US3089766A

US3089766A - Controlled chemistry cupola

Info

Publication number: US3089766A
Application number: US719179A
Authority: US
Inventors: Wald Elsie K De; Pota John B La
Original assignee: Chemetron Corp
Current assignee: Chemetron Corp
Priority date: 1958-01-27
Filing date: 1958-01-27
Publication date: 1963-05-14
Anticipated expiration: 1980-05-14

Description

United States Patent 3,tl8,766 CONTROLLED CHEMISTRY CUPOLA Lee H. De Wald, deceased, late of Aurora, 11].; Elsie K.

De Wald, administratrix, and John B. La Pota, Evanston, Ill., assignors to (Thernetron Corporation, Chicago, Ill., a corporation of Delaware No Drawing. Filed Jan. 27, 1958, Ser. No. 719,179 6 Claims. ((31. 7543) This invention relates to improvements in operating shaft type furnaces and more particularly in operating cupola furnaces, and is particularly concerned with increasing the overall thermal efficiency of such operation as Well as providing positive control of the chemistry within the cupola and of the chemistry of the metal produced by injecting high purity oxygen and/or fuel gas directly through the tuyeres, either continuously or intermittently, by a new and improved process.

The common practice in operating such furnaces is to discharge air from a blower into an annular chamber encircling the furnace shell and therefrom into the furnace through the tuyeres. It has been heretofore proposed to increase the thermal efficiency of cupola furnace operation by mixing oxygen with the air in the windboX to enrich the blast air used for the combustion of the coke in the furnace. Enrichment of this type is generally of a continuous nature and the most beneficial results have been obtained using oxygen in amounts of 3% to of the blast air. It has been found that in this process substantial primary oxygen, up to 5%, is lost through leaks in the windpipe system between the blower and the tuyeres, and that erosion of the refractories occurs from continued use of this process. Preheating of the blast air prior to its entry into the windpipe is also known. Nei-' ther of these processes make provision for a tool to control combustion within the furnace when rapid increases in temperature are required to overcome vagaries in the cupola operation or to regain temperature rapidly after shut-downs for various reasons, or a continuous process for providing a positive control of the chemistry within the cupola or of the resulting metal.

There also have been efforts recorded in the prior art directed toward utilizing air, oxygen and/or fuel gas injected directly into the tuyeres-of a shaft type furnace for various purposes. An early effort along these lines is recorded in French Patent No. 444,809, published on October 26, 1912. In this patent oxygen, air and fuel gas are injected solely for combustion purposes. jThe principal application of such process described in the patent is a smelting and refining operation in which no combustible such as coke is utilized in the charge. It appears that this patent represents a procedure for a continuous operation, but apparently contemplates enrichment initially at least of 30% to 40% oxygen, following which the patent states that the oxygen content is still further gradually increased. This is not believed to be commercially feasible today from an economic standpoint alone and it is doubtful that it could have been otherwise in 1 912. The French patent further states that Obviously, the procedure covered can be applied to any type of furnace, both stationary and oscillatory. In cases when the procedure is applied to treatment of ores, the fuel gas supply in the blast pipes is eliminated so as to limit the supplying of gas to the air and oxygen pipe alone. This French process apparently never reached commercial proportions as there is no other recorded information available in the literature or within the knowledge of presently living experts to indicate that such a process was ever successfully carried out commercially.

Still another effort along these general lines is recorded in the British specification, 453,552, bearing a priority date of December 10, 1934. This invention briefly involves the direct injection of oxygen into selected tuyeres of a shaft type furnace for the sole purpose of providing a means of insuring uniform descent of the charge and specifically to do so by burning out areas within the furnace whereat the coke bed may be hanging or bridging.

As is understood by those skilled in the art, in cupola furnaces alternate charges of metal and coke sometimes with the addition of limestone or other fluxing material above each coke charge are fed from the top of the cupola and the combustible material is burned at the bottom, with the resultant heat melting metal which rains down and is collected in the cupola well where it is ultimately tapped off, either batchwise or continuously. The height of the coke bed is relatively critical and if the bed is too high or too low unsatisfactory results are obtained. In conventional practice, to correct this condition, substantial periods of time elapse due to the fact that the charge must be adjusted from above and permitted to burn down before the corrected charge condition reaches the combustion area or coke bed.

It is desirable, therefore, toprovide an arrangement in i which maximum thermal efficiency and control of the metal chemistry may be achieved in a continuous fashion throughout the melting cycle as well as having available a high speed control for local thermal control and height of the coke bed in the event of shut-downs or other vagaries occurring in inoperation during the melting cycle. This applicants have achieved by the injection, both intermittent and continuous, of high purity oxygen and/ or fuel gas directly into the tuyeres of a shaft type furnace. The metal chemistry within the cupola will thus be maintained as uniform as possible, and consequently, the chemistry of the resulting product is likewise controlled so that consistently good results are obtainable.

It is accordingly the principal object of this invention to provide a process'by means of which high purity oxygen and/or fuel gas can be introduced into a cupola furnace directly through the tuyeres in an eflicient manner, either continuously or intermittently, to provide within practical limits a substantially uniform and at all times controlled melting cycle.

