HOW A BLAST FURNACE WORKS:

HOW A BLAST FURNACE WORKS:

INTRODUCTION:

The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called “ hot metal” .  The blast furnace is a huge, steel stack lined with refractory brick , where iron ore, coke and limestone are dumped into the top, and preheated air is blown into the bottom. The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.

The process:

Iron Oxides can come to the blast furnace plant in the form of raw ore , pallets or sinter. The raw ore is removed from the earth and sized into pieces that range from 0.5 to 1.5 inches. This ore is either Hematite ( Fe₂O₃ ) or Magnetite ( Fe₃O₄ ) and the iron content ranges from 50% to 70%.

This iron rich ore can be charged directly into a blast furnace without any further processing. Iron ore that contains a lower iron content must be processed or beneficiated to increase its iron content. Pallets are produced from this lower iron content ore. This ore is crushed and ground into a powder so the waste material called gangue can be removed. The remaining iron-rich powder is rolled into balls and fired in a furnace to produce strong, marble-sized pellets that contain 60% to 65% iron. Sinter is produced from fine raw ore, small coke, sand sized limestone and numerous other steel plant waste materials that contain some iron. These fine materials are proportioned to obtain a desired product chemistry then mixed together. This raw material mix is then placed on a sintering strand, which is similar to a steel conveyor belt, where it is ignited by gas fired furnace and fused by the heat from the coke fines into larger size pieces that are from 0.5 to 2.0 inches. The iron ore, pellets and sinter then become the liquid iron produced in the blast furnace with any of their remaining impurities going to the liquid slag.

The coke is produced from a mixture of coals. The coal is crushed and ground into a powder and then charged into an oven. As the oven is heated the coal is cooked so most of the volatile matter such as oil and tar are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time. The coke is cooled and screened into pieces ranging from one inch to four inches. The coke is very strong compared to coal . The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter.

The final raw material in the ironmaking process is limestone. The limestone is removed from the earth by blasting with explosives. It is then crushed and screened to a size that ranges from 0.5 inch to 1.5 inch to become blast furnace flux. This flux can be pure high calcium limestone, dolomite limestone containing magnesia or a blend of the two types of limestone.

Since the limestone is melted to become the slag which removes sulfur and other impurities, the best furnace operator may blend the different stones to produce the desired slag chemistry and create optimum slag properties such as a low melting point and a high fluidity.

All of the raw materials are stored in an ore field and transferred to the stock-house before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace.

The Iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a series of chemical reactions. These reactions occur as follows:

1-    3 Fe₂O₃  + CO = CO₂  +  2 Fe₃O₄  Begins at 850⁰ degree F

2-     Fe₃O₄  + CO = CO₂  +  3 FeO  Begins at 1100⁰ degree F

3-    FeO  +  CO = CO₂  +  Fe    or  FeO  + C = CO + Fe, Begins at 1300⁰ degree F

At the same time the iron oxides are going through these purifying reactions, they are also beginning to soften then melt and finally trickle as liquid iron through the coke to the hearth.

The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows:

        C + O₂   =  CO₂  +  Heat

Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows:

CO₂  +  C  =  2 CO

The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions.

The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows:

   Ca CO₃   =  CaO  +  CO₂

This reaction requires energy and starts at about 1600⁰ F. The CaO formed from this reaction is used to remove sulfur from the iron which is necessary before the hot metal becomes steel. This sulfur removing reaction is :

  FeS  +  CaO  +  =  CaS  +  FeO  +  CO

The CaS becomes part of the slag. The slag is also formed from any remaining Silica

( SiO₂ ), Alumina ( Al₂ O₃ ), Magnesia ( MgO ) or Calcia ( CaO ) that entered with the iron ore, Pellets, Sinter or Coke. The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense.

Another product of the iron making process, in addition to molten iron and slag, is hot dirty gases. These gases exit the top of the blast furnace and proceed through gas cleaning equipment where particulate matter is removed from the gas and the gas is cooled. This gas has a considerable energy value so it is burned as a fuel in the hot blast stoves which is used to preheat the air entering the blast furnace to become “hot blast”. Any of the gas not burned in the stoves is sent to the boiler house and is used to generate steam which turns a turbo blower that generates the compressed air known as “cold blast” that comes to the stoves.

In summary, the blast furnace is a counter-current realtor where solids descend and gases ascend. In this reactor there are numerous chemical and physical reactions that produce the desired final product which is hot metal. A typical hot metal chemistry follows:

Iron ( Fe )                    =      93.5   ---   95.0 %

Silicon ( Si )                 =      0.30  ----   0.90%

Sulfur ( S )                   =      0.025 ----  0.050%

Manganese (Mn)         =      0.55   ----  0.75%

Phosphorus (P )          =      0.03   ----- 0.09%

Titanium ( Ti )              =      0.02   ----- 0.06%

Carbon  ( C )               =      4.1    ------ 4.4%  

Now that we have completed a description of the iron making process, let us review the physical equipment comprising the blast furnace plant.

