IRON MAKING COURSE NOTE
RAW MATERIALS AND THEIR PROPERTIES
The blast furnace raw materials are: 1. 2. 3. &. . ,.
Iron ore (lump ore/Coarse ore) Sinter/pellets Coke oke !n"th n"thin in# # tha that prom promot ote e $ol $olume ume also also prom promot ote e pro% pro%uc ucti tion on 'imestone (flu) *ot +last air +- !%%iti$es (2 steam pul$erise% coal h"%rocarbon fuels) Functions
Iron ore (lump ore/Coarse ore) 0 is the source of iron bearin# minerals. Sinter Source of iron (flu) (f lu) 'imestone 0 Source of flu +last air 0 source of 2 (2 21 2 0 45) use% for combustion of coke at the tu"eres. Coke
i) ii) ii) iii) iii) i$) i$) $)
The function of coke in the blast furnace is fi$e fol% namel" !cts !cts as as fuel fuel b" pro pro$i $i%i %in# n# for for the ther therma mall re6u re6uir irem ement ents s in in the the furn furnac ace e the the rea react ctio ion n bein bein# # 2C 2 7 2C 8ro$ 8ro$ii%es %es perm permea eabi bili lit" t" (in (in the the %r" %r" as wel welll as the the the wet wet 9one 9one)) an% also also mech mechan aniical cal support to the lar#e char#e column 8ro$ 8ro$i% i%es es #ase #ases s (C (C)) for for re%u re%uct ctio ion n of of iro iron n oi oi%e %es s ;e%u ;e%uct ctio ion n of of oi oi%e %es s of of met metal allo loi% i%s s suc such h as as
Iron Ores The iron bearin# minerals containin# iron compoun%s are Tyes o! Iron Ore H$e#$tite M$'netite Li#onite Si(erite Pyrite
C"e#ic$% !or#u%$
&Fe in Pure St$te
-e23 -e3& 2-e23.3*2 -eC3 -eS2
4= 42.& ,= &5.3 &,.,
C"$r$cteristics C"$r$cteristics o! Iron Ore
The essential characteristics of the iron ore are: i) ii) iii) i$) $) $i)
;e%ucibilit" Si9e an% si9e %istribution Stren#th Temperature an% ran#e of softenin# Iron moisture an% #an#ue contents Swellin# an% $olume chan#e.
1
Re(uci)i%ity 0 is the eas" at which the o"#en combine% with ore can be remo$e%
in%irectl". ! hi#her re%ucibilit" means a #reater etent of in%irect re%uction that ma" be obtaine% in the blast furnace resultin# in lower coke rate an% hi#her pro%ucti$it". Si*e $n( Si*e Distri)ution
>iscosteel iron ore si9e ra ran#e 5 to 32 32mm 'ower si9e ran#e , 0 5mm ?pper si9e ran#e shoul% be 2 0 3 times the lower si9e ran#e A(+$nt$'es o! N$rro, Si*e Distri)ution
Increase Increase char#e permeabilit" permeabilit" an% uniform uniform #as %istribution %istribution resultin# resultin# in better utilisation utilisation of the chemical an% thermal ener#ies of the #ases.
-
Stren't"
re shoul% be able to resist the followin#: 1. 2. 3. &.
Compression !brasion Impact 'oss 'oss of of stren stren#th #th to $ol $olum ume e chan# chan#e e in re%u re%uci cin# n# atm atmos osph pher ere e Tumbler in%e @ 4= !brasion A Iron- #oisture $n( '$n'ue contents
Bener Beneral all" l" ;ich ;ich res res ha$e ha$e = = 0 ,,-e e (hae (haema mati tite te ma#n ma#neti etite te an% an% lim limoni onite) te) 'ean res ha$e 3= 0 = -e (si%erite an% p"rite) >isco re @ & -e
-
Swel Swellilin# n# an% $olu $olume me chan chan#e #e occu occurr when when ore ore is re%uc re%uce% e% in a re%uc re%ucin in# # atmosphere at 5== 0 1=== C The conse6uent loss of stren#th an% compaction un%er loa% %ecreases the $oi%a#e an% resist the flow of #ases.
2
So!tenin' te#er$ture $n( so!tenin' te#er$ture r$n'e -
-
This is temperature at which the ores start to soften or melt an% takes place o$er a temperature ran#e. -or ores the ran#e ma" be wi%e 4== 0 1 3=C an% that for sinters an% pellets much narrower 1=== 0 1 3=C. The smaller the temperature ran#e of softenin# i.e the shorter the $ertical %istance in which the hi#hl" $iscous semifuse% mass forms an% eists the better the #as flow an% hi#her the pro%ucti$it".
Li#estone Si9e ran#e , 0 &=mm Chemical properties
Ca <# Si2 A !l23
abo$e &= 11 1.= 7 =.3
Coke
Coke is the uni$ersal fuel use% in the blast furnace. It acts both as a re%uctant as well as a supplier of heat. It also comprises the maDor portion of iron pro%uction cost. /0
P"ysic$% roerties o! coke
a) Coke si9e Coke comprises about = 0 ,= of the $olume of the char#e material. The importance of coke si9e is for pro$i%in# permeabilit" in the %r" as well as the wet 9one. In the %r" 9one the si9e shoul% be compactable with the ore si9e an% in the wet bosh 9one the coke si9e shoul% be as lar#e as possible to minimise floo%in#. The coke shoul% be about 3 to times lar#er than the ore. b) Coke Stren#th It is the 6ualit" cohesion that pre$ents the coke from collapsin# an% ten%s to a$oi% the formation of small particles. Coke stren#th shoul% be hi#h enou#h to resist breaka#e b" impact compression or abrasion. Coke stren#th is %etermine% b" se$eral coke makin# properties $i9. the proper crushin# an% blen%in# a%e6uate swellin# an% cokin# %urin# carbonisation absence of strains an% shrinka#e cracks e.t.c. 10
C"e#ic$% roerties o! coke
(a)
Carbon content Since coke is use% as fuel an% re%uction its calorific $alue an% carbon content (@5=) shoul% be as hi#h as possible. Carbon content shoul% not $ar" to a #reat etent if it %oes $ariable amounts of heat will be #enerate% in the furnace which makes it %ifficult to control the 6ualit" of iron.
(b)
(c)
Coke !sh
3
Sulphur
!bout 5= 0 F sulphur input into the +/- comes from coke If a sulphur input is hi#h the remo$al is accomplishe% b" hi#her sla# basicit" hi#her sla# bulk ($olume) hi#her temperature an% %esulphurisation out si%e the furnace. !ll these woul% increase the cost of hot metal pro%uction. (%)
Coke ;eacti$it" Coke reacti$it" is measure% b" the reaction rate of: C (coke) C2 7 2C at %ifferent temperatures. Coke shoul% be of low reacti$it". In the +/- reacti$it" of coke increases b" the action of alkali metals (G a) an% the" act as catal"sts.