In keeping with the foregoing object, it is a further object of this invention to thereby control in a close degree the metal chemistry within the cupola as well as the chemistry of the resulting metal.

Further objects and advantages of this invention will become. more apparent as the following description proceeds, and the features of novelty which occur in the invention will be pointed out with particularity in the claims which follow.

In practicing the present invention, the charging and firing of the cupola furnace is carried out in a conventional manner as follows. The coke in the bottom of the cupola above the hearth is ignited and the depth of this coke bed is regulated by the coke charges supplied thereto from the top. Air for combustion of the coke is supplied to the cupola through the tuyeres. The cupola charge conventionally comprises a layer of coke and subsequent layers of metal and coke until the desired amount of material has been introduced. Additional quantities of metal and coke may be added as rapidly as the stock lowers within the cupola. Limestone or other fluxing material may be added to the top of each coke charge in order to reduce the viscosity of the cupola slag.

During the operation of the cupola furnace, drops of At this point the molten metal is tapped from the cupola and allowed to run out through the tapping spout which is located at the base of the crucible below the slag spout. Normally the cupola is operated intermittently for periods which may vary widely in time and it can be understood that the period required before any molten metal is obtained at the proper casting temperature represents a dead loss in time and cold metal. By the practice of this invention this period has been reduced materially by means of the injection of the high purity oxygen directly into the cupola through the tuyeres which accelerates combustion. The acceleration of this combustion and melting process is a decided advantage and renders the cupola available for more efficient utilization in the melting operation itself as a consequence of which more metal may be melted during a given period of time. Because of the intermittent nature of the batch cupola operation any decrease in the period of time before the metal is obtained at a proper casting temperature is of an obvious advantage in increasing the efiiciency of the process. As an additional consequence of the advantage of using this injection process, it has been found possible to use cheaper and inferior grades of coke due to the higher temperatures possible as a result of which the economy of the operation can be materially increased.

Further, many cupola shops shut down during certain periods in the midst of the operation. As a result of this down time, the cupola will tend to produce cold metal, and a high carbon monoxide concentration will tend to build up at or near the melting zone. This condition causin g cold metal production necessitates an excessive amount of pigging or casting into scrap ingot, until the desired metal temperature and gas composition is re-established. The judicious use of pure oxygen streams in accordance with the present invention directed into the cupola through the tuyeres substantially shortens the period during which the furnace temperature is brought back to the operating temperature thus reducing the time during which the operating crew is idle.

It has further been found advantageous to use the injection of high purity oxygen in conjunction with fuel gas or other combustible fluids to increase the local temperature in the neighborhood and to minimize the slag accumulation in the tuyeres. When such accumulation builds up in the tuyeres the amount of air that can be introduced is necessarily reduced and long delays are required to alleviate this condition. This slag accumulation can be due to several factors. The slag which is in a molten state at high temperatures tends to solidify when the air blast strikes it in the neighborhood of the tuyeres. This is caused by the relatively cool temperature of the air which contains four-fifths of inert nitrogen and any moisture associated with it. It will be understood that any moisture will produce an endothermic reaction, cooling off the coke and thus slowing up the process. The injection of oxygen will counter-act the endothermic reaction. However, if the coke cools below its ignition temperature, the addition of oxygen will not help to re-ignite it. Therefore the fuel gas is injected with the oxygen to reestablish the proper ignition temperature. This cooling of the coke adjacent to the tuyeres which might even drop its temperature below its ignition point produces a cold zone in this region which greatly favors the formation of slag. The molten slag is often cooled in the neighborhood of the tuyeres by the combined cooling action of the wet cold air and tends to solidify across the tuyere opening, thereby obstructing the passage of air and eventually causing a shut down in operations. By injecting oxygen at high velocities by the practice of this invention it has been found possible to correct this condition during operation through raising the temperature rapidly in the tuyere zone, if above the coke ignition temperature, thereby reducing the slag viscosity and permitting the air blast to erode and penetrate the slag accumulation. If the temperature is below the coke ignition temperature, oxygen and fuel gas 4 or other combustible fluids may be introduced through the tuyere opening to raise the temperature in this zone rapidly to that stage required to fully ignite the coke and promote combustion to the point where the slag becomes molten or of such a reduced viscosity that the gas penetration through the slag is made relatively simple. In the practice of this invention it is also possible to preheat the gases so as to minimize any cooling action on the coke in the furnace. Further, it has been found that operations can be carried on for a longer period than previously, even when the slag accumulation has reached a point which would have previously necessitated shutting down operations due to the insufficient delivery of air. By injecting the high purity oxygen, which may be accompanied with fuel gas, through the tuyeres it is possible to supply sufficient oxygen to maintain combustion within the cupola even when there is a deficiency of air due to partial clogging or obstructions in the tuyeres. Similarly, if there is an insufficiency of coke due to an oxidizing condition in the furnace, this is remedied by introducing only fuel gas. As an additional consequence of this process by means of which high temperatures may be uniformly maintained, there is a tendency to reduce silicon and manganese loss as well as sulfur pick-up. In other words the chemistry of the resultant product is controlled. It has further been found possible to reduce the carbon loss by the practice of this invention and also to economize on coke consumption.

