Application of DRI in steelmaking
There is a shortage of scrap in steelmaking around the world. Due to the small domestic self-sufficiency of scrap in China and the unstable quality of scrap available, it is necessary to find alternatives to scrap. Direct reduced iron (DRI) hasbeen used as a raw material for steelmaking for many years and has been gradually applied in China.
1. DRI performance
As a raw material for steel making, the main properties of DRI compared with scrap steel are as follows:
DRI | scrap steel | |
Compositional stability | stable | volatility |
Impurity content | Very low | High |
Gangue content | contains | Very few |
Carbon content | medium | low |
porosity | porous | No |
It can be seen that DRI is a kind of high-quality steel making raw material, and only high gangue content will lead to high energy consumption. In addition, the steel making furnace must be suitable for processing a large amount of slag. Due to poor thermal conductivity, the ideal DRI smelting process should have a high temperature heat source, such as electric arc furnace and converter, to maintain high temperature in the DRI and its surrounding area.
DRI can be spherical, clumpy (spongy iron) or lumpy (CDRI or HBI) and usually have the following physical properties:
pellets | Sponge iron | CDRI | HBI | |
Particle size（mm） | 8-16 | 6-40 | 37x45x24 | 30x55x106 |
porosity（%） | 52 | 49 | 24 | 24 |
Bulk density（t/m³） | 1.8 | 2.0 | 2.6 | 2.8 |
The chemical composition of DRI is very important for the smelting process and should be as close as possible to scrap steel. The chemical composition of DRI is as follows, which may vary with different raw materials and production processes.
full iron （TFe） 92%
Metallic iron 83%
Carbon content 0.25~1.0%
Sulfur content 0.025%max
Phosphorus content 0.06%max
SiO2 2-3%
High quality DRI properties suitable for steelmaking should be:
• The highest metallization rate possible (preferably greater than 85%)；
• The content of silica should be as low as possible
• The volume density of DRI should be greater than 3.5t/ m³, preferably around 5.5t/m³, and greater than 1.8t/m³. Since small particles with particle size less than 3mm are easily suspended in slag and difficult to melt, the less content is better.
2. Application of DRI in electric furnace steelmaking
Using DRI as raw material for steelmaking has the following advantages:
• Stable composition, low impurity content, especially low sulfur content
• Rapid melting
• Easy to manage, convenient transportation
• Continuous feeding can be realized
In theory, the proportion of DRI replacing scrap can reach 100%. When the amount of DRI is small, it can be fed by scrap hopper, and when the amount of DRI is large, it is very convenient to use continuous feeding system. At present, the application of DRI in eAF steel making has made the following progress:
① The productivity of eAF is improved. Due to the low content of sulfur and phosphorus in DRI, the refining time of eAF is reduced. In addition, due to the realization of continuous feeding, the feeding time is reduced and the melting speed is accelerated.
② It can produce high quality steel, because there are many impurities in scrap steel, it is difficult to produce high quality steel with 100% scrap steel in electric furnace, but the impurity content in DRI is very low, and high quality steel such as electrical steel can be produced in electric furnace.
③ Due to the stability of DRI composition, the furnace times of steel composition out of the ordinary can be reduced, especially when producing low carbon steel.
④ The electrode breakage is reduced and the electrode consumption is reduced.
However, melting DRI in an electric furnace increases the power consumption compared to melting scrap. The increased power consumption is used to reduce the residual iron oxide and the gangue in DRI melting and slagging to separate from the iron. It was also observed that when the proportion of DRI in the raw material exceeded 20%, the increased power consumption remained constant. The application of DRI will also increase the consumption of lime and slag. When the electric furnace designed for melting scrap steel is used to melt DRI, the lining material life will be reduced. To solve the above problems, a new type of electric furnace with continuous DRI charge should be designed, which has the following characteristics:
① Increase furnace shell diameter;
② Reduce the diameter of the electrode center, or adopt the inclined electrode column method;
③ use water cooled furnace wall instead of refractory materials;
④ In the melting period, use low power factor, short arc operation mode, and corresponding transformer output current, bus guide, flexible cable and electrode;
⑤ Select a spare low reactance for the furnace;
⑥ Melting under foam slag, covering arc.
Production practice shows that sponge iron up to 30% can be added to the charge of electric furnace without any operational problems, but it may cause nodules in the furnace when the proportion of higher ingredients is higher. This suggests that DRI-100 % replacing scrap is still difficult.
3. Application of DRI in oxygen converter steelmaking
Scrap steel is used as a coolant in oxygen converter steelmaking. As the amount of scrap returned decreases, it is necessary to find other types of coolant to replace scrap steel. There are usually two options:
① Use iron ore or limestone
② Use DRI
Using DRI as a coolant has the following advantages:
① Relatively pure, less impurities
② Price stability
③ The recovery rate of molten steel is higher than that of iron ore and limestone.
Theoretically, the continuous addition of DRI in oxygen converter steelmaking is suitable because the oxygen converter provides the very concentrated heat required to melt DRI, so DRI melts quickly and stays in the oxidizing atmosphere of the furnace for a short time. The following progress has been made in the application of DRI in oxygen converter.
① Can produce low - sulfur grade steel, such as electrical steel
② The service life of furnace lining is improved by reducing the slag temperature and eliminating the mechanical impact of weighted scrap steel on furnace lining
③ When the proportion of DRI is high, the feeding time through the hopper is shortened, thus reducing the steel smelting time per furnace.
DRI feeding mode in oxygen converter can be through scrap hopper (batch feeding) or continuous feeding through high bin. The latter has the following advantages:
① The DRI melts faster because it can be fed during the blowing process;
② By adjusting the feed speed, the blowing process can reach thermal balance at any time, so the temperature in the furnace is easier to control
③ FeO content in slag is reduced, thus spatter is reduced and recovery rate of molten steel is increased.
Compared with sponge iron, HBI or CDRI have obvious advantages. The test results of a steel mill are as follows:
① Recovery rate of molten steel decreased by 1%
② The single consumption of molten iron increased by 11kg/t
③ Lime consumption increased by 6kg/t
④ As a scrap replacement, HBI can be used up to 32% of scrap with no operational problems.
According to the test results, it can be estimated that 30% of scrap steel can be replaced by DRI without affecting the converter operation. If a higher proportion of DRI is to be used, the following measures must be taken:
① DRI has higher metallization rate
② Use DRI continuous feeding system
③ Increase the tonnage steel volume ratio of furnace body
④ Increase slag treatment equipmen
