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Troubleshooting industrial flotation columns
MineraLvEngineering,Vol. 8, No. 12, pp. 1593-1605,1995
Copy~ght O 1995 E|~vier Science Ltd Printed in Great Britain. All fights rc~rved
Pergamo n 0892-6875(95)00121-2
o892-6875/95 $9.5o+0.oo
TROUBLESHOOTING INDUSTRIAL FLOTATION COLUMNS
J. B. YIANATOS and L. G. BERGH Cheraical Engineering Department, Santa Marfa University, Valparaiso, Chile
(Received 16 June 1995; accepted 1 August 1995)
ABSTRACT
Since the early 1980s flotation columns have been progressively incorporated in milling operations all over the worM. Improvements in final concentrate grade, using single cleaning stages, have been in most cases the reason for using this new technology. However, the main disadvantage with respect to conventional mechanical cells is the large spread oJ results, mainly in terms of recovery, which is normally compensated by a high circulating load and high capacities (overdesign). Besides ,normal changes in feed grade and flowrate, and the need for periodic maintenance for gas spargers, the following common troubles in plant practice are discussea: i!mproper calibration and maintenance of instrumentation to measure froth depth and gas rate ,bias definition and estimation uneven wash water distribution uneven froth depth in baffled columns lack of robustness of control strategies Proper boundaries for superficial wash water rate (0.1-0.2 cm/s), froth depth (0.5-1.0 m) and superficial gas rate (1-2 cm/s), based on fundamental knowledge and experience in large size columns, are suggested for stable operation. Unfortunately, the lack of coordination between these variables is an important limitation for metallurgical improvements. The effect of altitude on gas rate and gas holdup in flotation columns operating at 1500-4500 m over the sea level is analysed. Keywords
Flotation column, bias, wash water, gas rate, froth depth, altitude
INTRODUCTION
Chilean experience in column flotation includes the operation of more than 40 industrial columns, mainly related to C u - M o bulk concentration and C u - M o separation, among them 18 of the largest columns in the world of 16 m 2 in cross-section and 14 m height. Plant experiences have shown significant advantages Presented at MineralsEngineering '95, St lves, Cornwall, England, June 1995
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related to metallurgical improvements, circuit simplification and savings in capital and operating costs. Industrial columns have been characterized from hydrodynamic and metallurgical tests providing useful information for column diagnosis, optimization and operator training [I-8]. Fundamental work has been developed in order to improve the knowledge of key variables for scale-up and design purposes. Mass balance adjustment and static and dynamic column flotation simulation have been implemented in software packages to process the experimentally collected data. Typical operating data for medium and large size columns are discussed, as well as the main column design characteristics and operating troubles.
COLUMN M E T A L L U R G I C A L PERFORMANCE Recovery A typical range of 50--80% overall recovery has been observed from several industrial column operations, particularly in Cu/Mo bulk cleaner circuits. In many cases, a significant spread of data (±10 % absolute recovery) was found, firstly because the columns are very sensitive to changes in feed flowrate, mineralogy and feed grade, and secondly because of the poor accuracy of the operational variable measurements (level, flowrates) and the lack of robustness of the control strategies. Figure 1 shows a typical spread of grade-recovery data from a large size column operated under stabilizing control conditions. I00 IIm
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Cu Incremental Grade, % Fig. 1 Recovery vs incremental grade in a large size column Column: 16 m 2 cross-section, 14 m height A key point is also the froth recovery. Plant tests developed in a column, 0.9 m diameter and 15 m height, operating as third cleaner in a Cu-Mo separation circuit, showed a molybdenite froth recovery of 70-a:l 1%, while the chalcopyrite froth recovery was 58±11%, averaged from 9 tests. These results confirm the impact of froth drop back, as well as the potential for molybdenite upgrading along the froth. For example, Figure 2 shows the grade profiles in the column operating with a shallow froth depth. Figure 3 shows the estimates of molybdenite and chalcopyrite recoveries, in the froth zone, derived from grade profile information and column operational conditions, i.e. overall mass flowrates and pulp density profile along the froth [7]. Figure 4 shows the grade profiles of a Cu/Mo bulk cleaner column [8]. In this case a significant molybdenite drop back was observed along the 1 m froth depth. The moly drop back can be attributed to preferential reagent conditioning for copper recovery, competition between copper minerals and molybdenite, as well as the high superficial wash water rate, Jw=0.3 cm/s.
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Grade A typical upgrade of 2--4% in final copper concentrates was achieved by replacing mechanical cells for columns in cleaner circuits.
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J. B. Yianatos and L. G. Bergh
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C O L U M N DESIGN C H A R A C T E R I S T I C S Column Size and Shape The largest columns installed in Chile are 16 m 2 in cross-section and 14 m height, and they are square (4x4 m) or rectangular (2x8 m). Chilean columns up to 3 m equivalent diameter, and 7-8 m 2 in cross-section, are circular, square or rectangular. Large size columns above 8 m 2 in cross-section are only square or rectangular. The main reasons for using a rectangular shape in large size columns, despite the larger weight and cost, are: to to to to
provide larger lip length and simpler internal launders simplify the design (single size), installation and removal of gas spargers simplify the design and installation of baffles and feed entrance minimize floor space
Gas Sparger Originally, gas spargers were made of perforated rubber or filter cloth. Metallurgical results using this type of sparger were good, however the main disadvantage was the need to shutdown and to empty the columns in order to remove the spargers for maintenance. According to the local water quality, experience showed that maintenance of spargers, because of plugging or failure, could range from a few days up to 2-3 months. Common failures, due to rupture of some sparger, can be detected by a significant pressure decrease in the gas manifold. Thus, after sparger checking, the damaged sparger can be isolated to recover the manifold pressure and to continue the operation temporary until maintenance. Subsequently, a new generation of gas spargers was introduced, called Turbo-Air USBM/CII and Cominco, typically consisting of 1 inch tubes, I-2 m length, where a pressurized (60-80 psi) gas-water mixture is introduced and delivered into the column through small orifices, about 1 mm diameter. This arrangement allowed on-line removal of the gas sparger tubes, during column operation. In most plants the gas spargers are removed periodically for inspection, every 1-2 weeks, in order to check orifice plugging or waste.
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Inspections have shown that a significant percentage of orifices, above 50%, can be plugged if no periodical maintenance is made [1]. The main causes of orifice plugging are the poor water quality (salt content and lack of filtering) and orifice obstruction with pulp due to improper operation of the air/water control system. Partial or total orifice plugging increases the sparger pressure significantly and sometimes causes the lost of the carbon-tungsten inserts used to prevent orifice waste. It was observed that the lost of inserts causes a rapid waste and generates fissures, 10--20 cm, that can finally break the sparger tube [1]. In several plants, the operators decided to shut down the water because of operational troubles with the air/water control sys'tem and the poor water quality that increases orifice plugging. The operators feel that water suppression does not reduce metallurgical performance. However, to the authors' knowledge, if columns are operating under a stabilizing control strategy with a significant spread of metallurgical results, no reliable information on the impact of these decisions can be found. However, plant tests developed in large size columns have confirmed that gas holdup increases by adding water into the spargers [1]. Figure 5 shows the air holdup profiles measured along the collection zone of an industrial column, operating at constant superficial gas rate, Jg=2 cm/s, and average pulp density of 1.1 g/cc. Here, it can be seen that average air holdup increased 3--4% by increasing the water addition to the spargers, 2-49 IJmia. The higher air holdup is related to the generation of smaller bubbles.
