US2735758A - strassburger - Google Patents

Description

Feb. 21, 1956 METHOD OF TREATING IRON BEARING MATERIAL IN METALLURGICAL FURNACES Filed Oct. 3, 1952 JULIUS H. STRASSBURGER BY%-- M ATTORNEY R o T N w c5 50. 0530 $5 mzzdo P35 2 $5562 m Emmmmwow99t99m m o @w wm mm Q +xq n w N 8 Q Y x m mm D 51 x m v x 0m w A A Om QM n G0 a \4 a. 1 1

U i d States Patent METHOD OF TREATING'IRQN BEARING MATE- "RIAL IN METALLURGICAL FURNACES Julius H. Strasshurger, Steubenville, (lhio, assiguor to National Steel Corporation, acorporation of Delaware Application October 3, 1952, Serial No. 312,373

6 (Zlaims. (Cl. 75-41) This invention relates to blast furnace operation and more particularly to improvements in the method of operating blast furnaces when an oxygen rich blast is used'to burn the carbonaceous material and smelt the iron ore to produce molten'pig iron.

Conventional blast furnaces comprise a lower hearth, a stack, and a bosh between the hearth and the stack. The blast, comprising-essentially compressed air, is blown through tuyeres mounted in the 'bosh into the upper portion of the hearth, and the burden, including specific proportions of limestone, ferrous bearing material and carbonaceous material, is charged into thefurnace at the topaof the stack. The ferrous bearing material is usually iron ore and may include some scrap metal, sinter or other material, 'and the carbonaceous material is usually coke. The charge moves down the shaftof the furnace, and when it reaches a zone adjacent the tuyeres, the coke is "burnejd by the incoming blast to smelt the iron ore producing moltenpig iron, and the hot gaseous products of combustion flow up through the stack and out of the .top of the furnace. As the-hot gases flow up the stack, they preheat the descending charge and reduce the iron ore as it approaches the combustion zone. The molten pig iron collects in the bottom of the furnace beneath a layer of molten slag. Blast furnace installations also include anumber of stoves for preheatingthe blast to an elevated temperature. The stoves are usually employed .ingroups andwhile some. of the stoves arebeing heated, by burning top gas from the top of the blast furnace for example, at least part of the air blast is heated -upon l-flbwing through the remaining stoves which has been previously heated. In many cas es part of the blast is by=passed:around the stoves and is then mixed with the .hOt. blast before the .blast..is}discharged through the .tuyeresintothe furnace. By varying the quantity of cold blastit is possible .to compensate for variations in stove temperatureand maintain the hot blast discharged into the furnace atasubstantiallyconstant temperature .which may, .for example, be at .1000 F. Turbo-blowers are usually employed for :providing the .air blast. The blowe'rsrnay .be controlled bya manually operable .or

automatically operable means to maintain the amount of air .blowninto the furnace substantially constant. The volume of air blown into the furnace will vary depending in partupon the size. ofthe. furnace and the charge and uponthe character of the ferrous bearing material. For example, ,a blast furnace intended to produce about 1000 .tons ,ofQpig ironper .day froma good grade, of iron ore will require :an air. blast at; the rate. of aboutfl5,000 cubic feet per minute. It is to be noted that the air constitutes the largest mass. of material introduced into the blast furnace. In blast furnace furnace. andflthe rate and .mass :of air. blown into the furnace are calculated and controlled to maintain the highest vpossible i rate of pig iron production of desired characteristics with a maximum permissible loss due to ,7 operation the quantity and physical character of the. materialdischargedinto the top of the flue dust production. The amount of'blast necessary for a givensetof conditions is'critical and must be-carefully controlled in order to maintain a-smooth'operating' blast furnace. It is-also well known thatjif the blast is insufiicient a low rate of pig iron production will be obtained while if the blast is excessive the rate of flue dust production will rapidly increase as a result of the increase in the amount of gases flowing up through the stack and will nullify anyincrease inscrap-free pig iron production.

