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Flotation column amenability and scale-up parameter estimation tests
Mineral~ Engineering, Vol. 5, No. 2, pp. 169-182, 1992 Printed in Great Britain
0892-6875/92 $5.00 + 0.00 © 1991 Pergamon Press pic
FLOTATION COLUMN AMENABILITY AND SCALE-UP PARAMETER ESTIMATION TESTS
R. DEL VILLAR§, J.A. FINCHt, C.O. GOMEZt and R.ESPINOSA-GOMEZt § Universit~ Laval, Ste-Foy, Qu6bec, Canada t McGill University, Montr6al, Qu6bec, Canada Mount Isa Mines, Queensland, Australia (Received 30 April 1991; accepted 28 June 1991) ABSTRACT A preliminary step in the decision to install flotation columns in an existing separation circuit is the so-called amenability testing. This testing consists of comparing metallurgical results from laboratory columns with, for example the performance of the existing circuit or with laboratory mechanical cells. The following step is to select the size and number of columns required for the duty. One way this is achieved is by using a computer simulator based on a scale-up model. This model requires, among other parameters, the flotation rate constants and the solids removal froth capacity, which have to be experimentally determined. This paper describes the apparatus and methodology used to conduct flotation column amenability and scale-up tests and discusses problems encountered during experience at a number of different cmwentrators. Amenability tests and scale-up procedure are illustrated using examples from two particular case studies: Mount lsa Mines and Falconbridge Ltd.
Keywords Amenability tests, scale-up test, simulation, modelling, column flotation. INTRODUCTION There is considerable interest in evaluating column flotation to augment or replace mechanical cell circuits, particularly in cleaning operations. This testwork involves two steps, amenability testing and scale-up parameter determination. Amenability testing here refers to determining if the flotation column offers improved metallurgy over that of the existing circuit. This requires establishing some test procedures and criteria for comparison. If the amenability tests prove favourable for columns the second step is to select the size and number of units using a scale-up procedure requiring a number of parameters, which are empirically determined. Over the past four years, equipment and methodology for both steps have been developing. It is the subject of this paper, drawing on experiences at Falconbridge Ltd., Kidd Creek Mines and Mount Isa Mines Ltd. Procedures based on those described here have been used in designing several installations (other than the three companies mentioned), for example: Simplot's PocateHo operation [ 1], Paddington Gold Mine [2], and Magma Copper [3]. This communication should prove especially useful to those who are new to column Notation testing. 169
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EXPERIMENTAL SET-UP The column The flotation column (Fig. 1) we normally use is 5.04 cm in diameter by 11 m in height, made of six 1.80 m Plexiglas sections, which allows for visual inspection of the sparger action at the bottom and the interface near the top. Feed is introduced in the second section at about two meters from the top. The top section allows for free overflow of the concentrate into a box (20 cm in diameter by 15 cm in height) provided with a vertical pipe exit for removal of material. Launder water is used to break the froth through a perforated 0.6 cm copper tube ring. Wash water is supplied in a similar manner through a smaller ring placed about I cm below the overflow lip.
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Air )to shut down railings flowrate
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Tailings pump Fig.2 Column bottom The feed tank Feed, either directly from the circuit or collected in batches, is suspended in a 60 L polyethylene feed tank using a 1/2 HP Lightnin variable speed stirrer. Feed is screened at a suitable mesh prior to being introduced into the feed tank to remove debris. The tank is baffled to prevent the development of fluid circulation patterns produced by the intense agitation. The tank bottom is designed to eliminate dead volumes which could lead to particle settling. The tank is equipped with two outlets at the bottom, one connected to the feed p u m p suction and the second for feed sampling. This design has proved to give good reproducibility and the second outlet has relieved the problem of feed sampling while on the run. Material handling Either tubing pumps (e.g., Cole Parmer Masterflex) or diaphragm pumps are used for feed, wash and launder water additions and for tails removal. These pumps give good control of the flowrates. Wash water and launder water are pumped at a known rate from separate plastic buckets (100 L). This helps reduce fluctuations in water flowrates which could disturb later water balance calculations and also permits frother addition, if necessary, into the wash water. Tails and concentrate from the column are sampled then discarded (e.g. sent back into the circuit). Reagent addition, when necessary, is done prior to the feed tank. Air pressure is set to a pre-determined value (e.g. 140 kPa) using a gas regulator. Air flowrate is manually controlled using a calibrated gas rotameter. AMENABILITY TESTS The desirable test consists of comparing the metallurgy obtained in the laboratory column with that of the existing circuit. Two convenient forms o f presenting this comparison are given in Fig. 3 and Fig. 4. If plant data is not available for the tested stream, then the comparison can be made against laboratory mechanical cells. To obtain the grade-recovery relationship on the column, the best option is to vary residence time by varying feed rate. Residence times must be chosen in such a manner that the obtained recoveries cover the plant or lab cell recovery range. MINE--5/2--D
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Flotation column anumability
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It sometimes proves necessary to dilute the feed to avoid overloading the column; any shift in the grade-recovery relationship on changing feed percent solids should be noted. If possible, some runs at a feed percent solids close to the plant conditions should be completed. If mechanical lab cell tests for comparison purposes are required, timed batch flotation tests can be used, or possible release analysis.
Test Procedure Start up In a c o m m o n approach, samples of a target stream are collected from an appropriate plant location in 15 L plastic buckets (about 40) and transported to the column; experience has shown this minimises sample oxidation. The column is filled with water from the top, air is admitted through the sparger and set to its desired pressure and flowrate. T h e tailings pump is switched on and the tails flowrate is set to give the desired liquid residence time in the column. Water is then fed from the feed tank at a rate that ensures a positive bias (the downward velocity of the interface level in absence of wash water provides a good estimate of the bias rate). Wash water is then added to the column and adjusted to maintain the level. At this moment, the feed tank can be loaded with pulp to start feeding the column. After the column is filled with pulp (i.e. once the pulp starts leaving through the tails outlet), the tails flowrate is readjusted to the desired residence time. Feed and wash water flowrates are then adjusted accordingly, keeping in mind that a positive bias should always prevail. In absence of other information good starting conditions are: gas rate 1.5 cm/s, bias rate 0.1 cm/s (giving wash water rate approx. 0.3 cm/s) and froth depth 100 cm.
