PRODUCTION OF FERROALLOYS IN A PLASMA ORE-SMELTING SHAFT FURNACE USING THE EPOS-PROCESS TECHNOLOGY
UDK 669.187.2.
I.A. Bezrukov, A.I. Bezrukov, O.B. Moiseev, S.N. Malyshev, V.V. Pavlov, V.N. Filimonenko NPPEPOS, ClosedJoint-StockCompany, office@epos-nsk.ru,
Novosibirsk, Russian Federation
PRODUCTION OF FERROALLOYS IN A plasma ore-smelting shaft furnace USING the epos-process technology
1. INTRODUCTION.
Production of ferroalloys in the Russian Federation and abroad is carried out in conventional ferroalloy furnaces of open or closed type than are far from the ideal production process and have a common set of problems: high specific energy consumption (many times higher than rated (theoretical) values); high consumption of a reducer (in some cases more than two times higher as compared with the theoretical consumption required for the reduction reaction); immense amount of gases and solid particles supplied from a furnace to gas cleaning system; bulky structure; high materials consumption; high complexity of maintenance and operation; high dependence of electric and thermal conditions, as well as quality of the product on composition and quality of raw materials and reducers; high complexity of mode setting; high complexity of process control, and other problems.
We have accumulated a great experience of designing, operating and updating ferroalloy furnaces and used it to formulate a conceptually new approach to preparation of raw materials for production of ferroalloys, structures of furnaces, and smelting process, which gives essential technological and business advantages. Based on this experience, a series of new plasma shaft furnaces was developed, produced and launched. The experience of operating plasma ore-smelting shaft furnaces in 2006-2014 suggests that this ferroalloy production technology is absolutely superior. It can be affirmed that a significant part of problems associated with ore-smelting furnaces and techniques are solved when plasma ore-smelting shaft furnaces are operated on the basis of the EPOS-process technology.
This article describes in short the features and advantages of the EPOS process and application of the plasma ore-melting shaft furnace by the example of ferroalloy MiS17 produced in compliance with GOST 4756-91 from ores mined in one of the Mangan's fields. The approaches described in this study can be used for estimating efficiency of the EPOS process on the basis of other ores. In particular, Mangan offers the materials specified in Table 1.1 for sale.
Table 1.1. Composition of the Ores Offered by Mangan.
|
No. |
Material |
Mean chemical composition. % |
|
|||||||
|
Fe |
SiO2 |
Mn |
Al2O3 |
CaO |
Mg |
S |
P |
|||
|
1 |
External sources |
4-5 |
12-16 |
38-40 |
1-2 |
5 |
1.5 |
0.02 |
0.12-0.14 |
|
|
2 |
Own sources, 1 |
7-9 |
20-22 |
13-20 |
- |
6 |
|
0.02 |
0.15 |
|
|
3 |
Own sources, 2, after beneficiation |
8-11 |
26 |
30 |
2 |
8 |
1.2 |
0.02 |
0.14 |
|
The material 1 is more suitable for melting MnS17 and production of ferromanganese or manganese ferrosilicon. Material 3 is suitable for production of manganese ferrosilicon only in the plasma shaft furnace.
2. PURPOSE OF PLASMA ORE-MELTING SHAFT ELECTRIC FURNACES OF TYPE RShPP-XX-I1
The plasma ore smelting shaft furnaces (RShPP) operated on the basis of the EPOS-process are designed for continuous production of ferroalloys (e.g., manganese ferrosilicon MnS17 in compliance with GOST 4756-91) by reducing metals contained in a ferroalloy from a single-component burden in the form of pellets, with the optimized content of components for metallurgical treatment in accordance with TU 0732-010-55978394-04.
The notation of electric furnaces RShPP-ХХ-I1: R – ore-smelting, Sh – type of furnace profile: shaft, P – type of heating: plasma heaters, P – furnace, ХХ – total power of plasmatrons, MW; I1 – first version.
3. SPECIFICATIONS.
The critical parameters of the electric furnaces are given in Table 1.1. The external view of the electric furnace is given in Fig. 1, the shaft section – in Fig. 2.