Inasmuch as it is the oxygen in the blast which burns the coke to smelt the iron, there has been considerable discussion of the advantages-and disadvantages of enriching the blast with oxygen as a means to increasepig iron production without a corresp'onding'increase in flue dust. Many of the articles and patent teachings are contradictory. This contradiction may result from the fact that substantially all of the discussion is based on theory and not on practice. In addition, and despite the long use of blast furnaces, those skilled in the art do not know for certainwhat takes .place in the blast furnaces. The expected increase in production with an oxygen enriched blast is based upon :the increased rate of combustion which would in turn develop a higher temperature in the-hearth. However, this performance increases the rate of downwardmovement of the burden in the shaft and the furnace will run cold, that is, the descending burden is insufiiciently heated in the stack. border to overcome this condition and transfer heat'from the hearth up into'the stack, it has been suggested that water he added to the blast'as the water would-enter into an exothermic reaction with the carbon in the hearth and produce hydrogen and carbon monoxide. Thus the use of oxygen and water would appear'to nullifyeach other, as oxygen is used to increase the rate of combustionand the 'hearth'temperature, and water would in turndecrease the rate of 'hearth combustion and the temperature.

I have discovered, however, that the addition of oxygen to the air blast does increase production but tends to increase the -variation in furnace operation. The addition of oxygen to the blast also causes the furnace to tend to runcold. I alsohave found that a smoothly operating furnace providing an increase in production of pig iron can be obtained while at the same time actually decreasingthe production of fluedustby the controlled additions ofpredetermined amounts of moisture in specific relation with the percentage of oxygen enrichment.

In actual operation of blast furnaces it is not uncommon to experience erratic operation even in the absence of substantial changes in the burden constituents or in the relationship between the burden and the blast. In such cases diificulty has been encountered when attempt ing to smooth out the operation of the furnace'since a change in the burden is not immediately effective as it requires a number of hours of the burden to reach the hearth. Also, while the amount of hot blast blown into the furnace can be changed, only minor adjustments'can be made in this manner due to eifect of the blast on pig iron and flue dust production. I have found that such erratic operations may be relatively quickly overcome without material reduction of scrap-free pig iron production by controlling the percentage of oxygen enrichment and moisture content of the blast gas and their relationship with each other.

It is therefore an object of the present invention to provide an improved method of using an oxygen enriched air blast in a blast furnace.

Another object, isto provide an improved method of using an oxygen enriched air blast in a blast furnace that will result in smoother and more uniform furnace operation. x

Another object is to provide an improved method of 3 using an oxygen enriched air blast in a blast furnace and thereby effect an increase in production and satisfactory reduction of the iron ore in the stack without increasing the rate of flue dust production.

Still another object of the present invention is to provide an improved method of using an oxygen enriched blast in a blast furnace and to add moisture to the oxygen enriched air blast in such a manner as to not lose the advantages obtained from the use of oxygen and without causing the furnace to perform erratically. 7

Other objects and features of the present invention will appear more fully below from the following detailed description considered in connection with the accompanying drawing. It is expressly understood that the drawing is designed for purpose of illustration only and not as a definition of the limits of the invention, reference for the latter purpose being had to the appended claims.

The single figure of the drawing is a graph illustrating the range of moisture in grains of water per cubic foot of blast for different percentages of oxygen enrichment in accordance with the teachings of the present invention.

The present invention provides an oxygen enriched blast including a specific quantity of moisture that bears a definite relation to the percentage of oxygen enrichment. In general, the quantity of moisture in the blast is established according to the percentage of oxygen enrichment to prevent the furnace from running cold due to the increased rate of combustion resulting from the oxygen enrichment and to thus maintain a smooth operating furnace and eliminate erratic tonnage and high flue dust and coke rates accompanying the use of oxygen enriched blasts not including specific moisture control.

In a theoretical application of the present invention the total moisture content in the blast for a given percentage of oxygen enrichment will equal a fixed value for optimum performance when all factors affecting furnace performance and operation are constant. For example, with a particular furnace fed with a uniform burden made up of the same type of ore and constant relative quantities of coke and limestone of similar characteristics and with other variables maintained constant, the optimum moisture content in the blast having a two percent oxygen enrichment may comprise seven grains of moisture per cubic foot of the blast. However, it is not possible as a practical matter to maintain constant the variable factors which affect furnace performance. Therefore, it may be necessary in order to obtain optimum results from the use of an oxygen enriched blast to adjust the quantity of moisture content in the blast following changes in furnace operation which may result from variations in the characteristics of the components comprising the burden or in their relative relation with respect to each other, or from other factors. Controlling the moisture content of the blast not only obtains full advantage from the oxygen enrichment but provides a means for readily smoothing out an erratically operating furnace resulting from changes in the factors affecting furnace operation or from undeterminable causes.

