‘Preferential’ flotation is a specialized application of the flotative principle in the separation of minerals from their ores. It gained its first wide use as a name for certain methods of floating minerals in connection with the Horwood process mentioned below. ‘Selective’ flotation has come to mean (by common consent) the flotation of valuable minerals (generally the metal sulphides) in the presence of undesirable gangue-minerals. ‘Preferential’ flotation is the flotation of one of the ordinary selectively flotative minerals in the presence of another similar mineral. Thus, a mixture of galena and sphalerite can be floated ‘selectively’ from a gangue of granite, limestone, or other common gangue-material, while galena may be floated ‘preferentially’ from a mixture of galena and sphalerite.

On account of the great interest manifested in this subject of late, I have thought that the following review of proposed or operating processes might be of interest. This review is largely a compilation of patent literature, but it might be well to call attention to the fact that, at present, patent literature is one of the best sources of information on the subject of flotation for one who does not have the opportunity to visit at first-hand the localities where the practice of flotation is being used or tested.

In 1904 Cattermole (U. S. Patent 763,259) made one of the earliest proposals for the preferential flotation of minerals. The method, as he described it, was not exactly a flotation method, but it involved most of the underlying principles of flotation, and hence is of interest in this connection. As stated by Cattermole: “The invention relates to the classification of the metalliferous constituents of ores which have been separated from gangue by oil or similar matter,” and “consists in fractionally removing the different constituents from the agglomerated masses by freeing the constituents in turn from oil, and thus obtaining them in a separable condition by the use of emulsifying agents of varying strength and activity, preferably in conjunction with an alkali.” “In carrying out the process, the metalliferous matter agglomerated fry oil is mixed and agitated with a solution of an emulsifying agent, such as a soluble soap-alkaline oleate, for example, to which a certain proportion of soluble alkali, preferably caustic potash or soda, has been added.” “It is found that the minerals vary in their affinity for oil employed in the above manner, and thus by treating the oily masses or granules in the first place with an alkaline emulsifying solution of a certain strength, the mineral of least affinity can be separated therefrom, and by increasing the strength or modifying the proportions of the breaking down solution step by step, the various constituents can be thrown out in the order of their increasing affinity.”

Cattermole’s patent came at a time when the process was truly ‘oil flotation,’ as the use of small amounts of oil had not been made successful, and the particular method of flotation which he had in mind in this patent was probably that described in one of his other patents, in which the minerals desired were flocculated or granulated by the use of oil in larger amounts than the present methods of flotation, that is, in amounts up to 5% by weight of the ore, and these granules would sink of their own weight in an upward moving current of water, such as that of a classifier, while the unflocculated gangue would rise. He made the wording of his patent, however, broad enough to cover the treatment of products as obtained by true flotation.

As an example of the working of his process he uses an ore consisting of a silicious gangue, zinc-blende, copper pyrite, and galena, which has been treated with an oil for the granulation of the mineral sulphide particles, and the latter separated. The oil is preferably one that is not readily emulsified, such as a hydro-carbon oil which will give a wide range of strength in the solutions used later in the breaking down of the granules. The compound granules are run into the first agitation apparatus where they are agitated with a solution containing, say, 0.75% alkali, by which the zinc-blende is “dropped out.” The remaining granules are passed into the next similar apparatus, in which a solution containing 1.5% soap and 1.5% alkali is used. Here the copper pyrite is freed and only the granules of galena remain. As Cattermole proposed the use of so much oil, it had to be recovered by the use of strong alkali solutions.

The rules for proportioning the solutions took into account the fineness of the ore, the relative proportions of the minerals, their physical condition and chemical composition, also the kind of oil and emulsifying agents used, and the alkali selected. The finer the ore the more compact and cohesive the granules formed, and hence the stronger the solution required to break them down. With granules largely of galena, which breaks down with difficulty, stronger solutions are necessary than for those consisting mainly of sphalerite. With animal or vegetal oils that emulsify easily the breaking down of the granules will be too rapid for convenience. The heavy residuum oils and the heavy hydro-carbon oils are the best. Oils may be blended advantageously for this purpose. An alkaline solution of the oil used in granulating is best for emulsifying.

