Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

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Zinc solvent extraction - impurity and entrainment problems (1 reply and 1 comment)

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hasliclimber
6 years ago
hasliclimber 6 years ago

Hello everybody. I’m working in the probably smallest zinc facility of the world… We do Zn recycling from fly ash of an incineration plant, means 100% secondary input. The process is like this: leaching of fly ash with HCL > Cementation with Zn-Powder > SX > Electrowinning of Zn. So our SX chances the medium from chloride to sulphate.

The PLS has about 12 g/L Zn. To take it out at once (3 counter current extraction states) we add NaOH in the mixer. We cannot put the raffinate/acid of SX extraction back to the leaching. This because our HCl is "produced" by our wet flue gas treatment of the incineration plant. So we give our raffinate to the wastewater treatment. This is the reason, because we add a base (NaOH) in the SX extraction, to get out all the Zn in one “cycle”.

The addition of NaOH in the SX-Extraction on the one hand induces some saponification reactions with/of our complexing agent Cyanex 272 (means a lot of cleaning work) and on the other hand creates some serious entrainment (HCl in direction of electrowinning…).

Does anyone out there is working with a similar (add of a base in the SX Extraction) system and has some hints for me, who to reduce entrainment and saponification reaction?

Best regards

J
Justin Lamb
6 years ago
Justin Lamb 6 years ago
1 like by David

Some observations based on previous experience with Cyanex 272 in solvent extraction and zinc extraction with D2EHPA both using ammonia gas for pH control.

  • Is the NaOH added to the organic phase prior to mixing with the aqueous? This seemed to work best with ammonia gas, and avoids precipitation of solids that can stabilise the saponified C272.
  • Is the PLS feed neutralised for iron removal prior to extraction as this can create some issues with precipitation in the extraction units.
  • Do you control pH in each extraction stage. The saponification reaction is very rapid and reversed by the acid generated by the extraction reaction. Over dosing the base means that the pH is too high and saponified extractant is not redissolved into the organic phase. The target pH should be around 4 to 5 for zinc from memory. If pH control is done correctly a pH probe submerged in the aqueous phase after the mix box can provide pH feed back and control of the NaOH dosing.
  • The loading of the organic can create viscosity issues with the organic
  • Carryover of the strip solution into the extraction stage is a real issue if targeting low raffinate levels. The use of an organic wash stage can be beneficial in controlling the pH in the last extraction stage and recovering entrained zinc.

Good luck with it.

I
hasliclimber
6 years ago

Hi Justin. Thanks for your quick answer!

1) No. The NaOH is added in the mixer directly. I will have a look on this idea!
2) Yes. Iron content is around 2 mg/L by entering the SX.
3) Yes we do. Moreover we controll the flow to. So I can calculate the used amount of NaOH and compare it with the loaded organic. The results show me, that NaOH dosage shoud be fine.
4) We use around 18 w% C272. I guess this is fine. Temperature is around 43°C.
5) There should be no carryover. We daily contorl the stripped organic. Normaly there is less then 200 mg/L Zn in the organic. Moreover the stripped organic even avter zentrifugation does not show any water entreinment.


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