In actual practice the optimum quantity of water moisture in the blast for a given percentage of oxygen enrichment will vary for different size furnaces and even for different furnaces of the same size and over-all design as well as for changes in the burden including changes in the relationship of the components comprising the burden and in the characteristics of the components themselves. While all of these factors affect the moisture content of the blast it has been discovered that the iron bearing material in the blast comprises the greatest factor which affects the moisture content next to the percentage of oxygen enrichment itself. It has been determined that iron ores obtained from different mining districts exhibit different physical and chemical characteristics which become manifest during the process of producing pig iron in a blast furnace. One of these characteristics comprises the temperature at which the ore begins to fuse or the temperature at which the ore becomes plastic. When the ore reaches this state, which may be referred to as the state of incipient fusion, the individual particles of the ore tend to stick together when brought in contact with each other. Thus if this temperature is reached as the burden is passing downwardly in the stack the furnace will become sticky and close up since the ore will tend to form a single mass preventing free downward flow to the combustion zone. In order to prevent the furnace from becoming sticky and closing up the relationship between the moisture content and the percentage of oxygen enrichment is controlled to maintain the temperature in the furnace stack below the incipient fusion temperature of the iron bearing material in the burden.

The range of the quantity of moisture in the blast relative to the percent of oxygen enrichment according to the present invention is illustrated in the single figure of the drawing. In this figure the percent of oxygen enrichment or added oxygen is plotted along the vertical coordinate X and the total moisture in the blast as measured in grains per cubic foot of the blast is plotted along the horizontal coordinate Y. It is observed from this illustration that the lower range of the total moisture in the blast varies as a linear function Y=4X3 as the percent of oxygen enrichment varies from 1.5 percent to 4.5 percent. It is also observed that the high range of the total moisture in the blast varies as a linear function Y:4X+l when the blast is enriched with 1.5 percent to 2.0 percent oxygen, and when the oxygen enrichment varies from 2.0 percent through 4.5 percent the top range of the total moisture content in the blast varies as a linear function Y=6X3.

The relationships as illustrated in the drawing between the percentages of oxygen enrichment and the moisture content in the blast including the permissible range of total moisture content were derived from results determined upon observation of blast furnaces in actual operation with different percentages of oxygen enrichment in the blast and with difierent burdens and upon the pro- It is apparent from these results that for optimum performance the moisture content in the blast bears a definite relation to the percentage of oxygen enrichment; the moisture content increasing as higher percentages of oxygen are added to the blast. It is also established that when the burden comprises iron ores characterized with a relatively low incipient fusion temperature, when the burden itself is so characterized, the higher ranges of moisture content are employed for high eflicient use of the oxygen enrichment. In situations where the iron bearing material has a relatively high incipient fusion temperature, in cases where the burden includes percentages of scrap and in the case of certain beneficiated burdens, optimum utilization of oxygen enrichment results when the lower ranges of moisture content are maintained. As mentioned above, high moisture content is necessary when the ore manifests low incipient fusion temperatures to prevent the furnace from sticking. It should be pointed out also that the use of an excess amount of moisture in the blast with burdens of high incipient fusion temperatures over that required to prevent the furnace from running cold" will result in erratic operation and will not permit the high rate of production that should result from the oxygen enrichment.

The following examples of actual furnace operation i illustrate .the critical relationship between the moisture content and the character of the iron bearing material feed to the furnace. An identical furnace was operated on different occasions. with a burden constituting the same character of limestone, coke and iron ore in fixed relations. The iron ore comprised a mixture of 55% Mesaba, 35% Old Range and Cuynna. In one run the furnace was blown with a blast having 2.47 percent oxygen enrichment anda moisture content of 7.4 grain per cubic footof theblast. On asecond run the oxygen enrichment comprised 2.2 percent and the moisture content was maintained at 9.1 grains per cubic foot. Notwithstanding the fact that a greater percentage of oxygen enrichment was employed in the first run, the second run showed a greater pig iron production, lower coke rate and flue dust production while employing smaller burdens per charge and a lessnumber of charges per day.

