A method for desulfurization of phosphogypsum with low carbon emissions and preparation of magnesium-modified low-carbon clinker

Abstract

The present invention discloses a method for desulfurizing phosphogypsum with low carbon emissions and preparing magnesium-modified low-carbon clinker. Phosphogypsum is proportioned with medium and low-grade pyrite, aggregate waste residue, and silico-aluminous raw materials according to the designed molar ratios of S:S and effective CaO:SiO2, mixed and ground, made into balls with water, and then dried. The dried balls are calcined in a rotary kiln lacking oxygen or oxygen-free after coal combustion, and cooled after calcination to obtain magnesium-modified 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 synergistically using aggregate waste residue to regulate the atmosphere, increase the magnesium content, and reduce the iron content. Through reasonable proportioning and processes, efficient and stable desulfurization of phosphogypsum is achieved, and magnesium-modified low-carbon clinker with high carbonation strength 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 magnesium-modified 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 instead of limestone as the CaO source to produce cement clinker is a challenging and significant 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 under 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 by the reaction of carbonaceous raw materials as reducing agents with CaSO4.

[0004] Some studies have tried to use sulfur as a reducing agent for the decomposition of CaSO4. It 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. Moreover, sulfur is less stable than coal and is prone to react with O2 and sublime in large quantities in practical applications, resulting in waste.

[0005] Due to the increasing rise of the manufactured sand industry that replaces natural sand and gravel, it is inevitable for aggregate plants to discharge a large amount of waste residue containing stone powder during the production of sand and gravel, which can also be called filter press soil. Its composition is complex, with low CaO content and high MgO content, and it is difficult to utilize. The aggregate waste residue is easily affected by wind and causes dust, resulting in air pollution. If it seeps into rivers, it will cause water pollution and river channel blockage. The stone powder in the aggregate waste residue is mainly calcite and dolomite, with its MgO content generally at 5%-15%, and it also contains clay minerals. Enterprises generally use landfill for treatment, occupying a large amount of land and having a greater impact on the environment.

[0006] Under the premise that China's future requirements for carbon emissions are becoming increasingly strict, can a more stable, effective, non-carbonaceous raw material reducing agent be found, and the aggregate waste residue be co-disposed to achieve efficient and stable desulfurization of phosphogypsum, produce environmentally friendly and high-value-added materials, and the preparation method should be suitable for industrial promotion and application, which has become an urgent technical problem to be solved. 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 magnesium-modified 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 when treating a large amount of phosphogypsum, and synergistically using aggregate waste residues to adjust the atmosphere, increase the magnesium content, and reduce the iron content. Through reasonable proportioning and processes, efficient and stable desulfurization of phosphogypsum is achieved, and magnesium-modified low-carbon clinker with high carbonation strength is simultaneously prepared.

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

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

[0010] (1) Obtain phosphogypsum, medium and low-grade pyrite, aggregate waste residues, and silico-aluminous raw materials, dry and crush them, and carry out batching according to the designed molar ratios of S:S and effective CaO:SiO2, and then mix and grind.

[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 magnesium-modified 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 aggregate waste residue in step (1) is the waste residue containing stone powder generated during the process of manufactured sand, and the main chemical components and contents are: CaO: 20 - 40%; MgO: 5 - 15%; SiO2: 15 - 25%; Al2O3: 3 - 6%; Fe2O3: 2 - 5%; loss on ignition: 25 - 35%.

[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, and the dosage of the aggregate waste residue is not less than 20% of the dosage of phosphogypsum.

[0017] According to the above scheme, the molar ratio of effective CaO:SiO2 in step (1) is 1.4 - 1.6, which is the molar ratio of CaO:SiO2 after subtracting the calcium and silicon in minerals C2AS and C2MS2 in the batching.

[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 in step (3) is 1150 - 1220 °C, the calcination time is 5 - 20 min, and the O2 concentration in the calcination environment is 0 - 10%.

