Among the trivalent hydroxides of iron group elements (including Fe, Co and Ni), Ni(OH) 3 is the most oxidizing, Co(OH) 3 is the second, and Fe(OH) 3 is the most oxidizing. weak. Co 2+ is oxidized to Co 3+ with Ni(OH) 3 .
In industrial production, black nickel (FeOOH) is a stable form of Ni(OH) 3 . Since nickel oxyhydroxide [Ni(OH) 2 ] is dark green and nickel hydroxide [Ni(OH) 3 and NiOOH] is black, it is named "black nickel". Black nickel as Cl 2, as it can be used as the oxidant in the water and release of cobalt. The response is as follows:
NiOOH+Co 2+ +H 2 O=Ni 2+ +Co(OH) 3
The black nickel required to purify the precipitated cobalt as an electrolyte is obtained by electrolysis. The basic process of electrolytic anodizing Ni(OH) 2 is to extract a part of the purified liquid from the electrolyte purification system, add Ni(OH) 2 by adding sodium hydroxide, and put the Ni(OH) 2 slurry into the electrolytic cell. Into direct current, Ni(OH) 2 oxidizes on the anode:
Ni(OH) 2 -e=NiOOH+H +
The mechanism of electrolytic oxidation of Ni(OH) 2 to NiOOH is not fully understood. However, it is generally believed that the oxidation process occurs in the solid phase, that is, Ni 2+ can be oxidized without entering the solution, that is, when the Ni(OH) 2 particles are in contact with the anode. The electrolytic oxidation bath must be agitated to cause the Ni(OH) 2 particles to collide with the anode. The anode material of the electrolytic oxidation tank is an outer length initial pole piece, and the cathode material may be a nickel- chromium wire or a stainless steel mesh, and the slurry in the electrolytic oxidation tank is stirred by bubbling air. The table below shows the technical operating conditions of the electrolytic oxidation tank.
The following table "black nickel" electrolytic oxidation tank technical operating conditions
project | unit | |
Electrolyte composition | | NaOH 0.1~0.15mol/L Ni30g/L |
Electrolyte temperature | °C | 50 |
Cell voltage | V | 2.3 |
Anode current density | A/m2 | 20 |
Current efficiency | % | ~50 |
The Hijavarta refinery in Finland uses "black nickel" oxidized water to remove cobalt in two stages of countercurrent flow in two air stirred tanks with a volume of 120 m 3 . In the first stage of purifying the cobalt removal, the solution is in contact with the partially reacted NiOOH, and about 50% of the cobalt in the solution is precipitated. The slurry is sent to the automatic filter press for filtration. The filter residue is pickled and sent to another factory for recovery of cobalt, and the filtrate is sent to the second stage for purification of cobalt. Add new NiOOH to the second reaction tank.
The removal of cobalt by NiOH, because of its inverse product, is nickel ion, consistent with the main component of the electrolyte, and does not contaminate the treated solution. In addition, the removal of cobalt by NiOOH, due to its strong oxidizing ability, can simultaneously remove trace trace impurities, such as copper , iron, manganese , arsenic, etc. in the solution, for the purpose of deep purification.
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