A high-content phosphogypsum cement clinker and preparation method thereof

Abstract

The invention discloses a high-dosage phosphogypsum cement clinker and a preparation method thereof, relating to the field of cement clinker and preparation thereof. The raw materials of the high-dosage phosphogypsum cement clinker comprise, by weight, 25-35 parts of modified phosphogypsum, 50-55 parts of limestone, 9-12 parts of mud cake, 2-6 parts of clastic rock, 0-2 parts of steel slag, and 1-2 parts of calcium-containing waste residue. The modified phosphogypsum is obtained by adding fly ash, steel slag, and microcrystalline carbon to phosphogypsum and performing high-temperature modification treatment. The addition of steel slag can effectively reduce the calcination temperature during the modification process. The prepared modified phosphogypsum can replace a portion of limestone and is applied to the preparation of cement clinker. At the same time, the modified phosphogypsum does not affect the physical properties and setting time of the cement clinker, effectively realizing low-cost treatment and large-scale reuse of the phosphogypsum.

Description

Technical Field

[0001] The present invention relates to the field of cement clinker and its preparation, in particular to a high-dosage phosphogypsum cement clinker and its preparation method. Background Art

[0002] Phosphogypsum is a byproduct of the sulfuric acid decomposition of phosphate rock and the extraction of phosphoric acid. Phosphoric acid is the primary raw material for the production of high-concentration compound fertilizers and calcium hydrogen phosphate for feed. The production of one ton of phosphoric acid produces approximately five tons of phosphogypsum. Currently, over 780 million tons of phosphogypsum are stored in my country. This massive accumulation not only occupies land but also pollutes water and underground water, impacting the environment and human health. Phosphogypsum is produced in large quantities and is relatively concentrated. Its composition is complex. In addition to water and calcium sulfate, it also contains impurities such as incompletely decomposed phosphate rock, residual phosphoric acid, fluoride, acid-insoluble matter, and organic matter. The presence of fluorine and organic matter poses the greatest threat to the resource utilization of phosphogypsum, making it a global challenge. To date, the effective utilization rate of phosphogypsum worldwide is less than 10%. The lack of proven technologies for the cost-effective disposal and large-scale utilization of phosphogypsum poses an urgent challenge to both ecological and environmental protection and the sustainable development of the phosphorus chemical industry.

[0003] Currently, the main uses of phosphogypsum are as a cement retarder, for external sale or supply, for gypsum board and gypsum blocks, for road construction or backfilling, and for building gypsum powder. Cement retarder is the most widely used application for phosphogypsum. However, because phosphogypsum contains harmful impurities such as phosphorus and fluorine, which adversely affect cement properties, the average dosage of phosphogypsum does not exceed 3%. Therefore, the low dosage for cement retarder makes it difficult to address the problem of phosphogypsum reduction and disposal. In the prior art, although CN115180846 B discloses the desulfurization of calcium sulfate by adding carbonaceous raw materials, the calcination temperature is as high as 1000-1100°C, resulting in low desulfurization efficiency. The residual sulfur trioxide affects the clinker properties and calcination process, resulting in the produced clinker failing to meet the technical requirements of Portland cement clinker, which can adversely affect cement properties, such as significant fluctuations. Furthermore, the process flow is relatively complex, making it difficult to achieve large-scale application in actual production.

[0004] Therefore, it is necessary to develop a technology that can efficiently decompose calcium sulfate contained in phosphogypsum and has non-critical modification conditions. It can be well applied in the field of cement clinker, which can not only realize the resource utilization of industrial waste, but also bring huge economic benefits to enterprises. Summary of the Invention

[0005] In response to the deficiencies of the above-mentioned prior art, the present invention provides a high-dosage phosphogypsum cement clinker and a preparation method thereof. Modified phosphogypsum is obtained by calcining phosphogypsum, fly ash, steel slag, and microcrystalline carbon. The modified phosphogypsum is applied to the production of cement clinker. The physical properties of the prepared cement clinker are the same as those of ordinary Portland cement clinker, thus achieving low-cost processing and large-scale reuse of phosphogypsum. At the same time, the addition of steel slag, fly ash, etc. can reduce the calcination temperature during the phosphogypsum modification process. The raw material consumption in the high-dosage phosphogypsum cement clinker prepared by the present invention is 50-60%, which effectively reduces production costs and greatly saves resources. This is specifically achieved through the following technologies:

