US2701194A

US2701194A - Process of recovering zinc metals and its alloys from zinc dross

Info

Publication number: US2701194A
Application number: US247897A
Authority: US
Inventors: Henry C Deterding
Original assignee: Individual
Current assignee: Individual
Priority date: 1951-09-22
Filing date: 1951-09-22
Publication date: 1955-02-01
Anticipated expiration: 1972-02-01

Description

United States Patent PROCESS OF RECOVERING ZINC METALS AND ITS ALLOYS FROM ZINC DROSS Henry C. Deterding, Kansas City, M0.

N0 Drawing. Application September 22, 1951, Serial No. 247,897

3 Claims. (Cl. 75-24) This invention relates to a flux composition and to methods of making and using the same. More particularly, the invention relates to a flux composition for use in the refining of zinc and zinc base alloys from mixtures thereof with metallic oxides.

The commercial production of high purity zinc (99.99% grade) has made possible the development of various zinc base alloys having desirable physical properties and excellent casting characteristics. While some types of zinc base alloys used extensively are cast in molds under gravity pressure, the widest application of the zinc base alloy is in connection with high pressure die casting methods. In that case, the Zinc alloy in molten form is forced into cavity dies under high pressure. The speed of casting, degree of pressure and size of the entry gates actually result in the alloy entering the cavities as a spray of the molten alloy, with the result that oxidation of some of the constituents of the alloy takes place. In a subsequent operation, the useful castings are removed from the cast forms and the remaining metal in the form of sprues, gates, runners, defective castings, and the like are remelted.

On the remelting of the scrap, it is an observed fact, peculiar to zinc base alloys, that the oxides present do not separate cleanly from the metal. Instead, the oxides trap or hold as much as several times their own weight of metal, forming a thick mushy dross that floats upon the surface of the molten metal. In the preparation of zincaluminum alloys, a similar dross is formed as a result of the agitation required to alloy the aluminum with the 21110.

Several methods of metal recovery from the dross are in current usage. In some plants the dross is skimmed from the metal, cast in convenient form and returned to zinc smelters for recovery by distillation methods. Other die casting plants flux the dross to recover whatever metal can be reclaimed. Either method represents a substantial economic loss to the die casting plant. Where fluxes are used, they commonly consist of zinc chloride, ammonium chloride or the double salt: zinc ammonium chloride. These salts may be used singly or in combination with varying amounts of other salts to reduce fumes. Such fluxes are not entirely satisfactory because of low metal recovery and because of the creation of objectionable fumes and dust due to the volatility of the fluxes.

In accordance with my present invention, I provide a relatively low melting point flux composition which does not attack the metallic zinc, itself, but separates the oxides from the metal holding them in suspension. The fluxoxides mixture that results floats on the surface of the molten metal and is skimmed therefrom as a thick paste. While the flux composition of my invention is a relatively low melting point mixture, having a freezing point not in excess of about 950 F., and preferably between 815 and 850 F., the constituents of my flux composition are so chosen that there is substantially no fuming or smoking of the flux composition during its use.

It is therefore an important object of this invention to provide a flux composition having a relatively low freezing point and suitable for use in the refining of zinc mixtures of zinc metal and metallic oxides, the constituents of which are sufficiently non-volatile that the composition does not smoke or fume substantially during use thereof for refining purposes.

It is a further important object of this invention to provide a flux composition mainly composed of a eutectic mixture of metal chlorides and containing an effective amount of an alkaline earth fluoride to improve the efficiency of the flux composition in its refining action.

It is a further object of this invention to provide an effective method of refining zinc and zinc base alloys from admixtures thereof with metallic oxides by the use of an inorganic, non-fuming flux.

Other and further important objects of this invention will become apparent from the following description and appended claims.

