Rectification and purification method for removing trace metal impurities in selenium dioxide

Abstract

The invention relates to the technical field of preparation of high-purity inorganic compounds, and discloses a rectification purification method for removing trace metal impurities in selenium dioxide, which comprises the following steps: dissolving crude selenium dioxide in an aqueous solvent to form a seleninic acid aqueous solution, adding a chemical reducing agent to regulate the valence state of the impurities, and adding a ferric nitrate auxiliary agent and a hydrolyzable alkaline precursor substance; heating to hydrolyze the precursor substance so as to generate alkali in situ, and realizing homogeneous co-precipitation of iron cations and impurities; the method comprises the following steps: separating precipitates, carrying out thermal dehydration on a purified seleninic acid aqueous solution obtained after separation, and carrying out thermal decomposition removal on residual nitrate anions by heating while crystallizing and separating out selenium dioxide. By selecting ferric nitrate and reusing the heating function of the thermal dehydration step, the problem of secondary pollution of anions caused by conventional ferric salt auxiliaries is avoided.

Description

Technical Field

[0001] This invention relates to a distillation purification method for removing trace metal impurities from selenium dioxide, belonging to the field of high-purity inorganic compound preparation technology. Background Technology

[0002] Currently, the industry consensus for the production of electronic-grade selenium dioxide (SeO2) is that wet processes have greater potential for removing trace impurities compared to traditional dry sublimation. Wet processes typically include core steps such as dissolution, chemical purification, and pyrolysis regeneration crystallization. In the chemical purification stage, to remove trace key impurities such as arsenic and iron from the selenite acid (H2SeO3) solution formed by dissolving crude selenium dioxide raw materials in water, targeted co-precipitation using sacrificial auxiliaries such as iron salts is considered an effective technical approach based on chemical principles. This relies on controlling the pH of the solution to hydrolyze the iron salt into hydroxide flocs. The flocs then utilize their efficient adsorption and encapsulation capabilities for specific impurities, which are then precipitated and removed together.

[0003] However, when this chemical principle is applied to large-scale industrial production, several engineering and physical constraints emerge. The mismatch between chemical reactions and physical operations in the process chain makes it difficult to achieve chemical efficiency and engineering operability in a coordinated manner. On the one hand, the conventional pH control operation of adding alkali in industrial reactors usually creates a momentary local strong alkaline environment at the point of alkali dripping. This pH inhomogeneity causes sacrificial iron salts to precipitate instantaneously and form dense particles due to local overconcentration before they can fully contact and mix with trace impurities in the bulk solution, thus reducing the chemical efficiency of impurity adsorption in industrial practice. On the other hand, even if chemical adsorption occurs, the resulting hydroxide precipitate is usually in the form of a high-viscosity, high-resistivity colloid. When this colloid comes into contact with industrial filter media, it will quickly clog its pores, causing the filtration flux to approach zero. This will cause the entire process to be interrupted in the physical separation step due to separation difficulties. Conventional improvement ideas in existing technologies, such as optimizing stirring or adding flocculants, cannot eliminate the local concentration mutation under the constraints of fluid dynamics, and the latter will introduce new impurities into the system aimed at producing high-purity products.

[0004] To avoid the bottleneck of uncontrollable physical morphology of precipitates in wet processes, the technical solution has shifted to a purification path coupled with pressure swing adsorption (PSA) and distillation. For example, Chinese invention patent CN112624050B discloses a deep defluorination and drying method for HCl gas containing low concentrations of HF using FTrPSA separation and purification. This method attempts to purify HCl gas through the complex coupling of multiple unit operations such as medium-temperature pressure swing adsorption (FTrPSA), membrane separation, and HCl distillation. However, this path connects processes with different physical principles, such as adsorption, filtration, and distillation, in series. Relying on complex circulation loops, the entire process is extremely sensitive to fluctuations in operating conditions. The dynamic balance between unit operations, such as the desorbed gas from pressure swing adsorption and the feed to the distillation column, is difficult to match and maintain, resulting in some batches of products failing to meet purity standards. Furthermore, the pyrolysis crystallization step is prone to instantaneous large-scale nucleation, causing crystals to encapsulate the mother liquor and form volatile impurities during rapid precipitation. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: A distillation purification method for removing trace metal impurities from selenium dioxide, comprising:

[0006] Step a: Dissolve crude selenium dioxide containing trace metal impurities in an aqueous solvent to form an aqueous solution of selenite.

[0007] Step b: Add a chemical reducing agent to the selenite aqueous solution to reduce the high oxidation state arsenic impurities in the trace metal impurities to a preset low oxidation state;

[0008] Step c: Add ferric nitrate as a sacrificial iron salt auxiliary to the aqueous solution of selenite, and add a hydrolyzable alkaline precursor.

[0009] Step d: Heating the selenite aqueous solution causes the hydrolyzable alkaline precursor to hydrolyze and generate alkali in situ, raising the pH of the selenite aqueous solution to a preset range. The iron cations of ferric nitrate hydrolyze to form ferric hydroxide precipitate, which in turn forms a coprecipitate flocculant with the preset low-oxidation-state arsenic impurities.

[0010] Step e: Separate the coprecipitated flocs from the aqueous selenite solution to obtain a purified aqueous selenite solution carrying nitrate anions derived from ferric nitrate.

[0011] Step f involves thermally dehydrating the purified selenite aqueous solution by heating within a preset temperature range to evaporate the aqueous solvent and precipitate high-purity selenium dioxide crystals, while also causing the nitrate anions to undergo thermal decomposition and escape as gaseous products.

