US2076712A - Cover for metal baths - Google Patents

Description

Pl'l 13, 1937 c. G. FUNK E-r AL 2,076,712

covER Fon MTAL BATHs' Filed Jan. 12, 1935 l* I use 2SAM. l l HM INVENTORS.

ATTORNEYS Patented Apr. 13, 1937 COVER, FOR METAL BATES Colin G. Fink, New York, N. Y., Frederic J. Kenny, Tenaiiy, N. J., and .lohn dH. Hurd, New York, N. Y., assignors, by mesne assignments, to Spowers Research Laboratories, Inc., .Hersey City, N. J., a corporation of New Jersey Applicationf January 12, 1935, Serial No. L'id 5 Claims.

This invention relates to a'molten or liquid cover or blanket for application to the top of metal baths such as zinc baths commonly employed for galvanizing iron.' While the invention has been developed in connection with such zinc baths, and while it is particularly useful in this connection, it may also find use in connection with other similar metal baths.

When a zinc bath is employed for galvanizlng,

l0 it is commonly maintained at atemperature in the order of 420 C to 475 C., or even somewhat higher, and the tendency of the zinc under these conditions is to rapidly oxidize with the result that a large proportion of the zinc is lost. Even 15 as much as 50 pounds of zinc per square foot of '30 more disadvantages, among which are the rapid volatilization of certain covers at operating temperatures, the necessity for keeping certain other types of covers in amoist condition, objectionable reactions between certain covers and the bath, and the high cost and frequent replacement required incident to the use of certain types of covers.

The prime feature of the present invention is,

or similar. metal baths which shall avoid these disadvantages.

A more specific feature of this invention is to provide a cover which is composed of inorganic material molten at the temperature of the bath, and which is capable of preventing oxidation of the zinc. Advantageously, the cover shall oat on the bath and at the same time have a consistency which prevents access of air to the bath.`

Still another feature of theinvention resides in the provision of sucha cover which does not decompose or volatilize at the temperatures employed, or which decomposes or volatilizes so slowly that the loss, for practical purposes, is not objectionable. i

Still another feature ofthe invention resides in the production of a cover or blanketpof the character indicated which is effective and lasting in Vuse, and not excessively expensive. A

An additional feature of the invention is to provide a process for preparing and employing a cover of this character.

Other features, objects and advantages of the invention will in part be pointed out and in part become apparent in connection Withthe following detailed description of o ne example of the invention, reference being had to the accomtherefore, to provide an improved cover for zinc- (Cl. iii-70.2)

sible variations in the preferred character 'of the invention, and wherein:

Fig. 1 is a perspective view of a solid, prismatic-shaped body indicating percentages of ingradients which may be employed to produce compositions which melt at certain constant temperatures; and

' Fig. 2 is a ternary diagram also illustrating, in a somewhat dierent manner, the proportions of certain compositions which melt at temperatures indicated.

In its broader aspects, the invention consists of a cover or blanket composed primarily of a, normally solid substance which melts at tempera- 4 tures below that of the bath oniwhich the cover is employed and which'does not volatilizeor decompose appreciably at the temperature of the bath. We have found that excellent results may be achieved by the employment of a salt and, more particularly, by the employment of a mixture of salts which have foregoing properties and, in addition, have the ability to form a cover of suiiicient consistency to prevent the access of air to the molten bath beneath the cover.

By way of example, we have found that a mixture of a chloride of an alkali metal with a chloride of the' second group in the periodic table of elements' having a comparatively low melting point, constitutes a particularly effectiveV and satisfactory blanket. The mixture of the salts employed is preferably such that a sufficient quantity of a salt, for instance, zinc chloride, is employed in combination with sodium chloride'to produce a mixture having a meltingl point lower than the melting point of sodium chloride alone. If desired, a eutectic mixture may b'e employed, although it is not in all cases necessary or advisable to employ a strictly eutectic mixture. That is to say, the mixture should be such that the melting point of the combined salts is sufficiently low to enable the mixture to `become Vmoltenat temperatures at or below the temperature of ,the bath. Moreover, the temperatures of decomposition or of volatilization -of the. several ingredients should be considerably above the temperature of the molten bath upon which the mixture is placed as a cover.

While the chlorides and zinc and sodium have been mentioned specifically, it will be understood that it may be feasibleto substltutefor` these salts other compounds or salts which are y stable at the operating temperatures and which,

at the same time, are capable of melting at la temperature below the temperature of the bath on which the material is employed as a cover.

However, the chlorides of metals of the groups indicated have been found to be particularly stableA and satisfactory for producing a cover which oats upon a moltenl metal bath and '40 may contain the following:

which is capable of lasting over a considerable period of time.

As to the matter of proportions of the ingredients, it may be said in general that the 5 mixture is desirably such that its melting point is -.within the range of customary temperatures employed for the bath. By properly adjusting the proportions,.the melting point of the cover may be reduced even further, or, on the other hand, the bath may be employed at a temperature somewhat in excess ofnormal.

