US2375199A - Purification of metals - Google Patents

Description

May 8A, 1945. P. P. ALEXANDER 2,375,199

PURIFICATION 0F METALS Filed April4 26, 1944 ATTORNEYS Patented May i945 -v UNITED STATES PATENT oFrlcE Peter P. Alexander, to Metal Hydrides Marblehead, Mass., assignor Incorporated, Beverly, Mass..

a corporation of Massachusetts Application April 26, 1944, Serial No. 532,735

, 19 Claims. (Cl. 'i5-63) This invention relates to the purification of metals contaminated with metal impurities having a lower boiling point, especially oxidizable metal impurities, and has for its object more particularly the purification of metals contaminated with one or more of the alkali metals.

The alkaline-earth metals, calcium, strontium and barium, are usually made by subjecting their fused chlorides to electrolysis. Since the salts of the alkali metals, sodium and potassium, are usually present in these chlorides, the resulting alkaline-earth metal is usually contaminated with a small amount of alkali metal, as well as of the chloride of the alkaline-earth metal, itself. To purify the alkaline-earth metal, it has been proposed to subject the metal to distillation in a closed retort that has been evacuated to remove objectionable air. The distilled metal is permitted to condense in a cooler part of the retort, usually in the form of a more or less spongy deposit. After cooling the retort, air is admitted, the retort is opened and the condense metal is removed. The resulting product is so reactive that it frequently oxidizes rapidly on the surface and produces heat in suiiicient amount to ignite the whole mass, with the result that fires are quite' common. 'I'his objectionable result is all the more apt to occur because of the presence of the more highly reactive sodium and potassium. These alkali metals are also distilled and condense in a cooler part of the retort. The small deposits of sodium and potassium are so reactive that they almost invariably catch re when the retort is opened and they come in contact -with the oxygen of air.

Magnesium is produced pyrometallurgically and electrolytically. It is common in both methods to encounter the alkali metals, particularly sodium and potassium because they are foundwith the magnesium in nature. With sea water or magnesium-bearing ores as the source of the magnesium, there is a marked tendency for sodium and potassium to be present in electrolytic magnesium in amounts that are objectionable for some purposes. When magnesium-bearing ores are subjected to reduction pyrometallurgically, for example, in a retort that hasbeen evacuated to remove objectionable air, and the reduced magnesium is distilled and condensed, sodium and potassium tend to follow the magnesium and are usually condensed in a cooler zone. Fires are not uncommon when the retort, or its associated condenser, is opened and the sodium and potassium rst contact the oxygen of the air.

Special precautions are taken to condense mag- 1 nesium separate from the sodium and potassiumand to remove the sodium and potassium without starting a re. A special condenser for the sodium and potassiumis usually provided at the coolest end of the retort, removed from the zone in which the magnesium is condensed; but the condensed sodium and potassium may nevertheless catch nre on exposure tothe air.

As a result of my. investigations I Ahave discovered that these disadvantages may 'be overcome for the most part. The alkali metals, sodium and potassium, may be segregated from the higher boilingpointmetal to be puried and converted into Vcompounds that are non-inflammable on exposure to the oxidizing influence of air. The metal to be puried may be separately collected and in some instances be freed of other impurities.

In accordance with the invention, the metal to be purified is heated in the absence of air to a temperature sufciently high to distill the alkali metal impurity. 'Ihe resulting alkalimetal vapor impurityis then selectively oxidized. This may be accomplished by bringing the alkali metal vapor impurity into reactive contact with a refractory metal oxide, such as titanium oxide, zirconium oxide and chromium oxide, and collecting the oxidized alkaliA metal impurity as a solid separate from the metal to be puriiied. The metal to be puried is itself advantageously distilledin some cases and condensed separate from the oxidized alkali metal impurity.

These and other features of the invention may be better understood by referring to the accompanying drawing, taken in conjunction with the 4following description, in which:

Fig. 1 is a diagrammatic representation, being a side elevation in section, of an apparatus with which the method of the invention may be practiced; and

Fig. 21s a horizontal section on the line 2 2 of Fig. 1.

The apparatus shown comprises an outer retort it supported Within a heating furnace il, the retort being held in position by means of two or more supports l2 resting on top of the heating furnace? The retort preferably |consists of heatresistant steel. It is provided with a removable cover I3 having attached thereto a vertical pipe it and lateral valved branches I5 and i6, connectable with a, source of vacuum, and air, respectively. A removable inner retort 20, having an open top, rests within the outer retort. A removable tray 2l rests over the open top of the retort, being supported thereon by means of three or more lugs 22 extending over the wall of the inner retort. The heating furnace il consists essentially of a rectangular chamber 23 having a refractory bottom 24, side and end walls 25, and a top 26 having an opening of a, size adapted to receive the outer retort. An expanding opening 21 is provided at or near a lower |corner of one of the side walls of the chamber, for the introduction of heating gases into the chamber. A similar opening 28 is preferably provided at or near an Opposite corner, as a, spare or auxiliary means for around the retort 2l, which is in turn placed vide asealed joint.

introducing heating, or cooling, gases into the chamber. When not in use, the opening 2l is closed. A nue opening 29 extends through theV same wall as the opening 21, preferably at a higher level, soithat heating gases* passed into the chamber through the opening 21 tend to pass before leaving the chamber through the ilue opening 29.

