A large number of industrial gases such as oxygen, nitrogen and argon are used in the smelting process of iron and steel enterprises. Oxygen is mainly used in blast furnace, melting reduction smelting furnace, converter, electric furnace smelting; Nitrogen is mainly used for furnace sealing, protective gas, steelmaking and refining, slag splashing in converter to protect furnace, security gas, heat transfer medium and system purging, etc. Argon gas is mainly used in steelmaking and refining. In order to meet the production requirements and ensure the safe and stable operation of production, large steel mills are equipped with special oxygen station and oxygen, nitrogen and argon power pipe network system.

Large-scale full-process steel enterprises are currently equipped with conventional processes: coke oven, sintering, blast furnace steelmaking, converter electric furnace steelmaking, rolling process, etc. Due to the emphasis on environmental protection and simplification of process flow, the international iron and steel industry has developed a short process process before iron in modern times – melting reduction iron making, which directly reduces iron ore raw materials into molten iron in a smelting furnace.

There is a big difference in the industrial gas required by the two different smelting processes. The oxygen required by conventional smelting blast furnace accounts for 28% of the total oxygen demand of the steel plant, and the oxygen required by steelmaking accounts for 40% of the total oxygen demand of the steel plant. However, the smelt-reduction (COREX) process requires 78% of the total amount of oxygen needed for iron production and 13% of the total amount of oxygen needed for steel making.

The above two processes, especially the melting reduction iron making process, have been popularized in China.

Steel mill gas requirements:

The main role of oxygen supply in blast furnace smelting is to ensure a certain high temperature in the furnace, rather than directly participate in the smelting reaction. Oxygen is mixed into blast furnace and mixed as oxygen-rich air into blast furnace. The oxygen enrichment efficiency of the blast air proposed in the previous process is generally below 3%. With the improvement of the blast furnace process, in order to save coke, after the use of large coal injection process, and to meet the requirements of the blast furnace production to promote output, the oxygen enrichment rate of the blast air is increased to 5∽6%, and the single consumption of oxygen is up to 60Nm3/T iron.

Because the oxygen mixture of blast furnace is oxygen-rich air, the purity of oxygen can be low.

The oxygen in the melting reduction steelmaking process needs to be involved in the smelting reaction, and the oxygen consumption is directly proportional to the steel production. The oxygen consumption in the melting reduction furnace is 528Nm3/t iron, which is 10 times of the oxygen consumption in the blast furnace process. The minimum oxygen supply required to maintain production in the melting reduction furnace is 42% of the normal production amount.

The oxygen purity required by the melting reduction furnace is greater than 95%, the oxygen pressure is 0.8∽ 1.0MPa, the pressure fluctuation range is controlled at 0.8MPa±5%, and the oxygen must be ensured to have a certain amount of continuous supply for a certain time. For example, for the Corex-3000 furnace, it is necessary to consider the liquid oxygen storage of 550T.

Steelmaking process is different from blast furnace and melting reduction furnace smelting method. Oxygen used in converter steelmaking is intermittent, and oxygen is loaded when blowing oxygen, and oxygen is involved in smelting reaction. There is a direct proportional relationship between the amount of oxygen needed and steelmaking output.

In order to improve the service life of the converter, nitrogen slag splashing technology is generally adopted in steel mills at present. Nitrogen is in intermittent use, and the load is large during use, and the required nitrogen pressure is greater than 1.4MPa.

Argon is needed for steelmaking and refining. With the improvement of steel varieties, the requirements for refining are higher, and the amount of argon used is gradually increasing.

The nitrogen consumption of the cold rolling mill is required to reach 50∽67Nm3/t per unit. With the addition of the cold rolling mill in the steel rolling area, the nitrogen consumption of the steel mill increases rapidly.

Electric furnace steel-making mainly uses arc heat, and the temperature in the arc action zone is as high as 4000℃. Smelting process is generally divided into melting period, oxidation period and reduction period, in the furnace can not only cause oxidation atmosphere, but also can cause reducing atmosphere, so the efficiency of dephosphorization, desulfurization is very high. Intermediate frequency electric furnace is a kind of will power frequency 50 hz alternating current into intermediate frequency (above 300 hz – 1000 hz) power supply device, the three-phase alternating current (ac) power frequency, after rectification into direct current, then laid adjustable intermediate frequency electric current, direct current supply by capacitance and induction coil in through the intermediate frequency alternating current, generate high density magnetic field lines in the induction coil, induction coil, and cutting in cheng fang of metal materials, produce a lot of eddy current in the metal materials. Single oxygen consumption up to 42∽45 Nm3/t.