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Fig.5 Effect of water added into air spargers on air holdup profiles Column: 16 m 2 cross-section, 14 m height Recently, pilot scale tests to simulate the effect of sparger plugging have been developed [9]. Here, a comparative study was made by decreasing the orifice section (scale simulation) and decreasing the number of holes (blockade :;imulation). In both cases, it was found that keeping a constant gas rate at a constant pressure, as is usual in plant practice, decreases the gas holdup. Alternatively, it seems that a better approach is to maintain a constant water/air flowrate ratio until the sparger pressure reaches a maximum and then to proportionally decrease both. In this way the bubble size will remain smaller during the time of sparger operation. An alternative air sparger, called Minnovex, has been introduced more recently. This sparger operates only with air and consisr~s of a short tube, 1 inch diameter, provided with a movable conical plug at the end. Thus, the annular section for air sparging can be manually regulated in order to keep a constant pressure, and an approximately constant gas flowrate, for each sparger. The sparger has been tested and incorporated in several plants. However, reliable data on comparative sparger performance have not yet been reported.
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J.B. Yianatos and L. G. Bergh
Wash Water Distribution
Design, location and operating conditions of the wash water distributor is critical. The degree of froth mixing, channelling and shortcircuiting is mainly produced by wash water maldistribution and improper air rate and froth depth control. Inefficient wash water distribution is a common fault which limits the column metallurgical performance. Two kinds of water maldistribution are found: a) temporary plugging of a well designed device; and b) poorly designed device. In large size columns, a bad design means that the water is not added in the right amount at the right place. In fact, regarding the cleaning action, the wash water requirement decreases with the distance to the overflow lip. Thus, an homogeneous water distribution does not seem to be the best approach. However, the operators are unaware of the problem because they have no information about the internal state of the froth. The original design of the wash water distributor, developed by the Canadian Flotation Column Co., consisted of a tubular arrangement located around 10-20 cm below the lip level. Tubes were provided with two series of orifices, 2-3 mm in diameter, which direct the water downwards in a 90 degree angle. The advantage of this arrangement is the lower wash water requirement to maintain a positive bias, or a net downward water flowrate, thus a higher solid percentage can be obtained in the concentrate. The main disadvantage of this arrangement is that orifices in direct contact with mineralized froth are easily plugged and water maldistribution can not be directly observed. An alternative design consists of a water rain from an open tank provided with a perforated bottom, for example a rubber bottom with orifices of 3 mm in diameter. The tank is located about 30 cm above the froth. This arrangement allows for on-line observation of the water distribution and the orifices are not contaminated with the froth. In some cases the use of recycled water containing chemical reagents and flocculant, together with the water delivery by gravity, contributes to plugging the orifices [1]. Also, it may not be appropriate for applications with weak, easily to collapsible froths [10]. Another disadvantage is that visual checking and access to the froth overflow is greatly restricted. Operators of large size columns claim they do not feel confident with arrangements that hinder the froth. An example is the packed column where the froth overflow has no visual access at all. It is common at present to observe the froth overflow by means of closed circuit TV, and new instrumentation is also being developed to provide on-line analysis of froth characteristics for operator support and control purposes [11, 12]. A better design for the water distributor consists of an horizontal tubular set, for example made of PVC, with orifices located in the lower part of the tubes or, in some cases, with orifices in the upper part of the tubes in order to get a rain of sprinkled water, like a fountain. The system can be operated above the froth, or submerged, because it is movable. It is also easy to remove for inspection and replacement of tubes. This kind of arrangement has been incorporated in several new projects in Chile and Brazil. Internal baffles and launders
The advantage of building a single large column instead of several small units in parallel is twofold, firstly savings in capital cost (column and instrumentation) and savings in operating costs (simplified circuits and control). However, in order to provide a good distribution of pulp, gas and wash water, large columns are divided in sections by means of internal baffles. Single sections of I-1.5 m equivalent diameter are typically used in industrial columns of 10-14 m height. Initially, large size columns only used baffles in the collection zone, above the gas spargers and below the pulp/froth interface. Later on, the significant effect that froth recovery has upon the overall column recovery was recognized. In order to enhance large column performance, baffles and internal launders have been incorporated in the froth zone. Thus, a maximum horizontal distance of about 1 m, from the central axis of each section to the overflow lip was provided. Some new designs consider continuous baffles along the whole column, as well as independent feed and gas entrances for each section. A column 2.5 m diameter, without baffles, was tested at MIM in Australia and results showed a nonhomogeneous feed water entrainment along the overflow lip [10]. This problem can be attributed to unbalanced gas distribution, considering that an homogeneous external wash water addition was provided.
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Some researchers h~tve suggested the use of temperature measurements to detect entrainment over the lip [13]. An additional problem, however, is related to the unbalanced level control between baffled sections, especially if working: with shallow froth depths. Improper design of baffles together with gas maldistribution, particularly in the froth zone, decreases the level control. Plant experience with large size columns, provided with continuous baffles showed that this problem can dramatically decrease the column recovery by 10--15% as well as increase reagent consumption. Another observation, from columns provided with discontinuous baffles in the collection and froth zones, consisted of sampling the column concentrate at different locations along the internal ard external launders [ 1]. The results showed differences of 5--6% in concentrate grades of values and insolubles. During these tests the observed external wash water distribution was homogeneous. Thus, it has been shown that gas maldistribution and level unbalance between baffled sections are responsible for a significant loss of recovery and grade. In order to prevent this problem a local pressure balance, near the inl:erface level, is required. Tailings flowrate control The use of pinch valves, for regulation of tailings flowrate, is a simple, economical and common practice. However, the correx:t selection of the valve and the proper tailings circuit design is also critical. Thus, a wrong selection of the valve, pipe diameter and/or elevation of the tailings discharge point has been a source of failures. Inadequate tailing flow control, limited capacity to withdraw the pulp and obstruction due to solid settling are common troubles. INDUSTRIAL COLUMN PARAMETERS
Gas rate In order to reach metallurgical targets of grade and recovery, there is a critical compromise between gas rate, froth depth and wash water rate. The typical range of superficial gas rate is 1-2 cm/s, and it must be referred to the local pressure conditions at the top of the column. This is an important point which has not been carefully discussed, particularly for columns located at altitudes of 15(gg-4500 m above sea level, quite common in Chile and Peru. For example, at sea level the local pressure is 100 kPa and the air expansion from the bottom to the top of a column is around 2. However, in the case of two columns located at 4340 m above sea level, in Peru, the local pressure was 58 kPa. Under these conditions, the air expansion from the bottom to the top of the columns was around 2.9 and the average gas holdup decreased. It was also observed that the larger air expansion decreased the maximum air flowrate that would be used to prevent fine particles entrainment. In plant practice, it is usual to find large errors in gas flowrate measurements. Typical sources of error are improper calibration and maintenance of the flowmeter, as well as inadequate knowledge of the pressure related to the measurement, and the lack of correction for standard conditions, for example conditions at the top of the coluran [14].
