INFLUENCE OF REACTOR DESIGN TO PROCESS PERFORMANCE IN

HYDROMETALLURGICAL APPLICATIONS

*M. Latva-Kokko, T. Hirsi, M. Lindgren and T. Ritasalo

Outotec (Finland) Oy

Kuparitie 10

Pori, Finland 28101

(*Corresponding author: marko.latva-kokko@outotec.com)

ABSTRACT

Process performance can be significantly influenced by proper design of hydrometallurgical

reactors. Optimal reactor configuration is case specific and depends on process requirements such as

reaction rate limiting factor and particle size distribution. Choice of materials and manufacturing methods

also play important roles in highly corrosive and abrasive environment often involved in

hydrometallurgical industry. In this paper performance of different reactor designs has been compared in

some typical hydrometallurgical applications. Results show that significant improvement in energy

consumption, gas utilization efficiency and equipment lifetime can be achieved with process specific

reactor design.

KEYWORDS

Mixing, reactor design, solids suspension, gas dispersion

INTRODUCTION

Stirred tank reactor (STR) lies at the core of many leaching, precipitation and metals recovery

processes. All of hydrometallurgical plant income is extracted in reactors and even marginal

outperformance matters to plant economy. The initial investment usually has a low impact on the lifetime

costs of typical reactors. Instead it is the process result and availability that makes the difference.

Hydrometallurgical reactor is far more than a mixing tank and a complete approach requires detailed

knowledge of both the process and the equipment. There are remarkable case by case variation in the

industry’s raw materials, processes and construction material requirements. The corner stone in successful

reactor design is identification of case specific critical agitation duties in order to overcome the factors

limiting the reaction rate. However, reactor design shouldn’t underestimate the importance of overall plant

unit design, which fulfills the operation, safety, availability, and maintenance aspects, not forgetting the

economical profitability.

Suitable specification material has to be collected to start reactor design. In Outotec’s experience

the minimum required reactor design parameters are solids particle size distribution, type of chemical

reactions, knowledge from reaction kinetics, need for gas dispersion, phases involved, the mode of

temperature and pressure control and the production capacity. Similar lists can be found in multiple

publications (Paul et al., 2004, Perry & Green, 1997). For reaction type it is important to identify the

reaction limiting factor that is usually the starting point for reactor design. Outotec has long experience on

dimensioning of stirred tank reactors for hydrometallurgical applications and some of the most notable

factors defining reactor design are expressed in table 1 below.

Table 1 - Primary defining factors in stirred tank reactor design

Factor Parameters

Reactor duty Reaction limiting factor, required mixing

intensity, critical agitation duty

Solution properties Mass flow, composition, temperature,

density, etc

Solids properties Mass flow, density, hardness, particle size

Gas dispersion Mass flow, density, solubility

Heat transfer Endothermic or exothermic reaction

Information collected in table 1 defines the reactor design leading to required vessel size and form,

agitator type, baffle configuration and possible accessories such as heating or cooling baffles. The design is

generated through dimensioning and scale-up equations. Design has to be able to fulfill the given process

task, however overdesign is to be avoided to avoid excessive costs. For the durability of the installation

material selection is done based on the information collected. In material selection all affecting factors,

including those that have minimal or no effect on the process performance, have to be taken in

consideration to avoid equipment malfunction during operation.

In many hydrometallurgical applications declining ore grades forces to use of larger through puts

leading to higher solution and slurry flows and larger equipment. Larger equipment further emphasizes the

importance of accurate specification of reactors to avoid excessive costs through over dimensioning. With

proper reactor design significant improvement in process performance in terms of energy consumption, gas

utilization efficiency and equipment lifetime can be achieved

When developing a hydrometallurgical plant the correct specifying of a hydrometallurgical reactor

is only a part of the work required for successful operation. When multiple reactors form a plant unit and

plant units form a complete plant, a comprehensive design is required to achieve solution that has required

operation and maintenance design factors implemented.