(e)
!lkali !ttack !lkali metals increase the reacti$it" of coke an% %ecrease its stren#th !lkali attack weakens the cell walls an% increases the susceptibilit" of coke break%own un%er loa%. The %e#ree of break%own affects be% permeabilit" in the +/- lea%in# to han#in# an% nonuniform operation. Coke stren#th is se$erel" affecte% at temperatures abo$e 1===oC because of weakness in the matri #enerate% b" preferential alkali attack accor%in# to: 2G 2C 2 7 GC (potassium c"ani%e) A''%o#er$tions
-ines are #enerate% %urin# minin# an% ore %ressin# operations an% cannot be char#e% %irectl" into the +/- it is necessar" to a##lomerate them into lumps b" sinterin# or pelletisin# process. !n importance prere6uisite of sinter or pellets is that there is a%e6uate re%ucibilit" an% stren#th to withstan% han%lin# ha9ar%s an% impact abrasion an% compression in the +/-
Tyes o! Sinter
a)
on 0 -lue% or !ci% sinter These are sinters where no flu is present (base Ca) in the ore or is a%%e% %urin# sinterin#.
b)
-lue% Sinter These are sinters where flu has been a%%e% or is present in the ore. -lue% sinters are further classifie%/cate#orise% into two subclasses accor%in# to basicit":
i)
Self -lue% Sinters
&
Those where sufficient flu has been a%%e% in sinter mi to pro$i%e a basicit" that is %esire% in the final sla# takin# into consi%eration onl" the bur%en aci%s. !n etra flu is a%%e% to the bur%en while char#in# to cater to coke ahs aci%s. ii)
Super -lue% Sinter !n a%%itional flu (at Sinter 8lant) is a%%e% to the mi to pro$i%e for the %esire% final basicit" takin# into account aci% content of both i.e ore as well as the coke ash. -lue% sinters ha$e better re%ucibilit" in comparison with aci% sinters. Eesulphurisation takes place %ue to the present of Ca. Hner#" sa$er %urin# calcinations process e.# nona%%ition of Ca i.e limestone. 'ess than A1= is preferre%. A(+$nt$'es o! Sinterin'
1. -
2. 3.
!ci%s sinters !##lomeration of fines into har% stron# an% irre#ular porous lumps which #i$es #oo% be% permeabilit" (hi#h $oi% a#e) Hlimination of ,=4= of all sulphur an% arsenic (in present) %urin# sinterin# (Ca -e(s) C Hlimination of moisture h"%rate% water an% $olatiles on the sinter stran% with a cheap fuel. (2-e23 7 3*2 Si2 !l23 7 *2)
&.
Increase in softenin# temperature an% narrowin# %own the softenin# ran#e. F%u2e( Sinter
(a)
Calcinations of limestone insi%e the +/- is $er" epensi$e of carbon !ppro 0 ,=4=k# carbons per 1==k# C2 are ser$e% b" transferrin# the calcinations to sinter stran%.
(b)
Since in cooperation with lime lowers the $iscosit" an% li6ui%ous temp of the bon%in# sla# less coke (low temp) can be use% of the same stren#th an% hence a hi#h %e#ree of oi%ation an% re%ucibilit" can be achie$e%
(c)
'ime increases the acti$it" coefficient of -e in the silicate an% increase sinter re%ucibilit".
(%) (e)
Super flu ser$es much in more coke in the furnace *i#h blast furnace pro%ucti$it" better than e$en with pellets i%ea blast furnace material because of b" far the best hi#h temps #as permeabilit"
OUTLINE OF 3LAST FURNACE PROCESS
-
-urnace reactions are %i$i%e% into three 9ones base% on temperatures. The three 9ones ma" mer#e / o$erlap %epen%in# on the materials %istribution. The three 9ones which are temperature %epen%e% 9ones are:
(1) (2) (3)
The upperpreheatin# or preparation 9one. The mi%%le in%irect re%uction thermal reser$e or isothermal 9one. The lower processin# meltin# or %irect re%uction 9one.
Re$ction in t"e uer *one0
The bur%en is rapi%l" heate% from ambient temperature to abo$e 5==oC with a %istance of &,m from the stock le$el. The #ases comin# from the mi%%le 9one cool %own from a F== oC to 1==2== as it lea$es the furnace top.
4)5
Chemicall" uncombine% water enterin# the furnace with mostl" ore coke an% limestone is %ri$en off an% consumes about 5= GCal/ k# *2. ater as steam can be %eliberatel" is a%%e% to ore to lower the top #as minimise %ust nuisance. !$era#e moisture content of coke an% ore is about an% can be as hi#h as 1 in $er" moist ores. Coke $olatiles shoul% be 1.2 in #oo% 6ualit" of coke an% also $olatilili9e in this 9one without alterin# the top #as composition %ue to their small amounts. Deco#osition o! Hy(r$tes $)o+e 677OC
*"%rates consists of chemical combine% water of cr"stallisation or %eh"%ration e.#. -e23 3*2!l23 2Si2 2*2. Eecomposition of h"%rates of iron oi%es start at aroun% 3== %ecomposition increases with increase in temperature. Eecomposition of h"%rates re6uire about F, kcal / k# of * 2 i.e the a%%itional 35kcal / k# of *2is use% for breakin# more bon%s of h"%ration. 8h"sicall" combine% *2 escapes unchan#e% as steam at lower temperature but h"%rate% water (release% at abo$e 3==C 0 &==C) ma" react with Co to pro%uce C 2 an% *2 #as known as water #as (water #as shift reaction) *2 C C2 *2 ater Bas shift reaction C2 *2 C2 *2 F,5= cal. (c)
ater #as shift reaction C *2 7 C2 F,5= cal It is possible to ha$e this reaction in the upper furnace to a certain etent while the reaction woul% be kineticall" more fa$ourable in the mi%%le 9one where temperature ran#es between 5== 0 1== oC. The reaction influence% b" catal"sts an% fleshl" re%uce% iron posses catal"sin# properties.
4(5
Re(uction o! Iron O2i(es
*aematite
3-e23 C 7 2-e3o&C2 1=.33 GCal
-e3 & C 7 3-e C2 0 5.4 GCal
ustite
-e C 7 -e C2 3.FF GCal
!ll the three are eothermic reactions (releases heat)
The" are referre% as in%irect re%uction reactions. The etent of actual re%uction %epen%s on the si9e an% nature of the ore an% intensit" of #as flow in the ra%ial cross section of the 9one. If ore is lar#e an% non porous i.e. results in outer shell bein# re%uce% to -e (wustite) while the inner core still containin# -e23
,
;e%uction of -e23 an% ma#netite (-e3&) to -e is not complete in this 9one. -e3& is re%uce% to -e mainl" at 4==F==C.
4e5
C$r)on Deosition
2C 7 C2 C it happens at low temperature an% in the re$erse of the solution loss reaction (CC2 2C
The reaction occurs in narrow temperature ran#e &=,== presence of free iron (-e) an% its oi%e catal"ses the reaction. -e C reaction.
-e C2 7 Sla# formation carbon %ecomposition 0in%irect
A(+$nt$'es o! C$r)on (eco#osition
i) ii) iii)
Eecomposition of carbon on the ore surface minimise stickin# of the ores an% enhances permeabilit" Hconomics fuel consumption since the reaction is eothermic. The %eposite% carbon can %escen% alon# with the bur%en an% replace e6ui$alent amount of coke there b" economise fuel consumption. Dis$(+$nt$'es o! c$r)on (eco#osition
Eeposite% carbon reacts with ore to liberate% C which causes burstin# of the ore lumps there b" %ecreasin# the mechanical stren#th of the ore. -e23C -e c. This therefore affects #as %istribution an% permeabilit". Causes formation of bri%#es an% accretions. Carbon react %irectl" with ore C-e -e C Deosition o! c$r)on$tes
Carbonates of -e
Temperature ran#e (5==1===C).
Hten%s from upper 9one ( &,m) below the stack le$el %own to 3m abo$e the tu"ere le$el. ccupies =,= of shaft hei#ht i.e about 4 of shaft $olume. The temperature ran#e 5==1===C is $er" suitable for in%irect re%uction of -e. !
Met"o(s o! i#ro+in' in(irect re(uction.