[0012] Preferably, after step d and before step e, a precipitation and maturation step is further included: maintaining the selenite aqueous solution containing co-precipitated flocs at a maturation temperature and for a maturation time; the precipitation and maturation step includes: running a stirrer at a constant speed to stir the selenite aqueous solution, the stirrer being driven by a motor; monitoring the torque of the motor in real time, and dynamically determining the endpoint of the maturation time based on the time change rate of the torque; wherein, the endpoint is determined when the time change rate of the torque is continuously lower than a preset torque change threshold within a preset time period.

[0013] Preferably, the chemical reducing agent in step b is oxalic acid.

[0014] Preferably, the hydrolyzable alkaline precursor in step c is urea.

[0015] Preferably, step d, heating the selenite aqueous solution, is performed in a closed container under a preset back pressure greater than atmospheric pressure, so that the carbon dioxide generated by the hydrolysis of the hydrolyzable alkaline precursor dissolves in the selenite aqueous solution.

[0016] Preferably, the curing temperature is 40°C. Up to 70 .

[0017] Preferably, the preset temperature range in step f is 105°C. Up to 130 .

[0018] Preferably, the thermal dehydration process in step f further includes: real-time monitoring of the purified selenite aqueous solution at the current moment. turbidity Compared to the previous moment turbidity ; Calculate the rate of change of turbidity And the heating power of the thermal dehydration process is controlled in a closed loop. ; in accordance with Calculations and controls follow these rules: If ,but ;like ,but ;in, To preset the rate of change threshold, The preset reduced heating power This is the preset base heating power.

[0019] Preferably, the preset range in step d is a pH value of 3.0 to 4.0.

[0020] Preferably, the chemical reducing agent in step b is sodium sulfite.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] 1. The homogeneous hydrolysis of alkaline precursors is combined with the closed-loop conditions of applying back pressure. Homogeneous hydrolysis causes the pH value of the solution to rise slowly and synchronously throughout the entire volume, avoiding local pH abrupt changes and instantaneous solidification failure of precipitates caused by conventional alkali addition. Applying back pressure forces the carbon dioxide generated during hydrolysis to dissolve, eliminating foam entrainment that leads to material loss. The dissolved products further construct a chemical buffer system in situ in the solution, smoothing the pH rise process. Under this method, the sacrificial additive can grow slowly in a chemical environment without foam interference, forming a highly adsorbent flocculated structure, ensuring that the chemical efficiency of the core mechanism of sacrificial coprecipitation is realized in industrial-scale production.

[0023] 2. After precipitation and before filtration, a precipitation maturation step is set up. By using a controlled temperature holding process, the thermodynamically unstable colloidal precipitate is guided to spontaneously transform into physically stable and easily filterable dense particles, thus solving the physical operability bottleneck of co-precipitated products that cannot be filtered in industry. Furthermore, the maturation process is coupled with the torque monitoring of the stirring motor. By utilizing the physical correspondence between torque consumption and rheological properties (viscosity) at a constant speed, the torque signal drops to a stable plateau region as the criterion for judging the maturation endpoint, thereby realizing online closed-loop dynamic control of the phase change of colloidal particles.

[0024] 3. A purification sequence linking pre-chemical reduction and targeted co-precipitation is constructed. Before adding sacrificial iron salt auxiliaries, a selective chemical reducing agent is introduced to pre-treat the selenite aqueous solution. This quantitatively regulates high-oxidation-state arsenic impurities in the raw materials that may be ineffective in subsequent co-precipitation steps into easily adsorbed low-oxidation-state impurities. This ensures that the subsequent sacrificial co-precipitation mechanism is no longer constrained by the valence state fluctuations between upstream raw material batches. The scavenger effect is broadly effective against trace impurities of all valence states, ensuring high stability of the final product purity in continuous industrial production. In the final thermal dehydration and crystallization step, a closed-loop feedback control based on turbidity signals is introduced. This control is independent of a fixed heating program and monitors the time-varying rate of solution turbidity in real time. Once the rate of change exceeds the threshold characterizing crystal nucleation, the controller immediately and automatically pulls back or pauses the heating power. This mechanism dynamically locks the crystallization rate with the heating input, ensuring that the crystals grow under controlled conditions at low supersaturation. This avoids the inclusion of small crystals in the mother liquor due to nucleation, ensuring that the final high-purity selenium dioxide product is free of trace volatile impurities. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the wet purification and process control flow of selenium dioxide according to the present invention;

[0026] Figure 2 This is a flowchart of the torque feedback closed-loop control process for the precipitation and ripening step of this invention. Detailed Implementation

