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 Rhrwerkslsungen fr die hydrometallurgische Erzverarbeitung

Atmosphrisch begaste Rhrkesselkaskaden in kontinuier-licher Fahrweise erfllen in vielen hydrometallurgischen Laugungsprozessen eine Schlsselfunktion. Neben der Kos-tendegression durch Bau immer grerer gerhrter Einhei-ten und Verbesserungen der Prozesse, ist die Gesamtkosten-optimierung ein wesentliches Ziel. Ein Nebeneffekt bei der Mastabsvergrerung (Scale-up) ist die Verringerung der Rhrwellendrehzahlen, wodurch das Risiko einer ber-flutung bei Verwendung konventioneller Rhrorgane steigt. Die Auslegungskriterien zur Dimensionierung der Motorleistung richten sich dann nicht mehr allein nach der Stoffbertragungsleistung, sondern nach den zustzlichen Leistungsanforderungen, die notwendig sind, um ein ber-fluten des Rhrorgans zu verhindern und den Feststoff in Suspension zu halten. In diesem Artikel wird eine neue Art von Rhrorgan zum Gasdispergieren und Feststoffsuspen-dieren vorgestellt, das sehr hohe Gasmengen handhaben kann, aber ein berdimensionieren umgeht. Bei vielen die-ser begasten Rhrkesselkaskaden ist die Kompressorleis-tung des Luftversorgungssystems oftmals signifikant hher im Vergleich zur Antriebsleistung der Rhrwerke. Eine de-taillierte Analyse zeigt, dass Hochdruckbegasungssysteme aus konomischer Sicht unwirtschaftlich sind und der Ge-samtwirkungsgrad im Vergleich zu modernen Rhrwerks-lsungen niedrig ist. Abrieb an den Dsenauslssen ist auf-grund der hohen Gasaustrittsgeschwindigkeiten ein hufig

anzutreffendes Problem. Im schlimmsten Fall kann es durch Verblocken einiger Auslsse zu einer ungleichmigen Gas-verteilung am Rhrorgan kommen. Die Folgen sind zustz-liche dynamische Beanspruchungen des Rhrwerks, aber auch eine Verringerung der chemischen Ausbeute bedingt durch einen reduzierten Stofftransport von der gasfrmi-gen in die flssige Phase. Um diese Nachteile zu berwin-den, ist eine gnzlich neue Lsung fr begaste Rhrwerke entwickelt worden. Eine einzige Gasleitung versorgt das Begasungsrhrorgan, wobei das Gas ber hohle Holme direkt zu den Rhrorganblttern in die Zonen mit hchster Scherung eingeleitet wird. Gleichzeitig wird der Gesamt-druckverlust minimiert, whrend die Gasdispergierleistung maximiert wird, weil der Gasstrom ungedmpft 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 Lslichkeit und damit das treibende Geflle fr den Stoff-transport zu erhhen. Die Wirtschaftlichkeit eines solchen Prozesses kann weiter optimiert werden, indem spezielle mehrstufige Rhrorgananordnungen in Kombination mit einer angepassten Behltergeometrie verwendet werden.

Schlsselwrter:

Rhren Hydrometallurgie kla-Stofftransportkoeffizient Erzlaugung Gasausbeute

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

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

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 m. 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