US2404461A - Method for operating blast furnaces - Google Patents

Description

Patented July 23, 1946 I, Stanley li. Sanford, Cleveland; I g V NoDrawing.*Applicationflctobcr'6,1942;a.

FURNACES Y c ic Serial No; 460,979.,

. C n (01; 75 1 I .--In.th'e production ofpigironbyblastfurnace operations; four cardinal factors are i'njvolved,

namely, the'blast, thefuehth charge: and the composition of the resultant slag: inter mediate and almost equally important factor; therefmay'be. mentioned thatthe chemical composition of the gases'comingufrom'the top of the furnace isfof importance, as: suclifc'omposition is'an indication of the'actual performance'of the furnace with'n'respect to the volatilization of all the elements which are:volatiliaable.v A consideration of standard text booksand prior art could lead thegalveragelayman'to "believe that all. ferrous metallurgy; including. blast furnace operations; has been reduce'd'ito' an exact science,

' that all'the'important"factorsinvolved have been compen-Sated' and that all of] these factors in actual performance customarily are dovetailed so completely that the entire operation of a blast furnace is wholly standardized on a basis upon which. anyone skilled in the art can duplicate of areespressedor are inherent in Tablefi hereinafter. set. forth 'in' this specification :1;

56.5.for: determining the weight. ica/ans t blast perminute; 51.0 for determining.the'weight'of carbon in; the

charge per minute; 1. 2 31.0 for determining the weight or ironinthe charge per minute; v

75.1 for determining the weight: of carbonthat will be consumed by the oxygen of the? blast per minute; 1

therebyestablishing and, maintaining chemical balance upon stoichoimetric ;proportioning, in which not more than 76' per cent of the total weight'of carbon in. the charge. is. consumed by v the total weight of. oxygen in the predetermined any furnace performance at will; However; this actuallyhas not been thecase, standard operations being accompanied by many variations in operating conditions in the same furnace, producing unbalanced chemical and thermal environments, and causing, generally, sub-standard 5 yields: which very often fall very substantially below even the rated tonnage: of the furnace.

The present invention has for'one of its ob jectsthe provision of an improved processor. blast furnace operation which obviates the irregularitiesof blast furnace operations which characterize the so-called standard 'practice,'the present process being distinguished from standard practice by proportioning and maintaining the carbon content of the. charge tothe oxygen content of" the predetermined blast volume in the ratio of more than one pound of carbon for each one pound of oxygen, proportioning and maintaining the iron content of" the charge to the than. 1.37 pounds but less than 1.82 pounds of iron to each one pound of carbon, proportioning and. maintaining the iron content of the charge of'more than 1.37 but less than 1.82 pounds of iron to each one pound of oxygenii n the predetermined blast, such proportidnin-g being readily accomplished by dividing the predetermined blast blast volume upon the chemical" equivalence ratio of 1.33. pounds of oxy en for each'one poundjof carbon, thus evidencing-that since, no eacess blast oxygenlis available, overblowing-of the furnace is impossible.

The empirical factors'565, 51.0, 31.0 set forth above arebased upon the fixed blowing. rateof 65,000 cubic feet of. wind'which'hasbeen found,

in'accordance with the present invention; to be the standard-requirement of blast for the production .of one-'grosseton of pig iron perminute.

These empirical factors areused as empirical divisions of 65,000 cubic feetz to determine the total pounds (per gross ton 'of'pig iron) ofthe carbon in the coke, iron in: the metallic mix of the burden, thus-standardizing all-critical factors of blast and burden solely upon the blast volume 1 of 65,000 cubic feet per gross ton of iron.

