THE BEIJING AXIS China Business in Perfect Strokes

In a typical scenario, an industrial process will use an extraction step in which solutes (in this case the metal ions) are transferred from the aqueous phase to the organic phase. This is often followed by a scrubbing stage in which unwanted solutes are removed from the organic phase, and then a stripping stage in which the wanted solutes are removed from the organic phase. The organic phase may then be treated to make it ready for use again. The process is used in multiple stages to increase the concentration of the metal (for example, copper) in solution to a level where it can be economically recovered from the solution by Electrowinning (EW), a process similar to electroplating.

While solvent extraction is often done on a small scale by synthetic lab chemists using a separatory funnel, it is normally done on the industrial scale using machines that bring the two liquid phases into contact with each other. Such machines include centrifugal contactors, spray columns, pulsed columns, and mixer-settlers.

For metal applications in the Western world, centrifugal contactors have been superseded by mixer settlers or pulsed columns. In these systems the aqueous and organic phases are brought into contact, then allowed to separate by gravity. Operational advantages of these systems are that they have little or no maintenance as compared to centrifugal contactors, they are less sensitive to contaminants and there is almost no risk of losing solutions due to emulsification. Disadvantages are the increased flow space requirements and the larger inventory of solutions in the system. This last point can also be considered an advantage as it makes the mixer settlers less sensitive to variations in the feed.

The BATEMAN SETTLER™

The Bateman Settler is a variation on the conventional mixer settler. The main difference is that in conventional mixer settlers, the mixer tanks feed directly into the settler, flow proceeds through the settler and discharge is at the opposite end. This requires an end-to-end layout. In the Bateman Settler the mixer tanks feed through a channel and flow proceeds back towards the mixer tanks. This provides a more compact layout and reduced construction and maintenance costs.

The BATEMAN SETTLER™ was initially introduced to the mineral industry in the early 1990s by Bateman Litwin NV, which patented the design in the United States, Mexico and Australia (US. Patent No. 5 558 780 and Australian Petty Patent No. 678 749 have been awarded for this reverse-flow settler for solvent extraction).

The Bateman Settler®, with its advanced design and enhanced efficiency, provides many advantages compared to conventional settlers used in SX circuits, including:

• High process efficiency with low organic loss
• Compact layout
• Short construction time
• Low capital cost Ease of operation and maintenance
• Bateman’s experience with designing, installing and commissioning SX plants using the Bateman Settler® enables it to supply the SX section of a copper SX plant on a LSTK basis and provide the client with full process and mechanical guarantees.

Advantages of the Bateman Settler® compared to similar designs include:

• Approximately a 20% reduction in capital cost for the SX plant Increased flux rate capability which can further reduce costs
• Decreased organic losses
• Operator-friendly physical layout
• Total automatic control capability
• Increased fire prevention and control features

The Bateman Reverse Flow Mixer Settler differs from conventional mixer settlers by having integral mixing channel running along the one side the settler. This enables positioning of all the mixers at the same end of a bank of settlers. This means that all piping, wiring, access ways and maintenance facilities can be located on one side of the bank only. Depending on the application, this can provide a significant reduction in capital and operating costs.

Bateman Pulsed Column (BPC)

The BPC technology was developed over 30 years ago and has been successfully applied to a range of industries. The BPC was developed to overcome the problems associated with conventional mixer-settlers as well as other continuous contact columns.

The column consists of an upper and lower decanter separated by a vertical cylindrical contacting section comprising a series of discs and doughnuts sized and arranged for the specific solvent extraction duties.

The solvent phase is introduced in the lower decanter and travels through the contacting section for collection in the upper decanter with the aqueous phase travelling in the reverse direction. Air pulses are delivered to the pulse leg, resulting in alternate periods of mixing and coalescence within the cylindrical contacting section of the column. This pulsating action provides continuous mass transfer from the aqueous phase to the solvent phase along the length of the vertical cylindrical contacting section.