[0027] To make the technical means, creative features, objectives, and effects of this invention readily understandable, the invention is further described below with reference to specific embodiments. It should be noted that these embodiments are merely illustrative of the technical solutions of this invention and not intended to limit it. The scope of protection of this invention is not limited thereto, and all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0028] This invention provides a distillation purification method for removing trace metal impurities from selenium dioxide, comprising: step a, dissolving a crude material to form an aqueous selenite solution; step b, pre-chemically reducing the trace impurities in the solution to regulate their valence states; step c, adding a sacrificial iron salt auxiliary agent and a hydrolyzable alkaline precursor; step d, initiating hydrolysis of the precursor by heating to generate an alkali in situ in a homogeneous phase, achieving controlled co-precipitation of the sacrificial iron salt and impurities; step e, separating the precipitate to obtain a purified aqueous selenite solution; step f, ... The purified solution is thermally dehydrated, and while high-purity selenium dioxide crystallizes out, residual anions from the additives are thermally decomposed and removed. In step a, the crude selenium dioxide is an industrial-grade product, containing trace metal impurities such as arsenic, iron, and tellurium. The aqueous solvent is preferably high-purity water, such as ultrapure water with a resistivity greater than 18 MΩ·cm, to avoid introducing external contamination. The dissolution operation is carried out in an inert material reactor equipped with a stirrer, such as an enamel-lined reactor or a PFA-lined reactor, and stirred at room temperature until the crude material dissolves to form selenite ( The aqueous solution can be optionally filtered after dissolution to remove insoluble impurities carried in the raw material beforehand. Arsenic impurities in industrial crude materials may coexist in two oxidation states, As(III) and As(V), and the subsequent co-precipitation mechanism of ferric hydroxide has extremely low adsorption efficiency for highly soluble As(V) (arsenic acid). In step b, a chemical reducing agent is added to the selenite aqueous solution to quantitatively reduce all high oxidation state arsenic impurities to a preset low oxidation state that is easily adsorbed, namely As(III) (arsenic acid). The chemical reducing agent is selected such that the reduction potential is sufficient to reduce As(V) but will not reduce the main product Se(IV) (arsenic acid). The chemical reducing agent can be high-purity oxalic acid or high-purity sodium sulfite. Taking oxalic acid as an example, it reacts with As(V) in an acidic solution to produce As(III), carbon dioxide, and water. The reaction products are easy to handle and do not introduce new metal or halogen contamination. After adding the reducing agent, it can be stirred and kept warm for a period of time, for example at 30°C. Up to 50 The reaction should be allowed to proceed for 15 to 30 minutes to ensure the complete reaction of the valence regularization step. When performing step b, the stoichiometry of the added chemical reducing agent, such as oxalic acid or sodium sulfite, should be determined. The procedure includes: taking a representative sample of the batch of crude selenium dioxide raw material and dissolving it according to step a to prepare a standard concentration of selenite acid aqueous solution; using analytical methods such as liquid chromatography-atomic fluorescence spectrometry or inductively coupled plasma mass spectrometry (ICP-MS), the initial molar concentration of key high-valence impurities (such as As(V) and Te(VI)) in the solution should be determined. Based on the chemical equation for the reduction reaction, calculate the... The theoretical stoichiometric amount of reducing agent required to completely reduce all impurities to a predetermined low oxidation state, such as As(III). In industrial production, to cope with fluctuations in raw material batches and ensure complete reaction, the actual amount added... Usually set to The concentration is 1.05 to 1.20 times higher to ensure stable and reliable valence state regulation in step b; in step c, two key materials are added to the solution after valence state regulation is completed, the first being ferric nitrate (…). Ferric nitrate was chosen as a sacrificial iron salt auxiliary agent, instead of ferric chloride or ferric sulfate as commonly used, in order to avoid the introduction of the latter, which are highly thermally stable and difficult to remove in subsequent crystallization steps. or Secondary pollution from anions, nitrate anions introduced by ferric nitrate ( ) is a thermally unstable anion, which is thermally decomposed and removed during the subsequent thermal dehydration process in step f. The second is a hydrolyzable alkaline precursor substance, which, in a preferred embodiment, is high-purity urea ( ).

[0029] In performing step c, the determination procedure for the amount of ferric nitrate additive and the hydrolyzable alkaline precursor (urea) added includes: the amount of ferric nitrate added (in Fe³⁺). + The addition of ferric hydroxide flocs is based on a dual criterion. To ensure the formation of ferric hydroxide flocs with good adsorption properties, the addition amount must reach a minimum engineering concentration threshold, which is determined through preliminary experiments (e.g., 300 ppm to 800 ppm). This addition amount must simultaneously meet the requirements of its concentration relative to the total molar concentration of the trace impurities to be removed. There exists a sufficient molar ratio, for example, Fe³⁺. + / The ratio should be no less than 10:1, and adjustments should be made during application. The analysis and verification were performed to confirm whether the set engineering concentration met the molar ratio requirement. The amount of urea added was determined based on the total acid equivalent of the system, which included the acidity of selenite itself and the proton equivalent released by the hydrolysis of ferric nitrate. The specific calibration method was to take a sample of selenite aqueous solution with the determined amount of ferric nitrate added, and perform potentiometric titration with a standard concentration of NaOH solution at room temperature to determine the total molar amount of NaOH consumed to bring the solution pH to the midpoint of the target range, such as 3.5. The actual amount of urea added in the project is set to be [amount missing]. The stoichiometric concentration is 1.2 to 1.8 times the chemical equivalent, ensuring that the pH value can stably enter the target precipitation range of 3.0 to 4.0 under the heating and back pressure conditions in step d. Step d is used to achieve homogeneous co-precipitation. The traditional operation of adjusting the pH by externally adding alkali solution will create a momentary local strong alkaline environment at the dropping point, causing the iron salt to precipitate before it is fully mixed with the impurities, which will greatly reduce the adsorption efficiency. This technical solution heats the selenite aqueous solution, for example, to 70°C. Up to 90 The range of urea added in step c is used to initiate a slow and uniform hydrolysis reaction, urea hydrolysis ( ) alkali is generated in situ throughout the entire solution volume. This causes the pH of the solution to rise slowly and synchronously from acidic. When the pH slowly rises to a preset range, preferably 3.0 to 4.0, the iron cations in the solution ( Hydrolysis, simultaneously and slowly forming high surface area ferric hydroxide (Fe(OH)₂). ) flocculents, As a highly efficient adsorbent, the flocculant quantitatively adsorbs and encapsulates the low-oxidation-state arsenic impurities (As(III)) after the regulation in step b, forming coprecipitated flocculants.

