If caustic fusion test results are encouraging, the evaluation advances to the collection of bigger samples designed to recover larger-sized stones. Diamonds in kimberlites typically occur in a statistical lognormal size distribution. This means that smaller non-commercial diamonds occur with much greater frequency than larger, commercial-size diamonds. Therefore, large samples are required to recover commercial-size diamonds.
The purpose of mini-bulk sampling is to establish whether "commercial-sized" diamonds are present in the kimberlite. In general these are diamonds that are larger than 1.00 mm using a square aperture screen. Given that a kimberlite cannot be mined economically if it contains only small diamonds and insufficient quantities of large diamonds, it is essential to determine the frequency of occurrence of larger stones.
Accordingly, greater volumes of material are involved at the mini-bulk sampling stage in order to achieve the goal of recovering larger diamonds from the sample. As a result, different processing techniques are required at the mini-bulk stage, including use of a dense media separation ("DMS") plant. This equipment is used to recover stones that are commercial in size. For mini-bulk sampling, Stornoway recovers stones greater than 0.85 mm using a square aperture screen. Recovering larger stones from a mini-bulk sample is very important, since the decision to proceed to a major bulk-sampling phase depends on confirming that commercial-sized diamonds are present.