Process method for extracting vanadium from vanadium titano-magnetite

Abstract

The invention relates to a process method for extracting vanadium from vanadium titano-magnetite, and belongs to the technical field of hydrometallurgy. In order to solve the technical problems of low production efficiency and high energy consumption caused by material sintering, equipment bonding, forced matching and returned material adding due to concentrated oxidation heat release of low-valence substances in the existing vanadium slag roasting process, the invention provides the method for pre-oxidizing the high-temperature vanadium slag in an atmosphere with specific oxygen concentration (5-100%) after vanadium is extracted from a converter, and the high-temperature vanadium slag is subjected to heat preservation for 0.5-60 hours at the temperature of 500-1200 DEG C for pre-oxidation. Low-valence ions such as Fe < + > and V < + > in the vanadium slag are fully converted into Fe < + > and V < 5 + > through pre-oxidation, spinel structure decomposition is promoted, and phase conversion is achieved. Carrying out crushing, iron removal and ore grinding on the pre-oxidized vanadium slag until the particle size is 1t; after the thickness is 0.2 mm, mixing with a calcium salt additive according to a CaO / V2O5 mass ratio of 0.3-1, and roasting at 800-950 DEG C for 30-360 minutes. Through the pre-oxidation step, the violent oxidation heat release process is dispersed, the problem of heat accumulation in the roasting process is fundamentally solved, and the production efficiency and economic benefits are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of hydrometallurgy, specifically relating to a process for extracting vanadium from vanadium-titanium magnetite. More specifically, it relates to a new method for pre-oxidizing vanadium slag obtained after vanadium extraction in a converter, followed by calcification roasting, in order to improve vanadium conversion rate and avoid material sintering and equipment adhesion during the roasting process. Background Technology

[0002] Vanadium is an important strategic rare metal, widely used in steel, aerospace, chemical, and energy storage industries to enhance the strength, toughness, and corrosion resistance of alloys. Approximately 90% of global vanadium production comes from vanadium-titanium magnetite. A typical extraction route involves smelting the vanadium-titanium magnetite in a blast furnace to obtain vanadium-containing molten iron. This molten iron is then subjected to oxygen blowing in a converter (selective oxidation), causing the vanadium in the molten iron to oxidize and enter the slag, forming converter vanadium slag. Therefore, converter vanadium slag is the primary raw material for vanadium extraction.

[0003] The core technical step in extracting vanadium from vanadium slag is roasting. The purpose of roasting is to break down the stable mineral phases containing vanadium (such as fir olivine (Fe₂SiO₄) and spinel ((Fe,Mn)O·(V,Cr,Fe)₂O₃), and to extract the low-valence vanadium (such as V³⁺) that is insoluble in water or acid. + ) is converted into high-valent vanadates (V) that are soluble in water or acid. 5+ Currently, the most widely used vanadium extraction processes from vanadium slag in industry mainly include: Sodium roasting-water leaching-ammonium salt precipitation process: Vanadium slag is mixed with sodium salts (such as Na2CO3, Na2SO4) and roasted to produce water-soluble sodium metavanadate (NaVO3), which is then leached and precipitated to obtain V2O5. This process has a high conversion rate, but the sodium-containing wastewater and waste gas (such as HCl, Cl2) generated cause serious environmental pollution.

[0004] The calcination-roasting-acid leaching-hydrolysis precipitation process (also known as the "Tula process") involves mixing vanadium slag with calcium salts (such as CaO and CaCO3) and roasting them to produce acid-soluble calcium vanadate (Ca(VO3)2). This is then followed by acid leaching and hydrolysis precipitation to obtain V2O5. This process is environmentally friendly, but it requires stringent control of roasting conditions, making it difficult to produce high-quality V2O5.

[0005] Calcination roasting-acid leaching-ammonium salt precipitation process: This is the mainstream process adopted by domestic enterprises such as Panzhihua Iron and Steel Group. It is an improvement on the Tula process, using ammonium salt precipitation, and the process is slightly different.

[0006] Other processes, such as submolten salt vanadium extraction and blank roasting-alkali leaching vanadium extraction, also have their own characteristics, but their application range is relatively narrow.