There is an ore storage yard that can also be an ore dock where boats and barges are unloaded. The raw materials stored in the ore yard are raw ore, several types of pellets, sinter, limestone or flux blend and possible coke. These materials are transferred to the “stock-house/hiline” complex by ore bridges equipped with grab buckets or by conveyor belts. Materials can also be brought to the stock-house/hiline in rail hoppers or transferred from ore bridges to self propelled rail cars called “ore transfer cars”. Each type of ore, pellets, sinter, coke and limestone is dumped into separate “storage bins”. The various raw materials are weighed according to a certain recipe designed to yield the desired hot metal and slag chemistry.

This material weighing is done under the storage bins by a rail mounted scale car or computer controlled weigh hoppers equipped with load cells and feed a conveyor belt. The weighed materials are then dumped into a “skip car” which rides on rails up the “inclined skip bridge” to the “receiving hopper” at the top of the furnace. The cables lifting the skip cars are powered from large winches located in the “hoist house”. Some modern blast furnace accomplish the same job with an automated conveyor stretching from the stock-house to the furnace top.

At the top of the furnace the materials are held until a “charge” usually consisting of some type of metallic ( ore, pellets or sinter ), coke and flux (limestone) have accumulated. The precise filling order is developed by the blast furnace operators to carefully control gas flow and chemical reactions inside the furnace. The materials are charged into the blast furnace through two stages of conical “bells” which seal in the gases and distribute the raw materials evenly around the circumference of the furnace “throat”. Some modern furnaces do not have bells but instead have 2 or 3 air lock type hoppers that discharge raw materials onto a rotating chute which can change placement inside the furnace. ( known as bell less top system ).  

Also at the top of the blast furnace are four “uptakes” where the hot, dirty gas exits the furnace dome. The gas flows up to where two uptakes merge into an “offtake”. The two offtakes then merge into the “downcomer”. At the extreme top of the uptakes there “bleeder valves” which may release gas and protect the top of the furnace from sudden gas pressure surges. The gas descends in the downcomer to the “dustcatcher” , where coarse particles settle out, accumulate and are dumped into a railroad car or truck for disposal. The gas then flows through a “ventury Scrubber” which removes the finer particles and finally into a “gas cooler” where water sprays reduce the temperature of the hot but clean gas. Some modern furnaces are equipped with a combined scrubber and cooling unit. The cleaned and cooled gas is now ready for burning.

The clean gas pipeline is directed to the hot blast “stove”. There are usually 3 or 4 cylindrical shaped stoves in a line adjacent to the blast furnace. The gas is burned in the bottom of a stove and the heat rises and transfers to refractory bricks inside the stove. The products of combustion flow through passages in these bricks, out of the stove into a high “stack” which is shared by all of the stoves. 

Large volumes of air, from 2265M³/min. to 6513M³/min, are generated from a turbo blower and flow through the “cold blast main” up to the stoves. This cold blast then enters the stove that has been previously heated and the heat stored in the refractory brick inside the stove is transferred to the “cold blast” to form “hot blast” .

The hot blast temperature can be from 1600⁰ F  to  2300⁰ F depending on the stove design and condition. This heated air then exits the stove into the “hot blast main” which runs up to the furnace. There is a “mixer line” connecting the cold blast main to the hot blast main that is equipped with a valve used to control the blast temperature and keep it constant. The hot blast main enters into a doughnut shaped pipe that encircles the furnace, called the “bustle pipe” . From the bustle pipe, the hot blast is directed into the furnace through nozzles called “ tuyeres” . These tuyeres are equally spaced around the circumference of the furnace. There may be fourteen tuyeres on a small blast furnace and forty tuyeres on a large blast furnace. These tuyeres are made of copper and are water cooled since the temperature directly in front of the them may be 3600⁰ F to 4200⁰ F. Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace at Tuyere level to combine with the coke to release additional energy which is necessary to increase productivity. The molten iron and slag drip past the tuyeres on the way to the furnace hearth which starts immediately below Tuyere level.

