Sustainable agitator and reactor design for demanding applications in hydrometallurgy

Transcription

1 Sustainable agitator and reactor design for demanding applications in hydrometallurgy Marko Latva-Kokko, Tuomas Hirsi and Teemu Ritasalo Outotec, Finland ABSTRACT Reactor design plays a very important role in the hydrometallurgical industry. Ultimately, all hydrometallurgical plant income is extracted in reactors. However, we tend to see the same sorts of reactor design related issues repeating themselves in our industry. Faulty selection or improper dimensioning of an agitator leads to a decrease in process performance and availability. Inadequate mechanical design and underestimated material selection may have a huge impact on the availability, lifetime, and operating cost of the whole reactor system. Hydrometallurgical processes set some special requirements for stirred tank reactor design. The process environment can be highly corrosive and abrasive, simultaneous solids suspension and gas dispersion is common, and scaling can be intensive. The most feasible reactor design is always application- and case-specific. In this paper, the performance of different agitators is compared in conditions relevant to hydrometallurgical processes. Results show that impellers, which have been specially designed for three-phase mixing, are able to suspend solids with higher gas feed amounts than impellers designed for merely solids suspension or gas dispersion. This is especially important in pressure oxidation autoclaves that operate with high oxygen feed rates, sometimes in intensive scaling conditions. In addition to agitator design, the performance of a hydrometallurgical reactor can also be improved with tank design. Experimental test results have shown that the mixing power required for solids suspension can be significantly reduced by utilizing a novel tank bottom structure that is designed for downward pumping axial impellers. Keywords: Agitation, mixing, suspension, dispersion 1

2 INTRODUCTION Reactor design plays a very important role in the hydrometallurgical industry. All hydrometallurgical plant income is extracted in reactors and even marginal outperformance can have an impact in terms of plant economy. The initial investment usually has a low impact on the lifetime costs or lifetime value of a reactor. In contrast, the process result and availability can make the difference. A hydrometallurgical reactor is far more than a mixing tank and a complete approach requires detailed knowledge of both the process and the equipment. The cornerstone in sustainable agitator and reactor design is the identification of case-specific critical agitation functions in order to optimize reactor performance, such as overcoming the factors limiting the reaction rate. Experimental testing can be utilized to determine these limitations and to optimize the reactor design. Selection of materials is a crucial part of reactor design due to the highly corrosive and abrasive conditions often present in the hydrometallurgical industry. Finally, when multiple reactors form a plant unit and plant units form a complete plant, a comprehensive design is required to achieve a solution that has implemented the required operation and maintenance design factors. REACTOR DESIGN IN HYDROMETALLURGY This chapter will point out some of the industrially proven aspects to be considered in successful hydrometallurgical reactor design. More than five hundred delivered Outotec OKTOP reactors, with decades of industrial experience from hydrometallurgical reactor operations, have proven the importance of a systematic approach in reactor design to obtain the best metallurgical results with the highest availability, reliability, and safety. Experimental testing Adequately performed chemical batch tests with authentic and representative raw material samples create the foundation for successful hydrometallurgical process design. Typically these tests are carried out in 2 to 5 liter reactors in conditions where the reaction rate is not limited by the mixing intensity or the available reactive gas. Thus, an excessive amount of gas feed and oversized mixing intensity is utilized. These batch tests provide the most feasible process conditions and basic kinetic data for mass and heat balance calculations. Batch test data is converted for a continuous process based on residence time distribution in completely mixed stirred tank reactor series. Systematic documentation of raw material specifications, test work parameters, and results obtained from the legacy, literature, and present provide a great tool to find out the ideal case-specific processing conditions with the minimum amount of batch test work. A hydrometallurgical reactor must fulfill the given process task, but overdesign should be avoided in order to minimize capital and operating costs. Reactor design can also set limitations on process conditions and kinetic behavior in the hydrometallurgical process, as shown by Latva-Kokko et al. (2014). Selection of agitation can have a significant effect on process performance, such as metals recovery and production rate. Usually limitations including gas feed capacity and solids suspension ability among others are well known by equipment vendors, but their actual effect on process kinetics should be tested experimentally. These tests must be done on a larger scale, approximately 20 liters, so that industrial process conditions can be scaled down representatively. Test equipment is ideally geometrically similar to the industrial design. In addition, the reaction rate limiting factor must be known, since scale-down routines differ by phenomenon. 2

