A High-Efficiency Purification Method for Crude Selenium Based on Alkaline Leaching

Abstract

This invention relates to the field of non-ferrous metal smelting and purification technology, and discloses a method for efficient purification of crude selenium based on alkaline leaching. The method includes: mixing raw acid sludge with a composite acidic leachate for leaching; reducing the qualified leachate to obtain refined crude selenium raw material; neutralizing the deselenized liquid with alkali and precipitating mercury with hydrazine hydrate to remove heavy metal impurities; mixing the refined crude selenium raw material with the alkaline leaching mother liquor to convert elemental selenium into soluble sodium selenide and separate solid lead slag; and adding hydrogen peroxide dropwise to the selenium-containing filtrate to oxidize and precipitate powdered selenium. The purification process utilizes three independently prepared specific reagents. By constructing a stepwise phase transfer system for deep impurity removal, the purity of the crude selenium purification is improved; pre-prepared reagents mitigate the risk of drastic fluctuations in the on-site reaction and gas escape; and a specific extraction environment optimizes the nucleation and growth process, improving the dispersion of the finished powder.

Description

Technical Field

[0001] This invention relates to the field of non-ferrous metal smelting and purification technology, and more specifically, to a method for efficient purification of crude selenium based on alkaline leaching. Background Technology

[0002] Selenium, an important rare and dispersed metal, has wide applications in industries such as semiconductors, optoelectronics, metallurgy, and chemicals. The main industrial method for obtaining selenium is extraction and recovery from acid sludge or anode mud generated during the smelting of metals such as copper and lead. Currently, conventional purification processes mostly employ acid oxidation leaching, followed by the addition of a reducing agent to reduce selenium ions in the solution to elemental selenium, which is then enriched in the filter residue.

[0003] However, in actual production processes, the composition of raw acid sludge is often quite complex. In addition to the target element selenium, it usually contains impurities such as iron, lead, and trace amounts of heavy metals like mercury. Conventional purification processes have limitations in separating these symbiotic impurities, causing some impurity ions to easily co-precipitate with selenium during the reduction stage, making it difficult for the purity of the final selenium product to meet high industrial standards.

[0004] In processing these complex components, high concentrations of acidic or alkaline liquids or strong oxidizing and reducing agents are often directly added to the reaction tank during production. This direct addition method can easily trigger violent local reactions, making it difficult to maintain stable material concentrations and resulting in the concentrated release of large amounts of reaction heat. This can lead to the escape of acid mist or harmful gases, posing safety hazards to the workshop working environment.

[0005] Furthermore, in the final selenium precipitation stage, due to the lack of targeted control over the liquid phase system and reaction rate, selenium ions in the solution are prone to amorphous aggregation due to excessively rapid nucleation when transforming into elemental precipitate. This results in poor physical dispersion of the generated powder, limiting subsequent processing and applications. Therefore, improvements are needed to the existing crude selenium purification process and the corresponding reagent addition methods. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a highly efficient purification method for crude selenium based on alkaline leaching. This method aims to improve upon the limitations in impurity removal, the risk of gas escape during production, and the poor dispersibility of purified products in existing crude selenium purification processes.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A highly efficient purification method for crude selenium based on alkaline leaching includes the following steps: S1: The raw acid mud is mixed with the compound acid leachate and leached to separate the primary crude selenium refining raw material and qualified leachate. S2: Add deselenic reducing agent A to the qualified leachate, filter to obtain elemental selenium filter residue and deselenic liquid, and combine the elemental selenium filter residue with the primary crude selenium refining raw material to form crude selenium refining raw material. S3: Add sodium hydroxide solution to the deselenate solution to adjust the pH and neutralize it, so that the impurities precipitate out and the neutralized solution is obtained. S4: Add hydrazine hydrate solution to the neutralized solution to precipitate mercurous chloride and separate the mercury-precipitated solution. S5: Mix the crude selenium refining raw material with the alkaline leaching mother liquor for crude selenium refining and leach to convert the elemental selenium in the crude selenium refining raw material into soluble sodium selenide, and separate lead slag and selenium-containing filtrate. S6: Add hydrogen peroxide dropwise to the selenium-containing filtrate to oxidize the selenium ions and precipitate them as elemental selenium precipitate. After separation and drying, powdered selenium is obtained.

[0008] By adopting the above technical solution, the present invention constructs a stepwise reaction system based on a specific redox potential in the crude selenium purification process, and the liquid-phase transformation mechanism of related materials is further clarified.

[0009] Specifically, the composite acidic leachate provides an acidic oxidizing environment in the early stages, which promotes the dissolution of elemental selenium filter residue, selenite, and most of the symbiotic metal impurities in the raw acid sludge into the liquid phase.

[0010] The sodium sulfite in the deselection reducing agent A, which is added subsequently, will be converted into sulfurous acid or sulfur dioxide, which have strong reducing properties, in the acidic system of the composite leachate. This will reduce high-valence selenium ions to elemental selenium filter residue, resulting in reduction enrichment. The main reaction equation is: H₂SeO₃ + 2SO₂ + H₂O → Se↓ + 2H₂SO₄. Most of the lead in the raw acid sludge remains as insoluble salts in the solid phase of the crude selenium refining raw material. After deselection, sodium hydroxide is added to the liquid phase to increase the pH value of the system. This causes the trace iron ions remaining in the solution to flocculate and precipitate as hydroxides. The reaction equation is: Fe 3+ +3OH − →Fe(OH)3↓; Simultaneously, trace lead impurities in the system undergo co-precipitation separation. To treat the more difficult-to-remove mercury impurities in the system, hydrazine hydrate, as a strong reducing agent, can typically specifically reduce divalent mercury ions to sparingly soluble mercurous chloride. The reaction equation is: 4HgCl2 + N2H4 → 2Hg2Cl2↓ + N2↑ + 4HCl. This step largely achieves the efficient removal and resource recovery of heavy metal mercury from selenium-depleted wastewater, yielding the byproduct mercurous chloride.

[0011] In the crucial refining stage, the alkaline leaching mother liquor used for crude selenium refining contains a high concentration of hydroxide and hydrazine hydrate. The synergistic effect of these two substances promotes the reduction and dissolution of elemental selenium filter residue in the crude selenium refining raw material, converting it into water-soluble sodium selenide. Simultaneously, heavy metal impurities such as lead tend to remain in the solid phase and separate as lead slag. The reaction equation is: 2Se + N₂H₄ + 4NaOH → 2Na₂Se + N₂↑ + 4H₂O. Finally, hydrogen peroxide at room temperature re-oxidizes the selenium ions, precipitating elemental selenium filter residue. The reaction equation is: Na₂Se + H₂O₂ → Se↓ + 2NaOH.

[0012] The above-mentioned approach of first alkaline reduction and dissolution followed by oxidation and precipitation helps to alleviate the problem of harmful acidic gas escape in traditional acid refining, and achieves deep retention of impurities during liquid-phase conversion, thereby obtaining a high-purity finished product.

[0013] Preferably, in step S1, a composite acidic leachate is added at a liquid-to-solid ratio of 2–4 L / kg, and leaching is carried out at a constant temperature of 25–45°C for 1–3 hours; in step S2, the amount of reducing agent A added for selenium removal is 110–130% of the theoretical required amount, and the reaction time is 1–2 hours; in step S3, the pH value of the system is adjusted to 3.0–4.5; in step S4, the mass fraction of hydrazine hydrate solution is 80.0%, the reaction temperature is 25–45°C, and the reaction is carried out in a closed system for 1.5–2.5 hours; in step S6, the mass fraction of hydrogen peroxide is 30.0%, the system temperature is maintained at 25–35°C, and the reaction is carried out for 1.5–3 hours.

