US3490900A - Method of preparing rare earth master alloys - Google Patents

Description

United States Patent C US. Cl. 75-135 8 Claims ABSTRACT OF THE DISCLOSURE A process for producing master alloys containing magnesium, rare earth elements, silicon and iron for use as additives in iron and steel production in which magnesium metal is melted in a crucible and one or more siliconbearing materials is dissolved in the melt to provide in the melt at least sufficient silicon to assure formation of magnesium silicide and those silicides which form in preference to it. Thereafter, a rare earth-containing material is dissolved in the melt.

This invention relates to a method for making master alloys for use in iron and steel production. More particularly, it relates to a novel process for making master alloys containing magnesium, rare earths, silicon and iron.

Magnesium-rare earth-silicon-iron alloys are useful in I promoting graphite spheriodization in cast iron. In addition, they have been used to remove oxygen and sulfur from molten steel. However, the existing processes for producing these alloys are inefiicient and expensive. Some processes utilize misch metal, an expensive material, as a source of rare earths. Others entail reducing rare earth compounds with magnesium, a relatively expensive reducer. Recently rare earth-silicon-iron alloys have been produced by carbon reduction. These alloys, which contain up to 40 percent rare earths and up to about 45 percent silicon with the balance iron and incidental impurities, provide a relatively cheap source of rare earths. However, attempts to produce master alloys of magnesium, rare earths, silicon and iron both by dissolving solid magnesium into a rare earth-silicon-iron melt, and by dissolving crushed rare earth-silicon-iron into molten magnesium have failed. In both cases, the magnesium burned to such an extent that usually less than 50 percent of the magnesium added was retained in the master alloys.

I have found a process for producing master alloys containing magnesium, rare earths, silicon and iron which is efficient and can be performed with a minimal amount of burning. Specifically, I have discovered that magnesium can be' alloyed with rare earth alloys efliciently if the silicon content of the melt is maintained above a minimum level. The silicon content of the melt should be high enough to assure that all the magnesium present forms magnesium silicide. High recoveries of magnesium are attained when the silicon content of the master alloy melt equals or surpasses this level. Magnesium burning occurs when the silicon content in the melt drops below this level and becomes excessive if the silicon content is more than 10 percent below this amount.

In practicing the method of my invention, magnesium metal is melted in a crucible; one or more silicon-rich alloys, such as ferrosilicon, is dissolved into the melt; and thereafter a material containing rare earths, such as a rare earth-silicon-iron alloy, is dissolved in the siliconized melt. If it is desired to add another metal to the master alloy, such as calcium, the metal or alloy containing the metal can be dissolved in the melt after the addition of the silicon-rich alloy.

It is preferable to use a carbonaceous or alumina crucible. Melting can be done by any suitable means, such as induction, oil-fired or gas-fired furnaces. Any siliconrich alloy can be used. For economy, I prefer to use a high silicon ferrosilicon. The silicon alloy should be crushed prior to dissolving it into the molten magnesium. Rare earth alloys suitable for use in my process are commercially available. I prefer to use a rare earth-siliconiron alloy which can be made by the process disclosed in United States patent application Ser. No. 616,162, filed Feb. 15, 1967. Such alloys contain up to about 40 percent rare earths, up to about 45 percent silicon and the balance iron and incidental impurities. The rare earth-siliconiron alloy should also be crushed before addition to the melt.

In order for my process to operate efiiciently it is essential that suflicient silicon is present in the melt to assure that at least a major portion of the magnesium forms magnesium silicide. Therefore, the total silicon in the melt must be sufficient to provide silicon to form magnesium silicide and silicides of all other elements in the melt which form in preference to magnesium silicide. I have found that a master alloy melt which does not contain at least this amount of silicon contains free magnesium which vaporizes and burns; however, a melt which does contain sulficient silicon has all of its magnesium combined as Mg 'Si which is less volatile and less easily oxidized.

