|Title of Invention||
A METHOD FOR PRODUCING HOT BRIQUETTED IRON
|Abstract||A method for producing hot briquetted iron comprising steps (a) reducing the ore in a pressurized reactor; in which (b) the reduced iron is cooled after the exit from the pressurized reactor; and (c) depressurized in a depressurizing bin before entering into a hot briquetted machine, characterized in that the reduced iron exiting from the depressurizing bin is heated by induction.|
THE PATENT ACT, 1970
(39 of 1970)
THE PATENT RULES, 2003
(See section 10 and Rule 13)
NOVEL PROCESS FOR PRODUCTION OF HOT BRIQUETTED IRON
GRASIM INDUSTRIES LIMITED
An Indian Company
of Birlagram, Nagda-456 331, Madhya Pradesh,
THE FOLLOWING SPECIFICATION DESCRIBES THE INVENTION
Field of Invention
This invention relates to a novel process for production of hot briquetted iron.
This invention envisages a novel process for production of hot briquetted iron with higher tumbler index.
Typically the production of sponge metal in moving bed vertical reduction reactors involves the reduction of pebble-sized particles or lumps of metal oxides, ores or the like which descend downwardly through a reduction zone counter to a suitable upwardly flowing stream of hot reducing gas, largely composed of carbon monoxide and hydrogen at temperatures on the order of 850 °C to 1100 °C and for iron oxide preferably 900 °C to 1000 °C and discharging product by means of a briquetting machine directly coupled to the reduction reactor to produce hot briquetted iron.
Tumbler index represent strength of hot briquetted iron and is measured by percentage of briquettes remaining unbroken on screen of size 20 mm after rotating hot briquetted iron in drum for 200 revolutions. Briquetting strength is extremely dependant on temperature of sponge iron prior to briquetting.
High tumbler index of hot briquetted iron is primarily a quality requirement and is supportive in reducing breakage during shipment or transportation of hot briquetted iron. Unbroken hot briquetted iron is desirable to steel manufacturers.
It is known that non homogenous temperature of sponge iron prior to its introduction in the briquetting machine results in the low tumbler index of hot briquetted iron. Reason for non homogeneous temperature of sponge iron is:
The shaft furnace is of the type wherein a bed of particles descends by gravity through said furnace which has an upper section where ore pellets, lumps, or the like are reduced by reaction with a hot reducing gas, and a lower section of a downwardly tapering, preferably, in a generally conical shape, which converges to an outlet discharge orifice of smaller cross sectional area than the rest of said furnace. To avoid solid flow related problems in conical bottom and for smooth operation reactor shell in conical portion is cooled by heat-exchanging means by cooling jacket(s).
Cooling of sponge iron in conical bottom of reactor results in non homogeneity of temperature of sponge iron prior to briquetting machine. Average temperature of sponge iron before briquetting is in the range of 680-720 °C but temperature range of sponge iron is from 300 °C to750 °C. Non homogeneous temperature of sponge iron prior to briquetting is found to be major cause of low tumbler index of HBI. Briquetting strength is extremely dependant on temperature of sponge iron prior to briquetting.
Major causes for low tumbler index
i) Non-homogeneous briquetting temperature
The drop in tumbler index can be attributed to many reasons. The important factor affecting the low tumbler index is the non-uniform temperature of direct reduced iron (DRI) prior to briquetting. The reduction of iron ore occurs at 900°C in the reduction zone of the reactor in a pack bed. After reduction hot DRI moves down through a conical portion as shown in figure 1 (Schematic diagram of distribution of hot and cold DRI inside the conical portion of the reactor) of the accompanying drawings. Sponge iron has very high friction factor at high temperatures. Hence sponge iron in contact with conical shell of the reactor has to be cooled to avoid chocking problems. Conical portion of the reactor is provided with cooling jackets to enhance the flow of hot DRI. In this region the material gets cooled to around 700°C. The DRI particles near the wall of the conical portion get cooled at a faster rate than material in the middle. It is calculated and observed that about 90% of the DRI is in the range of 68O-720°C while 10% is at 300-500°C as shown in
figure 2 (Color coding for temperature change for iron) of the accompanying drawings. The average temperature required for briquetting is 700°C. The DRI at lower temperature adversely affects the tumbler index of HBI. The non-homogeneity in DRI temperature is more severe when the plant is operated at partial load.
ii) Increased use of lump ore
Regression analysis has identified the role of increased lump ore consumption as one of the reasons for decrease in tumbler index. Lump ore has low crushing strength and generate ore fines which affect the flow rate of the burden inside the reactor. This also leads to drop in DRI discharge temperature. The DRI fines generation also increases with increased lump ore consumption.
iii) Presence of MgO in the HBI which is used as anti-sticking agent MgO which is added as anti-sticking agent in the iron ore during reduction remains in the briquettes to the level of ~0.6 %. The reduction potentials of iron oxides are far below magnesium oxide, which implies that the reduction of MgO does not occur during the reduction of iron oxide as shown in table 1. Hence MgO remains in the reduced DRI, which would act as an anti-caking agent. The presence of MgO could affect the strength of the HBI.