SOLIDS SUSPENSION TANK

There are several applications in hydrometallurgy where solids suspension is main or only duty

for the reactor. For example continuous stirred tank reactors (CSTR) in processes with chemically

controlled reaction rate and all kind of slurry storage and buffer tanks, such as filter feed tank, belong to

this category. In these applications process performance of the equipment can be improved by increasing

the homogeneity of the suspension. Uniform suspension utilizes the equipment volume fully providing

longer retention time for solids in CSTR series. In storage tank applications uniform solids suspension

generates stable feed that can have a significant effect downstream. For example in filtration process drier

filter cake and lower content of soluble metals can be obtained.

Maximizing the degree of solids suspension with minimal energy consumption has been widely

studied (Hosseini et al., 2010; Tahvildarian et al., 2011). Typical mixer arrangement today is a downward

pumping hydrofoil impeller, which generates strong axial flow that can carry solid particles close to the

surface. In figure 1 it is shown that off bottom and uniform solids suspension can be systematically

achieved with lower energy consumption with a wide blade hydrofoil impeller (OKTOP®3200) than with

a standard 45º pitched 4 bladed turbine (PBT). This is in good agreement with Wu et al. study on energy

efficiency on axial flow impellers, which concludes that pitch bladed turbines are approximately 7 % less

efficient than the commercial hydrofoil impellers (Wu et al., 2006). In addition to impeller type, tank

configurations including baffle plates and bottom shape have an impact to solids suspension.

Figure 1 – Power consumption per total mass of slurry required to achieve partial (Njs) and uniform

suspension with different impellers, solid concentrations (wt%) and particle size fractions. Tests were

made with quartz sand-water slurry in flat bottom tank with diameter of 362 mm. (Tervasmäki, 2013)

Monitoring and Control of Solids Suspension

As shown in figure 1 particle size distribution and concentration of solids have strong effect to

degree of suspension. In continuous industrial operation both of these properties typically fluctuate.

Outotec has been developing novel technology for continuous surveillance and control of solids suspension

degree inside a slurry tank. The system includes measurement probes, which are integrated to OKTOP®

reactor and positioned so that cloud height of the slurry, thickness of the settled layer of solids at bottom of

the tank, or both can be continuously monitored. Measurement is based on electric impedance tomography

(EIT) that is not sensitive to changes in composition of the slurry and does not need any calibration during

operation. For automated control mixer motor must be equipped with variable-speed drive.

Site tests of this technology were made with prototype equipment in a 34 m3 OKTOP®3200

slurry tank shown in figure 2. This tank is used as a feed tank for Outotec High Compression Thickener at

Siilinjärvi in Yara Suomi Oy’s tailings treatment pilot plant. Tailings slurry from apatite flotation process

has a wide particle size distribution and its solids content is close to 50 wt%. Based on these on-site

measurements roughly 54 % savings in mixing energy consumption could be obtained with active

measurement and control of solids suspension, compared to operation with nominal rotation speed of the

impeller. This is due to the fact that in normal continuous operation the slurry tank is operated with

approximately 75 % surface level. Nominal mixing performance is required only in short term periods

during process shutdown and start-up situations.

Figure 2 – OKTOP®3200 slurry tank equipped with novel solids suspension measurement

probes (circled in red)

In addition to energy savings integrated solids suspension measurement can provide beneficial

information of process performance. For instance formation of settled layer of solids at bottom of a

conditioning tank indicates possible malfunction in a milling circuit. Dramatic or sudden decrease in

mixing power demand can alarm that a block in the pipeline is starting to form, decreasing the density in

the tank. This kind of information is very valuable, since corrective actions can be made before they have a

massive effect to overall process.

