Boric acid-free composition for removing deposits containing cryolite

Abstract

Described herein is an aqueous composition for removing cryolitic deposits from plants or parts of plants which serve for the conversion treatment of metal surfaces, said composition including a) at least one mineral acid, and b) at least one dicarboxylic acid of the formula HOOC—(CH.sub.2).sub.x—COOH, where x is 0 to 3 and no borate-containing compounds have been added to the composition. Also described herein is a corresponding method for removing cryolitic deposits.

Claims

1. An aqueous composition for removing cryolitic deposits from plants or parts of plants serving for the conversion treatment of metal surfaces, which composition comprises a) at least one mineral acid having a total normal concentration in the range from 3.0 to 10 mol/l; and b) at least one dicarboxylic acid of the formula HOOC—(CH.sub.2).sub.x—COOH; wherein x is 0 to 3 and no borate-containing compounds have been added to the composition.

2. The aqueous composition according to claim 1, which comprises b) at least one dicarboxylic acid of the formula HOOC—(CH.sub.2).sub.x—COOH having a total concentration in the range from 0.07 to 1.7 mol/l.

3. The aqueous composition according to claim 1, which comprises the at least one mineral acid and the at least one dicarboxylic acid in a molar ratio in the range from 2.4:1 to 60:1.

4. The aqueous composition according to claim 1, wherein the at least one mineral acid comprises sulfuric acid.

5. The aqueous composition according to claim 4, which further comprises nitrate.

6. The aqueous composition according to claim 1, wherein the at least one dicarboxylic acid comprises malonic acid and/or oxalic acid.

7. The aqueous composition according to claim 4, which composition comprises a) sulfuric acid having a total normal concentration in the range from 3.0 to 6.0 mol/l, and b) oxalic acid having a total concentration in the range from 0.35 to 1.0 mol/l, and comprises the sulfuric acid and the oxalic acid in a molar ratio in the range from 4.0:1 to 12:1.

8. The aqueous composition according to claim 1, which additionally comprises at least one nonionic surfactant.

9. The aqueous composition according to claim 1, which additionally comprises at least one corrosion inhibitor.

10. The aqueous composition according to claim 9, wherein the at least one corrosion inhibitor is a mixture of a compound of the formula I
R.sup.1O—(CH.sub.2).sub.x—C≡C—(CH.sub.2).sub.y—OR.sup.2  (I), in which R.sup.1 and R.sup.2 are both H, and a compound of the formula I in which R.sup.1 and R.sup.2 each independently of one another are an HO—(CH.sub.2).sub.w group with w≥2, wherein for each of the two compounds of the formula I, x and y in each case independently of one another are 1 to 4.

11. A concentrate from which, by dilution with a suitable solvent and/or dispersion medium, an aqueous composition according to claim 1 is obtainable.

12. A method for removing cryolitic deposits from plants or parts of plants serving for the conversion treatment of metal surfaces, which comprises contacting the plants and/or parts of plants with an aqueous composition according to claim 1.

13. The method according to claim 12, wherein the aqueous composition has a temperature in the range from 40 to 80° C.

14. The aqueous composition according to claim 1, which comprises a) at least one mineral acid having a total normal concentration in the range from 3.0 to 6 mol/l, and b) at least one dicarboxylic acid of the formula HOOC—(CH.sub.2).sub.x—COOH having a total concentration in the range from 0.35 to 1.5 mol/l.

15. The aqueous composition according to claim 1, which comprises the at least one mineral acid and the at least one dicarboxylic acid in a molar ratio in the range from 2.6:1 to 12:1.

16. The aqueous composition according to claim 1, wherein the at least one dicarboxylic acid comprises oxalic acid.

17. The aqueous composition according to claim 4, which composition comprises a) sulfuric acid having a total normal concentration in the range from 3.5 to 4.5 mol/l, and b) oxalic acid having a total concentration in the range from 0.5 to 0.8 mol/l, and comprises the sulfuric acid and the oxalic acid in a molar ratio in the range from 5.0:1 to 8.0:1.

18. The aqueous composition according to claim 1, which additionally comprises at least one nonionic surfactant selected from the group consisting of ethoxylated fatty alcohol polyglycol ethers.

19. The aqueous composition according to claim 1, which additionally comprises at least one compound selected from the group consisting of urea derivatives, diols, and alkoxylated diols.

