Papermaking white mud desulfurization and gypsum crystal regulation and control process

Abstract

The invention discloses a papermaking white mud desulfurization and gypsum crystal regulation and control process. The process comprises the following steps: firstly, enabling coal-fired flue gas to react with white mud slurry through a desulfurization tower to generate CaSO4; the gypsum-containing slurry enters a gypsum crystal regulation and control pool, and the crystal morphology is regulated and controlled through stirring, oxidation air and a plate type catalyst; when the median particle size of the gypsum crystals reaches 30-50 [mu] m, dehydrating, and finally controlling the moisture content of the gypsum within 10%; on the basis that an original desulfurization device (limestone-gypsum method) is fully utilized, the gypsum crystal regulating tank is additionally arranged behind the desulfurization tower, the morphology of the gypsum crystals is regulated and controlled, and the gypsum crystals with the morphology close to a column shape and the median particle size larger than 40 microns are generated through regulation and control. Therefore, the dehydration performance of gypsum thick slurry is remarkably improved, the water content of gypsum is controlled within 10%, and the problem that white mud desulfurized gypsum is difficult to dehydrate is solved.

Description

Technical Field

[0001] The invention relates to the technical field of papermaking, and in particular to a papermaking white mud desulfurization and gypsum crystal regulation process. Background Art

[0002] In the papermaking industry, white mud is mainly formed in the alkali recovery process of the papermaking process. Its main component is CaCO3, which accounts for more than 90% by mass. Since its CaCO3 content is equivalent to that of limestone, white mud can theoretically be used as an absorbent to replace limestone for flue gas desulfurization. Existing applications have shown that due to the small particle size and high activity of white mud, its desulfurization efficiency is slightly higher than that of limestone under the same conditions, which can ensure that the desulfurization effect meets the original design indicators. The use of white mud for flue gas desulfurization can effectively solve the problems of limestone mine resource depletion and environmental pollution caused by mining while saving desulfurization operation costs. It is a typical model for achieving "waste treatment with waste", with extremely significant economic and social benefits, and urgently needs to be further promoted on a large scale.

[0003] However, in existing engineering applications and experiments, the difficulty in dehydrating white mud desulfurization gypsum slurry (CaSO4﹒2H2O) and the high water content of the finished gypsum have been concentrated. The water content of desulfurization gypsum has a great impact on its resource utilization production process. Desulfurization gypsum with high water content is extremely viscous and easy to block in transportation and production equipment. Therefore, some scholars have pointed out that desulfurization gypsum with a water content higher than 10% has no industrial utilization value. The difficulty in dehydrating white mud gypsum has become an important problem restricting the large-scale promotion of white mud desulfurization.

[0004] At present, the mainstream limestone-gypsum process mainly adopts a two-stage gypsum dehydration process. After the gypsum slurry extracted from the desulfurization slurry pool is dehydrated by centrifugation and vacuum suction, the moisture content of the gypsum is reduced to less than 10%, meeting the requirements of industrial gypsum raw materials. Among them, the vacuum suction dehydration process is the final link of gypsum dehydration and is also the key to determining whether the gypsum moisture content can meet industrial standards. During the vacuum suction process, the gypsum crystals are used as filter materials, and their morphological characteristics (size, appearance) directly determine the permeability and resistance of the filter layer, thereby affecting the final dehydration effect. Compared with limestone, white mud has the following two characteristics: First, the soluble salt content in white mud is high, which makes the CaSO3 in the slurry insufficiently oxidized, affecting the formation of gypsum crystals; second, because it is an industrial by-product, the salt composition is complex. In addition to Ca, it also contains Fe, Mg, Al, Cu and other elements. The introduction of impurity elements will change the morphology of gypsum crystals, making the gypsum particle size smaller; the crystal morphology changes from a quadrangular prism to a flake or noodle shape with a higher packing density. Therefore, the essence of the difficulty in dehydrating white mud gypsum is that the impurity components of white mud affect the formation and morphology of gypsum crystals, thereby affecting the vacuum filtration dehydration performance.

