The present Invention relates to a process of producing improved dolomite bricks for use as lining materials of L.D. converters in steel plants.
The invention relates more particularly to a process for producing dolomite/magnesia enriched dolomite bricks which are pitch-bonded, hot pressed at a high pressure and temperature, and tempered in a specified manner, so as to have an extended shelf, and service life, an increased cold crushing strength (CCS), and bulk density (B.D.), and do not generate tar fumes during their laying as lining materials in L.D. converters.
The conventional process for producing dolomite bricks comprises crushing and grinding of sintered dolomite into grains followed by screening of the grains into three different fractions according to sizes, such as 3-8 mm, 1-3 mm and 0-1 mm, preparing a batch of the said three fractions of grains, mixing the batch with heated tar used as binder, pressing the mix at a temperature of 45-55°C in a 800T hydraulic press, palletising and storing the bricks in an air-conditioned room.
The dolomite bricks produced in the conventional process have certain drawbacks such as having relatively low bulk density, cold crushing strength, shelf life and service life as lining materials of L.D. converters.
The object of the present Invention is to provide a process for producing dolomite/magnesia enriched dolomite brlcks having increased bulk density, cold crushing strength, shelf life, and service life as lining materials of L.D. converters.
The main distinguishing features of the invented
process from the conventional process are :
(i) optimisation of grain size distribution in the batch
composition of grains for increasing the packing density of the
grains in the bricks leading to reduced porosity, increased bulk
density, corrosion and erosion resistance, shelf life and
materials service life as lining/of L.D- converters;
(ii) enrichment of magnesia in the batch composition of
grains by replacing a part of the fine fractions of sintered
dolomite grains by sintered magnesia grains of same size;
{iii) heating of the coarse fraction of dolomite grains
before mixing the same with molten pitch (instead of heated tar)
used as the binder;
(iv) mixing the cold fine fraction of the grains of
dolomite alone or of both dolomite and magnesia with the hot
mix of coarse fraction of grains of dolomite and molten pitch;
(v) hot pressing of the mix of coarse grains, molten
pitch and cold fine grains and
(vi) tempering of the 'green' bricks formed following a
specified schedule.
The invention is described fully and particularly,
without restricting its scope in any manner, with reference to
the accompanying drawings, in which -
Figure 1 is a flow diagram of the invented process;
and
Figure 2 is the specified schedule followed for
tempering of 'green' bricks.
Referring to Fig. 1, sintered dolomite from a rotary
kiln is stored in bin 1 from which it is transferred to impact
mill 2 where necessary crushing of the material is performed.
The crushed material is discharged into D.D. screen 3 which
delivers grains of size 5-8 mm into bin 4 and grains of size
3-5 mm into bin 5, and transfers grains of size above 8mm to
roll crusher 6 from which the crushed grains are discharged
into T,D, screen 7 which delivers grains of size 1.6-3.0 mm
into bin 8, grains of size 0.5-1.6 mm into bin 9 and grains of
size 0-0.5 mm into bin 10. This fraction 0-0.5 mm is used for
making ramming mass. Fine graining of sintered dolomite/magnesia
is done in the Vibro Mill 13 which delivers dolomite/magnesia
into bin 12. , and.-then grains of size 0-0.3 mm partly into bin 11 and partly onto the
revolving belt conveyor 14 onto which the grains are discharged from
bins A to 10. The mix of grains of size 0 to 8 mm then is
transferred to any one of the scale cars 15A, 15B . Scale
car 15A or 15B delivers fine grains of size 0-0.3 mm into bin
16 or 19 and coarse grains in bin 17 or 18 which are heated in
grain heaters 20A and grain heater 20B to a temperature of
160-200°C, preferably 180°C, and delivered Into mixer 22A or
mixer 22B, into which the required quantity of molten pitch
is fed from the molten pitch containers 21. Cold fine grains
from bins 16 and 19 are then fed into mixers 22A and 22B
respectively in the required proportions of the batch
composition to be produced.
The preferred proportions of coarse and fine grains in the mix are 65% and 35% respectively. 35% fine grains may comprise 20% dolomite fines and 1596 magnesia fines or 5% dolomite fines and 30% magnesia fines. The preferred proportion of molten pitch in the mix is 5.0-5.596 of the total weight of the bricks.
