US2622977A - Desulfurization of iron and iron alloys - Google Patents

Description

Dec. 23, 1952 B. M. S. KALLING ETAL DESULFURIZATION OF' IRON AND IRON ALLOYS Filed Dec. .28, 1949 \\N\\\ mu\\\\ Patented Dec. 23, 1952 DESULFEJRIZATION F IRUN AND'IRON ALLOYS Bo Michael StureY Kaliing, Peiny ke Christel' Danielsson, and Ottar Johan Lorens Dragge,

Domnarvct, Sweden Application December 28, 1949, Serial No. 135;?66 In Sweden N ovelnber 14, 1947 11 Claims. 1

This application is a continuation-in-part with respect to our copending application Serial No. 85,704, led April 5th, 1949, now abandoned, and relates back thereto with respect to all common subject matter'.

This invention relates'to a method of desul'- furizing metals and alloys andv has particular reference to the desulfurization of pig iron.

It has previously been suggested, e. g., in U. S. Patent No. 2,258,850, to desulfurize iron and iron alloys in such a manner, that the molten metal is treated with av solid desulfurizing agent (lime) While heating directly with combustion gases in a substantially horizontal rocking orl rotating furnace. The treatment is carried out tov an end temperature of about 14004500'ov C. and' the desulfurization is thereby performed in so short a time as about 1-2 hours. During the desulfurization also other constitutents of thev starting material such as Si, Mn and P can be removed.

One object of the present invention is to improve the desulfurization so that it canJ be carried out in much shorter time of the order of only a fraction of an hour, thereby enabling the process to be" carried outwithout supplying heat other than that already present in the melt to be treated.

Another object is to carry through the desul'- furizing process at a considerably lower temperature than has hitherto been used.

In order to achieve the above objects andother advantages which will bel evident from the following specication, the invention is based on the discovery made by the inventors during extended research in this technical field that the reaction between fine granular desulfurizing oxides of metals taken from the group consisting of alkaline earth metals and magnesium and the sulfur content of a pig iron melt is substantially instantaneous but that the activity of the said oxide grains rather rapidly decreases with time in the temperature range in question, i. e. between about 1300 and 1400 C., even if the detrimental low melting slagconstitutents such as silica are not formed during the reaction.

Considering the older knowledge and the said discovery made by the inventors, the invention can be said to consist in the following measures.

1. Using the desulfurizing oxide in solid (dry) pulverized form.

2. Keeping the temperature with certainty below such a level at which sintering of the desulfurizing oxides and the corresponding suldes might occur, i. e. below 1400" C., and preferably when treating pig iron within the limits 1350-1300". C

3. Avoiding such oxidizing conditions which would cause formation of easily sintering slag constituents such as silica. In this connection it must be kept in mind that even carbon monoxide can act as oxidizing agent for silicon in the metal at the temperatures in question andV that consequently reducing or heating gases containing'v carbon oxides andy especially carbon dioxide should be avoided.

4. Creating a rapid contact between the diiferent desulfurizing grains and the melt by vivid and effective stirring in order to make use of the instantaneousreactionabove mentioned and thusY to complete the desulfurization without supplying external heat and before the activity of the desulfurizing oxides is appreciably decreased.

The measures according to points 2 and-3 above have the same object, viz. to avoid damaging of the surface of the desulfurizing grains, inthe rst case due to sintering of the surface layer themselves and in the-second case due to coating the grains with a layer of easily melting. constituents such as silicates.

When desulfurizing pig iron containing silicorr with an oXidic desulfurizing agent the formation of` silica can of course not be avoided as is seen from the following reaction formula:

