A method for desulfurization of phosphogypsum without reducing carbon emissions and preparation of low-heat Portland cement clinker

Abstract

The invention discloses a method for desulfurization of phosphogypsum without reducing carbon emissions and preparing low-heat Portland cement clinker. The phosphogypsum, medium- and low-grade pyrite, and silico-aluminous raw materials are proportioned according to the designed mass ratios of S:S, KH value, and total CaO:SiO₂, mixed and pulverized, and made into balls with water, and then dried. The dried balls are calcined in a rotary kiln lacking oxygen or oxygen-free after the combustion of coal, and after the calcination is completed, they are cooled to obtain low-heat Portland cement clinker without C₃S. Subsequently, low-heat Portland cement clinker containing C₃S can be obtained by adding calcareous raw materials for secondary proportioning and calcination. The invention uses medium- and low-grade pyrite to decompose phosphogypsum, further reducing the coal consumption cost and carbon emissions during the large-scale treatment of phosphogypsum, and realizing the efficient desulfurization of phosphogypsum through reasonable proportioning and process, and simultaneously preparing low-heat Portland cement clinker. The clinker does not contain γ-C₂S and C₃A.

Description

Technical Field

[0001] The present invention belongs to the technical field of building materials, and particularly relates to a method for desulfurizing phosphogypsum without reducing carbon emissions and preparing low-heat Portland cement clinker. Background Art

[0002] Low-heat Portland cement has the characteristics of low production energy consumption, low production cost, less harmful gas emissions, good workability, low heat of hydration, high late strength, small drying shrinkage, high crack resistance, high durability, and high corrosion resistance, and is particularly suitable for mass concrete and high-strength high-performance concrete.

[0003] The traditional process for preparing low-heat Portland cement clinker is to proportion limestone and silico-aluminous raw materials, control KH = 0.68 - 0.76, SM = 2.5 - 3.2, IM = 0.65 - 0.89, and the calcination temperature is 1250 - 1350 °C. Since it is required that the C2S content in the clinker is ≥ 40% and the C3A content is ≤ 6%, there are mainly two problems: (1) The high C2S content leads to the phenomenon that the hydrated active mineral β-C2S is easily pulverized and transformed into the anhydrous active mineral γ-C2S during the cooling of the clinker, which requires high process requirements and needs to increase the rotation speed of the kiln and the cooling speed of the clinker; (2) The proportioning requirements are harsh, and iron raw materials with high Fe2O3 content and low Al2O3 content are required for correction, and this raw material is not easily obtained and will increase the cost.

[0004] Phosphogypsum is a solid waste generated in the wet-process phosphoric acid process. Its comprehensive treatment is a worldwide problem and the biggest obstacle restricting the development of the phosphorus chemical industry and new energy materials industry. Using phosphogypsum instead of limestone as the CaO source to produce cement clinker is a challenging and meaningful direction for solving the resource utilization of phosphogypsum. However, compared with CaCO3, CaSO4 needs to be completely decomposed and desulfurized at a higher temperature, which will increase the operation difficulty and cost of the process, and even a large amount can still exist under high-temperature calcination, which undoubtedly restricts the effective application of phosphogypsum.

[0005] Many research reports use carbonaceous raw materials as the reducing agent for the decomposition of CaSO4, such as coke and pulverized coal, which are expensive and will increase the cost of treating phosphogypsum. Although the carbon emissions are lower than those brought by the decomposition of CaCO3, there will still be relatively high carbon emissions, that is, the part of the reduced carbon emissions brought by the reaction of the carbonaceous raw material as the reducing agent with CaSO4.

[0006] Some research has tried to use sulfur as the reducing agent for the decomposition of CaSO4, which requires a lower reaction temperature and reaction heat than C and is more likely to decompose CaSO4. However, the main product after the decomposition of CaSO4 is CaS, and high desulfurization rate cannot be achieved, and the stability of sulfur is worse than that of coal, and it is easy to react with O2 and sublimate in large quantities in practical applications, resulting in waste.

