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Column flotation: A selected review— part IV: Novel flotation devices
Minerals Engineering, Vol. 8, No. 6, pp. 587-602, 1995
Pergamon 0892-6875(95)00023-2
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875/95 $9.50+0.00
COLUMN FLOTATION: A SELECTED REVIEWm PART IV: NOVEL FLOTATION DEVICES*
J.A. FINCH Department of Mining and Metallurgical Engineering, McGill University, Montreal, Canada, H3A 2A7 (Received 20 October 1994; accepted 5 December 1994)
ABSTRACT
Some of the novel devices in flotation that have entered the market place recently are reviewed. They are grouped into three categories: developments in mechanical cells, developments in flotation columns, and new cell designs selected here to illustrate the reactor/separator concept. In the first category, froth washing (is this transfer of technology from columns a simple matter?) and froth boosting (an old idea re-visited) are considered. Developments in columns include height ("short" can be beautiful), sparging (a chronic problem that may now be solved) and baffling (an unresolved issue but one that packing may make redundant). Lastly, the reactor/separator concept is introduced and the Jameson, Pneumatic, Contact and Centrifloat R cells are analyzed as representatives of this category. A projected future design based on this concept is offered.
Keywords Column flotation; spargers; Jameson cell; pneumatic cell; contact cell; centrifloat
INTRODUCTION The author has attempted through reviews to keep abreast of the developments in the general area of column flotation [1, 2, 3]. This is the fourth review and focuses on "novel developments". The past 15 years has seen a surge in development of flotation devices reminiscent of the early days of flotation. This may be credited, in part at least, to the acceptance of flotation columns by industry over this period which seems to have created a climate encouraging to both developers and users. Flotation columns have directly driven some developments (e.g. in sparger systems), have spawned derivatives (e.g. the Packed column) and new concepts (e.g. the Jameson cell), and have provoked developments in mechanical cell technology (e.g. froth washing). The rapidly growing new areas of application of flotation technology, such as de-inking recycled paper pulp and de-oiling water (in industries ranging from petroleum to food) will no doubt spur more developments. Not all the developments will be successful for both technical and marketing reasons [4]. All are worth examirfing, however, if one accepts the argument that some feature may offer a genuine advance and a judicious combination of "best" features may usher in the next generation of flotation machines. It is difficult for practising engineers to keep up with these developments which prompted the writing of this paper. The choice of what to include in any review is problematic: To limit the scope and provide a focus, only developments which target fine particle processing and have had some industrial exposure are considered. *Basedon PlenaryLecture,CONAMET'94, Antofagasta,Chile,August9, 1994
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The devices are grouped "under three headings: mechanical cells, flotation columns, and reactor/separator cell designs.
MECHANICAL CELLS
Froth washing A key (some would claim unique) feature of column flotation is the use of wash water [5]. Wash water, providing it travels down through the froth and crosses the froth/pulp interface, called a "positive bias", replaces the contaminated water reporting to the froth from the pulp and consequently reduces the entrainment recovery of fine hydrophilic particles. There is no apparent reason why wash water should not be tried in mechanical cells. This has indeed been done, Klassen and Mokrousov [6] for instance, give examples of froth washing. What is different now is a better understanding of the mechanism of hydrophilic particle recovery, the availability of techniques to detect it (e.g. through recovery-by-size analysis [7]), and a realization that engineering the solution through wash water addition may require considerable experimentation [8]. The engineer who has determined that entrainment is a problem can more confidently persist with finding the solution than his less well prepared predecessors. An example of washing mechanical cell froths is at Zinc Corporation of America's Balmat operation [8]. Non sulphide gangue recovery was lowering zinc grades to unacceptable levels and entrainment was known to be a factor. Wash water addition to the zinc cleaners was targeted. From most experience on columns, wash water is generally added as a gentle "rain"; however, in this case wash water had to added with some force to penetrate the stiff froths encountered. Once this was implemented, zinc grades improved (Figure 1) and this success led to the current practice at Balmat of froth washing practically all the cells.
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Fig.1 An example of froth washing mechanical cells; wash water addition increased zinc grades at Zinc Corporation of America's Balmat operation (adapted from Kaya and Laplante [8]).
Novel flotationdevices
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There is no consensus on the best means of introducing/distributing wash water in mechanical cells (or columns) and some studies are being conducted (e.g. [9]). In any event, transferring froth washing practise from columns to mechanical cells may not be as straight forward as it seems. One target of column control is a positive bias and instrumentation is provided to that end. It is not clear how easily this control philosophy can be used in a mechanical cell without the necessary instrumentation and hence the potential benefit of wash water may not be realised. The action of wash water needs further investigation to determine how best to make the transfer. To date most work on froth washing in mechanical cells has involved "homemade" devices. Mechanical cells engineered to include wash water are now available and in combination with froth boosting, have introduced a new line of cells from Outokumpu [10].
Froth boosting Deep froths often give higher upgrading than shallow froths as they give extra time for drainage. Building a deep froth can be difficult in some cases. Reducing the cross-sectional area of froth relative to the pulp often increases froth depth probably by increasing particle loading and altering the relative phase velocities. It is known as froth crowding or boosting. Outokumpu [10] have recently engineered cells to include froth crowding. The HG tank cell uses an adjustable booster cone (Figure 2) to crowd the froth into the annular space. The position of the cone influences two w~ables, the grade (by changing froth depth) and solids removal rate or capacity (by changing surface :area). By making the cone adjustable, some control over both these variables is introduced.
ADJUSTABLE BOOSTER CONE
FROTH
FEED
Fig.2 Diagrammatic illustration of Outokumpu's HG tank cell showing the variable booster cone concept (adapted from Green and Cox [10]).
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Adjusting the cone position, it can be argued, alters the geometry of the cell: with the cone out, the cell has a low height to surface area ratio compared to that with the cone in. This is like changing the geometry from that typical of mechanical cells to that more akin to flotation columns and to emphasize this connection the cells are also called "virtual columns"[10]. This variable geometry may mean the same cell can take on different duties. The general experience to date is that mechanical cells remain the preferred choice for roughing with columns largely restricted to cleaning. If this difference in duty is related to geometry then an HG tank cell can adopt both duties by having the cone out and in, respectively. (Attributing the difference in duty to geometry is debatable but provided some of the initial impetus behind development of the cell.) In a bank, the cone position could be adjusted on individual cells.
FLOTATION COLUMNS The column that is referred to here is that developed in the early '60s [11]. It has been pointed out that cells with a columnar geometry were tried prior to 1920 [12,13]. Those earlier units, however, did not use wash water nor did they have a control philosophy based on bias reflecting an understanding of the problem of entrainment. Accordingly, they do not fall into quite the same category; if they did it would also mean that the industry took some 60 years to realize a good idea which is a little difficult to accept!
