A method for composite reduction desulfurization of phosphogypsum and preparation of low-carbon clinker

Abstract

The present invention discloses a method for composite reduction desulfurization of phosphogypsum and preparation of low-carbon clinker. The method comprises the following steps: phosphogypsum is mixed with carbonaceous raw materials, medium- and low-grade pyrite, and ferro-aluminum raw materials according to a designed C:S, S:S, KH value, and total CaO:SiO2 mass ratio; the mixture is ground; water is added to form pellets; and the pellets are then dried; the dried pellets are calcined in an oxygen-deficient or oxygen-free rotary kiln after combustion of coal; and after calcination, the pellets are cooled to obtain low-carbon clinker. The present invention uses carbonaceous raw materials and medium- and low-grade pyrite as reducing agents to treat phosphogypsum, further reducing the coal cost and carbon emissions when treating large amounts of phosphogypsum. Through reasonable proportioning and processes, efficient and stable desulfurization of phosphogypsum is achieved, and low-carbon clinker is simultaneously produced. Furthermore, the preparation method is more suitable for industrial promotion and application, and is of great significance to the economy, environmental protection, and carbon reduction.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials, and particularly relates to a method for composite reduction desulfurization of phosphogypsum and preparation of low-carbon clinker. Background Art

[0002] Phosphogypsum, a solid waste generated during the wet-process phosphoric acid production process, faces a global challenge and poses a significant obstacle to the development of the phosphorus chemical and new energy materials industries. Using phosphogypsum as a CaO source in cement clinker production, replacing limestone, is a challenging and significant approach to reclaiming phosphogypsum as a resource. However, CaSO₄ requires higher temperatures than CaCO₃ to fully decompose and desulfurize, increasing operational complexity and cost. Furthermore, it can persist in significant quantities even after high-temperature calcination, significantly limiting its effective application.

[0003] 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.

[0004] 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.

[0005] 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 to reduce the amount of carbonaceous raw materials used as reducing agents, and achieve efficient and stable desulfurization of phosphogypsum, to produce environmentally friendly, high-value-added materials, and the preparation method must be suitable for industrial promotion and application, has become a technical problem that needs to be solved urgently. Summary of the Invention

[0006] The object of the present invention is to provide a method for composite reduction desulfurization of phosphogypsum and preparation of low-carbon clinker, which adopts carbonaceous raw materials and medium- and low-grade pyrite as reducing agents 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 and simultaneously producing low-carbon clinker, and the preparation method is more suitable for industrial promotion and application.

[0007] In order to achieve the above purpose, the technical solutions adopted are as follows:

[0008] A method for composite reduction desulfurization of phosphogypsum and preparation of low-carbon clinker, comprising the following steps:

[0009] (1) obtaining phosphogypsum, carbonaceous raw materials, medium- and low-grade pyrite, and silicon-aluminum-iron raw materials, drying and crushing them, batching them according to the designed C:S, S:S, KH values, and total CaO:SiO2 mass ratios, and mixing and grinding them;

[0010] (2) adding water to the obtained mixture to form pellets, and then drying;

[0011] (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.

[0012] According to the above scheme, the carbonaceous raw material in step (1) is one or more of ordinary coal, high-sulfur coal, and coke.

[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, in step (1), C:S=aX, X is in the range of 0.7-0.9; S:S=bY, Y is in the range of 0.45-0.55; wherein a is in the range of 0.2-0.8, b is in the range of 0.2-0.8, and a+b=1; the KH value is 0.4-0.5, and the total CaO:SiO2 mass ratio in the chemical composition is 1.4-1.7.

[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 1150-1220° 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 ≤1%, 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 in the present invention to decompose phosphogypsum can be low-priced and widely distributed medium- and low-grade pyrite to replace part of the carbonaceous raw materials, further reducing the coal cost and carbon emissions when processing large amounts of phosphogypsum. In combination with the low KH and low CaO:SiO2 design, SiO2 is used to induce CaSO4 to decompose in the direction of CaO, thereby achieving efficient and stable desulfurization of phosphogypsum, with the SO3 content in the desulfurization product being ≤1%, and simultaneously producing low-carbon clinker with a total content of C2S, C3S2, and CS being ≥70%, which is of great significance to economy, environmental protection, and carbon reduction.