3 Reactor plant engineering It is common practice in hydrometallurgical reactor design to comply with industry-specific standards, so process-specific requirements are not the main focus. In Outotec, we view the reactors in a certain process area as part of a larger reactor plant unit that includes, in addition to the reactor solution, transfer systems, access platforms, piping, and electrification. A split scope delivery with many suppliers rarely allows development of a functional unit. This causes difficulties in providing the best metallurgical performance and functional design to secure high availability with safe and easy operation and maintenance. Common issues in the design and delivery phases of a project are long response times, unclear battery limits, exceeded costs and lead times, and case by case variations in quality. Outotec s approach to overcome these typical issues is to supply the reactor plant unit as a complete, highly predefined, but fine-tuned for the purpose, functional unit that is integrated into the process design and automation. The Outotec OKTOP reactor plant unit concept utilizes proprietary OKTOP agitators. Productized delivery with standardized plant modules provides a shorter response and delivery time, minimizing the site work and guaranteeing the process results and ramp-up. Decades of industrial and experimental knowledge from a wide range of hydrometallurgical reference projects are combined in the delivery of an entire OKTOP reactor plant unit. The construction materials are carefully considered, based on industrial experience and case-specific testing. Safety is an integral part of productized reactor plant delivery. Special requirements like internal cooling or slurry heating are handled with removable high efficiency heat exchanger baffles that are especially suitable for slurries that cause scaling. Agitator support with a service platform as an integral part of the delivery ensures safe and quick one-piece installation and maintenance of the agitator unit. Reactor-related instrumentation and sampling points are built into the product at the optimum point for maintenance access and process control. A slurry distribution system allows any reactor of the OKTOP reactor plant unit to be bypassed for periodic maintenance. Figure 1 illustrates some of the aspects of the modular OKTOP reactor plant unit delivery. Figure 1 OKTOP reactor plant unit. Agitator service center top left and OKTOP launder top right. 3

4 COMMON FAILURES IN REACTOR DESIGN Certain reactor design related issues repeat themselves in the hydrometallurgical industry. Faulty selection or improper dimensioning of an agitator leads to a decrease in process performance and availability. Inadequate mechanical design or the wrong material selection may have a considerable impact on the availability, lifetime, and operating cost of the whole reactor system. One of the factors leading to these errors is the cost savings made in the project implementation phase. Another reason may be the inability to see the whole picture of the required functions for agitation to perform in the process and focusing only on certain aspects of agitation that are considered critical. Agitator positioning The positioning of the agitator is critical to the solids suspension properties. Solid-liquid suspensions are very often present in hydrometallurgical processes and in turbulently agitated tanks they are present in 80% of the operations in the chemical, biochemical, and minerals processing industries (Spidla et al. 2005). Positioning that is too low will cause non-uniform suspension even if all the particles are suspended and if it is too high, it will also cause suspension deficiencies in the just suspended state. However, in some cases, such as a filter feed tank, where the live volume of the tank is an important factor for the equipment functionality, the positioning of the agitator can be in a nonoptimal position, i.e. low in the tank, in order to give the desired maximum live volume. In twoimpeller systems, the distance between impellers may cause separation of the flow patterns for each agitator, thus causing diminishing efficiency in the process. Solids suspension deficiencies can lead to sanding, i.e., solid particle accumulation at the bottom of the tank. (Beale et al. 1983) Ensuring good solids suspension by positioning the agitator too close to the bottom may also cause other problems. The reagent feed is often directed to the top of the tank, and positioning too low may lead to the inadequate mixing of the top level of the solution and therefore inadequate utilization of the reagents. Problems such as poor control of free acid and oxidation-reduction potential may also be observed. Gas dispersion In gas dispersion, the amount of gas feed has to be suitable for the agitator properties so as to avoid flooding of the agitator. Irregular turbulence generated by air injection may impose varying and potentially damaging loads on the shaft and the impeller. Common gas dispersion scale-up equations rarely take into account factors such as electrolyte strength, solids concentration, and temperature. If there are solid particles present in the fluid, the agitator has to be able to perform solids suspension duties at the same time as providing gas dispersion. Gas feed positioning has an effect on the gas dispersion properties of the tank. By using different kinds of spargers, the effect of the agitator on the gas dispersion can be diminished somewhat. One error that has been observed is to feeding the gas into the side of the tank in a downward pumping agitator setup. This will lead to a situation where the gas goes into the rising fluid flow. The use of spargers can also be challenged on the grounds of energy consumption. Good dispersion with sparger requires usage of high velocity in nozzles that causes abrasion issues and has been found to be less economical than direct dispersion with agitator (Jung & Keller 2015). Spargers also tend to have blocking problems especially when gas feed is stopped, which further diminishes the efficiency of sparger operation. 4