[0030] Furthermore, the hydrolysis of urea will produce carbon dioxide ( ). The gas, when heated and bubbled, is prone to foaming, which may entrain selenite solution and cause material loss. To solve this problem, in a preferred embodiment, the heating step d is performed in a closed container under a preset back pressure greater than atmospheric pressure. This back pressure can be set to a gauge pressure of 0.1 MPa to 0.5 MPa, and is forcibly increased according to Henry's Law. Its solubility in the aqueous phase allows it to preferentially dissolve in selenite aqueous solution rather than escaping as bubbles, thus inhibiting foam formation and promoting dissolution. With the generated An ammonium bicarbonate / ammonium carbonate buffer system can also be formed in situ in the solution. This buffer system synergistically enhances the smoothness and controllability of the pH rise process. Step e is a solid-liquid separation step, in which the solid co-precipitated flocs formed in step d are separated by conventional physical separation methods, such as plate and frame filtration or centrifugation. The -As complex was completely removed, and after separation, a purified aqueous solution of selenite was obtained, derived from the nitrate anion of the ferric nitrate adjuvant ( As a highly soluble ion, with purified Together, they are retained in the aqueous solution, awaiting processing in subsequent step f; step f is a thermal dehydration and product regeneration step, in which the purified selenite aqueous solution carrying nitrate anions obtained in step e is pumped into the crystallization reactor, and the aqueous solvent is evaporated by heating within a preset temperature range. and Equilibrium in water ( It will shift to the right due to the decrease in water activity, resulting in For the precipitation of high-purity crystals, the preferred preset temperature range is 105°C. Up to 130 The key feature of this technical solution is that it reuses the heating function of this step: at 105 Up to 130 Temperature and acidity ( Under concentration conditions, the residual nitrate anions ( As a thermally unstable anion, it undergoes thermal decomposition reactions, with the main decomposition products being nitrogen oxides. Gaseous products such as ) escape in the gaseous phase along with water vapor and are completely discharged from the reactor, passing through the cations in step e ( Physical removal and step f of anions ( Thermochemical removal is combined to construct a closed-loop removal process, solving the secondary pollution problem caused by sacrificial additives. The process includes a precipitation and maturation step after step d and before step e. The hydroxide coprecipitate flocs formed in step d are initially high-viscosity, high-resistivity colloids. These colloids easily clog filter cloth pores in industrial filtration, leading to filtration difficulties. The maturation step guides the precipitate to undergo a thermodynamic spontaneous transformation by maintaining the selenite acid aqueous solution containing the coprecipitate flocs at a maturation temperature and for a certain maturation time. This involves the dehydration and rearrangement of the amorphous colloid into a denser, more easily filtered particulate form. The maturation temperature can be 40°C. Up to 70 .

[0031] To achieve precise control over the maturation endpoint and avoid filter clogging due to insufficient time or energy waste due to redundant time, the precipitation maturation step can include dynamic control based on process parameters. This involves operating a stirrer at a constant speed to agitate the selenite acid aqueous solution. The stirrer is motor-driven, and the system monitors the motor torque in real time. The maturation process is a sudden change in the system's rheological properties, transforming it from a high-viscosity colloid to a low-viscosity particulate suspension. This transformation is directly reflected in the decrease in the torque required by the stirrer motor to overcome fluid resistance at a constant speed. Therefore, the system can be dynamically controlled based on process parameters. The time-varying rate of torque dynamically determines the endpoint of the maturation time. In the specific judgment logic, the endpoint is determined as follows: if the time-varying rate of torque remains below the preset torque change threshold for 5 consecutive minutes within a preset time period, it indicates that the system viscosity has decreased to a stable plateau region, and the maturation process is complete. Step f, thermal dehydration and crystallization, has a decisive impact on the physical purity of the final product. If the heating power is too high, resulting in an excessively fast evaporation rate, the supersaturation of the solution will increase dramatically, triggering a nucleus explosion (nucleation). The massive number of tiny crystals generated by the nucleation explosion are highly susceptible to encapsulating the mother liquor (i.e.,...) during rapid growth. The presence of an aqueous solution (resulting in trace amounts of volatile impurities in the final product after drying) prevents it from meeting high purity requirements. To achieve controlled growth during crystallization, in a preferred embodiment, step f, the thermal dehydration treatment, further includes closed-loop feedback control based on turbidity signals. An online turbidity meter is installed in the circulation pipeline or sight glass of the crystallizer to monitor the purified selenite aqueous solution in real time. turbidity Compared to the previous moment turbidity The system calculates the turbidity change rate based on this. Crystal nucleation bursts physically manifest as a transient, nonlinear spike in solution turbidity, i.e. The value increases sharply; this closed-loop control system utilizes As a characterization of crystallization rate, the heating power of the closed-loop control thermal dehydration process is used. , in accordance with The calculation and control follow these rules: a preset rate of change threshold is set to characterize the nucleation burst. Preset base heating power and the preset reduced heating power , It can be zero power or the basic power consumption to maintain system operation. During operation, if The controller will automatically adjust the heating power. Set as Suspend or significantly reduce the heating input so that the supersaturation in the solution is consumed by the existing crystal growth, thereby inhibiting the formation of new crystal nuclei; if The controller will then adjust the heating power. Restore to Continue evaporating the solvent to dynamically lock the crystallization process in place. The defined nucleation edge ensures that the crystal grows in a controlled manner under low supersaturation.

