- Email: [email protected]
Column flotation: A selected review, part II
Minerals Engineering, Vol. 4, Nos 7-11, pp. 911-923, 1991 Printed in Great Britain
0892-6875/91 $3.00+000 Pergamon Press plc
COLUMN FLOTATION: A SELECTED REVIEW, PART II
G.S. DOBBY § and J.A. FINCHt § Dept. of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S I A4 f Dept. of Mining and Metallurgical Engineering, McGill University, Montreal, Canada H3A 2A7
ABSTRACT
Selected topics in the rapidly evolving field of column flotation are reviewed. The status of column scale-up concerns is assessed, with particular emphasis on recent measurements of froth zone performance. Column circuits are reviewed, and some performance comparisons between column and column~mechanical cell cleaning circuits are made. The effect of bubble size and gas rate upon column operation is discussed in general terms, and a comprehensive categorization of bubble generation techniques is made, including those used for feed slurry aeration. Keywords Flotation; column flotation; scale-up; froth; entrainment; flotation circuits; bubble generation INTRODUCTION Industrial application of column flotation has gone from virtually zero in 1983 to wide acceptance in 1990. (The first significant installation was actually in 1980, at Noranda's Mines Gasp6 [1]; however, the Mines Gasp6 plant was shut down in 1983-84 due to poor copper market conditions.) As a consequence of the widespread plant trials that have occurred during the past seven years, there has been a rapid growth in practical knowledge related to column design, construction, operation, sparging and control. There are now several companies worldwide that specialize in column design and engineering, and relatively few new orebodies are brought to commercial production without at least consideration of columns as part of the flotation flowsheet. Research on column flotation is now conducted in many mineral processing laboratories. The other significant advance over the past few years has been the introduction of alternative flotation technologies to conventional column flotation. Of special interest are the feed slurry aeration systems, such as the Bahr cell [2], the Jameson cell [3] and Jet flotation [4]. While, in 1990, these devices do not have nearly the same level of industrial implementation as that of the flotation column, there is a good deal of industrial interest. It is clear that flotation cell technology will continue to develop and evolve over the coming years. The purpose of this paper is to provide a selective review of some developments in column flotation over the past few years. A previous review by the authors [5] addressed mixing in the collection zone, maximum gas rates and carrying capacity. Topics that are covered here include scale-up, circuits and testing, and bubble generation. The discussion on bubble generation also looks at some of the feed aeration concepts. 911
912
G.S. DOBBYand J. A. FINCH COLUMN S C A L E - U P
The approach to column scale-up taken by D o b b y and Finch [6,7] recognizes the importance of the individual performance of the two zones, collection zone and froth zone, and their interaction. Each zone has a recovery, for each mineral type, relative to the feed to the zone. This is illustrated in Figure 1, in which R e and Rf are collection zone and froth zone fractional recoveries, respectively. For any given mineral, the overall column recovery R m is then given by R m-- R c R f / ( R c R f + l - Rc)
(1)
T CLEANING ZONE
1
Re
Re(1-1~) 1
F=X
COLLECTION ZONE
l-R= Fig.1 Schematic illustration of interaction between the collection zone and the froth zone in a flotation column. The interaction of the two zones is not always appreciated. A unique aspect is that particles rejected from the froth zone always have 100% of the retention time for recollection. In a bank of mechanical cells, the time available for recollection decreases as the particle progresses down the bank. The net result of this difference is that the column arrangement will provide for improved selectivity between two hydrophobic species [7]. The column loses this advantage when it has a small height to diameter ratio, i.e. a residence time distribution that approaches that of a single stirred tank. Collection zone recovery is modelled by a first-order rate process and is therefore a function of three parameters: collection zone rate constant K c, particle retention time, and degree of axial mixing. A practical model of the froth zone has yet to be developed. To simplify the process of modelling, the column is sometimes considered to be a single stage [8,9]. In this approach an overall rate constant K f is used, and the froth zone is not treated as a separate stage, although froth zone effects are recognized in terms of an impact on Kf. While this approach has been reported to be satisfactory for modelling large scale columns [9], it can be very misleading in scale-up from small (pilot) columns to large columns. This is because the froth zone recovery is often substantially lower in large columns compared to that in pilot columns. Hence, in such a case the pilot scale measured
Column flotation--lI
913
rate constant Kf (that includes the froth effect) will be unrealistically high if it is applied to the large column without correction. It is believed that in some cases the difference in Rf between that of the pilot column and that of the large plant column is as high a factor of 3 to 4, i.e. Rf in the plant column being 3 to 4 times lower than that in the pilot column.
Froth Zone Recovery Rf To conduct scale-up from data using the two-zone approach requires knowledge of both K e and Rf. Rf must be assumed in order to measure Kc, unless elaborate testing is conducted, in which the collection zone/froth interface is eliminated [10,11], or a special column is constructed in order to isolate the mineral particle dropping from the froth zone back to the collection zone [ 12], or the column is operated in sequential countercurrent and concurrent modes on the same feed [13]. However, these techniques are not required; a satisfactory approach has been to assume a froth zone recovery and then set the froth zone recovery in the scaled-up column to a range of lower values. This results in a scale-up grade-recovery prediction, where the upper end of the curve (low Rf and hence high grade) represents column operation at low gas rate, deep froth bed, and/or high wash water addition, and the lower end (high Rf and high recovery) represents operation at higher gas rate, shallower froth bed and/or less wash water. The difficulty here lies in selection of the scaled-up R f values relative to those of the pilot column. The approach is quite robust, however, for two reasons. First, it has been shown [14] that the scale-up prediction is relatively insensitive to the initial choice of Rf in the pilot column, provided that Rf values for the scale-up are adjusted relative to the initial Rf chosen. Second, Rf appears to be quite insensitive to mineral hydrophobicity, at least for heavily loaded froths in which recollection within the froth cannot occur. (It must be remembered that Rf refers to particles collected by the gas bubbles only, and does not account for entrainment.) The results of froth zone recovery measurements made by Wilson [l 1] for copper sulphide and nickel sulphide in a 3.8 cm diameter column showed that there was no statistically significant difference in Rf between the two mineral species; some of these results are given in Table 1.
TABLE 1 Rf values for CuaS and Ni3S 2 measured in a pilot column by Wilson [11] at Inco's matte separation plant. Feed Stream Cu 1st clnr. Cu 2nd clnr. Middlings Ni scavenger
Gas Rate (cm/s) 2.4 2.9+.2 1.7 2.9
Number of tests 3 4 3 5
Cu2S
s.d.
29 43 16 52
3 12 4 10
Rf (%) Ni3S 2 30 53 13 48
s.d. 3 14 5 7
"s.d." is standard deviation
Further evidence that little or no upgrading occurs within the froth has been provided by the froth profile sampling on a l m diameter column at Southern Peru Copper Corp. [15], Figure 2, and the froth sampling on a 2.1 m diameter copper column by Falutsu [16], Figure 3. In Falutsu's tests, the probe was designed to sample gas bubbles only (and reject the slurry), so the profiles in Figure 3 are for the solids on the gas bubble. The work of Yianatos [17] on 0.45 and 0.9 m molybdenite columns indicated that some upgrading did occur; it is hypothesized that this was due to the selective recollection that may occur within a lightly loaded froth.
914
G.S. DOBBY and J. A. FINCH
40
Interface
El
3O
-(D c5
(..9
2O
o_
E oo 10 El
[]
[]
[] ra
0
[]
[]
i
i
1
2
Distance
down from column
lip (m)
Fig.2 Profile of copper content in samples extracted from a 0.9 m diameter column at Southern Peru Copper Corp. [15], showing no upgrading in copper content within the froth bed (obvious upgrading at the interface).
