Patented Apr. 20, 1937 2,077,568

UNITED STATES PATENT OFFICE 2,07,568 PROCESS FOR PURFYING. FERROUS Augustus B. Kinzel, Douglaston, N. Y., assignor, by mesne assignments, to Union Carbide and Corporation, a corporation of New York No Drawing. Application April 3, 1935, Serial No. 4440 2 Claims. (C. 75-60) The invention relates to the removal of oxidiz quently is had to stopping the blow as the end able impurities from ferrous metals by blowing point is approached and taking a coupon of the molten with an oxidizing blast, and has metal in order to determine by inspection its ap for its object the provision of means whereby proximate composition. But even this practice 5 control over the quality of the product may be leaves much to be desired, for the difficulty re- 5 greatly increased. mains of stopping the blow at exactly the right The method of the invention is particularly point during a necessarily short time interval. applicable to, and will be described in connection It WOuld therefore seem desirable to slow down with, the manufacture of by processes of the the end reactions by decreasing the blast rate as 10 Bessemer or converter type, wherein the oxida the end of the blow is approached; but in the ordi- 0 tion is customarily accomplished by means of an nary bottom blown converter this is impossible, air blast. W because a high blast rate is necessary in order It is generally believed that Bessemer steel is, to prevent the metal from running through the for some purposes, inferior in quality to steel bottom tuyeres. This is, in fact, the feature of the 15 made by other methods and that this inferiority Bessemer process, as usually carried out, which 5 results from characteristics inherent in the proc makes it so inflexible. ess. Chief among these characteristics are the On the other hand, it has been proposed to in inflexibility of the process, the great speed with crease the Speed of the process and to adapt non which the oxidation reactions take place, varia Bessemer pig to the Bessemer process by 20 tions in temperature of the bath as related to the using oxygen enriched blast. Such a method of 20 analysis of the pig blown, and the uncer blowing obviously adds very greatly to the diff tainty of relative rates of removal of the various culty of properly determining the end point, by impurities which are oxidized by the blast. simultaneously increasing the temperature of the In the acid Bessemer process, using low phos metal and the oxidizing power of the blast. 25 phorus , the end point is reached very I have found that these difficulties can be over- 25 suddenly and its approach is indicated by the come, the quality and uniformity of Bessemer "drop of the flame' as the last of the carbon burns steel greatly improved, and the economy and use off. Since it is usual to Oxidize the last 20% of fulness of the Bessemer process greatly enhanced, the original carbon during the last minute or SO by slowing down the end reactions in a controlled 30 of the blow, the rate of change of carbon content manner, as hereinafter described. 30 is very rapid and it is almost impossible to stop According to a preferred method of the inven the blow at the correct point. For this reason it tion, the process is slowed down to the desired is common practice to overblow the metal and degree and at a desired moment by diluting the then to deoxidize it by using a relatively large blast with an inert gas, such as , whereby 35 amount of deoxidizer. This practice has the dis the rate of oxidation of impurities from the steel 35 advantage, among others, of tending to produce may be decreased without decreasing the blast dirty steel. rate. The effect of such dilution of the blast is In the basic Bessemer process for converting two-fold. The rate of heat production from the high pig iron, the phosphorus is large exothermic reactions taking place is decreased 40 ly removed after the carbon has disappeared, and and at the same time the large quantity of cold 40 for this reason the tendency to overoxidation may inert gas blown through the bath maintains the not be so great as in the acid process. Neverthe rate of removal of sensible heat. The combined less, the proper end point is equally difficult to effect may be so great as to rapidly cool the metal, attain because there is no indicator, Such as the and therefore by proper control of the oxygen Con 45 drop of the flame, to show when the removal of tent of the blast it is possible to control both the 45 phosphorus is complete. temperature of the metal and the speed with Another inherent difficulty of the Bessemer which the end point is reached. process is that variations in temperature alter the The method is particularly advantageous in relative rates of Oxidation of impurities. Thus, combination with the previously mentioned ex 50 with very high temperature it is possible to re pedient of using oxygen enriched blast. In such 50 move all the carbon before the and man a combined process, oxygenated blast may be ganese have been completely oxidized, and in such used during the first part of the blow, whereby a case the “drop of the flame” will give an entirely the reactions are carried on very rapidly and erroneous idea, as to the condition of the heat. exceptionally high temperatures may be reached. 55 To overcome these difficulties recourse fre The increased rate of reaction is, of course, ads. 55 2 2,077,568 vantageous for reasons of economy; but exces to any desired extent by blowing with blast of sively high temperature may make it very difi decreased oxidizing power for a suitable period cult to reach the proper final analysis because of time. Similarly, if the metal cools off too of the tendency of side reactions to take place. rapidly during the finishing blow with blast of Thus, the method of the present invention may decreased oxidizing power, it may be reheated be applied with great benefit in a combined by blowing for a suitable period of time with a process by reversing the character of the blast blast of the usual oxidizing power, whereupon during the latter part of the blow, starting with the use of diluted blast may be resumed. an Oxygen enriched blast and ending with a The process of the invention, as has been O nitrogen diluted blast which permits close con stated, is directed particularly to the purifica O trol of the end point, tion of ferrous metals, but it may also be ap Further advantage of such a combination plied in processes of analogous character for the process results from the fact that the two types - removal of oxidizable impurities from non of blast can usually be adjusted as to composition ferrous metals, as for example in the Bessemer 5 and time So that the surplus nitrogen rejected ization of copper. 15 from air in producing the oxygenated blast for I claim: the first part of the blow will be balanced by the i. In a process of the Bessemer type for the requirement for nitrogen to dilute the blast for of ferroalloys, the method which the latter part of the blow. The total elapsed comprises blowing the bath of molten metal first 20 time for such a heat is considerably less than With a blast of Oxygen enriched air until a major 20 that of a heat blown throughout with ordinary portion of the oxidizable impurities has thereby air, whereby the advantages of the oxygenated been removed from the metal and subsequently blast and the diluted blast are both obtained, With a blast of nitrogen-enriched air until re It will be obvious that any gas which is in moval of the oxidizable impurities to the desired 25 effect chemically inert under the conditions of amount has been completed. 25 the process, is the equivalent of nitrogen within 2. Process which comprises separating air the intent and scope of my invention. into a nitrogen-rich fraction and an oxygen Although for the purpose of illustration the rich fraction; storing Said fractions Separately; invention has been described in connection with withdrawing said nitrogen-rich fraction and 30 the Bessemer process of manufacturing steel by mixing it with air to produce nitrogen-diluted 30 blowing molten pig iron with a blast of air in air; Subjecting a bath of molten metal contain a bottom blown converter, many modifications ing oxidizable impurities to a blast of said oxy and variations of the invention are possible. For gen-rich fraction until a major portion of the example, the method may be employed for the Oxidizable impurities has thereby been removed 35 decarburization of ferro alloys such as ferro from the metal and thereafter subjecting the 35 chromium, and the blast may be applied from bath of molten metal to a blast of said nitrogen the side of the containing vessel, Ör by jets im diluted air until purification of the metal is com pinging upon the surface of the metal. pleted to a desired degree; and controlling the A modification of the invention contemplates air fractionation step of the process to separate 40 blowing the metal at irregular intervals or in the air into Oxygen-rich and nitrogen-rich frac 40 alternate cycles with gas having relatively high tions in the proportions required in the metal and relatively low oxidizing power. For example, purification step of the proceSS. if at any time during the blowing operation the metal becomes overheated, it may be cooled off AUGUSTUS B. KNZE,