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Showing posts with label Iron and Steel Technology. Show all posts
Showing posts with label Iron and Steel Technology. Show all posts

Importance of Tundish Design and Flow Modifier Refractories in Steel Making | Refractory Industry Guru

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 23-Sept-2020


www.industry.guru - Steel Tundish labelled image
To transfer finished melt steel from a ladle to mould in a continuous casting process, an intermediate vessel is used which is called tundish. The role of tundish is to deliver the molten metal to the moulds evenly and at a designed throughput rate and temperature without causing contamination by inclusions. Inclusion float out, slag vortexing, till end slab volume and residual metal in tundish are a strong function of tundish hydrodynamics. Tundish design as well as flow control devices / modifiers are known to have strong influence on tundish hydrodynamics. 



www.industry.guru - images of Tundish Flow Modifier Refractories


One of the major functions of steel making tundish is to enhance inclusion floatability and thereby, produce clean steel. For the removal of inclusion through floatation, wall adhesion and agglomeration the flow patterns inside the tundish play an important role, which in turn

Melt flow in any given tundish can be favourably altered by incorporating suitable tundish flow modifiers (TFM) and/or changing the design of the tundish. The flow modifiers play an important role in promoting the floatation of nonmetallic inclusions in steel.

Now-a-days refractory makers are offering customized refractory solution. The new age tundish refractories facilitate temperature homogenization, removal of macro-inclusion, prevention of nozzle clogging etc. inside tundish. To streamline the flow and compress turbulence inside tundish various Flow Control Devices (FCD) are being used in place of traditional FCDs or tundish furniture like Dams, Weirs, Charge Pads, and Side Wall Pads etc. 


Industry Guru - Used Steel Tundish image
The next generation FCDs are popularly known as Tundish Flow Modifier (TFM), Tundish Flow Optimizer (TFO) etc. are precast refractory shapes made of Ultra Low Cement Castables (ULCC) having 85 - 90% alumina. The interior of tundish flow modifiers or flow optimizers as you say it, are designed in such a way that incoming steel gets a churning effect which results into inclusion flotation and subsequent absorption at the tundish powder level. Tundish argon diffusers are also being used to reduce inclusion in steel.

Eventually, it is tundish design from the viewpoint of metal flow and appropriate selection of refractory materials with their right positioning inside tundish that holds the key to the success of subsequent operations in steel making.


Iron Making in Mini Blast Furnace (MBF)

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The Blast Furnace ironmaking process had, until recently, been the unchallenged method of making hot metal on a large scale. Till 1990, the blast furnace route of ironmaking had about 97% (527mt) share of global iron production. Since then many other alternative processes of iron making have arisen e.g., Shaft Furnace DR processes (MIDREX, HyL), Rotary Kiln DR processes (SL/RN, CODIR, TDR), and recently the COREX Smelting Reduction process. The Mini Blast Furnace (MBF) is the most proven technology, as revealed by one recent global iron production data. While gas-based as well as coal-based DRI production routes produced 2.7% of total iron production in 1990 - 1991, the corresponding share held by MBF, operating mainly in Brazil, China and India, was 3.4%.

Iron and Steel Industry Guru
Mini Blast Furnaces (MBFs) are ideally suited to small scale operations. A Mini Blast Furnace (MBF), which can be viewed as is a miniature version of conventional large blast furnace, also has a few additional characteristic features known for their simplicity and economy. Since MBFs are small (working volume ranges between 100 - 370 m3 corresponding to production capacities of hot metal between 60000 - 200000 tpa) blast furnaces, the technology involved is not only well proven, but also very sophisticated. Smaller scales of operation allows the use of inferior grade coke and iron ore (sinter usage is difficult). Mini blast furnaces are becoming increasingly as an economic and reliable source of iron for foundries as well as for forward integration with steelmaking units in EAF / EOF (and sometimes even small BOF) based steel plants.

The products from mini blast furnaces are of the same quality as that of normal Blast Furnaces and are free of tramp elements - this is of particular advantage in steel making in mini steel plants. Use of 40 - 45% hot metal in EAF (Electric Arc Furnace) charge has thus become standard practice, which has helped to reduce the power consumption in Electric Arc Furnaces to 380 - 400 kwh/t liquid steel from 550 - 600 kwh/t. At the same time, sine the hot blast temperature in MBFs is lower than normal blast furnaces and the specific heat loss is more, the coke rate tends to be 100 - 150 kg/thm higher.

