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An Introduction to Froth Flotation

Note:

This is an introduction to the mechanics and operation of froth flotation equipment.  It is not a complete guide to the subject.  This article, and any subsequent articles, will deal with flotation equipment and its use.  Any reference to reagents will purely be in a general nature.  There will only be cursory information on the chemical aspects of flotation, as dealing with any particular ore could require a large manual on that ore alone.

 Development and use of froth flotation as a beneficiation process has been ongoing since the first part of the last century.  Initial study of the flotation concept was in the late 19th century.  The basic process involves the selective coating of a particle's surface to alter or enhance its surface chemical characteristics.  The flotation process is widely used for treating metallic and non-metallic ores.  A greater tonnage of ore is treated by flotation than by any other single process. Practically all the metallic minerals are being recovered by the flotation process and the range of nonmetallic minerals is steadily being enlarged.

 Flotation, or more specifically froth flotation, is a physicochemical method of concentrating fine minerals and coal.   The process involves chemical treatment of a pulp to create conditions favorable for the attachment of particles to air bubbles.   Some particles are not readily wetted by water (hydrophobic), while others are readily wetted by water (hydrophilic).  By the addition of chemicals these proper­ties can be enhanced.  Air bubbles are created by the rapid motion of the agitator mechanism which draws air down the hollow shaft and disperses the air into the pulp.  The air bubbles carry the hydrophobic particles to the surface of the pulp and form a stabilized froth which is skimmed off while the hydrophilic particles remain submerged in the pulp. 

 TYPICAL CIRCUIT

More than any other beneficiation process, there is probably no such thing as a “typical” flotation circuit.  Development of a flotation circuit is entirely dependent on the characteristics of the ore and what works at one operation may not work at a nearby operation.  And even in one operation the requirements can and will change over time.  This requires continual test work.  That said there are some fairly common flotation arrangements.

 One of the most common is the rougher-scavenger-cleaner arrangement; where the first couple of cells (roughers) are set to produce a high grade concentrate, which are followed by cells (scavengers) to make maximum recovery.  The break between rougher and scavengers is sometimes set by concentrate grade but can vary over time.  The rougher concentrate may be a final concentrate or a portion combined with the scavenger concentrate which goes to cleaner section.  Often there is a regrind mill between the scavengers and the cleaners.  The cleaner concentrate will often go to final concentrate while the cleaner tails and the scavenger tails are a final tail.  For a complex ore, or a multi-product ore this circuitry can get to re-cleaners, secondary flotation, and even more complex circuits.  All of which is beyond an introduction level article.

 EQUIPMENT DESCRIPTION

Most flotation cells operate in the same manner, although there are a few exceptions, but this section will deal with the general operation of the majority of cells. Later articles will deal with some of the specialized units such as column cells.

 

The following figure shows a generic float cell, with the operating zones and general flow pattern.  The following sections will discuss some of this.

 

 Pulp Flow and Circulation

The pulp flows by gravity into each cell through the feed pipe, from which it is fed into the impeller in the mixing zone.  As the pulp flows over the impeller blades it is thrown outward and upward from the impeller and diffuser by the centrifugal action of the impeller. The pulp is kept in complete circulation by the impeller action and as the flotation reaction takes place, the pulp is passed from cell to cell. Pulp flows to each succeeding cell through the tails section, which in small cells can be an overflow weir, or on larger cells be by valves controlling the outflow through the side or bottom. This gives accurate control of pulp level as the pulp passes through the machine.

 A flotation machine must not only be able to circulate coarse material (encountered in practically every mill circuit) but also must re-circulate and retreat the difficult middling products.

 It is not essential to have each individual cell with separate tails control; however, for most installations this is recommended. An alternate arrangement (for smaller rectangular type cells) is with gate control every two to four cells for pulp level control, and free pulp passage from cell to cell, by means of the ports, as well as cell to cell overflow. The arrangement is actually a "grouping" without sacrificing the positive circulation feature.

 Aeration and Mixing

The passage of pulp through the cell and the action created in the impeller zone draws air down the standpipe (or it may be a low pressure air system). The impeller zone thoroughly mixes the air with the pulp and reagents. As this action proceeds, a thoroughly aerated live pulp is produced and furthermore, as this mixture is mixed together by the impeller action, the pulp is intimately diffused with exceedingly small air bubbles which support the largest number of mineral particles.

 Aeration is accomplished by one of two methods; either natural or forced/induced aeration.  Under natural aeration, the design of the impeller and diffuser natural draw air down the standpipe into the mixing zone.  For some applications, this may not be sufficient or a simpler impeller design is desired, and for these a forced or induced aeration system is used, where low pressure (commonly under 10 psig (0.6 atmospheres) air is supplied to the cell.  This feature is accomplished by the introduction of air from a blower or turbo-compressor through the standpipe connection into the aerating zone where it is premixed with the pulp by the impeller action.  Induced air is of particular advantage for low ratio of concentration and slow-floating ores.

