introduction
Since the humidifier or waste heat boiler ash contains about 75% CaCO ∨, in the wet flue gas desulfurization process of cement kiln, the humidifier or waste heat boiler ash is used as the desulfurizer. The humidifier or waste heat boiler ash is mixed with process water through metering and transportation to produce kiln ash slurry. The kiln ash slurry reacts inversely with SO ₂ in the flue gas in the absorber, forming CaSO ∨ in the slurry, The formed CaSO ≮ is oxidized by the air blown in by Roots blower to generate CaSO ₄ and generate CaSO ₄ · 2H ₂ O. The gypsum slurry discharge pump pumps the reacted slurry in the absorption tower to the gypsum dehydration system for gypsum dehydration. The moisture content of dehydrated gypsum is generally controlled below 15%. In the whole process of desulfurization reaction, the quality of gypsum is affected by many factors, such as the content of CaCO ≮ in kiln ash, the type and content of impurities in kiln ash, the pH value of slurry, process water quality, amount of oxidation air, reaction time, etc. Desulfurized gypsum is used instead of natural gypsum for cement retarder and other construction industries. Its water content is an important quality indicator. If the water content of gypsum reaches more than 15%, it will have adverse effects on reuse. This paper analyzes the factors that cause abnormal gypsum dehydration in the wet desulfurization system of the whole cement kiln, and provides treatment suggestions for the cement plant.
1、 Gypsum dewatering process flow and operation
The gypsum slurry at the bottom of the desulfurization tower is discharged from the gypsum slurry pump and pumped into the gypsum hydrocyclone. The hydrocyclone has dual functions: pre dehydration of gypsum slurry and classification of gypsum crystal. The gypsum slurry entering the hydrocyclone is suspended in tangential flow to produce centrifugal movement, and the fine particles flow upward from the center of the hydrocyclone to form overflow, which flows into the desulfurization tower sump under normal conditions; Heavy solids in hydrocyclones The particles are thrown to the wall of the hydrocyclone and flow downward to form an underflow with a solid concentration of about 50%. The bottom flow of the gypsum hydrocyclone flows automatically to the feed box of the vacuum belt dehydrator and enters the vacuum belt dehydrator for secondary dehydration.
In May 2018, the flue gas desulfurization system of a cement plant in southern Hunan experienced difficulties in gypsum dehydration. The gypsum moisture content was 45%, the pH value of desulfurization tower slurry was 6.0~6.5, the vacuum degree of the vacuum belt dehydrator vacuum pump was - 30~- 50kPa, and the gypsum on the vacuum belt dehydrator was severely adhered, the gypsum color was darker, and the water content was large. Refer to Table 1 。
Table 1 Chemical Composition Analysis of Desulfurization Gypsum (%)
According to the test data, the increase trend of gypsum water content is consistent with the decrease trend of SO ≮ content in gypsum, which means that the greater the amount of gypsum generated, the easier it is to be dehydrated. The gypsum water content increases, and the content of calcium sulfate and calcium sulfite generated in gypsum decreases; In addition, the greater the content of sulfate generated, the smaller the ratio of residual calcium oxide and SO ≮ content in gypsum.
2、 Causes of Difficulty in Gypsum Dewatering
According to the operation condition of gypsum dewatering system, the following reasons are obtained after checking the factors affecting the water content of gypsum one by one.
2.1 Improper pH control of slurry
The pH of slurry has an important impact on the quality of gypsum. The theoretical pH of slurry should be controlled at 5.2~5.8. In actual desulfurization production, the pH of slurry is on the high side, because higher pH is conducive to the absorption of SO ₂ and is not conducive to the dissolution of CaCO ≮. During the test, the average pH of slurry in the desulfurization tower of the cement plant is 6.2; In addition, due to the lack of experience of the operators, the pH meter was not washed in time during the actual operation, so that the pH meter could not truly reflect the pH value of the slurry in the absorption tower, resulting in excessive addition of CaCCO ≮. Too much limestone enters into gypsum, which reduces the purity of gypsum and makes it difficult to dehydrate gypsum.