35.00
Assays, % $
S
-------------____x r~
Cu
30.00
25.00
interface 20.00
I
I
I
I
50
100
150
200
Distance from column top, cm
Fig.3 Profile of copper content in samples extracted from a 2.1 m diameter column at Falconbridge Ltd. [16]. Samples are of solids attached to the gas bubbles only, no solids from the liquid phase are in the sample. Entrainment For many column applications, solids recovery via entrainment can be safely ignored when modelling the process or when considering scale-up. However, this is not so for applications such as: a) columns operated without wash water, and
Column flotation--II
b)
915
columns used to treat gold ores, where the solids recovery by flotation is low (1 to4 % typically) and, hence, where even a very low recovery of solids due to entrainment is quite detrimental to the product grade.
This latter effect is shown in the results of Furey, Figure 4 on a gold telluride ore [18]. The work of Maachar et al. [19] on a 3.8 cm diameter column has shown feed water recovery, and hence entrainment-related recovery, to be a clear function of feed superficial velocity JF and bias velocity JB, Figure 5. Note that data in Figure 5 covers a wide range in gas rate, wash water rate, feed rate and froth depth. 300
200
i,
Negative Bias
-
lIFt,, $
"U
I I I I I I I I I I I I I
100
0 "60
*
I "40
*
I -20
•
|
Positive Bias
•
0
I 20
, 40
Tails F l o w B I ~ (%)
Fig.4 Column flotation concentrate grade of a gold telluride flotation on a pilot column as a function of bias [18]. Bias here is quoted as a tailings flow percentage greater than (+ bias) or less than (- bias) feed water flow. Scale-up examples The results of several scale-up predictions in comparison to actual performance of 1.8 m diameter columns has been reported by Wilson [11]. In this work, 3.8 cm and 10 cm diameter pilot columns were operated on various feed streams to the plant columns at Inco's matte separation plant in Sudbury, Canada. Each scale-up was well within the accuracy that would be required for design of a new installation. A similar study has been conducted by Castillo [14], in which pilot data from a 15 cm diameter column was used to predict the performance of a 1.5 m diameter column in a zinc sulphide final cleaning application. The plant column performance and the scale-up prediction are shown in Figure 6. Ultimately, any scale-up work is limited in its precision by two factors: variability in the ore characteristics, and variability within the test procedure. COLUMN FLOTATION CIRCUITS Unlike mechanical cell cleaning, column flotation cleaning circuits can be configured in either the conventional countercurrent cleaning configuration, as it has been done for molybdenite cleaning [1], or in the scavenger arrangement, in which the first column
916
G.S. DoBBY and J. A. FINCH
produces the final grade product, its tailings are refloated in a subsequent column(s) and scavenger column concentrate(s) may be recycled back to the first column feed. A commercial example of the latter is the installation at Gibraltar Mines, Canada [20]. The choice of configuration (cleaning versus scavenging) will generally depend on whether the emphasis is on the attaining of grade or recovery.
60
/ a
L-G
• L-G..S ~ •
/
/
0
40
Water recovery 30
(%)
20
10
0 0
10
30
20
exp(-13.1Jb)/Jf
Fig.5 Feed water recovery as a function of bias (cm/s) and feed rate (cm/s) for a range of operating conditions [19]. Jg = 1.95_+1.0; Jw = 0.19_+0.19; JF = 1.5_+1.0; JB from -0.15 to +0.15 cm/s
58
Nc "O 57 O e" O
13
O
56
0
i
50
i
60
13
70
Zinc recovery (%)
Fig.6 Plant performance (data points) and predicted scale-up (line) of zinc grade-recovery on a 1.5 m diameter column at Kidd Creek Mines [14]. Feed grade was 46% Zn and throughput was 7.5 tonnes per hour solids. Data for scale-up was obtained from a 15 cm diameter pilot column.
Column flotation--II
917
There are several examples of integration between columns and mechanical ceils in cleaning circuits. Falconbridge operates two copper cleaning circuits, each arranged in the scavengerclosed configuration, whereby column tailings are scavenged in a bank of mechanical cells and mechanical cell concentrate is recycled. In Inco's copper matte flotation circuit mechanical cells and a column are arranged in a cleaning configuration, with mechanical cells as a first cleaner and the column as the second cleaner. For lead flotation at Cominco's Polaris concentrator two flotation columns are employed in a very flexible mode along with mechanical cells [21]. The operators have a choice of three circuit arrangements, the selection being dependent on feed grade. Zinc rougher concentrate at the Cominco Sullivan concentrator is treated in a 2.4 m diameter column that produces final grade product, accounting for about one-third of the total concentrate production [22]; the balance of concentrate is produced by treating the column tailings in the conventional (existing) mechanical cell cleaning circuit. Several unique circuits involving integration of columns with mechanical cells in Australia have been presented [23,24]. The need for a careful assessment of options available in circuit design prompted development of a portable, fully controlled pilot flotation plant consisting of 15 cm diameter by 6 m high columns and a bank of mechanical cells [25]. Results of extensive testwork on zinc rougher concentrate and copper rougher concentrate were reported. Table 2a compares performance of three columns arranged in a cleaning circuit (CCC/cleaner) and in a scavenger circuit (CCC/scavenger); see Figure 7 for the two flowsheets. Wash water was used only on those columns producing final concentrate. As might be expected, the CCC/cleaner configuration produced a higher grade and somewhat lower recovery. Another set of results, Table 2b, shows the effect of using a column versus a bank of Denver mechanical cells as the last stage of the cleaning circuit. No wash water was used in the column. The data indicates that the column performed slightly better than the bank of mechanical cells in the role of a cleaner scavenger.
TABLE 2a Comparison of two configurations using three pilot columns for zinc cleaning application [251(see Figure 7 for flowsheets). Circuit
CCC/Cleaner CCC/Scavenger
Feed % Zn
% Zn
43.7 45.8
57.1 56.6
Concentrate c* Zn Rec'y 82 76
67 76
Number of Tests 3 4
Table 2b Flotation column (15 cm diameter) versus mechanical cells (one bank of four 30 L cells) for zinc cleaner scavenging 125l. Cell Type
Feed % Zn
% Zn
Concentrate c* Zn Rec'y
Nominal Retention Time (min)
A. Scavenger stage of C C C / o r CCM/cleaner configuration Column 46.4 51.3 36 86 Mechanical 46.2 51.7 40 73
26 22
B. Re-scavenger stage of C C C / o r CCM/scavenger configuration Column 36.3 48.9 53 58 Mechanical 40.3 49.2 45 54
25 21
c* is 100 x (conc grade - feed grade)/(60 - feed grade), where 60 is the m a x i m u m concentrate grade that can be achieved with this mineralization, c* then, is a percentage of maximum upgrading and is a way of correcting for slight variations in feed grade.
918
G.S. DOBBYand J. A. FINCH
I
I Conc
Feed
i
3
2
Y
.11
CCC/ct~-
Conc
I
I
I
t
L
Feed
1
2
3
(X~C/acav
Fig.7
Flowsheets for CCC/cleaner and CCC/scavenger circuits
[25].