Around the bottom half of the blast furnace the “casthouse” encloses the bustle pipe, tuyeres and the equipment for “casting” the liquid iron and slag. The opening in the furnace hearth for casting or draining the furnace is called the “iron notch”. A large drill mounted on a pivoting base called the “taphole drill” swings up to the iron notch and drills a hole through the refractory clay plug into the liquid iron. Another opening on the furnace called the “cinder notch” is used to draw off slag or iron in emergency situations. Once the taphole is drilled open , liquid iron and slag flow down a deep trench called a “trough”. Set across and into the trough is a block of refractory, called a “skimmer”, which has a small opening underneath it. The hot metal flows through this skimmer opening, over the “iron dam” and down the “iron runners”. Since the slag is less dense than iron, it floats on top of the iron, down the trough, hits the skimmer and is diverted into the “slag runners”. The liquid slag flows into “slag pots” or into slag pits and the liquid iron flows into refractory lined “ladles” known as torpedo cars or sub cars due to their shape. When the liquids in the furnace are drained down to taphole level, some of the blast from the tuyeres causes the taphole to spit. This signals the end of the cast, so the “mudgun” is swung into the iron notch. The mudgun cylinder, which was previously filled with a refractory clay, is actuated and the cylinder ram pushes clay into the iron notch stoping the flow of the liquids. When the cast is complete, the iron ladles are taken to the steel shops for processing into steel and the slag is taken to the slag dump where it is processed into roadfill or railroad ballast. The casthouse is then cleaned and readied for the next cast which may occur in 45 minutes to 2 hours. Modern, larger blast furnaces may have as many as four tapholes and two casthouses. It is important to cast the furnace at the same rate that raw materials are charged and iron/slag produced so liquid levels can be maintained in the hearth and below the tuyeres. Liquid levels above the tuyeres can burn the copper casting and damage the furnace lining.

CONCLUSION :

The blast furnace is the first step in producing steel from iron oxides. The first blast furnaces appeared in the 14th century and produced one ton per day. Blast furnace equipment is in continuous evolution and modern, giant furnaces produce 13000 ton per day. Even though equipment is improved and higher production rates can be achieved, the processes inside the blast furnace remain the same. Blast furnaces will survive into the next millennium because the larger, efficient furnaces can produce hot metal at costs competitive with other iron making technologies.