[0020] According to the above solution, the SO3 content of the magnesium - modified low - carbon clinker obtained in step (3) is ≤1%, and the total content of C2S, C3S2, and C2MS2 in the mineral composition is ≥70%.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) 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 during the large - scale treatment of phosphogypsum, and co - treating aggregate waste residues to regulate the atmosphere, increase the magnesium content, and reduce the iron content. Combining the design of low KH and low CaO:SiO2, 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 magnesium - modified low - carbon clinker with a total content of C2S, C3S2, and C2MS2 ≥70% is prepared, which is of great significance for economy, environmental protection, and carbon reduction.

[0023] The main chemical reactions of FeS2 for desulfurizing phosphogypsum are as follows:

[0024] Main desulfurization: 5CaSO4+FeS2→5CaO+7SO2+FeO

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

[0026] Secondary desulfurization: CaSO4+2Fe3O4→CaO+SO2+3Fe2O3

[0027] Total secondary desulfurization: CaSO4+2FeO→CaO+SO2+Fe2O3

[0028] Main SiO2 - induced formation of C2S for desulfurization: 2CaO+SiO2→2CaO·SiO2

[0029] Secondary SiO2 - induced formation of CS for desulfurization: 2CaO·SiO2+SiO2→2(CaO·SiO2)

[0030] Secondary SiO2 - induced formation of C3S2 for desulfurization: 3(2CaO·SiO2)+SiO2→2(3CaO·2SiO2)

[0031] (2) The preparation method of the present invention is more suitable for industrial promotion and application: Aggregate waste residue is incorporated into the ingredients, and its low-temperature decomposition generates CO2, protecting the FeS2 in medium and low-grade pyrite from reacting with O2 in the air and causing waste; Secondly, using only medium and low-grade pyrite as a reducing agent will make the reaction atmosphere mainly composed of SO2. If the SO2 concentration is too high, it will inhibit the desulfurization of CaSO4. Therefore, using medium and low-grade pyrite as a reducing agent and cooperating with aggregate waste residue 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 resistance to acid gases and strict requirements for SO3 content and working conditions; Thirdly, using medium and low-grade pyrite as a reducing agent will introduce too much iron. When the effective CaO:SiO2 molar ratio in the ingredients is 1.4 - 1.6, the firing range is relatively narrow compared to when the CaO:SiO2 molar ratio is relatively high. Too much iron will cause the firing range to be even narrower and more prone to melting. Incorporating aggregate waste residue can dilute the proportion of iron content, preventing the iron content in the clinker from being too high.

[0032] (3) The MgO content in the aggregate waste residue is high, which has a fluxing effect and improves the burnability of CaSO4 desulfurization and the formation of calcium silicate minerals C2S, C3S2, and C2MS2.

[0033] (4) The carbonation strength of the C3S2 mineral in the low-carbon clinker is high and its stability is good. Its comprehensive performance is significantly better than that of C2S, CS, and C2AS. However, the firing range of C3S2 is narrow and it is prone to melting during calcination. Therefore, it is impossible to obtain pure C3S2 in the actual preparation process, and there will inevitably be C2S, CS, and C2AS. By adding aggregate waste residue to introduce MgO in the present invention, the C2S, CS, and C2AS minerals are replaced by C2MS2 with high carbonation activity and low requirements for CO2 diffusion during carbonation, improving the carbonation activity, carbonation uniformity, and carbonation strength of the low-carbon clinker.

[0034] (5) Introducing more Fe and Mg from medium and low-grade pyrite and aggregate waste residue can activate and modify the minerals in the low-carbon clinker, improving its carbonation activity and carbonation strength.

[0035] (6) Using the -1 valence S in FeS2 of medium and low-grade pyrite for the desulfurization of 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 and the stability is better than that of C and S.

[0036] (7) The reducing property of the S-type reducing agent itself is stronger than that of C, and it is easy to cause a large amount of CaSO4 to decompose into CaS, making desulfurization impossible. Moreover, the concentration of SO2 in the decomposed gas is too high, which will also inhibit the desulfurization of CaSO4. The medium and low-grade pyrite contains more SiO2. In this invention, while using medium and low-grade pyrite as the reducing agent, a design with a low KH value and a low CaO:SiO2 ratio is adopted to provide more SiO2 to promote the desulfurization conversion of CaSO4 to CaO, thereby forming calcium silicate minerals such as C2S, C3S2, and C2MS2, rather than CaS.