My flux composition comprises, broadly, two components: one, a major component comprising a mixture of metal chlorides either largely or wholly in the form of a eutectic composition; and, the other a minor component comprising an alkaline earth fluoride selected from the group consisting of calcium, strontium and barium fluorides. The first component comprises calcium chloride, sodium chloride and barium chloride. The proportions of the ingredients of the first, or major component of my flux composition are as follows:

Per cent by weight 4 55 Calcium chloride 0 Barium chloride 2545 Sodium chloride 10-30 Preferably, the first component of my flux composition comprises a eutectic mixture of calcium chloride, sodium chloride and barium chloride, the specific composition of which is as follows:

Per cent Calcium chloride 48 Sodium chloride 21 Barium chloride 31 This eutectic mixture has a freezing point of about 830 alkaline earth fluoride, or a mixture of alkaline earth fluorides. The alkaline earth metals found to be satisfactory are calcium, strontium and barium. On the basis of parts by weight of the first component, the second component comprises an amount of calcium, barium or strontium fluorides, or mixtures thereof, equivalent stoichiometrically to from 1 to 5 parts by weight of calcium fluoride. Preferably, 3 parts by weight of calcium fluoride for each 100 parts by weight of the first component are used, but if barium fluoride is substituted for calcium fluoride, the equivalent, or 6% parts, of barium fluoride are employed. The stoichiometrical proportions of calcium fluoride, strontium fluoride and barium fluoride are l to 1.6 to 2.25, approximately. The amount of the alkaline earth metal fluoride, or mixture of fluorides, equivalent to from 1 to 5% of calcium fluoride by weight of the first component can be calculated from the stoichiometric proportions. In general, an amount of the second component should be used such that the freezing point of the resulting mixture is not over 950 F., and, preferably, not over 850 F.

In making up my flux composition, the first component is prepared at fusion temperatures and then the second component added thereto. For instance, if the preferred eutectic mixture of 48% calcium chloride, 21% sodium chloride and 31% barium chloride is used, the mixture is heated to a temperature of about 1000 F., or higher, and while in fused, or molten, condition, there are added to such mixture 3 parts by weight of calcium fluoride for each 100 parts of the molten mixture, or 4.8 parts of strontium fluoride, or 6% parts of barium fluoride. The melt is then stirred by any suitable agitating means until all of the fluoride salts are dissolved. The molten mass may be cast in pellets, or it may be cast in any suitable form, and then cooled and reduced to convenient particle size by crushing, grinding or the like. Where the first component comprises the preferred eutectic mixture aforesaid plus 3 parts of calcium fluoride for each 100 aromas parts of the eutectic mixture, the freezing point of the flux composition is about 830 F.

Instead of making up the first component separately and then adding the second component to it, it is possible to use a mixture of the ingredients of the first and second components, without pre-fusing the ingredients of the first component. However, substantially higher metal temperatures are required, and, further, the prefusing of the first component is a distinct advantage in that it starts to melt at its eutectic melting point.

Up to the equivalent of 3% of calcium fluoride by weight of the first component is substantially Without influence on the freezing point of the eutectic mixture, but as the amount of calcium fluoride, or its equivalent of the other fluorides, is added, the freezing point rises. EX- perimental data on additions of calcium fluoride to the preferred eutectic mixture constituting the first component are as follows:

F. freezing pgint The above listed freezing points were determined by the use of a commercial grade pyrometer, but it should be pointed out that freezing points are not sharply defined and exact determinations vary with the judgment of the observer. Since the change in freezing point is governed by the fluoride radical and not the metal radical, the changes in freezing point are roughly inversely proportional to the molecular weights of the fluorides used. Thus, almost double the amount of barium fluoride may be substituted for calcium fluoride to effect the same increase in freezing point temperature. No advantage is gained by the use of strontium or barium fluoride over the use of calcium fluoride, and the cost in each case is appreciably higher. Consequently, calcium fluoride is preferred.

Although the preferred composition of the first component is the eutectic composition previously given, it is possible to vary the percentages a few per cent either way from the preferred composition without noticeable changes in the freezing point. Thus, any specific composition within the broader composition heretofore given, which has a freezing point not over 950 F. and preferably between 815 F. and 850 F. can be used.