charcoal use in Blast Furnace

Use of charcoal in blast furnace operations
FAO STAFF
Paper prepared for a United Nations interregional Symposium on the Application of Modern Technical Practices in the Iron and Steel Industry in Developing Countries, 1983.
FOR a long time charcoal was the only fuel and reducing agent used in iron production. Scarcity of wood in some countries caused the development of coke as an alternative fuel and today coke dominates even in most countries with large forest resources. Coke has become generally thought of as associated with the large, highly productive blast furnaces of today and it is then easy to think of charcoal as less efficient. Although the general trend has certainly been away from charcoal, charcoal blast furnaces are in operation in several countries and there are even plans for expanding some of these operations. Economic conditions vary both between countries and within them and it would therefore be of value to study the cases where charcoal is preferred in order to determine the relevant economic and technical factors. Unfortunately, it has not been possible to make a comprehensive study of the subject but certain data have become available which indicate that technical progress in forestry, charcoal production, and blast furnace operations have been of great significance. Constantine 1 states that "under modern conditions charcoal can be as economic and efficient as coke for smelting iron ore in a standard blast furnace subject only to the provisions of adequate supply of raw materials." At the same time, several of the papers submitted to the present symposium point out the scarcity and high price of metallurgical coke, which often has to be transported over great distances.
1 Constantine, A. Charcoal blast furnace operations, Wundowie, W. Australia. International symposium, 1963.
About 200 years ago one ton of pig iron produced in Sweden ² required over two tons of charcoal, i.e., probably about 18 cubic meters of solid wood (softwood). A century ago the requirements were less than 1.6 tons of charcoal or 12 cubic meters of softwood. Today about 0.7 tons of charcoal (equal to coke) are normally required corresponding to only 5 cubic meters of softwood due to higher yields of charcoal. If hardwoods are used this may drop to 3.6 cubic meters or even less in the case of very dense wood. Very substantial economies have clearly been achieved in the use of wood and charcoal.
² Arpi, G. Den svenska jarnhanteringens trakolsforsorjning 1830-1950. Jernkontoret, Stockholm, 1951.
Present use of charcoal in blast furnaces
In assessing the value of charcoal in pig-iron production let us first look at some of the operations that are using charcoal today.
Charcoal iron industry, Wundowie, West Australia
This is an integrated sawmill, charcoal, and iron industry described in detail by Constantine. 1 The principal data can be summarized as follows.
The forest is of very uniform composition and only one eucalypt species is used. The forest residues and sawmill residues amount to 70 percent of the growing stock. The forest is managed on a sustained yield basis with a rotation of 100 years. Within a radius of about 40 kilometers (25 miles) there is sufficient wood for 170,000 tons of pig iron per year and within 65 kilometers (40 miles), which is considered the maximum economic transport distance, there is enough for 400,000 tons of pig iron. The project has been developed in two steps. A pilot plant of 10,000 tons of pig iron initiated operation in 1948 and was economically self-supporting in 1953. The operation was expanded to 50,000 tons in 1958, is economically successful, and there are plans for expansion to 250,000 tons per year.
Charcoal is produced in continuous kilns and the yield is 37 percent of the dry wood. The kilns are integrated with a blast furnace to form a very efficient unit with respect to heat and electric power economy. AD operations are highly mechanized, including the forestry operations and the cost of wood prepared for charging to the retorts is less than U.S.$4 per ton. The cost of the charcoal is about U.S.$20 per ton but is expected to fall to $18 with increased volume production.
The pig iron is very pure and contains less than 0.015 percent sulfur and about 0.03 percent phosphorus and practically no other tramp elements.
The wood used is of very high density and gives hard charcoal with a bulk density of about 300 kilograms per cubic meter (19 pounds per cubic foot). This and other factors are the basis for the statement in the paper that it is debatable whether there is any limit to the size of charcoal furnaces. Expansion plans call for the building of units of a capacity of 400 tons of iron per day with a 19.5-meter (65 foot) working height and a hearth diameter of 4.5 meters (15 feet).
Belgo-Mineira steel company, Monlevado, Brazil
This company is located on the Brazilian high plateau in Minas Geraes and there are no local resources of coke. It uses charcoal from second and third growth mixed tropical forest and from plantations of eucalyptus species. At present the plantations supply only 10 percent of the requirements but the percentage increases every year and is planned to have reached 100 by 1983, for an iron output of 500,000 tons. The total plantation area required for this production volume is 136,000 hectares (340,000 acres) yielding 2.5 million solid cubic meters per year. Replanting is done after 22 years with intermediate clear fellings after 8 and 15 years. The average yield is about 20 solid cubic meters per hectare (280 cubic foot per acre) per year which is higher than the yield from the natural forest. The species planted yield relatively dense wood. The charcoal from the mixed forest is understood to be quite satisfactory but the density and texture are variable and the plantations yield a more consistently high quality product.
The forestry operations are less mechanized than in the previously described operation and charcoal production has so far been in batch kilns built of brick. A continuous kiln is under construction with a capacity of about 60 tons per day. The cost of plantation wood at the charcoal kiln is estimated at U.S.$2.5 per ton of dry wood. The corresponding cost of the charcoal is about U.S.$8 per ton whereas charcoal from the native forest, partly purchased, is less expensive.
Other countries
In the U.S.S.R., an unknown number of charcoal blast furnaces are in operation and Japan produces some 30,000 tons of charcoal iron annually. In Sweden the use of charcoal has declined mainly because of its high cost caused by competition for the raw material from the pulp and paper industries but, as late as 1948, 31 percent of the pig iron production was made with charcoal. In Sweden, charcoal has been mainly produced from softwoods giving a product with relatively low density, whereas in Japan metallurgical charcoal is produced from oak and is of high density.
Technical properties of metallurgical charcoal
Two main properties of charcoal of importance in blast furnace operations are compression strength and chemical composition.
High compression strength is desirable in order to avoid crushing in the furnace which reduces the possible working height of blast furnaces as well as the specific production per unit of volume. Most metallurgical charcoal produced in the Northern Hemisphere has been of low density, produced from softwoods or from medium-density hardwoods. The charcoal production at Wundowie has quite different characteristics and it is stated that with charcoal of comparable quality it is the softening of the iron ore, not the charcoal, that sets the limit of blast furnace height and capacity. In addition, the specific output per unit of furnace volume is higher than for soft charcoal.
Special testing methods for the compression strength (hardness) of charcoal are used. Some hardness data for different types of charcoal as well as other properties are given in Table 1.
The great range in hardness can be seen from the table. Some hardness data for charcoal from European woods are given in an FAO study ³ but are not comparable to those contained in the table shown here.
³ FAO. Charcoal from portable kites and fixed installations. Rome, 1956.
Main factors that influence the density and hardness of charcoal are: wood species, size of the wood as fed to the kiln, and the charcoal production method. Dense wood generally gives dense and hard charcoal and is therefore to be preferred to lighter wood for the production of metallurgical charcoal. It follows that when a mixture of species is used, the hardness of the end product will not be uniform. If the density range is wide, it may be necessary to sort out species of low density in cases where the requirements with respect to hardness are very exacting.
Among the chemical properties of charcoal for pig-iron production the volatile content and the ash content are the most important. In modern retorts the carbon and volatile content can be satisfactorily controlled and should not present difficulties even if the wood is not of uniform composition.
