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Activated carbon fouling by calcium carbonate (7 replies)

Raje Singh
9 months ago
Raje Singh 9 months ago

What possible laboratory tests can be done to determine extent of calcium fouling on activated carbon?

9 months ago
Standartenfurer 9 months ago

The first thing that you need to do is to measure the adsorption capacity of gold onto the carbon, over an extended period. So the test should initially be done daily and after a month if the changes are gradual you can reduce the frequency.

Adsorption testing comes in two forms Freundlich and Langmuir. The latter is more precise at low concentrations so I suggest that one see http://is.gd/auPSwf

This plot of gold adsorption will give you an S curve that informs you of the rate of loading (kinetics) and maximum capacity. You should try this on fresh carbon as well as recycled (regenerated) carbon. This test will give you a rate constant or K value, if the k value of regenerated carbon is less than 60% of the fresh carbon then you may need to add more carbon, or if between 60 & 80% use more frequent acid wash cycles to remove the calcium carbonate.

Jean Rasczak
9 months ago
Jean Rasczak 9 months ago

The standard activity test won't give you only calcium loading, however you can conduct chemical analysis using ICPAES on activated carbon after drying, ashing and acid digest to determine your calcium (and Au, Ag, Cu, Fe, Ni, Mg and Na). Si and Al need a different pre-treatment route, if I recall correctly it is pulverizing, ashing, fusion with an alkaline flux and then a water leach before ICPAES. I have had this done at a commercial lab before, good idea to chat with the chief chemist so that they have an idea of what they are dealing with.

The standard kinetic activity test procedure is normally conducted as a routine by your assay lab, they will have the procedure that they use, my preferred method is to measure the rate at which the carbon removes gold from solution by placing 0.5 g of sized carbon (-2.36 +2.0 mm) in 500 millilitres of solution containing 10 ppm gold (I) and 250 ppm of sodium cyanide and adjusting to pH 10.5. The mixture is then bottle rolled at 100 rpm and samples of solution taken at 0.5, 1, 2 and 3 hours. Gold is determined by direct atomic absorption spectrophotometry (AAS) using normal gold cyanide standards. The resultant data is used to calculate a rate constant, k, and an exponent factor, n, based on the Fleming model for CIP adsorption kinetics.

From this value of k, a relative activity is calculated by arbitrarily setting a sample of fresh carbon to 100% activity and expressing the other rates relative to this standard. The percentage figure thus derived is based on the relative amount of gold adsorbed after 1 hour under the test conditions.

An adjunct to these is the thermo gravimetric analysis that I have had done for me by the Parker Centre Group at a local university, they provide you with organic fouling data. I am not sure if this method is widely practiced, but if you are going to speak with your local lab about this work, it may be worth asking them if they can do it.

9 months ago
Subhash-Kumar-Roy 9 months ago

If I understand your query, you would like to determine the extent of Ca fouling in your loaded/stripped (LC/SC) carbon. An addendum to valid procedural: in a lab setting, we (Goldfields,NV, 90's) would immerse the SC or LC in 3-5% HCl w/mild agitation (to minimize attrition besides those fines cemented w/scale) for 1-hr; vacuum filter thru' 0.45-micron pore & analyze filtrate for Ca by AAS. This is the 'removable' Ca that you should be concerned with following H+ washing in your circuit; You could attempt both activity determination following a thorough water wash (pH -8), drying at 100C followed by screening the size fraction that would be used in your adsorption circuit; an ideal scenario would have been to tumble the H+ washed, dried carbon through a lab tube furnace (rotating) at -700-750C in the presence of steam as this would have simulated your reactivation circuit, then follow it up w/an activity determination.

Raje Singh
9 months ago
Raje Singh 9 months ago

We currently do not test for calcium. I will definitely go over both your suggestions with our chemist.

Tony Verdeschi
9 months ago
Tony Verdeschi 9 months ago

You can assay for the calcium level - if the acid washing is done properly you will see a step change in the calcium level between the sample of loaded carbon and acid-washed carbon. Other indirect tests that can be done are the activity test and the apparent density, both of which will give an indication of the extent of the effect of the fouling by calcium carbonate, without measuring the calcium level directly.

9 months ago
Obersturmbann 9 months ago

Unless you have a specific reason to look at only calcium carbonate I would encourage the use of carbon activity testing as described earlier which provides an indication of carbon fouling resulting from contact with a number of foulants. The activity testing of carbon prior to acid soaking and after the acid soak may give an indication of the inorganic foulants and the activity following the regeneration may provide information on organic foulants that may be getting into your process. (Oil spillage at the mine and plant etc.). I think the importance of effective acid washing and carbon regeneration is extremely important.

Sugar Watkins
9 months ago
Sugar Watkins 9 months ago

Is the fouling of the carbon by calcium a hypothesis as a plausible explanation for the current circuit performance or a demonstrated fact?

A very simple comparison testing of the absorption (or removal of gold from solution) obtained with the regenerated carbon against that obtained with fresh carbon should indicate if fouling is present and if so its severity.

If this indicates a much reduce activity, then it could be due to fouling (and the investigative methods suggested in the previous comments should be considered) or that the regeneration circuit is not operating properly (and each step should be examined in details).

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