5 Mechanical issues Many things can go wrong in the mechanical design of an agitator. Shaft may be too narrow or too thin so that it bends in operation. On the other hand, a very hard and rigid shaft can totally break during heavy loads, such as those during start-up situations. Even a small dissymmetry in impeller blades or simply operating at resonance frequency may cause severe vibration of the whole reactor. Perhaps the most challenging aspect of mechanical design is the long-term endurance of the agitator. During operation, impeller blades are exposed to severe fatigue stress and wear by the combined effect of corrosion and abrasion. Welded joints can be considered the weakest link of the impeller, due to their decreased corrosion resistance and vulnerability to fatigue failure (Latva-Kokko et al., 2014). In order to overcome these problems, Outotec has developed a whole series of agitators that do not contain any welded joints. PERFOMANCE OF DIFFERENT AGITATORS IN THREE-PHASE SYSTEMS Simultaneous solids suspension and oxygen dispersion commonly occur in the hydrometallurgical industry. Due to poor oxygen solubility and high oxygen demand, several hydrometallurgical operations are controlled by the rate of oxygen transfer from the gas to the aqueous phase. It has been stated that among all the factors that affect the rate of oxygen mass transfer, the most important is the reactor configuration and geometry (Filippou et al., 2000). Ideal impellers for gas dispersion are considered to be those that induce radial flow, such as the radial disc turbine (RDT). However, the presence of gas affects the performance of the impeller and its ability to suspend solids. Thus, operation in three-phase gas-solid-liquid systems places some special demands on agitator design. The performance of different impellers; the OKTOP 2000, concave impeller, radial disc turbine (RDT), and pitched blade turbine (PBT), in a three-phase system was studied in a transparent 500 L reactor, as shown in Figure 2. The test reactor had a flat bottom, standard size baffles, and torque measurement to determine the absorbed mixing power. Air was fed to a single point below the impeller. The gas feed capacities of the impellers were determined visually in a water-air system. In these experiments, the agitator was considered to be flooding when the impeller was not able to evenly disperse air into the solution and bigger air bubbles started to burst to the surface. The results from these tests are shown in Figure 3. Figure 2 Transparent 500 L test reactor and different impellers. 5

6 Impeller tip speed, m/s Figure 3 Gas feed capacities of different agitators in water-air systems as a function of absorbed mixing power intensity in effective volume and impeller tip speed. As shown in Figure 3, gas feed capacity is strongly correlated with mixing power intensity in effective volume. This correlates well with the general equation for the estimation of the volumetric mass transfer coefficient (kla) that is frequently found in the literature. The equation is presented below and shows that kla is a function of mixing power intensity in effective volume (P/V) and superficial gas velocity vs. The letters A, B, and C refer to reactor- and application-specific coefficients. kla = A (P/V) B vs C (1) With large size industrial reactors, the speed of the impeller tips is usually the limiting factor, due to erosive wear. Thus, the highest gas feed capacities with decent gas dispersion are achieved with high power number impellers like OKTOP 2000 and RDT. Performance in a three-phase system was studied with quartz sand that had a particle size range of between 50 and 200 µm. The solids concentration in these tests was kept at 400 g/l and the amount of air feed was varied between 0 and 26.5 Nm 3 /h. The impeller rotation speed required for complete off-bottom suspension of all solid particles was determined visually. The results are shown in Figure OKTOP2000 concave PBT RDT Air feed amount, Nm3/h Figure 4 Required impeller tip speed for off-bottom suspension with different agitators in a water-air-sand system. 6