The apparatus may be used as follows: A charge of calcium, for example, contaminated with sodium, potassium, and calcium chloride, is placed in the bottom of the inner retort 20. The inner retort is then placed in the outer retort Ill. A layer of titanium oxide 2li is placed in the tray across the open top oi l The removable cover il is placed l and locked thereon to pro- The lateral branch Ii is then connected to a source of vacuum.v With the valve oi this lateral branch open, and the valve of the other lateral branch` I6 closed, the outer and inner retorts are placed under vacuum to remove objectionable air.

Heating gases are passed through opening 21 into the heating chamber, where they circulate the retort and then ilue opening 29 to the outside lower portion of the outer portion of the the inner retort. on the outer retort pass through the atmosphere., As the retort I0 is heated, the lower inner retort is also heated. As the temperature rises, a point is reached where the sodium and potassium present in the calcium are distilled.

'to the upper and cooler portion of the retort As the temperature at the bottom of the inner retort continues to rise, a point is reached where the calcium is tends to condense and settle out against the wall of the inner retort immediately below the tray I6. Some calcium vapor may of course rise sunlciently to come in contact with titanium dioxide in the tray and react with it in a .manner similar to sodium or potassium vapor. The bulk of the distilled calcium may, densed and collected, as shown. sisting essentially of calcium chloride, .perhaps with a small amount of sodium chloride or potassium chloride, or both, remains as a residue in the bottom oi the inner retort.

The introduction of heating gases to the heating chamber is stopped and the retort is permitted to cool. The cooling operation may be accelerated by passing cooling air through the heating chamber, for example, through the opening 28, or by its contents from admit air to the interior of the retorts, after which the cover I3 is removed. The inner retort and tray are vpulled out of the outer retort; the

distilled and its resulting vapor are advantageously removed by washing the inner retort with water.

It will be clear to those skilled in this art that the invention lends itself to numerous modiilcations. Other metals of higher boiling point, such as barium, strontium, magnesium, etc., may be similarly treated to remove metal impurities of lower boiling point, such as the alkali metals, so- 'dium and potassium, to convert them into harmless compounds. Instead. of titanium oxide other suitable refractory metallic oxide, such as zirconium oxide and chromium oxide,A may be employed, either separately or admixed. While l silicon or ferro-silicon and distilled in an taken oi the relative boiling points 410 elevated to a point where the specific example relates to the purifying of a primary metal, it will be clear` that the invention also lends itself which the metal to be purified is reduced from its compounds and the alkali metal impurity is selectively oxidized. This modification may be employed, for example, in conjunction with the above mentioned method of producing magnesium pyrometallurgically. According to a wellknown practice, magnesium oxide is reduced with externally heated retort placed under vacuum to remove objectionable air. The distilled magnesium is condensed in a cooler part of the retort or in a condenser specially provided for the purpose. The alkali metal impurities, sodium and potassium, are likewise reduced and distilled. In accordance with the present invention, they are converted into harmless compounds by selectively oxidizing them in the manner described.

In the practice of the invention, advantage is of the metals. The boiling points of the alkali metals, sodium and potassium, are substantially below those of the metals to be puriiled. The temperature of the metal to be puried is, therefore, gradually the metal impurity of is distilled. By heating its is caused to yield a graduated temperature range. Since the alkali metals have the lower boiling points, their vapors tend selectively to rise highest in the retort. Since the metals to be puried have the higher boiling points, their vapors do not tend to rise as high in the retort. They soon meet a temperature environment conducive to condensation. In other words, the` metals to be puried may be made to condense in an intermediate section of the retort where thetemperature is high enough to keep the alkali metal impurities in vapor form. The alkali metal vapors, on the other hand, tend to rise to a cooler portion of the retort, where they were heretofore condensed, but where they are now converted into harmless compounds in accordance with the invention.

This application is a continuation-impart of my co-pending application Serial No. 478,095, led March 5, 1943'.

I claim:

1. In the method of purifying metals of higher boiling point contaminated with an alkali metal impurity of lower boiling point, the improvement which comprises heating the metal to be puriiled in the absence of air to a temperature sufiiciently high to distill the alkali metal impurity but not the metal to be puried, selectively oxidizing the resulting alkali metal impurity vapor lower boiling point bottom, the retort ,in a zone removed from the main body oi' metal to be puriiled, and collecting the oxidized alkali metal impurity as a solid separate from the metal to be puriiled.

2. Method according to claim l, in which the to a smelting operation in Y to be purified is distilled.

selectively oxidized by bringing it into reactive p contact with titanium oxide in a zone removed from the main body of metal to be purified. A

4. Method according to claim 1, in Which the resultingvapor of the alkali meta1 impurity is selectively oxidized by bringing it into reactive contact with zirconium oxide in a zone removed from the main body of meta1 to be purified.

