GB2103249A

GB2103249A - Method of producing castings using reduced iron as raw material, melting furnace and briquette used as raw material for castings

Info

Publication number: GB2103249A
Application number: GB08215102A
Authority: GB
Inventors: Kenji Fujii; Kazuyoshi Ishii
Original assignee: Yoshida Iron Works Co Ltd
Current assignee: Yoshida Iron Works Co Ltd
Priority date: 1981-06-23
Filing date: 1982-05-24
Publication date: 1983-02-16
Also published as: FR2508062A1; DE3222130C3; FR2508062B1; DE3222130C2; DE3222130A1; GB2103249B

Abstract

A method of producing castings using reduced iron as a raw material employs a vertical type shaft furnace construction which is described, while a briquette which is produced by pressure molding reduced iron may be used as the raw material. The method comprises the steps of filling the hearth of a shaft furnace with a solid carbonaceous material, blowing hot air into the furnace adjacent the hearth of the shaft furnace, allowing the solid carbonaceous material to mix with the reduced iron, and utilizing the resulting waste gas heat for preheating said hot air. The briquette has a solid carbonaceous material and/or an inorganic binder incorporated therein. The surface of the briquette is formed with a coating of inorganic oxide having a melting point lower than that of the briquette. The density of the briquette is 4 g/cm<3> or above.

Description

SPECIFICATION Method of producing castings using reduced iron as raw material, melting furnace and briquette used as raw material for castings This invention relates to an energy- and material-saving method of producing castings using reduced iron as a raw material, and it also relates to a melting furnace, and a briquette used as a raw material for castings which is produced by pressure molding particles or powder of reduced iron.

Compared with pig iron and steel generally used as raw materials for a melt, reduced iron is generally small in lump size and is porous like a sponge, having a low density, so that it has a large surface area. Therefore, when it is desired to produce iron castings using reduced iron as a raw material, the cupola, which is usually used for production of iron castings, is not suitable since reduced iron is in the form of small particles whose crushing strength is so low that the furnace passage resistance is increased by their collapse inside the furnace and a large amount of slag is formed of unreduced oxides contained in the reduced iron, which slag is difficult to separate and remove, thus aggravating the efficiency of operation.If the induction furnace is used to melt reduced iron, the furnace wall will be heavily eroded by the slag, leading to the decrease of the melting energy efficiency, and causing troubles to the melting operation. Further, it may also be pointed out that there are problems about reduced iron itself for use as a raw material for iron castings, e.g., large variations in molten metal composition and increased tendency to oxidation.Recently, the amount of production of reduced iron has been sharply increasing worldwide, and it is believed that the demand for using reduced iron as a raw material in the production of iron castings will continue to increase since it meets the needs of the foundry industry today calling for the saving of material and energy, because of its features including less energy expended in production thereof than in the production of blast furnace, pig iron, which is economically advantageous, smaller amount of impurity elements contained, and hopefulness for use as a raw material metal particularly for the production of spheroidal graphite cast iron.

Thus, we have conducted basic research to make fullest use of reduced iron as a raw material for castings.

Our basic research is as follows:- a) Where reduced iron was placed in various atmospheres in contact with a solid carbonaceous material and held at high temperatures or melted, the behaviour of the reduced iron was investigated. More particularly, it was placed in contact with such atmospheres as CO, air and CO2, and such a solid carbonaceous material as coke and graphite lumps, and held in the solid state at 1 ,000 C and subsequently melted.As a result, the following findings have been obtained (1) In such a reducing gas atmosphere as CO gas, even when it is retained in the solid state at 1 ,0000C, carburization readily proceeds from the surface, thus increasing the carbon content; (2) In an oxidizing atmosphere principally of air, CO2, etc., oxidation proceeds from the surface, forming a thick oxidized layer around the outer periphery which is seen when its cross-section is examined; (3) Where reduced iron is placed in contact with a solid carbonaceous material, an oxidized coating forms on the surface of the reduced iron if the contact time is short or the particle size of the solid carbonaceous material is large;; (4) If reduced iron is melted in intimate contact with a solid carbonaceous material, the melting completes with heating to about 1 ,4000C, which means that carburization from a solid carbonaceous material extremely readily proceeds, subject to high temperatures above 1 ,0000C, and it has also become clear that the unreduced oxides contained in reduced iron are reduced.