However, if the addition of lime is not too small and the original sulfur content of the pig ironto be treated not too high the quantity of silica formed according to the above. equation is so small that it has no appreciable detrimental effect on the course of the process. If for instance it is supposed that the pig iron is refined with a lime quantity of 1 per cent of the weight of the iron and that 0.050 per cent of S is removed, the lime charge at the same time theoretically absorbs a percentage of silica of about 5 per cent, corresponding to a reduction of the silicon percentage of the pig iron by about 0.025 per cent. A silicon dioxide percentage in the lime of this order is, of course, not insignificant, but, if the lime is very finely grained so that the total surface of the particles becomes suciently great, it is possible to carry out a quick and effective removal of the sulfur up to said silica content in the lime and even somewhat more at a relatively low temperature. On the other hand it is evident from the above discussion that the further increase in silica should be avoided. Therefore, according to the invention it is necessary or desirable to carry out the reaction under non oxidizing conditions. Since carbon monoxide is eiective as an oxidizing agent for silicon at the prevailing temperature as was already stated above it is evident that the desulfurization must be carried out without the addition of heat by means of combustion gases. Thus, it is not suicient to maintain a reducing atmosphere in the reaction chamber by means of carbon monoxide containing gases but all gases containing oxygen which might react with silicon under the prevailing condition shouid be excluded. This action of course is more important at higher than at lower silicon contents in the melt to be treated. In such cases where carbon is the most easily oxidized constituent of the iron bath said carbon takes over the role of reducing agent in the process and no formation of solid or fluid oxides will then take place, but for a rapid and effective removal of sulfur it is also in such cases advantageous to use a non oxidizing atmosphere.

According to the invention the desulfurization is to be carried out in a substantially closed reaction chamber in order to prevent access of oxygen in any form which might cause formation of easily fusing slag constituents. In this connection, however, it should be kept in mind that when a substantially closed reaction chamber is used it is not always necessary to replace the air prevailing in the reaction chamber at the time of charging by a reducing atmosphere, as the oxygen content of said air at the desulfurizing temperature is too small in relation to the weight of the charge to be treated to create any appreciable oxidation of silicon. In this case the oxygen content of the air in the reaction chamber is consumed by reaction with the charge leaving an atmosphere consisting essentially of nitrogen.

Of course it is diiicult to perform the desulfurization at a high temperature of the pig iron in cases where the heating of the iron can not be carried out during the treatment by direct introduction of combustion gases. However, according to the invention such addition of external heat has been made superfluous, due to the very rapid reaction created, which is a very important feature of the present invention. The necessary reaction time according to the invention as a rule does not exceed about minutes.

The formation of silica may, to a certain extent, be counteracted by supplying hydrogen which, in an atmosphere free from water vapour, is a somewhat stronger reducing agent than silicon at the range of temperature in question, at least at a moderate silicon content in the iron. Furthermore, it has proved that the presence of hydrogen results in an increase of the reaction speed. The hydrogen may be added in pure form but also in the form of oil, natural gas or other hydrocarbons which are being cracked at the prevailing temperature. The hydrogen may also with advantage be added in the form of ammonia.

The effect of the reaction can also be increased by adding small quantities of other reducing agents having higher affinity to oxygen, such as forinstance aluminium or magnesium. An addition oi calcium carbide may also with advantage be made to the oxide powder. An added quantity of up to 0.1-0.2 per cent Al or Mg and about the same quantity of calcium carbide has a good effect on the sulphur removal.

The process is preferably carried out at the highest temperature that can be used while retaining the lime 'charge in powdered condition during the whole operation. A temperature of 1300 to 1350 C., or up to about 1400o C. at the most, has proved to give the best results. Higher temperature than 1400" C. should not be used.

The lime used in the process should not be too hard-burnt as the reaction capacity can thereby be strongly reduced. Furthermore, the contents of carbon dioxide and hydrate Water in the lime should be as low as possible. The

f particle size shall be small. rihe crushing, when such is necessary, should be made to below 28-35 mesh of the main part of the lime.

It has proved possible to increase the reactivity of the lime by admixing a small quantity of soda or some other alkali metal compound to the charge. As alkali metal compounds are easily fusible, the addition must not be too great, because in this case the lime charge may sinter together. The best result is obtained if lime and soda are heated together at about 800 C. before use. The addition of alkali metal compound should normally not exceed about 5-10 per cent of the lime quantity, but exceptionally somewhat higher admixtures may be advantageous. When soda is added, the temperature of the pig iron may advantageously be kept somewhat lower than when lime only is used.

It has appeared advantageous to mix the lime with some pulverized carbonaceous material preferably with a low ash content in order to reduce the risk of sintering of the lime powder if it is desired to carry out the process at the highest possible temperature. Carbon materials developing gases rich in oxygen during heating should be avoided. The carbon quantity may be varied as required. Addition of more carbon than half the lime quantity is only occasionally desired, and usually no carbon addition at all is necessary when the gas atmosphere is made quite nonoxidizing. When the iron to be treated is high in silicon, especially when the silicon content is about one per cent or more, there will be a separation of graphite from the bath during the process that has been found to be very advantageous for the carrying out of the process.