The coarse grains and molten pitch are first mixed in mixers 22A and 22B for 3-5 minutes. Then the cold fine grains are added to mixers 22A, 22Band mixed for another 5-7 minutes. By heating the coarse dolomite grains prior to mixing with molten pitch, a uniform coating and intimate mixing of dolomite grains with molten pitch is obtained.
Mixers 22A and 22B are also heated by the flue gases of the grain heaters 20A and 20B, so that the temperature of the mix is maintained at 110-130°C, preferably 120°C.
Molten pitch in container 21 is prepared by heating solid pitch by means of a thermic oil heating medium system comprising a thermic fluid e.g. Servotherm medium and Hightherm 500 medium which is a mineral oil, kept in a closed loop system which is completely sealed off from air to prevent degeneration of the medium owing to its contact with air at a temperature of around 300°C. This system is used not only for melting pitch, but also for maintaining the pitch in a molten state during its storage in 21, handling and mixing with the coarse dolomite grains. The hot mix in mixers 22A and 22B is transferred to either of the presses 23A and 23B, and pressed at an elevated temperature of 100-110°C at an increased pressure of 1100-1200 kg/cm2 using a hydraulic press of 1600T capacity.
The 'green' bricks produced in presses 23A and 23B are stacked in a tempering pallet 24,
The tempering of the 'green' bricks is performed following an optimum tempering schedule shown in Fig. 2 in which the green bricks are heated to a temperature of 250-350°C in a closed chamber in an atmosphere containing a limited amount of volatile gases evolved from the pitch contained in the bricks. During this heating process, the lower hydrocarbons in the pitch are volatilised forming a skeleton of polymerised carbon network of higher hydrocarbons, which provides a protective film of carbon surrounding the dolomite/magnesia grains through vapour phase diffusion/deposition and improves thereby the properties, such as hydration resistance, residual carbon retention, mechanical strength and pore morphology of the bricks produced.
The temperature cycle followed in the optimised tempering schedule (Fig. 2) comprises ; (a) fast heating of the bricks to a temperature of 180-230°C in about 1 hour to avoid any hydration of dolomite grains; (b) slow heating/maintaining the temperature of bricks in the range of 280-325°C for 7 hours to allow completion of the polymerisation process and uniform distribution of carbon in the bricks through vapour phase diffusion/deposition of carbon; and (c) slow cooling of the bricks to a temperature of about 80-90°C in about 11 hours. The ß -resin content which is higher in pitch than tar gets cured during the tempering process and binds the dolomite grains in the bricks firmly to make the bricks to be of higher cold crushing strength compared with those produced in conventional process. The higher
coking index of pitch than that of tar increases the residual
carbon in the bricks and thereby makes the bricks of higher
corrosion resistance in comparison with the bricks produced in
the conventional process.
The comparative characteristics of tar and pitch are
presented in Table II, from which it is noted that softening
point, content of benzene insolubles, quinolene insolubles and
ß -resin, coking index and specific gravity of pitch are higher
in comparison with tar.
materials The performance of the dolomite bricks as lining/of
L.D. converters depends largely on the packing density of grains
in the bricks. A denser packing of the grains makes the bricks
less porous, less shrinkable, and of higher cold crushing
strength and bulk density.
The first step for attaining an increased packing
density of the grains is to optimise the grain size distribution
in the bricks. For this purpose, six different batch compositions
marked A to F, were prepared, comprising varying percentage of
grains of different sizes, and their packing density was
determined in a known method. From the test results presented in
Table I it is noted that the batch composition marked B is of
the highest packing density i.e. 2.49 gms/cc and comprises grains
of different sizes (by weight %) as : 5.0 to 8.0 mm - 15;
3.0 to 5.0 mm - 17; 1.6 to 3.0 nan - 20; 0.5 to 1.6 mm - 13 and
0 to 0.3 mm - 35. So the batch composition marked B is selected
for the invented process.
The second step for increasing the packing density of dolomite grains in the brick is to press the hot mix at a high pressure of 1100 - 1200 kg/cm 2 and temperature of 100 -1100C.
under a hydraulic press of 1600T capacity.
at26 The tempered bricks are packed in foils/.for use in
steel melting shop 27.
