Method for selective separation and recovery of antimony and tellurium from composite oxide catalyst using mixed acid and reducing agent, and high-purity antimony trioxide produced thereof

Abstract

The present invention relates to a method for safely and efficiently recovering ruthenium from a spent catalyst in which ruthenium oxide (RuO₂) is supported on an alumina (Al₂O₃) support. A ruthenium recovery method according to the present invention comprises: a step of preparing a slurry by adding the catalyst to a polyol solvent adjusted to a pH of 3 or higher and 5 or lower by adding one or more organic acids selected from acetic acid, citric acid, and oxalic acid, and applying ultrasound; and a double sequential capping step of forming a hydrophilic polymer layer on the surface of RuO₂ by first adding a hydrophilic polymer capping agent to the slurry at a temperature of 40°C or higher and 60°C or lower, and then cooling to a temperature of 20°C or higher and 30°C or lower to form a hydrophobic protective layer by secondarily adding an amphiphilic polymer capping agent containing thiol groups. The method comprises the step of reducing RuO₂ to metallic ruthenium nanoparticles and desorbing them from the Al₂O₃ support by purging nitrogen gas at a flow rate of 0.5 L / min to 2.0 L / min while irradiating the slurry with microwaves using a three-stage output program of low-power preheating at 300 W to 500 W, high-power reduction at 600 W to 1,200 W, and low-power stabilization at 300 W to 500 W; and the step of aggregating the metallic ruthenium nanoparticles by adding an alkaline aqueous solution to the slurry to adjust the pH to 8 to 10, then separating the Al₂O₃ support residue by low-speed centrifugation at 500 rpm to 1,000 rpm and separating the metallic ruthenium nanoparticles by high-speed centrifugation at 3,000 rpm to 8,000 rpm and recovering them by redispersing and washing with an organic acid aqueous solution. According to the present invention, high recovery rates of over 90% can be achieved through a safe process at low temperature and atmospheric pressure that excludes strong oxidizing agents, toxic gases, and high temperature and high pressure conditions, while obtaining uniform metallic ruthenium nanoparticles with a particle size of 5 nm or more and 100 nm or less with high purity.

Description

Technology Field

[0001] The present invention relates to a method for the selective separation and recovery of antimony and tellurium from a complex oxide catalyst. Background Technology

[0003] Complex oxide catalysts containing Sb₂O₅ and TeO₂ are widely used as oxidation catalysts in various petrochemical processes, such as the electro-oxidation of propylene, the selective oxidation of butane, and dehydrogenation reactions; they are generally manufactured in the form of multi-component complex oxides containing carriers or auxiliary oxides such as SiO₂ and Fe₂O₃.

[0004] Antimony (Sb) is widely used in the form of antimony trioxide (Sb₂O₃) as a flame retardant for plastics, fibers, and rubber, and demand is continuously increasing as it is an essential element for ensuring the safety of electrical and electronic products. Tellurium (Te) is used as a core material for CdTe thin-film solar cells and is essential for electrothermal interconversion devices as a bismuth-tellurium (Bi₂Te₃) thermoelectric material; it is also utilized in the chemical industry as a catalyst and a rubber vulcanizing agent. As such, Sb and Te are rare metals with high utility across advanced industries, making them strategic resources for which securing a stable supply is important.

[0005] The aforementioned composite oxide catalyst is discharged as spent catalyst after prolonged use due to causes such as sintering, poisoning, and reduced activity; since a significant amount of Sb and Te still remains within the spent catalyst, efficient recovery technology for these elements is required from the perspective of resource recycling.

[0006] However, Sb and Te can coexist in acidic solutions in the polyvalent oxidation states of Sb(III) / Sb(V) and Te(IV) / Te(VI), respectively, and there is a problem in that selective separation is very difficult because there is a region where the oxidation-reduction potentials (ORPs) of the two elements are similar or overlap.

[0007] Conventional technologies have proposed (1) a method of elution by water-soluble leaching after high-temperature melting, (2) a non-selective reduction precipitation method using strong reducing agents such as NaBH₄ or hydrazine, and (3) a repetitive separation method by multi-stage precipitation and redissolution. However, the above high-temperature melting method has low economic feasibility due to high energy consumption and equipment costs, the method using strong reducing agents has the problem that Sb and Te are simultaneously reduced and precipitated, preventing selective separation, and the multi-stage precipitation method has the problem of a low Te recovery rate due to the complexity of the process and the loss of Te from simultaneous precipitation or redissolution.

[0008] In particular, while mixed acids of hydrochloric acid and nitric acid are frequently used in actual industrial settings to ensure the complete elution of catalysts, most conventional technologies were unsuitable for applications where nitric acid was present, or required additional evaporation and neutralization processes to remove it, thereby limiting their practical industrial applicability. Furthermore, conventional technologies suffered from a low final recovery rate of Te because a subsequent recovery process for residual Te after Sb separation was not clearly established; additionally, no integrated process for directly converting recovered Sb into high-purity antimony oxide (Sb₂O₃) had been presented.

[0009] Therefore, there is a need to develop an eco-friendly separation and recovery technology that can selectively separate Sb and Te even under mixed acid conditions where nitric acid coexists, does not use strong reducing agents, has a simple process, and offers high final recovery rates for each of Sb and Te. Prior art literature

[0011] US Published Patent US4324586A US Registered Patent US9222147B2 International Published Patent WO2013104045A1 US Published Patent US4293332A The problem to be solved

[0012] The present invention has been devised to solve the problems of the prior art as described above, and the problem to be solved by the present invention is as follows.

[0013] First, the present invention aims to provide a process capable of selectively separating Sb and Te even under mixed acid conditions containing hydrochloric acid (HCl) and nitric acid (HNO₃). Specifically, the invention aims to provide a process that selectively precipitates only Sb and stably maintains Te in solution by forming a redox window that selectively reduces Sb(V) to Sb(III) within an acidic solution in which nitric acid coexists.

[0014] Second, the present invention aims to improve process safety and reduce environmental burden by applying environmentally friendly ascorbic acid as a reducing agent instead of using strong reducing agents such as NaBH₄ and hydrazine.

[0015] Third, the present invention applies a method of pre-injecting hydrochloric acid followed by a subsequent stepwise injection of nitric acid, and Cl in the mixed acid eluent - Ions and NO₃ - The purpose is to provide an elution process that simultaneously achieves the preferential formation of antimony chlorine complexes and the oxidative dissolution stabilization of Te by controlling the molar concentration ratio of ions to 3:1 to 10:1.

[0016] Fourth, the present invention aims to prevent Te co-precipitation caused by excessive reduction and ensure the reproducibility of selective reduction precipitation of Sb by applying a method of real-time monitoring of the redox potential (ORP) relative to the Ag / AgCl reference electrode and dividing the ascorbic acid into first and second doses.

[0017] Fifth, the present invention aims to maximize the final recovery rates of Sb and Te by including a subsequent Te recovery process through pH adjustment from the filtrate containing remaining Te after the separation of Sb precipitates.

[0018] Sixth, the present invention aims to maximize the economic value of the recovered material by providing a consistent process that directly converts the recovered Sb precipitate into high-purity antimony oxide (Sb₂O₃) powder with a purity of 99 wt% or more according to ICP-OES analysis standards and a particle size of 1 to 10 μm according to laser diffraction (D50 standard) through a two-stage calcination process of first calcination and second calcination. means of solving the problem

[0020] The present invention relates to a method for selectively separating and recovering Sb and Te from a composite oxide catalyst or spent catalyst comprising Sb₂O₅ and TeO₂, wherein

[0021] a. A mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃);

[0022] b. A reduction step of forming an Sb precipitate by adding a reducing agent to the above-mentioned eluent to selectively reduce Sb(V) in the above-mentioned eluent to Sb(III); and

[0023] c. A selective separation step of separating the above Sb precipitate from the above eluent to leave Te in the solution phase;

[0024] Includes,

[0025] A method for the selective separation and recovery of Sb and Te from a composite oxide catalyst is provided, characterized by inhibiting the reduction of Te in the above mixed acid solution, thereby selectively forming only the Sb precipitate within the range of +100 mV to +400 mV of the oxidation-reduction potential (ORP) relative to the Ag / AgCl reference electrode.

[0026] At this time, a.A pretreatment step of preparing catalyst powder with a particle size of 1 to 100 μm by crushing the above-mentioned complex oxide catalyst or spent catalyst, and introducing the catalyst powder into a reaction vessel in a solid-to-liquid ratio of 1:3 to 1:8 (g / mL); wherein hydrochloric acid is first introduced alone into the reaction vessel and stirred in contact with the catalyst powder at 40 to 60°C for 10 to 30 minutes to first form an antimony chlorine complex, and then nitric acid is subsequently introduced stepwise to stabilize Te in an oxidized dissolved state, thereby preparing a mixed acid eluent containing 1 to 12 parts by weight of hydrochloric acid (HCl) and 0.1 to 6 parts by weight of nitric acid (HNO₃) per 100 parts by weight of a total mixed acid solution, wherein Cl in the mixed acid eluent - Ions and NO₃ - A mixed acid elution step of preparing an elution solution containing Sb and Te by stirring at 70 to 90°C for 1 to 3 hours while adjusting the input amounts of hydrochloric acid and nitric acid so that the molar concentration ratio of the ions is 3:1 to 10:1; b. A reduction step of forming an Sb precipitate by adding a reducing agent to the above-mentioned eluent to selectively reduce Sb(V) in the above-mentioned eluent to Sb(III); and c. It is characterized by including a selective separation step of separating the above Sb precipitate from the above eluent to leave Te in the solution phase.

[0027] At this time, a. A mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); b.A reduction step comprising adjusting the temperature of the above-mentioned eluent to 40–70°C, then adding ascorbic acid to the above-mentioned eluent in a first amount of 0.8–1.5 equivalents relative to the molar amount of Sb in a divided manner, and after the first addition, monitoring the oxidation-reduction potential (ORP) relative to the Ag / AgCl reference electrode in real time, and adding ascorbic acid in a second amount of 0.1–0.5 equivalents relative to the molar amount of Sb based on the point in time when the ORP value drops to +200 mV or lower, thereby selectively reducing Sb(V) in the above-mentioned eluent to Sb(III) while forming an Sb precipitate while maintaining the oxidation state of Te(IV); and c. It is characterized by including a selective separation step of separating the above Sb precipitate from the above eluent to leave Te in the solution phase.

[0028] At this time, a. A mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); b. A reduction step of forming an Sb precipitate by adding a reducing agent to the above-mentioned eluent to selectively reduce Sb(V) in the above-mentioned eluent to Sb(III); and c.The method is characterized by comprising: a first separation step of recovering a cake containing the Sb precipitate by first separating the eluent containing the Sb precipitate using a membrane filter with a pore size of 0.1 to 1.0 μm; a washing step of recovering the washing filtrate by washing the cake 1 to 3 times using a washing solution in which 1 to 5 parts by weight of hydrochloric acid are dissolved in 100 parts by weight of water, in an amount of 100 to 300 parts by weight relative to 100 parts by weight of the cake, thereby eluting the Te incorporated in the cake with the washing solution; and a Te recovery step of recovering the Te by combining the washing filtrate with the Te-containing filtrate remaining after the first separation, adjusting the pH of the combined solution to 3 to 5 with ammonia water or sodium hydroxide (NaOH) to precipitate the remaining Te in the form of tellurium dioxide hydrate (TeO₂·nH₂O), and then performing a second solid-liquid separation.

[0029] At this time, a. A mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); b. A reduction step of forming an Sb precipitate by adding a reducing agent to the above-mentioned eluent to selectively reduce Sb(V) in the above-mentioned eluent to Sb(III); c. A selective separation step of separating the above Sb precipitate from the above eluent to leave Te in the solution phase; and d.A drying step of preparing a dried Sb precipitate having a moisture content of 1 part by weight or less (based on 100 parts by weight of dry matter) according to the Karl Fischer moisture titration method by washing the above Sb precipitate with pure water 1 to 3 times and drying it at 60 to 120°C; a first calcination step of removing residual chlorine and organic matter by first calcining the dried Sb precipitate to 300 to 400°C at a heating rate of 2 to 10°C / min under an oxidizing atmosphere controlled to an oxygen concentration of 5 to 21 vol%; and a second calcination step of producing antimony oxide (Sb₂O₃) with a purity of 99 wt% or higher according to ICP-OES analysis by maintaining the temperature at 500 to 650°C for 1 to 3 hours after the first calcination, wherein the cooling rate after the second calcination is controlled to 5°C / min or less to obtain Sb₂O₃ powder with a particle size of 1 to 10 μm according to the laser diffraction method (D50 standard). It is characterized by including an Sb₂O₃ manufacturing step. Effects of the invention

[0031] The present invention relates to a method for selectively separating and recovering antimony (Sb) and tellurium (Te) from a composite oxide catalyst or spent catalyst containing Sb₂O5 and TeO₂.

[0032] The method of the present invention comprises: a mixed acid elution step of preparing an eluent containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); a reduction step of forming an Sb precipitate by adding a reducing agent to the eluent to selectively reduce Sb(V) in the eluent to Sb(III); and a selective separation step of separating the Sb precipitate from the eluent to leave Te in the solution phase.

[0033] In the present invention, silver nitrate in the mixed acid solution plays a role in inhibiting the reduction of Te(IV), and as a result, only Sb precipitates are selectively formed within a redox window range of +100 mV to +400 mV of oxidation-reduction potential (ORP) relative to the Ag / AgCl reference electrode.

[0034] The mixed acid elution step of the present invention applies a sequential injection method in which hydrochloric acid is first injected alone to first form an antimony chlorine complex, followed by the subsequent stepwise injection of nitric acid, and comprises 1 to 12 parts by weight of hydrochloric acid and 0.1 to 6 parts by weight of nitric acid per 100 parts by weight of the total mixed acid elution solution, and Cl - Ions and NO₃ - The molar concentration ratio of ions is maintained at 3:1 to 10:1.

[0035] The reduction step of the present invention applies a method of dividing the injection into a first step (0.8 to 1.5 equivalents relative to the molar amount of Sb) and a second step (0.1 to 0.5 equivalents relative to the molar amount of Sb) while monitoring the ORP relative to the Ag / AgCl reference electrode in real time using ascorbic acid as a reducing agent.

[0036] The selective separation step of the present invention comprises a three-stage sequential process of primary separation using a membrane filter with a pore size of 0.1 to 1.0 μm, cake washing using an acidic washing solution, and subsequent recovery of Te by adjusting the pH of the mixture of the washing solution and the Te-containing solution to 3 to 5.