Height The original flotation columns marketed by Column Flotation Co. of Canada were 40--45 ft high [5] and this became, in essence, the standard. One advantage was that height was then not a factor and scale up could focus on the number and diameter of the columns required. The need for this height has always been in question, however. Some justification was given from analysis of mixing by noting that the greater the height to diameter ratio the less mixed were the column contents and consequently recovery and selectivity should improve [14]. Some researchers, linking the need for such height with a low particle collection rate, developed new designs aimed at increasing flotation rates by providing more intense particle-bubble contacting [e.g. 15,16]. Others attributed the low rate to mixing and short circuiting effects associated with the "open" nature of the conventional column and proposed various forms of baffling [e.g. 17]; the Packed column (see below) falls into this category. Yet other investigators simply tested different heights, in some cases showing short columns sufficed [18], in some cases did not [19]. Recent installations in Canada have seen columns of 20 ft at Myra Falls and I0 ft at Les Mines Gasl~. A plant in Norway is being designed with short (5 m) and tall columns in series. As experience and confidence grow more deviations from the "conventional" height will be seen. Bubble-particle collision probability models have been used to analyze the height issue [20,21]. Defining the minimum height as that achieving at least one collision, the models show that height is a strong function of particle size with heights increasing from less than lm for > 50ttm particles to exceed 10m for <101arn particles. An engineering solution to the required height for a given process may develop from the work of Ityokumbul [ 13]. He has proposed replacing the common kinetic approach with a mass transfer approach (although this may mean replacing one difficult-to-measure set of parameters by another). The technique needs extending to include interaction between the collection and froth zones as froth dropback can dominate column design in some situations. To illustrate the need to consider these i n , actions, one possible argument against tall columns is that: the bubbles become too loaded and this may actually reduce the solids carrying rate by overloading the froth in a manner similar to that when a column is "overfed" [14]. The decrease in recovery above a certain height observed by Maksimovetal. [22] may have its origin in this effect. The "overfeeding" phenomenon has never been explained. The capacity limitations apparently imposed by the froth have led to testing "zero froth column flotation" [23].
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Spargers The first spargers (i.e. bubble generation devices) were made from porous material such as filter cloth and perforated rubber. These had a number of drawbacks: blockage due to particles and/or precipitates tearing and general deterioration with use the need to shut down to change the large number required to maintain bubble size below -2-3 mm Improved sparger design has been a recurring demand by operators and has tested the ingenuity of developers. With porous materials, bubble generation is by formation of individual bubbles at an orifice. Two other bubble generation principles are by jetting and shearing. Examples of spargers based on the first are those developed by the USBM/Cominco and Minnovex (Figures 3a,b, respectively), and examples based on the second are those used in the MicrocelTM column and the Pneumatic cell (Figures 3c,d, respectively).
air
water
I~i
~
~" JI" I
Fig.3a The USBM/Cominco type sparger; bubbles are created by jet that forms at the orifice enhanced by a small addition of water.
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MOVABLE HI
-,. j - . ~ - 0 0 - 4 0 0 m / s )
Fig.3b The Minnovex variable gap sparger; a jet forms at the annular orifice controlled by gap width and gas pressure.
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gas s/u
bubble - slurry dispersion Fig.3c Schematic of sparger on MicrocelTM column showing use of a static in-line mixer to create bubbles by shear.
slurry
gas
gas
bubble - slurry dispersion Fig.3d Design of sparger used in the Pneumatic cell. In jetting techniques, bubbles form as a result of instabilities on the jet surface. The number of bubbles (nB) and bubble size (dB) depend on jet length (Ij), which in turn is dependent on jet momentum (per unit volume, pjvj), in thefollowingmanner
nB~~
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dBo~lflJ
(2)
Ijocpjvj
(3)
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Since producing a large population of fine bubbles is the general objective, a long jet length (i.e. high jet momentum) is required. Jet momentum can be increased by increasing either the density (p j) or velocity (vj) terms. In the USBM and Cominco designs, the jet is formed at a -1 mm diameter (circular) orifice and pj is increased by a small addition of water with the gas (typically ~ 1% of the volume of the gas) (Figure 3a). The effect of water addition is quite dramatic, the jet length visibly increasing with fine bubbles being produced and distributed across the cell (Figure 4). With the Minnovex "variable gap sparger", the approach is to control vj by adjusting the annulus (Figure 3b) dimensions and gas pressure. A small annulus width (~1 ram) and gas pressures above ~45 psig combine to give velocities between 200 and 400 m/s [24] and produces at least a 50cm long jet (Figure 5).
(a)
(b) Fig.4 USBM/Cominco type sparger in action: a). view along the sparger, no water added; b). as a) but with water added with the gas; note the jet forming on both sides which was not evident in a). (In both situations there was no frother present otherwise the jet in b) is lost in the cloud of small bubbles produced.)
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Fig.5 The Minnovex variable gap sparger in action; note the jet length is at least equal to the sparger length, 50 cm in this case. (There was no frother present for the same reason as in Figure 4.) Shearing involves the rapid motion of a fluid relative to another. In the MicrocelTM column sparger, shear is achieved by forcing slurry and air over the blades of an in-line static mixer (Figure 3c). The slurry is drawn from the cell contents [25] (but other sources of slurry, e.g. the feed [26], t o d d be substituted). In the Pneumatic cell, feed slurry is forced through a constricted opening, reaching velocities of ~ 4--6 m/s, and shears the air, introduced at right angles from a slot, into fine bubbles (Figure 3d). The acceptance of new spargers will be based on reliability of operation rather than, for example, any ability to control bubble size. The latest device, the Minnovex variable gap sparger, is proving quite popular (including use to supplement air in self-aeration mechanical cells at one plant in Chile). The attractions appear to be: readily replaceable, wear resistant head (tungsten/silicon carbide) ability to change orifice size on-line short length protruding into the column facilitating withdrawal for on-line maintenance the need for relatively few spargers (e.g. typically four on a 2 m dia. column) no need to use sparger water, or to produce high velocity slurry jets Manufacturing these spargers has demanded the use of a computer-controlled lathe to ensure uniform annulus dimensions to further limit wear rates. Sparger devices using slots are being investigated [27] and gas evolution by electrolysis (to give hydrogen and oxygen) is still periodically pursued [28]. Future developments may be to combine bubble generation with particle collection in what might better be d e s e r i ~ as a "reactor" rather than a sparger. Appropriate designs may be able to exploit dissolved air [29], and eavitatiordnucleation to selectively "activate" particles by frosting them with fine bubbles [30]. Baffling/Packing The "open" nature of conventional columns has always raised the issue of mixing. The more mixed are the contents of a flotation machine the lower the recovery and selectivity. This is recognised in mechanical cell installations by placing the cells in series to form banks. Columns in series are recommended for the same
Novel flotation devices
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reason [14]. One of the initial attractions of columns, however, was the apparent ability to replace several stages of mechanical cells [5]. There was a belief that this was due to reduced mixing inherent in the column geometry, thus the mixing characteristics became - - and remain - - a subject of investigation. One way to decrease mixing is through baffling. Vertical baffles to divide the column into compartments are quite widely used (in part for structural integrity). As a working target, a compartment diameter approximately that of the original column (i.e. ~ 1 m) was suggested [14]. A decrease in mixing as a result of such baffling, however, has usually not been evident [3]. The reason is probably that identified by Moys et al. [31]: Uneven distribution of feed and gas to the compartments causes circulation between them compounding rather than easing the problem. Various forms of baffles have been tried, including horizontal ones [17,29], but perhaps the most promising approach is to go as far as "packing" the column. The Packed column, developed by Yang [32], uses stacks of corrugated plates to provide a tortuous path for both bubbles and particles. Reduced mixing is one claim, but others include the ability to support an almost unlimited froth depth (which should aid fine particle cleaning), and the absence of any sparger as the packing serves to disperse the gas. The performance advantage over open columns was difficult to detect at the laboratory scale (in a de-inking study), in part because mixing is not then an issue [33]. The advantages (e.g. increased capacity) should be realised on units larger than about 1 m diameter. A retrofit to pack an open column has been completed at a plant in Chile [34] and the preliminary oral reports sound encouraging; results are awaited with interest.
