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Sodium Metabisulfite Detoxification (13 replies)
Metabisulfite is useful in that its concentrated in its powder form so can be used as a cyanide destroyer for mill tailings that are being used for underground backfill. The cost of reagent (per unit of SO2) including transport generally favour SO2/Air but often within only 10%
We used the Inco SO2/air cyanide destruction reactor at Red Dome in NQ in the 1990s to detoxify tailings return waters with Copper cyanide concentrations in excess of 400ppm. We used quite a small reactor and processed approx. 4000m3/day discharging to an old heap leach barren pond. The first pass reduced cyanide copper/gold and other elements from over 400ppm to around 10ppm. Controlling factor being maximising air through the spargers in the cell. We used 0.6-0.7kg/lt Sodium meta-bisulphite but my memory is not good on this. We then introduced basic water cannon to the pond and re-circulated treated waters and were able to reduce cyanide levels to 1-2 ppm. In the process we produced a copper gypsum carbonate residue we were eventually able to sell to a smelter as flux containing 2-3%copper. The mine was Copper gold with large silver credits. As another story we detoxified the heap leach pads using in-situ bacteria selected breed and fed using cyanide milk protein and veggie extract.
We are treating 900 m3/hr tailings pulp of -200ppm free CN destructed to 0ppm free CN in compliance to ICMC with 180 ppm WAD CN destructed to >50 ppm WAD CN. We converted one of last CIL tanks to INCO tank for destruction to increase retention time. We are also introducing dissolved oxygen/hydrogen peroxide as additional reactant. Also have online cyanide monitors that are interlocked to control SMBS and CuSO4 addition rates to minimize consumption rates.
What would be the chemical sales and costs in working with calcium polysulfide? I have similar experience like that. 4 hrs retention time, 5-6 kg SO2/kg cyanide (SMBS normally have 65% SO2), also copper sulphate as catalyst (0.7 kg/kg cyanide)
You can use SMB to reduce CN from 500 to less than 1 ppm, maintain the higher than 1,5 CN/SMB stoichiometric and Cu higher than 50 ppm or Zn and air enough. The reaction to convert CN to CNO is about 1 hr. To optimize the kinetic you need to control the pH around 8.7 and the EmV. The potential is relative from your kind of solution you have and can be measure and control the process. Very nice control of the pH, enough SMB, Cu and air allow to attach a green solution color y there is CN higher than 1 ppm you get light brown.
The percentage of solids is also a consideration? Or does not affect the reaction kinetics! Also I'd appreciate if you comment me when response time if one fixes the reagent dosing parameters.
In Merrill Crowe we get clean solution but if you have high turbidity you need to run some test to find how is your consume from that variable. As much solid you have you will increase your consume. The parameters you have control are pH, EmV, SMB, O2, Cu+2 and check the CN.
See link below for polysulfide reaction w/free & metal cyanides note the formation of less toxic thiocyanate (not the even lesser toxic OCN-); simplified sequential rxn chemistry for complete CN destruction (see second & third links for edification) includes:
S2- + CN- + H2O + 1/2O2 = SCN- + 2OH-
SCN- + 2O2 + 2OH- = OCN- + SO42- + H2O
OCN- + H+ + H2O = CO2 + NH3
I could read in the second webpage, "Ferro-cyanide is not destroyed, but precipitates as a base metal ferro-cyanide complex. The precipitate can re-dissolve at basic pH (>9) and release ferro-cyanide back into solution" which is a disadvantage; it means that If I increase the pH of the pulp detoxified, increase the concentration of CN?
Fe & Co cyanide complexes are the most difficult to oxidize and heavy metal ferri & Ferro-cyanide complexes will solubilise above pH 8.5.
The iron cyanide complexes have the highest stability constants and are classified as very strong complexes. Mercury, copper, nickel and silver form medium strength complexes, whereas cadmium and zincform weak complexes some references included for your study below.
I have been out of the mining industry for over a decade so am not positive if commercially applicable technology has been developed to destroy these species. I do recall the use of Ti salts for this purpose see links below; in any event, if you are not an SME member (www.smenet.org) strongly suggest membership for access to the invaluable OneMine library.
"Stability of Metal-Cyanide and Hydroxide Complexes", J H Kyle, World Gold 1997 Conference, Singapore, September 1997;
"Improvements in the Cyanide Reduction Process at Chatree Gold Mine", K Niyomjinda, B Etschmann, Metallurgical Plant Design and Operating Strategies, 2004.
The INCO (SO2:Air or SMBS: Air) process is an economic and effective process for destroying cyanide; capable of reaching targets of less than 0.2ppm WAD cyanide. It should be able to treat your material however critical design parameters must be satisfied or the plant will not work at all.
The optimum conditions of residence time, reagent addition, pH and oxygen transfer are dependent on the minerals in the ore (or concentrate) and what other metals must be removed from the tailings. You can see this variability in the range of conditions being used by the other contributors; each suitable for the material being treated. Lab tests are the best way to determine these design parameters.
In your case, with relatively high cyanide level of 900ppm selecting the wrong design could lead to very high operating costs and unstable operation. Oxygen transfer is particularly critical when treating high cyanide levels. We perform testing, design and supply of cyanide detoxification processes.
We use double detoxification with hydrogen peroxide as the first stage and sodium bisulfite in a second stage. The first step down from 600 ppm to 300 ppm free cyanide, then metabisulfite worked from 300 to 50 ppm. In the first stage peroxide was added in the Tail box pumps and reactions occurring directly into the pump concluding in line pipe to reach the second stage of detoxification by metabisulfite.
Very straightforward stoichiometry: SO2: CN ratio is critical to efficiency. As well as pH, Cu+, residence time. Appropriate lab equipment simulating plant conditions is preferred for test work.
Cyanide Detoxification of tailings from gold and silver ore with sodium metabisulfite, I would like if you can tell me your experiences with this process, I want to reduce from 900 ppm to 25 ppm.