More ores are treated using froth flotation cells than by any other single machines or process. Non-metallics as well as metallics now being commercially recovered include gold, silver, copper, lead, zinc, iron, manganese, nickel, cobalt, molybdenum, graphite, phosphate, fluorspar, barite, feldspar and coal. Recent flotation research indicates that any two substances physically different, but associated, can be separated by flotation under proper conditions and with the correct machine and reagents. The DRflotation machine competes with Wemco and Outotec (post-outokumpu) flotation cells but are all similar is design. How do flotation cells and machinework for themineral processing industry will be better understood after you read on.
While many types of agitators and aerators will make a flotation froth and cause some separation, it is necessary to have flotation cells with the correct fundamental principles to attain high recoveries and produce a high grade concentrate. The Sub-A (Fahrenwald) Flotation Machines have continuously demonstrated their superiority through successful performance. The reliability and adaptations to all types of flotation problems account for the thousands of Sub-A Cells in plants treating many different materials in all parts of the world.
The design of Denver Sub-A flotation cells incorporates all of the basic principles and requirements of the art, in addition to those of the ideal flotation cell. Its design and construction are proved by universal acceptance and its supremacy is acknowledged by world-wide recognition and use.
1) Mixing and Aeration Zone:The pulp flows into the cell by gravity through the feed pipe, dropping directly on top of the rotating impeller below the stationary hood. As the pulp cascades over the impeller blades it is thrown outward and upward by the centrifugal force of the impeller. The space between the rotating blades of the impeller and the stationary hood permits part of the pulp to cascade over the impeller blades. This creates a positive suction through the ejector principle, drawing large and controlled quantities of air down the standpipe into the heart of the cell. This action thoroughly mixes the pulp and air, producing a live pulp thoroughly aerated with very small air bubbles. These exceedingly small, intimately diffused air bubbles support the largest number of mineral particles.
This thorough mixing of air, pulp and reagents accounts for the high metallurgical efficiency of the Sub-A (Fahrenwald) Flotation Machine, and its correct design, with precision manufacture, brings low horsepower and high capacity. Blowers are not needed, for sufficient air is introduced and controlled by the rotating impeller of the Denver Sub-A. In locating impeller below the stationary hood at the bottom of the cell, agitating and mixing is confined to this zone.
2) Separation Zone:In the central or separation zone the action is quite and cross currents are eliminated, thus preventing the dropping or knocking of the mineral load from the supporting air bubble, which is very important. In this zone, the mineral-laden air bubbles separate from the worthless gangue, and the middling product finds its way back into the agitation zone through the recirculation holes in the top of the stationary hood.
3) Concentrate Zone:In the concentrate or top zone, the material being enriched is partially separated by a baffle from the spitz or concentrate discharge side of the machine. The cell action at this point is very quiet and the mineral-laden concentrate moves forward and is quickly removed by the paddle shaft (note direct path of mineral). The final result is an unusually high grade concentrate, distinctive of the Sub-A Cell.
A flotation machine must not only float out the mineral value in a mixture of ground ore and water, but also must keep the pulp in circulation continuously from the feed end to the discharge end for the removal of the froth, and must give the maximum treatment positively to each particle.
It is an established fact that the mechanical method of circulating material is the most positive and economical, particularly where the impeller is below the pulp. A flotation machine must not only be able to circulate coarse material (encountered in every mill circuit), but also must recirculate and retreat the difficult middling products.
In the Denver Sub-A due to the distinctive gravity flow method of circulation, the rotating impeller thoroughly agitates and aerates the pulp and at the same time circulates this pulp upward in a straight line, removing the mineral froth and sending the remaining portion to the next cell in series. No short circuiting through the machine can thus occur, and this is most important, for the more treatments a particle gets, the greater the chances of its recovery. The gravity flow principle of circulation of Denver Sub-A Flotation Cell is clearly shown in the illustration below.
There are three distinctive advantages of theSub-A Fahrenwald Flotation Machines are found in no other machines. All of these advantages are needed to obtain successful flotation results, and these are:
Coarse Material Handled:Positive circulation from cell to cell is assured by the distinctive gravity flow principle of the Denver Sub-A. No short circuiting can occur. Even though the ore is ground fine to free the minerals, coarse materials occasionally gets into the circuit, and if the flotation machine does not have a positive gravity flow, choke-ups will occur.
In instances where successful metallurgy demands the handling of a dense pulp containing an unusually large amount of coarse material, a sand relief opening aids in the operation by removing from the lower part of the cell the coarser functions, directing these into the feed pipe and through the impeller of the flowing cell. The finer fraction pass over the weir overflow and thus receive a greater treatment time. In this manner short-circuiting is eliminated as the material which is bled through the sand relief opening again receives the positive action of the impeller and is subjected to the intense aeration and optimum flotation condition of each successive cell, floating out both fine and coarse mineral.
