- Email: [email protected]
On lifetime costs of flotation operations
Minerals Engineering 21 (2008) 846–850
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
On lifetime costs of flotation operations Antti Rinne, Aleksi Peltola * Outotec, Riihitontuntie 7, P.O. Box 84, Espoo, Finland
a r t i c l e
i n f o
Article history: Received 3 December 2007 Accepted 22 April 2008 Available online 24 June 2008 Keywords: Flotation machines Mineral processing Process optimisation Agitation
a b s t r a c t Overall economy of a flotation operation bears much more than investment costs. The lifetime operation and maintenance of a flotation machine may affect the economy of a project far more than a million saved in investment. Studying the long-term effects on investments is worthwhile as they often surpass any apparent savings in capital expenditure. The optimal solution does not even need to be more expensive. Besides, with high operating costs, time is never on one’s side. In the paper various flotation cell arrangements are compared in a simplified example. The example considers capital equipment investment costs, power delivery arrangements, energy costs, and maintenance costs throughout a 25-year ownership. Also effects on CO2 emissions are discussed. The paper will illustrate how proper choice of equipment and modern power delivery and control methods will result in significant benefits in lifetime costs and profitability of flotation operations. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction When an investment for a new flotation plant is evaluated, the emphasis is often in minimizing the capital expenditures. Until recently, life cycle cost (LCC) as a criterion for selecting beneficiation equipment has played only a small role in the final decision making. A quick analysis discussed below shows that roughly 60–80% of the total 25-year cycle costs for a large flotation machine are spent on energy while the initial investment comprises less than 10%. As a result, if a small saving in investment is achieved by compromising energy efficiency, it can quickly turn into big losses in operational costs. If one looks for savings in the long run, life cycle cost analysis shows that the importance of the investment cost is almost negligible. Modern technology can offer completely new solutions for optimizing flotation processes both in terms of efficiency and metallurgy. Mechanical flotation machines have traditionally been limited by their relatively narrow range of aeration rate, fixed mechanism dimensions and speed. New flotation machine designs allow much wider adaptability with speed control, shear adjustment and wider range of air feed. More attention should be paid on maintenance of critical components. Cases are known where significant metallurgical losses have been observed due to poor condition of critical wear parts. This paper discusses the economy of flotation projects from the life cycle cost perspective. The examples have been calculated based on generic average values. There is a wide variation in costs * Corresponding author. Tel.: +358 20 529 2772; fax: +358 20 529 2998. E-mail address: [email protected] (A. Peltola). 0892-6875/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2008.04.018
between locations and specific processes, but averages give a good starting point to more specific analysis. 2. Life cycle costs of flotation operations Life cycle cost analysis simply considers the lifetime operation and maintenance costs of a flotation operation in addition to the initial investment, in selecting the most economical equipment. It may be feasible to pay higher initial cost if one saves in operational expenditures. The relevant cost factors for a flotation plant are investment, energy and reagent consumption, and maintenance. All these should be quantified for the estimated service life of the equipment. In order to illustrate the typical deviation of the relevant cost factors, typical ownership costs of a large mechanical flotation machine (100–200 m3) are considered in brief. In this consideration, the investment costs consist of merely equipment costs since the deviation in infrastructure, installation, and assembly is significant. Power draw of the equipment considers the power required for agitation and aeration. Maintenance proportion is determined by studying failure rates, costs, and normalized maintenance times of the wearing parts. Unit prices for maintenance services, electricity, and reagents are presented in Table 1. Typical total ownership costs over the time span of 25 years are presented in Fig. 1. There is high variation in the cost factors; more specific results can be easily obtained by inserting the actual rates for electricity, reagents and labor as well as for the cost of capital in the calculation model. In order to achieve accurate costing, a sensitivity analysis should be performed to understand the relationships between
A. Rinne, A. Peltola / Minerals Engineering 21 (2008) 846–850
2.2. Operational expenditures
Table 1 Average rates for electricity, reagents, and maintenance labor Power draw Cost of electricity Annual operating hours Cost of capital Reagents Hourly rate of maintenance services
138.82 0.06 8300.00 10.00 13,800.00 50.00
847
kW €/kWh h % €/a €/a
Fig. 1. Breakdown of a large flotation cell expenses over the lifespan of 25 years.
total ownership costs and uncertainties of each activity. That is, issues such as inflation rate, expected efficiency of resources, expected variation in energy costs etc. should be considered. The breakdown strongly suggests that the most significant life cycle cost item in flotation operations is the cost of electricity. Thus the operational expenditures are heavily influenced by the energy price and the energy efficiency of the equipment used for production. 2.1. Capital expenditures A given requirement for flotation capacity may be satisfied by several scenarios which may differ significantly in terms of required footprint, investment cost and required maintenance resources, etc. The most significant decisions concern the implemented unit size and the principle of operation of the equipment. In general, larger flotation cell units lead to lower investment, energy and maintenance costs as measured in unit price per unit of installed volume.
