Increasing the scrap ratio in the converter will help reduce molten iron consumption and achieve energy conservation and consumption reduction.
Keyword: Converter steelmaking
In the steelmaking production process, using scrap steel to make 1 ton of steel can reduce 1.6 tons of carbon emissions, reduce 1 ton of raw coal consumption, reduce the use of 1.7 tons of iron concentrate, reduce wastewater emissions by 70%, and reduce 4.3 tons of raw ore mining.
In the process of smelting crude steel from scrap steel, high energy-consuming processes such as sintering, coking, and blast furnaces in the traditional long process are avoided. The energy consumption index is far better than that of traditional long-process smelting. Its CO2 emissions are only 27% of those of long-process smelting, dust emissions are 10%, and energy consumption is 41%. The comparison of energy consumption ratio between scrap steel smelting and iron ore smelting crude steel is shown in Figure 1.
As the resource and environmental situation becomes increasingly tense and the amount of scrap steel in society increases, it is crucial to seek an efficient and low-cost steelmaking process with a high scrap steel ratio, and improving the converter scrap ratio technology is an important technical direction to achieve this goal. Therefore, it is necessary to summarize and analyze past research on improving the scrap ratio in converters in order to seek a better technical route.
Methods to improve converter scrap ratio
For different regions, due to their different resource endowments, the driving forces for research on technologies to improve converter scrap ratio are also different. Its goals mainly fall into the following categories:
(1) Ordinary BOF increases the scrap ratio in smelting and improves economic and technical indicators;
(2) The productivity of blast furnace pig iron is much lower than the productivity of crude steel, so increase the scrap steel ratio to increase production;
(3) Realize the rationalization of the processes of the steel complex;
(4) Scrap melting in power-poor areas.
In the process of increasing the scrap ratio in the converter, the main problem to be solved is the heat balance of the converter. The framework of its solution is shown in Figure 3. To achieve heat balance under high scrap ratio, the heat compensation method is mainly to add fuel to perform heat compensation through fuel combustion heat release or to improve the secondary combustion efficiency in the converter to achieve heat compensation. From the perspective of the object of thermal compensation, the solution is mainly to increase heat to preheat the scrap steel or directly compensate the heat for the metal molten pool.
Increase fuel ratio for converter heat compensation
In the case of high scrap ratio, the most direct way to achieve a new converter heat balance is to increase the fuel combustion heat release in the furnace to achieve thermal compensation. Its main fuels are coal, hydrocarbons CmHn, etc. The main adding methods include input method and injection method. Among them, the adding method and amount of fuel are the focus and difficulty of the process, which have a huge impact on fuel combustion efficiency. Typical technologies that increase the fuel ratio for heat compensation include ALCI (Arbed Lance Coal Injection), KMS (KlOckner Maximilian Steel), TAPS (Triple Action Process of STB), etc.
Here we take the KMS process as an example to introduce
The main technical features of the KMS method are that coal and O2 are injected into the molten pool from the bottom of the furnace body, and the bottom-blown oxygen nozzle is protected by natural gas cracking and endothermic protection. Coal injection of 50-55kg per ton of steel increases oxygen consumption by 40-50m3/t, bottom-blown oxygen ratio reaches 60%-100%, bottom-blown oxygen supply intensity reaches 4.5m3/t·min, and the scrap ratio can be as high as 75%.
There are two main KMS high scrap ratio smelting process routes:
Process 1: Retain part of the molten steel after tapping, add scrap → inject coal and oxygen at the bottom of the furnace to melt the scrap → add scrap again → inject coal and melt → refining → tap. Process 2: In the middle stage of blowing, part of the high-carbon molten steel is poured out for use in lower package smelting.
Improve secondary combustion efficiency and achieve thermal compensation
The idea of improving secondary combustion efficiency thermal compensation is to use the sensible heat and latent heat of exhaust gas to preheat scrap steel to save energy. If the metal pool is heated by secondary combustion heat release, the thermal efficiency will be low. With the development of the current new secondary combustion oxygen lance, the secondary combustion efficiency has been significantly improved. The new secondary combustion oxygen gun improves the reaction efficiency between the sprayed oxygen and the CO2 generated by the decarburization reaction through the new design of the gun body and outlet, which will generate more secondary combustion heat. Research shows that the secondary combustion rate should be capped at 30%. Practice shows that for every 10% increase in the secondary combustion rate, the converter scrap ratio can increase by 3.4% to 4.0%.
But in general, the ability to increase the scrap ratio of the converter by improving the secondary combustion efficiency is limited. Typical technologies include the EOF (Energy Optimized Furnace) method and the Reactor method. Among them, the EO method has the characteristics of double blowing from the top to the bottom of the ordinary converter. The furnace combustion efficiency is high, the molten pool is heated by radiation, and the furnace gas is used to preheat scrap steel and air. The ratio of pig iron to molten steel is very flexible. It is suitable for areas where scrap steel is abundant and electricity is short, and the particle size requirements of coal are not high. The scrap ratio can reach 100%, and the smelting cycle is 70~8Omin.
Problems with high scrap ratio in converters
Compared with scrap smelting in electric furnaces, increasing the scrap ratio in converters eliminates the need for secondary conversion of energy. However, the main reasons why the current high scrap ratio is not as competitive as electric furnaces are:
(1) The metal yield is low. Scrap steel requires a large amount of fuel combustion heat to melt. Blowing excessive oxygen will peroxidize the molten iron and reduce the molten iron yield;
(2) Fuel contaminates molten steel. Impurity elements such as N, P, and S are inevitably present in the fuel. The addition of fuel destroys the cleanliness of molten steel;
(3) Technology safety and stability. Due to the instability of scrap steel quality and fuel quality, the entire process will be unstable, and the five major operating systems of the converter will be affected. This complicates the converter operation and is not conducive to the need for accurate and intelligent smelting upgrades of the converter.
In the context of the severe steel situation, increasing converter waste must be achieved by increasing fuel and improving secondary combustion efficiency. The steel ratio may be a feasible way to adapt to future development.
By summarizing the world’s research on improving the scrap ratio in converters over the past few decades, the following conclusions can be drawn:
(1) The application of medium to high scrap ratio in steelmaking is the general trend, and it is urgent to find a competitive smelting technology route with high scrap ratio;
(2) In the past few decades, increasing the scrap ratio in converters was mainly achieved by increasing fuel and improving secondary combustion efficiency. Adding fuel to the converter destroys the cleanliness and operational controllability of the molten steel, and improving the secondary combustion efficiency has limited ability to increase the scrap ratio of the converter;
(3) It is currently necessary to calculate the economics of improving converter scrap steel in order to find an economically feasible and technically controllable converter high scrap steel ratio process route.