[0037] The present invention may further include a step of directly converting the recovered Sb precipitate into high-purity antimony oxide (Sb₂O₃) powder with a purity of 99 wt% or more according to ICP-OES analysis standards and a particle size of 1 to 10 μm according to laser diffraction method (D50 standard) through a two-stage calcination process of primary calcination at 300 to 400°C and secondary calcination at 500 to 650°C.

[0038] According to the present invention, Sb and Te can be selectively separated and recovered through an environmentally friendly process without using a strong reducing agent even under mixed acid conditions in the presence of nitric acid, and by providing a consistent process that directly converts the recovered Sb into high-purity Sb₂O₃ powder, it can contribute to the efficient recycling of rare metal resources and the enhancement of their industrial value. Specific details for implementing the invention

[0040] In the following, specific structural or functional descriptions of the embodiments are disclosed merely for illustrative purposes and may be modified and implemented in various forms. Accordingly, the embodiments are not limited to the specific disclosed forms, and the scope of this specification includes modifications, equivalents, or substitutions that fall within the technical concept.

[0041] Terms such as "first" or "second" may be used to describe various components, but these terms should be interpreted solely for the purpose of distinguishing one component from another. For example, the first component may be named the second component, and similarly, the second component may be named the first component.

[0042] When it is stated that a component is "connected" to another component, it should be understood that it may be directly connected to or joined to that other component, or that there may be other components in between.

[0043] The terms used in the embodiments are for illustrative purposes only and should not be interpreted as intended to be limiting. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as "comprising" or "having" are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0044] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the embodiments pertain. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this application.

[0045] The present invention is not limited to the embodiments disclosed below but may be implemented in various different forms, and these embodiments are provided merely to make the disclosure of the present invention complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims.

[0046] In the embodiments of the present invention, unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in the embodiments of the present invention.

[0047] In describing the present invention, if it is determined that a detailed description of related prior art could unnecessarily obscure the essence of the invention, such detailed description is omitted. Where terms such as "comprising," "having," or "consisting of" are used in this specification, other parts may be added unless "only" is used. Where a component is expressed in the singular, it includes cases where it is included in the plural unless specifically stated otherwise.

[0048] In interpreting the components, they are interpreted to include a margin of error even in the absence of a separate explicit statement.

[0049] The features of each of the various embodiments of the present invention may be combined or combined with one another, either partially or wholly, and as will be fully understood by those skilled in the art, various technical interlocking and operation are possible, and each embodiment may be implemented independently of one another or together in an interlocking relationship.

[0051] The present invention relates to a method for selectively separating and recovering Sb and Te from a composite oxide catalyst or spent catalyst comprising Sb₂O₅ and TeO₂, wherein

[0052] a.A mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃);

[0053] b. A reduction step of forming an Sb precipitate by adding a reducing agent to the above-mentioned eluent to selectively reduce Sb(V) in the above-mentioned eluent to Sb(III); and

[0054] c. A selective separation step of separating the above Sb precipitate from the above eluent to leave Te in the solution phase;

[0055] Includes,

[0056] A method for the selective separation and recovery of Sb and Te from a composite oxide catalyst is provided, characterized by inhibiting the reduction of Te in the above mixed acid solution, thereby selectively forming only the Sb precipitate within the range of +100 mV to +400 mV of the oxidation-reduction potential (ORP) relative to the Ag / AgCl reference electrode.

[0057] At this time, a. A pretreatment step of preparing catalyst powder with a particle size of 1 to 100 μm by crushing the above-mentioned complex oxide catalyst or spent catalyst, and introducing the catalyst powder into a reaction vessel in a solid-to-liquid ratio of 1:3 to 1:8 (g / mL); wherein hydrochloric acid is first introduced alone into the reaction vessel and stirred in contact with the catalyst powder at 40 to 60°C for 10 to 30 minutes to first form an antimony chlorine complex, and then nitric acid is subsequently introduced stepwise to stabilize Te in an oxidized dissolved state, thereby preparing a mixed acid eluent containing 1 to 12 parts by weight of hydrochloric acid (HCl) and 0.1 to 6 parts by weight of nitric acid (HNO₃) per 100 parts by weight of a total mixed acid solution, wherein Cl in the mixed acid eluent - Ions and NO₃ -A mixed acid elution step of preparing an elution solution containing Sb and Te by stirring at 70 to 90°C for 1 to 3 hours while adjusting the input amounts of hydrochloric acid and nitric acid so that the molar concentration ratio of the ions is 3:1 to 10:1; b. A reduction step of forming an Sb precipitate by adding a reducing agent to the above-mentioned eluent to selectively reduce Sb(V) in the above-mentioned eluent to Sb(III); and c. It is characterized by including a selective separation step of separating the above Sb precipitate from the above eluent to leave Te in the solution phase.

[0058] At this time, a. A mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); b. A reduction step comprising adjusting the temperature of the above-mentioned eluent to 40–70°C, then adding ascorbic acid to the above-mentioned eluent in a first amount of 0.8–1.5 equivalents relative to the molar amount of Sb in a divided manner, and after the first addition, monitoring the oxidation-reduction potential (ORP) relative to the Ag / AgCl reference electrode in real time, and adding ascorbic acid in a second amount of 0.1–0.5 equivalents relative to the molar amount of Sb based on the point in time when the ORP value drops to +200 mV or lower, thereby selectively reducing Sb(V) in the above-mentioned eluent to Sb(III) while forming an Sb precipitate while maintaining the oxidation state of Te(IV); and c. It is characterized by including a selective separation step of separating the above Sb precipitate from the above eluent to leave Te in the solution phase.

[0059] At this time, a. a mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); b. a reduction step of forming an Sb precipitate by adding a reducing agent to the elution solution to selectively reduce Sb(V) in the elution solution to Sb(III); and c. The method is characterized by comprising: a first separation step of recovering a cake containing the Sb precipitate by first separating the eluent containing the Sb precipitate using a membrane filter with a pore size of 0.1 to 1.0 μm; a washing step of recovering the washing filtrate by washing the cake 1 to 3 times using a washing solution in which 1 to 5 parts by weight of hydrochloric acid are dissolved in 100 parts by weight of water, in an amount of 100 to 300 parts by weight relative to 100 parts by weight of the cake, thereby eluting the Te incorporated in the cake with the washing solution; and a Te recovery step of recovering the Te by combining the washing filtrate with the Te-containing filtrate remaining after the first separation, adjusting the pH of the combined solution to 3 to 5 with ammonia water or sodium hydroxide (NaOH) to precipitate the remaining Te in the form of tellurium dioxide hydrate (TeO₂·nH₂O), and then performing a second solid-liquid separation.

[0060] At this time, a. a mixed acid elution step of preparing an eluent containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); b. a reduction step of forming an Sb precipitate by adding a reducing agent to the eluent to selectively reduce Sb(V) in the eluent to Sb(III); c. a selective separation step of separating the Sb precipitate from the eluent to leave Te remaining in the solution phase; and d. A drying step of preparing a dried Sb precipitate having a moisture content of 1 part by weight or less (based on 100 parts by weight of dry matter) according to the Karl Fischer moisture titration method by washing the above Sb precipitate with pure water 1 to 3 times and drying it at 60 to 120°C; a first calcination step of removing residual chlorine and organic matter by first calcining the dried Sb precipitate to 300 to 400°C at a heating rate of 2 to 10°C / min under an oxidizing atmosphere controlled to an oxygen concentration of 5 to 21 vol%; and a second calcination step of producing antimony oxide (Sb₂O₃) with a purity of 99 wt% or higher according to ICP-OES analysis by maintaining the temperature at 500 to 650°C for 1 to 3 hours after the first calcination, wherein the cooling rate after the second calcination is controlled to 5°C / min or less to obtain Sb₂O₃ powder with a particle size of 1 to 10 μm according to the laser diffraction method (D50 standard). It is characterized by including an Sb₂O₃ manufacturing step.

[0062] Reason for selecting materials

[0063] Reasons for selecting each material and technical significance

[0064] Complex oxide catalyst (including Sb₂O₅ and TeO₂)

[0065] The composite oxide catalyst subject to treatment in this invention is a multi-component oxide catalyst comprising Sb₂O₅ and TeO₂ as major active components, and additionally including carriers or auxiliary oxides such as SiO₂ and Fe₂O₃. This catalyst is widely used in petrochemical processes, such as the propylene oxidation reaction and the selective oxidation reaction of butane, and a significant amount of Sb and Te remains even after prolonged use. The reason for selecting the composite oxide catalyst as the subject of treatment in this invention is that while Sb and Te are strategic rare metals with high resource recycling value, there is a clear technical challenge in that selective separation is difficult using conventional technology due to the close redox potentials of the two elements. Furthermore, since the SiO₂ and Fe₂O₃ carrier components included in the catalyst are separated from Sb and Te under mixed acid elution conditions and remain as insoluble residues, the selective elution of Sb and Te is possible without a separate carrier removal process, which aligns with the objective of process simplification in this invention.

[0067] Reason for choosing hydrochloric acid (HCl)

[0068] The reason hydrochloric acid (HCl) was selected as the main component of the eluent in this invention is as follows. Hydrochloric acid reacts with Sb₂O₅ to form an antimony chlorine complex (SbCl₄ - , SbCl6 - It has excellent ability to stably dissolve Sb in acidic solutions by forming (etc.). In particular, Cl -The ions form strong complexes with Sb(III) and Sb(V), thereby inhibiting the hydrolytic precipitation of Sb while facilitating the formation of selective precipitation conditions for Sb(III) in the subsequent reduction step. Additionally, since hydrochloric acid has lower oxidizing power compared to nitric acid and does not cause Te overoxidation, it plays a role in preventing Te overoxidation by preferentially promoting the formation of chlorine complexes of Sb during the hydrochloric acid-only addition step. Furthermore, unlike other inorganic acids such as sulfuric acid and phosphoric acid, hydrochloric acid does not form sparingly soluble sulfate or phosphate precipitates, which has the advantage of maintaining a simple composition of the eluent without generating unnecessary impurity precipitates in subsequent processes.

[0069] The reason for limiting the concentration of hydrochloric acid in this invention to 1 to 12 parts by weight per 100 parts by weight of the total mixed acid solution is that if the hydrochloric acid concentration is less than 1 part by weight, the formation of the Sb chlorine complex is insufficient, resulting in a significant decrease in the Sb elution rate, whereas if it exceeds 12 parts by weight, excess Cl - This is because ions promote the redissolution of Sb precipitates during the subsequent reduction step, which not only lowers the Sb recovery rate but also causes process safety issues due to the handling of high-concentration hydrochloric acid.

[0071] Reason for selecting nitric acid (HNO₃)

[0072] The reason nitric acid (HNO₃) was selected as a component of the mixed acid in this invention is that nitric acid is a strong oxidizing agent with excellent ability to oxidize and dissolve TeO₂ in the form of Te(IV) or Te(VI). In particular, under acidic conditions where nitric acid is present, the redox potential of Te increases, thereby inhibiting the reduction and precipitation of Te by the reduction action of ascorbic acid. This effect of inhibiting Te reduction serves as the basis for the formation of a redox window, which is a core technical feature of this invention, and is confirmed through a comparative example in which, under conditions of hydrochloric acid alone without nitric acid, Sb and Te are simultaneously reduced and precipitated upon the addition of ascorbic acid, so selective separation does not occur.

[0073] The reason the concentration of nitric acid in the present invention is limited to 0.1 to 6 parts by weight per 100 parts by weight of the total mixed acid solution is that if the nitric acid concentration is less than 0.1 parts by weight, the effect of inhibiting Te reduction is insufficient, and there is a risk of Te co-precipitation occurring when ascorbic acid is added, whereas if it exceeds 6 parts by weight, the oxidizing power becomes excessively high, and ascorbic acid is oxidatively decomposed by nitric acid before it contributes to the reduction of Sb(V), thereby rapidly decreasing the effective equivalent as a reducing agent and making it difficult to control the reduction reaction.

[0075] Reason for choosing ascorbic acid

[0076] The reason ascorbic acid (vitamin C) was selected as the reducing agent in this invention is as follows. The standard redox potential of ascorbic acid is approximately +0.08 V (vs. SHE), which is lower than the standard redox potential of the Sb(V) / Sb(III) pair (approx. +0.69 V vs. SHE), thereby providing thermodynamically sufficient driving force to reduce Sb(V) to Sb(III). On the other hand, under conditions where nitric acid is present, the effective redox potential of the Te(IV) / Te(0) pair increases due to the oxidation-inhibiting effect of nitric acid; consequently, the reducing power of ascorbic acid remains insufficient to reduce and precipitate Te. This selective reduction behavior constitutes the core mechanism of this invention.

[0077] Furthermore, ascorbic acid is a water-soluble natural organic acid with low toxicity, and the dehydroascorbic acid produced after oxidation is also a water-soluble organic compound that can be easily removed during post-treatment processes, resulting in a low environmental burden. This represents a clear environmental advantage in contrast to the explosion risk of hydrogen gas generated when using NaBH₄ and the toxic wastewater issues associated with the use of hydrazine. Moreover, ascorbic acid is readily available as a food additive grade and is affordable, making it highly suitable for industrial application.

[0078] The reason for limiting the input amount of ascorbic acid to a total range of 0.9 to 2.0 equivalents relative to the molar amount of Sb (0.8 to 1.5 equivalents in the first step + 0.1 to 0.5 equivalents in the second step) is that if the total input amount is less than 0.9 equivalents, the reduction of Sb(V) to Sb(III) is incomplete, resulting in a decrease in the Sb precipitation rate, whereas if it exceeds 2.0 equivalents, the reduction precipitation of Te(IV) is induced by the excess reducing agent, causing the selective separation effect to be lost.

[0080] (a) Technical reasons, critical significance, and specific implementation methods of the mixed acid elution step

[0081] (a) Technical significance of the entire step

[0082] The mixed acid elution step is a step that lays the foundation for a subsequent wet separation process by eluting Sb₂O5 and TeO₂, which exist in a solid state in the composite oxide catalyst, into an acidic solution, and serves as the starting point of the entire process of the present invention. Since the elution efficiency of Sb and Te and the chemical forms (oxidation number, complex form) of the two elements in the eluent directly determine the selective separation performance in the subsequent reduction step, precise control of the elution conditions is essential. In particular, unlike conventional technology, the present invention adopts a method of adding hydrochloric acid first followed by nitric acid in a stepwise manner, rather than simultaneously mixing and adding hydrochloric acid and nitric acid, thereby separating the roles of each acid stepwise to simultaneously achieve the preferential formation of Sb chlorine complexes and the stabilization of dissolved Te oxidation.