[0014] By employing the above-mentioned technical solutions, controlling the liquid-solid ratio and temperature can often ensure the efficiency of primary leaching. Using an excess of 110-130% reducing agent for selenium removal helps maintain the reduction conversion rate of selenium ions in the liquid phase. In the neutralization stage, controlling the pH within the range of 3.0-4.5 mainly utilizes the solubility product constant of iron ions, causing them to preferentially precipitate as hydroxides and adsorb and retain impurities such as lead co-precipitates, while avoiding excessive alkalization of the system that could lead to the co-precipitation of other components. Furthermore, setting 80.0% hydrazine hydrate and reacting under closed conditions during the mercury precipitation stage maintains the stability of the reduction system and prevents material splashing caused by rapid nitrogen generation. In the final extraction stage, controlling the hydrogen peroxide concentration and the selenium precipitation reaction temperature ensures that the selenium ion oxidation nucleation rate and crystal growth rate are relatively matched, promoting the formation of powder with a uniform particle size distribution.

[0015] Preferably, in step S4, a constant flow metering pump is used to add hydrazine hydrate solution dropwise into the reactor, and the dropwise addition process is controlled to last for 1 to 2 hours. After the dropwise addition is completed, the reactor is continuously sealed and kept at a constant temperature for stirring and aging. In step S5, an alkaline leaching mother liquor for crude selenium refining is added at a liquid-to-solid ratio of 3 to 6 L / kg, and the reactor is leached by closed stirring at a speed of 200 to 250 rpm for 0.5 to 2 hours.

[0016] By adopting the above technical solution, the slow dripping of hydrazine hydrate using a constant flow metering pump alleviates the explosive formation of crystal nuclei caused by excessively high local instantaneous concentrations. Combined with the subsequent aging process, this tends to yield mercurous chloride with intact crystal forms and high purity. In step S5, increasing the liquid-to-solid ratio and stirring speed dilutes the spatial concentration of sodium selenide in the system. In engineering practice, this prevents unplanned oxidation precipitation caused by contact between the high-concentration liquid phase surface and air, thus better ensuring the depth of conversion of elemental selenium filter residue.

[0017] Preferably, the present invention uses separately prepared specific reagents in conjunction with the purification method, including a composite acidic leachate, a deselenic reducing agent A, and an alkaline leachate mother liquor for crude selenium refining; wherein the composite acidic leachate is made from raw materials containing deionized water, hydrochloric acid, and sodium chlorate; the deselenic reducing agent A is made from raw materials containing deionized water, sodium sulfite, and sodium hydroxide; and the alkaline leachate mother liquor for crude selenium refining is made from raw materials containing deionized water, sodium hydroxide, and hydrazine hydrate.

[0018] By adopting the above technical solution, this invention pre-prepares the fluids required in the purification process into independent reagents, which helps to alleviate the problems of concentration fluctuations and reaction runaway caused by direct addition of chemicals on the production site. In terms of specific mechanism of action, the composite acidic leachate utilizes the free chlorine and chlorine dioxide components generated in situ from the reaction of sodium chlorate and hydrochloric acid to maintain a relatively high oxidizing capacity, promoting the transfer of sparingly soluble selenium compounds and symbiotic metals into the liquid phase. The sodium sulfite in the deselection reducing agent A is weakly alkaline after dissolution; by adding sodium hydroxide to form a buffer system, the spontaneous oxidation reaction of sulfite ions during storage and transport can be effectively limited.

[0019] The alkaline leaching mother liquor designed for the refining process uses sodium hydroxide to provide an alkaline matrix, which, together with the reducing properties of hydrazine hydrate, converts the solid phase of crude selenium into the liquid phase of sodium selenide, in order to successfully complete the phase transfer between solid and liquid.

[0020] Preferably, the raw material ratio for preparing the composite acidic leachate is as follows: the volume ratio of deionized water to hydrochloric acid with a mass fraction of 37.0% is 1.5:1 to form a dilute hydrochloric acid base solution; and 50g of sodium chlorate powder with a purity of 99.0-99.9% is added to each liter of the dilute hydrochloric acid base solution. The preparation method of the composite acidic leachate includes: injecting hydrochloric acid into deionized water under stirring to prepare a dilute hydrochloric acid base solution; turning on external circulating water cooling, controlling the system temperature within the range of 20-30℃, slowly adding sodium chlorate powder in batches to the dilute hydrochloric acid base solution, and continuing to stir at a constant temperature after the addition is completed to allow the mixture to react fully; the raw material ratio for preparing the deselenic reducing agent A is: sodium sulfite crystals with a purity of 98.0-99.9% are mixed with deionized water at a mass-volume ratio of 200g / L, and sodium hydroxide solution with a mass fraction of 5.0% is added to adjust the pH value of the system to 10.0.

[0021] By employing the above technical solution and setting a specific water-to-acid volume ratio as the base solution, a dynamic equilibrium between hypochlorous acid and hydrogen chloride can typically be formed within the system during the sodium chlorate dissolution and activation process. Controlling the temperature within the 20–30°C range and adding the material in batches controls the exothermic rate of the oxidation activation reaction and prevents the escape of oxidizing gases, which is often crucial for ensuring the effective chlorine content of the leachate. In the reducing agent A for selenium removal, the pH value is adjusted to approximately 10.0 using 5.0% sodium hydroxide, creating a sulfite-stable microenvironment that delays the chain oxidation reaction initiated by dissolved oxygen, thereby improving the potency retention rate of the reducing agent during standby.

[0022] Preferably, the raw material ratio for preparing the alkaline leaching mother liquor for crude selenium refining is as follows: the final concentration of sodium hydroxide in the mixture system is 3.0 mol / L; the hydrazine hydrate is a liquid hydrazine hydrate with a mass fraction of 80.0%, and the volume of the added hydrazine hydrate liquid accounts for 8.0% of the total solution volume; the preparation method of the alkaline leaching mother liquor for crude selenium refining includes: Sodium hydroxide with a purity of 99.0–99.9% was dissolved in deionized water. After the system cooled naturally to 30°C, the hydrazine hydrate liquid was injected at a uniform rate using a metering pump under nitrogen protection, and the mixture was stirred until homogeneous. By adopting the above technical solution, setting a final sodium hydroxide concentration of 3.0 mol / L is beneficial for suppressing the dissociation process of hydrazine hydrate. In the preparation process, the alkali was dissolved first and cooled to 30°C, and then hydrazine hydrate was added under nitrogen protection. This oxygen-isolation operation prevented the premature self-decomposition of hydrazine hydrate in an alkaline thermal environment, thus giving the refined mother liquor good reducing potential when injected into the crude selenium system.

[0023] Preferably, the method for efficient purification of crude selenium based on alkaline leaching further includes the following steps: the neutralization residue obtained in step S3 is discharged externally for centralized harmless treatment; the mercury precipitation liquid obtained in step S4 is subjected to evaporation and crystallization treatment, and the condensate generated during evaporation and crystallization is returned to step S1 as process makeup water. By adopting the above technical solution, a closed-loop circulation of water resources within the plant is achieved, reducing the discharge of waste liquid. At the same time, the standardized treatment of neutralization residue and the recovery of condensate further improve the environmental benefits and resource utilization rate of this purification process. This invention provides a highly efficient purification method for crude selenium based on alkaline leaching. It has the following beneficial effects: 1. This invention, by constructing a segmented impurity removal and phase transfer system, not only improves the purity of crude selenium but also achieves comprehensive resource utilization of the deselenization wastewater. After acidic deselenization, a neutralization and dedicated mercury precipitation step is introduced. pH control is used to flocculate and precipitate trace amounts of iron and lead components that have entered the liquid phase. Hydrazine hydrate is then used to reduce divalent mercuric chloride to insoluble mercurous chloride, thus achieving deep separation and high-value-added recovery of heavy metal impurities during wastewater treatment. In the refining section, crude selenium is treated with an alkaline leaching mother liquor containing hydrazine hydrate, selectively converting the elemental selenium filter residue into liquid sodium selenide, further separating it from the solid lead slag. This process path, which involves repeatedly changing the dissolution state of the target elements, makes the impurity removal process clearer, which is beneficial for obtaining a high-purity finished product.

