- Email: [email protected]
Leeds column performance evaluation
Minerals Engineering, Vol. 4, Nos 7-11, pp. 935-950, 1991
0892-6875/9Z $3.00 + 000 © 1991 Pergamon Press plc
Printed in Great Britain
LEEDS COLUMN PERFORMANCE EVALUATION V.R. DEGNER§ and P.L. PERSONt
§ Vice President, Research & Development, WEMCO, PO Box 15619, Sacramento, CA 95852-1619, U.S.A. ? Western Regional Sales Manager, WEMCO, Tucson, Arizona, U.S.A.
ABSTRACT The LEEDS Column, initially conceived as a metallurgical test device in the late 1950's, has progressed through a systematic development program which included bench, pilot, and full size machines. Comparative metallurgical performance testing repeatedly demonstrated the potential for the LEEDS Column to achieve a significant improvement in product grade while maintaining a recovery level comparable to conventional flotation technology. These successes led to the commercialization of the LEEDS Column by WEMCO in 1988-89. This paper describes the fundamental concept of the LEEDS Column, and will include a brief summary of the WEMCO R&D Technology Advancement Program. A comparison between the LEEDS Column, "conventional" mechanical flotation machines, and the "Conventional" Column will be made. Metallurgical performance tests, conducted on full size LEEDS Columns, functioning under plant operating conditions, will be compared to alternative flotation machines, where the metallurgical performance benefits in "cleaning" duty will be shown using actual plant test data. Keywords Froth flotation; columns; copper beneficiation; kinetics; Leeds column
INTRODUCTION The LEEDS Column concept consists of a series of "barrier" rods which are used to advantage, often together with countercurrent reflux water, to achieve a high product grade in relatively few cleaning stages. This performance feature was initially utilized by C. Dell in University lab flotation "release" testing. The release analysis [1] procedure is designed to establish the ultimate metallurgical performance level achievable by flotation for a particular degree of mineral liberation. Conducting a release analysis using conventional impeller-type flotation equipment requires extensive cleaning and re-cleaning steps, often repeated seven or eight times, to achieve the end point which establishes the maximum (practical) achievable performance. C. Dell found, experimentally, that the LEEDS column, applied to the release analysis procedure, could accomplish the same metallurgical performance objective as the conventional flotation concept in much fewer procedural steps. This suggested that the concept may have commercial value beyond the analytical benefits observed in the laboratory. Early coal flotation studies also showed that significant improvements in filtration rate would accompany product grade improvement. This further reinforced the beneficial economic features of this new concept [2]. 935
936
V.R. DEGNERand P. L. l~gsoN
Financial support towards commercialization was initially sponsored by the British Technology Group (BTG) beginning in 1973. The PhD studies of B. W. Jenkins (1973-1976) produced the rod-type barrier which was basic to the first successful plant (pilot unit) trial at Lofthouse Colliery (U.K.) in 1976 [1]. The WEMCO R&D program continued the LEEDS column development, and was designed to establish the basic practical machine design, and scale-up principles. This program was initiated in July 1984, and culminated in a series of in-plant tests in which the metallurgical performance of the LEEDS column was compared to existing flotation technology. WEMCO/LEEDS COLUMN DESCRIPTION Figure 1 illustrates the key elements of the WEMCO/LEEDS flotation column. Four horizontal barriers are arranged vertically, one above the other. This barrier design separates the pulp in the cell into a series of vertical compartments. Each barrier consists of two sets of movable rods in contact, as indicated in Figure 2. Rod design is such that the lower rod has a net buoyant force in the upward direction and the upper rod has a net buoyant force in the downward direction. This design creates a hydrostatic pinching force between the rods when submerged in the pulp.
~KIMMER FROTH ~ROWDER PLATES AIR MANIFOLD
INTERNAL TRANSVERSE LAUNDER ROD BARRIER
REFLU} WATEF
PULP DISCHARGE
PUL FEE
LAUNDER O/F DISCHARGE
Fig.l WEMCO/LEEDS Column Flotation System The WEMCO/LEEDS flotation column incorporates a mechanical impeller in the lower section of the cell. Air supplied to the impeller region via a blower is dispersed as fine air bubbles and mixed with the solids to establish an "air bubble-floatable specie" matrix, which rises toward the pulp surface. The rise of the "air bubble-floatable specie" matrix is impeded by the horizontal rod barriers. Air bubbles collect under the barrier when the upper and lower rods are touching. The rods separate once the buoyant air bubble forces
937
LEEDS Column performance evaluation
are sufficient to overcome the rod "hydrostatic pinching" forces, thereby allowing air bubbles to continue rising toward the surface. As the specie laden air bubbles pass through the openings between the moving rod barriers, the stripping process will physically remove the lightly held gangue material. This "stripping" action improves the grade of the concentrate by reducing the entrapped or lightly held, predominantly gangue, attached to the air bubble.
~ISITY
S Pu~p)
J
ROD ,RRIERS
Fig.2 WEMCO/LEEDS Column Rod Barrier Concept Analysis of pulp samples taken from the vertical sections between the rod barriers shows a progressively cleaner or higher grade solids concentration in going from the lower to the upper sections of the flotation column. Reflux water, introduced at the top of the column and percolating downward, serves to complement the beneficiation action of the flotation column rod barriers. The performance benefits of reflux water are best determined on a specific case basis. Experimental studies have shown that in some applications, the use of reflux water significantly improves product grade. Other cases, however, showed that the use of reflux water was not required to achieve the desired high grade product performance. In the design of the rod barrier assembly, consideration must include configuration details of both the rods and the rod carriers. Both the upper and the lower rods are mounted in a carrier which is designed to permit freedom of movement of the upper and lower rods in both the vertical and horizontal direction. The rods are also free to rotate with respect to each other. Design parameters, which influence the hydraulic pinching force per unit length of rod, include the rod diameter and contact angle, as well as individual upper and lower rod density. For a given pulp density, variations in these design parameters can result in the five distinct static force balance cases, illustrated in Figure 3. Keeping in mind the freedom of vertical movement of the rods contained within the carrier; Case A represents rods not in contact, resting on the lower carrier. Case B illustrates rods in contact, with the upper rod only resting on the bottom carrier. Case C represents neutral buoyancy, with the rods in contact and neither rod in contact with the rod carrier. Case D illustrates the rods in contact, with the lower rod contacting the upper carrier, while in Case E neither rod is in contact and both rods are contacting the upper carrier. In Case A and E, where the rods are not in contact, no stripping or beneficiation action would be expected; therefore, ME 4:7111-T
938
V.R. DEO~rEXand P. L. l~xsoN
designing for static operation at either Case A or Case E condition would be unsatisfactory. Alternatively, the design for a given application must be in the range of B to D.
) ii&.
~~
J
..,~i~
CASE A: ROD DENSITY > > SLURRY
CASE D: BOTTOM RODS RESTING ON BOTTOM OF TOP CARRIER
CASE B: TOP ROD RESTINGON TOP OF BOTTOM CARRIER
CASE E: ROD DENSITY < < SLURRY
UPPER ROD SET UPPERROD CARRIER LOWER ROD SET ~iii::iiiiii#i!i::iiii::#i::i::ii~iiii::ii:~: LOWER ROD CARRIER ~ \ ~
CASE C: NEUTRAL BUOYANCY
Fig.3 WEMCO/LEEDS Column Rod Design (Static Force Balance Cases) The dynamic action of the rods relative to one another during operation, must also be examined. Studies of the rod action in operating transparent columns demonstrated that the rods oscillate primarily in the vertical direction and also rotate relative to one another. This relative vertical and rotational rod motion has an important bearing on the gangue stripping action, and would reduce the effectiveness of the column beneficiation action if restricted. Figure 4 illustrates the cleaning action of the rods during passage of the specie-laden air bubbles rising through the rod contact or "pinch" zone. Disengaged gangue flows downward through the contact region to the pulp section below. The flow path of reflux water, if used, is also shown in Figure 4. The LEEDS Column differs from the conventional column in several important areas: (1)
The LEEDS Column cell configuration is similar to the familiar "mechanical" cell both in terms of floor space and overall height. The conventional column is a tall device having a relatively small floor area (i.e. footprint).
(2)
In the LEEDS Column, air bubble generation is accomplished using well established forced air/rotating impeller/stabilizer interaction principles which have been used in minerals industry flotation machines for the last 50 years. Bubble generation techniques for the conventional flotation column are still undergoing component development to improve the reliability and reduce the maintenance (nozzle wear...) problems.
LEEDS Columnperformanceevaluation
939
(3)
Solids suspension capability of the LEEDS column is excellent because the pulp circulation energy from the impeller acts on a smaller fraction of the total cell volume (i.e. the cell volume below the lower rod bank). The result of the high circulation intensity is a relatively high mixing intensity and reduced pulp short-circuiting. The conventional column fails to provide for any pulp, or solids, recirculation and is, basically, a o n e p a s s system. This feature is conducive to short-circuiting effects which reduce metallurgical "recovery".