It might occur that the iron to be refined from sulphur has not the temperature desirable for carrying through the process effectively. The iron can then be pre-heated by blowing oxygen gas into the iron container. This is preferably carried out while the iron is in the transport ladle immediately before the sulphur-refining process. The oxygen gas burns primarily the silicon of the iron and thus the percentage thereof is at the same time reduced, which is often valuable for the use of the iron in the subsequent steel process. A more or less complete elimination of the silicon can also be advantageous for the removal of the sulfur because the formation of silica thereby is prevented. It has proved that the desulfurizing capacity of the lime has a tendency to increase when the silicon percentage of the iron is reduced to values below about 0.2%.

The removal of the sulfur may be carried out in furnaces or containers of different kinds. A natural and effective way of carrying out the process is to use a revolving furnace with a horizontal or substantially horizontal axis. The iron and the lime may in this case be charged and discharged discontinuously for example through central openings in the end walls of the furnace. These openings should be closed during the process so that air or some other gas containing oxygen can not enter the furnace. In case hydrogen or some other gas containing hydrogen is to be supplied during the process this is preferably effected through a pipe inserted through one of these central openings. The gas is then preferably discharged at the opposite end of the furnace. In order `to facilitate the tapping of the furnace when the reaction is completed, the furnace should be tiltable also in the axial direction, and in order to facilitate the complete discharge of the lime charge the end walls should preferably be of conical form.

In order to obtain an effective stirring the speed of rotation of the furnace usually has to be very high. In a furnace having an internal diameter of 1.5 meter, smooth Walls and a charge to about 1/3 of the total Volume, a rotation speed of 40 revolutions per minute corresponding to a peripherie speed of 3 meters per second has proved satisfactory. It is of importance that the reaction takes place quickly not only because it saves time but also for the reason that the reactivity of the lime is quickly reduced during the treatment. Experiments have shown that Iat all events the reaction capacity of the lime usually rapidly decreases after a time of treatment exceeding about minutes. As a consequence it has been possible to save lime and/or get an improved sulfur refining if the lime is added in several intervals during the operation, the lime already used in the furnace preferably being removed before each addition.

n order to improve the mixing action the furnace interiorly may be given a section deviating from the circular and the walls may be provided with cams or recesses causing iron to be raised from the bath by the rotation, whereafter it may drop back through the lime charge. The furnace can also be rotated vat such a high speed that the molten iron will form a layer around the whole furnace or up to the top of the furnace, from where it will fall down vertically through the oxide layer.

Especially when treating large quantities of pig iron it may be ladvantageous to choose a continuous process, i. e. to let the pig iron continuously or possibly in portions pass through the furnace in which a suitable lime quantity has been charged or may be added continuously during the process. In this case the furnace can be of rather small dimensions in relation to the total quantity of pig iron to be treated. If the process is carried through continuously it is convenient to use a rather long furnace, which advantageously can be put in between the blast furnace and the pig iron ladle, or possibly between the mixer and the transport ladle to the steel furnace.

An important advantage, which is most easily reached through a continuous or semi-continuous performance of the process is that in that case the process can be carried through with a relatively large quantity of lime in the furnace in comparison with the quantity of pig iron present at the same time. It has proved difficult to obtain a good stirring of the lime charge without using a very high rotation speed if the lime appears only as a rather thin layer on top of the pig iron bath in the furnace. A much quicker reaction is obtained if the lime charge lls so large a part of the furnace volume that the mixing of the lime is more directly affected by the rotation of the furnace. In order to obtain a good result in this respect the lime charge should preferably iill at least about 15 per cent of the furnace volume. The lime charge may occupy from 15% to 30% of the furnace volume. At the same time the quantity of pig iron present in the furnace should be small and preferably not exceed 10% of the furnace volume. Under such circumstances the lime must of course be kept in the furnace a considerably longer time than the pig iron, which may be done by means of a suitable sluice arrangement allowing a continuous tapping of the pig iron but at the same time substantially preventing the discharge of lime from thel furnace. A continuous process can, however, also be performed in a furnace tiltable in axial direction `and in which the openings for charging and tapping are placed in the center of the two end walls of the furnace. The tapping of the pig iron is then preferably not carried out quite continuously but at suitable intervals by tilting the furnace whereafter a new portion of pig iron is immediately charged. The lime can then to the desired extent be kept back in the furnace making it possible to obtain a high lime level in the furnace during the operation compared With the level of the pig iron.