The charcoal retains all the ash content of the wood which varies considerably according to species. The total ash content should be as low as possible. A figure of 0.24 percent has been indicated as excellent and an upper limit of 1.5 percent has been given. At Wundowie the ash content is very low, 0.26 percent. Apart from total ash, the amounts of phosphorus and sulfur are of major interest. The content of sulfur is generally very low, making charcoal suitable for the production of iron of high purity with respect to sulfur. Phosphorus is variable within wide limits not only between species but also within a tree and with the soil composition. Swedish hardwoods were found to contain four times as much phosphorus as the softwoods; bark contains much more than wood and splintwood more than heartwood. However, even in hardwoods the content of phosphorus is generally low enough for the production of pig iron of high purity. Typical data for the phosphorus content of Swedish charcoal are about 0.01 percent for softwood charcoal and about 0.04 for hardwood charcoal (birch, alder, aspen).4 Data for the phosphorus and sulfur content of charcoal pig iron from Wundowie are given as 0.03 percent and 0.015 percent respectively.
TABLE 1. - ANALYSIS OF CHARCOAL
SOURCE: Forest Experiment Station, Meguro. Charcoal Section. Outline of Japanese charcoal and charcoal study, 1960.
1 Hardness was tested by Miura hardness tester for charcoal . ² Calorific value calculated from industrial analysis data. ³ The type used for metallurgical purposes in Japan.
TABLE 2. - WOOD REQUIREMENTS, FOREST AREA AND TRANSPORT DISTANCE FOR PIG-IRON OPERATIONS OF VARIOUS SIZE
NOTE:
(a) The following assumptions have been made: charcoal consumption 0.7 tons/ton; charcoal yield 36 percent; wood density 0.6 g/m³ one third of the land area around the plant available for timber supply.
(b) 1 m³/ha/year = approximately 14 cu. ft/acre/year.
There is considerable variation in the ash content of tropical hardwoods, some of these having high silica content. The ash content will no doubt require attention in any new project to utilize mixed tropical woods although this aspect has not been reported as causing difficulties in the use of Brazilian woods.
4 Bergstrom, E. Kolning i ugn. Jernkontoret, Uppsala, 1947.
Of other analytical data the moisture content is of importance to smooth furnace operations. Charcoal should therefore be suitably protected against moisture.
Availability of wood
As mentioned initially, local shortage of wood was the main reason for developing coke for pig-iron production. Although the lower wood consumption per ton of iron brought about by technical improvements has helped to reduce the wood requirements for an iron operation, the larger output of many modern furnaces has worked in the other direction. In order to obtain a picture of the wood consumption, the forest area required, and the corresponding transport distance for a pig-iron operation, Table 2 has been prepared.
A wide range of plant sizes has been included in Table 2. It is noted that with respect to wood requirement, pig iron and chemical pulp production are very similar with a consumption of close to 2 tons per ton of end product. From a forestry standpoint the 300,000-ton project is a very large one: there are not many pulp mills with a larger wood intake. The data presented for Wundowie mention the economic results and expectations at 50,000 and 300,000 tons per year respectively. Under the assumptions made in Table 2, a 50,000-ton plant would need an operating radius of about 18 kilometers (11 miles) at the lowest of the yields assumed. At 300,000 tons per year a minimum yield of 6 to 7 cubic meters per hectare (84 to 98 cubic feet per acre) would be required to keep within the present operating distance contemplated at Wundowie, i.e., 40 kilometers (25 miles). A yield of this magnitude should normally be possible to reach in locations with subtropical or tropical climate and adequate rainfall. It should be noted that the average yield assumed in the planting program at Monlevado is above 20 cubic meters per hectare (280 cubic feet per acre) per year.
Wood in sufficient quantity and for sustained yield operations should therefore be available or possible to produce in many locations even for relatively large pig-iron operations. Today, wood from mixed tropical forests and from plantations of fast-growing species for wood production are sources that may be most generally available for charcoal pig-iron production. In particular, opportunities may be at hand in some instances for the clear-cutting of tropical or subtropical mixed forests for which there is little use at present except for a few species, often representing only a small fraction of the standing volume. In view of the technical requirements it might be necessary to reject certain species because of low density or high ash content. The area would then be replanted with high-yielding suitable species so that eventually the mill would switch over to using plantation wood. Another possibility is natural regeneration with suppression of undesirable species, but this alternative is likely to be less attractive as plantations will yield a more uniform product and offer better opportunities for the mechanization of operations.
There are also a few oases where wood is available at little or no stumpage value, e.g., rubber and wattle plantations. For rubber plantations estimates have been made 5 about the amount of wood available on a sustained basis in Thailand. The data indicate that, after allowing for the need of 1.3 cubic meters per hectare (18 cubic feet per acre) per year of rubber wood for smoking the rubber, approximately 4 cubic meters per hectare (56 cubic feet per acre) per year could be obtained for industrial purposes. In wattle plantations, which may yield 5 to 10 cubic meters per hectare (70 to 140 cubic feet per acre) per year, there is often little use for the wood, which may even be left behind after the bark has been removed for tannin extraction.
5 United Nations/FAO. Pulp and paper prospects in Asia and the Far East. Vol. II. Bangkok, 1962.
Cost of wood and charcoal
A natural upper limit for the cost of charcoal in pigiron operations will be set by the price of metallurgical coke. When taking the price of coke as a basis for comparison, it is assumed that coke and charcoal are approximately equal with respect to the quantity required. It is further assumed that the charcoal produced is of such quality that its performance in the blast furnace is similar to that of coke. Even so a charcoal operation will require more capital: both for charcoal production and for the development of the forest and its exploitation. In case the charcoal is of lower quality this will have to be compensated by lower cost, otherwise the cost of the pig iron will be higher than if coke were used.
Several of the papers presented to the symposium mention the scarcity of coke which enters into international trade and is often transported over long distances. Brown 6 gives the price of coke At plants on the eastern seaboard of the United States as U.S.$16 per ton. The cost of charcoal given above for the Brazilian operation is well below this figure. Even in the Australian example it is obvious that transport costs would bring coke above the cost of locally produced charcoal.
6 Brown, H. R. and W. R. Hesp. Substitutes for coking coals in iron ore reduction, Interregional symposium, 1963.
It can, however, be argued that a maximum allowable cost of charcoal wood of about U.S.$6 to 6 per ton is a very low figure. It may be compared with roughly U.S.$20 to 30 per ton of coniferous pulpwood in Europe and about half this cost for broadleaved pulpwood in the United States. In 1953 studies were made on pulp mill projects based on tropical species in Amapa in the Amazon area of Brazil and in Yucatan.7 The cost of fuelwood was shown to increase somewhat with the size of the operation but was given as about U.S.$4.7 and 2.3 respectively at 200,000 tons per year.
7 United Nations/FAO. Pulp and paper prospects in Latin America. New York, 1955.
It will obviously take efficient organization and careful planning even under favorable conditions to reach the low cost required, but the Australian example cited above shows that the required cost level can be reached even where labor costs are high.
Conclusions
On the basis of information on experience recently gained on the use of charcoal in blast furnace operation the following conclusions appear justified.
1. Pig-iron production with charcoal in blast furnaces requires approximately the same amount as coke, 0.7 tons per ton. This corresponds to about 2 tons of dry wood per ton of iron. In view of the fact that pig iron production generally is conducted in comparatively large units, it follows that forestry operations to supply charcoal for blast furnaces must normally be of a size comparable to pulp mill projects.
2. Charcoal from dense woods performs as well as coke in blast furnaces and can be used in units up to a capacity of at least 400 tons per day. Softer charcoal reduces the maximum size of a unit as well as the capacity of a furnace of a given size.
3. It is possible, in certain locations, to produce charcoal at a cost comparable to or lower than that of metallurgical coke. This requires a large, well-organized forestry operation, mechanized according to local conditions as well as the use of continuous charcoal retorts.
4. The large volume of wood required on a sustained yield basis to serve a pig iron operation as well as the requirements with respect to the cost of the wood and charcoal produced and the quality of the product considerably limit the number of locations where conditions are favorable.
5. Such conditions could be at hand in areas where high yield tree species can be grown, where the population pressure is low and where transport costs offer a natural protection against competition from coke.
6. The forestry operation could be developed by gradually converting natural forests of mixed species into plantations of uniform composition. This offers one of the few possibilities for the large scale utilization of mixed hardwood species, including the very dense woods.

