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Determine free Cyanide (8 replies and 1 comment)
You can try to dilute the sample with water and then do the titration, clear that for the calculation of the concentration CN you must use the dilution factor.
Or you can use lead acetate or lead carbonate mixed with the sample, then filter and the concentration determined by titration of CN.
I will try the first onebut could please give me some detail about the second one? What is the use of Lead carbonate or lead acetate in such sample?
Lead acetate or carbonate reacts with soluble sulfides & precipitates out as PbS (Ksp=9x10-29)...hence the filtration; additionally, you likely have thiocyanates (reaction of soluble sulfides w/CN forming SCN-) that will interfere w/the AgNO3 titration endpoint. Cu & Zn cyanides also interfere w/this titration due to the large number of complexes CN ligands that are liberated during the titration skewing free cyanide results. Your best bet is Ion Chromatography...see linkhttp://www.saimm.co.za/Journal/v083n01p011.pdf
We have performed quite a bit of work on your ore types to assess the benefits of extensive pre-oxidation on the gold recovery from your Cu float tails. Are your problems related to actual sulphide in solution (causing discoloration due to AgS precipitation etc, or an undefined end-point because the high copper levels require slow dissociation and the solution is slightly yellow because of dissolved iron and thiocyanate levels (which will cause discoloration and difficult to observe visual end-points).
If so, either try IC as suggested (but I doubt whether MCM has such a facility at disposal in Mauritania) or automated potentiometric titration (which is better, but also not the final answer as the system can also struggle to find an end-point in a very flat curve of the titration).
My advice to you would be a bit of a cop-out:
- Analyze for CN WAD (which will include your Cu species, Ni species and CN free (MCM Ni levels are as I remember negligible).
- Then analyze for Cu in solution, assume it to be present as Cu(CN)3 mainly and subtract this copper bound cyanide from the WAD level.
- Assume the rest to be CN free.
This is not as quick as a simple titration, I know. But maybe the best you can do in your parts of the world.
I would like to remind you though that when we did speciation for your solutions, we found that the titrations vastly overestimated the CN free levels (which I put down to operator error in end-point determination but could be linked to overall solution composition). But furthermore, under certain conditions one could run a "cyanide starved" leach with MCM ores resulting in nearly no free cyanide at residue level but still appreciable levels of generated free copper towards the leach end. If this is the case, my cop-out system would not work as you would over-estimate the amount of CN free (there would be none!). One easy way to check is linking the HCN gas level and pH to HCN and CN- in solution.
Your solutions are not easy to quantify, that much I can confirm to you.
If you find ASTM analytical method 4500 it explains how to deal with many interferences in cyanide measurement. I believe that these measurement methods are the ones recommended by the International Cyanide Management Institute (cyanidecode.org).
If you are interested in on-line cyanide measurement at your process plant, Orica offer a range of potentiometric-titration cyanide analyzers that can be configured to add additional reagents and deal with the 'flat titration curve'.
More information can be found here:http://www.orica.com/Products---Services/Mining-Chemicals/Service-Solutions/Cyanide-Analysers#.Usn1QtIW1d0
I’m just return from my free days. I can see all the advices for your problem. Any way anything more i am here.
Remove the sulphide from your solution, which is interfering with your titration (see here), using either Jean Rasczak proposed solution (beware of lead acetate) or using FeCl3.
Then, you can use a free cyanide test that will give you semi-quantitative results within 60 seconds and without any special equipment.
http://www.cyanoguard.ch/technology
Also, this test will not suffer from thiocyanate-related interferences.
Hope this helps.
This is on the edge of shameless promotion, but.. OK.
If you tell us how much is the Test-Kit, maybe 911Met Admin would delete this.
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Anyone could help me by giving advices to determine free cyanide CN- in CIL solution if the solution contains sulphide?
We are titrating the solution by Silva nitric using CN indicator, but experiencing some difficulties because of the presence of heavy metals in solution. Does free cyanide vs wad cyanide matter?