[0007] Regardless of whether sodium-based or calcium-based roasting is used, the core scientific principle remains the same: vanadium valence state transformation and phase reconstruction are achieved through oxidation reactions during the roasting process. However, in addition to vanadium, vanadium slag also contains a large amount of Fe²⁺. + These are reducing components. During roasting, these low-valence components undergo violent oxidation reactions (e.g., 2FeO + 1 / 2O2 → Fe2O3), accompanied by a large amount of concentrated exothermic heat. This concentrated and violent exothermic effect causes a sharp increase in local temperature within the roasting system, far exceeding the material's softening point. This leads to localized softening and sintering of the material, causing it to adhere to the rotary kiln walls or multi-hearth furnace grates, resulting in severe "kiln clogging" or "furnace clogging." This not only disrupts the normal roasting temperature regime, causing fluctuations and decreases in vanadium conversion rate, but also forces frequent kiln shutdowns for cleaning, significantly reducing equipment operating rates and increasing energy consumption and production costs.

[0008] To address this global challenge, current industrial practices commonly employ methods such as "adding recycled materials" or "using inert diluents." This involves mixing a certain proportion of already roasted, non-oxidizing vanadium extraction tailings, or adding inert substances like quartz sand (already oxidized roasted clinker), before the vanadium slag enters the roasting process. The aim is to physically dilute the concentration of oxidizable substances in the material, thereby slowing down the overall oxidation and exothermic rate and controlling the peak exothermic temperature within a range that the equipment can withstand and that prevents the material from sintering.

[0009] Although this method alleviates the sintering adhesion problem to some extent, the cost is enormous: Low processing efficiency: A large amount of reacted tailings or ineffective inert materials occupy valuable kiln volume, resulting in a significant reduction in the amount of "fresh vanadium-containing material" processed per unit time and per unit volume, and a decrease in equipment efficiency.

[0010] Increased energy consumption: Heating and maintaining large amounts of inactive materials at high temperatures results in a huge waste of energy.

[0011] High production costs: Reduced efficiency and increased energy consumption directly drive up the cost of vanadium extraction. At the same time, the transportation, storage, and mixing of returned materials also increase operational complexity.

[0012] Therefore, developing a new technology that can fundamentally solve the problem of exothermic accumulation during the roasting process and achieve "no return material" or "low return material" high-efficiency roasting has become a long-term goal pursued by those skilled in the art. Through in-depth research into the mechanism of vanadium slag oxidation, the inventors of this invention have realized that the root of the problem lies in the "concentration" rather than the "total amount" of the exothermic oxidation. If the intense exothermic process concentrated in the roasting stage can be "pre-emptively" and "dispersed," it is hoped that the sintering adhesion problem can be completely solved without significantly increasing the total energy consumption. Based on this insight, this invention proposes an innovative "pre-oxidation calcination roasting" method. Summary of the Invention

[0013] The primary objective of this invention is to overcome the shortcomings of existing vanadium slag roasting technology, which relies on the addition of recycled materials or inert substances to prevent sintering and agglomeration, and to provide a process for extracting vanadium from vanadium-titanium magnetite.

[0014] Another objective of this invention is to achieve controllable and gradual oxidation of low-valence elements in vanadium slag before calcination roasting through a pre-oxidation step, thereby avoiding concentrated heat release during calcination roasting and ensuring that the roasted clinker is loose and does not stick to the equipment.

[0015] Another objective of this invention is to achieve a high vanadium roasting conversion rate (stable at over 90%) without the need for return material, while simultaneously improving kiln processing efficiency and reducing energy consumption and production costs.

[0016] To achieve the above objectives, this invention provides a method for pre-oxidation and calcination roasting of converter vanadium slag. The core of this method lies in introducing a carefully controlled "pre-oxidation" step before the traditional calcination roasting process. The method specifically includes the following steps: Pre-oxidation step: Raw material: High-temperature vanadium slag obtained after oxygen extraction in a converter. The vanadium slag typically exits the furnace at a temperature higher than 1300℃ and contains a large amount of sensible heat.

[0017] Atmosphere control: The high-temperature vanadium slag is placed in a controlled atmosphere with an oxygen concentration of 5% to 100%. A preferred oxygen concentration is 10% to 30% to balance the oxidation rate and equipment cost.