When carrying out the process in a rotating furnace, and especially when charging the furnace with lime to a relatively high level the iron may with advantage be continuously fed into, the furnace, possibly divided into dierent jets, above the lime charge which by rotating the furnace is kept in strong motion. The iron will then sink in the form of fine drops through said charge and collect at the furnace bottom from where it is discharged continuously or discontinuously from the furnace, while the lime may be more or less retained e. g. through a suitable sluice device.

The action of the lime may be furthermore improved by carrying out the process in several steps or preferably by letting the lime and the iron flow in countercurrent. For this purpose a rotating furnace with horizontal or substantially horizontal axis may be used. The iron is supplied into the furnace at one end through `a, central, relatively wide opening and is lcaused to flow slowly through the furnace to the discharge end where it is tapped o over a sill at such a level that a suitable, not too great depth of the iron bath is maintained in the furnace. The lime is fed through the furnace in the opposite direction, which is made possible through a sluice device preventing the lime from going along with the iron out of the furnace, and further by keeping the level of the lime in the discharge end of the furnace higher than the overflow in the feeding end. During the rotation of the furnace the lime will then Wander in countercurrent to the iron, while effectively removing the sulfur from the metal.

The lining of the furnace may preferably be basic but can also be acid. The lining material has little influence upon the course of the process due to the advantageous fact that no liquid slag is formed during the treatment which can attack the furnace lining. Ordinary reclay has for example proved to be usable as lining material in spite of the basic material to be handled.

An apparatus for carrying out the process according to the invention with continuous operation and counter flow is shown in the `accompanying drawing.

In the drawing Fig. 1 is a central axial section, and

Fig. 2 is a section on the line II-II of Fig. l.

The iron flows continuously into the furnace through the chute a, passes through the furnace and is continuously discharged at the other end of the furnace, where it flows over a sill b after having passed the partition wall c through the sluice opening d. Fine granular lime, possibly mixed with some pulverized carbonaceous material, is fed through the apparatus f in to the furnace inside the partition Wall c, is caused to pass through the furnace in counter-current to the iron and is discharged through the rotation of the furnace over the sill g. In order to prevent the lime from going along with the iron out of the furnace, the sluice channels d are not parallel With the axis in this case, but are directed obliquely to said axis in the manner shown in Fig. 2. During that part of a revolution at which the sluice opening contacts the lime layer, the lime thus Will not be able to enter and pass through said opening.

As examples of the results which may be obtained by desulfurization in a rotating furnace according to the invention, the following experimental trials may be mentioned. The trials were made in a rotating chamotte lined furnace with smooth walls with an interior diameter of 1.5 meters and an interior length of likewise 1.5 m. The treatment was made discontinuous. During the treatment the furnace was completely closed. No addition of soda or gas containing hydrogen was used.

Table 8 plied through one or more pipes which are immersed in the bath. Within the temperature range in question it has been possible to use common non-insulated iron pipes for the supply of the gas, without any considerable consumption of the pipes.

An intimate Contact between lime and metal can also be achieved by blowing the gas only into the lime charge in order to get a sufficient stirring action in same-a way that is especially advantageous when the process is carried out in rotat ing furnaces.

After the treatment the lime charge usually contains a considerable quantity of iron substantially in the form of small rounded grains. These grains can, to a great extent, be separated off through a simple wind separation of the lime charge and thereafter, if desired, be directly returned to the iron bath. If a more effective separation is desired, it is suitable to carry out same magnetically after the charge has been cooled down sufficiently, possibly in connection With a grinding operation. However, it has appeared that almost no iron grains at all remain in the lime charge if same contains soda or some other alkali metal compound in a content of e. g. up to l per cent.

The lime containing sulfur which is obtained as a reaction product in the process, may be used for other purposes, e. g. as an addition at the sintering of iron ore for the blast-furnace process, the sulfur substantially being driven off as SO2 and S03.

Besides being used for desulfurizing molten pig Heat Rotation speed, R. I. M Weight of pig iron Additions:

burnt lime coke powder 9 Lime in percent of pig iron wexght. Starting temperature Temperature drop during treatment. Pig iron analysis:

about 40 C...

about 3.94% 1.421.45% about 1.25%. about 0.05%...

Sulfur content of the p After 0 minutes After l minute. After 2 minutes After 3 miuutes After 4 minutes. After 5 minutes. After 6 minutes After 7 minutes. After 8 minutes. After 9 minutes. After 10 mnutes Alter ll minutes 2o. 2,393 kg C out C... about 50 C.