For instance, if a mixture of equal parts by weight of zinc chloride and sodium chloride is employed,-the melting point of'the mixture will be about 532 C. However, for higher percentages of zinc chloride as compared to sodium chloride, the melting point of the mixtures will be even lower.

instead of the larger quantity of zinc chloride a proportion of alkali metal chloride which will result in a mixture yhaving a melting -point-below the normal melting point of the bath.

Similarly, if a mixture of equal parts by weight of zinc chloride and potassium chloride is employed; the melting point of the mixture will be about 435 C. within the temperature ranges commonly employed for galvanizing baths.

However, while satisfactory results .may be produced by employing a zinc salt in combination with a single alkali metal chloride, even superior results are produced by the employment of a zinc salt in combination with a mixture of sodium and potassium chloride, for when sodium and potassium chloride are employed together, the latter in relatively small amount as compared to the former, the melting point of the mixture may be still lower. For instance, an excellent mixture merely by way of example,

Parts by weight Components Sodium chloride 31 Potassium chloride 17 Zinc chloride 52 While indicative proportions of the ingredients have been mentioned, other satisfactory and preferred ranges of the ingredients are shown in the accompanying drawing. The two figures in the drawing illustrate in somewhat different manners the temperatures producible by various percentages of the three ingredients mentioned. Fig. 1 represents a solid body of prismatic form, the three corners of the prism being represented by 100% zinc chloride; 50% zinc 55, ehloride'and 50% sodium chloride; and 50% f zinc chloride and 50% potassium chloride. Constant temperature lines are indicated as lines parallel to the base of the prism while points on the shaded surfaces represent, by their dis- 60 tances from the respective corners, proportionsof the respective ingredients.

With regard to Fig; 2, which is a ternary diagram, it may be noted that the individual lines each represent 5% by weight of given ingredi- 65 ents, the right and left-hand sides of the tri- However, for reasons .of economy.' and for other reasons, it is preferable to employ cated upon the ternary diagram are -given in degrees centigrade.

If it be desired to employ `a eutectic mixture, this mixture is represented by the point e and has a melting point of 171 C.1 In both Fig. 1 5 and Fig. 2 there are shown certain lines e-a, e-b and e--c which radiate from the eutectic point and indicate the minimum melting point for a mixture containing a constant proportion of one ingredient and a variation in the relative 10 proportions of the two other ingredients.

In general, we have found that 4a mixture of about 40-95 parts by weight of zinc chloride with a quantity of potassium chloride or sodium chloride or both, to make up 100 parts by weight, 15 gives a satisfactory mixture. However, we prefer to have the zinc salt present in proportions around 5060% by weight of a mixture of the same with sodium chloride or potassium'chloride or both. Moreover, as has been mentioned, the 20 zinc chloride may be further increased at the expense of the alkali metal chloride'or chlorideswhere an even lower melting point is desired. Furthermore, it will be noted that sodium chloride tends to produce a somewhat 25 higher melting point than potassium chloride. However, variations in the relative proportions lmay be made to produce a mixture having a melting point within the range of 'about 420 C.475 C., or, on the other hand, the 30 temperature of the bath may be slightly raised tov enable the`use of somewhat higher percentages of sodium chloride. Also where higher melting points are not objectionable, then the proportion of zinc chloride may be. somewhat 35 reduced although we prefer to employ a mixture containing at least 40% of zinc chloride or similar salt.

' When magnesium chloride is employed in addition to the three ingredients discussed, this 40 may be used in amounts ranging from 530% by weight of the mixture of the four ingredients, the other three being employed generally in the relative 'proportions indicated. iIhe magnesium chloride alone has a fairly high-melting point, but 45 it may also tend to reduce the melting point of the mixture, and in this respect it may be regarded as a possible substitute for at least a portion of the zinc chloride. However, we do not regard the use ofmagnesium chloride to be '50 in all cases essential, although it does appear to improve the sheen of the galvanized product somewhat, particularly where this product is drawn through the cover. Whether this be due to the fact that magnesium chloride or its magnesia works into the galvanized coating to some extent, or whether this result is attributable to catalytic eilect, we are not prepared to say. As illustrative example of a composition which has produced excellent results-we give the following:

Components Parts by weight Sodium chloride (NaCl) A26.4 Potassium chloride (KCl) s 14.5 Zinc chloride (ZnClz) 43.3

Magnesium chloride (MgCl2) 15.8. (35

However, while speciiic prODrtions are indicated the following additional possible variations are suggested in the .nature of the composition. For instance, the ratio of sodium chloride to potassium chloride may be varied within limits determined in part by therelative cost of the two materials, and also -by the temperature at which a vmixture of the ingredients should melt, a1- 75lthough the major portion of the alkali metal chlorides is preferably sodium chloride. In general, we have found that excellent results (in connection with zinc baths) may be produced when the ratio of the zinc chloride to sodium chloride to potassium chloride is about 3 to 2 to 1 by weight, whether or not other materials are also added. Our research has appeared to show that variations of at least plus or minus 10% inthe quantities of these salts are feasible and satisfactory, but in the broader aspects of the present development, other variations are contemplated.

