US2201181A - Method of decarbonizing carbonholding iron without melting - Google Patents

Description

y 1940- B. M. s. KALLING METHOD OF DEGARBONIZING CARBON HOLDING IRON WITHOUT MELTING Filed Oct. 15, 1937 Patented May 21, 1940 UNITED STATES METHOD OF DECARBONIZING CARBON- HOLDING IRON WITHOUT MELTING Bo Michael Sture Kalling, Djursholm, Sweden Application October 15, 1937, Serial No. 169,193 In Sweden October 19, 1936 11 Claims.

My invention refers to improvements in methods for producing for instance soft iron grains to be used principally as a substitute for ordinary low carbon scrap when making steel in open hearths and electric furnaces.

The invention has for its objects: first, to provide a cheap and rational method for decarbonising high or low carbon iron or other carbonholding metals and alloys; second, to provide a method of decarbonising iron according to the equilibrium diagram of FeC-O when heating iron at certain temperatures in an atmosphere of a given composition; third, to provide a method for decarbonising iron and other metals at a temperature below the melting point and substantially without oxidation of the metal; fourth, to provide a method, adapted for producing grains of iron holding 0.05% of carbon and less; fifth, to provide a method for decarbonising pigiron by means of heating grains of pig-iron in a special atmosphere and supplying substantially no other heat than that produced by a suitable combustion of the carbon in the iron which is to be decarbonised; sixth, to provide a substantially continuous and more or less automatic process for decarbonising carbon holding metals and particularly pig-iron.

The said objects of my invention and the means for attaining the desired results will be fully described here below.

The method of decarbonising for instance pigiron without melting it, which has been employed for producing cast malleable objects, consists in packing the cast iron parts in boxes or chambers together with certain oxidising materials such as hematite ore and then heating the charge during about a week's time, gradually raising the temperature to 900 to 1000 C. This old method is, however, not suitable when it is desired to rapidly decarbonise finely subdivided pig-iron or other metals because of the time of annealing being entirely too long on account of the poor heat conducting ability of the charge and also because of the process of annealing in this way being discontinuous and, consequently, too expensive and very cumbersome and unpractical. It is also impossible to decarbonise sufficiently low for many important purposes, where a minimum of carbon in the product is desired.

To make the method economical as a step in the ordinary process of steel making it is necessary, that the heat required for the process may be applied without causing an uneven temperature in the charge and, furthermore, that the process is sufliciently labour-saving and continuous, so that the running costs of the operation are sufliciently low.

To comply with these conditions it has been found necessary to keep the charge of small grains in motion in one way or other during. the heating treatment. A rapid removal of carbon,

according to practical experience, requires that the temperaturev be kept as high as possible, for instance at about, 900 to 1200 C. It is, however, difiicult at this temperature to'prevent that a mixture of pig-iron grains with oxygen producing materials such as iron ore concentrates sinters together to big lumps or sticks to the furnace walls or boxes, which will make an undisturbed, continuous feeding of such a charge diflicult or even impossible.

According to the present invention carbonholding iron or other metals are not decarbonised by means of solid, oxygen-producing materials, which are mixed into the charge but by means of introducing gaseous oxygen in a free or combined state into the decarbonising furnace. This makes it unnecessary to mix the grains with other, more easily sintered substances and the temperature may be kept higher without causing any serious disturbances. Another advantage is that the soft, grainy product of for instance iron does not contain any oxygenholding substances and is free, or practically so, from other impurities, that might, eventually, have been introduced with, for instance, the iron ore, if such were used as decarbonising means. A perfectly pure, metallic product may, consequently, be obtained according to this invention. The oxidising gases must, however, be admitted' in such a way, that carbon is removed without the iron being at the same time oxidised. In case the surface of the grains is substantially oxidised, they will more readily stick to each other and big lumps of sintered grains may be formed. A much poorer product will also be obtained in such a case. Free oxygen should, thus, not come in direct contact with the iron grains but should be previously changed to carbon dioxide. The reaction, according to which a charge of pig-iron particles is being decarbonised, must, thus, principally follow the equation: C+CO2 2CO.

But even carbon dioxide is strongly oxidising on the iron, unless the gas contains at the same time a considerable quantity of carbon monoxide. In order to cause a gas consisting ofcarbon dioxide and carbon monoxide to decarbonise for instance common pig-iron without simultaneously oxidising the same, it is a condition that the content of carbon dioxide is less than about 25% at 1000" C. and if the gas in the furnace is diluted with other gases such as nitrogen, hydrogen or steam of water, there must be still less of carbon dioxide.

As the process of decarbonising is endothermic, heat must be supplied to the charge. It has, however, been found possible to make the process of removing carbon from iron exothermic by supplying free. oxygen, for instance by admitting air into the furnace in which the decarbonising is being effected. The oxygen must, however, be supplied in such a way, that it is mixed with the CO-gas produced by the carbon of the iron and is, thus, completely changed to CO2, before it gets an opportunity to attack the iron. The

resulting reactions will then substantially follow the equations: C+CO2=2CO and CO+O=CO2.