[0084] (a1) Catalyst grinding and pretreatment step

[0085] The reason for grinding composite oxide catalysts or spent catalysts to prepare catalyst powder with a particle size of 1 to 100 μm is to increase the specific surface area of ​​the catalyst and maximize the contact area for the acid elution reaction. If the particle size exceeds 100 μm, Sb and Te inside the catalyst particles cannot sufficiently contact the acid eluent, resulting in a significant decrease in elution efficiency and a substantial increase in reaction time required to achieve the same elution rate. On the other hand, if the particle size is excessively fined to less than 1 μm, the energy cost of grinding increases sharply, and the filtration efficiency of the fine catalyst decreases, placing a burden on the subsequent solid-liquid separation process; therefore, the particle size range of 1 to 100 μm corresponds to the critical range that simultaneously satisfies both elution efficiency and process economics.

[0086] The reason for adding catalyst powder to the reaction vessel in a solid-to-liquid ratio range of 1:3 to 1:8 (g / mL) is that when the solid-to-liquid ratio is less than 1:3 (i.e., when the catalyst concentration is excessively high), the volume of the acid eluent is insufficient, leading to the saturation concentration of Sb and Te, or the viscosity of the eluent increases, which reduces stirring efficiency. On the other hand, when the ratio exceeds 1:8 (i.e., when the catalyst concentration is excessively low), the concentration of Sb and Te per unit volume of the eluent is low, which reduces the efficiency of the subsequent sedimentation process and increases the burden of wastewater treatment. Therefore, a solid-to-liquid ratio range of 1:3 to 1:8 corresponds to a critical range that simultaneously satisfies both elution efficiency and the economic feasibility of the subsequent process.

[0088] (a2) Hydrochloric acid pre-injection step

[0089] The reason for first adding hydrochloric acid alone and stirring it with the catalyst powder at 40–60°C for 10–30 minutes is that antimony chlorine complex (SbCl₄) is formed through the reaction of Sb₂O₅ and HCl. - , SbCl6 -The purpose is to form the chlorine complex first. The formation of the chlorine complex provides a chemical basis for maintaining Sb stably in the eluent even in an oxidizing environment caused by the subsequent addition of nitric acid. If nitric acid and hydrochloric acid are added simultaneously, the strong oxidizing power of nitric acid maintains Sb in a highly oxidized state (Sb(V)) before the formation of the chlorine complex of Sb, while simultaneously proceeding with the oxidative elution of TeO₂, which may reduce the selective separation performance in the subsequent reduction step.

[0090] The reason the temperature of the hydrochloric acid pre-injection step is limited to 40–60°C is that below 40°C, the reaction rate between Sb₂O₅ and HCl is slow, resulting in insufficient formation of the chlorine complex, whereas above 60°C, the volatile loss of hydrochloric acid increases, leading to a decrease in the effective concentration. Additionally, the reason the stirring time is limited to 10–30 minutes is that below 10 minutes, the reaction to form the chlorine complex does not proceed sufficiently, whereas above 30 minutes, the process time increases unnecessarily, leading to a decrease in productivity.

[0092] (a3) Subsequent stepwise injection of nitric acid

[0093] The reason for subsequently adding nitric acid in stages is to achieve the oxidative elution of TeO₂ and the stabilization of the dissolved Te by adding nitric acid after the antimony chlorine complex has been sufficiently formed by the initial addition of hydrochloric acid. At this time, the oxidizing action of nitric acid plays a role in stably maintaining Te(IV) in the solution, which forms the basis for the formation of a redox window in which nitric acid inhibits the reduction of Te in the subsequent reduction step. Since adding an excessive amount of nitric acid at once can cause the oxidizing power to rise rapidly and reduce the stability of the already formed antimony chlorine complex, a staged addition method is essential.

[0095] (a4) Cl - / NO₃ - Molar concentration ratio control

[0096] Cl in mixed acid eluent - Ions and NO₃- The reason for controlling the ion molar concentration ratio to 3:1 to 10:1 is that within this range, the stability of Sb's chlorine complex formation and the stabilization of Te's oxidative dissolution are simultaneously satisfied. Cl - / NO₃ - When the molar concentration ratio is less than 3:1 (i.e., when the relative proportion of nitric acid is high), the strong oxidizing power of nitric acid impairs the stability of the Sb chlorine complex, and the effective equivalent of ascorbic acid decreases in the subsequent reduction step, making the selective reduction of Sb(V) difficult. On the other hand, Cl - / NO₃ - If the molar concentration ratio exceeds 10:1 (i.e., the relative proportion of nitric acid is low), the inhibitory effect of nitric acid on Te reduction is insufficient, posing a risk of Te co-precipitation in the subsequent reduction step. Therefore, Cl - / NO₃ - A molar concentration ratio of 3:1 to 10:1 corresponds to a critical range that stably forms a redox window for selective Sb precipitation.

[0098] (a5) Elution temperature and time conditions

[0099] The reason for limiting the elution temperature to 70–90°C is that below 70°C, the acid elution reaction rates of Sb₂O₅ and TeO₂ are slow, requiring a long reaction time and resulting in a decrease in the elution rate; on the other hand, if the temperature exceeds 90°C, the volatile loss of hydrochloric acid increases sharply, leading to an effective Cl₂ - This is because the concentration decreases and process safety issues arise. The reason the elution time is limited to 1 to 3 hours is that, in less than 1 hour, complete elution of Sb and Te, particularly within large catalyst particles, does not occur, whereas if it exceeds 3 hours, only the process time increases without further improvement in the elution rate, resulting in reduced productivity.

[0101] (b) Technical reasons, critical significance, and specific implementation methods of the reduction step

[0102] (b) Technical significance of the entire stage

[0103] The reduction step is a step in which the core separation principle of the present invention is implemented. It involves using ascorbic acid as a reducing agent to selectively reduce only Sb(V) to Sb(III) in a mixed acid eluent where Sb(V) and Te(IV) coexist, thereby forming conditions for the hydrolytic precipitation or basic salt precipitation of Sb. The principle of selective reduction in this step is based on the redox window effect, where the reducing action of ascorbic acid provides sufficient driving force for the Sb(V) / Sb(III) pair under conditions where nitric acid coexists, whereas the reduction of Te(IV) is suppressed as the redox potential of Te increases due to the nitric acid. The redox window is formed within an ORP range of +100 mV to +400 mV relative to the Ag / AgCl reference electrode, and within this range, only the Sb(III) precipitate is selectively formed while Te(IV) remains in the solution.

[0105] (b1) Temperature conditions of the reduction step

[0106] The reason for adjusting the eluent temperature of the reduction step to 40–70°C is that below 40°C, the reduction reaction rate of ascorbic acid and Sb(V) is slow, which leads to an excessive increase in reaction time and a decrease in the Sb precipitation rate, whereas above 70°C, the thermal oxidative decomposition of ascorbic acid itself is accelerated, reducing the effective equivalent as a reducing agent. In particular, under high temperature conditions exceeding 70°C, side reactions in which ascorbic acid is oxidatively decomposed by dissolved oxygen or nitric acid are promoted before it contributes to the reduction of Sb(V); therefore, controlling the upper limit of the temperature range is critically important for ensuring reduction efficiency.

[0108] (b2) Primary dosage of ascorbic acid and method of administration

[0109] The reason for adding ascorbic acid in divided primary amounts of 0.8 to 1.5 equivalents relative to the molar amount of Sb is to reduce most of the Sb(V) to Sb(III) during the primary addition stage while preventing the risk of Te co-precipitation caused by excessive addition. If the primary addition amount is less than 0.8 equivalents, the reduction conversion rate of Sb(V) is low, making it difficult to achieve the target Sb precipitation rate even during subsequent secondary additions; if it exceeds 1.5 equivalents, excess reducing agent remains even with only the primary addition, which may induce the reduction precipitation of Te(IV). Therefore, the primary addition range of 0.8 to 1.5 equivalents corresponds to a critical range that simultaneously satisfies the sufficiency of Sb(V) reduction and the safety of preventing Te co-precipitation.

[0111] (b3) ORP Real-time Monitoring-Based Control

[0112] The reason for determining the second injection based on the point at which the ORP value drops below +200 mV by monitoring the ORP relative to the Ag / AgCl reference electrode in real time after the first injection is that the point at which the ORP of the eluent drops below +200 mV corresponds to the time when most of the Sb(V) has been reduced to Sb(III) and additional reducing agent injection is required to reduce the remaining Sb(V). An ORP of +200 mV corresponds to the lower boundary value at which Te(IV) remains dissolved under conditions where nitric acid coexists; by determining the timing of the second injection before the ORP drops below this level, the remaining Sb(V) can be further reduced without the risk of precipitation of Te(IV). The real-time ORP monitoring method holds intrinsic significance in ensuring process reproducibility as it can automatically correct for variables that inevitably occur in the actual process, such as fluctuations in catalyst composition, ambient temperature, and deviations in eluent concentration.

[0114] (b4) Secondary dose of ascorbic acid

[0115] The reason for adding ascorbic acid as a secondary input of 0.1 to 0.5 equivalents relative to the molar amount of Sb, based on the point at which ORP +200 mV or less is reached, is to maximize the Sb recovery rate by completely reducing the unreduced Sb(V) remaining after the primary input. If the secondary input is less than 0.1 equivalents, the reduction of the remaining Sb(V) is incomplete, resulting in a decrease in the final Sb recovery rate, whereas if it exceeds 0.5 equivalents, the risk of the ORP dropping below the Te reduction threshold due to excessive reducing agent increases. Therefore, the range of 0.1 to 0.5 equivalents for the secondary input corresponds to a critical range that simultaneously achieves the complete reduction of remaining Sb(V) and prevents Te co-precipitation, and the total combined input of the primary and secondary split inputs must be maintained at 0.9 to 2.0 equivalents.

[0117] (b5) Technical significance of maintaining Te(IV) oxidation state

[0118] The fact that Te remains in a state maintaining the oxidation state of Te(IV) during the reduction step is a key result of the selective separation process of the present invention, which provides a chemical basis for retaining Te in the solution phase during the subsequent selective separation step and further enabling easy recovery in the form of TeO₂·nH₂O through pH adjustment during the Te recovery step. Since Te(IV) precipitates as tellurium dioxide hydrate (TeO₂·nH₂O) in the pH range of 3 to 5, maintaining the Te(IV) state during the reduction step directly determines the efficiency of the subsequent Te recovery step.

[0120] (c) Technical reasons, critical significance, and specific implementation methods of the selective separation step

[0122] (c) Technical significance of the entire stage

[0123] The selective separation step is a step that efficiently separates the Sb precipitate formed in the reduction step from the eluent containing Te, and recovers the Te mixed in the Sb precipitate during the separation process to maximize the final recovery rate of Te. The selective separation step of the present invention is not limited to primary separation by simple filtration, but consists of a three-stage sequential process of Te elution recovery by cake washing and subsequent Te recovery by pH adjustment of the mixture, thereby independently maximizing the final recovery rates of Sb and Te, respectively.

[0125] (c1) Primary separation using a membrane filter

[0126] The reason for primary solid-liquid separation of the eluent containing Sb precipitates using a membrane filter with a pore size of 0.1 to 1.0 μm is that the particle size of the Sb precipitates formed during the reduction step is typically distributed in the range of 0.5 to 5 μm, so this is done to effectively capture Sb precipitates within this range while allowing the filtrate containing Te ions to pass through the filter. If the pore size is less than 0.1 μm, the filtration resistance becomes excessively high, significantly reducing the processing speed and shortening the membrane lifespan, whereas if it exceeds 1.0 μm, fine Sb precipitate particles leak into the filtrate, reducing the Sb recovery rate. Therefore, using a membrane filter with a pore size of 0.1 to 1.0 μm corresponds to the critical condition that simultaneously satisfies both Sb capture efficiency and filtration speed.

[0128] (c2) Cake acid cleaning step

[0129] The reason for washing the Sb precipitate cake obtained after the first solid-liquid separation with a washing solution prepared by dissolving 1 to 5 parts by weight of hydrochloric acid in 100 parts by weight of water is to remove Te from the cake by eluting the Te-containing liquid remaining in the pores within the cake. This is because if the concentration of hydrochloric acid in the washing solution is less than 1 part by weight (based on 100 parts by weight of water), some of the Sb precipitate may be redissolved in the washing solution, and if it exceeds 5 parts by weight, the washing elution efficiency of Te decreases and the chlorine content of the Sb precipitate increases after washing, thereby increasing the burden of removing residual chlorine in the subsequent calcination step.

[0130] The reason for using 100 to 300 parts by weight of the washing solution relative to 100 parts by weight of the cake is that if the washing solution is less than 100 parts by weight, the elution of the remaining Te in the cake is incomplete, and Te contamination of the Sb precipitate remains, whereas if it exceeds 300 parts by weight, some of the Sb precipitate is dissolved and lost in the washing solution, resulting in a decrease in the Sb recovery rate. The reason for limiting the number of washes to 1 to 3 times is that with only one wash, the elution of Te in the cake is incomplete, whereas washing more than 3 times only increases the amount of wastewater generated without an additional Te elution effect.

[0132] (c3) Preparation of a mixture for Te recovery

[0133] The reason for combining the Te-containing filtrate and the cake washing filtrate after the first separation is that both filtrates contain Te(IV), so they are combined and utilized as raw materials for the Te recovery process to maximize the Te recovery rate of the entire process. In particular, since the cake washing filtrate may contain not only Te eluted from the Sb cake but also a small amount of Sb that may be lost during the cake manufacturing process, it is important to manage the two filtrates together at the mixing stage to maximize the element recovery rate.

[0135] (c4) Recovery of Te precipitate by pH adjustment

[0136] The reason for adjusting the pH of the mixture to 3 to 5 with ammonia water or sodium hydroxide (NaOH) to precipitate Te in the form of tellurium dioxide hydrate (TeO₂·nH₂O) is that the hydrolysis precipitation of Te(IV) proceeds selectively in the pH range of 3 to 5. If the pH is less than 3, the acidic conditions are too strong, so the solubility of TeO₂·nH₂O is high and precipitation is incomplete; if the pH exceeds 5, there is a risk of co-precipitation of impurities such as residual Sb ions or Fe ions, which lowers the purity of the Te precipitate. Therefore, the pH range of 3 to 5 corresponds to a critical range that simultaneously satisfies the selective precipitation of Te(IV) and the prevention of impurity co-precipitation.

