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World of Metallurgy – ERZMETALL 69 (2016) No. 2108

Jochen Jung et al.: Process and Cost Optimized Agitator Solutions for Hydrometallurgical Base Metals Processing

Process and Cost Optimized Agitator Solutions for Hydrometallurgical Base Metals ProcessingJochen Jung, Wolfgang Keller

Stirred tank reactors under continuous atmospheric oper-ating conditions fulfill an important key function for many gassed hydrometallurgical leaching processes. Apart from realizing “economies of scale” and implementing advanc-es in process performance, achieving a cost optimization of such systems is a fundamental goal. One side effect of scale-up is that the agitator shaft speeds are reduced, which increases the risk of “flooding” with conventional impeller types. To prevent “flooding”, the design criteria for the in-stalled motor power of such agitators is then not driven by the gas mass transfer rate but by the additional power re-quirements for keeping the three-phase gas-slurry mixture suspended. This paper presents a new type of impeller for gas dispersion and solids suspension that can handle very high gas loads and that avoids unnecessary agitator overde-sign. – In many gassed applications, such as base metals processing in atmospheric leaching tanks, the compressor power of the air supply system often exceeds the agitator power. A detailed analysis shows that high pressure gas sparging is not justified from an economic standpoint and that the over-all efficiency is low when compared to mod-

ern agitator solutions. Abrasion at the nozzles is a common issue due to high gas velocities. In the worst case this can lead to an unbalanced gas distribution, which causes ad-ditional impeller loads and lowers the chemical yield due to the reduced mass transfer. A completely new gassed agitator solution has been developed to overcome these disadvantages. A single gas pipe feeds the gassed impeller where the gas is distributed by hollow spars directly to the impeller blades in the zones of highest shear. The overall pressure loss is thereby minimized and gas dispersion is maximized as no dampening of the gas flow occurs. – Mass transfer and gas utilization is also important if pure gases are used. One example is the use of pure oxygen instead of air to increase the solubility and therefore the driving force for the mass transfer. The economies of such processes can be further optimized by using special multiple impeller stage agitators together with adapted vessel geometries.

Keywords:

Agitation – Hydrometallurgy – kla-mass transfer coeffi-cient – Leaching of ores – Gas utilization

Prozess- und kostenoptimierte Rührwerkslösungen für die hydrometallurgische Erzverarbeitung

Atmosphärisch begaste Rührkesselkaskaden in kontinuier-licher Fahrweise erfüllen in vielen hydrometallurgischen Laugungsprozessen eine Schlüsselfunktion. Neben der Kos-tendegression durch Bau immer größerer gerührter Einhei-ten und Verbesserungen der Prozesse, ist die Gesamtkosten-optimierung ein wesentliches Ziel. Ein Nebeneffekt bei der Maßstabsvergrößerung (Scale-up) ist die Verringerung der Rührwellendrehzahlen, wodurch das Risiko einer „Über-flutung“ bei Verwendung konventioneller Rührorgane steigt. Die Auslegungskriterien zur Dimensionierung der Motorleistung richten sich dann nicht mehr allein nach der Stoffübertragungsleistung, sondern nach den zusätzlichen Leistungsanforderungen, die notwendig sind, um ein „Über-fluten“ des Rührorgans zu verhindern und den Feststoff in Suspension zu halten. In diesem Artikel wird eine neue Art von Rührorgan zum Gasdispergieren und Feststoffsuspen-dieren vorgestellt, das sehr hohe Gasmengen handhaben kann, aber ein Überdimensionieren umgeht. Bei vielen die-ser begasten Rührkesselkaskaden ist die Kompressorleis-tung des Luftversorgungssystems oftmals signifikant höher im Vergleich zur Antriebsleistung der Rührwerke. Eine de-taillierte Analyse zeigt, dass Hochdruckbegasungssysteme aus ökonomischer Sicht unwirtschaftlich sind und der Ge-samtwirkungsgrad im Vergleich zu modernen Rührwerks-lösungen niedrig ist. Abrieb an den Düsenauslässen ist auf-grund der hohen Gasaustrittsgeschwindigkeiten ein häufig