[0021] The main chemical reactions of FeS2 on 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 preparation method of the present invention is more suitable for industrial promotion and application: carbonaceous raw materials and medium-low grade pyrite are used as reducing agents to treat phosphogypsum, wherein the coal first reacts with O2 in the air to generate CO and CO2, protecting the FeS2 in the medium-low grade pyrite from reacting with O2 in the air and causing waste; secondly, if medium-low grade pyrite is used as a reducing agent, the reaction atmosphere is basically all SO2, and too high a SO2 concentration will inhibit CaSO4 desulfurization. Therefore, the use of medium-low grade pyrite as a reducing agent and in combination with the carbonaceous raw material can make the SO2 concentration in the atmosphere not to exceed 1%. Because the content of CaSO4 in the reducing agent is too high, it further improves the desulfurization effect of CaSO4, which is suitable for industrial environments with poor resistance to acidic gases and strict requirements on SO3 content and working conditions; thirdly, if medium and low-grade pyrite is used as a reducing agent, too much iron will be introduced. When the KH value in the ingredients is 0.4-0.5 and the total CaO:SiO2 mass ratio is 1.4-1.7, the firing range is relatively narrow when the KH value and the CaO:SiO2 value are high. Too much iron will lead to a narrower firing range and easier melting. Therefore, the use of medium and low-grade pyrite as a reducing agent and combined with carbonaceous raw materials can prevent the iron content in the clinker from being too high.

[0030] (3) 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.

[0031] (4) 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.

[0032] (5) The desulfurization of phosphogypsum is assisted by the +2-valent Fe of FeS2 in medium and low-grade pyrite, and a certain 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.

[0033] (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 two-transformation and two-absorption acid production. 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. The chemical composition of a batch of phosphogypsum and coal gangue is shown in Table 1:

[0037] Table 1 Chemical composition analysis

[0038] 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

[0039] The carbonaceous raw material used was subjected to conventional analysis and chemical composition analysis of the ash. Here, high-sulfur coal was used. The results are shown in Table 2 and Table 3 respectively:

[0040] Table 2 Conventional analysis of high sulfur coal

[0041] name Moisture ash content Volatile matter Fixed carbon content Total sulfur Qnet,ad Qnet,d High sulfur coal 1.42 23.48 10.25 64.85 2.99 25.25 26.33

[0042] Table 3 Chemical composition analysis of high sulfur coal ash

[0043] name Loss on ignition <![CDATA[SiO2]]> <![CDATA[Al2O3]]> <![CDATA[Fe2O3]]> CaO MgO <![CDATA[SO3]]> <![CDATA[K2O]]> <![CDATA[Na2O]]> Coal ash 0 47.36 26.12 15.21 4.16 1.55 2.11 1.03 0.40

[0044] The chemical composition of the low- and medium-grade pyrites used was analyzed, and the results are shown in Table 4:

[0045] Table 4 Chemical composition analysis of low-grade pyrite

[0046]

[0047] Example 1

[0048] 1. Ratio design

[0049] The SO3 in the chemical composition of phosphogypsum is calculated into its own ignition loss, according to the ignition loss 19.27% + 42.46% = 61.73%, and the remaining chemical composition is measured according to the substance mainly composed of CaO, and the molar ratio of C in carbonaceous raw materials to SO3 in phosphogypsum is 0.5*0.8146, the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum is 0.5*0.5, the KH value is 0.449, and the chemical composition is 0. The mass ratio of total CaO and SiO2 is 1.499 for design proportioning, that is, the amount of high-sulfur coal = the amount of phosphogypsum * 0.4246 * 0.5 * 0.8146 * 12 / (80 * 0.6485), the amount of medium and low-grade pyrite = the amount of phosphogypsum * 0.4246 * 0.5 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum: coal gangue: high-sulfur coal: medium and low-grade pyrite is 88.8: 11.2: 3.55: 12.04.

[0050] 2. Ball making

[0051] The phosphogypsum, coal gangue, carbonaceous raw materials, 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.

[0052] 3. Calcination

[0053] 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 1190°C and the time is 15 minutes.

