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Application of potassium monopersulfate in precious metal extraction process


Potassium monopersulfate offers a distinct advantage over less soluble, dry oxidants such as sodium perborate, particularly at relatively low temperatures. At the preferred proportions of one part sodium bromide to two parts sodium chloride by weight, optimum leaching values are obtained from the minus 250 mesh precious-metal-bearing ores at pH values from 3.2 to 3.6 and oxidation-reduction potential values from +750 to +850 millivolts. Bromine and chlorine are liberated, and the chlorine synergistically oxidizes the precious metal complexes enabling up to 98% of the precious metal to be extracted in the form of water soluble metal bromides. 

Below is an example illustrating a process using potassium monopersulfate as oxidant. 

First, add 500 gallons of water to a 1000 gallon capacity tank to which was also added 2000 pounds of precious-metal-bearing ore having a particle size reduced to minus 250 mesh. The ore had been assayed by atomic absorption, emission spectroscopy and fire assay and found to contain 5.8 ounces of gold, 13.2 ounces silver, 0.41 ounce platinum and 1.54 ounce rhodium per ton.

The resultant slurry was stirred and 40 pounds of sodium bromide (97% technical grade) and 80 pounds sodium chloride (technical grade) were slowly added and the resultant slurry was stirred for 30 minutes, after which 0.25 pound increments of potassium monopersulfate were added, while the oxidation/reduction potential was monitored. The addition of potassium persulfate was continued until the potential reached 800 millivolts and the pH between 3.2 and 3.6. After the addition of 1.5 pounds of potassium monopersulfate the oxidation/reduction potential increased to +900 millivolts, and hydrochloric acid was added to reduce the potential to +810 millivolts. The slurry was stirred for five hours, and monitored every 30 minutes to determine its oxidation/reduction potential and pH. Potassium persulfate and hydrochloric acid were added in slight amounts as necessary to maintain the potential at 800 plus or minus 50 millivolts, and the pH between 3.2 and 3.6.

After five hours, the slurry was filtered and the filtrate was pumped to a 750 gallon holding tank equipped with a stirrer.

The precious metals in the filtrate were recovered by adding one gallon of an aqueous solution of 6 weight percent sodium borohydride and 20 weight percent sodium hydroxide was added. The oxidation/reduction potential of the liquid was adjusted to and maintained at -600 millivolts, plus or minus 50 millivolts and the pH of the liquid was maintained at 8.3 to 8.7 by the addition of sodium borohydride and hydrochloric acid.

After two hours of stirring, the liquid was filtered to recover the precipitate of precious metals. The filtrate was reconstituted for reuse.

The filtered solids were dried and purified to determine that the recovery, per ton of ore, was: 5.64 ounces of gold, 12.6 ounces of silver, 0.38 ounces of platinum, and 1.48 ounces of rhodium.