7 Impellers that have a pure radial flow pattern, RDT and concave, required significantly high mixing power in order to suspend all the particles. Interestingly, air feed seems to slightly improve the suspension performance of the concave impeller. Impeller tip speeds on industrial scale with these two impellers would be extremely high, even with low air feed rates. The downward pumping PBT impeller suspends solids with the lowest mixing power when the air feed rate is low. However, air feed strongly decreases the PBT s solids suspension performance, making it applicable only with low gas feed rates. The OKTOP 2000 impeller, which is specially designed for three-phase systems, is able to suspend all the solids with a tolerable impeller tip speed at an air feed rate twice as high as any other impeller in this comparison. Agitation in pressure oxidation autoclaves Pressure oxidation (POX) autoclaves are operated in a three-phase system with high oxygen feed rates. In addition, conditions can be highly abrasive and favorable to scaling. This places some special demands on them as regards agitator selection and design. A radial disc turbine is commonly used in POX autoclaves. However, as shown earlier, it requires a significantly high impeller tip speed in order to suspend solid particles in gassed conditions. This may lead to extensive wear of the impeller blades and the blade carrier disc, as reported by Benefice et al. (2015). The Outotec OKTOP 2200 agitator that is designed for three-phase autoclave applications can be operated with a 12% lower tip speed than RDT, which significantly reduces the wear rate. With an equal power input, the OKTOP 2200 induces more flow, ensuring good solids suspension performance and decreased mixing time (Tiihonen et al. 2013). Scale formation can be a severe problem in POX autoclaves. Scale growth rate is linked to slurry flow velocity. At high flow velocities, scale growth is suppressed by erosion. A radial impeller with a high bottom clearance minimizes the axial flow component in an autoclave compartment, which may lead to heavy scaling of the agitator shaft and compartment walls. In this case, adding a downward pumping impeller to the agitator is a good option because it generates high vertical flow velocities. As shown in the single phase CFD calculation presented in Figure 5, there are no stagnant zones and flow velocities near the shaft and near the compartment walls exceed 1 m/s. Figure 5 Fluid flow velocities in a POX autoclave compartment equipped with an OKTOP 2200 agitator. Agitator rotation speed in the simulation is 118 rpm and impeller diameter 850 mm. 7

8 NOVEL TANK BOTTOM DESIGN In addition to the agitator design, the performance of a hydrometallurgical reactor can also be improved by means of tank design. In particular, the solids suspension performance of a mixing tank can be influenced by the shape of the tank bottom. Latva-Kokko et al. (2015) have developed a totally new tank bottom structure, shown in Figure 6, which is designed for downward pumping axial impellers. This patent pending tank bottom structure has a corrugated formation, comprising alternate consecutive ridges and valleys that extend radially in relation to the center of the bottom. The bottom structure straightens, concentrates, and channels the flow of the slurry and thus increases the velocity of the flow. The structure simultaneously turns the downward flow generated by the hydrofoil impeller smoothly upwards and channels it. This increases the liquid flow velocities near the tank bottom as well as in the upward flow direction, in comparison to a flat or dished bottom tank. (Latva-Kokko et al. 2015) Figure 6 Novel corrugated tank bottom structure developed by Outotec. Experimental tests were made in order to study the performance of the corrugated tank bottom structure. The tests were conducted with laboratory-scale equipment using an OKTOP 3200 axially downward pumping hydrofoil impeller with a diameter of 145 mm. A tank with a flat bottom was compared to a tank with the new corrugated bottom structure. Both tanks had a diameter of 362 mm and were loaded to the same surface height with quartz sand in water slurry. Both tanks had four baffle plates of standard width. Different solids contents and particle size distributions of the test slurry were tested. The impeller rotation speeds required for complete off-bottom suspension (Njs) and uniform suspension were determined visually. Transparent tanks were attached to a torque table that was used to measure the absorbed mixing power in each case. The results are shown in Figure 7. 8