anzutreffendes Problem. Im schlimmsten Fall kann es durch Verblocken einiger Auslässe zu einer ungleichmäßigen Gas-verteilung am Rührorgan kommen. Die Folgen sind zusätz-liche dynamische Beanspruchungen des Rührwerks, aber auch eine Verringerung der chemischen Ausbeute bedingt durch einen reduzierten Stofftransport von der gasförmi-gen in die flüssige Phase. Um diese Nachteile zu überwin-den, ist eine gänzlich neue Lösung für begaste Rührwerke entwickelt worden. Eine einzige Gasleitung versorgt das Begasungsrührorgan, wobei das Gas über hohle Holme direkt zu den Rührorganblättern in die Zonen mit höchster Scherung eingeleitet wird. Gleichzeitig wird der Gesamt-druckverlust minimiert, während die Gasdispergierleistung maximiert wird, weil der Gasstrom ungedämpft bleibt. Der Stofftransport und die Gasausbeute sind auch dann wichtig, wenn reine Gase verwendet werden. Ein Beispiel ist die Ver-wendung von reinem Sauerstoff anstelle von Luft, um die Löslichkeit und damit das treibende Gefälle für den Stoff-transport zu erhöhen. Die Wirtschaftlichkeit eines solchen Prozesses kann weiter optimiert werden, indem spezielle mehrstufige Rührorgananordnungen in Kombination mit einer angepassten Behältergeometrie verwendet werden.

Schlüsselwörter:

Rühren – Hydrometallurgie – kla-Stofftransportkoeffizient – Erzlaugung – Gasausbeute

World of Metallurgy – ERZMETALL 69 (2016) No. 2 109


1 Introduction

The process routes for the extraction of many metals now-adays are mainly hydrometallurgical [1]. Once the ores are milled and the valuable ore is separated and often con-centrated by flotation, agitated leaching vessels are crucial to suspend and to leach these solids. Further downstream processing can only be successful if the valuable components have been entirely dissolved during the leaching step. Gen-erally speaking, atmospheric leaching tanks are less complex when compared to autoclave technology. Although the re-action rates are lower, atmospheric leaching vessels can be operated economically in cascades of up to ten vessels and individual vessel volumes nowadays typically ranging from 250 to 1500 m³. Particularly challenging is the design of agi-tated leaching reactors for gassed applications, e.g. bioleach-ing reactors for refractory gold ore [2] or the atmospheric leaching of zinc [3]. Operators of hydrometallurgical plants are, among other challenges, confronted with increasing energy costs, material costs and reduced budgets for new investments. Therefore, this paper will address the process and technical relevant design issues as well as the economic aspects of advanced agitation solutions.

2 Process related design issues

Design requirements for gassed continuously stirred tank reactors (CSTRs) are manifold. These should be weighed up carefully during the early design phase of a project as the concept will not only have an influence on the invest-ment costs but an impact on the operational costs as well.

2.1 Process intensification (output orientated view)

From a metallurgical standpoint, a high leaching rate or yield is of utmost interest. Process intensification means that the volume-specific throughput of reactors increase, or in other words, the plant can process more ore at higher specific yields. Agitators must fulfill the following mixing tasks in order to achieve these goals:

• Homogeneous solids suspension keeps the solids per-manently in contact with the leaching agent. Solids should not accumulate over time inside the vessel and should not deposit at the vessel bottom.

• Efficient gas dispersion is necessary to maximize bub-ble breakup and to suppress bubble coalescence in the bulk. As a result the interfacial area and the mass transfer coefficient will increase – as expressed by the kla value. This again helps to keep the dissolved oxygen on a sufficiently high and stable level which is especially important for bioleaching processes and shown in [4].

• At gassed conditions the impeller has to maintain its level of absorbed power to ensure homogeneous sol-ids suspension and gas dispersion. Suitable gassing im-

pellers are characterized by high power retention and show no power drop under gassed conditions.

• Impeller flooding should not occur under any circum-stances, not even at very high gassing rates or reduced power inputs.

2.2 Process efficiency and costs (input orientated view)

Gassed CSTRs are complex agitated systems that include the agitators, vessels, gas supply units and gas dispersion systems inside the vessel. Other internals such as baffles, feed and discharge pipes are also part of an appropriate overall de-sign. The detailed analysis shows that the cost-benefit ratio depends on a correct configuration of the system. In the end, the overall cost is more than simply adding the costs of the single units. Two examples will be presented in this paper:

• optimization of overall power requirements for agitators and air compressors by using a direct gassing impeller,

• significant cost savings for processes using pure gases, e.g. oxygen, by increasing the gas utilization rate,

• engineering related design issues.

From an engineering point of view, it should be possible to transfer the results obtained from test work in the lab or pilot scale to the commercial scale. The nature of these challenges is often technical, but commercial aspects also play an important role.

