Froth Flotation (Sulphide & Oxide)

Froth Flotation (Sulphide & Oxide)2017-04-04T06:57:31-04:00
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High Arsenic in ILR bullion with peroxide addition (3 replies)

Gene Cheeseman
2 years ago
Gene Cheeseman 2 years ago

Hi Guys,

We have been experiencing an interesting phenomenon and am wondering if anyone with a stronger hydromet background could explain why.

We have been getting high levels of arsenic in our gravity ILR bullion.  We have been using hydrogen peroxide as an oxygen source in our leach at pretty typical intensive leaching conditions.

20,000 ppm NaCN

pH ~12 with NaOH

Peroxide addition to maintain DO from 16-22 mg/L

Our pregnant solution is almost totally clear, ICP scans on the solution show minimal arsenic but when we smelt the sludge we still sometimes have 5%+ As showing up in our bars.

We have done a few batch trials using SeproLeach as an alternative oxygen source (I believe this is the same chemical as "LeachAid") and have noticed significantly lower Arsenic grades in the first pour we did without peroxide.

Is it possible that peroxide somehow causes arsenic precipitation into our EW sludge but a chemical oxidant would not cause this same phenomenon?

If this is the case, it will definitely help our case in switching from peroxide to SeproLeach but if I had a scientific explanation as to why this is happening it would be easier to convince my boss.

 

Thanks!

Todd H
2 years ago
Todd H 2 years ago

I would check the IL concentrate for As first.  Identify source.

Since your ICP scans dont show arsenic in preg solution what is the source of the As? It cant precipitate if it is not in solution.

Any oxidant can cause arsenic precipitation as scorodite FeAsO4·2H2O or another oxide but the difference could be the oxidation potential. It may be that one oxidant has a higher oxidation potential leading to precipitation.  However, this still does not explain where the As is coming from. This arsenic would ppt in the leach not the EW.

If you can measure the ORP of the ILR with the two systems you may find a difference that explains why As is precipitating.  If the As is ppt in the leach it should not make it into the EW as the slurry is filtered.

If you identify the source then you can take steps to control.

 

Regards

Todd Harvey - Global Resource Engineering http://www.global-resource-eng.com

Gene Cheeseman
2 years ago
Gene Cheeseman 2 years ago

Hi Todd,

Thanks for the reply.  There definitely is As in the gravity concentrate we are leaching.  We have seen grades ranges from 1-5% As.

The leach slurry is thickened/clarified rather than filtered prior to EW.  The solution looks clear as water but I suspect there is some ultrafine arsenic suspended in the solution that reports to our sludge when we use peroxide rather than the chemical accelerant.

Aside from the arsenic levels we also notice a difference in density and consistency of our sludge using the two different oxygen sources. 

With peroxide the sludge seems much less dense and “sludgey/slimey” whereas with the chemical oxidant it feels much denser and almost sandy.

These first trials look very positive but it would be nice to understand WHY we are seeing such a difference?

We will look into the ORP of the two solutions next time we run comparison tests.

Thanks again for your input!

Dave Tahija
2 years ago
Dave Tahija 2 years ago
1 like by David

The arsenic is not precipitating as scorodite at pH 12 as scorodite is only stable in acid ranges. You're  getting adsorption of dissolved As onto some metal oxide/hydroxide ( most likely ferric hydroxide) and/or coagulating fine arsenic-bearing particles with metal hydroxide(s). That fits your physical description too, as it sounds like the peroxide tests are producing typically slimy hydroxide precipitates, with the metal being our friend ferric or aluminum or even manganese, depending on your water chemistry.

I'd suggest a proper solution analysis, as ICP scans aren't very precise especially at low concentrations. I suspect there is more arsenic in solution than you think.

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