A method for desulfurizing phosphogypsum with low carbon emissions and preparing low-carbon clinker

Abstract

The present invention discloses a method for desulfurizing phosphogypsum with low carbon emissions and preparing low-carbon clinker. Phosphogypsum is proportioned with medium and low-grade pyrite, low-grade limestone, and silica-aluminum-iron raw materials according to the designed S:S, KH value, and total CaO:SiO2 mass ratio, mixed and ground, and made into pellets with water, and then dried. The dried pellets are calcined in a rotary kiln with lack of oxygen or no oxygen after coal combustion, and cooled after calcination to obtain low-carbon clinker. The present invention uses medium and low-grade pyrite to completely replace carbonaceous raw materials as a reducing agent to treat phosphogypsum, further reducing the coal consumption cost and carbon emissions when treating a large amount of phosphogypsum, and adding low-grade limestone to adjust the atmosphere and reduce the iron content. Through reasonable proportioning and process, efficient and stable desulfurization of phosphogypsum is achieved, low-carbon clinker is simultaneously prepared, and the preparation method is more suitable for industrial promotion and application.

Description

Technical Field

[0001] The present invention belongs to the technical field of building materials, and particularly relates to a method for desulfurizing phosphogypsum with low carbon emissions and preparing low-carbon clinker. Background Art

[0002] 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 to replace 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 decomposed and desulfurized at a higher temperature, which will increase the operation difficulty and cost of the process. Even at high-temperature calcination, it can still exist in large quantities, which undoubtedly restricts the effective application of phosphogypsum.

[0003] Many research reports use carbonaceous raw materials as reducing agents for the decomposition of CaSO4, such as coke and pulverized coal. They 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 reduction carbon emissions brought after the reaction of carbonaceous raw materials as reducing agents with CaSO4.

[0004] Some research attempts to use sulfur as a reducing agent for the decomposition of CaSO4. It requires a lower reaction temperature and reaction heat than C, and it is easier to decompose CaSO4. However, the main product after the decomposition of CaSO4 is CaS, and high desulfurization rate cannot be achieved. Moreover, sulfur is less stable than coal and is prone to react with O2 and sublimate in large quantities in practical applications, resulting in waste.

[0005] Due to the low CaO content, high MgO and SiO2 content in low-grade limestone, it is difficult to be formulated with other conventional raw materials into silicate cement clinker with a relatively high KH. Also, due to the high calcination temperature of the clinker and the reduction of the eutectic point, serious molten crust problems will occur in the equipment. Therefore, its application range is narrow and it cannot be effectively utilized.

[0006] Under the premise that China's future requirements for carbon emissions will be increasingly strict, whether it is possible to find a more stable, effective, non-carbonaceous raw material reducing agent, achieve efficient and stable desulfurization of phosphogypsum, prepare materials with environmental friendliness and high added value, and the preparation method is suitable for industrial promotion and application has become a technical problem to be solved urgently. Summary of the Invention

[0007] The object of the present invention is to provide a method for desulfurizing phosphogypsum with low carbon emissions and preparing low-carbon clinker. Medium and low-grade pyrite is used to completely replace carbonaceous raw materials as a reducing agent to treat phosphogypsum, further reducing the coal consumption cost and carbon emissions during the large-scale treatment of phosphogypsum, and adding low-grade limestone to adjust the atmosphere and reduce the iron content. Through reasonable proportioning and processes, efficient and stable desulfurization of phosphogypsum is achieved, and low-carbon clinker is simultaneously prepared. Moreover, the preparation method is more suitable for industrial promotion and application.

[0008] To achieve the above object, the following technical solutions are adopted:

[0009] A method for desulfurizing phosphogypsum with low carbon emissions and preparing low-carbon clinker, comprising the following steps:

[0010] (1) Obtain phosphogypsum, medium and low-grade pyrite, low-grade limestone, and silico-aluminous raw materials, dry and crush them, and proportion them according to the designed S:S, KH value, and total CaO:SiO2 mass ratio, and then mix and grind them;

[0011] (2) Add water to the obtained mixture to make pellets, and then dry them;

[0012] (3) Calcinate the dried pellets in a rotary kiln with oxygen deficiency or no oxygen after the combustion of coal, and cool them after calcination to obtain low-carbon clinker.