Gassed CSTRs typically consist of the following main func-tional units:

• gas supply (compressor unit, piping and instrumentation),• agitators (motor, gearbox, shaft, impellers),• gas distributor (typically ring spargers),• vessel and internals, e.g. baffles and heat exchanger tube

bundles in bioleaching.

As an example, Figure 1 shows a state-of-the-art setup of a bioleaching reactor.

The power requirements of such gassed CSTRs are basi-cally driven by the agitator and compressor motor. As an example, the design installation power for primary bio-leaching reactors and vessel sizes of 1000 m³ are typi-cally between 250 to 315 kW for agitation and up to 300 to 500 kW for the compressor power.

In the light of the above, it would be useful to have correla-tions to calculate the net power use for the hydrometallurgi-cal process. As already discussed by Pieterse for autoclaves [5], it is helpful to break down the process net power into:

1. gas expansion or gas buoyancy power of the rising gas bubbles,

2. dissipation of the kinetic gas power at the nozzle outlets of the sparger system,

3. absorbed power by the agitator.

Solutions d’agitation pour le traitement hydrométallurgique des métaux de base avec une optimisation des processus et des coûtsSoluciones de agitadores para el procesamiento hidrometalúrgico de metales comunes con optimización de procesos y gastosPaper presented on the ocassion of the Lead-Zinc Conference Pb-Zn 2015, June 14 to 17, 2015, in Düsseldorf, Germany



3 Energy efficiency considerations

To understand and to optimize the process not just from the process side, it is also important to have a closer look at the energy efficiency for generating the net power. In Figure 2 the energy fluxes for the agitator and the compres-sor units are explained by a so-called Sankey diagram. The widths of the arrows are proportional to the flow quantity. These diagrams help to visualize and to analyze the energy flows. The amount of energy losses and potential for opti-mization can be more easily identified.

Electrical losses of the agitator and compressor motor are 5 to 15 % of the electrical power input dependent on the energy efficiency class of the motor and the operation mode (direct or with a variable frequency drive). Mechanical loss-es in the gearbox account for approximately 5 %. In case of the agitator, the output power of the gearbox is transferred directly to the impellers and can be used as absorbed power for the process. For the compression of air additional me-chanical, thermal and pneumatic losses must be considered. The mechanical friction, the generation of compression heat and the pneumatic losses in the pipes and air distributers usually amount to another 15 to 40 %. The exact values de-pend on the pressure level as more heat must be removed to generate highly compressed air. The pressure level will also have an impact on the type of compressor that must be selected, e.g. rotary blower or screw compressor, and the number of compression stages that must be used. Pneumatic losses can be minimized by correct selection of the pipe di-ameters, valves and the selection of optimal sparger systems.

4 Experimental work

There are numerous publications concerning the mass transfer in gassed agitated slurries, kla measurement meth-ods and the effect of sparger geometries in gassed stirred reactors as summarized by [6]. Surprisingly, little is report-ed about the technical, operational and the energetic impli-cations or overall optimizations of such systems.

Therefore, experiments have been performed to better understand the interactions between absorbed agitation power, gas expansion (buoyancy) power and kinetic gas power. The authors carried out an extensive test program in the 1 m³ scale to analyze the results in terms of energy efficiency and gas utilization. The agitation power was

Fig. 1: State-of-the-art design of a CSTR showing the main functional units

Fig. 2: Sankey diagram visualizing the energy flows of agitator and com-pressor power



varied in a range from 0.05 to 0.75 kW/m³. The lowest superficial gas velocity was 0.1 mm/s and at very high gas-sing rates up to 25 mm/s. The study also comprised three different ratios of ring sparger to impeller diameter dR/d2, ranging from 0.78 to 1.10. The amount of holes in the sparger rings was varied to study the effect of gas predis-persion. Besides the ring geometry tests with high pres-sure lance spargers, a single feed pipe with a diameter of 30 mm and a new rotating sparger were conducted. Figure 3 illustrates a schematic overview of the most common industrial gas distributors.

The torque of the agitator shaft, the shaft speed, the gas hold-up and the oxygen reaction rates were measured for every setup for each test run. The so-called sodium sulphite method was used [7] for the measurement of the oxygen reaction rates. In the presence of cobalt sulphate as cata-lyst, the oxidation reaction of sodium sulphite to sodium sulphate is very fast and the reaction rate is controlled by the gas-liquid mass transfer resistance. The coalescence behavior can be adjusted by setting the ion strength to the required conditions of real slurries. As the reproducibility of this test method is better than ±10 %, kla values can be calculated with good accuracy. This is important for comparative studies and agitator scale-up. In addition to the fundamental test work with model systems, hydrome-tallurgical tests with real slurries in the lab and pilot scale are recommended, especially when developing new hydro-metallurgical processes.

