Method and apparatus for high purity arsenic production by high temperature liquid medium anhydrous process

Abstract

This invention discloses a method and apparatus for preparing high-purity arsenic in an anhydrous liquid medium at high temperatures, belonging to the field of molten salt electrochemistry. The method uses a low-melting-point, highly stable calcium chloride (CaCl2)-based molten salt as the electrolyte, directly dissolving or dispersing calcium arsenate powder in the molten salt. Electrolysis is carried out under an inert atmosphere, with the electrolysis temperature controlled between 650-950℃. Arsenate ions (AsO4) are present at the cathode. 3‑ The arsenic is directly reduced to elemental arsenic. Because the operating temperature is much higher than the boiling point of arsenic (614℃), the elemental arsenic instantly vaporizes into arsenic vapor (As₄(g)) and escapes from the molten salt. The arsenic vapor is transported to a condensation system via a carrier gas, where it condenses into solid high-purity arsenic in a low-temperature zone of 200-300℃ for collection. Oxygen is released from the anode through an oxygen ion oxidation reaction. This invention achieves the direct conversion of stable arsenate into high-value-added, high-purity arsenic. The process is short, energy-efficient, and produces high-purity products, providing a novel pathway for the high-value utilization of arsenic-containing waste.

Description

Technical Field

[0001] This invention belongs to the field of molten salt electrochemical technology, specifically relating to a method and equipment for preparing high-purity arsenic in anhydrous high-temperature liquid medium. Background Technology

[0002] High-purity arsenic (purity > 99.999%) is an important raw material for the preparation of compound semiconductors (such as GaAs and InAs), special alloys, and pharmaceutical products, and has broad application prospects in the fields of electronics, new materials, and medicine. Currently, the raw materials for the industrial production of high-purity arsenic are mainly arsenic-containing sulfide ores (such as arsenopyrite and FeAsS). The preparation process requires multiple complex steps, including oxidation roasting, reduction, and sublimation purification. This process is not only energy-intensive but also generates large amounts of arsenic-containing fumes and wastewater, causing serious environmental pollution and contradicting the trend of green chemical engineering.

[0003] Calcium arsenate is a common stabilization product generated during the treatment of arsenic-containing wastewater in industries such as metallurgy and chemicals. While its solubility is low and its environmental risks are relatively controllable, large-scale accumulation of calcium arsenate still poses a potential environmental threat and wastes arsenic resources. Existing technologies for recovering arsenic from calcium arsenate (such as the high-temperature carbothermal reduction method) suffer from problems such as long processes, high energy consumption, and the potential for secondary pollution (e.g., CO2 and As2O3 dust), resulting in poor economic and environmental performance and hindering industrial-scale application. Summary of the Invention

[0004] Technical problems to be solved

[0005] The purpose of this invention is to overcome the shortcomings of existing high-purity arsenic production processes, such as high energy consumption and severe pollution, as well as the long process and poor economic efficiency of calcium arsenate treatment technology. It provides a method and system for preparing high-purity arsenic in anhydrous high-temperature liquid medium with a short process, low energy consumption, high product purity, and environmental friendliness. It directly electrolyzes stable calcium arsenate in molten salt and utilizes the volatility of arsenic to achieve in-situ separation and continuous collection of the product, providing a new path for the high-value utilization of arsenic-containing waste.

[0006] Technical solution

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing high-purity arsenic in an anhydrous high-temperature liquid medium.

[0008] The method includes the following specific steps:

[0009] 1. Electrolyte System Preparation: Anhydrous CaCl2 or a CaCl2-LiCl / NaCl eutectic mixture is used as the base electrolyte, which is thoroughly mixed with dried and ground industrial-grade calcium arsenate powder, wherein the calcium arsenate content is controlled at 1-10 wt%. To further increase the solubility and stability of arsenic compounds in molten salt, additives such as NaOH, LiOH, or Na2O can be selectively added to construct a strongly alkaline electrolyte environment. The mixing ratio of CaCl2 to LiCl or NaCl in the CaCl2-LiCl / NaCl eutectic mixture can be adjusted according to actual needs to obtain a lower melting point and reduce energy consumption.

