A method for preparing high-purity metallic arsenic by one-step reduction based on precursor purification

Abstract

The application discloses a method for preparing high-purity metallic arsenic based on one-step reduction of a precursor, and belongs to the technical field of high-purity metallic arsenic preparation. The method uses industrial arsenic acid complex salt as raw material, and the arsenic acid complex salt is slowly added with alkali liquor under the synergistic action of an ultrasonic field and strong stirring to make the arsenic acid complex salt controlled crystallization after washing with dilute alkali water, dissolving with dilute acid and filtering with microporous. The obtained crystallization liquid is aged, washed with dilute alkali water and filtered to obtain arsenic acid complex salt crystals, and high-purity arsenic acid complex salt precursors are obtained through multiple dissolution-crystallization cycles, and then dried and reduced to obtain high-purity metallic arsenic of 5N or above. The high-purity control link is moved forward to the arsenic acid complex salt precursor stage, the dependence on the rear-end purification processes such as multiple sublimation, vacuum distillation or chlorination rectification in the traditional crude arsenic / white arsenic route can be reduced, and the method has the advantages of short process, low cost, low equipment requirement and strong raw material adaptability.

Description

Technical Field

[0001] This invention relates to a method for preparing high-purity metallic arsenic, and particularly to a method for preparing high-purity metallic arsenic by using industrial arsenate complex salt as raw material through in-situ purification of precursor and one-step reduction, belonging to the field of metallic arsenic synthesis technology. Background Technology

[0002] High-purity arsenic typically refers to elemental arsenic with a purity of 5N (99.999%) or higher. It is a crucial raw material for preparing compound semiconductor materials such as gallium arsenide (GaAs) and indium arsenide (InAs), and is also widely used in photoelectric conversion, infrared detection, lasers, high-speed communication, and solar photovoltaics. As downstream semiconductor materials increasingly demand higher purity and batch stability from raw materials, the purity requirements for metallic arsenic in some applications have reached 6–7N, or even 7N5. Therefore, the impurity content in the arsenic source material and the difficulty of subsequent purification directly affect the quality of high-purity arsenic and related arsenic-based semiconductor materials.

[0003] Currently, the industrial preparation of high-purity arsenic mostly uses crude arsenic or white arsenic as starting materials, and then gradually removes impurities through multiple sublimations, vacuum distillations, chlorination-distillation, and other methods. Although this route is relatively mature, it is essentially still a terminal purification process after reduction or in the metallic arsenic stage, and is highly dependent on the initial purity and impurity composition of the raw materials. Crude arsenic and white arsenic usually come from non-ferrous smelting, flue dust treatment, or other arsenic-containing material processing, and often contain metallic impurities, sulfur, oxygen, and other non-metallic impurities, with complex impurity morphologies. These impurities are difficult to separate during subsequent high-temperature purification, resulting in a long process flow, high energy consumption, and high equipment requirements. Especially when further improving the purity to 6N level and above, product stability and production costs are both limited.

[0004] Arsenic-containing solid waste from non-ferrous smelting is a significant source of arsenic in industrial systems. In recent years, the selective separation of arsenic using arsenate double salt mineralization precipitation has attracted considerable attention, resulting in relevant engineering practices and patented technologies. For example, Chinese patent (authorization announcement number CN114873647B) discloses a method for the harmless and resource-based treatment of arsenic-containing solid waste from non-ferrous smelting. Specifically, the arsenic-containing solid waste from non-ferrous smelting undergoes oxidative alkaline leaching and magnesium ammonium reagent precipitation to obtain struvite-type arsenate double salt mineral precipitates. Due to the selectivity of arsenate ions with specific cations in terms of coordination structure, charge matching, and crystal configuration, the formation of arsenate double salts can, to some extent, reduce the simultaneous entry of similar elements such as antimony, selenium, and sulfur. Building upon this, existing technologies further propose using arsenate complex salts as precursors to prepare metallic arsenic via high-temperature carbon reduction. For example, Chinese patent (authorization announcement number CN114836635B) discloses a short-process method for preparing high-purity metallic arsenic from arsenic-containing solid waste in non-ferrous smelting. Specifically, it involves precipitating arsenic-containing alkaline leachate with a mixed magnesium ammonium reagent to obtain arsenate complex salt crystals; mixing the arsenate complex salt crystals with carbon powder for reduction roasting; and condensing and recovering metallic arsenic from the flue gas. However, existing arsenate complex salt routes mainly utilize the selective separation effect of the mineralization precipitation stage, i.e., obtaining relatively clean arsenic source raw materials through the precipitation process, while paying insufficient attention to further purification of the arsenate complex salt precursor before reduction and crystallization quality control. In actual industrial arsenate complex salts, small amounts of antimony, selenium, sulfur, and metallic impurities may still remain. These impurities can enter the arsenate complex salt crystals through surface adsorption, intercrystalline entrainment, encapsulation, or local co-crystallization. If direct reduction is carried out, the purity of the obtained metallic arsenic is usually difficult to reach the level of high-purity arsenic. To further obtain high-purity metallic arsenic, existing processes often require further deep purification treatments such as sublimation, vacuum distillation, or chlorination-distillation at the metallic arsenic stage. These processes remain lengthy and involve high equipment and energy costs. Therefore, it is necessary to start from the arsenate precursor stage to reduce the residue and entrainment of impurities in the precursor crystals, making them more suitable for subsequent reduction to prepare high-purity metallic arsenic. Summary of the Invention

[0005] Traditional methods for preparing high-purity metallic arsenic often use crude or white arsenic as starting materials and rely on multiple sublimation, vacuum distillation, or chlorination distillation processes for purification. This results in long processes, high energy consumption, demanding equipment requirements, and strong dependence on the initial purity of the raw materials. The present invention aims to provide a one-step reduction method for preparing high-purity metallic arsenic based on precursor purification. This method uses industrial arsenate complex salt as raw material. Before reduction, the industrial arsenate complex salt undergoes a special purification treatment, which significantly reduces the adsorption, entrainment, and encapsulation of antimony, selenium, sulfur, and other metallic impurities in the arsenate complex salt crystals. This allows for the production of high-purity metallic arsenic of 5N grade or higher through high-temperature reduction. This method shifts the main impurity control process in the reduction of arsenate complex salt to the arsenate complex salt precursor stage, reducing the reliance on complex deep purification of the reduced metallic arsenic. It features a shorter process, simpler operation, and lower equipment requirements.

