Method for jointly recovering zinc and cadmium from zinc-cadmium slag of zinc hydrometallurgy

Abstract

The invention discloses a method for jointly recovering zinc and cadmium from zinc-cadmium slag of zinc hydrometallurgy. The method comprises the following steps: S1, carrying out acid leaching on the zinc-cadmium slag and adding zinc powder for replacement; s2, sponge cadmium is subjected to vacuum melting; s3, adding clear water and calcium oxide into the cadmium alkaline residues for wet grinding, filtering to obtain clear liquid and filter residues, and returning sodium hydroxide obtained after the clear liquid is evaporated and crystallized to S2; s4, the filter residues are subjected to reduction roasting, recyclable zinc-cadmium smoke dust, calcium oxide residues and sulfur trioxide are obtained, the calcium oxide residues are returned to S3, sulfuric acid is prepared from sulfur trioxide, and a part of sulfuric acid is returned to S1; the other part of sulfuric acid enters the next step; s5, the smoke dust is dissolved with sulfuric acid, a sulfuric acid solution containing zinc and cadmium is obtained, then zinc powder is added for replacement, and sponge cadmium and a zinc sulfate solution are obtained; the sponge cadmium returns to S2, and the zinc sulfate returns to S1. According to the method, the cadmium alkaline residue treatment product can be effectively recycled and utilized, the cadmium alkaline residue treatment cost is reduced, no cadmium element flows out of the process to cause environmental pollution, and the method is an environment-friendly process method.

Description

Technical Field

[0001] The invention relates to the technical field of nonferrous metal smelting, and in particular to a method for jointly recovering zinc and cadmium from zinc-cadmium slag in hydrometallurgical zinc smelting. Background Art

[0002] Cadmium is a byproduct of zinc smelting. During the hydrometallurgical zinc smelting process, cadmium is primarily concentrated in the zinc-cadmium slag. After the zinc-cadmium slag is leached with sulfuric acid, the cadmium enters the sulfuric acid solution, where it is then displaced with zinc powder to produce sponge cadmium and zinc sulfate solution. The zinc sulfate solution is then treated by hydrometallurgical methods to recover the zinc. The sponge cadmium is die-cast into a cadmium sponge cake for subsequent smelting to recover metallic cadmium. Cadmium has a low melting point of only 321°C and is easily oxidized to cadmium oxide. During smelting, its surface is coated with 20-25% sodium hydroxide to prevent oxidation. Therefore, the reduction smelting process of sponge cadmium to produce crude cadmium produces a large amount of alkaline slag, primarily composed of cadmium oxide, metallic cadmium, sodium zincate, sodium carbonate, sodium hydroxide, and sulfate.

[0003] Chinese patent application CN107475524A discloses a method for treating cadmium alkali residue, which mainly involves adding water to dissolve the cadmium alkali residue, separating insoluble cadmium and cadmium oxide, and then adding acid to separate zinc and sodium elements.

[0004] However, this method requires the use of sulfuric acid to adjust the pH, which is costly. Furthermore, due to the extensive solid-liquid separation process, some zinc hydroxide exists in colloidal form during actual production, making filter pressing difficult and preventing full zinc recovery. Furthermore, the high impurity content of the raw materials can lead to poor cadmium reduction during the reduction of cadmium-containing filter residues, resulting in low metal recovery rates. This method is lengthy, involves numerous process control points, and cannot fully recover valuable elements, making industrialization challenging. Summary of the Invention

[0005] To address the deficiencies in the prior art, the inventors have provided a method for the combined recovery of zinc and cadmium from zinc-cadmium slag produced during hydrometallurgy. This method enables the effective recovery and utilization of cadmium alkali slag treatment products, shortens the process, reduces processing costs, and eliminates the outflow of cadmium elements from the process to cause environmental pollution.