5 Example 1: Bioleaching applications

Ring spargers of different diameter ratios are typically used in bioleaching applications. If designed correctly, this type of sparger provides a good gas predispersion at high gas flow rates. The pressure losses and the shear rate at the nozzle outlets are moderate when compared to high pres-sure sparger solutions.

Besides the gassing rates and agitator power, Kato et al. [8] studied the influence of the ring sparger diameter of Rushton Turbine impellers on the mass transfer rate (kla values). The diameter of the ring sparger had no measurable impact on the kla values as long as the power number of the impeller was not affected by the gas load. The advantage of a ring sparger diameter larger than the

diameter of the impeller was to avoid direct gas loading. In an earlier work, Birch & ahmed [9] already proposed to move the sparger closer to the vessel wall and even above the impeller section to avoid a direct gas loading of the impeller. However, they stated that this can lead to poorer gas dispersion and reduced gas hold-ups. Similar results were obtained by [10] while studying axial flow im-pellers. Although they found that widefoil impellers are less prone to flooding compared to pitched blade turbines (PBT), the hydrodynamic properties also depend on the sparger geometry and nozzle location.

Tests at EKATO in the 1 m³ scale showed that the effect of flooding could not be observed for the COMBIJET impeller (Figure 4), compared to widefoil impellers. This means, that the sparger ring geometry was of minor importance, which is in agreement with Kato’s observations for low power drop and non-flooded conditions. The power retention can also be characterized by the impeller’s ability to maintain a high pumping number under gassed conditions. This means that the agitator’s pumping flow is undisturbed by the gas.

Figure 5 shows a comparison of a PBT, a widefoil impeller type and the COMBIJET. The relative pumping capacity is plotted with the gas expansion power on the abscissa versus the agitator power on the ordinate. The PBT is characterized by a sudden drop in its pumping capacity under gassed conditions. Widefoil type impellers have ad-vantages in this regard. However, the COMBIJET, which is characterized by its very stable pumping capacity, shows the best performance. The advantages of the COMBIJET impeller for agitation and the overall process design will be discussed in the following.

Fig. 3: Schematic overview of different sparger solutions for agitated CSTRs

Fig. 4: 1 m³ test setup with ring sparger and EKATO COMBI-JET impeller



5.1 Advantages of the COMBIJET impeller

A good quality of gas dispersion, solids suspension, bulk blending and heat transfer in large bioreactors in miner-als processing is traditionally achieved by using widefoil impeller types. This group of impellers represents a good solution for moderate to high gassing rates if the power input by agitation is kept high enough. However, one effect of scale-up is that the agitator shaft speeds are reduced. This increases the risk of “flooding” of this impeller type, as described before by Keller in [11]. In large commercial scale, the design criteria for the agitator’s motor power might not be driven by mass transfer requirements to fulfill the process task but rather the prevention of flooding. In return, flooding does not just have a negative effect on the

gas dispersion but another consequence of flooded impel-lers is poor solid suspension as the circulation of the slurry is interrupted.

Figure 6 shows a comparison of the gas handling capacity of two different types of agitators in existing identical primary bioleaching reactors as an example. Even with a speed reduction to 80 % of the nominal speed, which is about half of the nominal agitation power, the COMBIJET never reaches the area of flooding (numbers below one). With a widefoil type of impeller, the very high gassing rates cannot be dispersed safely if the agitator power is reduced below 100 %. Even more, the solid suspension quality decreases because the pumping capacity of a widefoil im-peller breaks down.

Fig. 5: Relative pumping capacity under gassed conditions – comparison of different impeller types

Fig. 6: Gas handling capacity for widefoil and COMBIJET impeller for dif-ferent gassing rates



5.2 Example of electrical power and cost savings

The energy and cost savings are shown exemplarily for a primary bioleaching reactor with a tank size of 1500 m³. The authors compared two agitator setups:

1. Widefoil impeller: 330 kW motor power,2. COMBIJET impeller: 200 kW motor power.

The on-site results proved that with a COMBIJET impeller at reduced power input the DO levels are the same as for the 330 kW agitators with widefoil impellers. At the same time, all other physical and chemical parameters were iden-tical. Sulphur oxidation rates were even slightly higher. The COMBIJET also showed a significantly more stable power uptake rate under changing gassing rates.

