Detailed explanation of refractory materials for three types of furnaces used in non-blast furnace ironmaking: smelting reduction furnace, gasificatio

Refractory materials for non-blast furnace ironmaking

Non-blast furnace ironmaking methods refer to other methods of reducing iron ore other than blast furnace ironmaking. Non-blast furnace ironmaking methods can be classified into two categories: direct reduction and smelting reduction.

In recent years, the non-blast furnace ironmaking process has been vigorously developed. The main reasons are:

(1) As the reserves of coking coal are getting smaller and smaller, the price is getting higher and higher. Non-blast furnace ironmaking can use non-coking coal and other energy sources as fuel, and basically no or less use of coke for ironmaking.

(2) With the development of the iron and steel metallurgical industry, the demand for scrap steel has greatly increased, and the supply of scrap steel has become increasingly tight. Sponge iron produced without blast furnaces is a good substitute for scrap steel.

(3) Non-blast furnace ironmaking eliminates the need for coking equipment, resulting in low investment costs and less pollution.

These reasons provide favorable conditions for the development of non-blast furnace ironmaking.

Refractory materials for direct reduction method

The main products produced by the direct reduction method include solid sponge iron, iron granules and liquid pig iron. Among them, the sponge iron production method is the most mature and has the largest output. Sponge iron production mainly uses iron concentrate, iron oxide scale and other raw materials containing high iron oxide to be reduced into metallic iron under the action of reducing medium. This reaction is a solid phase reaction and releases a lot of gas; there are many gases in the generated solid iron. The pores are like sponge, so it is called sponge iron. The reaction temperature is generally between 800 and 1300°C, and the reducing media used mainly include non-coke reducing agents such as coal, natural gas and coal gas. The equipment used mainly includes vertical kiln, square kiln, ring kiln, rotary kiln, tunnel kiln, trolley bottom continuous furnace, etc.

At present, sponge iron is mainly produced in the world by vertical kilns using natural gas as the reducing medium, accounting for about 80%. Regardless of the type of kiln, the operating temperature is not high, so generally aluminum-silicon refractory materials as kiln linings can meet the temperature requirements. However, it should be noted that Fe2O3 in refractory materials will be reduced to metallic iron and Fe3C under CO atmosphere conditions. The presence of Fe3C will promote the deposition of carbon, leading to embrittlement and cracking of refractory materials. Therefore, in order to increase the service life of the equipment furnace lining, the Fe203 content in the refractory materials should be reduced, and at the same time, the material's structure should be made dense and the pores fine.

Refractory materials for smelting reduction furnaces

The new smelting reduction ironmaking process is a smelting method that does not use coke or uses less coke, reduces iron oxides in a high-temperature molten state, and separates slag and iron to obtain carbon-containing molten iron similar to a blast furnace. It has a metallurgical process It is short, small in scale and causes little environmental pollution. However, due to the vast and cumbersome development process, huge investment, and many engineering problems that require industrial production practice to draw correct conclusions, it was once at a standstill. As the understanding of its theoretical technology continues to deepen, and under the pressure of the ecological environment and natural resources, the development of smelting reduction and direct iron-making technology is accelerated. After decades of research and development, more than 30 smelting reduction ironmaking processes have been developed, but the COREX smelting reduction ironmaking process is currently the only cutting-edge technology in the modern steel industry that has achieved industrialized production and was the first to achieve success.

Advantages of the COREX smelting reduction ironmaking method:

(1) Molten iron can be produced directly without coke and sinter;

(2) Powdered ore and coal can be directly used to effectively utilize resources and reduce 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. It does not require sintering machines and coke ovens, reducing pollution sources and reducing pollution in ironmaking plants by 70%;

(5) Process investment is reduced and production scale is flexible;

(6) Improved production efficiency.

The melting and gasification furnace is an important equipment for the production of molten iron in the COREX process. The melting gasifier is located in the lower part of the COREX system. The upper part of the furnace is an enlarged hemisphere and the lower part is cylindrical. Coal, flux and reduced iron ore enter the top of the melting gasifier through a pressurized sealed silo. After the coal enters the furnace, it meets the gas at about 1000~1100°C, is rapidly dried, carbonized, and carbonized, and then drops to the cylindrical part of the furnace body. After that, it is affected by the oxygen flow sent from the lower tuyere to form a stable fluidized layer. The temperature of the lower part of the fluidized layer is 1600~1700℃. The carbonized coal particles react with oxygen to first produce CO2. .As the air flow rises, CO2 is reduced and converted into CO when encountering carbon. In order to improve the gas quality, enhance the reduction ability and protect the tuyere, steam is especially introduced from the tuyere. Therefore, the high-temperature gas discharged from the top of the melting gasifier contains 95% CO+H2. This high-temperature gas is mixed with cold gas to adjust the temperature to 900°C, and is sent to the hot cyclone dust collector. After purification, it enters the reduction vertical kiln through the reduction vertical ring pipe. The settled dust particles purified by the hot cyclone dust collector are sent back to the melting gasification furnace with cold gas through the dust hopper.