[0006] The present invention provides a high-dosage phosphogypsum cement clinker, the raw materials of which comprise, by weight, 25-35 parts of modified phosphogypsum, 50-55 parts of limestone, 9-12 parts of mud cake, 2-6 parts of clastic rock, 0-2 parts of steel slag, and 1-2 parts of calcium-containing waste residue. The modified phosphogypsum is obtained by calcining phosphogypsum, fly ash, steel slag, and microcrystalline carbon in certain proportions.

[0007] Furthermore, the raw materials include, by mass, 30 parts of modified phosphogypsum, 55 parts of limestone, 9 parts of mud cake, 4 parts of clastic rock, 1 part of steel slag, and 1 part of calcium-containing waste slag.

[0008] The above-mentioned mud cake is obtained by sedimentation and filtration of sludge generated in the construction material production line. It is a general industrial solid waste, and its main components are calcium oxide (CaO) and silicon dioxide (SiO2).

[0009] Furthermore, the preparation method of the modified phosphogypsum is as follows: 8-10 parts of fly ash, 7-10 parts of steel slag, and 15-25 parts of microcrystalline carbon are added to 55-70 parts of phosphogypsum, the mixture is ground and mixed evenly, and then placed in a silicon carbide furnace and calcined for 20-30 minutes to obtain the modified phosphogypsum.

[0010] The above calcination temperature is 900-1000°C.

[0011] The principle of the above modification of phosphogypsum is as follows: microcrystalline carbon is added to phosphogypsum because microcrystalline carbon contains a high C content, and the generated CO reducing gas can begin to decompose the calcium sulfate in the phosphogypsum at 1000°C and reduce it to sulfur dioxide gas for release.

[0012] The inventors have discovered through experimental research that adding a certain proportion of steel slag and fly ash to phosphogypsum not only effectively reduces the calcination temperature during the decomposition and modification of phosphogypsum, but also allows it to begin decomposing at 900°C, thereby improving desulfurization efficiency and reducing energy consumption and costs. The reaction mechanism is as follows: steel slag contains a molten complex of iron oxide and other trace metal oxides, which acts as a catalyst, lowering the reaction temperature and further promoting the decomposition of CaSO4. Furthermore, metal oxides such as iron oxide (Fe2O3) oxidize the intermediate product of phosphogypsum decomposition, CaS, into CaO, promoting the desulfurization and decomposition of phosphogypsum. Fly ash contains highly active free SiO2, which reacts with CaO to form calcium silicate (CaSiO3), which helps continuously promote the decomposition of phosphogypsum.

[0013] In addition, the synergistic effect of steel slag and microcrystalline carbon strengthens each other's functions: on the one hand, steel slag can promote the desulfurization decomposition reaction between microcrystalline carbon and phosphogypsum due to its catalytic activity; on the other hand, the reducing gas atmosphere provided by microcrystalline carbon creates conditions for the reaction process, making the iron oxide (Fe2O3) in the steel slag have a catalytic reaction effect.

[0014] Phosphogypsum contains a small amount of fluoride ions (F - ) and soluble phosphorus (content of about 0.1-0.5%) decompose into gas or transform into insoluble calcium phosphate during high temperature calcination, which has no adverse effect on the performance of cement clinker. - It acts as a mineralizer, promoting the formation of silicate minerals and reducing the firing temperature.

[0015] Preferably, the microcrystalline carbon is at least one of charcoal or activated carbon.

[0016] The clastic rock in the raw material formula is at least one of shale and sandstone; the calcium-containing waste slag is at least one of converter slag and granulated blast furnace phosphorus slag.

[0017] The mechanism of action of the various raw material components of high-phosphogypsum cement clinker is as follows: Replacing part of the limestone with modified phosphogypsum not only does not affect the initial and final setting times of cement, but the highly active CaO also promotes the production of C3S (tricalcium silicate), improving the early strength of the cement clinker. Furthermore, the calcium-containing waste residue in the raw material provides the calcium oxide required for clinker formation. This waste residue contains CS-type minerals and some trace metal elements and fluorides, which facilitate clinker calcination, improve burnability, and enhance the hydration activity of minerals.

