Air Injected Froth Flotation

Air Injected Froth Flotation

The manifestation of the force of surface tension is a phenomenon that shows itself as a tendency of any liquid body—we may confine ourselves to a liquid—to assume that shape in which it has the least surface. It is a well-known fact that in the form of a sphere the ratio of surface to volume is at the minimum. Therefore we can say that surface tension is that force or property which tends to cause a body of liquid to assume the spherical form, in order to make its surrounding surface as small as possible.Gabbett

We are familiar with manifestations of this force; when a drop of water falls upon a hot stove, we see it immediately come into the form of a little sphere. The explanation of that probably is that the stove, being hot, generates a little steam all around the particles, and that frees it from interference by other forces, so that it assumes the shape which surface tension tends to give it.

I think the ordinary shot-tower, where molten lead when poured or dropped assumes the spherical form of shot is probably another manifestation of surface tension. The lead, instead of dropping in a formless mass as it passes through the air, under the influence of the contractile force around its surface is drawn up into a spherical body.

Another illustration is the tendency which we observe when water is spilled, we will say, upon a smooth surface or table. Were it not for surface tension it would spread out in an infinitely thin layer; gravity would tend to pull it down flat. But surface tension causes it to assume the form of a little bulge of water on the table. Viscosity of the water probably also plays a part in that. It is difficult to disentangle all of these causes. But surface tension surely is one of the forces to enter into that effect.

Now, this surface tension exists not only at the free air-surface (for instance, the surface of the water in this glass) but it exists at every point where there is a change of medium, that is, where the water encounters another substance. Surface tension here is along the water surface, the air surface, but that surface tension exists clear around the inner surface of the glass and at the bottom of the glass; it has the same relation to the water around the glass and at the bottom of the glass as next the atmosphere above. So this surface tension exerts itself about the entire surface of a body of liquid, tending to draw it into a spherical form.

As we observe a bubble—for instance, a soap-bubble—the idea that we are apt to have is that the bubble bursts from an interior force; that is, that it explodes. We are accustomed to that thought in connection with any explosion or bursting. I apprehend from the testimony of these experts that a lessening of this contractile force, or the surface tension, tends to permanency of a froth; from that fact I apprehend that the force causing a bubble to burst is not an expansive force from the inside, but that it is due to surface tension, if the surface tension is strong enough. For instance, imagine a soap-bubble, we will say, three inches in diameter. It is surrounded by a very thin film of water contaminated or modified by soap. The amount of water in that film which surrounds the air inside of the bubble is, as we may well imagine, very small. The ordinary soap- bubble bursting upon this piece of paper would hardly leave a visible sign of water. Now, as that bubble exists as a bubble the surface of that very small amount of water is very large. It is the entire outer surface of that bubble and the entire inner surface, where the film comes in contact with the exterior air and where it comes in contact on the inside with the enclosed air. MiamiNow, over that immense surface, for it is truly immense in consideration of the small amount of water, there is this contractile force, as if around the bubble there were stretched a sheet of rubber constantly drawing inward to make that bubble smaller, and the effort of that force of surface tension to reduce the area of the water forming the film of the bubble, simply pulls it in, bursts it, reduces it to a drop, which has the minimum of surface. So I apprehend that the reason the bubbles in these froths burst is on account of that shrinking inward, that tendency of surface tension to gather the water into the smallest possible compass.

The experts who have testified in this case say (and their views are in harmony with the literature on the subject) that any substance which tends to lower or lessen the surface tension of water tends to make the bubbles or the froth more persistent or permanent; and in view of what I have said, I think the reason why these modifying agents which lower the surface tension of water also tend to make the froth more permanent is clear. In the case of pure water, having a surface tension which I think is—well, it is arbitrarily represented by some numeral—we can say 1. It makes no difference. Now, that force of surface tension in clear water is strong enough to burst these bubbles. It pulls in and bursts them. But if the water is modified by some agent, such as an oil in emulsion, or a soluble substance, such as phenol or cresol, that contractile drawing-in force is lessened, and therefore the bubble has greater longevity, can exist longer, because there is not this constant pulling in. So, while this surface tension manifests itself in various ways and has been utilized in various ways, so far as this bubble flotation or froth flotation is concerned, I think it is clear why a lowering of that surface tension tends to permit a bubble to exist longer. And all of the experts in this case are in perfect agreement that an insoluble oil mixed up, or emulsified, lowers the surface tension in precisely the same way that a dissolved substance does.

