Application of water quality analysis instruments in continuous casting circulating water in steelmaking workshop

With the development of steel production, continuous casting technology has been increasingly adopted by steel enterprises, and the continuous casting ratio in China has increased significantly. The continuous casting process eliminates the processes of mold casting and initial rolling. The molten steel flows directly into the crystallizer of the continuous casting machine, causing the liquid metal to cool rapidly. The billet pulled out from the tail of the crystallizer enters the secondary cooling zone, which is composed of rollers and water spray cooling equipment. The main water-consuming processes in continuous casting production include crystallizer cooling water, equipment indirect cooling water, and casting and equipment spray water. The water quality requirements for each process are shown in Table 1.

Continuous casting circulating water can be divided into clean circulating water and turbid circulating water. Among them, the clean circulating water system is mainly used for indirect cooling of the crystallizer and equipment. After use, the water temperature rises, and the water quality is not polluted. It is mainly used for cooling, concentration rate management and water quality stabilization. Turbid circulating water is mainly used for equipment and casting spray cooling, cutting slag granulation water and washing iron oxide scale water. After use, the water temperature rises, the water quality is polluted, and the water contains a large amount of iron oxide scale particles and a small amount of oil. Except for the water used to flush the oxidized iron scale (with low water quality and water temperature requirements), which can be recycled after the first-level precipitation, the rest of the water is generally recycled after the second-level precipitation, filtration, degreasing, and cooling.

Instruments in the clean circulating cooling water use

clean circulating water because of the high water quality requirements, generally use softened water, and the water quality stability problem must be handled well. The corrosion rate is accelerated by using soft water after de-hardening. If the water hardness and alkalinity are small in the area, industrial water can also be used. Table 2 is the control index of the circulating cooling water of the continuous casting crystallizer of a large steel plant.

Since the hardness and alkalinity of clean circulating water are low, it is easy to adsorb cations on the surface of the crystallizer, so it is easy to cause corrosion during the circulation process. In addition, the water temperature near the molten steel in the crystallizer is as high as 100~150℃, which is easy to cause scaling, affecting the heat transfer of the crystallizer, resulting in the inability of the molten steel to form a strong embryo shell, resulting in waste embryos or defective embryos. Therefore, ensuring the quality of clean circulating water is a necessary condition to ensure the production of qualified casting embryos. According to the above control indicators, the online water quality analysis instruments that can be used in continuous casting clean circulating water are shown in Table 3.

Instrument application in turbid circulating cooling water

Turbid circulating water refers to the direct cooling water of equipment and products, mainly refers to the wastewater generated in the secondary cooling zone. A large number of nozzles spray water to the steel billet pulled by the pulling roller, further cooling and solidifying the steel billet. This water is contaminated by heat and contains iron oxide and grease. The water consumption per ton of steel in the secondary cooling zone is generally 0.5~0.8m3. Containing iron oxide, oil and other impurities and high water temperature are the characteristics of secondary cooling water. The requirements for turbid circulating water of continuous casting machines are high, especially for secondary spraying and equipment spray cooling. The suspended solids content in the cooling water is required to be less than 20~50mg/L, and the nozzle diameter is generally less than 2mm. Therefore, whether the selection of water treatment process is reasonable directly affects the normal production of continuous casting. The main purpose of turbid circulating water treatment is to remove turbidity, remove oil, cool down and manage the concentration rate. Turbidity removal is mainly to remove iron oxide produced in the production process, which generally accounts for 0.2%~0.5% of the billet output; the main means of turbidity removal are precipitation and filtration.

The turbid circulating water treatment process generally has the following schemes:

secondary water treatment scheme, that is, primary sedimentation and filtration. The primary iron sheet pit needs to remove iron oxide above 0.2mm. The suspended solids content in the treated water is not more than 60mg/L, which can be used to flush the iron oxide and directly cool the equipment. Although the secondary iron sheet sedimentation tank and corresponding facilities can be omitted, the primary iron sheet pit has a large volume, a deep structure, high construction difficulty and high cost, so it is generally rarely used.