For further details concerning the use of this process in connection with oxygen and/or fuel gas (preferably natural gas) reference is made to our co-pending application filed February 5, 1952, Serial No. 270,196, now abandoned, and of which application this is a continuation in part. The process as thus described is primarily concerned with providing a metallurgical tool for use during unbalanced cupola melting conditions. In general, it involves a system in which correlated substantially pure oxygen and/or fuel gas streams are intermittently injected directly into selected tuyeres of a shaft type furnace at a velocity greater than that of the blast air. The use of fuel gas alone, oxygen alone, or in combination as described in that application was intended to provide: (a) correction of excessively oxidizing conditions in the eupola during the melting cycle by injecting a reducing gas such as natural gas through the tuyeres; (b) fuel gas such as natural gas alone to control the chemistry of the eupola melting by maintaining thermal balance in cupola melting, especially where a deviation in the cupola melting was occuring; (c) fuel gas such as natural gas and oxygen in suitable proportions as a torch to remove slag accumulation within the cupola blocking the tuyeres; (d) fuel gas and oxygen to light up the coke bed prior to melting to insure a uniform burning in of the bed; (0) the production, after melting is completed and just prior to dropping the bottom of the cupola, should it develop that additional metal is needed because extra molds are available, of satisfactory cupola metal for this purpose by simply charging the necessary scrap and other fluxing materials during this late stage of melting and utilizing substantially pure oxygen and natural gas with the coke remaining in the bed to provide the necessary heat for melting this supplemental metal; (f) pure oxygen alone for rapid thermal control of coke bed height and metal chemistry, and overall reduction of the melting cycle, all with related advantages and economies. For further details in connection with these and additional advantages of using natural gas and/or oxygen in various correlated quantities, reference is directed to our above referred to co-pending application. In that application we additionally noted our discovery that oxygen and/or fuel gas, preferably natural gas, could be utilized effectively to control the chemistry of the cupola melting process, and thus to produce a melted metal with a controlled analysis of carbon, silicon, manganese and sulfur.

In this connection tests have proven that to obtain rapid thermal control as well as to positively control the metal chemistry Within the cupola and the analysis of the resulting metal within close limits, substantially pure oxygen may be injected directly into the tuyere of shaft type furnaces at velocities greater than the air blast for from about 5 to 15 minutes before the initial tap in volumes from 1% to 4% of the air blast volume, and after the initial tap out for intervals of from about 1 to 6 minutes to restore to normal the operation of the furnace as vagaries arise in the melting process. These parameters apply to cupolas where tapping off of metal is continuous after the initial tap oif.

For cupola operation involving intermittent tapping, we have discovered that the injection of substantially pure oxygen as described above should extend for periods of from 1 to 5 minutes prior to each tap and in volumes of from about 1% to 4% of the accompanying blast air. These parameters for example Were found adequate to maintain uniform high ladle temperatures of above 2700 F. where converter action followed in a duplexing operation.

A comparison of analyses of resulting metal from heats run under similar conditions without oxygen injection as described above and With oxygen injection illustrated respectively below in Tables I and II expressed in percent by weight.

TABLE I Silicon Total Sulfur Phospho- Mangacarbon rous nese Difference..-" +0. 29 +0. 16 +0. 014 +0. 005 +0.09

TABLE II Silicon Total Sulfur Phospho- Mangacarbon rous nose Dfierence- +0.17 +0. 06 +0. 010 +0.01 +0. 09

It Will be observed from these two tables that a Wider variation in chemical analysis of the metal is obtained in the non-oxygen heats. It will be noted from a comparison of Table I and Table H that the average silicon content between the oxygen and non-oxygen heats varies about 0.12, the total carbon .10, sulfur and phosphorous pick-up have been reduced appreciably and there is no pronounced variation in manganese content.

For achieving rap-id thermal control by judicious use of pure oxygen we have discovered that a temperature increase of about 15 F. per minute of oxygen injection can be achieved following which such increased tempera tures can be maintained by blast air alone. Obviously the gradient of thermal control is steeper between 2600 and 2650 F. than between 2650 and 2700 F.

In typical non-oxygen heats in a cupola wherein the charge was about 6.3 to 1 iron to coke ratio and a melting rate of about 2.6 tons per hour, the average temperature throughout the melting cycle after initial tap out was 2590 F.

Under similar conditions with an injection of pure oxygen for 5 minutes just prior to initial tap out and at a volume of 2.8% of the blast volume, consistently higher temperatures were maintained throughout the melting cycle of from 2650 to 2690 F. This is believed to represent an average figure. Raising the volume of oxygen to a value of 4% to 6% of the blast air and continuing same for periods of from 5 to 10 minutes in the start up will achieve temperatures of up to 2730 F.

It can be said generally therefore that with applicants 6 process melting cycle temperatures can be maintained throughout about 2700 F. whereas under normal prac tice 2600" F. or less are the general experience.

