Sampling practice and theory

Sampling, What Does It Entail?
& How To Obtain Representative Samples

Page 2

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

The mass of a sample taken to represent a particular ore depends upon the type of ore. For instance, if a ore of a particular particle size, has a regular and highly distributed metal, such as copper, in it, the size of the sample would be small, it might only be 5-10 pounds from a lot. However if the ore has a very low grade of copper in it, is sparsely distributed, with little or no pattern, it will take a much larger sample, 200 - 400 pounds, to obtain a representative sample of the whole lot. The characteristics of the ore to be sampled can have an exponential effect on the volume of the material to be sampled.

This is caused by two major factors, one the mineral, in this case copper, may only be of a particle size of 1 mm or finer. Therefore, to obtain a reasonable statistical accuracy in a sampling program, the material to be sampled should be crushed to approximately 1 mm, to effectively expose the mineral or metal equally to the sampling process. And second, the mineral or metal may be widely present (such as in a rich ore body), or sparsely present (such as in a low grade ore body). In a low grade ore, there may only be 15 - 50 1 mm particles of mineral or metal in a cubic foot of ore (150 - 200 lbs).

So, it is easy to see, the finer the ore to be sampled is crushed BEFORE sampling, the easier it will be to obtain a representative sample, and the smaller the sample mass that would be required.

How to Obtain A Representative Sample.
In a process plant, or mine, the preferred method of obtaining sample material, is from a moving stream, such as a conveyor belt, a slurry pipe line, or perhaps from a chute that a stream of ore gravity flows. How the sample is obtained, the number of increments and the size of each increment, will often determine the degree of probability that a sample is indeed representative. Generally speaking, the Best way to take a incremental sample from a moving stream, is to take a Cross Section of that stream, with each cross section having a pre-determined mass, and taking this cross section incremental sample a pre-determined number of times per hour (or unit of time).

Selecting the proper equipment for the task, and using as much automation as possible will eliminate many potential errors, and is very important if the sample is to have a +90% probability of being representative. The best sample plan and the worst equipment will produce only mediocre results. As in all good plans, every piece must work together, to achieve a quality result.

Sampling is generally comprised of a number of steps, the primary sample, the secondary split, the tertiary split, and so on. For sampling ores with maximum particle sizes of 2 inches or less, a cross stream sampling device is usually the best choice. Depending upon the material being sampled, it is generally desired to crush the primary sample, to obtain a smaller particle size and increase the probability that succeeding splits will accurately represent of the whole mass being sampled.

For more information, continue to the Next Page, "Placer Sampling"

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