[0007] Under the premise that my country's requirements for carbon emissions will become increasingly stringent in the future, whether it is possible to find a more stable, effective, non-carbonaceous raw material reducing agent, and achieve efficient and stable desulfurization of phosphogypsum without the need for carbonaceous raw materials and without reducing carbon emissions, and to produce high-value-added materials, has become a technical problem that needs to be solved urgently. Summary of the Invention

[0008] The present invention aims to provide a method for desulfurizing phosphogypsum without reducing carbon emissions and preparing low-heat Portland cement clinker, which uses medium- and low-grade pyrite as a reducing agent to treat phosphogypsum, further reducing the coal cost and carbon emissions when processing large amounts of phosphogypsum, and achieving efficient desulfurization of phosphogypsum through reasonable proportions and processes, while simultaneously producing high-value-added materials.

[0009] In order to achieve the above purpose, the technical solutions adopted are as follows:

[0010] A method for desulfurizing phosphogypsum without reducing carbon emission and preparing low-heat Portland cement clinker comprises the following steps:

[0011] (1) Obtaining phosphogypsum, medium- and low-grade pyrite, and silicon-aluminum-iron raw materials, drying and crushing them, batching them according to the designed S:S, KH value, and total CaO:SiO2 mass ratio, and mixing and grinding them;

[0012] (2) adding water to the obtained mixture to form pellets, and then drying;

[0013] (3) calcining the dried pellets in an oxygen-deficient or oxygen-free rotary kiln after coal combustion, and cooling after calcination to obtain C3S-free low-heat Portland cement clinker;

[0014] (4) Subsequently, low-heat silicate cement clinker containing C3S can be obtained by adding calcium raw materials for secondary batching and calcination.

[0015] According to the above scheme, the main chemical components and contents of the medium and low-grade pyrite described in step (1) are: FeS2: 40-70%; FeSO4: 0-20%; Fe2(SO4)3: 0-10%; SiO2: 10-40%; Al2O3: 0-5%; CaO: 0-5%; MgO: 0-5%.

[0016] According to the above scheme, the silicon-aluminum-iron raw material in step (1) is one or more of sandstone, coal gangue, silica, clay, fly ash, and various types of waste residues.

[0017] According to the above scheme, the S:S in step (1) is 0.45-0.55, the KH value is 0.6-0.7, and the total CaO:SiO2 mass ratio in the chemical composition is 2.0-2.3.

[0018] According to the above solution, the diameter of the material balls described in step (2) is 1 - 3 cm.

[0019] According to the above solution, the calcination temperature described in step (3) is 1150 - 1250 °C, the calcination time is 5 - 30 min, and the O2 concentration in the calcination environment is 0 - 10%.

[0020] According to the above solution, the SO3 content in the low-heat Portland cement clinker without C3S obtained in step (3) is 2 - 3.5%, the content of β-C2S in the mineral composition is 55 - 70%, the total content of CF, C2F, and C4AF is 25 - 40%, and it does not contain γ-C2S and C3A.

[0021] According to the above solution, the calcareous raw material described in step (4) is limestone.

[0022] According to the above solution, in step (4), the secondary batching uses the liquid-phase minerals C2F and C4AF for KH design, and the KH value is 0.68 - 0.78.

[0023] According to the above solution, the calcination temperature described in step (4) is 1200 - 1300 °C, and the calcination time is 5 - 30 min.

[0024] According to the above solution, the SO3 content in the low-heat Portland cement clinker containing C3S obtained in step (4) is 2 - 3.5%, the content of β-C2S in the mineral composition is 40 - 65%, the content of C3S is 0 - 30%, the total content of C2F and C4AF is 25 - 40%, and it does not contain γ-C2S and C3A.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] (1) In the present invention, the reducing agent used for decomposing phosphogypsum can all adopt medium and low-grade pyrite with low price and wide distribution, further reducing the coal consumption cost and carbon emissions during the large-scale treatment of phosphogypsum. Through reasonable proportioning and process, efficient and stable desulfurization of phosphogypsum without carbonaceous raw materials and without reduction carbon emissions is achieved. The SO3 content in the desulfurization product is 2 - 3.5%, and at the same time, a low-heat Portland cement clinker without C3S is prepared, which can also be used as a highly active admixture and blending material. Subsequently, a low-heat Portland cement clinker with different mineral compositions and containing C3S can be prepared by adding calcareous raw materials and silico-aluminous-ferrous raw materials for secondary batching and calcination, which is of great significance to economy, environmental protection, and carbon reduction.