The present invention also provides a novel method forcontrolling the silicon content in the pig iron. Since the percentof silicon present in the pig iron is dependent upon temperature, the total moisture content in the blast may be controlled for this purpose. For example, when it .is desired to produce pig iron having a high percent of silicon, low ranges of moisture are employed, while the high ranges of moistureare followed when low silicon content pig iron is desired. The total moisture content in the blast for obtaining pig iron having silicon contents of 0.80 percent and 1.2 percent for difierentpercentages of oxygen enrichment are set forth in .the following schedule:

The percentage of oxygen enrichment, for all practical purposes, will depend upon the quantity of oxygen or oxygen-enriched gas that is available for this purpose. For all practical installations and in actual operating conditions, the percent of oxygen enrichment usually varies from 1.5 percent to 4.5 percent. After the determination of what percentage of oxygen enrichment may be employed, the moisture content in the blast is then fixed within the limits disclosed herein considering the con? trolling factors discussed above. The method and ap paratus disclosed in my copending application Serial No. 312,877 filed October 3, 1952 for Method of Operating Metallurgical Furnaces may be employed for establishing and maintaining the percentage of oxygen enrichment and the quantity of moisture in the blast. The natural moisture content in the atmosphere may vary between 0.5

and 10.0 grains per cubic foot of the atmosphere depending upon the location and period of the year. In some instances, therefore, it may be necessary to select the percentage of oxygen enrichment because of the natural moisture content in the atmosphere. If this procedure is not possible, due to an inadequate source of oxygen or oxygen enriched gas, then a source of dry air may be added to the blast by means of a dehumidifying process for example. In general, when there is suflicient oxygen the blast may be easily composed to meet the limits set by the present invention irrespective of the natural moisture content of the atmosphere, while in the case of a limited supply of oxygen or oxygen-enriched gas difficulties may arise when the atmosphere includes a high quan tity of moisture which may be easily overcome by a dehumidifying process.

- the iron bearing material in the burden.

There is thus provided by the present invention a novel method for operating metallurgical furnaces employing an oxygen enriched blast. According to the present invention in order to obtain optimum results from an oxygen enriched blast a fixed quantity of moisture is added to the blast in definite relationto the percentage of-oxygen enrichment while considering the characteristics of The added moisture bears a definite relation to the percentage of oxygen enrichment to prevent-the furnace from running cold, and is determined in accordance with certain characteristics of the iron bearing material in order to prevent the furnace from becoming sticky and clos ing up. Also, the quantity of moisture may be con trolled to adjust furnace operation, such as when furnace operation becomes erratic for known or unknown causes and a comparatively quick correction is required. Moreover, the method provided by the present invention allows the use of'a blast including a constant moisture content which may be maintained above the natural moisture content of the atmosphere. This performance insures more uniform furnace operation.

As well understood by those skilled in the art, chemical and physical reactions and changes that take place within a blast furnace are not fully understood or known in spite of the extensive studies and investigations that have been made in this field. Also, the factors that affect furnace performance and production are not understood or known. It is therefore not possible to explain or to offer explanations for the required oxygen enrichment and moisture content relations disclosed herein that may be considered complete or accurate as compared to new developments in this art. It is also expressly understood that the invention should not be limited by the specific-illustration and examples, but reference should be had to theappended claims for this water vapor per cubic foot of blast according to the equation Y=4X3; and a maximum content of aqueous fluid varying from 7 to 9 grains of water vapor per cubic foot of blast according to the equation Y=4X +1; where Yis the quantity of aqueous fluid in the blast and X the percentage of oxygen enrichment.

2. The method of operating a blast furnace in which iron bearing material is smelted and in which coke is burned, comprising the steps of continuously forming an air blast mixture enriched with oxygen and containing an aqueous fluid and discharging the blast mixture into the furnace, the blast mixture including from about 1.5% to 3.0% oxygen enrichment, and an aqueous fluid content in accordance with the following: a minimum content of aqueous fluid varying from 3 to 9 grains of water vapor per cubic foot of blast according to the equation Y=4X-3; and a maximum content of aqueous fluid varying from 7 to 9 grains of water vapor per cubic foot of blast according to the equation Y=4X+1 and varying from 9 to 15 grains of water vapor per cubic foot of blast according to the equation Y=6X3; where Y is the quantity of aqueous fluid in the blast and X the percent of oxygen enrichment.

3. The method of operating a blast furnace in which iron bearing material is smelted and in which coke 'isburned, comprising the steps ofcontinuously forming an air blast mixture enriched with oxygen and containing an aqueous fluid and discharging the blast mixture into:

the furnace, the blast mixture including from about 1.5% to 4.5% oxygen enrichment, and an aqueous fluid content in accordance with the following: a minimum content of aqueous fluid varying from 3 to 15 grains of water vapor per cubic foot of blast according to the equation Y=4X3; and a maximum content of aqueous fluid varying from 7 to 9 grains of water vapor per cubic foot of blast according to the equation Y=4X+1 and varying from 9 to 24 grains of water vapor per cubic foot of blast according to the equation Y=6X-3; where Y is the quantity of aqueous fluid in the blast and X the percentage of oxygen enrichment.