BUBBLE GENERATION The objective of bubble generation in column flotation, as in any froth flotation system, is to produce relatively small gas bubbles (bubble diameter d b -- 0.5 to 1.5 mm) at a moderate gas rate (typically, superficial gas velocity Jq -- 0.5 to 2.0 cm/s). The size of bubbles produced is determined by the type of bubble generation system, frother type and dosage, and gas rate. It is sometimes forgotten that the performance of both the collection zone and the froth zone are affected by the nature of the bubble system. Hence, bubble conditions that are optimum for one zone may not be optimum for the other zone, and the effect on column performance of changing either Jg or d b on an operating system will not always be predictable. Some situations that can occur include the following: reducing the bubble size results in an increase in the rate of particle collection and therefore increases recovery.
Column flotation--II
919
reducing the bubble size results in an increase in the rate of particle collection, but also increases the gas holdup in the column to the point that critical gas holdup is exceeded; the outcome is that the collection zone "froths-up" unless gas rate is reduced, thus negating the effect of the reduced d b. increasing the gas rate results in an increase in the rate of particle collection and/or froth removal rate (i.e. Rf) and therefore increases recovery. increasing the gas rate in order to increase Rf (especially important for heavily loaded froths) results in collection zone "froth-up" because small bubbles are being generated and the critical gas holdup is exceeded. While producing smaller gas bubbles in a given column operation will often increase the rate of flotation, it will not necessarily improve separability. Two examples of this are shown in Figures 8 [26] and 9 [27], with the relevant operating conditions provided in Table 3. For the Cu-Ni separation results in a 0.9 m diameter column, shown in Figure 8, nickel recovery plotted versus copper recovery is similar with both filter cloth and USBM type spargers. The bubble size produced with the cloth sparger was slightly larger and a higher gas rate was required to achieve a specific recovery. The same effect is seen in Figure 9, where a rubber sparger is compared with a cloth sparger on the flotation of zinc. Grade-recovery is similar, but in this case the rubber sparger required considerably less gas to achieve a specific recovery.
O
Sparger System O
Clolh
A
External
&
v
o/
0 r~
rr
.~ Z
1 A o
8/,, 0 40
I 50
o
i 60
7'0
80
90
Copper Recovery (%)
Fig.8 Effect of sparger type (filter cloth versus USBM "external") on nickel recovery versus recovery for a 0.9 m diameter pilot column [26]. This data should not be interpreted that improved selectivity is not possible with a bubble generation system that generates smaller bubbles. If a given sparger system is performing poorly, for example generating some very large bubbles that cause disruption of the froth, then an improved sparger system could produce an improved recovery and grade. In a study on copper flotation at Disputada in Chile [28] it was reported that the use of USBM type spargers produced better separation (higher recovery and grade) than that with filter cloth spargers.
920
G.S. DoBaY and J. A. FINCH
58.4 Feed:
48.1-48.9
%Zn
m
Rubber
•
Filter Cloth
58.2 '
c N
~:~
58.0 •
578.
c
57.6 • c 0
(,.) 57.4 .
57.2 20
i 30
i 40
i 50
i 60
70
Zinc recovery (%)
Fig.9 Effect of sparger type (filter cloth versus rubber) on zinc grade versus recovery for a 15 cm diameter pilot column [27]. TABLE 3 Conditions for the performance results in Figures 8 and 9. Feed Material
Column Diameter (cm)
Sparger Type
Jg (cm/s)
Figure 8 Cu Rougher Conc
90
USBM Cloth
0.7-1.0 0.8-1.3
Figure 9 Zn Cleaner Conc
15
Rubber Cloth
1.65 2.6-3.2
Thus, the improvement in metallurgy that is possible through the generation of smaller gas bubbles depends on the level of performance of the existing system, as well as the source of the existing rate limitation: particle collection or froth removal. From the forgoing it is clear that both bubble size and gas rate must be considered together when designing a system or making adjustments to an operating column. Categories of Bubble Generation Techniques In the past several years there has been a great deal of ongoing development and testing of bubble generation systems. The intent of this section of the paper is not to critically analyze each of the prominent methods, but rather to attempt a general categorization. 1. Mechanical Shear Contacting This is the method of bubble generation provided by conventional mechanical flotation machines, and is generally very effective at producing small gas bubbles. Mechanical flotation machines typically produce gas bubbles that are 0.5 to 1.0 mm in diameter.
Columnflotation--II
921
2. Static Shear Contacting High velocity contacting of slurry and gas in an appropriate manner will generate small gas bubbles. Some examples of this include: the Davcra cell [29], in which gas and feed slurry are contacted in a cyclone-type device immediately prior to entering the flotation cell. the Packed Column [30], in which gas entering the bottom of the column is sheared into bubbles by extended metal mesh packing that occupies most of the column volume. contacting of gas with slurry in a pipe containing in-line mixers, patented by Yoon et al. [31].
3(a). Sparging Through a Porous Media: Without High External Shear This method has been the most common approach for column flotation, and was virtually the only technique applied to columns until the introduction, in the mid-1980's, of the USBM developed method [32] (described in the next category). Industrial sparging material has typically been pierced rubber or fabric such as filter cloth. Generally, pierced rubber generates a smaller gas bubble (assuming equal gas flow per unit surface area of sparger), but is more difficult to fabricate and is sometimes prone to swelling. Yianatos [33] has examined the effect of filter cloth permeability on the effect of gas holdup (and hence db), and suggests an upper permeability level of 6 m3/m2/min. For small scale (laboratory) columns, the common approach has been to utilize an inflexible porous material, such as porous steel, bronze, glass or plastic. The early work of the Column Flotation Company of Canada Ltd. showed that an inflexible medium would plug with solids and/or precipitates within several hours or days, and was therefore unsuitable for industrial use. 3(b). Sparging Through a Porous Media: With High External Shear If a porous media sparger is placed in a high velocity slurry or wet line, bubble generation is controlled by both the nature of the media as well as the shear action created by the flowing slurry/water. Two, very different examples of this approach are: the Bahr cell [2], in which feed slurry is passed through many cylindrical, porous plastic sparging elements. High shear is generated by using small diameter sparging elements and thereby developing a very high slurry line velocity. This cell has been introduced as industrial units for coal flotation (Germany) and phosphate flotation (Brazil). the Air Sparged Hydrocyclone (ASH) [34], which utilizes centrifugal force to develop high shear at the surface of the porous media. the Flotaire gas sparging system, introduced in 1990. -
4. Jetting Bubble generation occurs when either a gas stream is jetted from an orifice into a liquid, or when a liquid (or slurry) is jetted from an orifice into a pool. Some industrial examples follow. (a) Jetting of Gas into Slurry in the technique first generated by the U.S. Bureau of Mines [32] a mixture of air and water is expelled through orifices located in the bottom of a column. The orifices are typically about 1 mm in diameter. Water content of the air/water mixture is generally quite low, less than 1%. For jetting to occur from an orifice, as opposed to bubbling from the orifice, the Reynolds number for air flow from the orifice must exceed about 2000 [35]. It is felt that the role played by water addition to the air is to increase the density of the air jet and thereby increase the jetting distance into the slurry. This has the effect of providing more surface area to the jet and consequently increases the number and decreases the size of the bubbles that are sheared away from the jet stream. The Tailings Oil Recovery (TOR) vessel [36], developed and used by Syncrude Canada Ltd. for secondary recovery of bitumen from tar sands, employs the jetting of gas and slurry streams through venturies. The venturies are located within the cell, and this action acts to pump more slurry into contact with the gas. -
-
ME.4:71II-S
922
G.S. DOaBY and J. A. FINCH
(b) Jetting of Slurry In the Jameson cell [3] and the Jet Flotation Cell [4], feed slurry is injected vertically downwards into the flotation cell through an orifice(s), and the reduced pressure created by the slurry jet acts to draw air into contact with the slurry. The Jameson cell has a single orifice and uses a 2 to 4 m high vertical pipe (downcomer) to provide gas/slurry contacting prior to entry into the flotation cell. In the Jet Flotation cell there are five 1 mm diameter orifices and no downcomer; the slurry/gas mixture impinges on a plate within the cell. The approach used by AFT flotation [37] is to spray feed slurry and recycled cell slurry onto the surface of the pulp. This is somewhat similar to cascade flotation, except the feed slurry contacting with the pulp is more of a jet stream with the AFT. Several of these techniques apply aeration of the feed stream prior to entry into the flotation cell, as opposed to independent aeration within the flotation cell. These feed aeration devices include the Davcra, Bahr, Jameson and Jet flotation cells. Contacting such as this app6ars to be very effective in terms of increased rates of particle collection. For this reason, research and industrial interest in feed aeration systems likely will continue to advance. Ultimately, the type of sparger system selected for plant use by a mill operator will have an essential requirement of reliability and "ease of use". ACKNOWLEDGEMENTS A major proportion of the funding for column flotation research conducted by the authors has been provided by the Natural Sciences and Engineering Research Council of Canada, which is gratefully acknowledged. REFERENCES
.