GAS SAFETY IN BLAST FURNACE

GAS SAFETY IN IRON INDUSTRY(Blast furnace)

1.      INTRODUCTION

The Most potential hazardous element in the process of our plant is MBF gas i.e. the CO content in it, Un less handled carefully it may cause havoc by damaging man and machine. To bring safe condition we must be knowledgeable about CO gas and its characters. More over we must always obey safety rules and adopt safely procedures, safe methods and safe practices while caring out our jobs for operation maintenance and other services. Only then we can have Excellency in our performance for production and productivity with quality. Obey the gas safety rules gas will obey you.

2.     GAS GENERATION, HANDLING AND USE

BF gas is a by-product of MBF operation having good fuel value. It is used for most of our thermal activities in the plant. We use it as fuel to heat the MBF and boiler. It is transported to consumption site by steel pipes and control at several places by butterfly valves and water seals.

Hourly generation of MBF Gas is about 77000-80000 Nm3/Hr with service line pressure 700-800 mmwc. The CO content of this gas is about 22-24% and its calorific value is 650-680 K.Cal/Nm3.

Other gases used in the plant are Oxygen, LPG, and Nitrogen etc in cylinders. All these gases can create danger when handling carelessly.

3.     CO GAS AND IT’S HAZARDNESS

This gas is colorless, test less, odorless with a peculiar smell. Unless one is very much familiar with gas, it is difficult to detect it’s presence without testing instruments. But it has its effect when breathed in along with atmospheric air. Because of affinity of hemoglobin to CO (about 200 to 300 times) the following symptoms in human body is felt within a short period:

·        Headache

·        Nausia

·        Vomiting

·        Feeling giddiness and oppression in chest

·        Mental confusion

·        Difficult in breathing

·        Unconscious and death

The effects of Carbon Monoxide on the human system in different concentrations in atmosphere