[0006] To achieve the above objectives, the present invention provides a one-step reduction method for preparing high-purity metallic arsenic based on precursor purification, comprising the following steps:

[0007] 1) The industrial arsenate double salt was washed and filtered with dilute alkaline water A to obtain a preliminarily purified arsenate double salt;

[0008] 2) After dissolving the preliminarily purified arsenate complex salt in dilute acid solution B, filter it through a microporous membrane to obtain the filtrate;

[0009] 3) Place the filtrate in an ultrasonic field and slowly add alkaline solution C to the filtrate under stirring conditions to achieve controlled crystallization of arsenate double salt and obtain a crystallized solution;

[0010] The controlled crystallization conditions are as follows: ultrasonic power density of 0.05~0.5W / mL, ultrasonic frequency of 20~100 kHz, stirring speed of ≥500rpm, alkali addition rate to control pH increase rate of ≤0.1 / min, final pH of 7~10, and temperature of 10~40℃.

[0011] 4) After aging, the crystallizing solution is washed and filtered with dilute alkaline water D to obtain arsenate double salt crystals;

[0012] 5) Use the obtained arsenate double salt crystals as the raw material for the next round, and repeat steps 2) to 4) more than three times to obtain a high-purity arsenate double salt precursor;

[0013] 6) The high-purity arsenate double salt precursor is dried and reduced to obtain high-purity metallic arsenic of grade 5N or higher.

[0014] Industrial arsenate double salts are usually obtained directly from the mineralization precipitation process. They still contain a small amount of antimony, selenium, sulfur and metallic impurities. These impurities can exist in the arsenate double salt crystals through surface adsorption, intercrystalline inclusion, encapsulation or local co-crystallization. The metallic arsenic obtained after direct reduction is difficult to meet the purity requirements of high-purity arsenic and still needs to rely on subsequent purification steps such as sublimation, vacuum distillation or chlorination distillation.

[0015] The key to this invention lies in the multiple dissolution-filtration-crystallization-aging-washing process of industrial arsenate complex salts. Utilizing the highly ordered lattice structure of arsenate complex salts during recrystallization and the differential distribution of impurities within the lattice, impurities are gradually distributed into the mother liquor during multiple cycles, thereby achieving the stepwise stripping of antimony, selenium, sulfur, and metallic impurities. More specifically, the crucial element is the synergistic control of multiple fields—ultrasonic, shear, and temperature—to achieve high crystallinity while effectively preventing impurities from entering the arsenate complex salt crystals, thus obtaining a high-purity arsenate complex salt precursor. Strong shearing (stirring) improves the macroscopic mixing uniformity of the system, reduces local high pH and local supersaturation regions formed during alkali addition, and avoids rapid precipitation and impurity entrainment caused by instantaneous high supersaturation. Ultrasonic fields induce relatively uniform and controllable nucleation behavior in the solution through acoustic nucleation, cavitation, and acoustic flow, and improve mass transfer conditions near the crystal growth interface through microscale mass transfer enhancement, reducing the retention, co-deposition, and non-selective doping of impurity ions on the crystal surface. Temperature can control the crystal growth rate, preventing excessively rapid crystal growth that could lead to impurities entering the crystal. Furthermore, slow alkali addition and subsequent aging and washing further promote crystal structure reorganization and the migration of surface and intergranular impurities to the mother liquor. Through the synergistic effect of these conditions, the crystallization process of arsenate complex salts can be transformed from a non-equilibrium rapid precipitation to a controlled nucleation-stable growth process, yielding high-purity arsenate complex salt precursors with lower impurity content, more regular crystal morphology, and more concentrated particle size distribution. After drying, the precursor can be directly used to obtain high-purity metallic arsenic of grade 5N or higher through one-step high-temperature reduction and low-temperature directional condensation in a high-purity reducing atmosphere. This reduces the reliance on complex deep purification processes such as traditional back-end sublimation, vacuum distillation or chlorination distillation, and forms a high-purity metallic arsenic preparation route that shifts from "back-end purification" to "front-end purification".

[0016] The controlled crystallization conditions of this invention are optimized. For example, the growth of arsenate double salt crystals during crystallization is highly sensitive to temperature. If the crystallization temperature exceeds 40°C, the solubility of arsenate double salt crystals increases sharply with increasing temperature. Under otherwise constant conditions, the yield of arsenate double salt at 60°C decreases from approximately 99% to only 50% compared to 25°C. If the crystallization temperature is too low, co-precipitation of impurity elements is likely to occur. Strong stirring is used during crystallization, with a stirring rate ≥500 rpm, more preferably 500~800 rpm. The main approach utilizes the synergistic control of strong stirring and ultrasound to regulate the nucleation and crystal growth behavior of arsenate double salts. More specifically, strong stirring enhances macroscopic mass transfer and system homogenization, facilitating the rapid elimination of localized supersaturated regions caused by the addition of alkali solution, thus preventing explosive nucleation and non-selective inclusion of impurities due to transient high supersaturation. Simultaneously, the introduction of the ultrasonic field, through acoustic cavitation and acoustic flow effects, creates a scale-controllable transient micro-supersaturated region in the solution, inducing uniform and repeatable nucleation behavior. Furthermore, ultrasound enhances boundary layer mass transfer near the crystal surface, suppressing the retention and co-deposition of impurity ions at the crystal growth interface. Through strict matching and control of stirring intensity, ultrasonic power density, and frequency, the crystallization process can be transformed from non-equilibrium rapid precipitation to a controlled nucleation-stable growth process, resulting in high-quality arsenate double salt crystals with regular crystal shapes, concentrated particle size distribution, and significantly reduced impurity content.