[0024] 2. This invention designs key reaction fluids as independent purification reagents, improving the problem of drastic reaction fluctuations easily caused by directly adding raw and auxiliary materials on-site. For example, the composite acidic leachate is pre-activated under temperature control, the deselenic reducing agent A is used to construct an anti-oxidation buffer system by adjusting the alkali, and the alkaline refining mother liquor is pre-mixed under cooling and nitrogen protection. This mode of transferring the highly exothermic and volatile solution preparation process to the front end and stabilizing it reduces the risk of acidic or toxic gas escape caused by excessively high local concentrations in the main purification reactor, and increases the process stability of the entire purification production line.

[0025] 3. This invention optimizes the crystallization environment during the extraction stage, improving the physical dispersion of the final selenium powder. During alkaline refining leaching, a larger liquid-to-solid ratio and higher stirring speed dilute the spatial concentration of sodium selenide in the liquid phase, reducing the probability of unplanned oxidation caused by the solution surface being exposed to air. In the subsequent selenium precipitation stage, by limiting the hydrogen peroxide concentration, dropping method, and reaction temperature, the liquid-phase oxidation rate of selenium ions is matched with the nucleation and growth process of crystals. This, to a certain extent, suppresses the amorphous agglomeration of powder in the early stage of precipitation, which is beneficial for obtaining powder with a uniform particle size distribution. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the production process and sewage discharge relationship for the comprehensive utilization of acid sludge in this invention; Figure 2 This is a block diagram of the original process flow of the crude selenium refining workshop on which the present invention is based. Figure 3 This is a schematic diagram of the original process material flow in the crude selenium refining workshop on which this invention is based; Figure 4 This is a block diagram of the core process flow of the present invention for the efficient purification of crude selenium based on alkaline leaching; Figure 5 This is a schematic diagram showing the specific equipment and material flow of the crude selenium high-efficiency purification method based on alkaline leaching according to the present invention. Figure 6 Here is a comparative process flow diagram for selenium extraction using acid leaching and sodium sulfite reduction in existing technologies: Figure 7 This is a graph showing the evolution of the concentration of major elements and the crystallization kinetics of mercury precipitation in the solution system of Example 1 of the present invention; Figure 8 The diagram shows the physicochemical purity and impurity distribution of mercurous chloride products in the embodiments and comparative examples of this invention. Figure 9 This is a graph showing the overall direct recovery rate and selenium particle size distribution of the powder throughout the entire process of this invention. Detailed Implementation

[0027] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. 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.

[0028] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0029] Hydrochloric acid, CAS number 7647~01~0, mass fraction 37.0%, density 1.19 g / mL.

[0030] Sodium chlorate, CAS number 7775~09~9, purity ≥99.0%.

[0031] Sodium sulfite, CAS number 7757~83~7, purity ≥98.0%.

[0032] Sodium hydroxide, CAS number 1310~73~2, purity ≥99.0%.

[0033] Hydrazine hydrate, CAS number 7803-57-8, mass fraction 80.0%.

[0034] Sodium sulfide, CAS number 1313~82~2, purity ≥60.0%.

[0035] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing a composite acidic leachate, including the following steps: Step 1: Add deionized water to the mixing tank, and then slowly inject hydrochloric acid with a mass fraction of 37.0% while stirring. Control the volume ratio of deionized water to hydrochloric acid to be 1.5:1, and set the stirring speed to 200 rpm to prepare a dilute hydrochloric acid base solution. Step 2: Keep the mixture in the mixing tank under constant stirring. According to the ratio of 50g sodium chlorate per liter of dilute hydrochloric acid base solution, slowly add sodium chlorate powder with a purity of 99.0-99.9% in batches into the mixing tank. During the feeding process, turn on the external circulating water cooling and strictly control the system temperature within the range of 20-30℃ to prevent excessive escape of the generated strong oxidizing gas. Step 3: After adding sodium chlorate, continue stirring at 200 rpm and react at a constant temperature for 45 minutes to completely dissolve the sodium chlorate and allow it to undergo a moderate activation reaction with hydrochloric acid, thus obtaining a composite acidic leachate with a high oxidation potential. This leachate is then directly pumped into a sealed storage tank in the chlorination workshop for later use.

[0036] Preparation Example 2: This preparation example provides a method for preparing reducing agent A for deselectrophoresis, including the following steps: Step 1: Pour deionized water into the dissolving vessel, turn on the heating jacket to raise the water temperature to 40°C, turn on the stirrer and set the speed to 200 rpm; Step 2: Add sodium sulfite crystals with a purity of 98.0-99.9% to the dissolving vessel at a mass-volume ratio of 200g / L. Keep the water temperature constant at 40℃ and stir continuously for 30 minutes to completely dissolve the crystals and form a colorless and transparent sodium sulfite aqueous solution. Step 3: Slowly add a 5.0% sodium hydroxide solution to the above sodium sulfite aqueous solution. Since the sodium sulfite aqueous solution is weakly alkaline, the pH of the system needs to be further adjusted and maintained at 10.0 to construct an alkaline anti-oxidation buffer system to inhibit the spontaneous oxidation and loss of sulfite ions in the air. After stirring evenly, the deselenic reducing agent A is obtained and stored in a sealed storage tank away from light for later use.

[0037] Preparation Example 3: This preparation example provides a method for preparing alkaline leaching mother liquor for crude selenium refining, including the following steps: Step 1: Pump deionized water into the reaction vessel, turn on the stirring device and set the speed to 200 rpm, slowly add sodium hydroxide tablets with a purity of 99.0-99.9%, control the final concentration of sodium hydroxide in the mixture to 3.0 mol / L, and stir until the sodium hydroxide is completely dissolved. Step 2: After the sodium hydroxide solution has cooled naturally to 30°C, use a corrosion-resistant metering pump to inject hydrazine hydrate liquid with a mass fraction of 80.0% into the reactor at a uniform rate, controlling the volume of hydrazine hydrate added to be 8.0% of the total solution volume. During this process, keep the reactor under nitrogen protection. Step 3: After the hydrazine hydrate is injected, continue to stir at 200 rpm for 45 minutes in a closed environment to ensure that the strong alkali and hydrazine hydrate are fully mixed and homogeneous, thus obtaining the alkaline leaching mother liquor for crude selenium refining. The hydrazine hydrate in this mother liquor can completely reduce the elemental selenium filter residue to selenium ions. After preparation, it is directly pumped to the crude selenium refining workshop for sealed use.