Sl. No.	‘CO’ CONCENTRATION IN AIR	EFFECT ON EXPOSURE
1	0.003% (30 PPM)	NO EFFECT ON EXPOSURE OF 8 HOURS
2	0.005% (50 PPM)	IMPERCEPTIBLE FOR 8 Hrs OF EXPOSURE
3	0.01% (100 PPM)	IMPERCEPTIBLE FOR 4 Hrs OF EXPOSURE
4	0.02% (200 PPM)	MILD& FRONTAL HEADACHE AFTER 2-3 Hrs. OF EXPOSURE
5	0.04% (400 PPM)	i. MILD HEADACHE AFTER 1 Hr. OF EXPOSURE ii. OCCIPITAL HEADACHE AFTER 2-3 Hrs. OF EXPOSURE
6	0.08% (800 PPM)	HEADACHE, DIZZINESS AFTER 3-4 Hrs OF EXPOSURE
7	0.16% (1600 PPM)	i. ALL ABOVE SYMPTOMS IN 20 Mins. OF EXPOSURE ii. COLLAPSE UNCONSCIOUSNESS & PROBABILITY OF DEATH IN 2 Hrs. OF EXPOSURE
8	0.32% (3200 PPM)	ALL ABOVE SYMPTOMS IN 05 Mins. OF EXPOSURE, DEATH IN 30 Mins. OF EXPOSURE
9	0.64% (6400 PPM)	ALL ABOVE SYMPTOMS IN 1-2 Mins. OF EXPOSURE, DEATH IN 30 Mins. OF EXPOSURE
10.	1.28% (12800 PPM)	INSTANTANEOUS DEATH IN 1-4 Mins.

Age and physical condition of the body also have their influences. Alcoholism and obesity lower body’s resistance to the age. There is also individual variation depending upon the certain factors influencing absorption. E.g. blood volume, circulatory deficiency, breathing rate, status of health, anemia, old age etc.

4. CO GAS DETECTION AND PROBLEMS

There must be a periodical inspection and checking for any gas leakage from gas handling equipments. More over the test for gas leaking must be conducted immediately after having doubt. This test and detection must be conducted by authorized person with standard detecting instruments. The following safety tools and applications are must for the purpose:

For Detection:-

·        Canapy birds

·        CO Detector

·        Digital CO indicator

·        Tox guard (for co presence in atmos)

·        Explosive Gas Detector

For Protection:-

·        Gas Mask- blower type

·        Oxygen type gas mask

·        Oxygen respiration

·        First Aid Box

·        Stretcher

·        Caution and warning boards

5. GAS HAZARDOUS LOCATIONS IN THE PLANT

The followings locations must be avoided always or be approached with safety precautions:

1.      MBF Top

2.      Cast house very near to Tuyere

3.      GCP, VDF, actuator of venture

4.      Dust Catcher and its bottom bell valve and fresh extracted flue from dust deposit

5.      “U” seals and drain pots in clean gas line

6.      Burners of Stoves and Boiler

7.      Bleeder valves in gas line

6. PREVENTION AND CONTROL OF EXPLOSION AND FIRE IN GAS LINE

EXPLOSION:-Explosion is the rapid and uncontrolled combustion of fuel air mixture in a confined space resulting sound and sudden build up of pressure

Explosion occurs when

1.      The gas and the air mixture are within their explosive range

2.      The heat must be enough to ignite the resulting mixture of gas and air

3.      The mixture of gas and air is not sufficient to ignite the explosive mixture, there will be no explosion

Gas	Lower explosive limit	Upper explosive limit
BFG	Of gas in the mixture 35%	Of gas in the mixture 73.5%

Prevention of Explosion:-

1.      Avoid the formation of explosive mixture by purging the gas main with steam

2.      Prevent the suction of air in the gas main

3.      Naked lights must not be used near any de-pressurized gas mains unless the same have been thoroughly purged

4.      It is prohibited to have source of ignition near gas valves, gas line joints, gas holders and gas impulse lines

5.      In case of furnace going down for the repair, gas pipes to the furnace should be disconnected wherever possible and water seal on blank plates should be inserted into the mains

6.      Welding and cutting jobs on the gas line should be done only after these are purged and line is isolated by water seal or blank plates before the place where the welding or cutting job is to done. It is necessary to take clearance from the Process Department before doing any welding or gas cutting job.

7.      Routine maintenance of gas lines, taking quick action for stopping leakages when found proper ventilation of enclosed spaces, quick steam purging arrangement and good house-keeping help to prevent explosion.

FIRE DUE TO BF GAS

Action to be taken in case of fire in Gas Line

If the gas line catches fire during work on through some cracks as because of some leakage, it should be extinguisher with water, co2 extinguisher. The portion of the gas main affected must be cooled down with water and only after the fire is extinguished should the affected main be isolated for carrying out the repairs. The leakage of gas, which has resulted in fire, must be stopped after the fire is extinguished.

          It should be remembered that the gas pressure in the mains must be reduced below 100 mmwc where fire is still there.