Operation The column is run steadily for at least one and sampling. Launder water flowrate is adjusted concentrate; it is advisable to keep launder water to the air flowrate should be continuously checked as
desirably two residence times before to provide an adequate removal of a minimum, however. During the test, it normally fluctuates.
The interface level is kept (at about 80-120 cm from the column lip) by small adjustments of the wash water flowrate; feed or tailings rate can also be used for level adjustments. Wash water rate should not be less than 0.30 cm/s to ensure an adequate bias rate. Excessive wash water is not desirable as it causes turbulence in the froth. Slurry level in the feed tank and water level in the launder and wash water tanks should be continuously monitored. Manual operation, at least for a single column, is quite feasible. It is recommended that all controls be at the feed level for convenience.
Sampling Pulp sampling involves collecting samples of concentrate and tails (both timed to estimate the f l o w r a t e ) a n d feed (from the feed tank). Wash water and launder water rates are also measured and air flowrate and interface level are recorded. Once all sampling is completed, conditions for the next residence time are set. The collected samples are processed to determine percent solids and prepare dried sub-samples for chemical analysis. A mass balance program is used to filter lab results and to estimate flowrates; grades and recoveries for all mineral species are then calculated.
Operating Problems Slurry pumps If tube pumps a r e used, the plastic tube inserted in the pump head must be periodically changed to avoid sudden leakages. It is advisable to change them every day. For
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particularly dense pulps or those containing abrasive or coarse particles, diaphragm pumps have shown greater reliability.
Concentrate samples Pulp density values of the concentrate samples are unreliable. They are calculated from the concentrate sample weight and the launder water flowrate. In those cases where the concentrate solids flowrate is low (less than 30 g/rain), the error in the launder water measurement is critical and the concentrate percent solids becomes subject to a large uncertainty. No clear solution to this problem exists as reducing the launder water flowrate (the error source) is deleterious to concentrate removal. A more generous size of launder than that currently in use may be necessary.
Interface stability Sometimes it is not possible to establish a sharp and stable interface. This problem seems to be associated with processing streams collected from t h e final stages of the circuit, which normally contain high dosages of chemical reagents, high gangue content and/or large particles. The operator is faced with a barely distinguishable interface. The only abnormal characteristics observed in all these tests was the small size of the air bubbles and the low solids coverage. A similar problem occurred in the second column of a two-column circuit operated in series. Two actions successfully overcame these problems: i) reduction of the air flowrate, or ii) reduction of the sparger surface area (the more effective).
Double interface An intriguing operating problem was found when testing some streams at the Strathcona concentrator (Falconbridge Ltd.), namely a second interface in the bottom section of the column. It was easily noticed because the control of the column became difficult:, the interface (the top one) started oscillating, and larger than usual air bubbles moved through the column. The problem was normally solved by decreasing the air rate a n d / o r diluting the feed. This problem is similar to the coalescence and "burping" problem reported by full size column operators [4]. This phenomenon was deemed to be related to an excessive gas holdup due to high dosage of frother in the plant aggravated by a high feed percent solids. High feed percent solids probably generates a bed of pulp near the feed inlet with increased local viscosity which is difficult for the small bubbles to cross. A build-up of solids and loss of performance in production columns has also been reported [5]. The "double interface" was never experienced at Kidd Creek or Mount Isa Mines.
Oxidation of mineral surfaces It was observed at Mount Isa Mines that galena surfaces (heavy medium plant slimes rougher concentrate) were susceptible to oxidation. This oxidation appears to increase during column testing as a result of the conditioning time in the feed tank and especially due to the relatively long residence time, often greater than 16 rain, in the column. Under such conditions, galena flotation was usually poor. This observation was born out in the production columns [6]. It can be generalized that flotation columns might not be a suitable device for separations involving minerals which are subject to oxidation.
Dispersion Conventional mechanical cells provide strong mixing due to the shearing forces resulting from the impeller action. This turbulence enhances the dispersion of fine particles. In those cases where the pulp minerals are heterocoagulated (agglomeration of particles of different minerals), dispersion is essential toachieve selectivity. F o r s u c h systems, columns
Flotation column amenability
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are not recommended as they do not provide the degree of mechanical dispersion. Use of dispersing aids, including ultra-sonic vibration, has so far proved of little benefit. In the situation where particles coagulate only with similar particles, e.g. selective flocculation, columns may be well suited as they may enhance fines recovery without requiring dispersants. Adjusting the pulp chemistry to achieve selective flotation, has been promoted, for example, in the column flotation of coal [7].
Examples of application The authors have used this experimental procedure in several concentrators: Mattabi Mines (Ont.), Kidd Creek Mines (Ont.), Falconbridge-Strathcona (Ont.), Mount Isa Mines (Australia), Les Mines Selbaie (Qu6,), La Mine Niobec (Qu6.). The results permitted the assessment of the metallurgical feasibility of columns with respect to the conventional mechanically agitated cells and led in many of those cases to scale-up studies and later to the commissioning of large scale columns [8,9]. Figures 3, 4, 5 and 6 illustrate the application of the described experimental procedure at two different concentrators. Figure 3 shows some of the results obtained during the amenability testing of the Cu retreatment concentrate at Mount Isa Mines. It is evident that the column outperforms the laboratory mechanical cell even though the feed percent solids was substantially lower in the case of the mechanical cell (which normally leads to improved selectivity). 45
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(Falconbridge Ltd.), may indicate that entrainment is less of a factor compared with particle locking or difficult selective flotation. Other unfavourable differences, such as pulp potential and oxygen content of the pulp, could also be responsible, but in any case, these results emphasise the need for this amenability testing prior to any scale-up calculations or, worse, full size column commissioning.