The operational costs of a flotation machine depend on the efficiency of the equipment. Process efficiency, energy efficiency and availability are discussed below. 2.2.1. Process efficiency The key mechanical aspect for good flotation process efficiency is the proper condition of critical wear components. Missing rotor or stator parts make the cell surface wavy and cause the froth to collapse. Air dispersion is reduced and decreased pumping causes sanding. The use of copied spare parts has often caused problems (see Fig. 2). Experience has shown that non-standard spare parts often give a shorter wear life and in some cases decrease the metallurgical efficiency. The real savings that can be achieved by using worn out or low quality parts are negligible when compared to energy costs of the same equipment. If even small metallurgical losses occur because of poorly working equipment the savings in maintenance quickly become expenses. In order to obtain optimal availability performance it is safest to use only original equipment manufacturer’s spare parts. Comparisons of flotation operations using different technologies are published in two recent papers (Froehling et al., 2005; Coleman et al., 2006). These papers give a good view of the effect of metallurgical performance as well as maintenance aspects on overall economics. The first paper discusses of a retrofit from self-aspirated flotation cells to forced air TankCellÒ mixing technology which resulted in significant improvements in performance. The second paper compares Escondida’s self-aspirated flotation cell circuit to the TankCellÒ circuit. The latter showed significantly better performance also in this comparison. 2.2.2. Energy efficiency The traditional drive mechanism of a flotation machine consists of a single-staged V-belt drive connected to a low-speed induction motor. Theoretically the efficiency of the V-belt drive is 97–98% provided that the belts are optimally sized, properly aligned and tightened to correct belt tension. Unfortunately in practice this situation is rather rare and the actual efficiency is therefore lower. As the belts wear and stretch readjustments are required. This is often impossible without shutdown of the equipment, which, in turn, would result in reduced availability. Similar challenges occur also
Fig. 2. Pirate flotation cell mixer spare parts after two weeks of use.
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A. Rinne, A. Peltola / Minerals Engineering 21 (2008) 846–850
Fig. 3. Misaligned V-belt pulleys.
with gearbox-driven flotation machines when the power from the electric motor is transferred to the gearbox through a V-belt drive. Fig. 3 shows an example of a misalignment. In addition to the power transfer ratio of the drive mechanism, the rotation speed of the rotor and the air feed equipment are important factor in the electrical energy consumption. Studies on variable speed drive mechanisms have been conducted indicating a certain range of regulation in the rotor speed where the metallurgical performance of the flotation machine is practically the same. However, the rotation speed has a significant effect on the power draw of the mixing mechanism. The relationship can be simply expressed as
P¼
1
g
p Q;
where P, is the power draw; g, is hydraulic efficiency of the mechanism; p, is pressure difference generated by the mixing mechanism, and Q, is the volume flow rate through the mixing mechanism. The pressure difference over the mixing mechanism is proportional to the rotation speed squared and the volume flow rate is directly proportional to it. Thus the power draw of the mixing mechanism is proportional to the third power of the rotation speed. Consequently, a minor reduction in rotation speed may have no effect on process performance but a significant effect on the energy consumption. For example, 10% reduction in rotation speed roughly equals to 27% reduction in power draw. A drive mechanism that enables the adjustment of the rotation speed may produce significant savings in electricity consumption. There are internal case studies, which have shown that a variable speed drive may have payback time of only few months, if the process allows optimization of rotation speed. The same principles apply also to air blowers. Significant savings may be achieved if the air blower is not driven at maximum power against a regulating valve but at a speed that is sufficient to maintain the required airflow. Such optimization can be done using cell-specific air blowers with variable speed drives. 2.2.3. Availability performance One cannot consider operational performance without also considering the availability of the equipment. According to Lyytikäi-
nen (1987), the process yield is product of process performance and availability performance. Availability performance can be further divided into three subcategories, as presented in Fig. 4. Reliability of the equipment refers to the probability of a unit functioning normally when used according to specific conditions for at least a given period of time. Reliability can be estimated from the failure rate of the equipment. Maintainability can be defined as the ability of an item to be retained in a state in which it can perform the required function, when maintenance is performed using stated procedures and resources. For flotation machines this means ease of maintenance or replacement of mixing or drive mechanisms. For example, if the drive mechanism can be removed from the cell as a complete unit and there is a reserve drive standing by, the unplanned downtime can be minimized and thus the availability is not compromised even by events of sudden malfunctions. Supportability is the availability of the material required to keep the system operational. In general, standard components are technically and economically optimized structural solutions, which have decent availability. In addition, manufacturers of standard components benefit from economies of scale and thus the costs of standard components are usually lower as compared to craft production. 3. The potential of new technology 3.1. Use new equipment Outotec’s flotation cells have always been customized for ore and process characteristics to optimize metallurgical results. Anyhow, ore properties change during equipment lifetime or the same equipment may be utilized for a new pit close to original operation. The function of a single cell in the flowsheet can be changed. Mines with different types of ore are also relatively common and change from one ore type to another often causes process hiccups. Especially in this kind of cases optimization of flotation machine parameters such as mixer speed or power consumption would be beneficial. But there is a lot to be done also at flotation operations with no particular troubles. The effect of optimal air dispersion, mixer speed and shear are discussed below. 3.2. Leading edge technology for process optimization
Availability Performance
Reliability Performance
Maintainability Performance
Supportability Performance
Fig. 4. Factors affecting the availability performance. (Lyytikäinen 1987).
