US3368885A - Methods of desulphiding a bath of metal - Google Patents

Description

Feb. 13, 1968 N. voLlANlK METHODS OF DESLPHIDING A BATH OF METAL Filed NOV. 17, 1964 United States Patent O ABSTRACT F THE DISCLOSURE 15 Molten metal is desulfurized by depositing powdered calcium carbide on the upper surface of the bath of molten metal and then blowing an inert gas into the bath below its surface at such a rate of ow as to cause violent turbulence in the bath producing eddies which draw the powdered calcium carbide from the upper surface and disperse it through the bath to elfect rapid desulfurizing thereof. The operation takes but a few minutes and does not require heat to be applied to the molten metal to keep it molten.

This application is a continuation-impart of my prior application Ser. No. 128,515, led Aug. 1, 1961, and now abandoned.

The present invention relates to methods of desulfurizing a bath of metal, and has for an object, improvements therein.

It is known that a bath of metal may be desulfurized by depositing an alkaline oxide or carbonate on this bath and by agitating the whole by blowing in an inert gas such -as nitrogen. However, this method can only be used if the bath of metal is contained in a container which has a neutral or basic coating, this condition beingabsolutely necessary to ensurethe reproducibility of the rate of desulfurization and to prevent the coating from being attacked, an occurrence likely to happen if this coating were Silico-aluminous.

It is also known that a metal bath may be desulfurized by the injection of calcium carbide with the aid of a gas such as nitrogen. In this case the vessel containing the 4D bath can have an acid, basic or neutral coating. But the operation of this method is relatively complicated since it necessitates the use of injectors; besides this, it can become irregular in operation if the injectors become blocked.

Previous workers in the field have attempted to improve the desulphurization of liquid metal by blowing gas therethrough. For example, in U.S. Patent No. 2,587,573, Wynne discloses a system in which gas is bubbled through a melt in order to promote an agitation which serves to increase the desulphurization. However, this patent fails to disclose the velocity at which this gas passes into the bath and so the degree of agitation obtained cannot be determined. Moreover, Wynne discloses that with his process it was necessary to bubble gas through the melt for 7() to 120 minutes in order to obtain a satisfactory desulfurization. Such long treatment periods present obvious drawbacks, notably in that a large supply of gas must be used and, in particular for small masses of liquid metal, means must be provided to heat the metal so as to maintain it in a liquid state.

Applicant has discovered that all of these problems can be overcome if gas is bubbled into the melt at such a velocity as to cause a turbulence suicient to produce eddies which are apparent at the surface of the liquid. No prior art process known to applicant suggests the produc- (3,368,885 Patented Feb. 13, 1968 tion of such a violent turbulence to enhance the desulphurization process.

This degree of turbulence is achieved by introducing the agitating gas into the melt at a velocity which is much higher than those used in the prior art. The result of this procedure is to reduce the time necessary to produce a satisfactory desulphurization (e.g., a reduction of the sulphur content of the melt by 80%) from the 70 to 120 minutes required by Wynne to a period of 3 to 5 minutes, and in some cases to as little as two minutes.

The advantages resulting from such a decrease in the desulphurization time include a reduction in the quantity of gas used, and the elimination of the need for auxiliary heating means for maintaining the melt in a liquid state.

It is therefore an object of this invention to radically decrease the time required for desulphurizing a mass of liquid metal.

It is another object herein to decrease the quantity of gas required in a process whereby desulphurization is accelerated by using a gas flow to agitate the liquid metal.

A still further object of this invention is to eliminate the need for auxiliary heating equipment to maintain a mass of metal in its liquid state during a desulphurizing process.

The present invention has for another object a method of desulfurizing a bath of metal the operation of which is very simple and can be carried out in a vessel with any type of covering which allows very high and regular rates of desulfurization to be obtained.

The method according to the invention essentially con. sists in depositing at one time all the required powdered calcium carbide directly on the surface of the bath, the granulation of the powder being'preferably comprised between 0.5 and l millimeter and in agitating very energetically by blowing in a gas by means of one or more tubes through one or more openings situated below the surface of the metal, arranged so as to bring about a random turbulence of the liquid metal. In this manner and with a particularly simple apparatus, all of the calcium carbide particles are literally enveloped, almost instantaneously, by the eddies brought about on the surface of the bath by the turbulence of the liquid metal, thus ensuring, by the intimate contact between the metal and the calcium carbide, the speed and eiciency of their reaction. They cannot become separated from the bath by a gaseous envelope as is the case with the injection-desulfurizing processes.

