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Design SX with CFD



Using CFD to Assist in the Design of Solvent Extraction Pumper-Mixers

Added to Post Mixing on Nov 1, 2003. Presented in part at Hydro 2003, Vancouver on Aug 27, 2003.

Introduction

A solvent extraction pumper mixer is a special class of mixing impeller. The impeller must create a suction when rotating, thus creating a change in pressure or head. Through this action, flow is generated much like a pump. This head and flow requires power. The solvent extraction (SX) pumpers is the heart of an SX circuit. All of these parameters are required in order to properly design an SX plant. In the past, very few companies took the time to study this experimentally and use the data to scale-up. It all started about 30 years ago with General Mills.  Then came Davy (now Kvaerner), Krebs, Avendeņo, Lightnin and Outokumpu. The approach for scale-up is as different as the companies that have done the lab studies. So are the pumper designs.

Experimentation requires tanks, false bottoms, orifice plates, entrance and exit pipes, piping, a settler, weirs, a torque cell, and a flow meter. When using the organic and aqueous phases, it can get rather messy. Because of the continuous flow conditions, large inventories of reagents are required for these studies. No wonder so few studies have been done.

A more elegant method is using CFD, computational fluid dynamics, to study the SX-pumper. This has not been done to our knowledge for a systematic and fundamental study because

  1. An SX circuit is asymmetric and therefore a complicated 360 degree model must be made from scratch, which includes the inlet and outlet pipes, the false bottom, the orifice, the pumper, the baffles, and the overflow-underflow connection to the auxiliary tank or the settler. 
  2. The close proximity of the pumper to the stationary false bottom requires a very tight rotating reference frame.
  3. Most commercially available CFDs take notoriously long times to converge to a solution.
  4. Furthermore, most commercially available CFD programs are too diffusive in order to assist in the convergence, and thus violates some rules of physics.
  5. For this reason, most CFD results need to be validated by comparing it and tweaking the code to an experiment, otherwise the results will be looked upon with skepticism.

This article will discuss how Post Mixing uses CFD to understand, study, and design SX circuits. You will probably be amazed at how accurate and quick CFD can be when using the right program, Acusolve, and the right resources, Keith Johnson.

Background: Solvent Extraction Systems



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