4. The method of operating a blast furnace in which iron bearing material is smelted and in which coke is burned, comprising the steps of continuously forming an air blast mixture enriched with oxygen and containing an aqueous fluid and discharging the blast mixture into the furnace, the blast mixture including from about 2.0% to 4.5% oxygen enrichment, and an aqueous fluid content in accordance with the following: a minimum content of aqueous fluid varying from to grains of Water vapor per cubic foot of blast according to the equation Y=4X3; and a maximum content of aqueous fluid varying from 9 to 24 grains of water vapor per cubic foot of blast according to the equation Y=6X3; where Y is the quantity of aqueous fluid in the blast and X the percent of oxygen enrichment.

5. The method of operating a blast furnace in which iron bearing material is smelted and in which coke is burned producing pig iron having about 0.80% to about 1.20% silicon, comprising the steps of continuously forming an air blast mixture enriched with oxygen and containing an aqueous fluid and discharging the blast mixture into the furnace, the blast mixture including from about 1.5% to 2% oxygen enrichment, and an aqueous fluid content in accordance with the following: a minimum content of aqueous fluid varying from 3 to 5 grains of water vapor per cubic foot of blast according to the equation Y=4X3; and a maximum content of aqueous fluid varying from 7 to 9 grains of water vapor per cubic foot of blast according to the equation Y=4X+1, where Y is the quantity of aqueous fluid in the blast and X the percentage of oxygen enrichment; the silicon content of the pig iron decreasing with an increase in the quantity of aqueous fluid in the blast.

6. The method of operating a blast furnace in which iron bearing material is smelted and in which coke is burned producing pig ironhaving about 0.80% to about 1.20% silicon, comprising the steps of continuously forming an air blast mixture enriched with oxygen and containing an aqueous fluid and discharging the blast mixture into the furnace, the blast mixture including from about 1.5 to 3.0% oxygen enrichment, and an aqueous fluid content in accordance with the following: a minimum content of aqueous fluid varying from 3 to 9 grains of water vapor per cubic foot of blast according to the equation Y:4-X3; and a maximum content of aqueous fluid varying from 7 to 9 grains of water vapor per cubic foot of blast according to the equation Y==4X +1 and varying from 9 to 15 grains of water vapor per cubic foot of blast according to the equation Y=6X-3; where Y is the quantity of aqueous fluid in the blast and X the percentage of oxygen enrichment; the silicon content of the pig iron decreasing with an increase in the quantity of aqueous fluid in the blast.

References Cited in the file of this patent UNITED STATES PATENTS 1,510,271 Gottschalk Sept. 30, 1924 1,518,854 Kirby Dec. 9, 1924 1,921,212 Brassert Aug. 8, 1933 2,175,517 Ditto Oct. 10, 1939 FOREIGN PATENTS 704,110 France May 13, 1931

Claims (1)

1. THE METHOD OF OPERATING A BLAST FURNACE IN WHICH IRON BEARING MATERIAL IS SMELTED AND IN WHICH COKE IS BURNED, COMPRISING THE STEPS OF CONTINUOUSLY FORMING AN AIR BLAST MIXTURE ENRICHED WITH OXYGEN AND CONTAINING AN AQUEOUS FLUID AND DISCHARGING THE BLAST MIXTURE INTO THE FURNACE, THE BLAST MIXTURE INCLUSING FROM ABOUT 1.5% TO 2.0% OXYGEN ENRICHMENT AND AN AQUEOUS FLUID CONTENT IN ACCORDANCE WITH THE FOLLOWING: A MINIMUM CONTENT OF AQUEOUS FLUID VARYING FROM 3 TO 5 GRAINS OF WATER VAPOR PER CUBIC FOOT OF BLAST ACCORDING TO THE EQUATION Y=4X-3; AND A MAXIMUM CONTENT OF AQUEOUS FLUID VARYING FROM 7 TO 9 GRAINS OF WATER VAPOR PER CUBUC FOOT OF BLAST ACCORDING TO THE EQUATION Y=4X+1; WHERE Y IS THE QUANTITY OF AQUEOUS FLUID IN THE BLAST AND X THE PERCENTAGE OF OXYGEN ENRICHMENT.

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