2.2 Content of Cl - in desulfurization wastewater exceeds the standard
The Cl - in gypsum slurry mainly comes from HCl in flue gas and process wastewater. The amount of Cl - in wastewater has a significant impact on the dehydration effect of gypsum. With the increase of Cl - concentration, the moisture content of gypsum gradually increases. CI concentration and gypsum moisture content data are shown in Table 2 。
Table 2 Detection Value of Cl - Concentration of Gypsum Grout and Moisture Content of Gypsum
from Table 2 As can be seen, The higher the content of Cl -, the higher the moisture content of gypsum. Because CI - is more ionic than carbonate, Cl - is easy to combine with Ca2+. As CaCI ₂ is stored in the slurry, the concentration of Ca2+in the slurry becomes larger. According to the same ion effect, the dissolution of calcium carbonate is inhibited, which is not conducive to the reaction of SO ₂ in the flue gas with Ca2+in the slurry, and gypsum generation is difficult. In the process of dehydration, because Cl - blocks the water free channel in the gypsum crystal, gypsum will be removed Water becomes difficult. In addition, high concentration of Cl - will affect the crystallization process of gypsum. The crystal structure of gypsum will change, producing more crystal nuclei. The gypsum crystals are diversified and not easy to grow, which is unfavorable for gypsum dehydration.
2.3 Unreasonable amount of oxidation air
The oxidizing air blown in by the slurry of the absorption tower participates in the whole chemical process of flue gas desulfurization, and the O ₂ 2 in the air oxidizes SO ₂ 2 - to SO ₂ 2. With the increase of O ₂ content in the flue gas, the formation of CaSOO ₄ · 2H ₂ O accelerates, reducing the content of SO ₂ 2 - in gypsum, thus improving the quality of gypsum. In the actual production process, due to the phenomenon that the inlet filter screen of the oxidation fan is blocked or the outlet pipeline is blocked and not found in time, the amount of air blown by the oxidation fan into the slurry of the absorption tower is insufficient. SO ≮ 2-Incomplete oxidation As a result, the gypsum slurry has a high content of SO ≮ 2 -, and CaSO ₂ · 1/2H ₂ O is difficult to dehydrate due to the small gypsum particles and high viscosity, which makes it difficult for the vacuum belt dehydrator to dehydrate.
2.4 Desulfurizer quality not up to standard
CaCO ≮ content and fineness in desulfurizer are the key indicators of desulfurization Standard, the increase of impurities or the decrease of CaCO ≮ content will seriously reduce the slurry quality Sometimes, the fineness of CaCO ≮ is not enough or the content of magnesium oxide and other impurities is high, which will reduce the reaction activity of CaCO ∨, resulting in excessive slurry supply to the absorption tower. CaCO ∨ and gypsum particles stick together, affecting the crystallization process of gypsum. The capillary water carried between gypsum crystal particles increases, and the overall size of gypsum particles is small, causing difficulty in gypsum dehydration.
2.5 Dehydration equipment abnormality
After the dehydration system has been running for a long time, scaling and abrasion are easy to occur inside the hydrocyclone and the sand settling nozzle, which makes the separation efficiency of the hydrocyclone worse. The water content of the underflow slurry entering the vacuum belt dehydrator increases, further causing the difficulty in dewatering the vacuum belt dehydrator.
The vacuum belt dehydrator is an important equipment for the secondary dehydration of gypsum. The filter cloth of the dehydrator becomes less clean after long-term operation due to unreasonable washing, or the small and medium-sized gypsum crystals in the slurry increase or the impurity content in the slurry changes due to the change of the nature of the gypsum slurry itself, which causes the filter cloth filter channel to be blocked, making the water in the slurry difficult It is difficult to dewater gypsum because it is separated from the pores of filter cloth.
