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Principle and method of purifying copper by nickel electrolyte
In the nickel electrolysis anolyte, copper is one of the main impurities, and its content is generally between 0.1 and 1 g/L. The anolyte purification process requires that the copper removal be reduced to 0.0003 to 0.003 g/L. Many copper-based metal element in the nickel electrolyte, the copper metal more typical inert metal, and wherein the copper sulfide solubility product is small, generally chemically very stable in weak acid solution, and thus generally in the production Both copper removal is carried out by displacement or sulfur precipitation.
1. Replacement precipitation method for removing copper
The principle of displacement precipitation is a solution purification method in which a relatively inert metal (positive potential) is reduced to a metal and precipitated in a metal salt aqueous solution. In the anolyte from the nickel electrowinning consisting of sulphates and chlorides, Ni 2+ standard electrode potential (-0.25V) to be cleaned of impurities than Cu 2+ (+ 0.34V) more negative so the available nickel metal powder deposit The copper is displaced from the solution and the following reaction occurs:
Ni+Cu 2+ =Ni 2+ +Cu↓
However, iron powder potential (-0.44V) is more negative than copper, iron why not make the displacer it? This is because the copper is replaced by nickel powder, which removes both copper and nickel, and does not bring the impurity iron into the solution.
In production, when nickel powder is less active, sulfur is added to accelerate the copper removal process:
Cu 2+ +Ni+S=CuS+Ni 2+
Nickel electrolysis plants in the country use nickel powder to remove copper. In order to obtain a satisfactory copper removal effect, the following requirements should be observed during operation:
(1) Nickel powder should have high activity. Nickel powder suitable for this requirement can be obtained by a method of reducing nickel oxide powder with hydrogen at a relatively low temperature.
(2) The temperature of the anolyte should be maintained at 80 ° C and the pH should be below 3.5. Increasing the liquid temperature can speed up the displacement reaction rate of the nickel powder, maintaining the pH value of the solution lower than the hydrolysis value of Fe 3+ , and eliminating the hazard of the colloidal iron hydroxide precipitated nickel powder.
(3) The equipment should be sealed to reduce the air entering the purification tank and prevent the copper sponge that has been replaced from being oxidized and re-dissolved.
In the late 1950s, the former Soviet Union Northern Nickel Company succeeded in replacing the copper with a mechanical agitation tank instead of a mechanical agitation tank. Practice has proved that when the anolyte contains Cu 0.40～0.56g/L and the flow rate is 160m 3 /h, after the fluidized displacement tank is treated, the residual copper in the solution is 1.2～4.0mg/L. Compared with the mechanical agitation tank, the copper concentration of the fluidized tank after copper removal is low, and the utilization rate of nickel powder is high. The plant replaces 56 mechanical agitation tanks with a volume of 80 m 3 with two fluidized displacement tanks with a volume of 25 m 3 .
In order to add nickel powder to the copper removal reaction, the Thompson plant in Canada added sulfur powder while adding nickel powder. The reaction precipitated product was CuS. The advantage of this method is that there is no strict requirement for the activity of nickel powder.
A research institute in China has used nickel powder obtained by liquid phase hydrogen reduction to carry out copper reduction research. The nickel powder used has nickel content greater than 99.8% and particle size <0.074 mm. The results show that when the amount of nickel powder is 1.4 times of the theoretical amount, the copper content in the copper removal solution drops below 0.4 mg/L, and the copper removal rate is over 99%. If the amount of nickel powder is reduced, even if it is reduced to the theoretical amount, a better copper removal effect can be obtained.
2. Sulfide precipitation method for removing copper
1. The basic principle of sulfide precipitation method
Sulfide precipitation is based on the number of elements of the sulfide is insoluble in water, and therefore, when the solution has M n + present, was added S 2-, the following precipitation reaction will occur:
2M n+ +nS 2- =M 2 S n ↓
Its solubility product KSP = [M n+ ] 2 [S 2- ] n . The solubility products of some sulfides are shown in Table 1. [next]
Table 1 Solubility product of some sulfides
Sulfide
Temperature / °C
Ksp
lgKsp
Sulfide
Temperature / °C
Ksp
lgKsp
MnS
25
2.8×10 -13
-12.25
CdS
25
7.1×10 -27
-26.15
FeS
25
4.9×10 -18
-17.31
PbS
25
9.3×10 -28
-27.03
NiS (a)
25
2.8×10 -21
-20.55
As 2 S 3
18
4×10 -29
-28.4
CoS (a)
25
1.8×10 -22
-21.74
Sb 2 S 3
18
1×10 -30
-30
SnS
25
1×10 -23
-twenty three
CuS
25
8.9×10 -36
-35.05
ZnS
25
8.9×10 -25
-24.05
Cu 2 S
18
2×10 -47
-46.7
Common vulcanizing agents are H 2 S and Na 2 S.