REACTOR/SEPARATOR DESIGNS Concept The argument is the fidlowing: Flotation has two basic functions~ to attach particles to bubbles and then to separate these bubble--particle aggregates from the slurry, and the current design of flotation machines is a compromise between these two functions which limits the overall performance. The two functions are recognised in the design of mechanical cells which provide a region of intense mixing around the impeller and a quiescent region above (e.g see Figure 2). Flotation columns in this regard are also compromise designs. The concept, therefore, is to isolate the two functions, as illustrated schematically in Figure 6, in anticipation that the two functions can be optimized individually and an overall improvement in machine performance be realized.
SLURRY
REACTOR
AIR
FROTH
SEPARATOR
SLURRY
Fig.6 "[he general concept of the reactor/separator class of flotation machine.
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The concept appears to have been in the mind of some cell developers [e.g. 35,36] but the present author believes it serves to group several of the recent novel cell designs. The following aims to show how the concept applies rather than offering any detailed commentary on the metallurgical performance of the cells.
Jameson cell Developed in Australia in the mid-'80s [15], the unit has been used in coal and mineral processing and deoiling [37]. The unit consists of two parts, the downcomer where slurry and air are mixed and the "ceU" (Figure 7). More than one downcomer can feed into one cell.
a/r
FEED
DOWNCOMER
wasi
CELL
CONCENTRATE
~TAILS TAILINGS VALVE
Fig.7 Schematic of Jameson cell. Slurry is introduced into the downcomer via a nozzle to form a jet. The jet entrains air giving a slight vacuum which simultaneously supports the pool in the downcomer and draws in air. The air is sheared into bubbles as the jet plunges into the pool and all particle collection occurs in the downcomer; hence the downcomer is the "reactor". The downcomer contents discharge into the cell where disengagement of bubble-particle aggregates from the slurry occurs and a froth forms (which is usually washed). The cell, therefore, is the "separator". There is a growing body of operational detail becoming available. Four features are: the gas to slurry rate should be about 0.3-0.9 for stable operation of the downcomer [38,39] gas holdup in the downcomer is sensitive to percent solids in the feed [39] the gas rate per unit area of the separation chamber (or superficial rate) should be above a certain value to maintain a stable froth (e.g. ~ 0.7cm/s in the absence of solids [40]) unless automatically controlled, gas rate changes as level in t h e ' s e i ~ t i o n chamber changes [40] From this it is evident that here are some interactions between the reactor and separator which the concept in its pure form does not have.
Novel flotationdevices
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The jet in the downcomer is confined by the walls but unconfined or free jet devices, i.e. where the jet plunges directly into an open vessel, have also been proposed [41]; indeed, the Cascade cells developed earlier this century [42], a version of which is in operation at one mine in Chile recovering chalcopyrite from the tailings, fail into the free jet category. Pneumatic cell This cell continues to be developed and has been around for some time in a variety of forms [43]. Perhaps the simplest version 1:o illustrate the principle is that in Figure 8. Feed is forced through the sparger (Figure 3c), down a vertical pipe and into a tank. The sparger plus pipe represent the reactor and the tank the separation chamber. By distributing the feed, several reactors can discharge into a common separator. To date the author is aware of testwork in base metals minerals and in soil ,decontamination [43].
[44] and commercial experience with industrial
FEED air
~TAIL Fig.8 Schematic of Pneumatic cell; bubble generation device (at top) is that in Fig. 3d. Contact cell Figure 9 shows a schematic [36]. Water and air are injected into essentially a USBM/Cominco type sparger and the bubbles are immediately contacted with slurry and flow down the "contactor'. The combination of sparger and contactor is the reactor; the separation vessel is a modified conventional column. The contactor is operated under pressure, typically 10-40 psi, which permits some independence of gas and slurry rates and gives some control over gas content (gas holdup) in the contactor in addition to that provide by air rate and bubble size control with the sparger (e.g. by manipulating water addition rate). The separator is said to respond in the usual way to wash water/bias control [36]. A unit is being installed at a plant in Indonesia.
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a/r
wash water water ~
~
FEED
(slurry) ~
~
USBMC , OMINCO/ TYPE SPARGER
FROTH
CONTACTOR TAIL
Fig.9 Schematic of Contact cell; P indicates contactor is under pressure. The Jameson, Pneumatic and Contact cells have certain similarities: a downward flow of aerated slurry and bubble-particle contact for only a few seconds emphasis on high collection kinetics (relative to that in conventional columns) high collection kinetics attributed to characteristics of the reactor, in particular the high gas content (- 50%) attainable relative to columns (10--20%) which translates to a short particle--bubble interaction distance (the high gas holdup is in part the result of forcing bubbles down against their buoyancy). In the case of the Jameson and Contact cells, wash water addition to the separator is specifically retained but the Pneumatic cell could be readily adapted to include this feature. From the experience of the last 15 years it would be wise to consider wash water and how to control a bias in all new flotation machines. In every case, the "chemistry" must be established prior to entry to the unit as retention times are too short for any in-cell conditioning (such as aeration raising the pulp potential to the required level as can occur down a bank of mechanical cells). This also applies to the last novel device considered, the CentrifloatR which differs from the above primarily in the design of the reactor. Centrifloat R
In this device (Figure 10) slurry is fed tangentially near the bottom of a cylindrical vessel (the reactor). Air is injected through the porous wall and sheared into bubbles by the tangential slurry flow. The centrifugal force field established causes the bubbles to move inwards against the motion of the particles and this, it is claimed, results in high collection rates (again relative to columns). In this aspect there are similarities to the Air-Sparged Hydrocyclone. The swirling upward motion is dampened at the top by the "mushroom" and directed outwards into the "calming region" (the separator),
Applications to date are mainly in coal, de-inking and de-oiling [45].
