Water plays a vital role in electric furnace steelmaking and continuous casting production. The continuous casting process of electric furnaces is meticulous, the heat exchange temperature is high, and the system structure is simple, which requires high water quality for cooling water. As early as the 1970s and 1980s, the steel industry implemented extensive management of production water. Electric furnace continuous casting generally underwent direct cooling or low circulation rate operation. In addition, the water treatment technology was relatively backward at that time, which not only caused the waste of water resources, but also the scaling problem of high-temperature heat exchange equipment such as continuous casting crystallizers and electric furnace bottom electrodes was particularly prominent, which seriously affected steel production and steel quality. With the increasing attention of enterprises to water quality management and the continuous advancement of water treatment technology, the cooling water treatment of electric furnace continuous casting has gradually moved towards a virtuous cycle of high concentration rate and high-quality operation.
1 Production process and cooling water for continuous casting of electric furnaces
At present, ultra-high power AC arc furnaces and DC arc furnaces are commonly used, equipped with advanced multi-stream small square billets, large square billets, slabs, and H-shaped billet continuous casting machines. The general process flow of electric furnace continuous casting is as follows:
Scrap steel is melted by high-voltage electric arc in the furnace, and after oxidation dephosphorization, argon blowing temperature adjustment, vacuum degassing and qualified refining outside the furnace, it is poured out and poured. The molten steel passes through the water-cooled crystallizer, and a chill layer is formed around it, which becomes a ladle water-type billet. When it is pulled out of the crystallizer, it is sprayed with high-pressure water to cool it. After secondary cooling, the billet is completely solidified and formed, and then cut into the required length by the cutting machine to make a finished product.
A large amount of cooling water is required in the production of electric furnace continuous casting, which can generally be divided into two parts: one is indirect cooling water, which is mainly used for equipment cooling (including electric furnace shell, furnace cover, flue, bottom electrode, oxygen gun, hydraulic air conditioner and care equipment) and the initial cooling of the crystallizer; the other is direct cooling water, also known as secondary cooling spray, which is used to cool the ladle water-type billet after the initial cooling of the crystallizer.
2 Indirect cooling water
2.1 Water for cooling the continuous casting tube
The crystallizer is the most critical equipment in the production process of electric furnace continuous casting. The quality of its cooling water directly affects the quality of the product and the continuous production of the casting machine. The cooling water of the continuous casting tube has the following characteristics:
1) High heat exchange intensity. The surface temperature of the copper sleeve of the crystallizer is as high as 1200℃, and the heat transfer intensity is 84X10sKJ/m2.h, which is dozens of times that of general heat exchange equipment.
2) High water flow rate. The water temperature at the interface between the cooling water and the molten steel side is about 100℃. In order to prevent the vaporization of water, it is necessary to maintain a high flow rate, generally up to 6-10 m/s.
3) Small gap. Usually, the water gap thickness of the crystallizer jacket is about 4mm. Once scale is formed, it will cause blockage immediately.
4) Simple material, easy to produce galvanic corrosion.
The crystallizer is a high heat load equipment. To prevent scaling, the supplementary water is usually softened first, and then the water system is designed according to the characteristics of the continuous casting machine. Domestic steel design institutes have adopted the following three design schemes for the cooling water of the crystallizer.
2.1.1 Soft water closed circulation
Soft water closed circulation is the most commonly used design scheme at present. The continuous casting crystallizer is subjected to a closed circulation of softened water (or desalted water), which is reused after cooling by a plate heat exchanger. Typical examples include the third steelmaking plant of Shanghai No. 5 Steel Plant, Ma Steel, Xinjiang Bayi Steel Plant, and Hangzhou Steel Electric Furnace. The characteristics of this scheme are small amount of softened water and simple water treatment. The disadvantage is that due to the addition of a plate heat exchanger, the circulating water volume increases and the operating power consumption is high.
2.1.2 Soft water open circulation
The continuous casting crystallizer and equipment cooling are combined into a large net circulation system, supplemented with softened water, and open circulation. The design of Jiangyin Xingcheng Steel Company by the North Steel Institute adopted this scheme. The characteristics of this scheme are centralized cooling water system, convenient on-site treatment, small circulation volume, and disadvantages are large consumption of softened water and difficulty in water quality treatment.