[0010] 2. Pretreatment before electrolysis: The prepared electrolyte mixture is loaded into a sealed electrolytic cell, preferably made of corundum material, which has good high-temperature resistance and corrosion resistance. The electrolytic cell is then evacuated and filled with a high-purity inert gas (preferably argon, with a purity of not less than 99.999%), repeated three times to thoroughly remove air from the cell and prevent impurities such as oxygen from affecting the electrolytic reaction and product purity. The electrolytic cell is then heated at a rate of 5℃ / min to 300℃ and stabilized for 8 hours, while maintaining a negative pressure under continuous vacuum. This is to completely remove the water of crystallization from the molten salt and prevent water from participating in the electrolytic reaction to produce highly toxic byproducts such as arsine. After the water of crystallization is removed, heating continues to 650-950℃ to completely melt the salts, and the temperature is stabilized for 2-10 hours to ensure uniform melting of the molten salt and stability of the electrolyte system.

[0011] 3. Molten Salt Electrolysis Operation: Graphite rods or molybdenum rods are used as cathodes, and high-density graphite or nickel-iron alloy is used as anodes. Both electrodes are inserted into the molten salt in the electrolytic cell, ensuring full contact between the electrodes and the molten salt. A cell voltage of 2.0-5.0V is applied for constant voltage electrolysis for 2-10 hours. Preferably, the electrolysis temperature is controlled at 700-850℃, the cell voltage at 3.0-4.0V, and the electrolysis time at 4-8 hours, which can further reduce energy consumption while ensuring product purity and yield. During electrolysis, a reduction reaction occurs at the cathode, producing arsenate ions (AsO4). 3- Arsenic is reduced to elemental arsenic by accepting electrons; the reaction equation is: AsO4 3- + 6e - → As + 4O 2- Because the temperature of the electrolysis system is much higher than the boiling point of arsenic (614℃), the generated elemental arsenic will instantly vaporize into As4(g) bubbles, escaping from the molten salt and entering the steam collection passage. An oxidation reaction occurs at the anode, producing oxygen ions (O2). 2- Oxygen is released by the loss of electrons. The reaction equation is: 2O 2- - 4e -→ O2 (g) (when using an inert anode); if the anode is graphite, a side reaction of graphite oxidation will occur, producing CO2 or CO gas, with the reaction equations as follows: C + 2O 2- -4e - → CO2 (g), C + O 2- - 2e - → CO(g).

[0012] 4. Product Separation and Collection: Using a high-purity inert gas (consistent with the protective gas before electrolysis) as the carrier gas, the flow rate is controlled at 30-80 mL / min (preferably 50 mL / min). Arsenic vapor generated during electrolysis is discharged from the electrolytic cell through the vapor collection passage and sent to the condensation system. The temperature of the condensation system is controlled at 200-300℃. This temperature range is below the boiling point of arsenic but above its melting point, allowing the arsenic vapor to condense rapidly into solid elemental arsenic, preventing excessive cooling, agglomeration, or re-evaporation. The condensed solid high-purity arsenic is collected in a sealed, high-temperature resistant container to prevent product contamination or oxidation. Oxygen (or CO, CO2) generated at the anode is separately discharged from the anode area and centrally processed in the anode gas treatment device, achieving separation and disposal of anode and cathode products and preventing arsenic oxidation and leakage of harmful gases. By continuously or intermittently adding calcium arsenate raw material to the molten salt, the entire preparation process can be semi-continuously operated, improving production efficiency.

[0013] A device for preparing high-purity arsenic in an anhydrous high-temperature liquid medium.

[0014] This equipment is used to implement the above-mentioned method for preparing high-purity arsenic. Specifically, it includes a sealed electrolytic cell, a heating device, an inert gas supply device, a vacuum device, electrolytic electrodes, a vapor collection passage, a condensation system, a receiving tank, and an anode gas treatment device. The connection relationships and functions of each component are as follows:

[0015] 1. Sealed electrolytic cell: As the container for the entire electrolysis reaction, it is used to hold the electrolyte mixture. The cell body is made of high temperature and corrosion resistant materials such as corundum to ensure stable operation in a high temperature environment of 650-950℃ and to prevent reaction with electrolytes and products.

