SAMPLING ORES

INTRODUCTORY

1. Object of Sampling.-The object of ore sampling is to obtain for chemical or mechanical tests a small quantity that will contain all the minerals in the same proportions as the original ore. If the sample is not correct, there will be a loss to either buyer or seller. In concentrating mills and leaching plants, samples are also taken of the tailings, and in smelters of the slag, in order to determine how much value is being lost. In concentrating mills, the different products of each machine are sampled, in order to know whether the machine is doing the work expected from it. In blast-furnace smelting, samples and analyses of the ores, fluxes, and fuel are necessary in order to calculate the proportion in the charge that will make the furnace run properly. Careful sampling is very often disregarded; but no furnace or mill manager can afford to guess at values when the exact knowledge can be so accurately obtained by sampling and assaying. This is especially true in these days of close competition, when values are being profitably saved that could not be recovered by the old methods. It may be stated, as a rule, that the best extraction cannot be attained unless checked by careful sampling and assaying.

2. Obtaining a Correct Sample.-To obtain a correct sample, a systematic method must be used. Selecting lumps of ore haphazard here and there will not answer, for however honest the sampler may be, it is impossible to

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judge the right proportion of rich and poor ore by the eye. The more thoroughly the ore is mixed and sized, the more certainly will a perfect sample be obtained.

3. Sampling is done either by hand or by machine; machine sampling, however, is seldom completed by machine, the final process being done by hand.

Although great improvements have been made in sampling machinery, metallurgical works usually do their own sampling by hand, while public sampling mills do their sampling by machinery, which seems to give satisfactory results to both buyer and seller. Public samplers really occupy the position of umpire between buyer and seller, the seller frequently believing that the buyer takes unfair advantage of him, especially if the latter is a public smelter or reduction mill. Miners have been known to send ore to smelters so lean that, if freight and smelting charges were added, the miner would be in debt to the smelter. The only way for a miner to be satisfied that he is getting full value for his ore is to sample his ore before he sends it to the smelter or else send it to a public sampling mill. pling is as important as assaying, and the hand sampler should have no interest whatever in the ore if he would obtain an average sample.

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4. General Consideration.-The method of sampling dumps or any large piles of ore depends on the character of the ore, the amount to be sampled, and the disposition that is to be made of the ore. If all the ore is to be moved, the first sample may be obtained by taking a certain portion of the ore as it is being moved, as, for instance, every fifth shovelful or every fifth car or wheelbarrow load. This is called fractional selection.

If the main portion of the ore is not to be moved, the first sample may be taken by digging trenches or channels through the mass and either taking all the ore from the channels or a certain proportion of it by fractional selection. This method is called channeling.

The first sample from a large mass may also be obtained by sinking shafts into the pile or by driving tunnels through it. The sample is sometimes obtained by taking small portions of ore from various parts of the surface of the pile. A sample taken in this manner is called a grab sample.

5. Grab Sample.-When it is desired to get an approximate idea of the value of a large ore heap, a sample is sometimes obtained by taking a shovelful of the material from various points, equally spaced, all over the surface of the heap. This method should be used only for materials that are pretty uniform in composition and of low value, such as iron ores, fuels, and fluxes. Even with these care must be used to take coarse and fine pieces as they come and not to take all lumps, for the fines are quite certain to differ from the lumps in composition.

An improved modification of this method is sometimes used when unloading iron ore from a vessel. When enough ore has been removed to expose a face of ore reaching to the bottom of the vessel's hold, small quantities of ore are taken from all over the face, the samples being taken in regular order from side to side and from top to bottom. When considerable more ore has been taken out, samples are taken from the new face and so on. This procedure has the advantage of taking portions from all parts of the heap instead of merely from the surface. The sample may be further reduced by fractional selection, by quartering (which will be described later), or by a machine.

6. Fractional Selection. When a large lot of ore is being shoveled from cars or elsewhere, a sample may be obtained by throwing aside every third, fifth, tenth, or twelfth shovelful. The richer the ore or the more unevenly the minerals are distributed through it, the oftener is a

sample shovelful taken. Each shovelful should be taken from the floor and from the bottom of the pile. When the ore comes in sacks and is of a fairly uniform character, every fifth or tenth sack may be taken for a sample. Fractional selection is probably the most accurate method of obtaining a sample from a large amount of ore.

7. Channeling.-Channeling, as applied to dumps or large piles of ore or other material, consists in shoveling channels through the mass and taking all or a portion of the ore from the channels as a sample. When only a portion of the ore is taken as a sample, the reduction is generally made by fractional selection; that is, by throwing every third, fifth, or tenth shovelful of ore from the channel aside as a sample. When sampling dumps by channeling, care must be taken to see that the ore from the sides of the cuts does not fall into the channel to such an extent as to give an unduly large proportion of ore from the upper part of the pile.

8. Size of Sample.—The size of the sample taken from a dump or any other large quantity of ore should depend on the manner in which the values are distributed through the ore. In the case of ores in which the values are uniformly distributed, such as iron ores, a grab sample may be all that is required or every twentieth shovelful may be taken; but if the values are not uniformly distributed, as in the case of ores carrying free gold or valuable minerals, the first sample must be larger and the ore must be crushed finer if correct results are to be obtained. In some cases the first sample must be at least one-third of the ore. After the first sample is taken it is reduced by fractional selection, channeling, quartering, or by a machine.

SAMPLING SMALL LOTS OF ORE

9. General Consideration.-The sampling of small lots of ore does not differ greatly from the sampling of dumps or large lots. In dealing with a carload or other small lot of ore, it is generally necessary to handle all the ore,