The phenomenon of the flotation of small particles upon the surface of water, as upon the surface of water in that glass, has been referred to as surface tension flotation or skin flotation. That is a matter of arbitrary nomenclature. The surface tension effect does not enter into that effect any more than it does in bubble flotation. This stretched membrane, as we picture it, surrounding a body of liquid and tending to draw it into a small compass, also has the property of supporting a small particle upon the upper surface of a body of water, but the name ‘surface tension’ should not properly be restricted to that kind of a flotation, because the surface tension phenomena enter into all flotations, and it is the lowering of that surface tension that leads to the formation of these froths which have more or less permanency.

Besides these various prior art patents and publications that I have referred to, in which the lowering of surface tension is utilized for the purpose of giving permanency to a froth or a bubble, we have other patents, I believe owned by the complainants in this case, patents issued to scientific men, technical men, the officials connected with these companies, in which they also explain the use of both soluble and insoluble reagents for the purpose of contributing efficiency to a bubble or froth process.


“Our process has for its object the separation of minerals from silicious or earthy matters of ores by means of soaps or similar compounds and is dependent upon the superior physical attraction exhibited by minerals for fatty or resin acids, or for certain other aromatic derivates, such as phenols, cresols, etc., which form soluble salts or compounds with alkaline hydrates.”

Then upon the first page of that same patent they state:

“The mineral particles now attached to or more or less coated or enclosed by films of fatty or resin acids and the like, are capable of being separated from the gangue or earthy particles by various methods, depending upon this altered physical condition. For example, the coated mineral particles may be removed by generating gaseous bubbles in the mixture, which preferentially attach themselves to the fatty or similar acid coated particles and raise them to the surface of the pulp, whence they may be removed by skimming or the like.”

There is a clear statement of the use of soluble agents, the very soluble agents which are mentioned in the patents here in suit, with their use in connection with gases for raising them to the surface, and the only way in which a gas can function is as a bubble, and this effect of lowering the surface tension was there brought about by the same substances which are in use today.

In patent 793,808 which is the patent disclosing the perforated spiral coil that we have had so much discussion about, the patentees state:

“The present invention relates to the concentration of ores by separation of the metalliferous constituents and graphite, carbon, sulphur and the like, from the gangue, by means of oils, grease, tar, or any similar substance which has a preferential affinity for metalliferous matter over gangue.”

The tar there mentioned is one of the substances in use today. Coal-tar is the principal source, I think, of phenol and cresol, and it is used in a crude state in flotation operations. I think it is one


of the substances which the answers to the interrogatories say has been used by the Miami company. It is partially soluble and partially insoluble. It is a mixture. And then this patent, after naming these substances, oil, grease, insoluble substances, and then mentioning in the same breath tar which is partially soluble and partially insoluble, the same as is the frothing agent used at Miami, after explaining the use of these substances goes on to say:

“According to one method of carrying out our invention suitably crushed ore is suspended in water. To this suspension a proportion of oil, grease, or tar (hereinafter referred to as ‘oil’) is added and duly mixed with the mass by any suitable means in quantity insufficient to raise the oil mineral by virtue of the flotation power of the oil alone. A suitable gas is now generated in or introduced into the mixture, such as air, carbonic acid gas, sulphureted hydrogen, or the like.”

Now here again we have a process in which a soluble agent is used. Tar is not completely soluble, but the complainant has taken the position, which I will accept for the purpose of argument at present, that if any constituent of a substance is soluble, then the substance is a soluble agent under the second and third patents in suit. Accepting, for the sake of argument, this construction of these patents, we have here disclosed, down to the minutest detail, every operation that is performed at Miami. We have tar, a mixture of soluble and insoluble agents; we have the admixture of that substance with the pulp; we have the introduction of that substance into a vessel provided at the bottom with means for the admission of air; that means being this perforated spiral coil.
MR. SCOTT : If the Court please, just before the recess I was speaking of the spiral coil-pipe machine, the perforated spiral, and had stated that this process was identical with the operations at Miami. In fact, I think it will be difficult for anyone to conceive of any different action on the part of air bubbles escaping through fine holes in a metal pipe or a sheet of metal, and the same bubbles escaping through similar holes in a canvas bottom. I do not think the thing needs any argument, the two processes are absolutely identical.

