US2602027A - Method of operating blast furnaces - Google Patents

Description

July 1, 1952 B. 5. OLD 2,602,027

METHOD OF OPERATING BLAST FURNACES Filed May 6, 1947 5 Sheets-Sheet l m F} m l a I WET WASHER FIG.

CATO H E R INVENTOR. BRUCE SCOTT OLD y 1952 B. 5. OLD 2,602,027

METHOD OF OPERATING BLAST FURNACES Filed May 6, 1947 a Sheets-Sheet 2 2.70 a x O 2.60 Q v 9 2.50 1 cc MEAN STACK VELOCITY OF FURNACE GASES, F'E/SEC.

FIG. 2

INVENTOR.

BRUCE SCOTT OLD B. 5. OLD

METHOD OF OPERATING BLAST FURNACES July 1, 1952 3 Sheets-Sheet 5 Filed May 6, 1947 O O 4 l. 2 2

.rmzo MD E MEAN VELOClTY,FT./SEC., OF GASES IN FREE SPACE ABOVE STOGKLINE INVENTOR.

BRUCE SCOTT OLD BY Patented July 1, 1952 METHOD OF OPERATING BLAST FURNACES Bruce Scott Old, Concord, Mass., assignor, by

mesne assignments,

to Republic Steel Corporation, Cleveland, Ohio, a corporation of New Jersey Application May 6, 1947, Serial No. 746,220

4 Claims.

This invention relates to the operation of blast furnaces under internal pressures substantiall greater than those normally employed.

In his U. S. Patent 2,131,031, patented September 2'7, 1938, Julian M. Avery has set forth the principles of blast furnace operation at increased internal pressures, and has described the manher of doing soand the advantages attained thereby. The procedure set forth in that patent includes, among other things, supplying'the blast feed gas at a sufficiently high superatmospheric pressure, and throttling the gas discharge from the furnace, to create the desired pressure within the furnace; This desired pressure may range anywhere from about one to about ten atmospheres gauge, with advantageous operating results.

The procedure of the said Avery patent has already been adopted in some existing blast furnaces, with excellent results. Those furnaces, as would be expected, were designed and built for the normal average internal operating pressures of A to 1 atmosphere gauge, and pressures within the range of the Avery patent are attained by increasing the blowing pressure and installing throttling means upon the exit gases from the furnace. Present practical considerations require that operations under the Avery patent, with presently available equipment, be carried out Within the lower part of his pressure range. This is accomplished by using blowers capable of delivering the blast at say 30 to 40 p. s. i. g. (pounds per square inch, gauge) and imposing a top pressure of up to say p. s. i. g. upon the exit gases. The resulting average internal pressures in the furnace are consequently between about 1-plus and 2-plus atmospheres gauge. While many existing furnaces can safely withstand somewhat higher internal pressures, it is at present either impossible, or exorbitantly expensive, to obtain blowers which will provide adequate wind at pressures in excess of about 40 p. s. i. g. Y

Although higher-pressure furnaces with greater blowing capacity may well be built in the future, practical considerations limit the procedure of the Avery patent for the present to the existing types of furnace, wherein suitable modifications in blowing capacity and in the throttling and handling-of the exit gases are employed for increasing furnace pressure. Nevertheless, it would be highly advantageous to attain in existing furnaces the increased benefits which would flow from the-operation of the Avery process understill hi her pressures and rates of blowing.

It is now found, in accordance with the present invention, that such benefits may be-attained, by proper control of operating conditions, while still operating the furnaces at the above-stated pressures of 30 to 40 p. s. i. g. blast and up to 20 p. s. i. g. top pressure, or thereabout. Also, and within the scope of the present invention, it is possible to improve production, operations, and economies over those set forth in the Avery procedure, when operating at higher pressures-within his range (e. g. up to 7 atmospheres gauge average static pressure), as well as at the lower pressures (e. g. 2 atmospheres gauge, average static pressure) at which the present type of furnaces can be operated.

The procedure of the present invention also makes possible the utilization of the finer particle size ores which increasingly constitute the major reserves because of the depletion of highergrade ores. And furthermore, even when using such finer ores, the procedure of this invention assures that the amount of solids in the exit gases (i. e. flue dust) is maintained well below that occurringwith normal operation.