No Choke-Ups or Lost Time:A Sub-A flotation cell will not choke-up, even when material as coarse as is circulated, due to the feed and pulp always being on top of the impeller. After the shutdown it is not necessary to drain the machine. The stationary hood and the air standpipe during a shutdown protects the impeller from sanding-up and this keeps the feed and air pipes always open. Denver Sub-A flotation operators value its 24-hour per day service and its freedom from shutdowns.
This gravity flow principle of circulation has made possible the widespread phenomenal success of a flotation cell between the ball mill and classifier. The recovery of the mineral as coarse and as soon as possible in a high grade concentrate is now highly proclaimed and considered essential by all flotation operators.
Middlings Returned Without Pumps:Middling products can be returned by gravity from any cell to any other cell. This flexibility is possible without the aid of pumps or elevators. The pulp flows through a return feed pipe into any cell and falls directly on top of the impeller, assuring positive treatment and aeration of the middling product without impairing the action of the cell. The initial feed can also enter into the front or back of any cell through the return feed pipe.
Results : It is a positive fact that the application of these three exclusive Denver Sub-A advantages has increased profits from milling plants for many years by increasing recoveries, reducing reagent costs, making a higher grade concentrate, lowering tailings, increasing filter capacities, lowering moisture of filtered concentrate and giving the smelter a better product to handle.
Changes in mineralized ore bodies and in types of minerals quickly demonstrate the need of these distinctive and flexible Denver Sub-A advantages. They enable the treatment of either a fine or a coarse feed. The flowsheet can be changed so that any cell can be used as a rougher, cleaner, or recleaner cell, making a simplified flowsheet with the best extraction of mineral values.
The world-wide use of the Denver Sub-A (Fahrenwald) Flotation Machine and the constant repeat orders are the best testimonial of Denver Sub-A acceptance. There are now over 20,000 Denver Sub-A Cells in operation throughout the world.
There is no unit so rugged, nor so well built to meet the demands of the process, as the Denver Sub-A (Fahrenwald) Flotation Machine. The ruggedness of each cell is necessary to give long life and to meet the requirements of the process. Numerous competitive tests all over the world have conclusively proved the real worth of these cells to many mining operators who demand maximum result at the lower cost.
The location of the feed pipe and the stationary hood over the rotating impeller account for the simplicity of the Denver Sub-A cell construction. These parts eliminates swirling around the shaft and top of the impeller, reduce power load, and improve metallurgical results.
TheSub-A Operates in three zones: in bottom zone, impeller thoroughly mixes and aerates the pulp, the central zone separates the mineral laden particles from the worthless gangue, and in top zone the mineral laden concentrate high in grade, is quickly removed by the paddle of a Denver Sub-A Cell.
A Positive Cell Circulation is always present in theSub-A (Fahrenwald) Flotation Machine, the gravity flour method of circulating pulp is distinctive. There is no short circulating through the machine. Every Cell must give maximum treatment, as pulp falls on top of impeller and is aerated in each cell repeatedly. Note gravity flow from cell to cell.
Choke-Ups Are Eliminated in theSub-A Cell, even when material as coarse as is handled, due to the gravity flow principle of circulation. After shutdown it is not necessary to drain the machine, as the stationary hood protects impeller from sanding up. See illustration at left showing cell when shut down.
No Bowlers, noair under pressure is required as sufficient air is drawn down the standpipe. The expense and complication of blowers, air pipes and valves are thus eliminated. The standpipe is a vertical air to the heart of the Cell, the impeller. Blower air can be added if desired.
The Sub-A Flexibility allows it tobe used as a rougher, cleaner or recleaner. Rougher or middling product can be returned to the front or back of any cell by gravity without the use of pumps or elevators. Cells can be easily added when required. This flexibility is most important in operating flotation MILLS.
Pulp Level Is Controlled in each Sub-A Flotation Cell as it has an individual machine with its own pulp level control. Correct flotation requires this positive pulp level control to give best results in these Cells weir blocks are used, but handwheel controls can be furnished at a slight increase in cost. Note the weir control in each cell.
High Grade Concentrate caused by thequick removal of the mineral forth in the form of a concentrate increases the recovery. By having an adjustment paddle for each Sub-A Cell, quick removal of concentrate is assured, Note unit bearing housing for the impeller Shaft and Speed reducer drive which operates the paddle for each cell
Has Fewer Wearing Parts because Sub-A Cells are built for long, hard service, and parts subject to wear are easily replaced at low cost. Molded rubber wearing plates and impellers are light in weight give extra long life, and lower horsepower. These parts are made under exact Specifications and patented by Denver Equipment Co.
TheRugged Construction of theSub-A tank is made of heavy steel, and joints are welded both inside and out. The shaft assemblies are bolted to a heavy steel beam which is securely connected to the tank. Partition plates can be changed in the field for right or left hand machine. Right hand machine is standard.
