US2823989A - Agent for treating molten metals - Google Patents

Description

2,323,989 Patented Feb. l i958 ire AGENT non TREATING MoLTEN METALS- Alden J. Deyrup and John R. Ferron, Niagara Falls,

N. Y., assignors to E. l. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application July 22, 1953 Serial No. 369,711

Claims. (Cl. 7558) This invention relates to the active and relatively volatile metals sodium and magnesium. More particularly, it relates to a method for rendering these metals less active and volatile and to forms of the metals more inert than the commonly encountered forms.

In some desirable applications sodium and magnesium are too reactive and also too easily volatilized for convenient use. The excessive reactivity is noticeable in such well-known processes as the generation of hydrogen gas by the replacement of the hydrogen in water with elementary sodium. The reactivity persists at elevated temperatures where, additionally, the low boiling points of the metals begin to cause trouble.

Metallic sodium is, for example, an effective agent for the removal of the sulfur sometimes carried over into iron from the ores, the metal scrap or the fuel employed. Sodium, however, boils at about 889 C., a temperature considerably below the melting point of iron. The sudden increase in volume which will result if metallic sodium is placed in molten iron or steel may result in violent explosions. Sodium, in fact, seems too volatile and active for safe employment in ferrous melts. Despite these drawbacks, sodium is relatively cheap, in abundant supply and perhaps effective to promote graphite nodularization in cast iron and increase the tensile strength of the final product as well as to remove the sulfur therefrom. A method of reducing the reactivity and volatility of metallic sodium so that it can safely be employed in ferrous metallurgy is, therefore, quite desirable commercially.

Magnesium has also been used with molten iron to achieve various results, including desulfurization of the melt. On a less violent scale, the problems associated with the reactivity and volatility of sodium are also evident with this metal. It is, therefore, sometimes desirable to curb the reactive and volatile nature of magnesium. This aim has been achieved heretofore to some extent by alloying the magnesium with nickel. Such a procedure is, however, expensive and, in any event, not applicable to sodium since the last-mentioned metal does not alloy with nickel, or for that matter, with any metal tolerable in iron or steel.

A primary object of this invention is consequently development of a method for reducing the volatility and reactivity of sodium and magnesium. Another object of the invention is provision of forms of the metals in question less reactive and volatile than the forms commonly encountered.

An additional object of the invention is development of a process and a composition of matter suitable for desulfurizing ferrous melts.

The above-mentioned and still further objects are achieved in accordance with this invention by a process in which sodium or magnesium is diluted by impregnation into an inert, porous refractory. metal diluted in this manner is much less active and volatile than the same metal in its pure, undiluted state, and can be added directly to a ferrous melt without danger of explosion. It will be readily understood that when the diluted metal is said to be less volatile, the rate of evaporation of the metal is lowered, its actual boiling point remaining unaffected.

A preferred refractory is lump quicklime, desirably retaining the rock structure of the limestone from which it was calcined. In the case of sodium, the lime may be impregnated with about 2025% by weight of the metal. Greater weights of the denser metal magnesium will be absorbed by the lime. The lumps utilized may vary from about one-eighth inch to two inches in diameter, but a range of one-half to one and one-half inches is preferred. In fact, lumps below one-half inch in diameter are quite difficult to handle physically, and in addition, tend to be too reactive.

Other porous lump refractories may be substituted for lime as the carrier or base including calcined dolomite, alundum and the like. It is essential, however, that the material employed as a base possess the following characteristics: (1) it must be porous to carry a substantial amount of metal internally; (2) it must be refractory and a thermal insulator, to afiord some protection and insulation to the metal and prevent flash evaporation of the entire metallic content of the lump at once; (3) it must be reasonably chemically inactive or not harmful to molten iron, to the metal absorbed and to the furnace linings utilized; (4) it must be strong enough, physically, though porous, to withstand disintegration in the ferrous melt; and (5) it must have a minimum particle size so that its thermal insulating property can effectively retard heat transfer to metal absorbed in its pores. The benefits of these various factors combine to reduce the reactivity of sodium in sodium-lime, as the product of sodium on porous lime may be called, to practicable levels. In effect, the metal is produced in a form having both a minimum thermal conductivity and a minimum surfaceto-volume ratio.