These several advantages are brought about, in accordance with the present invention, by the proper control of certain operating variables within specified limits, as set forth below. These operating variables are, in particular, the velocity of the gases through the furnace, and the burden ratio. a

The principal economies which can be realized in the operation of a blast furnace with the present invention are those incident to increasing iron production; decreasing coke rate; decreasing flue dust production; and obtaining more uniform iron quality with a lesser per cent of casts falling outside specification limits. Proper control of the mean linear stack velocity of thereducing gases within the interstices of the furnace charge makes possible a substantial increase above normal in the amount of wind blown 'pcr unit of time and hence in the iron production of the furnace; .it also increases the thermal efficiency of the furnace by increasing the efficiency of heat transfer and decreasing the heat losses per ton of -iron produced. In addition, I have found that this control'of gas velocity makes it possible to increase substantially the burden ratio with resulting fuel economy; to carry higher blast temperatures which .further effectscoke saving; to decrease thepressure dropthrough. the: furnace stock column to bring about smoother operation with lessslipping and hanging; and to minimize markedly "the dust blown out of the furnace; For

the greatest overall economy this control can best be exercised through enriching the air blast with oxygen while maintaining the highest average pressures that the shaft of the furnace is capable of withstanding, by throttling the exit gases. In accordance with the preferred embodiments of my invention, I now describe in what follows the operation of a modern type blast furnace. The normal amount of wind blown per minute (or normal blast rate) referred to above is the normal blast rate at which a given furnace is designed to operate using top pressures of not over about 2.5 p. s. i. g. Methods of calculating such blast rates are well known and are used by all steel companies.

This invention will now be described in detail with reference to the accompanying drawings, wherein:

Fig. 1 represents, largely diagrammatically, and partly in section, a side elevation of a blast furnace and associated equipment adapted to carry out the procedure of this invention;

-Fig. 2 is a chart showing mean stack velocity of furnace gases plotted against burden ration; and

Fig. 3 is a chart showing velocity of furnace gases in the free space at the top of the furnace just above the stock line plotted against flue dust produced per ton of iron.

Fig. 1 illustrates a blast furnace Iii and auxiliary equipment, having certain special features found to be desirable for operation at high average static pressure, but which are otherwise of more or less conventional design.

In operating the blast furnace according to the new method and with the equipment described, turbo blower l2 operates to supply the blast, and the average static pressure in the shaft of the furnace I is raised to the desired level by throttling the discharge of gases, as by means of the throttling valve 49. The blast pressure is increased an appropriate amount which is dependent on the pressure at the top of the furnace and the pressure drop through the stock column. which is dependent on several factors such as the velocity of the gases through the stock column, average static pressure in the shaft, and character of the charge. The wind blown, defined as cubic feet of blast per minute delivered to the furnace. is then regulated at the proper level by means of turbo blower speed control and any enrichment oxygen which is fed into the conduit [4 beyond the turbo blower. The blast is compressed by turbo blower l2 and passes through conduit [4, stove l5, and bustle pipe 16 to tuyres 18. Enrichment oxygen may be supplied to the blast in any desired manner, e. g. by the use of a separatorffl which may for example be a Linda-Frankl type separator. Byproduct nitrogen from the separator passes off at 22, while the oxygen is supplied to the blast at a-suitable place as determined by the relative pressures of the exit oxygen and of the blast. For example the oxygen may be passed via conduit 24 controlled by valve 26 to conduit 28 which supplies air to the turbo blower, or it may be passed via conduit 30 (controlled by valve 32) to conduit l4 after the turbo blower. Alternatively to the latter procedure, compressor may be used to raise the pressure of the oxygen supply to conduit M, the oxygen passing to the latter conduit through conduit 36 controlled by valve 38.

As already indicated, a static pressure is imposed upon the furnace shaft by any suitable means, for example a'pressure operated throttling valve 40, which as shown consists of three pipes with a butterfly valve in each, and which discharges into the Cottrell precipitator. To seal the top of the furnace l0 against leaks because of the pressure within the furnace, hopper 42 and lower bell 45 may each be made in one piece, with their contacting surfaces 48 and 41, respectively, being hard-surfaced. Alternatively, a flexible sealing member such as an internallywater-cooled hose may be associated with the lower surface 46 of hopper 42, to ensure a gastight fit against bell 44.