4
!n increase in in%irect re%uction can be achie$e% b":
(a) (b)
Increase% re%ucibilit" of ores Htension an% enlar#ement of the mi%%le 9one b" use of less reacti$e coke (solution loss). 'on#er / #as ore contact time throu#h the use of hi#h top #as pressure (#as resi%ence time to be increase%) ?se of h"%ro#en bearers (tar steam) 8resence of limestone in the bur%en char#e shortens the mi%%le 9one (en%othermic calcinations reaction) which starts to occur at F===c.
(c) (%)
( ii)
Carburetion of iron
(a)
2C7C2C 3-eC 7-e3C
(b)
3-eC7-e3C&C2
(iii)
ater Bas shift reaction
(a)
C *2 7 C2 *2
(b)
C*2 7 C*2 at hi#h temps Re$ctions in t"e %o,er *ones
Temp abo$e F==1===C Hten%s from 3m abo$e the tu"ere le$el to the hearth bottom. Temperature of molten materials reaches 1&==1&=C an% the #as cools %own to 5== 1===C. +urnin# of coke in front of the tu"ere create an empt" space aroun% the hearth peripher" 0 combustion 9one of %epth 12m. Bau#e in a%miture with the flu starts to fuse in the bell" re#ion. Important chemical reactions occurrin# in this 9one are:
(i) (ii) (iii) (i$) ($) ($i) ($ii) ($iii) (i)
Hn%othermic calcinations of limestone 0 CaC3 7 Ca C 2 Hn%othermic %irect re%uction of -e: -e C 7 -e C Hn%othermic %irect re%uction of Si2: Si2 2C 7 Si 2C Hn%othermic %irect re%uction of
C2 C (D)
Carbon %eposition
2co 2C 7
C2C
Combustion >one -lame temperature 7 15==2===C This is a 9one in front of tu"ere where coke is burns in air pro%ucin# heat #ases ash.
5
The burnin# of coke in front of a tu"ere is calle% race wa" 9one. ;ace wa" / hearth annulus or acti$e perimeter. The actual acti$e rin# area of 12m %epth aroun% the hearth peripher". The si9e an% shape i.e the %epth of the racewa" an% wi%th between nei#hbourin# tu"eres which %etermines the race wa" area are of #reat importance for : -urnace pro%ucti$it" an% the thermal state of the hearth. ;eaction of the Combustion >one
(i) (ii)
C 2 C2 C
C2 heat F&&= calories (1.2m) C 0 *eat &1=== cal
Eurin# the 1st reaction of 2 of the blast react with coke to initial form C2 . This 9one is referre% as oi%isin# 9one because an" metal or sla# iron inclu%in# metal oi%es fallin# in this 9one are reoi%e to $ar"in# %e#rees. 1 N( Re$ction
In this 9one C2 bein# unstable imme%iatel" reacts with carbon (coke) to pro%uce C. The reaction where C2 C is calle% re%ucin# 9one. It is here that a consi%erable re%uction of sla# an% metal oi%es. Cohesi$e 9one is the 9one where iron an% sla# fusin#. F$ctors $!!ectin' r$ce,$y *one :
-actors affectin# the si9e an% shape of the racewa": (a) (b) (c) (%) (e) (f)
+last parameters: $olume $elocit" pressure an% temperatures. Eiameter len#th an% shape of the tu"ere. Inter tu"ere spacin# is the number of tu"eres aroun% the hearth peripher". Coke si9e 0 narrow si9e ran#e. +last mo%ification 0 blast a%%iti$es i.e tar natural #as steam 2 enrichment pul$erise% coal fuel oil coke o$ens #as. ature of stock mo$ement. Furn$ce Irre'u%$rities
-urnace pro%uction is beset with numerous %ifficulties an% irre#ularities a successful counter action of which onl" can #i$e rise to smooth operation an% hi#h pro%uction is therefore re6uire%. The interference of smooth operation i.e smooth counter flow of soli%s li6ui%s an% #ases occurs %ue to abnormalities in the ph"sical character of the bur%en materials which themsel$es ma" arise on chemical #roun%s. The usual %ifficulties an% counter measure #enerall" a%opte% are %etaile% below. H$n'in'
This is a con%ition that takes place in the furnace when %escen%in# material meets with resistance in its %ownwar% flow or the #as in its upwar% flow. This arises from the followin# causes: 1) 2) 3)
bstruction to bur%en %ecent %ue to presence of scaffol%s scabs an% stick" bosh sla#. Increase% #as support %ue to fines. +lockin# of %rain holes in the bosh %ue to hi#hl" $iscous or resoli%ifie% sla# or fro9en iron.
F
A0
Sc$)s $n( Sc$!!o%(s
-ormation of scabs scaffol%s bri%#es arches an% the like on the walls offer ph"sical resistance an% obstruction to material %ownwar% mo$ement. +" $irtue of re%ucin# the passa#e si9e material %escent is slowe% %own or e$en brou#ht to a *olt. !s a result #aps between suspen%e% material that is mo$in# %own form which when the" collapse lea% to chillin# of the hearth an% %ama#e to the refractor" linin# flat coolers an% e$en the tu"eres. hen the scaffol%s / scabs come awa" the su%%en %escent of material is calle% bosh slip. 30
Peri"er$% O+er%o$(in'
Hcessi$e presence of ore fines in the peripher" of the furnace lowers its permeabilit". The furnace works har% stock %escent becomes erratic an% the blast $olume backpressure rises rapi%l". In etreme cases the furnace han#s as a result. Solution to this is usuall" alterin# the +/C ratio char#in# se6uences an% remo$al of fines (coke / ore) b" use of screens. C0
Hot H$n'in'
This comes about as a result of an increase in flame temperature i.e in front of the tu"eres an% causes the bur%en in the furnace to %escent irre#ularl" %ue to cause the furnace to han#. So%ution to "$n'in' (ue to $)o+e is usu$%%y:
i) ii) iii)
;e%uction of flame temperature b" steam inDection. +" lowerin# the hot blast temperature. ?se of low carbon containin# coke.
D0
Co%( H$n'in'
Col% han#in# is %ue to lime" sla#s which %ue to their poor flow properties block the coke pores in the bosh. 8oor sla# flow can be %ue to low bosh an% hearth temperatures which ma" arise from: i) ii) iii)
'eakin# flat coolers or tu"eres. Su%%en %ecrease in the coke carbon content. Su%%en increase in team inDection. So%ution9
i) ii) iii)
;eplacement of leakin# flat coolers. Char#e a li#ht bur%en i.e bur%en with a low bur%en to coke ratio. Increase the blast temperatures an% temporaril" re%uce the blast $olume. P%e$se Note :
! %ecrease in in%irect re%uction can also cause col% han#in#. The en%othermic heat re6uirement of the increase% %irect re%uction of wasta#e (-e )ma" result in free9in# of iron in the $oi%s thus hin%erin# #as throu#h flow. -ree9in# of the iron if it occurs on the walls woul% resemble scabs.
1=
So%ution
i) ii)
Increasin# blast temperatures an% Coke /re/ ratio of char#e temporaril" an% impro$in# #as %istribution throu#h char#in# control. SLIPS
! slip is a su%%en rapi% %escent of the stock column in the furnace resemblin# a collapse Their occurrence is %ue to: i) i$)
*an#in# in the upper furnace as a result of which material below continues to %escent an% smelte% while material abo$e remains suspen%e%. -ormation of arches insi%e the furnace. hen the suspen%e% material collapses as a result of its own wei#ht a slip is sai% to ha$e occurre%. H$n'in' $n( su)se:uent s%iin' #$y )e tre$te( to9
a)
Con%ensation of oi%ise% alkali metal an% 9inc $apours in the upper shaft an% formation of har% imper$ious mass protru%in# inwar%s.
b)
*in%rance of counter flow of soli%s an% #ases %ue to carbon %eposition or lan%in# of pow%er" coke or ore fines in the interstices of the lump" material.
c)
;esoli%ification of pre$iousl" fuse% sla#s %ue to operational $ariations.