Novel flotationdevices
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MUSHROOM H LAUNDER
PORC WALL
FEED FEED SLURRY FORMS INVERTED VORTEX IN THIS REGION
Fig.10 Schematic of CentrifloatR cell.
Summary There are other cells which fall into this reactor/separator category but as yet have not had sufficient industrial exposure to be included here. For example, in Australia there is the Filblast [46]; in Canada the Cyclo-Column [16]; and, most recently in Chile, the author encountered the FUD cell. In one version of this latter device, it thin layer of slurry flows down an inclined porous plate through which air is passed. The froth generated builds against a weir, where it is washed, to form overflow (floats) and underflow products. (This is different from conventional table flotation where the slurry is aerated by the act of feeding to the deck [47].) The author believes that the next generation of flotation cells will incorporate the reactor/separator concept. Future plants may :~eehorizontal pipe reactors with separators dispersed along their length (Figure 11). The reactor will be fed a mix of slurry and air (or other gases) under pressure and may exploit bubble formation by cavitation.
CONCLUSIONS Some of the latest devices that confront the flotation engineer have been reviewed. The list is by no means exhaustive and does not even include developments in sensors which are worthy of a separate review. We seem to be living in exciting times that may reflect the tight economic conditions or the impact of the growing number of trained researchers in the field, especially those with a "feel" for plant practise. The reader may detect some biases j n the, review. The author maintains, however, that all developments are worthy of consideration as that gem of an innovation may be waiting to be mined.
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FROTH
air~,, a/r TAIL wash
FEED (slurry)
FROTH Fig.ll Author's impression of flotation plant of the future employing the reactor/separator concept; P indictes reactor is under pressure, and C that a cavitation device is included.
ACKNOWLEDGEMENTS Financial support for the research which forms the background to this review is from the Natural Sciences and Engineering Research Council of Canada, Partnerships Program, with sponsorship from Into, Cominco, Falconbridge, Teck and CANMET (coordinated through MITEC, the Mining Industry Technology Council of Canada) and is gratefully acknowledged. Presentations on this topic have been given to personnel at Inco, Hudson Bay Mining and Smelting, and Noranda, and the feedback helped in writing this article. The paper formed part of a Plenary Lecture given by the author at CONAMET '94, Antofagasta, Chile, Aug. 9 1994.
REFERENCES .
2. 3. 4. 5.
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Finch, J.A. & Dobby, G.S., Column flotation: A selected review, part 1, Int. J. Min. Proc., 33, 343-354 (1991). Dobby, G.S. & Finch, J.A., Column flotation: A selected review, part 11, Minerals Eng., 4(7-11), 911-923 (1991). Finch, J.A., Uribe-Salas, A. & Xu, M. Column flotation: a selected review part III, in Flotation Science and Engineering, (ed. K. A. Matis), Marcel Dekker, in press (1994). Huls, B.J., When innovations occur: their effect on a large mining company, in Innovations in Mineral Processing, (ed. T. Yalcin), Acme Printers, Sudbury, Canada, 1-14 (1994). Wheeler, D.A., Column flotation - - the original column, in Froth Flotation, Proceedings of 2nd Latin-American Congress on Froth Flotation, (eds. S. H. Castro and J. Alvarez), Elsevier, 17--40 (1985). Klassen V.I. & Mokrousov, V.A., An Introduction to the Theory of Flotation, Butterworths, 377-383 (1963).
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10. 11. 12. 13. 14. 15. 16. 17. 18.
19.
20. 21. 22.
23. 24. 25. 26.
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28.
29. 30. 31.
14[ 8 : 6 4
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Lynch, A.J., Johnson, N.W., Manlapig, E.V. & Thorne, C.G., Mineral and Coal Flotation Circuits, Elsevier, 33-35 (1981). Kaya, M. & Laplante, A.R., Plant applications of froth washing in mechanical cells, in Proceedings - - 22nd Ann. Meet. Can. Min. Proc., CIM, 160-197 (1990). Beanchamp, J.B. Murphy, D.A. Schroeder, G.V. Suarez, D.E & Heinrich, G.W., Improved flotation at Strathcona mill by new wash water spray design, in Proceedings - - 26th Ann. Meet. Can. Min. Proc., CIM, paper 9 (1994). Green, D. & Cox, C., Flotation of copper with an Outokumpu HG tank cell prior to gold leaching, pres Randol Gold Forum Beaver Creek '93, (1993). Boutin, P. & Wheeler, D.A., Column Flotation, Mining World, 20(3), 47-50 (1967). Rubenstein, J. & Gerasimenko, M.P., Design, installation and operation of a new generation of column flotation machines, in XVIII Int. Min. Proc. Cong,. Aus. IMM, 785-792 (1993). Ityokumbul, M.K., Design and scale-up issues in column flotation, in Innovations in Mineral Processing, (ed. T. Yalcin), Acme Printers, Sudbury, Canada, 187-200 (1994). Finch, J.A. & Dobby, G.S., Column Flotation, Pergamon Press, (1990). Jameson, G.J., A new concept in flotation column design, in Column Flotation '88, (ed. K. V. S. Sastry) 281-286 (1988). Yalcin, T., Cyclo-Column flotation, in Innovations in Mineral Processing, (ed. T. Yalcin), Acme Printers, Sudbury, Canada, 201-208 (1994). Kawatra, S.K. & Eisele, T.C., The use of horizontal baffles to improve the effectiveness of column flotation of coal, in XVIII Int. Min. Proc. Cong., Aus. IMM, 771-778 (1993). Ounpuu, M. & Tremblay, R., Investigation into the effect of column height on the 1200 mm diameter column at Matagami, in Column '91, (eds. G. E Agar, B. J. Huls and D. B. Hyma) MITEC, CIM, CIM(CMP), 303-316 (1991). Nicol, S.K., Roberts, T., Bensley, C.N. Kidd, G. W. & Lamb, R., Column flotation of ultrafine coal: experience at BHP-Utah Coal Ltd.'s Riverside Mine, in Column Flotation '88, (ed. K. V. S. Sastry), 7--11 (1988). Oteyaka, ]3. & Soto, H., Air velocity and turbulence effects in flotation, pres. 33rtL Annual Conference of Metallurgists, The Metallurgical Society of CIM, Toronto, (Aug. 20-25, 1994). Zhou, Z.A. Xu, Z. & Finch, J.A., Minimum recovery zone height requirements in flotation columns from particle-bubble collision analysis, submitted to Minerals Eng. Maksimov, I.I. Borkin, A.D. & Emelyanov, M.E, The use of column flotation machines for cleaning operations in concentrating non-ferrous ores, in XVII Int. Min. Proc. Cong., 313-323 (1991). Cienski, T & Negeri, T., Zero froth column flotation as applied to roughing and scavenging flotation, in Proceedings - - 25th Ann. Meet. Can. Min. Proc., CIM, paper 22, (1993). Dobby, G.S., Presentation at Novel Flotation Devices seminar, May 13/14, 1994, McGill University (1994). Yoon, R.H. & Luttrell, G.H., Microcel column flotation scale-up and plant practice, in Proceedings - - 26th Ann. Meet. Can. Min. Proc., CIM, paper 12 (1994). Xu, M. Quinn, E & Stratton-Crawley, R., Graphite/chalcopyrite separation using a rapid column cell, in Innovations in Mineral Processing, (ed. T. Yalcin), Acme Printers, Sudbury, Canada, 181-186 (1994). Jara, J. & Harris, R., A new device to enhance oxygen dispersion in gold cyanidation, pres. 33rd. Annual Conference of Metallurgists, The Metallurgical Society of CIM,Toronto, (Aug. 20-25, 1994). Llerena, C. & Piron, D.L., Utlectroflottation ~ l'hydrog~ne d'un minerai sulfur6 de zinc, pres. 33rd. Annual Conference of Metallurgists, The Metallurgical Society of CIM,Toronto, (Aug. 20-25, 1994). Parekh, B K. & Groppo, J.G., U. S. Patent 5,116,487, (May 26, 1992). Zhou, Z.A., Xu, Z. & Finch, J.A., Gas nucleation in flotation, in Innovations in Mineral Processing, (ed. T. Yalcin), Acme Printers, Sudbury, Canada, 53-66. Moys, M.H., Englebrecht, J.A. & Terblanche, N., The design of baffles to reduce axial mixing in flotation columns, in Column '91, (eds. G. E Agar, B. J. Huls and D. B. Hyma) M1TEC, CIM, CIM(CMP), 275-288 (1991).
