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Feb 17, 2019
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Basic Principles for Smelting and Refining – Stirring of Steel Bath, Circulating Flow Rate and Mass Transfer Rate

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Figure(a) shows that an increase in stirring power density  applied to the steel bath intensifies circulation of the molten steel and reduces the uniform mixing time . Regardless of the stirring method employed, the relationship shown in this figure holds true, and the time required for uniform mixing can be reduced by as much as one order of magnitude if molten steel is stirred at 200 W/ton. How much the overall mass transfer rate increases with increased stirring of the steel bath and an increased circulating flow rate is shown next by several examples. Figure(b) shows a case in which the uniform mixing time  was significantly reduced by blowing gas into the steel 

bath from the furnace bottom of the BOF. When  is short and the circulating flow rate Q of the molten steel is high, the mass flux of carbon transferred to the zone where oxygen gas comes into contact with the steel bath also becomes large. Therefore, the mass transfer rate of carbon increases, carbon is preferentially oxidized and decarburized, and the degree of oxidization of molten steel due to the formation of iron oxide and of iron loss in the slag becomes low. This advantage is especially great when the target carbon content becomes low, as shown in Fig.(c). The index used in this figure is the value obtained by dividing the product of partial pressure Po2 of oxygen in the furnace and oxygen volume Vo2 supplied per unit time to the reaction interface (mass flux of oxygen) by the product of circulating flow rate Q of the molten steel and specific weight r of the molten steel (corresponding to the mass flux of carbon). This index is called the ISCO value (index for selective carbon oxidation).

Stirring of Steel Bath, Circulating Flow Rate and Mass Transfer Rate

The value  in a top-blown BOF is a little more than 100 seconds in Fig.(b), while the ISCO value corresponding to this value  is about 230 in Fig.(c). The value  in bottom-blown BOF is about 12 seconds, and the corresponding ISCO value is about 60. As shown in Fig.(c), with  decreasing from a little more than 100 seconds to 12 seconds and the corresponding ISCO value decreasing from 230 to 60, the iron oxide in slag that is formed by oxidation of steel decreases substantially from about 23% to 10% as the total iron content. As a result of stirring, shorter (higher Q), that is, a lower ISCO value, will result in a higher overall mass transfer rate of carbon for the oxidation, and hence smaller oxidation loss of molten steel, giving higher iron yield.

The relationship that the overall mass transfer increases substantially with increasing Q or 1/ holds true not only for the decarburization that occurs between gas and metal, but also for the dephosphorization and desulfurization that occur between slag and metal. Furthermore, this relationship also holds true for the deoxidation that removes oxide inclusions such as alumina formed and suspended in the steel bath after adding a suitable deoxidizing agent such as aluminum. The result of bottom-blowing argon gas into the aluminum-killed stainless steel bath in a vacuum oxygen decarburization (denoted VOD hereinafter) furnace is shown in Fig.(d) as an example of this. When the bottom-blown gas flow rate was increased from 20 to 70 N/min, Q increased from 10 to 40 ton/min and, as a result, the mass transfer coefficient for alumina inclusion removal (deoxidation) was doubled from 1 to 2/min. Vortices are generated by stirring molten steel, and due to the velocity gradient in the vortices, inclusions collide with each other, coalesce, and become large. When their critical diameter is exceeded, these inclusions float outside of the system. Higher Q and higher  result in a higher value of k, because the number of vortices and the velocity gradient in the vortices both increase.

These various blowing and stirring operations are used for smelting and refining reactions and, as a result, remarkable progress has been achieved in the iron and steelmaking processes. This procedure has been developed to the extent that a region of high oxygen potential and low temperature is intentionally formed in one part of a reactor, a region of low oxygen potential and high temperature is intentionally formed in another part, and dephosphorization and desulfurization are simultaneously promoted in the former and latter regions, respectively. This technique has been put into practical use as a simultaneous dephosphorization and desulfurization process for pretreating hot metal and has substantially reduced the dephosphorization and desulfurization load in BOF blowing.

Article author: JFE

More Original Articles: LMM GROUP

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