%)
Slips ma" result from %ifferential rates of stock %escent as a result of channellin#.
i) ii) iii) i$) $) $i)
Eisruption of uniform stock %escent Eeterioration of uniform #as %istribution. Increase of coke rate. Bi$es rise to inconsistent metal 6ualit". Eecreases pro%uction. !ffect the thermal state of the hearth. The emission of %ust containin# a lot of coke an% the collapse of thermall" unprepare% an% unre%uce% ore into the lower furnace results in furnace irre#ularities like chille% hearth an% $ariations in iron 6ualit". Sc$!!o%(s
11
Scaffol%s are accretions of materials which buil% up on the furnace wall an% proDect towar%s the furnace centre. ! bri%#e forms ri#ht across the furnace hori9ontal cross section restrictin# stock %escent an% #as ascent. This buil% up occurs from the bosh to the upper part of the stack. Benerall" the term scaffol% refers to buil% up in the low temperature re#ions where li6ui% phases %o not normall" occur i.e in the mi%%le an% upper shafts. The re%uction in cross sectional area at the place of scaffol% formation lea%s to %isruption of stock %ecent an% to an increase % #as mo$ement in unaffecte% re#ions. !s a result a) b) c) %) e)
*i#h top #as temperatures ri#ht up to the uptakes are eperience%. Top #as C2 le$el %rops Bas utilisation %rops In%irect re%uction of the ore is re%uce% Coke rate an% #as $elocit" increase. C$uses o! Sc$!!o%(s
The" are forme% as a result of %eposition of compoun%s of $olatile alkali metals (a G) lea% 9inc tin low an% wi%e softenin# ran#e ores fine sinters carbon %eposition ba% furnace operation an% non uniform coolin# with flat coolers. Contro% o! Sc$!!o%( For#$tion
i) ii)
Control of input materials (6ualit" control choosin# ore containin# less alkalis lea% 9inc an% tin). Impro$in# eit of the scaffol%in# formin# a#ents. M$ni!est$tions o! Sc$!!o%(s
i) ii) iii) i$) $)
Su%%en increase in flue %ust emission (1== 2==) or e$en more. *i#h top #as pressures 'ow #as temperatures abo$e scaffol% with low inwall temperatures in the re#ions of the scaffol%in#. *i#h C/C2 ratio in the top #as on uniform bur%en %escent i.e han#in# an% slippin#. Met"o( o! Contro%
i)
iii) i$) $) $i) $ii) $iii) i)
Eecrease alkali le$els in the 5==1===C 9one b" increasin# their eit in the sla# (i.e b" lowerin# Ca / Si2 ratio. N39 basicit" cannot be lowere% below a certain limit for optimum %esulphurisation unless eternal %esulphurisation is practise%. ii) Control of flame temperature 0 low flame temperature re%uces alkali buil% up an% facilitates their remo$al $ia sla#. (Coke rate #oes up). 'owerin# 9inc input throu#h use of hi#her sinter le$els in the bur%en. Steep boshes to pre$ent an" buil% up formation. ?se of compact bricks capable of resistin# alkali an% C seepa#e. (*i#h !l23 bricks fire% at hi#h temperatures less prone. Substantial wall workin# to %iscoura#e formation this can be achie$e% $ia stock line control J or < t"pes increase% tu"ere %iameter to re%uce blast penetration an% encoura#e peripheral #as flow. ?sin# raw materials with small softenin# ran#e. Screenin# of all char#e% materials. Suitable blen%in# to eliminate alkali le$els.
12
CHANNELLING
1.=
C"$nne%%in'
$ens when #as mo$in# up the stack flows une$enl" %ue to $arie% properties. In a mie% be% of si9es of materials the be% permeabilit" %ecreases as the proportions of small particles or fines %ecreases. !s a result of %ifferential #as flow in the be% une$en #as %istribution of %ifferential #as $elocities occurs. In areas where the #as $elocit" is hi#h flui%isations ma" start smaller particles lifte% up an% the #as will rush into those re#ions which become more an% more unstable until the maDorit" of the particles are suspension. The suspen%e% la"er ma" eten% to consi%erable %epths. Suspension of particles is the be#innin# of channellin# an% #as starts to show an acti$e influence on the structure of the bur%en in the affecte% areas. In characteristic cases channellin# is %irectl" relate% to aero 0%"namic transfer of particles. In(ic$tions O! C"$nne%%in' (a) (b) (c) (%) (e) (f) (#) (h)
Sharp rise on peripheral #as temperatures near the channel. Sharp fall on C2 content of the top #as. ;ise an% closer approach of the top #as temperature *i#h flue %ust losses. Collapse of one or both of the stock line in%ications into the suspen%e% la"er Se$ere erratic #as pressure pulsation %urin# lowerin# of the bi# bell. Erop on blast pressure (when channels are lar#e). 'ar#e blast $olumes enterin# the tu"eres close to the channels. C"$nne%%in' Contro%
(i) (ii) (iii) (i$)
Is a con%ition that takes place in the furnace when %escen%in# material meets with resistance in its %ownwar% flow or the #as in its upwar% flowK This arises from the followin# causes: (1) (2) (3) (&)
bstruction to bur%en %ecent %ue to presence of scaffol%s scabs an% stick" +osch sla#. Increase% #as support %ue to fines. +lockin# of %rain holes on the coke la"ers. +lockin# of #as passa#e in the bosh %ue to hi#hl" $iscous an% resoli%ifie% sla# or fro9en iron.
A.
SCA3S AND SCAFFOLDS
-ormation of scabs an% scaffol%s bri%#es arches an% the like on the walls offer ph"sical resistance an% obstruction to material %ownwar% mo$ement. +" $irtue of re%ucin# the passa#e si9e material %escent is slowe% %own or e$en brou#ht to a *olt. !s a result #aps between suspen%e% material that is mo$in# %own from which when the" collapse le% to chillin# of the hearth an% %ama#e to the refractor" linin# flat coolers an% e$en the tu"eres.
13
hen the scaffol%s/scabs come awa" the su%%en %escent of the material is calle% bosh slip. 30
C.
Peri"er$% O+er%o$(in'
Hcessi$e presence of ore fines in the peripher" of the furnace lowers its permeabilit". The furnace works har% stock %escent becomes erratic an% the blast $olume backpressure rises rapi%l". In etreme cases the furnace han#s as a result. Solution to this is usuall" alterin# the +/C ratio char#in# se6uences an% remo$al of fines (coke/ore) b" use of screens. Hot H$n'in'
This comes as a result of an increase in flame temperature i.e in front of the tu"eres an% cause the bur%en in the furnace to %escent irre#ularl" an% to cause the furnace to han#. Solution to han#in# %ue to abo$e is usuall":
i) ii) iii)
;e%uction of flame temperature b" steam inDection. +" lowerin# the hot blast temperature. ?se of low carbon containin# coke.
D.
Co%( H$n'in'
Col% han#in# is %ue to lime" sla#s which %ue to their poor flow properties block the coke pores in the bosh. 8oor sla# flow can be %ue to low bosh an% hearth temperatures which ma" arise from: i) ii) ii)
'eakin# flat coolers or tu"eres. Su%%en %ecrease in the coke carbon content. Su%%en increase in steam inDection. So%ution
i) ii) iii)
;eplacement of leakin# flatcoolers or tu"ere. Char#e a li#ht bur%en i.e a bur%en with a low bur%en to coke ratio. Increase the blast temperature an% temporaril" re%uce the blast $olume.
