Molten steel bottom purging is a highly efficient, resource-saving technology of gas treatment of molten metals in the small-bubble mode in order to improve their quality by using specialised purging devices.
Until now, purging devices with chaotic-order capillaries (porous plugs) or with locally positioned capillaries (slit plugs) have been used in metallurgy. In both cases, liquid ejection by bubble flow occurs along its periphery. At the same time, internal volume of the gas flow does not participate in molten metal ejection, which considerably reduces the efficiency of the mass transfer process.
The technology of molten steel bottom purging in the small-bubble mode is based on the following.
Parallel plumes of inert gas micro bubbles are generated by specialised purging monoblocks, which are installed in the ladle bottom lining, electric arc furnace and so on.
Due to optimum distance between the plumes of the inert gas micro bubbles they do not intersect or swallow one another. Therefore, the entire volume of gas contacts the molten metal and not only the outer surface of the flow as it occurs in all other modern purging devices. It results in considerably better efficiency of the mass transfer processes during purging.The purging monoblocks are fabricated from high-quality refractory concrete according to the specialised technology and have parallel specially directed micro pores which cannot be plugged with steel or slag.
The design of the purging devices allows formation of their working surface that can reach from 2 to 20 % of the entire surface area of the metallurgical vessel’s bottom (this value of the foreign purging devices does not reach even 0.5 % of the bottom’s surface area).
Purging monoblocks can be installed into all types of metallurgical vessels in combination with any type of linings – furnaces like electric arc furnace, induction furnaces; ladles like steel-teeming ladle, tundish, ladle furnace, drum-type ladle and so on. Such devices are most commonly used in electric steel-making furnaces and in steel-teeming ladles used for steel refining jointly with the equipment like ladle furnace due to shorter melting time by 10 to 20% (considerably more efficient mixing of molten metal) and due to corresponding lower consumption of electrical energy.
Main advantages of the technology:
1) Steel-teeming ladle and ladle furnace unit:
The monoblocks are optimal devices for blowing-in neutral gases into molten steel aimed at steel refining, degassing and homogenisation, namely:
- by using the monoblocks, the supplied gas is dispersed in the molten metal into maximum possible number of small bubbles. This intensifies purging with multi-fold increase of the surface area of active interaction without the risk of excessive wildness of splashing of the molten metal;
- the purging monoblocks can resist chemical and thermal impact of metal and slag or resist washing by metal flows generated during purging;
- the design of the monoblocks and gas-supply devices is simple in maintenance and can be easily replaced upon completion of the lifetime;
- the design of the monoblocks does not require using scarce materials and allows providing relatively low cost of the materials in combination with the saving effect that can be achieved.
The time of argon purging when using the monoblocks is reduced by 10 to 20% in comparison with the standard purging through ‘local’ purging plugs.
In some cases, trial industrial runs of the monoblocks have demonstrated that this technology allows two-fold reduction of the time of the ladle staying in the vacuum vessel, which guarantees considerable reduction of energy costs and, finally, lower self-cost of the steel produced.
2) Electric arc furnace (EAF)
In spite of the fact that EAF is the most ‘environment-friendly’ steelmaking unit, the level of environment pollution still remains high (dust 8.1 kg/t; СО 1.5 kg/t; NOx 0.29 kg/t). To meet strict ecological requirements the modernEAFs are equipped with fume extraction and gas treatment systems, their capacity is up to 15 – 20 % of total power consumption spent for steel production in the furnace.
The monoblocks installation in the hearth portion of the furnace in the area of electrodes waist isolates the arcing area from supply of air by inert gas plumes. This prevents iron burning and generation of nitrogen oxides. Moreover, due to air-lift effect, continuous feeding of cold lower volumes of the molten metal takes place directly into the hot region, while overheated molten metal is transferred to colder periphery, thus preventing iron overheating and evaporation.
Results of the electric arc furnaces operation with monoblocks have demonstrated considerable reduction of red fumes emission and shorter melting time by at least 10 to 20% along with corresponding lower self-cost of the products.
For each metallurgical unit, the monoblock design is customised in order to ensure maximum efficiency of the monoblock’s use.