[0013] According to the above scheme, the main chemical components and contents of the medium and low-grade pyrite 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 low-grade limestone in step (1) is limestone with a low CaO content, and the main chemical components and contents are: CaO: 40 - 48%; MgO: 2 - 5%; SiO2: 5 - 15%; Al2O3: 0 - 3%; Fe2O3: 0 - 2%; loss on ignition: 33 - 40%.

[0015] According to the above scheme, the silico-aluminous raw material in step (1) is one or more of shale, coal gangue, silica, clay, fly ash, and various waste residues.

[0016] According to the above scheme, the S:S in step (1) is 0.45 - 0.55, the KH value is 0.4 - 0.5, the total CaO:SiO2 mass ratio in the chemical components is 1.4 - 1.7, and the dosage of low-grade limestone is not less than 20% of the dosage of phosphogypsum.

[0017] According to the above scheme, the diameter of the pellets in step (2) is 1 - 3 cm.

[0018] According to the above solution, in step (3), the calcination temperature is 1150 - 1220 °C, the calcination time is 5 - 30 min, and the O2 concentration in the calcination environment is 0 - 10%.

[0019] According to the above solution, 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%.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] (1) The present invention uses medium and low-grade pyrite to completely replace the carbonaceous raw material as a reducing agent to treat phosphogypsum, further reducing the coal consumption cost and carbon emissions during the large-scale treatment of phosphogypsum, and adding low-grade limestone to adjust the atmosphere conditions and reduce the iron content. Combining with the low KH and low CaO:SiO2 design, SiO2 is used to induce the decomposition of CaSO4 in the direction of CaO, achieving efficient and stable desulfurization of phosphogypsum. The SO3 content in the desulfurization product is ≤1%, and at the same time, a low-carbon clinker with a total content of C2S, C3S2, and CS ≥70% is prepared, which is of great significance for economy, environmental protection, and carbon reduction.

[0022] The main chemical reactions of FeS2 for desulfurization of phosphogypsum are as follows:

[0023] Main desulfurization: 5CaSO4 + FeS2 → 5CaO + 7SO2 + FeO

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

[0025] Secondary desulfurization: CaSO4 + 2Fe3O4 → CaO + SO2 + 3Fe2O3

[0026] Total secondary desulfurization: CaSO4 + 2FeO → CaO + SO2 + Fe2O3

[0027] Main SiO2-induced generation of C2S for desulfurization: 2CaO + SiO2 → 2CaO·SiO2

[0028] Secondary SiO2-induced generation of CS for desulfurization: 2CaO·SiO2 + SiO2 → 2(CaO·SiO2)

[0029] Secondary SiO2-induced generation of C3S2 for desulfurization: 3(2CaO·SiO2) + SiO2 → 2(3CaO·2SiO2)

[0030] (2) The preparation method of the present invention is more suitable for industrial promotion and application: low-grade limestone is incorporated into the ingredients, and its low-temperature decomposition generates CO2, protecting FeS2 in medium- and low-grade pyrite from reacting with O2 in the air and causing waste; secondly, using medium- and low-grade pyrite as the reducing agent completely will make the reaction atmosphere basically all SO2. If the SO2 concentration is too high, it will inhibit the desulfurization of CaSO4. Therefore, using medium- and low-grade pyrite as the reducing agent and cooperating with low-grade limestone can prevent the SO2 concentration in the atmosphere from being too high, further improving the desulfurization effect of CaSO4, and it is suitable for industrial environments with poor acid-resistant gas properties and strict requirements for SO3 content and working conditions; thirdly, using medium- and low-grade pyrite as the reducing agent will introduce too much iron. When the KH value in the ingredients is 0.4 - 0.5 and the mass ratio of total CaO:SiO2 is 1.4 - 1.7, the firing range is relatively narrow compared with that of high KH and high CaO:SiO2. Too much iron will cause the firing range to be narrower and more likely to melt, while incorporating low-grade limestone can dilute the proportion of iron content, preventing the iron content in the clinker from being too high. Low-grade limestone is significantly superior to carbonaceous raw materials in terms of cost.