The impregnated lime of this invention can be formed quite readily, a satisfactory product being obtained by heating a rough mixture of the materials in an inert atmosphere. A temperature of around 600 C. is sufficient for the preparation of sodium-lime but lower temperatures can be employed if sodium-poor material is to be produced. Higher temperatures are, of course, required for magnesium which melts above 600 C. Reaction time for the impregnation is not critical but usually 45-60 minutes are employed to insure best results. Good product can generally be obtained by heating the reaction mixture to the requisite temperature and allowing it to cool.

In practice, lump quicklime from three different commercial sources was employed without any apparent difference in product quality. The sodium-lime produced retained the physical structure of the original lump but varied in color from gray-blue to nearly black. On exposure to air it absorbed oxygen and moisture liberating considerable heat and quickly turning white. The re action of this sodium-lime with water, even hot water, is very mild. When a lump is thrown into Water, a slight flash occurs at the surface. The lump then sinks to the bottom and steadily liberates hydrogen for a considerable period of time. The sodium-lime is also quite resistant to burning. If a lump is ignited in a Bunsen flame or soaked in kerosene and lit, it burns for a short time and then goes out. The oxides forming on the surface and residual lime apparently smother the flame. The relative mildness of the reaction between sodium-lime and air or water is distinctly advantageous in the safe handling of the metal and contrasts strongly with the behavior of the free alkali metal. In appearance magnesium-lime resembles sodium-lime but the lumps of the material exhibit a greater tendency to spall, probably because magnesium is a harder and less tractable metal than sodium.

There follow several examples which illustrate in greater 1 detail various aspects of the invention.

Example 1 a inch cube of metallic sodium weighing about 5 g.

A nickel cover containing a thermowell and inlet and outlet means for a gas was inserted in the pot and nitrogen bled therethrough.

The mixture was heated for about minutes, the temperature rising to 440 C. Heating was then stopped and the mixture cooled and covered with kerosene. All of the original lumps of lime retained their shape and hardness. About half of the lumps were colored a deep blue-gray over part of the surface and several others over the entire surface. Throughout the interior of these lumps extended a violet coloration. A small piece from one of the lumps gassed vigorously when initially placed in water and continued to gas for about minutes. At the end of this time the piece had turned white.

Example 2 The run of Example 1 was repeated except that the temperature was allowed to reach 600 C. The product was about the same as that of the first example.

As a result of the above-mentioned and similar experiments, it was foundthat the sodium is not absorbed homogeneously. 'If it'is assumed that the lime has a 50% void volume and is 100% CaO (specific gravity 3.32), then the saturated product contains about 23% sodium by weight. This assumption was borne out in practice. If less than 23% sodium was used, the product was composed of separate volumes of nearly saturated material and of barren lime containing no sodium. A sodium content of about 23% or slightly higher gave a product with excess sodium on the surface.

For the use of sodium-lime contemplated by this invention, a product with a minimum exposed sodium surface is preferred. Consequently, the preferred sodiumlime contains about 23% or less of sodium.

.Example A number of small scale tests were made to determine whether or not sodium-lime, prepared as described in the previous example, can safely be added to molten steel. Lumps weighing 1-3 grams were immersed in molten steel and in cast iron. In all cases, sodium vapor was released in a controlled manner and bubbled'vigorously through the melts. Excess vapor flashed and burned at the surface giving considerable smoke but noexplosions. The reactions were complete in 15-30 seconds, regardless of the size of the lump used. When the reaction had subsided, the lime was always recovered intact, no disintegration of the lump structure being apparent. Previous experience had taught that additions of pure sodium to molten iron result in violent and dangerous explosions.

Example 4 In a clean steel container provided with a nickel thermocouple were placed nineteen lumps of calcined limestone, each of which was Vt' /z inch in diameter. The total weight of the lime was grams. 17.1 g. of technical magnesium and about 0.5 g. of sodium were placed on the lime and the container purged with argon. The container was heatedforabout two hours, the temperature being allowed to reach around 1000 C. near the end of this period.

The container was cooled and dismantled. The metal had disappeared, as in the case of the sodium runs, but the lumps substantially retained their shape. The lumps possessed a yellow and green exterior but a dark gray interior. The external color was attributed to nickel 4- from the therrnowell. When a chunkof impregnated material was dropped in water, it gassed vigorously and rapidly disintegrated.

It should perhaps be pointed out that the thermodynamics of the system calcium-magnesium-oxygen favor the formation of lime. Hence, replacement of calcium by magnesium is not to be expected.