An attempt has been made to establish the appearance of identity between that fragile mass of bubbles which results from the Miami operation and from the patent 793,808, and the persistent froth that results from the violent agitation of the patents in suit.

Now certainly to the eye there is no identity whatever. And going further, looking to the real essence of the two operations, we find as great a distinction as there is in the appearance. The agitation froth results from violent agitation of the pulp, beating the air into very fine particles and then bringing the liquid to rest; as the Court has seen here in court, when the agitating mechanism was stopped, there arises this persistent froth which lasts for days. Even when shaken in a bottle in accordance with the directions of the California Technical Journal , we showed the Court a froth that had stood for some two weeks, I think Mr. Dosenbach testified. That froth was so strong that it remained as a bridge across the bottle, even after the water had evaporated out from beneath it. I think that matter was called to the attention of the Court, that the stopper had been left out, and that froth was so strong that it simply adhered to the sides of the bottle, and bridged across the bottle without any support whatever from below. That is characteristic of all these froths that are formed by this violent agitation.

Now contrasting with that we have what was exhibited to the Court in many experiments with the canvas-bottom machine, our Exhibit 53, and in the perforated spiral-pipe machine, our Exhibit 52.

The first difference that strikes one is that in this agitation process the liquid must be brought to rest before the froth will form. The froth will not form in the presence of and simultaneously with that violent agitation. The agitating mechanism either must be stopped, or the liquid must be conducted into a side vessel where it will be quiet. As it was exhibited in court, the agitating mechanism was stopped, that being merely for convenience in demonstration. Both complainant and defendant did it in that manner. Of course, that would make the operation intermittent if it was applied in practice. It would simply be agitated, and stop, and take off some froth, and then take more material, and agitate again. As I think the Court is informed, in practice the pulp flows in a continuous stream through these agitating vessels and then into quiet vessels where there is no agitator, vessels called ‘spitzkasten,’ and the current as it flows along is so slow and gentle that the froth rises in these quiet spitzkasten after having been previously agitated in the adjoining agitation vessel. And that is a sine qua non of this agitation process: that the pulp be subjected first to violent agitation and then be brought to rest for this coherent froth to rise.

Now, it is equally of the essence and vital to the process carried on at Miami that just the opposite conditions prevail. In the process as carried on at Miami the bubbles which carry the metal concentrate to the surface can rise and can exist only in the presence of these


incoming streams of air from the bottom of the vessel. The complainant has contended that the mere gentle entrance of these bubbles at the bottom of the vessel is the equivalent of the violent agitation which forms a vital and essential element in the process of the patents in suit.

There again to the eye there is no similarity and no equivalency. In the agitation-froth process of the patents in suit—and they are all alike in the mechanical treatment of the pulp by agitation—we have a mass of water that is beaten into a perfect vortex or maelstrom, as violent a movement as we can conceive of; whereas in the Miami process where the air is admitted through a permeable bottom, we have no more agitation than one would observe in a glass of charged liquor, soda water, or champagne. There are simply the rising bubbles coming through the liquid. So far there is no similarity.

Look at the principle of the thing. There is an even greater dissimilarity. In the first place, as I have just stated, in the agitation- froth process the froth or bubbles can rise only after the agitation has stopped, or after the pulp has been conducted to a quiet place away from the agitating vessel.

In the Miami process the moment the influx of air stops—and air is what is contended to be the equivalent of the agitation, that is, the incoming stream of air—the minute that stops in the Miami process, the entire body of bubbles carrying the concentrate collapses, and I think your Honor has a vivid impression of that demonstration in which Mr. Yerxa and Mr. Hunt first turned on the air in that spiral-coil machine, and in the canvas-bottom machine, and built up this mass of bubbles, and then suddenly turned the air off whereupon this all dropped.