The biggest limitation of mini blast furnaces is that coal injection is normally difficult and the higher specific heat requirement has to be met entirely by coke (normally purchased from external sources).

In India, with the recent increasing demand of pig iron and steel, mini blats furnace technology has proliferated. Kalinga Iron Works is successfully operating three small blast furnaces with volumes less than 100m3 each, an MBF of 175m3 capacities was commissioned in Goa in 1992 and nine more mini blast furnaces with installed capacity of 0.80 Mtpa of foundry pig iron and 0.10 Mtpa of basic grade are already operational. These units are spread all over the country. If this trend continues, which is more likely to happen, Mini Blast Furnace Technology would play an increasingly important role in the rapid and wide spread growth of iron and steel making capacity in this country.

Brazil has a large hot metal production through mini blast furnaces which use charcoal as a reducer and an energy source. These companies are not integrated and their final product is pig iron. The growth in this sector started in Brazil in the early 1970's as a result of the availability of cheap and good quality raw materials (native wood charcoal and granulated iron ore). In addition, the return of the investment in the construction of Mini Blast Furnace was very fast. Nowadays, this sector is consolidated and has a fundamental role in the national and international iron and steelmaking sector since Brazil is a major supplier of primary iron.

Plant availability as well as the perfection achieved in technology, made Mini Blast Furnaces a well accepted iron making route in China. The situation in India could be similar in future. Presently, about one fifth of China’s total iron production is through about 55 - 60 MBFs. The furnaces in China use metallurgical coke, and the coke rates vary between 500 - 630 kg/thm. Extensive innovations have been introduced in the Chinese Mini Blast Furnaces including:
  • Injection of pulverized anthracite to the extent of 60 kg/thm, to bring down the coke rate by about 40 - 50 kg/thm.
  • Heat recovery from stove waste gas at 250-300OC for increasing the hot blast temperature by about 80OC.
  • Incorporation of self-preheating process stoves, enabling the generation of hot blast with a temperature of more than 1200OC.
  • Dry cleaning of furnace gas.

According to a published report some typical characteristics of raw materials used in Chinese Mini Blast Furnaces (MBFs) are given below:
Chemical Analysis (%)
Iron Ore
Sinter
Fe
40-45
53-54
FeO
--
11-12
CaO
8
10.5-12
SiO2
13-14
8.5-9
Al2O3
2
---
MgO
1
2.5
S
0.03-0.04
0.03-0.04
P
0.02
---
Tumbler index, %
81-82
Basicity
1.2-1.4

A typical range of iron oxide feed done in Chinese MBF is as follows:
Size (mm)
Percent
+70
2
-70, +60
2
-60, +40
8
-40, +25
12
-25, +10
49
-10, +5
20
-5
7

The coke characteristics used in China are:
Ash = 13.5 - 14.0
V.M. = 1.1 - 1.4
Sulphur = 0.25 - 0.75
Moisture = 7.5 - 8.0
M10 index = 17
M40 index = 75
Size = 25 - 60 mm