Comparative properties of the tar-bonded and pitch-bonded dolomite bricks produced in the conventional and invented process respectively, are presented in Table III, from which it is noted that the pitch-bonded dolomite bricks produced in the
invented process are of higher bulk density, cold crushing
materials
strength, shelf life and service life as lining/of L.D. converters compared with the tar-bonded bricks produced in the conventional process.
The advantages of the invented process over the conventional process are :
1. The bricks produced in the invented process being of
longer lining life, the consumption of bricks and cost of production
of steel decrease appreciably.
2. The bricks produced in the invented process being
pitch-bonded,no undesirable tar fumes are generated during laying
of the bricks in L.D- converters causing pollution of
environment.
Table I.
(Table Removed)

We claim:
1. A process of producing dolomite bricks of improved quality for lining L. D. converters in steel plants, comprising the steps:
(i) granulating sintered dolomite and magnesia into grains of different sizes;
(ii) preparing an optimum batch composition of the grains of different sizes;
(iii) dividing the grains in the composition into two groups termed 'coarse' and 'fine' ;
(iv) heating the coarse grains to an elevated temperature in grain heaters;
(v) optionally replacing a part of the fine grains of dolomite by the fine grains of magnesia;
(vi) preparing molten pitch for use as binder;
(vii) mixing hot coarse grains of dolomite with the required quantity of molten pitch;
(viii)mixing cold fine grains of dolomite/magnesia with the mix of hot coarse grains of dolomite and molten pitch in mixers
(ix) pressing the hot mix to produce 'green' bricks at an elevated pressure and temperature;
(x) tempering the green brick in an optimum schedule and
(xi) packing the bricks in foils for use in steel plants.
Characterized in that:
(a) granulation of sintered dolomite/magnesia is done to produce grains of size 0 to 8 mm;
(b) the optimum batch composition contains grains of different sizes (by weight %) as : 5.0 to 8.0 mm - 15, 3.0 to 5.0 mm - 17, 1.6 to 3.0 mm - 20, 0.5. to 1.6 mm - 13 and 0
to 0.3 mm - 35;
(c) 'coarse' grains are of size 0.5 to 8.0 mm and 'fine' grains are of size 0 to 0.3 mm;
(d) coarse and fine grains are mixed in the weight ratio of 65:
35;
(e) coarse grains are heated in a grain heaters to a
temperature of 160 - 200° C;
(f)molten pitch is prepared by heating solid pitch by means of a thermic oil heating medium system at a temperature of 300° C;
(g) hot coarse grains are mixed with molten pitch of 5.0 -5.5% by weight of total mix in the bricks;
(h) the mix of hot coarse grains, molten pitch and cold fine grains is pressed at a pressure of 1100- 1200 Kg/cm2 and temperature of 100 -110° C under a hydraulic press of 1600T capacity; and
(i)the optimum schedule followed for tempering the green bricks comprises: fast heating in a closed chamber to a temperature of 180 - 230 ° C in 1 hour, slow heating maintaining the temperature at 280 - 325 ° C for 7 hours and slow cooling to 80 - 90° C in 11 hours.
2. The process as claimed in claim 1, wherein the fine grains comprise 20% by weight of dolomite grains of size 0 to 0.3 mm and 15% by weight of magnesia grains of size 0 to 0.3 mm.
3. the process as claimed in claim 1, wherein the fine grains comprise 5% by weight of dolomite grains of size o to 0.3 mm and 30% by weight of magnesia grains of size 0 to 0.3 mm.
4. The process as claimed in claim 1, wherein the coarse grains are heated to a temperature of 180°C.
5. The process as claimed in claim 1, wherein pressing of the mix is done at 120° C
6. The process as claimed in claim 1, wherein the mixers are heated by flue gases from grain heaters at 120°C.
7. The process as claimed in claim 1, wherein pitch is maintained in a molten state during its storage, handling and mixing with coarse grains by means of the thermic oil heating medium system.
8. The process as claimed in claim 1, wherein the pressing of the hot mix to produce green bricks is done by using a hydraulic press of 1600T capacity.
9. The process as claimed in claim 1, wherein hot coarse grains of dolomite and molten pitch are mixed for 3-5 minutes.
10. The process a claimed in claim 1, wherein hot coarse grains
of dolomite, molten pitch and cold fine grains of
dolomite/magnesia are mixed for 5-7 minutes.