[0017] As a preferred embodiment, the industrial arsenate complex salt includes at least one of ammonium magnesium complex salt, potassium magnesium complex salt, and potassium calcium complex salt. The industrial arsenate complex salts involved in this invention are obtained by leaching and arsenic precipitation of arsenic-containing solid waste from non-ferrous smelting, with a purity of 3N or lower. These industrial arsenate complex salts are "struvite-like" arsenate complex salts, which exhibit high selective enrichment of arsenic during the mineralization and precipitation stage. Their stable crystal lattice configuration and well-defined composition facilitate further amplification of the differences in crystallization behavior between arsenic and impurities during the subsequent "dissolution-recrystallization" process, providing a good phase basis for advanced purification.

[0018] As a preferred embodiment, the pH of the dilute alkaline solution A and the dilute alkaline solution D is not higher than 10. Within the preferred pH range, the structural stability of the arsenate double salt can be guaranteed, while also facilitating the migration and removal of impurities. Furthermore, controlling the pH range appropriately avoids overly acidic or overly alkaline environments, which can also reduce the amount of acid and alkali used and reduce unnecessary costs.

[0019] As a preferred embodiment, the dilute alkaline solution A and the dilute alkaline solution D are at least one of sodium hydroxide solution, potassium hydroxide solution, and ammonia solution.

[0020] The dilute alkaline solution of this invention is prepared by mixing softened water or industrial purified water with at least one of sodium hydroxide, potassium hydroxide, or ammonia. Softened water refers to water with a CaCO3 concentration ≤25 mg / L. In step 1), dilute alkaline solution A mainly serves as an initial coarse wash, therefore the water quality requirements are appropriately lowered, and softened water can be used to save on water production costs. Dilute alkaline solution A in step 1) can also be replaced by the filtrate after recrystallization or the waste liquid after washing. In step 4), dilute alkaline solution D washes the recrystallized arsenate crystals; its water quality requirements are relatively high, and it can be prepared with industrial purified water.

[0021] As a preferred embodiment, the pH of the dilute acid solution is 2-4. Within this preferred pH range, the full dissolution of the arsenate double salt is ensured, while also facilitating the migration and removal of impurities during the dissolution process. Furthermore, controlling the pH range appropriately avoids overly acidic or alkaline environments, reducing the amount of acid and alkali used and minimizing unnecessary costs.

[0022] As a preferred embodiment, the dilute acid solution is at least one of hydrochloric acid solution and nitric acid solution.

[0023] The dilute acid solution of this invention is prepared by mixing industrial purified water with at least one of hydrochloric acid and nitric acid. Industrial purified water refers to water with a resistivity ≥17 MΩ·cm (25℃). By controlling the content of alkaline earth metals and trace ions in the water, non-selective doping during crystallization can be avoided, which helps to ensure the stability of the preparation process of high-purity arsenate double salt.

[0024] As a preferred embodiment, the pore size of the microporous filter membrane is between 0.1 and 1 μm.

[0025] As a preferred embodiment, the microporous filter membrane is made of one of nylon, polytetrafluoroethylene, polyvinylidene fluoride, or polypropylene.

[0026] The present invention selects a suitable microporous filter membrane, which can effectively remove insoluble impurities while avoiding the loss of arsenate double salt crystals, thus providing a guarantee for multiple recrystallization operations.

[0027] As a preferred embodiment, the mass fraction of the alkali solution is 10-15%.

[0028] As a preferred embodiment, the alkaline solution is a sodium hydroxide solution, a potassium hydroxide solution, or an ammonia solution.

[0029] The alkaline solution of the present invention is prepared by mixing industrial pure water with at least one of sodium hydroxide, potassium hydroxide, and ammonia water.

[0030] As a preferred embodiment, the aging conditions are controlled as follows: temperature 10~40℃, time 12~24h, and stirring rate 20~80rpm. The aging (maturation) process plays a crucial role: under certain temperature and mother liquor conditions, the crystal system undergoes a "dissolution-redeposition" maturation behavior (also known as Ostwald maturation) driven by surface energy. This causes small grains or high-defect crystal regions to preferentially dissolve and migrate to larger grains or low-defect crystal faces for deposition, thereby achieving spontaneous repair of crystal defects and crystal face reshaping. This process reduces the retention probability of impurities adsorbed on the crystal surface and promotes the redistribution of impurities trapped in the crystal surface or grain boundaries to the mother liquor under the influence of concentration gradient and interfacial rebalancing. This improves the density, uniformity, and purity stability of the crystals, ensuring high-quality arsenate double salt crystals for subsequent filtration and separation. During the aging process, a slow stirring method is adopted. This is mainly because after the arsenate double salt enters the aging stage, the weak stirring conditions can reduce the risk of mechanical collision and shear breakage between crystals, and provide a stable environment for the crystals to grow in a direction along the preferred crystal plane.

[0031] As a preferred embodiment, the high-purity arsenate complex salt precursor is in the form of elongated, rod-shaped, or plate-shaped crystals with a crystal length of 50-100 μm and a D90 / D10 ≤ 1.5. The relatively regular crystal morphology and concentrated particle size distribution are beneficial for the stable execution of subsequent filtration, washing, drying, and reduction processes.