We will now describe our process and its concomitant advantages in connection with the continuous injection of oxygen and/or natural gas directly into the tuyeres of of shaft furnaces and at velocities greater than the air blast.

We have determined that by substituting natural gas or other fuel gas for part of the solid fuel in an otherwise standard cupola practice in proportions of from about 5% to about 20% of fuel gas, in either an acid or basic lined, or water cooled shaft type furnace, it is possible to reduce the height of the coke bed, and to reduce the size of the coke charge in an amount equivalent to the total B.t.u.s contained in the coke for which the fuel gas is substituted and control the composition of the metal more closely, it being noted that the composition of natural gas is substantially consistent as compared to the varying composition of foundry cokes. It has been found further possible by utilizing natural gas in the proportions specified above, to reduce and control the sulfur content of the resultant metal, thus improving the strength properties of the cast iron, and to reduce and control manganese and silicon loss resulting in higher alloy recovered. It

has been further found possible to reduce and control loss of iron in melting and to reduce the overall operating cost of the cupola melting by superheating the cupola Well or crucible with oxygen and/ or natural gas, which results in increased carbon and silicon pick-up with little or no reduction in manganese. It has been further discovered that the overall thermal efiiciency of the melting cycle is higher with a natural gas substitution in accordance with the ranges indicated above. These advantages are discussed in greater detail below.

Reduction of Coke Bed Height and Size of Coke Charge In standard cupola operation, Without the use of fuel gas, the proper adjustment of the bed height and the proper burn-in of the bed are among the most critical aspects of the cupola melting process.

There is no question about the importance of the function of the cupola bed in cupola melting. By maintaining a proper bed height, it is possible to control the temperature of the metal within narrow limits. Any unbalance in the bed height usually results in colder metal. The bed height also determines to a large extent the actual chemistry of the melted metal produced in the cupola.

Excessive loss of silicon and manganese together with low carbon pick-up and sudden changes in melting rate are almost sure signs of improper bed performance. Highly oxidized iron may readily result from an improper bed height. These are a few of the deleterious effects that may occur from an improper bed.

The addition or subtraction of coke is one method of correction, but this is time consuming. A more direct remedy is to use natural gas and/or oxygen as required by the cupola furnace atmosphere condition prevailing during melting.

It is apparent from the foregoing discussion that the adjustment of the proper bed height and the proper performance of the bed are essential to good cupola operation.

In standard cupola operation where coke alone constitutes the coke bed and where coke alone is used as a fuel additive, the coke bed should be of uniform height and the coke should be of proper size to permit free flow of air and to promote rapid and uniform ignition of the bed.

The method of lighting off and burning in the bed is a matter of personal preference; and the method used is generally governed by economics. One of the most effective methods developed is the use of oxygen and natural gas injected into the cupola through each tuyere. The even heat distribution obtained by this method promotes a uniform burn-in of the bed.

The coke bed in the cupolacan be termed the heart of cupola melting operation.

The composition of the coke, the size and shape of the coke, the amount of coke in the bed are all important factors in the production of iron in the cupola. The chemical and physical characteristics of the charged materials obviously have considerable influence on the succcssful operation of the cupola and on the production of a metal with the desired chemical composition but the coke bed is by far the most important single element.

In standard cupola operation, i.e., without the use of natural gas or other fuel gas or oxygen, the preparation or burning-in of the coke bed and establishing and maintaining the proper bed height are most important. If we disrupt either of these, we are inviting trouble in melting.

Under ordinary conditions, i.e., without the use of natural gas and/ or oxygen, a lowering of the bed height could result in a lower carbon, silicon, manganese and a higher sulphur in the metal. The analysis of the metal would also be inconsistent.

On the other hand, if fuel gas is employed as a substitute for part of the solid coke fuel, we can lower or readjust the bed and still produce a melted metal having the desired metal chemistry, without cause for concern about disrupting the cupola operation. This method, therefore, provides the cupola operator with a means of operating the cupola. satisfactorily, even though there is a deviation from the normal melting conditions.

Now, if a change in the bed height is desired, by lowering the bed, for example, and if we wish to reduce the amount of coke in the charge, this can be accomplished by substituting natural gas or some other suitable fuel gas for the coke. It should be noted that without the addition of natural gas, it is conceivable that the melting conditions in the cupola will be disrupted if the bed is. lowered and the coke charge is reduced.

However, by injecting the fuel gas with the cupola in correct amounts, the fuel gas volume that is added will compensate for the reduced amount of coke in the bed and for the reduced coke in the charge.

The addition of fuel gas to the cupola as a substitute for part of the solid fuel will provide the additional B.t.u.s that are required (1) for combustion in the cupola, (2) for control of thermo-chemical conditions in the cupola, and (3) for control of the chemistry of the melted metal. All of these benefits will be obtained when replacing some of the coke with fuel gas.

Another benefit obtained through the use of natural gas is:

Closer Control of Metal Composition With Fuel Gas It is possible through the use of natural gas and/or oxygen to produce metal to specification levels with consistent output, temperature and analysis. This is not possible to accomplish without the use of natural gas and/or oxygen in the face of raw material variables and inconsistencies in melting performance by the cupola.