[0006] A method for jointly recovering zinc and cadmium from zinc-cadmium slag in hydrometallurgical zinc smelting comprises:

[0007] Step S1: After the zinc-cadmium slag is acid-leached with sulfuric acid, it is replaced with zinc powder to produce sponge cadmium and zinc sulfate solution; the zinc sulfate solution is treated by wet method to recover zinc;

[0008] Step S2: adding sodium hydroxide to a vacuum melting furnace, and after the sodium hydroxide is melted, adding sponge cadmium, and vacuum melting the sponge cadmium. During the melting process, a reducing agent is added, and the sponge cadmium is vacuum melted to obtain cadmium alkaline slag and crude cadmium; and refining the crude cadmium to obtain refined cadmium;

[0009] Step S3: Add clean water and calcium oxide to the cadmium alkali residue for wet grinding, using water and calcium oxide to form OH - At the same time, the blocky alkali residue is broken into powder during the wet grinding process, and Na + OH formed with calcium oxide - The reaction is sufficient to regenerate NaOH. The slurry obtained in this step is filtered to obtain a clear liquid and filter residue, and the clear liquid is evaporated and crystallized to obtain sodium hydroxide; the sodium hydroxide is returned to step S2;

[0010] The chemical equation for the reaction in this process is:

[0011] 2Na2ZnO2+CaO+3H2O=2CaZnO2↓+4NaOH,

[0012] Na2CO3+Ca(OH)2=CaCO3↓+2NaOH;

[0013] SO4 2- +Ca(OH)2=CaSO4+2OH -

[0014] This step is mainly to regenerate NaOH and at the same time make the sodium zincate in the cadmium alkali residue form calcium zincate precipitation. A small amount of SO4 remains in the alkali residue. 2- , adding calcium oxide can convert SO4 2- The CaSO4 is precipitated to purify the NaOH solution. The separated NaOH is returned to step S2.

[0015] Step S4: The filter residue is subjected to reduction roasting to obtain recyclable zinc-cadmium dust, calcium oxide residue, and sulfur trioxide. The roasting process generates SO3, which is used for acid production. The SO3 is discharged through the tail gas of a vacuum pump and absorbed by concentrated sulfuric acid to produce fuming sulfuric acid. The fuming sulfuric acid can be diluted to obtain sulfuric acid. A portion of the sulfuric acid is returned to step S1 to leach the zinc-cadmium residue, and the remaining portion is sent to the next step to perform wet separation on the zinc-cadmium dust. The chemical equation for the reaction in this process is:

[0016] C+2CdO=2Cd+CO2↑, C+2ZnO=2Zn+CO2↑, CaCO3=CaO+CO2↑

[0017] CaSO4=CaO+SO3, SO3+H2SO4=H2S2O7

[0018] Step S5: After the zinc-cadmium smoke is recovered, it is subjected to wet separation, and the smoke is dissolved with sulfuric acid prepared in step S4 to obtain a sulfuric acid solution containing zinc and cadmium, and then zinc powder is added for displacement to obtain sponge cadmium and zinc sulfate solution; after the sponge cadmium is die-casted, it returns to step S2, and the zinc sulfate solution returns to step S1.

[0019] Furthermore, in step 3, the liquid-solid ratio during wet grinding is 2-2.5:1, the amount of calcium oxide added is 10%-15% of the mass percentage of the raw materials, and the wet grinding time is 1-1.5 hours.

[0020] Furthermore, in step 4, when reduction roasting is performed, carbon powder is used as a reducing agent, and reduction is performed in a vacuum environment at a reduction temperature of 1000-1400° C. After reduction, the valuable metals enter the volatile phase in the form of cadmium oxide, cadmium, and zinc. At the same time, calcium sulfate, calcium zincate, and calcium carbonate in the residue are reduced and decomposed to form calcium oxide and return to step A.

[0021] Beneficial effects:

[0022] The invention provides a method for recovering zinc and cadmium from zinc-cadmium slag produced by hydrometallurgy. In the whole recovery process, the added CaO, sulfuric acid and the like are recycled, thereby reducing the processing cost. - The regenerated NaOH can be returned to the sponge cadmium smelting process to produce crude cadmium. Zinc-cadmium dust can be treated using a wet process, where the dust is dissolved in sulfuric acid and then replaced with zinc powder. This reduces the amount of dust and therefore the consumption of sulfuric acid. The residue is decomposed to produce calcium oxide, which contains a small amount of zinc and cadmium that has not been fully volatilized. This is returned to the wet grinding process, recycling the valuable metals within the process. No cadmium escapes from the process to cause environmental pollution. This is an environmentally friendly process that shortens the process and reduces treatment costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of the method for jointly recovering zinc and cadmium from zinc-cadmium slag in hydrometallurgy zinc smelting in Example 1. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below by way of examples.

