What Is Direct Reduced Iron?

DRI-Direct Reduced Iron is a low-carbon porous substance formed by the reduction of fine iron powder or iron oxide in the furnace at low temperature. Its chemical composition is stable and the content of impurities is small. As a coolant for converter steelmaking, it can also be used for powder metallurgy if it is re-reduced.

The use of non-metallic materials and non-ferrous metals in steel has increased, resulting in continuous decline in the quality of scrap steel. Scrap steel is used as the raw material of electric furnace steel. Due to its different sources, the chemical composition fluctuates greatly, and it is difficult to grasp and control, which brings great difficulties to the electric furnace steelmaking operation. If a certain proportion of direct reduced iron (30-50%) is used as a diluent with scrap steel, it can not only increase the uniformity of the steel, but also improve and improve the physical properties of the steel, so as to achieve the purpose of producing high-quality steel. Therefore, DRI is not only a substitute for high-quality scrap steel, but also an essential raw material for the production of high-quality steel. In recent years, due to the development of steel products in the direction of small size, light weight, advanced functions, and composites, non-metal materials in steel The increase in the proportion of nonferrous metals and non-ferrous metals has led to the continuous decline in the quality of scrap steel.

According to foreign reports, during the smelting of high-power electric furnaces, the charge is matched with 30-50% direct reduced iron, the productivity is increased by 10-25%, and the operating rate is increased by 25-30%. On February 26, 1996, our tests on a three-ton electric furnace at the Luquan Steel Rolling Plant showed that the use of 30-50% direct reduced iron in the charge can save an average of 27% of electricity per ton of charge, save 28% of steelmaking time, and consume oxygen. The amount is reduced by 22%, and the physical properties of steel are significantly improved. In this regard, adding a certain amount of direct reduced iron to the steel charge of the electric furnace can not only improve the production capacity of the electric furnace, but also reduce power consumption and production costs.

1. Stable chemical composition, effectively dilute the content of residual and inclusion metal elements in the steel, and improve the quality of the steel;

2 , P, S harmful element content is low, can shorten the refining time;

3 , reduce the number of loading times, reduce power outages and heat losses, fast melting speed, low power consumption, can improve efficiency and reduce costs;

4. During the melting period, the power supply operation is stable, allowing high power supply, low accent, less smoke and dust, and a good working environment;

5. Low cost of use and high economic benefits.

There are many iron and phosphorus reduction methods and iron concentrate reduction methods used in industry. That is, the rolled steel iron oxide phosphorus or concentrate powder is pressed into a block by a reduced iron briquetting machine , and then loaded into a roasting tube and kiln roasted to produce. Out of high-quality reduced iron.

After the direct reduction iron is crushed (the direct reduction iron ingot is broken into blocks), the direct reduction iron is broken (the massive direct reduction iron is broken into particles of 0-15 mm), and then the magnetic separation is performed to remove SiO2, CaS and free carbon And other impurities. The user can use the reduced iron briquette machine to press the direct reduced iron particles again, so that the direct reduced iron particles are shaped and meet a certain specific gravity g / cm3 requirement. Directly reduced iron crushed particles directly affect the physical properties of the compact (compressibility, formability, bulk specific gravity g / cm3) and play a vital role in the production of special steel.

1. Iron-Phosphorus Reduction Method

The production process of DRI can be divided into the following five processes:

I. Raw material preparation and drying and crushing procedures:

The desulfurizing agent and the reducing agent are charged into a quantitative hopper. The quantitative hopper sends the materials to a drying room through a conveyor to dry and mix the two materials according to the weight ratio of the two materials. The moisture content of the dried material is less than 3%. The dried material is sent to the reducing agent crusher through the conveyor for pulverization, and the pulverized particle size is 1.5 mm or less. The crushed material is lifted to the high level silo by the conveyor, and then sent to the storage silo by the conveyor. Concentrate powder is sent directly to the drying unit by the conveyor for drying. After drying, the moisture content is less than 3%. The dried concentrate powder is sent by the conveyor to the high-level bin, and then sent by the conveyor to the storage material. warehouse.

2. Automatic charging process:

This process can produce barrel-shaped, columnar, or tile-shaped direct reduced iron as needed. The loading system is composed of a silo, a quantitative tube and a charging head. The concentrate powder and reducing agent pass through the storage silo, unload the material into the distribution silo, and then load the reducing head into each reduction crucible to achieve the Crucible cloth.

Three. Reduction roasting process:

This process is completed in a rapid reduction furnace. Appropriate and stable furnace temperature and reduction time are the keys to determine the quality of DRI. This process includes three stages: preheating, reduction, and cooling. First, the car passes the transmission mechanism and sends the car to the preheating section in the rapid reduction furnace. During this time, the moisture in the material completely evaporates, the desulfurizing agent decomposes, the temperature rises to the reduction temperature, and enters the high temperature reduction section. During this time, The iron oxide is fully reduced to form elemental iron, and then enters the cooling section for cooling. After cooling to below 200 ° C, it is released from the furnace.

