Effect of Sampling On Assays

By Charles Kubach, Mine-Engineer.Com
18 October, 2014

Sampling is defined as taking a small portion of a whole mass that accurately represents the whole mass.

Sample Preparation and Sampling Methods Are More Important Than The Actual Assay.

Perhaps many would argue with the above statement, but many more, such as the Father of Modern Sampling, Pierre Gy, and a host of metallurgical engineers and mine operators have presented plenty of conclusive evidence as to why the above statement is valid. There are hundreds of case study papers published that document the importance of sampling, and the process of preparing the sample, which will result in the few grams to be assayed.

When a few grams of material are going to represent many thousands of tons of ore, and the valuable mineral(s) they hold, it is easy to understand the importance of using scientific methods that are as accurate as conditions allow, to obtain the few grams for the actual assay ore. Many large companies have opted for the robotic, computer controlled assay lab, where human intervention is minimized as a solution. However these complex and expensive labs have their own built in errors, such as the ability to clean out disk pulverizers between samples, often leaving caked remnants of previous ores for the next batch, resulting in some degree of cross contamination.

It is very possible for a manual, human operated assay lab to be more accurate than these robotic labs, because humans can recognize potential problems and react to them, whether they are "programmed" to look for them or not.

It is critical that methods used to reduce the ore to those few grams be as accurate as possible, and equipment is available that will assist in doing this, in many cases. For instance, cone and quartering is the "old" method for splitting a large sample. Cone and quartering induces a margin of error of 19.2%. 1 Starting off with a 19.2% error is not the best game plan for more accurate assays.

The Rotary Sample splitter and Vezin have a margin of error of 0.1% 1 , and are capable of splitting large bulk samples of -1/4" ore. This simple changing of the equipment/methods used in splitting large samples for the lab can eliminate 19% error in the assay actually being representative. Further, since a riffle splitter is usually involved in reducing the final cut from the bulk sample to the one actually assayed, there is a more precise riffle splitter that should be used. A precision riffle splitter has a margin of error of 2.7% vs. a typical Jones riffle splitter's error margin of 3.7% 1 . So, by utilizing these two methods in the sample preparation procedure would eliminate 20% of the margin of error, and combined with the assay lab's experienced personal knowledge, mean that a human operated assay lab could be more accurate that a robotic assay lab. It is the combination of human experience and the ability to intervene when something is noticeably errant, and utilizing the most appropriate and accurate equipment that will make the difference in a most positive way.

1 - AA Khan-Critical Evaluation of Powder Sampling Procedures

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