US2416992A - Method and apparatus for condensing metallic vapors - Google Patents

Description

March 4, 1947- T. GRISWOLD, JR.,, ET AL. 2,415,992"

METHOD AND APPARATUS FOR CONDENSING METALLIC VAPO'RS Filed March 24, 1945 II; 14 lllllllll.. I

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INVENTORS Thomas Gn'swo/afjn BY Thomas H M Con/c0122 ATTORNEYS Patented Mar. 4, 1947 METHOD AND APPARATUS FOR CONDENS- ING METALLIC VAPORS Thomas Griswold, r., and Thomas H. McConica, HI, Midland, Mich., assignors to. The Dow Chemical Company, Midland, Mich., a corporation of Michigan Application March 24, 1945, Serial No. 584,629

Claims. r 1 This invention relates to improvements in the condensation of metal vapors. It also concerns the use of these improvements in the recovery of volatile metals from alloys containing them;

In a number of metallurgical processes in which a metal is first liberated as a vapor and is then condensed, economic success requires that there be little if any loss of valuable product due to incomplete condensation and that the condensed product be wholly in liquid form. However, under adverse operating conditions, such as when the metal vapor is diluted with fixed gas or when the liquid metal condensate has an appreciablevapor pressure, the condensation step, when carried out conventionally, may permit a material part of the vapor to escape uncondensed. This vapor then either is lost in the exhaust gases or may be converted in cooler parts of the apparatus to a fine powder which is difiicult to collect and which can be coalesced to liquid metal only by elaborate treatment. As a result of these diificulties, which have heretofore not always been overcome satisfactorily, the development of several otherwise promising metallurgical processes has been retarded.

It is accordingly an object of the present invention to provide an improved condensation method and apparatus which avoid the disadvantages mentioned and which permit virtually complete conversion of metal vapors to liquid form, even when the vapors are diluted with gas or the liquid condensate has an appreciable vapor pressure.

In metallurgical condensation processes embodying the invention, a major part of the metal vapor may first be recovered in conventional manner, by cooling the vapor to. a condensing temperature above the freezing point of the metal to produce a liquid condensate. The surviving vapors, which would be lost in previous processes, are then passed into a second zone into intimate contact with a substantially nonvolatile quenching liquid supplied at such a temperature that the vapor pressure of the metal, when con-i to condensing metal vapors, it is particularly useful in processes in which the vapor to be condensed is supplied under such conditions of temperature, pressure, or dilution that the vapor pressure of the liquid metal condensate is an appreciable fraction, say 3 per cent or more, of the partial pressure of the vapor prior to condensation. It is in these cases, especially when the metal vapor is at a drastically reduced .pressure or is diluted with fixed gases, that formation of powdery condensate is most troublesome and the invention has its greatest usefulness. Typical instances are met in the vacuum distillation of magnesium and of sodium from alloys con-- taining them, in the presence of inert gases, such as hydrogen or helium.

The invention may be explained in detail as applied to the vacuum distillation of magnesium from a lead-magnesium alloy containing 5 to 20 per cent by weight of magnesium and with reference to the accompanying drawing, in which the single figure is a vertical view, partly in section, of one form of suitable distilling apparatus.

In the equipment shown, the liquid lead-magnesium alloy to be-distilled is stored in an alloy steel tank 2 surrounded by insulating brick 3. The alloy is withdrawnby a sump pump 4 driven by a motor 5 and forced through a reed line 6 into a gas scrubbing tower 1 consisting of an alloy steel pipe into which splash plates 8 are welded jects through packing gland H in the scrubber Wall. The alloy, after falling through the scrubber, leaves by a drain line It! and flows into a stripper l3 in which part of the magnesiuminthe alloy is vaporized and condensed The stripper I3 is constructed of a large-diameter alloy steel pipe set vertically and closed at both ends, with a flanged clean-out M at the top and a drain pipe l5 at the bottom leading to the reservoir 2. The stripper is divided into a lower boiling section 16 and an upper condensing. section ll, the two zones being separated by radiation shielding baflles 18. The boiling region I6 is set in a brick furnace l9 heated by gas burners 20, and the condenser I1 is cooled externally by anair circulating jacket 2|.

The lead-magnesium alloy flowing from the scrubber I through the feed line I2 is introduced into the uppermost of a series of frusto-conical trays 22 which are welded to the stripper wall at regular intervals down the heated section It.