[0031] (3) Desulfurization of phosphogypsum is mainly carried out with -1 valence S in FeS2 of medium- and low-grade pyrite. The reducibility of -1 valence S is stronger than that of C and S. When -1 valence S is used as the 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 the reducing agent, the theoretical C:S is 0.5, and the actual optimal C:S is 0.7 - 0.9, and the desulfurization of CaSO4 by FeS2 is mainly a solid-solid reaction. Therefore, the dosage is more economical and the stability is better than that of C and S.

[0032] (4) The reducibility of S-based reducing agents is stronger than that of C, which is likely to cause a large amount of CaSO4 to decompose into CaS and unable to desulfurize. Moreover, the concentration of SO2 in the decomposition gas is too high, which will also inhibit the desulfurization of CaSO4; medium- and low-grade pyrite contains more SiO2. In the present invention, while using medium- and low-grade pyrite as the reducing agent, a design with a low KH value and a low CaO:SiO2 provides more SiO2 to promote the conversion of CaSO4 to CaO desulfurization, thus forming calcium silicate minerals C2S, C3S2, CS, rather than CaS.

[0033] (5) Desulfurization of phosphogypsum is carried out with +2 valence Fe in FeS2 of medium- and low-grade pyrite as a supplement. And a certain amount of Fe makes the fluxing effect more effective under reducing conditions, improving the burnability of CaSO4 desulfurization and the formation of calcium silicate minerals C2S, C3S2, CS.

[0034] (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 subsequent double conversion and double absorption sulfuric acid production. Specific embodiments

[0035] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with 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.

[0036] Obtaining and detecting raw materials in the following specific embodiments:

[0037] Obtain phosphogypsum, low-grade limestone, and silico-aluminous raw materials. After drying, crushing, and grinding, perform chemical composition analysis. It should be noted that the silico-aluminous raw material 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 implementation manner. The chemical compositions of a batch of phosphogypsum, low-grade limestone, and coal gangue are shown in Table 1:

[0038] Table 1 Chemical composition analysis

[0039]

[0040] Perform chemical composition analysis on the medium and low-grade pyrite used, and the results are shown in Table 2:

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

[0042]

[0043] Example 1

[0044] 1. Ratio design

[0045] 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 compositions are measured corresponding to the substances mainly composed of CaO. Design the ratio with low-grade limestone, silico-aluminous raw materials, and medium and low-grade pyrite according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum being 0.5, KH value being 0.441, the mass ratio of total CaO to SiO2 in the chemical composition being 1.5, and the dosage of low-grade limestone being 26.43% of the dosage of phosphogypsum. That is, the dosage of medium and low-grade pyrite = dosage of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64). The weight ratio of phosphogypsum: low-grade limestone: coal gangue: medium and low-grade pyrite is 70:18.5:11.5:18.98.

[0046] 2. Ball making

[0047] Mix and grind the above-mentioned phosphogypsum, low-grade limestone, coal gangue, and medium and low-grade pyrite measured according to the ratio, then add water to make material balls with an average diameter of 2 cm, and then naturally dry for 1 day.

[0048] 3. Calcination

[0049] The ball materials after natural drying are put into a rotary kiln lacking oxygen after the combustion of coal (coal providing temperature and heat) for calcination. The O2 concentration is 8%, and low-carbon clinker is obtained after cooling upon completion of calcination. The calcination temperature of the kiln is set at 1180°C, and the time is 15 min.

