Method for preparing fine cadmium from sponge cadmium

Abstract

This invention discloses a method for preparing refined cadmium from sponge cadmium, belonging to the field of cadmium smelting technology. The method includes the following steps: S1, directly oxidizing the sponge cadmium obtained by zinc powder replacement without briquetting to obtain an oxide material; S2, pulverizing the oxide material and mixing it with coking coal powder to form briquettes; S3, heating and reducing the briquettes obtained in step S2 under vacuum; S4, condensing the cadmium vapor obtained in step S3 in a condensation zone to obtain refined cadmium. The method provided by this invention changes the traditional process route of preparing refined cadmium from sponge cadmium via "briquetting-alkaline refining-distillation"; it proposes a new process of preparing refined cadmium from sponge cadmium via "oxidation-vacuum carbothermal reduction". This effectively avoids the generation of alkaline slag during the alkaline refining process of sponge cadmium, significantly reduces the dispersion loss of cadmium during smelting, shortens the process flow for preparing refined cadmium from sponge cadmium, and reduces production energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of cadmium smelting technology, specifically relating to a method for purifying sponge cadmium. Background Technology

[0002] In cadmium extraction, zinc powder displacement is commonly used to extract cadmium from solution in the form of sponge cadmium. However, during zinc powder displacement, because the standard electrode potentials of cadmium and zinc are quite similar, the displacement driving force for cadmium is insufficient, easily leading to a high zinc content in the displacement product, accompanied by impurities such as Pb, Mn, and Mg. The cadmium content in the displaced sponge cadmium is approximately 80%. Furthermore, because the boiling points and saturated vapor pressures of cadmium and zinc are quite similar, effective separation by distillation is difficult when the zinc content is high. On the other hand, the displaced sponge cadmium usually has a porous and loose structure with a large specific surface area, making it prone to oxidation in air. To prevent oxidation and deterioration, the sponge cadmium is usually compressed into clumps and temporarily stored in water. Subsequently, impurities such as zinc are further removed through an alkaline refining process, followed by vacuum distillation for purification, ultimately yielding refined cadmium. Alkaline refining is carried out under a molten caustic soda cover. The principle of zinc removal using caustic soda is as follows: The alkaline refining process consumes a large amount of caustic soda and generates hazardous gases. Statistics show that processing 1 ton of sponge cadmium requires approximately 150–200 kg of caustic soda. Simultaneously, this process ultimately produces a large amount of cadmium-containing alkaline slag; approximately 460 kg of cadmium-containing alkaline slag is generated for every ton of refined cadmium. Due to the complex composition of the waste alkaline slag and its status as hazardous solid waste, mature resource recovery technologies are currently lacking, and it is mostly disposed of through stockpiling during production. Furthermore, a certain amount of cadmium remains in the cadmium-containing alkaline slag, which not only reduces the overall cadmium recovery rate but also leads to further dispersion of cadmium in the solid waste system, thereby increasing the potential risk of cadmium pollution in the environment. Therefore, finding new purification processes and pathways for sponge cadmium has significant research value and engineering application potential.

[0003] Patent CN202110049969.8 discloses a method in which sponge cadmium obtained from a single displacement process is piled up and naturally oxidized, then subjected to a second acid extraction to obtain sponge cadmium with a higher cadmium content, followed by alkaline refining to obtain crude cadmium. The process is cumbersome and uses a large amount of acid.

[0004] Patent CN202411151391.7 mentions a process of oxidation, dissolution, re-displacement, and alkaline smelting of a cadmium-filled sponge. This process is also complicated and involves the use of concentrated sulfuric acid.

[0005] These methods are lengthy and require precise control, and they involve concentrated acids, increasing their inherent risks. Therefore, there is an urgent need for a method that is both shorter and safer in its preparation process. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing refined cadmium from sponge cadmium, which solves the problems of large amounts of alkaline residue generated during the alkaline refining process of sponge cadmium and the cumbersome process in existing technologies.