The BPC is continuous and a single column can replicate the effect of several separate stages of conventional mixer settlers. The BPC are ideally suited to fast reaction kinetics like those common in uranium treatment and base metals such as nickel and zinc.

The BPC has lower total installed cost due to the small footprint and thus reduced civil costs and the enclosed design that reduces fire risk and simplifies fire protection. Operating costs are also lower since the fully enclosed design reduces solvent losses and reagent oxidisation.

In 1997 the BPC system was successfully commercialised for uranium extraction on two BPCs at the Olympic Dam operation in Australia, followed by a full scale plant comprising 10 BPCs in 1998 and a further two column expansion in 2004. More recently Goro Nickel has installed 21 BPCs, making this the largest installation of BPCs so far.

The benefits of the BPC include:

Multi-stage contacting

A single BPC by its very nature provides multiple stages of contacting whereas a conventional mixer-settler can never exceed one theoretical stage of mass transfer and usually provides less. The number of stages for a particular BPC is achieved by selecting the height of the cylindrical contacting section in accordance with the theoretical height of a transfer unit (HTU) calculated for the specific application.

The benefit to operators is the reduced number of equipment items and footprint resulting in potential reductions in initial capital, operating and maintenance costs. Additionally, the organic inventory of a BPC is substantially less than the equivalent mixer-settler, further reducing capital expenditure.

Maintenance and operation

Maintenance items associated with the BPC circuit are limited to the air supply blower, three-way solenoids and associated control actuators and instruments. The ongoing maintenance of these items is significantly less than that associated with conventional mixer-settler systems, especially with pump mixers especially as more than one mixer in series is often required.

Crud formation in the BPC is often negligible. In contrast, conventional mixer settlers may require monthly floating-off of interface crud and yearly clean out of settled crud, incurring both maintenance and labour costs and loss of organic and product inventory.

The characteristics of the mixing in a BPC tend to be gentler movement than in mixer-settlers. Thus the likelihood of forming small bubbles of dispersed phase or stable emulsions is much reduced. Indeed, adjusting the pulsation intensity can control the bubble size. The net result of this behaviour is negligible losses of solvent in the raffinate, which has both economic and environmental benefits.

Once commissioned, the BPCs need little operating attention while practices such as sampling are simplified with a key control and sample collection point centralised for easy access and minimum maintenance.

SX auxiliary equipment

Because the operation of the BPC is such that there is negligible entrainment and crud formation, certain auxiliary equipment normally associated with mixer-settler plants is not required. Amongst these are flotation cells, crud removal equipment, crud handling and disposal equipment, intermediate tanks and pumps, after-settlers and surge and pump suction tanks. This has an impact on both capital and operating costs.

Safety and environment

The BPC circuit is an enclosed system and as such has negligible organic losses. This provides a safer operating environment in terms of personal health and fire hazards and eliminates the cost associated with the replacement of evaporative organic and ammonia loss. Reduced evaporation and entrainment losses also result in lower organic reagent consumption and lower operating costs.

Return on investment

The utilisation of BPCs provides the following benefits to ROI for the project: Installed capital cost of a BPC circuit is typically much less than that of a conventional mixer settler circuit The smaller footprint, especially in existing plants, where free area for expansion may be limited, provides secondary savings in terms of installing plant services (fire, water, lighting, process liquids etc) Higher process recoveries can be achieved Operating costs are lower with automatic control available in the control room.

Reduction of operating costs Based on Bateman’s experience, the utilisation of the BPC for the project will provide advantages regarding later additional operation costs: The high level of automation of BPC will avoid increasing the number of operators per shifts Solvent losses will be lower compared to mixer-settlers Maintenance cost will be much lower as the plant will have less rotating equipment and flanges Plant availability over the year will be much higher.

John Riordan (GM: Hydrometallurgy, Bateman Engineering)

Note: This article is republished here courtesy of Bateman Engineering.