COREX Process of Iron Making - its Merits and Demerits

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COREX Process and Technology of Ironmaking
The COREX technology is a cost efficient, environmentally friendly and industrially accomplished alternative to the blast furnace route for the production of hot metal from iron ore and coal. By fulfilling more stringent ecological regulations by law, utilization of low-cost, highly available low grade raw materials including fines, the COREX process has been accepted as a commercially proven technology for present and future iron making process. The COREX process was created by Siemens VAI and was elevated to industrial maturity by PRIMETALS. Primetals Technologies Limited, is a London (UK) based engineering and plant construction company established in 2015 by a joint venture of Siemens VAI Metals Technologies and Japan’s Mitsubishi Hitachi Metals Machinery (MHMM).
In a nutshell, COREX is a coal based SR (smelting reduction) process of making hot metal or pig iron by direct use of non-coking coal. The outputs can be used either by integrated mills or EAF (electric arc furnace) mills. The process gasifies non-coking coal in a smelting reactor, which also produces liquid iron. The gasified coal is then fed into a shaft furnace to remove oxygen from iron ore lumps, pellets or sinter and finally, this direct reduced iron (DRI) is fed to the smelting reactor. Compared with the traditional iron making process via the blast furnace route, the COREX process differs since non coking coal can be directly used for ore reduction and melting work, eliminating the need for coking plants. The use of lump ore or pellets also dispenses with the need for sinter plants.
COREX Process of Iron Making
Some Merits and Demerits of COREX Process
=> Conducive to production of high end and special steel required for sophisticated industrial and scientific applications with minimum damage to the environment at various stages of steel making and mining.
=> Unlike the conventional Blast furnace route for production of hot metal, it can accept high alkali containing ores without any build up inside the reactor.
=> It takes only half an hour to stop the plant and only four hours to restart it.
=> Specific melting capacity is higher than that in Blast Furnace; productivity around 3.5 t/m3/d can be achieved.
=> In 2009 Siemens first completed a life cycle assessment for pig iron production, looking at both conventional production in a blast furnace and the more environmentally friendly COREX and FINEX processes. Siemens claimed the COREX and FINEX processes can substantially reduce pollutant emissions when compared to traditional steel production, with its blast furnace and coking and sintering facilities.
=> The iron content of the feed should not be less than 50% as otherwise the slag volume produced will be too high.
=> As is the case in blast furnaces, over 90% of the phosphorous input reports to the hot metal. So, the phosphorous content of ore and coal should be as low as possible.
=> There are also reports that the COREX process cannot be operated without some amount of coke along with non-coking coal - at least around 10% of coke is required in the total reductant charge.
=> As far as coal is concerned, the non-coking coals having too high volatile matter (VM) or too low fixed carbon (FC) cannot be used in corex process of iron making.
=> The heat transfer plays a crucial role in the overall efficiency of the COREX process. This being a two stage process, i.e. reduction and smelting taking place in two separate units, post combustion of the gas generated in the smelting unit provides the heat to melt the DRI produced in the reduction unit. This calls for high heat transfer efficiency.
=> Unless the net export gas from any Corex plant (extent of generation around 1650 Nm3/thm) can be utilized, the process will not be economical. Because of many peripheral requirements, the total cost of a Corex project can be relatively high.   
=> The export gas generated in Corex technology can be used as a fuel gas in the downstream facilities to generate electricity or for the production of direct reduced iron in a region that has almost no resources of natural gas.
=> COREX plant emissions contain only insignificant amounts of NOx, SO2, dust, phenols, sulphides, and ammonium. Emission values are already far below the maximum values allowed by future standards. Also, waste-water emissions from the COREX process are far lower than those in the conventional blast-furnace route. These environmental features are key reasons for the attractiveness of the COREX process.
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FINEX® Process, smelting reduction technology of ironmaking - Features, Merits and Limitations

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28-June-2020
What is FINEX ?

The FINEX is the latest addition and an optimized fine-ore smelting reduction (SR) iron making process based on the direct use of the coal and iron ore fines. FINEX Process is a fluidized bed based process using ore fines instead using iron ore lumps and pellets. This is a process with great potential with regard to productivity and the low cost production of hot metal.
In 1992, POSCO and VAI, Austria signed an agreement to work together for a joint development of the FINEX Process. And accordingly, FINEX process was developed jointly by POSCO, Korea and Primetals Technologies to provide the iron making sector with the capability of producing (hot metal) at a reduced cost, lesser environmental pollutions and more flexibility in terms of operation and the choice of raw materials. Primetals Technologies Limited, is a joint venture of Siemens VAI Metals Technologies and Japan’s Mitsubishi Hitachi Metals Machinery (MHMM).
The present article contains about:
  • What is Finex process
  • Benefits or Merits of this technology
  • Some limitations or disadvantages of Finex technology
To know more about the Steel plants with FINEX process in operation as how these FINEX plants were started and subsequent developments and changes brought in those FINEX plants, read: 

https://www.industry.guru
Fig: FINEX Technology (Flowsheet)
FINEX Process of Iron Making - An Overview
In the FINEX process the iron production is carried out in two separate Process steps. In a series of fluidized bed reactors, fine-grained iron oxides are reduced to direct-reduced iron, compacted and then transported to a melter gasifier. Coal and coal briquettes charged to the melter gasifier are gasified, providing the necessary energy for melting in addition to the reduction gas. Fine ore and additives (limestone and dolomite) are dried and then charged to a 3 or 4 stage fluidized bed system where the iron ores are progressively reduced in counter current flow with the reducing gas to fine DRI and the fine additives are partly calcined.

