Treatment of copper and cadmium slag

1. Electrochemical production of metal cadmium

Copper electrowinning processes produce slags Cd Cd metal as shown in FIG.

Fig.1 Process flow of metal cadmium electrowinning method from copper cadmium slag

The composition of copper cadmium slag generally ranges from 2.5% to 12% Cd, 35% to 60% Zn, 4% to 17% Cu, and 0.05% to 2.0% Fe. Copper cadmium slag also contains a small amount of As, Sb, SiO ₂ , Co, Ni, T1, In and other impurities.

In order to accelerate the leaching process, some factories oxidize copper cadmium slag in the air before leaching. This also increases the loss of copper dissolution, which is only applicable when dealing with copper-cadmium slag with lower copper content. The copper slag composition obtained by the leaching process is: 30% to 50% Cu, 10% to 15% Zn, and 0.3% to 1.0% Cd.

In the leaching, in addition to the dissolution of zinc and copper, some Ni, Co, In, T1 enter the solution, and the obtained leaching liquid composition is: 120-130 g/L Zn, 8-16 g/LCd, 0.3-0.8 g/LCu, 3 to 9 g/LFe, 0.05 to 0.1 g/LCo, 0.05 to 0.1 g/L Ni. After the leachate is purified by adding zinc powder to remove copper, the zinc powder is sent to replace the precipitated cadmium. Displacement of precipitated cadmium is generally carried out in two stages. In the first stage, the temperature was maintained at 333 K to reduce the cadmium in the solution to 1 g/L. The second step of the solution after separating the copper cadmium slag by filtration can further reduce the cadmium content to 10-15 mg/L. The sponge cadmium (II) obtained in the second stage contains low cadmium and reverses the leaching process of copper cadmium slag. The solution after the second stage replacement contains Co, T1, In, etc., and the cobalt is removed by the xanthate to further recover T1 and In.

The sponge (I) obtained by replacing the precipitated cadmium in the first stage is leached with a cadmium electrolyte. The concentration of sulfuric acid in the solution is 200-250 g/L, the leaching temperature is 353-363 K, and MnO ₂ or KMnO _{4 is added} to accelerate the dissolution of the cadmium sponge. The pH value of the leaching is 4.8-5.2, and the copper is hydrolyzed into the slag.

The cadmium cotton leaching solution after separating the copper slag, adding SrCO ₃ to remove lead , adding zinc powder to replace copper, adding KMnO _{4 to} oxidize T1 and Fe, and then hydrolyzing and precipitating.

The electrowinning of cadmium solution generally adopts the system of non-circulating operation of electrolyte, and its operating conditions and technical indicators:

Add electrolyte component / (g · L ^-1 ) 160 ~ 220Cd, 20 ~ 30Zn, 12 ~ 15H ₂ SO ₄

Waste liquid component after electrowinning / (g·L ^-1 ) 15～20Cd, 150～180H ₂ SO ₄

Electrolyte temperature / K 303 ~ 308

Current efficiency /% 70 ~ 92

Cell voltage / V 2.5 ~ 2.6

Dielectric cycle / h 24

Power consumption / (kW·h·t ^-1 ) 1400～1700

Higher current efficiency can be obtained by using the electrolyte circulation production method.

Many wet zinc smelters in the former Soviet Union use the electrowinning process. China's wet zinc smelting plant uses the electrowinning method of the electrolyte circulation system. For example, the electrowinning process for treating such Cu-Cd slag at Zhuzhou Smelter is shown in Figure 2. The chemical classification of Cu-Cd slag is:

5.64% Cu, 14.31% Cd, 40.26% Zn, 1.27% Pb, 0.076% Ni, 0.0212% Co, 0.0075% In, 0.0024% Ge, 0.0029% Ga, 0.0329% T1, 4.07% Fe.

Figure 2 Process for producing cadmium from Cu-Cd slag at Zhuzhou Smelter

The technical conditions for the main smelting process for producing cadmium from copper and cadmium in Zhuzhou Smelter are as follows:

(1) Leaching of Cu-Cd slag

Leaching was carried out using a 50 m3 mechanical agitation leaching tank. Sulfuric acid is slowly added to the leaching tank containing Cu-Cd slag, and the maximum acidity for leaching is 10-15 g/L, and the temperature is 353-363K. When the pH dropped to 5 ~ 4g / L, soft manganese ore was added, at pH 4.8 to 5.0, milk of lime (now use ZnO powder) and then stirring was stopped when the to pH = 5.2 ~ 5.4. The entire leaching process lasts for 6-8 hours.