The Minerals Separation or M.S. Sub-aeration cells, a section of which is shown in Fig. 32, consists essentially of a series of square cells with an impeller rotating on a vertical shaft in the bottom of each. In some machines the impeller is cruciform with the blades inclined at 45, the top being covered with a flat circular plate which is an integral part of the casting, but frequently an enclosed pump impeller is used with curved blades set at an angle of 45 and with a central intake on the underside ; both patterns are rotated so as to throw the pulp upwards. Two baffles are placed diagonally in each cell above the impeller to break up the swirl of the pulp and to confine the agitation to the lower zone. Sometimes the baffles are covered with a grid consisting of two or three layers each composed of narrow wood or iron strips spaced about an inch apart. The sides and bottom of the cells in the lower or agitation zone are protected from wear by liners, which are usually made of hard wood, but which can, if desired, consist of plates of cast-iron or hard rubber. The section directly under the impeller is covered with a circular cast-iron plate with a hole in the middle for the admission of pulp and air. The hole communicates with a horizontal transfer passage under the bottom liner, through which the pulp reaches the cell. Air is introduced into each cell through a pipe passing through the bottom and delivering its supply directly under the impeller. A low-pressure blower is provided with all machines except the smallest, of which the impeller speed is fast enough to draw in sufficient air by suction for normal requirements.
The pulp is fed to the first cell through a feed opening communicating with the transfer passage, along which it passes, until, at the far end, it is drawn up through the hole in the bottom liner by the suction of the impeller and is thrown outwards by its rotation into the lower zone. The square shape of the cell in conjunction with the baffles converts the swirl into a movement of intense agitation, which breaks up the air entering at the same time into a cloud of small bubbles, disseminating them through the pulp. The amount of aeration can be accurately regulated to suit the requirements of each cell by adjustment of the valve on its air pipe.
Contact between the bubbles and the mineral particles probably takes place chiefly in the lower zone. The pumping action of the impeller forces the aerated pulp continuously past the baffles into the upper and quieter part of the cell. Here the bubbles, loaded with mineral, rise more or less undisturbed, dropping out gangue particles mechanically entangled between them and catching on the way up a certain amount of mineral that has previously escaped contact. The recovery of the mineral in this way can be increased at the expense of the elimination of the gangue by increasing the amount of aeration. The froth collects at the top of the cell and is scraped by a revolving paddle over the lipat the side into the concentrate launder. The pulp, containing the gangue and any mineral particles not yet attached to bubbles, circulates to some extent through the zone of agitation, but eventually passes out through a slot situated at the back of the cell above the baffles and flows thence over the discharge weir. The height of the latter is regulated by strips of wood or iron and governs the level of the pulp in the cell. The discharge of each weir falls by gravity into the transfer passage under the next cell and is drawn up as before by the impeller. The pulp passes in this way through the whole machine until it is finally discharged as a tailing, the froth from each cell being drawn off into the appropriate concentrate launder.
No pipes are normally fitted for the transference of froth or other middling product back to the head of the machine or to any intermediate point. Should this be necessary, however, the material can be taken by gravity to the required cell through a pipe, which is bent at its lower end to pass under the bottom liner and project into the transfer passage, thus delivering its product into the stream of pulp that is being drawn up by the impeller
Particulars of the various sizes of M.S. Machines are given in Table 21. It should be noted that the size of a machine is usually defined by the diameter of its impeller ; for instance, the largest one would be described as a 24-inch machine.
The Sub-A Machine, invented by A. W. Fahrenwald and developed in many respects as an improvement in the Minerals Separation Machine, from which it differs considerably in detail, particularly in the method of aerating the pulp, although the principle of its action is essentially the same. Its construction can be seen from Figs. 33 and 34.
In common with the M.S. type of machine, it consists of a series of square cells fitted with rotating impellers. Each cell, however, is of unit construction, a complete machine being built up by mounting the required number of units on a common foundation and connecting up the pipes which transfer the pulp from one cell to the next. The cells are constructed of welded steel. The impeller, which can be rubber-lined,if required, carries six blades set upright on a circular dished disc, and is securely fixed to the lower end of the vertical driving shaft. It is covered with a stationary hood, to which are attached a stand-pipe, a feed pipe, and the middling return pipes. The underside of the hood is fitted with a renewable liner of rubber or cast-iron. The pulp, entering the first cell through the feed pipe and sometimes through the middling pipes, falls on to the impeller, the rotation of which throws it outwards into the bottom zone of agitation. The suction effect due to the rotationof the impeller draws enough air down the standpipe to supply the aeration necessary for normal operation. A portion of the pulp, cascading over the open tops of the impeller blades, entraps and breaks up the entrained air, the resulting spray-like mixture being then thrown out into the lower zone of agitation, where it is disseminated through the pulp as a cloud of fine bubbles. Should this amount of aeration be insufficient, air can be blown in under slight pressure through a hole near the top of the stand-pipe, in which case a rubber bonnet is fastenedto the lower bearing and clamped round the top of the stand-pipe so as to seal the supply from the atmosphere.