[0038] Examples 1-5: Example 1

[0039] This embodiment provides a method for efficient purification of crude selenium based on alkaline leaching, including the following steps: Step 1: 100 kg of raw acid sludge is fed into the leaching reactor in the chlorination workshop via an automatic feeder. The composite acidic leachate prepared by the method in Preparation Example 1 is pumped in at a liquid-to-solid ratio of 3 L / kg. The stirrer is turned on and the speed is set to 150 rpm. Leaching is carried out continuously at a constant temperature of 35°C for 2 hours. After leaching, the slurry is discharged from the bottom of the reactor and pumped to a box filter press for solid-liquid separation. The resulting filter residue is the primary crude selenium refining raw material, and the filtrate is the qualified leachate. Step 2: Pump the qualified leachate into the reduction reactor, start stirring, and determine the concentration of free selenium ions in the qualified leachate through routine sampling and testing. Calculate the theoretical amount of reducing agent required based on the stoichiometric ratio of the reduction reaction. Then, slowly add 120% of the theoretical amount of reducing agent A prepared by the method in Preparation Example 2. During this process, trace amounts of sulfur dioxide and acid mist generated are collected and introduced into an alkaline absorption tower for treatment. After reacting for 1.5 hours, pump the reduced slurry into a box filter press for filtration. The resulting filter residue is elemental selenium filter residue. Combine it with the primary crude selenium refining raw material from Step 1 as crude selenium refining raw material. The resulting filtrate is the deselenized liquid. Step 3: The deselenized liquid is sent into the neutralization reactor. Under stirring at 150 rpm, a 30.0% sodium hydroxide solution is slowly added to adjust the pH of the material system to 3.5. After neutralization, the slurry is pumped into a box filter press for filtration. The resulting neutralization residue is collected and discharged separately for centralized harmless treatment. The resulting filtrate is the neutralized liquid. Step 4: The neutralized liquid is transferred to a mercury precipitation reactor. Under continuous stirring, an 80.0% hydrazine hydrate solution is added at a uniform rate as a reducing agent. The reaction temperature is controlled at 35°C, and the reaction is carried out in a sealed environment for 2 hours to reduce the divalent mercury ions in the solution to insoluble mercurous chloride precipitate. After the reaction is completed, the slurry is centrifuged and filtered to separate the mercurous chloride. The precipitated liquid is sent to the production wastewater treatment system for evaporation and crystallization treatment. The resulting condensate is returned to Step 1 as process makeup water for recycling. Step 5: The crude selenium refining raw materials collected in Step 2 are transported to the leaching stirring tank in the crude selenium refining workshop. The alkaline leaching mother liquor for crude selenium refining prepared by the method of Preparation Example 3 is injected at a liquid-to-solid ratio of 4 L / kg. Under normal temperature and pressure, the mixture is leached by closed stirring at a speed of 200 rpm for 1 hour, so that the elemental selenium filter residue is completely converted into soluble sodium selenide by the reducing agent in the alkaline system. The slurry is then pumped into a filter press for solid-liquid separation. The resulting filter residue is high-grade lead slag, which is collected separately and sent to the hazardous waste temporary storage room. The resulting selenium-containing filtrate is transported to the selenium precipitation stirring tank. Step 6: Turn on the stirrer of the selenium precipitation tank, and slowly and evenly add 30.0% hydrogen peroxide to the above filtrate to re-oxidize the free selenium ions in the solution and precipitate them as elemental selenium precipitate; maintain the system temperature at 30℃ and continue the reaction for 2 hours; after the reaction is completed, perform solid-liquid separation by centrifugal filtration, and wash the filter cake with pure water and dry it to obtain high-purity powdered selenium. Example 2

[0040] This embodiment provides a method for efficient purification of crude selenium based on alkaline leaching, including the following steps: Step 1: 100 kg of raw acid sludge is transported to the leaching reactor in the chlorination workshop via an automatic feeder. The composite acidic leachate prepared by the method in Preparation Example 1 is pumped in at a liquid-to-solid ratio of 2 L / kg. The stirrer is turned on and the speed is set to 150 rpm. Leaching is carried out continuously at a constant temperature of 25°C for 1 hour. After leaching, the slurry is discharged from the bottom of the reactor and pumped to a box filter press for solid-liquid separation. The resulting filter residue is the primary crude selenium refining raw material, and the filtrate is the qualified leachate. Step 2: Pump the above qualified leachate into the reduction reactor, start stirring, and slowly add the selenium-reducing agent A prepared by the method in Preparation Example 2 at 110% of the theoretical required amount; during this process, the trace amounts of sulfur dioxide and acid mist generated are collected and introduced into the alkaline absorption tower for treatment; after reacting for 1 hour, pump the reduction slurry into a box filter press for filtration, and the resulting filter residue is the elemental selenium filter residue, which is combined with the primary crude selenium refining raw material in Step 1 as the crude selenium refining raw material, and the resulting filtrate is the selenium-reduced liquid; Step 3: The deselenized liquid is sent into the neutralization reactor. Under stirring at 150 rpm, a 30.0% sodium hydroxide solution is slowly added to adjust the pH of the material system to 3.0. After neutralization, the slurry is pumped into a box filter press for filtration. The resulting neutralization residue is collected and discharged separately for centralized harmless treatment. The resulting filtrate is the neutralized liquid. Step 4: The neutralized liquid is transferred to a mercury precipitation reactor. Under continuous stirring, an 80.0% hydrazine hydrate solution is added at a uniform rate as a reducing agent. The reaction temperature is controlled at 25°C, and the reaction is carried out in a sealed environment for 1.5 hours to reduce the divalent mercury ions in the solution to insoluble mercurous chloride precipitate. After the reaction is completed, the slurry is centrifuged and filtered to separate the mercurous chloride. The precipitated liquid is sent to the production wastewater treatment system for evaporation and crystallization treatment. The resulting condensate is returned to Step 1 as process makeup water for recycling. Step 5: The crude selenium refining raw materials collected in Step 2 are transported to the leaching stirring tank in the crude selenium refining workshop. The alkaline leaching mother liquor for crude selenium refining prepared by the method of Preparation Example 3 is injected at a liquid-to-solid ratio of 3 L / kg. Under normal temperature and pressure, the mixture is leached by closed stirring at a speed of 200 rpm for 0.5 h, so that the elemental selenium filter residue is completely converted into soluble sodium selenide by the reducing agent in the alkaline system. The slurry is then pumped into a filter press for solid-liquid separation. The resulting filter residue is high-grade lead slag, which is collected separately and sent to the hazardous waste temporary storage room. The resulting selenium-containing filtrate is transported to the selenium precipitation stirring tank. Step 6: Turn on the stirrer of the selenium precipitation tank, and slowly and uniformly add 30.0% hydrogen peroxide to the above filtrate to re-oxidize the free selenium ions in the solution and precipitate them as elemental selenium precipitate; maintain the system temperature at 25℃ and continue the reaction for 1.5h; after the reaction is completed, perform solid-liquid separation by centrifugal filtration, and wash the filter cake with pure water and dry it to obtain high-purity powdered selenium. Example 3