Example 5 The run of Example 4 was substantially repeated except that the metallic sodium was omitted. The lumps were found to be almost white externally changing to yellow on exposure to air. There was less of the greenish color observed in the previous run. Split lumps again exhibited a two-layerstructure but this time the interior was very much the same color as the exterior. Gassing again resulted when the lumps were dropped in water. It may be noted that more fines were produced in the two runs with magnesium than in the sodium experiments. These contrasting results were attributed to the fact that sodium is much softer than magnesium and sets up fewer strains in the product.

It will be evident that the magnesium-lime of this invention can be substituted for magnesium metal in many usages. In particular, it can be used to treat molten iron or steel as can sodium-lime. Addition to the ferrous melt may be carried out with magnesium-lime in the same manner it was carried out with sodium-lime.

Various modifications in the details of this invention can be made without departing from the scope thereof. Other metals besides sodium and magnesium can, for example, be diluted and protected by lime or other porous refractory. Such additional metals include particularly the more active and volatile metals of groups I and H of the periodic table, lithium, potassium, rubidium, caesium, calcium, strontium and barium. I

The quantity of metal impregnated in a base will, furthermore, vary somewhat with both the metal and the porous refractory employed. In the case of sodiumlime, the amount of sodium retained will be about 2025% of the total weight of the product. The magnesium content of magnesium-lime may, in like manner, vary from that of the examples given, ca. 30%. Sufficient thermal insulation is provided at these low metallic contents to prevent too rapid vaporization even in molten iron or steel. Any excess of metal beyond that which can be absorbed, may, however, be quite undesirable because such unabsorbed metal can have the dangerous high activity normally expected with the free metals.

It will readily be apparent that the temperature required for optimum impregnation of the lime or other diluent may vary somewhat from metal to metal. In general, the temperature employed will be well above the melting point of the metal in question but below its boiling point. Thus, around 600 C. is quite satisfactory fcr impregnating lime with sodium but is not high enough for use with magnesium. Around 1000 C. is

desirable when dealing with the latter metal.

Having described our invention, we claim:

1. An article of manufacture for use in treatingferrous melts consisting essentially of a porous, refractory metal oxide in the form of integral lumps impregnated with at least one member of the group consisting of .rnagnesium and sodium, the refractory metal oxide being substantially inert to ferrous melts and to the aforesaid metals. 7

2. An article of manufacture for use in treating ferrous melts consisting essentially of lump quicklime impregnated with metallic sodium.

3. An article of manufacture for use in treating ferrous melts consisting essentially of lump quicklime impregnated with metallic magnesium.

'4. Quicklime in the form of lumps between about one-eighth and two inches in diameter impregnated with metallic sodium and adapted for treating ferrous melts.

5. Quicklime in the form of lumps between about oneeighth and two inches in diameter impregnated with between about 20 and 25% by weight of metallic sodium and adapted for treating ferrous melts.

6. Quicklime in the form of lumps between about onehalf and one and one-half inches in diameter impregnated with metallic sodium and adapted for treating ferrous melts.

7. The quicklime of claim 6 in which the sodium comprises between about 20 and 25% by weight thereof.

8. The method of making the article of claim 1 which comprises heating said metal with lumps of the porous, refractory metal oxide.

9. The method of making the article of claim 2 which comprises heating quicklime with about 20-25% by weight of metallic sodium.

10. The method of claim 9 during the heating is around 400600 C.

References Cited in the file of this patent UNITED STATES PATENTS Marino Nov. 4, Duparc Apr. 22, Hunt Oct. 6, Cross June 28, Norris Dec. 12, Espe et al. Apr. 10, Smith Mar. 14, Kroll Mar. 29, Gallinger et al. Jan. 2,

FOREIGN PATENTS Austria Apr. 10,

in which the temperature

Claims (2)

1. AN ARTICLE OF MANUFACTURE FOR USE IN TREATING FERROUS MELTS CONSISTING ESSENTIALLY OF A POROUS, REFRACTORY METAL OXIDE IN THE FORM OF INTEGRAL LUMPS IMPREGNATED WITH AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF MAGNESIUM AND SODIUM, THE REFRACTORY METAL OXIDE BEING SUBSTANTIALLY INERT TO FERROUS MELTS AND TO THE AFORESAID METALS.

8. THE METHOD OF MAKING THE ARTICLE OF CLAIM 1 WHICH COMPRISES HEATING SAID METAL WITH LUMPS OF THE POROUS, REFRACTORY METAL OXIDE.