In concentration by flotation, the soluble components of an ore may play an important role. Occasionally ores that are shown by preliminary test to be unsuitable for flotation may be treated by the process after the soluble ingredients have been removed by decantation. On the other hand, excellent results may be obtained on certain ores by flotation in fresh water; but when the water is fouled by successive contact with fresh lots of ore (as is often the case in mill-practice) the results may be far from satisfactory.
This article deals with a determination of the fouling agents in a certain ore, and outlines methods for overcoming such fouling efforts. Since all tests were made on ore from a single mine, the results cannot be regarded as generally applicable; however, it is hoped that the experience recorded here may be of some interest to others studying similar problems in flotation.
The tests were made on a silicified-rhyolite ore assaying silver 37 oz., gold 0.15 oz., lead 1%, copper 0.25%, and zinc 1.5%. The principal minerals were argentiferous sphalerite, argentiferous galena, and stromeyerite. The value lay almost entirely in silver. For this reason, only silver assays are here recorded. Sufficient analyses were made to indicate that the concentration of zinc, lead, and copper roughly paralleled that of silver.
Preparatory to making the tests, a large general sample of ore was ground to pass a 200-mesh screen, and thoroughly mixed. In each test, a 200-gram portion of the general sample was emulsified with one litre of water and 0.05% of crude pine-oil. The mixture was then treated for a half-hour in an experimental flotation machine, consisting of an agitation-chamber connected in such a manner with a concentrate-separation chamber, as to permit of repeated treatment of the tailing. Results in a flotation plant treating this ore roughly checked the work in the experimental machine.
Preliminary tests showed that when the ore was treated by flotation in fresh water, the tailing assayed 13 oz. silver and the concentrate assayed 440 oz. per ton. When the water used in the first test was removed by filtration and re-used on a second test, the tailing assayed 18 oz., and the concentrate 240 oz. When the same water was re-used a third time on a fresh sample of ore, the tailing assayed 27 oz.; the concentrate, 190 oz. Evidently some extremely deleterious substances had been dissolved from the ore. An analysis was made of the water filtered from the third test, with the following results:
Most of the soluble minerals were present as sulphates.
The next step was to determine the effect, on flotation, of the various sulphates.
Sodium and potassium sulphates, when added to fresh tests in the proportion indicated in the analysis, yielded a 13 oz. tailing and a 650 oz. concentrate; thus producing a marked increase in the grade of concentrate without detrimental effect on the tailing.
Manganese, magnesium, and ferric sulphates produced no effect when added in the proportions indicated; in larger amounts, magnesium sulphate was harmful and ferric sulphate beneficial.
Ferrous sulphate proved extremely injurious to flotation. When present as above recorded, a 20 oz. tailing and a 240 oz. concentrate were produced. When a small amount of copper sulphate was added to the same quantity of ferrous sulphate, the tailing assayed 25 oz., and the concentrate 200 oz. Evidently ferrous and copper sulphates were the principal fouling agents in the original tests.
It was necessary to devise means for correcting the effects of these sulphates.
First: sufficient sulphuric acid and hydrogen peroxide were added to a charge containing ferrous sulphate to convert the ferrous sulphate to the ferric state. The tailing from this charge assayed 11 oz., and the concentrate 800 oz. With the same amount of acid, but using no peroxide, the tailing assayed 14 oz., and the concentrate 780 oz. Evidently the acid increased the grade of concentrate and also decreased the injurious effects of the ferrous sulphate upon extraction.