The bottom part of the cell is protected from wear by renewable cast-iron or rubber liners. Four vertical baffles, placed diagonally on the top of the hood, break up the swirl of the pulp and intensify theagitation in the lower zone. The pumping action of the impeller combined with the rising current of air bubbles carries the pulp to the quieter upper zone, where the bubbles, already coated with mineral, travel upwards, drop out many of the gangue particles which may have become entangled with them, and finally collect on the surface of the pulp as a mineralizedfroth. One side of the cell is sloped outwards so as to form, in conjunction with a vertical baffle, a spitzkasten-shaped zone of quiet settlement, where any remaining particles of gangue that have been caught and held between the bubbles are shaken out of the froth as it flows to the overflow lip at the front of the cell. The baffle prevents rising bubbles from entering the outer zone, thus enabling the gangue material released from the froth to drop down unhindered into the lower zone. A revolving paddle scrapes the froth past the overflow lip into the concentrate launder.
Should the machine be required to handle more than the normal volume of froth, it is built with a spitzkasten zone on both sides of the cell. For the flotation of ores containing very little mineral the spitzkasten is omitted so as to crowd the froth into the smallest possible space, the front of the cell being made vertical for the purpose.
Circulation of the pulp through the lower zone of agitation is maintained by means of extra holes at the base of the stand-pipe on a level with the middling return pipes. An adjustable weir provides for the discharge of the pulp to the next cell, which it enters through a feed-pipe as before. Below the weir on a level with the hood is a small sand holeand pipe through which coarse material can pass direct to the next cell without having to be forced up over the weir. The same process is repeated in each cell of the series, the froth being scraped over the lip of the machine, while the pulp passes from cell to cell until it is finally discharged as a tailing from the last one. The middling pipes make it an easy matter for froth from any section of the machine to be returned if necessary to any cell without the use of pumps.
Table 22 gives particulars of the sizes and power requirements of Denver Sub-A Machines and Table 23 is an approximate guide to their capacities under different conditions. The number of cells needed
Onemethod of driving the vertical impeller shafts of M.S. Subaeration or Denver Sub-A Machines is by quarter-twist belts from a horizontal lineshaft at the back of the machine, the lineshaft being driven in turn by a belt from a motor on the ground. This method is not very satisfactory according to modern standards, firstly, because the belts are liable to stretch and slip off, and, secondly, because adequate protection againstaccidents due to the belts breaking is difficult to provide without making the belts themselves inaccessible. A more satisfactory drive, with which most M.S. Machines are equipped, consists of a lineshaft over the top of the cells from which each impeller is driven through bevel gears. The lineshaft can be driven by a belt from a motor on the ground, by Tex- ropes from one mounted on the frame work of the machine, or by direct coupling to a slow-speed motor. This overhead gear drive needs careful adjustment and maintenance. Although it may run satisfactorily for years, trouble has been experienced at times, generally in plants where skilled mechanics have not been available. The demand for something more easily adjusted led to the development of a special form of Tex-rope drive which is shown in Fig. 35. Every impeller shaft is fitted at the top with a grooved pulley, which is driven by Tex-ropes from a vertical motor. This method is standard on Denver Sub-A Machines, and M.S. Machines are frequently equipped with it as well, but the former type are not made with the overhead gear drive except to special order.
The great advantage of mechanically agitated machines is that every cell can be regulated separately, and that reagents can be added when necessary at any one of them. Since, as a general rule, the most highly flocculated mineral will become attached to a bubble in preference to a less floatable particle, in normal operation the aeration in the first few cells of a machine should not be excessive ; theoretically there should be no more bubbles in the pulp than are needed to bring up the valuable minerals. By careful control of aeration it should be possible for the bulk of the minerals to be taken off the first few cells at the feed end of the machine in a concentrate rich enough to be easily cleaned, and sometimes of high enough grade to be sent straight to the filtering section as a finished product. The level of the pulp in these cells is usually kept comparatively low in order to provide a layer of froth deep enough to give entangled particles of gangue every chance of dropping out, but it must not be so low that the paddles are prevented from skimming off the whole of the top layer of rich mineral. Towards the end of the machine a scavenging action is necessary to make certain that the least possible amount of valuable mineral escapes in the tailing, for which purpose the gates of the discharge weirs are raised higher than at the feed end, and the amountof aeration may have to be increased. The froth from the scavenging cells is usually returned to the head of the machine, the middling pipes of the Denver Sub-A Machine being specially designed for such a purpose. The regulation of the cleaning cells is much the same as that of the first few cells of the primary or roughing machine, to the head of which the tailing from the last of the cleaning cells is usually returned.
A blower is sometimes required with the M.S. Subaeration Machine. Since each cell is fitted with an air pipe and valve, accurate regulation of aeration is a simple matter. The Denver Sub-A, Kraut, and Fagergren Machines, however, are run without blowers, enough air being drawn into the machines by suction.
In the Geco New-Cell Flotation Cellthe pneumatic principle is utilized in conjunction with an agitating device. The machine, which is illustrated in Fig. 44, consists of a trough or cell made of steel or wood, whichever is more convenient, through the bottom of which projects a series of air pipes fitted with circular mats of perforated rubber. The method of securing the mat to the air pipe can be seen from Fig. 45. Over each mat rotates a moulded rubber disc of slightlylarger diameter at a peripheral speed of 2,500 ft. per minute. It is mounted on a driving spindle as shown in Fig. 46.
