Stomatal cause: ① because of the cathode reaction produced by the hydrogen bubble adhesion to the surface of the nickel plate, ② Nickel plate adsorption of mechanical inclusions of organic matter. Hydrogen bubbles and non-conductive organic matter occupy the surface position of the nickel plate, so that the resistance increases, forming the "0 charge zone" of nickel,Nickelcannot be precipitated into empty space, that is, stomata.
(1) Production of hydrogen bubbles
① is determined by the nature of the iron element. In the process of iron element electrolysis, one is that the phenomenon of "concentration polarization" is serious for iron metal, especially nickel, because the particle diffusion speed is less than the electron velocity, and the resulting nickel precipitation super potential is 0.3~0.8 V, while the general metal is less than 0.03 V, and the second is iron metal as cathode reaction 2h+ +2e- H2 has a good catalytic effect, greatly reducing the reaction speed is less than the electron motion speed caused by the "electrochemical polarization", so the ultra-voltage of hydrogen on the nickel cathode plate is much smaller than the average metal,NickelAnd the precipitation potential of hydrogen can be calculated.
NickelThe precipitated potential:
Hydrogen precipitation Potential:‰=ⅱ+ BLGDK (9-23) formula: n,6 is constant, n is related to the plate material, by the data check, nickel cathode plate 0.5 (general metal for 1.2-1.5), 6 take 0.12, calculate when the current density in the a/it12, the super potential of hydrogen is -0.8 v. It is seen from the calculation that the precipitation potential of nickel and hydrogen is similar in the process of nickel electrolysis, so the precipitation of hydrogen is inevitable. In practice, the ph of the diaphragm solution is higher than that of the new liquid, which confirms the existence of the discharge reaction of hydrogen ions.
Improper control of ② technical conditions
A PH: According to the formula, the precipitation potential of hydrogen is related to the concentration of hydrogen ions, V-analysis of =v0 +0.51gH+, from the formula, to increase the ph, is conducive to increasing the precipitation potential (negative value) of hydrogen. Low ph (2.6) electrolysis, hydrogen has a small super-potential, more hydrogen precipitation, this area due to large acidity, no hydroxide colloid produced in the solution, the solution viscosity is small, the cathode adsorption effect on hydrogen is small. Even if the temperature is around 55 ℃, hydrogen is unobstructed to escape the liquid surface and does not produce stomata.
High ph (4.5~5.0), this area has high hydrogen precipitation potential, produces less hydrogen, and produces a small amount of hydroxides mainly Ni (OH) 3. When the temperature is higher than 65 ℃, its viscosity is small, the migration of nickel has little effect, nickel does not produce stomata, if the temperature is less than 60 ℃, it is easy to produce stomata.
When the ph is about 3.5, the amount of hydrogen precipitated between the upper two regions, but this ph is just a trace of iron ions hydrolyzed into colloidal ferric hydroxide Fan, iron hydroxide suspended in the solution, its viscosity is quite large. On the one hand, the transfer speed of nickel ions is greatly reduced, and the diaphragm bag is blocked, which affects the cathode liquid circulation, thus speeding up the "concentration difference polarization" of nickel, on the other hand, the presence of iron hydroxide is also beneficial to the hydrogen bubble remaining on the cathode surface. Thus, the temperature of this area is maintained at more than 65 ℃, and the electro-nickel stomata are also produced in large quantities, which is a restricted area for the production of nickel
b Temperature: The temperature of nickel electrolyte increases while reducing the super potential of nickel and hydrogen. However, the temperature increases, the viscosity of the solution decreases, which is conducive to the precipitation of hydrogen, so the high viscosity of the solution, the requirements of high temperature. Conversely, the temperature can be reduced.
C Current density: Increase current density while increasing the super potential of nickel and hydrogen. But the potential is too high, and the super-potential of nickel increases faster than the super potential of hydrogen, which is beneficial to the precipitation of hydrogen. For the same piece of nickel, the cathode is not much wider than the anode, it produces a concentration of edge current density, nickel ions too late to replenish, the edge of the production of stomata. Widening the cathode eliminates this phenomenon.
For the nickel electrolytic solution of three systems, the conductive performance of the whole sulfuric acid medium is poor, generally can not use too high current density, must adopt the appropriate ph, temperature. Otherwise, it is easy to produce stomata.
D The main components of the solution: in the sulfate system mainly rely on nickel ions, sodium ions, sulfuric acid plasma conductive, nickel ions are in the cathode precipitated substances, if the concentration is low, will produce nickel dilution phenomenon. Nickel has different solubility in different electrolyte systems, but it is not high and easy to precipitate, which increases the viscosity of the solution. In addition, the conductivity of nickel is not very good, nickel ion concentration is too high too low will cause "concentration difference polarization", resulting in gas lone l produced. The general total sulfuric acid medium nickel ion concentration is controlled at 40~50/L.
Nickel ions can increase the solution ratio conductivity, but in the absence of nickel ions, sodium ions gather near the cathode, itself does not participate in the cathode reaction and hinder nickel precipitation, resulting in a large amount of hydrogen.
E Yin and Yang Polar liquid flow: the flow of yin and yang polar liquid directly affects the migration speed of nickel ions. In practice, the fluidity of cathode liquid is restricted by many factors, such as diaphragm bag material, new liquid flow and so on. However, the fluidity of the anode liquid is easily neglected. In the design of electrolytic cell, the electrolytic cell is long, wide and high, which is easy to cause the dead angle of anode fluid flow. When no measures are taken and additives are not used, nickel has a large number of stomata, and if several more inlet and outlet tubes are introduced from the cathode chamber and the anode chamber, the solution cycle is strengthened, and the gas lone L is significantly reduced or eliminated.
The above is due to hydrogen precipitation adhesion on the cathode to produce stomata, mostly distributed in the nickel plate upper and edge.
(2) Organic matter of various mechanical inclusions in solution
All kinds of organic matter brought into nickel electrolyte by equipment and process generally have two characteristics, one is that the solubility in water is small, most of them are in the solution with mechanical inclusions, and the other is that its conductivity is poor, and it is easy to be adsorbed by the interface, the mechanical inclusions are adsorbed by the cathode position, nickel can not precipitate to form stomata. If the content of 0272 extract is electrolytic, when its contents are higher than that of Mg/l, some mechanical inclusions of C272 are adsorbed by cathode to form bottom black. Due to the more kerosene, the solubility of kerosene in the solution is only 0.1 mg/L, which is adsorbed by the cathode and diffused to the surrounding area, forming a dense stomata.
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