Whether or not Cattermole’s process was ever applied is not known to me, but it is not impossible that its principles may be applied to modern flotation froths.

WENTWORTH. Following the U. S. patents in their chronological order, the next is No. 938,732, of 1909, taken out by H. A. Wentworth and assigned to the Huff Electrostatic Separator Co. It “relates to the separation of the ingredients which constitute ore mixtures, and particularly to the separation of sulphide ores from each other.” “The process consists in the preliminary treatment of ore mixtures containing several sulphides, which converts some of the sulphides, superficially at least, into metallic compounds which are differentiated in their behavior” with respect to flotation processes as commonly practised. To use the words of a later patentee, the surfaces of such minerals as galena, pyrite, and chalcopyrite are ‘deadened’ by a very short and slight roast in a roasting-furnace, while the sphalerite is unaffected. Thus the sphalerite can be removed by flotation from such an ore, leaving the other sulphide minerals to be removed by other means. A few minutes heating at a dull-red heat has been found to be sufficient.

This is a type of process that has been tried in Australia under the name of the Horwood. It is further described under that heading in this paper.

RAMAGE. The same idea underlies the next patent, which is No. 949,002, of 1910, taken out by A. S. Ramage and assigned to the Chemical Development Co., a Colorado corporation. “This process has for its object the separation of the valuable minerals from such ores as chalcopyrite, bornite, or erubescite, and mixtures of the same with pyrite, in which ores the copper is in chemical combination with the iron; and also from such ores containing zinc-blende. The method is also applicable to compound ores, such as those of the Cobalt district and other sulph-arsenides.” “The principle of the process is founded on the combination of fractional roasting with chemical floating.” Ramage’s introduction of the term “fractional roasting” is particularly felicitous, as it more accurately describes the method than does the term “preferential flotation,” used by Horwood.

Ramage described the process by the use of three examples, which are decidedly interesting. The first example is of an ore containing iron pyrite and chalcopyrite, with a content of about 5% copper and 30 to 40% sulphur. The ore is roasted at about a red heat long enough to decompose the pyrite slightly and not affect the chalcopyrite. “The burnt ore is then crushed to at least 15 mesh and passed through a solution of acid sulphate of soda and nitric acid (the solution being formed by adding nitric acid to sulphate of soda), which solution is kept near the boiling point. The copper sulphide immediately rises to the top of the bath and can be skimmed off.” The copper dissolved in the bath can be recovered in known ways. This method of flotation (hot acid bath) is not new, having been patented by De Bavay, Potter, Delprat, and others. The fractional roasting had been previously patented by Wentworth, and so the only thing that seems new is the combination of methods.

A second example is that of an ore containing pyrite, chalcopyrite, and zinc-blende in quantity. The ore is roasted at a temperature of not over 600° C., so that only the iron pyrite is deadened. The roasted ore is then subjected to the acid sulphate of soda solution for flotation of the unchanged sulphides of zinc and of copper. This product is then roasted at about 700° C. until all of the zinc sulphide is decomposed and the copper sulphide unchanged. This mixture is treated with a solution of dilute sulphuric acid for the dissolution of the zinc, to be recovered from solution by any familiar process, such as electrolysis, the copper sulphides being sent to the copper smelter. There are certainly most interesting facts disclosed in this patent. The great resistance of copper sulphides to the roasting process, as compared with the sulphides of zinc, is something new and will be a most valuable characteristic, if true.

The third example is that of the ores of the Cobalt district, Canada, where cobaltite, niccolite, chalcopyrite, pyrite, and native silver occur. All the sulphide and sulph-arsenide minerals are floated, leaving the silver in the gangue. The sulphides are roasted at about 800° C. and everything is decomposed except the copper sulphide, which can be floated from the calcine. Again we have mention of the almost incredible property of copper sulphides to resist roasting.