2.1.3 Semi-softened water open circulation
This scheme divides the continuous casting crystallizer and equipment cooling water into two independent net circulation systems, supplements semi-softened water, and open circulation. The water hardness of the crystallizer is generally controlled below 40dh.
Huaiyin Iron and Steel Company, Jiangsu Shagang Group, Sugang Plant, etc. accept this scheme. The characteristics of this scheme are small circulation volume and low operating power consumption. The disadvantage is that the system is decentralized and on-site treatment is difficult. In addition, due to the residual local hardness in the water, the scaling trend increases, and the difficulty of water quality treatment increases.
2.2 Equipment cooling water
The equipment cooling water system is relatively simple, and the heat exchange temperature of its main heat exchange equipment (such as electric furnace shell, furnace cover, flue) is 400-500℃, and the heat exchange temperature of air conditioning and hydraulic equipment is lower. Usually this is designed as an open net circulation water system, supplemented with semi-softened water or industrial water. The bottom electrode of the electric furnace has high requirements for the conductivity of the cooling water (<400μs/cm), so it is generally cooled directly with softened water (or demineralized water).
3 Direct cooling water (secondary cooling spray)
Secondary cooling spray is turbid circulating water. Secondary cooling heat transfer is the main link in the entire continuous casting process, accounting for 60% of the total solidification heat transfer of the billet. During the cooling process of the billet, the water is contaminated by oil and iron oxides on the surface of the billet. The suspended matter and oil content in the water are high. The water is recovered, precipitated, and filtered at high speed before cooling and reuse. Secondary cooling spray generally bears industrial water or sewage from the clean circulation system. After evaporation and concentration, the pH value rises, the scaling ions increase, and the high suspended matter and oil in the water are easily deposited at high temperatures, causing nozzle blockage. The iron filings in the water are also easy to cause wear to the nozzle. In addition, the high content of iron ions and oil in the return water is conducive to the reproduction of anaerobic bacteria such as iron bacteria and sulfate-reducing bacteria. Long-term operation will cause the water quality to become black and smelly.
Table: Commonly used water quality design values for each system
| Item | CMT | Equipment cooling water | Secondary cooling spray | ||
| Soft water closed type | Soft water open type | Semi-softened water open type | |||
| PH value | 7-9 | 7-9 | 7-9 | 7-9 | 7-9 |
| Carbonate hardness mg/L (CaC03) | <20 | <20 | <80 | <150 | <300 |
| Suspended solids mg/L | <20 | <20 | <20 | <20 | <60 |
| Chloride ion mg/L | <300 | <300 | <300 | <300 | <400 |
| Total iron mg/L | <3.0 | <3.0 | <3.0 | <3.0 | <6.0 |
| Oil content mg/L | <0.5 | <0.5 | <0.5 | <0.5 | <10 |
4 Water treatment scheme for continuous casting in electric furnaces
4.1 Continuous casting tube cooling water system
The main water quality obstacles in the crystallizer system are scaling and corrosion. The scaling factors mainly come from the following three aspects: hardness in water (CaCO scale), deposition of water treatment agents (organic calcium phosphate, zinc scale) and deposition of corrosion products (Fe2O3). Due to the widespread acceptance of softened water and semi-softened water, the former two have been definitely mastered. However, the content of divalent metal ions in softened water is low, the pH value is low, and the corrosiveness is higher than that of industrial water. In addition, the system’s care facilities and water pipelines are all carbon steel. If the corrosion inhibition does not meet the requirements, the deposition of Fe2O3 will become the main scaling factor. In addition, the biological slime caused by microbial reproduction will be deposited in the high temperature and low flow rate area, hindering heat transfer and accelerating corrosion under scale.
There are many kinds of water treatment agents used in continuous casting crystallizers. Commonly used corrosion and scale inhibitors include molybdates, nitrites, phosphates, organic phosphates, zinc salts, organic amines, BTA, acrylic acid copolymers, etc., and bactericides and algaecides include chloramine, active bromine, isothiazolinone, quaternary ammonium salts, etc. When choosing a water treatment solution, the principles of applicability, compatibility, and economy should be followed.
Molybdate has the characteristics of strong temperature resistance, good corrosion inhibition effect, and long lasting effect. It is suitable for closed water system treatment, but due to its high concentration and high price, it is rarely used alone. If it is used in combination with nitrite, BTA, polymers, etc., it can improve the corrosion and scale inhibition effect and reduce the treatment cost. Although nitrite is definitely toxic, it will not cause too much environmental pollution when used in closed water systems.