[0018] Temperature control: The pre-oxidation treatment temperature is 500℃ to 1200℃. The sensible heat of the vanadium slag itself can be utilized, or supplementary heating can be used. The preferred temperature is 600℃ to 1000℃.

[0019] Time control: The pre-oxidation holding time is 0.5 to 60 hours. The preferred time is 8 to 36 hours to ensure that the low-valence substances are fully oxidized without being too long and affecting efficiency.

[0020] Mechanism of action: In this step, Fe²⁺ in the vanadium slag + V³ + Low-valence ions react with oxygen and are oxidized to Fe³⁺. + and V 5+ This process not only changes the valence state of the elements, but more importantly, it disrupts the original stable spinel mineral structure, transforming it into an active phase that is more readily reacted with calcium salts in subsequent roasting. Crucially, this exothermic oxidation process occurs under relatively gentle conditions and over a longer period, allowing heat to be released and dissipated slowly, avoiding a concentrated burst of heat during the subsequent short-duration, high-temperature roasting process.

[0021] Preprocessing steps: The pre-oxidized vanadium slag undergoes a series of physical processing steps, including but not limited to crushing, magnetic separation for iron removal, ball milling, and air classification, to obtain "refined vanadium slag" that meets the requirements for subsequent roasting.

[0022] Particle size requirement: The particle size of the refined vanadium slag must be controlled below 0.2 mm. Preferably, the proportion of particles with a size below 0.096 mm (approximately 160 mesh) should account for more than 80% of its total mass to ensure sufficient contact with the additives and the reaction rate.

[0023] Iron content: The iron content of the refined vanadium slag should be less than 5%, preferably less than 3%. Excessive iron content will oxidize and release heat during roasting and may cause sintering, so it needs to be effectively removed by magnetic separation.

[0024] Mixing steps: The refined vanadium slag obtained above is mixed evenly with calcium salt additives. The calcium salt additives can be lime (CaO), limestone powder (CaCO3), etc.

[0025] Proportioning requirements: The mixing ratio, based on the mass ratio of CaO to V₂O₅ in the final mixture, should be between 0.3:1 and 1:1. A preferred ratio is between 0.4:1 and 0.7:1. This range ensures the formation of sufficient acid-soluble calcium vanadate while avoiding excessive calcium salts that would increase energy consumption and burden subsequent acid leaching.

[0026] Roasting steps: The uniformly mixed materials are calcined at high temperature in an oxidizing atmosphere.

[0027] Firing temperature: 800℃ to 950℃. The preferred temperature is 840℃ to 900℃.

[0028] Calcination time: 30 to 360 minutes. Preferred time is 100 to 200 minutes.

[0029] Oxygen concentration in exhaust gas: To ensure the completeness of the oxidation reaction, the oxygen concentration in the exhaust gas during the roasting process is controlled to be 8% to 20%, preferably 12% to 15%.

[0030] Results: In this step, the pre-oxidized material mainly undergoes a solid-state reaction between vanadium and calcium to form calcium vanadate, while the intense exothermic oxidation of low-valence elements has been largely completed in the previous step. Therefore, the temperature rise during roasting is stable, the resulting clinker is loose and porous, without sintered lumps, and there is no adhesion to the roasting vessel. The vanadium conversion rate is stable and high.

[0031] Compared with the prior art, the technical solution provided by the present invention has the following significant advantages and positive effects: To fundamentally solve the sintering adhesion problem: the "pre-oxidation" step moves and disperses the concentrated heat source that leads to sintering, making the calcination roasting process a relatively "mild" solid-phase reaction process, thereby completely eliminating the hidden dangers of material sintering and equipment adhesion.

[0032] Achieving efficient roasting without return material: Since the roasting process no longer requires the addition of vanadium extraction tailings or inert substances for dilution, the effective volume of the kiln is entirely used to process fresh vanadium-containing materials, which can increase the equipment processing efficiency by 20%-40% and significantly increase production capacity.

[0033] Energy consumption is significantly reduced: on the one hand, the energy consumption of reheating returned materials is eliminated; on the other hand, the sensible heat of the vanadium slag itself is fully utilized for pre-oxidation, reducing the input of additional energy. Overall energy consumption is expected to be reduced by 15%-30%.