20. The method according to claim 12, wherein the aqueous composition has a temperature in the range from 50 to 70° C.

Description

EXAMPLES

(1) In a spray phosphating plant for the treatment of metal surfaces which consist of 80 wt % of aluminum, 15 wt % of galvanized steel, and 5 wt % of steel, sparingly soluble deposits are observed in the nozzle assemblies, the composition of these deposits being as follows (all figures in wt %): 30.3% Na 12.4% Al 52.3% F 1.2% Zn 1.8% Fe 0.2% Mn 1.8% P.sub.2O.sub.5

(2) The deposits therefore consist to an extent of around 95 wt % of cryolite (Na.sub.3AlF.sub.6).

(3) In each case, one piece of cryolite crust was covered with a defined amount of solvent in a glass container. With gentle stirring (250 revolutions/min) and at the temperature reported in table 1 below, a determination was then made of the time taken for the crust to completely dissolve/disperse, initially using the naked eye. The solvent together with apparently dissolved/dispersed crust was transferred after the time reported in table 1 (requisite dissolution time), into a centrifuge tube. After around an hour, the cylinder tip of the centrifuge tube was observed to determine whether a sediment has formed. With regard to the results in table 1, no sediment was measured for the reported soluble amount of crust and requisite dissolution time.

(4) TABLE-US-00001 TABLE 1 Soluble Requisite amount of dissolution crust Solvent Temp. time (g/100 g (wt %) (° C.) (min) solvent) 15% HCl 20 150 1 20% H.sub.2SO.sub.4 20 75 1 20% H.sub.2SO.sub.4 60 ca. 20 1-2 15% NaOH/ 80 >100 1-2 complexing agents 45% NaOH 65 >150 1-2 20% AlCl.sub.3 20 220 1-2 20% AlCl.sub.3 70 50 3 15% NH.sub.2HSO.sub.3 60 >300 1 20% H.sub.2SO.sub.4/ 60 25 5-6 5% B.sub.2O.sub.3 20% H.sub.2SO.sub.4/ 60 20 1-2 9.9% adipic acid 20% H.sub.2SO.sub.4/ 60 35 5-6 9% glutaric acid 20% H.sub.2SO.sub.4/ 60 25 5-6 1% oxalic acid 20% H.sub.2SO.sub.4/ 60 35 5-6 14% oxalic acid 20% H.sub.2SO.sub.4/ 60 20 5-6 11% oxalic acid 20% H.sub.2SO.sub.4/ 60 20 5-6 6% oxalic acid

(5) The results compiled in the table show that with use of solvents based on hydrochloric acid, sulfuric acid, sodium hydroxide with or without complexing agents and in various concentrations, aluminum chloride, and amidosulfuric acid with the exception of sulfuric acid at 60° C. and of the sulfuric acid/boric acid combination relatively long dissolution times are required before the complete dissolution/dispersal of the crusts.

(6) Conversely, when using the method of the invention, the requisite dissolution time is comparatively short. Particularly striking, however, is that the amount of cryolitic deposit taken up by 100 g of solvent when applying the method of the invention is significantly higher than when using the other solvents with the exception of the sulfuric acid/boric acid combination.

(7) Measured against the majority of the comparative tests (borate-free variants), the amount dissolved is greater by a factor of 4 to 6, with the consequence of a considerable saving on solvent. The amount dissolved when using the method of the invention is comparable with the amount dissolved with the sulfuric acid/boric acid combination.

(8) Adipic acid is not soluble in 20% sulfuric acid and is therefore unable to act as a complexing agent for Al.sup.3+. The results are therefore the same as with 20% sulfuric acid alone. Consequently, the soluble amount of crust is significantly lower here as well than in the case of the sulfuric acid/boric acid combination.

(9) Glutaric acid, on the other hand, is soluble in 20% sulfuric acid although the dissolution procedure may take up to 30 minutes. Correspondingly, the soluble amount of crust here is already comparable with that for the sulfuric acid/boric acid combination.

(10) The best results in terms of the requisite dissolution time are achievable with 6% and 11% oxalic acid (in combination with 20% sulfuric acid). The use of 6% oxalic acid, however, also has the advantage that there is less sediment present after cooling than in the case of 11% oxalic acid.