(4)
The high air-pulp mixing intensity in the impeller-stabilizer region of the LEEDS Column contributes to good metallurgical (recovery) performance. The conventional column features lower intensity air bubble-floatable specie interception characteristics as the rising air bubbles migrate past the settling solids.
(5)
Operational control of the LEEDS Column follows the familiar practice of all conventional mechanical flotation machines, where the use of reflux water is not critical to good performance. In the conventional columns, froth depth and bias control are critical and must be maintained through proper flow control if good metallurgical performance is to be achieved.
j
AIR BUBBLE PATH
x
GANGUE = . VALUABLE SPECIE = • AIR BUBBLE = O HIGH DENSITY ROD = H LOW DENSITY ROD = L
Fig.4 LEEDS Column Beneficiation Mechanics The skimmer and froth crowder plates, shown in Figure 1, are utilized when the LEEDS Column is operating in applications having a relatively deep, persistent, froth such as coal flotation situations. In many metalliferous ore beneficiation application, the LEEDS Column skimmer is not required. The LEEDS Column is designed to permit easy removal, and cleaning, of the rod barriers in high calcium carbonate scaling situations. This maintenance step can be accomplished without shutting down the flotation row, since each barrier set can be lifted out of the cell without interfering with the mechanism. R&D TECHNOLOGY PROGRAM The WEMCO R&D philosophy, for the development of any new product, is to systematically study the new concept from bench size equipment, through pilot testing, to
940
V.R. DEG~R and P. L. PERSON
full-size equipment in the laboratory, prior to the initiation of in-plant evaluation tests. This procedure was followed for the LEEDS column development and began with a preliminary rod dynamics study. This study was confined to evaluating a wide variety of rod sizes and densities in small bench cells. These cells were transparent so that the action of the rods could be evaluated for varying gas transfer rates. Analysis of the test results led to the rod carrier design, rod geometry, and density selection. The W E M C O / L E E D S bench scale flotation column is made in two sections. The base acts as a rougher section, with the column portion acting as cleaner. The 3.5 liter base section features a b o t t o m - m o u n t e d , forced air, mechanical impeller and stabilizer, with a variable speed sheave assembly. The column portion, with seven rod barriers, attaches to the base cell to make a total cell volume of 13.5 liters. Air is introduced, at low pressure, into the central region of the impeller. In a typical "bench" flotation test, the products are collected during the various time increments and used for material balance calculations to produce grade/recovery curves and to determine flotation kinetic rate factors for metallurgical performance evaluations studies. Early testing of the LEEDS column bench machine was conducted in coal cleaning situations, where the combustible fraction is floated away from the non-combustible (ash) fraction. In all cases, the grade (product ash content) and combustible recovery performance was compared to the performance of (1) conventional "mechanical" type bench flotation machines and (2) to the release curve.* The results of a typical bench size test program, conducted using an eastern U.S. (Pennsylvania) coal feed, is shown in Figure 5. Note that the product grade achieved by the LEEDS Column was significantly above that of the conventional mechanical flotation machine both with and without a cleaning stage added. The LEEDS Column approached the maximum achievable flotation performance identified by the "Release Analysis" curve. This excellent performance comparison was a basis for continuing the product development to the next stage which featured continuous testing of pilot size equipment. Extensive bench tests, comparing the performance of the LEEDS Column to conventional flotation machine technology, included copper, molybdenum, gold, and iron ore applications, and all showed a significant performance advantage similar to Figure 5. The pilot W E M C O / L E E D S Column consisted of a four cell row with each cell having four major components: 1) cell tankage; 2) rotor and stabilizer mechanism; 3) froth skimmers; and 4) rod barriers. Each cell has a square footprint, and incorporated internal launders located transverse to the pulp flow direction. The machine is provided with launder height (weir) adjustment, with the lowest launder height giving an individual cell volume of 0.028 m 3. The pilot machine was plumbed for individual cell air and water addition, with the reflux water spray bars positioned at the pulp/froth interface and extending the width of the cell. Flowmeters, for the forced air and reflux water to each cell, are for monitoring and adjusting the flow rate. The pilot tests were conducted in the WEMCO Sacramento Laboratory using coal as the feed material. This test program was capable of providing a wide range of process test conditions for the LEEDS Column. Solid particle size distribution, solids concentration, * (1) (2) (3)
The release analysis test procedure, which represents the maximum grade/recovery c u r v e obtainable for a candidate ore, is as follows: Run the first flotation test to achieve the highest recovery until the "end" point is r e a c h e d (i.e., no more floatable specie reporting to the overflow). Subject the product from test (1) to a sequence (as many as seven or eight) of cleaning stage tests, while maintaining the value specie recovery at the maximum. Reagents a r e added a s required for recovery. The final product, after test phase (2) is re-pulped and fractionally floated, without adding reagents, by gradually increasing air and mixing intensity. Product is collected in stages until the cell is barren of floatable material.
941
LEEDSColumnperformanceevaluation
reagent strategy, as well as flotation test row pulp residence time are the typical process variables studied. LEEDS Column variables included pulp level, reflux water flow, number of rod barrier sets, vertical location of the rod barriers in the cell, air flow, impeller size, and impeller speed.
lc~n 9
~~::i::i::ii iii~:~' !;i!;~:;~;~........... :':~:::~
o
/ ? /~.[~":O 8O RELEASE
ANALYSIS
"ii~
70
i
=o O O
CONVENTIONAL ROUGHER (only)
/ 50
WEMCO,LEEOS f
,,o n-
COLUMN
|
~
50
CONVENTIONAL ROUGHER + CLEANER
~'~
4O i
~1~ 0
I
I
I
I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
i0
11
12
I 13
PRODUCTASH %
Fig.5 WEMCO/LEEDS Column: Batch Test Results (Coal) In the course of pilot plant testing, the number of rod barrier sets was varied from three to six. Conventional (pilot size) mechanical flotation machines were tested in parallel with the LEEDS Column. Test results, including a "release analysis", are shown in Figure 6, where the resulting metallurgical performance comparison follows the same trend as the batch "bench" tests. Analysis of the pilot machine tests concluded that the number of barrier sets, for most applications, would be four. Additional barrier sets produce progressively cleaner product but the product grade improvement, per additional barrier stage, becomes much less. The design of the full size LEEDS Column prototype machine followed the same conceptual design philosophy, of the pilot machine and was "scaled up" to an individual cell volume of 120 cu. ft. (3.4 m3). A photo of the three cell LEEDS Column prototype is shown in Figure 7. The barrier sets are assembled in banks to facilitate loading into the cell. The construction of each barrier set utilizes a frame that supports the upper and lower rod sets. Four sets of rods cover the width of the cell (see Figure 8). The reflux water distribution pipe, which disperses the reflux water across the pulp surface, is also shown above the top barrier in this figure. The cell is separated into two halves with the barriers flanking the central transverse launder. The launders are designed to provide for froth height variations by level control and launder weir bars.
942
V.R. DEGNERand P. L. PERSON
RELEASE - ANALYSIS
CONVENTIONAL MACHINE
LU
WEMCO LEEOS
60
COLUMN
0
I
i
I
~
I
3
I
~
I
~
I
6
I
7
I
8
I
9
I
13
I
!1
[
12
I
I3
I
la
PRODUCT ASH %
Fig.6 W E M C O / L E E D S Column: Pilot Test Results (Coal)
Fig.7 W E M C O / L E E D S Column (3 Cell R & D Prototype Machine) A rotary skimmer may be desirable for some applications, such as coal cleaning, but is usually not required in metalliferous ore flotation applications. The design of the cell, barriers, and mechanism provide ease of operation and service. The barrier sets are easily removed from the cell without having to remove the skimmer or mechanism. Once one barrier bank is removed, the mechanism can be lifted out of the cell in the space vacated. Early in-plant tests of the LEEDS Column were in coal flotation situations, and produced metallurgical performance results very similar to the performance benefits already summarized for the bench (batch test) and pilot (continuous test) size machines. (i.e: good recovery, comparable to s t a t e - o f - t h e - a r t mechanical flotation machines, but with a significant improvement in product grade). These results have been reported in reference
[31.