Table 1: Physical and electrochemical properties of iron ore and MgO
Formula MeltingPoint(°C) Boiling Point Specific (°C) (g/cm3) Gravity Reduction Potential (V)
Fe203 FeOFe304 MgO 1565 1377 1597 2825 decomposes 5.24 3414 6.0 decomposes 5.17 3600 3.65 -0.04 -0.447-2.37
Limitations of prior art
The hot briquetted iron produced by a typical prior art process of sponge iron production has a tumbler index less than 40.
Low tumbler index of hot briquetted iron primarily causes increased breakage during shipment or transportation of hot briquetted iron. Broken hot briquetted iron is not desirable to steel manufacturers. This also prevents securing overseas customers.
Particularly, this invention envisages induction heating method for production of hot briquetted iron which offers selective preheating of sponge iron before briquetting to improve uniformity of sponge iron temperature. This invention also provides an apparatus for the selective heating of sponge iron.
Object of the invention
The main object of this invention is to provide a process to produce improved
hot briquetted iron.
Another object of the present invention is to provide a process to produce
improved hot briquetted iron with higher tumbler index in reactor operating
at full capacity as well as at partial capacity.
It is yet another object of the present invention to provide a novel heating
method which offers selective preheating of sponge iron to improve
uniformity of sponge iron temperature before briquetting.
It is yet another object of the present invention to improve tumbler index of
HBI more than 90 at full as well as at partial load.
DESCRIPTION OF THE INVENTION
This invention identifies a method, process and apparatus for selective heating of sponge iron resulting in hot briquetted iron having tumbler index ranging from 45 to 95.
This invention discloses method, process and apparatus for selective heating of sponge iron so that cold sponge iron particles having temperature about 300-500°C would be heated at faster than rate of heating of sponge iron particles having temperature about 720°C.
In addition to induction heating other modifications done to improve the tumbler index were,
a) Increasing the temperature of depressurizing bin by removing the cooling arrangement and providing insulation,
b) Increasing the torque for briquetting by installing new gear boxes,
c) Using more porous pellets,
d) Improving method for coating of pellets,
e) Slow cooling of HBI,
f) Reducing the size of briquettes,
g) Lubricating the pressing rolls, etc.
On implementing all these modifications the tumbler index improved to 65.
Therefore in accordance with this invention the low tumbler index of HBI can be improved by employing two strategies, first one is the preventive method and second one corrective method.
The root cause of low tumbler index is the non-homogeneous temperature before briquetting. Removing the cooling jacket is practically impossible as it disturbs the flow of hot DRI through the conical portion due to high friction of hot DRI. Any alterations in the cooling rate have adversely affected the plant operation. A permanent preventive action would be to introduce the conical portion with refractory linings. Alternate technologies
are available to avoid cooling jacket in the conical portion of the reactor. It consists of refractory linings along the conical portion which enhances the
flow of DRI. This would reduce the friction of hot DRI on the inner surface and enhances the flow of hot materials. The cooling jackets can then be removed. Though this method would solve major source of problem, it has the following disadvantages.
• This modification is expected to be very expensive
• Risk involved in modifications of the critical equipment.
• The implementation would take long time.
• Additional operating cost during DRI production.
• Modification in the reactor would require about 60 days resulting in down time of sponge iron production by 60 days.
Considering above disadvantages of preventive method, a corrective method is suggested. The fast and cheaper method is to implement a corrective action, which would be to heat the DRI before briquetting. Corrective method has following advantages,
• Risk factor is low, as critical equipment and existing plant set would not be disturbed.
• This is a fast method and can be implemented quickly.
• Capital cost requirement is low
• Corrective method has following disadvantages,
• Corrective method has operating cost in the form of additional power consumption (~0.9MW).
Methods to achieve the objective
Following alternatives are identified to improve tumbler index
• Increasing the briquetting temperature
• Reduce addition of anti-sticking agents
• Alternate anti-sticking agents
• Slow cooling of HBI after briquetting
• Binding agent during briquetting
• Modification during briquetting
Out of the above mentioned alternatives, first alternative is short listed to develop methods and solutions. This is because of the reason that non-uniformity of temperature in DRI appeared to be the major cause of the problem.