[0034] High and stable vanadium conversion rate: Pre-oxidation homogenizes and activates the vanadium slag phase, creating more favorable thermodynamic and kinetic conditions for subsequent calcination roasting, enabling the vanadium conversion rate to remain stable at over 90%, and the process has good repeatability.

[0035] A win-win situation for both the environment and the economy: the process is clean and generates no additional pollutants. At the same time, due to increased efficiency and reduced energy consumption, the production cost of vanadium is significantly reduced, enhancing the product's market competitiveness.

[0036] Reasonable process integration: This process can be well integrated with the existing converter steelmaking process, and the pre-oxidation stage can be used as an optimization and upgrade of the vanadium slag cooling and storage process, which is easy to implement and promote in the existing production system. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic flowchart of a process for extracting vanadium from vanadium-titanium magnetite according to an embodiment of the present invention. Detailed Implementation

[0039] The embodiments of this disclosure will be further described in detail below with reference to the accompanying drawings and examples. The detailed description of the embodiments and the accompanying drawings are used to illustrate the principles of this disclosure by way of example, but should not be used to limit the scope of this disclosure. This disclosure can be implemented in many different forms and is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

[0040] These embodiments are provided to make the disclosure thorough and complete, and to fully express the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​set forth in these embodiments should be interpreted as exemplary only and not as limiting.

[0041] As used in this disclosure, the words “including” or “contains” or similar terms mean that the element preceding the word covers the element listed after the word, and do not exclude the possibility that it may also cover other elements.

[0042] All terms used in this disclosure have the same meaning as understood by one of ordinary skill in the art to which this disclosure pertains, unless otherwise specifically defined. It should also be understood that terms defined in general dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant art, and not as idealized or highly formalized, unless expressly defined herein.

[0043] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, they should be considered part of the specification.

[0044] Combination Figure 1 In an embodiment of the present invention, a method 100 for pre-oxidation calcination roasting of converter vanadium slag includes the following steps: Step 101, Pre-oxidation step: The high-temperature vanadium slag obtained by oxygen blowing and vanadium extraction in the converter is pre-oxidized by holding it at a temperature of 500℃ to 1200℃ for 0.5 to 60 hours in an atmosphere with an oxygen concentration of 5% to 100% to obtain pre-oxidized vanadium slag.

[0045] Step 102, Pretreatment step: The pre-oxidized vanadium slag is crushed, iron removed and ground to obtain refined vanadium slag with a metallic iron content of less than 5% and a particle size of less than 0.2 mm.

[0046] Step 103, Mixing step: Mix the refined vanadium slag with calcium salt additives to obtain a mixture, wherein the mass ratio of CaO to V2O5 in the mixture is 0.3:1 to 1:1.

[0047] Step 104, calcination step: calcin the mixture at a temperature of 800°C to 950°C for 30 to 360 minutes to obtain calcined cooked material.

[0048] The following is combined Figure 1 The present invention will be further described in detail with reference to specific embodiments, but the implementation of the present invention is not limited thereto. Process parameters not specifically specified in the following embodiments are generally implemented under conventional conditions.

[0049] Example 1 This embodiment is used to specifically illustrate the pre-oxidation and calcination roasting method for converter vanadium slag of the present invention.

[0050] Pre-oxidation step: The vanadium slag obtained after vanadium extraction from the molten iron converter (temperature approximately 1350℃) is quickly transferred to a controlled atmosphere insulated container. A nitrogen-oxygen mixture with a concentration of 10% is introduced into the container. The temperature inside the container is maintained at 1000℃ and held under these conditions for 24 hours to complete the pre-oxidation process.

[0051] Pretreatment steps: The pre-oxidized lumpy vanadium slag is removed and first coarsely crushed by a jaw crusher, then the impurities of metallic iron particles are removed by a magnetic separator. Subsequently, the iron-removed vanadium slag is fed into a ball mill for fine grinding, and finally classified by air classification to obtain refined vanadium slag that meets the requirements. Analysis shows that the refined vanadium slag contains 15.86 wt% V2O5, 96% of which are smaller than 0.096 mm, and 1.8% metallic iron.