LEEDS Column performance evaluation
943
Fig.8 Top View (Rod Barrier Assemblies, Reflux Water...) COMMERCIAL LEEDS COLUMN I N - P L A N T TESTS The 3 cell W E M C O - L E E D S Column prototype was installed in cleaner duty in an Arizona Copper Concentrator early in 1988. Plant testing was initiated early in June and continued to September. During this period, performance tests were conducted on the LEEDS Column prototype operating, initially, in second cleaner duty and later in first cleaner duty. The flotation test circuit was arranged so that parallel "metallurgical performance" data could be obtained. In both flotation duties, the performance comparison related a three cell row of 120 cu. ft. LEEDS Column machines to a six cell row of 100 cu. ft. A G I T A I R mechanical flotation machines. Details of this test program are reported in reference [4], and identified the importance of reflux water, as an operational process parameter, and also showed that the performance of the LEEDS Column was relatively insensitive to the pulp level setting. A summary of the prototype test results from reference [4] is shown in Table 1. TABLE 1 Assay Comparison of Prototype Leeds Column & Conventional Flotation Machine Concentrates [4] ASSAY (%) DUTY
SPECIE
LEEDS COLUMN
SECOND
COPPER
32.95
31.60
CLEANER
MOLYBDENUM
0.63
0.61
INSOL
3.39
5.79
FIRST
COPPER
30.76
31.09
CLEANER
MOLYBDENUM
1.83
1.40
INSOL
3.70
6.67
CONVENTIONAL FLOTATION
944
V.R. DEGNERand P. L. PERSON
The performance comparison of a short three cell flotation row to a longer six cell flotation machine row is difficult due to pulp short circuiting effects in the shorter row. Equally difficult, is a parallel flotation row performance comparison in cleaning duty, where the grade and flotation kinetics of the feed to the cleaners often changes dramatically in time. However, the LEEDS Column prototype test results showed sufficient promise toward achieving the Plant performance objective of raising the concentrate copper grade, thereby reducing freight and smelter treatment charges, to justify replacing one (1) of the three (3) cleaner banks with a similarly sized LEEDS Column bank. The initial commercial LEEDS installation was in second cleaner duty (see Fig. 9). In this installation, the feed to each of the cleaner rows is similar. This first phase of the LEEDS column installation plan replaced the existing second cleaners in Row #2, which were six 100 cu. ft. A G I T A I R cells, with six 100 cu. ft. LEEDS columns. FEED
1"CLEANER
O3 a W
0 0
~ CLEANER
FINAL CU-MO CONCENTRATE TO CLEANER SCAVENGERS
Fig.9 WEMCO/LEEDS Column Commercial Bank as Second Cleaner Shift samples of the reference [4] metallurgical data are listed in Table 2. A comparison of the concentrate grade of the LEEDS Column to conventional flotation technology (Table 2) follows a pattern similar to the prototype tests reported in Table 1. The average concentrate copper assay produced by the LEEDS Column (31.965%) compares very favorably with the conventional machine product grade (30.699 and 30.613°/o Cu). A significant reduction in Insol content in the concentrate was also achieved by the LEEDS Column (4.699% insol) compared to conventional flotation machine technology (6.911 and 6.9550/0 insol). It should also be noted, from Table 2, that these LEEDS Column improvements in concentrate grade are achieved without suffering a significant copper recovery penalty. The commercial LEEDS Column (row 2) copper recovery (99.466%) compares favorably with the copper recovery of the conventional flotation machines in rows 1 and 3 which produced copper recoveries of 99.613 and 99.468% respectively. A metallurgical performance comparison of different flotation machine concepts can also be based on a flotation kinetic analysis of the test data [5]. An analysis of this type was conducted on the second cleaner installation shown in Fig. 9. The results of this kinetic
945
LEEDS Column performanceevaluation
analysis clearly shows the beneficial effect of the LEEDS Column (rod barrier ... reflux water) concept in terms of the kinetic coefficients governing the copper and Insol specie flotation response for the different types of flotation machines. TABLE 2 Assay Comparison of Commercial Leeds Column & Conventional Flotation Machine Concentrates [4l Initial commercial installation, second cleaner duty) CONCENTRATEASSAY (%) AGITAIR
LEEDS
AGITAIR
Flotation Machine Type Flotation Bank Number
DATE
SPECIE
l
2
3
6/12/89
Copper
30.12
31.56
30.28
(AM)
'INSOL'
8.08
4.80
6.90
6/12/89
Copper
31.08
31.89
31.08
'INSOL'
5.66
4.42
6.00
Copper
30.98
32.69
31.13
'INSOL'
7.58
3.88
6.82
6/29/89
Copper
32.13
33.82
31.81
(AM)
'INSOL'
7.10
5.10
7.10
6/29/89
Copper
30.92
32.44
30.92
(PM)
'INSOL'
7.49
5.69
6.93
Copper
30.43
30.74
29.28
'INSOL'
5.22
3.92
6.76
Copper
29.65
'INSOL'
6.88
5.27
5.89
Copper
30.28
31.68
29.73
'INSOL'
7.28
4.51
9.24
(PM) 6/15/89
7/6/89 30.9
30.67
7/26/89
8/18/89 Copper
30.699
31.965
30.613
6.911
4.699
6.955
AVG. Cu RECOVERY(%)
99.613
99.466
99.468
AVG. FEEDCu ASSAY (%)
29.747
28.363
27.928
AVERAGE 'INSOL'
Between 1981 and 1985, WEMCO R&D conducted systematic laboratory tests to select the flotation model best suited to serve as the basis for kinetic analysis and computer software program development planned for subsequent years. After evaluation over t h r e e - h u n d r e d laboratory and pilot flotation tests, the Klimpel model: Specie Recovery = S [1 - ( I / K T ) ( 1 - e'Kr)] was selected because it contained relatively few (two) empirically determined coefficients (S & K), having quantitative physical significance [6].
946
V.R. DEGNER and P. L. PERSON
The coefficient "S" represents the "steady state" recovery factor, and is the final recovery value the flotation system will achieve at infininte time. The coefficient "K" is the "transient" coefficient and will influence the rate at which the specie reports to overflow, and approaches the "steady state" value at very long flotation residence times [6,7]. The metallurgical performance data, obtained from tests of the initial commercial LEEDS Column, in second cleaner duty was analyzed using the "Klimpel" flotation kinetic model. This analysis also included a flotation kinetics evaluation of the conventional mechanical (AGITAIR) machines operating in second cleaner duty, in parallel with the LEEDS Columns, in rows number (1) and (3). The objective of this flotation kinetic analysis is to quantify the fundamental difference in flotation kinetic behavior, of the copper and Insol, for the different machine types. The computer analysis of the test data shown in Table 2, together with the respective feed and tails copper and Insol assay values, produced the steady state (S) and transient (K) coefficient values shown in Table 3A. The kinetic coefficients summarized in Table 3A provide a good basis for comparing flotation characteristics of the LEEDS Column and the conventional mechanical flotation machine operating in second cleaner duty. An analysis of this kinetic behavior summary is simplified by an evaluation of the ratios of the kinetic coefficients reported for the two flotation machine types in Table 3A. These kinetic coefficient ratios are summarized in Table 3B, where the ratio of the LEEDS Column to AGITAIR kinetic coefficients are tabulated for: Copper specie reporting to concentrate Non copper specie reporting to concentrate Insol reporting to concentrate Table 3A Kinetic Analysis Summary of Leeds Column and Agitair Machines (Initial commercial installation, second cleaner duty)
SPECIE
COEFFICIENT
AGITAIR (AVG.)
LEEDS COL. (AVG)
COPPER TO CONCENTRATE
Sfc Kfc
29.193
27.253
NON-COPPER TO CONCENTRATE
Snc
99.296
99.596
Knc
0.7798
INSOL TO CONCENTRATE
Sfi
83.950
Kfi
lO0
0 .I 3903
lO0
0.6458 71.633 0.10919
Naturally, for high copper recovery, both the copper steady state (S) and Transient (K) coefficients should be high, and for good selectivity, the non-copper steady state and/or transient coefficients should be low. For reduced Insol in the concentrate, both the steady state and transient coefficients should be low.