[0041] This embodiment provides a method for efficient purification of crude selenium based on alkaline leaching, including the following steps: Step 1: 100 kg of raw acid sludge is transported to the leaching reactor in the chlorination workshop via an automatic feeder. The composite acidic leachate prepared by the method in Preparation Example 1 is pumped in at a liquid-to-solid ratio of 4 L / kg. The stirrer is turned on and the speed is set to 150 rpm. Leaching is carried out continuously at a constant temperature of 45°C for 3 hours. After leaching, the slurry is discharged from the bottom of the reactor and pumped to a box filter press for solid-liquid separation. The resulting filter residue is the primary crude selenium refining raw material, and the filtrate is the qualified leachate. Step 2: Pump the above qualified leachate into the reduction reactor, start stirring, and slowly add the selenium-reducing agent A prepared by the method in Preparation Example 2 at 130% of the theoretical required amount; during this process, the trace amounts of sulfur dioxide and acid mist generated are collected and introduced into the alkaline absorption tower for treatment; after the reaction for 2 hours, pump the reduction slurry into a box filter press for filtration, and the resulting filter residue is the elemental selenium filter residue. Combine it with the primary crude selenium refining raw material in Step 1 as the crude selenium refining raw material, and the resulting filtrate is the selenium-reduced liquid; Step 3: The selenium-depleted liquid is sent into a neutralization reactor. Under stirring at 150 rpm, a 30.0% sodium hydroxide solution is slowly added to adjust the pH of the material system to 4.0. After neutralization, the slurry is pumped into a box filter press for filtration. The resulting neutralization residue is collected and discharged separately for centralized harmless treatment. The resulting filtrate is the neutralized liquid. Step 4: The neutralized liquid is transferred to a mercury precipitation reactor. Under continuous stirring, an 80.0% hydrazine hydrate solution is added at a uniform rate as a reducing agent. The reaction temperature is controlled at 45°C, and the reaction is carried out in a sealed environment for 2.5 hours to reduce the divalent mercury ions in the solution to insoluble mercurous chloride precipitate. After the reaction is completed, the slurry is centrifuged and filtered to separate the mercurous chloride. The precipitated liquid is sent to the production wastewater treatment system for evaporation and crystallization treatment. The resulting condensate is returned to Step 1 as process makeup water for recycling. Step 5: The crude selenium refining raw materials collected in Step 2 are transported to the leaching stirring tank in the crude selenium refining workshop. The alkaline leaching mother liquor for crude selenium refining prepared by the method of Preparation Example 3 is injected at a liquid-to-solid ratio of 5 L / kg. The leaching is carried out under closed stirring at 200 rpm for 1.5 h at room temperature and pressure, so that the elemental selenium is completely converted into soluble sodium selenide by the reducing agent in the alkaline system. The slurry is then pumped into a filter press for solid-liquid separation. The resulting filter residue is high-grade lead slag, which is collected separately and sent to the hazardous waste temporary storage room. The resulting selenium-containing filtrate is transported to the selenium precipitation stirring tank. Step 6: Turn on the stirrer of the selenium precipitation tank, and slowly and uniformly add 30.0% hydrogen peroxide to the above filtrate to re-oxidize the free selenium ions in the solution and precipitate them as elemental selenium precipitate; maintain the system temperature at 35℃ and continue the reaction for 2.5 hours; after the reaction is completed, perform solid-liquid separation by centrifugal filtration, and wash the filter cake with pure water and dry it to obtain high-purity powdered selenium. Example 4

[0042] This embodiment provides a method for efficient purification of crude selenium based on alkaline leaching. Compared to Example 1, this embodiment focuses on changing the precipitation process conditions in the chlorination workshop, including the following steps: Step 1: 100 kg of raw acid sludge is fed into the leaching reactor in the chlorination workshop via an automatic feeder. The composite acidic leachate prepared by the method in Preparation Example 1 is pumped in at a liquid-to-solid ratio of 3 L / kg. The stirrer is turned on and the speed is set to 150 rpm. Leaching is carried out continuously at a constant temperature of 35°C for 2 hours. After leaching, the slurry is discharged from the bottom of the reactor and pumped to a box filter press for solid-liquid separation. The resulting filter residue is the primary crude selenium refining raw material, and the filtrate is the qualified leachate. Step 2: Pump the qualified leachate into the reduction reactor, start stirring, and slowly add the selenium-reducing agent A prepared by the method in Preparation Example 2 at 120% of the theoretical required amount; during this process, the trace amounts of sulfur dioxide and acid mist generated are collected and introduced into the alkaline absorption tower for treatment; after reacting for 1.5 hours, pump the reduction slurry into a box filter press for filtration, and the resulting filter residue is the elemental selenium filter residue, which is combined with the primary crude selenium refining raw material as the crude selenium refining raw material, and the resulting filtrate is the selenium-reduced liquid; Step 3: The deselenized liquid is fed into a neutralization reactor. Under stirring at 150 rpm, a 30.0% sodium hydroxide solution is slowly added to raise the pH of the material system to 4.5. This maximizes the precipitation of trace iron salts, lead salts, and other impurities in the solution as hydroxides, and also promotes the co-precipitation of lead salts. After neutralization, the slurry is pumped into a box filter press for filtration. The resulting neutralization residue is collected and temporarily stored for return to the batching section. The resulting filtrate is a high-purity neutralized liquid. Step 4: Transfer the neutralized liquid to the mercury precipitation reactor, control the system temperature at 40℃, and slowly add an 80.0% hydrazine hydrate solution dropwise to the reactor using a constant flow metering pump. The dropwise addition process lasts for 1 hour to reduce the explosive formation of crystal nuclei caused by excessively high local instantaneous concentrations. After the dropwise addition is completed, continue the reaction under sealed and constant temperature stirring for 2 hours to carry out sufficient crystallization and aging. After the reaction is completed, centrifuge to separate mercurous chloride with complete crystal form and higher purity. The precipitated liquid is sent to the production wastewater treatment system for evaporation and crystallization treatment, and the generated condensate is returned to Step 1 as process makeup water for recycling. Step 5: The collected crude selenium refining raw materials are transported to the leaching stirring tank in the crude selenium refining workshop. The alkaline leaching mother liquor for crude selenium refining prepared by the method in Preparation Example 3 is injected at a liquid-to-solid ratio of 4 L / kg. The leaching is carried out under closed stirring at 200 rpm for 1 hour at normal temperature and pressure to completely convert elemental selenium into soluble sodium selenide. The slurry is then pumped into a filter press for solid-liquid separation. The resulting filter residue is high-grade lead slag, which is collected separately and sent to the hazardous waste temporary storage room. The resulting selenium-containing filtrate is transported to the selenium precipitation stirring tank. Step 6: Turn on the stirrer of the selenium precipitation tank, and slowly and evenly add 30.0% hydrogen peroxide to the above filtrate to re-oxidize the free selenium ions in the solution and precipitate them as elemental selenium precipitate; maintain the system temperature at 30℃ and continue the reaction for 2 hours; after the reaction is completed, perform solid-liquid separation by centrifugal filtration, and wash the filter cake with pure water and dry it to obtain high-purity powdered selenium. Example 5

[0043] This embodiment provides a highly efficient purification method for crude selenium based on alkaline leaching. Compared to Embodiment 1, this embodiment focuses on changing the extraction process conditions in the crude selenium refining workshop, including the following steps: Step 1: 100 kg of raw acid sludge is fed into the leaching reactor in the chlorination workshop via an automatic feeder. The composite acidic leachate prepared by the method in Preparation Example 1 is pumped in at a liquid-to-solid ratio of 3 L / kg. The stirrer is turned on and the speed is set to 150 rpm. Leaching is carried out continuously at a constant temperature of 35°C for 2 hours. After leaching, the slurry is discharged from the bottom of the reactor and pumped to a box filter press for solid-liquid separation. The resulting filter residue is the primary crude selenium refining raw material, and the filtrate is the qualified leachate. Step 2: Pump the above qualified leachate into the reduction reactor, start stirring, and slowly add the selenium-reducing agent A prepared by the method in Preparation Example 2 at 120% of the theoretical required amount; the trace amounts of sulfur dioxide and acid mist generated are introduced into the alkaline absorption tower for treatment; after reacting for 1.5 hours, pump the reduction slurry into a box filter press for filtration, and the resulting filter residue is the elemental selenium filter residue. Combine it with the primary crude selenium refining raw material as the crude selenium refining raw material, and the resulting filtrate is the selenium-reduced liquid; Step 3: The deselenized liquid is sent into the neutralization reactor. Under stirring at 150 rpm, a 30.0% sodium hydroxide solution is slowly added to adjust the pH of the material system to 3.5. After neutralization, the slurry is pumped into a box filter press for filtration. The resulting neutralization residue is collected and discharged separately for centralized harmless treatment. The resulting filtrate is the neutralized liquid. Step 4: Transfer the neutralized liquid to the mercury precipitation reactor, add 80.0% hydrazine hydrate solution at a constant rate while continuously stirring, control the reaction temperature at 35℃, and react in a sealed environment for 2 hours; after the reaction is completed, centrifuge and filter to separate mercurous chloride, and send the precipitated liquid to the production wastewater treatment system for evaporation and crystallization treatment, and return the generated condensate as process makeup water to Step 1 for recycling. Step 5: The collected crude selenium refining raw materials are transported to the leaching stirring tank in the crude selenium refining workshop. To prevent the surface of the high-concentration sodium selenide solution from being accidentally oxidized by air, alkaline leaching mother liquor for crude selenium refining prepared by the method of Preparation Example 3 is injected at a relatively high liquid-to-solid ratio of 6 L / kg to dilute the spatial concentration of selenium ions in the system. Under normal temperature and pressure, leaching is carried out at a high speed of 250 rpm with closed stirring for 2 hours to ensure that elemental selenium is completely converted into sodium selenide. After pressure filtration, the resulting filter residue is high-grade lead slag, which is collected separately and sent to the hazardous waste temporary storage room. The resulting selenium-containing filtrate is transported to the selenium precipitation stirring tank. Step Six: Turn on the stirrer of the selenium precipitation tank and slowly add 30.0% hydrogen peroxide dropwise to the above large-volume filtrate. Extend the total reaction time of hydrogen peroxide addition and oxidation precipitation to 3 hours. By slowly oxidizing, a long-term precipitation equilibrium is established to avoid amorphous agglomeration of the selenium powder. Maintain the system temperature at 35°C. After the reaction is completed, separate the powder by centrifugation and filtration. After the filter cake is washed with pure water multiple times and vacuum dried, high-purity selenium powder with excellent dispersibility is obtained.