b) The proportion of reduced iron mixed with pig iron or casting scrap was changed to 20, 40 and 60%, and the mixtures were melted in the induction furnace to produce iron castings, the nature of which was investigated. As a result, the following have been found: (1) Of the components contained, phosphorus and sulfur, particularly the latter, can be rapidly decreased in amount by the addition of reduced iron, so that cast iron with a low sulfur content can be obtained; (2) When the amounts of carbon, silicon and manganese, which have major influences on mechanical properties, were adjusted so that the final components were the same in amount, and comparisons were made.

As a result, it has been found that tensile strength can be increased if the proportion of reduced iron is increased.

An object of the present invention is to provide a method of producing improved castings using reduced iron as a raw material, on the basis of the results of the basic research described above.

Another object of the invention is to provide a melting furnace used for embodying this production method.

A further object of the invention is to provide a briquette used as a raw material for castings, which can be charged into a melting furnace for the production of castings using reduced iron and which is effective for the saving of material and energy while producing castings of improved quality.

Another object of the invention is to provide a metal briquette used as a raw material for castings, particularly a briquette using reduced iron, said briquette having an oxide coating formed on the surface thereof for the prevention of oxidation of the briquette.

Another object of the invention is to provide a briquette used as a raw material for castings, which is pressure molded so that its density is 4 gloom3 or above and it has a suitable size and shape.

Still a further object of the invention is to provide a raw material for castings which is prepared by forming reduced iron into pellets or small or large briquettes enclosed in a paper tube.

The arrangement of the present invention will become more apparent from the following description of embodiments thereof shown in the accompanying drawings, in which: Fig. 1 is a complete elevation, partly in longitudinal section, of a melting furnace used in a method of producing castings using reduced iron as a raw material according to the invention; Fig. 2 is a sectional view of a briquette formed with an oxide coating used as a raw material for castings, according to the present invention; Fig. 3 is a sectional view of a raw material for castings, which comprises reduced iron pellets, a ferro-alloy and a solid carbonaceous material which are sealed in a paper tube; Fig. 4 is a sectional view of a raw material for castings, which is in the form of two reduced iron briquettes continuously sealed in a paper tube;; Fig. 5 is a view of a raw material for castings, which is in the form of a cylindrical reduced iron briquette inserted in a paper tube of the same length; and Fig. 6 is a view of a raw material for castings, with a solid carbonaceous material charged into a space between a cylindrical reduced iron briquette and a paper tube of the same length.

With the results of the basic research described above taken into account, the method of producing castings using reduced iron as a raw material according to the present invention is based on the following:- (1) As for the reduced iron melting furnace, a vertical type shaft furnace construction is suitable for reasons to be later described. (2) Since the interior of the furnace must be maintained in a substantially reducing atmosphere, a solid carbonaceous material is placed on the hearth, said solid carbonaceous material being low-sulfur coke or, if necessary, graphite lumps, and air is blown into the furnace adjacent the hearth to allow the two reactions C+02eCO2 (exothermic reaction) CO2+CoCO (endothermic reaction) to take place in the furnace, thereby providing for heating necessary for melting and also providing for reduction of the unreduced oxidized iron contained in the reduced iron. (3) To facilitate the melting of reduced iron, carburization must be allowed to proceed even in the solid state and the melting point must be lowered.To this end, it is essential that the solid carbonaceous material and the reduced iron be in intimate contact with each other in their solid state. To achieve this object, in placing the carbonaceous material on the hearth and charging the raw material into the melting furnace, as described above, the reduced iron and carbonaceous material are fully mixed together.

(4) The raw material reduced iron is generally small in particle size, as described above, and the air pressure would abnormally rise if a conventional melting furnace is used. To decrease the furnace passage resistance, therefore, the exhaust gas take-out port must be located at a relatively low level for a shaft furnace construction. (5) The exhaust gases taken out of the furnace contain a large amount of CO gas and are at high temperature.Thus, they are passed through a heat exchanger which is provided at its bottom with a combustion chamber into which secondary air is blown to cause the secondary combustion of this CO gas, the remaining heat and the heat of combustion of the exhaust gases being used to preheat the supply air. (6) Since the silicon and manganese are relatively small in amount as compared with pig iron, it is desirable to provide a device for blowing metal silicon powder and metal manganese powder into the furnace. (7) Since the raw material, is small in particle size, as described above, it is desirable to install a device which enables continuous charging thereof.