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Pentlandite recovery (%) Fig.6 Amenability test results (Ni grade vs pentlandite recovery) Cleaning of nickel scavenger concentrate at Falconbridge The amenability studies at Mount Isa Mines contemplated testing a total of six streams. Results showed that column flotation could be advantageous in processing five streams at least [10]. At the Strathcona Concentrator, twelve streams (from copper rougher concentrate to nickel scavenger tailings) were tested searching for the most promising ones for column processing. Column flotation was attractive in most of them, specially in the copper circuit and in treating nickel concentrates (secondary rougher or final concentrate), i.e. using the columns as cleaners. A similar conclusion, though not as firm, was reached when treating concentrates and tailings from the mags and non-mags flotation cells. The column failed to outperform the conventional cells when treating the mags flotation feed. In the case of nickel scavenger tailings, column testing was curtailed because of the coarseness of the material fed to the column, which caused the solids to settle in the pipes as well as in the column. The type of laboratory column and procedure described hereis not appropriate for feeds much coarser than about 150/Jm. At Mattabi Mines the column testing was used as diagnostic tool for determining whether the poor performance of the Pb/Cu separation circuit was due to entrainment or to locking. The column performance barely matched that of the mechanical cells suggesting entrainment was not the issue. There is a suspicion, however, that reagent dosage was too high for o p t i m u m column performance, b u t a change in dosage was not possible without upstream modifications. This iUustrates a p r o b l e m in t e ~ g in an existing plant, the chemical condition, including frother type, may not b e ideally suited to column flotation.
Flotation column amenability
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SCALE UP TESTS The step following amenability test is sizing the columns required for the duty. This is achieved using a computer simulator [ 11 ] based o n the scale-up model developed by McGill researchers [12], modified for carrying capacity constraints, and more recently for froth z o n e - collection zone interaction [13]. The simulator requires two experimentally determined parameters: mineral flotation rate constants and carrying capacity. The methodology for estimating these parameters and some of the difficulties encountered and sources of uncertainty are described in the following paragraphs.
Determination of Carrying Capacity Carrying capacity is the maximum solids rate that can be floated in the column. It is related to the maximum achievable particle coverage of the bubbles and constitutes an upper limit to the particle collection process. Carrying capacity can be normalized to the column crosssectional area (e.g., t/h/m:') [14], or lip length (e.g., t / h / m ) [5], the latter being favoured for large diameter columns. Experimental procedure The technique used to measure this parameter consists of a series of column runs with increasing feed solids rate until a maximum production rate of solids to the concentrate is reached [14]. This maximum concentrate rate divided by, for example, the column cross-sectional area gives the carrying capacity (e.g. g/min/cm2). The procedure involves running the column at constant residence time with different feed percent solids. Timed samples of the railings and concentrate streams are collected along with a feed sample. Pulp density is determined for all collected samples and used to calculate feed and concentrate solids rates. Figure 7 shows a typical curve obtained for copper rougher concentrate testing. Some tests have shown that increasing the solids feed rate beyond that giving the maximum concentrate rate, causes a decrease in solids production as shown in this figure. Work is in progress to define whether this decrease is a consequence of a progressive increase in concentrate fineness due to the action of the froth or to a change in the column hydrodynamics produced by the increase in solids content within the column.
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For a given stream the carrying capacity is a strong function of particle size. Concentrate production rate vs solids feedrate curves are a function of the chosen residence time, but they should all tend to the same limit, the carrying capacity. Repeated measure of carrying capacity suggest a precision of about 30% [14].
Determination of Flotation Rate Constants The assumption inherent in this determination is that the collection process is first order. The objective, therefore, is to determine first order flotation rate constants for each mineral species. Ideally, the material to be used should be the same as the one to be processed in the scaled columns. However, if the characteristics o f this material are such that the particle collection process in the column will be limited by carrying capacity, then the feed pulp must be diluted (with process water).
Experimental procedure The technique used to measure flotation rate constants assumes the collection process follows a first order kinetics which means that, for a batch (or plug flow) process, the flotation rate constants can be obtained from the slope of the recovery versus time plot (Fig. 8).
