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Slag Heat Demands
APPENDIX III
Slag Heat Demands This appendix describes methods by which the enthalpy effects of blast furnace slag can be included in blast-furnace enthalpy balances. As will be seen, the enthalpy required to form liquid slag (1800K) from its com ponent oxides (1200 K) can be represented in terms of a discrete heat demand, Z) slag ,kJ per kg of slag. For simplicity, the discussion is restricted to CaO—Si02 slags only, but the method can be extended to include Al 2 0 3 ,MgO,etc. For slags formed from CaO and Si0 2 only, the additional terms in the Slection 11.4 enthalpy balance are: Input enthalpy*
Ä' S i 0 · "H° ; 1 2 l200 00, 2
"
Si02
Ä
C a 0
· H\ 200? CaO
^'slag ' ^ 1 8 °o
Output enthalpy
liquid slag (kJ per kg of slag)
where η$ϊ§ , «CaO a n d wt-slag a r e kg moles of Si0 2 in slag; kg moles of CaO from limestone and burnt lime; and weight of slag, all per kg mole of product Fe. Of course, slag weight is given by: ^•slag^^oV
6 0
^^·
5 6
·
*4Ο82=4Ο2-^Λ slag _ i , "CaO "CaO
w
i CaC0 3
where f^oiQ , « i n , ηη*ΓΩ
^ 0 ^S m ° l e s of these components charged to the furnace
per kg mole of product Fe. 216
Slag Heat Demands
217
Inserting these items into the enthalpy balance (and changing signs as described in Section 11.4) leads from equation (11.11) to ^wrz
+ nl
. Dl
+
^
. Hi8oo
_ n*g
slag = EB'n*
. tf o2oo _ „slag . ^ ο ^ Si0 2
CaO
-π^·(0.7·7/ί2οο+0.3·7/ί2οο)-0.38·7·πΙ·//ί2Οο· CO C0 2 H20 (AIII.l)
In consolidated form, the slag-related enthalpy terms may be represented by wt
where u
-slag " D*** „slag "Si0 2 wt. , slag
= wt
; v=
slag * ^H^o - u 'H°l200 ~ v Ή°120ο) slag Si0 2 CaO slag "CaO
wt. , slag
(ΑΙΙΙ.2)
(kg moles per kg of slag)
and Z) slag is the slag heat demand, kJ per kg of slag. This demand can be put in a more convenient form by making the substitutions (per kg of slag): #1800
=
slag #1200
slag
# 1 2 0 0 + [#1800 ~ # 1 2 0 o ] s l a g >
slag
=
#1200
+ u
"#1200
slag
Si0 2
+^"#1200
CaO
(u and v as in equation (AIII.2)) which lead from equation (AIII.2) to £ s l a g =#[200 + [#i8oo - #i2oo] s l a g kJ (kg of slag)"1. (AIII.3) slag This equation shows that Z) slag is exactly the heat required to form 1 kg of liquid slag (1800 K) from solid CaO and solid Si0 2 , both at 1200 K (the thermal reserve temperature).
218
The Iron Blast Furnace
Suggested Z)
slag
Calculation
Consider, for simplicity, that blast-furnace slag is made up of CaO and Si0 2 only and that its CaO/Si02 weight ratio is 1.2 (Table 1.1). One kilogram of this slag (0.55 kg CaO, 0.45 kg Si0 2 ) contains: 0.0098 kg mole of CaO = v, 0.0076 kg mole of Si0 2 = u, which form a slag of the approximate composition: 0.0022 kg mole of 2CaO · Si0 2 , 0.0054 kg mole of CaO · Si0 2 . Heats of formation (1200 K) of 2CaO-Si02 and CaO-Si02 are -124 000 and —91 000 kJ (kg mole of compound)" 1 , respectively (Appendix V), so that the heat of formation of 1 kg of slag is given by HUoo = 0.0022 - (-124 000) + 0.0054 · (-91 000) slag
kJ (kg of slag)"1.
= -750
The second term in the slag heat demand (equation (AIII.3)) is calcu lated from the slag enthalpy data given by Foerster,* i.e. [#i8oo - # i 2 o o ] s i a g = 1000 kJ(kgof slag)"1 and the total demand is D
slag = _ 7 5 0
+ 1000
= +250 kJ (kg of slag) -1 .
(AIII.4)
The above calculations do not consider the effects of A1 2 0 3 or MgO, but it is estimated that the demand expressed by equation (AIII.4) is accurate to ±20% when the weights of all slag components are included in wt.siag.
♦Foerster, E. F. and Weston, P. L. (1967) Heat Content of Some Blast-furnace and Synthetic Slags, U.S. Bureau of Mines Report of Investigation 6886, Washington.