Table 2. Critical Parameters of Electrical Furnaces RShPP
|
|
Parameter |
Rated value |
|
|
||
|
1 |
Type of furnace |
RShPP-1.5 |
RShPP-3.0 |
RShPP-4.5 |
RShPP-6.0 |
RShPP-12.0 |
|
2 |
Total rated power of plasmatrons, kW |
1,500 |
3,000 |
4,500 |
6,000 |
12,000 |
|
3 |
Numberofplasmatrons, pcs |
3 |
3 |
3 |
3-6 |
3-6 |
|
4 |
Rated power of one plasmatron, kW |
500 |
1,000 |
1,500 |
1,500-2,000 |
2,000-4,000 |
|
5 |
Ferroalloyoutput, kg/h |
up to 1,000 |
up to 2,000 |
up to 3,000 |
up to 4,000 |
up to 8,000 |
Figure 1. Plasma Ore Smelting Shaft Furnaces RShPP-1.5I1.
Figure 2. Shaft Section.
4. SHORT DESCRIPTION OF ADVANTAGES OF FERROALLOY PRODUCTION IN A PLASMA SHAFT FURNACE.
One of the best and perfect iron production methods is considered to be the Midrex process. To our opinion, the OxiCap process is the best technology for recycling industrial waste of metallurgical plants. The advantages of the furnace shaft structure include effective heat recovery, fume recovery (fumes may reach up to 20% of output), a higher utilization rate of a reducer, less requirements for a burden, environmental safety (airtight system), optimal weight and dimensions (1-5).
NPP EPOS, Closed Joint-Stock Company, is specialized in development of high-power and superpower plasma plants and systems in the field of plasma chemistry and metallurgy on the basis of its own patented solutions. The Company has developed, produced and launched shaft electric surfaces with plasma burners of special design using the so-called EPOS-process (6-10). This production process is ideally suitable for processing ore mineral resources and industrial waste from integrated iron-and-steel works and mining enterprises.
Solid-phase restoration, the concept of a well-balanced pellet and the shaft furnace structure are used. New components of the EPOS process include: use of special plasma burners instead of gas ones, the special shaft profile, technology and flow chart of hot furnace gas recirculation along furnace circuits through plasmatron and recirculation system, full utilization of reduction properties of plasma forming gases and moisture from furnace atmosphere, without oxidizers. By experiment, the recovery ratio of useful ingredients from ores was increased from 70-75% to 90-95% as compared with the conventional method. This makes the EPOS process one of the most promising technologies in the field of ore processing and industrial waste utilization provided that this process is properly controlled. As compared with other types of furnaces (blast, ore-smelting, and others), the EPOS process is based on the combination of the following features:
- Recirculation of furnace gases is organized in a special way using the closed recirculation system which ensures a necessary chemical composition of gases in working zones and recycling of blast-furnace gas ingredients. Plasmatrons are operated by crude hot furnace gases supplied from a smoke exhauster and considered as one of the most important tools for controlling the production process. Hydrogen (produced from vapors and water of ore materials) and carbon oxide (produced from carbon-bearing material in the course of the process) are used as basic reducers;
- The processes can be implemented without additional excess oxidizer. Oxidizer is required almost exceptionally for reduction reactions and compensation for losses with off-gases. Content of СО2 as a main ingredient and a certain content of Н2О vapors need to be maintained in furnace gases at the furnace output. Content of CO in blast-furnace gas is minimized by controlling the process;
- The pellet containing ore materials and a carbon-bearing reducer in the proportions balanced for full reduction of ore ingredients, possibly in the presence of slag-forming constituents, is used. The properly selected and prepared ingredients contact each other;
- The high shaft with drying, pre-heating, solid-phase restoration, and recovery processes is used. The reaction zone of special shape is used for smelting produced metals and slags, as well as for completion of chemical reactions. The high shaft which actually operates as a gas filter and the absence of overheating in the upper layer of the burden lead to significant reduction of manganese losses;
- Metallurgical plasmatrons of the patented structure designed to be operated under burden layer, resistant to contact with electrically conductive burden and smelt, without any limitations on time of continuous operation, are used. The reduction processes begin in upper burden layers at burden temperatures of more than 500°C. As the burden lowers in hotter zones, the reduction processes run more and more intensely in solid phase (11-12).
5. ADVANTAGES OF PLASMA SHAFT FURNACE FOR PRODUCTION OF FERROALLOY MNS17.
Advantage No. 1. Yield of the basic ingredient is higher than in the conventional furnace. The yield of basic ingredient in production of ferroalloy MnS17 in ore smelting furnaces RKO-27 and RKO-63 from the raw materials stated in Table 3 is specified in Tables 4 and 5 (based on the data of the Bardin Central Research Institute of Iron Industry for 2009).