Froth depth In plant practice, it is quite common to find large errors in froth depth measurements. The authors have verified errors of -30% to +150% in froth depth estimates from industrial columns, without baffles, using a single pressure sensor for level control. The impact of gas rate and pulp density changes on the pressure sensor readings have been discussed elsewhere [3]. It seems trivial, but a common mistake is the confusion between the sensor calibration (i.e., rnA versus kPa) and the model adjustment relating the sensor signal to the process variables (i.e., mA versus froth depth). In order to obtain a reliable correlation, actual data from plant tests must be used. The froth depth is aormally regulated at 0.5--1.0 m, by operating the tailing discharge bottom valve. The effectiveness of the froth depth control depends upon the quality of the froth depth measurement, the proper
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J.B. Yianatos and L. G. Bergh
level balance between baffles and the proper operation of the tailings discharge circuit, including pipe design, valve selection and maintenance. Level equilibrium between baffled sections in the froth is critical, because a single sensor is used to measure the level of one section, i.e. 1 m 2, while the control action affects the whole column cross-section, i.e. 16 m 2. Wash water rate
The role of the wash water is to prevent pulp entrainment into the concentrate. In this sense, the minimum wash water should just supply the water to the concentrate. However, an excess of wash water increases froth mixing and shortcircuiting, thus decreasing froth cleaning, particularly for shallow froth depths. It is also important to verify the circuit water balance, because an excess of water will cause problems downstream [15]. For large size columns the typical range of superficial wash water rate is 0.1-0.2 cm/s, providing a water excess of 0.02-0.05 cm/s. These values are strictly related to the normal range of air rate, i.e. 1-2 cm/s measured at local atmospheric pressure, otherwise it is easy to produce a significant feed pulp entrainment into the concentrate. Bias definition and estimation
The definition of bias has been a matter of confusion between operators and researchers. The actual bias definition can be stated based on the following objectives: a)
stabilizing action by compensation of water drainage by gravity
b)
feed water replacement by fresh water in order to prevent solid entrainment
Thus, "bias" can be defined as the difference between the fresh water added near the top of the froth and the fresh water that goes into the concentrate. Ideally, the wash water should be just enough for mineral transport into the concentrate. However, the wash water split into the concentrate and the feed water displacement are not perfect. Consequently, a minimum fresh water excess must be provided. In a more simple definition the "bias" has been referred to as the net water flowrate passing across the pulp/froth interface, the water being either fresh or feed water. Accordingly, if the fresh water entering the column exceeds the water recovered into the concentrate there is a net downward water flowrate in the froth and this condition is normally referred to as "positive bias", while a "negative bias" corresponds to a net water flowrate moving upwards into the froth. Operation with "positive bias" is not sufficient to prevent mineral entrainment into the concentrate. The main problem is froth mixing and shortcircuiting, which is strongly related to the wash water distribution, wash water and air flowrates, and froth depth. The critical interaction between gas rate, wash water rate and froth depth on feed water entrainment into the froth has been observed. The methodology consisted of introducing a step KC1 tracer signal into the feed water, in an air-water system, at laboratory scale [2]. The presence of tracer along the froth was measured by electrical conductance, at steady state. The effect of gas holdup changes on electrical conductance was cancelled considering an adimensional entrainment factor relative to the same operation without tracer. The effect of tracer on gas holdup was measured independently by pressure difference and it was found negligible. Thus, an entrainment factor equal to one means that there is no contamination (fresh water). Figures 6 and 7 show the distributed characteristics of the feed water entrainment along the froth (0.7 m depth) at constant wash water rates, Jw--0.5 and 0.2 cm/s, respectively. Figure 6 shows that there was almost no contamination along the froth at gas rate Jg=l cm/s and bias rate Jb=0.2 cm/s. However, at Jg=2 ends, the feed water entrainment increased and the bias rate decreased to Jb----Ocrn/s.
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Fig.6 Effect of superficial gas rate on feed water entrainment, at constant wash water rate, Jw--0.5 cm/s Figure 7 shows a significant froth contamination at Jg=l cm/s, despite the froth overflow not being contaminated. At ./g=2 cm/s, the froth was strongly contaminated up to the overflow level, and the bias became negative, Jb= -0. I cm/s. Here, it is clearly shown that froth contamination with feed water is not only dependent upon the bias rate but also upon the gas rate and froth depth. The froth contamination becomes critical at Jb -< 0cm/s. These results are in good agreement with previous observations of the internal state of the froth, where the effect of gas holdup was not considered [16]. Discrete measurements of the overflow concentrate grades, as used in plant practice, are a little sensitive for predicting potential troubles, and the manual feed back control, without a good coordination between variables, cannot be expected to give a proper metallurgical performance. 5 4.5 eO~v" / /
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J.B. Yianatos and L. G. Bergh
into the concentrate [13]. Thus, a positive "pulp bias" provides much higher wash water than required. Also the normal error in pulp flowrate measurement gives a poor estimate of the "pulp bias" because the flowrate difference is strongly affected by error propagation [13]. Another source of trouble arises in estimating the instantaneous bias for control purposes, because the bias B is commonly defined as the difference between feed flowrate F and tailings flowrate T, assuming steady state, B = T-F
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where, A c is the column cross-section, H is the pulp height and ~g is the gas holdup in the pulp zone. Accordingly, an industrial experience with a control strategy consisting of the following interactive loops: 1)
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"pulp bias" control, i.e. T-F, manipulating the wash water flowrate, showed a critical disturbance on the wash water addition because the transient condition was not accounted for.