[0007] To achieve the above technical solution, the present invention adopts the following technical solution: A method for preparing refined cadmium from sponge cadmium includes the following steps: S1. The displaced cadmium sponge is directly oxidized by stacking without being compressed into briquettes to obtain the oxidized material. S2. The material obtained in step S1 is crushed and then mixed with coking coal powder in a certain proportion and pressed into blocks. S3. The blocky material obtained in step S2 is heated and reduced under vacuum. Cadmium volatilizes in the form of elemental vapor, and zinc remains in the slag in the form of zinc oxide along with other impurities. S4. The cadmium vapor obtained in step S3 is condensed in the condensation zone to obtain refined cadmium.

[0008] In the preparation method described above, the sponge cadmium in step S1 is obtained by zinc displacement during cadmium extraction, and the stacking oxidation time is 5-7 days.

[0009] Thermodynamic calculations show that under vacuum conditions, the carbothermic reduction temperature of cadmium oxide is lower than that of zinc oxide. Utilizing this temperature difference, cadmium oxide can be reduced to elemental cadmium, which then volatilizes as vapor, while zinc remains as zinc oxide in the residue along with other impurity metals. Based on this characteristic, this invention provides a method for the direct carbothermic reduction of sponge cadmium using a carbonaceous reducing agent under vacuum conditions, and the volatilized elemental cadmium is collected by condensation. This method not only effectively avoids the formation of alkaline slag but also shortens the process flow and improves product purity.

[0010] Preferably, in step S1, the sponge cadmium is directly piled up and oxidized without being compressed into lumps. In the presence of moisture and oxygen, elemental cadmium is converted into cadmium hydroxide, and a small amount of cadmium sulfide and elemental cadmium are also present. The mass fraction of cadmium is 60%-80%.

[0011] The coking coal has a carbon content of 70%-80%. Cadmium reacts with carbon to ultimately produce carbon dioxide, and the actual molar carbon ratio is 1-3 times the theoretical molar carbon ratio.

[0012] The theoretical molar carbon ratio is based on the reaction equation:

[0013] The calculations are based on the cadmium content in the cadmium-oxidized sponge and the carbon content in the coking coal, with a theoretical carbon-to-coal ratio of 2:1. Since the reaction is a solid-solid reaction, a larger carbon-to-coal ratio increases the contact area, thereby improving the reaction efficiency.

[0014] Preferably, in step S2, the particle size of both the cadmium sponge oxide and the pulverized coking coal is 150-180 μm.

[0015] Preferably, in step S2, the actual molar carbon ratio is 1-3 times the theoretical molar carbon ratio, and the briquetting pressure is 2-16 MPa.

[0016] Preferably, in step S3, the vacuum carbothermal reduction temperature is 600-800℃, the reduction time is 2-4h, and the system pressure is 1-10Pa.

[0017] Preferably, in step S4, the condensation temperature is 200-300℃, and the purity of the obtained elemental cadmium is higher than 99.995%.

[0018] Due to the adoption of the above technical solution, the present invention has the following beneficial technical effects: Compared with traditional processes, the preparation method of this invention completely eliminates the generation of alkaline slag, eradicating the risk of environmental pollution. Traditional alkaline refining produces 460 kg of cadmium-containing alkaline slag (a hazardous solid waste) for every ton of refined cadmium, and most of it is piled up untreated. This invention uses vacuum carbothermal reduction to directly treat sponge cadmium, eliminating the need for caustic soda, thus avoiding the generation of alkaline slag at the source and preventing the risk of cadmium spreading into the environment with the alkaline slag, significantly reducing the hidden dangers of heavy metal pollution. At the same time, the impurity removal and distillation purification processes are coupled to achieve one-step reduction and purification of sponge cadmium, simplifying the process flow. Attached Figure Description

[0019] Figure 1 This is a flowchart of a method for preparing refined cadmium from sponge cadmium according to the present invention; Figure 2 A flowchart of a traditional cadmium sponge treatment method; Figure 3 This is the XRD pattern of cadmium sponge oxide; Figure 4 This is a picture of the collected cadmium condensate; Figure 5 (a) Relationship between ΔG and T for cadmium oxide reduction at 10 Pa; (b) Mass spectrometry analysis of cadmium oxide in carbon reduction atmosphere. Detailed Implementation

[0020] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1 like Figure 1 The diagram shows the process flow for preparing refined cadmium according to this application. Figure 2 This refers to the process in existing technologies.