Table 3. Chemical composition of manganese raw materials for reduction in furnaces RKO-63.
|
Material |
Ingredients, % |
||||||||
|
Mn |
SiO2 |
CaO |
MgO |
Al2O3 |
Fe |
P |
Others |
Ignition loss |
|
|
Concentrate |
39.10 |
13.34 |
5.00 |
0.50 |
3.12 |
7.60 |
0.03 |
0.70 |
9.00 |
|
Charge slag |
27.00 |
39.13 |
14.67 |
1.47 |
9.15 |
1.07 |
0.02 |
2.05 |
– |
|
Pellets |
43.13 |
23.58 |
8.84 |
0.88 |
5.52 |
– |
0.03 |
1.24 |
– |
Table 4. Manganese balance in smelting of manganese silicon in furnaces RKO-27 and RKO-63, without losses at the first stage of charge slag production.
|
No. |
Manganese specified |
ths. tonnes |
% |
No. |
Manganese produced |
тыс.т. |
% |
|
1 |
Charge slag |
25.031 |
79.69 |
1 |
Alloy |
24.258 |
77.23 |
|
2 |
Manganese concentrate |
3.624 |
11.54 |
2 |
Waste slag |
6.518 |
20.75 |
|
3 |
Pellets |
2.756 |
8.77 |
3 |
Losses (dust) |
0.630 |
2.00 |
|
|
|
|
|
4 |
Discrepancy |
0.005 |
0.02 |
|
|
Total |
31.411 |
100.00 |
|
Total |
31.411 |
100.00 |
Table 5. Manganese balance in smelting of carbon-bearing ferromanganese.
|
№ No. |
Manganese specified |
ths. tonnes |
% |
№ No. |
Manganese produced |
ths. tonnes |
% |
|
1 |
Manganeseconcentrate |
106.335 |
100.00 |
1 |
Alloy |
79.176 |
74.46 |
|
|
|
|
|
2 |
Charge slag |
25.031 |
23.54 |
|
|
|
|
|
3 |
Losses (dust) |
2.124 |
2.00 |
|
|
|
|
|
|
Discrepancy |
0.004 |
0.00 |
|
|
Total |
106.335 |
100.00 |
|
Total |
106.335 |
100.00 |
In production of ferroalloy MnS17 from ore material 1 supplied by Mangan with content of Mn 40%, the yield of manganese in the plasma ore smelting shaft furnace reaches 92% as compared with 70-72% in conventional furnaces according to the experience of ferroalloy plants (12).
Advantage No. 2. Carbon consumption (breeze coke or lower-grade coal) in the shaft furnace is almost two times less as compared with the conventional shaft. The calculated data on consumption of breeze coke (or lower-grade coal) are specified in Table 6.
Table 6. The calculated data for production of MnS17 for compared types of furnaces
|
Parameter |
Standard |
Notes |
|
|
Shaft furnace |
Conventional furnace |
||
|
Manganeseyield, % |
92 |
72 |
|
|
Manganese losses, kg/tonne of alloy |
87 |
390 |
|
|
Consumption of carbon per tonne of ferroalloy, % (kg)* |
11.9 % (242 kg) lower-grade coal may be used |
17.3 % (480 kg) |
Melting of 1 tonne of ferroalloy requires less breeze coke or lower-grade coal (almost by two times) |
|
Amount of burden/pellets per tonne of ferroalloy, kg |
2,030 |
2,773 |
|
Advantage No. 3. Lower temperature of blast-furnace off-gases, lower gas volume. Temperature of blast-furnace off-gases in the shaft furnace reaches 200°С and less, in the conventional furnace – 700°С. In the conventional furnace, temperature additionally increases after burning off-gases. Amount of supplied air is dozens of times higher than amount of gases generated in the course of the reaction due to multiple dilution of air to operating temperatures of gas cleaning system (usually no more than 90-110°С, rarely to 180-190°С with risk of filter destruction). Required gas cleaning capacity considerably increases: hundred thousands of cubic meters per hour even for small furnaces. A large amount of solid particles enriched with manganese is lost with gases. In plasma furnaces operated on the basis of the EPOS process, amount of gases to be removed and cleaned coincides with amount of gases generated in the course of reduction reactions and does not exceed 1,500-2,000 cubic meters per hour for ferroalloy output of 1 tonne per hour, i.e. may be dozens and hundreds of times less than in conventional furnaces. Therefore, the less heat losses, the less capacity of the gas cleaning system. Amount of solid particles lost with removed gases is dozens of times less than in conventional furnaces.
Advantage No. 4. More heat energy is utilized in the shaft furnace. Heat energy of gases is actually used for heating a burden descended through the shaft. For the conventional furnace, heat efflux with 1,120 kg of off-gases reaches approximately 245,000 W*h. For the shaft furnace, heat efflux with 876 kg of off-gases reaches approximately 511,000 W*h.
