At present there are no machines that can practically take care of themselves for an hour at a time in a small plant with fluctuating feed. In addition there is the difficulty of training local labor to use a new process that is, to say the least, hard to understand.

In the Joplin district Missouri-Kansas-Oklahoma field-all the ore mined on a piece of land must be treated, under the leasing system in effect, on that land, and slime from several mills can not be removed to a common point for treatment. The landowners believe that the value of a piece of ground is increased by the presence of a mill, and as the land is generally leased in 40-acre tracts, the capacity of the mills is usually restricted to 100, 200, or 400 tons a day with about 3 to 10 per cent of slime formed by crushing. A number of such mills have tried flotation but, as far as is known, few of these have yet been able to run at a profit. Although the daily loss in any one plant is not large, yet as there are over 200 mills in the Joplin district, the aggregate losses of zinc in slime is enormous. Only a change in grinding methods, the development of a small automatic flotation machine, or the changing of the present leasing system in the Joplin district will permit the general use of flotation for the slimes of the district, and the saving of the zinc now lost in the "chats." It is highly probable that the development of a small unit will be easier than a change of the leasing system.

That the flotation of the Joplin slimes is not difficult was determined by a few tests in the laboratory at the Salt Lake City station. Material containing 8.54 per cent zinc, the sample being from one of the mills treating "sheet ground" ore, could be "roughed" and cleaned in either a mechanically driven or an air machine. "Rougher" concentrates containing 35 to 49 per cent zinc could be made with most of the commercial flotation oils, such as wood creosote, coal creosote, pine oil, and pine-tar oil, pine-tar oil seeming to be the best. The recoveries were 85 to 91 per cent of the total zinc. In recleaning samples of "rougher" froth containing 39 per cent zinc, it was possible to obtain a concentrate containing 52 per cent zinc. The addition of sulphuric acid in the flotation of this ore was not necessary, but as it made the line of separation between the yellow froth and the white gangue more easily seen through the glass sides of the test machine used, amounts of about 50 pounds per ton of ore were used for that purpose. Assays of concentrates made without addition of acid were practically as good as those of concentrates made with it. Thus there seems to be no technical difficulty in successful flotation of the zinc in the slimes of the Joplin district, although there is a serious doubt as to whether flotation can be made to pay in the present state of the art. The Bureau of Mines is not in a position to develop units of the size needed by the mills of the district, but there is evidently a good opportunity there for an inventor or a development company.

DIFFERENTIAL FLOTATION.

Differential flotation is the name given to that type of flotation in which one flotative mineral is floated in the presence of a second— the flotation of lead sulphide in the presence of zinc sulphide, for example. Some persons have called this "preferential" and others have called it "selective" flotation, but the term "preferential" has been used by the inventor of a patented process of flotation in which the ore receives a slight roast in order to deaden the surfaces of one of the flotative minerals, and the term "selective" has been applied to that type of flotation in which the oils and other flotation agents select the valuable minerals in the presence of the gangue, thus allowing the separation of a clean concentrate.

One of the most pressing problems of metallurgy at present is the treatment of complex sulphides of zinc, lead, iron, and occasionally copper. Differential flotation promises to be successful for some of these ores, and already a number of mills are using it. Two phases of the general problem are of major importance: (1) The separation of zinc and lead sulphides, and (2) the separation of zinc and iron sulphides. Possibly the latter is the more important, because the more difficult.

ZINC-LEAD SULPHIDES.

In general, mixed zinc and lead sulphides are separated by differential flotation, but usually the separation is only partial. The method in widest use is based on the fact that in a neutral or slightly alkaline solution, when an oil like pine oil is added in small enough quantity to a pulp containing both lead and zinc sulphides, galena is the only mineral that floats. Addition of more oil and acidifying the pulp causes the sphalerite to float. This method is only partly successful, and it is necessary to treat both concentrates by breaking down the flotation froth and passing the concentrates over tables in order to separate the lead in the zinc concentrate and the zinc in the lead concentrate. Even by this procedure the results are not always the most desirable, as 15 to 20 per cent of zinc is sometimes left in the lead concentrate and as much as 10 per cent lead in the zinc concentrate. Hence, differential flotation can not be regarded as a complete success. The authors know of only a few instances of better work. In fact, the authors, after running a number of tests to see if better work could be done in the laboratory, decided that there was more promise in making a mixed lead and zine concentrate, and then separating the two metals by chemical or other means. Work on the chemical separation of such mixtures is reported in other parts of this bulletin and in another paper.