[0027] The main chemical reactions of phosphogypsum desulfurization are as follows:

[0028] Main desulfurization: 5CaSO4 + FeS2 → 5CaO + 7SO2 + FeO

[0029] Secondary desulfurization: CaSO4 + 3FeO → CaO + SO2 + Fe3O4

[0030] Secondary desulfurization: CaSO4 + 2Fe3O4 → CaO + SO2 + 3Fe2O3

[0031] Total secondary desulfurization: CaSO4 + 2FeO → CaO + SO2 + Fe2O3

[0032] Desulfurization induced by main SiO2 to generate C2S: 2CaO + SiO2 → 2CaO·SiO2

[0033] Others are: desulfurization induced by Fe2O3 to generate CF, C2F, C4AF, and CF, C2F, C4AF promote the formation of C2S and thus promote desulfurization.

[0034] (2) Using the -1 valence S of FeS2 in medium and low grade pyrite as the main agent for desulfurizing phosphogypsum. The reducibility of -1 valence S is stronger than that of C and S. When -1 valence S is used as a reducing agent, the theoretical S:S is 0.4, and the actual optimal S:S is 0.45 - 0.55. When C is used as a reducing agent, the theoretical C:S is 0.5, and the actual optimal C:S is 0.7 - 0.9. Moreover, FeS2 makes the desulfurization of CaSO4 mainly a solid-solid reaction, so the dosage is more economical than that of C and S, and the stability is better.

[0035] (3) The reducibility of S-based reducing agents itself is stronger than that of C, which easily causes a large amount of CaSO4 to decompose into CaS and thus unable to desulfurize. And the concentration of SO2 in the decomposed gas is too high, which will also inhibit the desulfurization of CaSO4. Medium and low grade pyrite contains more SiO2. In this invention, while using medium and low grade pyrite as a reducing agent, a design with low KH value and low CaO:SiO2 is combined to provide more SiO2 to promote the transformation of CaSO4 to CaO for desulfurization and thus form C2S instead of CaS.

[0036] (4) Using the +2 valence Fe of FeS2 in medium and low grade pyrite as an auxiliary for desulfurizing phosphogypsum, and a large amount of Fe makes the fluxing effect more effective under reducing conditions, improving the burnability of CaSO4 desulfurization and forming C2S.

[0037] (5) For the desulfurization of phosphogypsum, a formulation suitable for medium- and low-grade pyrite as a reducing agent has been developed. Pelletizing and calcining phosphogypsum is more conducive to industrial applications. The encapsulation of pellets can greatly save the amount of reducing agent and improve the phenomenon of molten crusting. The disadvantage is that it is more difficult to burn and desulfurize than in powder form. Although the melting point is lower, it is easier to burn when the KH and the mass ratio of total CaO:SiO2 are 0.3 - 0.55 and 1.1 - 1.9 respectively, and the promoting effect of SiO2 is more obvious, which is more conducive to the desulfurization of CaSO4, and can make the SO3 content in the desulfurized product ≤ 2%, or even less than 1%. However, the iron content introduced by medium- and low-grade pyrite is too high, resulting in a too narrow firing range under this formulation, prone to melting, and not conducive to application. The present invention still combines the technology that a low KH value (KH is 0.3 - 0.7) is beneficial to the desulfurization of CaSO4 to form C2S, and controls the KH and the mass ratio of total CaO:SiO2 during the desulfurization of phosphogypsum to 0.6 - 0.7 and 2.0 - 2.3 respectively, so that the eutectic temperature is high and the firing range is wide at this time. Instead, it utilizes the strong reducibility of -1 valence S in medium- and low-grade pyrite and the promoting effect of a large amount of iron on the formation of C2S, so that even when the KH and the mass ratio of total CaO:SiO2 are increased during pelletizing and calcining, there will still be no phenomenon of being too difficult to burn and desulfurize. In addition, the aluminum content in medium- and low-grade pyrite is low, and the relative content of aluminum compared to iron is even lower. After batching, the main iron phases formed are more CF, C2F, and a small amount of C4AF, and there will be no aluminum phase minerals C3A, C 12 A7, and other aluminum phase sulfur-fixing minerals. When the KH value is 0.6 - 0.7, a certain amount of aluminum-iron phase minerals are required to make C2S easy to burn, but the inhibitory effect of Al2O3 in aluminum-iron phase minerals on the desulfurization of CaSO4 is obvious. Conventional low-cost non-sulfur iron raw materials contain more aluminum, while medium- and low-grade pyrite can be used for the desulfurization of phosphogypsum, reducing this inhibitory effect.

