US2458662A - Method for preserving the effectiveness of a metal cleaning bath - Google Patents

Description

Patented Jan. 11, 1949 METHOD FOR PRESERVING THE EFFEC- TIVENESS OF A METAL CLEANING BATH Hugh G. Webster, Detroit, Mich.

No Drawing; Application May 10, 1946,

, Serial No. 668,777

1 Claim. 1

This application relates to methods for preserving the effectiveness of metal cleaning baths, particularly baths of the molten alkali salt type useful in metal cleaning.

Users of such baths have found it necessary from time to time to dispose of the bath after it has been in use for some while, and replace it by a fresh bath, because, for reasons hitherto not known, the efiectiveness of the bath, which deteriorates as the bath is used, has dropped below a point of minimum usefulness.

I have discovered that the deterioration of the bath is due to the formation of excessive amounts of alkali salt carbonate in the bath as the bath is used. Such carbonates are salts of the metal alkali whose salts form the bath. The carbonate may be formed either by the effect of atmospheric conditions or by chemical reactions arising during the metal cleaning. In such reactions some of the salts, which may be hydroxides, chlorides, nitrates, aluminates, fluorides, etc., are transformed into carbonates. I have discovered that as the percentage of carbonate increases in a bath, the efieotiveness decreases.

I have also developed a method for removing the carbonate from a bath and thus removing the cause of the deterioration of the bath and this method I will now describe. A molten bath of the character under consideration, including molten sodium hydroxide, molten sodium nitrate, molten sodium chloride, etc., operates at a temperature considerably above its melting point; at such temperature, generally 850-900 F., a large amount of carbonate is soluble in the bath. The first step in removing the carbonate, therefore, is to reduce the temperature of the bath until the temperature drops to a point very close to or even below the bath solidfication point, approximately 600 F. Thereafter, the bath is reheated and brought up to the melting point or slightly above until it begins to become fluid. Temperatures in the range of 650 F. to 700 F. have been found sufficient for this purpose, it being merely required that the temperature be very slightly above the melting point of the bath, whatever that melting point is.

At this point, the carbonate will be found to have precipitated out of the bath and drop to the bottom, obviously due to the fact that less of the carbonate is soluble in the lower temperature bath than at the operating temperature of the bath, considerably higher, the drop from the operating temperature, around 850 F., to the melting point, 600 F., being sufficient to cause a very large amount of the carbonate to drop out of the bath as a precipitate.

In the bottom of the tub in which the bath is confined is a sludge pan into which the precipitated carbonate drops. Very quickly after the bath has been brought up to the melting point and the carbonate has been dropped from the bath, that is, very quickly after the sludge has formed, the sludge pan is lifted up and out of the bath, thus removing a substantial portion of the carbonate from the bath.

With the carbonate removed, the bath is r turned to operating temperature and its effectiveness is no longer impaired by the presence of excess carbonate.

In some cases, it may be necessary to replenish the supply of other salts to the bath after the carbonate is removed and this of course, is a simple matter. With the supply of other salts replenished, the bath is once again effective for use.

Now having described the method herein disclosed for preserving the effectiveness of a metal cleaning bath of the molten alkali salt type, reference should be had to the claim which follows.

I claim:

A method for preserving the effectiveness of a metal cleaning molten alkali salt bath of the type wherein an excess of alkali salt carbonate is formed, either by atmospheric effects, or by chemical reactions arising during metal cleaning, which comprises removal of the carbonate from time to time as by precipitating it out of the bath and then removing it as a precipitate as follows:

reducing the temperature of the bath to the solidification point, then increasing the temperature of the bath to the melting point, thus causing the carbonate, less soluble in the bath at the melting point than at higher temperatures, to precipitate out of the bath, and drop to the bottom, and quickly removing the carbonate precipitate.

HUGH G. WEBSTER.

No references cited.