[0032] Example 1: A batch of industrial crude selenium dioxide raw material used for purification was analyzed and found to contain trace amounts of trivalent arsenic As(III) and high concentrations of pentavalent arsenic As(V). The purification target was 6N level, i.e., an ultrapure product with a purity of 99.9999%. If a conventional wet process is used, such as using ferric chloride... As an adjuvant and by directly adjusting pH for precipitation, highly soluble As(V) cannot be precipitated. Effective adsorption and penetration of the purification process; simultaneously introduced... Anions, being thermally stable ions, cannot be removed during the thermal dehydration crystallization process and will be trapped within... Secondary anion contamination occurs within the crystals. Both factors prevent the product from reaching the 6N level. The specific operating steps of the method of this invention are as follows: Dissolve this batch of crude selenium dioxide raw material in high-purity water according to step a to form an aqueous solution containing As(III) and As(V); Perform step b, add high-purity oxalic acid to the solution, and heat to 40°C. After reacting for 20 minutes, the As(V) impurities were quantitatively normalized into the easily adsorbed As(III) form, and all arsenic impurities in the solution were in the preset low oxidation state that could be targeted in step d. Step c was then performed, adding a preset amount of high-purity ferric nitrate as a sacrificial iron salt auxiliary to the solution. Simultaneously, high-purity urea is added as a hydrolyzable alkaline precursor. Step d is performed, and this mixed solution is pumped into a sealed reactor. Under a preset back pressure of 0.2 MPa gauge pressure, the temperature is slowly increased to 85°C. Under these conditions, urea undergoes homogeneous hydrolysis, and the in-situ generated... This causes the pH of the entire solution to rise slowly and synchronously from its initial acidity, while hydrolysis-related processes... The gas is forced to dissolve under back pressure, eliminating foam entrainment, and the resulting buffer system makes the pH rise process smoother; when the pH value slowly and homogeneously reaches 3.5, the iron cations of ferric nitrate... Hydrolysis forms high surface area During the formation of flocs, all arsenic impurities in the solution in the As(III) form are efficiently adsorbed and encapsulated, forming co-precipitated flocs. Step e involves plate and frame filtration of the slurry, where the solid phase... -As complex flocs are trapped and removed, at which point the cationic sacrificial adjuvant is removed. The anions of the additives have been physically removed; It then penetrates the filter cloth and enters the next process together with the purified selenite aqueous solution.

[0033] Execute step f, and obtain the information from step e. The purified anion-containing selenite aqueous solution was pumped into a thermal dehydration crystallization reactor, and the reactor temperature was controlled at 115°C. Within this preset temperature range, the heating function is reused: firstly, the aqueous solvent evaporates, and secondly, the selenite solution is concentrated to achieve high purity. Crystals begin to precipitate; secondly, under the dual conditions of high temperature and acidic concentration, the residual crystals in the solution... The anions undergo thermal decomposition, producing gaseous products such as nitrogen oxides, which escape from the system along with water vapor, thus achieving the chemical removal of the additive anions. The regularization of the valence state in step b, combined with the homogeneous precipitation in step d, solves the problem of As(V) penetration. Step c uses ferric nitrate in combination with the thermal dehydration in step f, achieving secondary anion contamination of conventional additives through a relay of physical precipitation of cations and thermal decomposition of anions. From this batch of raw materials, ultrapure selenium dioxide crystals with both chemical and physical purity meeting the 6N level requirements are obtained.

[0034] Example 2: This example uses the same batch of industrial crude selenium dioxide as raw material, containing trace arsenic impurities of 215.4 ppm, of which As(V) account for approximately 48%, or 103.1 ppm, and iron impurities of 52.8 ppm. A comparative experiment was conducted using one sample group and seven control groups (A to G). Each group used 1 kg of the above crude raw material dissolved in 10 L of high-purity water to form a selenite acid aqueous solution as the starting material. The trace impurities in the final product included As, Fe, and... , The content of selenium was determined by inductively coupled plasma mass spectrometry (ICP-MS) or ion chromatography (IC), and the recovery rate of selenium was calculated by material balance. The process flow of the sample group of the present invention was carried out according to the preferred parameters in the specific embodiment: Step b, add stoichiometric amounts of oxalic acid to the feed solution, and at 40 Stir the reaction for 20 minutes; in step c, add high-purity ferric nitrate (as shown in the image). (Calculate to achieve a final concentration of 500 ppm) and add 50 g of high-purity urea; step d, in a sealed reaction vessel, apply a gauge pressure of 0.2 MPa and heat to 85°C. Maintain the temperature and use urea hydrolysis to slowly raise the pH to 3.5, then stop heating; step e: filter and separate the co-precipitated flocs; step f: pump the purified selenite aqueous solution into the crystallization vessel and heat at 120°C. Thermal dehydration treatment is performed to collect the precipitated water. Crystals; Control group A: Same process as the sample group of the present invention, except that step b, adding oxalic acid, is not performed; Control group B: Same process as the sample group of the present invention, except that urea is not added, and in step d, high-purity NaOH solution is added externally under normal pressure to adjust the pH value to 3.5; Control group C: Same process as the sample group of the present invention, except that in step c, ferric nitrate is replaced with equimolar iron-containing analytical grade ferric chloride. Control group D: The process is the same as the sample group of the present invention, except that in step d, heating is stopped when the pH value rises to 2.5. Control group E: The process is the same as the sample group of the present invention, except that in step d, heating is continued until the pH value rises to 5.0. Control group F: The process is the same as the sample group of the present invention, except that in step f, the temperature of the thermal dehydration treatment is set to 95°C. Control group G: The process is the same as that of the sample group of this invention, except that in step f, the temperature of the thermal dehydration treatment is set to 140°C. The purity index and process recovery rate data of the final high-purity selenium dioxide product obtained from each group of experiments are shown in Table 1.