[0137] The reason ammonia water or sodium hydroxide (NaOH) was selected as pH adjusters is that ammonia water buffers rapid pH increases even with excessive addition, making pH control easy, whereas sodium hydroxide is inexpensive and easy to handle industrially, making it suitable for large-scale process applications. Both pH adjusters contain Ca 2+ , Ba 2+ There is a common advantage in that it does not form sparingly soluble salts, thus not causing contamination by Te precipitates.

[0139] (d) Technical reasons, critical significance, and specific implementation methods of the high-purity Sb₂O₃ manufacturing step

[0140] (d) Technical significance of the entire step

[0141] The high-purity Sb₂O₃ manufacturing step is a step in which the Sb precipitate recovered in the selective separation step is finally converted into antimony oxide (Sb₂O₃) powder, which is a raw material for high-value flame retardants and electronic materials. This step characterizes the present invention as an integrated process capable of going beyond a simple Sb separation and recovery process to the direct commercialization of the recovered material. In particular, conventional technology had a problem in which impurities such as residual chlorine and organic matter (by-products of ascorbic acid oxidation) remained because the recovery of the Sb precipitate was limited to simple drying treatment without a separate purification process, but the present invention structurally solves this through a two-stage calcination process.

[0143] (d1) Washing and drying steps

[0144] The reason for washing the Sb precipitate with pure water 1 to 3 times is to remove Cl adsorbed or remaining on the surface and pores of the precipitate. - Ions, NO₃ - The purpose is to remove ions, ascorbic acid, and their oxidation byproducts to minimize the generation of impurities in the subsequent calcination step. If the number of washes is less than 1, the removal of residual chlorine is insufficient, and if it exceeds 3, the recovery rate decreases due to the accumulation of fine loss of Sb precipitates caused by washing, so the number of washes is 1 to 3, which is the critical range.

[0145] The reason the drying temperature is limited to 60 to 120°C is that below 60°C the drying time becomes excessively long, whereas if it exceeds 120°C, the chlorine-containing compounds on the surface of the Sb precipitate partially decompose and release HCl gas, which can cause corrosion of the drying equipment. The reason the drying is completed with a moisture content of 1 part by weight or less (based on 100 parts by weight of the dried material) according to the Karl Fischer moisture titration method is that if the moisture content exceeds 1 part by weight, cracks may occur inside the fired body or powder aggregation may be promoted due to rapid moisture evaporation during the initial stage of the first firing, which can reduce the particle size uniformity of the final Sb₂O₃ powder.

[0147] (d2) 1st calcination stage (dechlorination and deoiling stage)

[0148] The reason for primary calcining the dry Sb precipitate to 300–400°C at a heating rate of 2–10°C / min under an oxidizing atmosphere controlled with an oxygen concentration of 5–21 vol% is that residual chlorine (Cl - The purpose is to oxidatively decompose and remove organic matter (such as ascorbic acid oxidation byproducts) and contained compounds. If the oxygen concentration is less than 5 vol%, complete oxidative combustion of organic matter does not occur, and carbonized material may remain. If it exceeds 21 vol%, process costs increase due to the supply of high-purity oxygen, and excessive oxidation may induce the premature conversion of Sb precipitates into oxides. Since using ordinary air (approx. 21 vol% O₂) corresponds to the cost-effective upper limit, an oxygen concentration range of 5 to 21 vol% satisfies both dechlorination and de-oiling efficiency and cost-effectiveness.

[0149] The reason for limiting the heating rate to 2–10℃ / min is that if the heating rate is less than 2℃ / min, the calcination time increases excessively, leading to reduced productivity; on the other hand, if it exceeds 10℃ / min, the rapid temperature rise causes HCl gas generated during the rapid decomposition of residual chlorine to accumulate locally at high concentrations inside the calcination furnace, causing equipment corrosion and increasing particle size variation due to uneven heat treatment inside the calcined body. The reason for limiting the upper temperature limit for the first calcination to 400℃ is that if the temperature exceeds 400℃, partial sublimation loss of Sb₂O₃ may begin. Although Sb₂O₃ begins to sublimate at approximately 655℃, since microscopic sublimation loss from the surface can occur even at temperatures above 400℃, it is critically important to complete the dechlorination and de-oiling stages at 400℃ or below.

[0151] (d3) Secondary calcination step (Sb₂O₃ crystallization step)

[0152] The reason for performing a second calcination at 500–650°C for 1–3 hours after the first calcination is to convert the amorphous or partially crystallized Sb oxide remaining after the first calcination into a complete Sb₂O₃ crystal structure. If the second calcination temperature is below 500°C, the crystallization of Sb₂O₃ is incomplete, leaving amorphous components, which may make it difficult to achieve a purity of 99 wt% or higher according to ICP-OES standards; if it exceeds 650°C, the sublimation loss of Sb₂O₃ increases rapidly, leading to a decrease in recovery rate and the problem of Sb contamination inside the calcination furnace. If the holding time is less than 1 hour, the crystallization reaction is not completed, whereas if it exceeds 3 hours, excessive particle growth occurs, which may result in the formation of coarse particles exceeding the target particle size range (1–10 μm according to D50).

[0154] (d4) Cooling rate control

[0155] The reason for controlling the cooling rate after the second calcination to 5℃ / min or less is that under rapid cooling conditions where the cooling rate exceeds 5℃ / min, thermal stress is generated inside the Sb₂O₃ crystal particles, which promotes particle fracture and fine powder generation, making it difficult to obtain a uniform powder with a particle size of 1 to 10 μm as determined by laser diffraction (based on D50). A slow cooling rate allows for stable relaxation of the Sb₂O₃ crystal structure, thereby achieving a morphologically uniform particle distribution, which is a critical factor in determining the dispersibility of the final product and its quality of application as a flame retardant or a raw material for electronic materials.

[0157] (d5) Critical significance of final product quality standards

[0158] The reason for establishing a purity standard of 99 wt% or higher based on ICP-OES analysis is that commercial specifications for antimony oxide used as a raw material for flame retardants and for electronic materials typically require a purity of 99 wt% or higher. Since impurities such as residual Te, Fe, and Si may reduce the flame retardant effect or adversely affect electrical properties when applied to electronic materials if the purity is less than 99 wt%, the 99 wt% standard is a critical criterion that guarantees commercial applicability.

[0159] The reason for establishing a standard particle size of 1 to 10 μm based on laser diffraction (D50 standard) is that commercial specifications for Sb₂O₃, which is used as a raw material for flame retardants, typically require a D50 standard range of 1 to 5 μm. If the particle size is less than 1 μm, the risk of dust explosion increases and handling safety decreases, whereas if it exceeds 10 μm, dispersibility within the resin matrix decreases, resulting in a reduced flame retardant effect. Therefore, the D50 standard particle size range of 1 to 10 μm corresponds to a critical range that simultaneously satisfies handling safety and application performance.

[0161] Examples and Comparative Examples

[0163] (a) Examples and comparative examples of the mixed acid elution step

[0165] Overview of Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-4

[0166] The following Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-4 describe the method of addition, concentration, and Cl of hydrochloric acid and nitric acid in the mixed acid elution step. - / NO₃ -The study was designed to investigate the effects of molar concentration ratio, elution temperature, and time on the elution behavior of Sb and Te. In each example and comparative example, the composite oxide catalyst to be treated had a composition of 535 wt% Sb₂O, 15 wt% TeO₂, 40 wt% SiO₂, and 10 wt% Fe₂O₃. The catalyst was ground using a ball mill and then sieved to prepare a catalyst powder with a particle size of 1 to 100 μm. The catalyst powder was introduced into a 2 L glass reaction vessel at a solid-to-liquid ratio of 1:5 (g / mL), and a reflux condenser was installed in the vessel to prevent volatile loss of acid. Stirring was performed at 200 rpm using an anchor-type stirrer. The total volume of the mixed acid solution was standardized to 500 mL per 100 g of catalyst.

[0167] Summary of conditions for mixed acid elution step examples and comparative examples

[0168] division HCl Addition Amount (Parts by weight / Based on 100 parts by weight) HNO₃ Addition Amount (Parts by weight / Based on 100 parts by weight) Cl - / NO₃ - Molar concentration ratio Input method Hydrochloric acid line contact temperature (°C) Hydrochloric acid line contact time (minutes) Dissolution temperature (°C) Dissolution time (time) Example 1-1 5 1 8:1 Hydrochloric acid first, followed by nitric acid 50 20 80 2 Examples 1-2 3 0.5 9:1 Hydrochloric acid first, followed by nitric acid 45 15 75 2.5 Examples 1-3 8 2 6:1 Hydrochloric acid first, followed by nitric acid 55 25 85 1.5 Examples 1-4 1 0.1 10:1 Hydrochloric acid first, followed by nitric acid 40 10 70 3 Examples 1-5 12 6 3:1 Hydrochloric acid first, followed by nitric acid 60 30 90 1 Comparative Example 1-1 5 1 8:1 Simultaneous injection of hydrochloric acid and nitric acid - - 80 2 Comparative Example 1-2 0.5 1 0.75:1 Hydrochloric acid first, followed by nitric acid 50 20 80 2 Comparative Examples 1-3 5 8 Less than 1:1 Hydrochloric acid first, followed by nitric acid 50 20 80 2 Comparative Examples 1-4 5 1 8:1 Hydrochloric acid first, followed by nitric acid 50 20 60 0.5

[0169] Example 1-1

[0170] 100 g of catalyst powder was added to a reaction vessel at a solid-to-liquid ratio of 1:5 (g / mL), and hydrochloric acid was added alone in an amount of 5 parts by weight per 100 parts by weight of the total mixed acid solution. The mixture was then stirred at 50°C for 20 minutes to first form an antimony chlorine complex. Subsequently, nitric acid was added in stages over 5 minutes in an amount of 1 part by weight per 100 parts by weight of the total mixed acid solution to form Cl - / NO₃ - The molar concentration ratio was adjusted to 8:1. The remaining volume was supplemented with pure water to reach a total volume of 500 mL of mixed acid solution, and the temperature was raised to 80°C. The elution reaction was carried out for 2 hours under stirring conditions of 200 rpm. After the elution was completed, the eluent was filtered under reduced pressure using No. 5C filter paper to remove insoluble residues of SiO₂ and Fe₂O₃, and the filtered eluent was used as a raw material for the subsequent reduction step.

[0171] Examples 1-2

[0172] 100 g of catalyst powder was added to a reaction vessel at a solid-to-liquid ratio of 1:5 (g / mL), and hydrochloric acid was added alone in an amount of 3 parts by weight per 100 parts by weight of the total mixed acid solution. The mixture was then stirred at 45°C for 15 minutes to form an antimony chlorine complex. Subsequently, nitric acid was added in an amount of 0.5 parts by weight per 100 parts by weight of the total mixed acid solution to form Cl - / NO₃ - The molar concentration ratio was adjusted to 9:1, and after adjusting the total volume to 500 mL with pure water, elution was performed for 2.5 hours at 75°C under stirring conditions of 200 rpm. Examples 1-2 correspond to examples for confirming the elution behavior under low concentration conditions by setting the hydrochloric acid and nitric acid concentrations near the lower limit range of the present invention.

[0173] Examples 1-3

[0174] 100 g of catalyst powder was added to a reaction vessel at a solid-to-liquid ratio of 1:5 (g / mL), and hydrochloric acid was added alone in an amount of 8 parts by weight per 100 parts by weight of the total mixed acid solution, followed by stirring at 55°C for 25 minutes. Subsequently, nitric acid was added in an amount of 2 parts by weight per 100 parts by weight of the total mixed acid solution to Cl - / NO₃ - The molar concentration ratio was adjusted to 6:1, and elution was performed for 1.5 hours at 85°C under stirring conditions of 200 rpm. Examples 1-3 correspond to examples for confirming the elution behavior under high concentration conditions by setting the hydrochloric acid and nitric acid concentrations to be above the intermediate range of the present invention.

[0175] Examples 1-4

[0176] 100 g of catalyst powder was added to a reaction vessel at a solid-to-liquid ratio of 1:5 (g / mL), and hydrochloric acid was added alone at a concentration of 1 part by weight (lower limit of the present invention) per 100 parts by weight of the total mixed acid solution, followed by stirring at 40°C for 10 minutes. Subsequently, nitric acid was added at a concentration of 0.1 part by weight (lower limit of the present invention) per 100 parts by weight of the total mixed acid solution to Cl - / NO₃ -The molar concentration ratio was adjusted to 10:1 (upper limit of the present invention), and elution was performed for 3 hours (upper limit of the present invention) at 70°C under stirring conditions of 200 rpm. Examples 1-4 correspond to examples of extreme lower limit conditions in which each lower limit of the present invention is applied simultaneously.

[0177] Examples 1-5

[0178] 100 g of catalyst powder was added to a reaction vessel at a solid-to-liquid ratio of 1:5 (g / mL), and hydrochloric acid was added alone in an amount of 12 parts by weight (upper limit of the present invention) per 100 parts by weight of the total mixed acid solution, followed by stirring at 60°C for 30 minutes. Subsequently, nitric acid was added in an amount of 6 parts by weight (upper limit of the present invention) per 100 parts by weight of the total mixed acid solution to Cl - / NO₃ - The molar concentration ratio was adjusted to 3:1 (lower limit of the present invention), and elution was performed for 1 hour (lower limit of the present invention) at 90°C under stirring conditions of 200 rpm. Examples 1-5 correspond to examples of extreme upper limit conditions in which each upper limit of the present invention is applied simultaneously.

[0179] Comparative Example 1-1 (Simultaneous Mixing and Injection Method)

[0180] Under the same catalyst and input amount conditions as in Example 1-1, hydrochloric acid (5 parts by weight) and nitric acid (1 part by weight) were pre-mixed in pure water and simultaneously introduced into a reaction vessel, after which elution was performed for 2 hours at 80°C under stirring conditions of 200 rpm. Comparative Example 1-1 is a comparative example that applies the conventional simultaneous mixing and input method rather than the method of pre-injecting hydrochloric acid followed by the subsequent stepwise input of nitric acid, and serves as a comparative example to confirm whether there is a preferential effect on the formation of antimony chlorine complexes due to differences in the input method.

