Reduction of silica in coke with ash of increased basicity
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Reduction of Silica in Coke with Ash of Increased Basicity GEORG SICK, OTTO ABEL, and KLAUS SCHWERDTFEGER The chemical reactions occurring in the blast furnace have been investigated by many researchers and plant operators. Recently, the reactions involving silicon have become of particular interest since modern steelmaking practice requires low silicon contents in the hot metal. It is known that much of the silicon found in the hot metal originates from the ash of the coke which is rich in silica. The silica of the ash is reduced to volatile silicon monoxide corresponding to reactions Si02 + C = SiO + CO
[1]
2 SiO~ + SiC = 3 SiO + CO
[2]
and the S~O is then transferred via the gas phase to the iron droplets descending in the furnace.t-7 At the tuyere level the iron phase may have silicon contents of several percents, s9 Much of this silicon is oxidized again to silica by the slag in the hearth, but these reactions are sluggish and do not reach equilibrium. 2fl~ That is, the silicon content of the hot metal is higher than that at equilibrium with the slag, and depends considerably on the extent of the reactions of the coke ash. For instance, the silicon content of the hot metal decreases with increasing blast pressure, L2which is in accordance with reactions [1] and [2], and it increases with the content of the ash in the coke 4 and with the coke/ore ratio, as Sulfur is also transferred from the coke to the descer~ding metal droplets by a volatile species, namely, SiS. The relevant reactions are S (organic) + 2 C + SiO2 = SiS + 2 CO
[3]
FeS + 2 C + SiO2 = SiS + 2 C O + Fe
[4]
CaS + C ยง SiO2 = SiS + CaO + CO
[5]
(The other relevant reactions can be written down involving SiC instead of SiO2.) Similarly as with silicon, there is a maximum sulfur content at the tuyere level, and further downward the metal is desulfurized again by the slag. 9~3~4 GEORG SICK and 'KLAUS SC~WERDTFEGER are with ]nstitut fiir Allgemeine Metallurgie, and OTTO ABEL is wilh lnslilm far Chernigche Technc,logie und Brennstofftechnik, Technische Univer~itat Clausthal, D-3392 Clau~thal-Zellerfeld, Germany. Manuscript submitted April 13, 1984. METALLURGICAL TRANSACTIONS B
From the reactions [ 1] to [5], it is evident that the partial pressures of SiO and SiS are lowered by decreasing the activity of SiO_, in the coke ash. The decrease of SiO~ activity can be achieved by addition of a basic oxide, e.g~, CaO, to the ash. Hence, it may be expected that the rate of volatilization of s~licon and sulfur from the coke can be influenced by changing the basicity of the ash. In the present work, cokes wtth artificial ashes of increased basicity were prepared and the volatilization reactions were studied in laboratory experiments. Preparation of" Coke Samples: Coke samples with three different hasicities CaO/SiO2 were made by adding pure burnt CaO to the coal. The starting material was composed of 15 pct breeze ( < 100/xm), 80 pct coal mixture (Ruhr area Germany) (
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