[0035] Table 1: Comparison of experimental results of AG between the sample group and the control group of this invention.

[0036]

[0037] Table 1 shows that the sample group of this invention uses the complete process, and all trace impurities As, Fe, etc. in the final product are eliminated. , All were controlled below 0.1 ppm, and The recovery rate reached 98.2%, achieving high purity purification and high recovery rate; the As content of control group A (missing step b) and control group B (using external alkali addition) were 101.3 ppm and 35.8 ppm, respectively, which were much higher than the 0.08 ppm of the sample group of this invention, confirming the necessity of pre-chemical reduction in step b for treating high-valence As(V) and that the co-precipitation adsorption efficiency of homogeneous precipitation in step d is improved compared with the conventional alkali addition method; control group C (using ) and control group F (95) Data from low-temperature heat treatment confirmed the synergistic effect of the anion removal steps; control group C... The content (78.4 ppm) and the control group F were... The content (65.2 ppm) was severely exceeded, indicating that conventional ferric chloride additives can introduce substances that are difficult to remove. Secondary pollution, and the ferric nitrate additive used in this invention leaves residues. Effective thermal decomposition and removal are only possible within a specific temperature range in step f, indicating that the selection of the auxiliary agent and the heat treatment temperature need to be coordinated; control groups D (pH 2.5), E (pH 5.0), and G (pH 140) Data from high-temperature heat treatment confirms the rationality of the critical process parameter boundaries; excessively low pH (2.5) leads to... Incomplete precipitation resulted in a significant decrease in the removal rates of both As and Fe; excessively high pH (5.0) or excessively high heat treatment temperature (140°C) also contributed to the problem. All of these led to The recovery rates decreased to 94.5% and 91.3%, respectively, indicating that the pH range of 3.0-4.0 and the heat treatment temperature range of 105°C defined in this invention are within acceptable limits. -130 It is the result of a technical balance between achieving high purity and high recovery rate.

[0038] Example 3: This example combines Figures 1 to 2 A method for distillation purification to remove trace metal impurities from selenium dioxide is described, such as... Figure 1 As shown, the process begins with crude selenium dioxide raw material, which is dissolved to form a selenite aqueous solution, then enters the purification sequence. A pre-chemical reduction step is performed to achieve the transformation of ordered impurities from As(V) to As(III), followed by a homogeneous co-precipitation step. In this step, impurities are captured by in-situ alkali generation and ferric salt hydrolysis. The precipitated material then enters a precipitation maturation step, transforming the colloid into easily filterable particles. Solid-liquid separation is then performed to separate the co-precipitated flocs. The purified solution after separation finally enters a thermal dehydration and crystallization step. After crystal precipitation and pyrolysis of anions, the final product, high-purity selenium dioxide, is obtained. This process integrates two key closed-loop control systems: one for the precipitation maturation step, which uses torque monitoring and endpoint determination to achieve rheological-based closed-loop control; and the other for the thermal dehydration and crystallization step, which uses turbidity signal closed-loop feedback control to monitor the turbidity change rate in real time. This is to achieve closed-loop control of the heating power P, thereby preventing crystal nucleus eruption and ensuring physical purity.

[0039] like Figure 2 As shown in the diagram, this figure illustrates the collaborative working logic of four units—operator / system, reactor, stirrer and motor, and torque sensor—after the precipitation and maturation step begins. The process is as follows: The operator / system issues a set maturation temperature of 40°C to the reactor. Up to 70 The command sends an instruction to the agitator and motor to start the agitator at a constant speed, initiating a continuous monitoring phase. During this phase, the agitator and motor drive the agitator to generate torque by sending a stirring motion to the torque sensor, which then provides real-time torque values ​​to the operator / system. After receiving the signal, the operator / system executes an internal judgment procedure, including calculating the torque change rate. The system determines whether the rate of change is consistently below the threshold. If the rate of change is consistently below the threshold, the maturation process is considered complete and ends, proceeding to the next step. If the rate of change is above the threshold, the maturation conditions are maintained.