[0005] Due to the influence of soluble salts and impurity ions in white mud, the current use of white mud for desulfurization will reduce the size of gypsum crystals and change their shape from quadrangular prisms to flakes and noodles that are difficult to dehydrate. At the same time, the impurities in the white mud will also cause insufficient oxidation of CaSO3 in the slurry, which will also affect the formation of gypsum crystals. Ultimately, it is difficult to generate gypsum crystals that meet the dehydration performance requirements, making it difficult to dehydrate the white mud desulfurization gypsum, and the discharged gypsum has a high moisture content and cannot be used as a resource. Summary of the invention

[0006] The purpose of the present invention is to provide a papermaking white mud desulfurization and gypsum crystal regulation process. Its notable feature is that on the basis of making full use of the original desulfurization device (limestone-gypsum method), a gypsum crystal regulating pool is added after the desulfurization tower to regulate the morphology of the gypsum crystals, and the gypsum crystals with a morphology close to columnar and a median particle size greater than 40um are generated.

[0007] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0008] The present invention comprises the following steps:

[0009] S1: The flue gas discharged from the coal-fired boiler is subjected to desulfurization treatment. The white mud slurry reacts with the SO2 in the flue gas to generate CaSO4, which is then discharged after passing through a demister. The desulfurization treatment is that the flue gas discharged from the coal-fired boiler enters the desulfurization tower and flows from bottom to top. It contacts and reacts with the white mud slurry sprayed from the spray layer in the desulfurization tower to remove the SO2 contained in the flue gas. It then flows through the demister to remove the droplets contained in the flue gas and is then discharged from the outlet of the desulfurization tower and finally discharged into the atmosphere through the chimney.

[0010] S2: The papermaking white mud is prepared into white mud slurry for standby use; when the pH value of the slurry pool drops to the set value, the white mud slurry is sent to the absorber slurry pool for reaction; the prepared white mud slurry has a solid content of 10%-30%. Specifically: the papermaking white mud is first prepared into slurry in the white mud slurry tank, and then sent to the white mud slurry buffer tank for storage and standby use through the white mud delivery pump; when the pH in the desulfurization slurry pool is less than the set lower limit, the white mud slurry pump is started to deliver the white mud slurry to the absorber slurry pool.

[0011] S3: During the desulfurization process, CaCO3 in the white mud slurry reacts with SO2 to generate CaSO4, and the reaction byproducts are forced to be oxidized by the air oxidation device. The density and pH value of the white mud slurry are kept within the set range to prevent fouling and ensure reaction efficiency; specifically: the slurry in the desulfurization slurry pool is transported to the spray layer by the circulating slurry pump and sprayed from the nozzle arranged in the spray layer. The desulfurization slurry pool is equipped with an oxidation air pipe to disperse the air blown by the oxidation fan in the entire desulfurization slurry pool to force the reaction product of SO2 and CaCO3 to be oxidized into CaCO3 and CaSO4; the desulfurization slurry pool is equipped with a stirrer to stir the entire desulfurization slurry pool. During the desulfurization process, the pH value and density in the slurry pool are monitored and the pH value range is set to 4.5-5.2; the maximum slurry density is 1050-1150kg / m3. When the density of the desulfurization slurry pool exceeds the set value, the gypsum discharge pump is started to discharge the gypsum crystals from the desulfurization slurry pool, and the process water and white mud slurry are supplemented at the same time. The number of circulation pumps corresponds to the number of spray layers, which is 3-7 layers.