[0038] (6) The reduction and decomposition of phosphogypsum by medium- and low-grade pyrite increases the concentration of SO2 in the flue gas, which is beneficial to the subsequent double conversion and double absorption sulfuric acid production.

[0039] (7) Due to the high content of iron phase minerals, mainly more CF, C2F, and a small amount of C4AF, with a total content of 25 - 40%, it has a strong fluxing effect, making the calcination temperature of the prepared low-heat Portland cement clinker lower, reduced from 1250 - 1350 °C for the low-heat Portland cement clinker prepared from conventional limestone to 1200 - 1300 °C.

[0040] (8) The content of C2S in low-heat Portland cement clinker is high, requiring ≥ 40%. In the traditional process of preparing low-heat Portland cement clinker with limestone, there is a phenomenon that the hydrated active mineral β-C2S is easily pulverized and transformed into the non-hydrated active mineral γ-C2S during cooling. This places high requirements on the process, and it is necessary to increase the rotational speed of the kiln and the cooling rate of the clinker. However, in the present invention, the low-heat Portland cement clinker prepared with phosphogypsum and medium- and low-grade pyrite contains 2 - 3.5% of SO3, and SO3 can stabilize β-C2S and prevent its transformation into γ-C2S.

[0041] (9) The aluminum content in medium- and low-grade pyrite is low, and the relative content of aluminum compared to iron is even lower. After batching, the main iron phases formed are more CF, C2F, and a small amount of C4AF. Even if Al2O3 is introduced by adding calcareous raw materials or even aluminous raw materials for secondary batching and calcination to prepare low-heat Portland cement clinker containing C3S, Al2O3 will form C4AF with CF and C2F, and there will be no C3A or C 12 A7, which more easily meets the requirement of ≤ 6% for the C3A content in the national standard for low-heat Portland cement. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 : XRD pattern of the C3S-free low-heat Portland cement clinker obtained in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0044] Obtaining and testing of raw materials in the following specific embodiments:

[0045] Obtain phosphogypsum, aluminosilicate raw materials, and calcareous raw materials, and conduct chemical composition analysis after drying, crushing, and grinding. It should be noted that the aluminosilicate raw materials can be any one or more of shale, coal gangue, silica, clay, fly ash, and various waste residues. For the convenience of comparative analysis, only coal gangue is used in the specific embodiments; the calcareous raw material is limestone. The chemical compositions of a batch of phosphogypsum, coal gangue, and limestone are shown in Table 1:

[0046] Table 1 Chemical composition analysis

[0047] Name Loss on ignition <![CDATA[SiO2]]> <![CDATA[Al2O3]]> <![CDATA[Fe2O3]]> CaO MgO <![CDATA[SO3]]> <![CDATA[K2O]]> <![CDATA[Na2O]]> Phosphogypsum 19.27 6.6 0.23 0.52 29.66 0.03 42.46 0.37 0.08 Coal gangue 4.63 76.3 6.62 4.64 1.91 2.35 0.48 1.34 0.55 Limestone 39.84 5.66 1.29 1.02 48.33 2.64 0.05 0.29 0.15

[0048] Conduct chemical composition analysis on the medium- and low-grade pyrite used, and the results are shown in Table 2:

[0049] Table 2 Chemical composition analysis of medium- and low-grade pyrite

[0050]

[0051] Example 1

[0052] 1. Proportion Design

[0053] Calculate the SO3 in the chemical composition of phosphogypsum into its own loss on ignition. According to the loss on ignition of 19.27% + 42.46% = 61.73%, the remaining chemical components are measured corresponding to the substances mainly composed of CaO. Design the proportion with low- and medium-grade pyrite according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum being 0.5, KH value being 0.669, and the mass ratio of total CaO to SiO2 in the chemical composition being 2.245. That is, the dosage of low- and medium-grade pyrite = dosage of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum to low- and medium-grade pyrite is 100:27.11.