Hydrogen sulfide gas solubility in water less, can be dissolved in 4.7 volume of H 2 S gas under normal conditions per volume water at a concentration of about 0.1lmol.L -1. H 2 S has the following ionization effects in aqueous solution:
Many metal ions can react with H 2 S or S 2 - to form sulfides with low solubility (see Table 9-5). The relationship between the hydrogen ion concentration and the sulfur ion in the solution is [H + ] 2 [S 2- ] = 6.8 × 10 -24 , which is passed in the acidic ([H 2 S] = 0.1 mol.L -1 ) solution. Into H 2 S can only supply a low concentration of S 2 - can only precipitate those metal sulfides with a small solubility product from the solution; and into the alkaline solution H 2 S (to form an alkali metal sulfide), then By supplying a higher concentration of S 2- ions, it is possible to precipitate a metal sulfide having a large or small solubility. Therefore, under controlled acidity, different metal ions in the solution can be precipitated separately by H 2 S.
For the same metal ion, the solution P佱 increases, and the residual concentration of the metal ions decreases. At 25 ° C, the concentration of S 2- is 0.1 mol / L, the relationship between the residual concentration of some metal ions and the P value is shown in Figure 1.
As can be seen from Table 1 and Figure 1, among the various elements of the nickel electrolyte, the sulfide solubility product of copper is the smallest, and the difference is large with the main metal sulfides such as nickel and cobalt . The method of separating copper and nickel is a relatively reliable method.
2. Production practice of lead removal by sulfide precipitation method.
The vulcanization precipitation method generally uses H 2 S as a precipitant, and the pH of the process is 1.8 to 2.5, and the reaction formula is:
Cu 2+ +H 2 S=CuS+2H +
During the reaction, the hydrogen sulfide gas should be highly uniformly dispersed and dissolved in the Fe-free liquid, so that S 2- and Cu 2+ are in sufficient contact to form numerous CuS precipitated nuclei, which pass through the process itself. Movement and diffusion, collision, adsorption and growth, and finally precipitated at the bottom to achieve the purpose of separation of slag liquid.
China's Chengdu Electric Metallurgical Plant and Chongqing Smelter have adopted H 2 S to remove copper. The purification process is to first react the Na 2 S solution with a dilute H 2 SO 4 solution to produce H 2 S gas, and pass it into the anode solution, and the Cu 2+ in the solution reacts with H 2 S to form a CuS precipitate. After mashing, the copper is removed from the solution. In order to prevent the overflow of hydrogen sulfide, the copper should be operated under a negative pressure and the pH value below 2 is controlled to inhibit Ni 2+ and Co 2+ from entering the copper slag due to precipitation. The technical conditions of the copper removal process are shown in Table 2.
From the viewpoint of sulfide dissolution, the removal of copper by H 2 S has a better selectivity. When H 2 S gas is passed, the oxidation-reduction potential of the solution is controlled to be -50 to -80 mV, and lead slag having a copper-nickel ratio of up to 10:1 can be obtained, but H 2 S is highly toxic, and if there is a leak, it is liable to cause personal accident; 2 S access must also be strictly controlled, otherwise high-quality copper slag can not be obtained.
Na 2 S can also be used as a copper sinking agent. The reaction mechanism is the same as that of H 2 S, but it is generally not used because it causes the production system to increase in Na + .
Table 2 Technical conditions for the removal of Cu by H2S
project
Control condition
Except Cu process solution temperature
>60°C
H 2 S generator negative pressure
0 to 3 × 10 -2 M Pa
Reaction chamber negative pressure
0 to 2.5×10 -2 MPa
H 2 S generator temperature
33～55°C
Cu in addition to Cu
<1g/L
Cu contains Cu after Cu
≤0.005g/L
Na 2 S solution
200～240g/L
H 2 SO 4 concentration
55% to 57%