The next patent was that of H. A. Wentworth, amplifying on his former patent in claiming the superficial chemical change of minerals as a method of separating them preferentially by flotation. He had in mind particularly the treatment of the ore with chlorine, which would sink when subjected to a film-flotation process, while others would have their flotative properties enhanced. As an example, a mixture of zinc and iron sulphides, when treated with chlorine gas in a slightly damp state, is so altered that the blende will float on a film-flotation machine much better than before treatment, while the pyrite has a coating formed over its surface, which is much more easily wetted, so that it will sink. Still a further example is the application to the separation of pyrite and chalcopyrite. The latter is attacked much slower than pyrite; hence it can be floated when both are present. A similar behavior of the minerals is observed when they are suspended in water containing dissolved chlorine in the proper concentration, but the best work seems to be done with minerals fed onto one of the film-flotation machines, such as that of H. E. Wood of Denver, although Wentworth gives the design of one of his own in the specification. It is easy to see that with chlorine- water and one of the mechanical frothing methods of flotation the soluble coatings that are formed on the surfaces of the minerals would be simply washed off and the preferential part of the flotation lost. Tests in our laboratory seem to show this. So far as is known to me, this process is not being used.

HORWOOD. This process of preferential flotation is practically the same as that described under Wentworth and Ramage. It has been worked for some years in Australia and received careful testing by the Zinc Corporation. It depends upon the ‘deadening’ of galena and pyrite in a short roasting at 300 to 500° C., whereby the galena is coated with lead sulphate and the pyrite with iron oxide, while the sphalerite is unaltered. This allows a separation of the undesirable zinc from the lead-iron-silver product and allows their separate marketing. This process has received more careful attention than any other process, and reference to original articles is best. According to the data given in some of this literature, it appears that it is possible to take a flotation concentrate containing 36% Zn, 15% Pb, and 22 oz. Ag per ton, and divide it into a zinc product running as high as 50% Zn, 7% Pb, and 15 oz. Ag, and a lead product containing 38% Pb, 8% Zn, and 42 oz. Ag per ton. This is of great interest to all producers of ‘complex sulphide’ ores, as the milling of coarsely crystalline material has presented much difficulty in the past for the reason that some finely divided material (slime) is bound to form in crushing, and while the combined lead and zinc sulphides can be floated nowadays without much difficulty, the mixture is of far less value than the two minerals separated. This is important enough, not to speak of the possibility of treating the microcrystalline sulphide ores and those containing gangue of high specific gravity, such as barite. While flotation has been a boon to the concentration of all sulphide slimes, preferential flotation is much more important for the ores containing undesirable combinations of sulphides. Hence Horwood’s work should receive the highest praise.

Another detail, as regards this process, is that 35 lb. of sulphuric acid per ton of ore is necessary and 2 to 3 lb. of oleic acid for the flotation of the unaltered zinc. All of this appeared in Horwood’s first patent, No. 1,020,353, of 1912, and he later came out with improvements on the process in patent No. 1,108,440, of 1914. In this later patent he stated that he had found there was a tendency for the silver to follow the zinc, which is undesirable, but that this could be prevented by simply washing away all soluble salts on the concentrate before it was subjected to the deadening roast. This reduces the amount of oxidized zinc formed, and lost by solution in the dilute acid in the mill-water, as well as allowing the silver to become deadened to a greater extent. He also found that the most successful flotation took place with the pulp at a temperature of about 120° F.

It will be seen that the Horwood process has been applied only to concentrates from previous flotation or from other concentration processes. This is the logical place to apply it, as there is no object in leaving a non-flotative galena or other sulphide mixed with gangue, by using the process on crude ore. The same remark applies to many of the other processes. To be sure, there has been some success in the Australian mills as well as in the United States in the treatment of mixed galena-sphalerite concentrates from flotation machines on concentrating tables. As an example, the Timber Butte mill is treating the flotation concentrate of a zinc ore containing some zinc concentrate carrying 53% Zn, 1.5% Pb, and 4% insoluble. However, this method has not always met with the best results, and where the proportions of lead and zinc in ordinary complex sulphide concentrates are about equal it is quite hard to get two products that are sufficiently pure. Where it can be done, it is certainly more desirable than the more complex fractional roasting and preferential flotation processes of Horwood, Wentworth, and Ramage.