The tertiary water treatment scheme, that is, the scheme of primary sedimentation, secondary sedimentation and filtration. The primary iron sheet pit needs to remove iron oxide above 0.5mm, and the suspended solids content in the effluent is less than 150mg/L, which can be used to flush the iron oxide, and ensure that the water supply pump is not severely worn, so the primary iron sheet pit has a small volume and low cost. Secondary sedimentation usually uses a horizontal flow sedimentation tank, which requires the removal of iron oxide scale above 0.2mm, and the suspended solids in the effluent are less than 60mg/L. It is generally an above-ground structure, which is convenient for construction and production management, and there are oil removal and scraping equipment to match it. Therefore, the three-stage water treatment process is widely used in the continuous casting turbid circulating water system. [page]

According to the above characteristics of turbid circulating water and the treatment process, the online water quality analysis instruments that can be used are shown in Table 4.

Treatment of circulating water in continuous casting workshop

In the continuous casting process, the monitoring of both clean circulating water and turbid circulating water is to avoid corrosion and scaling problems to the greatest extent. Therefore, in addition to the necessary monitoring of water quality, it is also necessary to add corrosion/scale inhibitors to the circulating water to adjust the water quality of the circulating water. The corrosion/scale inhibitors usually used are mainly polyphosphate products, so it is necessary to monitor the phosphorus content in the water to adjust the content of corrosion/scale inhibitors in the circulating water.

Jigang Group currently has a total asset of 22.8 billion yuan, and its main production processes include coking, sintering, ironmaking, steelmaking, and steel rolling. After continuous technological innovation and business transformation in recent years, it has become a leading enterprise in the production and export of thick plates, and its product export ratio, medium and thick plate output, cost, market share, export volume and other indicators are among the best in the country. The group has also passed international certifications for quality, safety and environmental management, and has taken the lead in the industry in achieving three-system integration and marketing service system certification.

Jigang promotes clean production, safe production and the development of a circular economy. The purpose of the steel industry to develop a circular economy is to achieve sustainable development with the least resource consumption and the lowest environmental cost. The key to developing a circular economy is to do a good job in the large, medium and small cycles of industrial materials and energy. According to the above three levels of circulation, the steel industry can implement a circular economy mainly from four aspects: improving the utilization efficiency of iron resources, the recycling rate of energy, the recycling rate of water and the utilization rate of solid waste. Among them, the recycling of water in the steel industry is to establish a multi-level water recycling concept within the process, within the factory and between factories, improve the concentration rate of water circulation, achieve water resource consumption reduction, and reduce the discharge

of industrial wastewater in the circulation system. The main measures to achieve the recycling of water in the steel industry are to adopt efficient, safe and reliable advanced water treatment technologies and processes to improve the recycling rate of water and further reduce the consumption of new water per ton of steel. The use of advanced processes to effectively treat the sewage and other drainage of the circulating water system makes industrial wastewater a resource and achieves zero discharge of industrial wastewater.

With the cooperation of HACH Company in the United States, Jigang Power Plant has established a real-time water quality monitoring system, which provides scientific data for production water supply and improvement of production process, and realizes centralized management and optimized scheduling within the system. The system uses water quality analysis instruments produced by HACH Company, in conjunction with domestic data acquisition units and on-site operator stations. Water quality online analysis instruments are installed before the circulating water is supplied to users according to the water quality requirements and on-site conditions of the water treatment system. For example, the use of instruments such as solid suspended matter Solitax, hardness APA6000, conductivity E53, total phosphorus PhosPhax, alkalinity APA6000, pH P53, etc., plays a vital role in the water supply process of steel production.

Application in the automatic control project of circulating water monitoring The

use of the water quality online monitoring instrument hardness/alkalinity APA6000 in the clean water system of gas power plants can monitor the actual situation of scaling and corrosion of circulating water pipelines to avoid blockage, leakage and pipe burst accidents. For circulating cooling water systems, the serious problem often encountered is scale deposition. If the water quality treatment fails to meet the process requirements for a long time, it will cause pipeline corrosion and leakage, and may also cause blockage, resulting in water delivery difficulties and energy waste. Hardness and alkalinity are important parameters that reflect scaling and corrosion in water. In the past, Jigang relied on manual sampling and monitoring on a regular basis, which could not reflect the water quality of circulating cooling water in a timely manner. Often, when the monitored values ​​changed significantly, scaling and corrosion had already occurred in the pipeline, causing serious delays in the production process. The use

of Hach's hardness/alkalinity APA6000 tester in the system can complete real-time sampling of the instrument and accurately analyze the hardness and alkalinity changes in the clean water system online. The measurement results are displayed in graphical trend curves and digital formats. The SIO module built into the instrument can collect and store detailed historical data for 30 days. Through the APA6000 hardness and alkalinity meter, the water supply workshop can timely grasp the changes in the circulating water quality of the pipeline clean water, and use this as a basis to control the addition of stabilizers, effectively ensuring the normal operation of the gas power plant's power generation equipment.