3.2.1. Disperse the right amount of air Optimal air dispersion is one of the basic requirements for good metallurgical performance. Plants operating with forced air cells have often noticed that the best results are achieved using individual and varying air feed rate in each cell. In traditional flotation mechanisms the air feed is limited by the reduction of power draw
A. Rinne, A. Peltola / Minerals Engineering 21 (2008) 846–850
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Fig. 6. Typical power draw curves of VSD with FloatForceTM mechanism and standard drive with OK-mechanism. Fig. 5. Power draw versus air feed-curves of FloatForceTM and OK mechanisms.
and mixing, or by reduced dispersion of air making the froth surface unstable and causing the froth to collapse. Outotec’s new mechanism design, the FloatForceTM, pushes further the maximum air feed limit compared to other existing designs. As a result the cell surface is steady in all situations and the pumping rate of the mechanism is only slightly affected by air. Because of the flat power curve less power is needed when the mechanism is operated without air. This allows smaller motors and benefits both in investment and operating costs because of more efficient operation of the motor. Fig. 5 presents power draws of OK and FloatForceTM mechanisms in comparative conditions Grönstarnd et al. (2006). 3.2.2. Adjust mixer speed The easiest solution for adjusting mixer speed is a variable speed drive (VSD). A frequency converter can theoretically be installed in most of existing flotation cell drives but compatibility with existing equipment such as electric motor must be checked. Typical adjustment range of a flotation cell with VSD lies between 75% and 110% of original shaft speed if there are no special limitations in the existing equipment. Initial process test results indicate two different behaviors with moderate speed adjustments. More often changes in metallurgy are negligible but significant changes in power consumption can be seen. The initial results in full-scale plant test showed no difference in metallurgy even with 50% lower power consumption compared to starting point. When this is compared to the findings in Fig. 1 the significance of power draw estimation can be understood. More detailed test results will be reported later. In some cases optimal speed shows optimal metallurgical point of the examined flotation cell. It is still likely that the optimal speed of each cell even within the same plant varies. 3.2.3. Add more shear to boost recovery The role of shear, or rotor–stator gap, has also been investigated. Earlier testwork has shown that smaller rotor–stator gap is beneficial especially for fine and ultra fine particle flotation. However, recent results indicate that also recovery of coarse particles can be increased at the same time. It has also been observed that motor speed adjustment at the same time has a significant effect on the results (Bilney et al., 2006). 3.2.4. The benefits of the recent improvements The potential of the recent improvements can be illustrated in a simple chart. Typical power draw curves of a flotation cell equipped with VSD and FloatForceTM mechanism and a cell with conventional fixed speed drive with the old OK-mechanism are presented in Fig. 6. The main benefit of the new arrangement is
the possibility to adjust the cell during normal operation. The optimal flotation cell operation point with VSD and FloatForceTM rotor can be found in the two-dimensional control space whereas the operation range of the old drive is bound to one curve. For a selfaspirated cell the control range is only one point whose position depends on slurry density. The main benefit of the new arrangement is the possibility to adjust the cell during normal operation. Adjustment of the rotor–stator gap provides another offline parameter that creates a family of curves for each gap value. 3.2.5. Which parameters do actually make the difference? A lot of research work and discussion is going on in the search for optimum metallurgy of each ore type. Among the most common tasks is optimization of basic parameters like bubble size and installed power. But how do we actually generate an optimal bubble size in a modern large flotation cell? Controlling bubble size distributions in a laboratory scale flotation machine is a subject for many studies, but research done with large-scale cells is much more limited. Scale up of the findings to industrial cell sizes is not very straightforward either. How is a powerful flotation machine determined? Is installed motor power the same as high measured total power consumption of a mixer drive? By installing a 250 kW motor instead of 100 kW the measurable power consumption of the cell increases for sure due to unfavorable operating point of the motor. When the mixer is updated to correspond the larger motor, what is really sought after with the power increase? Optimal and higher speed for optimal bubble size distribution or more shear? Moving the slurry around in the tank does not necessarily make a big difference if the basic parameters remain the same. In some cases extra power may only make the situation worse by causing unstable cell surface and froth. Today, we have a possibility to find out answers to at least some of these questions by selecting new mechanism design and VSD. 4. Summary Let us look at the importance of the energy efficiency and selection of the optimal equipment size by considering a flotation plant requiring, for example, 1800 m3 of flotation volume. This requirement can be fulfilled via five possible scenarios: 18 individual 100 m3 cells in two rows of nine. Nine 200 m3 cells as a single line of nine. Nine 200 m3 cells with variable speed drive mechanisms enabling optimization of the rotation speed, say, 5% lower than the nominal rotation speed.