GAS-SOLID-LIQUID HYDROMETALLURGICAL REACTOR

Simultaneous solids suspension and oxygen dispersion commonly occurs in 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. Oxygen mass-transfer

kinetics are temperature and pressure dependent and reactor specific. 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 those that induce radial flow like

radial disc turbine (RDT). However the presence of gas affects the performance of the impeller and its

ability to suspend solids. Thus in gas-solid-liquid applications both gas dispersion and solids suspension

needs to be taken into account in reactor design in order to optimize the process performance. The

importance of this aspect is emphasized with following comparison.

Based on laboratory tests an effective atmospheric copper sulfide ore leaching process requires 50

kg of oxygen per one ton of ore. Prior to leaching ore need to be ground to particle size of 90 % below 125

μm. Optimal solids content for the leaching was found out to be below 400 g/L. Kinetic behavior of

leaching with different oxygen feed profiles in batch tests are presented in figure 3. Required retention time

to achieve decent copper recovery appears to be 8 hours. Leaching rate can be enhanced by feeding more

oxygen during the first hours of leaching. Thus it can be concluded that initially the leaching is controlled

by the rate of oxygen transfer from the gas to the aqueous phase.

Figure 3 – Kinetic behavior of a copper sulfide ore in atmospheric batch leaching tests with different

oxygen feed rates

If planned annual copper treatment capacity would be 60 000 tpa and average Cu content of the

ore 6.5 %, ore feed rate should be 7200 kg/h. Performance comparison for different reactor designs in this

application has been presented in table 2. Impeller size and rotation speed is kept constant in order to

achieve similar equipment lifetime with each design.

Table 2 – Comparison of different reactor designs in copper sulfide ore leaching stage

(PBT= 45º pitch 6 bladed turbine, RDT= radial disc turbine)

Reactor design Mixed

flow

Radial

flow

OKTOP® Design

Reactors 1-4 Reactors 5-8

Tank dimensions

Tank diameter mm 6500 6500 6500 6500

Tank height mm 9750 9750 9750 9750

Effective volume m3 324 324 324 324

Bottom shape Flat Flat Flat Flat

Agitator configuration

Upper impeller PBT PBT OKTOP®3005 OKTOP®3005

Lower impeller PBT RDT OKTOP®2100 OKTOP®2000

Absorbed mixing power kW 164 338 317 214

Impeller tip speed m/s 5.5 5.5 5.5 5.5

Process parameters

Amount of reactors pcs 8 9 4 4

Solids content g/L 356 317 356 356

Oxygen feed per reactor Nm3/h 834 527 970 323

Process performance

Total oxygen feed Nm3/h 6668 4746 5172

Total mixing power kW 1312 3040 2124

Copper recovery in leaching % 82.7 % 82.7 % 88.8 %

Relative comparison

Capital costs % 100 % 113 % 100 %

Operating expenses

(electricity and oxygen) % 102 % 112 % 100 %

Copper production % 93 % 93 % 100 %

With conventional mixed flow reactor design, that includes two downward pumping PBT

impellers, mixing is performed with the lowest energy consumption in this comparison. Due to PBT’s

lesser gas dispersion ability, oxygen must be fed in amount of excess to achieve the same gas to liquid

mass transfer rate (kLa value). With radial flow design the highest oxygen dispersion efficiency is obtained,

but capital and operating costs are highest since amount of the reactors needs to be increased from 8 to 9 in

order to achieve the required retention time for the solids. This is due to RDT’s worse solids suspension

efficiency in gassed conditions.

With mixed and radial flow reactor designs oxygen feed must be distributed evenly to all reactors

in order to keep all solids suspended in gassed conditions. OKTOP®2000 series impellers are specially

designed for three phase processes where gas dispersion is the limiting factor for the reaction rate. Thus

with OKTOP® design more of the oxygen can be fed to first four reactors, which enables utilization of

faster leaching kinetics shown in figure 3. Due to residence time distribution difference between batch and

continuous operation, influence to copper leaching recovery is even higher than shown in the batch test

leaching curves. The yearly monetary value of this 6.1% improvement in Cu recovery is approximately 18

million euro (60 000 tpa Cu treatment capacity and 5000 €/t Cu price). This example shows that with case

specific reactor design the process performance can be influenced and significant improvement in project

economy achieved. Initial investments have a very low impact on the life time cost of a typical leaching

reactor circuit in a hydrometallurgical plant. In leaching, the leaching rate, recovery and availability rule

the economics.