[0054] Example 2

[0055] 1. Ratio design

[0056] The SO3 in the chemical composition of phosphogypsum is calculated into its own ignition loss, according to the ignition loss 19.27% + 42.46% = 61.73%, and the remaining chemical composition is measured corresponding to the substance mainly composed of CaO, and the molar ratio of C in carbonaceous raw materials to SO3 in phosphogypsum is 0.25*0.8146, the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum is 0.75*0.5, the KH value is 0.437, and the chemical composition is 0. The mass ratio of total CaO to SiO2 is 1.496 for design proportioning, that is, the amount of high-sulfur coal = the amount of phosphogypsum * 0.4246 * 0.25 * 0.8146 * 12 / (80 * 0.6485), the amount of medium and low-grade pyrite = the amount of phosphogypsum * 0.4246 * 0.75 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum: coal gangue: high-sulfur coal: medium and low-grade pyrite is 90.3:9.7:1.81:18.36.

[0057] 2. Ball making

[0058] The phosphogypsum, coal gangue, carbonaceous raw materials, 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.

[0059] 3. Calcination

[0060] 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 1175°C and the time is 17 minutes.

[0061] Example 3

[0062] 1. Ratio design

[0063] The SO3 in the chemical composition of phosphogypsum is calculated into its own ignition loss, and the remaining chemical composition is measured according to the ignition loss 19.27% + 42.46% = 61.73%, and the remaining chemical composition corresponds to the substance mainly composed of CaO, and is measured with the silicon-aluminum-iron raw materials, carbon raw materials, and medium and low-grade pyrite according to the molar ratio of C in carbon raw materials to SO3 in phosphogypsum is 0.75*0.8146, the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum is 0.25*0.5, the KH value is 0.460, and the chemical composition is 0. The mass ratio of total CaO to SiO2 is 1.498 for design proportioning, that is, the amount of high-sulfur coal = the amount of phosphogypsum * 0.4246 * 0.75 * 0.8146 * 12 / (80 * 0.6485), the amount of medium and low-grade pyrite = the amount of phosphogypsum * 0.4246 * 0.25 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum: coal gangue: high-sulfur coal: medium and low-grade pyrite is 87.3: 12.7: 5.24: 5.92.

[0064] 2. Ball making

[0065] The phosphogypsum, coal gangue, carbonaceous raw materials, 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.

[0066] 3. Calcination

[0067] 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 6%. After calcination, they are cooled to obtain low-carbon clinker. The calcination temperature of the kiln is set to 1200℃ and the time is 14 minutes.

[0068] The SO3 content of the calcined product (clinker) is shown in Table 5, and the SO3 content is measured by total sulfur analysis. The main mineral content is shown in Table 6, and the main mineral content is obtained by analyzing the XRD pattern of the calcined product (clinker) using jade software.

[0069] Table 5 SO3 content of calcined product (clinker)

[0070] project <![CDATA[SO3 content after firing]]> Example 1 0.78％ Example 2 0.82％ Example 3 0.77％

[0071] Table 6 Main mineral content (%)

[0072] project <![CDATA[C3S2]]> <![CDATA[C2S]]> CS <![CDATA[SiO2]]> <![CDATA[Ca2Al2SiO7]]> <![CDATA[Ca2Fe9O 13 ]]> <![CDATA[CaFe4O6]]> <![CDATA[Fe3O4]]> <![CDATA[CaSO4]]> Example 1 60.5 18.8 3.8 0.7 6.6 5.2 2.0 2.4 0 Example 2 61.7 15.3 5.3 0.5 5.4 4.7 2.5 4.6 0 Example 3 60.6 20.5 3.4 0.4 7.5 4.5 2.7 0.4 0

[0073] The SO3 content after burning in each embodiment is less than 1%, and low-carbon clinker with a total content of C2S, C3S2, and CS ≥ 70% is simultaneously produced, wherein the C3S2 mineral content can exceed 60%.

[0074] 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 composite reduction desulfurization of phosphogypsum and preparation of low-carbon clinker, characterized in that The following steps are involved: (1) Obtain phosphogypsum, carbonaceous raw materials, medium- and low-grade pyrite, and silicon-aluminum-iron raw materials, dry and crush them, and mix and grind them according to the designed C:S, S:S, KH value, and total CaO:SiO2 mass ratio; the C:S molar ratio range is 0.20365-0.61095; the S:S molar ratio range is 0.125-0.375; the KH value is 0.4-0.5, and the total CaO:SiO2 mass ratio in the chemical composition is 1.4-1.7; 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%; (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 1150-1220°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 carbonaceous raw material in step (1) is one or more of ordinary coal, high-sulfur coal, and coke.

3. 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.

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

5. The method according to claim 1, wherein The O2 concentration in the calcination environment in step (3) is 0-10%.

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

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