Lyon, D. A., and Ralston, O. C., Innovations in the metallurgy of lead: Bull. 157, Bureau of Mines, 1918, pp. 123-169.

However, it is not impossible that better methods of differential flotation can be developed, and the Bureau of Mines intends to conduct investigations to that end.

ZINC-IRON SULPHIDE MIXTURES.

Zinc-iron sulphide mixtures offer one of the most pressing problems of differential flotation. In the past the grains of the two sulphide minerals, if too small to permit mechanical separation without fine grinding, could not be separated; nor was there any way of separating the blende and pyrite in the slime formed in the regular milling of zinc ores containing pyrite. Moreover, the two minerals can not be separated when mixed in a flotation concentration, as they have about the same specific gravity.

The only method that has proved at all successful is roasting at temperatures below 600° C., the "ignition point" of pyrite, long enough to destroy the sulphide surfaces of the pyrite particles without affecting the blende particles, followed by flotation treatment to remove the latter. The objection to this process is that in a gravity mill, where the fines are an incidental product, the fines must be dewatered, roasted, and then mixed with water for flotation.

For treating an ore that has never been milled successfully before, the installing of a process in which the ore is roasted before being mixed with water and ground for flotation is easy, for no mill product would have to be dewatered and dried before roasting, and only flotation would be used. Tests at the Salt Lake City station have shown that a simple roast to deaden the surfaces of pyrite does not suffice for coarsely ground ore, but by a longer roast the particles of pyrite are almost completely altered, much of the iron being burned to the red oxide without much oxidizing of the zinc. These tests showed, too, that sulphuric acid was almost essential in the flotation of the zinc from the roasted ore. Fine grinding after such a roast is much easier than before. This method has the advantage of a simple flow sheet for the mill. Its success, of course, is due to the fact that the roasting of zinc sulphide usually does not proceed much at a temperature below 600° C.

Addition of soluble oxidizing agents, such as potassium or other permanganates, bichromates, and chloride of lime, has been proposed by numerous inventors, but so far has been found of little value for most of the ores tested in the Salt Lake City laboratory. Usually the inventors propose to add these agents to the mill pulp in which the zinc and iron sulphides are suspended and to allow a sufficient time of contact before flotation. Whether the surfaces of one of the minerals is really oxidized seems debatable, as with mixtures of galena and blende the chromates cause the zinc mineral to float and the permanganates cause the lead mineral to float differen

tially. It is argued that if oxidation is all that takes place the two reagents should produce the same effect. Be that as it may, oxidizers seem to be among the most effective reagents for obtaining differential flotation.

In a number of flotation mills the zinc concentrate seems little contaminated with iron. In these mills either the pyrite has been naturally oxidized to a condition where it does not tend to float, or the slime (from a gravity mill) does not contain as much iron as the heading, because the pyrite slimes less than the galena and the blende. In a few instances, mostly tests on a laboratory scale, copper sulphate as an addition agent improved slightly the flotation of zinc sulphide from slimes containing some iron pyrite. Copper sulphate reacts with zinc sulphide, forming a coating of copper sulphide over the zinc and this is supposed to make the zinc sulphide more easily floatable. Although copper sulphate is excellent for correcting the poor flotation of zinc sulphide, it has not proved very acceptable for differential separation in at least one mill, although laboratory tests seemed to promise improved work.