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Flotation column amenability
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Column operation is similar to that conducted in the first part of the paper (amenability tests). After a m i n i m u m of two residence times of stable operation, samples of the feed, tailings, and concentrate streams are collected. Samples are then processed and assayed. A mass balance computer program is used for filtering the laboratory data and to estimate the flowrates. These values are then used to calculate the recovery for all the mineral species. Sources of uncertainty in estimating rate constants Plug flow
Assuming plug flow transport is inherent to the procedure used to calculate the mineral rate constants. The experimental procedure respects this constraint by using a 5 cm by 10 m flotation column which reasonably well approaches this transport pattern [15]. Collection zone / froth zone interaction
The rate constants ideally refer to the collection process. However, the experimental procedure as described, means the froth zone is present when the measurement is made. It is known that a solids dropback from the froth occurs and a recycle between the two zones is established. Finch and Dobby [I 3] presented a method of extracting the collection zone rate constants from the overall (measured) rate constant. Feed characteristics
Clearly it is desirable to test the feed "as is", i.e. as it was sampled from the plant. However this may mean that the system is at its carrying capacity limit, in which case the feeds needs diluting. An increasing slope of the recovery vs time plot reveals a system at its carrying capacity limit (upper curves of Fig. 8). It is important to establish that diluting the feed (i.e. change pulp density) does not influence the actual separation. Particle settling
A second consideration is related to the residence time determination. The scale-up model requires the residence time of the particles in the collection zone. However, in the experimental procedure, the liquid residence time is calculated. For large and/or dense particles, corrections for particle settling velocity may be required. The model proposed by Masliyah [16] is used in the column simulator to calculate particle settling velocity, usually using the 80% passing size as a measure of particle diameter. Gas hold-up Part of the collection zone is occupied by gas, thus the true liquid residence time is shorter than the nominal one. However, gas hold-up will be different for each residence time setting, since this requires different liquid velocities. Since no reliable method is currently available to predict the gas holdup in a flotation column, for a given set of conditions the simplification is to ignore the gas holdup and use nominal residence times. This implies that, if the experimental results are to be reproduced by the simulator (e.g. to check pilotscale results), a zero gas hold up should selected. Gangue rate constants
Assays are often not made for the gangue, it being estimated by difference. The mass balancing should be made on all mineral species (including the gangue estimation) and the computer program should provide the option "complete analysis" (i.e. sum of all mineral contents equal 100%). Otherwise gangue predictions must be treated with caution. One consequence of this is that experimental grades and recoveries generally cannot be simultaneously reproduced by the simulator. Since the amenability testing has established
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improved performance in the columns (otherwise scale-up studies are probably not warranted), the emphasis of the simulation should be rather focused in determining a configuration to achieve the desired recovery. By adjusting the froth dropback factor, simultaneous grade/recovery fitting is reported to be possible [17].
Non-first order In many cases, first order kinetics does not fit experimental data well, i.e. flotation rate constants are recovery dependent. Usually a two-slope straight line better represents this non-linear process. This can be interpreted as two species of the same material floating at different rates, one fast and one slow. Two options for scale-up are possible: i) to use the slowest rate constant values (conservative approach), or i i ) t o use the rate constants corresponding to the target recovery (usual approach). When a single rate constant does not represents the entire process, the simultaneous simulation of grade and recovery becomes even more difficult.
Examples of application As noted in the introduction, the procedures described here, have been used at a number of locations; perhaps the most well documented to date is the experience at Mount Isa Mines Ltd. (Australia). At Mount Isa Mines three column circuits, each having three columns in series, have been commissioned based on laboratory-scale column data and calculations performed on the scale-up simulator [11]. Details of the commissioning of one these circuits, the low grade middlings, are given elsewhere [9]. The scale-up was largely determined by the carrying capacity limitations. The downturn of the percent remaining (100-recovery) vs time curve obtained during rate constant determination for the low grade middlings tests is an evidence of overloading of the bubbles by the fine particles (Fig. 8). This limitation led to dilution of the feed prior to any rate constant determination test. It became obvious then that the carrying capacity tests must be completed before rate constant determinations in order to establish the feed percent solids which permits a kinetic process. Diluting the feed and processing in a single column gave slopes similar to that in the second column in the undiluted case (bottom curves of Fig. 8). The concentrate production rate (carrying capacity) was relatively independent of the feed rate, a typical finding for fine particles, and a value of 4.5 g / m i n / c m 2 was used in the simulator. Figure 9 shows the original low grade middlings circuit at Mount Isa Mines Ltd. and the current column flotation circuit. Amenability testing results are summarized in Figure 5, while large scale column results (after commissioning) are given in Figure 10. It can be seen that the required capacity and target performance, greater than 80% Zn recovery and greater than 45% Zn + Pb grade, were met. The new circuit has contributed about a 2% increase in Zn recovery to the bulk concentrate with a similar Pb recovery as before. At Falconbridge Ltd. (Strathcona Concentrator), scale-up tests on five streams were completed [18]. Two 2.1 m dia. columns have been commissioned for processing copper rougher concentrate (CRC). Due to the coarseness of these feeds (relative to those at Mount Isa Mines) the collection process was found not be limited by the carrying capacity but rather by the kinetics (the carrying capacity value found experimentally for the CRC stream was 7.5 g / m i n / c m 2). The coarseness of the particles made it necessary to correct for particle retention time.
Flotation column amenabifity
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REFERENCES
Polinsky, S., presentation at the Eng. Found. Conf., Florida (December 1989).
.
2.
Newell, A., Gray, D. & Alfred, R. The application of flotation columns to gold recovery at Paddington Gold Mine, W.A., presented at Precious Metals Syrup., Montreal (1989).
.
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.
Hoffert, J., Discussion on froth zone parameters. In Feasby (Ed.), Proceedings of Column Flotation Symposium, Trail, B.C., 84 (Nov. 2-4 1987). Amenluxen, R., private communications (1990).
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6.
Espinosa-Gomez, R., Johnson, N.W. & Finch, J.A., Evaluation of flotation column scale-up at Mount Isa Mines Limited. Minerals Engineering, 2, No.3,369-375 (1989).
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Del Villar, R. & Finch, J.A., Strathcona Column Flotation Project, Amenability and Scale-up Testing. Report submitted to Falconbridge Ltd. (October 1988).
.
Espinosa-Gomez, R., Johnson, N.W., Pease, J.P., & Munro, P.D., The commissioning of the first flotation columns at Mount Isa Mines Limited, in Dobby,G.S. and Rap, R.S. (Eds), Processing of Complex Ores, Pergamon Press, 293-302 (1989).
10.
Espinosa-Gomez, R., Finch, J.A., & Johnson, N.W., Column flotation of very fine particles, Minerals Engineering, 1, No.I, 3-18 (1988).
11.
Del Villar, R., Finch, J.A., Yianatos, J.B. & Laplante, A.R., Column Flotation Simulation. In Fytas, Collins and Singhal (Ed.), Computer Applications in the Mineral Industry, Balkema, Rotterdam, 233-239 (1988).
12.