9 Figure 7 Required mixing power intensities in effective volume to obtain off-bottom and uniform suspensions with different tank bottom structures. Based on these experiments, the mixing power required for off-bottom suspension was between 20 to 60 % lower with the corrugated tank bottom structure compared to the flat bottom tank. Uniform suspension was achieved with a 55 to 75 % lower power consumption. The best result of the novel tank bottom design was achieved with a coarse particle size, µm, and the highest solids content of 800 g/l. CONCLUSION A sustainable reactor design for demanding applications in hydrometallurgy is always case-specific and requires detailed knowledge of both the process and equipment. Experimental testing provides a solid foundation for successful design. Identification of case-specific critical agitation operations is essential for the proper choice and dimensioning of the agitator. Supplying the reactor plant unit from highly predefined standardized plant modules provides a short response and delivery time, minimizing site work, and guaranteeing process results and ramp-up. Faulty agitator positioning and inefficient gas feed arrangements are common failures in industrial reactor design. In addition, attention must be paid to the mechanical design details of the agitator in order to obtain a long lifetime. The performance of a hydrometallurgical reactor can be greatly influenced by agitator and tank design. For instance, an impeller that is specially designed for three-phase systems is able to suspend solids with twice as high air feed rates compared to impellers designed for gas dispersion solely. In applications where solids suspension is the main focus, more than 50 % savings in mixing energy can be achieved with the novel tank bottom structure designed for solids suspension with downward pumping axial impellers. NOMENCLATURE PBT RDT pitched blade turbine radial disc turbine 9

10 POX kla vs P V A,B,C pressure oxidation volumetric mass transfer coefficient superficial gas velocity power volume application-specific coefficients REFERENCES Beale, C., Oldshue, J. & Sternod, G. (1983), Mining and leaching in 1400 m 3 tanks at Rossing, Hydrometallurgy (roc. 3rf Int. Symp. Atlanta, 1983), Metallurgical Society of the AIME Benefice, F., Schmidt, R-O., Keller, W. & Jung, J. (2015), «Walking the gold path» - continuous improvement of agitation systems for gold processing, ALTA 2015 Gold-Precious Metals Proceedings Filippou, D., Cheng, T. & Demopoulos, G.P. (2000), Gas-liquid oxygen mass-transfer; from fundamentals to applications in hydrometallurgical systems, Mineral Processing and Extractive Metallurgy Review, 20, pp Jung, J. and Keller, W. (2015) Process and cost cptimized agitator solutions for hydrometallurgical base metals processing, Proceedings of Pb-Zn 2015, GDMB Latva-Kokko, M., Hirsi, T., Lindgren, M. & Ritasalo, T. (2014), Influence of reactor design to process performance in hydrometallurgical applications, Hydrometallurgy 2014 Volume II, Proceedings of the 7th International Symposium, Canadian Institute of Mining, Metallurgy and Petroleum, pp Latva-Kokko, M, Hirsi, T., Ritasalo, T. & Tiihonen, J. (2015), Improving the process performance of gold cyanide leaching reactors, World Gold Conference 2015 Proceedings, The Southern African Institute of Mining and Metallurgy, pp Spidla, M., Mostek, M., Sinevic, V., Jahoda, M. & Machon, V. (2005), Experimental assessment and CFD simulations of local solid concentration profiles in a pilot-scale stirred tank, Chem. Pap, 59, pp Tiihonen, J., Haakana, T. & O Callaghan, J. (2013), Outotec pressure oxidation More out of sulfide ore, World Gold Conference 2013 Proceedings, The AusIMM, pp