FI20021469A - CO compression of pellets during band sintering process - Google Patents

Description

Patent Application August 12, 2002Jyrki Noponen

CO-sealing of pellets in the strip sintering process

Pellet steel strip sintering is a method developed by LKAB for iron concentrate and iron ore agglomeration. The process has subsequently been applied to the agglomeration of raw materials for the production of some other steel alloys (ferrochrome, ferromanganese, ferroniobium, etc.). In steel strip sintering, the pelletized raw materials are sintered into hard balls (pellets) of about 12mm in diameter. Steel strip sintering is based on moving steel strip, recycle process gas, the bottom layer of the pellet that protects the steel strip, and the permeability of the pellet mattress gases. The principle of steel strip sintering is shown in figure1.

The steel strip is a thin and flexible sheet of steel with a longitudinal perforation elongated hole. Depending on its width, there are usually 4-9 perferring lines in the tape. Larger amounts of interfering may be used. There is an intact disk between the perferences. Uninterrupted strap rotates through the furnace through two shafts (drive and adjusting shafts). The purpose of the perforations is to allow gas flows through the strip and through the pellet mattress.

The bottom pellet mattress consists of bottom pellets. Sintered product pellets are used as bottom pellets. The bottom pellets are fed to the steel strip as the first even layer.The layer has a thickness of 15-25 cm. The function of the bottom pellets is to protect the steel strip from high temperatures and to initially recover the thermal energy from the process gases passing through the pellet mattress, transfer the thermal energy to the cooling zone and return the thermal energy back to the circulating gases. The target temperatures for sintered pellets are 1250-1350 ° C.The temperature of the upper edge of the pellet zone near the sintered pellets is approximately 200-300 ° C (against the steel strip). Thus, the bottom pellet mattress has a fairly strong temperature gradient.

On top of the bed pellet layer, sintered pellets (so-called wet or fresh pellets) are fed into a uniform layer.

The process gases cool the cooling zones by means of a pellet mattress (bottom pellets and product pellets) and transfer the heat they receive to the drying, heating and sintering zones (Figure 1).

About 40% of the energy required for sintering comes from oxidation of iron and external fuel. 60% of the energy is obtained by recirculating the energy inside the process furnace. The external fuel can be fine carbon, CO gas, LPG or fuel oil. Coke is added to the raw materials prior to pelleting and thus provides energy to the furnace with the pellets. CO and LPG gas are used in gas burners in the heating and sintering zone. The burners heat the circulating gas in the heating and sintering zone. The burner-heated recirculation gas flows above the pellet mattress (Figure 1).

Coke can also be added to the surface of the pellet mattress as a thin layer. In this case, the coke in the pellet mattress resembles somewhat the "" bottom combustion "discussed in this patent application. The above method (" "surface coke") is patented.

Even after reflection, the recycle gas contains 10-15% oxygen. The iron in the finished pellets is fully oxidized.

There are also other pellet sintering processes that utilize the gas-permeability of the pellet mattress and utilize sintered pellets as a protective layer. Generally, the thermal energy of the pellets from cooling to heating is also recycled. The principle is generally the same, but there may be a grate in place of the strip or some other solution in the process steps may differ slightly from the steel strip sintering.

Current Problems / Advantages of "CO-Compaction" Mechanical Properties of Pellets

The sintering of the pellets takes place in a strip sintering furnace with a steel strip in the process gas stream. The process gas is mainly oxygen and nitrogen (air). As the pellet temperature rises to sintering temperatures, the bonding phase will sinter with the "main". The binding phase is composed of bentonite and diffused components of the 'parent substance' (eg chromite or hematite). These are mainly alkali and the 'main component' (eg Ο203 and FeiCJ). Alkaline concentrates in the pellet to grain boundaries Where they contribute to an increase in the alkaline content of the binding phase. A typical analysis of bentonite is as follows

SiO2 59.5% ALO3 16.4%

MgO 3.4%

CaO 5.5%

Na20 2.71% K20 0.65%

Fe203 5.6%

MnO 0.2%

The sintering is based on the diffusion of the two phases and the solid-solid junctions, but when the melting point of the intermediate phase drops sufficiently, the binding phase becomes molten. This causes the oxygen to dissolve in the molten phase and the oxygen activity may rise to high levels in a strong gas stream. This is especially the case if no pellets are added to carbon and the process does not burn other fuels. With high oxygen activity, the following reactions may occur in the pellet: 2K20 (1) + 02 (g) = 2K202 (1) (1) K202 (I) + 02 (g) = 2CO2 (1) (2) 2Na20 (l) + 02 ( g) = 2Na202 (l) (3)

Na 2 O (l) + O 2 (g) = 2NaO 2 (l) (4) 2MgO (l) + O 2 (g) = 2MgO 2 (l) (5) 2MnO (l) + O 2 (g) = 2MnO 2 (l) (6 )