Dobby, G.S. & Finch, J.A., Flotation column scale-up and modelling, CIM Bull., Vol.79, N ° 889, 89-96 (1986).
13.
Finch J.A. & Dobby, G.S., Column Flotation, Pergamon Press, Oxford (1990).
14.
Espinosa-Gomez, R., Yianatos, J.B., Finch, J.A., & Johnson, N.W., Carrying capacity limitations in flotation columns. In K.V.S. Sastry (Ed.), Column Flotation '88, S.M.E., Littleton Co., 143-148 (1988).
15.
Laplante, A.R., Yianatos, J.B. & Finch, J.A., Mixing characteristics of collection zone, in K.V.S. Sastry (Ed.), Column Flotation '88, S.M.E., Littleton Co., 69-79 (1988).
16.
Masliyah, J.M., Hindered settling in Chem. Engng Sci., 34, 1166-1168 (1979).
17.
Castillo, D. & Dobby, G.S., presentation at the Annual Canadian Mineral Processors Meeting, Ottawa (January 1990).
18.
Gomez C.O. & Finch, J.A., Strathcona Column Flotation Project, Simulation of Flotation Column Circuits. Report submitted to Falconbridge Ltd. (April 1989).
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FLOTATION COLUMN AMENABILITY AND SCALE-UP PARAMETER ESTIMATION TESTS
R. DEL VILLAR§, J.A. FINCHt, C.O. GOMEZt and R.ESPINOSA-GOMEZt § Universit~ Laval, Ste-Foy, Qu6bec, Canada t McGill University, Montr6al, Qu6bec, Canada Mount Isa Mines, Queensland, Australia (Received 30 April 1991; accepted 28 June 1991) ABSTRACT A preliminary step in the decision to install flotation columns in an existing separation circuit is the so-called amenability testing. This testing consists of comparing metallurgical results from laboratory columns with, for example the performance of the existing circuit or with laboratory mechanical cells. The following step is to select the size and number of columns required for the duty. One way this is achieved is by using a computer simulator based on a scale-up model. This model requires, among other parameters, the flotation rate constants and the solids removal froth capacity, which have to be experimentally determined. This paper describes the apparatus and methodology used to conduct flotation column amenability and scale-up tests and discusses problems encountered during experience at a number of different cmwentrators. Amenability tests and scale-up procedure are illustrated using examples from two particular case studies: Mount lsa Mines and Falconbridge Ltd.
Keywords Amenability tests, scale-up test, simulation, modelling, column flotation. INTRODUCTION There is considerable interest in evaluating column flotation to augment or replace mechanical cell circuits, particularly in cleaning operations. This testwork involves two steps, amenability testing and scale-up parameter determination. Amenability testing here refers to determining if the flotation column offers improved metallurgy over that of the existing circuit. This requires establishing some test procedures and criteria for comparison. If the amenability tests prove favourable for columns the second step is to select the size and number of units using a scale-up procedure requiring a number of parameters, which are empirically determined. Over the past four years, equipment and methodology for both steps have been developing. It is the subject of this paper, drawing on experiences at Falconbridge Ltd., Kidd Creek Mines and Mount Isa Mines Ltd. Procedures based on those described here have been used in designing several installations (other than the three companies mentioned), for example: Simplot's PocateHo operation [ 1], Paddington Gold Mine [2], and Magma Copper [3]. This communication should prove especially useful to those who are new to column Notation testing. 169
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R. DEL VILLARet al.
EXPERIMENTAL SET-UP The column The flotation column (Fig. 1) we normally use is 5.04 cm in diameter by 11 m in height, made of six 1.80 m Plexiglas sections, which allows for visual inspection of the sparger action at the bottom and the interface near the top. Feed is introduced in the second section at about two meters from the top. The top section allows for free overflow of the concentrate into a box (20 cm in diameter by 15 cm in height) provided with a vertical pipe exit for removal of material. Launder water is used to break the froth through a perforated 0.6 cm copper tube ring. Wash water is supplied in a similar manner through a smaller ring placed about I cm below the overflow lip.
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Flotation column amenability
171
Air )to shut down railings flowrate
Valve for emptying the column
Tailings pump Fig.2 Column bottom The feed tank Feed, either directly from the circuit or collected in batches, is suspended in a 60 L polyethylene feed tank using a 1/2 HP Lightnin variable speed stirrer. Feed is screened at a suitable mesh prior to being introduced into the feed tank to remove debris. The tank is baffled to prevent the development of fluid circulation patterns produced by the intense agitation. The tank bottom is designed to eliminate dead volumes which could lead to particle settling. The tank is equipped with two outlets at the bottom, one connected to the feed p u m p suction and the second for feed sampling. This design has proved to give good reproducibility and the second outlet has relieved the problem of feed sampling while on the run. Material handling Either tubing pumps (e.g., Cole Parmer Masterflex) or diaphragm pumps are used for feed, wash and launder water additions and for tails removal. These pumps give good control of the flowrates. Wash water and launder water are pumped at a known rate from separate plastic buckets (100 L). This helps reduce fluctuations in water flowrates which could disturb later water balance calculations and also permits frother addition, if necessary, into the wash water. Tails and concentrate from the column are sampled then discarded (e.g. sent back into the circuit). Reagent addition, when necessary, is done prior to the feed tank. Air pressure is set to a pre-determined value (e.g. 140 kPa) using a gas regulator. Air flowrate is manually controlled using a calibrated gas rotameter. AMENABILITY TESTS The desirable test consists of comparing the metallurgy obtained in the laboratory column with that of the existing circuit. Two convenient forms o f presenting this comparison are given in Fig. 3 and Fig. 4. If plant data is not available for the tested stream, then the comparison can be made against laboratory mechanical cells. To obtain the grade-recovery relationship on the column, the best option is to vary residence time by varying feed rate. Residence times must be chosen in such a manner that the obtained recoveries cover the plant or lab cell recovery range. MINE--5/2--D
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Flotation column anumability
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It sometimes proves necessary to dilute the feed to avoid overloading the column; any shift in the grade-recovery relationship on changing feed percent solids should be noted. If possible, some runs at a feed percent solids close to the plant conditions should be completed. If mechanical lab cell tests for comparison purposes are required, timed batch flotation tests can be used, or possible release analysis.