Of. the above factors, factor 56.5 is obtained I by dividing 65,000 cubic feet of blast per minute by the' Weight of oxygen. in pounds perminute in that-blastgthe'factor 51.0'is obtained by dividing cubic feetfof blast per minute by the pounds of carbon in the charge required per minute to carbon content of the charge in the ratio of more to the oxygen content'of the blast in the ratio A pounds of carbon that are required to combine volume by the following empirical division, all

unite with the ox gen reaching the tuyeres'; the.

charged per minute; the factor 75.1 is obtained by dividing 65,000 cubic feet of blast by 865 with 1,150pounds of oxygen in the 65,000 cubic feet of blast,

It is a further objectfto provide a methodjof operation which will require a less total weight furnace operations? of blast than of charge, and a less total weight. of oxygen in the blast than of carbonjin the charge. 7 I x M It is a still further object to provide a method of operation whereby the fuel consumption is lowered and furnace production is increasedi It is an additionalobject to provide a sta- .7 bilized method of operation whichwill prevent hanging and slipping of the furnace charge and prevent the creationof hot tops and/or cold bottoms? r s i PRESENT IMPROVED METHOD Foe OPERATING BLAST FURNACES,

In accordance with the present invention, it has been discovered that the realtiming element 2 in. blast furnace operation-is the blast-andthat maximum temperature of the blast is an iming from the variations in the composition of the charge and combustion of the coke in the charge v(this excessblast requiring the addition of compensating carbon or, alternatively, reduc ing the amount of blast being introduced) are undesirable conditions which are to be avoided. This condition causes reduced efliciency, and, in accordance with the present invention thereis maintained continuously a chemical balance of the vital components inthe charge and in the blast. Such a chemical; balance insures maximum production at minimum 'cost. I

It is absolutely necessary that this, blast be, heated to a temperature as close as possible to 1300 E, in order to provide the indispensable back-log of sensible heat that is demanded for continuous operation. Therefore, the prerequisites for obtaining, the maximum thermal efiiperative adjunct of the blast; that it is equally requisite that the actual percentage of the total carbon that actually arrives at the tuyeres for mbusti0n by ygen of the blast, be'eh m;

ically balanced against the actual percentage Weight of the oxygen in the blast; and that,. very definitely, no blast volume should be blown for the combustion at the'tuyeres of the remaining percentage of, the solid carbonin'tthe charge, because it has been discovered that such remaining percentage weight of carbon undergoes direct combustion in the top half of theblast furnace,. and, consequently, it never arrives in the combustion zone at the tuyeres. Thus, by the present invention, there is eliminated the long standing universal custom of over-blowing. In fact, it has been found that the entire reduction of iron oxides is accomplished by the process of -.;ciency of a given furnace are:

(-1) Determining the maximum rate of carbon 1 combustion per. minute, maintained at the tuyeres, by blowing a constant volume of blast at maximum temperature.

-(2) Charging'only such total weight ofcarbon 7 weight of carbon. x-Excess blowing definitely the present invention, in the top third of the furnace.

In practicing the present invention, it is found that the actual'chemistry and the actual mode of reduction tend generally towards 100 per cent thermal efiiciency in blast furnace operation only I w when, insofar as is possible, there is maintained a constant burden ratio and a constant blast vol- 'ume at maximum temperature, for any given grade of pig iron product; and that any neces sary thermal adjustments can be made readily n by alteration of the blast temperature, which procedure is in sharp contrast with the long established, universal custom of variations and over-blowings which characterize the standard practice, and which continue In accordance with the present invention, the improved results are dueto the maintenance of ratios of essential materials in strict chemical equivalence with each other, and because, in the improved process every cardinal factor is established permanently upon a chemical balance of .the vital factors O/C/Fe. 7

-; It is apparent also that the constant'minimum' required volume of blast at the maximum temperature would produce the constant minimum products of combustion and, therefore, wouldconvey the constant minimum volume of tem-f perature out of the 'tuyre zone. Conjointly, the constant minimum fuel ratio would insure the to unstabilize blast constant minimum products of combustion.

Therefore, there is the necessity for the constant minimum required blast volume at maximum,

temperature and the constant maximum burden ratio, which are the important factors of the U present invention.

