Process route selection of environmental friendly blast furnace water slag system

　　The water slag system is an important part of the blast furnace ironmaking process. With the increasingly stringent environmental protection policies of local governments, the environmental protection problems of the water slag system have become increasingly prominent. In particular, the environmental protection departments of some local governments clearly require that the white plume generated by the blast furnace water slag system be treated. Therefore, the water slag system has gradually become the focus of environmental treatment of iron and steel enterprises. In recent years, with the transformation and upgrading of iron and steel enterprises, the elimination of backward and excess capacity, and the realization of green and low-carbon production, many iron and steel enterprises have built new modern blast furnaces through capacity replacement. In the construction of new blast furnace systems, they are bound to face the problem of how to choose the water slag system process. Many iron and steel enterprises have the following Puzzles: how to balance production operations, operation and maintenance costs, investment and land occupation Especially, on the premise of meeting the environmental protection requirements and other comprehensive factors, the most suitable water slag system process for the enterprise shall be selected. Through the comparative analysis of different water slag system processes, this paper hopes to provide useful help for iron and steel enterprises to reasonably select water slag system processes.

　　Process and operation characteristics of typical blast furnace water slag system

　　Typical blast furnace water slag system process can be divided into two types according to different ways of slag and water separation: one is sedimentation filtration method, including slag pool type, hydraulic transmission sedimentation tank, bottom filtration method, Lhasa method, etc; The second is mechanical filtration method, including drum filter method, wheel method, stirring cage method, disc method, etc. At present, among the above two methods, the bottom filtration method is the most widely used by precipitation filtration method, and the drum filter method is the most widely used by mechanical filtration method. These two methods are the most widely used and dominant in modern blast furnace system. This paper mainly introduces two methods: bottom filtration method and drum filter method.

　　Bottom filtration

　　The slag water mixture flows into the sediment pool, and the grab crane is used to catch the slag. The water in the slag pool is drained through the filter layer at the bottom of the slag pool. The bottom filtration method of water slag process is most widely used in small and medium-sized blast furnaces below 1500m3.

　　Advantages of bottom filtration

　　1) High operation rate: there is no mechanical equipment in the on-line facilities of the bottom filtration method, so the equipment failure rate is low and the operation rate is relatively high, but the filter material of the filter layer of the slag pool needs to be replaced regularly.

　　2) The circulating water of the system is clean: the slag water separation is realized through the filter layer, and the slag water separation effect is good. Therefore, the circulating water quality is good. The slag flushing pump can use a clean water pump with low maintenance cost. The circulating water can be more convenient to realize waste heat recovery and utilization, and the heat exchanger is not easy to be blocked.

　　3) Low maintenance cost: there is almost no mechanical equipment in the slag water separation main line, and the equipment maintenance cost is very low.

　　Disadvantages of bottom filtration method

　　1) It covers a large area, which is usually more than twice that of the drum mechanical method.

　　2) Poor environment, disordered steam discharge, harmful to human health.

　　The steam generated by slag flushing contains a variety of harmful elements and fine suspended solids, mainly composed of H2S, cyanide and steam. H2S is a flammable colorless gas. Repeated long-term inhalation will cause anosmia, neurasthenia syndrome and autonomic nerve dysfunction.

　　3) The steam causes great damage to the surrounding facilities. The steam contains H2S gas, which is weakly acidic and corrosive to steel equipment. The equipment has been in a humid, hot and weak acid environment for a long time, accelerating the corrosion aging of the equipment and frequent circuit failures.

　　4) The degree of automation is low, and the grab crane needs to be used to catch the slag. The water slag often needs to be transported twice, which increases the production cost of forklift and transport vehicle.

　　5) The working efficiency of grab bucket is low, especially for large blast furnaces. With the increase of blast furnace capacity, the slag removal time left for the bottom filtration water slag system is becoming less and less, but the amount of slag treated is becoming larger and larger. The manual slag grabbing and grab bucket equipment are increasingly unable to adapt to their working conditions, the labor intensity of workers is increasing, and the production risk is also increasing.

　　6) The filter is hardened, and the blast furnace slag has the characteristics of hydraulic gel, resulting in the hardening of fine fibers in the mixture of filter material and slag water, which seriously affects the water filtration performance of the filter layer. The long-term cyclic enrichment causes more and more gel substances, which is easy to cause hardening and block the filter layer.

　　7) High requirements on the quality of make-up water: due to environmental protection requirements, industrial wastewater is not allowed to be directly discharged. Therefore, many enterprises' industrial wastewater provide feed slag system as production make-up water, mainly from coking water and desulfurization water. The phenol cyanogen organic compounds in coking water can react with many acidic substances to form gel substances. These gel substances and water slag can cause scaling. The high content of suspended solids in desulfurized water reduces the filtering effect of the filter layer and accelerates the formation of hardening. It can be seen that industrial wastewater is not suitable to be used as the make-up water for the bottom filtration water slag system. The service life of the filter layer in the normal slag pond is generally 1.5~2a. If industrial wastewater is used, the service life of the filter layer is only 0.5~1a. Therefore, in order to improve the service life of the filter layer, many enterprises have to use industrial fresh water as the make-up water for the water slag system, which increases the production cost.