Hard surfaced bleeder valves 48, 49 are provided in the customary position, at the top of uptakes :i8a and 49a, respectively. Gas from the furnace passes through downcomers 50, 50a, thence into and through dry dust catcher 5|, and thence through conduit 52 to wet washer 53. The latter has a control system and overflow 54, 56, to maintain the water seal in the wet washer within predetermined limits in spite of variations in pressure due for example to slipping or hanging of the charge. 7

The gas discharged from washer 53 passes through conduit 55 to throttling valve 40 already referred to. Conduit 55 extends on downwardly to form a blind end for collecting any water and solids coming over with the gas; these collect at the bottom and may be removed from time to time through a suitable gate '51.

Instead of using a throttling valve M for controlling the gas pressure within the furnace, this gas pressure may be maintained by a power recovery system of the type set forth in the aforementioned Avery Patent 2,131,031 and in Avery Patent 2,192,885. In that event, 53 may be 'a dry dust separator auxiliary to dust catcher 5|. and will be a valveless conduit leading to the power recovery mechanism, which latter then serves as the throttling means to maintain the pressure in the furnace.

The wet washer (or dust separator) 53 isalso provided with relief outlet 58 communicating with another bleeder valve 59, the latter being located adjacent the bleeder valves 48 and 49 at the top of the furnace so that the gas discharge may be made at a safely high level away from the working space at the bottom of the furnace. The outlet line -58 and bleeder valvel 59 serve to bleed clean gas from the washer 53 during minor slipping of the furnace. H V I V Equalizer line 60 controlled by valve 62 permits the flow of washed gas fromoutlet 58 of wet washer 53 into the space between the 'bells, 'to equalize the pressure above and below lowerfbell M sufficiently to permit dumping the latter. Relief line 64 controlled by valve 65 provides a means for exhausting the space between the bells. so that the upper bell '68 may be dumped.

Among the structural features described above are a number which are of value in operating the furnace in accordance with the present invention. These are in particular the hard-surfaced contacts, or the flexible sealing members. 'forbell s4 and hopper 42, the wet washer control and overflow system 54, 56, and the system for bleeding clean gas from Wet washer'53 through outlet 58 to bleeder valve 59 and line 60. These features are not, however, claimed herein, but form the subject matters of other applications for patent as follows: Slater, Serial No. 775,016, filed September 19, 1947, now abandoned; Latham, Serial No. 763,821, filed July 26, 1947, now Patent No. 2,599,334; J anecek and Bahney, Serial No. 773,680, filed September 12,1947, now-Patent N0.

2,602,027 6 2,585,779; and Le Viseur and Larson, SerialNo'. at least 2.45 and even to 2.7 or 2.8, by throttling 771,870, filed September 3, 1947, now Patent No. the exit gases to decrease the mean stack gas 2,585,800, relating to hard-surfaced contacts, velocity to between about 45 and 55 ft./sec., which flexible sealing members, wet washer system, and latter figure is just below the aforementioned system for bleeding, respectively. 5 upper critical stack gas velocity. The saving of By way of example, assume that operation by over 200 lbs. of dry coke per ton of iron thus the new method herein set forth is planned for made possible by controlling mean stack gas a blast furnace and burden of the following charvelocity is of cardinal importance in view of the acteristics (which are commonplace): dwindling supply of metallurgical coke in this country.

g zi g fi figf" cubic ft By normal blast rate is meant the blast de- W rking hei ht k livered to the furnace when operating at normal Stack i ft top pressures and blowing with ordinary air (2l% Ferrous burden (56% Mesabi ore, 36% Old Range ii fi at liatestas as practlcible Whlch ore 8% Open Hearth slag) 50% F l isusualyse by he endency of he urnace 5% carbon to hang and produce a prohibitive amount of dust. This normal blast rate is su-flicient to pro- Calculations can then be made of the mean duce iron at a tonnage approximately equal to gas velocity of thereducing gases through the that of the rating of the furnace.

