What is Flotation

Flotation, as the term is applied to ore concentration, means the separation of one of the constituents of an ore from the remainder by causing it to float at or above the surface of a pulp consisting of the finely pulverized ore and water.

Minerals that float

Minerals that float have a metallic, adamantine, or resinous luster. Minerals with vitreous, pearly, or earthy luster do not float, as the term is at present used in the art of concentration. It must not be understood, however, that the float concentrate in a flotation operation is free from these latter minerals. As a matter of fact, in many flotation concentrates the minerals of non-metallic luster predominate. Their inclusion is due in part to their being mechanically entrapped and held, as on a screen, by the bubbles composing the froth, in part to the inclusion of pulp in thick bubble walls, and in part to the removal of some of the pulp, as such, with the floating concentrate.

Ores amenable to concentration by flotation

Almost any ore consisting of a mineral of metallic, resinous, or adamantine luster associated with the usual rock-forming minerals, can be concentrated by flotation. In the great majority of cases the part of the ore that floats is the valuable portion, but if the constitution of an ore were such that the valuable mineral had a non-metallic luster, and the gangue a metallic luster, the floated portion would constitute the tailing while the valuable portion would remain submerged and be drawn off as residual pulp.

The grade of concentrate, the ratio of concentration, and the percentage of recovery will depend upon the ore itself and the method of treatment employed. Thus a copper ore consisting of chalcocite in a gangue of rock-forming silicates will yield, under a given method of treatment, a higher grade of concentrate and show a higher ratio of concentration than another copper ore of identical copper content, in which the copper mineral is chalcocite, but in which pyrite is associated with the other gangue minerals. This, for the reason that the flotation methods ordinarily practiced do not differentiate to any considerable extent between the minerals of metallic luster in the ore, but bring up all such minerals in the concentrate.Froth Flotation Handbook

Methods of flotation

Methods of flotation may be classified for purposes of study into three varieties, on the basis of the force that acts to buoy the mineral of metallic luster, as: skin-flotation methods, oil-flotation methods, froth-flotation methods.

Skin-flotation

Skin is a methods depend for their operation on the comparative reluctance of the minerals of metallic lustre in an ore to become wetted by water and the resultant buoyant effect of the force of surface tension exerted on these particles at the upper surface of a body of liquid pulp. Apparatus for the practice of skin flotation is of one of two general classes, depending upon whether the ore is fed to the machine dry or wet. Crude methods employing dry feeding have been rather widely used in the graphite industry. The most elaborate apparatus employing dry feeding is that described in U. S. patent 1,088,050 issued to H. E. Wood, February 24, 1914, and described on page 104. The best known of the methods in which wet ore is presented to the flotation machine are the Macquisten and DeBavay, which are described on pages 106 and 108 respectively. In general, the methods that feed dry ore require the dust to be separated before the ore is fed to the machine on account of the fact that very finely divided gangue is as difficult to wet at the surface of a body of water as is the mineral of metallic luster. This limitation as to size is not so important in “wet” skin-flotation methods. Skin-flotation produces a high-grade concentrate at the expense of a low recovery. On lead-zinc ores, differential flotation of a part of the lead in the form of a high-grade lead concentrate may be accomplished. This fact is, in some cases, held to justify the use of such methods preceding froth flotation. Otherwise the use of skin-flotation methods at present is limited to the case where grade of concentrate produced is a more important consideration than the recovery obtained.

Oil-flotation

Oil methods effect the selection of the minerals of metallic luster from the gangue minerals in an ore by reason of the fact that the minerals of metallic luster are wetted preferentially by oil in the presence of water and, hence, pass from the aqueous pulp containing them into the oil, or, more accurately, into the interface between the oil and water; while the gangue, with the reverse tendency so far as wetting is concerned, remains in the water. The buoyant effect necessary to float the selected mineral of metallic luster is brought about by reason of the difference in weight between the system of oil effectively acting plus mineral of metallic luster carried thereby and the weight of pulp that it displaces. The metallic mineral is held in the oil by the viscosity and resistance to breaking of the film interfacial to the oil and the water of the pulp.