The composition of the melted metal that is produced is affected by the size and composition of the charged metal scrap, the composition of the charged non-metallics such as briquettes, etc., the physical and chemical properties of the coke (in the coke bed and in the charge), the air input rate and the physical size and shape of the tuyeres and the size and shape of shaft type furnace. Each of these factors influence the composition of the metal as it emerges from the cupola.

Let us consider the composition and physical and chemical properties of the coke (in the bed and in the charge) as it affects the composition of the melted metal, and then try to show a correlation between the coke and natural gas. The theory has been advanced that the carbon is picked up by the molten metal in several different ways: (1) by direct solution of carbon (from the coke) in the molten metal. This method seems to provide much of the carbon in the melted metal. (2) The other is by the process of carburization in which CO reacts with the metal to form (Fe C), iron carbide. The partial combustion of the natural gas provides some of the CO for this process of carbon pickup by the metal.

In efiluent gases carbon pickup is increased with decreased CO content of the gas and an increased CO content of the gas. It appears, therefore, that the carbon content of the iron can be controlled if the thermochemical conditions in the cupola can be satisfactorily controlled.

Variations in carbon pickup are definitely more pronounced when coke alone is used as a fuel than when a fuel gas such as natural gas is used in conjunction with the coke. We have discovered that with natural gas or fuel gas, it is possible to produce cast iron with a high carbon content from an all steel charge as well as low carbon cast iron from high carbon charges. This means that cupola operators can use generally any varied charge to produce an iron with a given analysis. This is particularly significant when taking into account the difference in scrap prices existing between cast iron scrap, steel scrap, pig iron, etc.

Because of the complex nature of the cupola process, due to the many variables involved in cupola. melting, it is obvious that generalization regarding the actual bed height or bed reduction zone as related to metal composition, must of necessity be qualitative. Nevertheless, it has been found that by adjusting the depth of the coke bed (or coke bed height) and by adjusting the size of the coke charge and by substituting natural gas or other fuel gas in direct B.t.u. proportion for the coke, it is possible to influence and control the carbon content and other constituents of the melted metal.

For example, if it is desired to produce an automotive grade iron which is to be used for automobile cylinder blocks or cylinder liners having the following average base composition:

Percent by weight Carbon 3.30

Silicon 1.80

Manganese 0.71 Phosphorous 0.15 Sulphur 0.15

It is possible through the use of natural gas and/or oxygen to produce this same base iron or, if desired, an iron having the same carbon content, a slightly higher silicon and manganese content and a lower sulphur content.

The latter iron thus produced will have essentially the same B.H.N. hardness (187-241) higher minimum transverse load (2500 lbs.) and a slightly higher minimum strength (36,500 psi.) than the automotive grade iron produced without the use of natural gas.

If we consider the carbon content of the automotive grade iron above as a normal iron, we can, by adjusting the amount of natural gas and/or oxygen produce cast iron containing:

(1) The same range in carbon.

(2) A higher than normal carbon iron.

(3) A lower than normal carbon iron.

(4) A very low carbon iron, i.e. A 2.40 to 2.50 carbon iron used to produce iron shot.

proximately 6 to 1, and a normal melting rate, that is,

without supplement or substitution with fuel gas and/or oxygen, of 6 tons per hour. With fuel gas and/or oxygen substituted as indicated above, the melting rate increased to approximately 6.25 tons per hour. The air blast pressure was maintained at approximately 22 ounces, delivering air into the cupola at a rate of approximately 4000 cubic feet per minute, and the nominal coke bed height was measured at approximately 50" above the tuyeres. The same type of metal charge was used for producing all types of carbon irons except very low carbon iron wherein a high steel charge was utilized.

Production of Iron With Same Range in Carbon Natural gas can be used to produce an iron having the same carbon content as that which is produced by the coke method. Fuel gas or natural gas is substituted in the amounts ranging from 5 to 10% of the coke charge. In this case, the fuel gas rate is adjusted to correspond with the amount of B.t.u.s that are contained in the displaced coke. The coke bed height remains the same as in normal cupola operation. The air input rate will also be the same as in normal operation.

It should be explained that the efficiency of combustion is related to the CO content of the effiuent gas. Also, the C content of the flue gas may be taken as an indication of the cpndition in the bed. Within certain limitations, effluent gas composition also serves as a direct indication of the oxidizing or reducing character of the atmosphere to which the molten metal is being subjected.

In normal cupola operation when producing automotive iron, the average analysis of the flue gas was found to be 13.3% CO 11.2% C0 and the balance nitrogen gas with small amounts of hydrogen. The CO /CO ratio is therefore, 13.3/11.2 or 1.19.

The average molten metal analysis coming out of the cupola spout when using 510% natural gas and other conditions described above is:

Percent by weight Carbon 3.26

Silicon 1.83

Manganese 0.77 Phosphorous 0.15 Sulphur 0.13

The average flue gas analysis of the stack gas was found to be 13.75 CO and 12.05 CO, balance N or 13.75 12.05 =1.15 For all practical purposes, this lower CO /CO ratio suggests a slightly lower oxidizing condition in melting.

