US1695953A - Method of preheating the charge in shaft furnaces - Google Patents

Description

uw 18,1928. i 1,695,953V

A. L. FEILD METHOD OF PREHEATING THE CHARGE IN SHA-FT, FURNACES Filed April 25, 1924 SHO: nur.

Patented Dec. 18, 1928.

UNITED STATES PATENT OFFICE.

ALEXANDER L. EEILD, or JACxsON HEIGHTS, NEW YORK, AssTGNon To THE EINDE f Am PRODUCTS COMPANY, A CORPORATION or OHTO.

METHOD OF IEREEIEIATIIIG' TEE CHARGE IN SHAFT FURNACES.

Application led Apr125, 1924. Serial No. 709,012.

This invention relates to metallurgical operations conducted in shaft furnaces, or the like, and comprises an improved method of preheating vthe charge in the furnace.

The invention is particularly applicable in the blast smelting .of iron ores.. In the following description reference will therefore be made primarily to this use, but it is to'be understood that the improved method is advantageous in other metallurgical processes.

In standard blast furnace practice the charge fed downwardly through the stack receives heat from two principal' sources. These are the heat of combustion in the tuyre zone of the furnace and the heat of the air passing to the furnace from thev preheating stoves. Theflatter source of heat is a very important factor in the proper working of the furnace. When air is supplied from the stoves at 600 C. or above, according to usual practice, its heat content may amount to 20% or more of the heat produced by combustion in the furnace. About 75% of the heat of the air blast is carried b the nitrogen which it contains. The weig t o-f nitrogen passing through the furnace is approximately three times the weight of the iron produced. Nevertheless, the importance of the heating effect 'of this gas does 3G not appear 'to have been .accurately evaluated heretofore.

I have discovered that any material decrease in the nitrogen content of the air will result in a serious vdepletion of the thermal units available for preheating the charge.

A failure to recognize this fact has been responsible for the ditliculties encountered in the use of oxygen or oxygenated air in blast furnace practice. v

Such proposals have been made at various times during the last twenty years. So far as I am aware, however, no continuous in-y dustrial, use ,has been made of them. It is true that an oxygenated blast produces a higher reaction temperature in the smelting zone than is obtainable with an ordinary air blast, but the advantages of this higher temperature are largely discounted by the fact that the charge' is fed into the smelting zone without sufficient preheating. l* This defect is a necessary result of operating with an inadequate volume of hot gases ascending through Vthe charge, which requires a very large amount of heat to bring it to the proper temperature, Heat supplied from combustion of coke or other source should raise the temperature of the burden (iron ore and flux) to about 1500o C., or

above, before it enters the smelting region,

if the furnace vis to operate yto the best advantage. The burden undergoes no strongly exothermic chemical reaction. The ore must ordinarily be dried, and the limestone flux calcined, in the shaft. The cooling effect of the descending charge is augmented by the water-cooling of `the furnace in the tuyre zone.

A higher combustion temperature does not itself compensate for these heat losses, or insure proper preheating. By lowering the burden ratio (pounds ofv ore and flux to pounds of coke) the action may be improved somewhat, but such procedure is uneconomical. No substantial advantage is derived from an increased rapidity of descent of the charge, caused by quicker smelting. Any tendency in this direction is largely impeded by the greater heat absorbing capacity of the charge when fed at a more rapid rate.'

In accordance with the present invention, difficulties arising from insufficient preheating are avoided by passing into the furnace, at a point or points above the smelting Zone,

a hot gas'incapable of producing any substantial smelting of the charge.

. Nitrogen, or nitrogen in admixture with some oxygen, is preferred, but other suitable gases or Vgaseous mixtures may be used. Considerable heat may be safelyderived by the combustion of fuel in the charge by oxygen of the preheatinggas. The carbon monoXid so generated is advantageous, as it assists in the reduction of the ore. In all cases, however, the oxygen content of that gasshould be materially less than thatl of atmospheric air. An' air blast at a point above. thel ordinary tuyre level is to be avoided, as coke would be needlessly consumed in large quantities and the resulting fusion of the charge above the lower tuyres would have serious disadvantages. By using a gas low in oxygen I am able to avoid these difficulties.`

The preheating gas as it enters the furnace may contain some carbon monoxid or other reducing gases. However, I do not use top gases fromv the stack, nor other gases containing a' high Vroportion of the oXids of carbon, for pre eating the charge. Such gases are sometlmes recirculated for the pur- .furnace by the present process. The

vIn the accompanying drawings, illustra tive of the invention,v

Fig. 1 is a diagrammatic partial'vertical section through a blast furnace provided with preheating means and Fig. 2 is a diagram of a. blast furnace plant using oxygenated air and a preheating gas rich in nitrogen.

Referring to Fig. 1, numeral 1 denotes the shaft of a furnace of usual type, having a hearth 2, cinder notch 3 and metal tap hole 4. Tuyres 5 are installed in the customary manner.