The processes utilizing oil to select and float metallic minerals are of two varieties, viz.: those in which the oil is mixed with the ore in the presence of little or no water and those in which the ore, before admixture of the oil, has been brought to the condition of a freely flowing pulp. The better known processes of the first class are those of Everson and Robson; of the second, Elmore and Wolf and Scammell. These processes are described on pages 110 et seq.

Froth flotation

Flotation comprises two entirely different types of processes which resemble each other only in the fact that in both the concentrate is removed in the form of a froth composed of gas, liquid, and solid matter preponderantly sulphide mineral. The processes differ fundamentally both in the place in which concentration is done and in the mechanism of the selection of sulphide from gangue. On the basis of the first difference the processes may be classified as pulp-body concentration processes and bubble-column concentration processes.

Pulp-body-concentration processes

This concentration processes may be subdivided, on the basis of the method of introducing the bubble-making gas, into four types: (1) chemical- generation, (2) pressure-reduction, (3) boiling, and (4) agitation. All four types depend upon the fact that in a pulp, the liquid part of which is saturated with a gas, preferential precipitation of the gas on the sulphide particles can be brought about by so changing the conditions of temperature and pressure that the liquid is, under the changed conditions, supersaturated. This preferential precipitation of gas from the supersaturated liquid is enhanced, if the sulphide particles are coated with an oily substance, and the presence of such a substance also makes greater the force of adherence between the precipitated bubbles and sulphide particles. As a result of this preferential precipitation of gas on sulphide particles in the pulp, and its adhesion thereto, there are formed in the body of the pulp agglomerates consisting of one or more gas bubbles with sulphide particles firmly cemented to them. These agglomerates later rise to the surface in the form of a froth which is separated as concentrate. Observation of any of the pulp-body-concentration processes shows clearly this phenomenon of rising agglomerates whose color indicates distinctly that concentration has been completed at the surfaces of the bubbles composing them, below the surface of the pulp, that is, within the pulp body.

Chemical-generation processes

Chemical processes are typified by the Froment process and the Elmore electrolytic process. The former is described in detail on page 114. In the Froment process, gas is produced in pulp in the presence of oiled sulphide particles by causing sulphuric acid to react with carbonates, either naturally or artificially present. In the Potter-Delprat process as most extensively practised the same method of gas production is employed, but no oil is present. In the Elmore electrolytic process, hydrogen and oxygen are produced by the decomposition of water caused by the passage of an electric current through a pulp in which an electrolyte is present. In all of these processes it seems to be essential that at least a part of the gas pass through the solution stage in order to effect adherence to the sulphide. Gas which is freed in the form of bubbles at the surface of carbonate particles in the pulp and which persists as a bubble in its passage through the pulp, will rarely, if ever, adhere, in such passage, to sulphide particles. Such bubbles may coalesce with other bubbles already present on sulphide particles and thus aid in flotation, but they play their principal part in providing additional bouyancy in the froth and in picking up the sulphide particles in the froth which are dropped by the breaking of other bubbles therein.

The change in condition effective in these processes to produce local supersaturation is one of “solution pressure.” At the surface of the dissolving carbonates there is pressure by the molecules of carbon dioxide evolved tending to drive them into solution in the water. Those which dissolve travel by diffusion and by reason of water currents away from the regions where the forces tending to drive them into solution exist. In these regions of lessened solution pressure the molecules tend to come out of solution, to precipitate, and they do so preferentially at the surfaces where the least resisting forces exist, which, in this case, are the sulphide surfaces.

