Abstract
Gaseous pre-reduction of preheated chromite ore inside charging chutes of submerged Arc Furnace (SAF) followed by low temperature dissociation of carbon monoxide (co) gas catalysed by the freshly reduced iron leading to carburisation of the later and solid state reduction of chromite takes place in the production of Ferro chrome.The above objects are met by drawing the off-gas from a closed type SAF through its charging chutes by means of a high suction hot gas fan so that the outgoing high temperature gas, which is rich in carbon monoxide content; (> 80% CO.), comes in intimate contact with the incoming preheated charge material at low temperatures (300-500 deg.C). The iron oxide part of the chromite ore is pre-reduced by the CO gas, yielding lower oxides of iron as well as metallic iron. Further, the metallic iron acts as catalyst for the low temperature dissociation of CO gas, yielding finely divided carbon and carbon dioxide gas. The intimately distributed carbon combines with the fresh metallic iron to form iron carbide, which subsequently acts as an efficient reductant for the chromite ores at high temperatures.For the drying and preheating of the charge materials, the gas drawn above is combusted in a combustion chamber by preheated air supplied by an air blower in stoichiometric ratio. The gas drawn from the moving bed is initially cleaned in dry condition by means of an electrostatic precipitator. An online heat-exchanger is provided to reduce the gas temperature in order to increase efficiency of the hot gas fan and reduce its capital cost. The hot flue gases emanating from the combustion chamber are split into two streams. One part is used to dry the incoming feed materials in counter-current direction of the flows to maximize the heat and mass transfers. The other part is used to preheat the descending charge materials inside the charging tube in parallel flow mode to minimize the carbon gasification, which is quite common in case of a counter flow heat exchanger deployed currently for the purpose.
Specification
IN SITU DRYING AND PREHEATING INCLUDING GASEOUS PRE-REDUCTION OF CHROMITE ORE INSIDE CHARGING CHUTE OF SUBMERGED ARC FURNACE IN FERRO-CHROME PRODUCTION PROCESS.
FIELD OF THE INVENTION
The present invention relates to a method of drying and preheating including gaseous pre-reduction of charging material applicable for industrial use. More particularly, the present invention relate to a method of drying and preheating including gaseous pre-reduction of chromite ore inside a charging chute of closed type submerged arc furnace in production of ferrochrome.
BACKGROUND OF THE INVENTION
In any typical Ferro-alloy manufacturing plant, the drying and/or preheating of raw material for the SAFs have hitherto been generally achieved in separate devices such a rotary kilns, shaft
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furnaces etc. These devices, besides necessitating high costs, need sizeable amount of space. Moreover use of rotary kiln calls for additional handling of raw materials and results in excessive size degradation due to tumbling action inside the kiln. Since input charging of small sized materials (fines) poses many problems inside SAFs, many plants had to discontinue its operation and operate without drying and preheating. Incorporation of shaft furnaces in an existing SAF is quite costly and necessitates long shutdown. The counteircurrent design of flow adopted in such furnaces results in loss of coke carbon by gasification in the high temperature zones. Moreover, the use of wet cleaned gas reduces the thermal efficiency besides giving rise to environmental problems.
In view of the spiraling cost of energy and the adverse environmental impacts of its inefficient use, an emphasise is made to utilize the SAF off-gas most efficiently and gainfully. The device should operate without tumbling action and hence without of fine generation such as that in rotary kilns.
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The loss of valuable carbon In coke due to endothermic gasification by the hot carbon dioxide and hydrogen in the flue gases, which takes place in the shaft furnaces due to counter-current flow of the gas with respect to the incoming charge materials is also to be minimised. Moreover, dry gas cleaning and waste heat recovery have to be adopted as far as possible to improve thermal efficiency and minimise adverse impacts on environment. Gaseous pre-reduction of chromite ores also need to be accomplished, which is exothermic and hence reduces electricity as well as coke consumption in addition to increasing the productivity of closed type SAFs.
The invention is applicable to all closed-type SAFs of any size/capacity producing ferrochrome in order to reduce their specific energy (electrical as well as coke) consumptions and increase productivity. The invention solves the problems associated with the use of wet charge materials (as received) in. such furnaces.
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OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a method for drying and preheating including gaseous pre-reduction of chromite ore inside the Charging Chute of a closed SAF type in Ferro-chrome production process which eliminates the disadvantages of prior art.
Another object of the present invention is to propose a method for drying and preheating including gaseous pre-reduction of chromite ore inside the Charging Chute of a closed type SAF in Ferro-chrome production process, which saves both energy and cost.
A still another object of the present invention is to propose a method for drying and preheating including gaseous pre-reduction of chromite ore inside the Charging Chute of a closed types SAF in Ferro-chrome production process, which reduces the environmental pollution.
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A further object of the present Invention is to propose a method for drying and preheating including gaseous pre-reduction of chromite ore inside the Charging Chute of a closed SAF in for Ferro-chrome production process, which increases productivity.