I. Increasing the briquetting temperature
It has shown that briquette density is a function of temperature. Figure 4 of the accompanying drawings is a plot of briquette density against
briquetting temperature. Briquette density increases sharply at temperatures above 600°C.
Similarly, the abrasion strength of HBI increases with temperature. The increase in abrasion strength from 550°C is more than two fold for an increase in temperature of 100°C. However, the strength after 700°C is only
marginal. Abrasion strength resembles tumbler index of HBI. Hence figure 5 (Effect of briquetting temperature on HBI strength) of the accompanying
drawings show that minimum temperature required for briquetting is 700°C.
The DRI particles at the inner surface of the water jacketed conical portion would get cooled at a faster rate as compared to centre. We have theoretically calculated the percentage of DRI particles that may get
cooled in the jacketed area as shown in table 2. The percentage of material in the first three layers is 4.92%, which is more susceptible for cooling.
Table 2: Percentage of material at the conical portion
Conical portion larger radius 3.78 m
Conical portion smaller radius 0.45 m
Vertical height of the cooling jacket 16.67 m
Average diameter of pellets 0.015 m
Density of DRI 2 MT/m3
Weight of DRI in the first outer layer 10.43 MT
Weight of DRI in the second outer layer 8.77 MT
Weight of DRI in the third outer layer 8.71 MT
Percentage of material comprising three layers 4.92 %
The homogeneity of DRI temperature before briquetting is essential requirement for improving the tumbler index. The design criteria that should be considered for heating equipment are,
• Uniform temperature of 680 to 720°C is required
• Flow rate of DRI is 60 tonnes/hour
• Flow rate will be more than double at full capacity
• Flow rate may vary by about 10% on shift-to-shift basis
• Product has variation in iron metallics between 84 to 88%
• Product has variation in particle size 2-40 mm, with average of 15 mm
• Heating DRI inside metallic pipe which itself may heat-up
• About 10% of DRI is at lower temperature of about 500°C
Various methods were evaluated in order to bring uniformity of DRI. They are
• Exothermic reactions
• Reduced flow rate of water for cooling the reactor bottom
• Change in cooling media from water to oil
• Heating by a jacket with molten:
• zinc (melting point 540°C)
• eutectic salt (melting point 800°C)
• Infrared heating
• Microwave heating
• Electrical resistance heating
• Direct contact with hot gases
• Flame heating
• Induction heating
The thermal conductivity of sponge iron is around 1/12 times less than that of pure iron. Hence, the possibility of transferring heat through conduction from a hot DRI particle to cold one appears to be very less. After the discharge of hot DRI from the reactor, it is collected in two bins (product pressurized bin and surge bin). The total residence time in these bins before briquetting is around 55 minutes. This much residence time won't be sufficient for attaining temperature uniformity for all particles. DRI particles of size 9mm and below are likely to get heated up through conduction as shown in table 3. It is obvious that bigger particles require another method of heating for attaining required uniform briquetting temperature.
Sponge iron flow rate = 30 tph in one line
Average hold up in surge bin = 20 tonnes
Residence time in product pressurized bin =15 minutes
Residence time in surge bin = 40 minutes
Total residence time = 55 minutes
Table 3: Residence time required for attaining uniform temperature by
pellet diameter, 3 6 8 9 15 20 30 40
Weight of one pellet, g 0.05 0.40 0.94 1.29 6.18 14.65 49.46 117.23
transfer rate, 0.12 0.06 0.04 0.04 0.02 0.02 0.01 0.01
Heat required, 3 22 53 73 348 824 2782 6594
Time hrs:min 0:00 0:06 0:19 0:30 4:02 12:47 64:47 204:44
Among the various methods studied, the induction heating is found to be the most suitable. Detailed study of the technical feasibility of induction heating for sponge iron was conducted. The advantages of induction heating are
• Time for heating is of the order of a few seconds
• Iron gets heated from 35°C to 700°C in about 45 seconds
• Preferential method of heating
• Non-contact method of heating
• Induction heating separately heats each piece of sponge iron
• Thermal efficiency of induction heating is over 60%
• Proven method for melting sponge iron
Principles of induction heating
Induction heating is based on magnetic property and electrical resistivity Heating by hysterisis loss - using the magnetic property
Alternating magnetic field inside the induction coil repeatedly magnetizes and de-magnetizes iron. This causes heating inside the material because of friction between the iron atoms while they orient to the external magnetic field. Heating by eddy current - using the electrical resistivity
Flow of electricity through metal generates heat due to electrical resistance.
