Method for extracting selenium from selenium-containing acid mud through oxidation and acid leaching

Abstract

The invention discloses a method for extracting selenium from selenium-containing acid mud through oxidation acid leaching, which comprises the following steps: adding mixed acid with set acidity into the selenium-containing acid mud and MnO2 in a certain mass ratio, carrying out normal-pressure oxidation leaching to obtain a leaching solution, proportionally adding anhydrous sodium sulfite into the leaching solution to carry out reduction reaction, washing and drying a precipitation product obtained by the reduction reaction, thereby obtaining the selenium from the selenium-containing acid mud. The selenium elementary substance is obtained. Carrying out manganese sulfate recovery on the solution after the reduction reaction, namely adjusting the pH value of the solution to 3-4, and then adding a P204 extracting agent to carry out primary extraction; and taking the water phase after the primary extraction, adding a P204 extraction agent for secondary extraction, taking the manganese-loaded organic phase after the secondary extraction, adding a dilute sulfuric acid solution for back extraction of manganese, and concentrating and drying a product obtained by back extraction to obtain a manganese sulfate solid. The method not only can realize high leaching rate and high recovery rate of selenium, but also has the remarkable characteristic of low energy consumption, can effectively reduce pollutant discharge, and has the advantages of low comprehensive cost of raw materials and few processes.

Description

Technical Field

[0001] This invention belongs to the field of selenium-containing acid mud recycling, specifically relating to a method for extracting selenium from selenium-containing acid mud through oxidative acid leaching. Background Technology

[0002] Selenium is a typical rare element, with extremely low abundance and highly dispersed distribution in the Earth's crust. According to the latest statistics, the total proven reserves of selenium resources globally are approximately 100,000 tons, but its geographical distribution is significantly uneven. Selenium is hailed as the "vitamin of modern industry," and its unique physicochemical properties make it an indispensable key functional material. Currently, selenium and its compounds have achieved significant applications in several strategic emerging industries, including electronics, metallurgy, glass, agriculture, and biomedicine. In recent years, the demand for high-purity selenium and its derivatives in the electronics industry has experienced exponential growth, a phenomenon directly stemming from the rapid development of the global semiconductor and new energy industries. Especially driven by emerging technologies such as photovoltaic power generation, 5G communication, and artificial intelligence, the application prospects of high-purity selenium materials in high-end electronics manufacturing are even broader. From a global consumption perspective, the metallurgical industry is the largest application area for selenium (accounting for 40%), with its application as an additive in electrolytic manganese production dominating; the glass industry follows closely (25%), mainly used for coloring and decolorizing glass products. The booming development of new energy and high-end manufacturing has brought new trends to the selenium consumption structure. Data from 2020 shows that selenium consumption in emerging fields has increased significantly, particularly in the advanced inorganic non-metallic materials manufacturing sector, where it accounted for 22%, and in new energy applications such as solar energy, where it accounted for 6%, both showing a sustained and stable growth trend.

[0003] In industrial production, selenium is primarily obtained through three channels. The most significant source (accounting for approximately 70%-80%) is the byproduct of electrolytic refining of major non-ferrous metals such as copper, lead, nickel, and zinc. The anode mud from these processes contains abundant valuable metals. This precious metal-rich anode mud can be processed using hydrometallurgical methods to recover strategic resources such as selenium, tellurium, gold, and silver. Currently, the mainstream methods for selenium extraction fall into four categories: pyrometallurgical processes, semi-hydrometallurgical processes, fully hydrometallurgical processes, and other recovery processes.

[0004] Pyrometallurgical selenium extraction technology has long dominated the field of selenium extraction from copper electrolysis anode mud due to its advantages of wide raw material compatibility, simple process, and mature industrial adaptability. For example, patent application number 202110837163.5 describes a one-step method for separating and recovering selenium and mercury from acid mud. The acid mud is mixed with a slurry and slurried for 30–120 minutes to obtain a slurry containing acid and an oxidant. The slurry is then microwave-roasted at 400–800℃ for 20–120 minutes to obtain a mixed vapor of Hg and SeO2 and roasting residue. However, pyrometallurgical selenium extraction technology suffers from structural defects such as high-temperature roasting leading to large flue gas volumes (co-emission of SO2 / SeO2) and high energy consumption (accounting for 65%–70% of the total process energy consumption), with unit product energy consumption exceeding the benchmark value by 42%, hindering its sustainable development.

[0005] Compared to traditional pyrometallurgical selenium extraction processes, wet selenium extraction technology demonstrates significant techno- and economic advantages in industrial production and is expected to become the mainstream process for selenium extraction in the future. Existing wet extraction processes include: 1. "Preparation Method of Selenium Extraction from Selenium-Containing Waste at Room Temperature," by Ou Gaoyu et al., discloses a process for recovering and preparing elemental selenium from selenium-containing waste at room temperature. First, under strongly acidic conditions, hydrogen peroxide is used to oxidize the selenium in the waste into ionic form. Then, appropriate amounts of sodium chloride and ammonia are added during the reaction. A selenium-containing filtrate is obtained through solid-liquid separation. The filtrate is then reduced with hydrazine hydrate to obtain elemental selenium, achieving a purity of over 90%. This method requires strict control of the raw materials and products in the three processes, leading to process instability and hindering effective industrialization. Furthermore, the method uses multiple raw materials, including hydrogen peroxide, hydrochloric acid, ammonia, hydrazine hydrate, and sodium chloride. Hydrazine hydrate currently costs approximately 14,500 yuan / ton, resulting in a high overall raw material cost.