2.
.
4. 5. 6. 7. 8.
. 10. II. 12.
Cienski T. and Coffin V.L. Column flotation operation at Mines Gasp6 molybdenum circuit, Proc. 13th Annual Meeting of the Canadian Mineral Processors,240, (1981). Bahr A., Imhoff R. and Ludke H., Application and sizing of a new pneumatic flotation cell, in Proc. International Mineral Processing Congress, Cannes, 314, (1985). Jameson G.J., A new concept in flotation column design, in Column flotation '88 (K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 281, (1988). Alizadeh A. and Simonis W., Flotation of finest and ultra-fine coal particles, Aufbereitungs Technik, 6, 363, (1985). Finch J.A. and Dobby G.S., Column flotation: A selected review, 2nd Workshop of Flotation of Sulphide Minerals, Tekniska Hogskolan, Lulea, Sweden, 149, (1990). Dobby G.S. and Finch J.A., Flotation column scale-up and modelling, C.I.M. Bulletin, 79(889), 89, (1986). Finch J.A. and Dobby G.S., Column Flotation, Pergamon Press, Oxford, 1990. del Villar R., Finch J.A., Yianatos J.B. and Laplante A.R., Column flotation stimulation, in Computer Applications in the Mineral Industry (K. Fytas, J.L. Collins and R.K. Singhal, eds.), A.A. Balkema, Rotterdam, (1988). Alford R.A., An improved model for the design of industrial column flotation circuits in sulphide applications, in Sulphide Deposits - Their origin and Processing, (P. Gray, ed.) IMM (Nov. 1990). Yu S. and Finch J.A., Froth zone recovery in a flotation column, Canadian Metall. Quarterly, 29(3), 237, (1990). Wilson S.W., Masters Thesis, Univ. of Toronto, (1990). Falutsu M. and Dobby G.S., Direct measurement of froth drop back and collection zone recovery in a laboratory flotation column, Minerals Engineering, 2(3), 377 (1989).
Column flotation--II
13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
923
Contini N.J., Wilson S.W. and Dobby G.S., Measurement of rate data in flotation columns, in Column Flotation '88, (K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 81, (1988). Castillo D., Masters Thesis, Univ. of Toronto, (1991). Amelunxen R., Llerena R, Dunstan P. and Huls B., Mechanics of column flotation operation, in Column Flotation '88(K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 149, (1988). Falutsu M., Ph.D. Research, Univ. of Toronto, (Thesis in 1991). Yianatos J.B., Finch J.A. and Laplante A.R., Selectivity in column flotation froths, Int. J. Mineral Processing, 15,279, (1988). Furey J.T., Rougher column flotation of gold tellurides, Proc. 22nd Annual Meeting of the Canadian Mineral Processors, 242 (1990). Maachar A., Lin D. and Dobby G.S., Measurement of water recovery and entrainment in flotation columns, presented at C.I.M. Conference o/Metallurgists, Hamilton, Canada, August, (1990). Amelunxen R.L., The mechanics of operation of column flotation machines, Proc. 17th Annual Meeting of the Canadian Mineral Processors, 13, (1985). Kosick G.A., Freberg M and Kuehn L.A., Column flotation of galena at the Polaris concentrator, C.I.M. Bulletin, 81(920), 54, (1988). Redfern M.A. and Egan J.R., Large diameter column optimization and scale-up, in Processing of Complex Ores (G.S. Dobby and S.R. Rao, eds.) C.I.M. Conference of Metallurgists, Halifax, Canada, 303, (1989). Subramanian K.N., Connelly D.E.G. and Wong K.Y., Commercialization of a column flotation circuit for gold sulphide ore, in Column Flotation '88 (K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 13, (1988). Plouf T. and Malhotra D., Mill design changes to incorporate new flotation equipment, in Advances in Coal and Mineral Processing using Flotation (S. Chander and R. Klimpel, eds.) Proc. Eng. Foundation Conf., Publ. SME, 327, (1990). Dobby G.S., Kosick G.A. and Matwijenko O., Pilot plant testing for column flotation design, Proc. 22nd Annual Meeting of the Canadian Mineral Processors, 198, (1990). Huls B.J., Lachance C.D. and Dobby G.S., Bubble generation assessment for an industrial flotation column, Minerals Engineering 4 (1),37, ( 1991 ). Kosick G.A. and Dobby G.S., Internal report to Kidd Creek Mines, (1989). Araya F.C. and Bosshardt H.J., Column flotation development at Los Bronces concentrator of Compania Disputada de las Condes, SME Ann. Meet., Salt Lake City, Preprint 90-133, (1990). Anon., Engng. and Mining J., 172(7), 78, (1971). Yang D.C., New packed column flotation system, in Column Flotation '88 (K.V.S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 257, (1988). Yoon R.H., Adel G.T. and Luttrell G.H., U.S. patent appl. no. 5761008 (1988). McKay J.D., Foot D.G.Jr. and Shirts M.B., Column flotation and bubble generation studies at the Bureau of Mines, in Column Flotation '88 (K.V.S. Sastry,ed.) SME Annual Meeting, Phoenix, Arizona, 173, (1988). Yianatos J.B., Marchase M., Hutchinson S. and Wiles R.J., Characterization of gas sparging media in the flotation column, in Processing of Complex Ores (G.S. Dobby and S.R. Rao, eds.) C.I.M. Conference of Metallurgists, Halifax, Canada, 359, (1989). Ye T., Gopalakrishnan S., Pacquet E. and Miller J.D., Development of the airsparged hydrocyclone: a swirl-flow flotation column, in Column Flotation '88 (K.V.S. Sastry, ed.) SME General Meeting, Phoenix, Arizona, 305 (1988). Perry R.H. and Chilton C.E., Chemical Engineers Handbook 5th Edition, McGrawHill, New York, 18:68 (1973). Cymerman G.J. and Cymbalisty L.M., Improved oil sand extraction process at Syncrude Canada Ltd., in Proc. International Mineral Processing Congress, Cannes, 454 (1985). Gorski M.J., Carlsson S.A., Patel M.L. and Szentlaszloi A., Utilization of the AFT beneficiation ceils to produce clean pulverized coal and/or coal water slurry, Proc. 14th Int. Conf. Coal & Slurry Technology, Washington, D.C. (1988).