Production of Iron With Extra Low Carbon To produce a lower carbon iron by the normal cupola melting process, a high steel charge is necessarily used. A higher steel charge material (because of its lower carbon content) has a higher melting point than an iron charge material. It becomes necessary, therefore, to increase the coke charge, in order to provide sufiicienrt heat units to melt steel scrap in the cupola. This condition is more particularly true in the operation of an acid lined cupola.

Now, since more coke is needed to melt the higher melting point scrap, the result is a higher pickup in carbon than normally; thus making it very diflicult to produce a lower carbon iron. Obviously, if a normal coke charge is used, we would have difliculty melting the changed matenial and the metal temperature at the spout would probably be below the practical limits required for good cupola operation.

This, then, indicates the use of natural gas. To obtain an iron with about 2.40 carbon, the size of the coke is increased to reduce surface area of contact between the coke and molten iron droplets. The coke bed is lowered to a pnactical level. Natural gas is then added in an amount of between 5 to of that which is required to replace the B.t.u.s of the reduced coke bed and the coke charge.

The larger coke, lowered cokebed and the lean fuel Sulphur gas addition will permit the lower carbon scrap to pick up less carbon than usual, resulting in a lower carbon, higher strength iron which may be used to produce an iron shot.

The production of 2.40-2.50 carbon iron can also be accomplished by injecting less fuel gas than that which is required to replace the B.t.u. value of the displaced coke and to supplement the fuel gas by reducing the air input rate and simultaneously injecting oxygen in the tuyeres, in an amount that is required to produce the lower carbon iron. The injection of oxygen and fuel gas with the adjustments in bed height and coke ch arge noted above result in the production of a cast iron with the following average analysis:

Percent by weight Oarbon 2.46 Silicon 1.65 Manganese .62 Phosphorous .10 .10

Production of an Iron Metal Containing More Than the Normal Amount of Carbon The coke bed is adjusted so that it is the same height as in normal melting. A normal coke charge is used, i.e. (an amount which is used normally to produce an iron of a given carbon content).

Natural gas is added, injected through the tuyeres, in an amount ranging from 15 to 20% of the total coke charge B.t.u. value. The natural gas will provide an additional reducing substance, thus producing an iron with a higher than normal car-hon content. This can be supplemented with a 2% enrichment of oxygen while reducing the air input to allow for the 2% O addition. The CO /CO ratio of the effluent gas is found to be 11.9/ 12.1 or less than 1. The average chemical composition of the iron is:

Percent by weight Carbon 3.58 Silicon 2.10 Manganese 0.89 Phosphorous 0.148 Sulphur 0.113

Production of Iron With Less Than Normal Amount of Carbon To produce this iron, it is necessary to lower the bed height and reduce the coke charge in a proportion equivalent to about 12% of the bed and charge. This 12% will be replaced with natural gas.

The lower bed and the reduced coke charge will reduce the amount of carbon picked up from the coke, but these will contain enough B.t.u.s with the combined natural gas to provide an coming out of the spout having a sufficient tapping temperature.

The input rate will be increased inan amount just suflicient to reduce carbon pickup and not enough to cause undue oxidation of silicon and manganese.

l The resulting cast iron has an average analysis as folows:

Percent by weight Carbon 3.15

' Silicon 1.77 Manganese 0.72 Phosphorous 0.15 Sulphur 0.132

The above then represents some quantitative data concerning the use of natural gas to control the chemistry of the iron.

Another advantage of the use of natural gas and/or 11 oxygen is that it is possible, within a very short time, to alter the composition of the metal (running out of the spout) if the condition should require a change in metal composition.

Even when using scrap charges in the cupola, which consist primarily of mild steel scrap and other miscellaneous scrap materials, it is possible to vary and control the carbon content of the iron within the range of approximately 2.5 to 4.0% or greater, and always with a lower sulphur content, whenever it is necessary to produce such an iron.

Having discussed the control of the chemistry of the melted metal and of the cupola melting process in connection with the carbon pick-up and carbon content of the resultant metal, the manner in which control may be had by use of applicants process to reduce the sulphur content of the metal with use of the fuel gas such as natural gas, and to control manganese and silicon loss is discussed below.

Reducing the Sulphur Content of the Metal With Natural Gas Most commercial foundry cokes contain up to 0.08% sulphur by weight. The sulphur content, the carbon content and the ash content of the coke will, however, vary from car to car. Natural gas and other fuel gas prescribed in this application contain less that 0.01% sulphur, and the natural gas or other fuel gas is more consistent in analysis than coke. Obviously, then, less sulphur will be picked up by the molten metal when part of the coke is replaced with natural gas or other fuel gas, since there is proportionally less sulphur contained in the fuel.

It might be explained that iron is melted in the cupola in several different stages. First, it is preheated by the ascending hot gases in the cupola stack. Then, the iron descends into the initial melting zone, from where it continues its descent into the cupola well successively through the coke bed or reduction zone, and through the oxidation zone (which is located immediately above the tuyeres) and on into the cupola well. While the molten iron descends through the coke bed, it absorbs much of its carbon and most of its sulphur that is found in the melted metal.

