A “preg robbing” test is an analytical technique where a known amount of gold in cyanide solution is contacted with the sample. After a specific time period, the solution is re‐measured for gold to determine if gold was adsorbed by the sample.

Preg robbing is a bit of a challenge but is not too difficult once you understand the mineralogy and how it affects the pulp. There are several possible tests to carry out which I will list below and you can research them individually.

Conventional preg robbing test: 10gold cyanide to a suspected preg robber and bottle roll for 24 hours. Assay at 1,2,4,8,24h to see if preg tenor decreases.

Diagnostic leach: Determine deportment in carbonaceous minerals.

Parallel bottle rolls (this is the most common test) one with carbon and one without. Assay solids, solutions, and carbons at the end and do a gold balance.

Google "preg robbing index". You will find that there are papers discussing the relative severity of preg robbing and benchmark yourself against other orebodies. 

I will go through the last test. Since i am currently designing an experimental procedure for the diagnostic leach test! I will also add the parallel bottle roll test and get back to you with the results for further questions. Will research on the preg robbing index too!

Carbon is added at the initiation of the leach period to determine if preg-robbing constituents were present in the material. In the leach tests, if the gold extractions from the direct leach and the carbon in leach (CIL) tests are similar = suggests that preg-robbing carbon is not a significant concern for the sample material tested.

Methods: Cyanidation of gold

The ore is prepared prior to leaching by drying, followed by crushing and ball milling to 80% passing 106 pm.Three sets of samples weighing 1 kg each of the ores from the various mines were prepared at 50% pulp density. The pulp for each sample was then conditioned with lime to a pH of 10.5-11.5. and leached with 1 kg/t cyanide strength in rolling bottles for 24 hours. For the first set of samples prepared, activated carbon was introduced into the leaching bottles. For the second set of samples leached, no activated carbon was introduced. Kinetic leaching studies was conducted on the third set of samples without carbon addition.

Each of the leaching studies is conducted in duplicates. For the kinetic studies, solution samples were taken for gold analysis after every 1, 2, 4, 8, 16 and 24 hour(s). For the carbon in leach studies, solution samples and activated carbon were taken at the end of the leaching studies for gold analysis. Tailing samples were taken from each of the bottles and fire assayed to determine the residual gold.

Preg-robbing test

Dried ores were taken through crushing and ball milling to 80% passing 106 pm prior to preg-robbing test. Duplicates of each sample weighing 10 g were used to contact 25 ml of 16 pg/mL gold solution and agitated at 200 rpm in an orbital shaker incubator for 24 hours. Each pulp was then filtered and the filtrate analysed for the gold content at the end of the contact time. The Preg-robbing Effect of Carbon(PEC) in the various ores was estimated with respect to the amount of ore used for the adsorption test based on Equation 1.

In Equation 1, ICAu and FCAu are the initial and final concentrations of gold in solution. W-ore is the weight of the ore used in the adsorption test and Vs is the volume of gold solution used.

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