Each spindle is supported and aligned by ball-bearings contained in a single dust- and dirt-proof casting, and each pair is driven from a vertical motor through Tex-ropes and grooved pulleys, a rigid steel structure supporting the whole series of spindles with their driving mechanism. The machine can be supplied, if required, however, with a quarter-twist drive from a lineshaft over flat pulleys.
The air inlet pipes are connected to a main through a valve by which the amount of air admitted to each mat can be accurately controlled. The air is supplied by a low-pressure blower working at about 2 lb. per square inch. It enters the cell through the perforations in the rubber mat and is split up into a stream of minute bubbles, which are distributed evenly throughout the pulp by the action of the revolving disc. By this means a large volume of finely-dispersed air is introduced withoutexcessive agitation. There is sufficient agitation, however, to produce a proper circulation in the cell, but not enough to cause any tendency to surge or to disturb the froth on the surface of the pulp. All swirling movement is checked by the liner-baffles with which the sides of the cell are lined ; their construction can be seen in Fig. 44. They are constructed of white cast iron and are designed to last the life of the machine, the absence of violent agitation making this possible.The pulp must be properly conditioned before entering the machine. It is admitted through a feed box at one end at a point above the first disc, and passes along the length of the cell to the discharge weir without being made to pass over intermediate weirs between the discs. The height of the weir at the discharge end thus controls the level of the pulp in the machine. The froth that forms on the surface overflows the froth lip in a continuous stream without the aid of scrapers, its depth being controlled at any point by means of adjustable lip strips combined with regulation of the air.The Geco New-Cell is made in four sizesviz., 18-, 24-, 36-, and 48-in. machines, the figure representing the length of the side of the squarecell. Particulars of the three smallest sizes are given in Table 27. Figures are not available for the largest size.
development of scale-up enabling technique using constant temperature anemometry for turbulence measurement in flotation cells - sciencedirect
The CTA measures the velocity fluctuations in flotation cells to calculate TKEDR.Good correlation between CTA measurements and Mean TKEDR from power input data.This measurement technique enables the scale-up of flotation process design.
The success of flotation scale-up depends on understanding of the hydrodynamic condition of flotation cells at different scales and design specifications. In order to achieve this, it is very important to measure hydrodynamic parameters such as fluid velocity, turbulent kinetic energy (TKE) and turbulent kinetic energy dissipation rate (TKEDR) in flotation cells. There are many methods that are available to measure velocity and velocity fluctuations in turbulent fluids, especially for single-phase flows or two-phase flows, however very few of them can be used for flotation cells. In this study, constant temperature anemometry (CTA) and power measurement methods were used to measure the parameters in two scales of flotation cells. Acceptable correlation between the CTA measurements and the TKEDR has been found under different hydrodynamic conditions in both cells. It shows that this measurement technique enables the scale-up analysis for turbulence in flotation cells.
basic principles & variables affecting froth flotation
The results to be obtained in flotation depend, more than in any other concentration process, on the proper coordination of a considerable number of operating variables. These variables are listed below:
The mineralogical character of the ore determines, in a way, the character of the principal flotation agent or oil to be used. From this statement it is not to be understood that for a given ore one and only one oil can be used or even, disregarding for the moment the commercial consideration of availability, can be best used. But the physical phenomenon upon which selection depends is specific to a considerable extent. By no means all minerals of metallic or adamantine luster are selected from all gangue minerals by a given oily substance, and by no means all oily substances will act successfully in all flotation processes. The mineralogical character of the ore in connection with the character of the principal flotation agent determines whether or not another agent is necessary. Thus ores containing a considerable portion of argillaceous matter will not respond successfully to flotation unless this material is, to a considerable extent, flocculated in the pulp. If the oil fails to produce this necessary flocculation another agent, usually an electrolyte, is necessary.
The mineralogical character of the ore determines also the extent to which grinding must be carried before flotation is applied. This matter has been previously discussed. In the agitation-froth process, the recoverable mineral content of an ore, the amount of a given oil necessary, the percentage of solids in the pulp treated, the grade of concentrate and the recovery attained are strictly dependent variables. This interdependence may be stated as follows:
These relations have been proven conclusively for the agitation-froth process and should, therefore, hold for the other pulp-body-concentration processes. Some similar relation is indicated for bubble-column processes, but the writer is aware of no exhaustive and conclusive work in this direction, and the dissimilarity in the mechanism of the two types of processes forbids reasoning across from the one to the other.
The size of the particles in a flotation pulp affects the percentage of solids and the amount of oil necessary. It is not unlikely, also, that it has some effect on the necessity for other agents. If the solids are coarse it is necessary to run with a thick pulp in order to attain a good recovery. A thick pulp, in general, results in a low-grade concentrate. Hence a coarse feed is likely to mean a low-grade concentrate.