[0181] Comparative Example 1-2 (hydrochloric acid concentration below lower limit)

[0182] Under the same conditions as in Example 1-1, hydrochloric acid is reduced to 0.5 parts by weight (less than the lower limit of 1 part by weight of the present invention) per 100 parts by weight of the total mixed acid solution, and nitric acid is maintained at 1 part by weight so that Cl- / NO₃ - Elution was performed under conditions where the molar concentration ratio was set to 0.75:1. Comparative Examples 1-2 correspond to comparative examples to confirm the effect on Sb elution when the hydrochloric acid concentration is less than the lower limit (1 weight part) of the present invention, as the antimony chlorine complex formation is insufficient.

[0183] Comparative Examples 1-3 (Exceeding upper limit of nitric acid concentration)

[0184] Under the same conditions as in Example 1-1, nitric acid was increased to 8 parts by weight (exceeding the upper limit of 6 parts by weight of the present invention) with respect to a total of 100 parts by weight of the mixed acid solution, and hydrochloric acid was maintained at 5 parts by weight so that Cl - / NO₃ - Elution was performed under conditions where the molar concentration ratio was set to less than 1:1. Comparative Examples 1-3 correspond to comparative examples to confirm the effects of excessive oxidative decomposition promotion of ascorbic acid and inhibition of Te reduction when the nitric acid concentration exceeds the upper limit (6 parts by weight) of the present invention.

[0185] Comparative Examples 1-4 (Dissolution temperature / time below lower limit)

[0186] Under the same conditions as Example 1-1, elution was performed by setting the elution temperature to 60°C (less than the lower limit of 70°C of the present invention) and the elution time to 0.5 hours (less than the lower limit of 1 hour of the present invention). Comparative Example 1-4 corresponds to a comparative example to confirm the effect on the Sb and Te elution rates when the elution temperature and time fall below the lower limit of the present invention.

[0188] (b) Reduction step examples and comparative examples

[0190] Overview of Examples 2-1 to 2-5 and Comparative Examples 2-1 to 2-4

[0191] The following Examples 2-1 to 2-5 and Comparative Examples 2-1 to 2-4 were designed to use the eluent of Example 1-1 as a common raw material to determine the effects of the ascorbic acid input method, primary and secondary input equivalents, reduction temperature, and whether ORP monitoring was applied during the reduction step on the prevention of Sb selective reduction precipitation and Te co-precipitation. ORP measurements were taken in real time by immersing an ORP composite electrode (measurement range: -1000 to +1000 mV) with an embedded Ag / AgCl reference electrode in the eluent, and ORP values ​​were recorded at 1-minute intervals using a digital multimeter.

[0192] Summary of conditions for reduction step examples and comparative examples

[0193] division Reduction temperature (°C) Types of reducing agents 1st input amount (equivalent to mole of Sb) ORP monitoring ORP based on second injection (mV, vs. Ag / AgCl) Secondary input amount (equivalent to mole of Sb) Reaction time (time) Example 2-1 55 Ascorbic acid 1.2 apply +200 or less 0.3 1 Example 2-2 40 Ascorbic acid 0.8 apply +200 or less 0.1 1.5 Examples 2-3 70 Ascorbic acid 1.5 apply +200 or less 0.5 0.5 Examples 2-4 55 Ascorbic acid 1.0 apply +200 or less 0.2 1 Examples 2-5 55 isoascoric acid 1.2 apply +200 or less 0.3 1 Comparative Example 2-1 55 Ascorbic acid 2.5 (Batch input) Not applied - - 1 Comparative Example 2-2 55 Ascorbic acid 0.4 apply +200 or less 0.1 1 Comparative Example 2-3 55 NaBH₄ 1.2 apply +200 or less 0.3 1 Comparative Example 2-4 80 Ascorbic acid 1.2 apply +200 or less 0.3 1

[0194] Example 2-1

[0195] 500 mL of the eluent prepared in Example 1-1 was transferred to a 2 L glass reaction vessel, and the temperature was adjusted to 55°C. After calculating the molar amount of Sb in the eluent through ICP-OES preliminary analysis, ascorbic acid (food additive grade, purity 99% or higher) was added to the reaction vessel in solid powder form as a primary input of 1.2 equivalents relative to the molar amount of Sb, and the ORP relative to the Ag / AgCl reference electrode was recorded in real-time at 1-minute intervals while stirring at 150 rpm. After confirming that the ORP value dropped below +200 mV, additional ascorbic acid was added as a secondary input of 0.3 equivalents relative to the molar amount of Sb, and the reaction was continued for 1 hour while maintaining the temperature at 55°C. After the reaction was completed, the reaction solution containing the white to light yellow Sb precipitate was transferred to the next separation step.

[0196] Example 2-2

[0197] Using the eluent from Example 1-1, the reduction temperature was set to 40°C (lower limit of the present invention), and ascorbic acid was first added at a ratio of 0.8 equivalents (first lower limit) relative to the molar amount of Sb. Then, when the ORP dropped to +200 mV or lower, an additional 0.1 equivalent (second lower limit) was added to use a total of 0.9 equivalents of ascorbic acid, and the reaction was carried out for 1.5 hours. Example 2-2 corresponds to an extreme lower limit example in which the reduction temperature and the amount of ascorbic acid added are set to the lower limit conditions of the present invention.

[0198] Examples 2-3

[0199] Using the eluent of Example 1-1, the reduction temperature was set to 70°C (upper limit of the present invention), and ascorbic acid was first added at a ratio of 1.5 equivalents (first upper limit) relative to the molar amount of Sb. Then, when the ORP dropped to +200 mV or lower, an additional 0.5 equivalents (second upper limit) were added to use a total of 2.0 equivalents of ascorbic acid, and the reaction was carried out for 0.5 hours. Example 2-3 corresponds to an extreme upper limit example in which the reduction temperature and the amount of ascorbic acid added are set to the upper limit conditions of the present invention.

[0200] Examples 2-4

[0201] Using the eluent of Example 1-1, the reduction temperature was set to 55°C, and ascorbic acid was first added at 1.0 equivalent (first median value) relative to the molar amount of Sb. Then, when the ORP dropped to +200 mV or lower, an additional 0.2 equivalent (second median value) was added to use a total of 1.2 equivalents of ascorbic acid, and the reaction was carried out for 1 hour. Example 2-4 corresponds to a representative example applying the intermediate range conditions of the present invention.

[0202] Examples 2-5

[0203] Using the eluent of Example 1-1, a reduction step was performed under the same conditions as in Example 2-1, except that isoascorbic acid (erythorbic acid, food additive grade) was used instead of ascorbic acid as the reducing agent. Example 2-5 corresponds to an example to confirm whether an equivalent Sb selective reduction precipitation effect is achieved even when isoascorbic acid, a stereoisomer of ascorbic acid, is used as the reducing agent.

[0204] Comparative Example 2-1 (Excessive batch administration of ascorbic acid, ORP monitoring not applied)

[0205] Using the eluent of Example 1-1, ascorbic acid was added to the reaction vessel in a single batch at a ratio of 2.5 equivalents relative to the molar amount of Sb without ORP monitoring, and the reaction was carried out at 55°C for 1 hour. Comparative Example 2-1 is a comparative example that applies the conventional method of adding excess ascorbic acid in a single batch without ORP monitoring, and is intended to confirm whether Te co-precipitation occurs due to excess reduction.

[0206] Comparative Example 2-2 (Ascorbic acid input amount less than the lower limit)

[0207] Using the eluent of Example 1-1, ascorbic acid was added at a ratio of 0.4 equivalents (first input) relative to the molar amount of Sb, and at the point when the ORP dropped to +200 mV or lower, 0.1 equivalents (second input) were added to perform a reduction reaction using a total of 0.5 equivalents of ascorbic acid (less than the total lower limit of 0.9 equivalents of the present invention). Comparative Example 2-2 corresponds to a comparative example to confirm the effect on the reduction conversion rate of Sb(V) and the Sb precipitation rate when the amount of ascorbic acid added is less than the lower limit of the present invention.

[0208] Comparative Example 2-3 (Using strong reducing agent NaBH₄)

[0209] Using the eluent of Example 1-1, NaBH₄ was used as a reducing agent instead of ascorbic acid, and 1.2 equivalents relative to the molar amount of Sb were added at 55°C, and the reaction was carried out under ORP monitoring. Comparative Example 2-3 is a comparative example intended to confirm whether selective separation of Sb and Te is possible and whether Te co-precipitation occurs when using NaBH₄, a strong reducing agent, and to compare the difference with the use of ascorbic acid of the present invention.

[0210] Comparative Example 2-4 (Exceeding upper limit of reduction temperature)

[0211] Using the eluent of Example 1-1, the reduction temperature was set to 80°C (exceeding the upper limit of 70°C of the present invention), and the reduction reaction was performed under the same as those in Example 2-1 as the ascorbic acid input conditions (1.2 equivalents in the first step, 0.3 equivalents in the second step). Comparative Example 2-4 is a comparative example to confirm the effect on the reduction of the effective reduction equivalent and the Sb reduction precipitation rate due to the promotion of thermal oxidative decomposition of ascorbic acid when the reduction temperature exceeds the upper limit (70°C) of the present invention.

[0213] (c) Examples and comparative examples of selective separation steps

[0215] Overview of Examples 3-1 to 3-4 and Comparative Examples 3-1 to 3-3

[0216] The following Examples 3-1 to 3-4 and Comparative Examples 3-1 to 3-3 were designed to verify the effects of membrane filter pore size, cleaning solution composition and amount, number of cleaning cycles, and pH adjustment conditions for Te recovery on Sb separation efficiency and Te recovery rate, using the reaction solution containing Sb precipitate prepared in Example 2-1 as a common raw material. In all examples and comparative examples, primary solid-liquid separation was performed by vacuum suction filtration, and a Buchner funnel and a vacuum pump were used as the filtration device.

[0217] Summary of conditions for selective separation step examples and comparative examples

[0218] division Membrane filter pore size (μm) Cleaning solution HCl concentration (parts by weight relative to 100 parts by weight of water) Cleaning solution usage (parts by weight relative to 100 parts by weight of cake) Cleaning frequency (times) pH adjustment value of the mixture pH adjuster Te secondary separation method Example 3-1 0.45 3 200 2 4 ammonia vacuum filtration Example 3-2 0.1 1 100 1 3 ammonia vacuum filtration Example 3-3 1.0 5 300 3 5 NaOH vacuum filtration Examples 3-4 0.22 2 150 2 4 NaOH vacuum filtration Comparative Example 3-1 5.0 3 200 2 4 ammonia vacuum filtration Comparative Example 3-2 0.45 10 200 2 4 ammonia vacuum filtration Comparative Example 3-3 0.45 3 200 2 7 NaOH vacuum filtration

[0219] Example 3-1

[0220] The reaction solution containing the Sb precipitate prepared in Example 2-1 was subjected to primary solid-liquid separation by vacuum filtration using a PVDF (polyvinylidene fluoride) membrane filter with a pore size of 0.45 μm, and the Sb precipitate cake was recovered. The recovered cake was washed twice using a washing solution prepared by dissolving 3 parts by weight of hydrochloric acid in 100 parts by weight of water, at a ratio of 200 parts by weight to 100 parts by weight of the cake. After each washing, the washing filtrate was separated and recovered through vacuum filtration. The Te-containing filtrate obtained from the primary filtration and the filtrate from the primary and secondary washing were combined. Then, ammonia water (25 wt%) was added dropwise in small amounts in a vacuum stirring tank to adjust the pH of the mixture to 4. The mixture was stirred at 50°C for 1 hour to form a Te precipitate in the form of TeO₂·nH₂O, and then Te was recovered by secondary solid-liquid separation using the same PVDF membrane filter.

[0221] Example 3-2

[0222] The reaction solution of Example 2-1 was first separated using a PVDF membrane filter with a pore size of 0.1 μm (lower limit of the present invention), and the washing solution was prepared by dissolving 1 part by weight of hydrochloric acid (lower limit) in 100 parts by weight of water, and washing was performed once (lower limit) at a ratio of 100 parts by weight (lower limit) relative to 100 parts by weight of cake. The pH of the mixture was adjusted to 3 (lower limit) with ammonia water to perform Te precipitation recovery. Example 3-2 corresponds to an extreme lower limit example in which all conditions of the selective separation step are set to the lower limit of the present invention.

[0223] Example 3-3

[0224] The reaction solution of Example 2-1 was first separated using a PVDF membrane filter with a pore size of 1.0 μm (upper limit of the present invention), and the washing solution was prepared by dissolving 5 parts by weight (upper limit) of hydrochloric acid in 100 parts by weight of water, and washing was performed three times (upper limit) at a ratio of 300 parts by weight (upper limit) relative to 100 parts by weight of cake. The pH of the mixture was adjusted to 5 (upper limit) using an aqueous NaOH solution (10 wt%) to recover the precipitate of Te. Example 3-3 corresponds to an extreme upper limit example in which all conditions of the selective separation step are set to the upper limit of the present invention.

[0225] Examples 3-4

[0226] The reaction solution of Example 2-1 was first separated using a PVDF membrane filter with a pore size of 0.22 μm, and the washing solution was prepared by dissolving 2 parts by weight of hydrochloric acid in 100 parts by weight of water, and washing was performed twice at 150 parts by weight relative to 100 parts by weight of the cake. The pH of the mixture was adjusted to 4 using an aqueous NaOH solution (10 wt%) to recover the precipitated Te. Examples 3-4 are examples in which the intermediate range conditions of the present invention are applied, but NaOH is used as a pH adjuster, and are examples intended to confirm the difference in Te precipitation efficiency depending on the type of pH adjuster.

[0227] Comparative Example 3-1 (Exceeding membrane filter pore size upper limit)

[0228] Primary solid-liquid separation was performed by changing the membrane filter pore size to 5.0 μm (exceeding the upper limit of 1.0 μm of the present invention) under the same conditions as in Example 3-1. Comparative Example 3-1 is a comparative example to confirm the decrease in Sb capture efficiency due to the filtrate outflow of fine Sb precipitated particles when the membrane filter pore size exceeds the upper limit of the present invention.

[0229] Comparative Example 3-2 (Exceeding upper limit of cleaning solution HCl concentration)

[0230] Cleaning was performed under the same conditions as in Example 3-1 by increasing the concentration of hydrochloric acid in the cleaning solution to 10 parts by weight (exceeding the upper limit of 5 parts by weight of the present invention) relative to 100 parts by weight of water. Comparative Example 3-2 is a comparative example to confirm the effect on the redissolution loss of Sb precipitates in the cleaning solution and the Te cleaning elution efficiency when the concentration of hydrochloric acid in the cleaning solution exceeds the upper limit of the present invention.

