ECONOMICS
Flotation processes have a restricted range of application. The development has been, and is, so rapid that no one can prophesy what the end will be. Up to the present comparatively few minds have been working on the idea, but with the wider human effort which within a few years will be brought to bear, it is quite likely that the applicability will be considerably increased. There is reason to believe, also, that the forces of surface tension and adhesion could be profitably employed in industrial operations other than ore concentration, and especially in operations where coagulation will give rise to new combinations. BuDyancyflotation has already been applied to the separation of imperfect oranges from perfect ones, and also to the separation of wood fibre from guyaule rubber.
LIMITATIONS. There are, in the present state of our knowledge, four main limitations to the applicability of these methods. The first is that in general they are limited in their usefulness to those ores where certain valuable metals are in the form of sulphides, which need to be separated from worthless gangue. Some flotation patents have claimed to be able to treat ores where the minerals are carbonates or oxides, but these claims are at the present time over-optimistic, and so far there is not any demonstrated method of concentrating carbonates and oxides by flotation. It is the metallic sulphides, as distinguished from the carbonates and oxides, lor which the oils and gases used in these processes have a marked selective action. The carbonates and oxides are not without some adhesive quality, but this quality in them is so feeble on the one hand as compared with the sulphides, and so little differentiated from the adhesive quality of the gangue on the other, that it has been impossible so far to found a commercial process on this quality. This is an important point for research and invention, as it it quite likely that if a process was found by which the carbonates and oxides could be separated from gangue by flotation it would also obviate the second dominant limitation of the process. The most likely line of research at present in sight that would seem as if it might accomplish the desired end is along the line of soluble frothing agents, and especially the multitude of little known products from the destructive distillation of various woods ; a method which aims at coagulation of the mineral by means of slow agitation may also be beneficial. The second limitation to the usefulness of the processes is that in general they are not applicable to ores which contain more than 5% of those carbonates which are soluble in dilute acid. For the acid-flotation processes a small proportion of calcium carbonate is a sine qua non for success in order that the acid may have something from which to generate carbon dioxide gas. Those processes which use air as the flotation gas do not need any carbonate present ; it is a detrimental element in this case because it unnecessarily consumes acid. Four to five per cent, of calcium carbonate is the maximum required for acid-flotation processes ; more than this unnecessarily consumes acid. The ideal ore for the acid-flotation processes is the Broken Hill tailing from the table concentrators, having the following analysis :
%
Pb . .. .. .. . 7
Zn . .. .. .. .20
Fe . .. .. .. .8
Mn . .. .. .. .3
S . .. .. .. .14
C02 . .. .. .. .3
SiO2 . .. .. .. .42
CaO . .. .. .. .1
O, etc. . . . . . . 2
An ideal ore for oil-air flotation methods is that of the Guernica mine in Bolivia, which has the following analysis :
%
Cu . . . .. . . .3
Fe . . . .. . . .12
S . . . .. . . .10
Al2O3 . . . .. . . .2
SiO2. . . .. . . .72
O, etc . . . .. . . .1
This latter ore would require the addition of calcite in order to get any result with the acid-flotation processes.
Some ores which have up to 10% COa can be successfully treated by the oil-air flotation processes by adding a small amount of acid at the proper moment of flotation ; whereas an attempt to treat these ores by mixing them with a comparatively strong acid solution, such as is used in the acid-flotation processes, results in the consumption of all the acid in the circuit without getting the desired result. Some recent patents claim useful results without acid and the present practice at the Great Fitzroy mine in Queensland is of this order.
Ores like that of the San Francisco del Oro mine and the Avino mine in Mexico have proved difficult to treat, because these deposits occur in a calcareous country-rock, some of which is necessarily mixed with the ore. The former also contains fluorite, and this fluorite shows a decided tendency to float, thus contaminating the zinc concentrate with an element which the zinc buyers penalize most stringently. In this point, then, we have the third limitation to these methods that is, they are limited to those ores which do not contain floatable minerals that are deleterious to the concentrate. Besides the fluorite ore mentioned above, the mixed sulphide ores of Scandinavia fall into this class because they contain such a large percentage of iron pyrite, which floats, that the tenor of the concentrate is thereby reduced to a point so low that the product is unmarketable. The market demand for concentrate also bears on the cases of graphite and molybdenite. There has been a great deal of unjustified optimism concerning the supposed revolution flotation processes would bring about in the treatment of these two substances. In the case of graphite the fine crushing tends to destroy one of the best qualities of the graphite, because the best prices are paid for the larger grains. There is a very limited market for slimed graphite, and in any case there is always some gritty quartz in the flotation concentrate which is deleterious for the chief purpose for which fine graphite is used, namely, lubrication. Graphite thus falls within the third limitation. Molybdenite is the most easily floated of all the minerals, but a plant of sufficient size to pay a profit on mining and milling would produce enough concentrate in a year to supply the present demand for a hundred years. It is true that a cheap high-grade molybdenite concentrate might build up a new market by making that metal available for new uses, but not at the present price of the concentrate.
The fourth limitation is one for which at present no adequate reason can be given. An ore in which the valuable minerals are wholly or partly bornite or chalcocite, as those of Bingham canyon, will probably give trouble to flotation processes, although not always, for among the many ores tested the one which gave the most uniformly satisfactory results was a copper ore assaying 2.8% copper, all in the form of microscopic specks of bornite. The gangue of this latter ore was an acid dike rock, and it is just possible the bornite was an original constituent of the rock. There was no appearance in the ore to suggest oxidation, the whole mass being singularly free from fractures. The recovery on this ore was 95%, and the concentrate assayed 48% copper. It may be that only those ores where bornite and chalcocite are of secondary occurrence give trouble. It is true that good hand specimens of bornite, when finely crushed, can be made to float with ease. As a general rule, however, it will be necessary to carefully test copper ores which contain any bornite or chalcocite before pronouncing an opinion as to the probability of successful flotationconcentration. The Bingham canyon ore contains so little bornite or chalcocite that its presence often escapes superficial observation.
