1) Electrolyte composition The electrolyte used for nickel sulfide electrolysis must have a sufficiently high nickel ion concentration and a very low impurity ion concentration; in order to avoid anode passivation, it must also contain chloride salts. Therefore, a plant using a nickel sulfide electrolysis process uses a mixed electrolyte containing sulfate and chloride.
In the anodic electrolysis process of nickel sulphide, it is very important to select a suitable electrolyte composition. The composition of the electrolyte does not only affect the chemical composition and physical specifications of the product, but also affects the anode and cathode potentials, affecting power consumption and various reagent consumption and other indicators.
(1) Nickel ion The precipitation level of nickel is related to the concentration of nickel ions in the electrolyte. In the nickel electrolyte is the main metal, in order to obtain the pure product requires only an electroless nickel content of impurities in the electrolyte below the specified range, the main metal ion concentration and there are certain requirements, because, according to Faraday's law, the current density is constant, The amount of electricity passed per unit time is constant, so the amount of metal precipitated is also constant. If the nickel ion concentration is too low, the nickel ion is depleted on the surface of the cathode, which will cause the hydrogen to be precipitated, so that the pH of the local electrolyte in the cathode region rises sharply, and the nickel is hydrolyzed to form a basic salt, which seriously affects the product quality. Of course, too high nickel ion concentration is uneconomical and unnecessary. The relationship between nickel ion concentration and current density can be roughly calculated according to the following empirical formula:
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Where D k — limiting current density, A/m2;
C—electrolyte containing nickel ion concentration, g/L.
In the electrolysis process, increasing the nickel ion concentration or the catholyte circulation speed of the catholyte is an effective method for increasing the nickel ion concentration in the cathode region. The general control ion concentration in production is 70-75g/L.
(2) Chloride ion The presence of chloride ions in the electrolyte is advantageous for the entire electrolysis process. First, chloride ions can lower the electrolyte resistance, improve the conductivity of the solution, and reduce the cell voltage and power consumption. For example, in a 60 ° C aqueous solution of NiSO 4 -NiCl 2 , the specific conductance of the solution increases proportionally with the addition of NiCl 2 . The conductivity of the NiCl 2 solution at a concentration of 2 mol/L was 2.5 times higher than that of the NiSO 4 solution at the same concentration.
Secondly, chloride ions can alleviate and even eliminate the passivation of the electrodes. Because chloride ions can be adsorbed on the interface between the electrode and the solution, the structure of the electric double layer on the surface of the electrode is changed, and the activation energy of the reaction of the electric plate is reduced, so that the electrode process is easy to accelerate the reaction speed of the anode and the cathode.
Chloride ions also make the precipitation of nickel ions easier than hydrogen ions, thereby reducing the precipitation of hydrogen and improving the quality of nickel.
Since the degree of melting of NiCl 2 is much larger than that of NiSO 4 , the concentration of nickel ions in the solution can be increased, thereby increasing the current density and enhancing the production, ensuring the quality of the electric nickel, and creating conditions for improving the current efficiency. It is advantageous to use a high chloride ion or a pure chloride electrolyte in consideration of the recovery rate of the precious metal. Generally, the chloride ion is controlled at 50 to 90 g/L.
(3) Sodium ion In the purification process of nickel sulfide electrolyte, especially when using chemical precipitation method, sodium carbonate is used as a neutralizing agent, and some factories use sodium sulfide to remove copper , so sodium ions are brought into the process. In the electrolyte, and as the electrolysis process progresses, sodium ions gradually accumulate.
It is believed that the Na + and Cl - of the electrolyte fly have only one charge, but because of their high migration rate in the electrolyte and a high concentration in the electrolyte, Na - and Cl - Same, each carrying 30% of the current through the electrolyte. In the electrolysis process, sodium ions can increase the conductivity of the electrolyte, reduce the resistance of the solution, and reduce the power consumption. However, if the sodium ion concentration is too high, the viscosity of the solution and the resistance of the separator bag will be increased, the filtration performance will be affected, the crystallization will be easily generated, the pipes and valves will be blocked, and the normal production will be affected.
In order to maintain the sodium ion balance of the solution system, a part of the solution is often extracted to produce NiCO 3 in the production, and the sodium ion remains in the liquid phase during the liquid-solid separation, and the sodium discharge is achieved by effluxing. Therefore, the Na + concentration is higher than 45 g/L.
(4) The pH value of the electrolyte is as described above. When the pH value of the solution is low, the precipitation potential of hydrogen is relatively positive, hydrogen is preferentially precipitated by m on the cathode to reduce the current efficiency, and a large number of pores are formed on the surface of the electroless nickel; When the pH is high, Ni(OH) 2 precipitates on the surface of the cathode, and thus a dense cathode nickel is not obtained. Generally, the pH of the catholyte is controlled between 4.6 and 5.0.