Reactors R4 and R3 are primarily used to preheat the ore fines to the reduction temperature, which can be adjusted by partial combustion of the off-gas (export gas) from R2. In R2 the fine ore is pre-reduced to reduction degree (RD) of about 30%. At the end of the production in R1, the final reduction to DRI takes place (RD about 90%). Operational pressure in R1 to R4 is approximately 4 - 5 bars. The fine DRI is compacted and then charged in the form of Hot Compacted Iron (HCI) into the melter gasifier. So, before charging to the melter gasifier unit of the FINEX unit, this material is compacted in a hot briquetting press to give hot compacted iron (HCI) since the melter gasifier cannot use fine material (to ensure permeability in the bed). Non-coking coal (lumpy and / or briquetted fines) is charged from the top of the melter gasifier, dried and degassed in the upper char bed area and finally the degassed coal (char) is gasified with pure oxygen which is blown in at the tuyere zone of the melter gasifier bed. The gasification supplies the energy required for the metallurgical reactions and for the melting of HCI and coal ash to hot metal and slag. Pulverized coal injection (PCI) system is provided to inject fine coal via the oxygen tuyeres. The gas generated in the melter gasifier of the FINEX unit is used to reduce the ore in the reactors preceding the melter gasifier. The generated FINEX off-gas is a highly valuable product and can be further used in power generation or heating processes. The DRI is charged in the melter gasifier in hot condition, where it is melted, fully reduced and carburized to hot metal. The hot metal and slag produced in the melter gasifier is frequently tapped from the hearth similar to the blast furnace and COREX operations. Also refer COREX Process of Iron Making - its Merits and Demerits.

FINEX Process - Merits and Benefits
In many respects FINEX process can be considered as an offshoot of COREX process and hence, bear the various advantages of the COREX and more as outlined below -  
Flexibility in Raw Materials
  • No blending of ore & coal. Rather direct utilization of coal.
  • Use of Low-grade ore & low-ranked coal. Integration of the coal briquetting technology increases the range of suitable coal blends for the FINEX application. Utilization of 100% coal briquettes offers the possibility to mix different coal qualities for the generation of coal briquettes.
Easy & Flexible Operation
  • Independent control of reduction & melting processes
  • Easy & hassle-free operational control
Environmental Friendliness
  • Far less emission of SOx, NOx, phenols, sulphides, ammonia & dust because the FINEX process does not need sinter plant and the coke oven battery which are the actual sources of emission in a conventional blast furnace route.
  • Applicability to the CO2 sequestration.
Cost Competitiveness
  • Lower cost in both capital investment & operation as compared to the blast furnace route, keeping the quality of the hot metal same.
  • According to POSCO, the capital cost & operating cost of FINEX process are less than by 20 and 15 percent respectively of that of Blast Furnace route. 
  • Need much less land as compared to conventional BF complex.
  • Similar to the Corex export gas, FINEX export gas (with calorific value of 5,500 – 6,250 kJ/m3 STP) can be used to substitute natural gas, oil, coke and coal for metallurgical applications and power generations etc. Depending on the composition of coal and the decision whether gas recycling is applied or not, the amount and the composition of the export gas can vary within definite limits.
Limitations (Demerits) of FINEX Process
As said FINEX, COREX, HISMELT are the latest alternative methods for producing liquid iron (Hot Metal) through Smelting Reduction (SR) process. Some of the limitations (disadvantages) are -
  • Ease of obtaining FINEX technology is uncertain though POSCO has started to extend it.
  • Both COREX and FINEX processes need a large amount of oxygen.
  • The major criteria for an initial evaluation of coals or coal blends for the FINEX Process are: 1. Fix carbon content at a minimum of 55%, 2. Ash content up to 25%, 3.Volatile content lower than 35%, 4. Sulphur content lower than 1%
  • Additional to these qualities the coal must have a good thermal stability to ensure the formation of a stable char bed in the melter gasifier. 
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