P%e$se Note
! %ecrease in in%irect re%uction can also cause col% han#in#. The en%othermic heat re6uirement of the increase% %irect re%uction of $ustite (-e) ma" result in free9in# of iron in the $oi%s thus hin%erin# #as throu#hflow. -ree9in# of the iron if it occurs on the walls woul% resemble scabs. So%ution
i) ii)
Increase blast temperature an% Coke/ore ratio of char#e temporaril" an% impro$in# #as %istribution throu#h char#in# control. SCAFFOLDS
Scaffol%s are accretions of materials which buil% up on the furnace wall an% proDect towar%s the furnace centre. ! bri%#e forms ri#ht across the furnace hori9ontal cross section restrictin# stock %escent an% #as ascent. This buil% up occurs from the bosh to
1&
the upper part of the stack. Benerall" the term scaffol% refers to buil%up in the low temperature re#ions where li6ui% phases %o not normall" occur i.e in the mi%%le an% upper shafts. The re%uction in crosssectional area at the place of scaffol% formation lea%s to %isruption of stock %escent an% to an increase% #as mo$ement in the unaffecte% re#ions. !s a result a) b) c) %) e)
*i#h top #as temperatures ri#ht up to the uptakes are eperience%. Top #as C2 le$el %rops. Bas utilisation %rops. In%irect re%uction of the ore is re%uce%. !n% coke rate an% #as $elocit" increase. C$uses o! Sc$!!o%(s
The" are forme% as a result of %eposition of compoun%s of $olatile alkali metals (a G) lea% 9inc tin low an% wi%e softenin# ran#e ores finesinters carbon %eposition ba% furnace operation an% non uniform coolin# with flat coolers. Contro% o! Sc$!!o%( For#$tion
i) ii)
Control of input materials (6ualit" control choosin# ore containin# less alkalis lea% 9inc an% tin). Impro$in# eit of the scaffol%in# formin# a#ents. M$ni!est$tions o! Sc$!!o%(s
i) Su%%en increase in flue %ust emission (1== 2==) or e$en more. ii) *i#h top #as temperatures. iii) 'ow temperatures abo$e scaffol% with low in wall temperatures in the re#ions of the scaffol%in#. i$) *i#h C/C2 ratio in the top #as. $) on uniform bur%en %escent i.e han#in# an% slippin#. Met"o(s o! Contro%
i) ii) iii) i$) $) $i) $ii) $iii) i)
Eecrease alkali le$els in the 5== 0 1==C 9one b" increasin# their eit in the sla# (i.e b" lowerin# Ca/Si2 ratio N039 basicit" cannot be lowere% below a certain limit for optimum %esulphurisation unless eternal %esulphurisation is practice%. Control of flame temperature low flame temperature re%uces alkali buil% up an% facilitates their remo$al $ia sla#. (Coke rate #oes up). 'owerin# 9inc input throu#h use of hi#her sinter le$els in the bur%en. Steep boshes to pre$ent an" buil% up formation). ?se of compact bricks capable of resistin# alkali an% C seepa#e. (*i#h !12 3 bricks fire% at hi#h temperature less prone) Substantial wall workin# to %iscoura#e formation this can be achie$e% $ia stockin# control J or < t"pes increase% tu"ere %iameter to re%uce blast penetration an% encoura#e peripheral #as flow. ?sin# raw materials with a small softenin# ran#e. Screenin# of all char#e% materials. Suitable blen%in# to minimise alkali le$els. SLIPS
! slip is a su%%en rapi% %escen% of the stock column in the furnace resemblin# a collapse.
1
Their occurrence is %ue to: i) ii)
*an#in# in the upper furnace as a result of which material abo$e remains suspen%e%. -ormation of arches insi%e the furnace. hen the suspen%e% materials collapse as a result of its own wei#ht a slip is sai% to ha$e occurre%. *an#in# an% subse6uent slippin# ma" be trace% to:
a) b) c) %)
Con%ensation of oi%ise% alkali metal an% 9inc $apours in the upper shaft an% formation of har% imper$ious mass protru%in# inwar%s. *in%rance of counter flow of soli%s an% #ases %ue to carbon %eposition or lan%in# of pow%er" coke or ore fines in the interstices of the lump" material. ;esoli%ification of pre$iousl" fuse% sla#s %ue to operational $ariations. Slips ma" result from %ifferential rates of stock %escent as a result of channellin#
i) ii) iii) i$) $) $i)
Eistribution of uniform stock %escent. Eistribution of uniform #as %istribution. Increase of coke rate. Bi$es rise to inconsistent metal 6ualit". Eecrease pro%uction. !ffect the thermal state of the hearth. The emission of %ust containin# a lot of coke an% the collapse of thermall" unprepare% an% unre%uce% ore into the lower furnace results in furnace irre#ularities like chille% hearth an% $ariations in iron 6ualit". CHOKING OF HEARTH
This is %efine% as %isturbance of furnace workin# a s a result of li6ui% %raina#e interference throu#h the coke #ri% into the hearth an% buil% of fro9en soli%s on the hearth walls. C$uses o! C"okin'
i) ii)
iii) i$) $)
*i#h blast rates that interfere with li6ui% %raina#e lea%in# to floo%in# con%itions. 'ime" sla#s which ha$e a hi#h meltin# point. (These ten% to free9e when the operatin# temperatures are below their li6ui%us temperature 0 i.e the temperature at which li6ui% is in e6uilibrium with soli% such that a minute %rop in temperature is followe% b" precipitation of soli% particles in the li6ui%). ?se of low stren#th coke which breaks %own %urin# %escent alterin# permeabilit" $ia the be%. ?se of lime" sinters 0 same reason as (ii). !n% %ue to accumulation of #raphite %urin# pro%uction of hi#h silicon iron.
1,
Stock %escent is slu##ish an% sla# appears in the tu"eres soon after tappin#. Iron carr" o$er in the sla# is pre%ominant an% loss of tu"eres an% sla# notches becomes ea##erate%. Eue to %eposition of soli%s on the hearth walls the hearth $olume %win%les an% the le$el of li6ui%s rises 6uickl" after tappin#. Contro% o! C"okin'
i) ii) iii) i$) $) $i)
Impro$e mechanical stren#th of coke 0 break%own of coke is the most important cause of chokin# an% burnin# of tu"eres. If break%own of coke is %ue to alkali attack lower the alkali input if possible an% operates at low flame temperatures an% operates at low flame temperatures an% use lean sla#s 'ower hot blast temperature in or%er to increase coke/ore ratio. Control bur%en %istribution to achie$e hi#h sinter le$els at the peripher" e .# SS CCC / char#e.
0
The use of coke (C) as re%uctant (en%othermic re%uction re6uires heat). In(irect re(uction
The use of #as (C) (L *2 Bas) as a re%uctant. Hothermic re%uction releases heat). Hothermic on the lower 9one takes place at the tu"ere 9one where 2 in the blast air reacts with carbon (C) in coke %urin# burnin#. T,o ,$ys o! Re(ucin' Tuyere Di$#eter $re9
i) ii)
Cla"in# an% holin# ;e%ucers Tu"ere spacin# shoul% facilitate a tu"ere racewa" 9one. Inter tu"ere spacin# shoul% be #enerall" 1.&m to 1.m 2 a%%ition facilitates hi#h flame to usuall" accompanie% b" steam inDection. 3F SLAG
Sla# 0 *omo#eneous melt consistin# of oi%es of Si <# !l Ca -e etc The
i) ii)
'ower the meltin# point of impurities (i.e Si2 !l23 etc). To form a me%ium throu#h which the impurities can be remo$e%.