[0032] As a preferred option, the obtained arsenate double salt crystals are used as the raw material for the next round, and steps 2) to 4) are repeated 3 to 5 times. The number of cycles can be adjusted according to the impurity content in the industrial arsenate double salt raw material. Generally, after 3 to 5 cycles, the obtained arsenate double salt precursor can meet the requirements for the subsequent reduction preparation of high-purity metallic arsenic of grade 5N or higher.

[0033] The dilute alkaline solution and dilute acid solution used in this invention primarily employ alkalis and acids that have minimal impact on the recrystallization of arsenate double salts. This effectively avoids introducing new and complex impurity sources during multiple recrystallization processes, thus maintaining the controllability of the purification pathway and the reproducibility of the results. The newly introduced potential Na... + K + NH4 + OH - Cl - NO3 - Plasma is the dominant ion in the original leachate and has little impact on the recrystallization purification process. The trace ions remaining can also be removed in one step during the subsequent reduction process due to their large separation coefficient with arsenic.

[0034] The sodium hydroxide and potassium hydroxide involved in this invention have a purity of not less than 99%. The ammonia solution has a mass fraction of not less than 25% (NH3). Concentrated hydrochloric acid and concentrated nitric acid are used. All these chemical reagents are of analytical grade or higher.

[0035] As a preferred embodiment, the drying process employs microwave drying at a temperature of 150–300°C, with the drying endpoint controlled to ensure that the water content of the high-purity arsenate double salt precursor does not exceed 0.1 wt%. By controlling the water content of the arsenate double salt, splashing, agglomeration, or secondary contamination caused by the instantaneous evaporation of moisture during the high-temperature reduction process can be effectively avoided, thus promoting the stable progress of the reduction process.

[0036] As a preferred embodiment, the reduction conditions are: reduction at a temperature of 1000–1200°C under a reducing atmosphere. As a more preferred embodiment, the reducing atmosphere consists of a non-reactive gas and a reducing gas, wherein the reducing gas is at least one of carbon monoxide and hydrogen, and the non-reactive gas is at least one of nitrogen, argon, and helium.

[0037] In this invention, the arsenic vapor volatilized during the reduction process is condensed and collected in a low-temperature condensation zone, with the temperature controlled between 260 and 360°C. Within this condensation temperature range, the supersaturation of the arsenic vapor and the driving force of phase transformation are effectively matched, which is conducive to the preferential nucleation and continuous growth of α-arsenic at the condensation interface, thereby inhibiting the formation of amorphous arsenic or other metastable phases. At the same time, the directional condensation process can reduce the possibility of impurity co-condensation, reduce inclusions and structural defects caused by rapid cooling, and thus improve the purity stability and crystal quality of the obtained high-purity metallic arsenic.

[0038] The purity of the reducing atmosphere in this invention is not less than 99.9999% (6N grade). The reducing atmosphere consists of a non-reactive gas and a reducing gas, with an oxygen content (O2) not exceeding 0.1 ppm, and the volume fraction of the reducing gas controlled within the range of 20-45%. The non-reactive gas is selected from at least one of nitrogen, argon, and helium. The reducing gas is selected from at least one of carbon monoxide and hydrogen. This invention, by employing a high-purity reducing atmosphere, provides a stable reducing environment for the conversion of arsenate complex salts to elemental arsenic, enabling highly controllable reduction rates of arsenate complex salts. It achieves precise synergy between front-end reduction and back-end directional α-arsenic condensation, obtaining high-purity metallic arsenic with a single crystal form. Since the arsenic source has already achieved high purification in the precursor stage, the above reduction and condensation processes can directly obtain high-purity metallic arsenic under relatively simplified process conditions.

[0039] The total content of metered impurities in the high-purity metallic arsenic prepared by this invention is no higher than 1×10⁻⁶. -3 wt%, preferably not higher than 1×10 -4 wt%.

[0040] Compared with the prior art, the beneficial technical effects of the technical solution of the present invention are as follows:

[0041] (1) The present invention moves the impurity control process forward to the arsenate complex salt precursor stage. Through dissolution, filtration, controlled crystallization, aging and washing and multiple cycles, the residual antimony, selenium, sulfur and metal impurities in industrial arsenate complex salt are reduced, so that the obtained precursor can be used to prepare high-purity metallic arsenic of grade 5N or above in one step.

[0042] (2) In the crystallization process, the present invention uses ultrasonic field, strong stirring and shearing and temperature field in combination, and controls the alkali addition rate and the final pH, which effectively improves the controllability and repeatability of the crystallization process, significantly reduces the risk of non-selective inclusion of impurities in the crystal, and makes the obtained arsenate double salt crystals have a stable high purity level and concentrated particle size distribution characteristics.

[0043] (3) Based on obtaining high-purity arsenate double salt, this invention can directly prepare 5N grade and above high-purity metallic arsenic by only one step of high-temperature reduction and low-temperature directional condensation, which significantly simplifies the traditional deep purification process of multiple sublimation and distillation, and reduces energy consumption and equipment complexity.

[0044] (4) The process system adopted in this invention has a simple composition, a clear parameter window, strong adaptability to raw materials, good engineering feasibility and scale-up potential, and provides a technical solution with practical application value for the short-process and stable preparation of high-purity arsenic and arsenide semiconductor materials. Attached Figure Description

[0045] Figure 1 This is a process flow diagram of the present invention.

[0046] Figure 2 The images show the physical samples of magnesium ammonium arsenate raw material, high-purity magnesium ammonium arsenate precursor, and metallic arsenic in Example 1 of this invention. Detailed Implementation

[0047] The present invention will be further described in detail below through specific embodiments, but the scope of protection of the claims of the present invention is not limited by the embodiments.

[0048] Example 1

[0049] Magnesium ammonium arsenate produced by the industrial disposal line of arsenic-alkali slag from Xikuangshan, Loudi, Hunan Province, was selected as the research object. Its main chemical components are shown in Table 1.