In accordance with the present invention, it;

has been determinedthat the excess blast resultraises the mantle .in the furnace and thereby lowers the overall: temperature of the hearth, because the increased volume of gases, upon leaving the hearth, carries excessive temperature out of the hearth where it is needed to maintain the indispensable back-log for insuring continuous operation, up into the stack where it performs no useful service.

-'Ihe ratio of carbon to oxygen, and the ratio of free oxygen to combined, or reducible, oxygen are vital'factors iniblast furnace operation. In the foregoing estimates there has :been made the very best use of these factors for obtaining the maximum results, wherein the burden ratio would be more than 2.25 and the free oxygen to reducible oxygen ratio should be less than 1.75;

In practice it is. found thatnothing is more fatal to the-thermal efiiciency of a blast furnace. than the long-established and universally used method ofpulling the'wind and decreasing the burden ratio every time the silicon content in the iron threatens to exceed the upper limit of the specification, because this condition of itself is a definite indication of the furnace being overblown and that the excess volume of hearth gases are carrying the high temperature of the hearth too high in the shaft, thereby inducing the reduction of silica to silicon at a level in the furnace -Which is considerably above the level at which the oxygenof the blast burns the hearth carbon, that being the level'at which the highest temperature should be confined.

Blast furnaces that are charged and blasted according to the method herein described obviate the necessity for frequent manipulation, by reason of their ability to'adjust themselvesautomat- .ically to changingcon'ditions within,- The following tables show a, comparison of the results obtained in practice and in the operation of the a present improved process, these furnaces being those to which reference was made previously here n.

Table? I;

Furnace" #5 D Western op- Operated .by #5. rated in acpresent Western oordance improved 1 w th present 7 process HIIDIOVed' process Volume of turnacein. cu. it ,ooov Hearthdiameter I 210 21'0 250 250 Area of the hearth in sq. ft. 346. 4 346. 4 490.9 490. 9

Charge Burdensratio--10-- 1. 92 2. 32 2. 50 2.85 Gross tons pig iron per day (1440).. 721 1,142. 1, 049 1, 440 Minutes per gross ton 2. 00 1. 26 1. 37 1.00 Total weight of charge per minute 3, 500 6,100 4. 690 6, 375 Total per cent of iron per minute. 29. 50 32. 32 32.55 33.00 Total pounds'of iron per minute. 925 1, 670 1, 580 2,100" TotaLper centoi carbon perminu 24. 20. 00 22.38 20. 00 Total pounds of carbon per minute- 817 1,078 l, 050 l. 275 Pounds of carbon burned per ininut 772 1,010 994 1,185. Same at tuyeres burned per minute 600 728. 730. 868 Per cent tot carbon burned tuyeres 73'. 00 66. 00 70500- 68.30 Pounds carbon per gr. ton pig iron. 1,634 1,360 1, 438 1, 275.

V Blast Total cu. ft. wind per minute 54, 470 54, 470 65, 000 65, 000 Total cu. ft. wind per minute required 44, 8 70 54, 470 54-, 850 65,000 Total cu. ft. wind per minute excess .9, 600 0 10, 150" 0 Total cu. ft. wind per minute excess per cent.-. 17. 0 15. 50" 0' Total cu. ft. wind per pound carbon 90. 8 75. 2 90.0 75. 2v Same required at tuyeres i 75. 2 75.2 75. 2 75. 2 Total pounds oxygen per minute- 968 968 -1, 150 1,150 Same required tuyres per minute. 798 968 970 1, 150 Pounds oxygen excess at tuyeres. 170 0 185 Per cent of excess 17.50 0 15. 50 Total pounds oxygen/gross ton pig iron. 1, 936 1, 320 1, 576. 1,150. O 1.11 1. 1. 46 1.64 38. 75 12. 32 1. 72 1. 321 L 83' 81.05 38.70 0/ 0.90 1.10 0. 90' Ratio O/C percentage decrease... l 23.75 18. 20 Total pounds of-blast per minute.- 4.165 4, 975 4, 975 Total pounds of blast per minute req. 4,165 4,100 4,975 Per cent of excess 0 18. 00' 0 Total wgt. blast per day (1440').. 5, 999,000 7, 164, 000. 7, 164.000 Total wgt. ohargeper day (1440) 7, 630,000 6, 815000" 911811.000 Per cent ol'increase in tot. chge 61. 50 35, v Increase of G. T. pig iron/day 391 Per cent of increase .45 .373 Fur. vol. wasted by excess bl., cu. ft. 13, 850,000 0 14, 616, 000 .0. Tot. wgt. of excess blast/day, lbs... 1. 0 0, 3 0 0 1, 118, 600 0 Per cent of total blast wgt 17- 0 27.50 .0