A still further object of the present invention is to propose a method for drying and preheating including gaseous pre-reduction of chromite ore inside the Charging Chute of a closed SAF in Ferro-chrome production process, which reduces the coke consumption in production.
An yet further object of the present invention is to propose a method for drying and preheating including gaseous pre-reduction of chromite ore inside the Charging Chute of a closed SAF in Ferro-chrome production process, which reduces the loss of chromium in slag.
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SUMMARY OF THE PRESENT INVENTION
Gaseous pre-reduction of preheated chromite ore inside charging chutes of submerged Arc Furnace (SAF) followed by low temperature dissociation of carbon monoxide (co) gas catalysed by the freshly reduced iron leading to carburisation of the later and solid state reduction of chromite takes place in the production of Ferro chrome.
The above objects are met by drawing the off-gas from a closed type SAF through its charging chutes by means of a high suction hot gas fan so that the outgoing high temperature gas, which is rich in carbon monoxide content (> 80% CO.)/ comes in intimate contact with the incoming preheated charge material at iow temperatures (300-500 deg.C). The iron oxide part of the chromite ore is pre-reduced by the CO gas, yielding lower oxides of iron as well as metallic iron. Further, the metallic iron acts as catalyst for the low temperature dissociation of CO gas, yielding finely divided carbon and carbon dioxide gas. The intimately distributed carbon combines with the fresh metallic iron to form iron carbide, which
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subsequently acts as an efficient reductant for the chromite ores at high temperatures.
For the drying and preheating of the charge materials, the gas drawn above is combusted in a combustion chamber by preheated air supplied by an air blower in stoichiometric ratio. The gas drawn from the moving bed is initially cleaned in dry condition by means of an electrostatic precipitator. An online heat-exchanger is provided to reduce the gas temperature in order to increase efficiency of the hot gas fan and reduce its capital cost. The hot flue gases emanating from the combustion chamber are split into two streams. One part is used to dry the incoming feed materials in counter-current direction of the flows to maximize the heat and mass transfers. The other part is used to preheat the descending charge materials inside the charging tube in parallel flow mode to minimize the carbon gasification, which is quite common in case of a counter flow heat exchanger deployed currently for the purpose.
8 BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig-1 Shows a schematic arrangement for in situ drying, preheating including gaseous pre-reduction in a closed type submerged arc furnace.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure-1 shows schematically the in situ a drying Zone-(l) and a preheating Zone-(2) of the charge materials (6) decending from a charge bin (17) and a gaseous pre-reduction Zone-(3) of chromite ores inside the charging tube (7) of a closed type SAF (8). The hot CO-rich gas (20) is drawn from the furnace (8) top through a heap of the charge materials (5) by means of a hot gas suction blower (9), and cleaned in a dry type electrostatic precipitator (10). The dust particles accumulate in the dust chamber (14) which is evacuated periodically. The hot gas (20) is partially cooled by an on-line heat-exchanger (11) to enhance the efficiency of the hot gas fan (15), while preheating the incoming combustion air (19) to the burner (16),
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The hot gas (20) drawn from the SAF (8) by the fan (9) is fed to the burner (16) after dust separation and combusted in the combustion chamber (12) in stoichiometric proportion by the preheated air (19) supplied by the forced air blower (15). The product of combustion gas (21) is fed back into the charging tube (13) where it splits into two streams. One stream flows in upward direction for drying and partial preheating Zone-(l) of the descending charge materials (6). The counter-current flow arrangement in this portion maximizes the heat and mass transfer efficiency. The other stream of the combustion product (21) flows downwards in the charging chute in Zone-(2). In this zone, the hot gases (21) as well as the charge materials (6) flow in parallel, resulting in moderate but uniform and well soaked charge (6) heating. Absence of overheating on surface of the charge coke results in minimum loss of coke carbon by gasification.
Upon further descend of the preheated charge (6) inside the charging tubes (7), it comes in contact with the upcoming hot CO-rich gas (20) in Zone-(3), which readily reduces the iron oxide components of the chromite
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ore to lower oxides or even metallic iron. These freshly reduced surfaces act as catalyst for the carbon monoxide dissociation reaction at low temperatures (400-600 deg.C), depositing carbon over such surfaces In Zone-(4). Both gaseous reduction of iron oxides and carbon deposition reactions are exothermic.
Upon further descend of the charge material (6) to Zone-(5) and its heating up by the aforesaid exothermic reactions and the upcoming hot gases (20), the deposited carbon is absorbed by the metallic iron to form iron carbide. This later works as a very efficient and low temperature solid-state reductant for the remaining oxides (chromium oxide) in the ore.
Thus, the entire drying, preheating, gaseous pre-reduction, carbon deposition and absorption reactions are accomplished in situ by drawing the CO-rich hot gas (20) through the stack of charge materials (6) inside the charging tubes (7) and burning & recirculating it subsequently resulting in tremendous advantages to the process. These are reduction in the specific
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electrical energy as well as coke consumptions and increase in productivity due to the pre-reduction and reduced electrical resistivity of the charge.