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A. Rinne, A. Peltola / Minerals Engineering 21 (2008) 846–850
100.0 100.0 86.1 76.6
80.0
67.5 62.3 60.0
40.0
20.0
0.0 100-m3 cells
200-m3 cells
200-m3 cells, optimized rotation speed
300-m3 cells
300-m3 cells, optimized rotation speed
Investment costs
11.9
7.4
8.2
7.0
7.7
Reagents
18.7
17.0
17.0
15.4
15.4
9.4
5.8
6.0
3.5
3.6
Energy
60.0
55.9
45.4
41.5
35.6
LCC
100.0
86.1
76.6
67.5
62.3
Maintenance
Fig. 7. Relative investment costs of different flotation options.
Six 300 m3 cells as a single line of six. Six 300 m3 cells with variable speed drive mechanisms enabling 5% lower rotation speed. Relative investment, energy, reagent and maintenance costs are presented in Fig. 7. The trend towards larger units is obvious when looking at the comparison. Another leap of similar magnitude can still be taken if the process enables lower mixer speed. The example has been calculated with 5% speed change; since the power consumption of a mixer is proportional to the third power of the rotational speed this results in 15% savings of the energy cost. As the life cycle is rather long, the effect of the salvage value to the overall costs is low. In addition the salvage value is difficult to determine and may even be negative – one must pay to get rid of the commodity. Consequently, salvage value is ignored in the evaluation.
industrial world’s energy is produced using fossil fuels that result carbon dioxide emissions. The significance of proper choice of equipment from this point of view can be illustrated by considering the fact that the annual difference in energy consumption between eighteen 100 m3 cells and six 300 m3 cells with optimized rotation speed is approximately 4.72 GWh/a which corresponds to approximately 4.32 million kilograms of carbon dioxide if the energy is produced in a fossil fuel power plant. The amount equals to one hundred average passenger cars driving approximately 270,000 km. To prevent climate change, governments are tightening emission standards and emission trading is implemented to control pollution. To conclude, proper choice of equipment and modern power delivery and control methods will result in significant savings in investment costs and lower energy consumption to compensate against rising energy price.
5. Conclusions and discussion References As presented in Fig. 7, the larger cells have significant advantages in capital costs. If one selects eighteen 100 m3 cells instead of six 300 m3 cells, the initial investment cost is approximately 50% higher. The economic advantages continue also in areas such as maintenance, instrumentation and building costs since there are fewer units and the required footprint is smaller. It is also suggested that a comprehensive life cycle cost analysis is a cost effective method for maximizing the life cycle profits of flotation operations. That is, in addition to just initial investment, all project costs and options should be taken into consideration in the analysis. As mentioned above, energy efficiency has a significant effect on the life cycle economy. One aspect of it is that at least 75% of the
Bilney, T., MacKinnon, S., Kok, J., Assessment of high shear stator performance at Kanowna Belle Gold Mine. In: Metallurgical Plant Design and Operating Strategies 2006, Perth, Australia, September 18–19, 2006. Coleman, R.G., Urtubia, H.E., Alexander, D.J., 2006. A comparison of BHPBilliton’s Minera Escondida flotation concentrators. In: Canadian Mineral Processors 38th Annual Operators Conference, Ottawa, Canada, January 17– 19, 2006. Froehling, M., Mohns, C., Roman, E., Grady, P., 2005. A history of improvements at the Kemess concentrator. In: Canadian Mineral Processors 37th Annual Operators Conference, Ottawa, Canada, January 18–20, 2005. Grönstrand, S., Niitti, T., Rinne, A., Turunen, J., 2006. Enhancement of flow dynamics of existing flotation cells. In: Canadian Mineral Processors 38th Annual Operators Conference, Ottawa, Canada, January 17–19, 2006. Lyytikäinen, A., 1987. Reliability Engineering Handbook. Technical Research Centre of Finland, Research Notes 678, 147 p. + app. 6 p., ISBN: 951-38-2633-3, Espoo, Finland, 1987.