In this example for one primary bioreactor tank, approx-imately 95,000 kWh of electrical power are saved per month. Therefore, the return on investment of an agitator retrofit is very short, typically 0.5 to 1.5 years. For green field projects another positive effect of less motor power consumption are lower dynamic forces and therefore re-duced mechanical loads on the agitator support structure. In addition, smaller generator stations for backup power might be required in remote areas. Although the example above is for the primary bioleaching tanks, this approach is also applicable for secondary and the tertiary bioleaching vessels as well as for other similar gassed applications.

5.3 New developments – direct gassing

The objective of ring spargers is to supply uniform gas sparging. Therefore, it is important to know the pressure distribution and the velocity profile inside the ring pipe. Joshi et al. [12] studied the influencing parameters such as the number of holes, the hole and the pipe diameter and the pipe length among others. They validated their experi-mental results with their own CFD simulations and showed the limitations of the existing analytical models. Although it is possible to optimize sparger geometries for a specified design flow rate, the extent of non-uniformity might change with lower or higher gassing rates.

5.3.1 Pressure losses and operational disadvantages of ring spargers

The pneumatic losses in a ring sparger typically mount up to 0.20 to 0.50 bars. Influencing parameters are:

• gas flow rate,• gas density due to static pressure head,• feed pipe diameter of the air supply,• inner diameter of the ring,• outlet nozzle diameters and geometry of the nozzle,• uniformity of gas velocity amongst nozzles.

The picture to the left in Figure 7 shows four single nozzles of a section of a ring sparger. The nozzles are equipped with non-return valves made of rubber to prevent backflow of slurry. Slurry should not be able to enter the nozzles or get inside the sparge ring in case of a power outage, a compres-sor failure, or an accidental shut off of the compressed air. If

this happens, solids will sediment and block the single noz-zles or even the complete ring. Figure 7 on the right shows a picture of a ring sparger after slurry has entered the pipe.

Abrasion issues must be considered as slurry is perma-nently sucked into the gas jet exiting the nozzles and slurry swirling around the gas outlets can cause slide erosion. High gas outlet velocities also generate strong local shear zones which promote the probability of destroying bio-mass.

5.3.2 Direct gassing with COMBIJET+

In addition to the pneumatic design issues and the opera-tional difficulties found with ring spargers, the mechanical design can become quite complex as well. Due to the dy-namic forces acting on internals near the impeller, the me-chanical structure and the nozzle tips have to be sufficiently robust, as described by [13]. On the other hand, geometries that are too massive would comprise the flow field around the impeller and be detrimental for the mixing task.

Because of the disadvantages of the prior art, EKATO de-veloped and patented a new gassing concept for impellers (patent number WO 2010/142406 A1). The gas is directly fed through one central feed pipe coming from the vessel bottom into a rotating gas distributer which is integrated into the impeller (Figure 8).

The non-contacting gap between the feed pipe and the rotating gas distributer has the function of a “pneumatic

Fig. 7: Additional pressure losses due to flow restricting nozzles and blocked ring sparger

Fig. 8: Commercial scale reactor with COMBIJET (optional with Di-rect Gassing = COMBIJET+)



seal” that is maintenance free. The gas flows directly to the outer areas of the impeller blades from the rotating distributer where it is discharged and dispersed in zones of highest shear. As an advantage, the COMBIJET+ has an up to 20 % increased gas dispersion performance compared to conventional gassing systems. Additionally, the direct gas-sing method considerably simplifies the mechanical design, while at the same time it minimizes the pneumatic pressure losses. Since gas sparger rings frequently get blocked, the new air feed via a single pipe to the COMBIJET+ helps to reduce the number of shut-downs caused by blockings of conventional gas sparger designs as well. Fig. 9 shows the excellent gas distribution in a coalescing air-water system for different agitator speeds from very low to moderate power inputs in a 1 m³ test vessel with the EKATO COM-BIJET+.

It can be summarized that the process efficiency and the overall power requirements can be optimized with optimal agitation and direct gassing solutions. Still, the process task of agitation is not limited to primary gas dispersion. Dead zones will occur if the gas is not mixed properly throughout the slurry volume. As a consequence of reduced effective reaction volumes, process conditions will vary and overall reaction rates will be lowered. This not only applies for gas dispersion but is also important for a homogeneous mixing of the slurry. It is important to achieve a uniform suspension of the solids since the reactors are operated with an overflow. In addition, the operational temperature has to be controlled in bioleaching due to the exothermic heat of reaction. Therefore, the third mixing task for this application is to maintain a uniform temperature level for the bacteria throughout the slurry volume and to provide flow to the tube bundle heat exchangers inside the reactor for heat removal.