working volume of the furnace (also referred to By burden ratio is meant the ratio of pounds hereinafter and in the claims as mean stack gas of ferrous material charged, such as ore, scale,

velocity) by the following method: open hearth slag, etc., to the pounds of dry coke Mean stack gas velocity in feet per second: charged, per unit of time such as per round or (wind delivered in cu. ft./sec. 60 F., 14.7 per day. This ratio is commonlyus'ed in the steel p. s. i. a.) X (absolute pressure and temperature industry. In cases where the charge contains corrections for the average of the conditions in appreciable amounts of carbonate ores, such as the stack) (gas expansion factor) (average siderite, the weight of combined CO2 in the eifective cross sectional areaof furnace'working ferrous material charged is to be excluded from volume). V the figures for pounds of ferrous material Specifically, as applied to a furnace having the charged, in calculating the burden ratio. dimensions and burden given above, blown with The increased burden ratios attained in the air at 75,000 C. F. M. wind, with a blowing prespractice of the present invention may be exsure of 21 p. s. i. g. and a top pressure of 2.5 pressed in burden ratio figures, as already indip. s. i. g., and having a tuyere temperature of cated herein, or they may be expressed as per cent 2800 F. and a top temperature of 300 F., all of increase over the burden ratios customary in prior which are normal conditions for such a furnace, practice, i. e. without pressure operation. A'fair the mean stack gas velocity is calculated as folvalue, somewhat on the high side, for the burden lows (note: p. s. i. a. pound-s per square inch, ratios attained in such prior practice is, as absolut already indicated, 2.3. Hence the burden ratio 75000 7 20l0 l 3 5 so 26.4 520 54 Mean stack gas velocity:

Where 26.4=average absolute static furnace pressure I blast pressure+top pressure =67.2 ft./sec.

=calculated wind delivered, cu. ft./sec. 60 F.,'l4.7 p. s. i. a; r.

2010 average absolute furnace temperature (R.) (W) 460 460 520 =abso1ute air temperature (R.) 60 +460 rm n 1,35-gas expansion factor in top gas-5&5

54=average effective cross-sectional area of furnace working volume working vo1ume 0. 10 X 44,346 V working height 82 It has been found, in accordance with the pres- 60 value of 2.45 attainable under the present invenent invention, that the mean stack gas velocity tion represents an increase of 6.5% over the said so calculated controls the burden ratio the furnormal value, while the still higher burden ratios nace can carry, 1. e., within limits, the lower this of 2.7 or 2.8 hereby attainable represent increases velocity, the higher the permissible burden ratio. of about 17% and about 22% respectively. Furthermore, when the calculated mean stack gas The solid line of Fig. 2 represents the calcu- =per cent voidsX velocity is plotted against burden ratio as in lated values for burden ratio vs. mean stack gas Fig. 2, an upper critical mean stack velocity of velocity, which values are also the average values gases in the furnace stock column of about 60 found in actual operation. Obviously, the conft./sec. is found below which the burden ratio'can trols, and conditions of operation, and constitube raised sharply, to a point where it levels off on 0 cuts of the charge, of any blast furnace can never reaching a limiting value as dictated by the therbe maintained with such exactness that the re-v mal requirements of the smelting process. Thus sults Will always fall upon the solid line of Fig. 2. the burden ratio of 2.3 or less carried by the fur- They do, however, fall within a fairly restricted nace at the normal mean stack gas velocity of range indicated by the dashed lines on either side about 65 ft,/se c. -can ,be increased markedly, to 7? of the solid line of Fig; 2.

2,602,027 7 8 The main reasons found, in carrying out the produced in the nearl vertical portion of the present invention, for thesaving in coke through curve of Fig. 3, as the mean stack gas velocity control of mean stack gas velocity in the ranges controls to a large extent the slipping and chanof pressure operation allowableby limitations in neling tendencies of :the charge by controlling the present turbo blower and blast furnace design pressure drop .through the stack. Thus the are: (1) More effic'ient use of the reducing power higher the mean stack gas velocity the rougher of the furnace gases'through better gas 'distributhe operation and the greater is the amount of tion in the stock column and-longer time of condust thrown into the space above the stock line tact, both of which are effected by lower gas where it can'be carried over into the downcomers velocity. The justification for this statement is 10 by free-space gas velocities in excess of the critifound in the decrease occurring in (IO/CO2 ratio cal value of about 4.5 ft./sec.