[0040] Example 4: This example is used to verify the effect of the sedimentation and maturation step on the physical filterability of coprecipitated flocs. Two slurries containing the same coprecipitated flocs were prepared according to steps a to d (homogeneous coprecipitation to pH 3.5) of the sample group in Example 2 of the present invention. Each sample had a volume of 5 L and was labeled as test group A and test group B, respectively. Test group A (sample group of the present invention): The slurry was subjected to the sedimentation and maturation step according to the specific implementation method, that is, at 55 °C... At the desired maturation temperature, the mixture was stirred at a constant speed of 60 rpm until the motor torque signal remained below the preset torque change threshold of 0.005 N·m for 5 consecutive minutes, indicating that maturation was complete. The total maturation time was 1.3 hours. Experimental Group B (control group): The sedimentation maturation step was not performed; the mixture immediately proceeded to the filtration process after step d. The slurries from Experimental Group A and Experimental Group B were pumped into the same laboratory-scale small plate and frame filter press. This filter press used polypropylene filter cloth of the same specification and was set to a constant feed pressure of 0.4 MPa. The time required for each group to filter all 5 L of slurry was recorded. The content of the filter cake was then analyzed. Solid content characterizes the dewatering effect. The slurry in test group A (after maturation) had a smooth filtration process with a total filtration time of 18.5 minutes, and the resulting filter cake was dense with a solid content of 32.8 wt%. The slurry in test group B (unmatured) showed a sharp drop in filtration flux about 2 minutes after the start of filtration, exhibiting typical filter cloth clogging, indicating that the precipitate was still in a high specific resistance colloidal form. The total filtration time for 5L of slurry was as long as 85.2 minutes, and the filter cake was soft with a solid content of only 11.2 wt%. The experimental data show that the sedimentation and maturation step shortens the filtration time by about 78% and improves the dewatering efficiency of the filter cake.

[0041] Example 5: This example describes a standardized calibration procedure for determining key process control parameters. The first calibration procedure targets the turbidity change rate threshold for thermal dehydration crystallization in step f. The threshold value is determined and used for closed-loop control of heating power. Under different operating conditions, this threshold is related to the reactor volume, stirring efficiency, and material concentration. This procedure adopts an offline calibration method based on induced nucleation: Take a portion of the purified selenite aqueous solution to be crystallized prepared according to steps a to e, and place it in a calibration vessel with geometry similar to the production equipment and equipped with the same online turbidimeter; start stirring, using a power much greater than the normal production baseline. The induced heating power impacts and heats the solution, causing it to reach a high supersaturation level in a short time, triggering crystal nucleation. During this process, the control system records the turbidimeter readings in real time at a high sampling rate (set to 1Hz in this procedure) and calculates the turbidity change rate. Analysis of the collected data shows that the turbidity curve exhibits the steepest inflection point at the nucleus site, corresponding to... The peak value, or the preset fraction of that peak value, is set to 80% in this procedure, which is calibrated as the threshold for the rate of change of turbidity in this specific system. ; the calibration procedure obtained A numerical value, such as 1.2 NTU / s, is input into the production control system and can be used as the basis for execution in subsequent batch production. or The quantitative basis for judging the rules.

[0042] The second calibration procedure addresses the dynamic determination of the maturation time endpoint in the sedimentation and maturation step, resolving the issue that using a fixed threshold is insufficient to accommodate the differences in rheological properties between different batches of materials. It employs an online judgment method based on signal statistical characteristics: at the start of the sedimentation and maturation step, the system operates the agitator at a constant speed, monitoring the motor torque signal in real time; the control system sets a sliding time window, the length of which... In this procedure, the standard deviation of the torque signal sequence acquired within the window is continuously calculated every 5 minutes. In the early stages of maturation, the precipitate is in a high-viscosity colloidal state, with high and fluctuating torque values. The viscosity is relatively high; as maturation proceeds, the colloid transforms into particles, the system viscosity decreases, and the torque value decreases accordingly; when maturation is complete, the system enters a stable particle suspension state, the viscosity no longer changes, and the torque signal correspondingly enters a low plateau region within the sliding time window. Torque signal standard deviation within It will approach a minimum value, which corresponds to the signal-to-noise baseline; the system sets the ripening endpoint judgment logic as follows: when Within a preset time period, which is set to be 1 minute in this procedure, if the temperature remains below the statistical threshold characterizing the signal noise baseline (set to 0.005 N·m), the system determines that the ripening process is complete and automatically triggers the separation process in step e.

[0043] Example 6: In industrial continuous production, fluctuations in the source of crude selenium dioxide raw materials between batches can lead to inconsistent valence state distributions of trace impurities. Besides the conventional As(V), it may contain tellurium impurities in a high oxidation state, such as Te(VI). Therefore, the selection of the chemical reducing agent in step b needs to be adjusted accordingly. To determine the appropriate chemical reducing agent for step b, before processing a new batch of crude selenium dioxide raw materials, a representative sample from that batch is taken, dissolved according to step a, and divided into three equal test solutions. High-purity oxalic acid is added as a chemical reducing agent in the first solution, high-purity sodium sulfite is added as a chemical reducing agent in the second solution, and the third solution serves as a blank control without any chemical reducing agent. All three solutions are processed according to the same... The subsequent process parameters are executed sequentially as follows: step c involves adding ferric nitrate and urea; step d involves homogeneous coprecipitation; and step e involves filtration. Three portions of the purified selenite aqueous solution obtained after step e are taken and their residual trace impurities, including the total concentration of As and Te, are detected by ICP-MS. The three sets of data are compared. If the total impurity concentration of the blank control group meets the purity requirements of the final product, then step b is not required for this batch of raw materials. If the total impurity concentration of the blank control group does not meet the requirements, then the chemical reducing agent that can reduce the total concentration of trace impurities between the first and second portions of the raw materials is selected as the process setting for step b of the entire batch of raw materials.

