US2373516A - Purification of magnesium - Google Patents

Description

Patented Apr. 1'0, 1945 Q 7 PURIFICATION OF MAGNESIUM Philip T. Stroup and George F. Sager, New- Kensington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application March 30, 1942,

. Serial No. 436,765

7 Claims. (01. 75-47) This invention relates to the removal of iron from magnesium and it concerns more particularlythe removal of iron withthe aid of zirco- The term "magnesium as used-herein and in the appended claims will be understood to refer not only to magnesium itself but also to alloys suspel ion throughout the magnesium matrix,

and this finely dispersed iron usually causesa marked reduction in the resistance of magnesium to corrosive attack. a

In our copending application Serial No. 436,764 filed Mar. 30, 1942, for Purification of magnesium, a method is described which employs zirconium chloride for removing iron from molten magnesium. The zirconium chloride introduced into the molten metal appears to react with magnesium, releasing zirconium into the melt and this zirconium, without the necessity of any further treatment oi the melt, removes iron from the magnesium in a manner more satisfactory than zirconium, introduced into magnesium by other methods.

The object of the present invention is to remove iron from magnesium. More particularly,

.the object is to make the aforesaid method of removing iron from molten magnesium with the aid of zirconium chloride as economical and asconvenientin actual plant practice as possible. For this purpose, we have sought to provide a way whereby a charge of zirconium chloride can be in th magnesium to form a magnesium-zirconium alloy, settles to the bottom of the melt, carrying iron also to the bottom it the magnesium contains iron. The supernatant magnesium, or the supernatant magnesium-zirconium alloy which may have been formed, then can be decanted or-otherwise removed from the crucible, leaving as a residue a heel of magnesium rich inzirconium.

The magnesium-zirconium alloy, which may be either the heel or the supernatant alloy separated from the zirconium-rich heel, can be'utilized as an addition alloy to introduce zirconium into subsequent melts of magnesium to remove iron from them. It is necessary, however, that enough of the addition alloy from the initial melt be added to provide the subsequent melt with at least 0.1

per cent of zirconium since about that amount of zirconium is required to effect a satisfactory removal of iron. 'Since a greater amount of zirconium generally is present in the heel than in the decanted magnesium-zirconium alloy, we find it more convenient in the practice of our invention to utilize the heel as the addition alloy for introducing zirconium and removing iron in thesubsequent melts than to use the decanted alloy for that purpose. Accordingly, the-invention will be described as embodied in the use of this heel for introducing zirconium and'removing iron from subsequent melts of magnesium, but it will be understood that the decanted purifled magnesium, when it contains enoughv zirconium after .the treatment with zirconium chloride to provide a subsequent melt with 0.1 per cent or more of zirconium, can be used in the same way. as the heeland for the same purpose.

When the zirconium-rich heel from the initial melt is introduced into another body of molten magnesium to provide 0.1 per cent or mo;'e of utilized to remove iron from more than one melt of magnesium. J

Our present invention is predicated upon our discovery that an alloy of magnesium .containing zirconium derived by reaction. of zirconium chloride with magnesium can itself be introduced into a melt of magnesium to provide that melt withzirconium eflective for removing iron.

We have found that when 0.25 per cent or more ma preferably when from 0.25 to 5 percent of zirconium chloride is charged into molten magnesium in the practice of the invention described and claimed in the aforesaid application for Letters Patent. the zirconium thereby introduced, except for a small amount which may be dissolved zirconium to that body of molten magnesium, the zirconium precipitates and in from about minute up, to 'about' 15 minutes, depending upon the a size of the melt, remove iron in that body of magnesium, forming another heel which may itself be used to remove iron and to form a heel in yet another melt of magnesium. Accordingly, our present invention also embraces utilizing the heel formed in each 01' a series of melts-to introduce zirconium and remove iron from the magnesium in the next succeeding melt.

Following treatment ot--the initial melt or of each succeeding melt of magnesium in the prac-- tice of our invention, the precipitated zirconium-v rich heel and the, purified magnesium, which-of course, may contain some zirconium, can easily be separated. We prefer to decant the upper purified portion of the melt from the crucible, leaving the lower precipitate-containing portion of the melt, or the heel, as a residue ready to be mixed into the next melt to be treated for removal of iron. Instead, however, the heel may,

be removed from the crucible by methods known to the art, leaving the purified magnesium, instead of the heel, in the crucible.

In utilizing the heel from one melt for purifying the following melt, it is possible to transfer the heel from the crucible in which it is formed to another crucible already containing the molten -magnesium to be purified, and thereby to introducezirconium into that melt. We find it more convenient, however, 'to leave the heel in the bottom of the crucible after the magnesium is decanted and to charge subsequent melts into that crucible and over the heel. Each melt, properly protected by a suitable flux or atmosphere, pref erably is agitated to stir the heel into the magnesium; and the heel re-iormed in each melt by the settling zirconium and iron is left in the crucible when the purified magnesium is removed and is used to purify the succeeding melt.

Instead of keeping the heel in the molten state after the purified magnesium is poured from the crucible, the heel may be allowed to solidify and then either melted down with the next charge of magnesium or added after the next charge is molten. In large scale melting practice, however, it is more convenient to have the heel already in the molten condition when the next charge of magnesium is introduced into the crucible.