[0050] Example 2

[0051] 1. Proportion Design

[0052] The SO3 in the chemical composition of phosphogypsum is included in 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, and are designed for proportion with low-grade limestone, silico-aluminous raw material, 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 0.440, the mass ratio of total CaO to SiO2 in the chemical composition being 1.506, and the dosage of low-grade limestone being 61.13% of the dosage of phosphogypsum. That is, the dosage of medium-low-grade pyrite = dosage of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum:low-grade limestone:coal gangue:medium-low-grade pyrite is 53:32.4:14.6:14.37.

[0053] 2. Ball Making

[0054] The above-mentioned phosphogypsum, low-grade limestone, coal gangue, and medium-low-grade pyrite measured according to the proportion are mixed and ground, then water is added to make ball materials with an average diameter of 2 cm, and then they are naturally dried for 1 d.

[0055] 3. Calcination

[0056] The ball materials after natural drying are put into a rotary kiln lacking oxygen after the combustion of coal (coal providing temperature and heat) for calcination. The O2 concentration is 7%, and low-carbon clinker is obtained after cooling upon completion of calcination. The calcination temperature of the kiln is set at 1190°C, and the time is 15 min.

[0057] Example 3

[0058] 1. Proportion Design

[0059] 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 mixing ratio with low-grade limestone, silica-aluminum-iron raw material, 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 0.437, the mass ratio of total CaO to SiO2 in the chemical composition being 1.484, and the dosage of low-grade limestone being 154.69% of the dosage of phosphogypsum. 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: low-grade limestone: coal gangue: medium-low-grade pyrite is 32:49.5:18.5:8.68.

[0060] 2. Pelletizing

[0061] Mix and grind the above-mentioned phosphogypsum, low-grade limestone, 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 d.

[0062] 3. Calcination

[0063] Put the naturally dried pellets into a rotary kiln lacking oxygen after the combustion of coal (coal providing temperature and heat) for calcination. The O2 concentration is 6%. After calcination, cool to obtain low-carbon clinker. The calcination temperature of the kiln is set at 1200 °C and the time is 12 min.

[0064] The SO3 content of the calcined product (clinker) is shown in Table 3. The SO3 content is measured by total sulfur analysis. The main mineral content is shown in Table 4. The main mineral content is obtained after analyzing the XRD pattern of the calcined product (clinker) by jade software.

[0065] Table 3 SO3 content of the calcined product (clinker)

[0066] Project <![CDATA[SO3 content after firing]]> Example 1 0.86％ Example 2 0.79％ Example 3 0.76％

[0067] Table 4 Main mineral content (%)

[0068] Project <![CDATA[C3S2]]> <![CDATA[C2S]]> CS <![CDATA[SiO2]]> <![CDATA[Ca2Al2SiO7]]> <![CDATA[Ca2Fe9O 13 > <![CDATA[CaFe4O6]]> <![CDATA[Fe3O4]]> <![CDATA[CaSO4]]> Example 1 59.2 16.9 5.5 0.5 6.2 4.6 2.4 4.7 0 Example 2 60.9 17.8 3.8 0.3 7.6 4.9 2.2 2.5 0 Example 3 60.3 18.5 3.6 0.5 9.3 4.7 2.4 0.7 0

[0069] 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 obtained.

[0070] The above-mentioned 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-substantive 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 low-carbon emission desulfurization of phosphogypsum and preparation of low-carbon clinker, characterized in that The following steps are involved: (1) Obtain phosphogypsum, medium- and low-grade pyrite, low-grade limestone, and silicon-aluminum-iron raw materials, dry and crush them, mix them according to the designed S:S, KH value, and total CaO:SiO2 mass ratio, and grind them together; 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 main chemical composition and content of the low-grade limestone are: CaO: 40-48%; MgO: 2-5%; SiO2: 5-15%; Al2O3: 0-3%; Fe2O3: 0-2%; loss on ignition: 33-40%; the S:S ratio is 0.45-0.55, the KH value is 0.4-0.5, the total CaO:SiO2 mass ratio in the chemical composition is 1.4-1.7, and the amount of low-grade limestone used is not less than 20% of the amount of phosphogypsum; (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 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 ≤1%, and the total content of C2S, C3S2, and CS in the mineral composition is ≥70%.

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