US2183420A - Purification of lead and alloys thereof - Google Patents

Description

Patented Dec. 12,- 19.3

UNITED STATES rnamcs'non or man AND armors manor Gustave E. Behr, New York, N. Y., assignorto National Lead Company, New York, N. Y a corporation of New Jersey 1 No Drawing. Application November 10, 1938, Serial No. 239,773

w '3 Claims. The present invention relatu; to lead and lead "alloys and has for its primary object, among' others, a simple and efilcient means for removing zinc from lead and lead alloys.

Lead and lead alloys which are contaminated with zinc are principally derived from two sources; (1) de-silvered Ilead bullion, and (2) secondary, or scrap lead and lead alloys which are returned to the trade for refining. From the first source the presence of the zinc in the lead I bullion results from the use of zinc as a de-silverizing agent; from the second source, the zinc apparently results --tlon 1 I have found that if lead orlead alloys containing zinc are melted and treated under controlled conditions hereinafter to be described with ammonium chloride the zinc is substantially wholly converted into zinc chloride which rises to the top of the molten lead or lead alloy and may easily be removed. Because of the volatility of ammonia and ammonium chloride no deleterious addition of occurs.

I am aware that it has been proposed to remove monial lead except, perhaps, in such small amounts as to render removal unnecessary. The zinc which occurs in de-silverized lead or sec ondary lead and lead alloys is not in the form of a zinc compound but as. the metal, either in solution in the lead or alloyed therewith. In such forms the zinc cannot be removed by the mere physical means of dressing but a chemical expedient as described in my present invention must a be employed. i

According to the present invention, the lead or lead alloy to be purified of zinc is melted. .The molten'metal is stirred, preferably'by means of a-stirrer which produces a good vortex and while maintaining the metal at a temperature above thesolidifying point, powdered solid ammonium chloride is gradually added. In a short time a thin liquid, more or less dark in color begins to collect on the surface of the molten metal. This liquid, I have found, is substantially'made up of from unintentional contaminaf contaminants to the lead alloy does not'ordinarily occur in lead bullion or anti-.

liquid zinc chloride. Addition of ammonium chloride is continued until no more liquid forms. The purified metal may be separated by any suitable means: it may be' drained from the bottom of the kettle, the liquid may be drained off the metal or the whole may be allowed to solidify 'and' the salt cake of zinc chloride removed from the surface of the solid metal. 4 a

The temperature at which I the ammonium chloride is added should preferably be as low as possible to avoid excessive losses. of the ammonium chloride by'volatilization but it should not be so low as unnecessarily to prolong the duration of the treatment. I have found temperatures between about 400 C. and about 475 C. are best adapted for the practice of the invention.

The quantity of ammonium chloride required in order to remove zinc from lead or lead alloys according to the invention will, of course, be dependent upon two factors: (1) e amount of zinc to be removed and (2) the loss of ammonium chloride by volatilization. Aside from that lost by volatilization, at y leastsuflcient ammonium chloride should be introduced into the molten .metal to convert the whole 01 the zinc present into ammonium chloride, according to the equation: Zn+2NH4Cl- ZnClz+2NH3. 0n the whole, however, my experience has been that about from one and one-half to ten pounds ammonium chloride should be used for the removal of one pound o'Lzinc. I Y

The addition of the ammonium chloride should,

preferably, although not necessarily, be made in 0 portions over a period of time in order to cut down as much aspossible volatilization losses and to permit observation of the efiect of each addition. Such observation enables the operator to determine whether or not the treatment is finished. If no more liquid is formed after an addition of ammonium chloride the zinc has been substantially completely removed.

The following example illustrates but does not limit the invention:

trample 59,252 pounds of'a lead-tin alloy consisting o proximately of 60% lead and 40% tin and c'ontaining 0.21% zinc was 'melted and agitated. 200 poundsot NHiCl was added over a periodof 40 minutes. The temperature during the operation was held at about 400 C. 335 pounds salt skim was recovered. Thejtreated alloy was en-. tirely free of zinc.

The foregoing descriptionhas been given for clearness of understanding and no undue limitatlons should be deduced therefrom but the appended claims should be read as broadly as possible in view of the prior art.

I claim:

1. A method for removing zinc from lead and lead alloys which comprises melting the metal to be purified and while agitating it at a temperature above the solidifying point thereof adding ammonium chloride thereto, in the absence oi 10 added chlorine gas, and removing the zinc chloride skim formed thereby.

2. A method for removing zinc from lead and lead alloys which comprises melting the metal to be purified and while agitating it at a temperature between about 400 C. and about 475 C.

gradually adding thereto, in the absence of added cblorinmw. at least a suiiicient amount of ammonium chloride to convert the zinc contained