[0012] S4: The white mud slurry containing gypsum crystals is transported to the gypsum crystal regulation pool, and the morphology of the gypsum crystals is regulated through forced oxidation, stirring and catalyst action; the particle size of the gypsum crystals after regulation is 30-50μm, and the morphology is close to columnar; forced oxidation is carried out by blowing air into the slurry through the oxidation fan, and the amount of air blown into each cubic liquid is 2-5L; at the same time, the agitator is started to prevent the slurry from settling; under the disturbance of the agitator, the slurry and the flat catalyst are promoted to achieve forced catalytic oxidation. The flat catalyst is a modified Co-MS type catalyst, which is made into a plate-like structure during the molding process. The gypsum slurry in the gypsum crystal regulation pool is sampled and analyzed for crystal morphology every 30-60 minutes. If the crystals are still in the form of flakes or noodles, citric acid is added for morphology adjustment. The sampling analysis of crystal morphology is as follows: after drying the analysis sample into gypsum crystal dry powder, using a laser particle size analyzer and a SEM scanning electron microscope to analyze and characterize, quantitatively obtain the particle size distribution of the gypsum crystal and observe the shape structure of the crystal.

[0013] S5: The regulated gypsum slurry undergoes primary dehydration, and then the wastewater enters the treatment system, and the concentrated slurry undergoes secondary dehydration, and finally gypsum with a moisture content controlled within 10% is obtained.

[0014] The beneficial effects of the present invention are:

[0015] The present invention is a papermaking white mud desulfurization and gypsum crystal regulation process. Compared with the prior art, the present invention is to make full use of the original desulfurization device (limestone-gypsum method), add a gypsum crystal regulation pool after the desulfurization tower, regulate the morphology of the gypsum crystals, and regulate the gypsum crystals with a columnar morphology and a median particle size greater than 40um. Thereby significantly improving the dehydration performance of the gypsum slurry, controlling the gypsum moisture content within 10%, and solving the problem of the dehydration of the white mud desulfurization gypsum. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the connection of the process equipment of the present invention;

[0017] Figure 2 : are SEM electron microscope images of the crystal of the present invention, (a) SEM electron microscope image of the crystal before regulation, (b) SEM electron microscope image of the crystal after regulation;

[0018] Figure 3 These are sample diagrams of the present invention; (a) sample before regulation; (b) sample after regulation. DETAILED DESCRIPTION

[0019] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0020] like Figure 1 As shown: the process flow of the present invention is as follows:

[0021] Flue gas: The coal-fired flue gas 10 drawn from the boiler and having completed denitrification and dust removal enters the desulfurization tower 3 from the desulfurization tower inlet 3-9, flows from bottom to top, and contacts and reacts with the white mud slurry sprayed from the spray layer 3-4 in the desulfurization tower 3 to remove the SO2 contained in the flue gas, then flows through the demister 3-5 to remove the droplets contained in the flue gas and then is discharged from the desulfurization tower outlet 3-10, and finally discharged into the atmosphere through the chimney 11.

[0022] Slurry: After the papermaking white mud is transported into the factory, it is first stirred into slurry in the white mud slurry tank 1 by the white mud slurry mixer 1-1, and the solid content of the slurry is 10%-30%; then it is sent to the white mud slurry buffer tank 2 through the white mud delivery pump 1-2 and stirred and stored by the buffer tank stirring paddle 2-1 for standby use; when the pH in the desulfurization slurry tank 3-1 is less than the set lower limit, the white mud slurry pump 2-2 is started to transport the white mud slurry to the desulfurization slurry tank 3-1. The slurry in the desulfurization slurry tank 3-1 is transported to the spray layer through the circulating slurry pump 3-7 and sprayed from the nozzles arranged on the spray layer. The number of circulating pumps corresponds to the number of spray layers, which is determined according to the designed L / G, and the number is generally 3-7 layers. An oxidation air pipe 3-2 is arranged in the desulfurization slurry pool 3-1, which disperses the air blown by the oxidation fan 3-6 in the entire desulfurization slurry pool 3-1 to force the reaction product of SO2 and CaCO3 to be oxidized into CaSO3 and then into CaSO4 (gypsum); a stirrer 3-3 is arranged in the desulfurization slurry pool 3-1 to stir the entire desulfurization slurry pool 3-1 to prevent scaling. During the desulfurization process, the pH value and density in the desulfurization slurry pool 3-1 are monitored. The pH value range is set to 4.5-5.2; the maximum slurry density is 1050-1150kg / m3. When the slurry pool density exceeds the set value, the gypsum discharge pump 3-8 is started to discharge the gypsum crystals from the slurry pool, and process water and white mud slurry are supplemented at the same time.