of D gases leaving the furnace as the velocity Although, as will be noted from the foregoing decreases; (2) More efficient heat transfer sample calculations, there isno direct connection through more uniform gas fiow and longer time between the mean stack gas velocity and the freeof contact of the'hot ascending gases'with the space gas velocity, the point of critical velocity of descending charge, which also results from deabout 4.5 ft./sec. of the latter corresponds roughly creased velocity. Thus the temperature of the to a velocity of about 45 ft./sec. for the former. exit gases from the top of the furnace has been There are three methods "of decreasing the found to be a direct function of the velocity for mean stack velocity of gases through the furnace a given average static furnace gas pressure; and charge:

(3) Utilization of higher blast temperatures at ('1) Increasingthe average s'ta'tic pressure withthe higher pressures in the shaft of the furnace. in the furnace Control of-gas velocity permits another impor- (2) Decreasing the wind volume blown tant saving to be effected in the blast furnace -03) Enriching the air blast with oxygen.

process. The fine dust produced per ton of iron It is obvious that method (2) is undesirable, has been found to be a function of the free-space except possibly during times of business depresgas velocity, as shown in Fig. 3. This free-space sion, as low iron tonnages areproduced-when low gas velocity is that of the gases in the free space wind volumes are utilized.

just above the stock at the stockline of the fur- In Table I, below, are shown data and calculanace. It is calculated by the following method: 39 tions for operations of a furnace having the par- Free-space gas velocity just above the stock at ticular dimensions already given, at normal presthe s'tockline, in feet per second: (wind delivered sure of 2.5 p. s. i. g., and at increased top presin cu. ft./sec. 60 R, 1417p. s. i. a.)' (absolute sures of 10 p. s. i. g. and 20 p. s. i. g., in each case pressure and temperature corrections for condiwith ordinary air, with 25% oxygen air, and tions at the stockline) (gas expansion factor) with oxygen-air. 'By 25% oxygen air and (cross-sectional area of furnac'e'at the stockline'). 40% oxygen air are meant'respectively air con- Specifically, as applied to afurnace having the taining'substantially 25% O2 and 74% N2, and dimensions and burden already specified above, air containing substantially 4=0% O2 and 59% blown with air at 75,000 0.1. M. wind, with a N2, all figures being by volume. In the column top pressure of 10 p. s. i. g. and a top tempera- 4.0 headed Wind (or 02) Delivered (C. F. M.) are ture of 300 F., the said free-space gas velocity is figures in parentheses showing the C. F. M. of calculated as follows: oxygen in the wind delivered, e. g. 90,000 C. F. M.

Free-space gas velocity= X =4.3 ft.'peI'-Sec.

Where 75000 o =calculated wind delivered, cu. ft./sec. 60 F., 14.7p. s. 1. a.

14.7 g1. 7 -abso1ute pressure correction for top pressure 10 p..s. 1. g.

460+300 7 60 o 460 60 -absolute temperature correction for top temperature @300 F.

l.35=gas expansion factor= Again it will be noted that a critical velocity is of 25% oxygen air contains 22,500 C. F. M. oxygen. reached, this being one below which the dust 60 Normal air isfigured at 21% oxygen. The showproduction of the furnace becomes-inconscqueningsof Figs. 2 and 3, and the figures under N0 tial. Thus if one controls thefree-space velocity Air *Enrichmen in Table I, are based upon exat below about-4.5 ft./sec. the flue dust produced tensive series of actual operatingrun while the will be of the order of 90 lbs. or less per ton of figures for the balance of Table I are obtained iron, instead of a normal value of over 200 by calculations basedupon the results practically lbs/ton of iron for operation at 75.000 C. F. 'M. attained insuchoperatingruns, upon experience delivered wind at 2.5 p. s. i. g. top pressure. The gained therefrom, and upon well-founded theoresulting saving in iron yield really raises the retical considerations.

effective burden ratio charged. In addition, As an example, with reference toTable I, assavings will result from having to sinter only the sume that it is desired to supply to this furnace smaller amount of flue dust for'rechargin into about 22,000 C. F. M. of oxyge in th wind, This the blast furnace. The mean stack gas velocity, is not practically feasible with ordinary air Withif so high that the free-spaceigas velocity is above out added top pressure, because among other the critical dust carry-over value found, is imthings, the gas velocity would be sogreat as to portant in determining how much dust will be blow much of the charge out of "the furnace.