Carburisation O the meltin# point for -e from 134C T 12,=C.
;;;
14
thers are -e Ca an%
3$sic o2i(es
Ca <# alkali oi%es (G2 a) -e an%
N3:
!n" oi%es that %onate 2 is a basic oi%e
10
Aci(ic o2i(es
Si2 an% !l23 !n" oi%e that accepts 2 is an !ci%ic oi%e. SLAG 3ASICIT<
-
Is the ratio of +asic i%es to !ci%ic i%es. *i#h basicit" 0 caters for Sulphur %urin# %esulphurisation in hot metal to sla# Tyes o! 3$sicity
1.
+1 0 Ca/Si2
2.
+3 0 Ca <# Si2
3.
(ores without !lumina)
+& 0 Ca <# Si2 !l23 (ores with !lumina) S%$' .iscosity
-
(a) (b)
The eas" with which sla# flows 'ow $iscosit" 0 hi#her an% flui%it" (meltin# point) *i#h $iscosit" 0 low flui%it" S%$' Li:ui( Te#er$ture
-
-
Temperature at which sla# melts to li6ui% form. Temperature abo$e sla# eists in li6ui% form/state Sla# temperature = 0 1==oC abo$e hot metal temperature e.#. *
The minimum hearth temperature necessar" for free runnin# of sla# is about 1== 1=oC in or%er to pro$i%e some superheat in the hearth an% ensure that both sla# an% iron are in li6ui% state un%er all operatin# con%itions. Eesulphurisation takes place when iron/hot metal passes throu#h sla# an% the reactions sulphur is release% b" hot metal an% collecte% b" sla#. St$'es o! S%$' For#$tion
1.
8rimar" sla# 0 -e Si2 !l23 <# (lower stark an% bell" 9one)
2.
+osh sla# 0 <# Si2 !l23 (bosh 9one)
15
3.
*earth or final sla# 0 Ca <# !l23 Si2 (tu"ere 9one to hearth 9one)
/0
Pri#$ry s%$'
Is the sla# that is forme% when the bur%en starts to soften with the partiall" re%uce% oi%e (-e) combinin# with the #an#ue (i.e !l23 Si2) at temperature abo$e 12==oCK -e is the onl" constituenc" bein# re%uce% %urin# sla# formation (low $iscosit") 8rimar" sla# constitute hi#h -e (content) an% has hi#h meltin# point. Ca substitute -e to react with Si to form CaSi an% #i$e out C2. -
10 -
-
3os" s%$'
-orme% when a primar" sla# %escents to hi#her temperature with the -e rapi%l" re%uce% b" carbon O as well as carbon monoi%e (C) an% continuall" absorbs Ca chan#in# to Ca !l23 Si2 with some minor impurities accompan"in#. The Si2 in the sla# is mainl" from iron ore minus the ash silica pro%uce% in the combustion 9one. The basicit" will be hi#h i.e Ca Si2 Si2 The remo$al of -e then we impro$e our $iscosit". Ca has hi#h affinities for silica so sla# nee%s or absorbs much silica than li6ui% hot metal. I#ort$nce o! 3os" s%$'
-
1. 2. 3. &. . N03:
The properties of bosh sla# are of utmost important for furnace pro%ucti$it" an% metal 6ualit". The most important properties are: *i#h flui%it" 0 it ensures free mo$ement of the bosh sla# throu#h the coke #ri% arrow fusion ran#e 0 it lower the pressure %rop in the lower furnace (e.#. 1=kpa 0 &= kpa 7 11= kpa (hi#h kpa %rop) 1=kpa 0 5= kpa 7 4= kpa (lower pressure %rop). *i#h fusion/meltin# 0 for a%e6uate preheatin# before entr" of the smeltin# pro%ucts into hearth (hi#h resi%ence time of sla# in the bosh). *i#h basicit": for lower Si an% S iron a%%ition of <# to thin an" ecessi$el" lime" sa#. Items (2 0 &) abo$e permit the use of hi#hest blast temperatures without affectin# the metal silicon or smooth furnace operation. +osh sla#s are more basic than final sla# because more aci%s from coke ash are absorbe% at the tu"ere an% below. FINAL SLAG OR HEARTH 4Te# /=>7 ? />77 OC5
*earth sla# is the forme% on %issolution (%issol$in# into) of lime which was not incorporate% in the bosh an% absorption of the coke ash release% %urin# combustion. This sla# run alon# with the molten iron into the hearth accumulates there an% forms a pool with the molten metal un%erneath. Eurin# the passa#e of the iron %roplets throu#h the sla# la"er the sla# reacts with the hot metal an% a transference of mainl" Si
(a) (a)
'ean sla# 0 more Si
1F
(b)
Too much sla# less coke burnt an% less pro%ucti$it"
Sulphur is the bi##est enem" in hot metal pro%uction causes fractures or breakin# of our pro%uct an% cost. The most important purpose of S%$' Contro% is the control of sulphur since it is the ke" to iron 6ualit". It is unecono#ic$% to pro%uce a low 8 low Si an% hi#h sulphur iron. Si (silica) cause #raphite in iron or promote #raphitisation but eas" to shape. 'ean sla# are short 0 no chan#e to chemical composition. 'ime sla#s are lon# 0 %ue to chan#e in temperature an% chemical composition.
T"e Purose o! Criteri$ o! Goo( 38F S%$'
! proper/#oo% sla# shoul% perform a $ariet" of function for maimum efficienc" both in fuel as well as furnace performance it shoul% %etermine control or posses the followin#: 1. 2. 3. &.
PPP
.
Sulphur retention potential 0 i.e hi#h sulphur in sla# an% hot metal if the ratio is hi#h means more S in sla# an% less in hot metal. 'ow $iscosit" hi#h flui%it" 0 i.e the sla# shoul% be able to run out of the furnace freel" an% rapi%l" at the operatin# temperature as the critical hearth temperature which enables the sla# to flow affects the coke rate the furnace runnin# characteristics of the sla# will affect fuel economical consumption. -ree mo$ement of the stock an% furnace #ases are %etermine% b" the ph"sical properties an% 9one melt of fusion of primar" an% bosh sla#s which #reatl" affects the pro%ucti$it" of the furnace an% the 6ualit" of the pro%ucts (i.e *T< an% sla# 6ualit"). !lkalis 0 sla# shoul% ha$e hi#h alkali remo$al capacit" (a G) Sla# $olume shoul% be as low as is compatible with other criteria as use% abo$e. Suitabilit" in for use in manufacture of cement an% for roa% use %urin# maintenances.
,.
4. 5. F. 1=.
Furn$ce Desu%"uris$tion in 3%$st
Is the remo$al of hi#h sulphur in the raw materials an% hot metals into sla#K
2=
Tyic$% 3F Su%"ur 3$%$nce
M$teri$% Coke 4/&S5 @77k'8t"# C"$r'e #$teri$% 4Ore- !%u25 Iron 7076&S F%ue (ust $n( '$s S%$'
Su%"ur Inut K' &
Su%"ur Outut K'
&
,= 1.
5= 2=
=.3 & =.34 ,.52 F1 Sulphur remo$al much of it occurs when hot metal passes throu#h the sla# in the hearth.