0892-6875/91 $3.00+000 Pergamon Press plc
COLUMN FLOTATION: A SELECTED REVIEW, PART II
G.S. DOBBY § and J.A. FINCHt § Dept. of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S I A4 f Dept. of Mining and Metallurgical Engineering, McGill University, Montreal, Canada H3A 2A7
ABSTRACT
Selected topics in the rapidly evolving field of column flotation are reviewed. The status of column scale-up concerns is assessed, with particular emphasis on recent measurements of froth zone performance. Column circuits are reviewed, and some performance comparisons between column and column~mechanical cell cleaning circuits are made. The effect of bubble size and gas rate upon column operation is discussed in general terms, and a comprehensive categorization of bubble generation techniques is made, including those used for feed slurry aeration. Keywords Flotation; column flotation; scale-up; froth; entrainment; flotation circuits; bubble generation INTRODUCTION Industrial application of column flotation has gone from virtually zero in 1983 to wide acceptance in 1990. (The first significant installation was actually in 1980, at Noranda's Mines Gasp6 [1]; however, the Mines Gasp6 plant was shut down in 1983-84 due to poor copper market conditions.) As a consequence of the widespread plant trials that have occurred during the past seven years, there has been a rapid growth in practical knowledge related to column design, construction, operation, sparging and control. There are now several companies worldwide that specialize in column design and engineering, and relatively few new orebodies are brought to commercial production without at least consideration of columns as part of the flotation flowsheet. Research on column flotation is now conducted in many mineral processing laboratories. The other significant advance over the past few years has been the introduction of alternative flotation technologies to conventional column flotation. Of special interest are the feed slurry aeration systems, such as the Bahr cell [2], the Jameson cell [3] and Jet flotation [4]. While, in 1990, these devices do not have nearly the same level of industrial implementation as that of the flotation column, there is a good deal of industrial interest. It is clear that flotation cell technology will continue to develop and evolve over the coming years. The purpose of this paper is to provide a selective review of some developments in column flotation over the past few years. A previous review by the authors [5] addressed mixing in the collection zone, maximum gas rates and carrying capacity. Topics that are covered here include scale-up, circuits and testing, and bubble generation. The discussion on bubble generation also looks at some of the feed aeration concepts. 911
912
G.S. DOBBYand J. A. FINCH COLUMN S C A L E - U P
The approach to column scale-up taken by D o b b y and Finch [6,7] recognizes the importance of the individual performance of the two zones, collection zone and froth zone, and their interaction. Each zone has a recovery, for each mineral type, relative to the feed to the zone. This is illustrated in Figure 1, in which R e and Rf are collection zone and froth zone fractional recoveries, respectively. For any given mineral, the overall column recovery R m is then given by R m-- R c R f / ( R c R f + l - Rc)
(1)
T CLEANING ZONE
1
Re
Re(1-1~) 1
F=X
COLLECTION ZONE
l-R= Fig.1 Schematic illustration of interaction between the collection zone and the froth zone in a flotation column. The interaction of the two zones is not always appreciated. A unique aspect is that particles rejected from the froth zone always have 100% of the retention time for recollection. In a bank of mechanical cells, the time available for recollection decreases as the particle progresses down the bank. The net result of this difference is that the column arrangement will provide for improved selectivity between two hydrophobic species [7]. The column loses this advantage when it has a small height to diameter ratio, i.e. a residence time distribution that approaches that of a single stirred tank. Collection zone recovery is modelled by a first-order rate process and is therefore a function of three parameters: collection zone rate constant K c, particle retention time, and degree of axial mixing. A practical model of the froth zone has yet to be developed. To simplify the process of modelling, the column is sometimes considered to be a single stage [8,9]. In this approach an overall rate constant K f is used, and the froth zone is not treated as a separate stage, although froth zone effects are recognized in terms of an impact on Kf. While this approach has been reported to be satisfactory for modelling large scale columns [9], it can be very misleading in scale-up from small (pilot) columns to large columns. This is because the froth zone recovery is often substantially lower in large columns compared to that in pilot columns. Hence, in such a case the pilot scale measured
Column flotation--lI
913
rate constant Kf (that includes the froth effect) will be unrealistically high if it is applied to the large column without correction. It is believed that in some cases the difference in Rf between that of the pilot column and that of the large plant column is as high a factor of 3 to 4, i.e. Rf in the plant column being 3 to 4 times lower than that in the pilot column.
Froth Zone Recovery Rf To conduct scale-up from data using the two-zone approach requires knowledge of both K e and Rf. Rf must be assumed in order to measure Kc, unless elaborate testing is conducted, in which the collection zone/froth interface is eliminated [10,11], or a special column is constructed in order to isolate the mineral particle dropping from the froth zone back to the collection zone [ 12], or the column is operated in sequential countercurrent and concurrent modes on the same feed [13]. However, these techniques are not required; a satisfactory approach has been to assume a froth zone recovery and then set the froth zone recovery in the scaled-up column to a range of lower values. This results in a scale-up grade-recovery prediction, where the upper end of the curve (low Rf and hence high grade) represents column operation at low gas rate, deep froth bed, and/or high wash water addition, and the lower end (high Rf and high recovery) represents operation at higher gas rate, shallower froth bed and/or less wash water. The difficulty here lies in selection of the scaled-up R f values relative to those of the pilot column. The approach is quite robust, however, for two reasons. First, it has been shown [14] that the scale-up prediction is relatively insensitive to the initial choice of Rf in the pilot column, provided that Rf values for the scale-up are adjusted relative to the initial Rf chosen. Second, Rf appears to be quite insensitive to mineral hydrophobicity, at least for heavily loaded froths in which recollection within the froth cannot occur. (It must be remembered that Rf refers to particles collected by the gas bubbles only, and does not account for entrainment.) The results of froth zone recovery measurements made by Wilson [l 1] for copper sulphide and nickel sulphide in a 3.8 cm diameter column showed that there was no statistically significant difference in Rf between the two mineral species; some of these results are given in Table 1.
TABLE 1 Rf values for CuaS and Ni3S 2 measured in a pilot column by Wilson [11] at Inco's matte separation plant. Feed Stream Cu 1st clnr. Cu 2nd clnr. Middlings Ni scavenger
Gas Rate (cm/s) 2.4 2.9+.2 1.7 2.9
Number of tests 3 4 3 5
Cu2S
s.d.
29 43 16 52
3 12 4 10
Rf (%) Ni3S 2 30 53 13 48
s.d. 3 14 5 7
"s.d." is standard deviation
Further evidence that little or no upgrading occurs within the froth has been provided by the froth profile sampling on a l m diameter column at Southern Peru Copper Corp. [15], Figure 2, and the froth sampling on a 2.1 m diameter copper column by Falutsu [16], Figure 3. In Falutsu's tests, the probe was designed to sample gas bubbles only (and reject the slurry), so the profiles in Figure 3 are for the solids on the gas bubble. The work of Yianatos [17] on 0.45 and 0.9 m molybdenite columns indicated that some upgrading did occur; it is hypothesized that this was due to the selective recollection that may occur within a lightly loaded froth.
914
G.S. DOBBY and J. A. FINCH
40
Interface
El
3O
-(D c5
(..9
2O
o_
E oo 10 El
[]
[]
[] ra
0
[]
[]
i
i
1
2
Distance
down from column
lip (m)
Fig.2 Profile of copper content in samples extracted from a 0.9 m diameter column at Southern Peru Copper Corp. [15], showing no upgrading in copper content within the froth bed (obvious upgrading at the interface).