Even with added top pressure, it is hardly practicable with presently available equipment, since the blowing capacity and pressures required are in general too high. However, by enriching the blast with oxygen, and imposing practicable pres sure and blowing conditions, 22,000 C. F. M, of oxygen in the wind can be readily provided. Thus, by blowing 90,000 0. F. M. of 2 oxygen air at p. s. i. g. top pressure, good operating results in accordance with the present invention are attainable. Similarly good results are attainable by blowing 60,000 C. F. M. of about 37% oxygen air, at about 10 p. s. i. g. top pressure.

TABLE I the increase in burdenfratio possible through gas velocity control. should I prove highly advantageous. Since the ore reserves oftheUnited States are now largely made up of. lean ores, the importance of gasvelocity control cannot be overemphasized. i

As is clear from this table and from the foregoing disclosure, as well as from Fig. 2, the desirable results of the present invention are attained by imposing a top pressure onthe exit gases, and blowing sufiicientair to efiect a mean stack velocity of the furnace gases of less than ft. per sec. and preferably not over 55 ft.

Burden ratio, iron production, dry coke rate and dry flue dust production for varying furnace gas velocities and degrees of air enrichment O W121i TOP P- Blast $233 02; Burden 3.34 553 iai 535 1?" Delivered sure Pressure Velocity R500 n51 Tons 1.55.0011 LbsJton (0.1. m.) (p. s. i. g.) (p. s. l. g.) (ft/Sec) Day Iron Iron NO AIR ENRICHMENT 2. 5 31. 3 79.1 1 105,000 10 38.8 53. 9 2. 315 1, 500 1, 530 255 22,050 20 4s. 3 50. s 2. 525 1, 935 1, 430 e 25% OXYGEN .4111 (BY VOLUME) 2. 5 21 70. 2 2. 26 1, 404 1, 720 310 75,000 10 2s. 5 54. 7 2. 515 1, 575 1, 544 13,750 20 3s. 5 42, 3 2. 75 1, 730 1, 412 34 2. 5 20. 3 7G. 4 2. 225 1, 650 1, 750 400 90,000 10 33. 8 G0. 6 2. 365 1, 765 1, 643 195 22,500 20 43. 3 47. 5 2. 595 2, 030 1, 440 57 40% OXYGEN AIR (BY VOLUME) 2. 5 21 80. 9 2. 215 2, 200 1. 754 400 75,000 10 2s. 5 53. 0 2. 325 2, 320 1, 570 150 30,000 20 3s. 5 4s. 7 2. 5s 2. 700 1, 450 50 X Probably inoperable.

The data and calculations given in Table I are based upon the results of operatin a blast furnace of the size hereinbefore described on socalled straight ore burden using untreated Lake ores, as previously outlined, or on containing amounts of sinter, scale or scrap such that the iron content of the ferrous materials in the burden is approximately 48-54%, and for coke containing about 86.5% C and about 3 to 8% moisture. For difierent types of burdens the values in Table I would have to be altered by methods familiar to one skilled in the art. Thus for higher iron content burdens, for exampl those having higher than 60% Fe in the ferrous materials charged, the normal burden ratios would be so high that relatively little increase would result from gas velocity control. For lower iron content burdens, for example ,25% Fe or lower,

per sec., while maintaining an average 1555 15 pressure within the furnace of- 50155555115 atmosphere gauge. The top pressure so imposed should be at least 5 p. s. i. g., and for most furnaces will, for practical purposes, be in the general order of 10 to 20 p. s. 'i'. g.' For furnaces operatmgunder say seven atmospheres out in Table I is its flexibility. I erator can adjust the gas velocity, and hence shown in the foregoingdisclosure and in Table I. It will be noted, especially from Table I, that the high iron production and the low'coke rate and low flue dust are attained with burden ratios in excess of 2.45, and mean stack gas velocities of not more than about 55 ft./sec. Preferably this stack velocity is maintained above about 35 ft./sec., although lower velocities may be used with some decrease in coke rate from what could be attained at a higher velocity (but below about 55 ft./sec.), but this will generally be ofiset by increase in blowing costs or decrease in productivity. In general, the best results from the operation of the present invention are attained with a burden ratio of about 2.7 to 2.8 (i. e.