In a master alloy melt which contains only magnesium, rare earths, silicon and iron, RESI (rare earth silicide) and FeSi (iron silicide) form in preference to Mg Si. Ifcalcium is present, for example, a calcium silicide, believed to be CaSi, forms in preference to Mg- Si. In order to form the various silicides, a master alloy melt containingmagnesium, rare. earths and calcium should contain at least 0.58 percent silicon per 1.0 percent magnesium, 0.40 percentsilicon per 1.0 percent rare earths, 0.70 percent silicon per 1.0 percent calcium, and 1.0 percent silicon per 1.0 percent iron. Thus, the minimum theoretical silicon percentage required to form the master alloy can be determined from the following equation:

Minimum percent Si=0.58% Mg+0.40%

RE+0.70% Ca+1.0% Fl (1) The iron content of the melt is percent minus the other elements. Allowing for the existence of residuals in the melt such as aluminum, carbon, etc., the iron percentage is:

Percent Fe=100.0% (percent Mg-i-percent RE-l-percent Ca+percent Si+percent residuals) (2) By residuals is means those elements in the melt, normally encountered in practice, which, because of their chemical behavior, their concentrations or the conditions existing in the melt do not tend to form silicides in preference to magnesium silicide.

By substituting Equation 2 into Equation 1, the minimum theoretical percentage of silicon required can be determined by the following equation:

Min. percent Si=50.0%-(0.21% Mg+0.30%

RE+0.15% Ca+0.50% residuals) Since residuals usually comprises about 2 percent of the melt, the total percentage of silicon theoretically required in the melt to avoid free magnesium is:

Min. percent Si=49.0%(0.2l% Mg+0.30%

RE+0.15% Ca) 3 Thus, in carrying out my process I prefer to have a minimum silicon content at least as high as determined from Equation 3; however, I have found that the magnesium loss is not excessive if the silicon content is at least 90 percent of the theoretical percentage calculated from Equation 3.

The master alloy produced by my process is tapped from the furnace, preferably at temperatures between 2000 F. and 2500 F.; it can be tapped directly into graphite, refractory or metal molds and subsequently crushed to desired size.

Master alloys can be made using my invention containing 3-30 percent magnesium, 3-30 percent rare earths, 30-50 percent silicon and l50 percent iron. In the master alloys the silicides are present as distinguishable phases.

The following specific examples illustrates the method I have invented.

EXAMPLE 1 To produce about 100 pounds of master alloy having about 7 /2 percent magnesium and 9 percent rare earths, pounds of magnesium were melted in an induction furnace and 20 pounds of crushed ferrosilicon (77.8 percent silicon) were dissolved in the melt. Thereafter 80 pounds of crushed rare earth-silicon-iron alloy were dissolved in the melt. The rare earth-silicon-iron alloy contained 12.25 percent rare earths, 41.98 percent silicon, 42.88 percent iron and 2.89 percent residuals. The required percentage of silicon in this melt, according to Equation 3, was 44.4 percent. The percentage of silicon added to this melt was 44.7 percent. There was very little burning associated with the heat. The master alloy contained 7.3 percent magnesium, 9.1 percent rare earths, 45.4 percent silicon, balance iron and residuals. Recoveries were as follows:

Alloy- Percent Magnesium 71 Rare earths 90 Silicon 90 Iron+residuals 93 EXAMPLE 2 To produce about 100 pounds of master alloy having about 7 /2 percent magnesium and 20 percent rare earths, 9% pounds of magnesium were melted in an induction furnace and 20 pounds of crushed ferrosilicon (77.8 percent silicon) were dissolved in the melt. Then 80 pounds of crushed rare earth-silicon-iron alloy were dissolved in the melt. The rare earth-silicon-iron alloy contained 28.97 percent rare earths, 37.68 percent silicon, 30.60 percent iron and 2.75 percent residuals. The required percentage of silicon in this melt, according to Equation 3, was 40.8 percent. The percentage of silicon added to this melt was 41.6 percent. There was very little burning associated with this heat too. The master alloy contained 8.0 percent magnesium, 20.3 percent rare earths, 41.8 percent silicon, balance iron and residuals. Recoveries were as follows:

Alloy Percent Magnesium 78 Rare earths 83 Silicon 87 Iron+residuals 91 EXAMPLE 3 To produce about 100 pounds of master alloy containmg about 9 percent magnesium, 6 percent rare earths and 2 pe nt c l um, 19-7 peunds of mag um e melted in an induction furnace and 13.9 pounds of crushed ferrosilicon (77.8 percent silicon) were dissolved in the melt. Then 12.6 pounds of crushed calcium-silicon-iron alloy (15.9 percent calcium, 56.5 percent silicon) were dissolved in the melt, and 51.3 pounds of crushed ferrosilicon (47.6 percent silicon) were dissolved in the melt. Finally, 20.3 pounds of crushed rare earth-silicon-iron alloy were dissolved in the melt. The rare earth-siliconiron alloy contained 28.97 percent rare earths, 37.68 percent silicon, 30.60 percent iron and 2.75 percent residuals. The required percentage of silicon added to this melt, according to Equation 3, was 45.0 percent. The percentage of silicon added to this melt was 46.1 percent. There was little burning associated with this heat. The master alloy contained 9.2 percent magnesium, 5.8 percent rare earths, 46.5 percent silicon, 2.1 percent calcium, balance iron and residuals. Recoveries were as follows:

Alloy- 'Percent Magnesium 78 Rare earths 89 Silicon 84 Calcium 89 Iron+residuals 82 While I have described the preferred embodiment of my invention, it may be otherwise described within the scope of the appended claims.

I claim:

1. A process for the production of alloys containing at least magnesium, rare earth elements, silicon and iron comprising:

(A) melting magnesium metal;

(B) dissolving in the magnesium melt at least one silicon-rich alloy, the alloy providing sufficient silicon to assure that throughout the melt at least 90 percent of the silicon theoretically required to form magnesium silicide and those silicides which form in preference to magnesium silicide is present; and

(C) dissolving in said siliconized melt a material containing rare earths.

2. A process as set forth in claim 1 wherein the material containing rare earths is a rare earth-silicon-iron alloy containing up to 40 percent rare earth and up to 45 percent silicon.

3. A process as set forth in claim 1 wherein the amount of silicon in the melt is not less than that theoretically required to form magnesium silicide and those silicides which form in preference to magnesium silicide.

4. A process as set forth in claim 1 wherein the percent-age of silicon in the melt is at least equal to where Mg is magnesium and RE is rare earths.

5. A process for the production of alloys containing magnesium, rare earth elements, silicon, calcium and iron comprising:

(A) melting magnesium metal;

(B) dissolving in the magnesium melt at least one silicon-rich alloy, the alloy providing sufiicient silicon to assure that throughout the melt at least 90 percent of the silicon theoretically required to form magnesium silicide and those silicides which form in preference to magnesium silicide is present;

(C) dissolving in the siliconized melt a calcium-containing material; and

(D) thereafter dissolving in the melt a material containing rare earths.

6. A process as set forth in claim 5 wherein the material containing rare earths is a rare earth-silicon-iron alloy containing up to 40 percent rare earth and up to 45 percent silicon.

7. A process as set forth in claim 5 wherein the amount of silicon in the melt is not less than that theoretically required to form magnesium silicide and those silicides wh ch orm n p e en e o m gnesium silicide. I

5 6 8. A process as set forth in claim 5 wherein the per- 3,306,737 2/1967 Muhlberger 75134 centage of silicon in the melt is at least equal to 3,328,164 6/1967 Muhlberger 75168 3,364,015 1/1968 Sump 75134 49%-(.21% Mg+.30% RE+15% Ca) where Mg is magnesium, RE is rare earths and Ca is 5 DEWAYNE RUTLEDGEPrimary Examiner calcium.

References Cited E. L. WEISE, Assistant Examiner UNITED STATES PATENTS US. Cl. Rn 3,250,609 5/1966 Bungardt et a1. 7584 75 152 168 3,264,093 8/1966 Sump 7 5 10