In operating the improved apparatus constructed with the above conclusions in mind, the following matters must be taken into account: (1) To avoid entry of the sulfur component from the solid carbonaceous material placed on the hearth and from the solid carbonaceous material mixed with the raw material, or reduced iron, and charged into the furnace, at least half the amount of the solid carbonaceous material is in the form of electrode graphite. (2) If carburization of the raw material, or reduced iron, in the solid state is effected, its melting temperature lowers, so that it melts at a relatively low temperature, facilitating the furnace operation. Thus, the particle size of the solid carbonaceous material to be charged is made smaller to achieve satisfactory mixing with the reduced iron before it is charged.The method of producing castings using reduced iron as a raw material which is performed by these operations and the apparatus arranged with said considerations taken into account is a method of producing castings which consists in compensating for the configuration of reduced iron difficult to handle from the standpoint of raw material, i.e., its drawback of being brittle as compared with blast furnace pig iron while making use of its feature of having a large surface area providing easy carburization thereof in the solid phase, thereby lowering the melting temperature to facilitate the furnace operation, enhancing the merit of being small particle size while withdrawing the exhaust gases from within the furnace at a relatively low level portion thereof to avoid the resistance to air blast in the furnace, making use of their high CO gas content for secondary combustion to heat the supply air so as to increase thermal efficiency, while advantageously using coke of small particle size and electrode graphite waste, which are of low commercial value, as a solid carbonaceous material, the grade adjustment of the molten metal being easy.

An embodiment of a melting furnace used in the method of the present invention is illustrated.

The numeral 1 denotes a melting furnace of vertical type shaft furnace construction, wherein the raw material charged through a charging port 2 descends in the furnace 1 and below an exhaust gas withdrawing port 3 it is melted red hot and taken out as a molten metal through a tap hole 3.

Air from a blower 22 is fed to a cool air inlet 9 in a radiation type heat exchanger 7 and downwardly moves through a clearance between the outer wall of the radiation type heat exchanger 7 and a pipe-like heat transfer plate 23, during which heat exchange is effected whereby it changes to hot air, which is then fed from a hot air outlet 8 via an air feed pipe 5 into the furnace through the tuyeres 4 of the vertical type shaft furnace 1.

On the other hand, the exhaust gases used for heat exchange are taken out from the exhaust gas withdrawing port 6 and these high temperature exhaust gases containing a large amount of CO gas are ignited by an ignition burner 10 and secondarily burns with the air fed in through a secondary combustion chamber holes 11, increasing their temperature, and undergo heat exchange in the radiation type heat exchanger 7, thereby changing the cool air to hot air. The exhaust gases cooled by heat exchange are fed through an exhaust gas outlet port 12 into part of the exhaust gases which has ascended while heating the raw material layer, and finally led into a chimney. In addition, 13 denotes a ground line and 1 4 denotes a second floor line.

A briquette used as a raw material for castings according to the present invention is made of small particles or powder of a metal with which, if necessary, small particles of a solid carbonaceous material and/or inorganic binder is mixed, the mixture being pressure molded into lumps of suitable shape and size.

For example, in the case of a melting furnace for producing castings using reduced iron as a raw material, a small amount of inorganic binder, such as water glass, is added to the raw material, or reduced iron, and small particles of a solid carbonaceous material serving as a fuel and they are thoroughly mixed and pressure molded into briquettes of suitable size, which are charged into the melting furnace, producing castings of good quality while saving material and energy.

As described above, reduced iron, as compared with pig iron, is generally small in lump size, has a spongy appearance, is porous and has a low apparant specific density, so that its surface area is large. Therefore, when reduced iron is heated in an oxidizing atmosphere, oxidation readily proceeds from the surface, but in a reducing atmosphere, such as CO gas, carburization readily proceeds from the surface, increasing its carbon content.