[0044] Example 7: This example describes a standardized engineering calibration procedure for determining the preset back pressure in step d and the maturation temperature in the precipitation maturation step between steps d and e; the first procedure is used to determine the preset back pressure during homogeneous hydrolysis in step d, which must be sufficient to suppress the hydrolysis of urea. Foam, but not too much, to avoid excess. Dissolution resulted in an excessively acidic carbonate buffer system, preventing the solution pH from rising to the target range of 3.0-4.0. Five portions of the same starting solution as in Example 2 were placed in laboratory autoclaves equipped with viewing windows, and the same amounts of ferric nitrate and urea were added to each sample according to step c. The five portions of solution were heated to 85°C under the same heating program at constant gauge pressures of 0.05 MPa, 0.1 MPa, 0.3 MPa, 0.5 MPa, and 0.6 MPa, respectively. The temperature was maintained; foam formation was observed through a viewing window, and the final pH value after each group reached thermal equilibrium was recorded. The experimental results showed that severe foaming occurred at a gauge pressure of 0.05 MPa, posing a risk of material overflow; slight foaming occurred at a gauge pressure of 0.1 MPa, with a final pH of 3.8; no foaming occurred at gauge pressures of 0.3 MPa and 0.5 MPa, with final pH values ​​of 3.6 and 3.4 respectively, both falling within the target preset range; no foaming occurred at a gauge pressure of 0.6 MPa, with a final pH value of only 2.9, failing to reach the target preset range required for precipitation; therefore, 0.1 MPa to 0.5 MPa was determined to be the preset back pressure working range suitable for this system.

[0045] The second procedure is used to determine the optimal maturation temperature for the precipitation maturation step, ensuring that the colloidal form of the precipitate transforms into easily filterable particulate form within a reasonable time, avoiding excessively high temperatures that could lead to increased energy consumption or potential side reactions. Four portions of the slurry containing co-precipitated flocculants obtained after step d are placed in maturation kettles equipped with constant-speed stirrers and torque-monitoring motors. The four portions of slurry are then heated to 30°C. 40 55 and 70 The temperature was kept constant at the curing temperature, and the motor torque was continuously monitored and the rate of change of torque over time was recorded until the curing endpoint was reached. The test results showed that at 30°C... If the motor torque fails to reach a stable plateau within 5 hours, it indicates incomplete ripening; at 40... 55 and 70 The times required to reach the ripening endpoint were 2.8 hours, 1.2 hours, and 0.8 hours, respectively; further analysis of the filtrate revealed that 70 Trace amounts of selenium were detected in the filtrate under aging conditions, while 40 With 55 No significant dissolution was observed in the group; considering both maturation efficiency and process stability, 40 was determined. Up to 70 The effective ripening temperature range, of which 55 Optimal parameters for balancing processing time and energy consumption.

[0046] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A distillation purification method for removing trace metal impurities from selenium dioxide, characterized in that, include: Step a: Dissolve crude selenium dioxide containing trace metal impurities in an aqueous solvent to form an aqueous solution of selenite. Step b: Add a chemical reducing agent to the selenite aqueous solution to reduce the high oxidation state arsenic impurities in the trace metal impurities to a preset low oxidation state; Step c: Add ferric nitrate as a sacrificial iron salt auxiliary to the aqueous solution of selenite, and add a hydrolyzable alkaline precursor. Step d: Heating the selenite aqueous solution causes the hydrolyzable alkaline precursor to hydrolyze and generate alkali in situ, raising the pH of the selenite aqueous solution to a preset range. The iron cations of ferric nitrate hydrolyze to form ferric hydroxide precipitate, which in turn forms a coprecipitate flocculant with the preset low-oxidation-state arsenic impurities. Step e: Separate the coprecipitated flocs from the aqueous selenite solution to obtain a purified aqueous selenite solution carrying nitrate anions derived from ferric nitrate. Step f involves thermally dehydrating the purified selenite aqueous solution by heating within a preset temperature range to evaporate the aqueous solvent and precipitate high-purity selenium dioxide crystals, while also causing the nitrate anions to undergo thermal decomposition and escape as gaseous products.

2. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, After step d and before step e, a precipitation and maturation step is also included: maintaining the selenite aqueous solution containing co-precipitated flocs at a maturation temperature and for a maturation time; the precipitation and maturation step includes: running a stirrer at a constant speed to stir the selenite aqueous solution, the stirrer being driven by a motor; monitoring the torque of the motor in real time, and dynamically determining the endpoint of the maturation time based on the time change rate of the torque; wherein, the endpoint is determined when the time change rate of the torque is continuously lower than a preset torque change threshold within a preset time period.

3. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, The chemical reducing agent in step b is oxalic acid.

4. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, The alkaline precursor that can be hydrolyzed in step c is urea.

5. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, Step d, heating the aqueous selenite solution, is performed in a closed container under a preset back pressure greater than atmospheric pressure, so that the carbon dioxide generated by the hydrolysis of the hydrolyzable alkaline precursor dissolves in the aqueous selenite solution.

6. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 2, characterized in that, The curing temperature is 40°C. Up to 70 .

7. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, The preset temperature range in step f is 105°C. Up to 130 .

8. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, Step f, the thermal dehydration process, also includes: real-time monitoring of the purified selenite aqueous solution at the current moment. turbidity Compared to the previous moment turbidity ; Calculate the rate of change of turbidity And the heating power of the thermal dehydration process is controlled in a closed loop. ; in accordance with Calculations and controls follow these rules: If ,but ;like ,but ;in, To preset the rate of change threshold, The preset reduced heating power This is the preset base heating power.

9. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, The preset range in step d is pH value from 3.0 to 4.

0.

10. The distillation purification method for removing trace metal impurities from selenium dioxide according to claim 1, characterized in that, The chemical reducing agent in step b is sodium sulfite.

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