about 17% to 22% above normal), a mean stack gas velocity of 35 to 45 ft./sec., and a free-space gas velocity of less than 4.5 ft./sec. Such operations are in the zone of the upper inflexion point of the curve in Fig. 2, and below the point in Fig. 3 at which the flue dust production mounts nearly vertically.

As is evident from Fig. 3, there is no critical lower limit for free-space gas velocity. However, other conditions being equal, as mean stack gas velocity decreases, so does free-space gas velocity, although not necessarily in the same ratio. At the lower ranges of mean stack gas velocity herein set forth, i. e. about 35 ft./sec., the free-space gas velocities will generally be in the order of 3 to 4 ft./sec.

From Table I it will be noted that, normal top pressure of about 2.5 p. s. i. g., nor-- mal blast pressure-of about 21 p. s. i. g., normal maximum wind volume delivered to the furnace of about 75,000 C. F. M., and normal gas velocity of about 67 ft./sec., the burden ratio is about 2.28, the iron tonnage is about 1187, the coke rate about 1710, and the flue dust about 260 lbs. per ton of iron. Ordinarily, as one increases the blast rate, the coke rate and flue dust production also increases. However, it will be seen that one can, through gas velocity control, increase the unenriched blast rate to 105,000 C. F. M. and yet decrease the coke rate from the normal value of 1710 to 1480, and decrease the flue dust by over 100 lbs. per ton of iron, and increase production of iron by around 700 tons per day. Furthermore, with 25% oxygen air at the otherwise normal operation of 15,000 C. F. M. u

blast rate at 2.5 p. s. i. g. top pressure it will be noted that the coke rate will actually increase unless gas velocity control is utilized to increase the burden ratio. When this is done a saving of about 300 lbs. of coke per ton of iron is made possible, as well as a substantial saving in flue dust. The importance of these savings will be recognized when it is realized they represent a manufacturing cost saving of over $1.00 per ton of iron.

Another feature of gas velocity control brought Thus the opburden ratio, to suit his operating conditions (turbo blower'and furnace top pressure limita tions, and prevailing economic situation and plant costs) so as to balance coke rate, iron production, flue dust production, and blowing costs in order to operate the blast furnace at the optimum point of economic balance.

By using the new variable controls of this invention, gas velocity and blast enrichment, the operator can achieve economies of different kinds and adapt the furnace operation to the conditions and demands of the overall steel plant, of

for the 12 which the blast furnace is usually an integral unit. For example, if the coke supply is limited and the demand for pig iron above that available from normal operation of the furnace, the operator can adapt the burden ratio (which controls coke consumption) to about 2.7 by decreasing gas velocities through the furnace to say 40 ft. per see. by going to a top pressure of 20 p. s. i. g. Under these conditions production would be increased about 280 tons per day with substantially the same total daily coke consumption; or if the furnace could not be operated for purely physical reasons at such internal pressures, then the operator could achieve the same results by going to about 10 lbs. top pressure and enriching the blast in oxygen. With this same top pressure limit and the demands for still greater production the operator could increase the blast with some sacrifice in coke economy or maintain the same desirable high burden ratio through blast enrichment, while continuing to operate under the maximum top pressure which can be safely used in the furnace. From Table I it is therefore evident that widely varying conditions of operations can be realized with any given furnace through the manipulation of the variables of gas velocity and blast enrichment while operating under higher than normal top pressures. The flexibility achieved by the procedure of the present invention is therefore of the greatest practical importance. In fact, with its provision for proper control of gas velocity in the furnace, this inventien opens up an entirely new order of magnitude of production to be attained from present day blast furnaces, at efficiencies not hitherto believed to be possible.