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
On lifetime costs of flotation operations Antti Rinne, Aleksi Peltola * Outotec, Riihitontuntie 7, P.O. Box 84, Espoo, Finland
a r t i c l e
i n f o
Article history: Received 3 December 2007 Accepted 22 April 2008 Available online 24 June 2008 Keywords: Flotation machines Mineral processing Process optimisation Agitation
a b s t r a c t Overall economy of a flotation operation bears much more than investment costs. The lifetime operation and maintenance of a flotation machine may affect the economy of a project far more than a million saved in investment. Studying the long-term effects on investments is worthwhile as they often surpass any apparent savings in capital expenditure. The optimal solution does not even need to be more expensive. Besides, with high operating costs, time is never on one’s side. In the paper various flotation cell arrangements are compared in a simplified example. The example considers capital equipment investment costs, power delivery arrangements, energy costs, and maintenance costs throughout a 25-year ownership. Also effects on CO2 emissions are discussed. The paper will illustrate how proper choice of equipment and modern power delivery and control methods will result in significant benefits in lifetime costs and profitability of flotation operations. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction When an investment for a new flotation plant is evaluated, the emphasis is often in minimizing the capital expenditures. Until recently, life cycle cost (LCC) as a criterion for selecting beneficiation equipment has played only a small role in the final decision making. A quick analysis discussed below shows that roughly 60–80% of the total 25-year cycle costs for a large flotation machine are spent on energy while the initial investment comprises less than 10%. As a result, if a small saving in investment is achieved by compromising energy efficiency, it can quickly turn into big losses in operational costs. If one looks for savings in the long run, life cycle cost analysis shows that the importance of the investment cost is almost negligible. Modern technology can offer completely new solutions for optimizing flotation processes both in terms of efficiency and metallurgy. Mechanical flotation machines have traditionally been limited by their relatively narrow range of aeration rate, fixed mechanism dimensions and speed. New flotation machine designs allow much wider adaptability with speed control, shear adjustment and wider range of air feed. More attention should be paid on maintenance of critical components. Cases are known where significant metallurgical losses have been observed due to poor condition of critical wear parts. This paper discusses the economy of flotation projects from the life cycle cost perspective. The examples have been calculated based on generic average values. There is a wide variation in costs * Corresponding author. Tel.: +358 20 529 2772; fax: +358 20 529 2998. E-mail address: [email protected] (A. Peltola). 0892-6875/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2008.04.018
between locations and specific processes, but averages give a good starting point to more specific analysis. 2. Life cycle costs of flotation operations Life cycle cost analysis simply considers the lifetime operation and maintenance costs of a flotation operation in addition to the initial investment, in selecting the most economical equipment. It may be feasible to pay higher initial cost if one saves in operational expenditures. The relevant cost factors for a flotation plant are investment, energy and reagent consumption, and maintenance. All these should be quantified for the estimated service life of the equipment. In order to illustrate the typical deviation of the relevant cost factors, typical ownership costs of a large mechanical flotation machine (100–200 m3) are considered in brief. In this consideration, the investment costs consist of merely equipment costs since the deviation in infrastructure, installation, and assembly is significant. Power draw of the equipment considers the power required for agitation and aeration. Maintenance proportion is determined by studying failure rates, costs, and normalized maintenance times of the wearing parts. Unit prices for maintenance services, electricity, and reagents are presented in Table 1. Typical total ownership costs over the time span of 25 years are presented in Fig. 1. There is high variation in the cost factors; more specific results can be easily obtained by inserting the actual rates for electricity, reagents and labor as well as for the cost of capital in the calculation model. In order to achieve accurate costing, a sensitivity analysis should be performed to understand the relationships between
A. Rinne, A. Peltola / Minerals Engineering 21 (2008) 846–850
2.2. Operational expenditures
Table 1 Average rates for electricity, reagents, and maintenance labor Power draw Cost of electricity Annual operating hours Cost of capital Reagents Hourly rate of maintenance services
138.82 0.06 8300.00 10.00 13,800.00 50.00
847
kW €/kWh h % €/a €/a
Fig. 1. Breakdown of a large flotation cell expenses over the lifespan of 25 years.
total ownership costs and uncertainties of each activity. That is, issues such as inflation rate, expected efficiency of resources, expected variation in energy costs etc. should be considered. The breakdown strongly suggests that the most significant life cycle cost item in flotation operations is the cost of electricity. Thus the operational expenditures are heavily influenced by the energy price and the energy efficiency of the equipment used for production. 2.1. Capital expenditures A given requirement for flotation capacity may be satisfied by several scenarios which may differ significantly in terms of required footprint, investment cost and required maintenance resources, etc. The most significant decisions concern the implemented unit size and the principle of operation of the equipment. In general, larger flotation cell units lead to lower investment, energy and maintenance costs as measured in unit price per unit of installed volume.