5. Method according to daim 1, in which the resulting vapor of the alkali metal impurity is selectively oxidized by bringing it into reactive contact with chromium oxide in a zone removed from the main body of metal to be purified.

6. Method according to claim 1, in which the metal to be purified is also` distilled, and condensing the resulting vapor of the metal to be purified as a solid separate from the oxidized alkali meta1 impurity in a zone intermediate that in which the alkali metal impurity is oxidized and that from which the main body of metal 7. In the method of purifying metals of higher boiling point contaminated withA an alkali metal impurity of lower boiling point, the improvement which comprises heating the, metal to be puriiiedl in the absence of air to a temperature su'ciently high to distill the alkalimetal impurity but not the metal to be puried, said alkali metal impurity being at least one of the group: sodium and potassium, selectively oxidizing the resulting alkali metal impurity vapor in a zone removed from the main body of metal to be puried, and collecting the'oxidized alkali metal impurity as a solid separate from the metal to be purified.

8. Method according to claim 7, in which the resulting vapor of the alkali metal impurity is selectively oxidized by bringing it into reactive contact with a refractory metal oxide in a zone removed from the main body of metal to be purihed..

9. Method according to claim 7, in which the metal to be purified is also distilled, and condensing the resulting vapor of the meta1 to be purified as a solid separate from the oxidized alkali metal impurity in a zone intermediate that in which the alkali metal impurity is oxidized and that from which the main body of metal to be puried is distilled.

10. In the method of purifying magnesium contaminated with an alkali metal`impurity, the improvement which comprises heating the magnesium in the absence of air to a temperature sufiiciently high to distill the alkali metal impurity but not the magnesium, selectively oxidizing the resulting alkali metal impurity vapor in a zone removed from the main body of magnesium to be puriiied, and collecting the 'oxidized alkali meta1 impurity as a solid separate from the main body of magnesium to be purified.

. 11. Method according to claim l0, in which the resulting vapor of the alkali metal impurity is selectively oxidized by bringing it into reactive contact with a refractory metal oxide in a zone removed from the main body of magnesium to be purlfled.

12. Method according to claim 10, in which the resulting vapor of the alkali metal impurity is selectively oxidized by bringing it into reactive contact with titanium oxide in a zone removed' from the main body of magnesium to be purified. 13. Method according to claim 10, in `which the resulting vapor of the alkali metal impurity isV lselectively oxidized by bringing it into reactive contact with zirconium oxide rin a zone removed from the main body of magnesium to be purified.

14.v Method according to claim 10, in which the resulting vapor of the alkali meta1 impurity is selectively oxidized by bringing it into reactive contact with chromium oxide in a zone removed from the main body of magnesium tobe puriiied.

15. Method according to claim 10, in which the magnesium to be purified is a1so.distilled, and condensing the resulting magnesium vapor as a solid separate from the oxidized alkali metal impurity in a zone intermediate that in which the alkali metal impurity is oxidized and that from which the main body of magnesium is distilled.

16. In the method of purifying magnesium contaminated with an alkali metal impurity, the improvement which comprises heating the magnesium in the absence of air to a temperature sufiiciently high to distill the alkali (metal impurity but not the magnesium, said alkali metal impurity being at least one of the group: sodium and potassium, selectively oxidizing the resulting 'alkali meta1 vapor impurity in a zone removed from the main body of magnesium to be purified,

and collecting the oxidized alkali metal impurity as a solid separate from the magnesium.-

\ 1'7. In the method of purifying magnesium contaminated with an alkali meta1 impurity, the improvement which comprises heating the magnesium in the absence of air to a temperature sufliciently high todistill the alkali metal impurity but not the magnesium, selectively oxidizing the resulting alkali metal impurity vapor by bringing it into reactive contact with a refractory metal oxide in a zone removed from the main body of magnesium to be purified, collecting theoxidized alkali metal impurity as a solid separate from the main body of magnesium to be purified, distilling the magnesium, and condensing the resulting magnesium vapor as a solid separate from the oxidized metal impurity in a zone intermediate that in which the alkali metal impurity is oxidized and that from which the main body of magnesium is distilled.

18. In the method of purifying metals of relatively low boiling points contaminated with readily oxidizable metal impurities of lower boiling points, the improvement which comprises heating the meta1 to be puriiied'in the absence of air to a temperature sufficiently high to distill the metal impurity but not the metal to be puriiiedpselectively oxidizing the resulting metal impurity vapor by bringing it into reactive .contact with a reducible refractory meta1 oxide in a zone removed from the mainbody of meta1 to be puriiied, and collecting the oxidized metal impurity as a solid separate from the metal to be purified.

19. Method according to the preceding claim, in which the metal to be puriiied is also distilled, and condensing the resulting vapor of the metal to be puried as a solid separate from the oxidized metal impurity in a zone intermediate that in which the metal impurity is oxidized and that from which the main body of metal to be purified l is distilled.

PE'I'ER P. ALEXANDER.

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