However, bias control strategies should be adjusted depending on the flotation objectives, for instance in rougher application the use of negative bias can be a good compromise [17], as well as in the ease of reverse flotation of iron from silica. Gas Holdup and Bubble Diameter Gas holdup profiles along industrial flotation columns have been determined from pressure profiles measurements [3]. Tests were developed in columns of 1, 13 and 16 m 2 in cross-section, and 14 m height. In all cases it was found that gas holdup increases, from the bottom to the top of the pulp zone, almost proportionally to the gas expansion because of the decrease in hydrostatic pressure. Gas holdup expansion was about 1:2 for columns located at sea level, while it was closer to 1:3 at 4500 m above sea level. Average gas holdup in the collection zone varies in the range of 10--20% at normal gas rates of 1-2 cm/s. Average bubble sizes in the range of 1-2 mm have been estimated from air holdup measurements in industrial columns. Direct observation of industrial bubble size distributions, by sampling and image analysis from video film, agreed well with simple theoretical estimates [5]. Residence Time Distribution and Mixing Liquid, solid and gas RTD have been measured in industrial flotation columns using radioactive tracer techniques [4,6]. The main results showed that residence time of solids is strongly dependent on particle size. For example, non-floatable particles of 100 microns showed an average residence time that was only a half that of the liquid. Also, the average residence time of floatable mineral reporting to the concentrate was closer to the gas residence time [6]. This result showed that most of the floatable mineral was rapidly collected near the feed entrance. However, the floatable mineral recovered into the tailings flowrate showed an average residence time similar to that of the gangue. The liquid mixing in a column of 0.9 m diameter and 15 m height, operating at 1.8 cm/s superficial air rate, was reported equivalent to 1.25 perfect mixers in series [18]. The liquid mixing in a column of 2.5m diameter and 14 m height, without baffles, was closer to a perfect mixer [10]. Move recently, the liquid mixing in a large size column of 16 m 2 cross-section and 14 m height, provided with baffles between the
Troubleshooting industrial flotation columns
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feed and air entrances, was found equivalent to 1.1 perfect mixers in series [ 1 ]. Figure 8 shows the liquid residence time distribution of this column, using LiCI as tracer, while operating at 1.9 cm/s superficial air rate. Here, the significant recovery of tracer into the concentrate showed that the column was operating under negative bias. 2.¢
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Fig.8 Residence time distribution of a large size column Column: 16 m 2 cross-section, 14 m height
Particle entrainment and shortcircuiting Entrainment of fine particles from the pulp to the froth zone was observed by using radioactive tracer techniques [4]. Fo:: this purpose, actual fine particles of gangue were irradiated and introduced with the feed flowrate. Direct measurement of the presence of tracer, just above and below the pulp/froth interface, showed.that fine gangue particles were almost completely washed down very close to the interface level, under normal operating conditions, i.e., Jg=l.8 cm/s and 1 m froth depth. Thus, it was demonstrated that a very effective cleaning action can be achieved if proper conditions of gas rate, wash water rate and froth depth are selected, from which a distinct pulp/froth interface can be obtained. Alternatively, the operation with a shallow froth depth (50 cm), high air rate (2 era/s) and low wash water rate (0.1 cm/s), will favour the feed water entrainment into the concentrate, as was the case shown in Figure 8. Good agreement between this result and entrainment conditions from Figures 6 and 7 can be observed. However, a significant gas shortcircuiting into the tailings flowrate of an industrial column was observed using radioactive gas tracer [6]. Similar tests in a pilot column did not show gas shortcircuiting. This problem seems to be related to the design of the column bottom and gas sparger location.
Control Strateg~ To operate a coltDmn, only pulp level control is needed. Rough stabilization can be achieved despite the presence of gross measurement error in froth depth and the constraints posed by PI controllers. Metallurgical optimization is a long way off unless improvements in measurement of metallurgical performance, i.e. online grade estimates, and development of flexible control strategies are made. Common PID distributed control cannot take up the challenge of a proper coordination between a non linear and strongly interactive multivariable profess. Process data of improved quality, normally communicated to a host computer, should be managed in an integrated environment where alternative control actions can be selected by hierarchical rules.
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J.B. Yilmtos a~ L. O. Bergh CONCLUSIONS
The main disadvantage of flotation columns with respect to conventional mechanical cells is the large spread of results, mainly in terms of recovery, which is normally compensated by a high circulating load and high capacities (overdesign). Presently, fundamental knowledge of column flotation and its constraints allows for more reliable design procedures as well as proposing new challenges in column flotation conU'ol and optimization. Besides the normal changes in feed composition and flowrates, and the need for periodic maintenance of gas spargers, the following common sources of troubles in plant practice are: •
improper calibration and maintenance of instrumentation to measure froth depth and gas rate
•
bias estimation and control
•
uneven wash water distribution
•
uneven froth depth in baffled columns
•
lack of robustness of control strategies
Proper boundary conditions for superficial wash water rate, 0.1-0.2 cm/s, froth depth, 0.5-1.0 m and superficial gas rate, 1-2 cm/s, are well established for large size columns. Unfortunately, the lack of coordination between these variables is an important limitation for metallurgical improvements.
ACKNOWLEDGEMENTS We would like to thank the Consejo Nacional de Investigaci6n Cientffica y Tecnol6gica (Projects 1950551 and FONDEF MI-17) and the Santa Mafia University (Project 952723) for their financial support.
REFERENCES
Nufiez, R., Estudio experimental de columna de flotaci6n de cobre de minera Escondida Ltd.,
°
2. 3. 4.
5. 6.
.
8.
.
Metallurgical Engineering Thesis, Santa Mafia University, Valparafso, Chile (1993). Pereira, M.J., O proceso de transporte na zona de espumas da coluna de flutuactao, Master Thesis, Technical University of Lisboa, Portugal (1993). Yianatos, J.B., Bergh, L.G., Sepdlveda, C. & Ndfiez, R., Measurement of axial pressure profiles in large-size industrial flotation columns, Minerals Engineering, 8, 1/2, 101-109 (1994). Yianatos, J.B. & Bergh, L.G., RTD studies in an industrial flotation column: use of the radioactive tracer technique, International Journal of Mineral Processing, 36, 81-91 (1992). Yianatos, J.B. & Pueile, P., Development and calibration of a flotation bubble size sensor, Avances en Tecnologfa Mineral, Vol. IV, S. Castro, Ed., University of Concepci6n, Chile, 157-164 (!994). Yianatos, J.B., Bergh, L.G., Dur(m, O.U., Dfaz, F.J. & Her~si, N.M., Measurement of residence time distribution of the gas phase in flotation columns, Minerals Engineering, 7, 2/3, 333-344 (1994). Sepdlveda, C., Diagntstico de variables de diseflo y operaci6n en columnas de flotaci6n, Chemical Engineering Thes/s, Santa Marfa University, Valparaiso, Chile (1994). Cort6s, F.L., Yianatos, J.B. & Urtubia, H.C., Characterization of the copper-moly collective flotation circuit at Andina Division, Codelco-Chile, COPPER'95, Nov. 26-29, Santiago, Chile (1995). Kappes, M., Estudio de sistema lmdmjeador, Internal Report, Chem. Eng. Dept., Santa Maria University, Valparaiso (1995).
Troubleshoe~agiadusldalflotationc.olunms 10.
II. 12.
13. 14. 15. 16. 17. 18.
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Espinos~ R.G., Johnson, N.W., Pease, J.D. & Munro, P.D., The commissioning of the first flotation columns at Mount Isa Mines, Processing of Complex Ores, Dobby, G.S. & Rao, S.R., Eds., Pergamon Press, 293--302 (1989). Guarini, M., Soto, A., Cipriano, A., Ohnedo, J., Guesalaga, A. & C_.6ceres,J., Caracterfsticas de espuma en celdas de flotaci6n, Minerfa Chilena, 14, N°166, 81-85 (1995). Moolman, D.W., Aldrich, C., Van Deventer, J.S.J. &; Stallge, W.W., Digitsd image processing as a tool for on-line monitoring of froth in flotation plants, Minerals Engineering, 7, 9, 1149-1164 (1994). Finch, J A. & Dobby, G.S., Column Flotation. 1st Edn. 148, Pergamon Press (1990). Falutsu, M., Direct measurement of gas rate in a flotation machine, Minerals Engineering, 7, 12, 1487-14.94 (1994). Huls, B.J. & Williams, S.R., Limitations in the Application of Column Flotation, XVIII Internat.;.onal Mineral Processing Congress, Sydney, Australia, 23-28 May, 779-784 (1993). Uribe-S~das, A., G6mez, C.O. & Finch, J.A., Bias detection in flotation columns, Colwnn'91 Conference, Agar, Huls & Hyma, Eds., Sudbury, Canada, 2, 391--407 (1991). Furey, J.T., Rougher column flotation of gold tellurides. CMP Conference, Ottawa, Canada, Jan. 1990). Mills, P.J.T., Yianatos, J.B. & O'Connor, C.T., The mixing characteristics of solid and liquid phases in flotation column, Minerals Engineering, 5, 10/12, 1195-1205 (1993).