This is another process that has received consideration by the Zinc Corporation for the year or two preceding the European war. Lyster’s process is carried on in neutral or alkaline solutions (never acid) of the sulphates, chlorides, or nitrates of calcium, magnesium, sodium, potassium, or of their mixtures, or solutions of manganese, zinc, iron, acid sodium, or sodium-potassium sulphates. Using eucalyptus oil or a similar frothing agent, the agitation of the pulp takes place in centrifugal pumps, throttled to give further agitation, and discharging into spitzkasten with constricted tops. It is said that a galena froth can be collected carrying 55 to 60% lead and that by sending the tailing to a second machine with further addition of oil, the sphalerite can be floated.

It will be noticed that this, with the possible exception of Wentworth’s second patent, is one of the first proposals to give a true ‘preferential’ flotation to a mixture of sulphides, as the roasting methods above mentioned involve an actual conversion of some of the minerals, so that sulphide surfaces are no longer presented to the oils and air bubbles in the flotation operation. Lyster’s process, however, involves the actual flotation of one mineral in preference to another, unless the chemicals used are chemically altering certain of the sulphides so that they cannot float. Anyone who has worked with mixtures of sulphides has doubtless noticed that greater care is necessary in the flotation of zinc sulphide than in floating galena; in fact, galena is one of the most easily floated minerals outside of molybdenite, and zinc sulphide is considerably more difficult. The fact that a froth running so high in lead as the Lyster process is reported to give would also tend to make one suspicious that rather poor flotation conditions are maintained, so that only the most easily floated material (galena), and only the purest of that, is coming up in the first product. This takes place even in the presence of considerable oil, whenever flotation conditions are poor on almost any type of machine, and while the grade of froth that is obtained is high, the extraction is poor on account of the fact that only the best mineral is floating. It is possible that some such combination of results as this has caused the process not to be considered unfavorably.

NUTTER AND LAVERS. Perhaps the most important disclosure of a process for preferential flotation of minerals is contained in the patent specifications taken out by E. H. Nutter and H. Lavers, U. S. Patent No. 1,067,485 of 1913. This patent was assigned to Minerals Separation, Limited, as the patentees are engineers in the employ of that company. The wording of the patent shows more actual contact with flotation work on the part of the patentees than perhaps any other single patent that has been granted. They have observed that while controlling conditions in a flotation plant, the varying of certain of these conditions has been accompanied by changes in the character of the froth coming off their machines, the metals coming off in various ratios to each other at different times, and for definite causes. Thus there is considerable difference in the sizes of the different minerals separated under various conditions. It is no uncommon experience while developing the machinery of a flotation mill to float all of the fine part of the gangue as well as the sulphide minerals. In like manner, the more easily flotative minerals are liable to come off in the first froth that issues from a machine accompanied by the more finely divided portions of the less easily flotative minerals. “This tendency is dependent upon several factors, such as the amount and character of the agitation and aeration, or of each singly, the chemical constitution of the solution employed as mill-water, the degree of dilution, the temperature and the amount and nature of the different frothing agents.” “The word aeration is used in this specification to mean the supplying of air or other gas or gases.” By sufficiently controlling all of these factors it is possible to obtain effective separation of galena and sphalerite as well as other sulphides and metals. By taking the various froths obtained from subjecting the pulp to varying conditions, and classifying on apparatus such as concentrating tables it is often possible to get good separation of the minerals contained.

One example cited is that of an ore containing sulphides of lead, copper, and zinc. From this can be obtained a froth containing most of the chalcopyrite, and not much of the galena or the sphalerite, by the use of cresylic acid (cresol) without the addition of mineral acid to the pulp. This froth can be re-treated under varying conditions to purify it. To the mill-pulp that has been depleted of its copper can be added sulphuric acid as well as the frothing agent, to obtain the major portion of the lead, and, finally, by the addition of such an oil as oleic, it is possible to float all of the zinc mineral as well as any coarse particles of chalcopyrite and galena. The re-treatment of these froths by further flotation, or on tables, makes it possible to get good products of the grade demanded by smelters.