Four. Automatic unloading process:

After the material comes out of the furnace, it is reduced to 100 50 at normal temperature, and then enters the discharging system for discharging. The ash is sucked away by the wind, and the direct reduced iron is removed from the crucible through the grasping forceps to realize automatic discharge.

V. Product processing procedures:

This process completes the direct reduction ferromagnetic separation, crushing, and passivation processing-briquetting. The unloaded vehicle enters the loading system for loading. The direct reduced iron enters the intermediate silo through the discharge device. The direct reduced iron in the intermediate silo is conveyed to the direct reduced iron crusher through the conveyor for crushing. The crushed direct reduced iron is magnetically separated by a dry magnetic separator. The direct reduced iron powder after magnetic separation is transported by the conveyor to HDYJ for reduction. The iron briquetting unit performs briquetting and is stored in the warehouse through a transmission mechanism.

The realization of cold briquettes effectively improves the oxidation resistance of DRI. It reduces the water absorption and volume of DRI, which is conducive to the long-term storage and long-distance transportation of DRI, and it also improves the yield of molten steel when it is used. Therefore, it is welcomed by steelmaking companies. Has strong vitality and competitiveness.

The adjustment of the metallurgical industry requires optimizing the structure of steel, changing from focusing on scale and output to focusing on quality and efficiency. To improve the quality of steel, it will be very difficult if the raw materials are still mainly scrap steel. This is because almost 100% of the residual impurities such as Sn, As, and Cu in the scrap steel remain in the steel. It will be very difficult to simply use scrap steel to improve the quality of steelmaking.

Application in oxygen converter

When hot molten iron is made in an oxygen converter, the heat of oxidation of the heating elements (Si, C, etc.) in the molten iron often exceeds the need to heat the molten steel to a suitable tapping temperature. This requires the addition of a certain amount of coolant to maintain Normal molten steel temperature. The amount of coolant added can account for 7% to 32% of the mass of molten steel according to the composition, temperature and type of the molten iron.

Direct reduced iron is used as a coolant, and its cooling effect is 1.2 to 2.0 times that of returned scrap steel, and about 1/3 of iron ore, because FeO + C = Fe + CO endothermic reaction increases the cooling effect . The cooling effect of DRI increases as the metallization rate decreases.

The gangue content will also slightly reduce its cooling effect.

Because the slag basicity of the oxygen converter is 3.5, when direct reduced iron is used as a coolant, its

The content requirement is less than 3%.

When there is a shortage of qualified scrap steel for cooling, when producing special steels with extremely low sulfur, low oxygen, and low manganese, direct reduction briquettes are used in systems that automatically adjust the coolant and when used as a post-blow coolant in a steel drum. It is more suitable as a coolant.

In ironmaking blast furnaces, a large number of experiments have used direct reduced iron; in small blast furnace tests, direct reduction iron powder was sprayed from the tuyere, and the results were not good and not complicated. When direct reduction iron is added to the blast furnace charge to increase the metallization degree of the blast furnace charge, the coke ratio is reduced, which increases the output and has a significant effect.

The test results show that the metallization rates of the various charge materials have the same effect, but the effects are slightly different. In fact, this is due to the difference in the carbon content of the coke used. If measured in terms of carbon savings, the results of each test are very close.

The main reason that direct reduction of iron can reduce the blast furnace coke ratio is to reduce the burden on the iron ore iron oxide reduction in the blast furnace, but about 50% of the blast furnace requires only a small amount of energy to reduce the reduction. "Direct reduction" of carbon, which consumes a lot of energy and has high energy consumption, is of practical significance. The part of the direct reduced iron that has been reduced can only reduce the "direct reduction", which has a significant impact in the blast furnace, in proportion to the "direct reduction" degree of the blast furnace. Therefore, theoretically, the total energy consumption of the blast furnace using metallized charge cannot be reduced a lot. In addition, the energy consumed by direct reduction is not much cheaper than coke. Therefore, the economics of using direct reduced iron in the blast furnace The benefits are not good, so they have not been widely used.

In ironmaking electric furnaces, the reduction reaction of iron ore is basically a "direct reduction" method of carbon, so the reduction work completed in direct reduction iron can be used to replace the reduction workload in ironmaking electric furnaces. Therefore, it can achieve better results than the blast furnace.

Application in casting

In pig iron casting, a certain amount of direct reduced iron can be added instead of pig iron, but the amount is limited by the content of Si and C in the cast iron, because increasing the amount of direct reduced iron will dilute the Si and C components in the cast iron. Direct reduced iron for cast iron is required to have the highest possible metallization ratio, because in the iron furnace, supplemental reduction of FeO consumes energy (coke) seriously. Iron furnaces do not have strict requirements for the content of gangue in direct reduced iron, because acid gangue only increases the amount of slag, and has a certain effect on the focus ratio and output. ^[1]