The heterogeneity test uses the principles of Pierre M. Gy's Sampling Theory.
The test work establishes appropriate sampling protocols for particular ores and allows for the design of sample flow sheets from the field through to sample preparation and assaying. Errors at each stage of sample size diminution can be calculated.
A full test involves the collection of between 300 and 500 kgs of material representing a cross-section of the type of samples expected to be encountered within a particular ore-type. For this example we will assume that the principal material is relatively coarse with an average size greater than 1.0 cm and less than 10 cm.
The material is jaw crushed to around 2 cm and then screened over 2 cm and 1 cm screens. Each fraction is riffle split and sampled for 1000 gram screen fire assay gold. The -2cm +1 cm fraction is spread evenly over the ground and 100 random samples are prepared. These are made up 15 random rock chips. These 100 samples are totally assayed using fire assay.
The heterogeneity test uses the behaviour of the coarsest fragments to calculate sample diminutions. This is done because Sampling Theory states that the coarsest fragments demonstrate the worst case characteristics of the samples. The screen fire assays are used to validate this argument. If we see changes in values greater than one order of magnitude between the different size fractions the test must be considered invalid.
The Fundamental Error of the lot is generated by Constitution Heterogeneity which consists of the average difference in gold content between individual fragments. So, this error depends on the sample weight, the maximum size of fragments for a given stage of comminution, the expected average gold content of the lot from which the sample is coming, the degree of liberation of the gold or the minerals that contain it, the shape and density of the fragments etc... The heterogeneity test intends to predict this error in a given sampling protocol.
Once the test is finished nomograghs are produced and appropriate sampling protocols can be recommended. See below for an example.
If the material in question is going through a gravity circuit we can produce nomographs without performing the full heterogeneity test. These would not be as accurate due to differences in performance from one gravity circuit to the next but could be used for approximations. We would need to know some qualities of the ore, the P80 of the feed to gravity and the percentage of total gold recovered to gravity.