MATERIALS AND MANUFACTURING METHODS

Highly corrosive and abrasive conditions are often present in the hydrometallurgical industry.

Therefore the selection of materials is a crucial part of the reactor design. The starting point for a case

specific material selection is corrosion resistance of the material at process conditions. Temperature and

chemical composition determine the corrosivity of the solution. In hydrometallurgical applications both

uniform and localized corrosion must be taken into account. Uniform corrosion typically occurs in acids

and hot alkaline solutions. Localized corrosion, like pitting or crevice corrosion is of concern in acidic

solutions that contain chloride ions and oxidizing ions like Fe3+

.

Corrosion

Stainless steels have traditionally been a common construction material for hydrometallurgical

reactors. Austenitic steel grades like 316L (1.4432) and 904L (1.4539) are typical choices for their good

fabrication characteristics. From these grades the higher alloyed 904L is more corrosion resistant. Duplex

steel grades that have austenitic-ferritic structure provide excellent corrosion resistance together with

higher mechanical strength and surface hardness (Ekman and Berqvist 2008). Results from some of the

corrosion tests made by Outotec are presented in table 3.

Table 3 – Results from corrosion tests conducted at 90 ºC with different stainless steel grades

Solution

Temp.

ºC

Cl-

mg/L

Austenitic

316L

LDX

2101

Duplex

2205

mm/a Loc mm/a Loc mm/a Loc

10 g/L H2SO4

90 200 0.80 yes

Wear by erosion

In some cases hydrometallurgical reactors are exposed to erosive wear due to high solid contents

and abrasive components found in many ores and concentrates. Especially impeller blades tend to wear

since the velocities of the particles are highest next to the impeller. Impact and sliding wear are the two

main wear mechanisms in impellers and the erosion rate is strongly dependent on the impeller tip speed

(Wolfgang 2008). In addition to impeller tip speed, factors that affect the most the intensity of erosive wear

are solids content, particle size and shape, their specific density and hardness.

Outotec conducted extensive material testing campaign together with Tampere University of

Technology. The wear rate of different impeller and tank materials as well as paints and coatings were

studied in a test device that simulates the process conditions in a hydrometallurgical reactor. In addition to

pure erosion, the simultaneous effect of corrosion and erosive was investigated. The effect of particle size

on erosive wear with different stainless steel grades is shown in figure 4.

Figure 4 – The effect of particle size of quartz on the weight loss of stainless steel grades in erosion tests

Manufacturing methods

Usually the corrosion resistance of a stainless steel weld is not as high as that of the base material.

Although corrosion resistance can be greatly influenced by proper welding procedures and environment,

both mechanical and chemical after-treatment measures are usually required in order to ensure the

corrosion resistance of welded joints in hostile conditions often present in hydrometallurgical applications.

(Anttila et al., 2013).

In addition to corrosion the welded joints of impellers are vulnerable to fatigue failure. Welded

joints can thus be considered as the weakest link of an impeller. Outotec has developed a whole series of

agitators; OKTOP®2100, OKTOP®3103, OKTOP®3105 and OKTOP®3300, that does not contain any

welded joints of impeller blades or their fastenings. This bolted design has proved to be superior to welded

structures in terms of corrosion resistance and mechanical strength. The design also enables easy and quick

replacement of wear-out blades to new ones and, as a result, the maintenance costs can be decreased.