[0006] 2. "Research on the Process of Selenium Recovery from Selenium-Enriched Sludge," by Pan Xijian et al. This study employed hydrogen peroxide oxidation-alkali leaching experiments to wet-leach selenium from selenium-enriched sludge in a smelter; sodium hypochlorite oxidation-acid leaching experiments and selective mercury precipitation experiments were used to study the separation and recovery process of selenium and mercury from the alkali leaching residue; and sodium sulfite reduction experiments were used to reduce and recover selenium from the leaching enrichment solution. This method also requires control over the raw materials and product results of the three processes, leading to process instability and difficulty in effective industrialization. Furthermore, this method uses multiple raw materials, including hydrogen peroxide, sodium hydroxide, hydrochloric acid, sodium hypochlorite, sodium sulfide, sulfuric acid, and sodium sulfite, resulting in high overall raw material costs.

[0007] 3. "Research on the Process of Mercury Removal and Selenium Extraction from Selenium-Containing Acid Sludge," by Wang Yongmei et al. This study investigated mercury removal and selenium fixation in acid sludge using a calcium-added selenium fixation method, and conducted acid leaching experiments on the roasted residue. Experimental results showed that under the conditions of 80% lime addition, a roasting temperature of 500℃, and a roasting time of 2 hours, the mercury volatilization rate and selenium fixation rate reached 99.03% and 85.69%, respectively. Under the conditions of a liquid-to-solid ratio of 5:1, a sulfuric acid concentration of 350 g / L, the addition of 6 ml of nitric acid per liter of leachate, a leaching temperature of 90℃, and a leaching time of 3 hours, the selenium leaching rate in the roasted residue could reach 91.03%. This research involves high-temperature roasting, which presents a problem of high energy consumption.

[0008] 4. "Experimental Study on Selenium Extraction from Selenic Acid Sludge," by Wang Xiaowu et al., investigated the separation of selenium and mercury from selenic acid sludge using a calcium-added selenium fixation and mercury removal wet process. The study examined the effects of roasting temperature, roasting time, and the mass ratio of material to lime on the selenium fixation rate, as well as the effects of sulfuric acid concentration and leaching time on the selenium leaching rate in the slag, determining suitable selenium fixation and leaching parameters. Experimental results showed that under the conditions of roasting temperature 350–500℃, roasting time 2 h, and a material-to-lime mass ratio of 100:40–100:55, the selenium fixation rate reached over 99%; under the conditions of sulfuric acid concentration 500 g / L and leaching at room temperature for 2 h, the selenium leaching rate reached over 90%. This study involves high-temperature roasting, which presents a problem of high energy consumption.

[0009] In addition, S. soda has been used to recover selenium from kiln dust, a byproduct of cement production, through chemical leaching followed by bacterial reduction precipitation. However, this method has drawbacks, including high environmental requirements and difficulty in controlling bacterial activity. Currently, there is a need to develop a low-energy-consumption, low-pollution, and easily controllable method for extracting selenium from selenium-containing acid sludge through oxidative acid leaching. Summary of the Invention

[0010] The purpose of this invention is to solve the above-mentioned technical problems and provide a method for extracting selenium from selenium-containing acid mud by oxidative acid leaching. This method not only achieves a high leaching rate and high recovery rate of selenium, but also has the significant characteristics of low energy consumption, which can effectively reduce pollutant emissions. At the same time, it has the advantages of low overall raw material cost and fewer processes, and is easy to realize industrial application.

[0011] To achieve the above objectives, the technical solution of the present invention is as follows: A method for extracting selenium from selenium-containing acid mud by oxidative acid leaching involves adding a mixed acid of a certain acidity to a certain mass ratio of selenium-containing acid mud and MnO2, oxidizing and leaching at normal pressure to obtain a leachate, adding anhydrous sodium sulfite to the leachate in a certain proportion to carry out a reduction reaction, and washing and drying the precipitate obtained from the reduction reaction to obtain elemental selenium.

[0012] The main reaction formula for its oxidative leaching is as follows: MnO₂ + 4HCl = MnCl₂ + 2H₂O + Cl₂ H₂O + Cl₂ = HCl + HClO Se + 2HClO + H₂O = H₂SeO₃ + 2HCl Se + 3HClO + H₂O = H₂SeO₄ + 3HCl SeO2 + H2O = H2SeO3 SeO2 + 4HCl = SeCl4 + 2H2O; The formula for its reduction reaction is as follows: 2Na₂SO₃ + H₂SeO₃ = 2Na₂SO₄ + Se + H₂O As a further technical solution, the oxidation leaching temperature is 70-90℃, the acidity is 6.0-7.0 mol / L, and the oxidation leaching time is 50-70 min.

[0013] As a further technical solution, the temperature of the oxidation leaching described above is 90°C.

[0014] As a further technical solution, the mass ratio of the selenic acid mud to MnO2 is 1:(2.2-2.5), and the solid-liquid ratio of the oxidative leaching is 1g:(10-30)mL.

[0015] As a further technical solution, the selenic acid mud mentioned above is obtained by washing the flue gas generated from the smelting of brittle antimony lead ore with water. The components of the selenic acid mud include Se, Pb, As and Ag, with Se and Pb being the main components.

[0016] As a further technical solution, the above-mentioned mixed acid is composed of concentrated hydrochloric acid and concentrated sulfuric acid. The concentration of the concentrated hydrochloric acid in the mixed acid is 3-6 mol / L, and the concentration of the concentrated sulfuric acid is 0.6-1.2 mol / L. The acidity is adjusted by changing the amount of concentrated hydrochloric acid and concentrated sulfuric acid added. The molar ratio of concentrated hydrochloric acid to concentrated sulfuric acid is (3-5):1.