602 32. 33. 34. 35. 36. 37. 38.
39. 40. 41.
42. 43. 44. 45. 46. 47.
J.A. FINCH Yang, D.C., U. S. Patent 4,592,834, (June 3, 1986). Petrie, B.M., M. Eng Thesis, Univ. Toronto, Toronto, Canada, (1994). Foreman, D., Presentation at Novel Flotation Devices seminar, McGill University, (May 13/14, 1994). Hood, G.D. & Jordan, C.E., In-line static mixer rapid flotation system for improved flotation kinetics, presentation at SME Ann. Meet., Reno, Feb. 1993, preprint 93-187 (1993). Amelunxen, R., The Contact cell: a future generation of flotation machines, promotional literature, Amelunxen-Wales Technologies Inc., (Jan. 26, 1993). Jameson, G.J. & Manlapig, E.V., Applications of the Jameson cell, in Column '91, (eds. G. E Agar, B. J. Huls and D. B. Hyma) MITEC, CIM, CIM(CMP), 673-688 (1991). Atkinson, B.W. Griffin, P.T. Jameson, G.J. & Espinosa-Gomez, R., Jameson cell test work on copper streams of Mount Isa Mines Ltd., in XVIII Int. Min. Proc. Cong., Aus. IMM, 823-828 (1993). Marchese, M.M., Uribe-Salas, A. & Finch, J.A., Hydrodynamics of a downflow column, in XVIII Int. Min. Proc. Cong., Aus. IMM, 813-822 (1993). Summers, A., M. Eng. Thesis, McGill Univ., Montreal, Canada, (1995). Giiney, A., 0nal, G. & Dogan, M.Z., Beneficiation of t.)~k6prii Chromite gravity tailings by free jet type flotation system, in Column '91, (eds. G. E Agar, B. J. Huls and D. B. Hyma) MITEC, CIM, CIM(CMP), 609-618. Taggart, A.E, A Manual of Mineral Dressing, John Wiley & Sons, Inc., 133-138 (1921). Imhof, R., Presentation at Novel Flotation Devices seminar, May 13/14, 1994, McGill University (1994). Changgen, L. & Bahr, A., Flotation of copper ore in a Pneumatic flotation cell, Minerals and Metallurgical Proc., 9(1), 7-12 (1992). Schneider, J., Presentation at Novel Flotation Devices seminar, May 13/14, 1994, McGill University, (1994). Finch, J.A., Presentation at Novel Flotation Devices seminar, May 13/14, 1994, McGill University, (1994). Moudgil, B.J. & Barnett, D.H., Agglomeration-skin flotation of coarse phosphate rock, Mining Engineering, 31(3), 263-289 (March 1979).
Pergamon 0892-6875(95)00023-2
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875/95 $9.50+0.00
COLUMN FLOTATION: A SELECTED REVIEWm PART IV: NOVEL FLOTATION DEVICES*
J.A. FINCH Department of Mining and Metallurgical Engineering, McGill University, Montreal, Canada, H3A 2A7 (Received 20 October 1994; accepted 5 December 1994)
ABSTRACT
Some of the novel devices in flotation that have entered the market place recently are reviewed. They are grouped into three categories: developments in mechanical cells, developments in flotation columns, and new cell designs selected here to illustrate the reactor/separator concept. In the first category, froth washing (is this transfer of technology from columns a simple matter?) and froth boosting (an old idea re-visited) are considered. Developments in columns include height ("short" can be beautiful), sparging (a chronic problem that may now be solved) and baffling (an unresolved issue but one that packing may make redundant). Lastly, the reactor/separator concept is introduced and the Jameson, Pneumatic, Contact and Centrifloat R cells are analyzed as representatives of this category. A projected future design based on this concept is offered.
Keywords Column flotation; spargers; Jameson cell; pneumatic cell; contact cell; centrifloat
INTRODUCTION The author has attempted through reviews to keep abreast of the developments in the general area of column flotation [1, 2, 3]. This is the fourth review and focuses on "novel developments". The past 15 years has seen a surge in development of flotation devices reminiscent of the early days of flotation. This may be credited, in part at least, to the acceptance of flotation columns by industry over this period which seems to have created a climate encouraging to both developers and users. Flotation columns have directly driven some developments (e.g. in sparger systems), have spawned derivatives (e.g. the Packed column) and new concepts (e.g. the Jameson cell), and have provoked developments in mechanical cell technology (e.g. froth washing). The rapidly growing new areas of application of flotation technology, such as de-inking recycled paper pulp and de-oiling water (in industries ranging from petroleum to food) will no doubt spur more developments. Not all the developments will be successful for both technical and marketing reasons [4]. All are worth examirfing, however, if one accepts the argument that some feature may offer a genuine advance and a judicious combination of "best" features may usher in the next generation of flotation machines. It is difficult for practising engineers to keep up with these developments which prompted the writing of this paper. The choice of what to include in any review is problematic: To limit the scope and provide a focus, only developments which target fine particle processing and have had some industrial exposure are considered. *Basedon PlenaryLecture,CONAMET'94, Antofagasta,Chile,August9, 1994
587
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J.A. FINCH
The devices are grouped "under three headings: mechanical cells, flotation columns, and reactor/separator cell designs.