[0050] Table 1. Composition of Magnesium Ammonium Arsenate in Arsenic-Alkali Slag Industrial Plant (wt%)

[0051]

[0052] The entire process was conducted in a precision cleanroom environment at 24℃. 10 kg of industrial magnesium ammonium arsenate (arsenic-alkali residue type) was added to 100 L of dilute alkaline solution A (prepared with sodium hydroxide and softened water) and washed for 30 min, with the pH of solution A controlled at 9. After washing, the solution was filtered using a clean industrial filter press to obtain preliminarily purified magnesium ammonium arsenate raw material. The obtained magnesium ammonium arsenate raw material was then added to dilute acid solution B (prepared with concentrated hydrochloric acid and industrial pure water), with the pH of solution B controlled at 3. After complete dissolution, the solution was filtered through a 0.22 μm PTFE microporous membrane to obtain the filtrate. The filtrate was transferred to a vertical stirring tank and subjected to vigorous stirring at 800 rpm, with the ratio of the stirring paddle diameter to the tank diameter being 1 / 3. Simultaneously, an ultrasonic device was activated with an ultrasonic power density of 0.2 W / mL and an ultrasonic frequency of 60 kHz. Subsequently, a 10% (w / w) alkaline solution C, prepared from concentrated ammonia and industrial purified water, was added, and the pH increase rate was controlled at 0.1 / min to raise the pH of the system from 3 to 9, thereby achieving controlled crystallization of magnesium ammonium arsenate. After crystallization, ultrasonication was stopped, and the stirring speed was reduced to 60 rpm. The mixture was then aged for 20 hours under these conditions. After aging, the mixture was washed three times with a weakly alkaline solution D (pH=9) prepared from concentrated ammonia and industrial purified water, and filtered to obtain recrystallized magnesium ammonium arsenate crystals.

[0053] Using the recrystallized magnesium ammonium arsenate crystals as the raw material for the next round, the dissolution-filtration-controlled crystallization-aging-washing steps were repeated twice, for a total of three cycles. This yielded a high-purity magnesium ammonium arsenate precursor that underwent four crystallizations, based on the original industrial magnesium ammonium arsenate production. The yield of the obtained high-purity magnesium ammonium arsenate precursor was 95% higher than that of the arsenic-alkali slag type industrial magnesium ammonium arsenate. Table 2 shows that the total amount of foreign impurities measured in the obtained high-purity magnesium ammonium arsenate precursor was <8.80 × 10⁻⁶. -5 The purity of the precursor meets the requirements for the subsequent reduction preparation of 6N grade high-purity metallic arsenic, with a purity of wt%.

[0054] The resulting high-purity magnesium ammonium arsenate precursor was placed in a clean air atmosphere treated for dust and moisture removal and microwave-dried at 200℃ for 6 h, then transferred to a high-temperature reduction furnace. A high-purity reducing atmosphere consisting of 35% carbon monoxide and 65% nitrogen (purity 6N) was introduced into the furnace, and a one-step high-temperature reduction was carried out at 1050℃ for 2 h. The arsenic-containing vapor generated during the reduction process entered a low-temperature directional condensation zone, with the condensation temperature controlled at 320℃. The high-purity metallic arsenic was collected after condensation. As shown in Table 3, the total content of quantified impurities in the obtained high-purity metallic arsenic is <6.40 × 10⁻⁶. -5 wt%, purity reaches 6N grade.

[0055] Table 2 Impurity content of high-purity magnesium ammonium arsenate crystals (×10) -5 wt%

[0056]

[0057] \ indicates no statistics are included, - indicates no detection was detected.

[0058] Table 3. Arsenic impurity content in high-purity metallic metals (×10⁻¹⁰) -5 wt%

[0059]

[0060] \ indicates no statistics are included, - indicates no detection was detected.

[0061] Example 2

[0062] Magnesium ammonium arsenate produced by a semi-industrial copper flue ash disposal line in Qinghai was selected as the research object, and its main chemical components are shown in Table 4.

[0063] Table 4. Composition of copper-ash type industrial magnesium ammonium arsenate (wt%)

[0064]

[0065] The entire process was conducted in a precision cleanroom environment at 24℃. 10 kg of copper-ash type industrial magnesium ammonium arsenate was added to 100 L of dilute alkaline solution A, prepared with sodium hydroxide and softened water, and washed for 30 min, with the pH of solution A controlled at 8. After washing, the solution was filtered using a clean industrial filter press to obtain preliminarily purified magnesium ammonium arsenate raw material. The obtained magnesium ammonium arsenate raw material was then added to dilute acid solution B, prepared with concentrated hydrochloric acid and industrial pure water, with the pH of solution B controlled at 3. After complete dissolution, the solution was filtered through a 0.22 μm PTFE microporous membrane to obtain the filtrate. The filtrate was transferred to a vertical stirring tank and vigorously stirred at 650 rpm, with the ratio of the impeller diameter to the tank diameter being 1 / 3. Simultaneously, an ultrasonic device was activated with an ultrasonic power density of 0.3 W / mL and an ultrasonic frequency of 80 kHz. Subsequently, a 10% (w / w) alkaline solution C, prepared from concentrated ammonia and industrial purified water, was added, and the pH increase rate was controlled at 0.1 / min to raise the pH of the system from 3 to 8, thereby achieving controlled crystallization of magnesium ammonium arsenate. After crystallization, ultrasonication was stopped, and the stirring speed was reduced to 60 rpm. The mixture was then aged for 12 h under these conditions. After aging, the mixture was washed three times with a weak alkaline solution D (pH=8) prepared from concentrated ammonia and industrial purified water, and filtered to obtain recrystallized magnesium ammonium arsenate crystals.