[0231] Comparative Example 3-3 (Te recovery pH exceeding upper limit)

[0232] Te precipitate recovery was performed by adjusting the pH of the mixture to 7 (exceeding the upper limit of 5 of the present invention) with an aqueous NaOH solution under the same conditions as Example 3-1. Comparative Example 3-3 corresponds to a comparative example to confirm the decrease in purity of the Te precipitate due to co-precipitation of impurity ions such as Fe and Si when the pH exceeds the upper limit (5) of the present invention.

[0234] (d) Examples and comparative examples of high-purity Sb₂O₃ manufacturing steps

[0236] Overview of Examples 4-1 to 4-4 and Comparative Examples 4-1 to 4-4

[0237] The following Examples 4-1 to 4-4 and Comparative Examples 4-1 to 4-4 were designed to investigate the effects of drying conditions, primary calcination temperature and heating rate, secondary calcination temperature and holding time, and cooling rate on the purity and particle size of the final Sb₂O₃ powder, using the Sb precipitate cake recovered from Example 3-1 as a common raw material. Calcination was performed using an electric tube furnace, and the oxidation atmosphere was maintained by supplying air into the furnace at a constant flow rate (1 L / min) (O₂ concentration in air approximately 21 vol%). The heating and cooling rates were precisely controlled using a PID temperature controller.

[0238] Summary of High-Purity Sb₂O₃ Manufacturing Steps, Examples, and Comparative Example Conditions

[0239] division Drying temperature (°C) Target moisture content (parts by weight or less based on 100 parts by weight of dry matter) Oxygen concentration (vol%) First firing temperature (°C) Heating rate (°C / min) Second firing temperature (°C) Duration (time) Cooling rate (°C / min or less) Example 4-1 90 1 21 350 5 580 2 3 Example 4-2 60 1 5 300 2 500 3 5 Example 4-3 120 1 21 400 10 650 1 2 Examples 4-4 80 1 10 350 5 550 2 3 Comparative Example 4-1 90 1 21 250 5 580 2 3 Comparative Example 4-2 90 1 21 350 15 580 2 3 Comparative Example 4-3 90 1 21 350 5 700 2 3 Comparative Example 4-4 90 1 21 350 5 580 2 15

[0240] Example 4-1

[0241] The Sb precipitate cake recovered in Example 3-1 was washed twice with pure water and dried in an electric drying oven at 90°C. The moisture content was confirmed to be 1 part by weight or less per 100 parts by weight of the dried material using a Karl Fischer moisture titrator. The dried Sb precipitate was placed into an electric tubular furnace and subjected to a first calcination to 350°C at a heating rate of 5°C / min under an air atmosphere (O₂ 21 vol%), and maintained for 1 hour to remove residual chlorine and organic matter. Subsequently, a second calcination was performed under the same air atmosphere, with the temperature raised to 580°C at a rate of 5°C / min and maintained for 2 hours. Afterward, the product was cooled to room temperature while controlling the cooling rate to 3°C / min to obtain Sb₂O₃ powder.

[0242] Example 4-2

[0243] After confirming that the moisture content of the Sb precipitate cake of Example 3-1 was 1 part by weight or less by drying at 60°C (lower limit of the present invention), it was first fired to 300°C (first lower limit) at a heating rate of 2°C / min (lower limit of the present invention) in a mixed gas (N₂ / Air mixture) atmosphere with an oxygen concentration of 5 vol% (lower limit of the present invention), and then secondarily fired at 500°C (second lower limit) for 3 hours (upper limit of the present invention) and cooled at a cooling rate of 5°C / min (upper limit of the present invention) or less. Example 4-2 corresponds to an extreme lower limit example in which the lower limit of the firing conditions is applied.

[0244] Example 4-3

[0245] The Sb precipitate cake of Example 3-1 was dried at 120°C (upper limit of the present invention) to confirm that the moisture content was 1 part by weight or less, then fired in an air atmosphere (O₂ 21 vol%, upper limit of the present invention) at a heating rate of 10°C / min (upper limit of the present invention) to 400°C (first upper limit), fired in a second firing at 650°C (second upper limit) for 1 hour (lower limit of the present invention), and then cooled at a cooling rate of 2°C / min. Example 4-3 corresponds to an extreme upper limit example in which the upper limit of the firing conditions is applied.

[0246] Examples 4-4

[0247] After drying the Sb precipitate cake of Example 3-1 at 80°C, it was first fired to 350°C at a heating rate of 5°C / min in a mixed gas atmosphere with an oxygen concentration of 10 vol%, and then secondarily fired at 550°C for 2 hours, followed by cooling at a cooling rate of 3°C / min. Example 4-4 is an example in which the oxygen concentration was set to an intermediate value in the range of 5 to 21 vol%, and it corresponds to an example for confirming the effect of applying a low-oxygen atmosphere.

[0248] Comparative Example 4-1 (Below lower limit of primary firing temperature)

[0249] Calcination was performed under the same conditions as in Example 4-1 by lowering the primary calcination temperature to 250°C (less than the lower limit of 300°C of the present invention). Comparative Example 4-1 is a comparative example to confirm the effect of incomplete removal of residual chlorine and organic matter on the purity of the final Sb₂O₃ powder when the primary calcination temperature falls below the lower limit of the present invention.

[0250] Comparative Example 4-2 (Exceeding upper limit of heating rate)

[0251] Calcination was performed by increasing the heating rate during the first calcination to 15℃ / min (exceeding the upper limit of 10℃ / min of the present invention) under the same conditions as Example 4-1. Comparative Example 4-2 is a comparative example to confirm the effect on the final powder quality of particle size non-uniformity and the risk of equipment corrosion caused by rapid HCl gas generation when the heating rate exceeds the upper limit of the present invention.

[0252] Comparative Example 4-3 (Exceeding the upper limit of the second firing temperature)

[0253] Calcination was performed by raising the secondary calcination temperature to 700°C (exceeding the upper limit of 650°C of the present invention) under the same conditions as Example 4-1. Comparative Example 4-3 is a comparative example to confirm the effect of increased sublimation loss of Sb₂O₃ and formation of coarse particles on the final powder recovery rate and particle size distribution when the secondary calcination temperature exceeds the upper limit of the present invention.

[0254] Comparative Example 4-4 (Cooling rate upper limit exceeded)

[0255] After secondary firing under the same conditions as Example 4-1, rapid cooling was performed by increasing the cooling rate to 15℃ / min (exceeding the upper limit of 5℃ / min of the present invention). Comparative Example 4-4 is a comparative example to confirm particle fracture and reduced particle size uniformity caused by thermal stress within Sb₂O₃ crystal grains under rapid cooling conditions where the cooling rate exceeds the upper limit of the present invention.

[0257] Experimental Example (Additional Experimental Example to Support Claim)

[0259] Experimental Example 1: Experiment to Confirm Selective Separation of Sb / Te in the Presence or Absence of Nitric Acid

[0260] This experimental example was designed to quantitatively confirm the effect of the presence of nitric acid on the precipitation behavior of Sb and Te after the addition of ascorbic acid under mixed acid elution conditions. As shown in Table 5 below, Experimental Example 1-A was a condition in which a total of 1.5 equivalents (1.2 in the first step + 0.3 in the second step) of ascorbic acid were added to an eluent prepared under the conditions of Example 1-1 (5 parts by weight of hydrochloric acid + 1 part by weight of nitric acid); Experimental Example 1-B was a condition in which ascorbic acid under the same conditions was added to an eluent prepared only with 5 parts by weight of hydrochloric acid without nitric acid; and Experimental Example 1-C was a condition in which the same ascorbic acid was added to an eluent prepared only with 1 part by weight of nitric acid. After the reaction was completed under each condition, the precipitate and filtrate were separated by filtration, and the Sb and Te content of each were analyzed by ICP-OES.

[0261] Experimental conditions for precipitation behavior depending on the presence or absence of nitric acid

[0262] division Hydrochloric acid (parts by weight) nitric acid (parts by weight) Ascorbic acid injection method Total input equivalents Reaction temperature (°C) Experimental Example 1-A 5 1 1st 1.2 + 2nd 0.3 equivalents (ORP-based splitting) 1.5 55 Experimental Example 1-B 5 0 (no nitric acid) 1st 1.2 + 2nd 0.3 equivalents (ORP-based splitting) 1.5 55 Experimental Example 1-C 0 (no hydrochloric acid) 1 1st 1.2 + 2nd 0.3 equivalents (ORP-based splitting) 1.5 55

[0263] This experimental example is intended to support the idea that the selection of mixed acid conditions has intrinsic chemical significance beyond simply increasing elution efficiency in the selective separation of Sb / Te, by independently verifying the role of nitric acid in inhibiting Te reduction and the role of hydrochloric acid in forming Sb chlorine complexes.

[0265] Experimental Example 2: Cl - / NO₃ - Experiment on Change in Selectivity According to Molar Concentration Ratio

[0266] This experimental example concerns Cl in the mixed acid eluent - / NO₃ - It was designed to quantitatively verify the effect of molar concentration ratio on Sb precipitation selectivity over a continuous numerical range. As shown in Table 6 below, the total HCl and HNO₃ inputs were adjusted, while Cl - / NO₃ - The precipitation rates of Sb and Te were measured by applying the same split ascorbic acid input conditions while varying the molar concentration ratio to five conditions from 0.5:1 to 15:1.

[0267] Cl - / NO₃ - Experimental conditions according to changes in molar concentration ratio

[0268] division Cl - / NO₃ - Molar concentration ratio HCl (parts by weight) HNO₃ (parts by weight) Total input equivalents of ascorbic acid Reaction temperature (°C) Experimental Example 2-A 0.5:1 1 3 1.5 55 Experimental Example 2-B 2:1 3 2.2 1.5 55 Experimental Example 2-C 3:1 (lower limit) 5 2.4 1.5 55 Experimental Example 2-D 6:1 (Medium) 5 1.2 1.5 55 Experimental Example 2-E 10:1 (upper limit) 5 0.7 1.5 55 Experimental Example 2-F 15:1 5 Less than 0.5 1.5 55

[0269] This experimental example is Cl - / NO₃ - This corresponds to an experimental example intended to support the significance of a critical range of molar concentration ratios of 3:1 to 10:1 in the selective precipitation of Sb with continuous numerical data, and to prove the criticality of the said numerical range through the results of comparative conditions Experimental Examples 2-A and 2-B (below range) and Experimental Example 2-F (above range).

[0271] Experimental Example 3: Experiment on Changes in Sb Precipitation Rate and Te Co-precipitation Rate According to Total Equivalents of Ascorbic Acid Input

[0272] This experimental example was designed to quantitatively verify the effect of the total input equivalent of ascorbic acid (relative to the molar amount of Sb) on the Sb precipitation rate and the Te co-precipitation rate. As shown in Table 7 below, the total input equivalent of ascorbic acid was varied from 0.3 to 2.5 across 7 conditions, and an ORP-based split input method (1st:2nd = 4:1 ratio) was applied under all conditions.

[0273] Experimental conditions according to changes in total input equivalents of ascorbic acid

[0274] division Total input equivalents 1st input equivalent Second input equivalent Reaction temperature (°C) ORP standard (mV, vs. Ag / AgCl) Experimental Example 3-A 0.3 0.25 0.05 55 +200 or less Experimental Example 3-B 0.6 0.48 0.12 55 +200 or less Experimental Example 3-C 0.9 (lower limit) 0.72 0.18 55 +200 or less Experimental Example 3-D 1.2 (Intermediate) 0.96 0.24 55 +200 or less Experimental Example 3-E 1.5 1.2 0.3 55 +200 or less Experimental Example 3-F 2.0 (upper limit) 1.6 0.4 55 +200 or less Experimental Example 3-G 2.5 (exceeds) 2.0 0.5 55 +200 or less

[0275] This experimental example is intended to support with numerical data that the critical range of a total input equivalent of 0.9 to 2.0 is the optimal range that simultaneously satisfies the Sb precipitation rate and the prevention of Te co-precipitation, and it is intended to prove the criticality of the said range through a comparison with the low Sb precipitation rate below the range (Experimental Examples 3-A, 3-B) and the occurrence of Te co-precipitation above the range (Experimental Example 3-G).

[0277] Experimental Example 4: Comparative Experiment on Final Sb₂O₃ Powder Quality Between Two-Stage and Single-Stage Calcination Processes

[0278] This experimental example was designed to compare the final Sb₂O₃ powder purity, residual chlorine content, and particle size distribution of the two-stage calcination process (1st stage 350°C + 2nd stage 580°C) and the single calcination process (direct calcination at 580°C) of the present invention. As shown in Table 8 below, three calcination conditions were applied using the same dry Sb precipitate as a raw material.

[0279] Conditions for the Experiment Example Comparative Test of Two-Stage Firing vs. Single Firing

[0280] division firing method First firing conditions Secondary firing conditions Cooling rate (°C / min) Experimental Example 4-A Two-stage firing (the present invention) 350℃, 1 hour, 5℃ / min temperature increase, Air 580℃, 2 hours, 5℃ / min temperature increase, Air 3 Experimental Example 4-B Single firing (direct high temperature) doesn't exist 580℃, 2 hours, 5℃ / min temperature increase, Air 3 Experimental Example 4-C Single firing (low temperature single stage) doesn't exist 350℃, 3 hours, 5℃ / min temperature increase, Air 3

[0281] This experimental example is intended to support the technical necessity and critical significance of the two-stage calcination process by quantitatively confirming the effect of omitting the first low-temperature calcination (dechlorination / de-oiling) step on the residual chlorine content and purity of the final product.

[0282] Results and Discussion

[0284] Experimental Example 1 Results and Discussion: Confirmation of Selective Separation of Sb / Te in the Presence or Absence of Nitric Acid

[0285] By comparing the results of Experimental Examples 1-A, 1-B, and 1-C, it was confirmed that the role of nitric acid under mixed acid conditions has intrinsic chemical significance beyond that of a simple elution aid.

[0286] In Experimental Example 1-A (mixed acid condition of 5 parts by weight of hydrochloric acid + 1 part by weight of nitric acid), a white to pale yellow Sb precipitate was clearly formed after the fractional addition of ascorbic acid (total 1.5 equivalents), and after filtration, the Te content in the precipitate was confirmed to be below the detection limit (0.1 mg / L), indicating that selective separation of Sb and Te was achieved. This is attributed to the formation of a redox window in which only Sb(V) is selectively reduced to Sb(III) within the ORP range of +100 mV to +400 mV relative to the Ag / AgCl reference electrode under conditions where nitric acid is present. In other words, it was confirmed that the effect of nitric acid on increasing the redox potential of Te(IV) acted as a chemical barrier that inhibited the reducing power of ascorbic acid from acting on Te.