　　8) It takes a long time to replace the filter material. Due to the hardening of the filter layer in the slag pond, it is particularly difficult to replace the filter layer, which takes a long time, generally 15~20d.

　　Drum filter method

　　The blast furnace slag is granulated and cooled by the high-speed water flow ejected from the granulation box. The slag water mixture is separated from the slag water in the drum filter. With the rotation of the drum filter, the dehydrated slag directly falls onto the belt conveyor and is transported to the slag product tank for storage. The filtered water is recycled.

　　Advantages of drum method

　　1) The floor area is small. Because there is no filter, there is only a small sedimentation tank, which greatly reduces the floor area.

　　2) High degree of automation, fully unattended, water slag filtration and transportation are integrated, and the filtered water slag is automatically transported by the belt conveyor. The rotating speed of the dewatering drum can be automatically adjusted according to the slag flow.

　　3) There are few mechanical equipment and a small amount of maintenance work.

　　4) The environmental protection effect is good. Compared with the bottom filtration process, the rotary drum process basically realizes full sealing, and the steam generated during slag flushing has a good collection effect. It can be centrally discharged through the chimney, greatly improving the environment in front of the furnace.

　　5) The dehydration drum process equipment is mature and suitable for different blast furnace types. At present, it has been applied to 5500m3 blast furnace.

　　Disadvantages of drum method

　　1) The filter screen of the dehydrator of the rotary drum method needs to be replaced after wear. In order not to affect the production, it is generally carried out during the two slag discharge intervals, which requires a high level of operation and proficiency of workers.

　　2) As the metal filter screen is used for slag water separation in the dehydrator, the filtration precision is low, and the suspended solids in the filtered water are relatively high. Therefore, the clean water pump cannot be used as the slag flushing pump, and the slurry pump needs to be used, which increases the investment and maintenance costs.

　　3) Due to the poor water quality after filtration, it is difficult to flush the slag water heating process, causing the heating process water filter to be easily blocked. The rotary drum slag flushing water needs a strict filtration process to be used for slag flushing water heating.

　　Water slag white plume treatment and environmental friendly water slag treatment process with the improvement of environmental treatment requirements, environmental friendly water slag treatment process has become the necessary configuration of blast furnace water slag system at present. The key object of environmental protection treatment of water slag system is the white plume generated during slag flushing.

　　Water slag white feather

　　Formation of white plume of water slag

　　A large amount of wet air with high humidity or flue gas with high moisture content is produced in the slag flushing system of iron making blast furnace. During the discharge process, the cold air is encountered to rapidly release heat and cool down. Part of the steam water in the wet air and flue gas condenses into small water droplets, which appear as white plumes (referred to as white plumes) under visible light and sunlight. In terms of thermodynamic mechanism, white plume refers to the rapid heat and mass transfer process after the unsaturated (no white plume) or saturated (critical point) wet air and wet flue gas are mixed with air. When the wet air and wet flue gas enter the supercooling state, white plume appears, as shown in Figure 1.

　

　　Harm of white feather

　　1) In winter, the production water slag system will produce a large number of white plumes, especially in the bottom filtration water slag process. The white plumes are discharged unorganized and diffuse everywhere, which has a great impact on the production environment of the blast furnace casting yard, and the operator's vision is greatly affected, resulting in potential safety hazards.

　　2) Baiyu is highly corrosive to the buildings and industrial pipelines around the cast iron yard. It is often necessary to replace the steel structure external wall sealing plate and industrial pipelines of the cast iron yard in about three years, which increases the production and maintenance cost.

　　3) The visual effect is poor, which destroys the aesthetics of the blast furnace system and greatly affects the effect of environmental protection production. Therefore, it is in urgent need of treatment.

　　How to eliminate white feather

　　In terms of mechanism, the treatment of white plume is to prevent the state point trajectory of the mixed gas from entering the visible area of white plume below the saturation line during the heat and mass transfer process after the wet flue gas is mixed with air. It can be seen from Figure 2 that the humidity of the treated flue gas operating point is less than the exhaust point, and the temperature is higher than the exhaust point. Therefore, the measures to eliminate white feather are dehumidification and heating. At present, the whitening process has been applied in the northern region, and achieved good environmental protection results, improving the operating environment in front of the blast furnace.

　　

　　Environmental protection type bottom filtration slag treatment process

　　On the basis of the traditional bottom filter slag treatment process, an environmental friendly bottom filter slag treatment process has been gradually developed, mainly through the following environmental protection improvement measures: a granulating tower is designed at the slag washing point of the slag melting ditch to fully collect a large amount of steam at the slag washing point, shorten the slag washing ditch, reduce the flow of steam, close the bottom filter, design a cooling tower for the slag washing water, reduce the temperature of the slag washing water, and reduce the evaporation capacity of the slag washing water, All steam generating points shall be collected for whitening treatment after being closed. After the adoption of environmental protection measures such as whitening process, the production environment of the bottom filter slag treatment process has been greatly improved. There is basically no visual white plume in summer, and the production environment has also been greatly improved in winter. However, the environment-friendly bottom filter process still has the problem of white plume flowing in winter, mainly in the bottom filter, which is still inevitable when grasping the slag, This is the inevitable defect of the bottom filtration process that the grab crane is used to manually grasp the slag at present.