[0044] Comparative Examples 1-4: Comparative Example 1: Compared with Example 1, the difference is that the wet purification process of the present invention, which consists of compound acid leaching, reduction neutralization and alkali leaching refining, is completely replaced by the conventional traditional pyrometallurgical / acid leaching process in the prior art (that is, the same batch of raw material acid mud is placed at 650°C for high-temperature oxidation roasting, and the selenium dioxide is collected by dust collection to produce crude selenium, and the bottom residue is then extracted by conventional acid leaching with sulfuric acid to extract metals). The other raw material batches and performance test conditions are the same.

[0045] Comparative Example 2: Compared to Example 1, the difference lies in the omission of the core "neutralization" stage in the process flow (i.e., step three is omitted). The selenium-depleted liquid obtained from solid-liquid separation in step two is directly transported to the mercury precipitation reactor in step four for subsequent operations without pH adjustment, resulting in the dissolution of Fe in the solution. 3+ Impurities such as lead salts failed to precipitate and retain them in advance as hydroxides (neutralization residue), and lead impurities also lost their co-precipitation carrier. All other steps, raw material ratios, and process parameters were the same.

[0046] Comparative Example 3: Compared to Example 1, the difference lies in the replacement of the core reducing agent used in the mercury precipitation stage in step four, instead of using the Hg-reducing agent. 2+ Instead of using a hydrazine hydrate solution with specific selective reducing properties, a conventional metal reducing agent (iron powder) with an equivalent theoretical reducing capacity is used, which alters the reduction specificity and impurity separation mechanism of the reaction system. All other steps, raw material ratios, and process parameters remain the same.

[0047] Comparative Example 4: Compared with Example 1, the difference is that the key parameters in step five exceed the protection boundary set by this process. The liquid-solid ratio of the alkaline leaching mother liquor added during crude selenium refining is reduced from 4L / kg to 1.5L / kg, resulting in an excessively high Se content in the refining leachate (extremely high local concentration). This makes the generated reduced sodium selenide (Na2Se) easily oxidized again by oxygen in the air at the gas-liquid interface. All other steps and process methods are the same.

[0048] Test Examples 1-3: Test Example 1: Testing the Mechanism of Ion Phase Evolution and Selective Transformation in a Full-Process Solution System This test case is used to verify the selective transformation law of key ions in different stages of the "alkaline leaching" process route of the present invention. It focuses on examining the phase control mechanism of each element in the acid leaching, deselenization and neutralization processes, thereby proving the practical feasibility of the chemical generation path of the present invention.

[0049] 100 mL each of the following samples from Example 1 during the process were used as test subjects: "qualified leachate" (after step one), "selenium-depleted solution" (after step two), and "neutralized solution" (after step three). Each group of filtrate samples was diluted to volume using a 2% nitric acid matrix and then injected into an inductively coupled plasma optical emission spectrometer (ICP-OES). The RF generator power was set to 1300 W, and the plasma gas flow rate to 15 L / min. Using the full-spectrum direct-reading function, the changes in the ion concentrations of total Se, total Hg, and the representative impurity total Fe in the solution were quantitatively analyzed. For the mercury precipitation reaction system in step four of Example 1, 5 mL of supernatant was periodically drawn at 30 min, 60 min, and 120 min of reaction time, and the residual Hg was monitored using an ion chromatograph (IC) or ICP-OES. 2+ Concentration was used to plot precipitation crystallization kinetics curves.

[0050] Table 1. Evolution of concentrations of major elements in the characteristic process fluids of each stage in Example 1

[0051] Note: In Table 1, “\” indicates that it was not measured.

[0052] Figure 7 This is a graph showing the evolution of the concentration of major elements and the crystallization kinetics of mercury precipitation in the solution system of Example 1 of the present invention. Sub-figure (a) shows the phased change trends of the total Se, total Hg, and total Fe concentrations in the solution system during the macroscopic process flow of the qualified leachate, the deselenized solution, and the neutralized solution in Example 1; Sub-figure (b) shows the continuous dynamic decay process of the residual total Hg concentration in the supernatant as the reaction time increases when the mercury precipitation reaction system of Example 1 proceeds to step four.

[0053] According to the data in Table 1, the process flow of Example 1 exhibits staged separation characteristics of the target element and impurity elements at each node. Analysis of the element concentration changes before and after selenium removal shows that the total Se concentration in the qualified leachate is 18.27 g / L. After treatment with the reducing agent, the total Se concentration in the deselenized solution decreases to 0.14 g / L, while the total Hg concentration changes from 4.13 g / L to 4.09 g / L. The fluctuation in the total Hg concentration is within the error range of conventional instrument testing, indicating that in the chemical system with the introduction of the deselenization reducing agent, sodium sulfite mainly undergoes a redox reaction with selenite acid in the system to generate elemental selenium precipitate, while free divalent mercury ions are retained in the solution phase, showing a relatively clear difference in phase distribution between the two.

[0054] In the data measured after neutralization, the total Fe concentration in the solution decreased from 1.15 g / L in the selenium-depleted solution to 0.04 g / L, while the total Hg concentration also decreased to some extent (from 4.09 g / L to 3.98 g / L). This result indicates that adjusting the pH of the material system to a specific weakly acidic range promotes the preferential precipitation of dissolved ferric ions as hydroxides. This separation process can retain some interfering metal ions that would cause impurities in the final product before the precipitation of divalent mercury ions, thus providing material conditions for improving the physicochemical purity of the subsequent target product. Dynamic sampling data from the supernatant of the mercury precipitation reaction show that after adding hydrazine hydrate solution, the residual total Hg concentration in the system shows a continuous decreasing trend with increasing reaction time. In the first 60 minutes of the reaction, the total Hg concentration decreased from the initial 3.98 g / L to 0.86 g / L. Subsequently, the rate of decrease slowed down, and the concentration was measured to be 0.05 g / L at 120 minutes.

[0055] This evolutionary pattern reflects the process by which divalent mercury ions are reduced by hydrazine hydrate and combine with free chloride ions to form a mercurous chloride solid phase. This process involves a relatively concentrated initial nucleation stage followed by a later crystal growth and aging stage. By maintaining a reaction cycle of 120 minutes, most of the soluble mercury in the solution was converted to the solid phase. The test results demonstrate that the settings for reagent dosage and process time parameters are feasible at the chemical transformation level and can well support the process mechanism of this invention regarding the separation of targeted substances.