Eurin# the %ecent of the stock the sulphur in the char#e% materials is absorbe% in the shaft molten iron an% sla# an% the final transfer of sulphur from the metal to sla# occurs in the hearth. Coke starts #i$in# off its sulphur in the shaft (2=) ai%e% b" an increase in *2 content of the stack #as an% %ecreasin# si9e of the coke. +ulk of sulphur in the coke transfer b" chemical reactions %urin# combustion of coke in front of the tu"ere.
Note: lean sla# has a hi#h alkali retention capacit" because Ca is less in lean sla# than the
alkalis (e.#. G a2) replaces Ca an% reacts in Sulphur silica an% e.t.c. 8art of Sulphur #oes to hot metal since %esulphurisation has been compromise%. CHEMESTR< OF SULPHUR REACTIONS NOTE9
Sulphur in iron %enote% as QSR Sulphur in sla# %enote% as 0 (S) 1.
Sulphur enters into the metal throu#h
2.
-eS 1= -e23 7 4 -e3& S2 S2 2C S2 2C Q-eR S2 (#) 7 Q-eSR
3.
CaS& Q-eR 3C Q-eSR Ca 3C
&.
CaS& c 7 CaS & C CaS -e 7 Ca Q-eSR
.
CaS Si (#) 7 SiS (#) Ca SiS (#) 2Q-eSR 7 Q-e 0 SiR Q-eSR
,.
Eesulphurisation b" lime Ca Q-eSR QCaR(s) 0 C 7 CaS C Q-eR 2Q-eR
2Q-eSR 2 Ca(#) QSiR 7 2 CaS Si2
Desu%"uris$tion in t"e S"$!t8 Su%"ur Re$ctions in t"e S"$!t
21
The sulphur mo$ement in the shaft compromises of liberation of sulphur from the coke an% its absorption b" the bur%en components from the #ases. !s sla# an% iron comes %own there is a pick in sulphur content but in sla# continue to rise while %roppin# in hot metal. =4= sulphur of the coke enters the #as phase before the tu"ere %ue to combustion of coke. Sulphur is liberate% b" the coke an% is absorbe% b" the bur%en components. Sulphur is ascen%in# tu"ere #as is absorbe% b" the %escen%in# bur%en an% re%uce% iron. Sulphur in both sla# an% hot metal rises continuousl" up to the tu"ere le$el. In front of the tu"ere le$el there is a sli#ht %rop of iron sulphi%e (-eS an% is foun% in hot metal) thereafter the -eS %rops #reatl" in the hearth up to the iron notch. The sulphur in sla# rise onl" a little in the tu"ere 9one thereafter there is rapi% of increase/rise of sulphur in the sla#. Di$'r$#s
-e shoul% not be more on sla# because nee%s more heat to keep sla# temp hi#h an% at hi#h flui%it" an% low $iscous. Thus we nee% less than 1= -e in sinter re6uires hi#h temp/ heat hence hi#h coke rate. -e C 0 shoul% complete in mi% 9one SULPHUR REACTION IN THE 3OSH
-or efficient %esulphurisation in the bosh sla# shoul% be flui% hi#h basic (hi#h basicit") an% low -e (i.e. re%ucin# sla#)
The hi#h the S in the bosh sla# the lower will be the loa% on the hearth sla# for the final %esulphurisation of iron.
If the bosh sla# is (hi#h) $iscous it ma" be ma%e flui% b" increasin# the man#anese (
If a low man#anese iron is sort for the sla# shoul% be ma%e (thin) flui% b" the use of <# (foun% in limestone).
N3:
-inal metal S %epen%s upon the etent of echan#e of S between the iron %roplets an% the hearth sla# %urin# the passa#e of the iron %roplets throu#h the hearth sla#. (*i#h basicit"/ ecess Ca) FACTORS DETERMINE THE RATE OF DESULPHURISATION
1.
Sulphur reaction rate of the sla# %epen%s on
(i) (ii) (iii)
+asicit" of temp of sla# Sla# bulk ($olume shoul% be low to allow %esulphurisation) Contact surface time
2.
;eaction surface between the sla# an% metal.
3.
Contact time 0 the #reater the thickness of the sla# la"er throu#h which the iron %roplets pass the #reater the contact time the #reater the %esulphurisation.
22
&.
Sla# compositions 0 sla# basicit" shoul% be hi#h in Ca (lime) but lower in -e content.
.
Sla# bulk sla# basicit" an% sulphur loa%low.
,.
Sla# an% metal $iscosities 0 if too hi#h affects %esulphurisation because of surface area an% temp to allow iron to pass throu#h sla# hence impro$e %esulphurisation.
4.
SUMMAR<
The control of sulphur in iron is base% on the followin# 1.
!n etensi$e prere%uction of ore before it reaches the bosh it ensures a hotter bosh (helps sla# flui%it").
2.
*i#h basicit" an% low -e content of bosh sla#.
3.
-lui% bosh sla# 0 thinnin# b" <# an% or
&.
-or rich ores an% conse6uent low sla# $olume the coke rate the coke sulphur an% the coke ash shoul% be low a low coke rate an% a low coke ash take care of ecessi$e bosh basicit" an% a low coke rate an% low coke sulphur %o so of sulphur solubilit" in sla#.
.
8art replacement of coke with h"%rocarbons e.#. tar lowers the input of coke ash (thus lowerin# from e.#. Fk# to ,k# coke input).
,.
'ower sulphur loa% b" %ecreasin# coke rate.
4.
?se of preflue% bur%en (e.#. sinter) 0 hi#h re%ucibilit" uniform basicit" attracts S #as to preflue% bur%en.
5.
'ower input of sla# -e into the hearth.
F.
Hcessi$e lime in bosh sla# to be replace% b" <# (impro$es flui%it" or lowerin# the $iscosit") as well as maintainin# the %esulphurisation capacit".
1=.
Increase in sla# basicit".
11.
-or hi#h ash coke a$oi% refractor" bosh sla# b" replacin# part of the coke b" h"%rocarbons e.#. tar.
12.
1. 2. 3. &. . ,.
Channellin# Scaffol%s Seabs *an#in# Slippin# Chockin# of hearth
23
4. 5. F. 1=.
Coke mess Chille% hearth +urnin# of tu"eres re shift STOCK DISTRI3UTION AND CHARGING
+last furnace pro%ucti$it" %epen%s upon the 6uantit" of air that can be blown per unit time. The 6uantit" of #as that can be accepte% is #o$erne% b" the uniformit" of $oi%a#e (pores) an% particle si9e both of which %epen%s upon proper material %istribution in the stock (bur%en) column. Eurin# char#in# materials ten% to se#re#ate ra%iall" accor%in# to nature si9e an% %ensit". onuniform ra%ial %istribution of the bur%en materials results in a nonuniform ra%ial %istribution of particle si9e $oi%a#e an% hence another of permeabilit" of the bur%ens. The stock line cross section can be %i$i%e% into 3 9ones i.e. (i) peripher" (ii) interme%iate (iii) Central. The interme%iate 9one occupies = of the total cross sectional area an% 3= 0 3 of the #as passes throu#h this area. Hach of the peripher" an% central occup" 2 of the area respecti$el" an% , 0 4= of the #as passes throu#h them. Char#in# se6uence is %enote% b" e.#. CC/ where %enotes ore ' 0 limestone S 0 sinter an% C 0 coke or roun% or batch. The %istribution of materials when %umpe% on a heap on a stationar" be% #enerall" beha$es as follows: (i)
The finer particles ten% to accumulate in a peak or ri%#e rubble an% lumps sli%e %own to %istance accor%in# to their kinetic ener#ies the lar#est #oin# furthest.