Moreover, if it is heated in contact with a solid carbonaceous material which is crushed into small particles, carburization from said carbonaceous material is effected at 1 ,0000C with great ease.

There is also a merit that this procedure lowers the melting point of said reduced iron and completes its melting at about 1 ,4000C, thus saving energy.

In producing castings using reduced iron as a raw material on the basis of the above fact, coke breeze is used rather than pulverizing large lumps of coke serving as a solid carbonaceous material, so that the energy needed for pulverization is saved. Further, coke breeze, which is a by-product from the production of coke by carburization of coal in a gas or coke producing factory, is of little commercial value or, rather, a nuisance, so that the use of coke breeze is very desirable from the standpoint of the saving of material.Further, in order to maintain the grade of the molten metal taken out from the furnace, it is not necessary for the operator to adjust the amount of raw material mixed in, and the raw material to be charged into the melting furnace is adjusted to such a proportion as corresponds to the quality of castings to be produced in sufficient amounts, and it goes without saying that in order to make the air passage condition in the melting furnace uniform and maintain it constant, it is very advantageous to make briquettes of constant size and constant shape by molding.

In molding, a suitable amount of binder such as water glass is added to the material and if there is another metal to be added, powder of such metal, e.g., ferro-silicon orferro-manganese, may be added and after they are fully mixed, the mixture is pressure molded. By selecting the type and size of briquettes, sufficient adjustment of ventilation in the melting furnace become possible.

If briquettes for use as a raw material for castings prepared by mixing small particles of reduced iron, and small particles of a solid carbonaceous material, e.g., coke breeze, or graphite particles, with a small amount of an inorganic binder, e.g., water glass, added thereto, and molding the mixture into a suitable size and suitable shape, is charged into the melting furnace, carburization is effected in the solid phase to lower the melting point for easy melting, while uniform ventilation is obtained, so that the red heat layer is made uniform and the operation is stabilized.

Thus, the briquette is useful as a raw material for castings, which is capable of producing castings of good quality while achieving the saving of material and energy.

Further, if the above embodiment is further improved by forming an oxide coating on said briquette, other advantages will be obtained.

These advantages will now be described with reference to an embodiment shown in Fig. 2.

In Fig. 2, the numeral 1 5 denotes a reduced iron briquette produced by pressure molding a mixture of reduced iron in particle or powder form, small particles of a solid carbonaceous material, and a small amount of inorganic binder, such as water glass, said briquette being formed into a suitable size and shape. The numeral 1 6 denotes an inorganic oxide, such as glass, in particle or powder form, applied to the surface of the reduced iron briquette, said inorganic oxide melting at a temperature lower than the melting point of said reduced iron briquette.The material which can be fusion bonded to the reduced iron briquette is applied to the latter with an inorganic binder such as water glass, whereby a coating of the inorganic oxide 1 6 consisting mainly of glass is formed on the reduced iron briquette 1 5.

In the case of melting this reduced iron briquette, e.g., in an induction furnace, the inorganic oxide on the surface melts at about 8000C to form a molten glass coating with the result that said reduced iron briquette is completely isolated from the surrounding atmosphere and thereby prevented from oxidizing while being heated, and the reduction of the iron oxide proceeds owing to the solid carbonaceous material in the reduced iron briquette, and it melts at about 1 ,4000C to become molten iron. Thus, after this melting, the glass in the molten state forming the coating floats over the molten iron owing to the difference in specific weight, thus isolating the molten iron and the surrounding atmosphere from each other, preventing oxidation of the molten iron.

Figs. 3 through 6 show raw materials for castings according to other embodiments of the invention, their arrangements being as follows.

In Fig. 3, the numeral 1 7 denoted reduced iron pellets, which may be small-size, reduced iron oriquettes, smaller than pellets, formed by molding reduced iron particles. The numeral 18 denotes a paper tube formed of inexpensive kraft paper adhesively wound with an inorganic adhesive, such as water glass, the outer surface of said paper tube being coated with a heat-resistant inorganic paint to prevent the paper tube from readily burn out. The numeral 20 denotes a ferroalloy, and 20 denotes a solid carbonaceous material.The raw material for castings shown in Fig. 3 comprises a mixture of the reduced iron pellets 17, ferro-alloy 19, such as ferro-silicon, added to adjust the composition of cast iron, and solid carbonaceous material 20, such as coke breeze, to fill the clearance between the pellets 17 and ferro-alloy 19, and the paper tube 18 for enciosing said mixture. The raw material for castings thus formed has the effect of preventing the reduced iron and alloying element such as Si from being oxidized in the furnace and of increasing the yield.