OUTOTEC OKTOP® REACTOR PLANT UNIT

Common practice in hydrometallurgical reactor design is to fulfill the industry specific standards

and the process specific requirements are not in the main focus. Reactors are normally considered as

simple tanks with agitation, and supply is divided either to general suppliers. Suppliers may not be familiar

with duty and process requirements of metallurgical reactors, or they have only limited impact to control

the total solution in data sheet type engineering. The split scope delivery rarely allows development of a

functional unit providing the best metallurgical performance and functional design to secure high

availability with safe and easy operation and maintenance. Common issues in design and delivery phases

of a project are long response times, unclear battery limits, exceeded costs and lead times, case by case

variation in quality and solutions that are not always as feasible as estimated.

Outotec’s approach to overcome these typical issues, and cover the hydrometallurgical application

specific reactor requirements, is to supply the reactor train as a complete, highly predefined, but still tailor-

made functional unit that is integrated with the process design and automation. 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 guarantees the process

results and ramp-up.

Decades of industrial and experimental knowledge from wide range of hydrometallurgical

reference projects are combined into the entire OKTOP® reactor plant unit delivery. Materials of

construction are carefully considered based on the industrial experience and case specific testing. Safety is

an integral part of productized reactor plant delivery. Special requirements like internal cooling or heating

of slurry are handled with removable high efficiency heat exchanger baffles that are especially suitable for

slurries causing scaling. Agitator support with 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.

Slurry distribution system, consisting of slurry risers, minimizing slurry bypassing, and gravity

flow launder with isolation valves, allows any reactor of OKTOP® reactor plant unit to be bypassed for

periodic maintenance. Access and service platforms are integrated to the modular launder product

minimizing the assembly work at the site and allowing transportation of modules in 40 feet standard sea

containers. Modular design maximizes the factory manufacturing, which enables high level of cost control

due to the controlled working environment and assures high quality of the products. An exhaust gas

venting and scrubbing system is easy to integrate into the standardized layout. Figure 5 illustrates some of

the aspects of modular OKTOP® reactor plant unit delivery, starting from case specific agitator selection

and scale-up, verified by case specific chemical testing, and leading to the complete plant unit delivery.

Figure 5 - From top left: OKTOP® 3200 agitator for energy efficient solids suspension and uniform

mixing. OKTOP® 3005 agitator for moderate mass transfer. OKTOP® 2000 agitator for efficient gas

dispersion to slurry. Removable heat exchanger baffles for slurry cooling or heating. One piece assembly

of OKTOP® agitator providing safe and easy installation and maintenance of agitator. Complete functional

OKTOP® reactor train connected with gravity flow launder providing easy bypass of reactors for

maintenance.

SUMMARY

Hydrometallurgical reactor must fulfill the given process task, but overdesign should be avoided

in order to minimize capital and operating costs. Thus it is important to specify the necessary defining

factors and parameters. When solids suspension is the main duty for the reactor, process performance can

usually be improved by increasing the homogeneity of the suspension. Since particle size distribution and

concentration of solids typically alter in time in an industrial process, significant savings in energy

consumption can be achieved with continuous surveillance and control of solids suspension degree.

In applications where simultaneous solids suspension and gas dispersion is required process

performance can be greatly influenced and significant economical improvements achieved with proper

reactor design. Selection of materials is a crucial part of reactor design due to highly corrosive and abrasive

conditions often present in hydrometallurgical industry. In addition to correct material equipment lifetime

and maintenance costs can be improved by equipment design and manufacturing methods.

In order to guarantee the process performance of reactors in case specific hydrometallurgical

applications Outotec has launched the OKTOP® reactor plant unit concept. The reactor train is delivered

as a complete, highly predefined, but still tailor-made functional unit. Productized delivery with

standardized plant modules provides a shorter response and delivery time, minimizes the site work and

guarantees the process results and ramp-up.

ACKNOWLEDGEMENTS

Authors would like to thank Jarmo Aaltonen and other personnel involved in Yara Suomi Oy for

cooperation in on-site measurements and authorization to publish the results.

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