[0017] As a further technical solution, the method for extracting selenium from selenium-containing acid mud by oxidative acid leaching described above is characterized in that the molar ratio of selenium to anhydrous sodium sulfite in the leachate is 1:(1-3).

[0018] As a further technical solution, the reaction temperature of the reduction reaction described above is 70-100℃, and the reaction time is 45-70 min.

[0019] As a further technical solution, the above method for recovering manganese sulfate from the solution after the reduction reaction involves adjusting the pH of the solution to 3-4, then adding P204 extractant for primary extraction; taking the aqueous phase after primary extraction, adding P204 extractant for secondary extraction; taking the manganese-loaded organic phase after secondary extraction, adding dilute sulfuric acid for back-extraction of manganese; and concentrating and drying the back-extraction product to obtain solid manganese sulfate.

[0020] As a further technical solution, in the first-stage extraction described above, the volume ratio of the organic phase to the aqueous phase is 2.5:1, the saponification rate is 30%, and the organic phase composition is 30% P2O4 and 70% sulfonated kerosene by volume, with the first-stage extraction time being 10-15 min; in the second-stage extraction, the volume ratio of the organic phase to the aqueous phase is 2:1, the saponification rate is 30%, and the organic phase composition is 30% P2O4 and 70% sulfonated kerosene by volume; in the back-extraction, the volume ratio of the organic phase to the aqueous phase is 4:1, and the acidity is 110 g / L.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The method of this invention achieves high leaching and recovery rates of selenium. This invention uses manganese dioxide as an oxidant to react with acid to oxidize and leach selenium-containing acid sludge. Strong oxidant Cl2 is released through the reaction of concentrated hydrochloric acid and MnO2. Cl2 reacts with water to generate hypochlorous acid, which is then used as an oxidant to oxidize and leach selenium from the acid sludge. The addition of concentrated sulfuric acid mainly serves to adjust the acidity. Under the parameters defined in this invention, the selenium leaching rate reaches over 90%; under the optimal ratio, the selenium leaching rate reaches over 99%, and the selenium recovery rate reaches over 97%.

[0022] 2. The method of this invention has low energy consumption. This invention adopts a wet process, and under the condition of adding MnO2 and a mixed acid system, efficient leaching of selenium can be achieved at 70-90℃, avoiding the technical problem of high energy consumption under high-temperature baking.

[0023] 3. The method of this invention can effectively reduce pollutant emissions. After the strong oxidant Cl2 is released from the reaction of concentrated hydrochloric acid and MnO2, the Cl2 can immediately react with water to generate hypochlorous acid without overflowing. This effectively avoids the flue gas emission problem in pyrometallurgical processes, meets the "dual carbon" target and environmental protection requirements, and is conducive to achieving green and clean production in the selenium extraction process.

[0024] 4. The method of this invention has low overall raw material costs and fewer processes, making it easy to implement industrially. The manganese dioxide in this invention can be made from pyrolusite. The raw materials required in the entire extraction process are manganese dioxide, concentrated hydrochloric acid, concentrated sulfuric acid, and anhydrous sodium sulfite, resulting in low overall raw material costs. Secondly, the extraction process of this invention consists of two steps: oxidation leaching and reduction. Compared with existing wet processes, this invention reduces the number of reaction steps, meaning a reduction in unstable factors (such as raw material factors, human factors, and environmental factors). In the verification tests of this invention, the leaching rate and recovery rate were basically consistent across three trials, demonstrating the stability and controllability of the process and making it easier to industrialize the selenium extraction process.

[0025] 5. This invention can also achieve comprehensive recovery of manganese and improve the comprehensive utilization rate of resources. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of a method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to the present invention; Figure 2 This is a graph showing the effect of MnO2 addition on selenium leaching rate in this invention. Figure 3 This is a graph showing the effect of the acidity of the mixed acid on the selenium leaching rate in this invention. Figure 4 This is a graph showing the effect of the solid-liquid ratio on the selenium leaching rate during the oxidative leaching process of this invention. Figure 5 This is a graph showing the effect of oxidation leaching time on selenium leaching rate in this invention. Figure 6 This is a graph showing the effect of oxidation leaching temperature on selenium leaching rate in this invention. Figure 7 The XRD characterization analysis spectrum of the selenium extracted in this invention; Figure 8 SEM image (scale bar 20 μm) of the selenic acid sludge used in this invention; Figure 9 This is an elemental surface distribution diagram of the selenic acid mud used in this invention; Figure 10 The image shows the energy dispersive spectroscopy (EDS) results of the selenic acid mud used in this invention. Figure 11 This is a SEM image (scale bar 20 μm) of the oxidative leaching residue of this invention. Figure 12 This is an elemental surface distribution diagram of the oxidative leaching residue of the present invention; Figure 13 The image shows the energy dispersive spectroscopy (EDS) analysis results of the oxidative leaching residue of this invention. Figure 14 SEM image of the selenium product of this invention (scale bar 20 μm); Figure 15This is an elemental surface distribution diagram of the selenium product of the present invention; Figure 16 This is a graph showing the energy dispersive spectroscopy (EDS) results of the selenium product of this invention. Figure 17 The graph shows the change of Se leaching rate over time at different temperatures. Figure 18 The graph shows the linear fitting curve of the Se oxidation leaching kinetics based on the contraction kernel model. Figure 19 The curve of the reaction rate of oxidative leaching of Se controlled by external diffusion, x, as a function of time; Figure 20 The rate of oxidative leaching of Se is governed by surface reaction equation 1 - (1 - X). 1 / 3 Fitted curves over time; Figure 21 The rate of oxidative leaching of Se is governed by the internal diffusion equation 1 - 2X / 3 - (1 - X). 2 / 3 Fitted curves over time; Figure 22 The rate of oxidative leaching of Se is governed by the mixing equation 1 - (1 - X). 1 / 3 -1 / 3ln(1 - X) as a function of time fitting curve; Figure 23 Arrhenius curve for oxidative leaching of Se. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited to the scope shown in the embodiments.