[0018] The present invention also provides a method for preparing high-dosage phosphogypsum cement clinker, comprising the following steps:

[0019] S1, drying the limestone, mud cake, clastic rock, steel slag and calcium-containing waste residue in the raw material respectively, mixing them uniformly, and grinding them to obtain a mixture;

[0020] S2. The mixture obtained in step S1 and the modified phosphogypsum are uniformly mixed, and water is added to prepare balls with a diameter of 8-10 mm. After drying, the balls are placed in a rotary kiln for high-temperature calcination at 1400-1450° C. for 30-45 min, and cooled to obtain high-dosage phosphogypsum cement clinker.

[0021] Compared with the prior art, the present invention is beneficial in that:

[0022] 1. By adding a certain proportion of steel slag and fly ash to phosphogypsum, the present invention can reduce the calcination temperature during the decomposition and modification of phosphogypsum from the typical 1100°C to 900-1000°C, while simultaneously improving desulfurization efficiency and reducing energy consumption and costs. Because steel slag contains a molten complex of iron oxide and other trace metal oxides, it has a catalytic effect, lowering the reaction temperature while further promoting the decomposition of CaSO4. Furthermore, metal oxides such as iron oxide (Fe2O3) can oxidize CaS, an intermediate product of phosphogypsum decomposition, into CaO, promoting the desulfurization and decomposition of phosphogypsum. The added fly ash contains highly active free SiO2, which reacts with CaO to form calcium silicate (CaSiO3), which helps to continuously promote the decomposition of phosphogypsum.

[0023] 2. The present invention adds industrial waste residues such as steel slag, mud cake, and calcium-containing waste residue in the clinker preparation process to increase mineral activity, which is beneficial to improving clinker quality, saving energy and reducing carbon emissions. The consumption of raw materials in the clinker preparation process is only 50-60%, which effectively reduces production costs and greatly saves resources.

[0024] 3. By modifying phosphogypsum and applying it to the production process of general-purpose Portland cement clinker, the present invention significantly improves the utilization rate of phosphogypsum in cement production. Furthermore, the quality, performance, and technical indicators of the prepared cement clinker are comparable to those of normal Portland cement clinker, and its performance is no different from that of general-purpose cement clinker. This enables the modified phosphogypsum to be widely used in Portland cement production, achieving low-cost processing and large-scale reuse of phosphogypsum. DETAILED DESCRIPTION

[0025] The technical solutions of the present invention are described clearly and completely below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0026] Unless otherwise specified, the following examples and comparative examples are all prepared using the following method for high-dosage phosphogypsum cement clinker:

[0027] S1. Dry the limestone, mud cake, clastic rock, steel slag and fly ash in the raw material respectively, mix them evenly, and grind them to obtain a mixture; the fineness of the mixture is controlled to have a residue of 13-15% on an 80 μm sieve.

[0028] S2. Evenly mix the mixture obtained in step S1 and the modified phosphogypsum, then add water to prepare pellets with a diameter of 8-10 mm. After drying, place the pellets in a rotary kiln for high-temperature calcination at 1400-1450° C., and cool the pellets to obtain high-dosage phosphogypsum cement clinker.

[0029] The chemical compositions of the raw materials used in the specific embodiments of the present invention are shown in Table 1 below:

[0030] Table 1 Main chemical composition of raw materials (wt%)

[0031]

[0032]

[0033] Example 1

[0034] The raw materials of the high-dosage phosphogypsum cement clinker of this embodiment include, by mass, 30 parts of modified phosphogypsum, 55 parts of limestone, 9 parts of mud cake, 2 parts of shale, 2 parts of sandstone, 1 part of steel slag, and 1 part of granulated blast furnace phosphorus slag.

[0035] In the preparation method of high-content phosphogypsum cement clinker in this embodiment, the material balls in step S2 are 8-10 mm, the calcination temperature is 1400° C., and the calcination time is 40 min.

