Pneumatic flotation is already fully established in a number of places and the results in comparison with the other and older schemes fully justify the opinion of your correspondent that it constitutes the most distinct advance in flotation in recent years.
The first pneumatic-flotation plant in this country was erected by me in February 1914 for the National Copper Company at Mullan, Idaho, a description of which has already been published. The results were fully up to expectations from the very start. This plant consists of 8 rougher and 2 cleaner cells and treated 500 tons per 24 hours. A 30-hp. motor furnished power for all the air necessary for both mixing and separating, and the oil consumption averaged as low as 0.13 lb. of refined pine-oil per ton of ore. It was an ideal floating ore. Since then the Callow scheme has been adopted by nearly all the other mills in the Coeur d’Alene for the treatment of their lead- zinc fine sand and slime-feed, and has been a means of simplifying their plants and adding greatly to their recoveries.
The other plants that I have since erected have followed the same general lines as were laid down at the National, namely, air or tube-mill mixing and emulsifying of the feed followed by the separator cells run in parallel. Recent results at the Inspiration mine would indicate some advantage (on that ore, at least) in running the cells in series of two instead of parallel, and without any sacrifice in capacity for a certain set number of cells. On an ore carrying a large percentage of mineral there is not much doubt that the series plan will give the best results and possibly dispense with the necessity for cleaning the concentrate.
The various elements that compose the scheme in general are illustrated in the accompanying diagram. The preparation of the pulp by any form of violent or propeller agitation is not necessary for good results with the pneumatic process. Where the oil can be introduced into the tube-mill no further refinement in mixing is necessary. This, of course, cannot always be done and in such cases air-mixing with the Pachuca becomes necessary.
A standard separatory cell has a capacity that will vary from 35 to 75 tons of feed per 24 hours when run in parallel, depending of course upon the characteristics of the feed. Two cells in parallel will have about the same capacity as two cells in series, but a slightly lower tailing may be expected with the series treatment.
The porous medium in the bottom of the cell is a loosely woven canvas-twill four layers thick, secured to the upper surface of a perforated plate with bifurcated rivets and washers. This is the present standard construction and has been adopted after a great many disappointing experiments with other materials. It is good for a three or four months’ continuous campaign, and becomes inoperative only through the filling up of the pores by dust that has been introduced by way of the blower. Four or five pounds of air-pressure is ample in all eases to give the proper aeration, the quantity of air varying from 6 to 10 cu. ft. per square foot of blanket- surface. When the cell is properly adjusted and doing its best work there should be no violent agitation with the cell, but only such agitation as is incident to proper aeration. Any violent agitation producing a surging of the liquid contents has proved detrimental to good results.
A slope of 3 in. per foot on the bottom has been found sufficient to treat all ordinary ores after crushing through 30 or 40-mesh. Treating coarser material or ores that contain a large percentage of heavy gangue (such as in the Coeur d’Alene), it has been found advantageous to increase this to as much as 4½ in. per foot to prevent the blanketing of the porous medium by coarse sand collecting on the bottom.
The air from the separatory cells has so far been furnished from positive blowers, but turbine-blowers would be preferable where the size of the installation justifies their adoption. An air-pressure of 15 lb. is ample for the Pachuca mixer and 25 cu. ft. of free air per minute is sufficient volume for mixing 250 tons of minus 48-mesh pulp, and of course should be furnished from a low-pressure compressor.
The density of the pulp may vary from 2½ :1 on a sandy feed up to 7 : 1 on a strictly slime-feed. The particular density is not a matter of so much importance as that the supply of pulp shall be uniform as to its density, since each variation in the density requires a readjustment of the oil-supply, the quantity of oil required increasing in proportion to the increased volume of the pulp independent of its solid content.
The advantages of the pneumatic scheme are greater recoveries,
less oil, greater simplicity, and less skill required to operate. The difference in wear and tear is obvious.
Proof of the increased recoveries is shown in Table I, which represents a 30-day competition. It is noticeable that this increase in recovery lies principally in the finer and slime portions of the feed.
The resulting concentrates from the same competition are shown in Table II, which indicates that the slightly lower grade of copper is more than offset by the lower insoluble and higher iron contents.
The proof of lower power-consumption is that the 10 cells of the National Copper Co. were run with a 30-hp. motor and treated 500 tons per day; the same tonnage treated by a propeller machine would have taken close on to 100 hp. This figure has also been confirmed at other places. The power required will vary from 2 to 3 kw.-hours per ton.
The froth produced from the pneumatic process is much more ephemeral than that produced by propeller-agitation machines, a self-evident advantage when it comes to collecting and handling the resulting concentrates.
Regarding oils, the remarks of your contributor with regard to cresylic acid and the methods of analysis given are extremely interesting and valuable. So far as my own experience goes. I have not found cresylic acid indispensable; at the present time and price it is out of the question as a flotation agent; it would seem, moreover, that the same results can be obtained by the use of less refined and expensive reagents. In the Coeur d’Alene on the zinc-lead ores, wood creosote seems to give the best results. The Inspiration uses a mixture of 80% El Paso coal-tar and 20% creosote. At one time we used a 2½ to 5% addition of pine-oil, but this has since been found unnecessary. A mixture of 20% pine-oil, 20% cresol, and 60% carbolic was tried experimentally at the Miami mine, but just as good results were obtained by substituting Salt Lake creosote for the cresol and carbolic. These are both chalcocite ores with some pyrite; the chalcocite is easier to float than the pyrite. The pulp is neutral. The recoveries will approximate 95% of the sulphide-copper contents.
I have tried a great many of the wood-oils, both the steam-distilled refined products and also the destructively distilled crude tars and creosotes, but have generally come back to the coal-tar mixtures. The pine-tar products are excellent frothers but the coal-tar products seem to act as collectors, and a combination of the two is often necessary. Acid sludge has been used with good success on Butte copper ores, but the disadvantages of this are that it requires heat and also additional quantities of acid to get the best results. This means working with an acid pulp and prevents the introduction of the oil into the crushing plant because of the destructive effect of the acid on anything but a silex-lined mill. Some ores work best in an alkaline pulp and others in a neutral one. My own opinion is that in most cases the same results can be obtained in alkaline or neutral pulp as can be obtained in an acid one and the advantage of an alkaline or neutral pulp is self-evident.
In one plant we had an interesting experience of this kind. The water-supply was limited and all the milling-water was returned back to the mill. The flotation results gradually deteriorated as the mill-water accumulated acid; the more acid it got, the poorer the results. Lime was then added in the tube-mill and the pulp made alkaline; this produced a tremendous increase in the volume of froth made, but without any definite improvement in the tailing. By reducing the lime and allowing the pulp to work back to the neutral point the results again became normal; and it is at the neutral point that we now do our best work.
There is a great deal of work yet to be done with oils, especially in compounding, modifying, and in making them miscible without beat. The right combination of oil is everything; the quantity per ton has less influence on the results than the right mixture. The effect with too much oil is mechanical rather than metallurgical; too much oil produces so much froth that it overflows everything and cannot be handled in the plant. Some experiments made in this direction gave the following results:
But this is a large subject and altogether beyond the scope of these random remarks.