Pressure-reduction processes

Pressure reducing processes depend upon a reduction, in external pressure to bring about the condition of gaseous supersaturation essential to preferential precipitation of gas bubbles on the surfaces of sulphide particles. These processes are of two kinds. In one variety the water of the pulp is saturated with air by being subjected to pressures greater than atmospheric. Upon subsequent discharge into the atmosphere the air dissolved under supernormal pressure is released at sulphide surfaces, and the bubbles adhering thereto eventually raise the sulphide mineral to form a froth. The Norris patent, U. S. 873,586, is the most promising of this group, which includes U. S. patent 835,479. The other variety of pressure-reduction process is typified by the vauum process invented by F. E. Elmore and described in U. S. patent 826,411. A detailed description is given on page 114. In the vacuum process a pulp pre- mixed with oil is subjected to a vacuum, which causes the air contained in the water to come out of solution. The air coming out of solution does so preferentially at the surface of the minerals of metallic luster and adheres thereto. When the system of bubble and sulphide has become sufficiently buoyant to rise, the sulphides are carried to the surface to form a froth. 

Boiling processes

Boiling depends upon heat to drive air and water vapor out of the water in a pulp. These gases form bubbles preferentially at the surfaces of the metalliferous minerals and the bubbles with their solid load rise to form a froth. U. S. patent 835,143 is typical of this type of process. The phenomenon is effective both with and without “oil” and actual boiling is not essential.

Agitation-froth process

Agitation depends upon local supersaturation of the water of a pulp with air by the mechanical action of a swiftly revolving beater, and the simultaneous precipitation of air in the form of bubbles, preferentially on the surface of the particles of metalliferous mineral, to effect the same result effected in the previously mentioned processes of the pulp-body-concentration type. Agitation-froth machines are described in detail on pages 115 et seq. They consist essentially of an agitation chamber in which a stirrer mounted on a vertical spindle rotates at high speed, and a froth-separating compartment in which the pulp is allowed to come to rest and the bubbles carrying the metalliferous mineral rise to the surface to form a froth which is skimmed off. The pulp in the agitating compartment, under the influence of the rotating agitator, is thrown from the center toward the side of the chamber. The result is that the surface of the pulp assumes the shape of an inverted cone. When the cone becomes sufficiently pronounced that the tip reaches down to the revolving beater arms, the tip is cut off and a large bubble of air is entrained. This bubble is immediately broken up by the direct impact of the impeller arms and by the violent swirling of the pulp into a large number of small bubbles. These bubbles, due to their minute size, are in the most favorable state to be taken into solution, and many of them are, at the time of their formation, subjected to considerably more than atmospheric pressure, due to the impact of the impeller blades. They have, also, on account of their small size, but slight vertical motion relative to the pulp mass, and are, therefore, kept for a comparatively long time in contact with the water and subject to the impact of the impeller blades. As a result, some of them go into solution in the water. At the same time there exists behind each impeller blade a volume of pulp on which the pressure is reduced by reason of the forward movement of the blade and the inertia of the pulp mass. Here air comes out of solution in the form of bubbles at the surfaces of the sulphide particles. The excess bubbles which never go through the solution stage, in this, as in the other pulp-body-concentration processes, in part coalesce with the bubbles already formed on sulphide surfaces; in part pass with the pulp into the froth-separating chamber and there, rising, add buoyancy to the froth and serve to pick up particles dropped by the bursting of other bubbles; in large part, however, they rise to the surface of the pulp in the agitating compartment and are lost to the process.

The froths produced in pulp-body-concentration processes are small-bubble, coherent and persistent, and characteristic. The volume of gas effectively utilized in floating the mineral is. of the order of 20 to 50 cu. ft. per cu. ft. of solid floated.

