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

Abstract

The present invention discloses a method for desulfurizing phosphogypsum without reducing carbon emissions and preparing low-carbon clinker. The method comprises the following steps: phosphogypsum is mixed with medium- and low-grade pyrite and ferro-aluminum raw materials according to a designed S:S ratio, KH value, and total CaO:SiO2 mass ratio, and the mixture is ground and water is added to form pellets, which are then dried. The dried pellets are calcined in an oxygen-deficient or oxygen-free rotary kiln after combustion of coal, and cooled after calcination to obtain low-carbon clinker. The reducing agent used to decompose phosphogypsum in the present invention can all be low-priced and widely distributed medium- and low-grade pyrite, further reducing the coal cost and carbon emissions when processing large amounts of phosphogypsum. Furthermore, through reasonable mixing and processing, efficient and stable desulfurization of phosphogypsum without carbonaceous raw materials and reducing carbon emissions is achieved, while simultaneously producing low-carbon clinker, which is of great significance to economy, environmental protection, and carbon reduction.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials, and in particular relates to a method for desulfurizing phosphogypsum without reducing carbon emission and preparing low-carbon clinker. Background Art

[0002] Phosphogypsum is a solid waste generated during the wet-process phosphoric acid process. Its comprehensive management is a global challenge and the primary obstacle to the development of the phosphorus chemical and new energy materials industries. Phosphogypsum has a complex composition. In addition to hydrated calcium sulfate, it also contains incompletely decomposed phosphate rock, residual phosphoric acid, fluoride, acid-insoluble matter, and organic matter. The presence of fluorine and organic matter has the greatest impact on the resource utilization of phosphogypsum. Stockpiling consumes significant land and pollutes water and land resources. Currently, my country's phosphogypsum stockpiles exceed 800 million tons, with an annual production of approximately 77 million tons, and a comprehensive utilization rate of 50.4% by 2022. However, the large stockpile, complex composition, low added value, and immature technology contribute significantly to the difficulties in its utilization.

[0003] Using phosphogypsum instead of limestone as a CaO source to produce cement clinker is a challenging and meaningful direction to solve the resource utilization of phosphogypsum. However, compared with CaCO3, CaSO4 requires higher temperatures to completely decompose and desulfurize, which increases the operational difficulty and cost of the process. It can still exist in large quantities even under high-temperature calcination, which undoubtedly restricts the effective application of phosphogypsum.

[0004] Many studies have reported the use of carbonaceous raw materials as reducing agents for the decomposition of CaSO4, such as coke and pulverized coal. These materials are expensive and will increase the cost of treating phosphogypsum. Although the carbon emissions are lower than those caused by the decomposition of CaCO3, there will still be higher carbon emissions, namely the reduced carbon emissions caused by the reaction of carbonaceous raw materials as reducing agents with CaSO4.

[0005] Some studies have attempted to use sulfur as a reducing agent for the decomposition of CaSO4. The reaction temperature and reaction heat required are lower than those of C, making it easier to decompose CaSO4. However, the product of CaSO4 decomposition is mainly CaS, which cannot achieve a high desulfurization rate. In addition, the stability of sulfur is worse than that of coal. In actual applications, it is easy to react with O2 and undergo large-scale sublimation, resulting in waste.

[0006] 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 environmentally friendly, high-value-added materials, has become a technical problem that needs to be solved urgently. Summary of the Invention

[0007] The object of the present invention is to provide a method for desulfurizing phosphogypsum without reducing carbon emissions and preparing low-carbon clinker, which uses medium and low-grade pyrite as a reducing agent to treat phosphogypsum, further reducing the coal cost and carbon emissions when treating large amounts of phosphogypsum, and through reasonable proportions and processes, achieving efficient and stable desulfurization of phosphogypsum without the need for carbonaceous raw materials and without reducing carbon emissions, while simultaneously producing environmentally friendly materials.

[0008] In order to achieve the above purpose, the technical solutions adopted are as follows:

[0009] A method for desulfurizing phosphogypsum without reducing carbon emission and preparing low-carbon clinker comprises the following steps:

[0010] (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;

[0011] (2) adding water to the obtained mixture to form pellets, and then drying;

[0012] (3) The dried pellets are calcined in an oxygen-deficient or oxygen-free rotary kiln after coal combustion, and cooled to obtain low-carbon clinker after calcination.

[0013] 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%.

[0014] 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.

[0015] According to the above scheme, the S:S in step (1) is 0.45-0.55, the KH value is 0.3-0.55, and the total CaO:SiO2 mass ratio in the chemical composition is 1.1-1.9.

[0016] According to the above scheme, the diameter of the ball in step (2) is 1-3 cm.

[0017] According to the above scheme, the calcination temperature in step (3) is 1150-1200° C., the calcination time is 5-30 min, and the O 2 concentration in the calcination environment is 0-10%.

