US1957819A - Refining metal - Google Patents

Description

Patented May 8, 1934 UNITED STATES REFINTNG METAL William A. Cowan,

National Lead Company,

Brooklyn, N. Y., assignor to New York, N. Y., a

corporation of New Jersey ApplcationJuly 19, 1930, Serial No. 469,251

s claims. gol. 'z5- 17) The invention is a process of rening so-called white metal alloys such as tinor lead-base alloys, for the purpose of eliminating therefrom, or reducing therein, the proportions of certain ingredient metals regarded as undesirable or as impurities and commonly such as antimony, copper, arsenic and iron, but including any others to which the process applies. Its object is to accomplish complete removal of the impurity, or

0 a desired reduction of proportions, with greater efficiency and at less operating cost than heretofore.

The process is carried out by first ascertaining,

by test or otherwise, the existing proportions of such impurity metal or metals in the metal or alloy to be treated arid then contacting such metal t aluminum with another with an appropriate amount of aluminum which is best added to the molten metal in the form of flat or substantially flat pieces, such as the cuttings from aluminum sheets. These are readily spread over the molten metal coating thereon and are used as a protective blanket on the metal more or less excluding air and thus preventing burning 2, during the raising of the temperature to the point at which the aluminum will combine therewith. Whether the aluminum is introduced in this form, or in another form, it is actively stirred intorthe metal, when the latter has lreached the proper temperature, say 600 C. or more, and`for this purpose a power-driven stirrer is used and which, to be suitable, must be capable of carrying the aluminum under and keeping it in motion through the metal until the reactionis complete. The process depends in large measure on the thoroughness of the stirring; satisfactory results have been obtained by a sc-called cone stirrer having a propeller in the cone which maintains a constant circulation downward at the center of the kettle and upwards along its side walls, thus keeping the aluminum in constant motion during the reaction. f

By introducing the aluminum in solid pieces .while the metal to be treated is being heated,'it is found that the desired'reaction takes place before the melting point of the aluminum addition has been reached, the pieces apparently dissolving in the bath, as the temperature approaches that point and the stirring is continued until they have disappeared. Heating the metal to a temperature higher than will thus dissolve the aluminum is not necessary, and advantage is taken of the fact to save time as well as fuel. The suitable temperature approximates 620 C.,

i5 which is less than the melting point of aluminum I (G50-660 0.), being lower if tin is present in the alloy or metal being treated.

The reaction, which is exothermic, resultsvin -the'formaton of definite chemical compounds impuri y metal or metals,

of aluminum with the having a composition following the general formulas, AlSb, AlCuz, etc., according as to which of the impurity metals were present in the original Ametal. So-called white metal scrap commonly contains a substantial portion of antimony which is thus, in most cares, the principal and sometimes the only impurity to be removed, but copper and arsenic and also iron are found to form compounds with aluminum separable on the same basis.

Instead of adding aluminum alone, an alloy of metal can be lused in those cases where the introduction of the companion metal is not incompatible with the process '75 or the result desired. That is to say, if the metal under treatment already contains tin, the process may be performed by adding a tin-aluminum as stated and to be alloy, for example, one-third aluminumand twothirds tin, in place of the tin thus introduced would merely be in addition to that already present and may not be inconsistent with the result aimed at. The use of such alloy, when permitted, has the vadvantage that it is then poss'ble to dissolve or stir it into the molten metal at a lower temperature than when aluminum alone is used, perhaps as much as C. lower, thus to some extent further economizing on fuel and time.

The amount of .aluminum added, whether as straight aluminum or alloy, depends on the amount of impurities to be removed, and for complete removal of the ordinary run of the impurities mentioned, should be from two to three ninths of the amount of such impurities by weight more straight aluminum, because .so

or less, which is slightly more than the theoretical combining power of aluminum with the particular impurity metal or metals, thus avoiding any substantial excess of al um after the reaction is complete. Ifthe proportion of impurity vvmetal or metals is merely to be reduced, as distinguished from complete elimination, the amount of aluminum addition is regulated accordingly. Where antimony is the principal impurity to be removed, proportion the aluminum so as not to reduce it below 0.05% or .1%, although a greater proportion willgive results. When added in proportion to-reduce the antimony to this extent, the other impurities will be reduced to traces and this will be found practical for most commercial purposes.