[0066] Using the recrystallized magnesium ammonium arsenate crystals as the raw material for the next round, the above dissolution-filtration-controlled crystallization-aging-washing steps were repeated three times, for a total of four cycles. Based on the original industrial magnesium ammonium arsenate, a high-purity magnesium ammonium arsenate precursor with five crystallizations was obtained. The yield of the obtained high-purity magnesium ammonium arsenate precursor was 94% higher than that of copper ash type industrial magnesium ammonium arsenate. As shown in Table 5, the total content of metered foreign impurities in the obtained high-purity magnesium ammonium arsenate precursor was <1.74 × 10⁻⁶. -4 The purity of the precursor meets the requirements for the subsequent reduction preparation of 5N grade high-purity metallic arsenic, with a purity of wt%.

[0067] The resulting high-purity magnesium ammonium arsenate precursor was placed in a clean air atmosphere treated for dust and moisture removal and microwave-dried at 200℃ for 6 h, then transferred to a high-temperature reduction furnace. A high-purity reducing atmosphere consisting of 25% carbon monoxide and 75% argon (purity 6N) was introduced into the furnace, and a one-step high-temperature reduction was carried out at 1100℃ for 2 h. The arsenic-containing vapor generated during the reduction process entered a low-temperature directional condensation zone, with the condensation temperature controlled at 320℃. The high-purity metallic arsenic was collected after condensation. As shown in Table 6, the total content of quantified impurities in the obtained high-purity metallic arsenic is <1.33×10⁻⁶. -4 wt%, purity reaches 5N grade.

[0068] Table 5 Impurity content of high-purity magnesium ammonium arsenate crystals (×10) -5 wt %)

[0069]

[0070] \ indicates no statistics are included, - indicates no detection was detected.

[0071] Table 6. Arsenic impurity content in high-purity metallic metals (×10) -5 wt%

[0072]

[0073] \ indicates no statistics are included, - indicates no detection was detected.

[0074] Comparative Example 1

[0075] Compared with Example 1, all other conditions remained the same, except that the ultrasonic enhancement process was removed, that is, the alkali crystallization, aging, washing and multiple cycles were carried out only under strong stirring conditions.

[0076] The magnesium ammonium arsenate precursor obtained after the final four crystallizations had Sb, Se, and S contents all > 8.50 × 10⁻⁶. -4 The total amount of foreign impurities measured at wt% is approximately 7.84 × 10⁻⁶. -3After drying, one-step high-temperature reduction, and low-temperature directional condensation, the total content of quantified impurities in the obtained metallic arsenic is approximately 3.92 × 10⁻⁶ wt%. -3 The purity reached 4N grade (wt%), failing to meet the requirement of 5N grade or higher for high-purity metallic arsenic. In the absence of an ultrasonic field, the nucleation and crystal growth of the arsenate complex salt were mainly controlled by macroscopic stirring and solution supersaturation, leading to localized supersaturated regions that resulted in heterogeneous nucleation and rapid localized growth during crystallization. During this process, impurity elements with similar chemical properties to arsenic, such as Sb, Se, and S, were prone to co-deposition, surface adsorption, or intergranular entrainment at the crystal growth interface, and were difficult to fully migrate back to the mother liquor during subsequent aging. In contrast, the introduction of an ultrasonic field in Example 1 promoted more uniform sono-induced nucleation through ultrasonic cavitation and acoustic flow, enhancing mass transfer near the crystal surface and reducing the risk of impurity ion retention and non-selective doping at the crystal growth interface. Therefore, the ultrasonic enhancement step plays a crucial role in improving the purification effect of the magnesium ammonium arsenate precursor and the purity of the metallic arsenic obtained in the subsequent reduction step.

[0077] Comparative Example 2

[0078] Compared to Example 1, all other conditions remained unchanged except for the aging step, which was omitted after crystallization. Instead of low-speed stirring and aging, the sample was directly washed, filtered, and entered the next cycle. The magnesium ammonium arsenate precursor obtained after the final four crystallizations had Sb, Se, and S contents all > 4.00 × 10⁻⁶. -4 The total amount of foreign impurities measured at wt% is approximately 2.51 × 10⁻⁶. -3 After drying, one-step high-temperature reduction, and low-temperature directional condensation, the total content of quantified impurities in the obtained metallic arsenic is approximately 1.58 × 10⁻⁶ wt%. -3 The magnesium ammonium arsenate precursor, with a purity of 4N (wt%), failed to meet the requirements for high-purity metallic arsenic of 5N or higher. Without the aging step, the crystallization process terminated after nucleation and initial growth, making it difficult to fully reform internal defects, grain boundaries, and high-energy sites on the surface. This allowed some impurities to remain in the magnesium ammonium arsenate crystals more easily through surface adsorption, intergranular inclusion, or local encapsulation. Even after multiple cycles of dissolution-filtration-crystallization-washing, impurities were still difficult to fully redistribute to the mother liquor, resulting in the magnesium ammonium arsenate precursor failing to meet the purity requirements for one-step reduction to prepare high-purity metallic arsenic of 5N or higher. Therefore, the aging step is not simply about extending the operation time, but a crucial step in promoting crystal structure reforming, impurity redistribution, and deep purification of the precursor.

[0079] Comparative Example 3

[0080] Compared with Example 1, with other conditions unchanged, only the number of cycles of dissolution-filtration-controlled crystallization-aging-washing was reduced, and the product quality was examined when the recrystallization cycle was insufficient.

[0081] When only one cycle of dissolution-filtration-controlled crystallization-aging-washing is performed, a secondary crystalline magnesium ammonium arsenate precursor is obtained based on the original industrial magnesium ammonium arsenate. The Sb, Se, and S contents in this precursor are all >7.80 × 10⁻⁶. -3 The total amount of foreign impurities measured at wt% is approximately 5.73 × 10⁻⁶. -2 After drying, one-step high-temperature reduction, and low-temperature directional condensation, the total content of quantified impurities in the obtained metallic arsenic is approximately 2.57 × 10⁻⁶ wt%. -2 wt%, purity reaches 3N level.