[0025] Example 1

[0026] like Figure 1 As shown, this embodiment provides a method for jointly recovering zinc and cadmium from zinc-cadmium slag in hydrometallurgy zinc smelting, comprising the following steps:

[0027] Step S1: After the zinc-cadmium slag is acid-leached with sulfuric acid, it is replaced with zinc powder to produce sponge cadmium and zinc sulfate solution; the zinc sulfate solution is treated by wet method to recover zinc;

[0028] Step S2: 10-20% sodium hydroxide is added into the sponge cadmium vacuum melting furnace, and sponge cadmium is added intermittently after the sodium hydroxide is melted. The generated water vapor is collected in the condenser, and the smoke is collected in the filter bin, and the tail gas is discharged in compliance with the emission standards.

[0029] Step S3 includes wet-grinding the cadmium alkali residue with clean water and calcium oxide, filtering to obtain a clear liquid and filter residue, wherein the liquid-to-solid ratio during wet grinding is 2-2.5:1, the amount of calcium oxide added is 10%-15% of the mass percentage of the raw materials, and the wet grinding time is 1-1.5 hours; and the clear liquid is evaporated and crystallized to obtain sodium hydroxide. The sodium hydroxide is returned to step S2 for crude cadmium production.

[0030] Step S4: The filter residue is subjected to reduction roasting to produce recyclable zinc-cadmium dust, calcium oxide residue, and sulfur trioxide. During the reduction roasting, carbon powder is used as a reducing agent, and the reduction is carried out in a vacuum environment at a temperature of 1000-1200°C. After reduction, the valuable metals enter the dust in the form of cadmium oxide, zinc, and zinc oxide. The calcium sulfate and calcium zincate in the residue decompose to form calcium oxide, which is then returned to step S3. The SO₃ generated during the roasting process is discharged through the vacuum pump exhaust and absorbed with concentrated sulfuric acid to produce fuming sulfuric acid. The fuming sulfuric acid is diluted to produce sulfuric acid. A portion of the sulfuric acid is returned to step S1 to leach the zinc-cadmium residue, while the remaining portion is transferred to the next step for wet separation of the zinc-cadmium dust.

[0031] Step S5: After the zinc-cadmium smoke is recovered, it is subjected to wet separation, and the smoke is dissolved with sulfuric acid prepared in step S4 to obtain a sulfuric acid solution containing zinc and cadmium, and then zinc powder is added for displacement to obtain sponge cadmium and zinc sulfate solution; after the sponge cadmium is die-casted, it returns to step S2, and the zinc sulfate solution returns to step S1.

[0032] Example 2

[0033] 2000 kg of zinc-cadmium slag, consisting of 64.27% Cd and 33.96% Zn, was added to a reactor and 20% dilute sulfuric acid was added. After the zinc-cadmium slag was completely dissolved, the pH was adjusted to 2-3. 730 kg of zinc powder was added for displacement, ultimately yielding 1548 kg of sponge cadmium. Compositions included 93.77% Cd, 2.15% Zn, and 0.073% Tl. After vacuum smelting, 1252 kg of crude cadmium and 245 kg of cadmium alkaline slag were produced.

[0034] Weigh 1000 g of a cadmium alkali residue sample, whose main components are: 13.3% Zn, 8.18% Cd, 16% water, 14.02% impurities, and 48.5% NaOH; place the above sample in a ball mill, add 2000 mL of clean water and 150 g of calcium oxide, stir for 1 hour, filter, and dry the filter residue to obtain 1840 mL of clear liquid and 542 g of filter residue; the mass percentage of Zn element in the filter residue is 24.2%, and the mass percentage of Cd element is 14.8%; the Zn content of the filtrate detected by chemical method is 32.7 mg / L, and the Cd content is 3.3 mg / L.