Slag Heat Demands This appendix describes methods by which the enthalpy effects of blast furnace slag can be included in blast-furnace enthalpy balances. As will be seen, the enthalpy required to form liquid slag (1800K) from its com ponent oxides (1200 K) can be represented in terms of a discrete heat demand, Z) slag ,kJ per kg of slag. For simplicity, the discussion is restricted to CaO—Si02 slags only, but the method can be extended to include Al 2 0 3 ,MgO,etc. For slags formed from CaO and Si0 2 only, the additional terms in the Slection 11.4 enthalpy balance are: Input enthalpy*
Ä' S i 0 · "H° ; 1 2 l200 00, 2
"
Si02
Ä
C a 0
· H\ 200? CaO
^'slag ' ^ 1 8 °o
Output enthalpy
liquid slag (kJ per kg of slag)
where η$ϊ§ , «CaO a n d wt-slag a r e kg moles of Si0 2 in slag; kg moles of CaO from limestone and burnt lime; and weight of slag, all per kg mole of product Fe. Of course, slag weight is given by: ^•slag^^oV
6 0
^^·
5 6
·
*4Ο82=4Ο2-^Λ slag _ i , "CaO "CaO
w
i CaC0 3
where f^oiQ , « i n , ηη*ΓΩ
^ 0 ^S m ° l e s of these components charged to the furnace
per kg mole of product Fe. 216
Slag Heat Demands
217
Inserting these items into the enthalpy balance (and changing signs as described in Section 11.4) leads from equation (11.11) to ^wrz
+ nl
. Dl
+
^
. Hi8oo
_ n*g
slag = EB'n*
. tf o2oo _ „slag . ^ ο ^ Si0 2
CaO
-π^·(0.7·7/ί2οο+0.3·7/ί2οο)-0.38·7·πΙ·//ί2Οο· CO C0 2 H20 (AIII.l)
In consolidated form, the slag-related enthalpy terms may be represented by wt
where u
-slag " D*** „slag "Si0 2 wt. , slag
= wt
; v=
slag * ^H^o - u 'H°l200 ~ v Ή°120ο) slag Si0 2 CaO slag "CaO
wt. , slag
(ΑΙΙΙ.2)
(kg moles per kg of slag)
and Z) slag is the slag heat demand, kJ per kg of slag. This demand can be put in a more convenient form by making the substitutions (per kg of slag): #1800
=
slag #1200
slag
# 1 2 0 0 + [#1800 ~ # 1 2 0 o ] s l a g >
slag
=
#1200
+ u
"#1200
slag
Si0 2
+^"#1200
CaO
(u and v as in equation (AIII.2)) which lead from equation (AIII.2) to £ s l a g =#[200 + [#i8oo - #i2oo] s l a g kJ (kg of slag)"1. (AIII.3) slag This equation shows that Z) slag is exactly the heat required to form 1 kg of liquid slag (1800 K) from solid CaO and solid Si0 2 , both at 1200 K (the thermal reserve temperature).
218
The Iron Blast Furnace
Suggested Z)
slag
Calculation
Consider, for simplicity, that blast-furnace slag is made up of CaO and Si0 2 only and that its CaO/Si02 weight ratio is 1.2 (Table 1.1). One kilogram of this slag (0.55 kg CaO, 0.45 kg Si0 2 ) contains: 0.0098 kg mole of CaO = v, 0.0076 kg mole of Si0 2 = u, which form a slag of the approximate composition: 0.0022 kg mole of 2CaO · Si0 2 , 0.0054 kg mole of CaO · Si0 2 . Heats of formation (1200 K) of 2CaO-Si02 and CaO-Si02 are -124 000 and —91 000 kJ (kg mole of compound)" 1 , respectively (Appendix V), so that the heat of formation of 1 kg of slag is given by HUoo = 0.0022 - (-124 000) + 0.0054 · (-91 000) slag
kJ (kg of slag)"1.
= -750
The second term in the slag heat demand (equation (AIII.3)) is calcu lated from the slag enthalpy data given by Foerster,* i.e. [#i8oo - # i 2 o o ] s i a g = 1000 kJ(kgof slag)"1 and the total demand is D
slag = _ 7 5 0
+ 1000
= +250 kJ (kg of slag) -1 .
(AIII.4)
The above calculations do not consider the effects of A1 2 0 3 or MgO, but it is estimated that the demand expressed by equation (AIII.4) is accurate to ±20% when the weights of all slag components are included in wt.siag.
♦Foerster, E. F. and Weston, P. L. (1967) Heat Content of Some Blast-furnace and Synthetic Slags, U.S. Bureau of Mines Report of Investigation 6886, Washington.