Flotation Controlling Factors

Flotation Controlling Factors

The object of this paper is to offer some suggestions to assist metallurgists to supervise the operations of flotation plants. We do not yet thoroughly understand what “flotation” is, or “why minerals float,” and all attempts that have been made to explain the phenomena have been based on theory or imagination, and have not been capable of substantiation by practical demonstra­tion, or even by laboratory tests ; but it is well known that par­ticular minerals will float in a liquid under certain specified conditions, and that some other minerals will not float under those conditions, and that, if a mixture of the two classes of minerals be subjected to those conditions, there will be separation of the minerals, of which one class will float and the other will remain unfloated as a residue. The duty of the flotation expert is to so control the operations and the conditions of the liquor as to obtain the best commercial result from the ore he is treating. In early 1900, it appeared the chief controlling factors in a flotation plant appear to fall under the following eight headings, all of which are important:silver flotation

  1. Crushing/Grinding.
  2. Rate and regularity of feed.
  3. Density of pulp.
  4. Temperature.
  5. Addition agents.
  6. Condition of circuit liquor.
  7. Agitation.
  8. Aeration.


The ideal feed for flotation treatment is the true slime which has been the bugbear of metallurgists throughout the ages. This fact should be remembered and taken advantage of wherever possible, although it would not be profitable, as a rule, to continue the crushing operation on crude ore to so great a degree as to make a slime of it. It may be generally accepted, however, that it is bad practice to have any particles remaining on a 40-mesh screen. This is not true in some exceptional cases, as sometimes, for instance, with laminated minerals such as molybdenite and certain copper ores.

Whether the degree of crushing is qualified by classifiers or screens, it is advisable for the metallurgist to assume personal control by having laboratory sizing tests made on regular samples of the crushed feed. In practice it will be generally found that the undersize of a 20-mesh revolving screen, with 45% screening area when fully loaded, will almost entirely pass through a 40-mesh laboratory screen (I.M.M. standard).
An efficient crushing plant is the first essential for flotation treatment.

There are two reasons why the ore requires to be crushed so that all particles will pass through a 40-mesh screen-

(1) Because the bubble of air or gas which becomes attached to a particle has not sufficient power to float larger particles of mineral which are produced by any other coarser crushing. If, moreover, a bubble of gas were large enough to float a larger particle, the resistance which it would offer to the flow of the pulp .would probably cause it to be wiped off and removed from the particle before it had time to reach the surface of the flotation bath.

(2) Large particles of ore, when being hurled around in the agitation zone of the flotation vessel, do considerable damage in hindering aeration by cutting bubbles of gas off other particles of mineral.

Rate and Regularity of Feed:

It is essential that the rate of feed going into the flotation plant should he as nearly regular as possible each second of time. There is nothing which interferes so much with treatment as irregularity in the rate of feed and irregularity in the quantity of liquor which passes through the plant from moment to moment. It is a simple matter to make the rate of feed and liquor regular by means of a large agitator or Dorr thickener placed at the head of the treatment vessels. This agitator or Dorr thickener can store quantities of feed supplied to it in an irregular manner by the preparatory plant and deliver it in a regular flow at its dis­charge to the treatment plant. This storage of slime also enables the flotation plant to be operated continuously through any small stoppages of the preparatory plant.

Density of Pulp:

It is necessary that the feed should he introduced to the treat­ment plant, in the form of pulp, at a predetermined density which is best suited to the process. The density should he maintained at a regular rate from moment to moment just as strictly as the rate of feed itself.

Extensive experiments were made on the dump material flotation plant at the Junction North mine for the purpose of determining a suitable density and rate of feed. It was found that variations of density had an important effect upon the rate of feed that could he treated. When the density was 56 % solids it was impossible to treat more than 8 tons of dump material per hour, and then only with poor metallurgical results. When the density was reduced to 35 % solids the same plant treated 24 tons of dump material per hour with excellent metallurgical results. This fact alone shows how simply a flotation plant can go wrong.