[0028] Selenic acid sludge: After grinding and screening, it comes from an environmental protection technology company; after chemical analysis, its specific chemical composition is shown in Table 1. As can be seen from Table 1, the main components of selenic acid sludge are Se and Pb, of which Se content is 83.47% and Pb content is 7.35%.

[0029] Table 1 Main components of raw materials / %

[0030] Other reagents included manganese dioxide, concentrated hydrochloric acid, concentrated sulfuric acid, anhydrous sodium sulfite, selenium standard solution, and P2O4 extractant, which were purchased from a chemical company on the market.

[0031] Example 1:

[0032] like Figure 1As shown, a method for extracting selenium from selenic acid mud by oxidative acid leaching involves weighing 1g of sieved selenic acid mud and 2.4g of MnO2, then sequentially adding the selenic acid mud and MnO2 to an agate mortar and grinding them evenly. A mixed acid solution of HCl-H2SO4 (containing 5mol / L concentrated hydrochloric acid and 1mol / L concentrated sulfuric acid) with a total acidity of 7mol / L is transferred to a 100mL round-bottom flask. The leaching temperature is set at 80℃, and the solid-liquid ratio is 1g:20mL. The flask is placed in a magnetically stirred pot, and the reaction is carried out for 50 minutes with cooling water. After the reaction is complete, the solution is cooled and filtered to obtain the leachate. The leachate is placed in an iodine flask, and 3.8g of Na2SO3 is added according to the ideal molar ratio (1.2 times the ideal molar ratio). A magnetic stir bar is added, and the flask is placed in a magnetically stirred pot at 90℃ for 60 minutes to reduce the selenium. The obtained solution was centrifuged, the precipitate was washed with distilled water until neutral, and then dried in a vacuum drying oven at 70°C to obtain elemental selenium. The leaching rate of selenium was measured to be 90.37%, and the selenium recovery rate was 87.56%.

[0033] Example 2: like Figure 1 As shown, a method for extracting selenium from selenic acid mud by oxidative acid leaching involves weighing 1g of sieved selenic acid mud and 2.5g of MnO2, then grinding and mixing them in an agate mortar. A mixed acid solution of HCl-H2SO4 (containing 3.6mol / L concentrated hydrochloric acid and 1.2mol / L concentrated sulfuric acid) with a total acidity of 6mol / L is transferred to a 100mL round-bottom flask. The leaching temperature is set at 90℃, and the solid-liquid ratio is 1g:30mL. The flask is placed in a magnetically stirred tank and stirred while purging with cooling water for 70 minutes. After the reaction is complete, the solution is cooled and filtered to obtain the leachate. The leachate is placed in an iodine flask, and 3.8g of Na2SO3 is added according to the ideal molar ratio (1.2 times the ideal molar ratio). A magnetic stir bar is added, and the flask is placed in a magnetically stirred tank and reacted at 90℃ for 60 minutes to reduce the selenium. The obtained solution was centrifuged, the precipitate was washed with distilled water until neutral, and then dried in a vacuum drying oven at 70°C to obtain elemental selenium. The leaching rate of selenium was measured to be 90.61%, and the selenium recovery rate was 88.31%.

[0034] Example 3: like Figure 1As shown, a method for extracting selenium from selenic acid mud by oxidative acid leaching involves weighing 1g of sieved selenic acid mud and 2.5g of MnO2, then grinding and mixing them in an agate mortar. A mixed acid solution of HCl-H2SO4 (containing 4.1mol / L concentrated hydrochloric acid and 1.2mol / L concentrated sulfuric acid) with a total acidity of 6.5mol / L is transferred to a 100mL round-bottom flask. The leaching temperature is set at 90℃, and the solid-liquid ratio is 1g:30mL. The flask is placed in a magnetically stirred tank and stirred while purging with cooling water for 50 minutes. After the reaction is complete, the solution is cooled and filtered to obtain the leachate. The leachate is placed in an iodine flask, and 3.8g of Na2SO3 is added according to the ideal molar ratio (1.2 times the ideal molar ratio). A magnetic stir bar is added, and the flask is placed in a magnetically stirred tank and reacted at 90℃ for 60 minutes to reduce the selenium. The obtained solution was centrifuged, the precipitate was washed with distilled water until neutral, and then dried in a vacuum drying oven at 70°C to remove surface moisture, yielding elemental selenium. The selenium leaching rate was measured to be 97.59%, and the selenium recovery rate was 95.72%.