Bubble-column process

In the bubble-column process substantially all of the concentration is done in a column of bubbles above and floating on the surface of the body of pulp. In this process the volume of gas effectively used to produce concentration is enormously greater than in pulp-body concentration, being of the order of 1000 to 2000 cu. ft. per cu. ft. of solid floated. The result is that the froth is fragile and evanescent and strikingly different from that characteristic of the other class of processes. Further investigation of the process, by observation of the operation in glass-sided machines, makes apparent the following facts: (1) The bubbles are much larger than in pulp-body processes; (2) they are more numerous; (3) they rise through the pulp more rapidly; (4) they arrive at the surface of the pulp with a solid load composed of sulphide and gangue in the same proportions that these exist in the pulp through which they have passed; (5) concentration begins at the bottom of the bubble column (i.e., the surface of the pulp body) and progresses upward. The actual mechanism of the concentration itself can be observed by studying the bubble column with a hand glass. Such study shows that in the bubble walls there is a differential draining of the gangue and sulphide particles; that the average downward velocity of the sulphide particles is less than the average upward velocity of the bubbles; that the average downward velocity of the gangue is greater than the average upward velocity of the bubbles; and that, as a result, the sulphides are lifted up and away from the gangue. It is apparent, also, from such study, that the sulphide particles in the bubble column are nowhere firmly adherent to bubbles, as they are in the pulp-body processes.

Machines in which the bubble-column process is practiced may be classified, on the basis of the method of introducing air, as plunging stream or cascade machines, pneumatic machines and centrifugal machines.

In plunging-stream type bubble-column machines, air is carried into the pulp by a plunging stream of pulp. The bubbles are relatively large and the disturbance of the pulp body relatively slight. Hence there is quick rise of relatively large bubbles through the pulp which does not cause supersaturation with subsequent precipitation and concentration below the pulp surface, but rather necessitates that such concentration as, takes place shall occur in the bubble column. One type of plunging-stream machine is described on page 133.

Pneumatic bubble-column machines are typified by the Callow cell, which is described on page 122. In this device air is introduced into the pulp through a porous medium. Canvas, cotton twill, blanket, carborundum, concrete and other porous substances are used as media for the distribution of the entering air. In pneumatic machines the pulp is relatively quiescent, the bubbles are larger than in agitation- type machines and hence rise rapidly. No pressure is exerted to force them into solution, nor is there any local release of pressure to cause air already in solution to precipitate. The result is that no selection of sulphide particles takes place beneath the pulp surface. The bubbles rushing upward through the pulp mechanically push a certain amount of pulp above them as they emerge, with the result that the walls of the emerged bubble contain a solid load of the same composition as that in the body of the pulp. At the pulp surface the speed of rise of the bubble abruptly lessens and the solid particles which now form a part of the bubble film begin to drain away rapidly. At the same time the bubble is lifted by the bubbles which follow it to the pulp surface. The solid particles drain away at different rates, the gangue particles much the more rapidly, so that, if the air supply and consequent rate of rise of bubbles is properly adjusted, the average downward velocity of the gangue will be greater than the average upward velocity of the bublbe, and it will largely settle back into the pulp, while the average downward velocity of the sulphide will be less than that of the bubble, with the result that the sulphide will be carried up and away from the gangue and may be separated as concentrate.

Centrifugal bubble-column machines include several in which air is drawn into the pulp due to the revolution of a disk or impeller on a vertical shaft and two in which the air-entraining mechanism is mounted on a horizontal shaft. In the former class are the Ruth and Groch hollow-shaft machines and the Hebbard sub-aeration machine. The Rork and K. and K. (Kraut and Kollberg) machines are of the horizontal-shaft type. These machines are described on pages 135 et seq. They appear on first inspection to be of the agitation type but study of their action shows that pulp is displaced so rapidly through the zone of the moving parts into the quiet zone that the time is insufficient to effect pulp-body-concentration. This conclusion is confirmed by study of the bubble column in the machines. Such study demonstrates clearly that practically no concentration has taken place at the bubble surface by the time the bubble reaches the pulp surface and that substantially all of the concentration takes place subsequent thereto.

Differential flotation

Differential flotation is the term applied to methods of concentration by flotation which seek to float one only of the minerals of metallic luster in a mixture of such minerals either in or out of the presence of the usual rock-forming gangue minerals.

Flotation of oxidized ores

Oxidized ores is a misnomer unless it is understood that the oxidized mineral containing the sought-for metals, usually lead or copper, is first changed by chemical means into some compound of the metal having a metallic luster. The latter product can then be floated with varying success. In some of these methods the transformation of the oxidized mineral requires complete solution of the metal and subsequent precipitation as metal or sulphide; in other methods a surface transformation only, usually to sulphide, is attempted. No generally applicable and commercially successful method for the treatment of oxidized ores by flotation has yet been devised.

