Design optimisation for Stock Houses of large Blast Furnaces
By
Neeraj Mathur, S.K.Singh, A.R.Dasgupta and S.K.Bose
Centre For Engineering & Technology
Steel Authority of India Limited
Ranchi. Jharkhand – 834002
[email protected]; [email protected]; [email protected];
[email protected]
----------------------------------------------------------------------------------------------------------ABSTRACT
Blast Furnaces (BFs) are installed in steel plants for production of hot metal. Raw
materials like iron ore, sinter, pellet, coke & additives are charged in the BFs to produce
hot metal. These raw materials are stored in the stock house comprising of bins, feeders,
screens, weigh hoppers, belt conveyors, etc. to feed required quantity of raw materials in a
proper sequence to the BF. Earlier, BFs were generally small compared to today’s new
generation BFs of large capacities. In small sized BFs, raw materials are generally fed
through skips. However, there are instances, where, even in small BFs, raw materials are
charged through conveyors. But with large BFs, the required skip capacities become
extremely large which are difficult to be installed. As a result, conveyor charging has been
adopted for large BFs. Earlier, big bell & small bell was provided at the top of the BFs to
receive raw materials. Subsequently, bells were replaced by Bell Less Top (BLT)
technology. In today’s large BFs, BLT is used.
The equipment sizing for BF stock houses, whether, skip charging with bells or skip
charging with BLT or conveyor charging with BLT need detail study considering various
charging sequences with certain force filling factor. Designing a stock house with large BF
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further require feeding of different sizes of ore, different sizes of sinter and different sizes
of coke.
Recently, SAIL has installed three Nos. large size BFs each having production capacity of
about 8000 tpd of hot metal. These stock houses have been studied and an endeavour has
been put in order to standardise the equipment sizing for future bigger BFs to be installed
in SAIL for inventory optimisation.
Keywords: Blast Furnace, Stock House, Cyclogram, sizing of equipment
INTRODUCTION
BF-BOF route is a worldwide accepted route for hot metal & steel production, although,
DRI-EAF routes are also in practise now-a-days. The BFs consume various raw materials
like iron ore, sinter, pellet, coke & additives. Production of desired level of hot metal
largely depends on the quality and quantity of the raw materials and the sequence of
feeding. Designing a stock house is as important as designing the Blast Furnace itself since
improper design of a stock house may lead to starvation of the BF due to non-availability
of raw materials.
DESCRIPTION OF STOCK HOUSE
In a stock house, raw materials from various plants like Raw Material Handling Plant,
Sinter Plant, Coke Ovens and Lime and Dolomite Plants are received. These materials are
stored in different bins in the stock house. Usually, the top area of these bins is known as
“highline” and the bottom area is known as “stock house”. These Bins are provided at the
bottom with gates, vibrating feeders, vibrating screens and weigh hoppers. For bigger BFs,
these raw materials are transported in a measured quantity in a proper sequence through a
series of belt conveyors located inside the stock house and finally through a single
charging belt conveyor feeding to the top of the BFs. However, in earlier days and even
today in older BFs of SAIL, which are relatively smaller BFs compared to today’s sizes,
burden materials were/are fed through skips at the top of BFs. All the equipment of a stock
house is sized based on calculations comprising of control philosophy & timing
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cyclograms. The stock houses are provided with all necessary pollution control equipment
to control dust as per pollution control norms.
Fig. 1.1 Typical flow diagram of a stock house
Various steps to be followed for sizing the equipment of a stock house are mentioned
below. In this regard, a typical BF of 4500 m3 useful volume has been considered.
1. Hot metal (thm/d) capacity is based on useful volume (m3) of the Blast Furnace and
its productivity (t/m3/day). Hot metal = 9000 tpd has been considered in this case.
2. Types of raw materials used are lump ore, sinter, pellets, additives coke and nut
coke. In bigger BFs, two sizes of sinter are used, namely, sinter (10 – 40 mm) and
small sinter (5 – 10 mm). For coke also, two sizes are used, namely, centre coke (60
– 80 mm) and surface coke (34 – 60 mm). The surface coke is charged on the belt
conveyor first with the centre coke following immediately. The BLT equipment
indexes from the BF walls to the centre of the furnace, distributing the surface coke
to the walls and the centre coke to the centre of the furnace as required. Sizes of
lump ore are 10 – 40 mm. Sizes of pellets are 8 – 16 mm. Sizes of nut coke are 8 –
34 mm. Sizes of limestone are 6 – 30 mm. Sizes of quartzite are 6 – 30 mm.
3. A typical ratio of ore: pellet: sinter = 10:10:80 has been considered.
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4. Total fuel rate (kg/thm) is to be finalised. Typical values considered are as below:
 Coke rate without CDI injection (i.e. all coke):
495 kg/thm
 Coke rate with CDI injection @ 150 kg/thm:
330 kg/thm
 Nut coke rate
30 kg/thm
:
5. Specific consumption of each of the iron bearing materials (kg/thm) based on Fe
content and the ratio of ore: pellet: sinter is to be calculated. Typical specific
consumption values considered are as below:
Sl. No.