[0035] Example 4: like Figure 1 As shown, a method for extracting selenium from selenic acid mud by oxidative acid leaching involves weighing 1g of sieved selenic acid mud and 2.5g of MnO2, then sequentially adding the selenic acid mud and MnO2 to an agate mortar and grinding them evenly. A mixed acid solution of HCl-H2SO4 (containing 5mol / L concentrated hydrochloric acid and 1mol / L concentrated sulfuric acid) with a total acidity of 7mol / L is transferred to a 100mL round-bottom flask. The leaching temperature is set at 90℃, and the solid-liquid ratio is set at 1g:30mL. The flask is placed in a magnetically stirred pot, and the reaction is carried out for 60 minutes with cooling water. After the reaction is complete, the solution is cooled and filtered to obtain the leachate. The leachate is placed in an iodine flask, and 3.8g of Na2SO3 is added according to the ideal molar ratio (1.2 times the ideal molar ratio). A magnetic stir bar is added, and the flask is placed in a magnetically stirred pot and reacted at a constant temperature of 90℃ for 60 minutes to reduce the selenium. The obtained solution was centrifuged, the precipitate was washed with distilled water until neutral, and then dried in a vacuum drying oven at 70°C to remove surface moisture, yielding elemental selenium. The selenium leaching rate was measured to be 99.66%, and the selenium recovery rate was 97.63%.

[0036] In the above embodiments, the solution after reduction reaction can be used to recover manganese sulfate. The steps are as follows: the pH of the reduced solution is adjusted to 3.5, and then P204 extractant is added for primary extraction. The ratio of organic phase volume to aqueous phase volume (O / A) is 2.5:1, the saponification rate is 30%, and the organic phase composition is 30% P204 and 70% sulfonated kerosene by volume. The primary extraction time is 10 min. The aqueous phase after primary extraction is used for secondary extraction of manganese. The ratio of organic phase volume to aqueous phase volume (O / A) is 2:1, the saponification rate is 30%, and the organic phase composition is 30% P204 and 70% sulfonated kerosene by volume. The manganese-loaded organic phase after secondary extraction is added to dilute sulfuric acid for back-extraction of manganese. The ratio of organic phase volume to aqueous phase volume (O / A) is 4:1, the acidity is 110 g / L, and the obtained product is concentrated and dried to obtain solid manganese sulfate.

[0037] The following is the research and development experiment process of this invention, which proves the feasibility of the process and the correlation of the control parameters.

[0038] 1 Experimental Methods 1.1 Selenium leaching experiment Weigh 1 g of sieved selenic acid mud and 2.2–2.6 g of MnO2. Add the selenic acid mud and MnO2 sequentially to an agate mortar and grind them thoroughly. Transfer the mixed acid (5–7 mol / L total acidity) to a 100 mL round-bottom flask. Set the leaching temperature to 50–90 °C and the solid-liquid ratio to 1:10–1:50 g / mL. Place the flask in a magnetically stirred tank and circulate cooling water to carry out the reaction. After the reaction is complete, cool the solution and then filter and dilute it. Each leaching experiment was performed in triplicate, and the average value was taken as the final leaching efficiency. The leaching rate is calculated using the following formula: The Se content in acid mud leachate, C0 = C1 × dilution factor. Leaching rate of Se in acid mud leachate: (1-1) In the formula, C0 is the selenium content in the test solution (mg / L); C1 is the selenium content in the diluted solution (mg / L); V is the volume of the leachate (L); m(g) and Wx are the mass of the raw material and the weight content of the "Se" element in the initial raw material, respectively.

[0039] 1.2 Leaching principle This invention uses an HCl-H2SO4 mixed acid system to react with manganese dioxide to generate chlorine gas. The main reaction formula is as follows: MnO₂ + 4HCl = MnCl₂ + 2H₂O + Cl₂ (1-2) H₂O + Cl₂ = HCl + HClO (1-3) Se+2HClO+H2O=H2SeO3+2HCl (1-4) Se+3HClO+H2O=H2SeO4+3HCl (1-5) SeO2 + H2O = H2SeO3 (1-6) SeO2 + 4HCl = SeCl4 + 2H2O (1-7) 1.3 Selenium Reduction Experiment The leachate obtained from the oxidative leaching was placed in an iodine flask. Following the ideal molar ratio of 1.2 times (i.e., 3.8 g of Na₂SO₃) as indicated in the reaction equation, a magnetic stir bar was added, and the mixture was placed in a magnetically stirred pan and reacted at a constant temperature of 90 °C for 60 min to reduce selenium. The resulting solution was centrifuged, and the precipitate was washed with distilled water until neutral. The precipitate was then dried in a vacuum drying oven at 70 °C to obtain elemental selenium.

[0040] 2Na2SO3+H2SeO3=2Na2SO4+Se+H2O (1-8) Weigh and record the mass of the recovered selenium precipitate, and calculate the selenium recovery rate using the following formula (1-9): (1-9) In formula (1-9), E is the selenium recovery rate, W is the mass fraction of selenium in the selenium-containing acid sludge, m1 is the weight of the selenium-containing acid sludge, and m2 is the weight of the reduction product selenium.

[0041] 1.4 Manganese sulfate recovery experiment The pH of the reduced solution was adjusted to 3.5, and then P204 extractant was added for primary extraction. The organic phase volume to aqueous phase volume (O / A) ratio was 2.5:1, the saponification rate was 30%, and the organic phase composition was 30% P204 and 70% sulfonated kerosene by volume. The primary extraction time was 10 min. The aqueous phase after primary extraction was used for secondary extraction of manganese. The organic phase volume to aqueous phase volume (O / A) ratio was 2:1, the saponification rate was 30%, and the organic phase composition was 30% P204 and 70% sulfonated kerosene by volume. The manganese-loaded organic phase after secondary extraction was used for back-extraction of manganese with dilute sulfuric acid. The organic phase volume to aqueous phase volume (O / A) ratio was 4:1, and the acidity was 110 g / L. The obtained product was concentrated and dried to obtain solid manganese sulfate.