Phosphates and organic phosphates are generally used in combination with zinc salts, BTA, and polymers. However, since phosphates are easily hydrolyzed under high temperature and long-term residence conditions, they are not suitable for use in closed systems. When used in open system treatment, the concentration of Ca2+ in the water must be strictly controlled to prevent the formation of calcium phosphate and organic calcium phosphate. In the system supplemented with semi-softened water, the concentration of phosphorus and zinc must be strictly controlled. A steel plant in Jiangsu Province had a problem with excessive concentration of reagents in the water, which caused phosphorus and zinc scaling.
The selection of bactericides also plays a key role in the treatment effect. The corresponding bactericide varieties are selected according to different water quality conditions. The closed-loop circulating water system is not suitable for chlorine-containing bactericides due to its long residence time and strong accumulation. Usually, non-oxidizing bactericides with long effective time and no foaming, such as isothiazolinone and organic aldehydes, are selected. When using nitrite for corrosion inhibition, bactericides that are effective against nitrifying bacteria and nitrite bacteria must be selected. The open system can withstand the sterilization method that combines oxidizing and non-oxidizing agents. Common sterilization schemes include chlorine + quaternary ammonium salt, bromine + quaternary ammonium salt.
4.2 Equipment cooling water system
Equipment cooling water is a commonly used open circulating cooling water. The main water quality obstacles are scaling, corrosion and microbial reproduction. The commonly used scale and corrosion inhibition formula is: organic phosphonic acid + polymer + zinc salt + copper corrosion inhibitor. This formula has the characteristics of good treatment effect, strong operability and economy. Sterilization and algae removal treatment can withstand the above oxidizing and non-oxidizing biocides.
4.3 Secondary cooling spray water system
Poor water quality treatment of secondary cooling spray can easily cause scaling, blockage and wear of nozzles, corrosion of pipe networks and rollers. And microbial reproduction. The treatment of secondary cooling spray water is generally divided into two steps. The first step is oil removal and flocculation treatment to ensure that the suspended solids and oil content of the effluent meet the control requirements. Flocculants are generally anionic polyacrylamide, with a molecular weight of more than 8 million and a concentration of 0.5-2.0 mg/L. When the oil content in the return water exceeds 10 mg/L, oil removal treatment is required. Oil removal flocculants can be added at the entrance of the sedimentation tank, or special integrated oil removal equipment can be used for oil removal. There are also reports of oil removal using SGO (biological enzymes) in China.
After ensuring that the suspended solids and oil content of the effluent meet the requirements, the next step of scale inhibition, corrosion inhibition and sterilization treatment is carried out. The water quality requirements of turbid water are lower than those of clean water, and high suspended solids in the water are easy to adsorb agents. Generally, several mg/L of organic phosphorus and acrylic acid copolymers are added. Phosphonic acid has strong adsorption resistance and is suitable for scale inhibition and corrosion inhibition treatment of this system. The storage capacity of turbid water is large. From an economic point of view, the sterilization and algae removal treatment alone accepts oxidizing biocides, such as chloramine and active bromine, and the addition frequency should also be lower than that of the clean water system.
5 Others
During the treatment of cooling water for continuous casting of electric furnaces, the following issues should be noted.
1) Establish reasonable water quality control indicators. The hardness, total iron and other indicators in the water should be strictly evaluated, and the corrosion and scaling trends of the water quality should be monitored by means of bypass hanging plates and monitoring heat exchangers.
2) Protective treatment for short-term parking. The cycle of steelmaking production is short. Before parking, the concentration of corrosion inhibitors should be increased to reduce the corrosion of pipelines by buffered water. When restarting, the total iron content is high and the water turns yellow, so replacement is required first.
3) Strengthen the operation monitoring of the crystallizer, pay attention to the water flow and temperature rise of each channel of the crystallizer, and prevent scaling caused by too low local flow rate.
4) Carry out regular chemical cleaning. The corrosion rate of cooling water for continuous casting of electric furnaces is generally high, and long-term operation is prone to pitting and accumulation of corrosion products. Regular chemical cleaning can remove dirt, improve the operation of the system, and ensure the safe, economical and efficient operation of the device.