Realize the automatic control of the dosage of phosphorus corrosion/scaling inhibitors.

Like most domestic circulating cooling water treatment processes, the clean circulating water of gas power plants uses polyphosphorus corrosion/scaling inhibitors to prevent pipe scaling, blockage and corrosion, because phosphorus corrosion/scaling inhibitors have both corrosion and scale inhibition effects and are non-toxic and economical. However, phosphorus corrosion/scaling inhibitors are easy to hydrolyze, and the corrosion/scaling effect is reduced after hydrolysis, and the pipes will be corroded or blocked. However, after increasing the dosage, it is necessary to increase the discharge volume to prevent the orthophosphate generated by hydrolysis from generating calcium phosphate scale, which will also cause damage to the pipes; on the other hand, the larger the dosage, the higher the concentration rate, and the more severe the pipe corrosion will be. Therefore, it is very important to strictly control the phosphorus content in the circulating water.

After installing the total phosphorus PhosPhax analyzer of Hach Company on the dosing system of the clean circulating water, the gas power plant has fully realized the real-time monitoring of the total phosphorus content in the water, reflecting the concentration of phosphorus-based agents in the circulating water. According to the input of the electromagnetic flowmeter, the output module of the PLC controls the dosage of the phosphorus-based corrosion and scale inhibitor in the regulating tank (controls the quantitative dosing pump), forming a closed-loop control. On the one hand, the scaling and corrosion of the pipeline are strictly controlled; on the other hand, due to the use of automatic monitoring and control, a large amount of manual detection time and labor are saved, and the working mode of manual dosing based on experience in the past is completely changed. While improving work efficiency, it also achieves the purpose of saving medicine and electricity, greatly reducing production costs.

Make the circulating water concentration ratio clear at a glance

The concentration ratio indicates the concentration degree of salt in the system. Its value is equal to the ratio of the salt content of the circulating water to the salt content of the supplementary water. It is often calculated based on the concentration of some ions that are not easy to volatilize, not easy to precipitate, and not added during the treatment process, such as Cl-, SO42-, etc. Increasing the concentration ratio of circulating cooling water can reduce the amount of make-up water, thereby saving water resources, reducing the amount of sewage discharged, reducing environmental pollution and the amount of wastewater treated, while increasing the concentration ratio and saving the consumption of water treatment agents. However, too high a concentration ratio will also increase the scaling tendency of water, thereby increasing the difficulty of scaling control. Therefore, controlling a reasonable concentration ratio according to actual conditions is of great significance to the entire circulating water treatment process.

Since it is difficult and expensive to measure Cl- and SO42- online, at the suggestion of Hach Company, Jigang Power Plant uses E53 conductivity analyzer to reflect the concentration ratio of circulating water by measuring the conductivity in water, and the measurement is more convenient and intuitive. This ensures that users are well aware of the concentration ratio of circulating cooling water and can reasonably and timely replenish water and discharge sewage.

Summary

In the continuous casting process of the steelmaking workshop, circulating cooling water is very important. Clean circulating water is mainly used in the indirect cooling water supply of continuous casting crystallizers and equipment. This part of water is in a closed cycle, so the water quality requirements are mainly based on anti-corrosion and anti-scaling. It is necessary to focus on monitoring its pH, conductivity, hardness, alkalinity, suspended solids and other indicators. Turbid circulating water is mainly used in casting and spraying water. It is in direct contact with steel billets and contains a large amount of impurities such as iron oxide and oil. Therefore, reducing turbidity and oil content is the key. To control the concentration multiple, it is necessary to focus on monitoring parameters such as conductivity, suspended solids and oil in water. The water quality analyzers produced by HACH, in conjunction with domestic data acquisition units and on-site operator stations, play a vital role in the production water supply process by online monitoring of important parameters in the water. (end)