Now, looking at the matter of equivalency, it seems to be an impossible construction of the facts and law to urge that the incoming air-streams in the bottom of the permeable-bottom cell, whether it be the spiral pipe or the canvas bottom, is the equivalent of the agitation, when their action is precisely opposite in respect to the formation of the froth. In one case the froth forms only when the agitation ceases. In the other the floating or rising bubbles exist only while the so-called agitation is going on. The principle of the two things is as different as the manifestation of that principle. The manifestation differs in this respect that I have pointed out. In one the froth rises when the agitation stops. In the other the bubbles can rise only when this so-called agitation is going on.

Now as to the principle. The two processes attack the problem in completely different ways. In the agitation-froth process the thought is so to treat this pulp by violent agitation that a froth will form and exist after agitation, that froth to be of so permanent and lasting a character that it can be manipulated, can be floated or skimmed off, as the Court has seen witnesses do in this case. The idea there was by this agitation to effect a separation more or less permanent between the gangue and the concentrate, to get the gangue at the bottom and the concentrate at the top—to stratify them, as it were. We have the two strata with an intervening stratum of water. And then to take off that froth in any way which may be convenient, either by simply flowing off or by skimming, as has been explained by complainants’ witnesses in that instance where they had a revolving paddle something after the fashion of the stern-wheel on some of the river-steamers. That paddle would revolve and scrape off this froth.

In the Miami process the mode of attack upon the problem is completely different. There is no idea in the Miami process of stratifying these materials and making a permanent float, which can be scraped off or floated off. The thought there, and the process as actually carried out, is to admit at the bottom of the vessel containing the pulp with the agent that is used, a stream of air bubbles which act as Dr. Liebmann has said, simply like an upcast. These air bubbles, rising by the force of gravity through the water, collect the mineral particles by reason of the fact that they do collect them. That is about all that we can say, that the mineral particles adhere to these bubbles while the gangue does not.

Then the bubbles come to the surface of the water and break almost instantly. There is a constant succession of breaking bubbles, but the influx of air at the bottom of this vessel manufactures these bubbles at a slightly more rapid rate than they break, and for that reason the upper layer of bubbles is overflowed from the top of the vessel and saved, with their burden of mineral. The only reason that the Miami process is a success, or that the process of patent 793,808 is a success, is that it is possible by these rising bubbles to make the bubbles a little faster than they break. If they broke as

Now, where in the complainant’s process does the bubble attach itself to the mineral—below the surface or above ?
I think that must be below the surface, too fast as they were made, we would never have any appreciable amount of bubbles on top. They would simply break, each one in time for the next one to come up; but as it is there is a small interval of time before they break, and more bubbles are being formed at a rate so rapid that some of those bubbles are raised to the top and carried over the top before they have broken, and that is the only reason that it is possible to concentrate ores in that way.

The frothing process, on the other hand, is not dependent upon any such principle at all. The thing is simply agitated violently, and when the agitation stops, this froth rises and floats much the same as a board would. It may not be as long-lived as a piece of wood, but it lasts for weeks, and it simply rises there and floats. Speaking with regard to the purpose of concentration, it is permanent, within the limitations that permanency is necessary or desirable.

Now in view—
THE COURT- Let me see if I get your idea. Do you draw a distinction between a process which results in the formation of what is termed a permanent froth, and a process in which the bubbles come up in rapid succession, but not in such quantities or in such close proximity as to form a permanent froth, but as soon as they get to the surface they float away; is that what you mean?
MR. SCOTT: They float over, if we get them over before they break. It is a kind of a race.
THE COURT : If you get them over before they break? But suppose they break before you get them over, what becomes of the mineral ?
Mr. SCOTT : Then, as is shown in these demonstrations, if they do break, the mineral drops, and it is caught by the bubbles below.
THE COURT: And brought up again?
Mr. SCOTT : And brought up again. The bubbles must be brought up fast enough so that it will gradually be raised.
THE COURT : That is, above the surface,
MR. SCOTT : Yes. There must be new bubbles coming fast enough so that it is gradually carried up over.
THE COURT: And they float off?
MR. SCOTT: And finally float off.
THE COURT : That is what I meant. I did not express it in that way. You expressed it in two stages, a first mass of bubbles and a succeeding mass of bubbles, but that was what was in my mind.
Mr. SCOTT : The same idea, I think.
THE COURT : That there is no permanence ?
MR. SCOTT: NO permanence in the bubble process at Miami. Now, if the Court remembers the mass of bubbles was probably about 6 inches high above the water, as I remember.
MR. KENYON: Ten or twelve inches high.
MR. SCOTT: Ten or twelve inches high. It was quite high. Now, if a particle should be allowed to drop by reason of the bubble
breaking, it would be caught on a bubble below, and thus constantly raised up by new bubbles coming into the bottom, so that it is, as you might say, a case of stepping back an inch and going forward two inches, and gradually getting over, despite the breaking of the bubbles.