[0056] Test Example 2: Comparison Test of Impurity Targeting Retention and Reactant Specificity Weigh 10.0g of the mercurous chloride solid products obtained from Example 1, Comparative Example 2 and Comparative Example 3 after their respective precipitation separation and drying treatment, place them in a mortar and grind them into a uniform powder with no obvious visible particles, and use them as the powder to be tested.

[0057] Accurately weigh 0.500g of powder from each of the test powders and place them in a polytetrafluoroethylene digestion vessel. Add a mixed acid solution consisting of nitric acid and hydrochloric acid in a volume ratio of 1:3. After sealing, place the vessel in a microwave digestion instrument for programmed temperature digestion. After the reaction system cools to room temperature, transfer it to a volumetric flask and dilute to 100mL with deionized water to prepare the test digestion solution for metal impurity analysis.

[0058] The digested solution to be tested after being brought to a constant volume was introduced into a flame atomic absorption spectrophotometer. The characteristic absorption wavelengths of iron and lead were selected respectively. Under the set atomization temperature and combustion gas ratio, the absorbance of each digested solution was tested. The mass fraction of iron and lead impurities in each powder object was calculated by combining the standard curve.

[0059] In addition, 2.000 g of powder was accurately weighed from each of the powders to be tested and added to a reagent bottle containing excess standard iodine solution and potassium iodide. The mixture was shaken under light-protected conditions to allow mercurous chloride to react with the iodine. Then, using starch solution as an indicator, the remaining unreacted iodine was slowly titrated with sodium thiosulfate standard titration solution. The mass fraction of the main component mercurous chloride in each powder was calculated based on the actual volume of sodium thiosulfate consumed.

[0060] Table 2. Physicochemical index determination data of mercurous chloride products in the examples and comparative examples

[0061] Figure 8 These are physicochemical purity and impurity distribution diagrams of mercurous chloride products from the embodiments and comparative examples of the present invention. Sub-figure (a) shows the scatter plot test data of iron and lead impurity content in the products of Example 1, Comparative Example 2, and Comparative Example 3; sub-figure (b) shows the test data of the mass fraction of the main mercurous chloride component in the products of Example 1, Comparative Example 2, and Comparative Example 3.

[0062] According to the data in Table 2, different purification processes and the selection of chemical reducing reagents resulted in differences in the component distribution of the final solid product. In Example 1, the mercurous chloride purity of the product was measured to be 99.82%, with iron and lead impurities controlled at 0.014% and 0.021%, respectively, exhibiting relatively high overall physicochemical purity. In Comparative Example 2, due to the absence of a neutralization and pH adjustment step, the acidic leachate was directly reduced, resulting in a mercurous chloride purity of 96.38%, and increases in iron and lead impurities to 1.764% and 1.153%, respectively.

[0063] This phenomenon indicates that ferric iron and other metallic impurities dissolved into the liquid phase during the acidic oxidative leaching process, if lacking a pre-treatment hydroxide crystallization separation path before the precipitation of divalent mercuric ions, will physically entrain or exhibit trace co-precipitation with the generated mercurous chloride, thereby increasing the proportion of non-target metal compounds within the byproduct precipitate. This suggests that independently setting a neutralization and separation stage plays a certain auxiliary role in reducing the background impurities in the subsequent mercury precipitation wastewater system and improving the physicochemical purity of the recovered mercurous chloride byproduct.

[0064] Comparative Example 3 used conventional iron powder to replace the specific hydrazine hydrate as the reducing agent in the precipitation stage. Test results showed that the purity of the main mercurous chloride component further decreased to 91.24%, and the mass fraction of iron impurities surged to 5.632%, while the mass fraction of lead impurities remained at a low level of 0.026% due to the pre-neutralization and impurity removal process. From the reaction conversion logic analysis, when solid elemental iron undergoes displacement and reduction reactions in the system, an excess of iron powder is usually required to ensure complete reduction of mercury ions. This results in a large amount of unreacted iron powder skeleton remaining directly as insoluble solid impurities in the mercurous chloride precipitate, which is difficult to remove through conventional washing processes. Furthermore, the redox potentials of different reducing agents vary. The iron powder reduction system tends to cause some mercury ions to undergo incomplete reaction or excessive dechlorination, transforming them into free mercury metal, leading to a double decline in the yield and purity of the crystalline substrate. These data characteristics confirm that the reagent ratios and separation logic used in the examples help maintain the phase specificity of the product and reduce the introduction of external heavy metal elements.

[0065] Test Example 3: Comprehensive Direct Recovery Rate of Target Elements and Physicochemical Properties of Selenium Powder The total mass of raw acid sludge fed into Examples 1, 5, Comparative Example 1, and Comparative Example 4 throughout the entire process, the volume and element concentration of the transfer liquid in each process, and the actual dry weight of the final produced powders of selenium and mercurous chloride were retrieved.

[0066] Based on the material balance data of the above groups, the mass flow of Se and Hg elements during the entire system operation cycle is calculated. After deducting the sampling and mechanical losses in the intermediate process, the direct recovery rate of Se and the precipitation recovery rate of Hg for each sample group under the corresponding process are calculated.

[0067] Weigh 5.00g of the selenium powder obtained from each process and place it in a beaker containing a 0.1% sodium hexametaphosphate aqueous solution. Start the ultrasonic cleaner and treat it at room temperature for 10 minutes to fully disperse the powder particles in the liquid medium and prepare a suspension for particle size analysis.

[0068] The prepared suspensions were sequentially transferred into the sample cell of the laser scattering particle size analyzer. The optical parameters of the instrument and the speed of the disperser were set. The particle size distribution of the equivalent selenium sphere volume of each group of powders was measured. The median particle size D50 corresponding to the cumulative distribution percentage reaching 50% was recorded, and the complete frequency distribution curve data was output.

[0069] Take 1.00g of each group of powdered selenium, add it to a mixed oxidant composed of nitric acid and perchloric acid and heat to dissolve it. After eliminating the interference of nitrogen oxides, the final mass fraction of elemental selenium in each group of products is determined by iodometric titration.

[0070] Table 3. Comprehensive Direct Recovery Rate and Test Data of Selenium Powder

[0071] Figure 9 This is a graph showing the overall direct recovery rate and particle size distribution of selenium powder throughout the entire process of this invention. Sub-figure (a) shows the macroscopic test data of the direct recovery rate of Se, the precipitation recovery rate of Hg, and the corresponding mass fraction of the main selenium component in the powder during the production cycle of Examples 1, 5, Comparative Example 1, and Comparative Example 4. Sub-figure (b) shows the relative frequency distribution curves of the selenium powder produced by the above four experimental groups under a laser scattering particle size analyzer. The horizontal axis is logarithmic, and the triangles indicated by the peaks of each curve in the figure correspond to the median particle size D50 of their respective samples.

[0072] According to the data in Table 3, the selection of the entire process route and key parameters showed differences in the control of target element yield and powder morphology. Comparative Example 1 used conventional high-temperature roasting pyrometallurgical separation combined with subsequent acid leaching, and the measured direct Se recovery rate was 81.79%, and the comprehensive Hg recovery rate was 84.65%. The test results reflect the thermodynamic evolution law of the material in the pyrometallurgical roasting section. Under high temperature conditions, selenium in the raw material acid sludge is oxidized and converted into gaseous selenium dioxide volatilization. At the same time, mercury in the material is also dissociated in large quantities during the heating process and released in gaseous form.

[0073] This separation process, which relies on gas-phase transfer, is prone to the escape of a large amount of volatiles with the flue gas, and the condensation and capture efficiency of mercury vapor is greatly affected by fluctuations in operating air volume and temperature gradient. In addition, when the bottom ash after roasting is subjected to secondary acid leaching, some residual metals encapsulated in silicate or complex oxide matrices are difficult to completely dissolve. The superposition of multiple gas-solid two-phase loss pathways results in a low overall recovery rate.