A list of the most important American and British patents covering differential flotation is as follows:

United States patents: H. A. Wentworth, No. 938732, Jan. 2, 1909; E. J. Horwood, No. 1020353, Feb. 16, 1909; A. S. Ramage, No. 949002, July 2, 1909; E. H. Nutter and Henry Lavers, No. 1067485, Sept. 1, 1911; H. H. Greenway and A. H. P. Lowry, No. 1102738, May 17, 1913; E. J. Horwood, No. 1108440, Dec. 4, 1913; Henry Lavers, No. 1142821, May 28, 1914; T. M. Owen, No. 1157176, Feb. 27, 1914; G. S. Meikle, No. 1182290, Oct. 9, 1914; R. F. Bacon, Nos. 1198589 and 1197590, June 28, Nov. 10, 1915; F. J. Lyster, Nos. 1203372, 1203373, 1203374, and 1203375, May 8, 1913.

English patents: H. L. Sulman and H. F. K. Picard, No. 8650, Apr. 9, 1910; Henry Lavers, No. 23870, Oct. 14, 1910; H. H. Greenway, No. 11471, May 16, 1913; F. J. Lyster, No. 11939, May 22, 1913; Minerals Separation (Ltd.), No. 16141, July 12, 1913; Leslie Bradford, No. 21104, Sept. 18, 1913; Amalgamated Zinc (De Bavay's) (Ltd.), Ore Treaters, No. 9049, Apr. 9, 1914; Minerals Separation and De Bavay's Processes Australia Proprietary (Ltd.), Metallurgists, Nos. 19373 and 19374, Sept. 2, 1914; Leslie Bradford, No. 21880, Nov. 2, 1914; Minerals Separation (Ltd.), Metallurgists, Nos. 5650, 8746, and 10478, Apr. 14, June 14, July 19, 1915.

At the plant of a leasing company, which is working over the tailing of the concentration mill at the Midvale (Utah) smelter of the United States Smelting Co., a mixed concentrate of zinc and lead was separated from an ore, which also contains some pyrite, by the use of 8 pounds of lime per ton of ore. The pyrite was said to be dropped from the flotation froth, and the mixed concentrate of sphalerite and

galena was separated on concentrating tables. While marketable zinc concentrate can be separated and likewise marketable lead concentrate, the separation is none too clean and leaves much to be desired.

DRY CONCENTRATION PROCESSES.

There are three accepted dry methods of separating zinc minerals or zinc compounds from zinc sulphide ores, as follows: "Igneous" concentration, electrostatic separation, and magnetic separation.

IGNEOUS CONCENTRATION.

By igneous concentration is meant heating zinc ores with a blast of air to a temperature high enough to cause the zinc to volatilize and later be oxidized by the oxygen of the blast. This method is largely used in treating oxidized zinc ores and roasted sulphide ores. The usual procedure is to roast the sulphide ores to the oxide condition, mix them with fuel, such as anthracite or coke breeze, and place the mixture on a grate through which a blast of air is forced. The burning of the fuel heats the ore to a temperature at which the zinc can be reduced by the carbon or the carbonaceous gases. The zinc distills and is oxidized in the stream of air, the white zinc oxide produced, known as "American process" zinc oxide, being caught in a bag house. A discussion of igneous concentration and an account of some experiments made by the authors will be found in a subsequent section of this book.

The sulphide zinc ores now sent to igneous concentration plants are usually complex sulphides of zinc, iron, and lead which can not be separated by the standard methods of separation; hence the prices paid for these ores are usually lower than what would be paid for the several minerals if these could be mechanically separated. Many complex mixtures of zinc and iron sulphides not mechanically separable are used, the iron not interfering with the recovery of the zinc. Any lead present forms lead sulphate, which as fume is caught with the zinc oxide in the bag house, the white pigment being then known as a "leaded zinc" pigment. All of these sulphide ores are usually roasted and the sulphur dioxide gases are used for sulphuric acid manufacture. Consequently those igneous concentration plants that accept sulphide ores prefer the ores to contain some pyrite, because of its facilitating the preparation of gases suitable for making sulphuric acid. Direct "blasting" of zinc sulphide ore in making zinc oxide pigment is not in use in any part of the world, so far as the authors are informed. It is said that if sulphide ores are blasted without previous roasting, the pigment will contain too much zinc sulphate, which is undesirable. Also the zinc oxide absorbs sulphur dioxide with some avidity, which causes "livering" of the paints prepared from the