[0287] In Experimental Example 1-B (condition of 5 parts by weight of hydrochloric acid alone without nitric acid), the ORP rapidly decreased after the addition of ascorbic acid and reached below the reduction potential of Te; as a result, Sb and Te precipitated simultaneously, resulting in a mixture of Sb and Te within the precipitate. This implies that under conditions where nitric acid is absent, the reducing power of ascorbic acid provides sufficient reduction driving force not only for Sb(V) but also for Te(IV), thus preventing the formation of a redox window. Therefore, the absence of nitric acid acts as a fundamental cause of failure in the selective separation of Sb / Te, and it has been proven that the adoption of mixed acid conditions in the present invention is not merely for improving elution efficiency but is an essential condition for implementing the core mechanism of selective separation.

[0288] In Experimental Example 1-C (conditions of 1 part by weight of nitric acid alone and no hydrochloric acid), although the oxidative dissolution stabilization of Te was achieved, the elution of Sb itself was incomplete, resulting in a significantly low measured Sb concentration in the eluent. Consequently, the amount of precipitate formed was extremely minimal, confirming that substantial Sb recovery was impossible. This indicates that for the effective elution of Sb, Cl... - This implies that the formation of antimony chlorine complexes by ions is essential, and indicates that conditions using only nitric acid without hydrochloric acid are unsuitable for achieving the purpose of the present invention.

[0289] Synthesizing the above results, it has been clearly proven that the mixed acid condition of hydrochloric acid and nitric acid adopted in this invention is an essential and critical condition that enables selective separation of Sb and Te, as the two chemical functions of preferential formation of Sb chlorine complexes by hydrochloric acid and inhibition of Te reduction by nitric acid act complementarily.

[0291] Results and Discussion of Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-4: Mixed Acid Elution Step

[0292] Example 1-1 (5 parts by weight of hydrochloric acid, 1 part by weight of nitric acid, Cl - / NO₃ -In the case of the hydrochloric acid pre-injection method (8:1, hydrochloric acid pre-injection method, 80°C, 2 hours), high elution rates of Sb and Te were achieved at 98.5% and 97.2%, respectively, and the appearance of the eluent was transparent yellowish-green, confirming the complete dissolution of Sb and Te. Insoluble residues (SiO₂, Fe₂O₃) were easily removed by vacuum filtration using No. 5C filter paper, and the remaining amounts of Sb and Te in the residue after filtration were only 1.5% and 2.8%, respectively, relative to the input amount. This indicates that the hydrochloric acid pre-injection followed by nitric acid injection method of the present invention creates high-efficiency elution conditions by prioritizing the formation of chlorine complexes of Sb.

[0293] In Comparative Example 1-1 (under the same acid concentration conditions as Example 1-1 or a method of simultaneous mixed injection of hydrochloric acid and nitric acid), the Sb elution rate was 91.3%, which is about 7% lower than that of Example 1-1. This is attributed to the fact that under simultaneous injection conditions, the oxidizing power of nitric acid competed with the chlorine complex formation reaction, thereby reducing the efficiency of Sb's chlorine complex formation. Additionally, in Comparative Example 1-1, the co-precipitation rate of Te in the subsequent reduction step was higher than that of Example 1-1, confirming that the difference in the injection method directly affects not only the elution efficiency but also the subsequent selective separation performance.

[0294] Comparative Example 1-2 (0.5 parts by weight of hydrochloric acid, 1 part by weight of nitric acid, Cl - / NO₃ - At a ratio of 0.75:1), the Sb elution rate decreased significantly to 72.4%, and due to insufficient formation of antimony chlorine complexes in the eluent, a phenomenon was observed where some of the Sb prematurely reprecipitated as a white precipitate during the elution process. This is because when the hydrochloric acid concentration is less than 1 part by weight (lower limit of the present invention), Cl - This implies that the ion concentration is insufficient to stably maintain the antimony chlorine complex, and at the same time, Cl - / NO₃ - As the molar concentration ratio dropped to less than 1:1, a double problem occurred in which the relative excess of nitric acid reduced the effective equivalent of ascorbic acid.

[0295] Comparative Examples 1-3 (5 parts by weight of hydrochloric acid, 8 parts by weight of nitric acid, Cl - / NO₃ - In the case of (less than 1:1), the Te elution rate was high at 99.1%, but in the subsequent reduction step, the rate at which ascorbic acid was oxidized and decomposed by nitric acid was faster than the rate of reduction of Sb(V), so the actual effective reduction equivalent was significantly reduced relative to the input amount, and consequently, the Sb precipitation rate was lowered. This result directly demonstrates that exceeding the upper limit of nitric acid concentration (6 parts by weight) has critical significance in hindering the efficient utilization of the reducing agent.

[0296] In Comparative Examples 1-4 (dissolution temperature 60℃, dissolution time 0.5 hours), the dissolution rates of Sb and Te were significantly low at 68.2% and 71.5%, respectively, and it was confirmed through SEM analysis of the residue that undissolved components remained, particularly in the center of catalyst particles with large particle sizes. This indicates that the lower limit of the dissolution temperature (70℃) and the lower limit of the dissolution time (1 hour) are critical minimum conditions for complete dissolution of Sb and Te.

[0297] Examples 1-2 (3 parts by weight of hydrochloric acid, 0.5 parts by weight of nitric acid, Cl - / NO₃ - = 9:1, 75℃, 2.5 hours) and Examples 1-4 (1 part by weight of hydrochloric acid, 0.1 part by weight of nitric acid, Cl - / NO₃ - When comparing the conditions (= 10:1, 70℃, 3 hours), good dissolution efficiency was achieved in Example 1-4, which is close to the lower limit condition, with an Sb dissolution rate of 94.1% and a Te dissolution rate of 93.8%, confirming that the lower limit condition of the present invention is effective as the minimum condition for practical process operation. However, since the dissolution time in Example 1-4 is 1 hour longer than in Example 1-1, which is disadvantageous in terms of productivity, it was found that the conditions of Example 1-1 or 1-3 are more desirable for practical processes.

[0298] Examples 1-5 (12 parts by weight of hydrochloric acid, 6 parts by weight of nitric acid, Cl - / NO₃ - At (= 3:1, 90℃, 1 hr), the highest elution efficiency was achieved with an Sb elution rate of 99.2% and a Te elution rate of 98.9%; however, steam generation and an increase in the reflux condenser load were observed due to the handling of high-concentration hydrochloric acid. Additionally, a tendency for ascorbic acid consumption to increase in the subsequent reduction step was observed, which is due to Cl - / NO₃ - As the molar concentration ratio approaches 3:1 (lower limit), the relative concentration of nitric acid increases, suggesting that controlling the input amount to secure the effective equivalent of ascorbic acid becomes more important.

[0300] Results and Discussion of Examples 2-1 to 2-5 and Comparative Examples 2-1 to 2-4: Reduction Step

[0301] In Example 2-1 (55°C, 1.2 equivalents of ascorbic acid first + 0.3 equivalents of ORP-based second), the ORP dropped to +200 mV or lower approximately 35 minutes after the first addition. At this point, the second addition was performed, resulting in excellent selective separation after a total reaction time of 1 hour, with an Sb precipitation rate of 98.7% and a Te filtrate retention rate of 99.3% (i.e., Te co-precipitation rate of 0.7%). The formed Sb precipitate was in the form of a fine powder ranging from white to pale yellow, and X-ray diffraction (XRD) analysis confirmed that it was a mixture of Sb(III) oxide and basic antimony salt. This result directly indicates that the reduction of Sb(V) to Sb(III) by ascorbic acid was successfully carried out.

[0302] In Comparative Example 2-1 (lump-sum addition of 2.5 equivalents of ascorbic acid, no ORP monitoring applied), the Sb precipitation rate was high at 99.1%, but the Te co-precipitation rate reached 38.5%, resulting in a large amount of Te being incorporated into the Sb precipitate. This was because the ORP in the reactor rapidly dropped below the Te reduction threshold due to the lump-sum addition of excess ascorbic acid, causing Te(IV) to become Te(0) or Te₂ 2- It is determined that this is because it was reduced and precipitated in this form. The above results clearly demonstrate that the split injection method based on real-time ORP monitoring is a structurally essential process design for preventing Te co-precipitation, and indicate that Te co-precipitation cannot be completely prevented by simply adjusting the injection amount.

[0303] In Comparative Example 2-2 (total ascorbic acid 0.5 equivalents, 0.4 in the first step + 0.1 in the second step), the Sb precipitation rate was significantly low at 61.3%, and analysis of the filtrate confirmed that a large amount of unreduced Sb(V) remained. This implies that complete reduction of Sb(V) does not occur when the total input amount of ascorbic acid is less than 0.9 equivalents (lower limit of the present invention), and this result directly demonstrates the critical significance of the lower limit of the total input amount. In particular, it was observed that the ORP did not drop below +200 mV after the first input; this is judged to be because the effective equivalent amount contributing to the reduction of Sb(V) was insufficient after the input ascorbic acid was partially consumed by reaction with dissolved oxygen and residual nitric acid in the eluent.

[0304] In Comparative Example 2-3 (using strong reducing agent NaBH₄, 1.2 equivalents), the Sb precipitation rate was high at 99.5%, but the Te co-precipitation rate reached 82.3%, resulting in complete non-selective co-precipitation of Sb and Te. Since NaBH₄ has a very low standard redox potential (-1.24 V vs. SHE), it provides sufficient driving force to completely reduce Te(IV) to Te(0) even under conditions where nitric acid is present; thus, the formation of the redox window required by the present invention is structurally impossible. The above results clearly indicate that the selection of ascorbic acid in the present invention is not merely for environmental friendliness, but is based on the precise selection of appropriate reducing power that enables selective reduction.

[0305] In Comparative Example 2-4 (reduction temperature 80°C), the rate of ORP decrease after the first addition of ascorbic acid was actually slower than in Example 2-1. This is attributed to the fact that under high temperature conditions of 80°C, thermal oxidative decomposition and oxidation reactions caused by dissolved oxygen and nitric acid proceeded rapidly before ascorbic acid could contribute to the reduction of Sb(V), thereby reducing the effective reduction equivalent. As a result, the Sb precipitation rate decreased to 78.2%, and the total amount of ascorbic acid required to achieve the same target precipitation rate increased by more than twofold compared to Example 2-1. Consequently, the risk of Te co-precipitation due to excess ascorbic acid also increased. This result indicates that exceeding the upper limit of the reduction temperature (70°C) causes a dual negative effect of reduced ascorbic acid reduction efficiency and increased risk of Te co-precipitation.

[0306] In Examples 2-5 (using isoascorbic acid), selective separation performance similar to that of Example 2-1 was achieved with an Sb precipitation rate of 97.9% and a Te co-precipitation rate of 0.9%. This is attributed to the fact that isoascorbic acid is a stereoisomer having an equivalent redox potential to ascorbic acid and exhibits the same selective reduction mechanism. The above results indicate that the range of reducing agents usable in the present invention is not limited to ascorbic acid alone but can be extended to organic acids of equivalent reducing power including isoascorbic acid.

[0308] Experimental Example 2 Results and Discussion: Cl - / NO₃ - Change in selectivity according to molar concentration ratio

[0309] Experimental Example 2-A (Cl - / NO₃ - In the case of (= 0.5:1), extremely poor selectivity was observed, with the Te co-precipitation rate reaching 55.3% after the addition of ascorbic acid. This is believed to be because the excess nitric acid rapidly oxidized and decomposed ascorbic acid, significantly reducing the effective reducing equivalent, while the formation of the chlorine complex of Sb was insufficient, thereby lowering the elution stability of Sb itself.

[0310] Experimental Example 2-B (Cl - / NO₃ - At = 2:1), the Te co-precipitation rate improved slightly to 23.1%, but it still did not meet practical selective separation criteria. The above results are Cl - / NO₃ - Under conditions below the lower molar concentration ratio limit (3:1), the selective separation performance is rapidly degraded, which supports the fact that a ratio of 3:1 or higher is the critical lower limit for preventing Te co-precipitation.

[0311] Experimental Example 2-C(Cl - / NO₃ -In the 2-D (6:1, intermediate value) and 2-E (10:1, upper limit of the present invention) samples (= 3:1, lower limit of the present invention), the Sb precipitation rates were 96.8%, 98.7%, and 97.5%, respectively, showing favorable results of over 96% in all cases; meanwhile, the Te co-precipitation rates were 2.1%, 0.7%, and 1.2%, respectively, confirming excellent selectivity of less than 3% in all cases. This indicates Cl - / NO₃ - This result indicates that the selective separation process of the present invention operates stably within a molar concentration ratio range of 3:1 to 10:1.

[0312] Experimental Example 2-F(Cl - / NO₃ - = 15:1 (exceeding the upper limit of the present invention), the Te co-precipitation rate increased to 8.7%, which is judged to be because the relative concentration of nitric acid decreased, reducing the oxidative dissolved stabilization effect of Te(IV) and causing the reducing action of ascorbic acid to begin partially affecting Te as well. The above results are Cl - / NO₃ - This indicates that the effect of inhibiting Te reduction becomes insufficient when the upper limit (10:1) of the molar concentration ratio is exceeded, directly proving the critical significance of setting the upper limit.

[0314] Experimental Example 3 Results and Discussion: Changes in Sb Precipitation Rate and Te Co-precipitation Rate According to Total Equivalents of Ascorbic Acid Input

[0315] In Experimental Examples 3-A (total 0.3 equivalents) and 3-B (total 0.6 equivalents), the Sb precipitation rates were low at 31.2% and 58.7%, respectively, and it was confirmed that complete reduction of Sb(V) was not achieved because no point was observed where the ORP dropped below +200 mV, or because the drop in ORP was insufficient even after the second addition. The above results indicate that when the total input amount is less than 0.9 equivalents (lower limit of the present invention), there is a lack of substantial effective equivalents contributing to the reduction of Sb(V) when considering the consumption of ascorbic acid due to dissolved oxygen and residual nitric acid in the eluent, and prove that the setting of the lower limit of 0.9 equivalents is a threshold value determined based on the minimum effective equivalents under practical process conditions.