Copy~ght O 1995 E|~vier Science Ltd Printed in Great Britain. All fights rc~rved
Pergamo n 0892-6875(95)00121-2
o892-6875/95 $9.5o+0.oo
TROUBLESHOOTING INDUSTRIAL FLOTATION COLUMNS
J. B. YIANATOS and L. G. BERGH Cheraical Engineering Department, Santa Marfa University, Valparaiso, Chile
(Received 16 June 1995; accepted 1 August 1995)
ABSTRACT
Since the early 1980s flotation columns have been progressively incorporated in milling operations all over the worM. Improvements in final concentrate grade, using single cleaning stages, have been in most cases the reason for using this new technology. However, the main disadvantage with respect to conventional mechanical cells is the large spread oJ results, mainly in terms of recovery, which is normally compensated by a high circulating load and high capacities (overdesign). Besides ,normal changes in feed grade and flowrate, and the need for periodic maintenance for gas spargers, the following common troubles in plant practice are discussea: i!mproper calibration and maintenance of instrumentation to measure froth depth and gas rate ,bias definition and estimation uneven wash water distribution uneven froth depth in baffled columns lack of robustness of control strategies Proper boundaries for superficial wash water rate (0.1-0.2 cm/s), froth depth (0.5-1.0 m) and superficial gas rate (1-2 cm/s), based on fundamental knowledge and experience in large size columns, are suggested for stable operation. Unfortunately, the lack of coordination between these variables is an important limitation for metallurgical improvements. The effect of altitude on gas rate and gas holdup in flotation columns operating at 1500-4500 m over the sea level is analysed. Keywords
Flotation column, bias, wash water, gas rate, froth depth, altitude
INTRODUCTION
Chilean experience in column flotation includes the operation of more than 40 industrial columns, mainly related to C u - M o bulk concentration and C u - M o separation, among them 18 of the largest columns in the world of 16 m 2 in cross-section and 14 m height. Plant experiences have shown significant advantages Presented at MineralsEngineering '95, St lves, Cornwall, England, June 1995
1593
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J.B. Yianatos and L. G. Bergh
related to metallurgical improvements, circuit simplification and savings in capital and operating costs. Industrial columns have been characterized from hydrodynamic and metallurgical tests providing useful information for column diagnosis, optimization and operator training [I-8]. Fundamental work has been developed in order to improve the knowledge of key variables for scale-up and design purposes. Mass balance adjustment and static and dynamic column flotation simulation have been implemented in software packages to process the experimentally collected data. Typical operating data for medium and large size columns are discussed, as well as the main column design characteristics and operating troubles.
COLUMN M E T A L L U R G I C A L PERFORMANCE Recovery A typical range of 50--80% overall recovery has been observed from several industrial column operations, particularly in Cu/Mo bulk cleaner circuits. In many cases, a significant spread of data (±10 % absolute recovery) was found, firstly because the columns are very sensitive to changes in feed flowrate, mineralogy and feed grade, and secondly because of the poor accuracy of the operational variable measurements (level, flowrates) and the lack of robustness of the control strategies. Figure 1 shows a typical spread of grade-recovery data from a large size column operated under stabilizing control conditions. I00 IIm
90
i
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Cu Incremental Grade, % Fig. 1 Recovery vs incremental grade in a large size column Column: 16 m 2 cross-section, 14 m height A key point is also the froth recovery. Plant tests developed in a column, 0.9 m diameter and 15 m height, operating as third cleaner in a Cu-Mo separation circuit, showed a molybdenite froth recovery of 70-a:l 1%, while the chalcopyrite froth recovery was 58±11%, averaged from 9 tests. These results confirm the impact of froth drop back, as well as the potential for molybdenite upgrading along the froth. For example, Figure 2 shows the grade profiles in the column operating with a shallow froth depth. Figure 3 shows the estimates of molybdenite and chalcopyrite recoveries, in the froth zone, derived from grade profile information and column operational conditions, i.e. overall mass flowrates and pulp density profile along the froth [7]. Figure 4 shows the grade profiles of a Cu/Mo bulk cleaner column [8]. In this case a significant molybdenite drop back was observed along the 1 m froth depth. The moly drop back can be attributed to preferential reagent conditioning for copper recovery, competition between copper minerals and molybdenite, as well as the high superficial wash water rate, Jw=0.3 cm/s.
Troubleshooting industrial flotationcolumns
1595
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C o l u m n Depth, m Fig.3 Froth recovery estimates in a molybdenite cleaner column Column: 0.9 m diameter, 15 m height
Grade A typical upgrade of 2--4% in final copper concentrates was achieved by replacing mechanical cells for columns in cleaner circuits.
1596
J. B. Yianatos and L. G. Bergh
0.8
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Column Depth, m Fig.4 Grade profiles in a bulk copper/moly cleaner column Column: 13 m 2 cross-section, 14 m height
C O L U M N DESIGN C H A R A C T E R I S T I C S Column Size and Shape The largest columns installed in Chile are 16 m 2 in cross-section and 14 m height, and they are square (4x4 m) or rectangular (2x8 m). Chilean columns up to 3 m equivalent diameter, and 7-8 m 2 in cross-section, are circular, square or rectangular. Large size columns above 8 m 2 in cross-section are only square or rectangular. The main reasons for using a rectangular shape in large size columns, despite the larger weight and cost, are: to to to to
provide larger lip length and simpler internal launders simplify the design (single size), installation and removal of gas spargers simplify the design and installation of baffles and feed entrance minimize floor space
Gas Sparger Originally, gas spargers were made of perforated rubber or filter cloth. Metallurgical results using this type of sparger were good, however the main disadvantage was the need to shutdown and to empty the columns in order to remove the spargers for maintenance. According to the local water quality, experience showed that maintenance of spargers, because of plugging or failure, could range from a few days up to 2-3 months. Common failures, due to rupture of some sparger, can be detected by a significant pressure decrease in the gas manifold. Thus, after sparger checking, the damaged sparger can be isolated to recover the manifold pressure and to continue the operation temporary until maintenance. Subsequently, a new generation of gas spargers was introduced, called Turbo-Air USBM/CII and Cominco, typically consisting of 1 inch tubes, I-2 m length, where a pressurized (60-80 psi) gas-water mixture is introduced and delivered into the column through small orifices, about 1 mm diameter. This arrangement allowed on-line removal of the gas sparger tubes, during column operation. In most plants the gas spargers are removed periodically for inspection, every 1-2 weeks, in order to check orifice plugging or waste.