[0054] 2. Pelletizing

[0055] Mix and grind the above-mentioned phosphogypsum and low- and medium-grade pyrite measured according to the proportion, then add water to make pellets with an average diameter of 2 cm, and then dry naturally for 1 day.

[0056] 3. Calcination

[0057] Put the naturally dried pellets into a rotary kiln lacking oxygen after the combustion of coal (coal providing temperature and heat) for calcination, with an O2 concentration of 8%. After calcination, cool to obtain low-heat Portland cement clinker without C3S. The calcination temperature of the kiln is set at 1200 °C and the time is 10 min.

[0058] The XRD pattern of the low-heat Portland cement clinker without C3S obtained in this example is shown in the appendix Figure 1 As shown, it can be observed that the mineral composition is mainly β-C2S and CF, followed by C2F and C2AS, and also contains a small amount of undissolved CaSO4.

[0059] Example 2

[0060] 1. Proportion Design

[0061] Calculate the SO3 in the chemical composition of phosphogypsum as part of its own loss on ignition. According to the loss on ignition of 19.27% + 42.46% = 61.73%, the remaining chemical components are measured corresponding to the substances mainly composed of CaO. Design the mixing ratio with silico-aluminous raw materials and medium-low grade pyrite according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum being 0.5, KH value being 0.629, and the mass ratio of total CaO to SiO2 in the chemical composition being 2.123. That is, the dosage of medium-low grade pyrite = dosage of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64). The weight ratio of phosphogypsum: coal gangue: medium-low grade pyrite is 99:1:26.84.

[0062] 2. Pelletizing

[0063] Mix and grind the above-mentioned phosphogypsum, coal gangue, and medium-low grade pyrite measured according to the ratio, then add water to make pellets with an average diameter of 2 cm, and then dry naturally for 1 day.

[0064] 3. Calcination

[0065] Put the naturally dried pellets into a rotary kiln lacking oxygen after the combustion of coal (coal providing temperature and heat) for calcination, with an O2 concentration of 6%. After calcination and cooling, a low-heat Portland cement clinker without C3S is obtained. The calcination temperature of the kiln is set at 1190 °C and the time is 15 min.

[0066] Example 3

[0067] Perform secondary batching on the low-heat Portland cement clinker without C3S obtained in Example 1 and limestone, and design KH using the liquid-phase minerals C2F and C4AF. The dosage of limestone is 17% of the dosage of phosphogypsum in Step 1 of Example 1. At this time, the KH value is 0.724. Calcinate at 1270 °C using a new dry process kiln. After calcination and cooling, a low-heat Portland cement clinker containing C3S is obtained. The secondary batching ratio values and theoretical mineral compositions are shown in Table 3:

[0068] Table 3 Secondary batching ratio values and theoretical mineral compositions

[0069]

[0070] Example 4

[0071] Perform secondary batching on the low-heat Portland cement clinker without C3S obtained in Example 1 and limestone, and design KH using the liquid-phase minerals C2F and C4AF. The dosage of limestone is 21% of the dosage of phosphogypsum in Step 1 of Example 1. At this time, the KH value is 0.759. Calcinate at 1280 °C using a new dry process kiln. After calcination and cooling, a low-heat Portland cement clinker containing C3S is obtained. The secondary batching ratio values and theoretical mineral compositions are shown in Table 4:

[0072] Table 4 Secondary batching ratio values and theoretical mineral compositions

[0073]

[0074] The SO3 content of the obtained clinker is shown in Table 5. The SO3 content is measured by total sulfur analysis. The main mineral contents are shown in Table 6 and are obtained by analyzing the XRD pattern of the clinker with jade software.