The above demonstrates that the Western furnace, when operated in accordance with th'epresent improved procedure, will have a. burden. ratio of 2.85 or greater than 2.25, the burden ratio being the ratio of the total Weight of the metallic,

It Will also in the charge to free oxygen in the blast greater than 1.00 (1275 lbs. of carbon in charge per gross ton of pig iron+ll50 lbs. of oxygen in the blast. per gross ton of pig iron=1.1l); and a ratio of total Weight of oxygen in the blast to the total weight of the reducible oxygen in the charge less than 1175 (1150 lbs. of oxygen in the blast per gross ton of pig iron- 900 lbs. of reducible oxygen in the charge=1.28). v

Generally speakingthe methodiof the present invention comprises preparing a charge of hema tite ore in natural hydrated condition and containing not less than 4.0 per cent iron, coke, and limestone, introducing the resulting mixture into a blast furnace and blastin the said: mixture with super-heated air, While maintaining a-burden ratio greater than 2.25 but less than 3.25 a

of fixed-carbonin-the charge to the free oxygen.

' in the blast, greaterthan 1.00, butless than 3.00,

and a ratio of total Weight of oxygen in the blast to reducibleoxygen in the charge less than 1.75, but not less than 0.75.

In takingover any operating furnace, the first consideration, in operating in accordance With-the improved process, would be the ascertaining of .the

establishedproduction rate and the maximum available blast, or; preferably, the established maximum operating blast. Practically every charging unit has an unused charging capacity-of from aboutB'O per cent 130.50 per cent, sothat there is sufiicientcapacity available to meet the require men-ts for charging according to the improved process. After these determinations are made then Table'II'would be consulted for ascertaining thecorrectweights of carbon and of'metalliciron m also by the factor 51.00, for the purpose of ascertaining; first; the total eight of the iron in the burden, and,.second, the total weight of carbon in the burden. Then there would be determined the respective. requirements 'of the number of charging rounds and the total weight of each charged in one regular operating round).

Coke: 100% 0101530 153433 into thefurround required for the operation in accordance with the improved procedure. The procedure,

then, in taking over a given furnace, is as follows:

First, maintain the blast at the established maximum operating blast rate, at not less than 1200 F.;

Second,charge one coke blank', the'total weight of which is equivalent to the total weight of the coke in two regular operating rounds plus a total weight of stone which is'equivalent to the total weight of stone that is'charged in one'regular op.-

erating round; I 1

In normal furnace operation the furnace is charged in rounds each "round. being com- Now, in accordance with the second step of changing a conventionallyoperating furnace tofthe present improved process as described above, wherein there have been charged intothe furnace 26,000 lbs. of coke (one coke blank, 111300031 weight of whichis equivalent to the total weight of the coke in two regular. operating rounds) plus 7,000 lbs. of stone (total-weight of stone that is This gives the very large back-log of heat required 'for operation of the furnace. No additional coke blanks are necessary.