LEEDS Column performance evaluation
947
Table 3B Kinetic Analysis Summary of Leeds Column and Agitair Machines (Initial commercial installation, second cleaner duty)
KINETIC COEF. RATIO*
SPECIE
STEADY STATE (S)
I [
TRANSIENT (K)
COPPER TO CONCENTRATE
l.O00
0.934
NON-COPPER TO CONCENTRATE
1.003
0.8282
INSOL TO CONCENTRATE
0.85328
0.78537
*LEEDS/AGITAIR These desirable "relative" flotation kinetic features are ratios" listed in Table 3B. A review of this table regarding the inherent flotation characteristics of conventional mechanical flotation machines in second
displayed in the "kinetic coefficient leads to the following conclusions the LEEDS Column compared to cleaner duty:
Value specie recovery (copper) for the LEEDS Column is comparable to conventional flotation machine technology by virtue of a steady state coefficient ratio of 1.000 and a transient coefficient ratio of 0.934 in Table 3B. (Note: Actual copper recovery in Table 2 shows the LEEDS Column recovery of 99.466% comparing favorably with conventional flotation machines at 99.467 and 99.613%). The selectivity of the LEEDS Column is superior to conventional flotation machine technology as seen in the significantly reduced transient coefficient ratio (0.8282) for the n o n - c o p p e r specie reporting to the concentrate. This corresponds to the concentrate copper grade (31.965) compared to conventional flotation machine technology (30.613 & 30.699%) shown in Table 2. The LEEDS Column operating principle, which incorporates a set of barrier rods and reflux water to remove lightly held gangue material from the "air-bubble flotable specie" matrix, is believed to be a significant factor in reducing the Insol transport to concentrate as evidenced by the dramatically reduced kinetic coefficient ratios (0.85328 for "S" and 0.78537 for "K") in Table 3B. Again, this metallurgical performance benefit is seen in the test results in Table 2 where the Insol assay in the LEEDS Column concentrate (4.699%) compares very favorably with that of the conventional mechanical flotation machine (6.911 & 6.955%). Most recently, the second commercial installation phase has been completed. This consisted of an addition of a bank of first and a bank of second cleaner LEEDS machines. These machines were installed in parallel to an existing row of first and second cleaner A G I T A I R machines. In both rows, the six cell first cleaner bank and the six cell second cleaner bank are 100 cu.ft.machines. The preliminary plant test data analysis concluded that the overall first and second cleaner performance of the LEEDS Column was superior to the conventional mechanical flotation machine; however, again, the direct comparison of performance on the basis of the assay data was complicated by variations in feed conditions in tilne and between machine rows. The average of the test data for the initial plant test series, in first and second cleaner duty, is summarized in Table 4A. Average assay of the first cleaner feed and second cleaner product is shown for copper, molybdenum, and insol for both the LEEDS Column and the A G I T A I R flotation machine rows.
948
V.R. DEGNERand P. L. PERSON
TABLE 4A Assay Comparison, Leeds Column & Conventional Flotation Machine (First and second cleaner duty) ASSAY (%) i
TRACE SPECIE
SECOND CLEANER CONC.
OVERALL RECOVERY %
MACHINE TYPE
FIRST CLEANER FEED
LEEDS
15.55
31.24
96.439
2.009
AGITAIR
16.58
31.92
97.280
1.925
LEEDS
0.7583
1.485
95.657
1.958
AGITAIR
O. 7883
1.435
95.031
1.820
LEEDS
36.57
5.047
6.718
0.1380
AGITAIR
34.21
4.953
7.669
0.1448
CONC.* RATIO
COPPER
MOLYBDENUM
INSOL *Conc. Ratio = Product Assay/Feed Assay
A direct comparison of machine metallurgical performance, based on the assay values shown in Table 4A, is not possible because of the difference in feed conditions to each of the flotation test rows. For example, the average assay test data shows that the LEEDS Column cleaner system achieves a final product grade of 31.24% copper, while the final product grade for the A G I T A I R system is somewhat higher (i.e., 31.92% copper). However, it must be noted that the average copper assay of the feed to each of these test rows differs significantly. The LEEDS Column achieves a concentration ratio of 2.009 with a final product grade of 31.24% from a feed of 15.55% copper, which compares favorably to the A G I T A I R metallurgical performance, where a copper concentration ratio is over 4% less (i.e., 1.925) with a final product grade of 31.92%, and the higher feed grade of 16.58% copper. The comparison of machine performance on the basis of the molybdenum metallurgy is somewhat more favorable to the LEEDS Column, where it is shown (Table 4A) that the concentration ratio of the LEEDS row (1.958) is over 7% higher than the A G I T A I R machine row (1.82). Finally, the Insol rejection of the LEEDS Column, with a concentration ratio of 0.1380, is over 4% better than that of the A G I T A I R , with a concentration ratio of 0.1448. An alternate way of comparing the metallurgical performance of these two types of flotation machines is to follow the kinetic analysis procedures described earlier. In this analysis, the data shown in Table 4A is used to determine a set of kinetic flotation coefficients which model the overflow response for the trace species (i.e., copper, molybdenum, and Insol) for both the LEEDS Column and the A G I T A I R test data. The resulting coeficients are then used to predict the metallurgical performance of both the LEEDS Column and the A G I T A I R machine rows having the same feed (assay) conditions and processing the same dry tonnage. Results of this analysis, which more closely represents the comparative metallurgical performance under similar process conditions, is summarized in Table 4B. One of the particular benefits of the first commercial plant test was the severe "scaling" characteristics of process water, which provided an excellent basis for identifying LEEDS Column maintenance requirements under severe environmental conditions. Somewhat unique to this installation, a significant calcium carbonate scale builds upon wetted surfaces over a period of time. Naturally, significant scale build-up oil the LEEDS Column barrier
LEEDS Column performance evaluation
949
rods can ultimately inhibit rod movement and lose the performance benefits of the LEEDS Column over conventional mechanical flotation machines*. Plant operating experience has shown that, even in this severe water chemistry situation, a six month rod barrier maintenance cycle is sufficient to maintain consistently good LEEDS Column operation. Plant experience has shown that a clean time of 1-2 hours (steam clean plus hydrochloric bath) with turnaround time of one shift is sufficient to return the "scaled" rod barrier assembly to its original, as manufactured, condition. This maintenance can be accomplished without shutting down the LEEDS Column mechanism. TABLE 4B Performance Comparison, Leeds Column & Conventional Flotation Machine (First & second cleaner duty) ASSAY (%) TRACE SPECIE
MACHINE TYPE
FIRST CLEANER FEED
SECOND CLEANER PRODUCT
OVERALL RECOVERY %
LEEDS
16.07
35.69
96.075
AGITAIR
16.07
34.75
96.768
LEEDS
0.7733
l .7365
95.322
AGITAIR
0.7733
I .5259
95.884
LEEDS
35.39
4.223
5.537
AGITAIR
35.39
4.324
5.971
COPPER
MOLYBDENUM
I NSOL
CONCLUSIONS A review of the prototype LEEDS Column test programs at CYPRUS-BAGDAD [4], Kerr McGee-Galacia [3] and other concentrators, as well as the operational experience obtained via the first commercial LEEDS Column in second cleaner duty at CYPRUS-BAGDAD, is the basis for the following general comments relative to the LEEDS Column concept: (l)
The LEEDS Column metallurgical performance is relatively insensitive to pulp level. Tests have shown little statistical difference in operating at froth depths from 2-5 inches.
(2)
The reflux water flow rate is a significant operational process parameter. However, the amount of reflux water which is optimum may vary depending on the application. In metalliferous ore "flotation cleaning" applications, reflux water is generally used to advantage while in fine coal washing situations, reflux water use is often not required.
* Tests have shown that the metallurgical performance of the L E E D S in the barrier assembly is similar to the mechanical flotation machine.
Column
with "stationary" roads
950
V. R. DEor,n~gand P. L. PEI~SON
(3)
Air transfer to the LEEDS Column is a metallurgical performance parameter which is best determined for each specific application. The LEEDS Column performance is not critically dependent on air transfer; however, tests have shown that optimum metallurgical performance is often achieved when using less air than is commonly used by the conventional mechanical flotation machine.
(4)
The effect of number of rod barriers on metallurgical performance was studied during the WEMCO R&D program and is reported in reference [3]. Test results showed that, while increasing the number of rod barriers led to improved concentrate grade, a good economical design would be based on four (4) barriers. The metallurgical performance benefits of more than four barriers become progressively less significant with additional rod barriers.
The LEEDS Column flotation concept has been shown to have an excellent application in (metalliferous ore) "flotation cleaner" situations and in fine coal washing applications. WEMCO believes the LEEDS Column concept represents a significant flotation technology advancement for flotation "cleaner" duty. WEMCO is prepared to support client modernization or new plant design studies using the extensive technology base established for the LEEDS Column during the past six years.
REFERENCES
.
Dell, C. C., and Jenkins, B. W., The LEEDS Flotation Column, Seventh International Conference on Coal Processing, Sydney, Paper (J.3), (1976)
.
Dell, C. C., Column Flotation of Coal - The Way to Easier Filtration, Mine and Quarry, March, pp. 36-40 (1978)
.
Degner, V. R., and Sabey, J. B., WEMCO/LEEDS Flotation Column Development, Column Flotation '88, K.V.S. Sastry Editor, Society of Mining Engineers, Littleton, Colorado, pp 267-279 (1988)
.
Dworatzek, J. J., Testing of a WEMCO-LEEDS Column Flotation Machine as a Copper Cleaner at Bagdad, Annual Meeting of the Arizona Conference of the AIME, Dec. 3, (1989)
.
Degner, V. R., Flotation Machine Selection for Sulfide and Non-Sulfide Applications, Design and Installation of Concentration and Dewatering Circuits, Mular, A.L., and Anderson, M.A., editors, AIME, Littleton, Colorado, pp 56-75 (1986)
.
Klimpel, R. and Hansen, R., Some Factors Influencing Kinetics in Sulfide Flotation, AIME Annual Meeting, February, Preprint 81 - 14 (1981)
.