More oil is, in general, necessary, if the feed is coarse. This is probably due to the fact that, owing to the lesser covering power of the coarse material, more of the stabilization of the froth must be done by the oil. The necessity for flocculation of very fine material is not present in the case of coarse feed. Hence the necessity of an electrolyte to produce such flocculation is lacking and the conclusion followsthat a coarsely-ground pulp from a given ore is less likely to require the use of acid or alkali than a finely-ground pulp from the same ore.
Oil is a generic term which, in flotation terminology, is used to designate the organic substance that is used to produce frothing and to effect selection of the metalliferous mineral. The oil employed is usually an oily substance, but it may be a non-oleaginous organic substance. Rarely, as in the Potter-Delprat process and in certain applications of the agitation-froth and pneumatic processes an inorganic compound may take the place of the oil. The character or kind of oil used depends upon:
The purpose of the oil in froth-flotation is (1) to form, together with the water and solid of the pulp and the gas introduced into the pulp, a froth; and (2), to aid in the selection of the particles of mineral of metallic, resinous or adamantine luster in the pulp from the gangue minerals. Not all oils will perform both of these functions with all ores in all processes. Newly refined paraffin hydrocarbons, if pure, will not froth to a sufficient extent to make them efficient flotation agents in the agitation-froth or pneumatic processes. Certain other substances, although possessing the property of froth formation in these processes, exclude practically all solid matter from the froth. Saponin is such a substance. Certain other agents, such as soap, cause the formation of a froth containing solid matter, but this froth results in no useful concentration. Finally, a considerable number of oily substances such as essential oils and coal-tars and wood-tars and their fractions and derivatives cause not only copious frothing but, with certain ores, efficientselection of metalliferous mineral from gangue. With other ores the selection is nil or wholly inefficient. It may be put down as an axiom of the art that no one substance is universally applicable as an oil in froth-flotation concentration of all ores.
The fineness to which the ore is ground has a considerable effect on the kind of oil necessary. A certain degree of stability is essential in every froth, enough to allow for the removal of the froth from the flotation machine. Stability of froth is affected by the extent to which the bubble walls are covered by solid matter and by the strength of the liquid films of the bubbles themselves. The more closely particles of solid are packed together in the bubble walls and the thinner the layer of liquid between two adjacent pieces of solid, the stronger will be the bubble film. In order to get the greatest covering power from a given lot of ore, it should be ground as finely as possible. With such finely-ground ore little or no aid in stabilizing the bubble films is needed from the oil. If, however, the ore is not so finely pulverized and its covering power is thereby decreased, some agent capable of
adding stiffness to the froth must be used, in order to obtain the necessary stability. Pine-tar oil and some petroleum oils are usually used for this purpose.
The percentage of solids in the pulp affects the character of oil necessary in a way similar to that in which it is affected by the degree of pulverization of the ore. A low percentage of solids in a pulp ground to a given degree of fineness means a greater distance between individual particles. It will result from this greater spacing of particles that a bubble will arrive at the surface less heavily loaded than in a thick pulp and will be, therefore, less stable. Such being the case, it will require the addition of an oil with froth-stabilizing power to make up for the lack of stability.
Another factor enters here also. If a given amount of solid matter is to be passedthrough a given machine in a given interval of time, say 24 hours, with a thin pulp agreater volume must pass through in each unit of time than with a thick pulp. Thismeans that less time is afforded for the dispersion of the oil through each unit ofvolume of the thin pulp. The rate at which an agent can be dispersed is a specificfunction of the agent itself, depending principally on its viscosity and its solubilityin water, and is also a more or less direct function of the agitation. With the degreeof agitation and the rate of solid feed fixed, it follows that with a thin pulp a moremobile or more easily soluble oil, or both, will be necessary than with a thick pulp.The method of treatment of the pulp following the introduction of the oil determines also the kind of oil most suitable. This follows from a consideration of dispersion. If the pulp is to be subjected to a considerable amount of agitation, eitherviolent or of long duration, or both, after the introduction of the oil and beforeflotation is to be attempted, then a viscous or only slightly soluble oil will be effective. If, on the other hand, flotation is to be attempted with but little agitation intervening following the introduction of the oil, a mobile and relatively soluble oil mustbe used.