The production of heat according to these reactions is sufficient for the technical employment of the process, if the heat losses may be kept low enough by a careful heat insulation and other precautions. If, on the contrary, the oxidation takes place by the use of iron ore, which must be simultaneously reduced, the process will under all circumstances be endothermic, demanding, thus, a relatively big heat supply from the outside, which is one of the reasons, why decarbonising in that way is rather difiicult. Normally the reaction gas should be completely combusted to CO2, before it leaves the furnace, in which the iron is being decarbonised. If, however, the heat produced is insufflcient for both heating and decarbonising of the iron, the production of heat may be increased by supplying some suitable fuel, for instance carbon powder, oil or a gas. Blast furnace gas may advantageously be used for such purpose. It is possible also to use electric heat for supplying additional heat if required. In such a case electrically produced heat should be supplied in the first instance to the zone of final reaction, where it is most difficult to maintain the required temperature by means of the combustion of the reaction gas alone and without causing the furnace atmosphere to become too strongly oxidising.

An additional heat supply is needed, especially when it is required to decarbonise for instance common, soft scrap holding about 0.25% of carbon down to still lower values, whereby the carbon content of the charge is not high enough to produce the required heat supply, even under favorable conditions.

In order to accelerate the reaction it may be suitable to supply hydrogen to the furnace, either in free state or in the form of steam.

Several different types of furnaces may be employed for the process. Decarbonising will for instance be effected by using a furnace of the pyrites roasting type, in which the charge travels in a counter direction to an upwards rising flow of hot gases of suitably composed amounts of CO and CO2 etc.

The process is, however, in general effected in the simplest wayin a substantially horizontal or slightly inclined, rotating furnace, for instance of the cement burning type but suitably modified.

It has been found, that it is possible to decarbonise iron at temperatures above 1000 C. in such furnaces without any tendency of the grains of iron to stick or ball together, even at about 1200 C. in case the iron grains are smooth and rounded and not oxidised. A continuous feeding of the charge along a rotating furnace is easily effected and the rotation causes a very eflicient and desirable stirring of the grainy charge, so that a reasonably uniform temperature without undue overheating will be attained, even if the charge is being heated only by combustion of the reaction gases above the same.

The accompanying drawing shows in Fig. I a longitudinal section of a rotating, tubular furnace, in which heat is produced by combustion of the reaction gas and, eventually, of some other fuel, for instance blast furnace gas, which is admitted from the outside. Fig. II shows a crosssection at AA in Fig. I and Fig. III is a crosssection at BB in Fig. I. The charge is introduced at an opening I and is passing along the furnace, which is being rotated sufficiently fast to cause a rapid and efficient mixing of the charge, the oblique surface of which is being exposed to the immediate action of the hot, decarbonising atmosphere as indicated in Fig. II.

Before the iron grains are discharged at the opposite end of the rotating furnace they will have to pass through a special container 2 of a kind, that is well known in the art and so arranged, that the grains but not thegases may escape and pass continuously over a lip through the central discharge opening at the same rate as at the entrance end of the furnace. By means of outside cooling of the locking device 2 the temperature of the iron grains may be lowered sufficiently to prevent oxidation of the surface of the particles, if this is especially desired.

The air required by the process is introduced through a pipe 3 and the furnace gases are passing in an opposite direction, leaving the furnace at the opening I. It has been found somewhat difiicult to transmit the heat from the gases in the furnace in an efficient way without making the furnace rather elongated. In order to improve upon the heat transmission to the charge a plurality of separating shields 4 may be advantageously arranged crosswise in the furnace, pref erably at the entrance end of the furnace chamber. The shields may be provided with openings 5, which permit the gas to pass along the furnace and are so disposed, that the gas is compelled to change direction of flow during its passage through the furnace. This arrangement will cause an efiicient transmission of heat from the gas to the separating shields, which in their turn on account of the rotation give off the stored heat to the charge. The mixing of the charge on account of the rotation is favorable to a uniform heating being attained, but it might cause also a mixing in the lengthwise direction as particles, that have already been decarbonised, might move backwards to the entrance end, while other particles might pass along and leave the furnace before they have been sufficiently decarbonised. The capacity of the furnace and the efficiency of the process might, thus, be lowered in a certain degree. To diminish this difficulty it may be suitable to provide a plurality of separating shields 6 also in the reaction zone. Said shields are provided with holes and are intended to control the movement of the charge so that the mixing in the lengthwise direction, mentioned above. is prevented.

The V combustion air is introduced through openings 1 on the pipe 3, which are so many and so disposed. that the supply of oxygen does not at any place become great enough to cause a substantial oxidation of the iron grains. In case an additional supply of gaseous or other fuel is requircd, this may be effected for instance through a pipe 8 at the discharge end of the furnace or by means of a pipe parallel to or inside of the pipe 3. The grain size of the iron is of a con-- charged from a blast furnace or an electric reducing furnace.

It is desirable that the granulated iron is not any coarser than required for passing a sieve having 5 mm. meshes.

