Table optimising

Many years ago I was involved with a project to maximise gold recovery from a milled ore by using a small (3') wilfley table to clean up concentrates and middlings from a bank of larger tables.

Due to environmental constraints chemical processing was not allowed on site.

A short period of operation revealed that running the table in its standard format was leading to losses of fine gold where the particle size was less than 50 micron screen size.

The gold particles were flattened from the milling and appreciable quantities were also lost from the plus 100 micron screen sizes.

Despite doing all of the standard optimisings of adjusting feed size screening, feed rate onto the table, table side tilt and water flow I could not get a major recovery improvement.

I went into the literature and found that the largest number and most useful papers were those from the British tin industry.

Basically they said that keeping a tight sizing on the feed was vital, I was already doing that so OK there.

Making sure that the table was level on the longitudinal axis was a fundamental which I was also doing.

Feed rate was best when a loose bed was set up along the table, don't put too little feed on the table nor try to put too much feed on the table.

Keeping the feed rate constant was also very important, I was feeding from a wet sump with a screw feeder so OK there also.

Side tilt was to be such that clay fraction particles were washed over the side of the table but the tilt was to be little enough that a middlings product could be readily separated at the end of the table.

Even when I had the table set up to cover all of the above parameters I was still losing fine gold.

The only area where I did not have full control was the side water coming onto the table, no matter how much I tried I could not get a perfectly even flow across the table.

I decided that I needed to improve the delivery of water to the table.

I did so by running a length of 3/4" copper pipe suspended about 4" above the side of the table where the water exited the original flow boxes.

The pipe was blanked off at the table exit end and was connected to a hose and ball valve at the feed supply end.

Ever 1" along the pipe was drilled a 1/8" hole.

When the water was turned on a curtain of water sprayed down onto the top edge of the table and delivered an even flow of water which could be easily adjusted for flow rate with the ball valve.

Even this change only partly improved the recovery, it was evident that there needed to be a difference in the water flow rate supplied to various parts of the table.

After a lot of testwork I settled on having the water delivery pipe as two pipes.

The holes remained the same size and spacing but the delivery was split into two parts.

The first part was as above but only extended 2/3 of the table.

The second part covered the last 1/3 of the table, each part was as a separate length of pipe so that adjustments could be made to either part without affecting the other part's flow rate.

In order to keep the water holes at the 1" spacing the pipe for the last 1/3 of the table was fed from the bottom of the table and the pipe ends almost touched.

Each pipe length had its own ball valve for separate flow adjustment.

The side wash water pipes were fed from an overflow overhead tank so that a constant head was maintained.

This was important on a mine site where valves were being opened and closed in other parts of the circuit, this would affect the pressure to the wash pipes.

This setup allowed recovery of free gold down to 25 microns, the disappointing part was the low weight of the 25 to 50 micron gold recovered, it looked a lot as a sheet like paint on the table but weighed far less.

On the plus side there was a substantial improvement in the plus 50 micron gold which did weigh well.

If run from a municipal water supply the overhead header tank may not be necessary depending on the vagaries of the particular supply.


Deano
 
This thread just gets better and better, one nugget of information at a time.

Thanks for sharing!

Göran
 
I started off these threads in the chemical processes section because they originally were mainly chemically focused.

I am keeping them there as I expect that the bulk of material to come will also be chemically focused.

If someone wants to post these more mechanically mining parts in the actual mining section I am happy to have them do so.

I can see potential for scrap treatment with a table but only in the separation of metallics from non-metallics.

This would require a consistent milling format for the material, with the variations of plastic types and thicknesses I see this as the hardest part to get right.

If the material can be milled so that the metallics are freed from the plastics then I would expect a very good separation to be achieved.

Note that some awfully cheap tables are available from China, the drives are really good but the actual table surface is pretty ordinary and should be replaced in total.

Deano
 
My mistake. :oops:

When I read Finn's post, I thought he was intending to add a link to something on the forum he mentioned that might have additional information. Now that I've re read it, I believe he's asking if he can link to this article from the other forum. Finn, I'm sorry for any misunderstanding, but I don't have the authority to answer your question. :oops:

Dave
 
I can't see any problems with linking from outside sources. I link heavily from my wiki and gogle links to everything.

Linking between sites is what makes the web what it is today.

Göran
 
As far as I am concerned the decision is not mine to make, I have no objection to the proposal but once I have posted information it is not my position to say what links may be put in place. Refer the point to the site ownership.

Deano
 
Few hundred kilos of the material with 1% AU? So there were several pounds of AU precipitated?
 
Sometimes when precipitating gold the product is fairly slimy and difficult to filter.

One way to improve the gold floc size is to put some silica flour in the precipitating liquor.

About a teaspoon in a litre of the precipitation liquor is a good working amount.

Stir it to distribute it evenly.

The silica will report to the slag when the gold precipitates are smelted.

Deano
 

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