Second: efforts were made to precipitate the ferrous and copper sulphate. A test, containing these sulphates in the proportions indicated in the analysis, was rendered slightly alkaline by the addition of lime hydrate. The tailing assayed 12 oz., and the concentrate 450 oz. When sodium hydrate was used in place of lime, the tailing assayed 8 oz., and the concentrate 600 oz. With the iron precipitated by sodium carbonate, the tailing assayed 6 oz., and the concentrate 800 oz. With a combination of lime hydrate and sodium carbonate, the concentrate assayed 800 oz., and the tailing 3 oz. Evidently the use of hydrates and carbonates produces much better results than can be secured by acid. The cost of sodium hydrate and sodium carbonate, and the injurious effects of the latter upon settling and filtration, restricts the use of these chemicals. Lime hydrate, on the other hand, presents a cheap and efficient means for preventing the accumulation of ferrous sulphate in mill- solutions. The calcium sulphate resulting from the reaction is somewhat detrimental to flotation; a saturated solution yielding a 16 oz. tailing and a 450 oz. concentrate. In ordinary practice the solution would be far from saturated, and the results much more satisfactory.
The lime-hydrate method has been successfully used on this ore in continuous mill-tests. During flotation the alkalinity was maintained as nearly as possible at 0.02 lb. lime-oxide per ton of water. After flotation, half the circuit-water was wasted, the remaining half being supplemented by fresh water, at the head of the mill; lime sulphate thus being prevented from accumulating in the solution. For a month during which the process was used, the concentrate from the flotation plant averaged 600 oz., and the tailing 6 oz. This compares favorably with a 20-oz. tailing from gravity concentration and a 12-oz. tailing from flotation in an acid solution. Aside from effecting better concentration, the lime method is cheaper than the acid process and is not injurious to subsequent cyanidation.
When lime is used in flotation, extreme care must be exercised in maintaining the proper alkalinity. The table submitted herewith shows that the best results, both as regards extraction and grade of concentrate, are secured when the alkalinity during flotation is extremely low (between 0.01 and 0.02 lb. CaO per ton of solution). Experiments indicate that alkalinity is beneficial to flotation but that the coagulating effect of high lime upon slime increases the affinity of the slime for the froth, lowering the grade of concen-trate. “When the coagulating effect is counteracted by the addition of sodium carbonate, or when sodium hydrate is used in place of lime, an alkalinity equivalent to a half-pound of CaO per ton of solution may be maintained without harmful effects on flotation.
Usually a bare alkalinity maintained with quicklime produces equally good results.
In order further to study the effects of copper sulphate on flotation, a solution containing 0.01% CuS04 was employed in another series of tests. When this solution was used alone the separation was extremely poor, the tailing assaying 32 oz. and the concentrate 100 oz. When sufficient quicklime was added to produce a slight alkalinity, the tailing was reduced to 24 oz., while the concentrate increased to 180 oz. per ton. A further improvement was effected by the use of sodium hydrate, a 340 oz. concentrate and an 11 oz. tailing being secured. This was still far from satisfactory.
An attempt was made next to precipitate the copper as sulphide. By employing hydrogen sulphide in a solution rendered alkaline by lime hydrate, a 6 oz. tailing and a 450 oz. concentrate were obtained. When sodium sulphide and sodium hydrate were used, the tailing-assay was reduced to 3 oz., and the concentrate increased to 700 oz. These results show that the injurious effect of soluble copper may be overcome by the use of hydrogen or sodium sulphide in conjunction with lime or sodium hydrate.
The following conclusions were established for the ore tested:
- Sodium, potassium, and ferric sulphates are rather beneficial to flotation than otherwise.
- Manganese sulphate has practically no effect on flotation.
- Magnesium and calcium sulphates are slightly harmful, while ferrous and copper sulphate are extremely harmful.
- The effect of magnesium and calcium sulphates may be overcome by the use of sodium carbonate in an alkaline solution.
- The effect of ferrous sulphate can be overcome by the use of sulphuric acid or, better still, by employing quicklime, caustic soda, or sodium carbonate.
- The effect of copper sulphate may be overcome by the use of hydrogen sulphide or sodium sulphide in an alkaline solution.
- The use of sodium carbonate, though aiding materially in flotation, is of doubtful utility in plants where the pulp must be dewatered.
- Lime hydrate is slightly less satisfactory metallurgically than sodium hydrate or sodium carbonate, but the use of it is inexpensive and aids materially in settling and filtering.