The amount of carbon and sulphur that is absorbed by the metal is dependent to a large extent on the size of the coke and coke bed height. Of course, the prevailing furnace atmosphere conditions (i.e. oxidizing or reducing) as well as the temperature in the cupola will also have some influence on the amount of carbon and sulphur absorbed by the metal. Nevertheles, the less sulphur introduced into the cupola bed and charge, the less sulphur will be picked up by the molten metal. Lower sulphur content in most irons show a tendency to improve the strength properties of the iron.

To Control the Manganese Loss Through the Use of Natural Gas For a given charge, when producing iron with a given analysis, it was found that the use of natural gas as a substitute for part of the solid coke fuel permitted the control of the manganese content of the iron Within a narrower range than that obtainable with a standard coke practice (i.e. without the use of natural gas).

A useful guide for cupola melting and more particularly a guide to manganese and silicon loss in the cupola, is the determination of the CO /CO ratio in the stack gases. i

A correlation between stack gas composition and the loss of manganese was obtained from an analysis of the effluent gases.

Based upon the data taken during the tests, it can b shown that the CO /CO ratio can be more closely controlled with natural gas than with the coke practice alone,

thus permitting not only better control of the manganese content of the iron, but also better recovery of the manganese, resulting in a savings in alloy addition.

it is also possible to obtain a higher manganese recovery by reducing the amount of sulphur in the cupola charge. Since the manganese reacts with the sulphur to produce manganese sulfide in the slag, the less sulphur that is charged, the less manganese will be tied up with the sulphur. Since the coke contains most of the sulphur in the cupola charge, it follows that the more coke that is replaced by the natural gas, the lower will be the sulphur in the charge, the greater will be the manganese recovery, and the greater will be the savings in manganese alloy addition.

Control and Reduction of Silicon Loss by the Use of Natural Gas The thermochemical principles governing the loss of manganese will apply substantially to the principles governing the loss of silicon. Thus, the influence of fuel gas addition on silicon recovery and metal composition, as described for manganese will apply here. A dicussion of these principles therefore will not be repeated for silicon.

A brief discussion follows in connection with the overall costs of operating a cupola with applicants process of directly injecting into the tuyeres selected quantities of fuel gas and/ or oxygen on a continuous basis as set forth above.

Reduction of Overall Operating Cost With Fuel Gas and/ or Oxygen (a) First of all, the cost of a unit B.t.u. of fuel gas is less than the cost of a unit B.t.u. of coke. The savings in fuel is proportional to the amount of fuel gas used.

(b) Second, there is no melting loss of iron, silicon, and manganese when fuel gas and/or 0 is used which results in reduced metal loss and higher alloy recovery.

(c) Third, lower percentage of casting rejects are obtained with fuel gas and/or 0 due to more uniform metal temperature and uniform metal composition.

((1) An increase in production output is obtained with natural gas, and/ or 0 (e) A coke of lower quality and thus lower cost can be used if desired.

( Overall operating costs such as costs above, man

hours per ton, etc., are reduced with fuel gas and/or oxygen.

Control and Reduction of Iron Melting Loss The higher temperature of the slag and iron reached in a cupola fired with some fuel gas and the lower coke burden in the charge, promotes desulfurization as well as iron oxide reduction. This in turn is reflected in less oxidation of the iron in the metal and thus, a lower metal loss in melting.

In a cupola which uses natural gas as a substitute for part of the solid fuel, the constant stream of iron droplets falling through a constant depth of slag of constant composltion, results in conditions which for all practical purposes are consistent with chemical equilibrium and thus good control of the chemical composition of the metal.

Representative heats are shown below in connection with the sustained injection directly into the tuyeres of a cupola furnace of oxygen alone for various percentage ranges from 2% to 6%, together with an average for an equivalent number of heats without oxygen for a comparison; which is representative of standard cupola heats utilizing no oxygen or fuel gas additions. In these various heats the underlined numbers indicate the analysis of scrap materials utilized in the charge, and the non-underlined numbers indicate the actual chemistry of the resulting metal. The substantial differences in percent of change in the various components of the resultant metal from the composition of the initially charged scrap is significant as noted.

AVERAGES FOR NON-OXYGEN HEATS Metal chemistry Si Total S P Mn as w 1 n i 2.10 3 37 .10 .086 .71

Percent change 18 +36 023 009 13 SLAG CHEMISTRY S105 0110 F020, MgO A1 0 MnO REPRESENTATIVE OXYGEN HEATS Metal chemistry Percent Heat numbers oxygen Si Total S P Mn 2 w m m n I 2.10 3.39 .015 .058 .58 2

222 an E z H 2.12 3.31 .100 .093 .76 3

22 w m m a m 2. 22 3. 50 .113 .103 .64 4

an an 11 m at V 2.17 3 34 .133 .093 .67 4

L 2 m2 m a VI 2.06 3. 20 .033 .037 .03 4

Avg. (acid s1ag). 2.12 3. 37 .101 .090 .66 Avg. scrap 2.24 2.99 .083 .085 .81

Percent change -.12 +.38 +.O18 +.005 .15

Slag chemistry Heat numbers Percent oiygen S102 OaO F620 MgO A130; MnO

Avg. (acid slag)". 46. 50 29.90 3.80 1.97 14.74 3.03

Having described our invention in detail, it will be apparent -to those skilled in the art that certain modifications and adjustments will be suggested, and it is intended that all such as are within the spirit of this invention are intended to fall within its scope as best defined in the claims appended.