Troubleshootingindustrial flotationcolumns
1597
Inspections have shown that a significant percentage of orifices, above 50%, can be plugged if no periodical maintenance is made [1]. The main causes of orifice plugging are the poor water quality (salt content and lack of filtering) and orifice obstruction with pulp due to improper operation of the air/water control system. Partial or total orifice plugging increases the sparger pressure significantly and sometimes causes the lost of the carbon-tungsten inserts used to prevent orifice waste. It was observed that the lost of inserts causes a rapid waste and generates fissures, 10--20 cm, that can finally break the sparger tube [1]. In several plants, the operators decided to shut down the water because of operational troubles with the air/water control sys'tem and the poor water quality that increases orifice plugging. The operators feel that water suppression does not reduce metallurgical performance. However, to the authors' knowledge, if columns are operating under a stabilizing control strategy with a significant spread of metallurgical results, no reliable information on the impact of these decisions can be found. However, plant tests developed in large size columns have confirmed that gas holdup increases by adding water into the spargers [1]. Figure 5 shows the air holdup profiles measured along the collection zone of an industrial column, operating at constant superficial gas rate, Jg=2 cm/s, and average pulp density of 1.1 g/cc. Here, it can be seen that average air holdup increased 3--4% by increasing the water addition to the spargers, 2-49 IJmia. The higher air holdup is related to the generation of smaller bubbles.
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.
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Fig.5 Effect of water added into air spargers on air holdup profiles Column: 16 m 2 cross-section, 14 m height Recently, pilot scale tests to simulate the effect of sparger plugging have been developed [9]. Here, a comparative study was made by decreasing the orifice section (scale simulation) and decreasing the number of holes (blockade :;imulation). In both cases, it was found that keeping a constant gas rate at a constant pressure, as is usual in plant practice, decreases the gas holdup. Alternatively, it seems that a better approach is to maintain a constant water/air flowrate ratio until the sparger pressure reaches a maximum and then to proportionally decrease both. In this way the bubble size will remain smaller during the time of sparger operation. An alternative air sparger, called Minnovex, has been introduced more recently. This sparger operates only with air and consisr~s of a short tube, 1 inch diameter, provided with a movable conical plug at the end. Thus, the annular section for air sparging can be manually regulated in order to keep a constant pressure, and an approximately constant gas flowrate, for each sparger. The sparger has been tested and incorporated in several plants. However, reliable data on comparative sparger performance have not yet been reported.
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J.B. Yianatos and L. G. Bergh
Wash Water Distribution
Design, location and operating conditions of the wash water distributor is critical. The degree of froth mixing, channelling and shortcircuiting is mainly produced by wash water maldistribution and improper air rate and froth depth control. Inefficient wash water distribution is a common fault which limits the column metallurgical performance. Two kinds of water maldistribution are found: a) temporary plugging of a well designed device; and b) poorly designed device. In large size columns, a bad design means that the water is not added in the right amount at the right place. In fact, regarding the cleaning action, the wash water requirement decreases with the distance to the overflow lip. Thus, an homogeneous water distribution does not seem to be the best approach. However, the operators are unaware of the problem because they have no information about the internal state of the froth. The original design of the wash water distributor, developed by the Canadian Flotation Column Co., consisted of a tubular arrangement located around 10-20 cm below the lip level. Tubes were provided with two series of orifices, 2-3 mm in diameter, which direct the water downwards in a 90 degree angle. The advantage of this arrangement is the lower wash water requirement to maintain a positive bias, or a net downward water flowrate, thus a higher solid percentage can be obtained in the concentrate. The main disadvantage of this arrangement is that orifices in direct contact with mineralized froth are easily plugged and water maldistribution can not be directly observed. An alternative design consists of a water rain from an open tank provided with a perforated bottom, for example a rubber bottom with orifices of 3 mm in diameter. The tank is located about 30 cm above the froth. This arrangement allows for on-line observation of the water distribution and the orifices are not contaminated with the froth. In some cases the use of recycled water containing chemical reagents and flocculant, together with the water delivery by gravity, contributes to plugging the orifices [1]. Also, it may not be appropriate for applications with weak, easily to collapsible froths [10]. Another disadvantage is that visual checking and access to the froth overflow is greatly restricted. Operators of large size columns claim they do not feel confident with arrangements that hinder the froth. An example is the packed column where the froth overflow has no visual access at all. It is common at present to observe the froth overflow by means of closed circuit TV, and new instrumentation is also being developed to provide on-line analysis of froth characteristics for operator support and control purposes [11, 12]. A better design for the water distributor consists of an horizontal tubular set, for example made of PVC, with orifices located in the lower part of the tubes or, in some cases, with orifices in the upper part of the tubes in order to get a rain of sprinkled water, like a fountain. The system can be operated above the froth, or submerged, because it is movable. It is also easy to remove for inspection and replacement of tubes. This kind of arrangement has been incorporated in several new projects in Chile and Brazil. Internal baffles and launders
The advantage of building a single large column instead of several small units in parallel is twofold, firstly savings in capital cost (column and instrumentation) and savings in operating costs (simplified circuits and control). However, in order to provide a good distribution of pulp, gas and wash water, large columns are divided in sections by means of internal baffles. Single sections of I-1.5 m equivalent diameter are typically used in industrial columns of 10-14 m height. Initially, large size columns only used baffles in the collection zone, above the gas spargers and below the pulp/froth interface. Later on, the significant effect that froth recovery has upon the overall column recovery was recognized. In order to enhance large column performance, baffles and internal launders have been incorporated in the froth zone. Thus, a maximum horizontal distance of about 1 m, from the central axis of each section to the overflow lip was provided. Some new designs consider continuous baffles along the whole column, as well as independent feed and gas entrances for each section. A column 2.5 m diameter, without baffles, was tested at MIM in Australia and results showed a nonhomogeneous feed water entrainment along the overflow lip [10]. This problem can be attributed to unbalanced gas distribution, considering that an homogeneous external wash water addition was provided.
Troubleshooting industrial flotationcolumns
1599
Some researchers h~tve suggested the use of temperature measurements to detect entrainment over the lip [13]. An additional problem, however, is related to the unbalanced level control between baffled sections, especially if working: with shallow froth depths. Improper design of baffles together with gas maldistribution, particularly in the froth zone, decreases the level control. Plant experience with large size columns, provided with continuous baffles showed that this problem can dramatically decrease the column recovery by 10--15% as well as increase reagent consumption. Another observation, from columns provided with discontinuous baffles in the collection and froth zones, consisted of sampling the column concentrate at different locations along the internal ard external launders [ 1]. The results showed differences of 5--6% in concentrate grades of values and insolubles. During these tests the observed external wash water distribution was homogeneous. Thus, it has been shown that gas maldistribution and level unbalance between baffled sections are responsible for a significant loss of recovery and grade. In order to prevent this problem a local pressure balance, near the inl:erface level, is required. Tailings flowrate control The use of pinch valves, for regulation of tailings flowrate, is a simple, economical and common practice. However, the correx:t selection of the valve and the proper tailings circuit design is also critical. Thus, a wrong selection of the valve, pipe diameter and/or elevation of the tailings discharge point has been a source of failures. Inadequate tailing flow control, limited capacity to withdraw the pulp and obstruction due to solid settling are common troubles. INDUSTRIAL COLUMN PARAMETERS
Gas rate In order to reach metallurgical targets of grade and recovery, there is a critical compromise between gas rate, froth depth and wash water rate. The typical range of superficial gas rate is 1-2 cm/s, and it must be referred to the local pressure conditions at the top of the column. This is an important point which has not been carefully discussed, particularly for columns located at altitudes of 15(gg-4500 m above sea level, quite common in Chile and Peru. For example, at sea level the local pressure is 100 kPa and the air expansion from the bottom to the top of a column is around 2. However, in the case of two columns located at 4340 m above sea level, in Peru, the local pressure was 58 kPa. Under these conditions, the air expansion from the bottom to the top of the columns was around 2.9 and the average gas holdup decreased. It was also observed that the larger air expansion decreased the maximum air flowrate that would be used to prevent fine particles entrainment. In plant practice, it is usual to find large errors in gas flowrate measurements. Typical sources of error are improper calibration and maintenance of the flowmeter, as well as inadequate knowledge of the pressure related to the measurement, and the lack of correction for standard conditions, for example conditions at the top of the coluran [14].