[0042] 1.5 Sample Characterization and Analytical Methods Phase analysis was performed on finely ground selenium-containing acid mud and its leaching residue using X-ray diffraction (XRD). The selenium concentration in the solution was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Errors were controlled to be within ±5%. The surface morphology and elemental composition of the samples were examined using an energy-dispersive X-ray spectroscopy (EDS) scanning electron microscope.

[0043] 2 Results Analysis 2.1 Characterization and Analysis of Raw Materials The selenic acid mud of this invention has been chemically analyzed and its specific chemical composition is shown in Table 1 above.

[0044] 2.2 Single-factor experiment 2.2.1 Effect of MnO2 addition on selenium leaching rate Depend on Figure 2 It can be seen that when the MnO2 addition amount is in the range of 2.2 g to 2.5 g, as the MnO2 addition amount increases from 2.2 g to 2.5 g, the Se leaching rate increases from 86.78% to 98.12%. When the MnO2 addition amount is greater than 2.5 g, the increase in the Se leaching rate is no longer significant. This indicates that in the initial stage, the higher the MnO2 addition amount, the more oxidant is generated, and the Se in the acid sludge is oxidized. A large amount of elemental Se is oxidized to H2SeO3 and enters the leachate, thus more effectively dissolving the Se in the selenium-containing acid sludge, and the leaching rate continuously increases. When the MnO2 addition amount reaches 2.5 g, the Se leaching rate tends to stabilize, at which point the system has reached a leaching equilibrium state. Therefore, it is preliminarily determined that the optimal MnO2 addition amount is 2.5 g.

[0045] 2.2.2 Effect of leaching acidity on selenium leaching rate Depend on Figure 3 It can be seen that within the acidity range of 5–7 mol / L, the leaching rate of Se increases rapidly with increasing mixed acidity, reaching 98.22% at an acidity of 6.5 mol / L, and then stabilizing. This indicates that within this concentration range, MnO2 can fully react to generate chlorine gas, thus increasing the Se leaching rate. However, when the mixed acid concentration increases further, the increase in Se leaching rate becomes less significant and remains stable because the oxidant in the system is already sufficient. With increasing acidity, the concentration of free acid in the leachate increases accordingly, which on the one hand promotes the dissolution of impurity elements, and on the other hand increases the difficulty of subsequent processing. Therefore, the optimal acidity of the mixed acid is preliminarily determined to be 6.5 mol / L.

[0046] 2.2.3 Effect of leaching solid-liquid ratio on selenium leaching rate Depend on Figure 4It was found that when the solid-liquid ratio was in the range of 1g:10mL to 1g:30mL, the leaching rate of Se rapidly increased from 92.31% to 98.13%. When the solid-liquid ratio was low, the contact between the selenic acid sludge and the solution was insufficient, making it difficult for the oxidant and selenic acid sludge to break through the diffusion layer and migrate to the surface of the solid film. As the solid-liquid ratio increased above the critical value, the ion mass transfer and diffusion process in the solution reached a dynamic equilibrium, and the selenium leaching rate jumped to a high and stable stage. However, the energy consumption required for subsequent solution treatment also increased simultaneously. Based on a comprehensive evaluation of leaching efficiency and energy consumption cost, a solid-liquid ratio of 1g:30mL was determined to be the optimal parameter for the system.

[0047] 2.2.4 Effect of leaching time on selenium leaching rate Depend on Figure 5 It can be seen that when the leaching time is in the range of 40 to 60 minutes, the leaching rate of selenium increases rapidly with the increase of leaching time. After the leaching time reaches 60 minutes, the leaching rate of selenium tends to stabilize with the increase of leaching time. Therefore, the optimal leaching time is initially determined to be 60 minutes.

[0048] 2.2.5 Effect of leaching temperature on selenium leaching rate Depend on Figure 6 It can be seen that as the temperature increases from 50 ℃ to 90 ℃, the leaching rate of Se increases from 93.03% to 99.36%. During the leaching process, the increase in temperature increases the average velocity of molecular motion and the frequency of intermolecular collisions, which is conducive to the occurrence of chemical reactions. Since the increase in leaching rate from 80 ℃ to 90 ℃ is very small, but the energy consumption at 90 ℃ is high, considering all factors, the optimal temperature is initially determined to be 80 ℃.

[0049] 2.3 Orthogonal Experiment 2.3.1 Orthogonal Experiment for Selenium Leaching Rate Referring to the results of the single-factor experiment, three factors were selected: temperature (A), acidity (B), and time (C). The L9 (3) model was used. 4 The orthogonal array was used to conduct three repeated experiments to determine the optimal process conditions for the oxidative acid leaching process in the selenium extraction process. The orthogonal experiments were performed according to Tables 2 and 3.

[0050] Table 2. Factor Level Table for Orthogonal Optimization Experiment

[0051] Table 3. Arrangement of orthogonal experiments

[0052] As shown in Table 4, the optimal leaching process combination for selenium, determined by range analysis of the orthogonal experiment, is A3B3C2, which is an acidity of 7 mol / L, a leaching temperature of 90 ℃, and a leaching time of 60 min.

[0053] Table 4. Results of orthogonal experiments and range analysis

[0054] From the orthogonal experiment results and range analysis in Table 5, the optimal parameter combination is A3B3C2. Furthermore, from the variance analysis of the orthogonal experiment in Table 5, the influence of the three factors on the selenium leaching rate is as follows: leaching temperature (extremely significant, P<0.01) > acidity (significant, P<0.05) > leaching time (not significant, P>0.05). The order of influence of the three factors on the selenium leaching rate is A (temperature) > B (acidity) > C (time).

