A method for desulfurizing phosphogypsum without reducing carbon emission and preparing high-iron phase γ-C2S clinker

Abstract

The present invention discloses a method for desulfurizing phosphogypsum without reducing carbon emissions and preparing a high-iron phase γ-C2S clinker. The method comprises the following steps: preparing phosphogypsum, 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; mixing and grinding the mixture; adding water to form pellets; and then drying the pellets. The pellets are calcined in an oxygen-deficient or oxygen-free rotary kiln after combustion of coal. After calcination, the pellets are naturally cooled to obtain the high-iron phase γ-C2S clinker. The present invention uses medium- and low-grade pyrite to decompose phosphogypsum, further reducing the coal cost and carbon emissions when processing large amounts of phosphogypsum. Furthermore, efficient desulfurization of the phosphogypsum is achieved through a reasonable proportion and process, reducing the SO3 content in the calcined product to below 1%, and simultaneously producing a high-iron phase γ-C2S clinker.

Description

Technical Field

[0001] The present 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 high-iron phase γ-C2S 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] γ-C2S (gamma-type dicalcium silicate) has excellent CO2 absorption capacity, high carbonization activity and self-pulverization characteristics, and can be used to prepare high-performance materials to reduce carbon emissions. However, SO3 will stabilize β-C2S and prevent it from converting to γ-C2S. Therefore, it is difficult to prepare γ-C2S using phosphogypsum. The SO3 content of the desulfurized clinker made from phosphogypsum must be as low as possible.

[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, so that the SO3 content of phosphogypsum clinker after desulfurization is ≤1%, and a high-carbonization active low-carbon clinker mainly composed of γ-C2S is produced, has become a technical problem that needs to be solved urgently. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for desulfurizing phosphogypsum without reducing carbon emission and preparing high-iron phase γ-C2S 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 ratios and processes, reducing the SO3 content in the calcined product to below 1%, and simultaneously producing high-iron phase γ-C2S clinker.

[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 high-iron phase γ-C2S 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 then naturally cooled to obtain γ-C2S clinker with a high iron phase.

[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, iron ore soil, 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 0.6-0.7, and the total CaO:SiO2 mass ratio in the chemical composition is 2.0-2.3.

[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 1300-1400° 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 in the high-iron phase γ-C2S clinker obtained in step (3) is ≤1%, the γ-C2S content in the mineral composition is 55-70%, and the total content of CF, C2F, and C4AF is 25-40%.

[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 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. Through reasonable proportions and processes, efficient and stable desulfurization of phosphogypsum is achieved without the need for carbonaceous raw materials and without reducing carbon emissions. The SO3 content in the desulfurization product is ≤1%, and high-iron phase γ-C2S clinker 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] Others are: Fe2O3 induces the generation of CF, C2F, and C4AF for desulfurization, and CF, C2F, and C4AF promote the formation of C2S and thus promote desulfurization.

[0028] (2) For the desulfurization of phosphogypsum, a formulation suitable for using medium- and low-grade pyrite as a reducing agent has been developed. Pellets of phosphogypsum are more conducive to industrial application. The encapsulation of the pellets can greatly save the amount of reducing agent and improve the phenomenon of melt crusting. However, the disadvantage is that it is more difficult to burn and desulfurize than powder. Although the melting point is lower and easier to burn when the KH and total CaO:SiO2 mass ratios 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 CaSO4 desulfurization and can make the SO3 content in the desulfurization product ≤2%, or even less than 1%, the iron content introduced by medium- and low-grade pyrite is too high, resulting in a narrow firing range under this ratio, easy to melt, and not conducive to application. The present invention still combines the technology of low KH value (KH is 0.3-0.7) which is conducive to CaSO4 desulfurization and thus forming C2S, and controls the KH and total CaO:SiO2 mass ratios during phosphogypsum desulfurization at 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 reducing property 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 the phenomenon of being too difficult to burn and difficult to desulfurize will not occur after the KH and total CaO:SiO2 mass ratios are increased during ball calcination; in addition, the aluminum content in medium and low grade pyrite is low, and the relative content of aluminum to iron is even lower. After batching, the main iron phase formed is more CF, C2F and a small amount of C4AF, without aluminum phase minerals C3A,

[0029] --C 12 A7, C4A3S, C4AS and other aluminum-phase solid sulfur minerals require certain aluminum-iron phase minerals to make C2S easy to burn when the KH value is 0.6-0.7. However, the Al2O3 in the aluminum-iron phase minerals has a significant inhibitory effect on CaSO4 desulfurization. Low-priced conventional non-sulfur iron raw materials contain more aluminum, and medium- and low-grade pyrite can be used for phosphogypsum desulfurization, which reduces this inhibitory effect.