The annular space between each tray and the adjacent boiler wall serves as a boiling trough, and is in communication with the like space of the next lower tray by an overflow pipe 23. These pipes 23 are preferably set at 180 intervals from tray to tray, so that the alloy being boiled is caused to flow successively through the entire extent of each tray, finally leaving the bottom tray and returning to the reservoir 2.

Magnesium vapors liberated in the boiling section [6 escape upwardly into the condensing section H, where they come into contact with the cooled walls and are condensed to liquid magnesium. The condensate trickles down into a collector trough 24 welded to the wall of the condenser at the bottom, and from there runs through a product line 25 into a holding pot 26 mounted in a brickwork setting 21 fired by a gas burner 28.

The scrubber 1 and stripper l3 are maintained under vacuum by an exhaust pump 29 connected by a valved vacuum line.30 to a pipe riser 3| on the top of the scrubber, which is also provided with a valved clean-out .32. The scrubber and condenser I! are .in open gas communication through a large equalizing line- 33, which extends from a point below the lowestscrubber splash plate 8 through the stripper wall to the top of the condenser.

The alloy tank 2, scrubber 1, condenser l1, and associated pipes 6, l2, I5, 25, and 33 are all provided with heaters, not illustrated, to raise them to the appropriate temperatures when beginning operation and with thermal insulation, not shown, to prevent freezing of. the lead-magnesium alloy and the magnesium product once circulation is established.

In operation of the apparatus shown in the drawing, the lead-magnesium alloy in the tank 2 is kept under a protective saline flux or inert gas atmosphere. The tank, feed line 6, and scrubber I are maintained at a temperature which is above the freezing point .of the alloy, but is sufficiently low that the partial pressure of magnesium above the alloy is inappreciable, preferably below 600 C. The fuel supply to the burners 20 is adjustedto maintain the heated portion 16 of the stripper ata boiling temperature, usually 775 to 875 (3., and the cooling air to the condenser I'! is controlledso thatthe temperature of the magnesium condensate is above its freezing point (651 C.) preferably 655 to 665 C. The exhaust system is operated to hold the scrubber 1 and stripper l3 at a pressure which is suflie ciently reduced to permit boiling to take place at a high rate, but is above the vapor pressure of the magnesium condensate; absolute pressures between 0.1 and about 0.5 inch of mercury are most satisfactory. At this pressure, the alloy in the feed line 6 and the return drain [5, and the magnesium in the product line 25 rise to levels well above those of the tank 2 and pot 26, forming barometric seals.

The pump 4 is then operated, so that'the leadmagnesium alloy enters the scrubber 1, showering' down over the splash plates, and then passes to the stripper l3, flowing in succession through the boiling trays 22 and finally returning to the tank 2. The feed valve 9 is adjusted so as to regulate the alloy flow in accordance with the desired production rate without flooding the .trays 23. During passage of the alloy through the trays, part of the magnesium is vaporized. The vapors rise into the condenser I! where they are in large part condensed to liquid magnesium, which leaves as product through the line 25. Uncondensed magnesium vapor, and any inert gases boiled out of solution in the feed alloy, then escape from the condenser through the equalizing line 33 into the base of the tower I. There they flow upwardly countercurrent to the shower of feed alloy, being thoroughly scrubbed. As a result of this action, magnesium vapor in the condenser exhaust is almost wholly absorbed or quenched by the feed alloy and is returned to the stripper for reboiling. Practically, only inert gases ever reach the riser 3| and are removed by the vacuum pump 29. In this way, virtually no loss of valuable magnesium takes place.

Inasmuch as the lead-magnesium alloy in the tank 2 is gradually depleted of magnesium during circulation through the stripper l6, operation would necessarily stop unless magnesium were constantly added to the alloy. For this reason, the apparatus illustrated finds its greatest use in carrying out the distillation step of a continuous magnesium production process in which the metal is liberatedfrom its ore in vapor form, and the vapor is then quenched in lead or a lean leadmagnesium alloy to form a richer alloy from which the magnesium is recovered by distillation. One such process is described in detail in a co-pending application, Serial No. 554,975, filed September 20, 1944, by Thomas H. McConica. In

such a process, the alloy in the tank 2 is circulated to the quenching step, as well as to the distillation step in the stripper l6, and the two operations are co-ordinated so that the magnesium content of the alloy remains roughly constant.

has been described with reference to the distillation of magnesium, it is equally useful in the distillation of metallic sodium from lead-sodium alloys containing 5 to 15 per cent by weight 01 sodium. In this case the alloy in the tank 2 and scrubber I is maintained molten at a temperature below 450 0., and the furnace i9 is regulated to hold a temperature of 675 to 750 C. Under these conditions, absolute pressures as low as 0.02 inch of mercury may be used, although pressures up to 0.5 inch are operable.