Third, begin charging in rounds as follows: Round 1, which is composed as follows:

' Pounds nace in regular rounds for normal fur- Round'2, which is composed as follows:

. Pounds Coke: 100% of that charged into the furnace in regular rounds'for normal fur-' nace operation 13,000 Stone: 60% of that charged into the furnace in regular rounds for normal fur-- nace operation 10 Ore: 40% of that charged into the furnace in regular rounds 'for normal fur nace operation 16,000

Total 33,200

Coke Stone Ore Total This last-is a regular round for normal furnace operation, andis reachedgradually, as indicated above. The increased burdening of the blast furnace is based. upon a constant weight of coke, thence increasing the weight of ore and stone in successive rounds. Decreased burdening is ac, complished by maintaining a constant weight of coke and decreasing the weight of stone and ore. Stated in another way, the percentages referred to in the above tables are percentages of the various quantities of the materials, coke, stone and ore, that are charged into the furnace in regular nace operation 13 000 rounds for normal furnace operat1on. Stone: of that chargedinto the fur- The charg1 ,b ld Operatlng Ofa blas 'nace 1n regular rounds for normal fur- 40 In accordance wlth the chemically balanced presnace operat1on -1.- 3,500 ent process 1s illustrated in the following table Ore: 25% of that charged 1nto the furnace (Table II), the values 1n the said table being n re u ar rounds for a furn e predicated on the use of hematite ore in natural operatlon-e 0, 0 hydrated cond1t1on andcontammg not less than 4 V Total -3 26,500 40 per cent won: 5 5

Table II 3 1 Blowper min. Charge per min. M t P d Hill 68 01.111 S per ggg carbon ggg i Blast gr. ton burned cu ft Pounds Pounds Pounds iron per day by blast carbon oxygen carbon iron 100,000 1,774 1,900 3,235 0.050 2,215 1,333 70.0 95,000 1, 030 1, 333 3,000 0. 035 2, 1, 207 70. 0 90,000 1,593 1,705 2, 900 0.725 1, 030 1, 200' 4 70.0 35, 000 1, 505 1, 003 2, 750 0. 705 1, 332 1, 133 70. 0 000 1, 420 1, 570 2, 530 0. 303 1, 730 1, 003 70. 0 75,000 1,330 1,472 2,420 0,900 1,033 1,000 70.0 72, 500 1, 235 1, 421 2, 335 0. 912 1, 592 900 70. 0 70,000 1, 240 1, 375 2, 200 0. 944 1, 525 932 70. 0 07,500 1, 195 1, 325 2, 0. 932 1, 433 900 70. 0 Gr. '1 05,000 1, 1, 275 2, 097 1.000 1,440 305 70. 0 3 I 02,500 1,105 7 1,225 2,015 1.042 1,335 330 70.0 00,000 1, 002 1, 1,932 1. 033 1, 330 300 70. 0 N. T 57, 350 1, 024 1, 135 1, 300 1.103 1, 231 700 70. 0 57, 500 1, 013 1, 125 1, 355 1. 132 1, 275 700 70. 0 55, 000 974 1, 075 1,772 1. 131 1,200 732 70. 0 52,500 r 923 1,025 1, 000 1.240 1,105 700 70.0 50,000 335 930 1,015 1.300 1,107 004 70.0 47, 500 340 930 1,532 1. 372 1, 054 031 70. 0 45, 000 790 332 1, 450 1. 444 1, 000 000 70. 0 42,500 752 333 1,371 1.535 950 500 70.0 40, 000 r 703 734 1, 291 1. 025 900 532 70. 0 37, 500 004 732 1, 210 1. 750 343 493 70. 0 7 35,000 020 033 1,129 1.375 775 404 v 70.0 32, 500 570 033 1, 050 1. 921 720 432 70. 0 30,000 539 533 974 2.107 005 400 1 70.0 27.500 435 541 333 2.334 v 010 303 70.0 25,000 442 490 307 2.000 555 330 V 70.0 22, 500 393 440 720 2. 930 500 300 70. 0 20,000 .354 392 045 3.250 445 205 70.0

Empirical 53. 5 51. 0 31.0 .75. 0 70. 0

dlVlSOl'S Norm-Divide O. F. M; blast by empirical divisors for desired duotients.