In Fig. 4, two reduced iron briquettes 21, performed into cylindrical briquettes and arranged side by side, are inserted in a paper tube 1 8 having a thickness corresponding to the outer diameter of the briquettes, the opposite ends of said paper tube being closed. With the raw material for castings thus formed, since the paper tubes 18, when burning in the furnace, undergoes incomplete combustion, the reduced iron briquettes are maintained in a reducing gas atmosphere during melting, so that oxidation thereof in the solid state before melting is prevented and hence the yield is increased.

Further, the briquettes are protected by the paper tube against breakage due to impact during handling. Therefore, the apparatus for forming briquettes does not need to have a high output capacity, and it is possible to use briquettes of low crushing strength which can be produced by an inexpensive, high-productivity briquetting machine, such as of the roll type. In addition, in molding briquettes having a solid carbonaceous material and a composition adjusting alloy added thereto or large briquettes, an inorganic adhesive, such as water glass, is added to provide a suitable strength.

Fig. 5 shows a melting raw material, wherein a paper tube 1 8 having a thickness touching the outer diameter of a cylindrical, reduced iron briquette 21 is cut to the cylinder length of the briquette and the reduced iron briquette 21 is put therein, while in Fig. 6, the paper tube 18 of Fig. 5 is increased in thickness to exceed the outer diameter of the briquette and a carbonaceous material is charged into a clearance between the reduced iron briquette 21 and the paper tube 1 8.

In Fig. 5, said breakage is prevented and the briquette is kept from being oxidized in the solid state until it is melted, while in Fig. 6, the solid carbonaceous material charged around the periphery further increases this effect.

It is also possible to increase the fire resistance of the paper tube by mixing powder of a refractory material into the inorganic adhesive or into inorganic paint which is used in producing the paper tube.

Since the raw material for castings of the invention is enclosed in a paper tube, as described above, the grade of the raw material briquette can be adjusted very easily by the selection of the quality of the paper tube, which is another effect besides those described above.

The effects of this embodiment will now be described on the basis of test results. Table 1 below shows the relationship between the amount of reduced iron mixed in and the amount of slag formation as found when a cast iron material and reduced iron were melted in a high frequency induction furnace. The use of the briquettes of the invention suppressed the oxidation o. reduced iron; the iron oxide in the reduced iron is reduced by the solid carbonaceous material in the briquettes and the amount of slag formation decreases as indicated by the broken line in the graph.

Table 1 shows the relationship between the amount of reduced iron mixed in and the amount of slag formation in melting operations using a high frequency furnace.

Note A B: the amount of slag formation (calculated value) in cases where the FeO in reduced iron is 100% reduced.

A C: the amount of slag formation (calculated value) in cases where the FeO in reduced iron is not reduced at all.

In these calculations, the amount of slag formed owing to attacks on the furnace wall and other causes is not included, and the amount of slag where reduced iron is not mixed in is taken as 0.

In addition, such oxide coating as described above may be applied to other briquettes than those using reduced iron in the above embodiment.

Thus, it is possible to apply to the surface of a briquette pressure-molded of particles or powder of a metal with a binder added thereto if necessary, and inorganic oxide which will melt and adhere to said metal briquette, by the use of an inorganic adhesive such as water glass, whereby an oxide coating is formed on the surface of the briquette; such coating serves to prevent oxidation of the briquette where the latter is stored for a long time or during melting of the briquette said coating will melt first to form a molten oxide coating on the surface of the briquette, isolating the latter from the surrounding atmosphere to prevent oxidation of said metal briquette, thereby providing for improved yield of molten metal and improved efficiency of operation.