Although the decarbonising of iron according to the present invention is to be effected principally by means of gaseous oxygen, it is not excluded that a simultaneous oxidation by means of mgen in a solid form, such as for instance an oxide of iron is effected. It may be of advantage to add a certain quantity of an iron ore concentrate to the iron grains, although the amount thereof must be so small, that the oxide becomes reduced fairly rapidly and, thus, is prevented from disturbing the main process, which is characterized by the oxygen being admitted as a gas and preferably in the form of free oxygen, for instance with air.

The process is not limited to the decarbonising of pigiron but may be advantageously employed also for decarbonising other kinds of iron low in carbon and finely subdivided such as steel granules, turnings, sponge iron and the like. It is, however, especially important and valuable, that decarbonising according to the present invention may be efi'ected not only very rapidly but also down to contents of carbon, which are considerably lower than has been possible to attain by means of any industrial decarbonising method so far known in the art. It has, thus, been ascertained, that if the iron grains are heated to 1200 C. in an, atmosphere of CO and C0: in such proportions, that the carbon in the iron but not the iron itself becomes oxidised, a rapid removal of carbon to below 0.01% is quite possible. If the gas mixture is diluted with hydrogen or nitrogen or both, or if a vacuum is maintained in the furnace chamber, the percentage of carbon may be lowered still more according to the theories.

An increase of the temperature up to about 1200 C. facilitates the decarbonising process but the use of a still higher temperature does not seem to be especially desirable.

I claim:

1. Process of decarbonizing ferrous metals which comprises feeding a carbon containing ferrous metal in the form of granules into a reaction chamber, maintaining a mass of said granules in the reaction chamber, agitating said mass to expose fresh surfaces, discharging granules from said reaction chamber, maintaining a temperature within the reaction chamber of from about 900 C. to 1200 C., maintaining in the raction chamber in contact with the mass of granules an atmosphere containing CO and CO2 in such a ratio that the carbon content of the granules is oxidized without substantial oxidation of the metal and introducing an oxygen containing gas into said reaction chamber at a point or points removed from the surface of said mass of granules so that the oxygen content of the gas reacts only with the CO flowing from the surface of said mass to maintain said ratio of CO to C02.

2. Process as defined in claim 1 in which the temperature within the reaction chamber is maintained by the exothermic reaction of the introduced oxygen with the carbon content of the metal.

3. Process as defined in claim 2 in which a fuel is introduced into the reaction chamber for reaction with the introduced oxygen to assist in maintaining the temperature within the reaction chamber.

4. Process as defined in claim 2 in which the ratio of CO2 to CO in the gas atmosphere within the reaction chamber is less than 1 to 3.

5. Process as defined in claim 2 in which the granules are less than 5 mesh in size and in which their carbon content is reduced to less than 0.1%.

6. Process as defined in claim 2 in which the ferrous metal granules introduced into the reaction chamber contain at least 2.00% of carbon.

7. Process for decarbonizing a ferrous metal which comprises introducing granules thereof into the upper end of an inclined rotary reaction chamber, accumulating a body of said granules in the chamber and preventing unequal forward movement .of different portions of the mass by interposing heat conducting transverse baflies within the chamber, said baflies serving to carry heat from the gases in the chamber into the mass of granules, discharging granules from the lower end of the reaction chamber, maintaining a temperature of from 900 C. to 1200 C. within the reaction chamber, maintaining a gas atmosphere within the reaction chamber in contact with said mass of granules consisting essentially of CO and CO2, the ratio of CO: to CO being not greater than 1 to 3, and introducing an oxygen containing gas into said atmosphere in such a way that it mingles with said gas atmosphere before contacting said mass of granules.

8. Process as defined in claim 7 in which the oxygen containing gas introduced into the reaction chamber .is distributed along the length thereof and gas is discharged from the chamber adjacent the upper end thereof.

9. Process as defined in claim 1 in which the granules have rounded surfaces.

10. Process for decarbonizing a ferrous metal which comprises introducing granules thereof into the upper end of a slightly inclined rotary reaction chamber. accumulating a body of said granules in the chamber and discharging granules from the lower end of the reaction chamber, maintaining a maximum temperature of from 900 C. to 1200" C. within the -reaction chamber, maintaining a gas atmosphere within said mass of granules containing C0 and CO2. the ratio of CO: to C0 being not greater than 1 to 3, and introducing an oxygen containing gas into said atmosphere in such a way that it mingles with said gas atmosphere before reacting with said mass of granules.

11. Process as defined in claim 10 in which the oxygen containing gas introduced into the reaction chamber is distributed along the length thereof and gas is discharged from the chamber adjacent the upper end thereof.

30 MICHAEL STORE KAILING.

CERTIFICATE OF CORRECTION.

Patent No. 2,201,181 May 21, 191 .0.

BO MICHAEL STURE KALLI'NG.

It is hereby certified that error appears in the printed specification of the above nunbered patent requiring cerrection as follows; Page 5, first 'column, line 611., claiml, for "reaction" read "reaction"; same page, seeclaims 5, 1L, 5, and 6 respectively, d --1; and that the said Letters ion therein that the same may con- 0nd column, lines 9, 11;, 17, and 21, for the claim refehenc e numefal 2' rea Patent should be read with this correct form to the record'of the case in the Patent Office.-

Signed and sealed this 25th da 0r June, A. D. 19L 0.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.

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