Reduced loss of chromium in the slag is also expected due to the gaseous per-reduction and availability of additional reductant (deposited carbon and iron carbide formed) which is in intimate contact with the charge material (6).
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KEY FEATURES
Reference Character Features
1 Drying Zone
2 Preheating Zone
3 Pre-reduction Zone
4 A Zone where carbon deposits over
reduced surfaces.
5 A heap of charge materials
6 Charging materials
7 Submerged Arc furnace
8 Charging chute
9 Hot gas suction blower
10 Electrostatic precipitator
11 Heat-exchanger
12 Combustion chamber
13 Point Entry of combustion gas
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14 Dust Chamber
15 Hot gas fan
16 Duel fuel burners of combustion
chamber
17 Charge bin
18 Flue gas out let
19 Air
20 Fuel gas
21 Hot product gas
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WE CLAIM
1. A device for drying, preheating including gaseous pre-reduction of chromite ore inside charging chute of a closed type submerged arc furnace in ferro chrome production, the device comprising
- an inclined cylindrical log charging chute (7) having a top
charge storage (17) from where the charge of chromite ores
(6) are fed and gradually the charge (6) descending down via
plurality of zones (1, 2, 3,) and finally from the bottom of chute
the charges are passed on to the submerged furnace (8) and
- the charging chute is provided with one inflow hot gas (21) line
(13) to ingress hot combustion products (21) which inturn acts
on drying in zone-1, preheating in zone 2 including gaseous
pre-reduction in zone 3 of chromite ore charge (6) and one
outlet connector (18) for exhaust gas (20) from chute (7) and
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a suction means (9) to draw combustible gas (20) from top surface of submerged Arc furnace (8) through an electrostatic precipitator (10) to clean it from dust particle which is being deposited in a dust chamber (14) and the clean combustible gas (20) directly send to the gas burner of duel fuel burner (16).
a combustion chamber (12) is provided in the system from which the combustion products (21) is supplied to the chute
(13), the combustion gas (21) product is generated by the duel fuel burner (16) provided with the chamber (12) and incoming air (19) is supplied to the burner from a fan (15) via a heat-exchanger (11) to the burner (16).
2. The device as claimed in claim 1 wherein the chute (7) comprises a vertical top part (1) for drying the charge, a middle part in which the preheating (2) and gaseous pre-reduction (3) take place and a bottom part to pour the charge material (6) directly to the submerged furnace (8)
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3. The device as claimed in claim 1 where in a gas (20) out let (18) on
the chute (7) is provided to allow exit gas (20) and dust particle from
the chute (7), which directly transferred to an electrostatic preceptors
(10) for cleaning the dirt particles, and! subsequent recycling.
4. The device as claimed in claim 1 wherein the dust partical are
periodically evacated out from dust chamber (14)
5. A device for drying, preheating including gaseous pre-reduction of
chromite ore inside charging chute of closed type submerged Arc
furnace in Ferro chrome production as substantially described and
illustrated herein with reference to the accompanying drawing.
Gaseous pre-reduction of preheated chromite ore inside charging chutes of submerged Arc Furnace (SAF) followed by low temperature dissociation of carbon monoxide (co) gas catalysed by the freshly reduced iron leading to carburisation of the later and solid state reduction of chromite takes place in the production of Ferro chrome.
The above objects are met by drawing the off-gas from a closed type SAF through its charging chutes by means of a high suction hot gas fan so that the outgoing high temperature gas, which is rich in carbon monoxide content; (> 80% CO.), comes in intimate contact with the incoming preheated charge material at low temperatures (300-500 deg.C). The iron oxide part of the chromite ore is pre-reduced by the CO gas, yielding lower oxides of iron as well as metallic iron. Further, the metallic iron acts as catalyst for the low temperature dissociation of CO gas, yielding finely divided carbon and carbon dioxide gas. The intimately distributed carbon combines with the fresh metallic iron to form iron carbide, which subsequently acts as an efficient reductant for the chromite ores at high temperatures.For the drying and preheating of the charge materials, the gas drawn above is combusted in a combustion chamber by preheated air supplied by an air blower in stoichiometric ratio. The gas drawn from the moving bed is initially cleaned in dry condition by means of an electrostatic precipitator. An online heat-exchanger is provided to reduce the gas temperature in order to increase efficiency of the hot gas fan and reduce its capital cost. The hot flue gases emanating from the combustion chamber are split into two streams. One part is used to dry the incoming feed materials in counter-current direction of the flows to maximize the heat and mass transfers. The other part is used to preheat the descending charge materials inside the charging tube in parallel flow mode to minimize the carbon gasification, which is quite common in case of a counter flow heat exchanger deployed currently for the purpose.