[0018] According to the above scheme, the SO3 content of the low-carbon clinker obtained in step (3) is ≤2%, and the total content of C2S, C3S2, and CS in the mineral composition is ≥70%.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The reducing agent used to decompose phosphogypsum in the present invention can be all low-priced and widely distributed medium- and low-grade pyrite, which further reduces the coal cost and carbon emissions when processing large amounts of phosphogypsum, and achieves efficient and stable desulfurization of phosphogypsum without the need for carbonaceous raw materials and without reducing carbon emissions. The SO3 content in the desulfurization product is ≤2%, and low-carbon clinker with a total content of C2S, C3S2, and CS ≥70% is simultaneously produced, which is of great significance to economy, environmental protection, and carbon reduction.

[0021] The main chemical reactions of phosphogypsum desulfurization are as follows:

[0022] Main desulfurization: 5CaSO4+FeS2→5CaO+7SO2+FeO

[0023] Secondary desulfurization: CaSO4+3FeO→CaO+SO2+Fe3O4

[0024] Secondary desulfurization: CaSO4+2Fe3O4→CaO+SO2+3Fe2O3

[0025] Total desulfurization: CaSO4+2FeO→CaO+SO2+Fe2O3

[0026] Main SiO2 induces C2S desulfurization: 2CaO+SiO2→2CaO·SiO2

[0027] Secondary SiO2 induces CS desulfurization: 2CaO·SiO2+SiO2→2(CaO·SiO2)

[0028] Secondary SiO2 induces C3S2 desulfurization: 3(2CaO·SiO2)+SiO2→2(3CaO·2SiO2)

[0029] (2) The desulfurization of phosphogypsum is mainly carried out with -1 valence S of FeS2 in medium and low grade pyrite. -1 valence S has stronger reducing power than C and S. The theoretical S:S ratio when -1 valence S is used as a reducing agent is 0.4, and the actual optimal S:S ratio is 0.45-0.55. When C is used as a reducing agent, the theoretical C:S ratio is 0.5, and the actual optimal C:S ratio is 0.7-0.9. Moreover, the desulfurization of CaSO4 by FeS2 is mainly a solid-solid reaction, so the dosage is less than that of C and S, and the stability is better.

[0030] (3) The reducing property of S-type reducing agents is stronger than that of C, which can easily cause a large amount of CaSO4 to decompose into CaS and fail to desulfurize. Moreover, the concentration of SO2 in the decomposition gas is too high, which will inhibit the desulfurization of CaSO4. Medium and low-grade pyrite contains more SiO2. The present invention uses medium and low-grade pyrite as a reducing agent and combines a low KH value and a low CaO:SiO2 design to provide more SiO2 to desulfurize CaSO4 to CaO, thereby forming calcium silicate minerals C2S, C3S2, and CS, rather than CaS.

[0031] (4) The desulfurization of phosphogypsum is assisted by the +2-valent Fe of FeS2 in medium and low-grade pyrite, and the large amount of Fe makes the fluxing effect more effective under reducing conditions, thereby improving the desulfurization of CaSO4 and the susceptibility to burn of calcium silicate minerals C2S, C3S2, and CS.

[0032] (5) 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 two-transformation and two-absorption acid production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 : XRD pattern of the calcined product of Example 1. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0035] The acquisition and detection of raw materials in the following specific examples:

[0036] Obtain phosphogypsum and ferro-aluminum raw materials, dry, crush, and grind them for chemical composition analysis. It should be noted that the ferro-aluminum raw materials can be any one or more of sandstone, coal gangue, silica, clay, fly ash, and various types of waste residues. For the convenience of comparative analysis, only coal gangue is used in the specific implementation. For example, the chemical composition of a batch of phosphogypsum and coal gangue is shown in Table 1:

[0037] Table 1 Chemical composition analysis

[0038]

[0039]

[0040] The chemical composition of the low- and medium-grade pyrites used was analyzed, and the results are shown in Table 2:

[0041] Table 2 Chemical composition analysis of low-grade pyrite

[0042]

[0043] Example 1

[0044] 1. Ratio design

[0045] The SO3 in the chemical composition of phosphogypsum is included in its own loss on ignition, and the remaining chemical composition is measured according to the loss on ignition 19.27% + 42.46% = 61.73%. The remaining chemical composition is measured corresponding to the substance mainly composed of CaO, and the proportioning is designed with the ferrosilicon raw materials and medium and low-grade pyrite according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum of 0.5, the KH value is 0.426, the SM value is 1.511, and the mass ratio of total CaO to SiO2 in the chemical composition is 1.496. That is, the amount of medium and low-grade pyrite = the amount of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum: coal gangue: medium and low-grade pyrite is 91.9:8.1:24.92.

[0046] 2. Ball making

[0047] The phosphogypsum, coal gangue, and medium- and low-grade pyrite measured in the above ratio are mixed and ground, and then water is added to form balls with an average diameter of 2 cm, and then naturally dried for 1 day.

