Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-04-21T02:33:06+00:00
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Elution Efficiency (14 replies)

Rahil Khan
1 year ago
Rahil Khan 1 year ago

What might be the some of the possible causes of high Au on Barren Carbon after elution (AARL), when all your temperatures, pressures and flow rates are good!

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

What are the temperature, pressure and flow rates?
Consider that you are not leaving your pre-soak long enough, or that possibly your CN/Caustic mix in not strong enough, or that you are not putting enough bed volumes of water through the column.

1 year ago
OberstGruppen 1 year ago

What are your "good" parameters?
Temperature, pressure, flow rates, total flow (in bed volumes), %NaOH and %NaCN?

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

AARL elution relies on low hardness (or ionic strength) of hot water used for elution. Demin water is not necessary, but normally some form of soft water with low ionic strength works well. Some mines make the mistake of using process water to cut costs.

AARL elution is favoured by absence of Ca and Mg. The capacity of activated carbon for gold increases with increasing concentration of cations of solution in the order Ca > Mg >. H > Na > K. Hence the use of caustic soda to convert the Ca[Au(CN)]2 ion pair to Na[Au(CN)].

Rahil Khan
1 year ago
Rahil Khan 1 year ago

Our column temperature and pressure during elution are around1135 'C & 360KPa respectively. Our flow is around 80 m3/h.

We currently doing about 7 bed Volumes of solutions for the elution at the and the NaOH= 1.6% and NaCN =0.17%. Our stock solutions are usually around 24% for NaOH and 31% for NaCN. Currently our flow rate cannot be varied since the pump is a fixed speed pump.

We always analyse our solutions before the strip and ensure a low ionic strength before the process starts.

1 year ago
Unterstarm 1 year ago

Some parameters, to keep in mind:

Solution Flow: 38 m3/ H - 42 m3 /H
Heating Temperature: 119 °C -125°C
Free cyanide concentration in pre-soak tank: 5000 ppm - 8000 ppm
Caustic sodium concentration: 50 kg/m3 - 80 kg/m3
Operation time: 7- 8.5 hours
Solution pH in pre-soak tank: 12.75 - 13.20
Bed volume: 2- 4
Strip time operation: 7 -8.5 hours

Rahil Khan
1 year ago
Rahil Khan 1 year ago

Temperature usually 135'C
Pressure = 350 kPa (+-10)
Elution flow rate around 78-82 m3/h
Total volume of solution =7 bed volumes
%NaOH during pre-soak & heating = 2%
%NaCN during pre-soak & heating = 0.15%

We always analyse the solution to ensure low ionic strength before we start the strip.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Has this always been a problem or has it just recently started to occur?

If it has just recently started to occur- something has changed. You may say nothing has changed but this problem does not just out of the blue occur without there being something different somewhere taking place. Examine what has been happening from the early stages of processing, has there been a change of carbon type or supplier Process of elimination.

How the acid wash before elution goes, is your acid of sufficient strength. Is your pre-treatment (pre-soak) long enough? Combined, acid wash and pre-treatment steps convert the gold-cyanide ions from being strongly adsorbed to the carbon to being weakly adsorbed to the carbon and hence easily desorbed in the elution stage.

I would just add that if your carbon has any silver or copper on it, it will desorb first and if you run the strip to a timetable it may not have eluted the gold before the strip finished.

Also, if you were not having this trouble in the past, something has changed to make this problem occur. Process of elimination, go back through the whole process and determine what has changed since the ore has been introduced into the plant. There has been a change somewhere. Things don't just change without a reason.

1 year ago
Gruppen 1 year ago

Importantly, the bed volume must calculate the volume from the elution solution of this over 120 ° C. In my case, we have good results with 800 ppm of free cyanide in the solution strip

Paul Morrow
1 year ago
Paul Morrow 1 year ago

Your problem seems to be pressure related predominantly and also ensure your caustic level pH is high and your cyanide strength is good. I would also check your elution column auto valving and screen systems for any problems and possible leakage which should also show up in your pressure column attainment during stripping.

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

You can equally consider looking at the cation exchange capacity of your activated carbon, look at the series of efficiency critically for Gold. Also look at your water type (soft and hard water) as well as their chemical properties with respect conductivity (ionic strength). Hope you get the issue solved.

1 year ago
Obersturmbann 1 year ago

Our Caustic stock solution strength usually ranges between 22-26%.
We increased the solution Pre-soak (recirculation and heating) time from 120mins to 180 mins since we cannot reduce the pump speed (elution flow rate) and it seem things are getting better now. Our last strip had a barren carbon of 68ppm Au. The Loaded carbon was 4000 ppm+.

What we have not been able to look at is the Ca & Mg content on the loaded carbon. Any quick method of checking!

Kumar Choudhry
1 year ago
Kumar Choudhry 1 year ago

XRF or digest in aqua regia followed by AAS on the filtrate.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

You have the same set-up as Ahafo, that works ok and Akyem has been running a long time now, Lyco have all the info in the Plant Philosophy, Criteria and description. Sounds like something (equipment) or someone created the problem.

1 year ago
Standartenfurer 1 year ago

Poor acid washing resulting in inorganic fouling can definitely increase your barren grades. At a previous mine we had many problems with acid washing such that we were not acid washing for almost one year. During this time the barren grades steadily climbed to 150-300 ppm gold, 100-300 ppm silver. As soon as the acid washing was consistently resumed, calcium levels on carbon dropped, carbon apparent density dropped dramatically and within a couple of weeks the barren grades dropped to 20-30 ppm gold, 30-50 ppm silver and they stayed there consistently thereafter. Thankfully the split AARL elution circuit was well run, achieved 128 degrees in hot elution, 8 bed volumes per cycle and kept up healthy cyanide and caustic doses.

Other contributors to poor barren values are usually inadequate stripping temperature and reagent starvation, in that order. Also you could check the filters in the bottom of the column and do a few eluate profiles to check if there is any channelling of eluate in the carbon column, as this can also be a reason for high barren values.

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