Mobile and highly soluble oils can be employed in smaller quantity, all otherconditions being equal, than viscous and relatively insoluble oil. This follows naturally from the preceding discussion. Mobile and highly soluble oils are easily dispersed in an extremely high state of subdivision, while viscous and slightly solubleoils are dispersed more slowly and to no such high degree. In pulp-body concentration the function of the oil is to coat the mineral particles. In bubble-columnprocesses it is essential that the rising bubbles become oiled. An extremely thinfilm is all that is necessary. But in order to insure that the sulphide particles in theone case and the air bubbles in the other shall come into contact with oil, a certainminimum spatial distribution of the oil in the pulp is necessary. In order to insurethis minimum spatial distribution with a viscous and relatively insoluble oil, necessarily in relatively large masses as compared with the particles of a mobile andhighly soluble oil, a greater amount of the former must be used. Owing to thegreater size of the masses of the viscous and insoluble oil the films on the particles and the bubbles will exceed the effective minimum, and further the amount ofexcess oil which does no coating but which is necessarily present in order toaccomplish the required spatial relation will, in this case, exceed in bulk that unused in the case of the mobile or highly soluble agent.Above a certain minimum quantity, all other conditions being constant, theamount of oil necessary in the. agitation-froth process varies directly with theamount of recoverable mineral in the ore. This is easily proven experimentally andcan be predicted from theoretical considerations as follows: The maximum surfacethat can be covered by a given quantity of a given oily substance is measured bythe area of the film, one molecule thick, which can be obtained from the givenamount of agent. In any successful agitation-froth flotation operation it is essentialthat all of the sulphide mineral particles be coated to at least this extent. This coating cannot be accomplished without the presence of an excess of the agent in thepulp. Hence the minimum quantity of agent necessary is some probably fixed excess over that required to coat the sulphide particles with a layer one moleculedeep, which excess depends upon the degree and duration of agitation, the kind ofagent and the thickness of the pulp. Any increase in the amount of metallic mineralin the pulp means an increase in the area to be covered by the oil and hence an increase in the amount of oil that must be provided.
Above a certain minimum quantity, all other things being constant, the amountof oil necessary in the agitation-froth process to make a given recovery from agiven ore with a given grade of concentrate varies directly with the percentage ofmoisture in the pulp, within the efficient working range of moisture percentageswhich is from, say, 65 or 70 per cent to 90 or 95 per cent. This is confirmed by experimental data and follows logically from a theoretical analysis. As has been previously stated, a certain minimum spatial distribution of the particles of oil in thepulp is necessary in order that the metalliferous mineral particles may be coatedduring the time that the pulp is under treatment. If the volume of pulp carrying agiven amount of solid matter is increased, then the number of particles of oilnecessary to produce the minimum spatial distribution of the same throughout the
total volume of pulp will likewise be increased.
If a pulp containing solid matter ground to agiven degree of fineness is being concentrated by flotation with a given minimumquantity of a given agent, the same metallurgical results can be obtained with asmaller quantity of agent, if the solids are more finely ground. Conversely more oilmust be used, if the grinding is so changed that the product to be floated is coarser. The explanation of this observed phenomenon is, probably, that a certain degree of stability is essential in the froth and that this stability may be provided byeither oil or solid matter. If the covering and hence stabilizing power of the solid isincreased by finer subdivision, the oil is relieved of part of its duty and less of it,therefore, is necessary. Vice versa, if the covering and stabilizing power of the solidis decreased, as by coarser grinding, more burden is placed on the oil and it mustbe increased in quantity.The treatment to which the pulp is subjected in the interval between the additionof the oil and the actual operation of flotation has a considerable influence on thequantity of agent used. A certain minimum degree of dispersion of the agent isessential, as has been previously explained, in order to assure such spatial distribution of the agent that every air bubble or sulphide particle shall become coated.
It is necessary further that the agent be dispersed to such a degree that gravity willhave little or no effect to cause it to collect together and that the tendency of theparticles of the agent to coalesce shall be overcome. Otherwise the agent will collect on the surface of the pulp and break down the froth. Hence, if an oil of a givencharacter is employed, the amount of agitation required to disperse it must be increased, as the amount of oily agent is either decreased or increased from a certainamount corresponding to a minimum degree of agitation for mixing.
The quantity of oil necessary and the duration of the flotation operation, all otherconditions being constant, bear a close relation to each other. With any givenminimum amount of agent and minimum duration of the flotation operation corresponding thereto required to attain a certain result, a lessening of the duration ofthe operation will require an increase in the amount of oil and vice versa.
The composition of the ore, particularly as regards the gangue, affects the quantity, of oil necessary. Clean, hard ores require less oil, all other conditions being constant, than do soft clayey ores. One reason for this difference probably lies in the fact that the clayey gangue matter from the soft ores acts to emulsifyoil and thus prevents a certain amount of the latter from playing any part in theprocess. Some sulphides, also, require a less quantity of oil to separate them froma given gangue than do others. Thus galena can be separated from a given ore withless oil than can blend, and chalcocite is separated with the use of a less quantityof oil than pyrite. Some methods of differential flotation are founded on this phenomenon.
Pulp-body-concentration processes requires more oil to be employed,all other things being equal, than do bubble-column methods. This is due to thedifference in the phenomena acting in the two cases. In the pulp-body processesusing oil it is necessary that the sulphide particles become coated in the pulp before effective selection takes place and this necessitates thorough and quick dispersal of the oil. Further, a given particle of oil is effective only during the shorttime that it is below the pulp surface. In the bubble-column processes, where all ofthe work of the flotation agent is done above the pulp surface, coating of all thebubbles in the pulp is not necessary (since those that rise without coating will becoated in the bubble column itself) and hence the need for so large a number of oilglobules floating around therein does not exist. Further, each oil-coated bubbleserves to separate a considerably greater bulk of solid than is done by a corresponding area of bubble surface in the pulp-body processes. It should be notedalso in this connection that, on account of the much larger size of the bubbles inthe bubble-column processes, a bubble rising through a pulp containing particlesof flotation agent in a given spatial distribution, is much more certain to meet suchparticles than the very much smaller bubble in the agitation process.