[0048] 3. Calcination

[0049] The naturally dried pellets are placed in an oxygen-deficient rotary kiln after burning coal (coal that provides temperature and heat) for calcination. The O2 concentration is 7%. After calcination, they are cooled to obtain low-carbon clinker. The calcination temperature of the kiln is set to 1175°C and the time is 12 minutes.

[0050] The XRD pattern of the low carbon clinker obtained by calcining in this embodiment is shown in the attached Figure 1 As shown, it can be observed that the mineral composition is mainly C2S, C3S2, and CS, followed by C2AS and iron-containing substances.

[0051] Example 2

[0052] 1. Ratio design

[0053] The SO3 in the chemical composition of phosphogypsum is calculated into its own loss on ignition, and the remaining chemical composition is measured according to the loss on ignition 19.27% + 42.46% = 61.73%. The remaining chemical composition corresponds to the substance mainly composed of CaO, and the proportioning is designed with the ferrosilicon raw materials and medium and low-grade pyrite according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum of 0.5, the KH value is 0.352, the SM value is 1.719, and the mass ratio of total CaO to SiO2 in the chemical composition is 1.267, that is, the amount of medium and low-grade pyrite = the amount of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum: coal gangue: medium and low-grade pyrite is 88:12:23.86.

[0054] 2. Ball making

[0055] The phosphogypsum, coal gangue, and medium- and low-grade pyrite measured in the above ratio are mixed and ground, and then water is added to form balls with an average diameter of 2 cm, and then naturally dried for 1 day.

[0056] 3. Calcination

[0057] The naturally dried pellets are placed in an oxygen-deficient rotary kiln after burning coal (coal that provides temperature and heat) for calcination. The O2 concentration is 5%. After calcination, they are cooled to obtain low-carbon clinker. The calcination temperature of the kiln is set to 1150°C and the time is 15 minutes.

[0058] Example 3

[0059] 1. Ratio design

[0060] The SO3 in the chemical composition of phosphogypsum is calculated into its own loss on ignition, and the remaining chemical composition is measured according to the loss on ignition 19.27% + 42.46% = 61.73%. The remaining chemical composition is measured corresponding to the substance mainly composed of CaO, and the proportioning is designed with the ferrosilicon raw materials and medium and low-grade pyrite according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum of 0.5, the KH value is 0.529, the SM value is 1.294, and the mass ratio of total CaO to SiO2 in the chemical composition is 1.815. That is, the amount of medium and low-grade pyrite = the amount of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum: coal gangue: medium and low-grade pyrite is 96:4:26.03.

[0061] 2. Ball making

[0062] The phosphogypsum, coal gangue, and medium- and low-grade pyrite measured in the above ratio are mixed and ground, and then water is added to form balls with an average diameter of 2 cm, and then naturally dried for 1 day.

[0063] 3. Calcination

[0064] The naturally dried pellets are placed in an oxygen-deficient rotary kiln after burning coal (coal that provides temperature and heat) for calcination. The O2 concentration is 8%. After calcination, they are cooled to obtain low-carbon clinker. The calcination temperature of the kiln is set to 1190°C and the time is 10 minutes.

[0065] The SO3 content of the calcined product (clinker) is shown in Table 3, and the SO3 content is measured by total sulfur analysis. The main mineral content is shown in Table 4, and the main mineral content is obtained by analyzing the XRD pattern of the calcined product (clinker) using jade software.

[0066] Table 3 SO3 content of calcined product (clinker)

[0067] project <![CDATA[SO3 content after burning]]> Example 1 0.94％ Example 2 1.35％ Example 3 1.20％

[0068] Table 4 Main mineral content (%)

[0069]

[0070]

[0071] The results of each embodiment show that the reducing agent used to decompose phosphogypsum can all be low-priced and widely distributed medium- and low-grade pyrite. Through reasonable proportions and processes, the SO3 content after burning is less than 2%, achieving efficient and stable desulfurization of phosphogypsum without the need for carbonaceous raw materials and without reducing carbon emissions, and simultaneously producing low-carbon clinker with a total content of C2S, C3S2, and CS of ≥70%.

[0072] The above embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and replacements made by technicians in this field on the basis of the present invention fall within the scope of protection required by the present invention.

Claims

1. A method for desulfurizing phosphogypsum without reducing carbon emission and preparing low-carbon 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 0.3-0.352, and the total CaO:SiO2 mass ratio in the chemical composition is 1.1-1.267; (2) Add water to the obtained mixture to form pellets, and then dry; (3) The dried pellets are calcined in an oxygen-deficient or oxygen-free rotary kiln after coal combustion. The calcination temperature is 1150-1200°C and the calcination time is 5-30 minutes. After calcination, the pellets are cooled to obtain low-carbon clinker.

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 O2 concentration in the calcination environment in step (3) is 0-10%.

5. The method according to claim 1, wherein The SO3 content of the low-carbon clinker obtained in step (3) is ≤2%, and the total content of C2S, C3S2, and CS in the mineral composition is ≥70%.

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