Froth depth In plant practice, it is quite common to find large errors in froth depth measurements. The authors have verified errors of -30% to +150% in froth depth estimates from industrial columns, without baffles, using a single pressure sensor for level control. The impact of gas rate and pulp density changes on the pressure sensor readings have been discussed elsewhere [3]. It seems trivial, but a common mistake is the confusion between the sensor calibration (i.e., rnA versus kPa) and the model adjustment relating the sensor signal to the process variables (i.e., mA versus froth depth). In order to obtain a reliable correlation, actual data from plant tests must be used. The froth depth is aormally regulated at 0.5--1.0 m, by operating the tailing discharge bottom valve. The effectiveness of the froth depth control depends upon the quality of the froth depth measurement, the proper
1600
J.B. Yianatos and L. G. Bergh
level balance between baffles and the proper operation of the tailings discharge circuit, including pipe design, valve selection and maintenance. Level equilibrium between baffled sections in the froth is critical, because a single sensor is used to measure the level of one section, i.e. 1 m 2, while the control action affects the whole column cross-section, i.e. 16 m 2. Wash water rate
The role of the wash water is to prevent pulp entrainment into the concentrate. In this sense, the minimum wash water should just supply the water to the concentrate. However, an excess of wash water increases froth mixing and shortcircuiting, thus decreasing froth cleaning, particularly for shallow froth depths. It is also important to verify the circuit water balance, because an excess of water will cause problems downstream [15]. For large size columns the typical range of superficial wash water rate is 0.1-0.2 cm/s, providing a water excess of 0.02-0.05 cm/s. These values are strictly related to the normal range of air rate, i.e. 1-2 cm/s measured at local atmospheric pressure, otherwise it is easy to produce a significant feed pulp entrainment into the concentrate. Bias definition and estimation
The definition of bias has been a matter of confusion between operators and researchers. The actual bias definition can be stated based on the following objectives: a)
stabilizing action by compensation of water drainage by gravity
b)
feed water replacement by fresh water in order to prevent solid entrainment
Thus, "bias" can be defined as the difference between the fresh water added near the top of the froth and the fresh water that goes into the concentrate. Ideally, the wash water should be just enough for mineral transport into the concentrate. However, the wash water split into the concentrate and the feed water displacement are not perfect. Consequently, a minimum fresh water excess must be provided. In a more simple definition the "bias" has been referred to as the net water flowrate passing across the pulp/froth interface, the water being either fresh or feed water. Accordingly, if the fresh water entering the column exceeds the water recovered into the concentrate there is a net downward water flowrate in the froth and this condition is normally referred to as "positive bias", while a "negative bias" corresponds to a net water flowrate moving upwards into the froth. Operation with "positive bias" is not sufficient to prevent mineral entrainment into the concentrate. The main problem is froth mixing and shortcircuiting, which is strongly related to the wash water distribution, wash water and air flowrates, and froth depth. The critical interaction between gas rate, wash water rate and froth depth on feed water entrainment into the froth has been observed. The methodology consisted of introducing a step KC1 tracer signal into the feed water, in an air-water system, at laboratory scale [2]. The presence of tracer along the froth was measured by electrical conductance, at steady state. The effect of gas holdup changes on electrical conductance was cancelled considering an adimensional entrainment factor relative to the same operation without tracer. The effect of tracer on gas holdup was measured independently by pressure difference and it was found negligible. Thus, an entrainment factor equal to one means that there is no contamination (fresh water). Figures 6 and 7 show the distributed characteristics of the feed water entrainment along the froth (0.7 m depth) at constant wash water rates, Jw--0.5 and 0.2 cm/s, respectively. Figure 6 shows that there was almost no contamination along the froth at gas rate Jg=l cm/s and bias rate Jb=0.2 cm/s. However, at Jg=2 ends, the feed water entrainment increased and the bias rate decreased to Jb----Ocrn/s.
Troubleshootingindustrialflotationcolumns
1601
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Fig.6 Effect of superficial gas rate on feed water entrainment, at constant wash water rate, Jw--0.5 cm/s Figure 7 shows a significant froth contamination at Jg=l cm/s, despite the froth overflow not being contaminated. At ./g=2 cm/s, the froth was strongly contaminated up to the overflow level, and the bias became negative, Jb= -0. I cm/s. Here, it is clearly shown that froth contamination with feed water is not only dependent upon the bias rate but also upon the gas rate and froth depth. The froth contamination becomes critical at Jb -< 0cm/s. These results are in good agreement with previous observations of the internal state of the froth, where the effect of gas holdup was not considered [16]. Discrete measurements of the overflow concentrate grades, as used in plant practice, are a little sensitive for predicting potential troubles, and the manual feed back control, without a good coordination between variables, cannot be expected to give a proper metallurgical performance. 5 4.5 eO~v" / /
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Froth Depth, m Fig.7 Effect of superficial gas rate on feed water entrainment~ at constant wash water rate, Jw--0.2 cm/s In plant practice no instrumentation is available for on-line measurement of the actual bias. Therefore a "pulp bias", defined as the difference between the railings and feed volumetric flowrates, has been commonly used for control purposes. Here, it is important to note that a negative difference in such flowrates does not necessarily represent a "negative water bias", because it depends upon the solid recovery E Ibl2-g
1602
J.B. Yianatos and L. G. Bergh
into the concentrate [13]. Thus, a positive "pulp bias" provides much higher wash water than required. Also the normal error in pulp flowrate measurement gives a poor estimate of the "pulp bias" because the flowrate difference is strongly affected by error propagation [13]. Another source of trouble arises in estimating the instantaneous bias for control purposes, because the bias B is commonly defined as the difference between feed flowrate F and tailings flowrate T, assuming steady state, B = T-F
(1)
However, if the froth depth is changing the actual transient pulp balance around the collection zone is,
B_+Ac dH(l-cg) = T - F
(2)
Ot
where, A c is the column cross-section, H is the pulp height and ~g is the gas holdup in the pulp zone. Accordingly, an industrial experience with a control strategy consisting of the following interactive loops: 1)
froth depth control manipulating the tailings flowrate
2)
"pulp bias" control, i.e. T-F, manipulating the wash water flowrate, showed a critical disturbance on the wash water addition because the transient condition was not accounted for.