[0022] A method for preparing refined cadmium from sponge cadmium includes the following steps: (1) Take sponge cadmium directly without pressing and then oxidize it by stacking and crushing it. The oxidizing sponge cadmium was stacked for 6 days. The particle size of the oxidized sponge cadmium after crushing was 150-180μm. The coking coal was crushed and the particle size of the coking coal powder was 150-180μm.

[0023] (2) Take cadmium oxide sponge powder and coking coal powder and mix them at 2.3 times the theoretical molar carbon ratio. Take about 20g of the mixed powder material and press it into blocks under a pressure of 4 MPa.

[0024] (3) The blocky material obtained in step (2) was reduced for 4 h under the conditions of system pressure of 10 Pa and temperature of 725 °C. Cadmium was reduced to elemental form and volatilized in the form of vapor.

[0025] (4) The cadmium vapor generated in step (3) is condensed on graphite paper in a condensation zone at a temperature of 200-300℃ to obtain metallic cadmium, such as Figure 4 The image shown is a diagram of cadmium condensate collected in this application.

[0026] The direct recovery rate of cadmium and the residual rate of zinc in this invention are calculated using the cadmium and zinc content in the raw materials and the cadmium and zinc content in the residue, respectively:

[0027]

[0028] In this embodiment, the direct recovery rate of cadmium was 97.98%, the residual rate of zinc was 85.96%, and the zinc content in the condensate was less than 0.0005%.

[0029] The cadmium oxide sponge obtained in this embodiment was subjected to XRD analysis, such as... Figure 3 As shown, in the presence of moisture and oxygen, elemental cadmium forms cadmium hydroxide, and a small amount of cadmium sulfide and elemental cadmium also exist, with a cadmium mass fraction of 60%-80%.

[0030] The restoration process in step (3) is analyzed, based on... Figure 5 (a) It can be seen that the initial temperature at which cadmium oxide reacts with carbon to produce carbon dioxide is lower than the initial temperature at which carbon monoxide is produced. Therefore, the carbon reduction of cadmium oxide mainly produces carbon dioxide. Figure 5 (b) By further detecting the atmospheric composition of the carbon reduction of cadmium oxide, carbon dioxide rose sharply at 800°C, indicating that cadmium oxide reacts with carbon to produce carbon dioxide.

[0031] The condensate from Example 1 was analyzed for impurity elements using inductively coupled plasma optical emission spectrometry (ICP-OES), and the results are shown in the table below:

[0032] Under these conditions, the direct recovery rate of cadmium was 97.98%, the residual rate of zinc was 85.96%, and the cadmium purity of the collected condensate was 99.998%.

[0033] like Figure 1 As shown, the carbon reduction residue produced by this method can be returned to the copper-cadmium slag leaching process to recover residual cadmium and other valuable metals, without generating any solid waste. Existing technologies, such as the alkaline residue mentioned in patent CN118326171A, have the following composition:

[0034] The alkaline residue was not recycled and contained a large amount of cadmium; while the reduction residue of the present invention has a low cadmium content and low mass, and consists of residual carbon and polymetallic compounds, which can be directly returned to the copper-cadmium slag or zinc leaching process for recycling. Therefore, the process of this application improves the utilization efficiency of cadmium resources in sponge cadmium.

[0035] Example 2 A method for preparing refined cadmium from sponge cadmium includes the following steps: (1) Take sponge cadmium and directly pile it up for oxidation without blistering, then crush it. The particle size of the oxidized sponge cadmium is 150-180μm. Crush the coking coal and the particle size of the coking coal powder is 150-180μm. Pile it up for oxidation for 7 days.