35.00
Assays, % $
S
-------------____x r~
Cu
30.00
25.00
interface 20.00
I
I
I
I
50
100
150
200
Distance from column top, cm
Fig.3 Profile of copper content in samples extracted from a 2.1 m diameter column at Falconbridge Ltd. [16]. Samples are of solids attached to the gas bubbles only, no solids from the liquid phase are in the sample. Entrainment For many column applications, solids recovery via entrainment can be safely ignored when modelling the process or when considering scale-up. However, this is not so for applications such as: a) columns operated without wash water, and
Column flotation--II
b)
915
columns used to treat gold ores, where the solids recovery by flotation is low (1 to4 % typically) and, hence, where even a very low recovery of solids due to entrainment is quite detrimental to the product grade.
This latter effect is shown in the results of Furey, Figure 4 on a gold telluride ore [18]. The work of Maachar et al. [19] on a 3.8 cm diameter column has shown feed water recovery, and hence entrainment-related recovery, to be a clear function of feed superficial velocity JF and bias velocity JB, Figure 5. Note that data in Figure 5 covers a wide range in gas rate, wash water rate, feed rate and froth depth. 300
200
i,
Negative Bias
-
lIFt,, $
"U
I I I I I I I I I I I I I
100
0 "60
*
I "40
*
I -20
•
|
Positive Bias
•
0
I 20
, 40
Tails F l o w B I ~ (%)
Fig.4 Column flotation concentrate grade of a gold telluride flotation on a pilot column as a function of bias [18]. Bias here is quoted as a tailings flow percentage greater than (+ bias) or less than (- bias) feed water flow. Scale-up examples The results of several scale-up predictions in comparison to actual performance of 1.8 m diameter columns has been reported by Wilson [11]. In this work, 3.8 cm and 10 cm diameter pilot columns were operated on various feed streams to the plant columns at Inco's matte separation plant in Sudbury, Canada. Each scale-up was well within the accuracy that would be required for design of a new installation. A similar study has been conducted by Castillo [14], in which pilot data from a 15 cm diameter column was used to predict the performance of a 1.5 m diameter column in a zinc sulphide final cleaning application. The plant column performance and the scale-up prediction are shown in Figure 6. Ultimately, any scale-up work is limited in its precision by two factors: variability in the ore characteristics, and variability within the test procedure. COLUMN FLOTATION CIRCUITS Unlike mechanical cell cleaning, column flotation cleaning circuits can be configured in either the conventional countercurrent cleaning configuration, as it has been done for molybdenite cleaning [1], or in the scavenger arrangement, in which the first column
916
G.S. DoBBY and J. A. FINCH
produces the final grade product, its tailings are refloated in a subsequent column(s) and scavenger column concentrate(s) may be recycled back to the first column feed. A commercial example of the latter is the installation at Gibraltar Mines, Canada [20]. The choice of configuration (cleaning versus scavenging) will generally depend on whether the emphasis is on the attaining of grade or recovery.
60
/ a
L-G
• L-G..S ~ •
/
/
0
40
Water recovery 30
(%)
20
10
0 0
10
30
20
exp(-13.1Jb)/Jf
Fig.5 Feed water recovery as a function of bias (cm/s) and feed rate (cm/s) for a range of operating conditions [19]. Jg = 1.95_+1.0; Jw = 0.19_+0.19; JF = 1.5_+1.0; JB from -0.15 to +0.15 cm/s
58
Nc "O 57 O e" O
13
O
56
0
i
50
i
60
13
70
Zinc recovery (%)
Fig.6 Plant performance (data points) and predicted scale-up (line) of zinc grade-recovery on a 1.5 m diameter column at Kidd Creek Mines [14]. Feed grade was 46% Zn and throughput was 7.5 tonnes per hour solids. Data for scale-up was obtained from a 15 cm diameter pilot column.
Column flotation--II
917
There are several examples of integration between columns and mechanical ceils in cleaning circuits. Falconbridge operates two copper cleaning circuits, each arranged in the scavengerclosed configuration, whereby column tailings are scavenged in a bank of mechanical cells and mechanical cell concentrate is recycled. In Inco's copper matte flotation circuit mechanical cells and a column are arranged in a cleaning configuration, with mechanical cells as a first cleaner and the column as the second cleaner. For lead flotation at Cominco's Polaris concentrator two flotation columns are employed in a very flexible mode along with mechanical cells [21]. The operators have a choice of three circuit arrangements, the selection being dependent on feed grade. Zinc rougher concentrate at the Cominco Sullivan concentrator is treated in a 2.4 m diameter column that produces final grade product, accounting for about one-third of the total concentrate production [22]; the balance of concentrate is produced by treating the column tailings in the conventional (existing) mechanical cell cleaning circuit. Several unique circuits involving integration of columns with mechanical cells in Australia have been presented [23,24]. The need for a careful assessment of options available in circuit design prompted development of a portable, fully controlled pilot flotation plant consisting of 15 cm diameter by 6 m high columns and a bank of mechanical cells [25]. Results of extensive testwork on zinc rougher concentrate and copper rougher concentrate were reported. Table 2a compares performance of three columns arranged in a cleaning circuit (CCC/cleaner) and in a scavenger circuit (CCC/scavenger); see Figure 7 for the two flowsheets. Wash water was used only on those columns producing final concentrate. As might be expected, the CCC/cleaner configuration produced a higher grade and somewhat lower recovery. Another set of results, Table 2b, shows the effect of using a column versus a bank of Denver mechanical cells as the last stage of the cleaning circuit. No wash water was used in the column. The data indicates that the column performed slightly better than the bank of mechanical cells in the role of a cleaner scavenger.
TABLE 2a Comparison of two configurations using three pilot columns for zinc cleaning application [251(see Figure 7 for flowsheets). Circuit
CCC/Cleaner CCC/Scavenger
Feed % Zn
% Zn
43.7 45.8
57.1 56.6
Concentrate c* Zn Rec'y 82 76
67 76
Number of Tests 3 4
Table 2b Flotation column (15 cm diameter) versus mechanical cells (one bank of four 30 L cells) for zinc cleaner scavenging 125l. Cell Type
Feed % Zn
% Zn
Concentrate c* Zn Rec'y
Nominal Retention Time (min)
A. Scavenger stage of C C C / o r CCM/cleaner configuration Column 46.4 51.3 36 86 Mechanical 46.2 51.7 40 73
26 22
B. Re-scavenger stage of C C C / o r CCM/scavenger configuration Column 36.3 48.9 53 58 Mechanical 40.3 49.2 45 54
25 21
c* is 100 x (conc grade - feed grade)/(60 - feed grade), where 60 is the m a x i m u m concentrate grade that can be achieved with this mineralization, c* then, is a percentage of maximum upgrading and is a way of correcting for slight variations in feed grade.
918
G.S. DOBBYand J. A. FINCH
I
I Conc
Feed
i
3
2
Y
.11
CCC/ct~-
Conc
I
I
I
t
L
Feed
1
2
3
(X~C/acav
Fig.7
Flowsheets for CCC/cleaner and CCC/scavenger circuits
[25].