(ii)
Two %ifferent materials %umpe% un%er the same con%itions ten% to ha$e their surfaces at %ifferent an#les with the hori9ontal i.e. the" ha$e %ifferent an#les of repose 0 ore: 3= 0 3o coke: 3& 0 &=o. Smaller particles ha$e #reater an#les than lar#er particles. !n#le of repose %epen%s upon the moisture content coefficient of friction an% si9e as well as the %ensit" of the materials.
(iii)
An'%e o! reose o! )%$st !urn$ce c"$r'e #$teri$%s M$teri$% An'%e o! reose Me$n si*e 4##5 $+er$'e si*e R$n'e 4##5
Sinter
Pe%%ets
Coke
R$, Ore
313& 2= 3=
2,25 11 51&
335 & 24
333 15 1=3=
Two profiles forme% when materials are char#e% into the furnace are (i) < 0 shape (ii) J 0 shape < 0 shape% profile forme% when the material or bur%en particles hit the bur%en material or stock column. J 0 shape% profile forme% when the bur%en material/ particles hit the furnace walls.
2&
In a J 0 shape% profile the peripher" is packe% an% partiall" blocke% %ue to the presence of fine materials near the furnace wall an% more $olume of #as will ten% to pass throu#h the centre resultin# in low #as utilisation. The m 0 profile obtaine% when the material strikes the surface of the material an% results in both peripher" an% central #as flow. CHARGING C
The material place% first on the bi# bell will accumulate on the inwall of the furnace an% as a result e.#. if ore is place% first on the bell the material near the wall will be mainl" ore an% the same will happen if coke is place% first. Since the coke be% is more permeable than ore be% the #as %istribution in the furnace can be altere% b" char#in# ore 0 coke (C) or coke 0 ore (C). -or mie% bur%en of ore sinter an% pellet the stock %istribution pattern chan#e accor%in# to their proportions an% placement on the lar#e bell. T
Nor#$% Fi%%in' Or Positi+e Fi%%in'
re is place% in the bell first an% coke comes o$er it. This t"pe of fillin# helps the peripher" to return the ore an% coke tra$ellin# to the centre hence makin# the centre more permeable. The char#e is epresse% as 'CCC/ 'CC/ 'CC. This fillin# is use% when the amount of fines is small an% their nearness to wall will not cause the formation of accretions on the wall. ! lower coke rate is usuall" attaine% because of better peripheral utilisation of #ases (C #as). 10
Re+ers$% Fi%%in'8 Ne'$ti+e c"$r'in'
Coke is place% first in the bell with ore comin# o$er coke e.#. CC or CC'. The coke falls ahea% of ore formin# a slope an% ore rolls to the centre loosenin# the peripher". This t"pe of fillin# promotes peripheral #as %istribution results in a hi#her coke rate %ue to low utilisation of C #as e.#. the fillin# can be use in bur%ens where the" ten% to form accretions on the walls. ccasional char#e o$er from the normal fillin# to re$erse fillin# helps in the cleanin# up of the inwalls. 60
Mi2e( Fi%%in'
H.#. C'C/C'CC. It is especiall" suitable for ores or sinters with a lot of fines. It helps in the %istribution of ore both the peripher" an% centre. Some of the fines are also carrie% to the centre alon# with the rollin# coke. =0
Separate -illin#/ 'a"erin#/ +lock -illin# H.#. /CCCC or '/CCC This is when ore an% coke are char#e% separatel". This fillin# is $er" suitable for closel" #ra%e% material i.e. narrow si9e ran#e because this metho% has a ten%enc" to cause si9e se#re#ation if use% where the si9e ran#e is wi%e. Coke R$te ? coke consu#tion r$te
(i)
Coke rate 0 The amount of coke consume% for a unit of iron. Coke rate 7 k#/thm. Coke constitutes a maDor portion of the pro%uction cost of iron (hot metal). +esi%es pro$i%in# mechanical support an% permeabilit" coke is necessar" in the blast furnace for the followin# purposes: Carburisation of iron Cc (k#)
2
(ii) (iii) (i$)
8ro%uction of heat on burnin# at the tu"eres for co$erin# the thermal re6uirements on combustion it pro$i%es C for #aseous in%irect re%uction of iron oi%es Ct (k#) (tu"ere (C) carbon). Eirect re%uction of iron oi%es C% (k#) (%irect re%uction carbon). Eirect re%uction of metalloi%s Cm (k#). Coke rate 7 Cc Ct C% Cm (k# carbon/thm !ll reactions in the blast furnace (ecept #aseous re%uction of iron oi%es 0 in%irect re%uction carbon %eposition an% sla# formation) #i$e thermal %eficienc" (ore en%othermic) i.e. %irect re%uction of iron oi%e an% metalloi%s (meltin# of sla# an% iron) sensible heat of the flue %ust an% top #as ra%iation an% coolin# looses calcinations e$aporation of moisture an% h"%rate% water. Sensible heat lost in the top #as $aries between 0 1= of the total heat #enerate% at the tu"ere an% F= 0F of heat is utilise% showin# the +- to be an efficient counter current apparatus.
N3:
Eirect re%uction accounts for 3 0 = of the hi#h potential heat re6uire% in the lower furnace an% about 2= 0 3 of the total heat re6uire% in the blast furnace process e.#. %irect re%uction of iron oi%es. Re(uction o! Coke R$te
The coke consumption rate per T*< can be %ecrease% b" an" metho% which: (i) (ii) (iii) (i$)
Increases %irect re%uction Eecreases the thermal loa% (those that re6uire heat e.#. moisture cal nation) Increases the sensible heat suppl" increase hot blast temp. Eecreases the sensible heat out flow or replace coke with other fuel. These can be ba%l" realise% b":
(a) (b) (c) (%) (e) (f)
Increase the blast temp +ur%en preparation (si9in#) Stack #as inDection at aroun% 1===C Super bur%en (Eirect re%uce% iron pre 0re%uce% ores) +last a%%iti$es *i#h top #as pressure (*T8) 0 utilisation of chemical ener#" 0 C #as ?tilisation of thermal ener#" temp
Note9
with cheaper an% easil" a$ailable fuels. The increase in in%irect re%uction an% the use of hi#h blast temps are possible but partiall" at the cost of inferior (low) c$ of top #as lower top #as $olume cost of bur%en preparation an% the epen%iture of fuel for %e$elopment of hi#her hot blast temps (e.#. throu#h the use of sinters hi#h softenin# an% low softenin# ran#e).
3%$st Te#er$ture
?se of hi#h blast temperatures results in sa$in# of coke an% in an increase in pro%ucti$it" the sa$in# is mainl" %ue to an increase% suppl" of sensible heat throu#h the blast inor%er to re%uce coke consumption.
2,
3ur(en Pre$r$tion
The primar" purpose is to use blast furnace char#e material which are optimum in si9e easil" re%ucible passes a thermal loa% as low as possible in or%er to achie$e maimum pro%uction with a minimum of coke rate pro%uction cost an% capital outla". 3%$st Furn$ce Pro(ucti+ity
8ro%ucti$it" 7 Total hot metal pro%uce% per %a" 7 orkin# $olume
2=== t/%a" 1==m3
7 (Tonnes) t/m3 per %a" 8ro%ucti$it" (8) 7
coke burne% (U) Coke rate (G)
8ro%ucti$it" %epen%s upon the amount of coke burnt in unit time at the tu"eres an% tu"ere carbon (coke) consume% for pro%ucin# a unit of iron. Note9 The hi#her the blast $olume i.e. the hi#her the 2 input the #reater will be the amount of
coke burnt an% therefore the lar#er will be the pro%uction.
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