It will be seen from this that the rate of feed and density are allied with each other and complementary, and should be-kept under constant control. The following is a quick and reliable method for controlling the density of the pulp in a flotation plant:—

A tin is made to hold 1000 ccs. of water up to a mark, say, about half an inch below the top. The tin is dipped quickly into the flowing pulp and filled up to the mark. It is then weighed, using a counterpoise for the tin. The shift boss or operator should make such a density test at intervals not greater than 15 minutes. The weight of the pulp would be, say, 1350 to 1400 grm., which represents about 30 to 34 % of solids, in the pulp at the dump material treatment plant at the Junction North mine. If the weight is greater than 1400 grm. the operator adds water to the pulp as it leaves the Dorr thickener.


With some ores flotation can be carried out in the cold, but a good many ores require heat to a limited extent. Sometimes, with ores containing several minerals, it is necessary to have a little heat for the flotation of one mineral and a greatly increased amount of heat for the flotation of the second mineral. At the dump treatment plant at the Junction North mine in the selective flotation treatment of lead and zinc sulphides by either the Bradford process or the P.-S.-N. process, it has always been necessary to watch the temperature of the pulp very closely. In the lead section the most suitable temperature was 89° to 90° F. Even a single degree over 90° F. has been detrimental to the grade of the lead concentrate, as zinc sulphide commenced to float with the lead concentrate with the rise in temperature. In the zinc section the best results were obtained at a temperature of 135° F. with the Bradford process, and at a temperature of 125° F. with the P-S.-N. process. If the temperature is allowed to drop to 130° F. in the former case, the zinc concentrate is not completely floated, and a loss is made in the residues. No benefit is gained by increasing the tempera­ture above 135° F. which are harmful to flotation, and it is advisable to let these have time to disappear before the pulp reaches the flotation machine.

Oil is not an essential reagent in flotation, but when it is used it should be added far enough back in the flow to ensure its emulsification in the pulp before it reaches the flotation machine. When it is not thoroughly emulsified, oil is distinctly detrimental to flotation. Oil should never be added directly to the flotation machine, but may be added in the tube mills or other grinding machines or in the return liquor pump or in the agitation machine, as may be found best in practice.

Condition of Circuit Liquor:

It is usually convenient to employ the ordinary mill-feed water for making up flotation circuit liquor. The first essential is that this water shall be clean—that is, free from sediment. Such feed water is usually drawn from the underground drainage system, to which has been added some fresh water from the surface rainfall supplies. The mine water would probably contain a certain amount of salts in solution, and in some cases the proportion of salts in solution would render the water unsuitable for flotation.

The mill water coming in contact with the ore usually dissolves a good deal of the soluble impurities from it, and by the time the water reaches the flotation plant it might be too heavily charged with salts to make it suitable for acting as a flotation circuit liquor.

The flotation circuit liquor requires some properties other than cleanliness to ensure its successful use in a flotation plant, but it is not clear what those properties are. If or a long time it was thought that it was necessary to have at least a comparatively large definite quantity of salts in solution, and that this was the main controlling factor in the constitution of a good circuit liquor for flotation. From many tests carried out at the Junction North mine, it would appear even yet as if the quantity of salts, Metallurgists like, wherever possible, to treat the ores with as little added heat as possible, to save cost. Wherever heat is required, attempts are made to utilize exhaust steam of engines and other sources of heat, if possible. It is not always possible, however, to make use of exhaust steam in this way, even if same may be available. The favourite method of heating, although perhaps the most expensive method, is by the introduction of high-pressure steam direct from the boilers to the pulp, which is always very convenient and easy to manipulate, and has quick and effective results, requiring only the operation of a valve from time to time for keeping the temperature of the pulp constant. The ease with which high-pressure steam can be obtained by the operator, and the fact that extra temperature does no harm in zinc treatment, makes it liable to be wasted very considerably. A close watch requires to be kept on the steam consumption in this way, as it might, besides being wasteful in costs, tend to over­load the boiler plant if it is otherwise working at high pressure.

