Products and main technical and economic indicators of nickel sulfide electrolysis (2)

1. Current efficiency 1) Basis for calculating current efficiency (1) Faraday's law When the same amount of electricity is used to decompose various compounds in the electrolyte, the mass of the decomposition product is proportional to its chemical equivalent. That is, when the amount of electricity of 96,500 C (Coulomb) is passed through various electrolyte solutions, 1 gram equivalent of any substance will be obtained on the electrode regardless of the nature of the substance. The 96500C electric quantity is called the Fara one-speaking unit, which is represented by F, that is, 1F=96500C=26.8Ah
Thus, when 1 Faraday to a unit of electricity through the electrolyte, has a cathode on the gram equivalent of a metal or hydrogen evolution, while dissolving 1 gram equivalent of a metal or a gram-equivalent of oxygen evolution on the anode.
(2) Electrochemical equivalent The mass of the product obtained by introducing the unit electric quantity is called electrochemical equivalent, and the number of kilograms (kg) per ampere per hour (Ah) is usually expressed in the industry to represent the electrochemical equivalent:

Where n is the number of valence states of the metal atom.
Therefore, Faraday's law can be expressed as the relationship between the amount of precipitated material and the current intensity and time:
G=qIt
Where G is the mass of the precipitated (deposited) material, kg;
Q—Electrification equivalent of precipitated (deposited) material, kg/(Ah);
I—current intensity, A;
t—Power-on time, h.
According to the atomic weight of the metal and the valence of the atom, the electrochemical equivalent of various metals (elements) can be calculated. For example, the q value of nickel is 1.0954×10 ^-3 (kg.A ^-1 .h ^-1 ); the atomic weight of cobalt is slightly larger than that of nickel. Its q value is 1.1000×10 ^-3 (kg.A ^-1 .h ^-1 ).
2) Calculation formula of current efficiency and influencing factors of current efficiency In production practice, the amount of precipitated substances in the electrolysis process is often inconsistent with the calculation according to Faraday's law. For example, in nickel electrorefining, when the amount of electricity passing through the electrolytic cell is 1000 A.h, the amount of nickel deposited on the cathode is less than 1.0954 kg. Practice has proved that this is not the fact that Faraday's law itself is not rigorous, but in the process of electrolysis, other undesired reactions occur, that is, side reactions (such as ion discharge) or electrolytic cell leakage.

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Where I - the current intensity through the electrolytic cell, A;
N—the number of electrolytic cells;
T—electrolytic energization time, h;
G—the amount of product actually deposited in the cathode of the N electrolytic cells during the energization time, kg;
q—Electrification equivalent, 10 ^-3 × kg.A ^-1 .h ^-1 .
For example, there are 384 nickel sulfide electrolyzers in a factory with a current intensity of 13000A and a cathode cycle of 5d. The actual output of electrolytic nickel is 643t. Find current efficiency.
Solution: G=643×10 ³ kg, I=13000A, N=384, t=5×24h, q=1.0954×10 ^-3 kg/(Ah),
In industrial production, the actual nickel output is always less than the theoretical precipitation, and the nickel sulfide electrolytic refining current effect is generally 95% to 98%. The reasons why the current efficiency is less than 100% are:
(1) Short circuit. Due to the improper placement of the plates, the surface of the cathode is embossed and the edges are agglomerated with a cathode and a cathode, and the anode is short-circuited.
(2) Leakage. The electric current flows into the earth due to poor insulation between the electrolytic cell and the electrolytic cell, the electrolytic cell and the ground, the conductive plate circuit system, and the solution circulation system, thereby causing leakage.
(3) A side reaction such as hydrogen evolution occurs on the cathode.
In order to improve current efficiency in production, it is necessary to use higher electrolyte temperature, higher current density and higher pH electrolyte to strengthen workshop management, prevent short circuit, open circuit and leakage, and strengthen equipment insulation.
3) Anode current efficiency The anode current efficiency of nickel electrorefining has a direct impact on the electrolytic refining process. For soluble anodic electrolysis, anode current efficiency is the ratio (in percent) of the actual amount of a metal dissolved from the anode to the theoretical amount that should be dissolved from the anode by Faraday's law under the same conditions. The nickel sulfide anode is obtained by the secondary nickel concentrate casting, and the impurity metal content is high, which leads to the occurrence of a variety of impurity anode dissolution side reactions, thus causing the anode current efficiency to be lower than the cathode current efficiency, so that the nickel ions in the electrolyte are depleted, so it is required Electrolytic rehydration supplements nickel ions.
Since visible, current efficiency is actually a measure of the degree of deviation of Faraday's law from the electrolysis process. Generally, the current efficiency is less than 100%. The reason is because the theoretical calculation assumes that "the cathode (or anode) only precipitates (or dissolves) the metal ions (n ​​= 2 for Ni ²⁺ ions) that determine the valence of a certain atom. No other substances are precipitated (or dissolved). The actual electrolysis conditions are that in addition to the main metal precipitation (or dissolution) reaction, there may be other side reactions to precipitate (or dissolve) other substances, and correspondingly consume a part of the electricity.