CH641207A5 - Feed salt for low-fuming hot galvanising - Google Patents

Description

The invention has for its object to find a litter salt, which gives the galvanizer the opportunity to keep the advantages of litter galvanizing, such as the avoidance of rinsing baths, flux baths and thus the prevention of wastewater problems, and at the same time to keep the prescribed ejection limitation within the permissible limits .

It was found that these requirements can be met with perfect galvanizing at the same time if a litter salt of a certain composition is used.

The litter salt according to the invention consists of 60-90% by weight of potassium chloride and / or sodium chloride, 5-20% by weight of zinc oxide, which can be replaced in whole or in part by equivalent amounts of basic zinc carbonate, zinc hydroxide or zinc oxychloride and 5-20% by weight. -% ammonium chloride.

It is particularly advantageous to use the potassium chloride / sodium chloride mixture in its eutectic composition. This is about 56% by weight of potassium chloride and 44% by weight of sodium chloride.

A litter salt which contains 8-12% by weight of zinc oxide or the abovementioned equivalent amounts of basic zinc carbonate, zinc hydroxide or zinc oxychloride, which entirely or partly replace the zinc oxide, is particularly preferred.

Furthermore, a litter is preferred which contains 8-12% by weight ammonium chloride.

Particularly good results are obtained with a litter salt, which consists of

80% by weight potassium chloride and sodium chloride in a weight ratio of 56:44,

10 wt .-% zinc oxide and 10 wt .-% ammonium chloride.

Basic zinc carbonate is formed when alkaline carbonates are added to zinc salt solutions. It consists of varying amounts of ZnCÜ3 and Zn (OH) 2, e.g. 2 ZnCOi, 3 Zn (OH) 2.

Zinc oxychlorides are the basic chlorides of zinc, e.g. of the composition ZnCk ■ ZnO • 1.5 H2O.

It is assumed that the zinc oxide neutralizes the adhering excess pickling acid with the formation of the corresponding zinc salt, usually zinc chloride. The water that forms and adheres from the pickling acid creates a film that contains potassium chloride and sodium chloride and the flux-active components zinc chloride and ammonium chloride. During the galvanizing process, the zinc chloride / ammonium chloride mixture takes over the tasks described above. The potassium chloride / sodium chloride mixture makes it possible to keep the concentration of ammonium chloride so low during the littering process that the emissions during the galvanizing process are kept within the prescribed limits.

The following litter salts, given as examples, have proven themselves in practice:

a)

44.8% by weight potassium chloride 35.2% by weight sodium chloride 10.0% by weight zinc oxide

10.0% by weight ammonium chloride (according to claim 5)

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b)

80.0% by weight potassium chloride 10.0% by weight zinc oxide 10.0% by weight ammonium chloride c)

77.8% by weight potassium chloride 12.2% by weight zinc hydroxide 10.0% by weight ammonium chloride d)

42.5% by weight potassium chloride 33.5% by weight sodium chloride 14.0% by weight basic zinc carbonate 10.0% by weight ammonium chloride e)

41.8% by weight potassium chloride 32.8% by weight sodium chloride 15.4% by weight zinc carbonate 5 10.0% by weight ammonium chloride

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40.4% by weight potassium chloride 31.8% by weight sodium chloride 10 17.8% by weight zinc oxychloride 10.0% by weight ammonium chloride

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Claims (5)

641207 2nd PATENT CLAIMS 1. Bedding salt for low-smoke hot-dip galvanizing, consisting of 60-90% by weight of potassium chloride and / or sodium chloride, 5-20% by weight of zinc oxide, which can be replaced in whole or in part by equivalent amounts of basic zinc carbonate, zinc hydroxide or zinc oxychloride and 5-20% by weight of ammonium chloride. 2. Bedding salt according to claim 1, containing 60-90 wt .-% of a eutectic potassium chloride / sodium chloride mixture. 3. Litter salt according to claim 1 or 2, containing 8-12 wt .-% zinc oxide or equivalent amounts of the substitute products mentioned in claim 1. 4. bedding salt according to claim 1, 2 or 3, containing 8-12 wt .-% ammonium chloride. 5. bedding salt according to claim 1, consisting of 80% by weight potassium chloride and sodium chloride in a weight ratio of 56:44, 10% by weight zinc oxide and 10% by weight ammonium chloride. When galvanizing metal objects consisting essentially of iron, various methods are used in order to improve the wettability of the surfaces of the metal objects by the molten zinc and to promote the formation of alloys in the boundary layer. In general, the surfaces of the metal objects are first freed from oxides and adhering scale by an acid pickling process. In the case of dry galvanizing, the pickled metal object is immersed in an aqueous solution of zinc chloride and ammonium chloride. The metal object is then removed from the solution. The solution adhering to the metal object is then dried, so that a coating of zinc chloride and ammonium chloride forms. The metal object thus prepared for the galvanizing is now immersed in a zinc melt, a zinc layer forming on the metal object to form an iron-zinc alloy as an intermediate layer. The ammonium chloride on the metal object forms a metal surface that is completely free of oxides. The metal oxides are converted into the corresponding metal chlorides, which are taken up by the zinc chloride. In the case of wet galvanizing, the pickled metal object is immersed in the molten zinc by a molten salt which floats on the molten zinc. The molten salt usually consists of ammonium chloride and zinc chloride. It is also possible to sprinkle the metal object, which is still wet from the pickling, with a flux. This can be done by hand or with a powder spray gun. The so-called sprinkling salt is used for sprinkling or spraying. The galvanizing material is then immersed in an uncovered zinc melt at around 450 ° C. The ammonium chloride used as litter salt converts the remaining metal oxide on the surface of the galvanized material into metal chloride in a known manner. Zinc and zinc oxide from the zinc bath form zinc amine chlorides with ammonium chloride, which absorb the metal chlorides formed. Excess ammonium chloride sublimes or is thermally split. The resulting ammonia and hydrochloric acid vapors or the ammonium chloride mist pollute the environment. In addition, any excess pickling acid, usually hydrochloric acid, still adhering to the galvanized material is released into the environment. This leads to a so-called ejection during hot-dip galvanizing, the amounts of which are far above the permissible concentrations of 80 mg / Ncbm. Values of 200-1000 mg / Ncbm were measured.