To carry'out the purposes of this invfention,'one or more tuyres or sets of tuyres are provided, preferably above those already referred to. As illustrative, four tuyres 6 (of which three only are shown) are placed symmetrically somewhat above the tuyeres 5. This .location is deemed especially advantageous, as substantially the maximum preheating effect of the gases is obtained. However, the tuyres 6 may be otherwise arranged, if desired, both with respect to their level and'to their circumferential position.

The preheating gas (nitrogen, vnitrogenoxygen mixture, or the like) is heated to the proper temperature in stoves'and forced into the shaft under suitable pressure, say 12 to 15 lbs. per sg. in. The volume of pre-4 heated gas required may be readily determined theoretically for given conditions by means of published data familiar to those skilled in the art.

The smelting is preferably effected by heated air blown in through tuyres 5. However, .the present inventlon is e ually applicable to cold blast smelting. t has special advantages in connection w1 th the use of an oxygenated blast, as hereinafter -described in connection withFig. 2.

In that figure, reference numerals 5 and 6 denote as before the tuyres respectively serving to introduce the smelting and preheating gases into the furnaces. Stoves 7, 7 are respectively connected by pipe line 8 with tuyres 5 and by pipe line 9 with tuyres 6. While the stoves are indicated as units, it will be' understood that subdivisionsy are provided, as well understood in the art, to permit the heating of one section while another is receivin 4the air passingto the furnace. A plant or preparing from air a vfraction enriched in oxygen is indicated by ,numeral 10. 'Ihis fraction is passed through pipe 11 to stoves-7 and thence to tuyres 5. The residual fraction of the air, containing a high proportion of nitrogen, is passed through pipe 12 to stoves 7 and -thence to tuyres 6. A blowing engine forces the air fractions to the stoves. I A preheating gas is led to the stoves from the blast furnace stack as indicated or from some other suitable source.

Important improvements in the operation of the furnace are produced by this distributed introduction of the air. The tempervature inthe hearth and bosh regions is very high, owing to the augmented oxygen concentration of the smelting gas. Hence the rate of smelting, and consequently the rate ofl feed of the charge, is much increased. The charge is properly preheated by the non-smelting gas, so that thesmelting takes place without' the handicap of a' cooling charge. The output of the furnace per' unit weight of fuel consumed is materially. raised by the new process.

There are also numerous incidental advani tages inherent in the process. For example, `it is practicable to use low grade ores or flux which can not be satisfactorily worked known. I generally prefer to have at least 30% of oxygen by weight in the smelting gas, but more or less may be used under particular conditions. If this gas is prepared from air, the residual gas will contain about 18% of oxygen, and is excellently suited for the preheatin gas. The advantages of the invention, as gdescribed above, are in no way dependent upon the fractionation of air to which reference has been particularly made. Oxygenated and nitrogenated gas or the like may be obtained from any suitable source. The preheating gas, as well as the smelting gas, may be dried, if desired. While I regard the sensible heatl of the gas blown in at the upper tuyre level as the primary source of heat, more or less heat may be generated various other connections, with or without modification, and is therefore defined broadly in the appended claims:

I claim: 1. Process of preheating the charge in shaft furnaces, comprising passing a hot,

non-smelting gasl ycomprising an oxygenfl nitrogen mixture in whlch the ratio of oxy- -gen to nitrogen is lessthan that in atmosi pheric air into the furnace above the smelting zone. f

2. Process of preheating the ore-iluX-carbon charge in a blast furnace, comprising lpassing into the furnace another gas, comprising an oxygen-nitrogen mixture contalning less oxygen than atmospheric air, but carrying sufficient heat to preheat the charge at least in part.

4. Process of `operating shaft furnaces,

comprising passing into the charged furnace a gas adapted to produce the desired reduction reaction in the charge, preheating another gas, comprlsing an oxygen-nitrogen mixture contaimng less oxygen than atmospheric air, and passing it in heat-interchanging relation to the charge.

5. Process of operating shaft furnaces, comprising passing the furnace charge downwardly in Contact with an ascending current-of a hot gas introduced into the furnace at a pointl above the site of the main reaction therein, said gas being substantially incapable of producing the desired reaction in the charge and. comprising an oxygennitrogen mlxture containing less oxygen than atmospheric air, and separately introducing'adjacent to the base of the furnace a gas adapted to effect such reaction.

6. Process of operating blast furnaces for the smelting of ores, comprising passing into the charged'furnace heated air divided into a plurality of fractions with different contents of oxygen, the portion high in oxygen passing to the smeltlng zone, and the portion low in oxygen ascending through the charge to preheat the same.

7. Process of operating blast furnaces for the smelting of iron ores, comprising passing into the charged smelting zone a gas containing a proportion of oxygen higher than that in atmospheric air, and passing through the bulk of the charge a hot gas containing a proportion of oxygen lower l than that in. atmospheric air and ha :ing low reducing power.

8. Process of operating blast furnaces for the smelting of iron ores, comprising pre'- paring from air a gas containlng a materially increased proportion of oxygen, passing said gas into the charged smelting zone of the furnace, prehcating the residue of the air treated, and passingl it through the charge, whereby the charge is heated to the proper temperature before entering the smelting zone.'

In testimony whereof, I ax my signature.

ALEXANDER L. FEILD.y

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