As for a method of using reduced iron as a raw material for castings more advantageously, this reduced iron in pellet form usually having a density of 4 g/cm3 or below may or may not be crushed and together with small particles of a solid carbonaceous material and an inorganic binder it may be pressure molded into a suitable size and shape so that the density is above 4 g/cm3. Production of iron castings using this reduced iron briquette will now be described.

Reduced iron pellets having a density of 2-3 g/cm3 is first crushed into small particles of 3 mm or below.

The energy required for crushing the reduced iron pellets is only 1.2-1.5 KWH/T in cases where 80% of them are 9 mm or below and must be crushed so that 80% are 2.5 mm or below. The reduced iron thus crushed, as such, may be pressure-molded into briquettes or, if necessary, small particles of a solid carbonaceous material such as electrode graphite waste and an inorganic binder such as water glass may be added thereto and the mixture may be pressure molded.

The mixing of a solid carbonaceous material with briquettes is effected in cases where it is necessary to reduce the unreduced iron oxide contained in the reduced iron and to carburize the latter because of its low carbon content. If electrode graphite waste is used as a solid carbonaceous material, its little tendency to absorb sulfur results in low-carbon molten metal, so that if a spheroidizing treatment is applied thereto, spheroidal graphite cast iron can be readily obtained without requiring any desulfurization.

The purpose of the addition of a binder is to prevent crumbling of the briquettes since the moldability of briquettes is poor where a solid carbonaceous material is added. For example, the use of an inorganic binder such as water glass makes it possible to produce a strong briquette.

Further, if this inorganic binder, e.g., water glass, is added in an amount of 6%, the amount of slag formation is 30% or below of the amount of the addition, i.e., 1.8% or below of the total amount of the charges; thus, the amount of the slag is small and it can be removed, so that there is no danger of degrading the quality.

When briquettes are formed by pressure molding so that their density is 4 g/cm3 or above, the porosity is decreased and so is the surface area; therefore, even if they are held at high temperatures in the furnace, the ioss due to oxidation is little, the melting yield is high, and the amount of slag formation due to oxidation is not large. Further, briquetting greatly increases the crushing strength, so that the briquettes will not be broken or powdered during handling. Since the unit weight of reduced iron increases, there will be no drop in the melting rate when the raw material, or reduced iron, is charged. Further, since these briquettes are molded to have a suitable size for use as a charge into a cupola there is no danger of increasing the furnace passage resistance.

On the basis of the results of tests on this embodiment, the effects thereof will now be described.

The following tables indicate the relationship of the amount of reduced iron mixed in to the furnace passage resistance, melting yield, and amount of slag formation, respectively, as found when a cast iron material and reduced iron were melted in a cupola. It was found that the use of the reduced iron briquettes according to the invention, as compared with the use of reduced iron pellets, resulted in decreased furnace passage resistance and decreased oxidation which, in turn, resulted in increased yield and decreased slag formation. Further, the use of a briquette made of a mixture of reduced iron and electrode graphite waste resulted in a further decrease in slag formation, reducing substantially all of the unreduced iron oxide in the reduced iron.Further, in the relationship of the amount of reduced iron mixed in to the amount of slag formation and the melting yield as found when a mixture of a cast iron material and reduced iron was melted in a high frequency induction furnace, the use of a briquette consisting of reduced iron alone suppressed oxidation of the reduced iron, decreased the slag formation and improved the melting yield of reduced iron.

Table 2 shows the relationship between the amount of reduced iron mixed in and furnace passage resistance (blast box air pressure) in Cupola melting operations.

Table 3 shows the relationship between the amount of reduced iron mixed in and melting yield in Cupola melting operations.

Table 4 shows the relationship between the amount of reduced iron mixed in and the amount of slag formation in Cupola melting operations.

Table 5 shows the relationship between the amount of reduced iron mixed in and the amount of slag formation in a high frequency melting furnace.

In these calculations, the amount of slag formation due to attacks on the furnace wall and other causes is not included, and the amount of slag formation where reduced iron is not mixed in is taken as 0.

Table 6 shows the relationship between the amount of reduced iron mixed in and the yield of melting materials in high frequency melting operations.

As described above, according to this embodiment, oxidation of reduced iron during melting can be minimized, and the increased melting yield and decreased slag formation contribute much to improvement of efficiency of operation enchancing the value of reduced iron as a raw material for castings.