Advantage No. 5. More chemical energy of off-gases is utilized in the shaft furnace. Furnaces gases of conventional ore-smelting furnaces mainly contain carbon oxide (up to 92%), a certain amount of hydrogen and carbon dioxide. The carbon oxidation reaction is accompanied with heat release. In case of the shaft furnace, this energy is used for maintaining reduction reactions and heating a burden, which allows more electric energy and reducer to be saved. In case of the conventional furnace, this energy is consumed for heating off-gases and actually excluded from the process. In case of the conventional furnace, 1,120 kg of CO is burnt and 3.146 Mw*h of energy is emitted outside the burden under canopy. This energy is actually lost due to emission of diluted furnaces gases to the atmosphere. Certainly, lost heat energy of gases can be partly used outside the shaft, but this would require oversized and expensive devices. Efficiency of the furnace inevitably decreases (14).
Advantage No. 6. Plasmatrons and the production process. In order to understand the advantage of using plasmatrons in the shaft furnace, let us consider them in comparison with characteristics and disadvantages of a three-phase ore-smelting furnace. We shall not describe the well-known structure and operating principles of an ore-smelting furnace. It should be only noted that these furnaces are usually operated by high-ampere current, dozens or even hundreds of thousands of amperes at low transformer voltage (120-250 V). However, arc voltage rarely exceeds 90-100 V and makes from one fourth to half of power supply voltage.
Localization of the energy release zone and load compensation by phase are very challenging tasks. They can be solved by a designer due to accurate calculation and design and by a metallurgist due to complex work. However, uniformity of power in arc discharges and phase-to-phase energy release in a burden, uniformity of power density in the chemical reaction zone can never be reached in continuous mode of operation. Also note another negative condition in terms of local control of smelting zone parameters: there is no possibility to vary power under one of electrodes without changing a smelting mode under other two electrodes. Therefore, modes and powers can be changed under all electrodes, even if only one electrode is shifted. This is very inconvenient for setting and controlling the furnace run process. Positions of electrodes cannot be changed without affecting a number of other important furnace indicators. Any change in load or smelting parameters (in some cases only by 5%) may result and, in a number of cases inevitably results, in significant deterioration of all smelting indicators – yield ratio, specific energy consumption, consumption of charge-adjusting material, etc. Deterioration of quality, changing selectivity of recovery, termination of metal reduction, yield of only liquid slags are also possible.
In order to expand possibilities of process control, designers and technologists use single-phase transformers, multi-electrode systems, direct current, etc. However, they fail to overcome a number of important negative factors mentioned above, especially the impossibility to make the energy release mode and positioning of an electrode in a bath, electric conditions and chemical aspects of the process, as well as thermal and electric conditions independent of each other. There seems to be no solution to overcome the limitations mentioned above.
The problem of power supply source and its proper use did not allow one to implement the most important advantages associated with independent power supply. It was possible to use a plasma jet without contacting a burden or an arc positioned on a burden with a plasmatron isolated from a burden in a breast or otherwise. In both cases, an arc could operate only in open position rather than under a burden – close to surface of lining or a cooled panel. Any contact of a plasmatron body with a burden or contact of a plasma jet with lining caused emergency condition of the furnace. For this and a number of other reasons we do not consider in detail, plasmatrons were very limited in use and often did not meet expectations. Plasma furnaces were considered as exotics; plasmatrons were used only in exceptional cases.
We have resolved the basic contradictions described above for this type of furnaces in the implemented power range and overcome the technical problems. The plasma assembly was designed in a qualitatively new way to be operated independently, under a burden. This allows one to implement the advantages of the plasma furnace: energy release from a plasmatron arc ensures spatial, thermal and electric independence from other sources; location for energy release can be selected on a case-by-case basis; energy release can be completely localized in a burden almost without losses; control of electric arc shape allows one to generate a required shape of the energy release zone and, therefore, to remove a number of essential limitations on developing process techniques and metallurgy. Now, it is possible to use local reduction processes in working zone of the plasmatron, their synergetic action for the common smelting and reduction zone, as well as to bring in and out the plasmatron, or to change its location without disturbing the production process. In other words, the process becomes a flexible controlled technology.
If we add the implemented mode control technology based on the special algorithms we have developed for this process for accurate identifying and controlling the energy release zone, the plasma shaft furnace becomes a multi-purpose, process flexible electrometallurgical unit than can be accurately adapted to a specific process with achievement of optimal technical and economic advantages. Under equal installed powers of energy sources, it is possible to bring up to 95-98% of supply power and voltage to the energy release zone at significantly lower currents and low heat losses.