[0036] The modified phosphogypsum in this embodiment is prepared by adding 10 parts of dried fly ash, 10 parts of steel slag, and 20 parts of charcoal to 60 parts of phosphogypsum, grinding the mixture until all the powder passes through a 200 μm square hole sieve, thoroughly mixing the mixture, and then calcining the mixture in a silicon carbide furnace at 950° C. for 25 minutes to obtain the modified phosphogypsum.

[0037] The main chemical components of the modified phosphogypsum and the prepared clinker obtained in this example are:

[0038]

[0039] The KH value of the clinker is 0.931, the n value is 2.41, and the p value is 1.21. The calculated desulfurization efficiency of the modified phosphogypsum is 94.2%.

[0040] Example 2

[0041] The raw materials of the high-dosage phosphogypsum cement clinker of this embodiment include, by mass, 25 parts of modified phosphogypsum, 55 parts of limestone, 10 parts of mud cake, 6 parts of shale, 2 parts of steel slag, 1 part of converter slag, and 1 part of granulated blast furnace phosphorus slag.

[0042] In the preparation method of high-content phosphogypsum cement clinker in this embodiment, the material balls in step S2 are 8-10 mm, the calcination temperature is 1400° C., and the calcination time is 45 min.

[0043] The modified phosphogypsum in this embodiment is prepared by adding 8 parts of dried fly ash, 7 parts of steel slag, and 15 parts of activated carbon to 70 parts of phosphogypsum, grinding the mixture until all the powder passes through a 200 μm square hole sieve, thoroughly mixing the mixture, and then calcining the mixture in a silicon carbide furnace at 1000°C for 20 minutes to obtain the modified phosphogypsum.

[0044] The main chemical components of the modified phosphogypsum and the prepared clinker obtained in this example are:

[0045]

[0046]

[0047] The KH value of the clinker is 0.931, the n value is 2.50, and the p value is 1.31. The calculated desulfurization efficiency of the modified phosphogypsum is 91.5%.

[0048] Example 3

[0049] The raw materials of the high-dosage phosphogypsum cement clinker of this embodiment include, by mass, 35 parts of modified phosphogypsum, 50 parts of limestone, 12 parts of mud cake, 2 parts of shale, and 1 part of granulated blast furnace phosphorus slag.

[0050] In the preparation method of high-dosage phosphogypsum cement clinker in this embodiment, the pellets in step S2 are 8-10 mm in size, the calcination temperature is 1450° C., and the calcination time is 30 minutes.

[0051] The modified phosphogypsum in this embodiment is prepared by adding 10 parts of dried fly ash, 10 parts of steel slag, and 25 parts of charcoal to 55 parts of phosphogypsum, grinding the mixture until all the powder passes through a 200 μm square hole sieve, thoroughly mixing the mixture, and then calcining the mixture in a silicon carbide furnace at 900°C for 30 minutes to obtain the modified phosphogypsum.

[0052] The main chemical components of the modified phosphogypsum and the prepared clinker obtained in this example are:

[0053]

[0054] The KH value of the clinker is 0.925, the n value is 2.17, and the p value is 1.27. The calculated desulfurization efficiency of the modified phosphogypsum is 90.3%.

[0055] Comparative Example 1

[0056] The raw materials of the high-content phosphogypsum cement clinker in this comparative example include, in parts by mass: 55 parts of modified phosphogypsum, 55 parts of limestone, 9 parts of mud cake, 2 parts of shale, 2 parts of sandstone, 1 part of steel slag, and 1 part of granulated blast furnace phosphorus slag.

[0057] The preparation method of the high-dosage phosphogypsum cement clinker in this comparative example is the same as that in Example 1, and the preparation method of the modified phosphogypsum is the same as that in Example 1.

[0058] Comparative Example 2

[0059] The raw materials of the high-content phosphogypsum cement clinker of this comparative example include, in parts by mass: 15 parts of modified phosphogypsum, 55 parts of limestone, 9 parts of mud cake, 2 parts of shale, 2 parts of sandstone, 1 part of steel slag, and 1 part of granulated blast furnace phosphorus slag.

[0060] The preparation method of the high-dosage phosphogypsum cement clinker in this comparative example is the same as that in Example 1, and the preparation method of the modified phosphogypsum is the same as that in Example 1.