　　Environmental friendly rotary drum slag treatment process

　　Compared with the bottom filtration process, the rotary drum slag treatment process has inherent advantages in environmental protection. However, in winter, local steam is diffused. At the same time, a large number of white plumes are discharged from the chimney above the dehydrator, resulting in poor visual effect. Specific environmental protection measures for the environmental protection rotary drum method: the cooling tower is designed for the slag flushing water to reduce the temperature of the slag flushing water and the evaporation capacity of the slag flushing water. At the same time, in terms of the existing good sealing effect, the rotary drum method further improves the unified collection of water steam generating points after sealing for whitening treatment, so as to completely eliminate the visual white plume. After adopting such environmental protection measures as whitening, the rotary drum method slag treatment process has a better environmental protection effect.

　　It can be seen from table 1 that the two water slag processes have their own advantages and disadvantages. Due to the small investment gap between the two processes at present, the existing enterprises mostly choose the slag treatment process based on the production operation rate and the continuity of production operation. However, under the new form of environmental protection, how to adapt to the environmental protection policies of the national and local governments and meet the survival and development needs of enterprises is a problem faced by iron and steel enterprises.

　　

　　Selection of environmental friendly slag treatment process

　　It is suggested that iron and steel enterprises choose slag treatment process according to the following principles:

　　1) Selection of slag treatment process for large blast furnaces (with an effective volume of more than 2500m3): it is recommended to select the rotary drum slag treatment process. The large blast furnace has 3~4 tapholes, so it is necessary to design two sets of slag treatment systems, select the rotary drum slag treatment process, give full play to the small floor area of the rotary drum slag treatment system, and realize the compact and beautiful process layout in front of the furnace. Large blast furnaces have high process equipment configuration and high automation requirements. The slag treatment process by mechanical drum method represents the development direction of ironmaking technology. However, the bottom filtration method is not suitable for the process layout of large blast furnaces, which is reflected in the large floor area, crowded process layout in front of the furnace, low automation, poor environmental protection facilities, and does not match the high standard configuration of large blast furnaces.

　　2) Selection of slag treatment process for iron and steel enterprises in the South: the south is a non heating area, and the demand for slag flushing water heating is not large. Therefore, the rotary drum slag treatment process is preferred, which will not be limited by the quality of slag flushing. The slag flushing water is mainly used for heating in northern winter. The heating water has certain requirements for water quality. If the impurities in the water do not meet the standard, the impurities in the water are easy to deposit and block the pipes, valves and heat exchangers in the heating system. At present, the bottom filtration method is adopted for the heating process. After being filtered by the water slag itself and the pebbles, it enters the filter to filter out the small suspended solids, and then enters the main heating pipeline. From the construction point of view, the south is rich in water resources, the groundwater level is relatively shallow, the slag pool of the bottom filtration method covers a large area, and the underground foundation is deep, which will lead to an increase in the cost of civil construction (waterproof support). The bottom filtration method is uneconomical in the south.

　　3) Selection of slag treatment process for iron and steel enterprises in northern China: if the local government has strict requirements on white plume treatment of water slag system, the rotary drum slag treatment process is preferred. Especially for iron and steel enterprises in northern China, it is difficult to control white plume in winter, while the rotary drum slag treatment process has better whitening effect and is easier to implement. If a proper amount of white feather is allowed to be discharged, the environment-friendly bottom filtration process can be selected, which can give full play to the advantages of bottom filtration process suitable for slag flushing water heating on the premise of meeting the requirements of white feather treatment.

　　Concluding remarks

　　1) The water quenching treatment of blast furnace slag is still the main treatment method of blast furnace slag at present. Therefore, under the increasingly strict environmental protection situation, whether it is the bottom filtration method or the mechanical drum method, it is necessary to further improve the environmental protection design and realize green production. Through the efforts of ironmaking workers, the environmental protection improvement of high furnace water slag system has made great technical progress.

　　2) For large blast furnaces (with an effective volume of more than 2500m3), it is recommended to choose the rotary drum slag treatment process, which has a high level of automation and small land occupation; It is suggested to choose the rotary drum slag treatment process in southern China because the demand for slag flushing water heating is not large and will not be limited by the quality of slag flushing water, and the construction cost of rotary drum method is low; According to the requirements of local government for white plume treatment of water slag system and the requirements of slag flushing water heating process, the appropriate water slag treatment process is selected in northern China.

　　3) Actively develop new treatment forms of blast furnace slag, replace traditional water quenching with dry method, completely eliminate the production disadvantages of water quenching method, and strive to recover sensible heat of blast furnace slag and improve energy recovery efficiency.