BUBBLE GENERATION The objective of bubble generation in column flotation, as in any froth flotation system, is to produce relatively small gas bubbles (bubble diameter d b -- 0.5 to 1.5 mm) at a moderate gas rate (typically, superficial gas velocity Jq -- 0.5 to 2.0 cm/s). The size of bubbles produced is determined by the type of bubble generation system, frother type and dosage, and gas rate. It is sometimes forgotten that the performance of both the collection zone and the froth zone are affected by the nature of the bubble system. Hence, bubble conditions that are optimum for one zone may not be optimum for the other zone, and the effect on column performance of changing either Jg or d b on an operating system will not always be predictable. Some situations that can occur include the following: reducing the bubble size results in an increase in the rate of particle collection and therefore increases recovery.
Column flotation--II
919
reducing the bubble size results in an increase in the rate of particle collection, but also increases the gas holdup in the column to the point that critical gas holdup is exceeded; the outcome is that the collection zone "froths-up" unless gas rate is reduced, thus negating the effect of the reduced d b. increasing the gas rate results in an increase in the rate of particle collection and/or froth removal rate (i.e. Rf) and therefore increases recovery. increasing the gas rate in order to increase Rf (especially important for heavily loaded froths) results in collection zone "froth-up" because small bubbles are being generated and the critical gas holdup is exceeded. While producing smaller gas bubbles in a given column operation will often increase the rate of flotation, it will not necessarily improve separability. Two examples of this are shown in Figures 8 [26] and 9 [27], with the relevant operating conditions provided in Table 3. For the Cu-Ni separation results in a 0.9 m diameter column, shown in Figure 8, nickel recovery plotted versus copper recovery is similar with both filter cloth and USBM type spargers. The bubble size produced with the cloth sparger was slightly larger and a higher gas rate was required to achieve a specific recovery. The same effect is seen in Figure 9, where a rubber sparger is compared with a cloth sparger on the flotation of zinc. Grade-recovery is similar, but in this case the rubber sparger required considerably less gas to achieve a specific recovery.
O
Sparger System O
Clolh
A
External
&
v
o/
0 r~
rr
.~ Z
1 A o
8/,, 0 40
I 50
o
i 60
7'0
80
90
Copper Recovery (%)
Fig.8 Effect of sparger type (filter cloth versus USBM "external") on nickel recovery versus recovery for a 0.9 m diameter pilot column [26]. This data should not be interpreted that improved selectivity is not possible with a bubble generation system that generates smaller bubbles. If a given sparger system is performing poorly, for example generating some very large bubbles that cause disruption of the froth, then an improved sparger system could produce an improved recovery and grade. In a study on copper flotation at Disputada in Chile [28] it was reported that the use of USBM type spargers produced better separation (higher recovery and grade) than that with filter cloth spargers.
920
G.S. DoBaY and J. A. FINCH
58.4 Feed:
48.1-48.9
%Zn
m
Rubber
•
Filter Cloth
58.2 '
c N
~:~
58.0 •
578.
c
57.6 • c 0
(,.) 57.4 .
57.2 20
i 30
i 40
i 50
i 60
70
Zinc recovery (%)
Fig.9 Effect of sparger type (filter cloth versus rubber) on zinc grade versus recovery for a 15 cm diameter pilot column [27]. TABLE 3 Conditions for the performance results in Figures 8 and 9. Feed Material
Column Diameter (cm)
Sparger Type
Jg (cm/s)
Figure 8 Cu Rougher Conc
90
USBM Cloth
0.7-1.0 0.8-1.3
Figure 9 Zn Cleaner Conc
15
Rubber Cloth
1.65 2.6-3.2
Thus, the improvement in metallurgy that is possible through the generation of smaller gas bubbles depends on the level of performance of the existing system, as well as the source of the existing rate limitation: particle collection or froth removal. From the forgoing it is clear that both bubble size and gas rate must be considered together when designing a system or making adjustments to an operating column. Categories of Bubble Generation Techniques In the past several years there has been a great deal of ongoing development and testing of bubble generation systems. The intent of this section of the paper is not to critically analyze each of the prominent methods, but rather to attempt a general categorization. 1. Mechanical Shear Contacting This is the method of bubble generation provided by conventional mechanical flotation machines, and is generally very effective at producing small gas bubbles. Mechanical flotation machines typically produce gas bubbles that are 0.5 to 1.0 mm in diameter.
Columnflotation--II
921
2. Static Shear Contacting High velocity contacting of slurry and gas in an appropriate manner will generate small gas bubbles. Some examples of this include: the Davcra cell [29], in which gas and feed slurry are contacted in a cyclone-type device immediately prior to entering the flotation cell. the Packed Column [30], in which gas entering the bottom of the column is sheared into bubbles by extended metal mesh packing that occupies most of the column volume. contacting of gas with slurry in a pipe containing in-line mixers, patented by Yoon et al. [31].
3(a). Sparging Through a Porous Media: Without High External Shear This method has been the most common approach for column flotation, and was virtually the only technique applied to columns until the introduction, in the mid-1980's, of the USBM developed method [32] (described in the next category). Industrial sparging material has typically been pierced rubber or fabric such as filter cloth. Generally, pierced rubber generates a smaller gas bubble (assuming equal gas flow per unit surface area of sparger), but is more difficult to fabricate and is sometimes prone to swelling. Yianatos [33] has examined the effect of filter cloth permeability on the effect of gas holdup (and hence db), and suggests an upper permeability level of 6 m3/m2/min. For small scale (laboratory) columns, the common approach has been to utilize an inflexible porous material, such as porous steel, bronze, glass or plastic. The early work of the Column Flotation Company of Canada Ltd. showed that an inflexible medium would plug with solids and/or precipitates within several hours or days, and was therefore unsuitable for industrial use. 3(b). Sparging Through a Porous Media: With High External Shear If a porous media sparger is placed in a high velocity slurry or wet line, bubble generation is controlled by both the nature of the media as well as the shear action created by the flowing slurry/water. Two, very different examples of this approach are: the Bahr cell [2], in which feed slurry is passed through many cylindrical, porous plastic sparging elements. High shear is generated by using small diameter sparging elements and thereby developing a very high slurry line velocity. This cell has been introduced as industrial units for coal flotation (Germany) and phosphate flotation (Brazil). the Air Sparged Hydrocyclone (ASH) [34], which utilizes centrifugal force to develop high shear at the surface of the porous media. the Flotaire gas sparging system, introduced in 1990. -
4. Jetting Bubble generation occurs when either a gas stream is jetted from an orifice into a liquid, or when a liquid (or slurry) is jetted from an orifice into a pool. Some industrial examples follow. (a) Jetting of Gas into Slurry in the technique first generated by the U.S. Bureau of Mines [32] a mixture of air and water is expelled through orifices located in the bottom of a column. The orifices are typically about 1 mm in diameter. Water content of the air/water mixture is generally quite low, less than 1%. For jetting to occur from an orifice, as opposed to bubbling from the orifice, the Reynolds number for air flow from the orifice must exceed about 2000 [35]. It is felt that the role played by water addition to the air is to increase the density of the air jet and thereby increase the jetting distance into the slurry. This has the effect of providing more surface area to the jet and consequently increases the number and decreases the size of the bubbles that are sheared away from the jet stream. The Tailings Oil Recovery (TOR) vessel [36], developed and used by Syncrude Canada Ltd. for secondary recovery of bitumen from tar sands, employs the jetting of gas and slurry streams through venturies. The venturies are located within the cell, and this action acts to pump more slurry into contact with the gas. -
-
ME.4:71II-S
922
G.S. DOaBY and J. A. FINCH
(b) Jetting of Slurry In the Jameson cell [3] and the Jet Flotation Cell [4], feed slurry is injected vertically downwards into the flotation cell through an orifice(s), and the reduced pressure created by the slurry jet acts to draw air into contact with the slurry. The Jameson cell has a single orifice and uses a 2 to 4 m high vertical pipe (downcomer) to provide gas/slurry contacting prior to entry into the flotation cell. In the Jet Flotation cell there are five 1 mm diameter orifices and no downcomer; the slurry/gas mixture impinges on a plate within the cell. The approach used by AFT flotation [37] is to spray feed slurry and recycled cell slurry onto the surface of the pulp. This is somewhat similar to cascade flotation, except the feed slurry contacting with the pulp is more of a jet stream with the AFT. Several of these techniques apply aeration of the feed stream prior to entry into the flotation cell, as opposed to independent aeration within the flotation cell. These feed aeration devices include the Davcra, Bahr, Jameson and Jet flotation cells. Contacting such as this app6ars to be very effective in terms of increased rates of particle collection. For this reason, research and industrial interest in feed aeration systems likely will continue to advance. Ultimately, the type of sparger system selected for plant use by a mill operator will have an essential requirement of reliability and "ease of use". ACKNOWLEDGEMENTS A major proportion of the funding for column flotation research conducted by the authors has been provided by the Natural Sciences and Engineering Research Council of Canada, which is gratefully acknowledged. REFERENCES
.