In the use of my flux composition, the molten metal and its dross, as for instance zinc metal and zinc base alloys and zinc dross in admixture, are heated to a temperature of approximately 900 F., and an amount of the fused flux composition approximately equal to of the weight of the dross is added to the surface of the molten mass.

The flux is preferably allowed to melt, or at least to preheat thoroughly, and then is stirred or rabbled into the mass of dross. A gradual separation of oxides from the metal occurs with a progressive thickening of the fused flux mass as the oxide content increases. If the dross is of high oxide content, more flux should be added to effect proper consistency and effect clean separation of the metal from the dross. In general, one part by weight of flux will take care of two parts by weight of metallic oxides. When a satisfactory separation has been made, the oxide-saturated flux is skimmed from the surface of the metal. The percentage recovery of the zinc or zinc base alloy is normally between 75 and 80% theory, as compared with around 50 to 60% by heretofore known methods.

Under some conditions, as for instance where the dross has a high iron content and this iron is present as an ironaluminum compound, the flux, if within the above specified formulas, will not react properly with the dross. If it is known that the dross contains iron, or previous experrence with a similar dross indicates that the dross is given a preliminary treatment by rabbling with a small amount of a suitable oxidizing agent, such as sodium or potasslum nitrate in the ratio of about 0.1 to 1.0% by weight of nitrate to weight of dross, and then treated with the flux of my composition in the manner already descrlbed. In iron-bearing drosses so treated, a substantially larger ratio of flux to dross is required, say up to 25% of flux by weight of the dross. These iron-bearing drosses are not usually encountered in the melting of gates, sprues and the like, but are more likely to be encountered in the case of drosses formed by alloying alurrlllinum with the zinc in the preparation of zinc base a oys.

The flux composition of my preferred formula has been heated to 1300 F. without any fumes being observed. This is a higher temperature than that at which my flux composition would normally be used in the refining of zinc dross. Consequently, my flux composition may properly be said to be substantially non-fuming and nonsmoking under normal operating conditions.

The treatment of iron-bearing drosses is preferably a two-step process, as described above, in which the dross is first treated with the nitrate, or nitrates, and then with the flux of my composition. Iron-bearing drosses are only infrequently encountered and should not occur in good zinc base alloy shop practice. The nitrates cannot be added in the preparation of my flux, since they break down at the fusion temperature, around 1000 F., that is required to form the eutectic melting point composition.

My method of refining by the use of the flux composi tion herein described is particularly adapted for the refining of zinc and zinc base alloys. By the term zinc base alloys is meant any zinc alloy containing at least 85% of zinc. Usually a zinc base alloy contains aluminum, with or without copper and magnesium, plus the usual impurities. Zinc base alloys commonly used for die casting and known to the trade as Zamak are described in The American Society For Testing Materials Specification B24049T.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

I claim as my invention:

1. The method of recovering zinc metal and zinc base alloy from a zinc dross containing zinc metal, zinc base alloys and oxides of metals present in said zinc base alloys, which comprises agitating said dross in a molten state at a temperature of around 900 F. with an admixture of a flux composition consisting essentially of a mixture of from 40 to 55% of calcium chloride, from 25 to 45% of barium chloride, a sufficient amount of sodium chloride to make a total of and a sufficient amount of an alkaline earth metal fluoride selected from the group consisting of calcium, strontium and barium fluorides to be stoichiometrically equivalent to between 1 and 5% calcium fluoride by weight of said mixtures, continuing such agitation until a substantial separation of molten zinc metal and zinc base alloys from the metallic oxides has been effected, with the metallic oxides held in suspension in said flux on top of said molten zinc metal and zinc base alloys, and skimming off said flux-metallic oxide suspension to recover zinc and zinc base alloys substantially free from metallic oxides.