The resulting per- and superoxides lower the melting point of the silicate-based bonding phase to dissolve an additional '' major component '' in the melt. The amount of molten phase in the bonding phase is thus clearly increased. As a result, amorphous areas may be formed which are hard but noticeably brittle. At the same time, the porosity of the pellet path decreases due to the dense layer on the surface. The release of gases generated during sintering from the pellet may become more difficult and the oxygen diffusion into the pellet will be retarded. The pellet becomes inhomogeneous. In addition, the peroxides and superoxides mixed in the melt phase also reduce the heat resistance of the pellets. Similar problems can also be caused by "" normal "alkali oxides (Na 2 O and K 2 O) when enriched in pellets at the boundary phase of the grain boundaries.

CO-sealing can prevent the formation of the aforementioned oxides and reduce the peroxides and superoxides formed. The reducing gas CO reduces oxides according to the following formulas.

2K02 + CO (g) = K202 + CO2 (g) (7) K202 + CO (g) = K20 + CO2 (g) (8) 2NaO2 + CO (g) = Na2O2 + CO2 (g) (9)

Na 2 O 2 + CO (g) = Na 2 O + CO 2 (g) (10)

MgO 2 + CO (g) = MgO + CO 2 (g) (11)

MnO 2 + CO (g) = MnO + CO 2 (g) (12) CO-sealing makes the pellets more porous and improves the mechanical properties of the pellets.

Alkaline content of pellets

Sodium and potassium are very problematic in the blast furnace process. They cause a so-called time cycle, which causes the blast furnace walls to overgrow and cause blast furnace '' hangs ''. Blast furnace overflow refers to a situation in which blast furnace gas flows are stopped by increasing the pressure in the lower part of the furnace. About half of the blast furnace's alkaline load comes from iron-containing raw materials. In pellets, sodium and potassium are present as oxides (for example, Na 2 O and K 2 O).

In CO-condensation, alkali is reduced to pure elements.

K20 + CO (g) = 2K (g) + CO2 (g) (13)

Na 2 O + CO (g) = 2Na (g) + CO 2 (g) (14)

At sufficiently high temperatures (1032 K potassium and 1156 K sodium) they evaporate from the pellets to the surrounding gas. This reduces the alkali content of the pellets and reduces the other disadvantages of alkali (mainly by increasing the heat resistance). In addition, the use of manganese in the blast furnace can be reduced.

Oxygen content of pellets

The pellets are iron ore or concentrate agglomerated in block form. Agglomeration is carried out as this facilitates the reduction of iron ore. Because the ultimate purpose of the reduction is to remove oxygen, complete oxidation of the pellets in the sintering process slows down the reduction process to hematite (Fe 2 O 3). Similarly, the reduction of the metal is retarded by the other oxides mentioned above.

CO-sealing can reduce hematite. The oxides of iron then react as follows.

3Fe 2 O 3 + CO (g) = 2Fe 3 O 4 + CO 2 (g) (15)

Fe304 + CO (g) = 3FeO + CO2 (g) (16)

The degree of reduction in hematite depends on the CO content of the CO gas and the temperature of the pellet mattress.

Reduction of iron oxides during sintering achieves the following benefits: Reduction of carbon consumption in the blast furnace (less reductant and less heat energy required to heat the batch, more efficient utilization of the blast furnace carbon)

Claims (9)

A process for compacting (pre-reducing) pellets or the like containing metal or metal compounds with reducing gas in the context of sintering the pellets or the like in the same furnace. An independent claim A method for sealing pellets according to claim 1, characterized in that the reducing gas is introduced into the pellet mattress below the substrate (e.g. grate or steel strip). An independent claim Method for sealing pellets according to claim 1, characterized in that the reducing gas is simultaneously used for cooling the pellet mattress. An independent claim Method for sealing pellets according to one of Claims 1 to 3, characterized in that the method is used in steel strip sintering. An independent claim A method for sealing a pellet mattress according to any one of claims 1 to 3, characterized in that the method is used in a pellet sintering process which utilizes gas permeability of the pellet layer (pellet mattress) and / or where thermal energy is recycled. An independent claim Method for sealing a pellet mattress according to any one of claims 1 to 5, characterized in that the reducing gas is CO gas from an electric furnace or a blast furnace. An independent claim Method for compacting pellets according to one of Claims 1 to 6, characterized in that the method is used in the production of raw iron in the manufacture of iron pellets. An independent claim Process for compacting pellets according to one of Claims 1 to 6, characterized in that the process is used for the production of manganese in the manufacture of a manganese sinter or pellets. An independent claim Method for compacting pellets according to one of Claims 1 to 6, characterized in that the process is used in the production of a metal for the production of sinter or pellets.