However, bias control strategies should be adjusted depending on the flotation objectives, for instance in rougher application the use of negative bias can be a good compromise [17], as well as in the ease of reverse flotation of iron from silica. Gas Holdup and Bubble Diameter Gas holdup profiles along industrial flotation columns have been determined from pressure profiles measurements [3]. Tests were developed in columns of 1, 13 and 16 m 2 in cross-section, and 14 m height. In all cases it was found that gas holdup increases, from the bottom to the top of the pulp zone, almost proportionally to the gas expansion because of the decrease in hydrostatic pressure. Gas holdup expansion was about 1:2 for columns located at sea level, while it was closer to 1:3 at 4500 m above sea level. Average gas holdup in the collection zone varies in the range of 10--20% at normal gas rates of 1-2 cm/s. Average bubble sizes in the range of 1-2 mm have been estimated from air holdup measurements in industrial columns. Direct observation of industrial bubble size distributions, by sampling and image analysis from video film, agreed well with simple theoretical estimates [5]. Residence Time Distribution and Mixing Liquid, solid and gas RTD have been measured in industrial flotation columns using radioactive tracer techniques [4,6]. The main results showed that residence time of solids is strongly dependent on particle size. For example, non-floatable particles of 100 microns showed an average residence time that was only a half that of the liquid. Also, the average residence time of floatable mineral reporting to the concentrate was closer to the gas residence time [6]. This result showed that most of the floatable mineral was rapidly collected near the feed entrance. However, the floatable mineral recovered into the tailings flowrate showed an average residence time similar to that of the gangue. The liquid mixing in a column of 0.9 m diameter and 15 m height, operating at 1.8 cm/s superficial air rate, was reported equivalent to 1.25 perfect mixers in series [18]. The liquid mixing in a column of 2.5m diameter and 14 m height, without baffles, was closer to a perfect mixer [10]. Move recently, the liquid mixing in a large size column of 16 m 2 cross-section and 14 m height, provided with baffles between the
Troubleshooting industrial flotation columns
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feed and air entrances, was found equivalent to 1.1 perfect mixers in series [ 1 ]. Figure 8 shows the liquid residence time distribution of this column, using LiCI as tracer, while operating at 1.9 cm/s superficial air rate. Here, the significant recovery of tracer into the concentrate showed that the column was operating under negative bias. 2.¢
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Fig.8 Residence time distribution of a large size column Column: 16 m 2 cross-section, 14 m height
Particle entrainment and shortcircuiting Entrainment of fine particles from the pulp to the froth zone was observed by using radioactive tracer techniques [4]. Fo:: this purpose, actual fine particles of gangue were irradiated and introduced with the feed flowrate. Direct measurement of the presence of tracer, just above and below the pulp/froth interface, showed.that fine gangue particles were almost completely washed down very close to the interface level, under normal operating conditions, i.e., Jg=l.8 cm/s and 1 m froth depth. Thus, it was demonstrated that a very effective cleaning action can be achieved if proper conditions of gas rate, wash water rate and froth depth are selected, from which a distinct pulp/froth interface can be obtained. Alternatively, the operation with a shallow froth depth (50 cm), high air rate (2 era/s) and low wash water rate (0.1 cm/s), will favour the feed water entrainment into the concentrate, as was the case shown in Figure 8. Good agreement between this result and entrainment conditions from Figures 6 and 7 can be observed. However, a significant gas shortcircuiting into the tailings flowrate of an industrial column was observed using radioactive gas tracer [6]. Similar tests in a pilot column did not show gas shortcircuiting. This problem seems to be related to the design of the column bottom and gas sparger location.
Control Strateg~ To operate a coltDmn, only pulp level control is needed. Rough stabilization can be achieved despite the presence of gross measurement error in froth depth and the constraints posed by PI controllers. Metallurgical optimization is a long way off unless improvements in measurement of metallurgical performance, i.e. online grade estimates, and development of flexible control strategies are made. Common PID distributed control cannot take up the challenge of a proper coordination between a non linear and strongly interactive multivariable profess. Process data of improved quality, normally communicated to a host computer, should be managed in an integrated environment where alternative control actions can be selected by hierarchical rules.
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J.B. Yilmtos a~ L. O. Bergh CONCLUSIONS
The main disadvantage of flotation columns with respect to conventional mechanical cells is the large spread of results, mainly in terms of recovery, which is normally compensated by a high circulating load and high capacities (overdesign). Presently, fundamental knowledge of column flotation and its constraints allows for more reliable design procedures as well as proposing new challenges in column flotation conU'ol and optimization. Besides the normal changes in feed composition and flowrates, and the need for periodic maintenance of gas spargers, the following common sources of troubles in plant practice are: •
improper calibration and maintenance of instrumentation to measure froth depth and gas rate
•
bias estimation and control
•
uneven wash water distribution
•
uneven froth depth in baffled columns
•
lack of robustness of control strategies
Proper boundary conditions for superficial wash water rate, 0.1-0.2 cm/s, froth depth, 0.5-1.0 m and superficial gas rate, 1-2 cm/s, are well established for large size columns. Unfortunately, the lack of coordination between these variables is an important limitation for metallurgical improvements.
ACKNOWLEDGEMENTS We would like to thank the Consejo Nacional de Investigaci6n Cientffica y Tecnol6gica (Projects 1950551 and FONDEF MI-17) and the Santa Mafia University (Project 952723) for their financial support.
REFERENCES
Nufiez, R., Estudio experimental de columna de flotaci6n de cobre de minera Escondida Ltd.,
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Espinos~ R.G., Johnson, N.W., Pease, J.D. & Munro, P.D., The commissioning of the first flotation columns at Mount Isa Mines, Processing of Complex Ores, Dobby, G.S. & Rao, S.R., Eds., Pergamon Press, 293--302 (1989). Guarini, M., Soto, A., Cipriano, A., Ohnedo, J., Guesalaga, A. & C_.6ceres,J., Caracterfsticas de espuma en celdas de flotaci6n, Minerfa Chilena, 14, N°166, 81-85 (1995). Moolman, D.W., Aldrich, C., Van Deventer, J.S.J. &; Stallge, W.W., Digitsd image processing as a tool for on-line monitoring of froth in flotation plants, Minerals Engineering, 7, 9, 1149-1164 (1994). Finch, J A. & Dobby, G.S., Column Flotation. 1st Edn. 148, Pergamon Press (1990). Falutsu, M., Direct measurement of gas rate in a flotation machine, Minerals Engineering, 7, 12, 1487-14.94 (1994). Huls, B.J. & Williams, S.R., Limitations in the Application of Column Flotation, XVIII Internat.;.onal Mineral Processing Congress, Sydney, Australia, 23-28 May, 779-784 (1993). Uribe-S~das, A., G6mez, C.O. & Finch, J.A., Bias detection in flotation columns, Colwnn'91 Conference, Agar, Huls & Hyma, Eds., Sudbury, Canada, 2, 391--407 (1991). Furey, J.T., Rougher column flotation of gold tellurides. CMP Conference, Ottawa, Canada, Jan. 1990). Mills, P.J.T., Yianatos, J.B. & O'Connor, C.T., The mixing characteristics of solid and liquid phases in flotation column, Minerals Engineering, 5, 10/12, 1195-1205 (1993).