[0016] 2. Heating device: It is installed outside the electrolytic cell and adopts a gradient heating design. It can achieve a heating rate control of 5℃ / min and stable temperature maintenance in two temperature ranges: 300℃ and 650-950℃, which meets the temperature requirements of dehydration before electrolysis and electrolysis reaction.

[0017] 3. Inert gas supply device: connected to the sealed electrolytic cell, used to fill the electrolytic cell with high-purity inert gas (such as argon) with a purity of not less than 99.999%, to remove air from the electrolytic cell, protect the electrolysis reaction, and at the same time serve as a carrier gas to transport arsenic vapor.

[0018] 4. Vacuum device: Connected to the sealed electrolytic cell, it is used to perform vacuum operation on the electrolytic cell. In conjunction with the inert gas supply device, it can completely remove air. At the same time, it maintains negative pressure in the electrolytic cell during the dehydration stage before electrolysis, promoting the volatilization and removal of crystal water in the molten salt.

[0019] 5. Electrolytic electrodes: These include the cathode and anode, both of which are inserted into the molten salt within the electrolytic cell. The cathode is made of graphite or molybdenum rods, which have good conductivity and high-temperature resistance. The anode is made of high-density graphite or nickel-iron alloy. High-density graphite is suitable for applications requiring the generation of CO or CO2, while nickel-iron alloy is suitable for inert anode applications, avoiding the side reaction of graphite oxidation.

[0020] 6. Steam collection passage: One end is connected to the cathode area inside the electrolytic cell, and the other end is connected to the inlet of the condensation system. It is made of quartz tube, which is heat resistant and does not react with arsenic vapor. It is used to remove the arsenic vapor generated at the cathode along with the carrier gas from the electrolytic cell.

[0021] 7. Condensation system: Used to condense arsenic vapor into solid high-purity arsenic. It adopts stainless steel condenser tubes or quartz condenser tubes, and the temperature can be precisely controlled at 200-300℃ to ensure efficient condensation of arsenic vapor and easy collection of solid arsenic after condensation.

[0022] 8. Receiving tank: Connected to the outlet of the condensation system, it is a sealed, high-temperature resistant container used to collect solid high-purity arsenic after condensation, preventing the product from being oxidized or contaminated by air.

[0023] 9. Anode Gas Treatment Unit: Connected to the anode area inside the electrolytic cell, it is used to collect oxygen (or CO, CO2) generated at the anode and treat it centrally to prevent the leakage of harmful gases and ensure the environmental safety of the production process.

[0024] Beneficial effects

[0025] 1. Achieving high-value recycling of arsenic resources: Using calcium arsenate, a hazardous waste generated by the metallurgical and chemical industries, as raw material, it is directly converted into high-value-added high-purity arsenic, turning waste into treasure. This not only solves the environmental hazards of calcium arsenate accumulation, but also achieves efficient utilization of arsenic resources. Moreover, calcium arsenate has a melting point as high as 1455℃, and there is no volatile loss within the operating temperature range of this invention. It also has strong compatibility with electrolyte systems.

[0026] 2. Short process and high efficiency: Calcium arsenate can be directly converted into elemental arsenic in one electrolysis step, eliminating multiple processes such as oxidation roasting, reduction, and sublimation purification in the traditional high-purity arsenic production. This simplifies the production process, improves production efficiency, and reduces material loss during production.

[0027] 3. High product purity: The gas phase volatilization-condensation process itself is a highly efficient physical purification process, which can effectively separate most of the impurities in the molten salt without the need for additional purification processes. The product purity can directly reach more than 99.95%, meeting the requirements for industrial-grade high-purity arsenic.

[0028] 4. Significantly reduced energy consumption: The electrolysis operating temperature of this invention is 650-950℃, which is much lower than the operating temperature of the existing calcium arsenate carbothermal reduction technology (>1200℃), greatly reducing energy consumption in the production process and improving the economic efficiency of the process.