When using an ore containing only copper and zinc sulphides they state that with the use of cresylic acid or eucalyptus oil, without the addition of any mineral acid, it is possible to get a froth containing a portion of the copper minerals, some fine zinc, and some still finer gangue. They also state that if the remaining pulp is acid, the froth obtained will contain an additional amount of more coarse zinc and copper minerals and that the zinc minerals are finer than the copper minerals. If oleic acid is added to clean the tailing, the froth obtained will carry much gangue, but most of the sphalerite and chalcopyrite are very coarse-grained. The treatment of these froths on vanning machines or tables gives the desired products.

Consciously or unconsciously, a number of operators have applied methods more or less like those claimed in this patent. By restricting the amount of oil used, it seems to be possible to float galena in the presence of sphalerite, though the lead product obtained always carries a good deal of zinc, and it is impossible to get all of the lead out before the sphalerite is floated by the addition of further oil. This is practically an application of Lyster’s process, except that pure water is used instead of the solutions recommended by him. However, there can be no doubt that the addition of certain substances to the mill-water does help in this type of flotation. In another plant where an ore containing pyrite and chalcopyrite is being treated, the first froth contains most of the chalcopyrite in a finely divided form, while only a small amount of the pyrite, in large pieces, comes to the surface. The property of chalcopyrite to disintegrate into very fine flakes on crushing has bothered mill- men in the old days when the production of slime was kept down to a minimum. Now it seems to be an advantage. These two instances of “controlling flotation” conditions are somewhat different from the ones implied in the Nutter and Lavers patent, and it is doubtful if it could be held to cover these cases, at least more than in part. However, too much attention cannot be given to their patent, as it discloses the methods by which preferential flotation will be first developed successfully, as far as I am able to see.

GREENWAY AND LOWRY. A further development of the idea of using a solution of some chemical that will permit true preferential flotation of one mineral in the presence of another flotative mineral is contained in the patent of H. H. Greenway and A. H. P. Lowry, No. 1,102,738 of 1914. They discovered that ‘‘if a salt of chromium (such as sodium bichromate or potassium bichromate) is introduced in solution into the circuit liquors, or if the material to be treated is subjected to the action of such chromium salt solution by digestion or otherwise, the sulphides are affected in such a way as to leave certain of them amenable to flotation, whereby products are obtained relatively high in certain sulphides on the one hand, and relatively high in the other sulphides on the other.”

Three examples are cited: (1) A molybdenum ore containing 15% molybdenite and 25% iron pyrite was crushed to pass 100-mesh screen and treated in a froth-flotation apparatus with four times its weight of water containing 0.25% sodium bichromate, and heated to 120° F. One pound of eucalyptus oil per ton of slime was used and the flotation product consisted of 93% MoS2 and 4.9% iron pyrite. Attention should be called to the fact that this example does not tell as much as it would seem to say, for the reason that molybdenite is one of the most easily floated minerals. I believe that work of a character more nearly comparable with this result could be obtained without the use of chromates.

The second example cited is of a copper ore containing 6.5% copper and 35% iron. This, likewise, was crushed to pass a 100-mesh screen and digested in a hot solution of 1% sodium chromate for about 30 minutes, the liquor decanted and the mineral treated in a flotation machine with one pound of eucalyptus oil per ton of dry slime. The flotation product contained 19% copper and 30.2% iron, while the residue contained 0.7% copper and 36.2% iron. When we remember the case cited above of separating the chalcopyrite from the pyrite by virtue of the fact that fine grinding takes the copper down to a much finer product than it does the iron, we are led to wonder if this process is really necessary for this kind of ore. It may be that a better grade of product and a higher extration can be obtained by this method than without the addition of bichromate, but otherwise it is doubtless possible in most cases to do the same work with proper control of ordinary conditions.

