Back in the 90s, two coalescing devices were considered in the engineering stage for several SX-EW projects in Northern Chile. Both the AIMCO Pacesetter and the Codelco "Chuqui" coalesce performed as planned. Prior to these "first applications from plant conception" these units had been successfully retrofitted in some operating plants (Not sure if it was at the then Cyprus's Miami operation where they retrofitted one Pacesetter unit, and at the Chuqui Ripios they designed one coalesce and retrofitted it into the SX circuit).

In my opinion, both of these coalesces performs well in these duty, but the application of the Pacesetter for organic removal from the raffinate does not seem to pay off.

A third device was retrofitted at the Michilla SX plant also in North Chile: packs of HDPE "shavings" are introduced inside the organic tank, where these packs promote coalescence.

To sum up; any of these devices works, some better than others. They can be retrofitted, or considered as part of the design -Better results are achieved if coalesces are included from the beginning. But of course, a good coalescing device is a conservative design of the SX train.

I have been involved in several Ni SX projects, being partly responsible for the design of the circuit and equipment. In my opinion the best and simplest, but possible most expensive is a deep ( approx. 1-1.5 metre wetted depth) after settler for the organic with at least 60 minutes residence time and automatic removal of settled aqueous from the bottom of the after settler. The after settler should also be fitted with coalescing media packs, e.g. from Spintek.

Aqueous entrainment removal seems problematic and we haven't really cracked it. Pacesetters do a reasonable job as does knit mesh which comes from an interesting background in removing aqueous phase from jet fuel. Hydro cyclones have been very effective in organic entrainment removal down to 5-8 ppm of physical entrainment and there is work going on to make a similar device to remove aqueous entrainment.

We know that the dispersed phase is the one that has less entrainment and separates the clearest. In copper systems, the difference is easy to see. So, what if you had one additional stage where the aqueous, say raffinate, was mixed with the organic, say the recycle organic, at an O/A ratio of 2 or something that will maintain organic continuous conditions. It doesn't even have to be the extractant mixture. Probably just diluents will work even better. As long as the phases still separate reasonably well (and in some systems they might not, e.g. U-amine), the aqueous should have very low entrained organic. The organic will have some entrained aqueous, but the organic is just going around and around in that stage, so it doesn't really matter. It will just build up a steady-state load of entrained aqueous. And you will want to slowly bleed some of this used organic into the system as diluents makeup, allowing for the low concentration of extractant that will in turn build up in it. I'm sure that someone has thought of and tried something like this before and it would be interesting to hear of any experiences. (Even simpler...if possible - run the stage where the aqueous of concern leaves the circuit, with organic phase continuous).

P.S. I'm always reluctant to put anything in the system that can act as a nucleation or collection site for solids, either carrying through the circuit or precipitating out. There are effective coalescing media but I'd worry about using them in a system where solids might be present or generated, for example through pH control additions. You can end up with an awful mess.

In 1993 we retrofitted a coalescing system using a proprietary bagged media at the Girilambone copper operation in Australia. It worked very well and various papers were published on it. Two years later the same thing was done at the Nifty Operation in NW Australia. There were also papers published on this one. It was so successful that instead of a major electrolyte chloride problem they ended up adding salt to the electrolyte to maintain > 30 ppm Cl.

Since then there have been 35 SX plants that we have designed that have had the loaded organic coalesce included in the circuit. This has allowed the plants to operate in any continuity and maintain manageable levels of aqueous entrainment. It is sufficiently crud tolerant that it requires cleaning only every 2 or 3 months; not because of hydraulic blockage but the solids accumulation over time slowly increases the entrainment levels. The cleaning can be down during a planned shut off. We design in a bypass that allows cleaning while the plant operates.

Thank you all for your prompt comments. If anyone has contacts for pacesetters or specifically successful knit mesh media (entrained aqueous removal), please let me know (through this forum or privately). And i agree that any solids that might be present will plug the media.

One additional comment. If the aqueous entrainment is due to micro emulsions then mechanical devices such as settling tanks and coalesces are unlikely to help. Micro emulsions are often detected by unexplainable transfer of impurities in SX. A wash stage on the organic may change the chemical composition of the micro emulsion and reduce impurity transfer. Damaged organics are more prone to micro emulsion formation than fresh organics.

Your comments remind me of the diluents wash stage CSIRO suggested should be installed at Bulong to overcome transfer of Cyanex 272 to their Versatic 10 circuit and the subsequent gypsum precipitation problems that caused. This wash stage worked really well for them.

In terms of reducing aqueous entrainment, I found a coalesce-free way that works in the lab (for all lab samples and plant samples we've tested), but I've never tried it in a commercial operation.

Some additional comments:

I tried the diluents wash route several years back, similar to what has mentioned above, in a Ni SX pilot plant (a few years back) and it worked well.

A coalescing medium that has worked for me in other pilot plants is using polypropylene screen, as in the screen that is used for test sieves. These can be purchased off-the-shelf and in-bulk with whatever aperture size you find to be optimal. Possibly it is feasible for the scale of plant you are referring to.

In your situation, is it feasible to adjust the mixing conditions/equipment so to further reduce formation of ultra-fine droplets?

A left-field idea that has some application in niche operations is to use an electrostatic coalesce. Some will baulk at this idea, although with correct design and sensible engineering, the electrostatic hazard is not any more significant than in a regular SX plant. (the frictional charging from organic and plastic/rubber contact generates higher volts that an electrostatic coalesce uses).

Yes, the diluents wash is a good way to remove entrained organic (and extractant) from the product aqueous solution, however, the task I need addressing is to remove fine aqueous.

Few specifics that I have in mind to address aqueous carryover with Organic streamed are appended.

•At the Mixer, a trade-off is available: Radial orientation of Mixers produce more shear. Mass transfer is promoted at the expense of droplet size. An Axial orientation of agitator produces more flow; results in larger average droplet size, mass transfer could be compromised.

Agitators combine both radial and axial elements and hence a choice is available in terms of agitator selection vis a vis the "acceptable" mass transfer.

•In terms of Wet ability, as a basis for aiding separation of aqueous dispersions, Metallic meshes tend to be more Hydrophilic . Metals tend to be more polar compared to Plastics and hence more likely to wet the aqueous droplets. For Nitrate based and few sulfate based aqueous systems, stainless steel grades of coaleaser pads should be workable. For a Fluoride based system (often encountered in Ta/Nb circuits) and Chloride based systems, constrains in material selection could imply opting for aqueous continuity instead of organic continuity.

•For small units it is possible to manipulate the phase continuity by recycling a phase from the settler back to the Mixer. Recycling of a phase back to mixer to manipulate the continuity has a limitation as it limits the solvent train capacity to the extent of recycle and this does not go well with Process Managers.

To remove aqueous micro emulsion from organic phase one should choose granulated hydrophilic media. Granulated form allows cleaning the media from entrapped suspended solids by backwash operation with fluidization of the media. There are many materials available in the market: quartz sand, ion exchange resins, granulated silica.