This turbulence, which becomes evident at the surface of the bath, must not become a regular movement of the liquid metal which would simply have as its effect to renew the particles of liquid metal in order to put them successively into contact with the calcium carbide which would remain on the surface; it must cause eddies such that the carbide is driven into the middle of the bath and regularly distributed within the liquid mass.

To this end the gas must be blown in through one or more tubes having openings of a section of the order of a square centimeter without any diaphragm, porous stoppers or any other regulating device being inserted.

The gas to be used may be an inert gas such as nitrogen or argon, but air may be used since, when calcium carbide is used, oxygen will not change the chemical composition ofthe molten metal.

The velocity with which gas must be blown into the melt in order to produce the required degree of turbulence `does not have a unique value for all cases, but varies according to the quantity of metal being treated. However, the velocity required to achieve, for example, an 80% reduction in the sulphur content of the metal within a period of 3 to 5 minutes can be expressed as a linear function of the quantity of metal being treated.

3 4 This function may be given as: Another feature of the present invention rsides in the fact that the powdered calcium carbide is eposited n 1 h V/gT-l-SO the surface of the bath only after the molten metal has W er@ been poured into its treatment vessel, whereas previous V eqllllS the Velocity 0f the gas in meters/SCC 3S 1l leaves 5 experimenters have taught that the powdered desulfuriz its tube, and ing agent should be placed on the bottom of the vessel T equals the quantity 0f metal bemfe treated, 1n OIIS prior to the pouring of the molten metal. Applcants (l ton=2200 lbs.). novel procedure represents a distinct improvement be- It should be understood that this equation is not in- Cause lt Perllllts the addltlen of exactly the quallllty of tended to be limiting inasmuch as the velocity could be Calcium Carbide needed after the quantity of metal'in the slightly lower and still produce the improved results de- Vessel has been delermmed alle because lt flemme the scribed herein. It must also be noted that the process mollen metal le be deslagged pllo te llle aeldltlen of the hereunder discussion is intended to be Carried out with calcium carbide, thus further ensuring an intimate contact the melt at a temperature of at least 1350 C. and using between the latter alle llle metal', a quantity of calcium-carbide which is between 0.009T The method eeeordmg to tlle mvellllol'l leads te Surprls' and 0.015T (where T is the quantity of melt in tons). mg reel-me particularly regardmg the bleh degrees of .de' When a gas introduction tube or tubes having an open- Sullurlallon Obtamedu and llez Speed of the operano ing of the order of a square centimeter are used, the Wlllch Oeellol nolllla yexeee. w3 lllllllltes' volumetric ow rate of the gas for each tube can be eX- NOW thls Operatlonalspeed ls esselltlal lll Order lo pre ressed by the relation, vent the bath from cooling. Moreover, the treatment conp tainer or Crucible may with advantage be heat-insulated in @ZK order to slow down this cooling process.

3 (2) The invention will now be described with reference t0 here Q is in rue/hmm the following specific examples.

These relations clearly show that both the gas velocities Example 1 and the volumetric ow rates are far greater than those employedin Similar prior art processes' 800 kilograms of a liquid. fount having a sulphur The most significant differences presented by the pres contentof 0.09%? were melted in a container. 8 kilograms ent process may clearly be Seen fromme following table of calcium carbide were scattered on the surface of the comparing the values of various parameters according to pathymd I/mrogeildllmbleldthough mba .veloclty efceed the present process with those of similar parameters acjngl fm' Seg' W1. e al o .one tu e lmmersed lll llle cording to Wynnes process. It should be mentioned that ttlthor al huratlont oft two lglles' it was then found inasmuch as the Wynne patent fails to set forth the diadb e755; ur con en was l'e had been 10W' mensions of his gas introduction pipe, the cross section ere y o E l 2 thereof was assumed to be 1 cm2. All of the remaining 35 xamp e figures were taken directly from the patent. 800 kilograms of a liquid ount having a sulphur con- TABLE i 1901155. of melt 13.6 tons of melt 31.7 tons of melt Wynne Volianik Wynne Vollanik Wynne Volianik Volumctric rate of gas-flow per ton of melt (litnrs/miiL/toii) 20 2, 700 14 185 15. 7 165 Actual flow rate (in.3/hour) 0.08 10.2 11. 32 152.5 29. 7 315 Gas exit velocity (M/sec.) 0. 22 30. 5 10. 5 152. 5 27. 5 315 Duration of process (min.) S90 4 59o S4 90 S4 Quantity of gas consumed (111.3) 0.12 0. 68 17 10. 16 44.6 21.0