[0036] (2) Take cadmium oxide sponge powder and coking coal powder and mix them according to the theoretical molar carbon ratio of 3. Take about 20g of the mixed powder material and press it into blocks under a pressure of 4MPa.

[0037] (3) The blocky material obtained in step (2) is reduced for 4 hours under the conditions of system pressure of 10 Pa and temperature of 800℃. Cadmium is reduced to elemental form and volatilized in the form of vapor.

[0038] (4) The cadmium vapor generated in step (3) is condensed on graphite paper in a condensation zone at a temperature of 200-300℃ to obtain metallic cadmium.

[0039] The condensate from Example 2 was analyzed for impurity elements using inductively coupled plasma optical emission spectrometry (ICP-OES), and the results are shown in the table below:

[0040] Under these conditions, the direct recovery rate of cadmium was 99.98%, the residual rate of zinc was 11.39%, and the cadmium purity of the collected condensate was 99.54%.

[0041] Example 3 The method and feed amount are the same as those in Example 1. The only difference is that the temperature in step (3) is 775℃. Under this experiment, the direct recovery rate of cadmium is 99.94%, the residual rate of zinc is 27.10%, and the zinc content in the condensate is 0.23%.

[0042] This indicates that increasing the temperature has a significant impact on the residual rate of zinc and the purity of cadmium in the condensate, but it can also achieve a certain purification effect.

[0043] Comparative Example 1 The comparative example and Example 1 follow the same method steps and feed amounts. The only difference is that the temperature in step (3) is 675°C. Under this experiment, the direct recovery rate of cadmium was 49.8%, and the residual rate of zinc was 92.45%. This indicates that lowering the temperature will have a significant impact on the direct recovery rate of cadmium.

[0044] Comparative Example 2 The comparative example and Example 1 follow the same method and feed amount. The only difference is that in step (2), the carbon ratio is increased to 1 times the theoretical molar ratio. Under this experiment, the direct recovery rate of cadmium was 69.05%, and the residual rate of zinc was 86.51%. This indicates that reducing the carbon ratio will have a significant impact on the direct recovery rate of cadmium.

[0045] Comparative Example 3 The method and feed amount are the same as those in Comparative Example 1. The only difference is that the reduction time in step (3) is changed to 1.5h. Under this experiment, the direct recovery rate of cadmium is 86.16% and the residual rate of zinc is 88.87%. This shows that reducing the reduction time will have a significant impact on the direct recovery rate of cadmium.

[0046] The above are all preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing refined cadmium from sponge cadmium, comprising the following steps: S1. The sponge cadmium replaced by zinc powder replacement method is directly piled up and oxidized without being lumped to obtain the oxidized material; S2. After the oxide material is crushed, it is mixed with coking coal powder and pressed into blocks; S3. The blocky material obtained in step S2 is heated and reduced under vacuum; S4. The cadmium vapor obtained in step S3 is condensed in the condensation zone to obtain refined cadmium.

2. The method for preparing fine cadmium from sponge cadmium according to claim 1, characterized in that: In step S1, the sponge cadmium is obtained through zinc displacement during cadmium extraction, and the stacking oxidation time is 5-7 days.

3. The method for preparing refined cadmium from sponge cadmium according to claim 1, characterized in that: In step S2, the particle size of the cadmium oxide sponge is 150-180 μm, and the particle size of the coking coal is 150-180 μm; the mass fraction of cadmium in the cadmium oxide sponge is 60%-80%, and the carbon content of the coking coal is 70%-80%; the actual molar carbon ratio of the cadmium oxide sponge is 1-3 times the theoretical molar carbon ratio; and the briquetting pressure is 2-16 MPa.

4. The method for preparing refined cadmium from sponge cadmium according to claim 1, characterized in that: The vacuum carbothermal reduction temperature in step S3 is 600-800℃, the reduction time is 2-4h, and the system pressure is 1-10Pa.

5. The method for preparing refined cadmium from sponge cadmium according to claim 1, characterized in that: In step S4, the temperature of the cadmium condensation zone is 200-300℃; the purity of the obtained refined cadmium is greater than 99.995%.

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