[0061] Comparative Example 3

[0062] The raw material ratio of the high-dosage phosphogypsum cement clinker in this comparative example is the same as that in Example 1, and the preparation method of the high-dosage phosphogypsum cement clinker is also the same as that in Example 1.

[0063] The modified phosphogypsum in this comparative example was prepared by adding 10 parts of dried fly ash and 20 parts of charcoal to 60 parts of phosphogypsum, grinding the mixture until it completely passed a 200 μm square mesh sieve, thoroughly mixing the mixture, and calcining the mixture in a silicon carbide furnace at 950°C for 25 minutes to obtain the modified phosphogypsum. This means that no steel slag was used in the preparation of the modified phosphogypsum in this comparative example.

[0064] Comparative Example 4

[0065] The raw material ratio of the high-dosage phosphogypsum cement clinker in this comparative example is the same as that in Example 1, and the preparation method of the high-dosage phosphogypsum cement clinker is also the same as that in Example 1.

[0066] The preparation method of the modified phosphogypsum in this comparative example is as follows: 10 parts of dried fly ash, 10 parts of steel slag, and 20 parts of ordinary coal are added to 60 parts of phosphogypsum, and the powder is ground until all the powder passes through a 200 μm square hole sieve, and the mixture is thoroughly mixed. The mixture is then placed in a silicon carbide furnace at 950° C. and calcined for 25 minutes to obtain the modified phosphogypsum.

[0067] Performance test of cement clinker prepared in application examples 1-3 and comparative examples 1-4

[0068] The high-content phosphogypsum cement clinker prepared in the above Examples 1-3 and Comparative Examples 1-4 was subjected to various performance tests. The compressive strength and flexural strength test methods were determined according to GB / T 17671-2021 "Test method for strength of cement mortar (ISO method)" to measure the 3-day and 28-day flexural strength and compressive strength of the high-content phosphogypsum cement clinker. The loss on ignition, sulfur trioxide, free calcium oxide and other chemical components of the clinker were determined according to GB / T 176-2017 "Cement Chemical Analysis Methods". The standard consistency water requirement, setting time and stability of the high-content phosphogypsum cement clinker were tested according to GB / T 1346-2011 "Test method for water requirement, setting time and stability of cement at standard consistency". The specific results are shown in Table 2 below:

[0069]

[0070]

[0071] Combined with the physical property data in Table 2, analysis is performed: Comparing Examples 1-3 of the present invention with common Portland cement clinker, it can be seen that the sulfur trioxide content of the cement clinker prepared in Examples 1-3 has increased but meets the technical requirements, the initial and final setting times have increased slightly but are within the allowable normal range, and the 3-day and 28-day flexural strengths and 3-day and 28-day compressive strengths are close to or slightly higher than those of normal Portland cement clinker. This shows that when the high-dosage modified phosphogypsum provided by the present invention replaces about 30% of the raw materials, the properties of the produced cement clinker will not be affected, and the low-cost processing and resource recycling of phosphogypsum can be effectively achieved.

[0072] The cement clinkers produced in Comparative Examples 1-4 have different degrees of extension in initial and final setting time compared with normal Portland cement clinker, with the longest extension exceeding 80 minutes. Their 3-day and 28-day flexural strengths and 3-day and 28-day compressive strengths are also reduced. This indicates that the preparation method of the modified phosphogypsum or its dosage or the cement clinker preparation method of Comparative Examples 1-2 and Comparative Examples 3-4 has a great influence on the various properties of the produced cement clinker, resulting in a significant decrease in performance indicators.

[0073] Specifically, comparing the test results of Example 1 and Comparative Examples 1-2, the initial setting time and final setting time of the cement clinker in Comparative Example 1 are greatly increased compared with those in Example 1, and the 3-day and 28-day compressive strengths are significantly reduced, indicating that excessive use of modified phosphate rock will affect the setting time of the cement clinker, which is not conducive to actual use in production. Although the amount of phosphogypsum is significantly reduced in Comparative Example 2, the lime saturation coefficient (KH) of the clinker is too high due to the small proportion of phosphogypsum, and the free calcium is too high, resulting in a significant decrease in the 28-day strength of the cement clinker.