Degner, V. R., Flotation Machine Size Selection for Coal Applications, Coal Prep '86, Industrial Presentations West, Aurora, Colorado, pp 319-347 (1986)
0892-6875/9Z $3.00 + 000 © 1991 Pergamon Press plc
Printed in Great Britain
LEEDS COLUMN PERFORMANCE EVALUATION V.R. DEGNER§ and P.L. PERSONt
§ Vice President, Research & Development, WEMCO, PO Box 15619, Sacramento, CA 95852-1619, U.S.A. ? Western Regional Sales Manager, WEMCO, Tucson, Arizona, U.S.A.
ABSTRACT The LEEDS Column, initially conceived as a metallurgical test device in the late 1950's, has progressed through a systematic development program which included bench, pilot, and full size machines. Comparative metallurgical performance testing repeatedly demonstrated the potential for the LEEDS Column to achieve a significant improvement in product grade while maintaining a recovery level comparable to conventional flotation technology. These successes led to the commercialization of the LEEDS Column by WEMCO in 1988-89. This paper describes the fundamental concept of the LEEDS Column, and will include a brief summary of the WEMCO R&D Technology Advancement Program. A comparison between the LEEDS Column, "conventional" mechanical flotation machines, and the "Conventional" Column will be made. Metallurgical performance tests, conducted on full size LEEDS Columns, functioning under plant operating conditions, will be compared to alternative flotation machines, where the metallurgical performance benefits in "cleaning" duty will be shown using actual plant test data. Keywords Froth flotation; columns; copper beneficiation; kinetics; Leeds column
INTRODUCTION The LEEDS Column concept consists of a series of "barrier" rods which are used to advantage, often together with countercurrent reflux water, to achieve a high product grade in relatively few cleaning stages. This performance feature was initially utilized by C. Dell in University lab flotation "release" testing. The release analysis [1] procedure is designed to establish the ultimate metallurgical performance level achievable by flotation for a particular degree of mineral liberation. Conducting a release analysis using conventional impeller-type flotation equipment requires extensive cleaning and re-cleaning steps, often repeated seven or eight times, to achieve the end point which establishes the maximum (practical) achievable performance. C. Dell found, experimentally, that the LEEDS column, applied to the release analysis procedure, could accomplish the same metallurgical performance objective as the conventional flotation concept in much fewer procedural steps. This suggested that the concept may have commercial value beyond the analytical benefits observed in the laboratory. Early coal flotation studies also showed that significant improvements in filtration rate would accompany product grade improvement. This further reinforced the beneficial economic features of this new concept [2]. 935
936
V.R. DEGNERand P. L. l~gsoN
Financial support towards commercialization was initially sponsored by the British Technology Group (BTG) beginning in 1973. The PhD studies of B. W. Jenkins (1973-1976) produced the rod-type barrier which was basic to the first successful plant (pilot unit) trial at Lofthouse Colliery (U.K.) in 1976 [1]. The WEMCO R&D program continued the LEEDS column development, and was designed to establish the basic practical machine design, and scale-up principles. This program was initiated in July 1984, and culminated in a series of in-plant tests in which the metallurgical performance of the LEEDS column was compared to existing flotation technology. WEMCO/LEEDS COLUMN DESCRIPTION Figure 1 illustrates the key elements of the WEMCO/LEEDS flotation column. Four horizontal barriers are arranged vertically, one above the other. This barrier design separates the pulp in the cell into a series of vertical compartments. Each barrier consists of two sets of movable rods in contact, as indicated in Figure 2. Rod design is such that the lower rod has a net buoyant force in the upward direction and the upper rod has a net buoyant force in the downward direction. This design creates a hydrostatic pinching force between the rods when submerged in the pulp.
~KIMMER FROTH ~ROWDER PLATES AIR MANIFOLD
INTERNAL TRANSVERSE LAUNDER ROD BARRIER
REFLU} WATEF
PULP DISCHARGE
PUL FEE
LAUNDER O/F DISCHARGE
Fig.l WEMCO/LEEDS Column Flotation System The WEMCO/LEEDS flotation column incorporates a mechanical impeller in the lower section of the cell. Air supplied to the impeller region via a blower is dispersed as fine air bubbles and mixed with the solids to establish an "air bubble-floatable specie" matrix, which rises toward the pulp surface. The rise of the "air bubble-floatable specie" matrix is impeded by the horizontal rod barriers. Air bubbles collect under the barrier when the upper and lower rods are touching. The rods separate once the buoyant air bubble forces
937
LEEDS Column performance evaluation
are sufficient to overcome the rod "hydrostatic pinching" forces, thereby allowing air bubbles to continue rising toward the surface. As the specie laden air bubbles pass through the openings between the moving rod barriers, the stripping process will physically remove the lightly held gangue material. This "stripping" action improves the grade of the concentrate by reducing the entrapped or lightly held, predominantly gangue, attached to the air bubble.
~ISITY
S Pu~p)
J
ROD ,RRIERS
Fig.2 WEMCO/LEEDS Column Rod Barrier Concept Analysis of pulp samples taken from the vertical sections between the rod barriers shows a progressively cleaner or higher grade solids concentration in going from the lower to the upper sections of the flotation column. Reflux water, introduced at the top of the column and percolating downward, serves to complement the beneficiation action of the flotation column rod barriers. The performance benefits of reflux water are best determined on a specific case basis. Experimental studies have shown that in some applications, the use of reflux water significantly improves product grade. Other cases, however, showed that the use of reflux water was not required to achieve the desired high grade product performance. In the design of the rod barrier assembly, consideration must include configuration details of both the rods and the rod carriers. Both the upper and the lower rods are mounted in a carrier which is designed to permit freedom of movement of the upper and lower rods in both the vertical and horizontal direction. The rods are also free to rotate with respect to each other. Design parameters, which influence the hydraulic pinching force per unit length of rod, include the rod diameter and contact angle, as well as individual upper and lower rod density. For a given pulp density, variations in these design parameters can result in the five distinct static force balance cases, illustrated in Figure 3. Keeping in mind the freedom of vertical movement of the rods contained within the carrier; Case A represents rods not in contact, resting on the lower carrier. Case B illustrates rods in contact, with the upper rod only resting on the bottom carrier. Case C represents neutral buoyancy, with the rods in contact and neither rod in contact with the rod carrier. Case D illustrates the rods in contact, with the lower rod contacting the upper carrier, while in Case E neither rod is in contact and both rods are contacting the upper carrier. In Case A and E, where the rods are not in contact, no stripping or beneficiation action would be expected; therefore, ME 4:7111-T
938
V.R. DEO~rEXand P. L. l~xsoN
designing for static operation at either Case A or Case E condition would be unsatisfactory. Alternatively, the design for a given application must be in the range of B to D.
) ii&.
~~
J
..,~i~
CASE A: ROD DENSITY > > SLURRY
CASE D: BOTTOM RODS RESTING ON BOTTOM OF TOP CARRIER
CASE B: TOP ROD RESTINGON TOP OF BOTTOM CARRIER
CASE E: ROD DENSITY < < SLURRY
UPPER ROD SET UPPERROD CARRIER LOWER ROD SET ~iii::iiiiii#i!i::iiii::#i::i::ii~iiii::ii:~: LOWER ROD CARRIER ~ \ ~
CASE C: NEUTRAL BUOYANCY
Fig.3 WEMCO/LEEDS Column Rod Design (Static Force Balance Cases) The dynamic action of the rods relative to one another during operation, must also be examined. Studies of the rod action in operating transparent columns demonstrated that the rods oscillate primarily in the vertical direction and also rotate relative to one another. This relative vertical and rotational rod motion has an important bearing on the gangue stripping action, and would reduce the effectiveness of the column beneficiation action if restricted. Figure 4 illustrates the cleaning action of the rods during passage of the specie-laden air bubbles rising through the rod contact or "pinch" zone. Disengaged gangue flows downward through the contact region to the pulp section below. The flow path of reflux water, if used, is also shown in Figure 4. The LEEDS Column differs from the conventional column in several important areas: (1)
The LEEDS Column cell configuration is similar to the familiar "mechanical" cell both in terms of floor space and overall height. The conventional column is a tall device having a relatively small floor area (i.e. footprint).
(2)
In the LEEDS Column, air bubble generation is accomplished using well established forced air/rotating impeller/stabilizer interaction principles which have been used in minerals industry flotation machines for the last 50 years. Bubble generation techniques for the conventional flotation column are still undergoing component development to improve the reliability and reduce the maintenance (nozzle wear...) problems.
LEEDS Columnperformanceevaluation
939
(3)
Solids suspension capability of the LEEDS column is excellent because the pulp circulation energy from the impeller acts on a smaller fraction of the total cell volume (i.e. the cell volume below the lower rod bank). The result of the high circulation intensity is a relatively high mixing intensity and reduced pulp short-circuiting. The conventional column fails to provide for any pulp, or solids, recirculation and is, basically, a o n e p a s s system. This feature is conducive to short-circuiting effects which reduce metallurgical "recovery".