          Inform MBF Control Room in case of all fire in Gas line

7. GENERAL SAFETY PRECAUTIONS

1.      No unauthorized person is allowed to enter I the gas hazard area.

2.      Unless any demand, always avoid to go to near to gas handling lines and equipments such as MBF top, Dust Catcher, GCP, gas lines and Bleeder valves. Entrance to gas Hazard area is normally prohibited

3.      No body must take rest or be wormed near MBF

4.      No single person must not go to gas hazardous area for inspection and work

5.      No body with empty belly or with body ailment should be allowed to work in the position with gas hazard

6.      No naked flame is allowed to gas leaking suspected area.

7.      Test the suspected leaking point with soap water only

8.      Report Shift In charge immediately about the gas leakage and arrest the leakage without time loss by safety procedure

9.      go for any maintenance in gas line after getting the clearance from proper authority

10. .Go away from the place where you smell a peculiar smelling with uneasy feeling to a different clear air site

11. When you see somebody unable to move with gas poisonings do not help him alone. Seek the help of others to remove him immediately from that place. If necessary use gas mask

12. Safety appliances must be in good working order and be kept it an easily approachable marked area.

13. Follow the standard instructions for maintaining MBF gas pressure at different positions and it’s critical higher or lower limit

14. Conditions and sequence must be maintained before all owing BFG in burner of stove and boiler to avoid explosion

15. Gas line pressure and sequence of operations must be maintained for bleeding or stem purging.

16. While bleeding or purging, see that there is no man in the direction that gas air mixture blows

17. Cordon off gas leaking area and fix caution and danger board

8.     SAFETY PRECAUTIONS AND PROCEDURE TO WORK IN GAS HAZARD LOCATIONS

Special safety precautions and procedure must always be followed for any job to be done in gas hazard locations earlier. Before taking clearance from process department no body should go in the gas hazard location. Near any de-pressurized gas mains or equipment naked light must not be used. Steam purging is necessary to avoid formation of explosive mixture before welding or cutting job. Keep open the corresponding relief valve to avoid sudden build up of pressure in a confined space

9.     CAUSES LEADING TO GAS EXPOSURE AND EXPLOSION

1.      Ignorance about the gas safety, CO gas and its effect

2.      New man arrival (shift changing)

3.      During shut down and start up of MBF

4.      During inspection and maintenance

5.      Faulty gas pressure control (beyond critical limit)

6.      Confusion about preventive method and procedure

7.      Disobedience, carelessness and hurries

8.      Non adherence to safety rules

9.      Non adherence to gas handling procedure

10. Ignorance of using life saving appliances

11. Negligent supervisor for judgment and watchfulness during work carried on gas hazard area

12. Doubtful supervisor not consulting with superiors the event of new conditions arises

13. Non protection of safety appliances and wrong testing tools

10. CARBON MONOXIDE EXPOSURE - FIRST AID:-

DO’S

1.       The affected person should immediately removed from the gaseous atmosphere to FIRST AID

2.     Any tight fitting clothes, shoes, and belt must be REMOVED

3.      Any PHYSICAL EXERTION by the affected person must be avoided

4.     If conscious, the affected person must be REASSURED

5.      MEDICAL UNIT must be informed and the affected person should be sent in an AMBULANCE ONLY.( Medical Unit: DIAL   )

6.      Oxygen inhalation if required should be started immediately by a trained person

7.      Concerned supervisor and safety personal should be informed

DON’TS

1.      Do not give warm milk or food to the affected person

2.      Do not crowd around the affected person. Give him fresh air

3.     Do not Panic

11.  CARBON MONOXIDE POISONING – CLINIC SIDE

The air we breathe contains 21% of Oxygen at normal atmosphere pressure (760 mm Hg). With respiration air enters into the minutest microscope sacs (units) of lungs called “Alveoli”. It is in the alveoli that the oxygen of the air gets absorbed by the hemoglobin of blood to form a compound called oxy hemoglobin (O2+Hb=OHb). Oxygen requirement of the tissues of the body is thereby met oxy hemoglobin.

          Unfortunately, hemoglobin has 200 to 300 times more affinity for CARBON MONOXIDE than Oxygen. Carbon Monoxide when breathed along with air into the lungs, combines readily with hemoglobin to form CARBOXY HEMOGLOBIN (CO+Hb=COHb) and thereby deprives the blood of its oxygen carrying capacity. Thus the body tissues suffer from lack of oxygen (ANOXAEMIA). The symptoms, which occur in carbon monoxide poisoning, is directly proportional to the degree of anoxaemia.

          According the percentage of CO in the atmosphere and the period of exposure, percentage of COHb in blood increases giving rise to certain symptoms. They are HEADACHE, NAUSEA, DIZZINESS, COLLAPSE, UNCONSCIOUS and even death. Other factors that lower body resistance to the gas are OBESITY and ALCOHOLISM. Circulatory deficiencies, anaemia, rate of respiration are some other factors influencing CO absorption

          An Instrument called MINI SMDKERLYZER marked by Bedfont helps in assessing the percentage of COHb in blood by simple breath test.