2.
.
4. 5. 6. 7. 8.
. 10. II. 12.
Cienski T. and Coffin V.L. Column flotation operation at Mines Gasp6 molybdenum circuit, Proc. 13th Annual Meeting of the Canadian Mineral Processors,240, (1981). Bahr A., Imhoff R. and Ludke H., Application and sizing of a new pneumatic flotation cell, in Proc. International Mineral Processing Congress, Cannes, 314, (1985). Jameson G.J., A new concept in flotation column design, in Column flotation '88 (K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 281, (1988). Alizadeh A. and Simonis W., Flotation of finest and ultra-fine coal particles, Aufbereitungs Technik, 6, 363, (1985). Finch J.A. and Dobby G.S., Column flotation: A selected review, 2nd Workshop of Flotation of Sulphide Minerals, Tekniska Hogskolan, Lulea, Sweden, 149, (1990). Dobby G.S. and Finch J.A., Flotation column scale-up and modelling, C.I.M. Bulletin, 79(889), 89, (1986). Finch J.A. and Dobby G.S., Column Flotation, Pergamon Press, Oxford, 1990. del Villar R., Finch J.A., Yianatos J.B. and Laplante A.R., Column flotation stimulation, in Computer Applications in the Mineral Industry (K. Fytas, J.L. Collins and R.K. Singhal, eds.), A.A. Balkema, Rotterdam, (1988). Alford R.A., An improved model for the design of industrial column flotation circuits in sulphide applications, in Sulphide Deposits - Their origin and Processing, (P. Gray, ed.) IMM (Nov. 1990). Yu S. and Finch J.A., Froth zone recovery in a flotation column, Canadian Metall. Quarterly, 29(3), 237, (1990). Wilson S.W., Masters Thesis, Univ. of Toronto, (1990). Falutsu M. and Dobby G.S., Direct measurement of froth drop back and collection zone recovery in a laboratory flotation column, Minerals Engineering, 2(3), 377 (1989).
Column flotation--II
13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
923
Contini N.J., Wilson S.W. and Dobby G.S., Measurement of rate data in flotation columns, in Column Flotation '88, (K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 81, (1988). Castillo D., Masters Thesis, Univ. of Toronto, (1991). Amelunxen R., Llerena R, Dunstan P. and Huls B., Mechanics of column flotation operation, in Column Flotation '88(K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 149, (1988). Falutsu M., Ph.D. Research, Univ. of Toronto, (Thesis in 1991). Yianatos J.B., Finch J.A. and Laplante A.R., Selectivity in column flotation froths, Int. J. Mineral Processing, 15,279, (1988). Furey J.T., Rougher column flotation of gold tellurides, Proc. 22nd Annual Meeting of the Canadian Mineral Processors, 242 (1990). Maachar A., Lin D. and Dobby G.S., Measurement of water recovery and entrainment in flotation columns, presented at C.I.M. Conference o/Metallurgists, Hamilton, Canada, August, (1990). Amelunxen R.L., The mechanics of operation of column flotation machines, Proc. 17th Annual Meeting of the Canadian Mineral Processors, 13, (1985). Kosick G.A., Freberg M and Kuehn L.A., Column flotation of galena at the Polaris concentrator, C.I.M. Bulletin, 81(920), 54, (1988). Redfern M.A. and Egan J.R., Large diameter column optimization and scale-up, in Processing of Complex Ores (G.S. Dobby and S.R. Rao, eds.) C.I.M. Conference of Metallurgists, Halifax, Canada, 303, (1989). Subramanian K.N., Connelly D.E.G. and Wong K.Y., Commercialization of a column flotation circuit for gold sulphide ore, in Column Flotation '88 (K.V.S. Sastry, ed.) SME Annual Meeting, Phoenix, Arizona, 13, (1988). Plouf T. and Malhotra D., Mill design changes to incorporate new flotation equipment, in Advances in Coal and Mineral Processing using Flotation (S. Chander and R. Klimpel, eds.) Proc. Eng. Foundation Conf., Publ. SME, 327, (1990). Dobby G.S., Kosick G.A. and Matwijenko O., Pilot plant testing for column flotation design, Proc. 22nd Annual Meeting of the Canadian Mineral Processors, 198, (1990). Huls B.J., Lachance C.D. and Dobby G.S., Bubble generation assessment for an industrial flotation column, Minerals Engineering 4 (1),37, ( 1991 ). Kosick G.A. and Dobby G.S., Internal report to Kidd Creek Mines, (1989). Araya F.C. and Bosshardt H.J., Column flotation development at Los Bronces concentrator of Compania Disputada de las Condes, SME Ann. Meet., Salt Lake City, Preprint 90-133, (1990). Anon., Engng. and Mining J., 172(7), 78, (1971). Yang D.C., New packed column flotation system, in Column Flotation '88 (K.V.S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 257, (1988). Yoon R.H., Adel G.T. and Luttrell G.H., U.S. patent appl. no. 5761008 (1988). McKay J.D., Foot D.G.Jr. and Shirts M.B., Column flotation and bubble generation studies at the Bureau of Mines, in Column Flotation '88 (K.V.S. Sastry,ed.) SME Annual Meeting, Phoenix, Arizona, 173, (1988). Yianatos J.B., Marchase M., Hutchinson S. and Wiles R.J., Characterization of gas sparging media in the flotation column, in Processing of Complex Ores (G.S. Dobby and S.R. Rao, eds.) C.I.M. Conference of Metallurgists, Halifax, Canada, 359, (1989). Ye T., Gopalakrishnan S., Pacquet E. and Miller J.D., Development of the airsparged hydrocyclone: a swirl-flow flotation column, in Column Flotation '88 (K.V.S. Sastry, ed.) SME General Meeting, Phoenix, Arizona, 305 (1988). Perry R.H. and Chilton C.E., Chemical Engineers Handbook 5th Edition, McGrawHill, New York, 18:68 (1973). Cymerman G.J. and Cymbalisty L.M., Improved oil sand extraction process at Syncrude Canada Ltd., in Proc. International Mineral Processing Congress, Cannes, 454 (1985). Gorski M.J., Carlsson S.A., Patel M.L. and Szentlaszloi A., Utilization of the AFT beneficiation ceils to produce clean pulverized coal and/or coal water slurry, Proc. 14th Int. Conf. Coal & Slurry Technology, Washington, D.C. (1988).