MECHANICAL CELLS
Froth washing A key (some would claim unique) feature of column flotation is the use of wash water [5]. Wash water, providing it travels down through the froth and crosses the froth/pulp interface, called a "positive bias", replaces the contaminated water reporting to the froth from the pulp and consequently reduces the entrainment recovery of fine hydrophilic particles. There is no apparent reason why wash water should not be tried in mechanical cells. This has indeed been done, Klassen and Mokrousov [6] for instance, give examples of froth washing. What is different now is a better understanding of the mechanism of hydrophilic particle recovery, the availability of techniques to detect it (e.g. through recovery-by-size analysis [7]), and a realization that engineering the solution through wash water addition may require considerable experimentation [8]. The engineer who has determined that entrainment is a problem can more confidently persist with finding the solution than his less well prepared predecessors. An example of washing mechanical cell froths is at Zinc Corporation of America's Balmat operation [8]. Non sulphide gangue recovery was lowering zinc grades to unacceptable levels and entrainment was known to be a factor. Wash water addition to the zinc cleaners was targeted. From most experience on columns, wash water is generally added as a gentle "rain"; however, in this case wash water had to added with some force to penetrate the stiff froths encountered. Once this was implemented, zinc grades improved (Figure 1) and this success led to the current practice at Balmat of froth washing practically all the cells.
62 60 >
o~~
,~
NO WASH WATER
58
54
WITH WASH WATER
52 50
3
11 APRIL 89
,."
.
I
16
.
I
.
I
.
I
.
I
.
JUNE 89
I
.
I
.
29
Fig.1 An example of froth washing mechanical cells; wash water addition increased zinc grades at Zinc Corporation of America's Balmat operation (adapted from Kaya and Laplante [8]).
Novel flotationdevices
589
There is no consensus on the best means of introducing/distributing wash water in mechanical cells (or columns) and some studies are being conducted (e.g. [9]). In any event, transferring froth washing practise from columns to mechanical cells may not be as straight forward as it seems. One target of column control is a positive bias and instrumentation is provided to that end. It is not clear how easily this control philosophy can be used in a mechanical cell without the necessary instrumentation and hence the potential benefit of wash water may not be realised. The action of wash water needs further investigation to determine how best to make the transfer. To date most work on froth washing in mechanical cells has involved "homemade" devices. Mechanical cells engineered to include wash water are now available and in combination with froth boosting, have introduced a new line of cells from Outokumpu [10].
Froth boosting Deep froths often give higher upgrading than shallow froths as they give extra time for drainage. Building a deep froth can be difficult in some cases. Reducing the cross-sectional area of froth relative to the pulp often increases froth depth probably by increasing particle loading and altering the relative phase velocities. It is known as froth crowding or boosting. Outokumpu [10] have recently engineered cells to include froth crowding. The HG tank cell uses an adjustable booster cone (Figure 2) to crowd the froth into the annular space. The position of the cone influences two w~ables, the grade (by changing froth depth) and solids removal rate or capacity (by changing surface :area). By making the cone adjustable, some control over both these variables is introduced.
ADJUSTABLE BOOSTER CONE
FROTH
FEED
Fig.2 Diagrammatic illustration of Outokumpu's HG tank cell showing the variable booster cone concept (adapted from Green and Cox [10]).
590
J.A. FINCH
Adjusting the cone position, it can be argued, alters the geometry of the cell: with the cone out, the cell has a low height to surface area ratio compared to that with the cone in. This is like changing the geometry from that typical of mechanical cells to that more akin to flotation columns and to emphasize this connection the cells are also called "virtual columns"[10]. This variable geometry may mean the same cell can take on different duties. The general experience to date is that mechanical cells remain the preferred choice for roughing with columns largely restricted to cleaning. If this difference in duty is related to geometry then an HG tank cell can adopt both duties by having the cone out and in, respectively. (Attributing the difference in duty to geometry is debatable but provided some of the initial impetus behind development of the cell.) In a bank, the cone position could be adjusted on individual cells.
FLOTATION COLUMNS The column that is referred to here is that developed in the early '60s [11]. It has been pointed out that cells with a columnar geometry were tried prior to 1920 [12,13]. Those earlier units, however, did not use wash water nor did they have a control philosophy based on bias reflecting an understanding of the problem of entrainment. Accordingly, they do not fall into quite the same category; if they did it would also mean that the industry took some 60 years to realize a good idea which is a little difficult to accept!
Height The original flotation columns marketed by Column Flotation Co. of Canada were 40--45 ft high [5] and this became, in essence, the standard. One advantage was that height was then not a factor and scale up could focus on the number and diameter of the columns required. The need for this height has always been in question, however. Some justification was given from analysis of mixing by noting that the greater the height to diameter ratio the less mixed were the column contents and consequently recovery and selectivity should improve [14]. Some researchers, linking the need for such height with a low particle collection rate, developed new designs aimed at increasing flotation rates by providing more intense particle-bubble contacting [e.g. 15,16]. Others attributed the low rate to mixing and short circuiting effects associated with the "open" nature of the conventional column and proposed various forms of baffling [e.g. 17]; the Packed column (see below) falls into this category. Yet other investigators simply tested different heights, in some cases showing short columns sufficed [18], in some cases did not [19]. Recent installations in Canada have seen columns of 20 ft at Myra Falls and I0 ft at Les Mines Gasl~. A plant in Norway is being designed with short (5 m) and tall columns in series. As experience and confidence grow more deviations from the "conventional" height will be seen. Bubble-particle collision probability models have been used to analyze the height issue [20,21]. Defining the minimum height as that achieving at least one collision, the models show that height is a strong function of particle size with heights increasing from less than lm for > 50ttm particles to exceed 10m for <101arn particles. An engineering solution to the required height for a given process may develop from the work of Ityokumbul [ 13]. He has proposed replacing the common kinetic approach with a mass transfer approach (although this may mean replacing one difficult-to-measure set of parameters by another). The technique needs extending to include interaction between the collection and froth zones as froth dropback can dominate column design in some situations. To illustrate the need to consider these i n , actions, one possible argument against tall columns is that: the bubbles become too loaded and this may actually reduce the solids carrying rate by overloading the froth in a manner similar to that when a column is "overfed" [14]. The decrease in recovery above a certain height observed by Maksimovetal. [22] may have its origin in this effect. The "overfeeding" phenomenon has never been explained. The capacity limitations apparently imposed by the froth have led to testing "zero froth column flotation" [23].
Novel flotationdevices
591
Spargers The first spargers (i.e. bubble generation devices) were made from porous material such as filter cloth and perforated rubber. These had a number of drawbacks: blockage due to particles and/or precipitates tearing and general deterioration with use the need to shut down to change the large number required to maintain bubble size below -2-3 mm Improved sparger design has been a recurring demand by operators and has tested the ingenuity of developers. With porous materials, bubble generation is by formation of individual bubbles at an orifice. Two other bubble generation principles are by jetting and shearing. Examples of spargers based on the first are those developed by the USBM/Cominco and Minnovex (Figures 3a,b, respectively), and examples based on the second are those used in the MicrocelTM column and the Pneumatic cell (Figures 3c,d, respectively).
air
water
I~i
~
~" JI" I
Fig.3a The USBM/Cominco type sparger; bubbles are created by jet that forms at the orifice enhanced by a small addition of water.