The process of the invention is not limited to the specific illustrations given, but is useful in separating any relatively volatile metal from a molten alloy with amuch less volatile metal. Still other ways of applying the principle of the invention will doubtless occur to those familiar with the general field, it being intended that the invention be limited in scope only by the following claims.

What is claimed is:

1. In a process for the recovery of a volatile metal from a molten alloy thereof with a much less volatile metal, the steps which comprise: feeding the alloy through a scrubbing zone at a temperature at which the vapor pressure of volatile metal in the alloy is negligible and thence into a distilling zone and therein heating it to vaporize volatile metal; conveying the evolved metal vapors into a condensing zone and cooling them to a temperature such as to condense a major part of the vapors as liquid metal product; and withdrawing surviving vapors from the condensing zone and passing them through the aforesaid scrubbing zone into intimate contact with the alloy therein, thus effecting substantially complete recovery of the vapors of volatile metal escaping the condensing zone for return to the distillation step with the feed alloy.

.. "2- In a process for the recovery of magnesium While operation of the apparatus illustrated from a lead-magnesium alloy containing 5 to 20 per cent by weight of magnesium, the steps which comprise: feeding the alloy through a scrubbing zone at a temperature below 600 C., and thence into a distilling zone maintained at a pressure between 0.1 and about 0.5 inch of mercury absolute and therein heating it to vaporize magnesium; conveying the magnesium vapors into a condensing zone at substantially the same reduced pressure and cooling them to a condensing temperature above 651 C. to condense a major part of such vapor as liquid magnesium product; and Withdrawing any surviving vapors from the condensing zone and passing them through the aforesaid scrubbing zone into intimate contact with the alloy therein, thus effecting substantially complete recovery of magnesium escaping the condensing zone for return to the distillation step with the feed alloy. H

'3. In a process for the recovery of sodium from a lead-sodium alloy containing 5 to 15 per cent by weight of sodium, the steps which comprise: feeding the alloy through a scrubbing zone at a temperature below about 450 C. and thence into a distilling zone maintained at a pressure below 0.5 inch of mercury absolute and therein heating it to vaporize sodium, conveying the sodium vapors into a condensing zone at substantially the same reduced pressure and cooling them to a condensing temperature to condense a major part of such vapors as liquid sodium product; and withdrawing any surviving sodium vapors from the condensing zone and passing them through the aforesaid scrubbing zone into intimate contact with the alloy therein, thus efiecting substantially complete recovery of sodium escaping the condensing zone for return to the distillation step with the feed alloy.

4; Condensing apparatus comprising, in combination with a gas-flow system including a source of metal vapor, a condenser for the vapor, and an exhauster for removing gas from the condenser, apparatus for receovering any uncondensed metal vapor from the condenser exhaust gas comprising a gas-liquid contact chamber interposed in the gas-flow system between the condenser and the exhauster, a reservoir for a molten metal absorbent for recovering metal from the exhaust gas, circulat ing means for withdrawing liquid from the reservoir and supplying it to the contact chamber into intimate contact with gas flowing through the chamber. and distillation means for recovering condensed metal from the absorbent after its passage through the contact chamber.

'5. Apparatus for the recovery of a volatile metal from an alloy thereof with a less volatile metal comprising: a reservoir for the alloy; a boiler for heating the alloy to vaporize volatile metal; a condenser for metal vapor in communication with the boiler; an exhauster for removing fixed gases from the condenser; a gas-liquid contact chamber for recovering uncondensed metal vapor from the condenser exhaust gas; gas conduits connecting the condenser with the gas inlet of the contact chamber and the exhauster with the gas outlet thereof; and liquid conduits connecting the alloy reservoir with the liquid inlet of the contact chamber and the boiler with the liquid outlet of the chamber.

THOMAS GRISWOLD, JR. THOMAS H. MCCONICA, III.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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