2. 72-3.14%. G35-0.36%. 0.50-0.50%. about 1.70%.

the importance oi a. suillciently vivid stirring.

The intimate contact which is necessary between the iron and the lime powder, in order that the reaction shall proceed quickly and completely, may be obtained also in other ways than in a rotating furnace. According to the invention it is also possible to effect the stirring of part of it by blowing an inert or reducing gas into the bath, whereby an effective contact action between the metal and the lime charge can be obtained. Such a treatment may of course also be carried out in a rotating furnace but can also be performed in a bessemer converter of the ordinary type or in some other suitable furnace or transport container in which case the gas is supiron, the process described above may in appropriate parts be used also for other metals or alloys which are in molten state in the temperature range of 700 to l400 C. and do not give rise to other reactions disturbing the desulfurizaticn. If the reducing action of the metal or alloy in question is not sufficiently high, a suitable reducing agent, e. g. silicon or carbon, or highly reducing gases may be added.

We claim:

l. A method of desulfurizing a metal of the group consisting of iron and iron alloys having a melting point below 1400o C. which comprises Y vigorously stirring a molten metal of said group in a closed relation chamber in contact with a solid deulfurizing agent of the group consisting of the oxides of the alkaline earth metals and the oxide of magnesium in ne granular form at a temperature below 1400 C. at which said metal remains in the molten state and said desulfurizing agent remains in the solid state Without sintering and in a non-oxidizing atmosphere consisting essentially of a gas of the group consisting of nitrogen and hydrogen.

2. The method as defined in claim 1 in which said non-oxidizing atmosphere is provided by supplying a substantially oxygen-free hydrogen containing gas to the chamber.

3. The method as dened in claim l in which a substantially oxygen-free hydrogen containing compound of the group consisting of hydrocarbons and ammonia is supplied to the chamber.

4. The method as dened in claim 1 in which the solid desulfurizing agent is burnt lime.

5. The method as defined in claim 1 in Which the solid desulfurizing agent is burnt dolomite.

6. The method as dened in claim 1 in which the solid desulfurizing agent contains up to about percent of an alkali metal oxide but insufficient to cause sintering of the desulfurizing agent.

7. The method as defined in claim 1 in which the non-oxidizing atmosphere is provided by continued operation in a chamber closed to the atmosphere.

8. The method as dened in claim 1 in which the stirring is effected by operation in a cylindrical chamber with its axis substantially horizontally disposed, said cylinder being rotated at a circumferential speed of about 1.5 meters per second.

9. The method as dened in claim 1 in which a reducing agent having a greater affinity for 10 oxygen than the metal being desulfurized is in= troduced into the chamber.

l0. The method as defined in claim 1 in which the desulfurizing agent occupies 15-30 percent and the metal occupies up to about 10 percent of the volume of the chamber.

11. The method as dened in claim 1 carried out in a rotating cylindrical reaction chamber the axis of which is substantially horizontally disposed by supplying the metal and the desulfurizing agent at opposite ends of said chamber and withdrawing metal and desulfurizing agent at opposite ends of the chamber after countercurrent passage through said chamber.

Bo MICHAEL sTURE KALLING.

PEHR .limev cHRIsTER DAmELssoN.

OTTAR JOHAN LoRENs DRAGGE.

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

UNITED STATES PATENTS Great Britain Sept. 17, 1938

Claims (1)

1. A METHOD OF DESULFURIZING A METAL OF THE GROUP CONSISTING OF IRON AND IRON ALLOYS HAVING A MELTING POINT BELOW 1400* C. WHICH COMPRISES VIGOROUSLY STIRRING A MOLTEN METAL OF SAID GROUP IN A CLOSED RELATION CHAMBER IN CONTACT WITH A SOLID DEULFURIZING AGENT OF THE GROUP CONSISTING OF THE OXIDES OF THE ALKALINE EARTH METALS AND THE OXIDE MAGNESIUM IN FINE GRANULAR FORM AT A TEMPERATURE BELOW 1400* C. AT WHICH SAID METAL REMAINS IN THE MOLTEN STATE AND SAID DESULFURIZING AGENT REMAINS IN THE SOLID STATE WITHOUT SINTERING AND IN A NON-OXIDIZING ATMOSPHERE CONSISTING ESSENTIALLY OF A GAS OF THE GROUP CONSISTING OF NITROGEN ANDD HYDROGEN.

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