JUSTER
a/r
)LUMN WALL /
MOVABLE HI
-,. j - . ~ - 0 0 - 4 0 0 m / s )
Fig.3b The Minnovex variable gap sparger; a jet forms at the annular orifice controlled by gap width and gas pressure.
592
J.A. FINCH
gas s/u
bubble - slurry dispersion Fig.3c Schematic of sparger on MicrocelTM column showing use of a static in-line mixer to create bubbles by shear.
slurry
gas
gas
bubble - slurry dispersion Fig.3d Design of sparger used in the Pneumatic cell. In jetting techniques, bubbles form as a result of instabilities on the jet surface. The number of bubbles (nB) and bubble size (dB) depend on jet length (Ij), which in turn is dependent on jet momentum (per unit volume, pjvj), in thefollowingmanner
nB~~
(1)
dBo~lflJ
(2)
Ijocpjvj
(3)
Novel flotationdevices
593
Since producing a large population of fine bubbles is the general objective, a long jet length (i.e. high jet momentum) is required. Jet momentum can be increased by increasing either the density (p j) or velocity (vj) terms. In the USBM and Cominco designs, the jet is formed at a -1 mm diameter (circular) orifice and pj is increased by a small addition of water with the gas (typically ~ 1% of the volume of the gas) (Figure 3a). The effect of water addition is quite dramatic, the jet length visibly increasing with fine bubbles being produced and distributed across the cell (Figure 4). With the Minnovex "variable gap sparger", the approach is to control vj by adjusting the annulus (Figure 3b) dimensions and gas pressure. A small annulus width (~1 ram) and gas pressures above ~45 psig combine to give velocities between 200 and 400 m/s [24] and produces at least a 50cm long jet (Figure 5).
(a)
(b) Fig.4 USBM/Cominco type sparger in action: a). view along the sparger, no water added; b). as a) but with water added with the gas; note the jet forming on both sides which was not evident in a). (In both situations there was no frother present otherwise the jet in b) is lost in the cloud of small bubbles produced.)
594
J.A. FINCH
Fig.5 The Minnovex variable gap sparger in action; note the jet length is at least equal to the sparger length, 50 cm in this case. (There was no frother present for the same reason as in Figure 4.) Shearing involves the rapid motion of a fluid relative to another. In the MicrocelTM column sparger, shear is achieved by forcing slurry and air over the blades of an in-line static mixer (Figure 3c). The slurry is drawn from the cell contents [25] (but other sources of slurry, e.g. the feed [26], t o d d be substituted). In the Pneumatic cell, feed slurry is forced through a constricted opening, reaching velocities of ~ 4--6 m/s, and shears the air, introduced at right angles from a slot, into fine bubbles (Figure 3d). The acceptance of new spargers will be based on reliability of operation rather than, for example, any ability to control bubble size. The latest device, the Minnovex variable gap sparger, is proving quite popular (including use to supplement air in self-aeration mechanical cells at one plant in Chile). The attractions appear to be: readily replaceable, wear resistant head (tungsten/silicon carbide) ability to change orifice size on-line short length protruding into the column facilitating withdrawal for on-line maintenance the need for relatively few spargers (e.g. typically four on a 2 m dia. column) no need to use sparger water, or to produce high velocity slurry jets Manufacturing these spargers has demanded the use of a computer-controlled lathe to ensure uniform annulus dimensions to further limit wear rates. Sparger devices using slots are being investigated [27] and gas evolution by electrolysis (to give hydrogen and oxygen) is still periodically pursued [28]. Future developments may be to combine bubble generation with particle collection in what might better be d e s e r i ~ as a "reactor" rather than a sparger. Appropriate designs may be able to exploit dissolved air [29], and eavitatiordnucleation to selectively "activate" particles by frosting them with fine bubbles [30]. Baffling/Packing The "open" nature of conventional columns has always raised the issue of mixing. The more mixed are the contents of a flotation machine the lower the recovery and selectivity. This is recognised in mechanical cell installations by placing the cells in series to form banks. Columns in series are recommended for the same
Novel flotation devices
595
reason [14]. One of the initial attractions of columns, however, was the apparent ability to replace several stages of mechanical cells [5]. There was a belief that this was due to reduced mixing inherent in the column geometry, thus the mixing characteristics became - - and remain - - a subject of investigation. One way to decrease mixing is through baffling. Vertical baffles to divide the column into compartments are quite widely used (in part for structural integrity). As a working target, a compartment diameter approximately that of the original column (i.e. ~ 1 m) was suggested [14]. A decrease in mixing as a result of such baffling, however, has usually not been evident [3]. The reason is probably that identified by Moys et al. [31]: Uneven distribution of feed and gas to the compartments causes circulation between them compounding rather than easing the problem. Various forms of baffles have been tried, including horizontal ones [17,29], but perhaps the most promising approach is to go as far as "packing" the column. The Packed column, developed by Yang [32], uses stacks of corrugated plates to provide a tortuous path for both bubbles and particles. Reduced mixing is one claim, but others include the ability to support an almost unlimited froth depth (which should aid fine particle cleaning), and the absence of any sparger as the packing serves to disperse the gas. The performance advantage over open columns was difficult to detect at the laboratory scale (in a de-inking study), in part because mixing is not then an issue [33]. The advantages (e.g. increased capacity) should be realised on units larger than about 1 m diameter. A retrofit to pack an open column has been completed at a plant in Chile [34] and the preliminary oral reports sound encouraging; results are awaited with interest.
REACTOR/SEPARATOR DESIGNS Concept The argument is the fidlowing: Flotation has two basic functions~ to attach particles to bubbles and then to separate these bubble--particle aggregates from the slurry, and the current design of flotation machines is a compromise between these two functions which limits the overall performance. The two functions are recognised in the design of mechanical cells which provide a region of intense mixing around the impeller and a quiescent region above (e.g see Figure 2). Flotation columns in this regard are also compromise designs. The concept, therefore, is to isolate the two functions, as illustrated schematically in Figure 6, in anticipation that the two functions can be optimized individually and an overall improvement in machine performance be realized.
SLURRY
REACTOR
AIR
FROTH
SEPARATOR
SLURRY
Fig.6 "[he general concept of the reactor/separator class of flotation machine.
596
J.A. FINCH
The concept appears to have been in the mind of some cell developers [e.g. 35,36] but the present author believes it serves to group several of the recent novel cell designs. The following aims to show how the concept applies rather than offering any detailed commentary on the metallurgical performance of the cells.