Their third example is of a lead-zinc slime containing 18.6% lead and 32.3% zinc, which was digested for 30 minutes in a warm solution of 1% sodium bichromate. The solution was decanted and the material subjected to froth-flotation with one pound of eucalyptus oil per ton of slime. The flotation product contained 47.2% zinc and 6% lead, while the residue contained 31.6% lead and 16.3% zinc. The solution was heated to 120° C. These are interesting figures, but there is too much zinc in the lead concentrate. Here again I feel that the work is not much better than it would be without the aid of bichromates. By taking advantage of the fact that galena floats more easily than blende, it is possible to get a galena froth from an ore that will contain 54% lead and 15% zinc, while the blende-froth that follows will contain 37% zinc and 20%- lead. This type of work errs in the other direction, that is, in having too much lead in the zinc product, but the point is that the addition of bichromates does not make a separation which is any more advantageous than does preferential flotation by other methods. However, the fact that the galena can be deadened by treatment with a weak solution of sodium bichromate is most interesting in that it shows that a true preferential flotation is possible. It is assumed that the action of the bichromate solution must be that of oxidation of the surfaces of the galena to insoluble sulphate, while such an action on the sphalerite could not be possible, as the zinc sulphate would dissolve. The treatment of the high lead-zinc product of this last mentioned preferential flotation product by the bichromate process might be a useful method of cleaning this kind of concentrate. It is probable that successful preferential flotation will develop along such lines, though bichromates are not the only chemicals that will be used. While the results obtained by this process have been shown to be capable of duplication otherwise than by the use of bichromates, this fact of the peculiar action of weak bichromate solutions is thankfully accepted and further work is urged to discover if it can have a field of application peculiarly its own.

BRADFORD. Another process along these lines is that revealed in British Patent No. 21,101 of 1913, by L. Bradford of Broken Hill. He claims the use of a solution that will wet one of the sulphides which it is desired to separate from the other preferentially without chemically altering the same. A medium which will wet galena particles and allow sphalerite and pyrite to float unaffected is a solution of one or more of the alkaline chlorides, slightly acidulated and heated to about 120 to 160° F. On account of its low cost a solution of sodium chloride is used, and Bradford states that there are no definite requirements as to how concentrated the solution shall be, but suggests a 10% NaCl solution as about the right strength to use.

The acidity should be about 0.1 to 0.2%, for a higher amount than 1% will cause the flotation of galena on account of the formation of hydrogen sulphide bubbles on its surface. He states that if the process is applied directly to crude ores the use of a frothing agent is not necessary, although it will cause no harm if added. Any well-known apparatus can be used for the flotation.

The invention may be applied either to the crude ore or to the mixed flotation concentrate from the ordinary method of flotation. Where the plant is used on crude ore the tailings from the flotation of the sphalerite and the pyrite are agitated again in pure water with a frothing agent in order to float the galena. On account of this requirement it is thought better to make a mixed concentrate first by ordinary flotation and then separate preferentially as above described. This latter method, however, makes a higher-grade zinc concentrate.

When treating ordinary mixed flotation concentrate, it is best to remove the oil on the surface by the use of an alkaline or an alkaline carbonate solution, or by either. Further, finely ground material is liable to agglomerate too quickly, so that some of the galena will be entrained mechanically with the agglomerated sphalerite and decrease its value. In these cases it is desirable to add an agent that will retard flotation. Substances suitable for this purpose are sulphites or thiosulphites of alkalies or sulphur dioxide, but they must be used sparingly and with care, or they will entirely spoil all flotation. He thought so much of this latter step that he later incorporated it in a separate patent (Brit. Pat. No. 19,844 of 1914.) No further description of this patent is necessary.

There are many other proposed methods in the patent literature of England, Germany, France, and the United States, concerning which I am not fully informed, but it is believed that most of the important ones have been reviewed. Many interesting details are disclosed, such as the fact that galena and sphalerite will not float in a solution containing zinc chloride of the right concentration and acidified by hydrochloric acid (German patent, No. 282,234). Aniline compounds are said to allow flotation of galena in preference to sphalerite, etc.

It will doubtless be noticed that little mention is made of the kinds of machinery used in the above methods of preferential flotation and there will doubtless be some question as to whether or not these principles can be applied equally well in the mechanically-agitated and in the pneumatic-agitation machines. Most of the above processes where not specifically stated, have been worked out with the aid of mechanically-agitated machines, but it is possible to apply most of them to the pneumatically-agitated machines, such as the Callow or the Towne. Such machines, owing to the economy of power in making froth, and the easy control of flotation conditions, will doubtless materially assist in the development of preferential flotation.

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