Flotation agents

Theses agents include oils and certain other organic compounds, and many inorganic compounds. All of these substances will act alone to effect concentration by froth-flotation. Usual practice, however, is to use an oil or mixture of oils as the principal agent, with or without the addition of some inorganic substance. The oils commonly used are (1) essential oils, of which class pine oils are most frequently employed; (2) coal-tar oils; (3) wood-tar oils; and (4), petroleums. Sulphuric acid is the commonest inorganic agent; lime, salt cake and soda are less frequently added.

Conditions of operation of froth-flotation

Froth-flotation is in general effective only on ores ground to pass 0.3-mm. aperture or less and the agitation-froth and bubble-column processes are most effectively practiced when the bulk of the feed will pass a 0.074-mm. screen. In these latter processes the finely pulverized ore must be in the condition of a freely flowing pulp with water. The most favorable percentage of solids lies within the range of 15 per cent, to 30 per cent. The operation of the process is affected by the mineralogical character of the ore, the grade of feed, the kind and quantity of flotation agent and, to a less extent, by the temperature of the pulp, the place and method of adding the flotation agent, the type of flotation machine used, and the method of removing concentrate.

Typical flow-sheets in flotation practice

Flotation enters into mill flow-sheets in two capacities, viz.: (1) as a method of treatment accessory to gravity concentration; (2) as the principal method of concentration. Which of these flow-sheets is followed in a given mill should be determined primarily by the character of the ore, bearing in mind that gravity concentration is more economical than flotation when the valuable mineral occurs in such sizes that it can be suitably treated on shaking-tables or jigs. Hence, when the valuable mineral occurs in the ore in aggregates more than one or two millimeters in diameter and the projected mill tonnage is such that both gravity-concentration and flotation machines can be fed to capacity, a flow-sheet in which flotation is accessory to gravity-concentration should be investigated. Otherwise flotation should be the principal process, with gravity-concentration subordinate, if, indeed it be employed at all.

Step treatment

It is inherent in most froth-flotation processes that a high grade of concentrate and a high percentage of recovery cannot be made in one and the same cell at one and the same time. It is necessary to first treat the pulp in one flotation machine, usually called a “rougher.” In this machine the attempt is to produce a clean tailing and a dirty concentrate. The dirty concentrate is then re-treated in a second machine, usually called a “cleaner,” which makes a clean concentrate and a high-grade tailing. Common practice is to re-treat the tailing from the cleaner cell. In another scheme of treatment a clean concentrate and a high-grade tailing, or middling, are made on the first treatment cell or cells. The high-grade tailing is further treated in other cells which make a low-grade concentrate and a finished, low-grade tailing. The low-grade concentrate from the later machines is re-treated in cleaner machines as above described, or is returned to the head of the first machine.

Flotation results

Since, in flotation, only minerals of metallic, resinous, or adamantine luster are selected, investigation and judgment as to the efficiency of the process should be based on a consideration of the recoverable mineral only. Thus in an ore containing both sulphide and oxide copper the recovery credited to the process should be based on the sulphide copper content of the feed, concentrate and tailing, if a true measure of the efficiency of the process is sought. Froth-flotation, properly practiced, will recover from 60 to well over 95 per cent, of the sulphide mineral content of an ore in the form of a concentrate containing from 10 to 40 per cent insoluble matter, i.e., gangue.


A reproduction of Arthur Taggart’s 1922 A Manual of Flotation Processes


 

Basic Principles & Variables Affecting Froth Flotation

Laboratory Flotation Equipment & Testing

Flotation Equipment and their Test Procedures

Metallurgical Testwork for Process Development

Flotation Process Development in Laboratory

Testing Flotation Oils and Frothers

Laboratory Scale Flotation Tests & Flotation Plant Results

Froth Flotation and its Machines

Flotation Flowsheets

Pulp Formulas & Metallurgical Formulas