Gold Flotation Tests

Gold Flotation Tests

The first gold flotation testing should be confined to an all-flotation method so that the experimenter can become familiar with the ore. Careful observation, coupled with experience in testing, will enable the operator to interpret the various phenomena.

It is desirable that he shall have had practical operating experience so that he may know the limitations of each step under consideration. Fortunately, it is generally true that, if the laboratory procedure follows sound operating practice, the flotation-mill results will equal or surpass those attained in the laboratory.

The simplest silver or gold flotation testing results when a single mineral or concentrate is desired. This is usually the case in copper ores when the copper occurs as one or more of the sulphide minerals. Such ores generally contain more or less pyrite. If the pyrite be present in such an amount that it would affect the grade of the concentrate, then differential methods of flotation are employed whereby the copper metals are caused to float and the pyrite is prevented from floating.

Differential methods are also used in separating two or three minerals such as galena and sphalerite or galena, sphalerite, and pyrite. When making differential separations, it is important to know the degree with which one mineral may be inter-grown or locked with another. This is determined by microscopic examination and chemical analysis.

The distribution of the precious metals or their association with different base metals should be determined, as this has an important bearing upon the net value of the concentrate produced. For example, gold or silver associated with a lead concentrate yields a greater net return than an equivalent amount in a zinc concentrate.

Within limits, the type of laboratory cell used is not a matter of great importance as far as obtaining indicative results is concerned. That this is so is shown by the fact that different operators using different types of laboratory machines with which each has become familiar will arrive at practically the same procedure. The important thing is to obtain a machine which is mechanically reliable and easy to clean and which permits quick work.

The amount of material taken for a test will be governed by the amount of ore available, size of machine, amount of floatable material in the ore, and various other factors. If the amount taken for a test is too small, any errors or effects of manipulation are unduly magnified. On the other hand, if the amount is too large, each test may consume too much time—not so much the actual flotation itself but the grinding or other preparation of the pulp and the drying and sampling of the products. Probably 500 to 1000 grams is the best all-round size of charge. If possible, however, several different sizes of machines should be available so that charges of 50 to 2500 grams can be used if desired, and more latitude is permitted for cleaning operation, especially where the ratio of concentration is high.

Where differential separations are made or several cleaning operations seem called for, the amount of pulp taken should be sufficiently large to allow the various operations to be made and to ensure resultant products large enough for assay.

Broadly speaking, the reagents used in flotation may be divided into three classes. Various writers on flotation extend the division, and the nomenclature differs somewhat. However, the three following classes are generally recognised and accepted by flotation operators without confusion:

    These include such compounds as lime, soda ash, sodium silicate, sodium sulphide, sodium cyanide, sodium sulphate, sodium bichromate, copper sulphate, and zinc sulphate, which are in general use. There are others less frequently used.
    Under this heading are found the various xanthates, dithiophosphates (aerofloat), thiocarbanilid, fatty acids, and others classed as chemical promoters and also certain oils and tars.
    This class embraces such compounds as the various pine oils, cresols, alcohols, and other petroleum and coal- or wood-tar derivatives.

It is not within the scope of this chapter to enter into a detailed discussion of the effects and use of the many reagents used in flotation. To those unfamiliar with the subject, the suggestion is made that they obtain from the manufacturers or distributors of reagents data concerning their use. Then they should consult various books and publications in which details of flotation-plant practice are described.

The U.S. Bureau of Mines, in cooperation with various mining companies, has issued a number of technical papers covering in great detail the flotation operations at most of the important milling plants in the United States. Publications of the various mining and metallurgical societies and technical journals contain many valuable articles. Manufacturers catalogues also are well worth study.

The amount of reagents used can be determined either by weight (dry reagent) or by volume (definite-strength solution), and the amount used is usually expressed in equivalent pounds per ton (2000 lb.) of original ore even though the operation involved is a cleaning step involving only a small percentage of the original feed. Lime is prepared as described under the sections on cyanide testing. Soda ash may be used either dry or in solution. If dry, it should be thoroughly dried to constant weight and kept in a stoppered bottle. The moisture content should be checked occasionally, as soda ash may absorb 15 to 18 per cent water and still appear quite dry.

Water-soluble salts such as zinc and copper sulphates and sodium cyanide should be used as solutions of definite strength—1, 5, or 10 per cent—and measured by pipettes. Xanthates are best used as 1 per cent solutions and freshly prepared each day as needed.

Oils are measured as drops from calibrated pipettes or medicine droppers. A dropper is used for each oil and is calibrated by weighing 10 or more drops. It is convenient with some oils—particularly steam-distilled pine oil—to have several droppers delivering different-size drops. This can be done by drawing out the tip of the dropper to a capillary and then breaking it off to obtain a small orifice. In this way it is possible to obtain a variation of 50 to 250 drops of pine oil per gram.