[0037] (8) Using the +2 valence Fe in FeS2 of medium and low-grade pyrite for the desulfurization of phosphogypsum, 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 such as C2S, C3S2, and C2MS2.

[0038] (9) 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. Detailed implementation manners

[0039] In order to make the purpose, technical solutions and advantages of this invention clearer, the following examples are used to further elaborate on this invention. It should be understood that the specific examples described here are only used to explain this invention and are not used to limit this invention.

[0040] Obtaining and detecting raw materials in the following specific examples:

[0041] Obtain phosphogypsum, aggregate waste residue, and aluminosilicate raw materials. After drying, crushing, and grinding, conduct chemical composition analysis. It should be noted that the aluminosilicate 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. For example, the chemical compositions of a batch of phosphogypsum, aggregate waste residue, and coal gangue are shown in Table 1:

[0042] Table 1 Chemical composition analysis

[0043] 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 Aggregate waste residue 31.5 20.99 4.64 3.13 28.29 8.76 0.93 0.12 0.71 Coal gangue 4.63 76.3 6.62 4.64 1.91 2.35 0.48 1.34 0.55

[0044] Conduct chemical composition analysis on the medium and low-grade pyrite used, and the results are shown in Table 2:

[0045] Table 2 Chemical composition analysis of medium and low-grade pyrite

[0046]

[0047] Example 1

[0048] 1. Ratio design

[0049] 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 aggregate waste residue, silicon-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, the molar ratio of effective CaO:SiO2 being 1.495, and the dosage of aggregate waste residue being 24.83% 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:aggregate waste residue:coal gangue:medium-low grade pyrite is 72.5:18:9.5:19.66.

[0050] 2. Pelletizing

[0051] Mix and grind the above-mentioned phosphogypsum, aggregate waste residue, 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 naturally dry for 1 d.

[0052] 3. Calcination

[0053] 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 8%. After calcination, cool to obtain magnesium-modified low-carbon clinker. The calcination temperature of the kiln is set at 1180 °C and the time is 9 min.

[0054] 4. Carbonation

[0055] Grind the magnesium-modified low-carbon clinker to R45μm ≤ 20%, make cylindrical specimens with a diameter of 2 cm and a height of 2 cm at a forming pressure of 10 MPa and a water-solid ratio of 0.15, and carbonize for 24 h under the conditions of a carbon dioxide concentration of 100%, a temperature of 25 °C, a humidity of 60%, and an air pressure of 0.3 MPa, and then measure the carbon fixation rate and compressive strength.

[0056] Example 2

[0057] 1. Mixing ratio design

[0058] 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 aggregate waste residue, silicon-aluminum-iron raw material, and low- to medium-grade pyrite according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum being 0.5, the molar ratio of effective CaO:SiO2 being 1.494, and the dosage of aggregate waste residue being 81.80% of the dosage of phosphogypsum. That is, the dosage of low- to medium-grade pyrite = dosage of phosphogypsum * 0.4246 * 0.5 * 32 * 120 / (80 * 0.5873 * 64). The weight ratio of phosphogypsum:aggregate waste residue:coal gangue:low- to medium-grade pyrite is 50:40.9:9.1:13.56.

[0059] 2. Pelletizing

[0060] Mix and grind the above-mentioned phosphogypsum, aggregate waste residue, coal gangue, and low- to medium-grade pyrite measured according to the ratio, then add water to make pellets with an average diameter of 2 cm, and then air dry for 1 day.

[0061] 3. Calcination

[0062] Put the air-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 7%. After calcination, cool to obtain magnesium-modified low-carbon clinker. The calcination temperature of the kiln is set at 1190 °C and the time is 8 minutes.

