BROGIM®: A new three-phase mixing system testwork and scale-up

doi:10.1016/j.hydromet.2006.03.040

Hydrometallurgy

Volume 83, Issues 1–4, September 2006, Pages 97-105

https://doi.org/10.1016/j.hydromet.2006.03.040 Get rights and content

Abstract

BROGIM® is a mixing concept for bioleach tanks that provides high oxygen transfer performances, by means of an air dispersing flat blade turbine at the bottom, and mixing, with low power-consuming propellers at the upper part of the rotating shaft. Oxygen transfer performances of this system have been firstly determined in a small-scale testwork in an 850-l tank. A pilot-scale study in a 65-m³ tank enabled to define rules for the scaling-up procedure. The paper describes the philosophy for the scale-up of the mixing system installed in industrial tanks of up to 1250 m³ operating volume.

At industrial scale, it has been shown that the BROGIM® system exhibits little power differences between gassed and ungassed conditions, for air flowrates up to 24,000 Nm³ h^− 1 comparatively to what was found at smaller scales. The agitator is able to run with or without air with no need for oversizing the power of the electrical driving motor.

A procedure for oxygen transfer coefficient (k_la) determination in real conditions has been tested. It is essentially based on gas balance measurement for establishing the oxygen uptake rate and dissolved oxygen measurement at different levels in the tank.

Introduction

Recovery of metals from mineral sulphides by bioleaching in agitated tanks at industrial scale means mixing large volumes of aerated slurry with residence times as long as several days.

The principle of the treatment is to oxidise sulphide compounds to sulphate by using oxygen from air and the catalytic effect of bacteria able to use energy from this reaction. This treatment is an alternative to sulphide roasting and pressure leaching. The main advantages of bioleaching over other processes are to be cost-effective and to provide the same result in a simpler way in terms of operability.

The key factor of bioleaching in agitated tanks is mixing. It has to be efficient in order to have a high level of oxygen transfer by air dispersion to comply with oxygen uptake rates of about 1500 mg l^− 1 h^− 1, and even distribution of the various components of the slurry throughout the tank.

The solid phase mainly consists of sulphide minerals (pyrite; FeS₂, chalcopyrite; CuFeS₂, arsenopyrite; FeAsS, etc.) and oxidised minerals (silica, gypsum, and iron hydroxides). Sulphide minerals are relatively heavy (specific gravity ca 4) and oxidised minerals are relatively light (specific gravity ca 2.5). Both types of minerals are fine solids (minus 100 μm). The solids concentration of the pulp is generally around 20% (w/w).

The aqueous solution is highly concentrated in sulphate salts (typically in the range of 30 to 40 g l^− 1 Fe and of 100 to 150 g l^− 1 SO₄⁼) and contains a low concentration of biomass (a few grams per litre).

Air is sparged into the medium at a flowrate of about 0.2 vvm (volume of air per volume of slurry per minute). For cost reason, air is preferably provided by means of blowers rather than compressors at a maximum relative head pressure of about 150 kPa. The highest attainable head pressure of the blowers restricts the operating height of the tanks to a maximum value of about 10 m.

Biomass is a mixed culture of sulphide/sulphur-oxidising and iron-oxidising bacteria isolated from sulphide orebodies. The optimal efficiency of mesophilic bacteria is obtained by running the process in continuous mode at an operating temperature in a range of 35 °C to 45 °C.

Substrate feed flowrate is fixed by the objective of maintaining the maximum bacterial growth in chemostatic conditions in a series of tanks.

The volumes of the tanks in the existing industrial installations are ranging from 75 to 1350 m³ put together in a series of several stages of tanks in parallel. Obviously, the configuration and unit volumes of the tanks are the result of a compromise between an objective of process efficiency and the capital and operating costs of the equipment.

The main application of sulphide ores bioleaching in agitated tanks is for gold recovery. Bioleaching process applied to refractory gold ores actually liberates gold from a pyrite/arsenopyrite gangue making it free to solubilising treatment by cyanidation. A recent application [1], [2], [3] has been launched for cobalt production in Kasese, Uganda. In this case, bioleaching results in dissolving cobalt, which is originally finely disseminated in a pyrite matrix. In subsequent process steps, cobalt is recovered from the bioleach solution by solvent extraction and electrowinning. Other projects in progress aim to apply bioleaching in agitated tanks to copper and nickel bearing minerals [4].

This paper presents the results of a R&D work undertaken by Robin Industries (now MILTON ROY MIXING) and BRGM aiming to define a new mixing system named BROGIM® able to meet the requirement of a high oxygen demand while minimising the cost for both mixing and aeration.

Section snippets

Scope of the study

There are two mixing systems currently in use at industrial scale for processing refractory gold sulphide concentrates.

The first system is a Rushton turbine, which is the traditional impeller used in processes requiring high gas dispersion rates. It is known that a Rushton turbine has a high power draw. It is also significantly sensitive to gas flowrate. Moreover, the radial flow can produce separate mixing zones above and below the horizontal axis where different solids suspension, heat

k_la measurement at lab-scale

The method used for measuring k_la follows the unsteady-state or dynamic procedure. In this method, the liquid is deoxygenated by sparging with an inert gas and k_la is determined by monitoring the oxygen dissolved in solution from the start of air inflow.

In the experimental system developed by BRGM for this purpose the gas inflow sequence and the operating conditions was monitored by a PC and the dissolved oxygen concentration was measured by means of an electrochemical probe, see the

Testwork in the 850-l tank

As shown in Fig. 4, k_la values for the BROGIM® system measured in the 850-l tank vary in the range between 0.007 and 0.04 s^− 1 with a specific power varying from 100 to 500 W m^− 3 and superficial velocities of gas from 0.002 to 0.01 m² s^− 1.

These values of specific power and air flowrates are typical of industrial applications especially for the largest working volumes where power consumption is the key point for the selection of the agitator. The fact that the values are below 0.1 s^− 1 validates

Industrial unit scale-up philosophy

In the frame of the cobalt plant design, the BROGIM® system was scaled up for tanks with an operating volume of 1250 m³.

It was expected that the ratio between k_la values in process and cold water conditions was above or equal to 1, especially resulting from the high sulphate concentration in the process solution making it less coalescing than water, as was evidenced by finer air bubbles in these conditions.

With a minimum value of dissolved oxygen fixed at 2 ppm at the top of the tank, which was

Conclusion

The results of the testwork at different scales show the technical validity of the BROGIM® system as being a particularly flexible device to comply with the mixing requirements in bioleaching of sulphide ores in agitated tanks with minimised power consumption. Actually, the main quality of the BROGIM® system in this matter is to be quite customisable.

At industrial scale, it has been already shown that the BROGIM® system exhibits little power differences between ungassed and gassed conditions,

Acknowledgements

The research and development work of the BROGIM® system has been supported by a grant from the ADEME. Agence De L'Environnement et de la Maîtrise de l'Energie (Convention No 4 02 0007).

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Determination of gas–liquid mass transfer and solids suspension parameters in mechanically agitated three phase slurry reactors
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Cobaltiferous pyrite beneficiation by biohydrometallurgy

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