[0075] Table 5 SO3 content of the obtained clinker

[0076] Project <![CDATA[SO3 content after burning <!-- 5 -->]]> Example 1 3.04％ Example 2 2.88％ Example 3 2.32％ Example 4 2.25％

[0077] Table 6 Main mineral contents (%)

[0078] Project <![CDATA[β-C2S]]> <![CDATA[γ-C2S]]> <![CDATA[C3S]]> <![CDATA[SiO2]]> <![CDATA[Ca2Al2SiO7]]> CF <![CDATA[C2F]]> <![CDATA[C4AF]]> <![CDATA[C3A]]> <![CDATA[Fe3O4]]> <![CDATA[CaSO4]]> Example 1 62.3 0 0 0.1 2.1 29.5 3.4 0.1 0 0.2 2.2 Example 2 63.1 0 0 0.2 2.3 28.7 2.0 0.2 0 1.4 2.0 Example 3 52.7 0 12.6 0 0 0 28.0 5.6 0 0 1.1 Example 4 45.5 0 20.8 0 0 0 26.8 5.9 0 0 1.0

[0079] The results of each embodiment show that the reducing agent used for decomposing phosphogypsum can all adopt medium and low grade pyrite with low price and wide distribution. Through reasonable proportioning and process, the SO3 content after burning is 2 - 3.5%, realizing efficient and stable desulfurization of phosphogypsum without carbonaceous raw materials and without reduction carbon emissions, and preparing low heat Portland cement clinker. The SO3 in it stabilizes β-C2S, no γ-C2S is generated, and C3A with large heat release, large dry shrinkage and low absolute strength is not generated, which is easier to meet the requirements of production and national standards. Among them, in Examples 3 and 4 of subsequent secondary batching, while adding calcareous raw materials, silico-aluminous raw materials with high Al2O3 content can also be added to convert CF and C₂F into C4AF with higher hydration activity.

[0080] The above implementation manners are only the preferred implementation manners of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention belong to the scope required to be protected by the present invention.

Claims

1. A method for desulfurizing phosphogypsum without reducing carbon emission and preparing low-heat Portland cement clinker, characterized in that The following steps are involved: (1) Obtain phosphogypsum, medium- and low-grade pyrite, and ferrosilicon-aluminum raw materials, dry and crush them, prepare the ingredients according to the designed S:S, KH value, and total CaO:SiO2 mass ratio, and mix and grind them; the main chemical components and contents of the medium- and low-grade pyrite are: FeS2: 40-70%; FeSO4: 0-20%; Fe2(SO4)3: 0-10%; SiO2: 10-40%; Al2O3: 0-5%; CaO: 0-5%; MgO: 0-5%; the S:S is 0.45-0.55, the KH value is 0.6-0.7, and the total CaO:SiO2 mass ratio in the chemical composition is 2.0-2.3; (2) Add water to the obtained mixture to form pellets, and then dry; (3) calcining the dried pellets in an oxygen-deficient or oxygen-free rotary kiln after combustion of coal, and cooling the pellets after calcination to obtain C3S-free low-heat Portland cement clinker; the C3S-free low-heat Portland cement clinker has a SO3 content of 2-3.5%, a β-C2S content of 55-70% in the mineral composition, a total content of CF, C2F, and C4AF of 25-40%, and does not contain γ-C2S or C3A; (4) Adding calcium raw materials for secondary batching and calcining to obtain low-heat silicate cement clinker containing C3S; the secondary batching uses liquid minerals C2F and C4AF for KH design, and the KH value is 0.68-0.78; the calcination temperature is 1200-1300°C, and the calcination time is 5-30 minutes; the low-heat silicate cement clinker containing C3S has an SO3 content of 2-3.5%, a β-C2S content of 40-65% in the mineral composition, a C3S content of 0-30%, a total content of C2F and C4AF of 25-40%, and does not contain γ-C2S or C3A.

2. The method according to claim 1, wherein The ferro-aluminum raw material in step (1) is one or more of sandstone, coal gangue, silica, clay, and fly ash.

3. The method according to claim 1, wherein The diameter of the ball in step (2) is 1-3 cm.

4. The method according to claim 1, wherein The calcination temperature in step (3) is 1150-1250° C., the calcination time is 5-30 min, and the O 2 concentration in the calcination environment is 0-10%.

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