,operationandas they areoper i s h tll t i (that is, the pounds of oxygen in the wvind) in it balanced pre en unmoved pr cess for .m l rthe blast e "m riruic; 111g maxlmu the ma e en i V The following additional data, is presented as Table III showing furtherthe advantages arising from the c .r ioperation of blast iurnaces in accordancewith gg x the present inventi n. J'Ihis ope at ng ata wa D Western obtained on two blast {furnaces 'opera hat Donora, Pennsylvania. Blast furnacesNo. '1 .and' g wind per i 5 4, 6?, No. ZWere identical in all measurements. The 5 con soxygen per-min H, d c Pounds Garbo permlznum H 817 1,050 s m r w materlals w re used, e am ow g 01 carbon er m mte should bc 1, 1, g inventions were employed and the same operating f{" f f ff jjjjj 1,768 7 21100 p rs nnel w s' d ,11 bo h f a utilizing Grossgons iron per (my w 1 i}, the same technique and producing the same kind Minutes 075a?15511:: cg of iron under identical conditions,'inso-far as was 0 .2 1. it g V Cu. it. wind per gr. ton. 108,940 8 mss-lbleh method of the -pres-e-nt*mventl9p o P l); .0 caggg was used on furnace No. '1, butfurnace No. 2 x 'en 4 g I 01m 1,130 1,150 Was operated accord ng. to the conventional 1 01 carbon per gr. to method characterized by many blast volume g s; gggggggfig 'g '23 changes, extra coke charged ever fourth charge ounsoxygcn per gr. onsaving V l On. it.wind per gr. ton saving. 43,940 24,050 freqlilent burden 'F c On. it. wind per gr. ton savmg p. 0 27 ferencesm the operation Qf;the;two furnaces ar 1 2,404,451 9 10 intermediateblowingcapacitiesrequire the portantly, the operation of a blast, furnaceby respective, proportionate, intermediate weights of blowing it with a less total weight or blast metallic iron and carbon, as indicated herein; (pounds of wind perv .net ton of iron) than its Additionally, the following table (TableIII) total weight of, the solid charge gpoundsof wind I shows the complet chemical balan Of t e v a :15 per netton of all solids, ore, coke, 1' nd limefaetors conce n d in t o ion f t blast stone). per net ton of'iron, and that in thesolid .iurnaces shown in Tab s i g a a is charge t totai weigh-t of, amma iemambetween the conditions as they were in st da .tained greater than total weightof oxygen 7 shown clfifllly-byjlihg following on rat ng d ta.-

Iiable IV No.1 No.2

Period covered by the test July, 1942, 31 days July, 1942, 31 days. Total net tons pig iron (basic)- 24,552 19,778; 1 Total net tons pig iron per day 792 038. Total net tons pig iron increase/day 0. Total net tons pig iron percent/day 2 .10% 0. i I Cu. ft. blast per min 43,200(c0nstant) 40,500 to 46,500. Weight of blast per mi 3,305# (constant). 3098# to 3590#. Minutes per net ton pig iron.-. 1.83. r 2.28. Cu. ft. blast per net ton pig iron 92,380 to 106,700. Weight blast per net ton pig iron- 7,067# to 8,163#= 7,9961%. Weight of blast oxygen per net ton 1,640# to 1,894#=1,820#, Weight of solid carbon per net ton Aver. 1,56 Weight of solid carbon per net ton percent increase. 9.50 Weight of solid carbon ton pig iron burned at tuyres 1,l67#=74%. Wgt. oxygen req. for comp. combustion 1,550#. Excess blast oxygen 269#. Over-blast equal to 15,000 cu. ft, Wgt of solid chge. per net ton pig ll'Oll. 6, 0#; v Wgt of blast chge. per net ton pig ll'Oll '7,996#. Cu. ft. of blast per net ton pig iron 104,500.