Jameson cell Developed in Australia in the mid-'80s [15], the unit has been used in coal and mineral processing and deoiling [37]. The unit consists of two parts, the downcomer where slurry and air are mixed and the "ceU" (Figure 7). More than one downcomer can feed into one cell.
a/r
FEED
DOWNCOMER
wasi
CELL
CONCENTRATE
~TAILS TAILINGS VALVE
Fig.7 Schematic of Jameson cell. Slurry is introduced into the downcomer via a nozzle to form a jet. The jet entrains air giving a slight vacuum which simultaneously supports the pool in the downcomer and draws in air. The air is sheared into bubbles as the jet plunges into the pool and all particle collection occurs in the downcomer; hence the downcomer is the "reactor". The downcomer contents discharge into the cell where disengagement of bubble-particle aggregates from the slurry occurs and a froth forms (which is usually washed). The cell, therefore, is the "separator". There is a growing body of operational detail becoming available. Four features are: the gas to slurry rate should be about 0.3-0.9 for stable operation of the downcomer [38,39] gas holdup in the downcomer is sensitive to percent solids in the feed [39] the gas rate per unit area of the separation chamber (or superficial rate) should be above a certain value to maintain a stable froth (e.g. ~ 0.7cm/s in the absence of solids [40]) unless automatically controlled, gas rate changes as level in t h e ' s e i ~ t i o n chamber changes [40] From this it is evident that here are some interactions between the reactor and separator which the concept in its pure form does not have.
Novel flotationdevices
597
The jet in the downcomer is confined by the walls but unconfined or free jet devices, i.e. where the jet plunges directly into an open vessel, have also been proposed [41]; indeed, the Cascade cells developed earlier this century [42], a version of which is in operation at one mine in Chile recovering chalcopyrite from the tailings, fail into the free jet category. Pneumatic cell This cell continues to be developed and has been around for some time in a variety of forms [43]. Perhaps the simplest version 1:o illustrate the principle is that in Figure 8. Feed is forced through the sparger (Figure 3c), down a vertical pipe and into a tank. The sparger plus pipe represent the reactor and the tank the separation chamber. By distributing the feed, several reactors can discharge into a common separator. To date the author is aware of testwork in base metals minerals and in soil ,decontamination [43].
[44] and commercial experience with industrial
FEED air
~TAIL Fig.8 Schematic of Pneumatic cell; bubble generation device (at top) is that in Fig. 3d. Contact cell Figure 9 shows a schematic [36]. Water and air are injected into essentially a USBM/Cominco type sparger and the bubbles are immediately contacted with slurry and flow down the "contactor'. The combination of sparger and contactor is the reactor; the separation vessel is a modified conventional column. The contactor is operated under pressure, typically 10-40 psi, which permits some independence of gas and slurry rates and gives some control over gas content (gas holdup) in the contactor in addition to that provide by air rate and bubble size control with the sparger (e.g. by manipulating water addition rate). The separator is said to respond in the usual way to wash water/bias control [36]. A unit is being installed at a plant in Indonesia.
598
J.A. FINCH
a/r
wash water water ~
~
FEED
(slurry) ~
~
USBMC , OMINCO/ TYPE SPARGER
FROTH
CONTACTOR TAIL
Fig.9 Schematic of Contact cell; P indicates contactor is under pressure. The Jameson, Pneumatic and Contact cells have certain similarities: a downward flow of aerated slurry and bubble-particle contact for only a few seconds emphasis on high collection kinetics (relative to that in conventional columns) high collection kinetics attributed to characteristics of the reactor, in particular the high gas content (- 50%) attainable relative to columns (10--20%) which translates to a short particle--bubble interaction distance (the high gas holdup is in part the result of forcing bubbles down against their buoyancy). In the case of the Jameson and Contact cells, wash water addition to the separator is specifically retained but the Pneumatic cell could be readily adapted to include this feature. From the experience of the last 15 years it would be wise to consider wash water and how to control a bias in all new flotation machines. In every case, the "chemistry" must be established prior to entry to the unit as retention times are too short for any in-cell conditioning (such as aeration raising the pulp potential to the required level as can occur down a bank of mechanical cells). This also applies to the last novel device considered, the CentrifloatR which differs from the above primarily in the design of the reactor. Centrifloat R
In this device (Figure 10) slurry is fed tangentially near the bottom of a cylindrical vessel (the reactor). Air is injected through the porous wall and sheared into bubbles by the tangential slurry flow. The centrifugal force field established causes the bubbles to move inwards against the motion of the particles and this, it is claimed, results in high collection rates (again relative to columns). In this aspect there are similarities to the Air-Sparged Hydrocyclone. The swirling upward motion is dampened at the top by the "mushroom" and directed outwards into the "calming region" (the separator),
Applications to date are mainly in coal, de-inking and de-oiling [45].
Novel flotationdevices
599
MUSHROOM H LAUNDER
PORC WALL
FEED FEED SLURRY FORMS INVERTED VORTEX IN THIS REGION
Fig.10 Schematic of CentrifloatR cell.
Summary There are other cells which fall into this reactor/separator category but as yet have not had sufficient industrial exposure to be included here. For example, in Australia there is the Filblast [46]; in Canada the Cyclo-Column [16]; and, most recently in Chile, the author encountered the FUD cell. In one version of this latter device, it thin layer of slurry flows down an inclined porous plate through which air is passed. The froth generated builds against a weir, where it is washed, to form overflow (floats) and underflow products. (This is different from conventional table flotation where the slurry is aerated by the act of feeding to the deck [47].) The author believes that the next generation of flotation cells will incorporate the reactor/separator concept. Future plants may :~eehorizontal pipe reactors with separators dispersed along their length (Figure 11). The reactor will be fed a mix of slurry and air (or other gases) under pressure and may exploit bubble formation by cavitation.
CONCLUSIONS Some of the latest devices that confront the flotation engineer have been reviewed. The list is by no means exhaustive and does not even include developments in sensors which are worthy of a separate review. We seem to be living in exciting times that may reflect the tight economic conditions or the impact of the growing number of trained researchers in the field, especially those with a "feel" for plant practise. The reader may detect some biases j n the, review. The author maintains, however, that all developments are worthy of consideration as that gem of an innovation may be waiting to be mined.
600
J.A. FINCH
FROTH
air~,, a/r TAIL wash
FEED (slurry)
FROTH Fig.ll Author's impression of flotation plant of the future employing the reactor/separator concept; P indictes reactor is under pressure, and C that a cavitation device is included.
ACKNOWLEDGEMENTS Financial support for the research which forms the background to this review is from the Natural Sciences and Engineering Research Council of Canada, Partnerships Program, with sponsorship from Into, Cominco, Falconbridge, Teck and CANMET (coordinated through MITEC, the Mining Industry Technology Council of Canada) and is gratefully acknowledged. Presentations on this topic have been given to personnel at Inco, Hudson Bay Mining and Smelting, and Noranda, and the feedback helped in writing this article. The paper formed part of a Plenary Lecture given by the author at CONAMET '94, Antofagasta, Chile, Aug. 9 1994.
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