[0082] When a two-cycle process of dissolution-filtration-controlled crystallization-aging-washing is performed, a tertiary crystallized magnesium ammonium arsenate precursor is obtained based on the original industrial magnesium ammonium arsenate. The Sb, Se, and S contents in this precursor are all >4.30 × 10⁻⁶. -4 The total amount of foreign impurities measured at wt% is approximately 1.92 × 10⁻⁶. -3 After drying, one-step high-temperature reduction, and low-temperature directional condensation, the total content of quantified impurities in the obtained metallic arsenic is approximately 1.35 × 10⁻⁶ wt%. -3 The purity is 4N, but it still does not meet the requirements for high-purity metallic arsenic of 5N or above.

[0083] The above results indicate that while a single or small number of recrystallization cycles can reduce the content of some impurities, they are insufficient to fully utilize the distribution differences between arsenate complex salts and impurities during multiple dissolution-recrystallization processes. Only through a sufficient number of cycles can Sb, Se, S, and metallic impurities gradually migrate to the mother liquor and be washed away, thereby obtaining a precursor suitable for one-step reduction preparation of high-purity metallic arsenic.

[0084] Comparative Example 4

[0085] Compared to Example 2, all other conditions remained unchanged except that the operating temperature of the controlled crystallization process was increased to 55°C. The yield of the magnesium ammonium arsenate precursor obtained after five crystallizations was only 22% relative to the copper ash type industrial magnesium ammonium arsenate, significantly lower than the 94% yield in Example 2. Magnesium ammonium arsenate is highly sensitive to crystallization temperature. When the operating temperature rises to 55°C, its solubility in acidic and weakly alkaline systems increases, making it difficult to form a stable and effective supersaturated state during the dissolution-recrystallization process, thus affecting the nucleation and crystal growth of magnesium ammonium arsenate. Simultaneously, during subsequent aging, washing, and filtration processes, the formed crystals are also prone to secondary dissolution, causing the arsenic component to be lost in dissolved form with the mother liquor and washing liquid. Therefore, while excessively high temperatures may facilitate the migration of some impurities, they significantly reduce the recovery rate of the magnesium ammonium arsenate precursor, making it difficult to balance purification efficiency and material yield. This makes it unsuitable as a one-step reduction process for preparing high-purity metallic arsenic in the precursor purification of this invention.

[0086] Comparative Example 5

[0087] Compared to Example 2, all other conditions remained the same, except that the 2-3 washing steps between aging and filtration were omitted; that is, after aging, the product was directly filtered into the next cycle or subsequently dried and reduced. In the final magnesium ammonium arsenate precursor obtained after five crystallizations, the contents of Sb, Se, and S were all >1.50 × 10⁻⁶. -4 The total amount of foreign impurities measured at wt% is approximately 5.31 × 10⁻⁶. -3 After drying, one-step high-temperature reduction, and low-temperature directional condensation, the total content of quantified impurities in the obtained metallic arsenic is approximately 3.42 × 10⁻⁶ wt%. -3 The purity of the magnesium ammonium arsenate reached 4N grade, but failed to meet the requirements for high-purity metallic arsenic above 5N grade. After aging, a certain amount of mother liquor components and dissolved impurity ions may still remain on the crystal surface and in the intercrystalline region. These impurities include elements with similar chemical properties to arsenic, such as Sb, Se, and S, as well as alkali metal ions, alkaline earth metal ions, and other soluble inorganic salt impurities from the reaction system. These impurities mostly exist in adsorbed, entrained, or intercrystalline residual forms, and are difficult to completely eliminate by the crystallization process alone. In Example 2, the crystals were washed multiple times with weak alkaline water, which can remove the mother liquor residue and amorphous impurities attached to the crystal surface and promote the migration of intercrystalline impurities into the washing solution. If the washing step is omitted, the above impurities will enter the subsequent high-temperature reduction process with the magnesium ammonium arsenate precursor, and ultimately affect the purity of the metallic arsenic obtained by low-temperature directional condensation. Therefore, multiple washing after aging is an important step to ensure the purification effect of the precursor and the quality of the final high-purity metallic arsenic.

[0088] Comparative Example 6

[0089] Compared to Example 1, all other conditions remained unchanged except that the rate of addition of alkali solution C in step 3) was increased, thereby increasing the rate of pH increase from 0.1 / min to 1 / min, still from pH=3 to pH=9. The magnesium ammonium arsenate precursor obtained after the final four crystallizations had Sb, Se, and S contents all >4.75 × 10⁻⁶. -3 The total amount of foreign impurities measured at wt% is approximately 2.35 × 10⁻⁶. -2 After drying, one-step high-temperature reduction, and low-temperature directional condensation, the total content of quantified impurities in the obtained metallic arsenic is approximately 1.18 × 10⁻⁶ wt%. -2The purity reached 3N grade (wt%), but failed to meet the requirements for high-purity metallic arsenic above 5N grade. When the pH increase rate is too rapid, localized high pH and supersaturation environments easily form in the alkali addition area, causing magnesium ammonium arsenate to precipitate rapidly within a short time. This process easily leads to explosive nucleation and non-uniform growth, resulting in a wider crystal size distribution, increased crystal defects, and increased probability of mother liquor entrainment, surface adsorption, and intergranular encapsulation. Impurities such as Sb, Se, and S, as well as some metallic impurities, are more likely to precipitate along with the magnesium ammonium arsenate crystals or be encapsulated within the crystals during rapid crystallization, making them difficult to completely remove during subsequent aging and washing. In contrast, controlling the pH increase rate to ≤0.1 / min in Example 1 allows the system to gradually establish supersaturation, which is beneficial for controlled nucleation and stable growth of magnesium ammonium arsenate, reducing the risk of non-selective impurity inclusion. Therefore, slow alkali addition and controlled pH increase rate are crucial conditions for ensuring the purification effect of the precursor and the purity of the metallic arsenic obtained in the subsequent reduction step.