[0055] Table 5. Variance Analysis of Orthogonal Experiments

[0056] 2.3.2 Optimal Solution Verification Experiment The optimal combination of oxidative acid leaching processes for selenium-containing mud was determined and verified through experiments. The optimal combination obtained from the orthogonal experiment was used in three verification experiments (A3B3C2), and the verification results are shown in Table 6. The average leaching rate obtained from the verification experiments was 99.73%, which is better than the leaching rate of 99.66% obtained from the orthogonal experiment, indicating that the results obtained from the orthogonal experiment are reliable.

[0057] Table 6 Verification Results

[0058] 2.4 Selenium Reduction Since selenium exists in the leaching solution as sodium selenite, which has certain oxidizing properties, it can be reduced to selenium using a reducing agent. Sodium sulfite was used as the reducing agent in the selenium reduction experiment. The experiment was performed in triplicate, and the average selenium recovery rate was 97.21%. The results are shown in Table 7.

[0059] Table 7 Selenium recovery rate / %

[0060] 2.5 Manganese sulfate recovery The experimental results are shown in Table 10. The total extraction rate of manganese reached 98.67%. The manganese-containing organic phase was back-extracted with dilute sulfuric acid, achieving a manganese back-extraction rate of 99.1%. The resulting manganese sulfate solution was concentrated, dried, and filtered to obtain the solid product. The experiment was performed in three parallel trials, with an average manganese recovery rate of 90.87%.

[0061] Table 8 Manganese recovery rate / %

[0062] 2.6 XRD Analysis 2.6.1 XRD Analysis of Leaching Residue 2.6.2 XRD analysis of reduced selenium from Figure 7 It can be seen that the XRD pattern closely matches the standard characteristic diffraction peaks of elemental Se. Comparing the diffraction peak positions (2θ values) of the reduced Se from the selenic acid mud leachate with the standard PDF card data for the selenium phase, the peak positions are basically consistent, further confirming the presence of the selenium phase. Simultaneously, observing the peak intensity ratio, which is consistent with the standard card, provides stronger evidence of the phase composition, closely matching the characteristic peak spectrum of the substance, thus proving the feasibility of the process for extracting Se from selenic acid mud.

[0063] 2.7 SEM Analysis Figure 8 , Figure 14 These are SEM images of selenium-containing sludge and extracted selenium products, respectively. Figure 8 The selenoic acid mud raw material exhibits irregular particle shapes, significant size variations, and a rough surface with numerous pores and uneven areas. This complex structure is likely due to the mixing and reaction residues of various substances, providing space for selenium and other elements to exist. Figure 14 The SEM images of selenium show uniformly spherical particles. The reduced selenium particles have relatively regular morphology and a well-developed porous structure, which may be formed during the material transformation and gas escape process. This structure increases the specific surface area and affects the physicochemical properties of selenium, such as catalytic activity and adsorption capacity.

[0064] From the elemental surface distribution map Figure 9 It is evident that the distribution of elements is uneven. Selenium is relatively enriched in some areas, while other impurity elements also exhibit their own specific distribution areas, reflecting the differences in the aggregation state of different elements within the selenium-containing sludge raw material. Figure 15 It is evident that the reduced selenium content reached 100%, with other impurities almost invisible. The selenium element was relatively evenly distributed in the reduced sample, demonstrating the consistency of the selenium composition at the microscopic scale, indicating that the reduced elemental purity was good.

[0065] Based on the results of energy spectrum analysis Figure 10 As shown, the main component of the raw material is selenium, with a content of 83.47%. Other impurities mainly include Pb, As, and Ag, with contents of 7.35%, 3.59%, and 2.76%, respectively. The coexistence of these elements in the selenium-containing acid mud indicates that the raw material has a complex composition.

[0066] 3.1 Establishment of the condensation nuclear dynamics model In this invention, manganese dioxide reacts with mixed acid to generate chlorine gas, which then reacts with water to generate hypochlorous acid. Hypochlorous acid then oxidizes selenium-containing sludge. This reaction process belongs to a gas-liquid-solid reaction system. Since the concentrations of the liquid-phase reactant components can be considered essentially constant, a contraction core model can be used to describe this process. Based on the different control steps of the leaching process, it can be divided into four types: external diffusion control, internal diffusion control, chemical reaction control, and mixing control, as shown in the following equations: The reaction rate is governed by the external diffusion equation: (3-1) The reaction rate is governed by the internal diffusion equation: (3-2) The reaction rate is governed by the surface reaction control equation: (3-3) The reaction rate is governed by the mixed control equation: (3-4) In the formula: x represents the Se leaching rate, % K1 is the external diffusion rate constant, with dimensions min. -1 ; K2 is the internal diffusion rate constant, with dimensions min. -1 ; K3 is the surface reaction rate constant, with dimensions in min. -1 ; K4 is the hybrid control rate constant, with dimensions min. -1 ; t is the reaction time of the process, with dimensions in min.

[0067] The apparent activation energy of the reaction process can then be calculated using the Arrhenius equation.

[0068] (3-5) In the formula: k is the reaction rate constant for a certain reaction process, with dimensions in min. -1 ; A is a pre-exponential factor, which is dimensionless; E a Let be the apparent activation energy of a certain reaction process, with dimensions in kJ / mol; T is the thermodynamic temperature, with dimensions in K. R is the molar gas constant, with dimensions of kJ / (mol•K).

