Refractory Materials for Molten Reduction Furnace

Refractory materials for smelting reduction furnaces. Smelting reduction ironmaking is a novel process that uses little or no coke, reducing iron oxides in a high-temperature molten state and separating slag from iron to obtain carbon-containing molten iron similar to that produced in a blast furnace. It features a short metallurgical process, smaller scale, and less environmental pollution. However, due to its extensive and complex development process, huge investment, and the fact that many engineering problems could only be correctly resolved through industrial production practice, it was once stagnant. With a deeper understanding of its theoretical technology and pressure from ecological environment and natural resources, the development of smelting reduction and direct ironmaking technologies has accelerated. After decades of research and development, more than 30 smelting reduction ironmaking processes have emerged, but the COREX smelting reduction ironmaking process is currently the only one to have achieved industrialized production and is one of the leading technologies in the modern steel industry.

Rongsheng Refractory Bricks for Furnace Lining

Advantages of the COREX smelting reduction ironmaking method:

(1) It can produce molten iron directly without coke and sinter.

(2) It can directly utilize ore fines and pulverized coal, effectively utilizing resources and reducing energy consumption per ton of iron by more than 20%.

(3) The production line and equipment are simple.

(4) It is beneficial to environmental protection, as it eliminates the need for sintering machines and coke ovens, reducing pollution sources and decreasing pollution in ironmaking plants by 70%.

(5) It reduces process investment and allows for flexible production scale.

(6) It improves production efficiency.

The molten gasifier is a crucial piece of equipment in the COREX process for producing molten iron. Located at the bottom of the COREX system, it has an enlarged hemispherical upper section and a cylindrical lower section. Coal, flux, and reduced iron ore enter the top of the molten gasifier through a pressurized, sealed hopper. Upon entering the furnace, the coal encounters gas at approximately 1000–1100°C, rapidly drying, carbonizing, and descending into the cylindrical section of the furnace. Thereafter, it is subjected to an oxygen flow from the lower tuyeres, forming a stable fluidized bed with a temperature of 1600–1700°C at its lower part. The carbonized coal particles react with oxygen, initially producing CO2. As the gas rises, the CO2 is reduced to CO upon contact with carbon. To improve gas quality, enhance reducing capacity, and protect the tuyeres, steam is introduced through them. Therefore, the high-temperature gas exiting from the top of the molten gasifier contains 95% CO and H2. This high-temperature coal gas, mixed with cold coal gas and heated to 900°C, is sent to a hot cyclone dust collector for purification before entering the reduction vertical kiln through a reduction vertical ring pipe. The dust particles purified and settled by the hot cyclone dust collector are sent back to the melting gasifier via a dust hopper using cold coal gas.

The highly metallicized pre-reduction charge entering the furnace from the spherical top of the melting gasifier is heated and melted during its descent, ultimately becoming molten iron and slag. The reduction vertical kiln, located at the top of the COREX system, is cylindrical. The reducing gas (coal gas) generated by the melting gasifier, after being heated and purified, enters the kiln from the lower middle tuyeres, rises through the solid material layer (ore and flux added from the top of the kiln), and falls under its own weight, being heated and reduced by the high-temperature reducing gas. The reduced metallicized iron material continuously and evenly falls into the melting gasifier through the lower discharge device and discharge pipe of the kiln. The COREX melting reduction ironmaking process consists of two stages: pre-reduction and smelting. The pre-reduction stage is the process of reducing the solid phase of iron ore into metallic iron or sponge iron in a vertical kiln. Then, the sponge iron directly enters the melting gasification furnace to be refined into molten iron.

COREX Molten Gasification Furnace Refractory Materials

Molten gasification furnaces are generally divided into four parts: the drying zone, the fluidized combustion zone, the tuyeres zone, and the hearth.

The slag produced by the molten reduction process contains a large amount of FeO, which severely corrodes refractory materials. Furnace lining corrosion is mainly caused by slag melting, chemical erosion from alkaline vapors and molten iron, thermal melting, thermal spalling due to temperature fluctuations, impact from the furnace charge, and scouring by furnace dust and gases. These factors severely damage the furnace lining refractory materials, resulting in a very short service life for the equipment. This is one of the main reasons why the molten reduction process is difficult to put into practical use and widely adopted. Therefore, furnace lining refractory materials have a significant impact on the development of this new ironmaking method.

Because the temperature in the drying zone is 1000~2000℃, coal decomposes and volatiles are removed. The furnace lining in this area is subjected to very strong mechanical impact from the furnace charge, as well as scouring and corrosion from dust-laden gases. Therefore, the furnace lining must be wear-resistant. High-alumina bricks with an Al2O3 content of 55%–65% are sufficient for this area. Similar to the lining of a reduction vertical kiln, the Fe2O3 content must be as low as possible to prevent lining embrittlement or pulverization and extend service life.

Due to coal combustion in the fluidized combustion zone, temperatures can reach 1600–1700℃. The fluidized charge causes severe erosion of the lining, subjecting the refractory lining to high heat loads and high-temperature abrasion. Significant temperature fluctuations during blast and shutdown further contribute to spalling. A South African company used magnesia-carbon bricks in this area, which suffered severe spalling due to frequent shutdowns and restarts. SiC bricks with Si3N4 bonding, exhibiting excellent thermal shock resistance and thermal stability, and erosion resistance, should be selected for the lining.

The tuyeres of the molten gasifier operate under oxygen conditions, resulting in high heat loads and harsh working conditions for the tuyeres bricks. This has a strong oxidizing effect on carbonaceous refractories. Slag and molten iron form here, thus the tuyeres bricks are subject to severe corrosion at high temperatures. The tuyere uses SiC bricks, while the lining from the top of the tuyere to the inspection hole uses magnesia-carbon bricks. However, these bricks exhibit hydration; therefore, it is recommended to use SiC bricks bonded with Si3N4 as the inner lining. For the tuyere lining, p-SiC bonded SiC bricks work very well. Erosion tests on Al2O3-Cr2O3 bricks showed excellent erosion resistance and spalling resistance. Therefore, silicon carbide bricks and chrome corundum bricks should be used in the tuyere and the area above it. Sialon bonded corundum bricks are also a very good choice.

The refractory linings of the furnace bottom, hearth, and taphole of the molten gasifier are constantly in contact with high-temperature molten iron and slag, and the resulting erosion is the main cause of refractory damage. The refractory materials used are comparable to those used in the blast furnace, mainly microporous carbon bricks with a layer of ceramic cups. Al2O3-SiC-C materials are still used at the taphole. These refractory materials are the same as those used in the blast furnace bottom and hearth; please refer to the section on blast furnace refractory materials.