Addition Agents:

In all flotation plants, it is necessary to add some chemicals. The point at which such chemicals should be added is an important one, and should be determined carefully. It is a matter of vital importance that the valuable minerals in the pulp should be in a properly prepared condition suitable for floating before the pulp enters the flotation machine. For the purpose of ensuring this, it is obvious that whatever chemicals are required should be added to the pulp at least before it reaches the flotation machine. Frequently the purpose will be served if the chemicals are added in the agitation machine, but some­times it is advisable to add them to the pulp before it reaches the agitation machine.

When acid is required, generally speaking the best place to add’ it is in the agitation machine, although it may sometimes be added directly to the flotation machine. Frequently acid produces gases, such as sulphuretted hydrogen, by action upon the pulp, and kind of salts, in solution exerted an important influence on the flotation operation. It was noticed sometimes, when the flotation plants were not working well, that the quantity of salts in solution had either dropped below 1400 gr. per gal. or had risen above 2600 gr. per gal. It would seem from the various tests that the best results were obtained when there were, about 2000 gr. of salts per gal. in the solution. The salts were mainly salts of manganese, zinc, iron, and calcium. It would be a very difficult matter to carry out a set of tests either in the laboratory or in an operating plant to prove the effect of salts in solution. The ore itself containing soluble salts would, as soon as it comes in contact with pure water, immediately provide some salts in solution.

A good deal of work has been done at the Junction North mine and by other investigators to endeavour to determine the actual effect of certain specified salts in solution, and a good deal of interesting information has been collected ; but it has never wet been shown that any of the common salts which naturally/ go into solution in the Broken Hill treatment have a very marked influence upon results, unless present in excessive quantities, and none of the salts known to exist in the circuit liquors in Broken Hill have anything like so great an influence upon the treatment as to explain the reason why some flotation liquors are energetic and others dull in their action. It is distinctly notice­able, when flotation work is going on, successfully in the plant, that the flotation liquor has some property which has never yet been thoroughly defined, and it seems to be a physical property denoting energy which has been instilled by some means into the circuit liquor. Sometimes the nature of the liquor can be corrected by alteration to the oiling, sometimes by alteration to the application of acid. The new P.-S.-N. process claims to correct the conditions of the liquors in many cases by addition of elemental sulphur.

If oil exists in the circuit liquor it must be emulsified. Free oil is very detrimental to flotation work, as can easily be seen in the Cascade process. If a few drops of oil are admitted to the first Cascade vessel its effect will be seen on each of the fol­lowing Cascade vessels of the series in killing the float which was just, previously there.


Agitation is beneficial in all methods of flotation of all classes of ore. Some classes of ore, however, do not require so much agitation as others, and calcitic ores treated by purely acid processes usually require only a moderate degree of agitation before treatment. The quantity of calcite present in the ore need not be more than a few pounds per ton. The acid readily sets free the C02 gas which, being nascent, rapidly attaches itself to the mineral particles and floats them. The intimate associa­tion of the calcite in the ore ensures successful gassing. Such ores as do not contain calcite or any other carbonates require treatment by agitation/aeration, by which the atmospheric air is beaten into the pulp, to cause the attachment of bubbles of air to mineral particles. The M.S. machine was originally designed for this purpose, and later on the Owen patent and Lyster patent showed other means by which it could be carried out more effectively and more completely. The agitation should be carried out in the treatment vessel so that the mineral can rise to the surface of the vessel and be removed without having to travel a greater distance than is absolutely necessary, or, in other words, to avoid overtaxing the ability of the bubble to retain its hold on the mineral particle for leading it out of the flotation bath.

The author is, however, strongly in favour of providing some kind of machine at the head of the treatment; for instance., a vortex mixer or a mineral separation mixer, or a larger agitation machine, to provide that the pulp shall be thoroughly agitated and partially aerated before it enters the treatment vessel. This pre-agitation aeration incorporates oils, reagents, acids, & c., and probably causes some beneficial effect by oxidation of some of the sub­stances, and so saves the first-treatment vessel from being wasted on this work. It might be, too, that some deleterious, substances are formed immediately upon application of the acid and reagents, such as noxious gases like H2S, which, in this preliminary partial agitation/aeration, are either driven off or altered to make them innoxious, and in this preliminary agitation machine chemicals, or acid or perhaps oil may be added, the temperature being con­trolled so that when the pulp enters the flotation machine it is ready for separation of the valuable minerals. It is even advisable to add the chemicals or oil or acid to the pulp before’ it enters the preliminary agitation machine.