[0090] Comparative Example 7

[0091] Compared to Example 1, other conditions remained unchanged, except for the ultrasonic field conditions (ultrasonic power density of 0.2 W / mL and ultrasonic frequency of 60 kHz). However, the strong stirring condition was eliminated, and only ordinary low-speed stirring was used, with the stirring rate controlled at 150 rpm. The magnesium ammonium arsenate precursor obtained after the final four crystallizations had Sb, Se, and S contents all >9.00 × 10⁻⁶. -4 The total amount of foreign impurities measured at wt% is approximately 5.68 × 10⁻⁶. -3 After drying, one-step high-temperature reduction, and low-temperature directional condensation, the total content of quantified impurities in the obtained metallic arsenic is approximately 2.95 × 10⁻⁶ wt%. -3 The purity reached 4N grade (wt%), failing to meet the requirements for high-purity metallic arsenic above 5N grade. Under conditions of ultrasound alone without strong stirring, while ultrasonic cavitation and acoustic flow can promote nucleation and mass transfer in localized areas, they are insufficient to completely eliminate the macroscopic concentration gradient and local pH differences formed during alkali addition. Especially in larger volume solutions, localized supersaturation easily occurs near the alkali addition area, leading to rapid crystallization in some regions, while nucleation and growth processes in areas far from the alkali addition zone are relatively delayed, resulting in an inhomogeneous crystal growth environment. This inhomogeneous crystallization process increases surface adsorption, intercrystalline entrainment, and local encapsulation of impurity ions, making it difficult for the magnesium ammonium arsenate precursor, even after multiple recrystallizations, to meet the requirements for one-step reduction to prepare high-purity metallic arsenic above 5N grade. Therefore, an ultrasonic field cannot completely replace strong stirring; only a combination of ultrasonic field and strong stirring can simultaneously improve microscale nucleation and mass transfer as well as macroscopic mixing uniformity, thereby achieving better precursor purification results.

Claims

1. A method for producing high purity metallic arsenic by one step reduction based on precursor purification, characterized by: Includes the following steps: 1) The industrial arsenate double salt was washed and filtered with dilute alkaline water A to obtain a preliminarily purified arsenate double salt; 2) After dissolving the preliminarily purified arsenate complex salt in dilute acid solution B, filter it through a microporous membrane to obtain the filtrate; 3) Place the filtrate in an ultrasonic field and slowly add alkaline solution C to the filtrate under stirring conditions to achieve controlled crystallization of arsenate double salt and obtain a crystallized solution; The controlled crystallization conditions are as follows: ultrasonic power density of 0.05~0.5W / mL, ultrasonic frequency of 20~100 kHz, stirring speed of ≥500rpm, alkali addition rate to control pH increase rate of ≤0.1 / min, final pH of 7~10, and temperature of 10~40℃. 4) After aging, the crystallizing solution is washed and filtered with dilute alkaline water D to obtain arsenate double salt crystals; 5) Use the obtained arsenate double salt crystals as the raw material for the next round, and repeat steps 2) to 4) more than three times to obtain a high-purity arsenate double salt precursor; 6) The high-purity arsenate double salt precursor is dried and reduced to obtain high-purity metallic arsenic of grade 5N or higher.

2. The method for preparing high purity metallic arsenic by one step reduction based on precursor purification according to claim 1, characterized in that: The industrial arsenate complex salts include at least one of ammonium magnesium complex salt, potassium magnesium complex salt, and potassium calcium complex salt.

3. The method for preparing high-purity metallic arsenic in one step based on precursor purification and reduction according to claim 1, characterized in that: The pH of the dilute alkaline solution A and the dilute alkaline solution D is not higher than 10; The dilute alkaline solution A and the dilute alkaline solution D are at least one of sodium hydroxide solution, potassium hydroxide solution, and ammonia solution.

4. The method for preparing high-purity metallic arsenic in one step based on precursor purification and reduction according to claim 1, characterized in that: The pH of the dilute acid solution B is 2-4; The dilute acid solution B is at least one of hydrochloric acid solution and nitric acid solution.

5. The method for preparing high-purity metallic arsenic in one step based on precursor purification and reduction according to claim 1, characterized in that: The pore size of the microporous filter membrane is between 0.1 and 1 μm; The microporous filter membrane is made of one of the following materials: nylon, polytetrafluoroethylene, polyvinylidene fluoride, or polypropylene.

6. The method for preparing high-purity metallic arsenic in one step based on precursor purification and reduction according to claim 1, characterized in that: The mass fraction of the alkaline solution C is 10-15%; The alkaline solution is at least one of sodium hydroxide solution, potassium hydroxide solution, and ammonia solution.

7. The method of claim 1, wherein the method is characterized by: The aging conditions are controlled as follows: temperature 10~40℃, time 12~24h, and stirring rate 20~80rpm.

8. The method for preparing high-purity metallic arsenic in one step based on precursor purification and reduction according to claim 1, characterized in that: The drying process employs microwave drying at a temperature of 150–300°C, with the drying endpoint controlled to ensure that the water content of the high-purity arsenate precursor does not exceed 0.1 wt%.

9. The method of claim 1, wherein the method is characterized by: The reduction conditions are: reduction at a temperature of 1000–1200°C in a reducing atmosphere.

10. The method for preparing high-purity metallic arsenic in one step based on precursor purification and reduction according to claim 9, characterized in that: The reducing atmosphere consists of a non-reactive gas and a reducing gas, wherein the reducing gas is at least one of carbon monoxide and hydrogen, and the non-reactive gas is at least one of nitrogen, argon, and helium.

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