The operational costs of a flotation machine depend on the efficiency of the equipment. Process efficiency, energy efficiency and availability are discussed below. 2.2.1. Process efficiency The key mechanical aspect for good flotation process efficiency is the proper condition of critical wear components. Missing rotor or stator parts make the cell surface wavy and cause the froth to collapse. Air dispersion is reduced and decreased pumping causes sanding. The use of copied spare parts has often caused problems (see Fig. 2). Experience has shown that non-standard spare parts often give a shorter wear life and in some cases decrease the metallurgical efficiency. The real savings that can be achieved by using worn out or low quality parts are negligible when compared to energy costs of the same equipment. If even small metallurgical losses occur because of poorly working equipment the savings in maintenance quickly become expenses. In order to obtain optimal availability performance it is safest to use only original equipment manufacturer’s spare parts. Comparisons of flotation operations using different technologies are published in two recent papers (Froehling et al., 2005; Coleman et al., 2006). These papers give a good view of the effect of metallurgical performance as well as maintenance aspects on overall economics. The first paper discusses of a retrofit from self-aspirated flotation cells to forced air TankCellÒ mixing technology which resulted in significant improvements in performance. The second paper compares Escondida’s self-aspirated flotation cell circuit to the TankCellÒ circuit. The latter showed significantly better performance also in this comparison. 2.2.2. Energy efficiency The traditional drive mechanism of a flotation machine consists of a single-staged V-belt drive connected to a low-speed induction motor. Theoretically the efficiency of the V-belt drive is 97–98% provided that the belts are optimally sized, properly aligned and tightened to correct belt tension. Unfortunately in practice this situation is rather rare and the actual efficiency is therefore lower. As the belts wear and stretch readjustments are required. This is often impossible without shutdown of the equipment, which, in turn, would result in reduced availability. Similar challenges occur also
Fig. 2. Pirate flotation cell mixer spare parts after two weeks of use.
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A. Rinne, A. Peltola / Minerals Engineering 21 (2008) 846–850
Fig. 3. Misaligned V-belt pulleys.
with gearbox-driven flotation machines when the power from the electric motor is transferred to the gearbox through a V-belt drive. Fig. 3 shows an example of a misalignment. In addition to the power transfer ratio of the drive mechanism, the rotation speed of the rotor and the air feed equipment are important factor in the electrical energy consumption. Studies on variable speed drive mechanisms have been conducted indicating a certain range of regulation in the rotor speed where the metallurgical performance of the flotation machine is practically the same. However, the rotation speed has a significant effect on the power draw of the mixing mechanism. The relationship can be simply expressed as
P¼
1
g
p Q;
where P, is the power draw; g, is hydraulic efficiency of the mechanism; p, is pressure difference generated by the mixing mechanism, and Q, is the volume flow rate through the mixing mechanism. The pressure difference over the mixing mechanism is proportional to the rotation speed squared and the volume flow rate is directly proportional to it. Thus the power draw of the mixing mechanism is proportional to the third power of the rotation speed. Consequently, a minor reduction in rotation speed may have no effect on process performance but a significant effect on the energy consumption. For example, 10% reduction in rotation speed roughly equals to 27% reduction in power draw. A drive mechanism that enables the adjustment of the rotation speed may produce significant savings in electricity consumption. There are internal case studies, which have shown that a variable speed drive may have payback time of only few months, if the process allows optimization of rotation speed. The same principles apply also to air blowers. Significant savings may be achieved if the air blower is not driven at maximum power against a regulating valve but at a speed that is sufficient to maintain the required airflow. Such optimization can be done using cell-specific air blowers with variable speed drives. 2.2.3. Availability performance One cannot consider operational performance without also considering the availability of the equipment. According to Lyytikäi-
nen (1987), the process yield is product of process performance and availability performance. Availability performance can be further divided into three subcategories, as presented in Fig. 4. Reliability of the equipment refers to the probability of a unit functioning normally when used according to specific conditions for at least a given period of time. Reliability can be estimated from the failure rate of the equipment. Maintainability can be defined as the ability of an item to be retained in a state in which it can perform the required function, when maintenance is performed using stated procedures and resources. For flotation machines this means ease of maintenance or replacement of mixing or drive mechanisms. For example, if the drive mechanism can be removed from the cell as a complete unit and there is a reserve drive standing by, the unplanned downtime can be minimized and thus the availability is not compromised even by events of sudden malfunctions. Supportability is the availability of the material required to keep the system operational. In general, standard components are technically and economically optimized structural solutions, which have decent availability. In addition, manufacturers of standard components benefit from economies of scale and thus the costs of standard components are usually lower as compared to craft production. 3. The potential of new technology 3.1. Use new equipment Outotec’s flotation cells have always been customized for ore and process characteristics to optimize metallurgical results. Anyhow, ore properties change during equipment lifetime or the same equipment may be utilized for a new pit close to original operation. The function of a single cell in the flowsheet can be changed. Mines with different types of ore are also relatively common and change from one ore type to another often causes process hiccups. Especially in this kind of cases optimization of flotation machine parameters such as mixer speed or power consumption would be beneficial. But there is a lot to be done also at flotation operations with no particular troubles. The effect of optimal air dispersion, mixer speed and shear are discussed below. 3.2. Leading edge technology for process optimization
Availability Performance
Reliability Performance
Maintainability Performance
Supportability Performance
Fig. 4. Factors affecting the availability performance. (Lyytikäinen 1987).