[0029] 5. Environmentally friendly and highly safe: The entire production process adopts an anhydrous molten salt electrolyte system, eliminating the highly toxic arsine byproducts that may be generated during the electrolysis of molten salt; the gases (O2 / CO / CO2) generated at the anode can be collected and treated centrally, and no wastewater or waste residue is generated throughout the process, avoiding environmental pollution; at the same time, the inert atmosphere protection can effectively prevent the oxidation of arsenic and improve the safety of the production process. Attached Figure Description

[0030] Figure 1 This is a process flow diagram for the anhydrous electrolytic preparation of high-purity arsenic according to the present invention;

[0031] Figure 2 This is a schematic diagram of the molten salt electrolysis-condensation collection device used in this invention.

[0032] Figure 1 In this process, the materials and steps are as follows: calcium arsenate, calcium chloride basic salt, NaOH, Na2O, LiOH and other additives → mixing and melting → molten salt electrolysis → condensation and collection → high-purity arsenic;

[0033] Figure 2 The diagram shows the connection relationships between the sealed electrolytic cell, heating device, electrodes, steam collection passage, condensation system, receiving tank, and anode gas treatment device, clearly presenting the complete device structure of molten salt electrolysis and condensation collection. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] like Figure 1-2 As shown,

[0036] Example 1:

[0037] like Figure 1As shown, firstly, in a glove box, 500g of anhydrous CaCl2, 50g of dry calcium arsenate powder, and 10g of Na2O were mixed evenly and placed into an alumina crucible electrolytic cell. The electrolytic cell was sealed, evacuated, and filled with argon gas. This process was repeated three times to remove air from the electrolytic cell. Heating was then initiated at a rate of 5°C / min, reaching 300°C and stabilizing for 8 hours, while maintaining a negative pressure under continuous evacuation to remove the water of crystallization in the molten salt. The temperature was then raised to 850°C and held. A molybdenum rod was used as the cathode, and a high-density graphite column as the anode. Electrolysis was performed under a constant voltage of 3.2V. During electrolysis, As4 vapor was carried by argon gas (flow rate 50 mL / min) from the cathode area to a quartz collecting tube, and then through a pipeline into a stainless steel condenser maintained at 250°C. After 6 hours of electrolysis, heating was stopped. After cooling, the inner wall of the tube was loaded with silvery-gray crystalline arsenic with a metallic luster. The product in the condenser was weighed, yielding 17.6g of solid arsenic. ICP-MS analysis showed that its purity reached 99.97%.

[0038] Example 2:

[0039] A CaCl2-40wt%LiCl eutectic mixture was used as the electrolyte, and 10g of LiOH was added as an additive to lower the operating temperature to 650℃. Other conditions were the same as in Example 1. A schematic diagram of the electrolysis and condensation apparatus is shown below. Figure 2 As shown. After 6 hours of electrolysis, volatile arsenic vapor was successfully obtained and condensed for collection. The product in the condenser was weighed, yielding 13.4 g of solid arsenic. This indicates that the lower melting point CaCl2-LiCl system is also effective and helps to further reduce energy consumption, but the yield of electrolysis products decreased slightly due to the reduced electrochemical reaction rate and electrolyte mass transfer rate at lower temperatures.

[0040] This embodiment demonstrates that using nickel-iron alloy as an inert anode can avoid the occurrence of graphite oxidation side reactions. The anode only releases oxygen, which facilitates centralized gas treatment and further improves the environmental friendliness of the process.