As stated above, the use of magnesium chloride is not considered essential to the production of a satisfactory cover for vzinc baths, but to some extent at least it may be considered as a substitute for zinc chloride although it does not produce as low a resulting temperature as does the latter when mixed with alkali metal chlorides. It may also have the effect of making the cover more of the proper consistency for excluding air from lthe bath. Y

While the particular salts named have been found especially advantageous, it will be understood that the invention in its broader aspects is not limited to these details.

4In preparing the mixture to be employed as a cover, we preferably mix the ingredients in finely divided form and then melt the mixture at a temperature of around 500 C. The mixture given in the second table will commence to melt at around 390 C., but we prefer to heat it to the higher temperature mentioned because this appears to produce a superior, uniform composition. During the heating some sediment may form, and it is desirable to continue the heating until the liquid is clear. The liquid may be drawn off from the sediment and passed directly to the bath which is to be covered, or the liquid may be allowed to solidify and may be put up in any suitable form for marketing to concerns desiring this type of product.

The thicknessof the material employed as a cover on the bath may vary considerably but ordinarily a cover of less than an inch in thickness produces satisfactory results. Those skilled in the art will understand the limits within which the thickness may vary, and they can readily determine, taking into account considerations of economy, the desired amount of cover to use in order to reduce oxidation of the bath to a satisfactory point.

Should the bath, after a considerable time, become less effective, small additional quantities 1 of zinc chloride may be added. iHowever, it is preferable when the cover eventually loses its efiiciency, to remove it, cool it and remelt it together with suiiicient additional ingredients to make up the desired proportions.

It is also possible in the employment of a cover of this type to intermingle therewith certain additional materials of solid character, such as charcoal or slag from iron furnaces. Such materials appear to aid in prolonging the life of the wipes of galvanizing baths.- Preferably, the proportions of these solid ingredients are not sufiiciently large to alter the liquid character of the cover, for a liquid cover appears to present great advantages over covers formed of particles of solid material.

Through the present invention, therefore, there is produced a cover ,which not only prevents the access of air to the bath, but also does not require any material attention during the use of the bath. Inother words, the cover is not destroyed or adversely affected by the temperature of the bath but remains in molten form with sufiicient continuity, density or viscosity, to form an eiective seal. At the same time, the cover does not adversely aect the character of the zinc bath and does not tend to alloy with the container for the bath, or stick to the surface thereof. Furthermore, inasmuch as the cover lasts a long time, the initial cost of the same is not excessive.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the usev of such terms and expressions, of excluding any equivalents of the features shown and described, or portions thereof, but it is recognized that various modiiications are possible within the scope of the invention claimed.

We claim:

1. A method of preventing oxidation of a zinc bath which comprises applying thereto a mix-Y ture containing zinc chloride, sodium chloride and potassium chloride in the relative proportions of about 3 to 2 to l, together with magnesium chloride.

2. A cover for zinc baths comprising a mixture of inorganic salts having a melting pointA below the melting point of the bath and being non-volatile and stable at the temperature of the bath, said mixture comprising about 26% of sodium chloride, about 14% of potassium chloride, about 43% of zinc chloride and about l15% of magnesium chloride.

3. A molten bath blanket comprising a mixture of 40-95 parts by weight of zinc chloride with about 5 to 60 parts by weight of sodium and potassium chlorides, the latter being present in a relatively small amount as compared to the former, together with magnesium chloride.

4. Process of galvanizing which comprises providing a bath of molten zinc at a temperature of about 420 C. to about 475 C., dipping thelarticles to be galvanized into said hot bath, and maintaining a molten blanket of substantial thickness upon the surface of said zinc bath, said blanket comprising a mixture of zinc chloride, sodium chloride and potassium chloride, the zinc chloride constituting the major ingredient of the mixture, the sodium chloride in the order of one-quarter to a third of the mixture and the potassium chloride in substantial amount but less than the sodium chloride, a substantial proportion of magnesium chloride, said mixture having a melting point substantially lower than the melting point of the zinc bath and being capable of forming a stable, substantially air-excluding cover over the surface of the zinc bath for substantial operating periods of time without decomposing or vaporizing.

5. A cover for zinc baths and for wiping the galvanized articles passing therethrough, comprising a mixture of inorganic salts having a melting point below the melting point of the bath and being non-volatile and stable at the temperature of the bath `and acting to remove impurities and scum after galvanizing, said mixture containing zinc chloride, sodium chloride, and potassium chloride in the relative proportions of about 3 to 2 to l, together with magnesium chloride.

COLIN G. FINK.

FREDERIC J. KENNY. JOHN nI- I. HORD.

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