3、 Control countermeasures
3.1 Strengthen the maintenance and calibration of instruments
The instrument is the key to control the desulfurization process indicators, which is related to the desulfurization efficiency of the desulfurization system and up to standard emissions. At least every Check and maintain the online instruments every week, wash and calibrate the pressure transmitter and pH meter in time, and try to avoid the difficulty of gypsum dehydration caused by the distortion of slurry pH value in the desulfurization tower. The pH value of absorption tower slurry is a key process index to ensure the desulfurization efficiency and gypsum dehydration. When pH<5.2, it is difficult to ensure the desulfurization efficiency, and the corrosion and wear of absorption tower slurry on slurry pipeline cannot be ignored. When the pH of the slurry of the absorption tower is more than 5.8, the gypsum slurry contains unreacted CaCO ≮, which aggravates the wear and blockage of the pipeline. During the actual operation of desulfurization system, the slurry pH of absorption tower is controlled at 5.2~5.8.
3.2 Control of Cl - content in wastewater
There is no waste water in limestone gypsum desulfurization system of cement kiln The waste water can only be discharged to the grate cooler or raw mill in a small amount, which aggravates the high content of CI - in the desulfurized gypsum slurry. In order to control the difficulty of gypsum dehydration caused by high CI content, the external discharge of system wastewater shall be increased as much as possible. The slurry of absorption tower shall be replaced once a week, and the CI content in desulfurization slurry shall be reasonably controlled to be less than 109/L.
3.3 Ensure reasonable oxidation air volume
Always pay attention to the current and pressure of roots blower, if the current and If the pressure is abnormal, it is estimated that the oxidation air duct has been severely scaled. Clean the oxidation air duct network in time, reduce the limestone slurry supply and gypsum slurry discharge, reduce the pH control requirements, and temporarily control the slurry pH of the absorption tower at 4.8~5.5.
3.4 Strictly control the quality of desulfurizer
Wet desulfurization and desulfurizing agent in cement plant uses humidification tower or waste heat boiler ash, and the content of CaCO ≮ in humidification tower and waste heat boiler ash fluctuates relatively With large amount of impurities, a strict desulfurizer management system should be developed to ensure that the CaCO ≮ content of desulfurizer humidifier tower or waste heat boiler ash is ≥ 75%, and control that the 80 μ m sieve residue of desulfurizer humidifier tower or waste heat boiler ash is less than 10%.
3.5 Strengthen patrol inspection and maintenance of dehydration equipment
Strengthen the patrol inspection of gypsum hydrocyclone, and check the liquid flow state at the bottom flow port of gypsum hydrocyclone. The desulfurization effect is best when the spray from the grit chamber is in the form of mist, and the desulfurization effect has become worse when it is close to the DC. At this time, the grit chamber should be replaced, and the concentration of gypsum slurry bottom flow of the hydrocyclone can also be measured. If the concentration of gypsum slurry bottom flow is less than 50%, the hydrocyclone should be replaced. Secondly, the flushing time should be prolonged after the gypsum cyclone is shut down To prevent the slurry from settling and scaling in the hydrocyclone.
For the vacuum belt dehydrator, pay attention to the condition of the filter cloth. After the filter cloth is blocked, its dewatering effect becomes worse; Secondly, maintain a stable pressure of filter cloth flushing water, and timely clean up the sundries in the vacuum tank and pipeline in case of abnormal vacuum degree.
4、 Conclusion
There are many reasons that affect the difficulty of gypsum dewatering, such as pH value of slurry in absorption tower, Cl - content in desulfurization wastewater, amount of oxidized air, quality of desulfurizer, abnormality of gypsum dewatering system equipment and other factors Mutual restriction and influence; In the process of wet FGD operation in cement plant, gypsum dehydration is difficult, so the quality of FGD gypsum should be controlled by comprehensively considering various influencing factors in combination with the actual working conditions. At the same time, improve the operation level, and summarize experience and analyze reasons in the actual operation process to ensure the long-term stable operation of the entire desulfurization system.
Authors: Sang Shenghuan, Yan Wei, Luo Chao, Xu Lianchun, Wu Tao
Source: Nanjing Kaisheng International Engineering Co., Ltd
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