Hydrometallurgy

Pre-oxidation and leaching

Modern extraction of precious and base metals usually involves one or all of the following processes: pre-oxidation of the concentrate, cyanidation or leaching.

Depending on the minerals extracted, the media is either acidic or alkaline. Depending on this the suitable material and/or lining is selected.

If air or oxygen is introduced into the tank, suitable gas handling impellers are selected to prevent impeller “flooding”. Additionally, Mixtec can offer “gas down the shaft” configuration of the mixer to introduce gas in a tank where no sparging system is installed.

For this application Mixtec’s EDICT system is the agitator configuration of choice. It allows efficient mass transfer, gas retention and enhanced tank overflow in leaching circuit.

Adsorption (CIP / CIL)

In gold and silver refining the leaching step is usually followed by adsorption of dissolved metal from pregnant leach solution onto activated carbon. Two variations of process are most widely implemented: “carbon in pulp” (CIP) for standard ores or “carbon in leach” (CIL) when there is risk of mild “preg-robbing” phenomenon.

The main challenge for agitators in adsorption circuits if finding the delicate balance between sufficient pumping rate to allow uniform mixing, mass transfer and solids suspension and gentle agitation to prevent carbon attrition.

Mixctec’s EDICT system is the optimal choice for adsorption circuits which has been proved by hundreds of successful installations in CIP and CIL circuits worldwide.

Detox / cyanide destruction

Most gold plants around the world are required by law to destroy cyanide and metal cyanide complexes in their tailings, prior to discharge from the metallurgical site into the tailings dam. Many plants destroy the cyanide in a contained area within the metallurgical site, to minimise the exposure of wildlife, particularly birds, to these toxic solutions.

Experience and CFD backed design enable us to optimally size the agitator to best suit the chosen process specifically the applications with demandingly high gassing rates. At the heart of the solvent extraction plant are the primary pump mixers and secondary mixers. Depending on the chemical content of the pregnant leach solution stream, stainless steel, titanium or special non-metallic lined wetted parts are used.

PROCESS EXAMPLE USING SO2 AND O2

Process slurry from adsorption tanks discharges by gravity into the Cyanide Detox Feed Box and is distributed to the Cyanide Detox Tanks prior to it being sent to a tailings pond. The tanks are mechanically agitated with the use of Mixtec’s custom impellers specifically designed for gas dispersion applications.

Typically, compressed oxygen gas (O2) and sulphur dioxide gas (SO2) can be injected into the detoxification tanks. The agitator will disperse the bubbles evenly throughout the tank contents to promote efficient transfer of O2 and SO2 and into the slurry. During this process, agitators will also provide uniform suspension of slurry solids.

Cyanide Destruction can be achieved in many ways. In order to achieve our client’s process results, we disperse the gas being introduced into the process. The volume of gas can be substantial, requiring the use of Mixtec’s HA736 and HA724 impellers. The standardised, HA736 has been specifically designed for high pumping rates required for cyanide destruction. Although there are many ways cyanide destruction can be achieved, the use of agitators is a preferred standard amongst many plant designers. This is an area where Mixtec specialises in and adds significant value through tried and true design procedures accompanied with an impressive installation list of some of the largest cyanide detox agitators in the world today.

Solvent Extraction

Liquid–liquid extraction (LLE), also known as solvent extraction (SX) and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an organic solvent (non-polar). There is a net transfer of one or more species from one liquid into another liquid phase, generally from aqueous to organic. The transfer is driven by chemical potential, i.e. once the transfer is complete, the overall system of chemical components that make up the solutes and the solvents are in a more stable configuration (lower free energy). The method’s greatest advantage is its ability to selectively separate out even very similar metals.

At the heart of the solvent extraction plant are the primary pump mixers and secondary mixers. Depending on the chemical content of the pregnant leach solution stream, stainless steel, titanium or special non-metallic lined wetted parts are used.

It is clear that in order to improve the reaction kinetics, reaction surfaces play an important role, therefore droplet size is a key factor on the overall efficiency and valuable metal recovery.

Extensive mixing and pumping test work both in the laboratory and field has been essential to fully develop superior Mixtec solvent extraction technology.

Bacterial oxidation

Decades ago, along with Goldfields - the pioneers of BIOX® process (currently registered trademark of Metso Outotec), Mixtec has developed impellers specifically for bacterial oxidation of refractory concentrates.

This biohydrometallurgical process also known as biological leaching and its variations were developed for pre-cyanidation treatment of refractory gold ores or concentrates. It comprises contacting refractory sulfide ROM ore or concentrate with a strain of the bacterial culture for a suitable treatment period under an optimum operating environment. The bacteria oxidize the sulfide minerals, thus liberating the occluded gold for subsequent recovery via cyanidation.

There are quite some challenges for mixing equipment in this application. The slurry in reactor is both corrosive and abrasive. This is why rubber lined or austenitic stainless steels will not be a suitable solution. The steel grade (usually lean duplex, duplex or super duplex steels) is selected according to the acidity and the temperature of the mixed environment.

The other challenge is possible flooding of the impellers. Usually, large volumes of air are introduced to the slurry. If not properly evaluated and designed, the impellers might easily get flooded canceling out the mixing effect of the agitator. This is why Mixtec is well known for its gas handling impellers.

Mixtec’s Biological Leaching certified HA736 is the ideal choice for this application. Mixtec has produced various agitators for the largest biological leach plants. The design functions of our efficient impeller system for biological leach application include:

• Providing a high primary flow volume
• Producing good in-tank flow patterns
• Ensuring high fluid velocities over the heat transfer and gas inlet surfaces

All these features were incorporated into the effective development of the HA736 to ensure excellent solids suspension, heat transfer and gas dispersion are achieved, whilst shear is minimised to prevent biological damage.

Autoclaves

Mixtec knows it’s way around pressure oxidation (POX) and high-pressure acid leaching (HPAL) processes.

Autoclave mixing requires a specialized agitator design because of the high pressure, corrosive and abrasive media at high temperatures inside the autoclave vessel. From the selection of the drive and mechanical seal, Mixtec will ensure that all considerations have been taken into account to provide the highest possible quality agitator for your autoclave application.

Over Mixtec's history we have worked side by side with industries, such as platinum, to design and manufacture the most efficient, effective and reliable autoclave design possible.

Rugged cartridge seal housings ensure that minimum deflection and extended seal life are guaranteed. Autoclave seals are available in two variants. One with a bearing to steady the shaft and impart the upward axial force into the mounting plate and the other without a bearing that allows the axial force up into the gearbox.

Because of the extreme conditions, the wetted parts can be manufactured of super duplex steels, special alloys and titanium.

Albion Process™

The Albion Process™ is an atmospheric leaching process for processing zinc concentrate, refractory copper and refractory gold. The process is important because it is the most cost-effective method currently in use for extracting both the zinc and lead from concentrates that contain high lead levels (7% or greater).

The media in Albion Process™ reactors is corrosive and abrasive, thus special metallurgy is required for the wetted parts. The impellers have to be properly selected to handle the oxygen injected at supersonic rates in the slurry without flooding.

Extensive R&D work has been conducted by Mixtec in order to design an optimal mixer for Albion Process™.