[0023] The slurry containing gypsum crystals discharged from the desulfurization slurry pool 3-1 enters the gypsum crystal regulating slurry pool 4; after the gypsum crystal slurry enters the equipment, the oxidation fan 4-4 is started, and air is blown into the aeration pipe 4-1 to force oxidation of the calcium sulfite contained in the slurry, and 2-5L of air is blown into each cubic meter of liquid; at the same time, the agitator 4-2 and the horizontal agitator 4-3 are started to prevent the slurry from settling; under the disturbance of the agitator, the contact between the slurry and the flat plate catalyst is promoted to achieve forced catalytic oxidation; the flat plate catalyst is a modified Co-MS type catalyst, which is made into a plate structure during the molding process, and the filling amount in the pool is 0.05-0.15kg (catalyst) / (liquid) m3. Due to the porous structure of the catalyst matrix, it has the ability to absorb Fe 3+ , Fe 2+ 、Al 3+ Mg 2+Functions such as these can reduce the content of impurity ions in the slurry pool, slow down and eliminate the development of the crystal shape towards noodles and flakes. Through the above process flow, the calcium sulfite content in the slurry can be effectively reduced, the content of gypsum crystals in the slurry pool can be increased, and the risk of dehydration difficulties caused by calcium sulfite wrapping the surface of gypsum crystals can be eliminated. The timing starts from the time the slurry enters the regulating pool 4, and the slurry in the regulating pool 4 is sampled and analyzed every 30-60 minutes; the analysis refers to drying the analysis sample into a dry powder of gypsum crystals, and then analyzing and characterizing it using a laser particle size analyzer and a SEM scanning electron microscope to quantitatively obtain the particle size distribution of the gypsum crystals and observe the shape structure of the crystals. When the gypsum crystal particle size grows to 30-50um (measured in D50 particle size) and the shape is close to columnar, the gypsum crystal discharge pump 4-5 is turned on to transport the gypsum slurry to the gypsum cyclone 5 for primary dehydration; the wastewater discharged from the gypsum cyclone 5 is transported to the wastewater cyclone 6 through the wastewater feed pump 5-1 for further liquid-solid separation; the wastewater discharged from the top of the wastewater cyclone 6 enters the wastewater treatment system 8, and the slurry discharged from the bottom enters the gypsum slurry tank 7. The gypsum slurry discharged from the bottom of the cyclone enters the gypsum slurry tank 7 by gravity, and is finally transported to the empty belt dehydrator 9 through the gypsum slurry pump 7-1 for deep dehydration, and the gypsum moisture content is controlled within 10%.

[0024] If after two samplings, the overall crystal morphology is still in the form of thin flakes, noodles, etc., then add citric acid to the slurry pool, and use the promoting effect of citric acid on specific crystal faces to restore the crystal morphology to short columns; the amount of citric acid added should be such that the slurry pH is between 3-4. At the same time, continue to monitor the gypsum crystal morphology according to the set sampling analysis and characterization process. When the gypsum crystal particle size grows to 30-50um (measured in D50 particle size) and the shape is close to columnar, dehydrate the gypsum slurry according to the above-mentioned primary and secondary dehydration steps to obtain a gypsum finished product with a moisture content of less than 10%.