[0030] (3) SO3 will stabilize β-C2S and prevent it from converting to γ-C2S. The KH and total CaO:SiO2 mass ratios used in the ingredients of the present invention are 0.6-0.7 and 2.0-2.3, respectively. The calcination temperature is controlled at 1300-1400°C, so that the desulfurization rate of phosphogypsum when medium and low-grade pyrite is used as a reducing agent is further improved compared with 1150-1250°C, thereby achieving a SO3 content of ≤1% in the desulfurization product, which is beneficial to the conversion of β-C2S to γ-C2S. Secondly, the high Fe content in medium and low-grade pyrite can activate and modify γ-C2S, thereby improving its carbonization activity and carbonization strength.

[0031] (4) 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.

[0032] (5) 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 C2S instead of CaS.

[0033] (6) 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 susceptibility of CaSO4 to desulfurization and forming C2S.

[0034] (7) 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

[0035] Figure 1 : XRD pattern of the high iron phase γ-C2S clinker obtained in Example 1. DETAILED DESCRIPTION

[0036] 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.

[0037] The acquisition and detection of raw materials in the following specific examples:

[0038] Obtain phosphogypsum and ferro-aluminum raw materials, dry, crush, and grind them for chemical composition analysis. Ferro-aluminum raw materials can be any one or more of sandstone, shale, coal gangue, silica, clay, fly ash, iron ore, and various types of waste residues. For example, the chemical composition of a batch of phosphogypsum and iron ore is shown in Table 1:

[0039] Table 1 Chemical composition analysis

[0040] 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 Iron ore soil 3.27 48.9 15.55 15.9 7.56 4.41 0.01 0.78 2.79

[0041] The chemical composition of the low- and medium-grade pyrites used was analyzed, and the results are shown in Table 2:

[0042] Table 2 Chemical composition analysis of low-grade pyrite

[0043]

[0044] Example 1

[0045] 1. Ratio design

[0046] 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 with medium and low-grade pyrite is designed according to the molar ratio of -1 valence S in FeS2 to SO3 in phosphogypsum is 0.5, the KH value is 0.669, and the mass ratio of total CaO to SiO2 in the chemical composition is 2.245. 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: medium and low-grade pyrite is 100:27.11.

[0047] 2. Ball making

[0048] The phosphogypsum 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.

[0049] 3. Calcination

[0050] 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 naturally cooled to obtain γ-C2S clinker with a high iron phase. The kiln calcination temperature is set to 1350°C and the time is 14 minutes.

[0051] The XRD pattern of the high iron phase γ-C2S clinker obtained in this example is shown in the attached Figure 1 As shown, it can be observed that the mineral composition is mainly γ-C2S and CF, followed by C2F, C4AF, and β-C2S.

[0052] Example 2

[0053] 1. Ratio design

[0054] 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 0.639, and the mass ratio of total CaO to SiO2 in the chemical composition is 2.170. 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: iron ore soil: medium and low-grade pyrite is 99:1:26.84.

[0055] 2. Ball making

[0056] The phosphogypsum, iron ore soil, 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.

[0057] 3. Calcination

[0058] 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 naturally cooled to obtain γ-C2S clinker with a high iron phase. The calcination temperature of the kiln is set to 1340°C and the time is 15 minutes.

[0059] Example 3

[0060] 1. Ratio design

[0061] 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 0.48, the KH value is 0.626, and the mass ratio of total CaO to SiO2 in the chemical composition is 2.138, that is, the amount of medium and low-grade pyrite = the amount of phosphogypsum * 0.4246 * 0.48 * 32 * 120 / (80 * 0.5873 * 64), and the weight ratio of phosphogypsum: iron ore soil: medium and low-grade pyrite is 98:2:25.51.

[0062] 2. Ball making

[0063] The phosphogypsum, iron ore soil, 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.

[0064] 3. Calcination

[0065] 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 naturally cooled to obtain γ-C2S clinker with a high iron phase. The calcination temperature of the kiln is set to 1340°C and the time is 15 minutes.

[0066] The SO3 content of the obtained clinker is shown in Table 3, and the main mineral content and the conversion rate of γ-C2S are shown in Table 4.

[0067] Table 3 SO3 content of the obtained clinker

[0068] project <![CDATA[SO3 content after burning]]> Example 1 0.78％ Example 2 0.83％ Example 3 0.76％

[0069] Table 4 Main mineral contents and γ-C2S conversion rate (%)

[0070]

[0071] The results of each embodiment show that the reducing agent used to decompose phosphogypsum can all be low-priced, widely distributed medium- and low-grade pyrite. Through reasonable proportioning and process, the SO3 content after burning is within 1%, achieving efficient and stable desulfurization of phosphogypsum without the need for carbonaceous raw materials and without reducing carbon emissions, and producing high-iron phase γ-C2S clinker. The conversion rate of γ-C2S during natural cooling (that is, the proportion of γ-C2S in the total of all C2S) is high.

[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 high iron phase γ-C2S 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 naturally after calcination to obtain γ-C2S clinker with a high iron phase; the calcination temperature is 1300-1400°C, the calcination time is 5-30 minutes, and the O2 concentration in the calcination environment is 0-10%; the SO3 content in the γ-C2S clinker with a high iron phase is ≤1%, the γ-C2S content in the mineral composition is 55-70%, and the total content of CF, C2F, and C4AF is 25-40%.

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

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

Images