3.2.1. Disperse the right amount of air Optimal air dispersion is one of the basic requirements for good metallurgical performance. Plants operating with forced air cells have often noticed that the best results are achieved using individual and varying air feed rate in each cell. In traditional flotation mechanisms the air feed is limited by the reduction of power draw
A. Rinne, A. Peltola / Minerals Engineering 21 (2008) 846–850
849
Fig. 6. Typical power draw curves of VSD with FloatForceTM mechanism and standard drive with OK-mechanism. Fig. 5. Power draw versus air feed-curves of FloatForceTM and OK mechanisms.
and mixing, or by reduced dispersion of air making the froth surface unstable and causing the froth to collapse. Outotec’s new mechanism design, the FloatForceTM, pushes further the maximum air feed limit compared to other existing designs. As a result the cell surface is steady in all situations and the pumping rate of the mechanism is only slightly affected by air. Because of the flat power curve less power is needed when the mechanism is operated without air. This allows smaller motors and benefits both in investment and operating costs because of more efficient operation of the motor. Fig. 5 presents power draws of OK and FloatForceTM mechanisms in comparative conditions Grönstarnd et al. (2006). 3.2.2. Adjust mixer speed The easiest solution for adjusting mixer speed is a variable speed drive (VSD). A frequency converter can theoretically be installed in most of existing flotation cell drives but compatibility with existing equipment such as electric motor must be checked. Typical adjustment range of a flotation cell with VSD lies between 75% and 110% of original shaft speed if there are no special limitations in the existing equipment. Initial process test results indicate two different behaviors with moderate speed adjustments. More often changes in metallurgy are negligible but significant changes in power consumption can be seen. The initial results in full-scale plant test showed no difference in metallurgy even with 50% lower power consumption compared to starting point. When this is compared to the findings in Fig. 1 the significance of power draw estimation can be understood. More detailed test results will be reported later. In some cases optimal speed shows optimal metallurgical point of the examined flotation cell. It is still likely that the optimal speed of each cell even within the same plant varies. 3.2.3. Add more shear to boost recovery The role of shear, or rotor–stator gap, has also been investigated. Earlier testwork has shown that smaller rotor–stator gap is beneficial especially for fine and ultra fine particle flotation. However, recent results indicate that also recovery of coarse particles can be increased at the same time. It has also been observed that motor speed adjustment at the same time has a significant effect on the results (Bilney et al., 2006). 3.2.4. The benefits of the recent improvements The potential of the recent improvements can be illustrated in a simple chart. Typical power draw curves of a flotation cell equipped with VSD and FloatForceTM mechanism and a cell with conventional fixed speed drive with the old OK-mechanism are presented in Fig. 6. The main benefit of the new arrangement is
the possibility to adjust the cell during normal operation. The optimal flotation cell operation point with VSD and FloatForceTM rotor can be found in the two-dimensional control space whereas the operation range of the old drive is bound to one curve. For a selfaspirated cell the control range is only one point whose position depends on slurry density. The main benefit of the new arrangement is the possibility to adjust the cell during normal operation. Adjustment of the rotor–stator gap provides another offline parameter that creates a family of curves for each gap value. 3.2.5. Which parameters do actually make the difference? A lot of research work and discussion is going on in the search for optimum metallurgy of each ore type. Among the most common tasks is optimization of basic parameters like bubble size and installed power. But how do we actually generate an optimal bubble size in a modern large flotation cell? Controlling bubble size distributions in a laboratory scale flotation machine is a subject for many studies, but research done with large-scale cells is much more limited. Scale up of the findings to industrial cell sizes is not very straightforward either. How is a powerful flotation machine determined? Is installed motor power the same as high measured total power consumption of a mixer drive? By installing a 250 kW motor instead of 100 kW the measurable power consumption of the cell increases for sure due to unfavorable operating point of the motor. When the mixer is updated to correspond the larger motor, what is really sought after with the power increase? Optimal and higher speed for optimal bubble size distribution or more shear? Moving the slurry around in the tank does not necessarily make a big difference if the basic parameters remain the same. In some cases extra power may only make the situation worse by causing unstable cell surface and froth. Today, we have a possibility to find out answers to at least some of these questions by selecting new mechanism design and VSD. 4. Summary Let us look at the importance of the energy efficiency and selection of the optimal equipment size by considering a flotation plant requiring, for example, 1800 m3 of flotation volume. This requirement can be fulfilled via five possible scenarios: 18 individual 100 m3 cells in two rows of nine. Nine 200 m3 cells as a single line of nine. Nine 200 m3 cells with variable speed drive mechanisms enabling optimization of the rotation speed, say, 5% lower than the nominal rotation speed.