Since iron as well a zirconium is contained in the heel which forms following treatment of each body of molten magnesium, the introduction of each heel into a subsequent melt introduces iron as well as zirconium into that melt. This iron and the iron already in the magnesium then settle out with zirconium. During treatment of a succession of melts with a single charge of zirconium chloride, the iron content of the heel increases as the iron removed from each successive melt of ma nesium becomes added to the heel. Eventually the iron content in the heel becomes so great that zirconium in the heel is incapable of settling both the iron impurity in the magnesium being treated and the iron introduced from the heel. At this point, the iron content is usually about 1.5 per cent of the weight of the heel and it is necessary either that the heel be augmented with more zirconium introduced as zirconium chloride in order to prolong the effectiveness of the heel for removing iron or that the heel be discarded and. a new heel formed with zirconium chloride. Even without the addition of supplementary amounts of zirconium chloride, however, a single heel formed in an initial melt of magnesium by the introduction of zirconium chloride equivalent to 0.25 to 5 per cent of the total weight of the initial melt normally can be used to remove iron from a succession of up to about 12 subsequent melts. An even greater number of melts of magnesium may be treated for the removal of iron it fresh amounts of zirconium chloride are added from time to time or if a greater amount of zirconium chlorde originally is added to the initial melt to form the heel.

The operation of our invention described and claimed herein is efiective for the removal of iron impurity from both magnesium itself and from magnesium containing the usual amount of common alloying elements. For example, iron may be removed from magnesium base alloys containing from 1 to 15 per cent aluminum. or 0.2 to 10 per cent zinc, or 0.1 to 2.5 per cent manganese, or 0.5 to 8 per cent tin, or combinations of these elements. Other elements also may be present as alloying ingredient in magnesium treated by the method of our invention.

"mens were cast in sand molds from each melt and these were analyzed to determine. their iron contents. The first melt then was charged with zirconium chloride in briquette form in an amount and the molten alloy was poured off, leaving a heel containing magnesium, zirconium, and, iron at the bottom of the crucible. The decanted metal then was analyzed, ,and its iron content was found to have been diminished by the treatment with zirconium from 0.028 to 0.005 per cent.

The heel left after the pouring of the first melt then was admixed with the second melt and allowed to settle. The purified magnesium then was decanted and test specimens cast in sand from the decanted portion were analyzed-to determine the extent of the iron reduction by treatment with the zirconium-rich heel. Thereupon, the heel was admixed with a third melt to remove it iron and the heel formed in this third melt was charged into the fourth melt. Altogether, the heel was charged successively from one portion to the next until all eight melts had been treated. The following table shows the amount of iron in each melt before and after the treatment with zirconium.

Mon Amount of Amount of iron before iron af ter treatment treatment e 1 Treated with 2 ZrCli P r f Per (05 2 Treated with heel from No. l. 022 .009 3 Treated with lice] from No. 2. .031 .006 4 Treated with heel from No. 3. .017 .005 5 Treated with heel from No. 4. .016 .008 f i Treated with heel from No. 5. .012 .007 I Treated with heel from No. 6. .027 .015 S Treated with heel from No. 7. 020 .011

This example will be understood to be illustrative of the practice 01' our invention and it will be appreciated that modificationsw'hich will appear to those skilled in the art will be within the scope or our invention.

We claim:

1. The method of removing iron impurity from a melt of magnesium which comprises mixing said melt with magnesium containing zirconium previously provided therein by reaction of zirconium chloride with magnesium, permitting the zirconium and iron in said melt to ettle to the bottom of the container, and separating from each other the upper purified portion and the lower precipitate-containing portion of the resultant melt.

2. The method of removing iron impurity from a melt of magnesium which comprises mixing said melt with magnesium containing zirconium previously provided therein by reaction of at least about 0.25-per cent by weight of zirconium chloride with magnesium, permitting the zirconium and iron to settle to the bottom of the container, and separating from each other the upper purified portion and the lower precipitate-containing portion of the resultant melt.

3. The method ofremoving iron impurity from a melt of magnesium which comprises mixing said melt with magnesium containing zirconium previously provided therein by reaction of between about 0.25 and 5.0 per cent by'weight of zirconium chloride and subsequent settling of iron and of the zirconium'with magnesium, permitting said melt of magnesium to stand for a time between about one-half minute and about 15 minutes while precipitation of zirconium and iron to the-bottom of the container takes place, and decanting the purified magnesium from the precipitated zirconium and iron.

4. The method of removing iron impurity from a melt of magnesium which comprises mixing said melt with an initial body of magnesiumcontaining zirconium derived by reaction therein of zirconium chloride with magnesium, the zirconium content of said initial body of magnesium being suificientto provide said melt of magnesium with at least 0.1 per cent of zirconium. permitting the zirconium and iron in said melt to settle to the bottom of the container, and separating from each other the upper purified portion and the lower precipitate-containing portion of the resultant melt.

5. The method of removing with a heel derived from an initial body of magiron from a melt of magnesium which comprises mixing said melt nesium containing zirconium provided by reaction therein or zirconium chloride with mag-.

nesium, the zirconium content of said heel being suflicient to introduce at least 0.1 per cent of zirconium into said melt, permitting the zirconium and iron in said melt to settle to the bottom of the container, and separating from each other the upper purified portion and the lower precipi- 1:.

tate-containing portion of the resultant melt.

6. The method of removing iron impurity from a succession of melts of magnesium which comprises introducing zirconium chloride in an initial melt of magnesium in an amount suificient to remove iron and to form a heel containing zirconium in said initial body of magnesium, separating said heel from said initial melt, charging the heel into a second melt of magnesium to cause precipitation of iron and zirconium and the formation of another heel in said second melt adapted to be charged into a third melt to remove iron impurity from said third melt.

7. In combination with the method of removing iron from a body of molten magnesium by charging. at least about 0.25 per cent of zirconium chloride into said body of molten magnesium to cause precipitation of iron and zirconium and i'ormation of a residue containing said precipitate-containing portion of the resultant melt to provide a heel comprising said precipitate-containing portion adapted to be mixed in yet another body of molten magnesium to provide zirconium and remove iron in that body of magnesium.

PHILIPVT. STROUP. GEORGE F. SAGER.