[0041] 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing high-purity arsenic in an anhydrous high-temperature liquid medium, characterized in that, Includes the following steps: S1 Electrolyte System Preparation: Anhydrous CaCl2 or a CaCl2-LiCl / NaCl eutectic mixture is used as the base electrolyte, and thoroughly mixed with dried and ground industrial-grade calcium arsenate powder, wherein the content of calcium arsenate is 1-10 wt%; NaOH, LiOH or Na2O can be selectively added as additives to construct a strongly alkaline environment to increase the solubility and stability of arsenic compounds. S2 Pretreatment before electrolysis: Place the mixture prepared in step S1 in a closed electrolytic cell, evacuate the electrolytic cell and fill it with high-purity inert gas, repeat three times to remove air from the cell; then heat at a rate of 5℃ / min, raise it to 300℃ and stabilize it for 8 hours, while continuously evacuating to maintain a negative pressure state to remove the water of crystallization in the molten salt; continue heating to 650-950℃ to melt the salt, and stabilize it for 2-10 hours to ensure complete melting and uniform electrolyte; S3 molten salt electrolysis: A graphite rod or a molybdenum rod is used as the cathode, and a high-density graphite or nickel-iron alloy is used as the anode. A constant voltage electrolysis of 2.0-5.0V is applied for 2-10 hours. During the electrolysis process, an arsenate ion reduction reaction occurs at the cathode to generate elemental arsenic. The elemental arsenic is instantly vaporized into As4(g) bubbles that escape from the molten salt. An oxygen ion oxidation reaction occurs at the anode to release oxygen. If the anode is graphite, a graphite oxidation side reaction will occur. S4 Product Separation and Collection: Using high-purity inert gas as the carrier gas, the arsenic vapor generated in step S3 is exported from the electrolytic cell and sent to a condensation system with the temperature controlled at 200-300℃. The arsenic vapor is condensed into solid high-purity arsenic in this temperature range and collected in a receiving tank. The gas generated at the anode is exported separately for centralized treatment. Calcium arsenate raw material is added to the molten salt continuously or intermittently to achieve semi-continuous operation of the process.

2. The method according to claim 1, characterized in that, The CaCl2-LiCl / NaCl eutectic mixture mentioned in step S1 is a low-melting-point eutectic system formed by mixing CaCl2 with LiCl or NaCl in any proportion.

3. The method according to claim 1, characterized in that, The high-purity inert gas mentioned in step S2 is argon, with a purity of not less than 99.999%.

4. The method according to claim 1, characterized in that, In step S3, the preferred electrolysis temperature is 700-850℃, the preferred cell voltage is 3.0-4.0V, and the preferred electrolysis time is 4-8h.

5. The method according to claim 1, characterized in that, In step S4, the flow rate of the carrier gas is 30-80 mL / min, preferably 50 mL / min.

6. An apparatus for implementing the method of any one of claims 1-5, characterized in that, It includes a sealed electrolytic cell, a heating device, an inert gas supply device, a vacuum device, electrolytic electrodes, a steam collection passage, a condensation system, a receiving tank, and an anode gas treatment device; The sealed electrolytic cell is used to contain the electrolyte mixture, and the cell body is made of high-temperature and corrosion-resistant materials. The heating device is installed outside the electrolytic cell to achieve gradient heating and stable temperature control. The inert gas supply device is connected to the electrolytic cell and is used to fill the electrolytic cell with high-purity inert gas. The vacuum device is connected to the electrolytic cell and is used to perform vacuuming operations. The electrolytic electrodes include a cathode and an anode, which are respectively inserted into the molten salt inside the electrolytic cell. The cathode is made of graphite rod or molybdenum rod, and the anode is made of high-density graphite or nickel-iron alloy. One end of the steam collection passage is connected to the cathode area inside the electrolytic cell, and the other end is connected to the inlet of the condensation system to export arsenic vapor and carrier gas. The condensation system is used to condense arsenic vapor into solid high-purity arsenic, with the temperature controlled at 200-300℃. The outlet of the condensation system is connected to a receiving tank. The anode gas treatment device is connected to the anode area inside the electrolytic cell to collect and treat the gas generated at the anode.

7. The device according to claim 6, characterized in that, The sealed electrolytic cell is made of corundum, and the condensation system is made of stainless steel condenser tubes or quartz condenser tubes.

8. The device according to claim 6, characterized in that, The steam collection passage is made of quartz tube, and the receiving tank is a sealed high-temperature resistant container.

Images