Test Procedure Start up In a c o m m o n approach, samples of a target stream are collected from an appropriate plant location in 15 L plastic buckets (about 40) and transported to the column; experience has shown this minimises sample oxidation. The column is filled with water from the top, air is admitted through the sparger and set to its desired pressure and flowrate. T h e tailings pump is switched on and the tails flowrate is set to give the desired liquid residence time in the column. Water is then fed from the feed tank at a rate that ensures a positive bias (the downward velocity of the interface level in absence of wash water provides a good estimate of the bias rate). Wash water is then added to the column and adjusted to maintain the level. At this moment, the feed tank can be loaded with pulp to start feeding the column. After the column is filled with pulp (i.e. once the pulp starts leaving through the tails outlet), the tails flowrate is readjusted to the desired residence time. Feed and wash water flowrates are then adjusted accordingly, keeping in mind that a positive bias should always prevail. In absence of other information good starting conditions are: gas rate 1.5 cm/s, bias rate 0.1 cm/s (giving wash water rate approx. 0.3 cm/s) and froth depth 100 cm.
Operation The column is run steadily for at least one and sampling. Launder water flowrate is adjusted concentrate; it is advisable to keep launder water to the air flowrate should be continuously checked as
desirably two residence times before to provide an adequate removal of a minimum, however. During the test, it normally fluctuates.
The interface level is kept (at about 80-120 cm from the column lip) by small adjustments of the wash water flowrate; feed or tailings rate can also be used for level adjustments. Wash water rate should not be less than 0.30 cm/s to ensure an adequate bias rate. Excessive wash water is not desirable as it causes turbulence in the froth. Slurry level in the feed tank and water level in the launder and wash water tanks should be continuously monitored. Manual operation, at least for a single column, is quite feasible. It is recommended that all controls be at the feed level for convenience.
Sampling Pulp sampling involves collecting samples of concentrate and tails (both timed to estimate the f l o w r a t e ) a n d feed (from the feed tank). Wash water and launder water rates are also measured and air flowrate and interface level are recorded. Once all sampling is completed, conditions for the next residence time are set. The collected samples are processed to determine percent solids and prepare dried sub-samples for chemical analysis. A mass balance program is used to filter lab results and to estimate flowrates; grades and recoveries for all mineral species are then calculated.
Operating Problems Slurry pumps If tube pumps a r e used, the plastic tube inserted in the pump head must be periodically changed to avoid sudden leakages. It is advisable to change them every day. For
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particularly dense pulps or those containing abrasive or coarse particles, diaphragm pumps have shown greater reliability.
Concentrate samples Pulp density values of the concentrate samples are unreliable. They are calculated from the concentrate sample weight and the launder water flowrate. In those cases where the concentrate solids flowrate is low (less than 30 g/rain), the error in the launder water measurement is critical and the concentrate percent solids becomes subject to a large uncertainty. No clear solution to this problem exists as reducing the launder water flowrate (the error source) is deleterious to concentrate removal. A more generous size of launder than that currently in use may be necessary.
Interface stability Sometimes it is not possible to establish a sharp and stable interface. This problem seems to be associated with processing streams collected from t h e final stages of the circuit, which normally contain high dosages of chemical reagents, high gangue content and/or large particles. The operator is faced with a barely distinguishable interface. The only abnormal characteristics observed in all these tests was the small size of the air bubbles and the low solids coverage. A similar problem occurred in the second column of a two-column circuit operated in series. Two actions successfully overcame these problems: i) reduction of the air flowrate, or ii) reduction of the sparger surface area (the more effective).
Double interface An intriguing operating problem was found when testing some streams at the Strathcona concentrator (Falconbridge Ltd.), namely a second interface in the bottom section of the column. It was easily noticed because the control of the column became difficult:, the interface (the top one) started oscillating, and larger than usual air bubbles moved through the column. The problem was normally solved by decreasing the air rate a n d / o r diluting the feed. This problem is similar to the coalescence and "burping" problem reported by full size column operators [4]. This phenomenon was deemed to be related to an excessive gas holdup due to high dosage of frother in the plant aggravated by a high feed percent solids. High feed percent solids probably generates a bed of pulp near the feed inlet with increased local viscosity which is difficult for the small bubbles to cross. A build-up of solids and loss of performance in production columns has also been reported [5]. The "double interface" was never experienced at Kidd Creek or Mount Isa Mines.
Oxidation of mineral surfaces It was observed at Mount Isa Mines that galena surfaces (heavy medium plant slimes rougher concentrate) were susceptible to oxidation. This oxidation appears to increase during column testing as a result of the conditioning time in the feed tank and especially due to the relatively long residence time, often greater than 16 rain, in the column. Under such conditions, galena flotation was usually poor. This observation was born out in the production columns [6]. It can be generalized that flotation columns might not be a suitable device for separations involving minerals which are subject to oxidation.
Dispersion Conventional mechanical cells provide strong mixing due to the shearing forces resulting from the impeller action. This turbulence enhances the dispersion of fine particles. In those cases where the pulp minerals are heterocoagulated (agglomeration of particles of different minerals), dispersion is essential toachieve selectivity. F o r s u c h systems, columns
Flotation column amenability
175
are not recommended as they do not provide the degree of mechanical dispersion. Use of dispersing aids, including ultra-sonic vibration, has so far proved of little benefit. In the situation where particles coagulate only with similar particles, e.g. selective flocculation, columns may be well suited as they may enhance fines recovery without requiring dispersants. Adjusting the pulp chemistry to achieve selective flotation, has been promoted, for example, in the column flotation of coal [7].