[0035] The filtrate was evaporated and crystallized for 2 h to obtain 492 g of white powdered sodium hydroxide with a mass percentage of 94.8%, which was collected and returned to the sponge cadmium smelting process.

[0036] 542 g of the filter residue was added with 54 g of carbon powder and placed in a vacuum distillation furnace. The temperature was controlled at approximately 1000° C. and distilled for 1.5 hours, with a stable vacuum degree of less than 50 Pa and a maximum temperature of 1075° C. After the distillation, zinc-cadmium smoke volatiles and off-white powdery calcium oxide residue were obtained. The calcium oxide residue weighed 189.7 g and the smoke mass was 158 g. Chemical testing showed that the zinc-cadmium smoke volatiles had a Zn content of 42.86% and a Cd content of 51.33%. The calcium oxide residue had a Zn content of 0.074%, a Cd content of 0.057%, and a Ca content of 39.15%.

[0037] 20% sulfuric acid was added to the smoke, and the pH was adjusted to 2. After the smoke was completely dissolved, 48 g of zinc powder was added. Sponge cadmium gradually precipitated in the solution. After the reaction was completed, 96 g of sponge cadmium was obtained. After testing, the water content of the sponge cadmium was 12%, the dry weight cadmium content was 99.87%, and the zinc content was 0.11%.

[0038] Example 3

[0039] Weigh 2000g of the cadmium alkali residue sample obtained in Example 2, whose main components are: Zn13.3%, Cd8.18%, moisture 16%, impurities 14.02%, and NaOH 48.5%; the above sample is placed in a ball mill, 4000mL of clear water and 250g of calcium oxide are added, and the mixture is filtered after stirring for 1.2h. The mixture is washed with water on a Buchner funnel until the pH value is 7, and the filter residue is dried to obtain 3670mL of filtrate and 1250g of zinc-cadmium filter residue; wherein the mass percentage of the Zn element contained in the filter residue is 19.9%, and the mass percentage of the Cd element is 12.2%; and the Zn content and Cd content of the filtrate detected by chemical method are 30.2mg / L and 2.7mg / L respectively.

[0040] The filtrate was evaporated and crystallized for 3.5 h to obtain 995 g of white powdered sodium hydroxide with a mass percentage of 93.7%, which was collected and returned to the sponge cadmium smelting process.

[0041] 1250 g of filter residue was added to 130 g of carbon powder and mixed, and then placed in a vacuum distillation furnace. The temperature was controlled at about 1050° C. and distilled for 1.5 hours, with a vacuum degree stable at less than 50 Pa, and the maximum temperature was 1110° C. After the distillation, zinc-cadmium smoke volatiles and off-white powdery calcium oxide residue were obtained. The calcium oxide residue weighed 440 g, and the smoke weighed 329 g. Chemical testing showed that the zinc-cadmium smoke volatiles had a Zn content of 43.74% and a Cd content of 50.69%. The calcium oxide residue had a Zn content of 0.062%, a Cd content of 0.071%, a Ca content of 40.08%, and a Mg content of 6.13%.

[0042] 20% sulfuric acid was added to the smoke, and the pH was adjusted to 2. After the smoke was completely dissolved, 102 g of zinc powder was added. Sponge cadmium gradually precipitated in the solution. After the reaction was completed, 198 g of sponge cadmium was obtained. After testing, the water content of the sponge cadmium was 12%, the dry weight cadmium content was 99.89%, and the zinc content was 0.13%.

[0043] Example 4

[0044] Weigh 1000g of the cadmium alkali residue sample obtained in Example 2, whose main components are: Zn15.43%, Cd10.21%, moisture 17%, impurities 10.9%, and NaOH 46.3%; the above-mentioned dry sample is placed in a ball mill, 2000mL of clear water and 160g of calcium oxide are added, and the mixture is filtered after stirring for 1h. The mixture is washed with water on a Buchner funnel to a pH of 7, and the filter residue is dried to obtain 1850mL of filtrate and 521g of zinc-cadmium filter residue; wherein the mass percentage of the filter residue containing the Zn element is 27.2%, and the mass percentage of the Cd element is 18.8%; and the Zn content of the filtrate detected by chemical method is 42.1mg / L, and the Cd content is 2.8mg / L.