Pressed through a 28m ² gel filter press, the obtained filter residue composition: 20% to 30% Cu, <1% Cd, copper smelting treatment to recover copper. The filtrate component: 8-15 g/LCd, 80-140 g/L Zn, 0.050 g/LCu.

(two) replacement

The displacement was carried out in a mechanical stirring tank of 50 m ³ . Before the replacement, H ₂ SO _{4 is} added to acidify the leached filtrate to pH=3 to 4, and the zinc powder is slowly added for the displacement reaction. When the solution contains cadmium less than 100 mg/L, it is sent to pressure filtration.

The sponge cadmium obtained by the replacement contains 60% to 80% of Cd, and is then stacked for 7 to 10 days for natural oxidation and then sent to the reconstitution solution. When the poor liquid after the replacement contains 15 to 30 g (T1)/m ³ , the zinc powder may be added to replace the crucible and then sent to the wet zinc smelting system.

(3) Rehydration

The liquid was formed in a mechanical agitation tank of 9 m ³ . Add sponge cadmium and concentrated sulfuric acid to the tank, maintain the dissolution of 85 ~ 90 ° C, after 2 ~ 3h to reduce the acidity of the solution to 0.5 ~ 1g / L, then add KMnO4 oxidation to remove iron , then add cadmium cotton to reduce the pH to 3.8 ~ 4.0, and then neutralized with lime milk to pH = 5.4, and sent to filter.

(four) purification

Purified in a 17 m ³ mechanical agitation tank. After 50 ° C, add fresh cadmium cotton to replace copper, and then add KMnO ⁴ to remove iron. The composition of the purified solution is 200-250 g/LCd, 20-30 g/L Zn, less than 0.05 g/LFe, less than 0.0005 g/LCo, and less than 0.001 g/L (As+Sb).

(5) Electron accumulation

The electrolyte is circulated in an electrolytic cell lined with reinforced concrete. The size of the groove is 2800×850×1250mm, and each slot can hold 26 anodes and 25 cathodes. Powered by a 2000A and 0 to 36V selenium rectifier.

The technical conditions of the electrowinning process are as follows:

The same name is 10mm

Electrolyte circulation amount 0.103m ³ /min

Electrolyte temperature 298～305K

Current density 45～75A/m ²

Slot voltage 2.4~2.5V

Electrolysis cycle 24h

Electrolyte composition / (g · L ^-1 ) 60 ~ 70Cd,

120～145H ₂ SO ₄

(6) Refining and casting

Refining in a 1 t cast iron pan.

The casting temperature is 723～823K, the surface is covered with a layer of NaOH, and 7.5kg of cadmium ingot is cast. Its composition: cadmium is more than 99.99%, lead is less than 0.004%, zinc is less than 0.002%, copper is less than 0.001%, and iron is lower than 0.002%. The first grade rate of cadmium is 100%.

Second, the replacement method to produce metal cadmium

Due to the large power consumption of cadmium produced by electrowinning, many factories have changed the electrowinning method to the replacement method.

The electric zinc plant of the American Smelting and Refining Company originally used the electrowinning method to treat the cadmium residue from the second purification of zinc production to produce cadmium. Now it is replaced by the replacement method. The process flow is shown in Figure 3.

Figure 3 Process for the production of cadmium from cadmium slag by US smelting and refining companies

The Finnish Kokkola Electric Zinc Plant uses the second stage of purification to produce cadmium slag to produce cadmium, which is also used in the displacement process. The composition of cadmium slag treated by Kokkola Electric Zinc Plant is as follows: No. 1 15%~25% Cd, about 1% Cu, 0.05% Co, 0.005%~0.05% Ni, 60% Zn; No. 2 22.4% Cd, 0.7% Cu , 54.5% Zn.

The electro-zinc plant of the former Soviet Union Usk-Kaminnogorsk lead-zinc joint company processes Cu-Cd slag by displacement precipitation in a centrifugal reactor. The production process is shown in Fig. 4.

Figure 4 Tungsten-Kaminnogorsk electric zinc plant processing copper cadmium slag production process

The centrifugal reaction separator has a cylindrical shape and a hollow shaft at the center. The shaft is equipped with a special structure separation disc, and the rotation speed of the hollow shaft reaches 3000r/min.

The rate of displacement of the precipitate in the centrifugal reactor was more than 300 times the precipitation rate of the general displacement sedimentation tank, and the productivity per liter volume reached 200 L/h. The low-cadmium cotton obtained in the second-stage centrifugal reactor is dissolved with zinc waste electrolyte, heated to 343K, and the final pH of the reaction is 4.5-5.5, and then purified by KMnO ₄ to remove the ruthenium, and then sent to the centrifugal reactor for replacement. cadmium.

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