Material
Specific consumption
(kg/thm)
1
Surface coke
372
(all coke case)
2
Centre coke
123
(all coke case)
3
Sinter
1206
4
Small sinter
63
5
Lump Ore
159
6
Pellet
159
7
Nut coke
30
8
Additives (LS/QZ)
50
6. Charging sequence:
 A typical option is 1 charge = 2 batches (CS+CC / S+P+O+M+Cn), where
CS = Surface coke
CC = Centre coke
S = Sinter + small sinter
P = Pellet
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O = Lump Ore
M= Additives
Cn = Nut coke
 1 batch comprises of surface coke + centre coke. Surface coke is 75% of
total coke and centre coke is 25% of total coke.
 1 batch comprises of sinter + small sinter + pellet + ore + additives + nut
coke. Sinter is 95% of the total sinter and small sinter is 5% of the total
sinter.
7. Ore layer thickness (cm) at BF throat diameter is selected. Generally, it is
considered as 70 cm.
8. Coke layer thickness (cm) at BF throat diameter is selected. Generally, it is
considered as 70 cm considering “all coke” charge. However, considering CDI
injection, the coke layer thickness is considered as 50 cm.
9. Retention time (h) in the bins for each raw material is finalised. A typical sizing of
the bins for different raw materials are as below:
Sl.
Material
No. of Bins
Useful volume of
each bin (m3)
No.
1
Surface coke
6
850
2
Centre coke
2
850
3
Sinter
8
1200
4
Small sinter
2
850
5
Lump Ore
4
850
6
Pellet
2
850
7
Nut coke
2
200
8
Additives (LS/QZ)
2
400
Bins are also provided for the fines generated in the stock house after screening.
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10. “Catch-up rate” or “force filling factor”, is defined as a ratio between no. of charges
available per day based on design of equipments of stock house/ no. of charges
required per day for the target hot metal production. Typical value considered is
1.3.
11. Quantity (t/batch) of coke (CS+CC) for “all coke” as well as coke along with CDI
injection is calculated based on the volume required at throat diameter for the
desired thickness of coke at throat.
12. Quantity (t/batch) of iron bearing material including nut coke (S+P+O+M+Cn) is
calculated based on the volume required at throat diameter for the desired thickness
of iron bearing materials at throat.
13. Batches of coke per day is calculated as (Specific consumption of coke x hot metal
per day)/ (quantity per batch). In this case, normal batches = 130 per day.
14. Batches of iron bearing materials including nut coke per day is calculated as
(Specific consumption of iron bearing materials x hot metal per day)/ (quantity per
batch). In this case, normal batches = 130 per day.
15. Timing (secs.) for each batch formation for coke as well as iron bearing material is
calculated as (24 x 3600)/ no. of batches.
16. Total no. of equipment (vibrating feeder, vibrating screen, weigh hoppers etc.)
operating at a time out of total nos. installed is decided.
17. Development of a cyclogram considering operation of equipment for Bell Less Top
(BLT) (bigger BFs are generally provided with BLT system) and operating time of
all stock house equipment is carried out. Cyclograms are prepared after calculating
time of operation of all the equipment in the stock house and total time required for
each batch. Two cyclograms are prepared. One with “normal rate” and the other
with “catch-up rate”.
18. Sizing of all equipment like gates, vibrating feeders, vibrating screens, weigh
hoppers, belt conveyors including main charging conveyor finally feeding at the top
of the BF in the receiving hoppers of BLT is done. The equipment is designed
based on coke rate along with CDI injection option and providing consideration for
“all coke” burden operation.
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Fig. 1.2 Typical Cyclogram
Typical capacities for vibrating feeders, vibrating screens, weigh hoppers and belt
conveyors may be as below:
 Vibrating feeders/vibrating screens
Sl.
Material
No.
1
Surface
Nos.
Nos. in
Capacity
installed
operation
(tph)
6
4
75
coke
2
Centre
Remarks
Feeder + screen
2
1
100
coke
Feeder + screen
3
Sinter
8
6
250
Feeder + screen
4
Small
2
1
60
Feeder only. No
sinter
screen
5
Lump Ore
4
3
150
Feeder + screen
6
Pellet
2
1
150
Feeder + screen
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Sl.
Material
No.
Nos.
Nos. in
Capacity
installed
operation
(tph)
2
1
30
7
Remarks
Feeder only. No
Nut coke
8
screen
Additives
2
1
45
Feeder only. No
(LS/QZ)
screen
 Weigh hoppers
Sl.
Material
No.
Nos.
Nos. in
Effective volume (m3)
installed
operation
for each
1
Surface coke
6
4
45
2
Centre coke
2
1
45
3
Sinter
8
6
35
4
Small sinter
2
1
20
5
Lump Ore
4
3
25
6
Pellet
2
1
25
7
Nut coke
2
1
10
8
Additives
2
1
10
(LS/QZ)
 Belt conveyors
Sl.
Material
Capacity (tph)
Main charging conveyor
5000
No.
1
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Fig. 1.3 Typical general layout of a stock house
CONCLUSION
In near future, all steel plants will be installing bigger BFs with above mentioned type
stock houses. In this regard, an endeavour may be made for a uniform design of the stock
house equipment which will lead to standardisation of equipment and less inventory.
Symbols used:
Symbol
Description
thm/d
ton of hot metal per day
t/m3/day
ton per cubic metre per day
tpd
ton per day
kg/thm
kilogram per ton of hot metal
LS
Limestone
QZ
Quartzite
tph
ton per hour
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