(4)
The high air-pulp mixing intensity in the impeller-stabilizer region of the LEEDS Column contributes to good metallurgical (recovery) performance. The conventional column features lower intensity air bubble-floatable specie interception characteristics as the rising air bubbles migrate past the settling solids.
(5)
Operational control of the LEEDS Column follows the familiar practice of all conventional mechanical flotation machines, where the use of reflux water is not critical to good performance. In the conventional columns, froth depth and bias control are critical and must be maintained through proper flow control if good metallurgical performance is to be achieved.
j
AIR BUBBLE PATH
x
GANGUE = . VALUABLE SPECIE = • AIR BUBBLE = O HIGH DENSITY ROD = H LOW DENSITY ROD = L
Fig.4 LEEDS Column Beneficiation Mechanics The skimmer and froth crowder plates, shown in Figure 1, are utilized when the LEEDS Column is operating in applications having a relatively deep, persistent, froth such as coal flotation situations. In many metalliferous ore beneficiation application, the LEEDS Column skimmer is not required. The LEEDS Column is designed to permit easy removal, and cleaning, of the rod barriers in high calcium carbonate scaling situations. This maintenance step can be accomplished without shutting down the flotation row, since each barrier set can be lifted out of the cell without interfering with the mechanism. R&D TECHNOLOGY PROGRAM The WEMCO R&D philosophy, for the development of any new product, is to systematically study the new concept from bench size equipment, through pilot testing, to
940
V.R. DEG~R and P. L. PERSON
full-size equipment in the laboratory, prior to the initiation of in-plant evaluation tests. This procedure was followed for the LEEDS column development and began with a preliminary rod dynamics study. This study was confined to evaluating a wide variety of rod sizes and densities in small bench cells. These cells were transparent so that the action of the rods could be evaluated for varying gas transfer rates. Analysis of the test results led to the rod carrier design, rod geometry, and density selection. The W E M C O / L E E D S bench scale flotation column is made in two sections. The base acts as a rougher section, with the column portion acting as cleaner. The 3.5 liter base section features a b o t t o m - m o u n t e d , forced air, mechanical impeller and stabilizer, with a variable speed sheave assembly. The column portion, with seven rod barriers, attaches to the base cell to make a total cell volume of 13.5 liters. Air is introduced, at low pressure, into the central region of the impeller. In a typical "bench" flotation test, the products are collected during the various time increments and used for material balance calculations to produce grade/recovery curves and to determine flotation kinetic rate factors for metallurgical performance evaluations studies. Early testing of the LEEDS column bench machine was conducted in coal cleaning situations, where the combustible fraction is floated away from the non-combustible (ash) fraction. In all cases, the grade (product ash content) and combustible recovery performance was compared to the performance of (1) conventional "mechanical" type bench flotation machines and (2) to the release curve.* The results of a typical bench size test program, conducted using an eastern U.S. (Pennsylvania) coal feed, is shown in Figure 5. Note that the product grade achieved by the LEEDS Column was significantly above that of the conventional mechanical flotation machine both with and without a cleaning stage added. The LEEDS Column approached the maximum achievable flotation performance identified by the "Release Analysis" curve. This excellent performance comparison was a basis for continuing the product development to the next stage which featured continuous testing of pilot size equipment. Extensive bench tests, comparing the performance of the LEEDS Column to conventional flotation machine technology, included copper, molybdenum, gold, and iron ore applications, and all showed a significant performance advantage similar to Figure 5. The pilot W E M C O / L E E D S Column consisted of a four cell row with each cell having four major components: 1) cell tankage; 2) rotor and stabilizer mechanism; 3) froth skimmers; and 4) rod barriers. Each cell has a square footprint, and incorporated internal launders located transverse to the pulp flow direction. The machine is provided with launder height (weir) adjustment, with the lowest launder height giving an individual cell volume of 0.028 m 3. The pilot machine was plumbed for individual cell air and water addition, with the reflux water spray bars positioned at the pulp/froth interface and extending the width of the cell. Flowmeters, for the forced air and reflux water to each cell, are for monitoring and adjusting the flow rate. The pilot tests were conducted in the WEMCO Sacramento Laboratory using coal as the feed material. This test program was capable of providing a wide range of process test conditions for the LEEDS Column. Solid particle size distribution, solids concentration, * (1) (2) (3)
The release analysis test procedure, which represents the maximum grade/recovery c u r v e obtainable for a candidate ore, is as follows: Run the first flotation test to achieve the highest recovery until the "end" point is r e a c h e d (i.e., no more floatable specie reporting to the overflow). Subject the product from test (1) to a sequence (as many as seven or eight) of cleaning stage tests, while maintaining the value specie recovery at the maximum. Reagents a r e added a s required for recovery. The final product, after test phase (2) is re-pulped and fractionally floated, without adding reagents, by gradually increasing air and mixing intensity. Product is collected in stages until the cell is barren of floatable material.
941
LEEDSColumnperformanceevaluation
reagent strategy, as well as flotation test row pulp residence time are the typical process variables studied. LEEDS Column variables included pulp level, reflux water flow, number of rod barrier sets, vertical location of the rod barriers in the cell, air flow, impeller size, and impeller speed.
lc~n 9
~~::i::i::ii iii~:~' !;i!;~:;~;~........... :':~:::~
o
/ ? /~.[~":O 8O RELEASE
ANALYSIS
"ii~
70
i
=o O O
CONVENTIONAL ROUGHER (only)
/ 50
WEMCO,LEEOS f
,,o n-
COLUMN
|
~
50
CONVENTIONAL ROUGHER + CLEANER
~'~
4O i
~1~ 0
I
I
I
I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
i0
11
12
I 13
PRODUCTASH %
Fig.5 WEMCO/LEEDS Column: Batch Test Results (Coal) In the course of pilot plant testing, the number of rod barrier sets was varied from three to six. Conventional (pilot size) mechanical flotation machines were tested in parallel with the LEEDS Column. Test results, including a "release analysis", are shown in Figure 6, where the resulting metallurgical performance comparison follows the same trend as the batch "bench" tests. Analysis of the pilot machine tests concluded that the number of barrier sets, for most applications, would be four. Additional barrier sets produce progressively cleaner product but the product grade improvement, per additional barrier stage, becomes much less. The design of the full size LEEDS Column prototype machine followed the same conceptual design philosophy, of the pilot machine and was "scaled up" to an individual cell volume of 120 cu. ft. (3.4 m3). A photo of the three cell LEEDS Column prototype is shown in Figure 7. The barrier sets are assembled in banks to facilitate loading into the cell. The construction of each barrier set utilizes a frame that supports the upper and lower rod sets. Four sets of rods cover the width of the cell (see Figure 8). The reflux water distribution pipe, which disperses the reflux water across the pulp surface, is also shown above the top barrier in this figure. The cell is separated into two halves with the barriers flanking the central transverse launder. The launders are designed to provide for froth height variations by level control and launder weir bars.
942
V.R. DEGNERand P. L. PERSON
RELEASE - ANALYSIS
CONVENTIONAL MACHINE
LU
WEMCO LEEOS
60
COLUMN
0
I
i
I
~
I
3
I
~
I
~
I
6
I
7
I
8
I
9
I
13
I
!1
[
12
I
I3
I
la
PRODUCT ASH %
Fig.6 W E M C O / L E E D S Column: Pilot Test Results (Coal)
Fig.7 W E M C O / L E E D S Column (3 Cell R & D Prototype Machine) A rotary skimmer may be desirable for some applications, such as coal cleaning, but is usually not required in metalliferous ore flotation applications. The design of the cell, barriers, and mechanism provide ease of operation and service. The barrier sets are easily removed from the cell without having to remove the skimmer or mechanism. Once one barrier bank is removed, the mechanism can be lifted out of the cell in the space vacated. Early in-plant tests of the LEEDS Column were in coal flotation situations, and produced metallurgical performance results very similar to the performance benefits already summarized for the bench (batch test) and pilot (continuous test) size machines. (i.e: good recovery, comparable to s t a t e - o f - t h e - a r t mechanical flotation machines, but with a significant improvement in product grade). These results have been reported in reference
[31.