 Throttling of air is of benefit when suppressed flotation is required. This is accomplished by cutting off or decreasing the size of air inlet on the standpipe.  Suppressed flotation finds its chief use in certain nonmetallics and occasionally in cleaner or recleaner operations.

 The aeration and mixing of the pulp with reagents all takes place in the lower zone of the cell. Thorough mixing is to a considerable degree responsible for the metallurgical efficiency of the cell.

 Separation

The aerated pulp, after leaving the mixing zone, passes upward by displacement to the central section of the cell. This is a quiet zone and is free from cross currents and agitation. In this zone, the mineral-laden air bubbles separate from the gangue and pass upward to the froth column without dropping their load, due to the quiescent condition. The gangue material follows the pulp flow and is rejected at the discharge weir or valve.

 It is in the separation zone that effective aeration is essential and the air is broken up into minute bubbles. These finely diffused bubbles are essential for carrying a maximum load of mineral.  

 Concentration

The mineral-laden bubbles move from the separation zone to the pulp level and are carried to the overflow by the crowding action of succeeding bubbles. To facilitate the quick removal of mineral-laden froth, some cells are equipped with froth paddles. Froth removal can be further facilitated by the use of crowding panels which create a positive movement of froth to the overflow.   Cells normally have the overflow along the outside edges, while larger circular cells may have additional overflows running towards the center.  These additional overflows do cause issues during agitator maintenance.

 Coarse Material Handling

Positive circulation of all pulp fractions from cell to cell is important. Minimizing short circuiting, which can occur through the machine is important; so that every particle is subject to positive treatment. In instances where successful metallurgy demands the handling of a dense pulp containing an unusually large percentage of coarse material, the use of bottom mounted valves provides additional sand relief in the machine operation. This opening removes from the lower part of the cell the coarse fractions and passes them through the feed pipe to the impeller of each succeeding cell. The sand relief openings assure the passage of slow floating coarse mineral to each impeller and therefore it is subject to the intensive mixing, aeration and optimum flotation condition of each successive cell.  The passage of the coarse fractions through each impeller minimizes short circuiting and thus, both fine and coarse mineral are subject to positive flotation.

 EQUIPMENT SIZING

Flotation cells are normally set up in rows or banks of equal sized cells.  The size of the cells and number of cells of a flotation bank or row depends upon facts and conditions which can best be determined by test work and modified by experience.  At a given/desired pulp density and reagent combination, a certain flotation contact period/residence time is required to obtain the desired recovery and grade.  This contact time and pulp density determines the volume required for a given feed rate in tons per unit time.

 Residence/Contact Time

Flotation contact time required for the ore is one of key factors in calculating capacity.  If an ore is slow floating and requires twelve minute treatment time, and another ore is fast floating and requires but six minute treatment, the second ore requires only half the capacity of the first.  With the residence time and knowing the pulp density and specific gravity of dry solids the cubic feet of pulp handled by the flotation machine, so are determining factors in calculating the flotation contact period.  

Results

Metallurgical results required from the flotation machine will have considerable bearing on the installed capacity. Several stages of cleaning may be required to give a high grade concentrate. Results with cells of equal volume will not necessarily be equal because they may not be equally efficient.

 Volume

The volume of the flotation cell determines the time available for flotation to take place. Therefore, the capacity of any flotation machine is dependent on the volume.  All flotation cells having the same volume will have approximately the same capacity, with allowance made for horsepower, the efficiency of the impeller and aeration. As the flotation contact period is very important in any flotation machine, the actual cubical content of any machine should be carefully checked as well as accurate determinations on average pulp specifications.

 To determine the number of cells required, firs determine the volumetric flow (cubic feet/minute, cubic meters/minute, or similar) of pulp and multiply by the desired residence time, this gives the required flotation volume.  Based on the desired operating philosophy of one, two or more rows/banks of cells divide the total volume by the number of rows/banks to get the volume per row/bank.  Then divide this by the desired number of cells per row/bank.  

 In order to secure the maximum positive treatment of the mineral, and to produce a· desired concentrate grade, it is best to have the necessary total volume divided into at least four cells and preferably five or six separate cells, so that they may be used for roughing, cleaning, or recleaning purposes.  Alternatively the cleaners/recleaners can be a separate row/bank preceded by scrubbing, attrition, or modifying reagents.

 Recovery

Recovery in flotation is of prime importance. In studying recoveries it is essential also to investigate thoroughly the intermediate products produced. It is a simple matter to make a high recovery or a low tailing if no thought is given to the nature of the concentrate produced or circulating load.

 A 'comparison of product assays does not give true and complete information with respect to the performance of a flotation machine. . Product assays for two flotation machines operating in parallel could quite conceivably be identical, yet the physical characteristics of the products recovered and discarded would be entirely dissimilar. Wide differences which would be obvious in detailed investigation might not be indicated by a cursory examination.  

 Higher recoveries have been possible in many instances by changes in grinding and removal of coarse primary concentrates. Recovery at a coarser grind means a decreased amount of slime mineral in the pulp. Absence of slime in concentrates is reflected in the analysis of the insoluble fraction.