[0025] The regulating tank 4 is provided with a stirrer 4-2, 1-4 of which are arranged as needed; an oxidation air pipe is arranged at a position 0.5-1m from the bottom of the tank, and the oxidation fan 4-4 blows air to disperse it in the entire slurry tank. The stirrer 4-2 is located above the oxidation air pipe; a plate catalyst 4-3 is arranged above the stirrer; the plate catalyst is preferably arranged vertically, with a spacing of 50-100mm between each plate, and 1-3 groups are arranged as needed; the plate catalyst is a Co-MS type catalyst prepared by extrusion molding, which can greatly promote SO3 2- Oxidized to SO4 2- At the same time, due to the porous structure of the catalyst matrix, it has the ability to absorb Fe 3+ , Fe 2+ 、Al 3 + Mg 2+And other functions. During the operation of the slurry pool, slurry samples are regularly drawn, and after drying, the gypsum crystal morphology and particle size are tested; among them, the crystal morphology test adopts SEM electron microscope scanning, and the particle size test adopts laser particle size tester for analysis. The sampling period is 30-60 minutes. According to the sample morphology, reagents such as citric acid can be added to the slurry pool to achieve the regulation of the growth morphology and growth rate of the crystal, and the reagent addition concentration is 10-300ppm. When the gypsum crystal particle size grows to 30-50um (measured in D50 particle size) and the shape is close to columnar. The slurry in the gypsum crystal regulation slurry pool 4 is passed, and the gypsum discharge pump 4-4 is transported to the gypsum cyclone 5 for the first dehydration; the wastewater discharged from the cyclone 5 is transported to the wastewater cyclone 6 through the wastewater feed pump for further liquid-solid separation; the wastewater discharged from the top of the cyclone enters the wastewater treatment system 8, and the concentrated slurry discharged from the bottom enters the gypsum concentrated slurry box 7. The gypsum slurry discharged from the bottom of the cyclone enters the gypsum slurry box 7 by gravity, and is finally transported to the empty belt dehydrator through the gypsum slurry pump 7-1 for deep dehydration to control the moisture content of the gypsum within 10%.

[0026] like Figure 2 As shown in (a), before regulation, affected by the impurity ions of white mud, the crystals are long columnar, the gaps between crystals are small, and the dehydration resistance is large; Figure 2 (b) After the regulation, the crystals are restored to short columns, the crystal morphology is restored to short columns, and the overall size of the crystals is significantly increased (as shown in Table 1), the porosity between the crystals is increased, and it is easier to remove water. It can be seen intuitively from Table 1 that when there is no regulation process, the moisture content of the gypsum is 32.1%, and after adding the regulation process, the moisture content of the gypsum decreases by 9.8%. The above results show that the gypsum regulation process is effective.

[0027] Table 1 Particle size changes before and after adding control process

[0028]

[0029] The on-site experiment also shows that the moisture content in the gypsum has been significantly improved after adjustment. Figure 3 As shown, before adjustment, the gypsum sample after secondary dehydration still contains a lot of water, and all the gypsum particles stick together; in contrast, the gypsum sample after adjustment is basically dusty, with no obvious water.

[0030] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A papermaking white mud desulfurization and gypsum crystal control process, characterized in that: The following steps are involved: S1: The flue gas discharged from the coal-fired boiler is desulfurized, and the white mud slurry reacts with the SO2 in the flue gas to generate CaSO4, which is then discharged after passing through the demister; S2: Papermaking white mud is prepared into white mud slurry for standby use; when the pH value of the slurry pool drops to the set value, the white mud slurry is sent to the absorption tower slurry pool for reaction; S3: During the desulfurization process, CaCO3 in the white mud slurry reacts with SO2 to generate CaSO4, and the reaction by-products are forced to be oxidized by the air oxidation device. The density and pH value of the white mud slurry are kept within the set range to prevent fouling and ensure reaction efficiency; S4: The white mud slurry containing gypsum crystals is transported to the gypsum crystal regulation pool, and the morphology of the gypsum crystals is regulated through forced oxidation, stirring and catalyst action; the particle size of the gypsum crystals after regulation is 30-50 μm, and the morphology is close to columnar; S5: The regulated gypsum slurry undergoes primary dehydration, and then the wastewater enters the treatment system, and the concentrated slurry undergoes secondary dehydration, and finally gypsum with a moisture content controlled within 10% is obtained.