As has been described so far, according to the present invention, the utilization of reduced iron which meets the needs of the foundry industry today for the saving of material and energy can be made more effectively.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

Claims

1. A method of producing castings using reduced iron as a raw material, comprising the steps of using a vertical type shaft furnace, charging a solid carbonaceous material onto the hearth of said shaft furnace, blowing hot air into the furnace adjacent the hearth of said shaft furnace, and charging reduced iron as a raw material into said shaft furnace, said reduced iron having a solid carbonaceous material mixed thereinto, the heat of the resulting exhaust gases being utilized to preheat said hot air.

2. A method as set forth in Claim 1, wherein said exhaust gas heat is taken out adjacent the hearth above the position where said hot air is supplied.

3. A method as set forth in Claim 1 or 2, wherein the heat of secondary combustion of the CO gas contained in said exhaust gases is used as said exhaust gas heat.

4. A method as set forth in any of Claims 1 through 3, wherein said solid carbonaceous material is low-sulfur coke.

5. A method as set forth in any of Claims 1 through 3, wherein said solid carbonaceous material is graphite.

6. A method as set forth in any of Claims 1 through 3, wherein at least half the amount of said solid carbonaceous material is electrode graphite.

7. A method as set forth in any of Claims 1 through 6, wherein metal silicon powder and metal manganese powder are blown into said shaft furnace.

8. A melting furnace for producing castings using reduced iron as a raw material, comprising a main body having a vertical shaft furnace construction, means for bringing a solid carbonaceous material into contact with the raw material, or reduced iron, placed on the hearth of said main body, hot air blowing and feeding means disposed adjacent said main body hearth, and preheating means for preheating the hot air from said feeding means by utilizing the exhaust gas heat produced as a result of the heating action of said hot air.

9. A melting furnace as set forth in Claim 8, wherein the take-out port for said exhaust gases is located adjacent the hearth above said feeding means.

1 0. A melting furnace as set forth in Claim 8 or 9, wherein said preheating means comprises means for secondary combustion of the CO gas contained in said exhaust gases, and radiation type heat exchange means for extracting the heat of combustion from said combustion means for the purpose of said preheating.

11. A briquette as a raw material for castings, formed by pressure-molding a metal of small particle size into a suitable shape and size.

12. A briquette as set forth in Claim 11, wherein said metal is reduced iron.

13. A briquette as set forth in Claim 12, wherein a solid carbonaceous material is mixed in prior to the pressure molding.

14. A briquette as set forth in any of Claims 11 through 13, wherein an inorganic binder is mixed in prior to the pressure molding.

15. A briquette as set forth in Claim 14, wherein said solid carbonaceous material is coke breeze.

16. A briquette as set forth in Claim 14, wherein said inorganic binder is water glass.

1 7. A briquette as set forth in Claim 14, wherein another metal powder is mixed in.

1 8. A briquette as set forth in any of Claims 11 through 17, wherein the surface of the briquette is formed with a coating of an inorganic oxide of small particle size which will melt at a temperature below the melting point of the briquette.

19. A briquette as set forth in Claim 18, wherein said inorganic binder is water glass.

20. A briquette as set forth in Claim 18, wherein when said coating is formed, said inorganic oxide is bonded with an inorganic adhesive.

21. A briquette as set forth in Claim 18, wherein when said coating is formed, a mixture of said inorganic oxide and inorganic adhesive is applied.

22. A briquette as set forth in any of Claim 12 through 14, wherein the density is 4 g/cm3 or above.

23. A briquette as set forth in any of the Claims 11 through 14, wherein one or more briquettes are in large cylindrical form and enclosed in a paper tube.

24. A briquette as set forth in any of Claims 11 through 14, wherein the briquette is in large cylindrical form and enclosed in a paper tube with both ends opened, in such a manner as to leave no clearance at the lateral surfaces of the briquette.

25. A briquette as set forth in Claim 24, wherein a solid carbonaceous material is packed into a clearance between the briquette and the paper tube.

26. A raw material for castings comprising reduced iron pellets or briquetted, small-size, reduced iron which, together with a small amount of a solid carbonaceous material and a composition adjusting alloy, is enclosed in a paper tube.