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100.0 100.0 86.1 76.6
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Fig. 7. Relative investment costs of different flotation options.
Six 300 m3 cells as a single line of six. Six 300 m3 cells with variable speed drive mechanisms enabling 5% lower rotation speed. Relative investment, energy, reagent and maintenance costs are presented in Fig. 7. The trend towards larger units is obvious when looking at the comparison. Another leap of similar magnitude can still be taken if the process enables lower mixer speed. The example has been calculated with 5% speed change; since the power consumption of a mixer is proportional to the third power of the rotational speed this results in 15% savings of the energy cost. As the life cycle is rather long, the effect of the salvage value to the overall costs is low. In addition the salvage value is difficult to determine and may even be negative – one must pay to get rid of the commodity. Consequently, salvage value is ignored in the evaluation.
industrial world’s energy is produced using fossil fuels that result carbon dioxide emissions. The significance of proper choice of equipment from this point of view can be illustrated by considering the fact that the annual difference in energy consumption between eighteen 100 m3 cells and six 300 m3 cells with optimized rotation speed is approximately 4.72 GWh/a which corresponds to approximately 4.32 million kilograms of carbon dioxide if the energy is produced in a fossil fuel power plant. The amount equals to one hundred average passenger cars driving approximately 270,000 km. To prevent climate change, governments are tightening emission standards and emission trading is implemented to control pollution. To conclude, proper choice of equipment and modern power delivery and control methods will result in significant savings in investment costs and lower energy consumption to compensate against rising energy price.
5. Conclusions and discussion References As presented in Fig. 7, the larger cells have significant advantages in capital costs. If one selects eighteen 100 m3 cells instead of six 300 m3 cells, the initial investment cost is approximately 50% higher. The economic advantages continue also in areas such as maintenance, instrumentation and building costs since there are fewer units and the required footprint is smaller. It is also suggested that a comprehensive life cycle cost analysis is a cost effective method for maximizing the life cycle profits of flotation operations. That is, in addition to just initial investment, all project costs and options should be taken into consideration in the analysis. As mentioned above, energy efficiency has a significant effect on the life cycle economy. One aspect of it is that at least 75% of the
Bilney, T., MacKinnon, S., Kok, J., Assessment of high shear stator performance at Kanowna Belle Gold Mine. In: Metallurgical Plant Design and Operating Strategies 2006, Perth, Australia, September 18–19, 2006. Coleman, R.G., Urtubia, H.E., Alexander, D.J., 2006. A comparison of BHPBilliton’s Minera Escondida flotation concentrators. In: Canadian Mineral Processors 38th Annual Operators Conference, Ottawa, Canada, January 17– 19, 2006. Froehling, M., Mohns, C., Roman, E., Grady, P., 2005. A history of improvements at the Kemess concentrator. In: Canadian Mineral Processors 37th Annual Operators Conference, Ottawa, Canada, January 18–20, 2005. Grönstrand, S., Niitti, T., Rinne, A., Turunen, J., 2006. Enhancement of flow dynamics of existing flotation cells. In: Canadian Mineral Processors 38th Annual Operators Conference, Ottawa, Canada, January 17–19, 2006. Lyytikäinen, A., 1987. Reliability Engineering Handbook. Technical Research Centre of Finland, Research Notes 678, 147 p. + app. 6 p., ISBN: 951-38-2633-3, Espoo, Finland, 1987.