6 Example 2: Atmospheric leaching with pure gases

Atmospheric leaching has been developed for metals such as zinc, copper, gold, nickel, manganese and rare earth metals to name only a few. Typical operating temperatures are close to the boiling point and usually range from 80 to 100 °C in order to maximize the reaction rates. From a process point of view, it is advantageous to use e.g. pure oxygen instead of air since the solubility in the slurry and therefore the driving force for mass transfer increases. On

the other hand, under these conditions the sparged gas saturates quickly with water vapor diluting the reactive gas which in return reduces the mass transfer. The gas utiliza-tion should be high as additional costs arise for the supply or the on-site generation of oxygen.

6.1 Basics on oxygen mass transfer rate and gas utilization in CSTRs

In gassed CSTRs, the mass transfer rate of the gas into the liquid bulk phase is often the controlling mechanism and limits the leaching rate. The aim is to maximize the interfacial area and the mass transfer coefficient which is expressed by the kla value. The overall mass transfer rate OTR can be calculated with the knowledge of the kla value and the driving concentration gradient c* – cbulk of oxygen.

( )-⋅ ⋅2 2

bulk*l O OOTR = k a V c c (Equation 1)

Where:

OTR overall mass transfer rate of oxygen [kg/h],kla mass transfer coefficient [1/h],V agitated volume [m³],c*O2

physical solubility of oxygen in the slurry [kg/m³],cbulk

O2 concentration of dissolved oxygen in the slurry [kg/m³].

The reaction rate under steady-state conditions, e.g. sul-phur oxidation rate, is proportional to the mass transfer rate of oxygen. As long as the chemical reaction is fast, the level of dissolved oxygen in the bulk is low and the reaction rate is limited by the oxygen mass transfer. The reaction rate can be accelerated by increasing the mass transfer rate (kla value). kla values can be calculated using Equation 2. This equation is fundamental for an accurate scale-up from the pilot to the commercial scale, as explained in more details in [14].

α

β ⋅ ⋅

agl sg

Pk a = c v

V (Equation 2)

Where:c, α, β constants dependent on impeller/agitator geometry

and sparger arrangement, [c] = 1/h,Pag/V volume specific absorbed agitator power,vsg superficial gas velocity.

Increasing the gas utilization means that the gassing rate is lowered as close as possible to the stoichiometric require-ments for the chemical reaction. This has the negative ef-

Fig. 9: COMBIJET+ (Direct Gassing) – 1 m³ laboratory scale 100 rpm 150 rpm 200 rpm



fect that the superficial gas velocity is decreased, kla values are lower (Equation 2) and the oxygen transfer rate drops (Equation 1). Of course this is in contradiction with higher utilization rates and leads to a “vicious cycle”.

6.2 Influence of kinetic gas power

The expansion power (buoyancy) of the gas is considered by the superficial gas velocity in Equation 2. The absorbed agitator power is accounted for by the term P/V and the exponent. Unclear is how the kinetic gas power contributes to the mass transfer.

Figure 10 shows the experimental setup with the COM-BIJET impeller and four single lances for sparging air or oxygen at high pressure in a 1-m³ test vessel at EKATO. The sparger nozzles had a diameter of 2.5 mm, a length of approximately 10 mm and a smooth transition at the nozzle inlet towards the larger feed pipe diameter.

The gas flow imparts its kinetic energy near the nozzle exit when it hits the slurry. someya [15] studied the impact and free jet length of high pressurized gas flows into water. He found the effective penetration depths of the underex-panded gas jets in water in a range of 50 mm up to 125 mm at a maximum stagnation pressure of 8 bars. Using kla measurements, the authors found that the kinetic power of the gas is negligible compared to the agitator power and the gas buoyancy for ring spargers and open pipes where the typical outlet velocities are below 50 m/s. The picture changes for outlet velocities above 100 to 150 m/s.

A physical limitation to augment the gas flow velocity is the sonic speed that is reached at the critical pressure ratio. The gas density and the mass flow through the nozzle in-creases above the critical pressure ratio, but under choked conditions the gas exit velocity will not exceed the sonic velocity. More details about the theory of compressible gas flows can be found in standard textbooks, such as [16]. Tests results show that the pneumatic characterization of such pressurized spargers is in excellent agreement with the theory and the results of [15] (Figure 11).