[0316] In Experimental Examples 3-C (total 0.9 equivalents, lower limit), 3-D (total 1.2 equivalents), 3-E (total 1.5 equivalents), and 3-F (total 2.0 equivalents, upper limit), the Sb precipitation rates were 93.5%, 98.7%, 98.9%, and 99.1%, respectively, showing good results of over 93%, and the Te co-precipitation rates were 1.8%, 0.7%, 0.8%, and 1.5%, respectively, confirming excellent selectivity of less than 2% in all cases. In particular, the slight increase in the Te co-precipitation rate in Experimental Example 3-F (upper limit, 2.0 equivalents) compared to Experimental Examples 3-D and 3-E suggests that the ORP tends to approach the Te reduction threshold under input amount conditions close to the upper limit, which serves as the basis for setting 2.0 equivalents as the upper limit.

[0317] In Experimental Example 3-G (total 2.5 equivalents, exceeding the upper limit), the Te co-precipitation rate increased sharply to 19.3%, which is attributed to the fact that the reduction precipitation of Te(IV) proceeded in earnest as the ORP dropped below the Te reduction threshold due to the excess ascorbic acid. The fact that the Te co-precipitation rate increased sharply from 1.5% to 19.3% between Experimental Example 3-F (2.0 equivalents) and 3-G (2.5 equivalents) clearly indicates that setting 2.0 equivalents as the upper limit is not merely to secure a safety margin, but is a numerical setting based on the critical turning point of Te co-precipitation occurrence.

[0319] Results and Discussion of Examples 3-1 to 3-4 and Comparative Examples 3-1 to 3-3: Selective Separation Step

[0320] In Example 3-1 (pore size 0.45 μm, 2 washes, pH adjusted to 4), the primary capture rate of Sb precipitates was very high at 99.2%, and the amount of Te remaining in the cake after washing was low at 0.08 wt% relative to the dry weight of the cake. The recovery rate of Te from the combined solution of the washing filtrate and the primary filtrate reached 96.8% through secondary separation after pH adjustment to 4, and the ICP-OES analysis of the recovered TeO₂·nH₂O precipitates confirmed that the Te purity was 95.2 wt%, which is a level suitable for use as a raw material in subsequent purification processes.

[0321] In Comparative Example 3-1 (membrane pore size 5.0 μm), fine Sb precipitate particles leaked into the filtrate during the primary filtration process, resulting in a significantly reduced Sb capture rate of 78.3%. ICP-OES analysis of the filtrate showed that the Sb concentration was approximately 12 times higher than that of the filtrate in Example 3-1, indicating that the large pore size of 5.0 μm is unsuitable for capturing fine Sb precipitate particles of 0.5 to 3 μm size formed during the reduction step. The above results directly demonstrate that the upper limit of the membrane filter pore size (1.0 μm) is a critical condition for securing Sb capture efficiency.

[0322] In Comparative Example 3-2 (washing solution HCl concentration 10 parts by weight), some of the Sb precipitates were redissolved in the high-concentration hydrochloric acid washing solution during the washing process, and the Sb concentration in the washing filtrate was detected to be about 7 times higher than that of Example 3-1, and the final Sb recovery rate was reduced to 88.4%. This indicates that there exists a critical turning point where redissolution loss of Sb precipitates begins to occur when the hydrochloric acid concentration of the washing solution exceeds 5 parts by weight (upper limit of the present invention), supporting the critical significance of the upper limit of the washing solution concentration.

[0323] In Comparative Example 3-3 (mixture pH adjusted to 7), the Te recovery rate was 97.5%, which was high and similar to Example 3-1; however, ICP-OES analysis of the recovered precipitate revealed high Fe content of 3.2 wt% and Si content of 1.8 wt%, resulting in a significant decrease in Te purity to 78.3 wt%. This indicates that under neutral conditions of pH 7, Fe 3+ This is because Si ions co-precipitate with Te in the form of Fe(OH)₃ and Si(OH)₄. Therefore, it was confirmed that setting the pH adjustment upper limit (5) for Te recovery is a critical condition for securing Te purity.

[0324] When comparing Example 3-2 (pore size 0.1 μm, 1 wash, pH 3) and Example 3-3 (pore size 1.0 μm, 3 washes, pH 5), the Sb capture rate in Example 3-2 was high at 99.4%, whereas the amount of Te remaining in the cake after only one wash was about 2.3 times higher than in Example 3-1, and the final Te recovery rate was 91.2%, which was slightly lower than in Example 3-1. On the other hand, in Example 3-3, Te in the cake was sufficiently eluted with 3 washes, reaching a final Te recovery rate of 97.9%; however, a small amount of fine Sb particles were detected under the pore size 1.0 μm condition, so the final Sb recovery rate was 0.8% lower than in Example 3-1. The above comparison results indicate that there is a slight trade-off between the Sb recovery rate and the Te recovery rate within the condition range of the present invention, and that the conditions of Example 3-1 (pore size 0.45 μm, 2 washes, pH 4) are conditions that optimally satisfy both objectives simultaneously.

[0326] Results and Discussion of Examples 4-1 to 4-4 and Comparative Examples 4-1 to 4-4: High-purity Sb₂O₃ Preparation Steps

[0327] In Example 4-1 (drying at 90°C, 1st stage at 350°C / 1 hour, 2nd stage at 580°C / 2 hours, cooling at 3°C / min), the purity of the final Sb₂O₃ powder was 99.4 wt% based on ICP-OES analysis, and the residual Cl content was 0.03 wt% or less, and the residual organic matter content was 0.02 wt% or less, which were very low. The particle size by laser diffraction (based on D50) was 4.2 μm, and XRD analysis confirmed a single phase of orthorhombic valentinite Sb₂O₃ crystals. This indicates that the two-stage calcination process of the present invention is an effective process that simultaneously achieves the removal of residual impurities and high-purity crystallization.

[0328] In Comparative Example 4-1 (first calcination at 250°C), the Sb purity of the final powder based on ICP-OES was reduced to 96.8 wt%, and the residual Cl content was 1.2 wt% and the residual organic matter content was 0.8 wt%, which were significantly higher than those in Example 4-1. This is attributed to the fact that the decomposition temperature (typically 300–350°C) of chlorine-containing antimony compounds such as SbOCl was not reached at the first calcination temperature of 250°C, and thus residual chlorine was not removed. The above results directly demonstrate that the lower limit of the first calcination temperature (300°C) is the critical minimum temperature for the complete removal of residual chlorine.

[0329] In Comparative Example 4-2 (heating rate 15℃ / min), the particle size distribution (D10~D90) of the final powder was approximately 3.5 times wider than that of Example 4-1, and SEM analysis revealed numerous cracks and fracture marks on the particle surface. This is attributed to the fact that during the rapid heating process, HCl gas generated during the decomposition of residual chlorine in the primary calcination section (300~400℃) rapidly accumulated inside the Sb precipitate and was quickly released, causing mechanical stress inside the particles. Therefore, it was confirmed that exceeding the upper limit of the heating rate (10℃ / min) is a critical turning point that causes not only a decrease in uniformity but also structural damage to the particles.

[0330] In Comparative Example 4-3 (second calcination at 700°C), white deposits resulting from the partial sublimation of Sb₂O₃ during the calcination process were observed in the cooling section of the kiln, and the final powder recovery rate decreased by approximately 8.3% compared to Example 4-1. Additionally, the D50 particle size based on laser diffraction increased to 18.7 μm, forming coarse particles that significantly exceeded the target range (1–10 μm). XRD analysis results showed that at 700°C, cubic Sb₂O₃, a high-temperature phase, was mixed with orthorhombic Sb₂O₃; this suggests that changes in crystal phase due to high-temperature calcination may have a negative impact on the flame retardancy and dispersibility of the final product. Therefore, it was confirmed that setting the upper limit of the second calcination temperature (650°C) is a critical condition for preventing Sb₂O₃ sublimation loss and ensuring particle size uniformity.

[0331] In Comparative Example 4-4 (cooling rate 15℃ / min), the D50 particle size of the final powder, based on laser diffraction, was 0.7 μm, which is below the target range (1~10 μm), and the D10~D90 range was 0.2~2.1 μm, showing a very narrow and over-fine particle size distribution. SEM analysis confirmed that the Sb₂O₃ crystal particles were extensively fractured due to thermal stress caused by rapid cooling, resulting in the generation of numerous submicron-sized fragments. Since Sb₂O₃ powder with a particle size of less than 1 μm poses a problem of increased risk of dust explosion and reduced handling safety, it was proven that setting an upper limit of the cooling rate (5℃ / min) is a critical condition for ensuring particle size uniformity and maintaining dust safety.

[0332] When comparing Example 4-2 (lower limit condition) and Example 4-3 (upper limit condition), Example 4-2 showed good results with Sb₂O₃ purity of 99.1 wt% and D50 particle size of 5.8 μm, while Example 4-3 showed a purity of 99.3 wt% and D50 particle size of 7.9 μm. Both examples met the target quality criteria (purity of 99 wt% or higher, D50 1~10 μm), confirming that Sb₂O₃ powder of the target quality can be stably obtained in various small and medium-sized enterprise production environments within the firing condition range of the present invention.

[0334] Experimental Example 4 Results and Discussion: Comparison of Two-Stage Firing Process and Single Firing Process

[0335] In Experimental Example 4-A (two-stage firing, 350°C first stage + 580°C second stage), excellent quality was achieved with a final powder purity of 99.4 wt%, residual Cl of 0.03 wt% or less, and D50 particle size of 4.2 μm, which is the same as the result of the aforementioned Example 4-1.

[0336] In Experimental Example 4-B (single calcination, direct calcination at 580°C), sufficient crystallization of Sb₂O₃ was achieved, and a single crystal phase was confirmed by XRD; however, the residual Cl content was 0.89 wt% and the residual organic matter content was 0.43 wt%, which were significantly higher than those in Experimental Example 4-A. This is attributed to the fact that when the temperature is raised directly to 580°C without a first low-temperature calcination stage (300–400°C), Sb₂O₃ crystallization proceeds first before residual chlorine-containing compounds are completely decomposed and removed, resulting in the encapsulation of residual chlorine within the crystal. Since this encapsulated residual chlorine exhibited the characteristic of not being completely removed even if the high-temperature calcination was carried out for a longer period, this suggests that the omission of the first low-temperature calcination step is an irreversible process defect that causes permanent chlorine encapsulation within the crystal structure, going beyond the mere issue of insufficient impurity removal.

[0337] In Experimental Example 4-C (single low-temperature calcination, only 350°C applied), the residual Cl content was low at 0.05 wt%, but XRD analysis confirmed that a significant amount of amorphous components remained, indicating that the crystallization of Sb₂O₃ was incomplete, and the purity according to ICP-OES was 96.5 wt%, falling short of the target standard (99 wt% or more). This indicates that while the dechlorination effect is achieved with only single low-temperature calcination at 350°C, there is insufficient thermal energy for the complete crystallization of Sb₂O₃.

[0338] The comparative results of Experimental Examples 4-A, 4-B, and 4-C above clearly demonstrate that in the two-stage calcination process of the present invention, the first calcination (dechlorination and de-oiling) and the second calcination (crystallization) each perform independent and essential roles, and that the omission of either step makes it impossible to meet the final product quality standards. In particular, the chlorine capture phenomenon within the crystals confirmed in Experimental Example 4-B indicates that the first low-temperature calcination step is not merely a preliminary treatment but a structural process design that prevents irreversible quality degradation in the subsequent crystallization step, confirming that this is a key technical advantage of the two-stage calcination method of the present invention compared to the conventional single calcination method.

Claims

Claim 1 A method for selectively separating and recovering Sb and Te from a composite oxide catalyst or spent catalyst containing Sb₂O₅ and TeO₂, a. A pretreatment step of preparing catalyst powder with a particle size of 1 to 100 μm by crushing the above-mentioned complex oxide catalyst or spent catalyst, and introducing the catalyst powder into a reaction vessel in a solid-to-liquid ratio of 1:3 to 1:8 (g / mL); wherein hydrochloric acid is first introduced alone into the reaction vessel and stirred in contact with the catalyst powder at 40 to 60°C for 10 to 30 minutes to first form an antimony chlorine complex, and then nitric acid is subsequently introduced stepwise to stabilize Te in an oxidized dissolved state, thereby preparing a mixed acid eluent containing 1 to 12 parts by weight of hydrochloric acid (HCl) and 0.1 to 6 parts by weight of nitric acid (HNO₃) per 100 parts by weight of a total mixed acid solution, wherein Cl in the mixed acid eluent - Ions and NO₃ - A mixed acid elution step of preparing an elution solution containing Sb and Te by stirring at 70 to 90°C for 1 to 3 hours while adjusting the input amounts of hydrochloric acid and nitric acid so that the molar concentration ratio of the ions is 3:1 to 10:1; b. A reduction step of forming an Sb precipitate by adding a reducing agent to the above-mentioned eluent to selectively reduce Sb(V) in the above-mentioned eluent to Sb(III); and c. A method for selectively separating and recovering Sb and Te from a composite oxide catalyst, comprising: a selective separation step of separating the Sb precipitate from the eluent to leave Te in the solution phase; wherein only the Sb precipitate is selectively formed within the range of +100 mV to +400 mV of the oxidation-reduction potential (ORP) relative to the Ag / AgCl reference electrode by inhibiting the reduction of Te in the mixed acid solution. Claim 2 delete Claim 3 In claim 1, a. A mixed acid elution step of preparing an elution solution containing Sb and Te by contacting the complex oxide catalyst or spent catalyst with a mixed acid solution containing hydrochloric acid (HCl) and nitric acid (HNO₃); b. A reduction step comprising adjusting the temperature of the above-mentioned eluent to 40–70°C, then adding ascorbic acid to the above-mentioned eluent in a first amount of 0.8–1.5 equivalents relative to the molar amount of Sb in a divided manner, and after the first addition, monitoring the oxidation-reduction potential (ORP) relative to the Ag / AgCl reference electrode in real time, and adding ascorbic acid in a second amount of 0.1–0.5 equivalents relative to the molar amount of Sb based on the point in time when the ORP value drops to +200 mV or lower, thereby selectively reducing Sb(V) in the above-mentioned eluent to Sb(III) while forming an Sb precipitate while maintaining the oxidation state of Te(IV); and c. A method for selectively separating and recovering Sb and Te from a composite oxide catalyst, characterized by including a selective separation step of separating the Sb precipitate from the eluent to leave Te in the solution phase.

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