The term ‘flotation’ seems inaccurate as applied to this Miami process. In the agitation-froth process, after the agitation is stopped, this froth actually does float. It will float for hours, and days, and weeks, and stay on the top of the water; but in the Miami process, as soon as the incoming current of air stops, everything drops, and we have the clear water-surface on the top of the cell. Now, that demonstrates absolutely that that mass of bubbles several inches deep, which we see in the Miami process operation, or that of the patent 793,808, is not really floating. It is held up there by the current of air, which holds it in place. That current of air not only manufactures these bubbles so fast that we have that mass of bubbles there, but it actually holds them up in position, and the minute the air is turned off everything drops. The mineral goes right to the bottom, and the bubbles break. So that instead of a floating mass of concentrate, it seems to me that it is best described, as I think I did once before in opening the case, by saying that this Miami operation is similar to these devices we have seen, where a stream of air is blown out of a pipe and a ball floats above it. The minute the stream of air is turned off the ball drops. In popular language, we may say that ball is floating in the air, but obviously that is a misnomer, if we attach any exact meaning to our words.Drawing

Now, the Miami process is analogous to this ball held up on that stream of air. The agitation-froth process is the ball floating in the water. The two things operate upon absolutely different principles, and the difference is so great that it cannot escape anyone’s notice.

Dr. Liebmann has characterized this Miami cell, or Callow cell, as he calls it, as an upcast, and the figure of speech is very happy. We took an ordinary upcast with water, such as that which was used for separating the Cattermole granules; and the Court will well remember that the mixture, the granules and gangue, was brought downward into a pipe, and water was flowing upward in the pipe, and that upward stream of water carried off the light tailing, and the heavy granules sank to the bottom. Now, ‘upcast’ is the term that is ordinarily used, and the action which takes place in this Miami process is absolutely analogous to that. It is not a floating operation. It is an operation in which, in a rising current of air, gravity is strong enough to pull some of the particles down, but the other particles are of such gravity and shape that the rising current of air carries them up against the force of gravity.

THE COURT : Let me ask you, where is the gangue separated from the metal?
MR. SCOTT: In the Miami process?
MR. SCOTT : The gangue comes out at the bottom and the mineral is carried over the top.
THE COURT: Yes. How is that the result of this upcast?
Mr. SCOTT: Of air?
THE COURT : What is it that separates the gangue from the particles ?
Mr. SCOTT: The rationale of the thing is evidently this: We have in this cell, or tank with the porous bottom, water carrying in suspension both gangue particles and mineral particles, and the mixture has been treated with some of these agents—tar, or coal tar, or what not—mixed up with it. In the first place, those air bubbles attract to themselves the mineral particles and do not attract the gangue particles; so that at that stage of the operation we have in the water a series of bubbles with the mineral particles sticking to them, and we have the gangue particles free in the water itself. Now, those bubbles rise, of course, by gravity and carry with them those mineral particles. The combination of the bubble and the mineral particle is together lighter than water, so it goes to the top.
THE COURT : I understand.
MR. SCOTT: And the gangue particle has had no assistance whatever from the air. It is still heavier than water, the way it always was and it goes to the bottom.
THE COURT : I understand, then, that in the Miami process the bubble attaches itself to the mineral below the surface.
Mr. SCOTT: Below the surface, yes.
THE COURT : And carries the mineral—
THE COURT : And it carries it up, does it ?
MR. SCOTT : It carries it up to the top.
THE COURT : Is it not pretty much a question of froth, rather than of concentration—the difference between the two processes? I understand that the purpose of the patents is to effect not a froth, but a concentration.
Mr. SCOTT :Precisely.
THE COURT : A separation ?
Mr. SCOTT :Precisely.
THE COURT : Now, you may assume that I do not know anything about this. I want to have it put to me as plainly as you can express yourself. What is the distinction in principle between those two processes? You have explained the difference in point of actual operation. Now, what is the distinction in principle, when it comes to the formulation of any principle, between these two operations, as bearing upon the question of the separation and saving of the metal particles ?