(5) Boron <br> boric acid (H 3 BO 3) is not a tribasic acid, but a monobasic acid, is a weak acid. H 3 BO 3 solution in H + ionized only BO 3- 3, and both ions, if the PH value of the electrolyte decreased (i.e. increased acidity) m H + is excessive on the following reaction: 3H + + BO 3 3- →H 3 BO 3 thus lowers the H+ concentration in the solution, so that the pH value is maintained as much as possible. If the pH in the solution rises (ie, the acidity decreases), the following reaction is carried out: H 3 BO 3 →3H + +BO 3 3-, H + +OH - →H 2 O, thereby lowering the OH - concentration and keeping the pH as constant as possible. [next]
In order to improve the quality of the product during the electrolysis process, boric acid is often added to the electrolyte as a buffer. After the addition of boric acid, the pH of the electrolyte on the surface of the cathode can be kept stable to a certain extent, which means that it is possible to reduce the hydrolysis of nickel and the formation of a basic salt, which is advantageous for the improvement of current efficiency. In addition, the presence of boric acid can also reduce the brittleness of the cathode electrolytic nickel, making the surface of the electrolytic nickel smooth and smooth. In order to maintain the pH of the electrolyte = 4.6 to 5.2, the amount of H 3 BO 3 added is generally 5 to 20 g/L.
(6) Impurity ions The potentials of Cu and Pb are lower than those of Ni, Fe, and Co, but their overvoltage is smaller than that of Ni. Therefore, they may be deposited together with Ni to affect the quality of nickel. Taking into account the difficulty of deep purification in production and its production cost, generally the control does not affect the chemical composition of electro-nickel.
The hero model nickel factory has different requirements for the catholyte composition required for No. 1 nickel and No. 0 nickel (according to the original GB6516-86 standard) according to the grade of electrolytic nickel products, see the table below. The important conditions for the operation of the nickel sulphide anodic electrorefining production technology control shall not affect the chemical composition of the electro-nickel.
The following table  Catholyte composition (g/L) required by a nickel plant for the production of No. 1 and No. 0 nickel | |||||||||
Catholyte | Ni | Cu | Fe | Co | Zn | Pb | Na + | Cl - | H 3 BO 3 |
Production of No. 1 nickel | 70 | <0.003 | <0.004 | <0.01 | <0.00035 | <0.0003 | <45 | >50 | 6 |
Production of nickel No. 0 | 70 | <0.0003 | <0.0004 | <0.001 | <0.0001 | <0.00007 | <45 | >50 | 6 |
(7) Organic substances The sources of organic substances are various, such as engine oils, organic extractants, and the like. When the organic matter in the electrolyte exceeds a certain value, it is disadvantageous to the electrolysis process. First, due to the presence of organic matter, the surface of the cathode becomes hydrophobic, so that hydrogen bubbles deposited on the cathode are firmly retained on the surface of the cathode, affecting the normal deposition of nickel, causing long pores on the surface of the cathode and affecting the appearance quality of the nickel. In addition, the presence of organic matter causes an increase in the internal stress of the deposit, and in severe cases, the deposit becomes black and cracks. Therefore, it is necessary to prevent organic matter from entering the electrolysis system as much as possible. Generally, the organic content in the electrolyte is controlled to be less than 1 g/L.
2) Electrolyte circulation The diaphragm electrolytic solution circulation method is that the catholyte flows into the cathode chamber at a certain speed, and the depleted liquid after electrolytic deposition is infiltrated into the anolyte through the diaphragm and sent to the purification process for impurity removal treatment.
The purpose of the electrolyte circulation is: 1 to continuously replenish nickel ions in the cathode chamber to meet the requirements of nickel deposition in electrolytic deposition; 2 to promote the solution flow in the cathode chamber, increase the ion diffusion speed, and reduce the concentration polarization.
The electrolyte cycle rate is related to the current density, the concentration of the nickel ion in the electrolyte, and the temperature of the electrolyte.
Under the condition that the electrolyte contains nickel ion concentration and temperature, the current density is larger, and the circulation speed of the electrolyte is required to be larger. Therefore, under different current densities and different solution compositions, the higher the current density, the more the electrolyte circulation should be adjusted. The circulating amount of the electrolyte is too small, which affects the quality of the nickel. If the amount of circulation is too large, the material consumption is increased and the cost is increased. Therefore, the catholyte circulation rate is generally controlled to be 380 to 420 mL / (bag. min). The amount of circulation can also be calculated from the current intensity passed in unit time, expressed in m/L (Ah).
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