[0063] 4. Carbonation

[0064] Grind the magnesium-modified low-carbon clinker to R45μm ≤ 20%, make cylindrical specimens with a diameter of 2 cm and a height of 2 cm at a forming pressure of 10 MPa and a water-solid ratio of 0.15, and carbonize for 24 hours under the conditions of a carbon dioxide concentration of 100%, a temperature of 25 °C, a humidity of 60%, and an air pressure of 0.3 MPa, and then measure the carbon fixation rate and compressive strength.

[0065] Comparative Example 1

[0066] Use limestone to replace phosphogypsum, design the mixing ratio according to the KH value of 0.472, the SM value of 7.195, the IM value of 0.771, and the mass ratio of total CaO to SiO2 in the chemical composition being 1.453. Calcinate at 1250 °C using a new dry process kiln to obtain low-carbon cement clinker mainly composed of C3S2, and then carry out the carbonation in Step 4 of Example 1, and measure the carbon fixation rate and compressive strength.

[0067] 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) with jade software; the carbonation conditions of the clinker and the strength after carbonation are shown in Table 5.

[0068] Table 3 SO3 Content of Calcined Product (Clinker)

[0069] Project <![CDATA[SO3 content after burning]]> Example 1 0.83％ Example 2 0.76％

[0070] Table 4 Main Mineral Content (%)

[0071] Project <![CDATA[C3S2]]> <![CDATA[C2MS2]]> <![CDATA[C2S]]> CS <![CDATA[C2AS]]> <![CDATA[SiO2]]> <![CDATA[Ca2Fe9O 13 > <![CDATA[CaFe4O6]]> <![CDATA[Fe3O4]]> <![CDATA[CaSO4]]> Example 1 56.6 23.5 2.7 1.0 4.3 0.2 4.8 2.4 4.5 0 Example 2 38.4 50.5 1.8 0 0 0.1 4.9 2.0 2.3 0 Comparative Example 1 66.8 0 18.2 6.4 8.5 0.1 0 0 0 0

[0072] Table 5 Carbonation Conditions of Low-Calcium Clinker and Strength after Carbonation

[0073]

[0074] The SO3 content after burning in each embodiment is less than 1%, and magnesium-modified low-carbon clinkers with a total content of C2S, C3S2, and C2MS2 ≥ 70% are simultaneously obtained. Compared with Example 1, in Example 2, as the dosage of aggregate waste residue increases, the MgO content increases, causing C2S, CS, and C2AS to gradually transform into C2MS2 with high carbonation activity and low requirements for CO2 diffusion during carbonation; the relatively high amounts of Fe and Mg in Examples 1 and 2 can also activate and modify the minerals in the low-carbon clinker, improving its carbonation activity and carbonation strength; when the C3S2 content in Examples 1 and 2 is lower than that in Comparative Example 1, a higher carbon sequestration rate and carbonation strength than those in Comparative Example 1 are exhibited.

[0075] The above embodiments are only the 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 substitutions made by those skilled in the art based on the present invention fall within the scope of protection required by the present invention.

Claims

1. A method for low-carbon emission desulfurization of phosphogypsum and preparation of magnesium-modified low-carbon clinker, characterized in that The following steps are involved: (1) Obtain phosphogypsum, medium- and low-grade pyrite, aggregate waste, and silicon-aluminum-iron raw materials, dry and crush them, mix them according to the designed S:S and effective CaO:SiO2 molar ratios, 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 aggregate waste residue is stone powder-containing waste residue produced in the machine-made sand process, and its main chemical components and contents are: CaO: 20-40%; MgO: 5-15%; SiO2: 15-25%; Al2O3: 3-6%; Fe2O3: 2-5%; loss on ignition: 25-35%; the S:S ratio is 0.45-0.55, and the amount of aggregate waste residue used is not less than 20% of the amount of phosphogypsum used; the effective CaO:SiO2 molar ratio is 1.4-1.6, which is specifically the CaO:SiO2 molar ratio after deducting the calcium and silicon in the minerals C2AS and C2MS2 from the ingredients; (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 5-20 minutes. After calcination, the pellets are cooled to obtain magnesium-modified 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 magnesium-modified low-carbon clinker obtained in step (3) is ≤1%, and the total content of C2S, C3S2, and C2MS2 in the mineral composition is ≥70%.

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