[0074] Comparative Example 4 employed a fully wet process but altered the process parameters during crude selenium refining, resulting in a liquid-to-solid ratio exceeding the set limits and causing an excessively high selenium content within the system. Under these conditions, the direct Se recovery rate was 88.92%, the mass fraction of the main selenium component in the powder was 94.86%, and the median particle size D50 reached 39.73 μm. The test data and the corresponding distribution curve broadening characteristics in sub-figure (b) indicate that a high degree of local supersaturation was formed in the high-concentration mother liquor system when an alkaline reducing agent was added to generate readily soluble sodium selenide.

[0075] When the reaction solution comes into contact with trace amounts of air at the gas-liquid interface, the free selenium-containing intermediate undergoes an unplanned secondary oxidation reaction, causing rapid heterogeneous nucleation of elemental selenium at the interface. The instantaneous precipitation process causes the newly formed grains to fail to grow according to the crystallographic spacing rules, instead rapidly agglomerating in an amorphous aggregate state. The larger particle clusters encapsulate part of the mother liquor and coexisting impurities, leading to a decrease in product purity and a reduction in the overall direct recovery rate.

[0076] Both Examples 1 and 5 exhibited relatively stable yield parameters during testing. In Example 1, the direct Se recovery rate and Hg precipitation recovery rate reached 98.14% and 99.46%, respectively. Due to the use of a closed-loop liquid-phase circulation system throughout the process, the high-temperature volatilization path was avoided, and the liquid-solid phase mass transfer process was thermodynamically controllable, achieving targeted element retention. In Example 5, by further adjusting relevant physicochemical parameters, the main selenium component mass fraction of the produced powder reached 99.97%, and the median particle size decreased to 8.14 μm. In sub-figure (b), the relative frequency distribution curve of this sample exhibits a narrow peak shape and concentrated distribution.

[0077] This characteristic indicates that, under suitable liquid-to-solid ratios and controlled oxidation rates, the conversion of sodium selenide to elemental selenium maintains a long-term precipitation-dissociation equilibrium. Crystal nuclei achieve slow, homogeneous growth under a relatively stable concentration gradient, reducing the probability of amorphous agglomeration and impurity encapsulation, resulting in a smaller and more dispersed powder structure at the macroscopic level. The aforementioned data trends demonstrate that the wet conversion mechanism and parameter boundary settings involved in this process scheme play a practical role in maintaining the stability of the material system.

Claims

1. A method for efficient purification of crude selenium based on alkaline leaching, characterized in that, include: The raw acid mud is mixed with a compound acid leachate and leached to separate the primary crude selenium refining raw material and qualified leachate. Add reducing agent A for deselection to qualified leachate, filter to obtain elemental selenium filter residue and deselection liquid, and combine elemental selenium filter residue with primary crude selenium refining raw material to form crude selenium refining raw material. Add sodium hydroxide solution to the deserved liquid to adjust the pH value, and separate the neutralized residue and the neutralized liquid. Add hydrazine hydrate solution to the neutralized solution to precipitate mercurous chloride, and separate the mercury-precipitated solution to obtain the mercury-precipitated solution. The crude selenium refining raw material is mixed with the crude selenium refining alkaline leaching mother liquor and leached to separate lead slag and selenium-containing filtrate. Hydrogen peroxide was added dropwise to the selenium-containing filtrate to precipitate elemental selenium, which was then separated and dried to obtain powdered selenium.

2. The method for efficient purification of crude selenium based on alkaline leaching according to claim 1, characterized in that, When mixing and leaching the raw acid mud with the composite acid leachate, add the composite acid leachate at a liquid-to-solid ratio of 2-4 L / kg, and leach for 1-3 hours at a constant temperature of 25-45℃. When adding deselenochemical reducing agent A to a qualified leachate, the amount of deselenochemical reducing agent A added is 110-130% of the theoretical required amount, and the reaction time is 1-2 hours. When adjusting the pH value of the deselenate solution by adding sodium hydroxide solution, adjust the pH value of the system to 3.0-4.

5.

3. The method for efficient purification of crude selenium based on alkaline leaching according to claim 1, characterized in that, When adding hydrazine hydrate solution to the neutralized liquid, the mass fraction of hydrazine hydrate solution is 80.0%, the reaction temperature is 25-45℃, and the reaction is carried out in a closed environment for 1.5-2.5 hours. When hydrogen peroxide is added dropwise to a selenium-containing filtrate to precipitate elemental selenium, the mass fraction of hydrogen peroxide is 30.0%, the system temperature is maintained at 25–35℃, and the reaction time is 1.5–3 hours.

4. The method for efficient purification of crude selenium based on alkaline leaching according to claim 1, characterized in that, When adding hydrazine hydrate solution to the neutralized liquid, a constant flow metering pump is used to add the hydrazine hydrate solution dropwise into the reaction vessel. The dropwise addition process is controlled to last for 1 to 2 hours. After the dropwise addition is completed, the vessel is sealed and kept at a constant temperature for stirring and aging. When leaching crude selenium refining raw materials with alkaline leaching mother liquor for crude selenium refining, add the alkaline leaching mother liquor for crude selenium refining at a liquid-to-solid ratio of 3-6 L / kg, and leach with closed stirring at a speed of 200-250 rpm for 0.5-2 hours.

5. The method for efficient purification of crude selenium based on alkaline leaching according to claim 1, characterized in that, in, The raw material ratio of the composite acidic leachate is as follows: The volume ratio of deionized water to 37.0% hydrochloric acid is 1.5:1 to form a dilute hydrochloric acid base solution; For each liter of the dilute hydrochloric acid solution, add 50g of sodium chlorate powder with a purity of 99.0-99.9%.

6. The method for efficient purification of crude selenium based on alkaline leaching according to claim 5, characterized in that, The preparation process of the composite acidic leachate includes: Hydrochloric acid was injected into deionized water under stirring to obtain a dilute hydrochloric acid base solution. Turn on the external circulating water cooling and control the system temperature within the range of 20-30℃. Slowly add sodium chlorate powder to the dilute hydrochloric acid base solution in batches. After the addition is completed, continue stirring at a constant temperature.

7. The method for efficient purification of crude selenium based on alkaline leaching according to claim 1, characterized in that, in, The raw material ratio of the reducing agent A for deselelation is as follows: Sodium sulfite crystals with a purity of 98.0–99.9% were mixed with deionized water at a mass-volume ratio of 200 g / L, and a sodium hydroxide solution with a mass fraction of 5.0% was added to adjust the pH of the system to 10.

0.

8. The method for efficient purification of crude selenium based on alkaline leaching according to claim 1, characterized in that, in, The raw material ratio of the alkaline leaching mother liquor used for crude selenium refining is as follows: The final concentration of sodium hydroxide in the mixture system is 3.0 mol / L; The hydrazine hydrate is an aqueous solution with a mass fraction of 80.0%, and the volume of the added hydrazine hydrate solution accounts for 8.0% of the total volume of the alkaline leaching mother liquor used for crude selenium refining.

9. The method for efficient purification of crude selenium based on alkaline leaching according to claim 8, characterized in that, The preparation process of the alkaline leaching mother liquor for crude selenium refining includes: Sodium hydroxide with a purity of 99.0-99.9% was dissolved in deionized water. After the system cooled naturally to 30°C, hydrazine hydrate solution was injected at a constant rate using a metering pump under nitrogen protection, and the mixture was stirred and mixed thoroughly.

10. The method for efficient purification of crude selenium based on alkaline leaching according to claim 1, characterized in that, Also includes: The neutralized residue obtained from the separation will be discharged externally for centralized harmless treatment; The separated mercury precipitation liquid is subjected to evaporation and crystallization treatment. The condensate generated during evaporation and crystallization is returned as process water to the raw material acid mud and the composite acid leachate for leaching.

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