MR. SCOTT: The broad principles are the same in both. In both we have the pulp, consisting of ore held in suspension in water. In both the water is modified to lower its surface tension. In both the buoyancy comes from air bubbles. The difference comes after the air bubbles have attached themselves to the mineral particles. In the agitation-froth process the air is beaten into very minute bubbles, and when they rise with these mineral particles they form this permanent froth. The permanent froth is then floated off or skimmed off. Now, in the Miami process there is no beating up of the liquid, of the pulp. The bubbles are larger and more fragile, and instead of forming a permanent froth, which will float, the thing is simply pushed off by the current of air. The basic principles are the same in both of them. The method practised at Miami is the older of the methods. It is the method of the patent 793,808, with the perforated spiral coil.

Now, speaking so far as patents go, departing from the Court’s question as to the general principle of the thing, which is identical up to the point I have explained—departing from broad explanations and approaching it from the patent side purely, the patents in suit must necessarily, if they have any validity whatever, be restricted to this permanent froth formed by this mechanical agitation, or they must confessedly be invalid by the prior existence of the perforated coil-pipe machine.

THE COURT : Let me ask you another question.
MR. SCOTT: Certainly, I am very glad to have you do so.
THE COURT : When was the perforated-coil machine first used ?
MR. SCOTT :As far as the record in this ease shows—
THE COURT: I mean with respect to the first patent in suit.
MR. SCOTT : The date of the application was about a couple of years before, and I think the patent was granted about a month before the application in this country. I will give you the exact date.
THE COURT : I mean, was it before ?
MR. SCOTT: Oh, absolutely.
THE COURT: Before the first patent in suit?
MR. SCOTT : Before the first patent in suit.
THE COURT : Now, let me ask you, what were the ingredients that were used, and what proportions were used ?
MR. SCOTT : In this patent the ingredients—and, of course, in all of these—are the ore-pulp in the first place, the water and the ore.
MR. SCOTT : And the ingredients for effecting bubble formation in this patent 793,808 are oils, grease and tar, or any similar substance, which has a preferential affinity for metalliferous matter over gangue.
THE COURT : And in what proportions ?
Mr. SCOTT : The proportions are stated in the patent in this language—there are no figures—in line 89 of the first page of the patent:

“We have also found that a particle of metalliferous mineral, if coated with a minute film of oil, grease or the like ”
And so forth. That expression, “minute film” certainly gives us indication of a very small quantity, because the word ‘film’ without any qualification whatever, conveys to the mind the idea of a very small amount: and when you say “a minute film” we are getting about the strongest expression of the necessity of a small quantity that words can convey.

There is one other place in this patent where I think the quantity is similarly characterized.
Mr. SHERIDAN :At the top of page 2.
Mr. SCOTT: “A small proportion of oil,” following the “minute film.”

This brings us, of course, to what I regard as the only real question in the case, and that is as to whether there is any distinction in principle between the froth which is formed with a very small quantity of oil and one which is formed with a larger quantity; in other words, whether there is a difference between an air-froth and an oil-froth or emulsion. The first patent in suit, 835,120, states that less than 1% of oil is used; and then in describing the action of that oil it states that it coats the mineral particles. Well, they cannot be coated otherwise than by a minute film. The descriptive language is precisely the same, and then, as shown by the demonstrations in this Court, we have produced the same result ocularly, and metallurgically, with these large quantities of oil as with the minute quantity.