[0052] Mixing Step: Accurately weigh 100g of the above-mentioned refined vanadium slag and mix it thoroughly with 14g of finely ground industrial limestone powder (containing 52.96 wt% CaO) in a mixer for 30 minutes to ensure homogeneity. The calculated mass ratio of CaO to V2O5 in this mixture is (14g × 52.96%) / (100g × 15.86%) ≈ 0.62, which falls within the preferred range.

[0053] Calcination procedure: The mixture is placed in an alumina crucible and then placed in a muffle furnace for calcination calcination. The calcination regime is set as follows: the temperature is programmed to rise from room temperature to 900℃ at a rate of 10℃ / min, and timing begins after reaching the target temperature, with constant temperature calcination for 120 minutes. During the calcination process, air circulation is maintained inside the muffle furnace (equivalent to an oxygen concentration of approximately 21% in the exhaust gas).

[0054] Results and Observations: After roasting, the crucible was removed. The roasted clinker was observed to be a uniform, dark brown, loose powder, without any visible large sintered pieces. When the crucible was lightly tapped, the clinker easily detached without any sticking. The roasting conversion rate of the clinker was analyzed, and the result was 92.24%.

[0055] Example 2 This embodiment is used to further illustrate the effect of the present invention under different pre-oxidation parameters.

[0056] Pre-oxidation step: Take the converter vanadium slag and place it in a controlled atmosphere container. Introduce a mixed gas with an oxygen concentration of 30% and control the temperature inside the container to 600℃. Under these conditions, carry out the pre-oxidation treatment for 36 hours.

[0057] Pretreatment steps: Same as in Example 1, after crushing, magnetic separation, ball milling, and air classification, refined vanadium slag is obtained. Its composition is: V2O5 content 18.92 wt%, particle size <0.096mm accounts for 96%, and metallic iron content 2.1%.

[0058] Mixing steps: Weigh 100 grams of this refined vanadium slag and mix it with 20 grams of industrial limestone powder (CaO content 53.28 wt%). The calculated CaO / V2O5 mass ratio is (20g × 53.28%) / (100g × 18.92%) ≈ 0.70.

[0059] Calcination step: The mixture is calcined in a muffle furnace at a constant temperature of 880°C for 180 minutes.

[0060] Results and observations: The obtained clinker was also loose, without sintering or agglomeration. The vanadium calcination conversion rate reached 91.98%. This indicates that the present invention can achieve excellent results even at a relatively low pre-oxidation temperature (600℃) and a relatively long time.

[0061] Example 3 This embodiment is used to demonstrate the performance of the present invention under intermediate process parameters.

[0062] Pre-oxidation step: The converter vanadium slag is pre-oxidized for 12 hours in an atmosphere with an oxygen concentration of 20% and a temperature of 800℃.

[0063] Pretreatment steps: The composition of the refined vanadium slag after treatment is as follows: V2O5 content 17.14 wt%, particle size <0.096mm accounts for 96%, and metallic iron content 1.5%.

[0064] Mixing steps: Take 100g of refined vanadium slag and mix it with 10g of industrial limestone powder (CaO content 54.02 wt%). The CaO / V2O5 mass ratio is (10g × 54.02%) / (100g × 17.14%) ≈ 0.45.

[0065] Firing procedure: Firing at 860℃ for 200 minutes.

[0066] Results and observations: The roasted clinker was in good condition, loose and non-sticky. The roasting conversion rate was 90.76%.

[0067] Comparative Example 1 For comparison, unoxidized raw vanadium slag from the same batch as in Example 1 was taken. Following conventional processes, it was crushed, beneficiated, and ground to the same particle size as in Example 1. Then, 50 grams of this raw vanadium slag was mixed and diluted with 50 grams of pre-roasted oxidized clinker (inert material) from another batch. An equivalent amount of calcium salt (equivalent to the amount of calcium salt required to process 50 grams of fresh vanadium slag, with the CaO / V₂O₅ ratio maintained at 0.62) was added to this 100 grams mixture. Roasting was then carried out under the exact same conditions as in Example 1 (900°C, 120 minutes).