According to this invention the metal is next cooled to a point below that at which the aluminum compounds referred to are ,soluble in it and which is found to be in general about 300 or less than 350 C., being variable according to the tin content, if any, in the alloy and being lower as such tin content is larger. The practical rule is to cool to within about 100 C. of the freezing point of the metal. Cooling to this extent or to an appropriate low degree is an important factor of the process and causes the products of the reaction between the aluminum and the mentioned impurity metal or metals to separatesharply, and float high on the molten. metal in the form of a thick mush, somewhat similar to the zinc crust of the Parkes process of desilverizing lead. Thus produced, the mush will be found to hold all`the compounds of aluminum with the impurity metal or metals as stated, and if aluminum has been added for complete elimination and properly stirred in, will be found to have cleared the original metal of such impurities down to a small fraction of a per cent. or' no more than a trace.

The mush and liquid metal are separated after 'the metal has reached the low temperature referred to and before the melt has been allowed to solidify, separation at this particular stage being important not only to avoid the cost of remelting but to avoid any tendency for the impurity metal in the mush to pass back and re-alloy itself with the purified metal. The separation may of course be begun before the low temperature has been reached but it is in any event completed, and the last particles of mush taken out of the liquid, before the latter has had opportunity to solidify and While it still remains liquid at the reduced temperature.

The'separation can be accomplished by ordinary skimming and draining, but in such case a considerable portion of the mother liquor metal which clings to or wets the crystals or particles of the mush will be removed with it. Accordingly, and if there is a considerable'mush production, the ventire content of the kettle, with the mush still in it, is passed through a hot metal filter, which latter may consist of any suitable container having a rforaminous bottom, preferably removable, and having a charging entrance which can be closed so that uid pressure, such as air pressure, can be used to force the metal through. The mush itself compacts against the lter bottom and forms the real filter medium. If4 not separated in this way the mush, otherwise removed, is placed in the filter; it may be poured into it or deposited therein in the form of pigs and partially re-melted to form a mush in the filter. In either case the mass of separated mush becomes located in the lter and by then introducing compressed air, into the top of the fllte'r, the wetting metal is blasted off and driven out through theholes in the filter bottom, then to be restored to the main body of metal of which it was originally a part. After such air blast treatment, the bottom of the lter is removed or opened and the filter cake of dried mush ejected, by the application of further air pressure if necessary. This method gives a thorough separation of the mush and liquid and is in accordance with the invention of G eorge Thompson and E. 'H. Sheaff disclosed in S. N. 469,252, filed evenly herewith.

The portion of aluminum, remaining in the metal after filtering or separation from the mush, is small, under .10%, as the result of the proper aluminum reaction and can be readily eliminated by introducing or mixing with the still liquid metal, a suitable amount of a material capable of selective reaction with aluminum in preference to the tin or lead or metals desired to re- -main in the product, and adapted to produce a dross thereof which can then be removed. Sulphur is a suitable drossing agent for this purpose and should be introduced while the metal is close to its freezing point, after which the dross is skimmed off, leaving the product free of any substantial trace of the aluminum and ready to be poured for marketing. It is found that Where the value of the product metal is not impaired by so doing, elimination ofantimony down to only .1% or .2% makes for easier removal of residuary aluminum than if the antimony is more completely removed. In place of sulphur, sal ammoniac, caustic soda, coal, vegetable materials like wood saw-dust, and also CO2 gas and steam, are found suitable for removing the residual aluminum.

'I'he successive steps of one form of the process are indicated in the drawing in which Fig. 1 represents a kettle containing the molten metal to be treated, covered with fiat pieces of aluminum so as to be protected thereby. Fig. 2 represents the cone stirrer applied to the kettle and engaged in the act of stirring the at pieces into the melt, the temperature of which is now suitable for melting or dissolving the aluminum pieces therein. Fig. 3 represents the kettle and contents after the reaction and while cooling to the necessary temperature within, say, 100 C., of the freezing point of themelt. In Fig. 4 the mush and the liquid metal, still cool but liquid, are being separated in a filter the lower end of the filter being submerged in the receiving kettle to prevent excessive exposure to oxidation;` this step is a substitute for skimming and quicker but other methods can be used as already stated. In Fig. 5 the separated mush is being cleared of the liquid metal which is retained by the mush adhering to or wetting its particles. Fig. 6 shows the removed liquid metal in course of treatment, for the elimination of residual aluminum therein.

The values in the skimmings or the filter cake material are recoverable by ordinary processes of smelting in a blast' furnace and the lter cake material in particular lends itself to specially satisfactory smelting in a reverberatory furnace in contact with oxides of metals such-for example as the metals, other than aluminum, which the lter cake material holds in chemical combination; this reaction results in the production of oxides of aluminum as slag and a metal or alloy which is available for use or separation treat ment, according to its particular composition.