Examples of application The authors have used this experimental procedure in several concentrators: Mattabi Mines (Ont.), Kidd Creek Mines (Ont.), Falconbridge-Strathcona (Ont.), Mount Isa Mines (Australia), Les Mines Selbaie (Qu6,), La Mine Niobec (Qu6.). The results permitted the assessment of the metallurgical feasibility of columns with respect to the conventional mechanically agitated cells and led in many of those cases to scale-up studies and later to the commissioning of large scale columns [8,9]. Figures 3, 4, 5 and 6 illustrate the application of the described experimental procedure at two different concentrators. Figure 3 shows some of the results obtained during the amenability testing of the Cu retreatment concentrate at Mount Isa Mines. It is evident that the column outperforms the laboratory mechanical cell even though the feed percent solids was substantially lower in the case of the mechanical cell (which normally leads to improved selectivity). 45
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R. DEL VILLAR et al.
(Falconbridge Ltd.), may indicate that entrainment is less of a factor compared with particle locking or difficult selective flotation. Other unfavourable differences, such as pulp potential and oxygen content of the pulp, could also be responsible, but in any case, these results emphasise the need for this amenability testing prior to any scale-up calculations or, worse, full size column commissioning.
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Flotation column amenability
177
SCALE UP TESTS The step following amenability test is sizing the columns required for the duty. This is achieved using a computer simulator [ 11 ] based o n the scale-up model developed by McGill researchers [12], modified for carrying capacity constraints, and more recently for froth z o n e - collection zone interaction [13]. The simulator requires two experimentally determined parameters: mineral flotation rate constants and carrying capacity. The methodology for estimating these parameters and some of the difficulties encountered and sources of uncertainty are described in the following paragraphs.
Determination of Carrying Capacity Carrying capacity is the maximum solids rate that can be floated in the column. It is related to the maximum achievable particle coverage of the bubbles and constitutes an upper limit to the particle collection process. Carrying capacity can be normalized to the column crosssectional area (e.g., t/h/m:') [14], or lip length (e.g., t / h / m ) [5], the latter being favoured for large diameter columns. Experimental procedure The technique used to measure this parameter consists of a series of column runs with increasing feed solids rate until a maximum production rate of solids to the concentrate is reached [14]. This maximum concentrate rate divided by, for example, the column cross-sectional area gives the carrying capacity (e.g. g/min/cm2). The procedure involves running the column at constant residence time with different feed percent solids. Timed samples of the railings and concentrate streams are collected along with a feed sample. Pulp density is determined for all collected samples and used to calculate feed and concentrate solids rates. Figure 7 shows a typical curve obtained for copper rougher concentrate testing. Some tests have shown that increasing the solids feed rate beyond that giving the maximum concentrate rate, causes a decrease in solids production as shown in this figure. Work is in progress to define whether this decrease is a consequence of a progressive increase in concentrate fineness due to the action of the froth or to a change in the column hydrodynamics produced by the increase in solids content within the column.
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For a given stream the carrying capacity is a strong function of particle size. Concentrate production rate vs solids feedrate curves are a function of the chosen residence time, but they should all tend to the same limit, the carrying capacity. Repeated measure of carrying capacity suggest a precision of about 30% [14].
Determination of Flotation Rate Constants The assumption inherent in this determination is that the collection process is first order. The objective, therefore, is to determine first order flotation rate constants for each mineral species. Ideally, the material to be used should be the same as the one to be processed in the scaled columns. However, if the characteristics o f this material are such that the particle collection process in the column will be limited by carrying capacity, then the feed pulp must be diluted (with process water).
Experimental procedure The technique used to measure flotation rate constants assumes the collection process follows a first order kinetics which means that, for a batch (or plug flow) process, the flotation rate constants can be obtained from the slope of the recovery versus time plot (Fig. 8).
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Flotation column amenability
179
Column operation is similar to that conducted in the first part of the paper (amenability tests). After a m i n i m u m of two residence times of stable operation, samples of the feed, tailings, and concentrate streams are collected. Samples are then processed and assayed. A mass balance computer program is used for filtering the laboratory data and to estimate the flowrates. These values are then used to calculate the recovery for all the mineral species. Sources of uncertainty in estimating rate constants Plug flow
Assuming plug flow transport is inherent to the procedure used to calculate the mineral rate constants. The experimental procedure respects this constraint by using a 5 cm by 10 m flotation column which reasonably well approaches this transport pattern [15]. Collection zone / froth zone interaction
The rate constants ideally refer to the collection process. However, the experimental procedure as described, means the froth zone is present when the measurement is made. It is known that a solids dropback from the froth occurs and a recycle between the two zones is established. Finch and Dobby [I 3] presented a method of extracting the collection zone rate constants from the overall (measured) rate constant. Feed characteristics
Clearly it is desirable to test the feed "as is", i.e. as it was sampled from the plant. However this may mean that the system is at its carrying capacity limit, in which case the feeds needs diluting. An increasing slope of the recovery vs time plot reveals a system at its carrying capacity limit (upper curves of Fig. 8). It is important to establish that diluting the feed (i.e. change pulp density) does not influence the actual separation. Particle settling
A second consideration is related to the residence time determination. The scale-up model requires the residence time of the particles in the collection zone. However, in the experimental procedure, the liquid residence time is calculated. For large and/or dense particles, corrections for particle settling velocity may be required. The model proposed by Masliyah [16] is used in the column simulator to calculate particle settling velocity, usually using the 80% passing size as a measure of particle diameter. Gas hold-up Part of the collection zone is occupied by gas, thus the true liquid residence time is shorter than the nominal one. However, gas hold-up will be different for each residence time setting, since this requires different liquid velocities. Since no reliable method is currently available to predict the gas holdup in a flotation column, for a given set of conditions the simplification is to ignore the gas holdup and use nominal residence times. This implies that, if the experimental results are to be reproduced by the simulator (e.g. to check pilotscale results), a zero gas hold up should selected. Gangue rate constants
Assays are often not made for the gangue, it being estimated by difference. The mass balancing should be made on all mineral species (including the gangue estimation) and the computer program should provide the option "complete analysis" (i.e. sum of all mineral contents equal 100%). Otherwise gangue predictions must be treated with caution. One consequence of this is that experimental grades and recoveries generally cannot be simultaneously reproduced by the simulator. Since the amenability testing has established
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improved performance in the columns (otherwise scale-up studies are probably not warranted), the emphasis of the simulation should be rather focused in determining a configuration to achieve the desired recovery. By adjusting the froth dropback factor, simultaneous grade/recovery fitting is reported to be possible [17].