Figure 12 shows the gas distribution in a 1-m³ test vessel with a non-coalescing water-salt mixture and four high pressure gas spargers. The gas leaves the nozzles at sonic speed.

The results of the kla measurements with the high pressure lances can be summarized as follows:

• The kla values can be further increased by the use of ki-netic gas power. However, the use of kinetic gas power only makes sense if no additional compressor power is required to supply the pressurized gas because the power efficiency is much lower compared to agitation.

• For a specified gas flow rate and excess pressure, the maximum kinetic gas power is fixed in the same way as the gas expansion power. It is not an adjustable parameter.

• The agitator power can be freely adjusted. This means that the kla values can be augmented independently of the gassing rate.

Fig. 10: COMBIJET with high pressure spargers Fig. 11: Pressure–flow rate curves of high pressure spargers

Fig. 12: Gas mixing and dispersion with high pressurized spargers

50 rpm 100 rpm 150 rpm 200 rpm



• The penetration length of the gas jets is restricted to the zone close to the nozzle outlets. Therefore, the task of the agitator must be also to mix and to homogenize the gas over the reactor volume. Figure 13 shows that the gas distribution is not uniform at low stirrer speeds, although the impeller is not flooded.

Figure 13 shows the analysis of kla measurements in terms of energy efficiency. Therefore the measured kla values were divided by the volume specific total power input of the specific test run. The power ratio of agitation power to the overall gas power is plotted on the abscissa.

The key results of Figure 13 are as follows:

1. The energy efficiency of the process result is dependent on the gassing principle.

2. The Combijet impeller demonstrates that it is the most effective gassing system. The measured kla values are higher than for ring spargers.

3. Generally speaking, the energy efficiency of all setups improves with increasing ratio of agitator power versus gas power.

6.3 Optimizing the process efficiency: kla, residence time and solubility

Thin and tall reactors are common for fermenters, differ-ent chemical reactor types and alumina precipitators [14]. Obviously the advantages of the higher filling height to vessel diameter ratio are a prolonged residence time of the reactive gas, slightly higher superficial gas velocities and an augmented over-all oxygen solubility that is gained from the pressure head of the elevated filling height.

Classical fermenters are equipped with Rushton turbines and/or PBTs. Flooding issues occur easily if these agita-tors are underpowered. With purely radial flow impellers such as the Rushton turbine, individual flow zones (known as compartmentalization) tend to form in each impeller

section. On the other hand, multiple up-pumping widefoil impellers have the tendency to develop regions of very high gas fractions in the upper part of the vessel as they might get flooded. More information about this topic can be found at roB et al. [17] and in [18].

As discussed for the bioleaching reactors, the classical type of impellers have disadvantages regarding the gas dispersion and power retention behavior under gassed conditions. Additional attention must be paid to solid sus-pension and mixing time for thin and tall gassed CSTRs.

Fig. 13: Power efficiency of mass transfer; green and gray areas represent typical bioleaching conditions

Fig. 14: Gas mixing and dispersion with COMBIJET

Test setup with multiple COMBIJET stages, kla measurement with sodium sulphite method

Test setup with multiple COMBIJET stages, tests with original slurry



The authors conducted an extensive test program to gain more information about the geometric effects in a tall and thin gassed reactor.

In Figure 14 the test setup is presented using the multiple COMBIJET stages. The test scale was 125 litres. The picture on the left hand side shows a test run with a non-coalescing sodium sulphite water mixture used for kla measurements. The kla values were calculated by measuring the oxydation times for oxydizing sodium sulphite to sodium sulphate. Besides the DO levels and temperature, the off-gas oxygen content was also recorded. These measurements were in good agreement with the models for calculating the gas utilization. Supplemental test work with original slurry had the objective to carry out oxydization reactions monitoring e.g. the DO, pH and Redox values in order to assess the sys-tem’s reactivity. Another objective of tests with real slurry was to assess the solids suspension at gassed conditions as shown in Figure 14 on the right hand side.

The standard filling height to diameter ratio of approximate-ly 1.2 was compared with the “thin and tall” vessel geometry as shown in Figure 15. Surprisingly, this multistage COMBI-JET setup showed that the kla values are furthermore in-creased because the gas recirculation is augmented and the bubble coalescence is minimized. Figure 15 shows that the kla values are significantly improved by a factor higher than 1.3 independent of the absorbed power. The correlation of the slope with Equation 2 shows an increased exponent. This means that the power efficiency of the COMBIJET impeller is superior to traditional impellers.