Open hearth steelmaking process with raw materials: (1) iron and steel materials such as pig iron or molten iron, scrap; ② oxidants such as iron ore, industrial pure oxygen, artificial rich ore; ③ slagging agent such as lime (or limestone), fluorite, ettringite, etc.; ④ deoxidizer and alloy additives.

Oxygen effect to provide oxidizing atmosphere, open hearth smelting indoor combustion gas (furnace gas) contains O2, CO2, H2O, etc., at high temperature, strong oxidizing gas to the molten pool oxygen supply up to 0.2 ~ 0.4% of the weight of the metal per hour, oxidation of the molten pool, so that the slag always has a high oxidation.

Tip: oxygen supply by furnace gas alone, the speed is slow, adding iron ore or oxygen blowing can accelerate the reaction process.

Features of oxygen used in steel mills: oxygen release and peak adjustment with oxygen.

How to meet the oxygen demand of steel mills? Generally, the following ways are adopted to meet the requirements:

* Adopts variable load, high degree of automation of advanced control, to reduce the oxygen release, can be multiple sets of combination

* Multiple groups of peak-regulating spherical tanks are used in the traditional way to increase the buffering strength, so that the total amount of oxygen used in a certain period of time is stable, which can reduce the amount of oxygen release and reduce the size of the device

* At the low point of oxygen use, the excess oxygen is extracted by liquid oxygen extraction; When the oxygen peak is used, the amount of oxygen is compensated by vaporization. When the external pumping capacity of liquid oxygen is not limited by the cooling capacity, the external liquefaction method is adopted to liquefy the released oxygen and the vaporization method is adopted to vaporize the liquid oxygen

* Adopt a number of steel mills connected to the grid for gas supply, which makes the total oxygen supply scale stable according to the different time points of gas consumption

Matching process of air separation unit

In the development of oxygen station process plan needs to unit capacity, product purity, conveying pressure, booster process, system security, overall layout, noise control to do special certification.

Large steel mills with oxygen, for example, the annual output of 10 million tons of steel blast furnace process with oxygen to achieve 150000 Nm3 / h, the annual output of 3 million tons of steel smelting reduction furnace process with oxygen to achieve 240000 Nm3 / h, form a complete set of mature very large air separation devices are now a 6 ∽ 100000 grade, when choosing device size should be from the total investment in equipment and operation energy consumption, maintenance spare parts, covers an area of consideration.

Oxygen calculation for steelmaking in a steel mill

For example, a single furnace has a cycle of 70min and a gas consumption time of 50min. When the gas consumption is 8000Nm3/h, the (continuous) gas production of the air separation unit is required to be 8000× (50/60) ÷ (70/60) =5715Nm3/h. Then 5800Nm3/h can be selected as air separation device.

The general tonnage of steel with oxygen is 42-45Nm3/h(per ton), the need for both accounting, and this shall prevail.

At present, the production capacity of China’s iron and steel enterprises has jumped to the forefront of the world, but special steel, especially some important fields related to the national economy and people’s livelihood of steel is still dependent on imports, so the domestic iron and steel enterprises led by Baowu Iron and Steel Factory still have a long way to go, for the breakthrough of advanced and sophisticated fields is particularly urgent.

In recent years, the demand for air separation products in the steel industry has become more and more diversified. Many users need not only oxygen, but also high-purity nitrogen and argon gas, or even other rare gases. At present, Wuhan Iron and Steel Co., Ltd., Shougang and other major steel mills have several sets of fully extracted air separation devices in operation. The by-product noble gas of air separation devices can not only meet the demand of national production, but also bring great economic benefits.

With the large-scale development of steel mills, instead of supporting air separation unit is toward large-scale and air separation industry after decades of development, domestic air separation companies are also positive to catch up with the world’s leading enterprises, domestic suppliers, represented by hangyang co and other air separation plant has developed 8-120000 grades of large air separation equipment, domestic rare gas device has also been successful research and development, the electronic Air China started relatively late, but also is in intensify research and development, believe that with the progress of science and technology, gas separation industry in China will go abroad, towards the world.

Post time: Nov-03-2021