The role of the minor agents (pH Modifiers) is to increase the grade of concentrate, i.e., aid in selection, and to a lesser extent, aid recovery. Various theories havebeen advanced to explain their action. In general they are electrolytes, and ingenious hypotheses have been based on assumed accentuation, due to their ions, inthe difference in magnitude of the electrical charges said to exist at the surfaces ofthe solid particles in the pulps. Excluding for the present the cases in which theminor agent reacts chemically with the principal agent or oil, it is a commonly observed experimental fact that successful use of a minor agent is accompanied byincreased flocculation of the flotation pulp, particularly of the flotation tailing. It isfurthermore usually true that the tailing from an unsuccessful flotation operation isslow-settling, indicating a lack of flocculation. Hence we may set down as anempirical rule that a suitable minor agent will be one that flocculates the pulp.
When the pH Modifierreacts chemically with the principal agent, the cause towhich its effect is to be ascribed is masked. It is not improbable that in such casesany improvement is as much due to the changed character of the principal agent asto the independent flocculating effect of the minor agent. Particularly is this truewhen the addition of the minor agent results in increased recovery accompanyingincreased frothing. Such chemical reaction is to be looked for where the principalagent is a vegetable or animal oil, and the minor agent an alkali.
The necessity for the use of a minor agent depends upon,(1) the mineralogicalcomposition of the ore, and (2) the character of principal agent employed. If theore is unaltered, hard and silicious, it is certain that with some principal agents nominor agent will be necessary. With other principal agents, however, the grade ofconcentrate may be improved by the use of a minor agent. If the ore is much altered with a resultant large amount of kaolinized gangue, a minor agent is likely tobe necessary, although here, also, the character of the principal agent may be such that the use of a minor agent can be dispensed with.The minor agent most widely used is sulphuric acid. Sodium hydroxide, sodiumcarbonate, sodium sulphate, sodium silicate and lime are not uncommonly employed. It is the writers experience that so far as flotation itself is concerned, whena minor agent is needed, sulphuric acid is the cheapest and most effective. Its usemay, however, as is the case with partially oxidized copper ores, be prohibited onaccount of the destructive effect of the resultant copper sulphate solution on theiron with which the pulp comes in contact in the mill, in which case an alkaline orneutral electrolyte may be effective.
Percentage of solids affects kind and quantity of principal flotation agent necessary, the quantity of minor flotation agent, the fineness of grinding, the grade ofconcentrate and the recovery attained. It is determined principally by the characterof the ore. The effect on principal flotation agent and fineness of grinding has already been discussed.
The relation between percentage of solids and quantity of minor flotation agentseems to point to the conclusion that it is the concentration of the aqueoussolution of the minor agent, rather than the quantitative relation between the minoragent and the solid, which determines its effectiveness. No systematic experimental work along these lines is reported but mill experience and laboratory experiment both indicate that the amount of minor agent necessary increases with decrease in the percentage of solids.
Grade of concentrate is, in general, improved by decrease in the percentage ofsolids in the pulp, and such improvement may be accompanied by improvement inrecovery, but continued decrease in percentage of solids eventually results in decrease in recovery.
The character of the ore determines the maximum economic percentage ofsolids. With clean, sandy ores this is about 30 per cent., with clayey or slimy ores,about 15 per cent. Slimy pulps as thin as 5 per cent, solids can be treated, althoughgenerally at the expense of a low recovery.
Heat aids gas precipitation from the water of the flotation pulp; italso aids in dispersion of the more viscous oils. Temperatures above normal are,therefore, of distinct advantage in pulp-body concentration. Heating can be dispensed with, however, by increasing some other factor such as the concentrationof gas in chemical-generation processes, the duration or intensity of agitation inthe agitation-froth process or the pressure difference in pressure-relief processes.In bubble-column processes, where bubble precipitation from solution is not theessential phenomenon in furnishing the effective gas, heat is not one of the important factors, and the temperature of the pulp is, therefore, unimportant, exceptinsofar as heat may be desirable to aid dispersion of the oil.
In the agitation-froth process, it is, in general, more economical to attainenhanced bubble precipitation by greater intensity and duration of agitation than byheating the pulp, and in bubble-column processes difficulties in dispersion of theoil are surmounted by adding the oil to the grinding mills, or using an oil easy todisperse. Heating is, therefore, rarely resorted to in present-day practice.
* Throughout the following discussion the phrases mineral of metallic luster,metallic mineral, metalliferous mineral and sulphide, or sulphide mineralwill be used interchangeably, as is the usual case in flotation terminology, to designate minerals of metallic, adamantine or resinous luster.