Advantage No. 7. Working Conditions and Operation of the Equipment
The conventional ore-smelting furnace is associated with physically demanding work, a great number of process operations with equipment and a burden, high personnel costs. It is necessary to provide staff for delivery and preparation of burden materials, supply of them to intermediate bins, control of supply to the furnace and distribution by charging pipes, control of a blast-furnace mouth, preparation of a self-baking electrode – control instrumentation engineers, mechanics, electricians, workers of the pouring section, etc. The staff involved in operation of a medium-power furnace includes 150-180 persons. Site and premises where the furnace is installed are always contaminated with dust and gas emissions from the equipment. Noise level from conventional equipment makes it almost impossible to talk on a site.
The plasma shaft furnace makes a real difference. The current plasma shaft furnace is operated by a prepared pellet burden. Pellets are transported from the automatic pelletizing section by a closed conveyor and loaded to measuring hoppers of the furnace section. Then, they are automatically supplied by a conveyer to the receiving hopper of the furnace through a closed passage, as they are consumed. The furnace is hermetically sealed for pressure of 3-5 thousand Pa. The furnace and all units operate quietly and allow workers to talk even under their breath. There are no gas emissions on a site and external signs of powerful arcs in the furnace. The furnace mantle is warm in the stationary mode, complies with applicable standards and completely safe in terms of contact. A worksite is free of dust and smoke. There is no need in workers during operation of the furnace: supply of a burden, descent, gas removal, electric conditions, position of operating members, cooling system etc. are automatically controlled from a control room according to the specified algorithm. An operator has available several monitors: for furnace control (operating modes of the furnace and equipment, burden charge, cooling system), as well as control of the gas removal and cleaning system. There is a standby computer which can be used for controlling the process in case of failures. One operator and his/her assistant can ensure operation of the furnace prior to metal yield. Workers are needed only for maintenance and handling liquid metal. Number of workers is minimized since there is no need to carry out such works on a regular basis.
6. PREPARATION OF BURDEN MATERIALS TO BE USED IN A SHAFT FURNACE
Any metallurgical process and any furnace unit are known to require high-quality preparation of burden materials. This means accurate dispensing of burden ingredients; repeatability of fraction composition, good mixing of burden ingredients, uniformity of the chemical composition, agglomerating, pelletizing or ball milling, strength of pelletized burden materials (cold and hot), gas tightness, open and close porosity. and some other requirements. All of them must be fulfilled for proper preparation of a burden. These are nuts and bolts of the production process, essential elements of the successful smelting process. Insufficient attention to these aspects adversely affects the key technical and economic indicators. The results of our own experiments conducted in shaft, plasma shaft and special experimental furnaces designed for these purposes correlate with the data (1-4, 9): Residence time of pellets in the furnace from loading to melting is usually about 90-100 minutes, while residence time of pellets in the working zone with temperature range of 1,000-1,450°C is up to 25 minutes. Provided that a pellet is properly composed of and produced, the reduction process is completed within the time interval specified above.
A pellet must be mechanically strong and resistant to cracking in cold and hot conditions. Its gas permeability must be sufficient for reactions to proceed throughout the pellet during the time of passing through the reduction zone, a properly selected composition, with certain specified fractions of ingredients. Its pores must not be completely closed. Pellets must not aggregate into a gas-tight formation in the furnace since this deteriorates permeability of the shaft and gas recycling. Non-observance of these properties, or violation of the process and the chemical composition may significantly deteriorate technical and economic indicators of the furnace and, in some cases, make it impossible to implement the process (as the practice shows). Therefore, a pellet is one of the most important problems, and the solutions we have found are one of the most essential know-how for development of the high-quality technology and the furnace structure of such type. Use of a pellet eliminates the human factor for preparation of a burden and smelting. The pellet production process and the ferroalloy production process will be disclosed to the Customer after putting the section into operation.
7. Conclusions.
Using ores supplied by Mangan as an example, the advantages of the EPOS process in plasma ore-smelting shaft furnaces produced by NPP EPOS, CJSC were considered. The implementation of the advantages in terms of the equipment and the process enables high-tech limited manning automated production. This guarantees a significant increase in ferroalloy yield per unit mass of ore (by 35-40%) and reduction of power consumption and consumption of reducers by two times, as well as high profitability of ferroalloy production.
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I.A. Bezrukov, A.I. Bezrukov, O.B. Moiseev, S.N. Malyshev, V.V. Pavlov, V.N. Filimonenko. ©
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