Oxidation Prevention

Over the years, a number of operations have experienced significant operational problems due to oxidation of the plant organic.  If the EMF of the aqueous solution in contact with the organic is greater than 800mV (Ag/AgCl reference) the diluent (and in some cases the oxime) will begin to degrade.  Oxidative degradation products tend to be very interfacially active and will significantly impact phase separation times.  This is most apparent under organic continuity and may not be fully recognized in operations which run under aqueous continuity,   The presence of oxidative degradation products also impact Cu:Fe selectivity as well as kinetics/stage efficiency.  Operations which have experienced severe degradation have had to significantly reduce flows (negatively impacting production), increase electrolyte bleeds, and take additional steps to remove the impurities via aggressive clay treatment.

Most often the oxidation is caused by an imbalance of iron and manganese in the electrolyte solution.  The problem has been experienced world wide but is most common in Africa due to the ore type.  Manganese is entrained from extract into the strip liquor, while iron is transferred both physically (via entrainment) and chemically.  Within the tankhouse, the valence state of the iron varies between ferrous Fe+2 and ferric Fe+3 (oxidizing at the anode and reducing at the cathode).  Manganese which reaches the electrolyte can be oxidized to higher valence states (Mn+5) which will then attack the organic.  Any ferrous ions in the electrolyte will oxidize in preference to the Mn thereby preventing the solution from reaching the higher redox potential.  As a rule of thumb, operations often try to maintain a ratio of iron to manganese of 10:1 in the electrolyte to ensure there is always sufficient ferrous present to reduce manganese back to a lower valence state.  Note:  Even a 10:1 ratio is not always sufficient to ensure manganese is reduced, so it is recommended that operations monitor the redox potential of the electrolyte on a frequent basis.

If the EMF of the aqueous solution ever exceeds 1000mV, the reaction is immediate and the results can be devastating.  An operation may require a new reagent fill and/or significant time to clay treat the entire organic inventory in order to return to full production.

To avoid Mn oxidation in the electrolyte, operations need to minimize A/O entrainment to strip.  This can be accomplished with proper house keeping, running sufficient organic depths, use of picket fences,  continuous decantation of the loaded organic tank, use of wash stages and or use of coalescers.

Operations which have leach solutions with a high concentration of Mn (or a feed with an Fe:Mn ratio of less than 5:1,  should always have a sufficient amount of ferrous sulfate in stock to add to the electrolyte when/if required.

Note:  Addition of ferrous sulfate is the most common methodology for reducing the solution potential, but has the downside of negatively impacting current efficiency.  Other reductants can be used including the use of a copper scrap tower to reduce the potential of the solution prior to contact with the organic phase.

Although operations can not continuously / successfully run at these higher redox potentials, Syensqo has developed a range of formultulations which provide a degree of protection against transient oxidative conditions.  If during a shift, an event leads to an imbalance and a high potential, the formulations contain species which will oxidize in preference to the oxime buying the operation some time to correct the situation.  There may still be some impact on the phase separation time but the oxime will be protected and remain available for copper transfer.

Any operation with elevated levels of manganese in the PLS should consider the use of Syensqo's Acorga OR (oxidation resistant) series of extractants as an additional level of protection.   The use of oxidative resistant formulations together with operational audits and implementation of recommendations to minimize aqueous entrainment, has allowed operations to consistently achieve their production targets.

Please contact your local Syensqo representative to discuss concerns around oxidation and to ensure the correct formulation, operational conditions, and analytical techniques are in place.