An ore is a metal-bearing substance from which a metal, alloy, or metallic compound can be extracted at a profit. Ores are the aggregates containing the minerals of economic value. Gangue is the portion of the ore not desired or which must be wasted in the recovery of the metal. Gangue removed by a fusion is called slag. The gangue of one century or decade is not infrequently the ore of the succeeding period.
The economically valuable minerals are chiefly native metals, oxides, sulphides, carbonates, silicates, or chlorides, as grouped in Table III.
It is vitally important to distinguish an ore deposit from an occurrence of a few handsome mineral specimens. The universally distributed and often pure and massive sulphides of iron are never directly ores of iron; the stupendous amounts of fairly pure silicate of aluminum are not ores of aluminum; dolomite is not an ore of magnesium; sea water, although it may contain more gold than does a gravel which is actually worked, yet is not an ore of gold; strictly speaking, we have no ores of cadmium, palladium, or iridium, for these metals are recovered only as by-products in the working-up of other metals.
Again, we must keep in mind that, although the metallic content of a rock figured into dollars at market metal prices may amount to $50 or $75 a ton, yet that rock may be without value, either if the metals are inseparable, or some inconspicuous deleterious element is present, or if the cost of treatment is greater than the recovered values.
All modern metallurgical operations are under chemical control. Before a chemical analysis can be made, the material must be sampled properly. Obviously, then, accurate sampling is equally important with correct analysis.
The sampling of materials is found in all stages of metallurgical operations, from raw material to finished product. It is essential from the buying and selling viewpoint, for economy and recovery, for purity and composition of final product.
Sampling may be accomplished in the mine. There it will be done by sinking shafts, by churn drilling, by core drilling, by twist drilling, by exposure cuttings, by grab sampling, or by selecting certain units such as a small car-full or an entire load in a railroad car.
Fig. 6. Bruton Sample Cutter Courtesy of Amerion of Mining Engineers.
Many mines and metallurgical plants main-tab special mills in which systematic sampling is effected by scientific divisions after finer and finer crushings. If the material is already fine, a portion will be selected by coning and quartering, by fifth-shovel sampling, or by the use of stationary cutters. If the material is coarse and needs to be kept so, the sampling will be by selecting portions from a falling stream, crushing, and again cutting the stream. In this way, after four to six selections, a small sample is obtained which accurately represents the entire lot of many tons.
Fig. 5 gives, in outline, the operation of such a mill. The student should follow in detail the course of the material from the in-coming to the out-loading car. Note how many times the ore stream is divided before the sample finally is received in the safe. Observe the methods for reducing the size of the chunks.
The actual sampling cutter in this mill is seen in Fig. 6 just as it is placed in the mill. The single collecting hopper A guides the ore stream through the narrower channel B to above the fractionating oscillator D, which separates it partly into the discard spout E, and partly into the sample spout F. G is the oscillating shaft; K an eccentric gear; I a gear shift.
Fig. 7 illustrates a simple mechanism to effect the cutting-out of a sample on a much smaller scale. It is seen that the feeding spout has mechanical shaking to work the material in uniformly. The discard is cleared away on the belt conveyor, while the selected smaller sample is caught in the bucket. Inside the main chamber is a revolving segmented cutter which throws a large portion of the ore stream out through the large discharge, and throws a smaller but impartially selected portion into the funnel over the bucket.
In the crush-and-cut method of mill sampling three very distinct ratios have to be maintained properly. These are: (1) size of largest particle to total weight of lot, with an ample factor to allow for increasing in homogeneity; (2) number of selections to uniformity of the lot - the more dissimilar, the more cuts are required; (3) size of opening to size of largest particle - which properly may be about 10.
Fig. 8 pictures one of the most convenient laboratory cutters yet designed for fine dry materials. When material is poured into the hopper with a shaking motion, it will receive numerous cuts and be divided into two portions. The cutting then can be repeated until the sample selected is of small enough size.
Fig. 7. Braun Laboratory Sampler.
Where mechanical methods are not available, excellent results can be obtained by coning and quartering. This venerable method is discredited abundantly but is wonderfully serviceable when necessity demands.
In Fig. 9 is shown this practice in a large Mexican smeltery. One by one the piles will be spread out, until they are not over 10 or 12 inches thick; with a marker the lot then will be quartered, and men will shovel away opposite quarters as discard and again cone up the two remaining quarters. This will be continued, with much breaking of the lumps, until the sample is small enough to go to the laboratory for grinding and for further division on a cutter like that of Fig. 8. In the illustration the Mexicans in the background are at work shoveling the discarded quarters into the wheelbarrow. Such a sampling floor is kept scrupulously clean; the dust is kept down with sprinkling.
Fig. 8. McCann Sampler for Fine Dry Materials Courtesy of Mine and Smelter Smelter Company, New York City.
Well-mixed molten materials are sampled accurately by dipping out small units, each approximately of the right size for the chemist or assayer without further division.
Punch sampling of cakes, slabs, and ingots is much used. We have learned that all metals crystallize on cooling to assume the solid state; this means that the mother magma will be enriched with impurities and solidify last; in other words, all ingots show segregation. Sometimes the irregularities can be kept small; often they are surprisingly large.
Sampling a finished product by selecting and testing a few units is practiced commonly. Obviously this presupposes a uniform product; such sampling has the same inconclusiveness as there is in the case of grab sampling.
Fig. 9. Sampling Floor of a Mexican Smeltery.