LEEDS Column performance evaluation
943
Fig.8 Top View (Rod Barrier Assemblies, Reflux Water...) COMMERCIAL LEEDS COLUMN I N - P L A N T TESTS The 3 cell W E M C O - L E E D S Column prototype was installed in cleaner duty in an Arizona Copper Concentrator early in 1988. Plant testing was initiated early in June and continued to September. During this period, performance tests were conducted on the LEEDS Column prototype operating, initially, in second cleaner duty and later in first cleaner duty. The flotation test circuit was arranged so that parallel "metallurgical performance" data could be obtained. In both flotation duties, the performance comparison related a three cell row of 120 cu. ft. LEEDS Column machines to a six cell row of 100 cu. ft. A G I T A I R mechanical flotation machines. Details of this test program are reported in reference [4], and identified the importance of reflux water, as an operational process parameter, and also showed that the performance of the LEEDS Column was relatively insensitive to the pulp level setting. A summary of the prototype test results from reference [4] is shown in Table 1. TABLE 1 Assay Comparison of Prototype Leeds Column & Conventional Flotation Machine Concentrates [4] ASSAY (%) DUTY
SPECIE
LEEDS COLUMN
SECOND
COPPER
32.95
31.60
CLEANER
MOLYBDENUM
0.63
0.61
INSOL
3.39
5.79
FIRST
COPPER
30.76
31.09
CLEANER
MOLYBDENUM
1.83
1.40
INSOL
3.70
6.67
CONVENTIONAL FLOTATION
944
V.R. DEGNERand P. L. PERSON
The performance comparison of a short three cell flotation row to a longer six cell flotation machine row is difficult due to pulp short circuiting effects in the shorter row. Equally difficult, is a parallel flotation row performance comparison in cleaning duty, where the grade and flotation kinetics of the feed to the cleaners often changes dramatically in time. However, the LEEDS Column prototype test results showed sufficient promise toward achieving the Plant performance objective of raising the concentrate copper grade, thereby reducing freight and smelter treatment charges, to justify replacing one (1) of the three (3) cleaner banks with a similarly sized LEEDS Column bank. The initial commercial LEEDS installation was in second cleaner duty (see Fig. 9). In this installation, the feed to each of the cleaner rows is similar. This first phase of the LEEDS column installation plan replaced the existing second cleaners in Row #2, which were six 100 cu. ft. A G I T A I R cells, with six 100 cu. ft. LEEDS columns. FEED
1"CLEANER
O3 a W
0 0
~ CLEANER
FINAL CU-MO CONCENTRATE TO CLEANER SCAVENGERS
Fig.9 WEMCO/LEEDS Column Commercial Bank as Second Cleaner Shift samples of the reference [4] metallurgical data are listed in Table 2. A comparison of the concentrate grade of the LEEDS Column to conventional flotation technology (Table 2) follows a pattern similar to the prototype tests reported in Table 1. The average concentrate copper assay produced by the LEEDS Column (31.965%) compares very favorably with the conventional machine product grade (30.699 and 30.613°/o Cu). A significant reduction in Insol content in the concentrate was also achieved by the LEEDS Column (4.699% insol) compared to conventional flotation machine technology (6.911 and 6.9550/0 insol). It should also be noted, from Table 2, that these LEEDS Column improvements in concentrate grade are achieved without suffering a significant copper recovery penalty. The commercial LEEDS Column (row 2) copper recovery (99.466%) compares favorably with the copper recovery of the conventional flotation machines in rows 1 and 3 which produced copper recoveries of 99.613 and 99.468% respectively. A metallurgical performance comparison of different flotation machine concepts can also be based on a flotation kinetic analysis of the test data [5]. An analysis of this type was conducted on the second cleaner installation shown in Fig. 9. The results of this kinetic
945
LEEDS Column performanceevaluation
analysis clearly shows the beneficial effect of the LEEDS Column (rod barrier ... reflux water) concept in terms of the kinetic coefficients governing the copper and Insol specie flotation response for the different types of flotation machines. TABLE 2 Assay Comparison of Commercial Leeds Column & Conventional Flotation Machine Concentrates [4l Initial commercial installation, second cleaner duty) CONCENTRATEASSAY (%) AGITAIR
LEEDS
AGITAIR
Flotation Machine Type Flotation Bank Number
DATE
SPECIE
l
2
3
6/12/89
Copper
30.12
31.56
30.28
(AM)
'INSOL'
8.08
4.80
6.90
6/12/89
Copper
31.08
31.89
31.08
'INSOL'
5.66
4.42
6.00
Copper
30.98
32.69
31.13
'INSOL'
7.58
3.88
6.82
6/29/89
Copper
32.13
33.82
31.81
(AM)
'INSOL'
7.10
5.10
7.10
6/29/89
Copper
30.92
32.44
30.92
(PM)
'INSOL'
7.49
5.69
6.93
Copper
30.43
30.74
29.28
'INSOL'
5.22
3.92
6.76
Copper
29.65
'INSOL'
6.88
5.27
5.89
Copper
30.28
31.68
29.73
'INSOL'
7.28
4.51
9.24
(PM) 6/15/89
7/6/89 30.9
30.67
7/26/89
8/18/89 Copper
30.699
31.965
30.613
6.911
4.699
6.955
AVG. Cu RECOVERY(%)
99.613
99.466
99.468
AVG. FEEDCu ASSAY (%)
29.747
28.363
27.928
AVERAGE 'INSOL'
Between 1981 and 1985, WEMCO R&D conducted systematic laboratory tests to select the flotation model best suited to serve as the basis for kinetic analysis and computer software program development planned for subsequent years. After evaluation over t h r e e - h u n d r e d laboratory and pilot flotation tests, the Klimpel model: Specie Recovery = S [1 - ( I / K T ) ( 1 - e'Kr)] was selected because it contained relatively few (two) empirically determined coefficients (S & K), having quantitative physical significance [6].
946
V.R. DEGNER and P. L. PERSON
The coefficient "S" represents the "steady state" recovery factor, and is the final recovery value the flotation system will achieve at infininte time. The coefficient "K" is the "transient" coefficient and will influence the rate at which the specie reports to overflow, and approaches the "steady state" value at very long flotation residence times [6,7]. The metallurgical performance data, obtained from tests of the initial commercial LEEDS Column, in second cleaner duty was analyzed using the "Klimpel" flotation kinetic model. This analysis also included a flotation kinetics evaluation of the conventional mechanical (AGITAIR) machines operating in second cleaner duty, in parallel with the LEEDS Columns, in rows number (1) and (3). The objective of this flotation kinetic analysis is to quantify the fundamental difference in flotation kinetic behavior, of the copper and Insol, for the different machine types. The computer analysis of the test data shown in Table 2, together with the respective feed and tails copper and Insol assay values, produced the steady state (S) and transient (K) coefficient values shown in Table 3A. The kinetic coefficients summarized in Table 3A provide a good basis for comparing flotation characteristics of the LEEDS Column and the conventional mechanical flotation machine operating in second cleaner duty. An analysis of this kinetic behavior summary is simplified by an evaluation of the ratios of the kinetic coefficients reported for the two flotation machine types in Table 3A. These kinetic coefficient ratios are summarized in Table 3B, where the ratio of the LEEDS Column to AGITAIR kinetic coefficients are tabulated for: Copper specie reporting to concentrate Non copper specie reporting to concentrate Insol reporting to concentrate Table 3A Kinetic Analysis Summary of Leeds Column and Agitair Machines (Initial commercial installation, second cleaner duty)
SPECIE
COEFFICIENT
AGITAIR (AVG.)
LEEDS COL. (AVG)
COPPER TO CONCENTRATE
Sfc Kfc
29.193
27.253
NON-COPPER TO CONCENTRATE
Snc
99.296
99.596
Knc
0.7798
INSOL TO CONCENTRATE
Sfi
83.950
Kfi
lO0
0 .I 3903
lO0
0.6458 71.633 0.10919
Naturally, for high copper recovery, both the copper steady state (S) and Transient (K) coefficients should be high, and for good selectivity, the non-copper steady state and/or transient coefficients should be low. For reduced Insol in the concentrate, both the steady state and transient coefficients should be low.