[0074] Comparing the test results of Example 1 and Comparative Example 3, the sulfur trioxide content of the cement clinker produced in Comparative Example 3 is higher than that in Example 1, the 3-day and 28-day flexural strength and the 3-day and 28-day compressive strength of Comparative Example 3 are reduced, and the calcination temperature required in the preparation process of the modified phosphogypsum in Comparative Example 3 is 100°C higher than the calcination temperature in Example 1. This is because steel slag is not used to assist charcoal in modifying the phosphogypsum in Comparative Example 3, which results in a higher calcination temperature to maintain the reaction and the desulfurization reaction is not as sufficient as in Example 1, resulting in a relatively high residual sulfur trioxide content in the modified phosphogypsum, which has an adverse effect on the physical properties of the cement clinker.

[0075] Comparing the test data of Example 1 and Comparative Example 4, the sulfur trioxide content of the cement clinker produced in Comparative Example 4 is slightly higher than that in Example 1, indicating that the use of ordinary coal and steel slag to modify phosphogypsum in Comparative Example 4 will greatly reduce the desulfurization efficiency, thereby affecting the flexural strength and early strength. This may be due to the large difference in physical structure between charcoal and ordinary coal. Coal has a high ash content and a high combustion temperature, so it is easy to form a eutectic with the intermediate product of phosphogypsum, which hinders the continuous decomposition of phosphogypsum. Charcoal or activated carbon has a higher carbon content and better reaction activity, and is more suitable for working with steel slag, which can effectively promote the desulfurization reaction, thereby making the modified phosphogypsum well applied to cement clinker.

[0076] From the above analysis, it can be seen that the present invention uses steel slag and microcrystalline carbon to modify phosphogypsum, and selects the optimal dosage of the modified phosphogypsum through a large number of experiments, which can effectively complete the desulfurization reaction, and the calcination temperature required in the modification process is also lower. The setting time and physical properties of the cement clinker prepared by the method are relatively close to the corresponding properties of ordinary Portland cement clinker, thereby realizing the effective and low-cost treatment and large-scale reuse of phosphogypsum.

[0077] The above specific embodiments describe the implementation of the present invention in detail, but the present invention is not limited to the specific details of the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

Claims

1. A high-dosage phosphogypsum cement clinker, characterized in that: The raw materials of the high-dosage phosphogypsum cement clinker are as follows: 25-35 parts of modified phosphogypsum, 50-55 parts of limestone, 9-12 parts of mud cake, 2-6 parts of clastic rock, 0-2 parts of steel slag, and 1-2 parts of calcium-containing waste residue in parts by mass; The modified phosphogypsum is prepared by adding 8-10 parts of fly ash, 7-10 parts of steel slag, and 15-25 parts of microcrystalline carbon to 55-70 parts of phosphogypsum, grinding the mixture and mixing the mixture evenly, and then calcining the mixture in a silicon carbide furnace for 20-30 minutes to obtain the modified phosphogypsum; the calcination temperature is 900-1000°C; The microcrystalline carbon is at least one of charcoal and activated carbon, and the calcium-containing waste slag is at least one of converter slag and granulated blast furnace phosphorus slag.

2. The high-content phosphogypsum cement clinker according to claim 1, characterized in that: The raw materials are as follows in parts by mass: 30 parts of modified phosphogypsum, 55 parts of limestone, 9 parts of mud cake, 4 parts of clastic rock, 1 part of steel slag, and 1 part of calcium-containing waste residue.

3. The high-content phosphogypsum cement clinker according to claim 1, characterized in that: The clastic rock is at least one of shale and sandstone.

4. A method for preparing high-dosage phosphogypsum cement clinker according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1, drying the limestone, mud cake, clastic rock, steel slag and calcium-containing waste residue in the raw material, mixing them uniformly, and grinding them to obtain a mixture; S2. Evenly mix the mixture obtained in step S1 and the modified phosphogypsum, add water to prepare pellets, dry and calcine at high temperature, and cool to obtain high-dosage phosphogypsum cement clinker.

5. The method for preparing high-content phosphogypsum cement clinker according to claim 4, characterized in that: In step S2, the diameter of the ball is 8-10 mm.

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