2. The papermaking white mud desulfurization and gypsum crystal control process according to claim 1, characterized in that: In the step S1, the desulfurization treatment is that the flue gas discharged from the coal-fired boiler enters the desulfurization tower and flows from bottom to top, contacts and reacts with the white mud slurry sprayed from the spray layer in the desulfurization tower to remove the SO2 contained in the flue gas, then flows through the demister to remove the droplets contained in the flue gas and then is discharged from the outlet of the desulfurization tower, and finally discharged into the atmosphere through the chimney.

3. The papermaking white mud desulfurization and gypsum crystal control process according to claim 2, characterized in that: The white mud slurry prepared in step S2 has a solid content of 10%-30%.

4. The process for desulfurization of papermaking white mud and regulation of gypsum crystals according to claim 3, characterized in that: The step S2 is specifically as follows: the papermaking white mud is first prepared into a slurry in a white mud slurry tank, and then sent to a white mud slurry buffer tank for storage and standby via a white mud delivery pump; when the pH in the desulfurization slurry pool is lower than the set lower limit, the white mud slurry pump is started to deliver the white mud slurry to the absorption tower slurry pool.

5. The papermaking white mud desulfurization and gypsum crystal regulation process according to claim 4, characterized in that: The step S3 is specifically as follows: the slurry in the desulfurization slurry pool is transported to the spray layer by a circulating slurry pump and sprayed from the nozzle arranged in the spray layer; an oxidation air pipe is arranged in the desulfurization slurry pool to disperse the air blown by the oxidation fan in the entire desulfurization slurry pool to forcibly oxidize the reaction product of SO2 and CaCO3 into CaCO3 and CaSO4; a stirrer is arranged in the desulfurization slurry pool to stir the entire desulfurization slurry pool; during the desulfurization process, the pH value and density in the slurry pool are monitored, and the pH value range is set to 4.5-5.2; the maximum slurry density is 1050-1150kg / m3. When the density of the desulfurization slurry pool exceeds the set value, the gypsum discharge pump is started to discharge the gypsum crystals from the desulfurization slurry pool, and process water and white mud slurry are supplemented at the same time.

6. The papermaking white mud desulfurization and gypsum crystal regulation process according to claim 5, characterized in that: The number of circulation pumps corresponds to the number of spray layers, which is 3-7 layers.

7. The papermaking white mud desulfurization and gypsum crystal regulation process according to claim 6, characterized in that: In the step S4, the forced oxidation is carried out by blowing air into the slurry through an oxidation blower, and the amount of air blown into each cubic meter of liquid is 2-5L; at the same time, the agitator is started to prevent the slurry from settling; under the disturbance of the agitator, the contact between the slurry and the flat catalyst is promoted to achieve forced catalytic oxidation.

8. The papermaking white mud desulfurization and gypsum crystal regulation process according to claim 7, characterized in that: The flat plate catalyst is a modified Co-MS type catalyst, which is formed into a plate-like structure during the molding process.

9. The papermaking white mud desulfurization and gypsum crystal regulation process according to claim 8, characterized in that: The gypsum slurry in the gypsum crystal adjustment pool is sampled every 30-60 minutes to analyze the crystal morphology. If the crystals are still in the form of thin flakes or noodles, citric acid is added to adjust the morphology.

10. The process for desulfurization of papermaking white mud and regulation of gypsum crystals according to claim 9, characterized in that: The sampling analysis of crystal morphology is as follows: after drying the analysis sample into gypsum crystal dry powder, using a laser particle size analyzer and a SEM scanning electron microscope to analyze and characterize, quantitatively obtain the particle size distribution of the gypsum crystal and observe the shape structure of the crystal.

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