I claim:

1. The method of producing iron from iron ore in a blast furnace utilizing a burden ratio which is high for the grades of coke and ore, with consequent decrease in coke consumption, comprising blowing air containing approximately 25% to 40% by volume of oxygen at a high blast rate sufficient to produce a mean stack velocity through the stock exceeding 55 linear feet per second at a top pressure of about 2.5 pounds per square inch gauge, throttling the outlet of the furnace to impose suficient top pressure on the exit gases to limit the mean stack velocity to the range of 35 to 55 linear feet per second through the stock while holding the blast at said high rate, said top pressure being adjusted in the range of 5 to 20 pounds per square inch gauge to impose an average internal static pressure in the furnace of greater than 1 atmosphere gauge.

2. The method as defined in claim 1 wherein the mean stack velocity of the gas through the furnace is limited to the range of 35-45 linear feet per second.

3. The method of producing iron from iron ore in a blast furnace utilizing a burden ratio which is high for the grades of coke and ore, with consequent decrease in coke consumption, comprising blowing air containing approximately 25% to 40% by volume of oxygen at a high blast rate sufiicient to produce a mean stack velocity through the stock exceeding 55 linear feet per second at a top pressure of about 2.5 pounds per square inch gauge, throttling the outlet of the furnace to impose sufficient top pressure on the exit gases both to limit the mean stack velocity to the range of 35 to 55 linear feet per second through the stock and to limit the linear gas velocity above the stock to less than 4.5 feet per second, while holding the blast at said high rate, said top pressure being adjusted in the range of about to pounds per square inch gauge to impose an average internal static pressure in the furnace of greater than 1 atmosphere gauge.

4. The method of producing iron from iron ore in a blast furnace utilizing a burden ratio which is high for the grades of coke and ore, with consequent decrease in coke consumption, comprising blowing gas containing at least approximately by volume of oxygen at a blast rate sufiicient to produce a mean stack velocity through the stock substantially exceeding 55 linear feet per second at a top pressure of about 2.5 pounds per square inch gauge, throttling the outlet of the furnace to impose suflicient top pressure on the exit gases to limit the mean stack velocity to the range of to linear feet per second through the stock, said top pressure being adjusted to above about 5 pounds per square inch gauge to impose an average internal static pressure in the furnace of greater than 1 atmosphere gauge.

BRUCE SCO'I'I' OLD.

14 REFERENCES CITED The following references are of record in the file of this patent:

UNITED sTATEs PATENTS Blast Furnace and Steel Plant, June 1945,

pages 699 to 707.

Iron and Steel Engineer, February 1946, pages 88 to 94, and 101.

Transactions, American Institute of Mining and Metallurgical Engineers, vol. 131, pages 108, 109, and 110.

Claims (1)

1. THE METHOD OF PRODUCING IRON FROM IRON ORE IN A BLAST FURNACE UTILIZING A BURDEN RATIO WHICH IS HIGH FOR THE GRADES OF COKE AND ORE, WITH CONSEQUENT DECREASE IN COKE CONSUMPTION, COMPRISING BLOWING AIR CONTAINING APPROXIMATELY 25% TO 40% BY VOLUME OF OXYGEN AT A HIGH BLAST RATE SUFFICIENT TO PRODUCE A MEAN STACK VELOCITY THROUGH THE STOCK EXCEEDING 55 LINEAR FEET PER SECOND AT A TOP PRESSURE OF ABOUT 2.5 POUNDS PER SQUARE INCH GAUGE, THROTTLING THE OUTLET OF THE FURNACE TO IMPOSE SUFFICIENT TOP PRESSURE ON THE EXIT GASES TO LIMIT THE MEAN STACK VELOCITY TO THE RANGE OF 35 TO 55 LINEAR FEET PER SECOND THROUGH THE STOCK WHILE HOLDING THE BLAST AT SAID HIGH RATE, SAID TOP PRESSURE BEING ADJUSTED IN THE RANGE OF 5 TO 20 POUNDS PER SQUARE INCH GAUGE TO IMPOSE AN AVERAGE INTERNAL STATIC PRESSURE IN THE FURNACE OF GREATER THAN 1 ATMOSPHERE GAUGE.

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