Effect of High Al2O3 Slag on the Blast Furnace Operations

Kohei SUNAHARA, Takanobu INADA, Shusaku KOMATSU and Takaiku YAMAMOTO

(Corporate Research & Development Laboratories Sumitomo Metal Industries, Japan.)

(January 22, 2008)

Increasing the Al2O3 content in the blast furnace slag, the blast furnace operations tend to make troubles such as excess accumulation of molten slag in the blast furnace hearth and increasing pressure drop at the lower part of the blast furnace. So, it will be important to keep good slag fluidity at the blast furnace operations such as, drainage of tapping and keeping good permeability.

In order to clarify the effect of high Al2O3 slag fluidity on the blast furnace, high Al2O3 slag (20%) test operations of experimental blast furnace have been carried out. Investigation results of the test operation are as follows;

1) Slag MgO improves the hearth drainage rate at high Al2O3 slag operation.

2) Permeability of the dripping zone is improved by decreasing slag CaO/SiO2, at high Al2O3 slag operation of the blast furnace.

3) It was verified that the slag drainage phenomena were able to be described by the fluid model.

4) The optimum composition of high Al2O3 slag of the blast furnace is high MgO and low CaO/SiO

 

The slag fluidity in a blast furnace affects softening-melting behavior in a cohensive zone, permeability in the lower part of a furnace due to liquid hold-up in a dripping zone, liquid flow in a furnace hearth, and the ability of the drainage slag by a taphole.

The slag fluidity is affected by temperature and composition, with the latter influenced by ore gangue minerals and ash materials of coke and pulverised coal. CaO/SiO2 in the slag, even in slag compositions, is adjusted in Japan to a range of 1.2 to 1.3 in concentration by an auxiliary material in terms of fluidity and desulfurization ability.

The Al2O3 concentration in the slag is considered to be a factor that degrades the slag fluidity, which is semi-empirically set in Japan at the upper limit of about 15 to 16% in order to avoid the accumulation of the iron slag and the deterioration of permeability in the lower part of the furnace.

Meanwhile, in recent blast furnace operations, slag-reduction is promoted from a viewpoint of SiO2-reduction in a sintered ore, facilitating improvement of permeability in the blast furnace, a reduction in slag volume, and environmental- response. However, as slag volume decreases, the concentration of Al2O3 in the slag increases relatively, even if the Al2O3 in the ore is not changed. Moreover, from the viewpoint of the supply and demand of raw materials and fuels that have recently become in tight supply, the increase in concentration of Al2O3 is inevitable. The issue now is to present clear guidelines with regards to the upper limit of Al2O3 concentration in the slag.

In formulating the guidelines, it is important to understand the behavior of slag in the blast furnace. Various kinds of studies of the behavior have been undertaken. On the matter of slag fluidity in the blast furnace, there are studies on the estimation of viscosity as liquid physical properties, quantitative evaluations of molten pig iron and slag tapping phenomena in the furnace hearth by experiments with models and by numerical analysis, and evaluations of effects on the pressure drops in liquid flow in the dripping zone. It is also pointed out that the increase of ore-Al2O3 affects not only the strength of a sintered ore, but also its characteristics at high-temperatures in the cohensive zone.

For evaluation in an actual blast furnace, a test to increase (20%) Al2O3 in the slag for about one month was carried out at Muroran1BF in 1968. Only the relationship between desulfurization performance and viscosity in the slag was considered, but there was no reference with respect to the upper limit of Al2O3 concentration in the slag in the blast furnace.

The examination described above was limited to a single phenomenon. The in-furnace phenomena were not systematically analyzed. Thus, for the effects of slag fluidity, focusing Al2O3 on each area in the blast furnace, we focus on permeability in the lower part of the blast furnace and slag drainage in the furnace hearth, and conduct a highAl2O3 slag operation test with an experimental blast furnace in order to systematically evaluate permeability and drainage.

Based on the results, the effects of the viscosity and crystallization temperature governing slag fluidity on the permeability in the lower part of the blast furnace and slag drainage were evaluated for each area.

For slag drainage, the iron slag remaining in the actual blast furnace was considered from the evaluation of fluidity from the experimental results by numerical analysis. For permeability in the lower part of the blast furnace, an evaluation of results from the experimental blast furnace was conducted together with a dripping experiment for a coke-bed layer of the cohensive slag and the softeningmelting test results of high Al2O3 sinter, and thought was given to the dripping zone and the cohensive zone. In addition, the slag design was considered, including

CaO/SiO2 and MgO from the comprehensive blast furnace evaluations of the high Al2O3 slag operation.

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