6.4 Example of oxygen savings with multiple stage COMBIJET

The test work demonstrated that excellent gas dispersion leads to a significantly higher gas utilization. In the test scale

of 125 liter the gas utilization went up by a factor of nearly three which is in absolute agreement with the kla and the off-gas oxygen measurements. Another result of the trials was that the mixing tasks solids suspension and fast blending are also fulfilled for the thin and large reactors and that these results can be scaled-up safely to the commercial scale.

From Table 1 it can be concluded that the volume flow rate of oxygen can be reduced by approximately 360 Nm³/h for Setup 2. This means that 362.9 t of pure oxygen can be saved per month by adapted vessel geometry and with a multistage COMBIJET agitator.

7 Conclusions

In addition to various stirred applications in minerals pro-cessing, the atmospheric leached processes are of particular importance. Especially challenging are gassed operations. Typical examples are bioleaching and the atmospheric leaching of non-ferrous metals.

Since the quality of ore bodies often decreases as rich re-sources get depleted, one trend is to process more slurry volume in one plant or module which leads to further increased vessel sizes for current and future projects. For applications as described above, widefoil impellers have

Fig. 15: Gas mixing and dispersion with multiple stage COMBIJET

Table 1: Scale-up of test results – comparison of two 800 m³ reactor designs with COMBIJET

Setup 1 Setup 2

Impeller setup COMBIJET COMBIJET multistage

Tank diameter T 9.5 m 7.5 m

Filling height H 11.3 m 18.1 m

H/T ratio H/T = 1.19 2.41

Gassing rate to stoichiometric

160 % 123 %



represented the standard in the mining industries so far as these agitators combine solid suspension with a good gas dispersion performance compared to common hydro-foil impellers. However, limitations occur when scaling-up gassed processes to very large reactor sizes and with in-creasing gas flow rates; especially when using gases with inert volumes such as air. Another issue is the demand to minimize the energy consumption which leads to reduced power input and lower agitator shaft speeds. This further increases the risk of “flooding”. When seen together it is obvious that new concepts are required. The answer to this challenge is the EKATO COMBIJET. It offers higher gas handling capacities and therefore provides stable flow and pumping conditions in the vessel for an extended range of operation conditions. The motor power of the agitator can be designed adequately for the mass transfer requirement and can be adjusted to the actual demand. Costly over designs are avoided and significant electricity costs savings are realized at the same time.

A further improvement can be achieved if the gas sparging device is integrated into the impeller itself. A typical pro-cedure is to add and predisperse the gas using a gas sparge ring with many small holes in the millimeter diameter range. One problem with this design is that solids are likely to block these holes. This means that the vessel has to be emptied so that the sparge ring can be cleaned and results in production losses. EKATO developed the COMBIJET+ which requires only one single large diameter gas feed pipe. The gas is di-rectly added via the hollow impeller spars into the high shear zones at the impeller blade tips. This results in an increased mass transfer performance. The pressure drop in the gas feed system is very low due to the large pneumatic cross sections. Therefore, another benefit of this agitator solution is the optimization for required compressor power.

Typically agitator power, gas expansion power (buoyancy) and kinetic gas power contribute to the desired process results of gas dispersion and mass transfer. The effective-ness of these net powers on the mass transfer was studied in an extensive test program. One fundamental result of the studies was that the ratio of agitator and gas power could be optimized. As a general rule of thumb, the energy induced by agitation was utilized more efficiently.

Additional prospects to improve the process and/or reduce operational costs arise if pure gases are utilized. If pure gases are available at an increased excess pressure, then thin and tall reactor concepts in combination with multiple impeller stage agitators are possible and advantageous. The increased filling height results in an augmented residence time of the gas in the vessel resulting in higher gas utili-zation rates. Even small improvements in the percentage range can lead to huge cost savings.

As presented by two examples, new technologies and de-sign concepts help to optimize processes and reduce oper-ational costs. Discussions at an early project stage with all partners are very important and helpful to design applica-tions in an appropriate way. As was shown remarkably by one example, even an audit of existing installations might reveal potential for improvements.

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Dipl.-Ing. Jochen JungDipl.-Ing. Wolfgang KellerBoth:EKATO Rühr- und Mischtechnik GmbHHohe-Flum-Straße 3779650 SchopfheimGermany

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