[0045] The filtrate was evaporated and crystallized for 2 h to obtain 471 g of white powdered sodium hydroxide with a mass percentage of 95.1%, which was collected and returned to the sponge cadmium smelting process.

[0046] 521 g of zinc-cadmium filter residue was added to 52 g of carbon powder and mixed. The mixture was then placed in a vacuum distillation furnace and distilled at a temperature of approximately 1050° C. for 1.5 hours with a stable vacuum degree of less than 50 Pa. The maximum temperature was 1070° C. After the distillation, zinc-cadmium smoke volatiles and off-white powdery calcium oxide residue were obtained. The calcium oxide residue weighed 181.308 g and the smoke weighed 153 g. Chemical testing showed that the zinc-cadmium smoke volatiles had a Zn content of 41.79% and a Cd content of 52.45%. The calcium oxide residue had a Zn content of 0.078%, a Cd content of 0.064%, a Ca content of 40.15%, and a Mg content of 5.82%.

[0047] 20% sulfuric acid was added to the smoke, and the pH was adjusted to 2. After the smoke was completely dissolved, 46 g of zinc powder was added. Sponge cadmium gradually precipitated in the solution. After the reaction was completed, 94 g of sponge cadmium was obtained. After testing, the water content of the sponge cadmium was 12%, the dry weight cadmium content was 99.79%, and the zinc content was 0.14%.

[0048] The above examples are used to illustrate the present invention, which are only used to help understand the present invention and are not intended to limit the present invention. Those skilled in the art can make several simple deductions, modifications or substitutions based on the concept of the present invention.

Claims

1. A method for jointly recovering zinc and cadmium from zinc-cadmium slag in hydrometallurgy, characterized in that: include: Step S1: After the zinc-cadmium slag is acid-leached with sulfuric acid, it is replaced with zinc powder to produce sponge cadmium and zinc sulfate solution; the zinc sulfate solution is treated by wet method to recover zinc; Step S2: adding sodium hydroxide to a vacuum melting furnace, and after the sodium hydroxide is melted, adding sponge cadmium, and vacuum melting the sponge cadmium. During the melting process, a reducing agent is added, and the sponge cadmium is vacuum melted to obtain cadmium alkaline slag and crude cadmium; and refining the crude cadmium to obtain refined cadmium; Step S3: adding clean water and calcium oxide to the cadmium alkali residue for wet grinding, filtering to obtain a clear liquid and a filter residue, and evaporating and crystallizing the clear liquid to obtain sodium hydroxide, which is returned to step S2; Step S4: The filter residue is subjected to reduction roasting to obtain recyclable zinc-cadmium smoke, calcium oxide residue and sulfur trioxide, wherein the calcium oxide residue is returned to step S3, the sulfur trioxide is used to prepare sulfuric acid, a portion of the sulfuric acid is returned to step S1, and the other portion of the sulfuric acid is fed to the next step; Step S5: After the zinc-cadmium smoke is recovered, it is subjected to wet separation, and the smoke is dissolved with sulfuric acid prepared in step S4 to obtain a sulfuric acid solution containing zinc and cadmium, and then zinc powder is added for replacement to obtain sponge cadmium and zinc sulfate solution; after the sponge cadmium is die-casted, it returns to step S2, and the zinc sulfate solution returns to step S1.

2. The method according to claim 1, wherein In step S3, the liquid-to-solid ratio during wet grinding is 2-2.5:1, the amount of calcium oxide added is 10%-15% of the mass percentage of the raw materials, and the wet grinding time is 1-1.5 hours.

3. The method according to claim 1, wherein In step S4, reduction roasting is performed using carbon powder as a reducing agent in a vacuum environment at a temperature of 1000-1200°C. After reduction, the valuable metals are released into the dust as cadmium oxide, zinc, and zinc oxide. Calcium sulfate and calcium zincate in the residue decompose to form calcium oxide, which is then returned to step S3.

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