2. The method of recovering zinc metal and zinc base alloy from a zinc dross containing zinc metal, zinc base alloys and oxides of metals present in said zinc base alloys, which comprises agitating said dross in a molten state at a temperature of around 900 F. with an admixture of a flux composition consisting essentially of a eutectic mixture of about 48% of calcium chloride, 21% of sodium chloride and 31% of barium chloride, and an amount of an alkaline earth metal fluoride selected from the group consisting of calcium, strontium and barium fluorides stoichiometrically equivalent to between 1 and 5% of calcium fluoride by weight of said eutectic mixture, continuing such agitation until a substantial separation of molten zinc metal and zinc base alloys from the metallic oxides has been effected, with the metallic oxides held in suspension in said flux on top of said molten zinc metal and zince base alloys, and skimming off said flux-metallic oxide suspension to recover zinc and zinc base alloys substantially free from metallic oxides.

3. The method of recovering zinc and zinc base alloys from a zinc dross containing zinc metal, zinc base alloys and oxides of metals present in said zinc base alloys, which comprises agitating such zinc dross in a molten state in the presence of a flux of the composition consisting essentially of a mixture of from 40 to 55% of calcium chloride, from 25 to 45% of barium chloride, a suflicient amount of sodium chloride to make a total of 100%, and a suflicient amount of an alkaline earth fluoride selected from the group consisting of calcium, strontium and barium fluorides to be stoichiometrically equivalent to between 1 and 5% of calcium fluoride by weight of said mixture, said flux being in the proportion of about 1 part of flux to 2 parts by weight of metallic oxides present, continuing such agitation until a substantial separation of molten zinc metal and zinc base alloys from said metallic oxides has taken place, with said metallic oxides held in suspension in said flux floating on said molten zinc metal and zinc base alloys so separated, and skimming ofi the flux-metallic oxide suspension to recover said molten zinc metal and zinc base alloys substantially free from metallic oxides.

References Cited in the file of this patent UNITED STATES PATENTS 79,701 Stevens July 7, 1868 2,148,664 Wile Feb. 28, 1939 2,472,025 Peake May 31, 1949 2,474,674 Holden June 28, 1949 FOREIGN PATENTS 450,690 Great Britain July 23, 1936 114,482 Australia Dec. 29, 1941 OTHER REFERENCES Metals and Alloys, vol. 5, page 561, December 1934. Handbook of Non-Ferrous Metallurgy, by Liddell, vol. 2, 2nd ed., pages 208, 430. Published 1945.

Claims

1. THE METHOD OF RECOVERING ZINC METAL AND ZINC BASE ALLOY FROM A ZINC DROSS CONTAINING ZINC METAL, ZINC BASE ALLOYS AND OXIDES OF METALS PRESENT IN SAID ZINC BASE ALLOYS, WHICH COPRISES AGITATING SAID DROSS IN A MOLTEN STATE AT A TEMPERATURE OF AROUND 900* F. WITH AN ADMIXTURE OF A FLUX COMPOSITION CONSISTING ESENTIALLY OF A MIXTURE OF FROM 40 TO 55% OF CALCIUM CHLORIDE, FROM 2.5 TO 45% OF BARIUM CHLORIDE, A SUFFICIENT AMOUNT OF SODIUM CHLORIDE TO MAKE A TOTAL OF 100%, AND A SUFFICIENT AMOUNT OF AN ALKALINE EARTH METAL FLUORIDE SELECTED FROM THE GROUP CONSISTING OF CALCIUM, STRONTIUM AND BARIUM FLUORIDES TO BE STOICHIOMETRICALLY EQUIVALENT TO BETWEEN 1 AND 5% CALCIUM FLUORIDE BY WEIGHT OF SAID MIXTURES, CONTINUING SUCH AGITATION UNTIL A SUBSTANTIAL SEPARATION OF MOLTEN ZINC METAL AND ZINC BASE ALLOYS FROM THE METALLIC OXIDES HAS BEEN EFFECTED. WITH THE METALLIC OXIDES HELD IN SUSPENSION IN SAID FLUX ON TOP OF SAID MOLTEN ZINC METAL AND ZINC BASE ALLOYS, AND SKIMMING OFF SAID FLUX-METALLIC OXIDE SUSPENSION TO RECOVER ZINC AND ZINC BASE ALLOYS SUBSTANTIALLY FREE FROM METALLIC OXIDES.