Non-first order In many cases, first order kinetics does not fit experimental data well, i.e. flotation rate constants are recovery dependent. Usually a two-slope straight line better represents this non-linear process. This can be interpreted as two species of the same material floating at different rates, one fast and one slow. Two options for scale-up are possible: i) to use the slowest rate constant values (conservative approach), or i i ) t o use the rate constants corresponding to the target recovery (usual approach). When a single rate constant does not represents the entire process, the simultaneous simulation of grade and recovery becomes even more difficult.
Examples of application As noted in the introduction, the procedures described here, have been used at a number of locations; perhaps the most well documented to date is the experience at Mount Isa Mines Ltd. (Australia). At Mount Isa Mines three column circuits, each having three columns in series, have been commissioned based on laboratory-scale column data and calculations performed on the scale-up simulator [11]. Details of the commissioning of one these circuits, the low grade middlings, are given elsewhere [9]. The scale-up was largely determined by the carrying capacity limitations. The downturn of the percent remaining (100-recovery) vs time curve obtained during rate constant determination for the low grade middlings tests is an evidence of overloading of the bubbles by the fine particles (Fig. 8). This limitation led to dilution of the feed prior to any rate constant determination test. It became obvious then that the carrying capacity tests must be completed before rate constant determinations in order to establish the feed percent solids which permits a kinetic process. Diluting the feed and processing in a single column gave slopes similar to that in the second column in the undiluted case (bottom curves of Fig. 8). The concentrate production rate (carrying capacity) was relatively independent of the feed rate, a typical finding for fine particles, and a value of 4.5 g / m i n / c m 2 was used in the simulator. Figure 9 shows the original low grade middlings circuit at Mount Isa Mines Ltd. and the current column flotation circuit. Amenability testing results are summarized in Figure 5, while large scale column results (after commissioning) are given in Figure 10. It can be seen that the required capacity and target performance, greater than 80% Zn recovery and greater than 45% Zn + Pb grade, were met. The new circuit has contributed about a 2% increase in Zn recovery to the bulk concentrate with a similar Pb recovery as before. At Falconbridge Ltd. (Strathcona Concentrator), scale-up tests on five streams were completed [18]. Two 2.1 m dia. columns have been commissioned for processing copper rougher concentrate (CRC). Due to the coarseness of these feeds (relative to those at Mount Isa Mines) the collection process was found not be limited by the carrying capacity but rather by the kinetics (the carrying capacity value found experimentally for the CRC stream was 7.5 g / m i n / c m 2). The coarseness of the particles made it necessary to correct for particle retention time.
Flotation column amenabifity
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REFERENCES
Polinsky, S., presentation at the Eng. Found. Conf., Florida (December 1989).
.
2.
Newell, A., Gray, D. & Alfred, R. The application of flotation columns to gold recovery at Paddington Gold Mine, W.A., presented at Precious Metals Syrup., Montreal (1989).
.
Clingan, B.V. & MacGregor, D.R., Column flotation experience at Magma Copper Co., Mineral and Metallurgical Processing, 3 (3), 121-125 (1987).
.
Hoffert, J., Discussion on froth zone parameters. In Feasby (Ed.), Proceedings of Column Flotation Symposium, Trail, B.C., 84 (Nov. 2-4 1987). Amenluxen, R., private communications (1990).
.
6.
Espinosa-Gomez, R., Johnson, N.W. & Finch, J.A., Evaluation of flotation column scale-up at Mount Isa Mines Limited. Minerals Engineering, 2, No.3,369-375 (1989).
.
Attia, Y.A. & S. Yu, Feasibility of separation of coal flocs by column flotation, in Sastry, K.V.S. (Ed.), Column Flotation '88, SME, 249-256 (1988).
.
Del Villar, R. & Finch, J.A., Strathcona Column Flotation Project, Amenability and Scale-up Testing. Report submitted to Falconbridge Ltd. (October 1988).
.
Espinosa-Gomez, R., Johnson, N.W., Pease, J.P., & Munro, P.D., The commissioning of the first flotation columns at Mount Isa Mines Limited, in Dobby,G.S. and Rap, R.S. (Eds), Processing of Complex Ores, Pergamon Press, 293-302 (1989).
10.
Espinosa-Gomez, R., Finch, J.A., & Johnson, N.W., Column flotation of very fine particles, Minerals Engineering, 1, No.I, 3-18 (1988).
11.
Del Villar, R., Finch, J.A., Yianatos, J.B. & Laplante, A.R., Column Flotation Simulation. In Fytas, Collins and Singhal (Ed.), Computer Applications in the Mineral Industry, Balkema, Rotterdam, 233-239 (1988).
12.
Dobby, G.S. & Finch, J.A., Flotation column scale-up and modelling, CIM Bull., Vol.79, N ° 889, 89-96 (1986).
13.
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14.
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