The highly metallized pre-reduced charge entering the furnace from the spherical top of the melting gasifier is heated, melted and eventually becomes molten iron and slag during its descent. The reduction shaft kiln is located in the upper part of the COREX system and is cylindrical in shape. The reduction gas (coal gas) generated by the melting gasifier is adjusted and purified, enters the kiln from the middle and lower tuyere of the reduction shaft kiln, and rises through the solid material layer (the ore and flux added from the top of the shaft kiln). It falls by its own weight and is heated and reduced by high-temperature reducing gas. The reduced metallized iron material falls continuously and evenly into the melting gasification furnace through the discharge device and discharge pipe at the bottom of the shaft kiln. The COREX smelting reduction ironmaking process is divided into two stages: pre-reduction and smelting. The pre-reduction stage is a melting stage in which the solid phase of iron ore is reduced to metallic iron or sponge iron in a vertical kiln, and then the sponge iron is directly fed into the melting gasifier to become molten iron.

COREX refractory materials for melting and gasification furnaces

Generally, the melting and gasification furnace is divided into four parts: drying area, fluidized combustion area, tuyere area and furnace.

The slag produced by the smelting reduction method contains a large amount of FeO, which seriously corrodes refractory materials. Furnace lining corrosion is mainly due to slag corrosion, chemical erosion of the furnace lining caused by alkali vapor and molten iron, thermal melting loss, thermal spalling due to temperature fluctuations, impact of furnace materials, and erosion of furnace dust gas. Due to these series of factors, the serious damage to the furnace lining refractory material has resulted in a very short service life of the equipment. This is one of the main reasons why the smelting reduction method is difficult to be practical and popularized. Therefore, furnace lining refractory materials have an important impact on the development of this new ironmaking method.

Because the temperature in the drying zone is 1000~2000°C, the coal decomposes and removes volatile matter. The furnace lining in this area is strongly affected by the mechanical impact of the charge, and is also subject to erosion and corrosion by dust-containing gases. Therefore, the furnace lining is required to be wear-resistant. In this area, the use of high-aluminum bricks with an Al2O3 content of 55% to 65% can meet the requirements. It is the same as the kiln lining of the reduction shaft kiln. In order to increase the service life, the Fe2O3 content is required to be as low as possible to prevent the lining from embrittlement or Chalk.

Since coal is burned in the fluidized combustion zone, the temperature can reach 1600~1700℃. The furnace charge is fluidized and the furnace lining is severely eroded. The refractory lining is subjected to a large thermal load and high-temperature wear. During air supply and air break, the temperature in this area fluctuates greatly and causes peeling. The ISCOR Company in South Africa used magnesia carbon bricks in this area, but they fell off seriously due to frequent furnace shutdowns and furnace restarts. Si3N4 combined SiC bricks with excellent thermal shock resistance and thermal stability and erosion resistance should be used as the furnace lining.

The tuyere of the melting gasification furnace is operated by oxygen. The heat load of the tuyere bricks is high and the working conditions are harsh. It has a strong oxidizing effect on carbon-containing refractory materials. Molten slag and molten iron are formed here, so the tuyere bricks at high temperatures are subject to strong corrosion. The tuyere is made of SiC bricks, and the furnace lining from the upper part of the tuyere to the manhole is made of magnesia carbon bricks. It has hydration phenomenon. It is recommended to use Si3N4 combined SiC bricks as the lining. For the tuyere combined set of bricks, the SiC bricks combined with p-SiC have a very good effect. Through corrosion tests on Al2O3-Cr2O3 bricks, it was found that it has very good corrosion resistance and spalling resistance. Therefore, silicon carbide bricks and chrome corundum bricks should be used in the tuyere and the area above it. Sialon combined with corundum bricks should also be a very good choice.

The refractory linings of the furnace bottom, hearth, and taphole of the melting and gasification furnace are always in contact with high-temperature molten iron and molten slag, and the resulting erosion is the main cause of damage to the refractory materials. The refractory materials used are equivalent to those of blast furnaces, mainly using microporous carbon bricks and a layer of ceramic cups. Al2O3-SiC-C materials are still used in the taphole. These refractory materials are the same as the refractory materials used in the blast furnace bottom and hearth. Please refer to the section of refractory materials for blast furnaces.