[0068] Results and Observations: After roasting, some slightly sintered small pieces were still visible in the clinker, and they were slightly adhered to the bottom of the crucible. The roasting conversion rate was analyzed to be 89.41%. This result is slightly lower than that of Example 1 of the present invention. This proves that although the traditional reprocessing can alleviate sintering to some extent, it comes at the cost of sacrificing processing efficiency (in this example, the effective vanadium slag processing volume is halved).

[0069] Comparative Example 2 Take 100 grams of unoxidized vanadium slag from the same batch as in Example 1, without adding any recycled material, and mix it directly with an equal amount of calcium salt from Example 1 (CaO / V2O5=0.62). Roast under the same conditions (900℃, 120 minutes).

[0070] Results and Observations: The calcined product was severely sintered, forming a hard, large block that firmly adhered to the crucible and could not be detached on its own. Only strong knocking was required to remove even a portion of the sample. Due to the severe sintering, the conversion rate was extremely low, measured at only 78.35%. This comparative example clearly demonstrates the disastrous consequences of concentrated exothermic reactions without pre-oxidation and without material return, highlighting the criticality and superior effectiveness of the pre-oxidation step in this invention.

[0071] Through a thorough comparison of the above embodiments and comparative examples, it is clear that the converter vanadium slag pre-oxidation calcination roasting method provided by the present invention successfully solves the long-standing roasting, sintering and bonding problem that has plagued the vanadium extraction industry by introducing a controllable pre-oxidation step. It achieves clean production with high conversion rate, high efficiency and low energy consumption without relying on recycled materials. The technical effect is significant and has great industrial application value.

[0072] The embodiments of this disclosure have now been described in detail. To avoid obscuring the concept of this disclosure, some details known in the art have not been described. Those skilled in the art will fully understand how to implement the technical solutions disclosed herein based on the above description.

[0073] While specific embodiments of this disclosure have been described in detail by way of examples, those skilled in the art should understand that the examples are for illustrative purposes only and are not intended to limit the scope of this disclosure. Those skilled in the art should understand that modifications can be made to the above embodiments or equivalent substitutions can be made to some technical features without departing from the scope and spirit of this disclosure.

Claims

1. A process for extracting vanadium from vanadium-titanium magnetite, characterized in that, Includes the following steps: a) Pre-oxidation step: The high-temperature vanadium slag obtained by oxygen blowing and vanadium extraction in the converter is pre-oxidized by holding it at a temperature of 500℃ to 1200℃ for 0.5 to 60 hours in an atmosphere with an oxygen concentration of 5% to 100% to obtain pre-oxidized vanadium slag. b) Pretreatment step: The pre-oxidized vanadium slag is crushed, iron removed and ground to obtain refined vanadium slag with a metallic iron content of less than 5% and a particle size of less than 0.2 mm; c) Mixing step: The refined vanadium slag is mixed with calcium salt additives to obtain a mixture, wherein the mass ratio of CaO to V2O5 in the mixture is 0.3:1 to 1:1; d) Calcination step: The mixture is calcined at 800°C to 950°C for 30 to 360 minutes to obtain calcined cooked material.

2. The method according to claim 1, characterized in that, In step a), the oxygen concentration of the pre-oxidation treatment is 10% to 30%.

3. The method according to claim 1, characterized in that, In step a), the pre-oxidation treatment takes 8 to 36 hours.

4. The method according to claim 1, characterized in that, In step a), the temperature of the pre-oxidation treatment is 600°C to 1000°C.

5. The method according to claim 1, characterized in that, In step b), the proportion of the refined vanadium slag with a particle size of less than 0.096 mm accounts for more than 80% of its total mass.

6. The method according to claim 1, characterized in that, In step b), the metallic iron content of the refined vanadium slag is less than 3%.

7. The method according to claim 1, characterized in that, In step c), the mass ratio of CaO to V2O5 in the mixture is 0.4:1 to 0.7:

1.

8. The method according to claim 1, characterized in that, In step d), the roasting temperature is 840°C to 900°C and the roasting time is 100 to 200 minutes.

9. The method according to claim 1, characterized in that, In step d), the roasting process is carried out in an exhaust gas atmosphere with an oxygen concentration of 8% to 20%.

10. The method according to claim 9, characterized in that, The oxygen concentration in the exhaust gas from the roasting process is 12% to 15%.

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