The determination of the pH value of the water of the pulp should always be made for reference, and the effects of changing the pH noted. A good procedure is to take a sample of the dry crushed ore and grind it with distilled water in a porcelain jar mill with flint pebbles. The pulp is filtered, and the pH value of the filtrate determined. At the same time the water is examined in more or less detail for soluble salts.

In order to obtain an idea of the grade of concentrate as well as the recovery that may be obtainable in practice, the following tests may be made:

Consider the simplest case where a one-mineral concentrate is desired. After a few preliminary tests so that the operator has a fairly comprehensive idea of such factors as degree of grinding, pH, reagents, and other pertinent data, two tests are made. In the first one such conditioning reagents as may be required are added, and then three or four successive froths are removed and kept separate, staged additions of promoters and frothers being used. The several froths and the tailing are dried, weighed, and assayed, and the distribution of the valuable mineral determined. All products should be examined under the microscope to obtain an idea of the nature and amount of any locked or true middling grains.

A comparative test is then made under the same conditions, but only one froth is made, the same kinds and amounts of reagents being used as before. After the tailing has been discharged from the cell and the cell cleaned, the froth is returned and refloated. Further addition of reagents to this cleaning operation may or may not be made; only the judgment and experience of the operator can determine. This operation of cleaning the froth may be repeated as often as deemed necessary. All products are finally dried, weighed, assayed, and examined as before.

The information so obtained may then be studied for the purpose of arriving at an idea of a possible flow sheet, and the advantages and disadvantages of the various types of circuits may be determined. However, all such deductions should be made with caution and regarded only as indicative of what to expect in plant practice.

As flotation tests progress, the operator will obtain an idea of the nature of the results being obtained by visual examination both of the froth and of the tailing. This is best done by vanning samples on a white enameled vanning plaque. The plaque is then placed under a microscope, and the product examined.

The froth sample is readily obtained by scraping it on to the plaque. The tailing or pulp sample may be drawn from the cell by means of a glass tube about ¼ in. in diameter and 12 to 15 in. long, the ends of which have been closed sufficiently to hold the pulp after the operator has sucked it into the tube.

Complete notes on every test are essential

The operator should also record in detail observations such as the character of the froth, the effect of each addition of reagents, and other phenomena. Nothing is too unimportant to be noted, so that months after a test has been made, the original experimenter or another can repeat the test and obtain substantially the same results; by reading the notes he can get an accurate and comprehensive picture of just what happened.

After the tests have been completed and a formal report is written, all pertinent data may be abstracted and assembled in a form designed to convey the necessary information, readily and accurately to the reader.

With some ores the removal of the slime portion before flotation is extremely beneficial. Therefore, should an ore be encountered that seems to be difficult to float by usual methods, the effect of removing the slimy portion ahead of flotation should be investigated. Practically, slime may be considered as that portion of the pulp finer than about 50 microns.

In practice the slime is usually removed by means of a Dorr bowl classifier or a Dorr hydroseparator or by centrifugal means, such as the Bird centrifuge. In the laboratory the usual method is to agitate the pulp thoroughly and then siphon off the suspended portion at a rate corresponding to the settling rate of the largest particle desired in the overflow. This may be repeated if two-stage operation seems called for.

Sometimes a “mud” or a “talc” froth may first be removed by flotation. Such froth will contain the greater part of the deleterious components with a negligible amount of valuable mineral. The tailing from this operation will then generally respond to usual methods of flotation.

The removal of slime from a pulp often markedly improves differential separations, although the results may not economically justify the added step. Caution should be used in recommending this procedure.

After the laboratory work has revealed methods by which it is believed the ore may be successfully treated by flotation, the work should be reviewed in the light of the relationship of flotation to other phases of the entire milling process. There is, for instance, the question of the economics of direct cyanidation versus flotation and cyanidation of the concentrates only, and such studies involve many other phases of the treatment as a whole.

For example, there are grinding, classifying, concentration (tables or blankets and other gold-saving devices), thickening, and filtering. In the case of ores containing gold or silver it may be found that a combination of flotation and cyaniding may yield a greater net return than either one alone. The effect of the various reagents on classifying, thickening, and filtering should be investigated, particularly if it is desired to recover the water for reuse.

The equipment of a flotation laboratory may range from something extremely simple to quite an elaborate installation. The minimum requirements include some means of wet grinding and a flotation cell. More complete equipment may include various types of dry-grinding machines, wet-grinding units employing steel rods, steel balls and flint pebbles, several types and sizes of flotation cells, testing sieves, microscope, air compressor, and vacuum pump.

The arrangement of the laboratory should be carefully considered so that there will be no lost motion in conducting a test. If possible, all wet work should be done on a table or bench covered with sheet zinc and sloping to a drain, so that any spills may be cleaned up readily.

Consumption of Flotation Reagents

The following information concerning reagents and quantities commonly used in the flotation of gold ores of no particular refractoriness is furnished by the American Cyanamid Company.

Addition agents

Cyanidation and concentration of gold and silver ores