Induction heating is because of
Magnetic property of sponge iron
(Heat produced)m α (Magnetic permeability)2
Electrical resistance of sponge iron
(Heat produced)e = (Applied voltage)2 / Resistance
Total heat produced = (Heat produced)m + (Heat produced)e
Total heat produced
Temperature rise =
Weight of sponge iron x Specific heat
Figure 6 of the accompanying drawings is a plot of magnetization against temperature for iron. The magnetization decreases as a function of temperature and the drop in magnetic property is very sharp as it reaches 700°C. The temperature at which the material looses its magnetic property completely is called Curie temperature. The heating by hysterisis would totally absent above Curie temperature.
The electrical resistance increases sharply with temperature as shown in figure 7 of the accompanying drawings. The heating of iron above Curie temperature becomes difficult because of the high electrical resistance.
The specific heat also increases with temperature as shown in figure 8 of the accompanying drawings. There is a abrupt increase in specific heat as iron reaches its Curie temperature. This property would help in designing power requirements for induction heater.
Relative rate of increase in temperature
Table 4: Comparison of relative contribution of magnetic, electric and
specific heat during induction heating
Initial Effect of change Effect of change Effect sponge iron in magnetic in electrical change temperature property conductivity specific heat Of Temperature in rise (total effect)
500°C 100(Base case) (Base case) 100(Base case) 100(Base case) 100(Base case)
750°C 15 60 71 20
The detailed study of principles induction heating provided valuable inputs for the selective heating of cold DRI over hot DRI particles. The selective heating is the result of change in magnetic property of DRI with increase in temperature as shown in table 4. When the DRI particles reach Curie temperature (~700°C), it looses its magnetic property completely. Any further heating above Curie temperature is the result of Eddy currents. The power required for heating above Curie temperature is comparatively very high. Hence, for selective heating DRI to 700°C induction method can be used.
After establishing technical feasibility of induction heating, bench scale trials were conducted. Results showed that DRI can be heated to 700°C with in 9 seconds. But heating above 700°C becomes very slow. It took additional 51 seconds to heat DRI from 700 to 1000°C. Further, pilot scale trials were carried out to establish the selective heating of hot and cold materials in mixture. The major conclusions from the pilot scale trials are:
• Preferential heating of cold DRI over hot DRI has been observed
• Faster rate of heating of cold DRI than hot DRI
• Very slow heating of DRI above 700°C
• Fast heating of sponge iron by induction furnace
• Very low heat transfer among sponge iron pellets by conduction
• Large size sponge iron pellets are heated at faster rate than smaller
The feasibility of induction heating for sponge iron was established during the bench scale trials. Two locations were identified for installing the induction heater as shown in figure 9 of the accompanying drawings. One is the lower portion of the surge bin and second the spool piece connecting the surge bin to the briquetting machine.
Advantages of the selected locations
• Induction coil should be as close as possible to briquetting machine so that heated sponge iron used can be used immediately for briquetting.
• Equipment is filled with DRI continuously.
• Operated at atmospheric pressure. The reactor and product pressuring bin has pressure about 4 bars.
• Area of around lm around selected portion is available free. Any metallic part should not be present in the electromagnetic field
generated by induction coil.
• The lower portion of the surge bin can be replaced with retort having
induction coils wound around it.
Other important requirements relating to the successful design and
· installation of induction heater are, Power requirements
· Thermal efficiency
· Frequency and voltage used in the induction coil
· Homogeneity of temperature of DRI at outlet
· Process controls
· Number of retorts and coils
· Utilities requirements
Selective heating of sponge iron
• Number of retorts and coils
• Utilities requirements
Selective heating of sponge iron
Non-homogeneous temperature before briquetting is the major cause for low tumbler index of HBI. Various methods have been studied and evaluated to attain uniform temperature.
The applicability of induction heating has been established by the bench scale and pilot scale trials to attain uniform temperature for DRI before briquetting.
According to this invention induction heating is the best corrective action which would perform selective heating for our objective. Cold particles would be heated at a faster rate than hot ones.
Induction heater is capable of improving tumbler index of HBI even when the
reactor is operated at partial load.
Induction heater achieves desired uniformity of temperature of DRI before
While considerable emphasis has been placed herein on the process of the preferred invention, it will be appreciated that many modifications can be made in the preferred process without departing from the principles of the invention. These and other changes in the preferred process as well as other process of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.
|Indian Patent Application Number||2119/MUM/2006|
|PG Journal Number||28/2010|
|Date of Filing||22-Dec-2006|
|Name of Patentee||GRASIM INDUSTRIES LTD.|
|Applicant Address||BIRLAGRAM, NAGDA 456 331,|
|PCT International Classification Number||C22B1/24|
|PCT International Application Number||N/A|
|PCT International Filing date|