Cyanide Leaching Of Gold, Laboratory Bottle Tests

Gold Cyanide Solution, Laboratory Bottle Test Method

This is one method for doing cyanide bottle leaching tests for determining the 'leachability' of gold and silver by leaching with cyanide.

Take the ore to be leached and crush it to a size that will fit into a pulverizer (usually -1/4"). Then feed the ore to a pulverizer and grind to -100 mesh. If doing multiple samples, a rotary sample splitter will split the sample in 12 or more splits with 99.9% accuracy. If rotary splitters are not available, or the sample is small (such as small core section), use a riffle splitter to split 50% until approximately a 100 gram sample is obtained. Weigh a 100 g portion on a balance with at least 0.01 g sensitivity. Place this 100 g in a 500 ml wide mouth jar with 300 ml of the prepared 0.5% cyanide solution, prepared as shown below.

Cyanide Solution. Prepare a 0.5% Cyanide solution, by weighing out the appropriate weight of cyanide for the weight of the water. Three hundred grams of water plus 1.6 grams of KCN is a 0.5% solution. For 5 liters, add 260 g KCN for a 5% solution. Now, add lime, to bring the pH of the solution up to 11. Use a lab mixer or stirrer to continuously mix the solution for about 30 minutes. Using a 500 ml graduated cylinder, measure out 300 ml of the solution for use with the leach test.

Now, with the 300 ml cyanide solution and the 100 g of ore in the bottle, place the lid on firmly, making a seal. Repeat the above steps for as many tests as need to be done. If hundreds of leach tests are to be run a day, and each initial sample is a Kg or larger, it is well worth while to invest in some automated sample splitters, since this is the most tedious and error prone part of the operation. If the sample is not representative, the rest of the work is not worth much.

Take the 500 ml jars with the leach solution and ore in them and place them on jar roller(s). These are machines with one, or multiple tiers of rollers, typically ranging from 1 foot long to 6 feet long, that will turn and agitate the slurry of solids and cyanide solution. The bottles need to be agitated around 25-30 rpm's, and most good roller drives have variable speed on them to allow a wide range of adjustment. Typical leach times are 24 to 48 hours. It may be feasible to take a small sample after each 12 hour period, checking the pH of the solution and conducting a assay on the liquid withdrawn. A AA Spectrophotometer is probably the easiest machine to do the assay on dissolved gold in a cyanide solution, although for the hearty assayer, there is a fire assay method for this test, as well. A AA assay gives results in parts per million. 34 PPM equals one ounce per ton, 3.4 PPM equals 0.1 ounce per ton and so on. Some newer AA's give results in PPB (parts per billion), but for bold and silver, if you don't have parts per million, you don't have ore, so it is not necessary.

Normally, there will not be much need to agitate past 24 hours, but occasionally ores do have chemical reactions that slow down the leach process. So it is recommended that the sample be agitated for at least 12 hours past 24 hours, and if the assay does not increase, then it has probably leached all that will be leached in cyanide solution.

Variables to change in bottle leach tests are the strength of the leach solution, the lowest strength solution that will leach the precious metal is always the most economic solution. Other variables are the pH of the solution, making it more alkaline. Acid should be avoided, since if any acid is used prior to the cyanide, and is still present on the ore (as evidenced by a pH lower than 7 in water) it will produce HCN, a very toxic gas. It is good to take a sample of the ground ore and place it in water and agitate it overnight to see if the pH changes. If it gets acidic, lime should be added to neutralize the acid producing potential of the rock. Also, the CN-AU reaction requires oxygen to be present, so if initial recoveries are poor, try using a larger 1 liter bottle. This will give a larger volume of air, and hence more oxygen for the reaction.

A common method for neutralizing any cyanide left in the tailings is to wash with a solution of Hydrogen Peroxide (H2O2). This neutralizes any unspent CN in the solids, and they may be disposed of, without regards to cyanide content.

Disc Plate Pulverizer
Rotary Sample Splitter
Bottle Roller
Information provided by Charles Kubach, Mining and Mineral Processing Engineer
Reference: Chemistry of Cyanidation, American Cyanamid  

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