What is claimed is,

1. The process of reducing the content of Sb, Cu, As or Fe as they may be present as impurities in alloys containing a predominatng portion of Pb or -Sn or both, which comprises as,- certaining the amount of such impurity metal or metals to be removed, raising the temperature of the alloy to a point at which aluminum willv react therewith, mixing aluminum with a body of such alloy at substantially said temperature in proportion not substantially greater than the theoretical combining power of aluminum with said impurity metals to be removed, thereby producing an aluminum compound or compounds 'of a mush, and the'stirring soluble in the liquid metal, separating the resulting mush from said liquid metal at such reduced temperature, completing such separation before the melt has solidified, separated metal for the removal aluminum therein.

2. In the purification of metals of the class consisting of tin, lead, and tin-lead alloys composed mainly of tin or lead or both andhaving therein as impurity antimony, arsenic, iron or copper; the process which comprises intimately stirring aluminum into a'molten` body of such metal at a temperature not substantially greater than that required for melting the aluminum, the latter being presentin the metal in proportions substantially equal to the combining power of the contained impurity to be removed and forming therewith a compound of aluminum in the form being suchas to carry the aluminum downwards under the liquid surface and being continued until the liquid part of the melt when cooled contains onlya fractional per cent of aluminum, then cooling the melt to a temperature within about 100 C. of the freezing point of the melt, separating said mush at such reduced temperature from the metal still of any residual remaining liquid, the latter then containing only said fractional per cent of aluminum, completing the separation before the melt solidies, and treating such liquid metal to remove such small amount of remaining aluminum.

3. The process of claim 2 in which the aluminum is added in the form of an alloy with a metal forming a desirable constituent of the puried metal.

4. In the purcation of metals of the class consisting of tin, lead, 'and tin-lead alloys composed mainly of lead or tin and containing as impurity antimony, arsenic, copper or iron; the process which comprises intimately contacting such a metal with solid pieces of aluminum at temperatures at which the aluminum becomes somewhat soluble in the melt but below the melting point of aluminum, cooling the melt to the point where the compounds formed between the aluminum and the impurities in said metal become insoluble and form a mush of crystals in the still liquid metal and while in such cooled condition separating and completing the separation of the mush from the body of metal still remaining liquid.

5. In the purication of metals of a class consisting of tin, lead, and tin-lead alloys, composed mainly of tin or lead, with antimony as and finally treating thean impurity, the process which comprises intimately contacting said metal at elevated temperatures and aluminum than that required to combine with all the antimony present, cooling the melt to the point where the compounds formed betweenthe aluminum and antimony become substantially inin molten state with slightly less f soluble in the melt and removing such compounds from the said melt.

' 6. The process of claim 5 in which the amount of aluminum added is proportioned so that from 0.1 to 0.2 per cent of antimony remains in the melt after the compounds of aluminum with the various impurities present are removed, and thereafter dressing out the residual antimony and aluminum from the partially purified metal.

. 7. In the purification of metals of the class consisting of tin, lead-, and tin-lead alloys composed mainly of tin or lead and having as impurity antimony, arsenic, copper or iron; the process which comprises intimately contacting aluminum with a molten body of such metal by thorough stirring at a temperature not substantially greater than that which is required to melt the aluminum, the latter being added to the melt in proportions substantially equal to the combiningl power of the impurity to be removed and forming therewith a compound of aluminum in the form of a mush, cooling the melt to a temperature at which the liquid part of the melt contains only a fraction of a per cent of aluminum, withdrawing or separating the body of liquid metal from the mush, while at such reduced temperature, completing the separation before solidiilcation of the melt, then removing the liquid metal that adheres 'to and wets the mush, and ilnally treating such removed liquid metal, then containing only said fractional percent of aluminum, to remove said aluminum therefrom.

8. The process of reducing the content of Sb, As, Cu or Fe present as impurity in an alloy containing a predominating portion of tin which consists in mixing a tin-aluminum alloy with a body' of the alloy to be treated at a temperature less than the melting point of aluminum dissolving the aluminum-tin alloy therein to produce a mush compound or compounds of the aluminum therein with said impurity metal or metals, cooling the mixture to a point at which such compound is substantially insoluble in the liquid metal, and separating the latter from the mush thereby. produced.

- WILLIAM A. COWAN.

Images