Comparison between present process and that of Wynne tent of 0.08% were melted in a container. 11.2 kilograms of calcium carbide were scattered on the surface of th This table clearly shows the distinctly lower gas ow e rates and velocities (assumed) used by Wynne which bath and nitrogen bubbled therethrough for three minvalues have been found to be clearly incapable of inducuthte ltl a veilolny excteedme :,'ll/sec' .It was then found ing the eddy-Producing turbulences characteristic of the adb 695515137111' con ent was l'e' had been 10W process of the present invention. errh y o' insofar as concerns the number of tubes used in the e accompanylng drawmgs llluslrale by Way 0f eX' practice of the present process, it has been discovered empltwo embodlmellls of apparatus for performlng the that satisfactory results will be obtained when using one mvlglonl l tube for masses of liquid metal weighing less than one 1 d ls a dlagrammalle lllustrallon of the PrmelplC ton and three tubes for larger masses. These tubes may mlgle h b d, l preferably be submerged to a depth of 20 to 30 cm. above h s OWS an em o lment of a more eommefelal the bottom of the liquid container. c racF-' l th h h When the llow rates and velocities are chosen accordh n ere ls s own at. 1 l e ladle er emellle'm ing to Equations 1 and 2 above, one may determine the l. mhh? lath .of molten east en ls Continued' Dllpmg approximate duration of treatment and percentage of Lnto l .ls.d allls e lue 2 connected by e cellar 3 Wllh a CaCg (with respect to the weight of metal treated) reeniblgl btuse 4dw Ich m tum 1s connecte? through a quired to reduce the sulphur content of the metal by pslse 2:7 Slh ai iggerfo a contamer of e0m` 80 accordin" to the ollowin e nations: 1L

c g q In the embodiment of FIG. 2, the ladle or Crucible is DUraUOH (mmUeS)?17-90010 (3) again shown at 1, the compressed gas container at 7, Weight of C302 and the ow meter device at 6. This device itself is PGICGH CCi--mlXlOO7-'O4@ connected to the container 7 through an intermediate tube 8 and a pressure reducer valve 9. Three graphite (4) tubes 2 are here provided and are carried from a common where 0 equals the temperature of melt in degrees centi- 75 support 10 provided with a clamping arrangement 11, the

grade at the start of the process.

support 10 being in turn secured to one end of the piston rod 12 of a ram actuator 13 carried by a fixed bracket 14 above the crucible 1. The respective tubes 2 are connected through tubes 15 with a feeder head 16 connected by a common flexible pipe 17 with the container and flowmeter 6. The tubes 15 nally are connected at 19 with the tubes.

Further in this ligure there is shown at 18 a hollow union for assembling two adjacent tube elements 2. The ram 13 provides the possibility of controlling at will the downward and upward movements of the tubes 2 into the ladle and crucible and thereby accurately determining the duration of the process.

It will be understood that the apparatus may admit of a large number of modified embodiments, particularly as concerns the means for raising and lowering the tubes.

I claim:

1. A method of desulfurizing a bath of molten metal, comprising the steps of depositing powdered calcium carbide on the top surface of said bath, and blowing a gas into said bath below the surface thereof, for a period not exceeding l minutes, through at least one opening having an unobstructed cross sectional area of the order of 1 crn.2 at a rate of ow suflicient to cause violent turbulence in said bath producing eddies which draw the powdered calcium carbide from said top surface and disperse the same throughout the bath for ensuring the rapid desulfurizing of all the molten metal prior to cooling the latter.

Z. A process as recited in claim 1, wherein said calcium carbide consists of particles in the size range between approximately 0.5 and 1.0 mm.

3. A process as recited in claim 1, wherein said gas is blown into said bath for a period not exceeding minutes.

4. A process as recited in claim 1 wherein said gas is introduced into said bath at a velocity in meters per second which is numerically superior to six times the quantity of molten metal being treated, in tons, plus ten meters per second.

5. A method of desulfurizing a bath of molten metal,

comprising the steps of depositing powdered calcium car-l bide on the top surface of said bath, and blowing a gas into said bath below the surface thereof and at a velocity V, in meters per second, superior to 6T+1O where T is the quantity of molten metal being treated, in tons, through at least one unobstructed opening at a rate of ow sufficient to cause violent turbulence in said bath for producing eddies which draw the powdered calcium carbide from said top surface and disperse the same throughout the bath for ensuring the rapid desulphiding of all of the molten metal prior to cooling of the latter.

6. A process according to claim 5 wherein said calcium carbide is deposited directly on the top surface of said molten metal after the latter has been poured into its treatment vessel.

References Cited UNITED STATES PATENTS 2,577,764 12/1951 Hulme 75-93 X DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner.

N. P. BULLOCH, H. W. TARRING,

Assistant Examiners.

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