Present improved Ram 1 I ra'ios Ratio burden wgt. of metallic mix to coke 1.91 2.25 to 3. 25 Ratio Wgt. of solid chge. to wgt. of blast 0.80 1.00 to 3.00 Ratio wgt. of solid carbon to blast oxygen 0.86 1.00 to 3. 00 Ratio wgt. of blast oxygen to reducible oxygen. 2.28 .75 to 1. 75 Ratio pounds of Fe per 1# C 1.17, Ratio pounds of Fe per 1# G percent increase 0. Ratio 00 to CO2 in top gases 2.26. Ratio reduction efliciency shown by top gase 1.00. Ratio overall thermal eflicieucy as shown by th tio of C to blast 0.62.

oxygen at the tuyres.

In accordance with the present invention, Summary of operations on No. 1 and No. 2 there is provided a, stabilized operation wherein blast furnaces at Donora, Pennsylvania, J mg, the reactive elements are all maintained in chem- 1942. I I r ical equivalency throughout the operation, pro- Both blast furnaces producing basic iron. 1 v V ducing substantially greater yields of metal with No. 1 blast furnace operated on present improved substantially higher efficiencies of operation and, method characterized by constancy. consequently, reduction in quantities of the re- No. 2 blast furnace operated on conventional actants, with attendant lowered costs. method characterized by frequent changes in It will be seen from the foregoing that the imblast volume and also in burden ratiodue to proved process .of this invention comprises, imthe charging of extra coke every fourth charge.

5 No.1 produced;

magic 1 v 24.10% more pig iron product, using 9.50% less solid carbon V 25,450 cu. ft. less blast and eliminated 15,000 cu. ft. of overblast thus demonstrating r 38.00% more overall thermal efficiency and 60.00% more reduction efiiciency as shown by the ratio of CO to CO2 in the top gases and falsoin 11.10% more metallic iron per pound of solid Carbon *flln'the present improved method, all the burden 1 I 1 ingredients are weighed in chemical equivalency against the fixedweight of oxygen that is contained in the chosen blast volume inithe ratio of i.1.33#.of'oxygen for each 1# of carbon that ar- *1 Obviously, these ratios are effective only on the fbasis of constant maintenance of the chosen blast j volume ancl'upon a--' constant burden ratio.

" Iii-the ---conventional' method, no similar or equivalent computations are provided, nor in- ;deed can they be because of inconstant blast vol-.

Iume and burden ratio, consequently, chemical f equivalency control is never attempted because {"of the many variables'that all conventional opfijatorsrbelive are nt l ble in blast furnace Operations. 4

, It maybe pointediout inthisconnection' that in customary operations blast furnaceefficiencyis based upon the ratio of net tons of pig iron per square foot'of hearth area, and the ratio of 2.65 is considered to be representative of-good practice.

-Inaccordance with the present invention, regular attainment of ratios of from between 2.90 and 4.90 is experienced by the use of the operating ratios'set forth above herein.

This application is a continuation-in-part of co-pending application No. 391,211, filed April 30,

I claim:

The method of operating a blast furnace for producing pig iron, which comprises preparing a charge including iron ore, coke and limestone, in-

troducing the resulting mixture into a, blast furnace, blasting the said mixture to metallic iron with a constant blast of superheated air, while maintaining a burden ratio greater than 2.25, but less than 3.25, a ratio of total Weight of charge to blast greater than 1.00, but less than 3.0, a ratio of total weight of fixed carbon in the charge to the free oxygen in the blast greater than 1.00, but less than 3.00, and a ratio of total weight of oxygen in the blast to reducible oxygen in the charge less than 1.75, but not less than 0.75,,the

said ore-being hematite me in natural hydrated condition and containing not less than 40 per cent iron, and maintaining a constant volume of blast through the tuyeres during the entire operation.

' STANLEY A. SANFORD.