NPP KVALITET, Moscow, Russia:

M. N. Naftal, Deputy Director for Metallurgy and Beneficiation, e-mail: qualitetmet@mail.ru

L. V. Antonenko, Lead Specialist at the Department of Beneficiation and Metallurgy

Ural Federal University named after the First President of the Russian Federation B. N. Eltsin, Ekaterinburg, Russia:
S. S. Naboychenko, Visiting Professor at the Department of Non-Ferrous Metallurgy

Norilsk Nickel Harjavalta Oy, Harjavalta, Finland:
N. S. Kuznetsov, Chief Planning Engineer

This paper demonstrates that the redox potential Eh of oxidized slurry in pressure oxidation leaching (POL) characterizes not only the rate and depth of pyrrhotite decomposition but also the degree of oxidation decomposition of the surfactant used. By changing Eh one can increase the extractive effect produced by liquid sulphur on relic sulphides (pentlandite, chalcopyrite, etc.) and precious metal minerals, in particular due to their selective occlusion with liquid sulphur. A relationship was established between the recovery of platinum group metals and gold by POL into the sulphide sulphur phase grades –150+44 μm (which are most favourable for sulphide sulphur flotation) and the Eh of oxidized slurry. It was found that for different surfactants used in POL the effective Eh ranges vary conside rably. When using commercial lignosulfonates with Eh rising from 421 to 495 mV, the recovery of all the precious metals into the –150+44 μm grade will naturally decrease leading to higher losses downstream. For a combination surfactant, the curves showing the recovery — Eh relationship differ for various precious metals. For Pt, Pd and Au, such relationship is of bimodal nature, whereas the Rh, Ir and Ru curves are parabolic. In the low Eh range (420–440 mV), a slight rise in the recovery of Pt, Pd and Au into the –150+44 μm grade was observed. When Eh was raised to 455–460 mV, the recovery of the metals would go down. Following that, higher Eh values would go with a steady increase in the recovery of Pt, Pd and Au into coarser grades of the sulphide sulphur phase. The highest recovery of the above metals into the  –150+44 μm grades was observed in a higher Eh range (480–494 mV). For Rh, Ir and Ru, as Eh rose from 420 to 460 mV, their recovery into the –150+44 μm grades would steadily go down. As Eh kept rising, the recovery of the above metals rose too reaching its maximum at Eh = 494 mV. When Eh rose from 423 to 494 mV, the concentration of iron in the –150+44 μm grades would steadily go down while the concentration of elemental sulphur increased. The obtained experimental relationships were considered from the perspective of sulphide decomposition during POL and the impact of process factors on the interaction between liquid sulphur and the oxidized slurry components in the presence of different surfactants. It is shown that the control over oxidation leaching of sulphide materials, which is based on continuous measurements of the Eh of oxidized slurry using an electrode flow cell, opened up new prospects for optimizing the nickel refining process at Norilsk Nickel Harjavalta.

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