[0069] When E a When E <20 kJ / mol, the leaching rate is diffusion-controlled and mass transfer dominates; when Ea At >40 kJ / mol, the leaching rate is controlled by surface chemical reactions, requiring the breaking of chemical bonds; when E a At a concentration of 20-40 kJ / mol, the leaching rate is influenced by both diffusion mass transfer and surface chemical reactions, resulting in a mixed control.

[0070] To investigate the rate-controlling steps in the leaching process of the MnO2-HCl-H2SO4 system, kinetic analysis was conducted. Under optimal conditions, the leaching kinetics of Se were studied at different reaction temperatures (323.15, 333.15, 343.15, 353.15, 363.15 K) and different leaching times (10, 20, 30, 40, 60 min).

[0071] 3.2 Kinetic Analysis of Leaching Process The leaching rate of Se at different temperatures versus time is shown in the following curves. Figure 17 As shown, the leaching rate of Se gradually increases with increasing reaction time and temperature. Considering that increasing the temperature can accelerate the solute diffusion rate and reaction rate, leading to a decrease in boundary layer thickness and accelerated liquid-solid mass transfer, thus enabling rapid leaching of Se in the initial stage, a reaction temperature of 363.15 K is deemed appropriate.

[0072] Based on the established contraction kernel model, multi-model linear regression analysis was performed on the oxidative leaching kinetics data of Se. For example... Figures 18-23 As shown, in the fitting curves of each dynamic equation, Figure 18 The hybrid control model exhibits optimal linear correlation (R² = 0.998). Its fitted curve is shown below. Figures 19-23 ,from Figure 19 The linear fitting curves show that the oxidation and leaching of Se in the MnO2-HCl-H2SO4 system better matches the mixed governing equation in the shrinking core model, with the highest degree of fit R0. 2 The value is greater than 0.99, which indicates that the oxidation and leaching of Se in the MnO2-HCl-H2SO4 system is controlled by mixing.

[0073] A linear relationship between lnk and 1 / T is constructed based on the Arrhenius equation, such as... Figure 23 As shown, linear regression analysis revealed a correlation coefficient R = 0.91209. Based on this slope, the kinetic parameters for the oxidative leaching of selenium in the MnO2-HCl-H2SO4 system were calculated: apparent activation energy E... a =29.075 kJ / mol, pre-exponential factor A=e 5.56807 The activation energy value is within the typical range (10-40 kJ / mol) of the diffusion-reaction mixed control mechanism, indicating that the leaching process is synergistically regulated by interfacial reaction and mass transfer.

[0074] The above embodiments are merely specific examples to further illustrate the objectives, technical solutions, and beneficial effects of the present invention, and the present invention is not limited thereto. Any modifications, equivalent substitutions, improvements, etc., made within the scope of the disclosure of the present invention are included within the protection scope of the present invention.

Claims

1. A method for extracting selenium from selenium-containing acid mud by oxidative acid leaching, characterized in that: A certain mass ratio of selenium-containing sludge and MnO2 is mixed with a certain acidity, and oxidative leaching is carried out under normal pressure to obtain a leachate. Anhydrous sodium sulfite is added to the leachate in proportion to carry out a reduction reaction. The precipitate obtained from the reduction reaction is washed and dried to obtain elemental selenium.

2. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 1, characterized in that: The oxidative leaching temperature is 70–90°C, the acidity is 6.0–7.0 mol / L, and the oxidative leaching time is 50–70 min.

3. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 2, characterized in that: The oxidation leaching temperature is 90°C.

4. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 1, characterized in that: The mass ratio of the selenium-containing acid mud to MnO2 is 1:(2.2-2.5), and the solid-liquid ratio of the oxidative leaching is 1g:(10-30)mL.

5. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 1, characterized in that: The selenic acid mud is obtained by washing the flue gas produced from the smelting of brittle antimony lead ore with water. The components of the selenic acid mud include Se, Pb, As and Ag, with Se and Pb being the main components.

6. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 1, characterized in that: The mixed acid is composed of concentrated hydrochloric acid and concentrated sulfuric acid. The concentration of concentrated hydrochloric acid in the mixed acid is 3-6 mol / L, the concentration of concentrated sulfuric acid is 0.6-1.2 mol / L, and the molar ratio of concentrated hydrochloric acid to concentrated sulfuric acid is (3-5):

1.

7. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 1, characterized in that: The molar ratio of selenium to anhydrous sodium sulfite in the leachate is 1:(1-3).

8. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 1, characterized in that: The reduction reaction is carried out at a temperature of 70–100°C for a time of 45–70 min.

9. The method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 1, characterized in that: The manganese sulfate recovery process for the solution after reduction reaction involves adjusting the pH of the solution to 3-4, then adding P204 extractant for primary extraction; taking the aqueous phase after primary extraction, adding P204 extractant for secondary extraction; taking the manganese-loaded organic phase after secondary extraction, adding dilute sulfuric acid for back-extraction of manganese; concentrating and drying the back-extraction product to obtain solid manganese sulfate.

10. A method for extracting selenium from selenium-containing acid mud by oxidative acid leaching according to claim 9, characterized in that: In the first-stage extraction, the volume ratio of the organic phase to the aqueous phase is 2.5:1, the saponification rate is 30%, and the organic phase composition is 30% P2O4 and 70% sulfonated kerosene by volume. The first-stage extraction time is 10-15 min. In the second-stage extraction, the volume ratio of the organic phase to the aqueous phase is 2:1, the saponification rate is 30%, and the organic phase composition is 30% P2O4 and 70% sulfonated kerosene by volume. In the back-extraction, the volume ratio of the organic phase to the aqueous phase is 4:1, and the acidity is 110 g / L.

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