LEEDS Column performance evaluation
947
Table 3B Kinetic Analysis Summary of Leeds Column and Agitair Machines (Initial commercial installation, second cleaner duty)
KINETIC COEF. RATIO*
SPECIE
STEADY STATE (S)
I [
TRANSIENT (K)
COPPER TO CONCENTRATE
l.O00
0.934
NON-COPPER TO CONCENTRATE
1.003
0.8282
INSOL TO CONCENTRATE
0.85328
0.78537
*LEEDS/AGITAIR These desirable "relative" flotation kinetic features are ratios" listed in Table 3B. A review of this table regarding the inherent flotation characteristics of conventional mechanical flotation machines in second
displayed in the "kinetic coefficient leads to the following conclusions the LEEDS Column compared to cleaner duty:
Value specie recovery (copper) for the LEEDS Column is comparable to conventional flotation machine technology by virtue of a steady state coefficient ratio of 1.000 and a transient coefficient ratio of 0.934 in Table 3B. (Note: Actual copper recovery in Table 2 shows the LEEDS Column recovery of 99.466% comparing favorably with conventional flotation machines at 99.467 and 99.613%). The selectivity of the LEEDS Column is superior to conventional flotation machine technology as seen in the significantly reduced transient coefficient ratio (0.8282) for the n o n - c o p p e r specie reporting to the concentrate. This corresponds to the concentrate copper grade (31.965) compared to conventional flotation machine technology (30.613 & 30.699%) shown in Table 2. The LEEDS Column operating principle, which incorporates a set of barrier rods and reflux water to remove lightly held gangue material from the "air-bubble flotable specie" matrix, is believed to be a significant factor in reducing the Insol transport to concentrate as evidenced by the dramatically reduced kinetic coefficient ratios (0.85328 for "S" and 0.78537 for "K") in Table 3B. Again, this metallurgical performance benefit is seen in the test results in Table 2 where the Insol assay in the LEEDS Column concentrate (4.699%) compares very favorably with that of the conventional mechanical flotation machine (6.911 & 6.955%). Most recently, the second commercial installation phase has been completed. This consisted of an addition of a bank of first and a bank of second cleaner LEEDS machines. These machines were installed in parallel to an existing row of first and second cleaner A G I T A I R machines. In both rows, the six cell first cleaner bank and the six cell second cleaner bank are 100 cu.ft.machines. The preliminary plant test data analysis concluded that the overall first and second cleaner performance of the LEEDS Column was superior to the conventional mechanical flotation machine; however, again, the direct comparison of performance on the basis of the assay data was complicated by variations in feed conditions in tilne and between machine rows. The average of the test data for the initial plant test series, in first and second cleaner duty, is summarized in Table 4A. Average assay of the first cleaner feed and second cleaner product is shown for copper, molybdenum, and insol for both the LEEDS Column and the A G I T A I R flotation machine rows.
948
V.R. DEGNERand P. L. PERSON
TABLE 4A Assay Comparison, Leeds Column & Conventional Flotation Machine (First and second cleaner duty) ASSAY (%) i
TRACE SPECIE
SECOND CLEANER CONC.
OVERALL RECOVERY %
MACHINE TYPE
FIRST CLEANER FEED
LEEDS
15.55
31.24
96.439
2.009
AGITAIR
16.58
31.92
97.280
1.925
LEEDS
0.7583
1.485
95.657
1.958
AGITAIR
O. 7883
1.435
95.031
1.820
LEEDS
36.57
5.047
6.718
0.1380
AGITAIR
34.21
4.953
7.669
0.1448
CONC.* RATIO
COPPER
MOLYBDENUM
INSOL *Conc. Ratio = Product Assay/Feed Assay
A direct comparison of machine metallurgical performance, based on the assay values shown in Table 4A, is not possible because of the difference in feed conditions to each of the flotation test rows. For example, the average assay test data shows that the LEEDS Column cleaner system achieves a final product grade of 31.24% copper, while the final product grade for the A G I T A I R system is somewhat higher (i.e., 31.92% copper). However, it must be noted that the average copper assay of the feed to each of these test rows differs significantly. The LEEDS Column achieves a concentration ratio of 2.009 with a final product grade of 31.24% from a feed of 15.55% copper, which compares favorably to the A G I T A I R metallurgical performance, where a copper concentration ratio is over 4% less (i.e., 1.925) with a final product grade of 31.92%, and the higher feed grade of 16.58% copper. The comparison of machine performance on the basis of the molybdenum metallurgy is somewhat more favorable to the LEEDS Column, where it is shown (Table 4A) that the concentration ratio of the LEEDS row (1.958) is over 7% higher than the A G I T A I R machine row (1.82). Finally, the Insol rejection of the LEEDS Column, with a concentration ratio of 0.1380, is over 4% better than that of the A G I T A I R , with a concentration ratio of 0.1448. An alternate way of comparing the metallurgical performance of these two types of flotation machines is to follow the kinetic analysis procedures described earlier. In this analysis, the data shown in Table 4A is used to determine a set of kinetic flotation coefficients which model the overflow response for the trace species (i.e., copper, molybdenum, and Insol) for both the LEEDS Column and the A G I T A I R test data. The resulting coeficients are then used to predict the metallurgical performance of both the LEEDS Column and the A G I T A I R machine rows having the same feed (assay) conditions and processing the same dry tonnage. Results of this analysis, which more closely represents the comparative metallurgical performance under similar process conditions, is summarized in Table 4B. One of the particular benefits of the first commercial plant test was the severe "scaling" characteristics of process water, which provided an excellent basis for identifying LEEDS Column maintenance requirements under severe environmental conditions. Somewhat unique to this installation, a significant calcium carbonate scale builds upon wetted surfaces over a period of time. Naturally, significant scale build-up oil the LEEDS Column barrier
LEEDS Column performance evaluation
949
rods can ultimately inhibit rod movement and lose the performance benefits of the LEEDS Column over conventional mechanical flotation machines*. Plant operating experience has shown that, even in this severe water chemistry situation, a six month rod barrier maintenance cycle is sufficient to maintain consistently good LEEDS Column operation. Plant experience has shown that a clean time of 1-2 hours (steam clean plus hydrochloric bath) with turnaround time of one shift is sufficient to return the "scaled" rod barrier assembly to its original, as manufactured, condition. This maintenance can be accomplished without shutting down the LEEDS Column mechanism. TABLE 4B Performance Comparison, Leeds Column & Conventional Flotation Machine (First & second cleaner duty) ASSAY (%) TRACE SPECIE
MACHINE TYPE
FIRST CLEANER FEED
SECOND CLEANER PRODUCT
OVERALL RECOVERY %
LEEDS
16.07
35.69
96.075
AGITAIR
16.07
34.75
96.768
LEEDS
0.7733
l .7365
95.322
AGITAIR
0.7733
I .5259
95.884
LEEDS
35.39
4.223
5.537
AGITAIR
35.39
4.324
5.971
COPPER
MOLYBDENUM
I NSOL
CONCLUSIONS A review of the prototype LEEDS Column test programs at CYPRUS-BAGDAD [4], Kerr McGee-Galacia [3] and other concentrators, as well as the operational experience obtained via the first commercial LEEDS Column in second cleaner duty at CYPRUS-BAGDAD, is the basis for the following general comments relative to the LEEDS Column concept: (l)
The LEEDS Column metallurgical performance is relatively insensitive to pulp level. Tests have shown little statistical difference in operating at froth depths from 2-5 inches.
(2)
The reflux water flow rate is a significant operational process parameter. However, the amount of reflux water which is optimum may vary depending on the application. In metalliferous ore "flotation cleaning" applications, reflux water is generally used to advantage while in fine coal washing situations, reflux water use is often not required.
* Tests have shown that the metallurgical performance of the L E E D S in the barrier assembly is similar to the mechanical flotation machine.
Column
with "stationary" roads
950
V. R. DEor,n~gand P. L. PEI~SON
(3)
Air transfer to the LEEDS Column is a metallurgical performance parameter which is best determined for each specific application. The LEEDS Column performance is not critically dependent on air transfer; however, tests have shown that optimum metallurgical performance is often achieved when using less air than is commonly used by the conventional mechanical flotation machine.
(4)
The effect of number of rod barriers on metallurgical performance was studied during the WEMCO R&D program and is reported in reference [3]. Test results showed that, while increasing the number of rod barriers led to improved concentrate grade, a good economical design would be based on four (4) barriers. The metallurgical performance benefits of more than four barriers become progressively less significant with additional rod barriers.
The LEEDS Column flotation concept has been shown to have an excellent application in (metalliferous ore) "flotation cleaner" situations and in fine coal washing applications. WEMCO believes the LEEDS Column concept represents a significant flotation technology advancement for flotation "cleaner" duty. WEMCO is prepared to support client modernization or new plant design studies using the extensive technology base established for the LEEDS Column during the past six years.
REFERENCES
.
Dell, C. C., and Jenkins, B. W., The LEEDS Flotation Column, Seventh International Conference on Coal Processing, Sydney, Paper (J.3), (1976)
.
Dell, C. C., Column Flotation of Coal - The Way to Easier Filtration, Mine and Quarry, March, pp. 36-40 (1978)
.
Degner, V. R., and Sabey, J. B., WEMCO/LEEDS Flotation Column Development, Column Flotation '88, K.V.S. Sastry Editor, Society of Mining Engineers, Littleton, Colorado, pp 267-279 (1988)
.
Dworatzek, J. J., Testing of a WEMCO-LEEDS Column Flotation Machine as a Copper Cleaner at Bagdad, Annual Meeting of the Arizona Conference of the AIME, Dec. 3, (1989)
.
Degner, V. R., Flotation Machine Selection for Sulfide and Non-Sulfide Applications, Design and Installation of Concentration and Dewatering Circuits, Mular, A.L., and Anderson, M.A., editors, AIME, Littleton, Colorado, pp 56-75 (1986)
.
Klimpel, R. and Hansen, R., Some Factors Influencing Kinetics in Sulfide Flotation, AIME Annual Meeting, February, Preprint 81 - 14 (1981)
.
Degner, V. R., Flotation Machine Size Selection for Coal Applications, Coal Prep '86, Industrial Presentations West, Aurora, Colorado, pp 319-347 (1986)