With some simple ores very little agitation is required before the pulp passes to the flotation machine, and, in such cases, if the pulp is lifted by bucket elevators direct to the flotation machine, sufficient agitation will he provided by the elevators for successful separation. In that case the chemicals and other reagents may he added, perhaps, at the hoot of the elevator. The bucket elevator may he looked upon as a first-class agitator, although the period of agitation provided by it is very short.

If the pulp requires much acid or heat added to it, it is advisable to have the agitation vessel at the head of the flotation machine besides the elevator, because in such cases the elevator belt, would suffer severely if it handled hot or acid liquors.

Agitation is always beneficial to flotation, and in most cases it is absolutely necessary. It has a cleaning effect on the particles, and, if oil is used, causes a greater emulsification and distribution of the oil. To ensure a more complete flotation of the particles, it breaks, up the bubbles of air and distributes them in a very free state of division throughout the pulp in such a manner as to give every particle of mineral a chance to become attached to air bubbles.


Aeration means the introduction of ah into the pulp in a finely- divided condition so as to form a froth, in order that the particles of mineral may become attached to finely-divided air bubbles, by means of which they are floated up to the surface of the flotation bath and removed across the periphery as a separate concentrate. Air for aeration was originally introduced into the mineral separation machine by being sucked down to the impeller from the surface through the vortex and disturbance caused by the agitation. The quantity of air so converted into froth was always indefinite and not under control, and for this reason flotation results were very changeable. It was necessary to have the agitation/aeration zone in another vessel separated from the flotation bath, because the latter required a quiescent surface, which could not be provided by the former. This was a very severe handicap, as it required the bubble to travel a very great distance before it could be rescued at the surface; consequently, only the most powerful methods of flotation could be used with it, such as would develop combined flotation and not permit of any selective action.

It was found by Owen that if agitation/aeration were carried out in. the flotation bath itself the bubble would have a very much shorter travel, and much more delicate flotation work could be earned out. The effect of this was to permit of selective flotation on one mineral in preference to another. By means of the Owen process, air was either sucked in by impellers or pumps, or was introduced as compressed air and delivered to the impeller in such a manner that it could be most effectively beaten up into a foam.

Later on, at the Junction North mine, it was found that definite control could be obtained to govern the quantity of aeration introduced into any pulp, and measurements could be obtained of the amount of aeration by means of a steelyard or an ordinary water-gauge tube, graduated and placed outside the flotation vessel and connected with it. It was afterwards found, by means of the Cascade process, that aeration could be adequately obtained by means of entanglement with pulp as it fell down open pipes.

It is essential that the air should be completely atomized in the pulp and not allowed to be distributed through the pulp in the form of comparatively large bubbles. This must be care­fully watched when aeration by compressed ah is adopted. If the supply of compressed air is too great through any one delivery pipe, then the air is distributed throughout the pulp in the form of large bubbles, which are unsuitable for flotation. These large bubbles float rapidly to the surface and disturb the quiescent zone, so that some of the float already there is caused to sink again and some of the gangue is caused to overflow the periphery and spoil the concentrate. Compressed air must be added in such a way that it enters the pulp in the maximum zone of agitation of the impellers. The force of agitation at that point will deter­mine the amount of air which may be effectively taken from the supply pipe. If it is desired to atomize a still greater quantity of air than can be dealt with from one supply pipe, it is possible to gain such an increase in any particular flotation machine by increasing the number of air-supply pipes around the impellers or by increasing the speed of the impellers.

In the Cascade process the air is drawn in and entangled by the pulp as it drops down the vertical pipe and becomes atomized by the splashing of the pulp in the impact, which occurs when the pulp hits the surfaces of the liquor and metallic parts of the flotation machine. It is generally advisable in the Cascade process to have a cup placed below the bottom of the vertical pipe to intensify the impact.