We claim:

1. A process for maintaining a normal and substantially constant operation in a shaft-type furnace utilizing a solid combustible in the charge to obtain uniform melting of metals therein and to increase the overall efficiency of furnace operation which comprises the steps of injecting, directly into one or more radially spaced tuyeres of the furnace at a velocity greater than that of the air blast, high purity oxygen for from five to fifteen minutes before the initial tap in volumes of from 1% to about 4% of the air blast volume to obtain and maintain uniform temperatures, gas compositions and metal temperatures within the furnace.

2. A process for maintaining a normal and substantially constant operation in a shaft-type furnace utilizing a solid combustible in the charge to obtain uniform melting of metals therein and to increase the overall 14 efficiency of furnace operation Which comprises the steps of injecting, directly into one or more radially spaced tuyeres of the furnace at a velocity greater than that of the air blast, high purity oxygen for from five to fifteen minutes before the initial tap in volumes of from 1% to about 4% of the air blast volume, and subsequent to the initial tap, for intervals of from one to five minutes at such times as vagaries arise in the melting process to rapidly restore the operation of the furnace to normal, to obtain and maintain uniform temperatures, gas compositions and metal temperatures within the furnace.

3. Process according to claim 2, wherein tapping is continuous after the initial tap off.

4. A process for maintaining a normal and substantially constant operation in a shaft-type furnace utilizing a solid combustible in the charge to obtain uniform melting of metals therein and to increase the overall eficiency of furnace operation which comprises the steps of injecting, directly into one or more radially spaced tuyeres of the furnace at a velocity greater than that of the blast air, high purity oxygen for from one to five minutes prior to each tap in an intermittent tapping operation and in volumes from about 2% to 4% of the air blast volume, to maintain uniformly high metal temperatures above 2700 F.

5. A process for maintaining a normal and substari tially constant operation in a shaft-type furnace utilizing a solid combustible in the charge to obtain uniform melting of metals therein and to increase the overall efficiency of furnace operation which comprises the steps of reducing the coke charge used in ordinary practice from about 5% to about 20% and injectingcontinuously throughout the melting cycle directly into the tuyeres of the furnace at velocities greater than the air blast, fuel gas in volumes equivalent to the B.t.u. content by which the coke charge is reduced, and at the sametirne inject ing continuously high purity oxygen from about 1% to about 5% of the air blast volume directly into the tuyeres of the furnace and at velocities greater than the air blast,- to maintain uniform and high temperatures to reduce sulfur pick-up and loss of silicon and manganese.

6. A process for maintaining a normal and substantially constant operation in a shaft-type furnace utilizing a solid combustible in the charge to obtain uniform melting of metals therein and to increase the overall eificiency of furnace operation which comprises the steps of maintaining the ratio of CO to CO in the efiiuent gas of the furnace to about 1 or less, by reducing the coke charge used in ordinary practice from about 5% to about 20% and injecting continuously throughout the melting cycle directly into the tuyeres of the furnace at velocities greater than the air blast, fuel gas in volumes equivalent to the B.t.u. content by which the coke charge is reduced, and at the same time injecting continuously high purity oxygen from about 1% to about 5% of the air blast volume directly into the tuyeres of the furnace and at velocities greater than the air blast, to maintain uniform and high temperatures to reduce sulfur pick-up and loss of silicon and manganese.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Transactions of the American Foundrymens Society,

vol. 56, pp. 250, 254, 255, 256, published by the Society at Chicago, Illinois.

Claims

1. A PROCESS FOR MAINTAINING A NORMAL AND SUBSTANTIALLY CONSTANT OPERATION IN A SHAFT-TYPE FURNACE UTILIZING A SOLID COMBUSTIBLE IN THE CHARGE TO OBTAIN UNIFORM MELTING OF METALS THEREIN AND TO INCREASE THE OVERALL EFFICIENCY OF FURANCE OPERATION WHICH COMPRISES THE STEPS OF INJECTING, DIRECTLY INTO ONE OR MORE RADIALLY SPACED TUYERES OF THE FURNANCE AT A VELOCITY GREATER THAN THAT OF THE AR BLAST, HIGH PURITY OXYGEN FOR FROM FIVE TO FIFTEEN MINUTES BEFORE THE INITIAL TAP IN VOLUMES OF FROM 1% TO ABOUT 4% OF THE AIR BLAST VOLUME TO OBTAIN AND MAINTAIN UNIFORM TEMPERATURES, GAS COMPOSITIONS AND METAL TEMPERATURES WITHIN THE FURNACE.