Treatment of nickel-containing waste water on phosphating

Abstract

A process for treating a nickel-bearing aqueous solution consisting of phosphating-bath overflow and/or of rinsing water after phosphating, phosphating being effected with an acidic aqueous phosphating solution that contains 3 to 50 g/l phosphate ions, reckoned as PO 4 3− , 0.2 to 3 g/l zinc ions, 0.01 to 2.5 g/l nickel ions, optionally further metal ions as well as, optionally, accelerators, whereby the phosphating-bath overflow and/or the rinsing water after phosphating is conducted across a weakly acidic ion-exchanger, characterised in that the acid groups of the ion-exchanger are neutralised with alkali-metal ions to an extent amounting to no more than 15% and in that when it is fed to the ion-exchanger the nickel-bearing aqueous solution has a pH value within the range from 2.5 to 6.0.

1 . A process for treating a nickel-bearing aqueous solution consisting of phosphating-bath overflow and/or of rinsing water after phosphating, phosphating being effected with an acidic aqueous phosphating solution that contains 3 to 50 g/l phosphate ions, reckoned as PO 4   3− , 0.2 to 3 g/l zinc ions, 0.01 to 2.5 g/l nickel ions, optionally further metal ions as well as, optionally, accelerators, whereby the phosphating-bath overflow and/or the rinsing water after phosphating is conducted across a weakly acidic ion-exchanger, characterised in that the acid groups of the ion-exchanger are neutralised with alkali-metal ions to an extent amounting to no more than 15% and in that when it is fed to the ion-exchanger the nickel-bearing aqueous solution has a pH value within the range from 2.5 to 6.0.  
     
     
         2 . Process according to  claim 1 , characterised in that the phosphating-bath overflow and/or the rinsing water after phosphating is subjected to a membrane filtration in the form of an ultrafiltration, a nanofiltration or a reverse osmosis or to a different filtration process which is selected from a screen filtration or bag filtration or a filtration across a particle bed and the aqueous solution is conducted across a weakly acidic ion-exchanger after the filtration.  
     
     
         3 . Process according to one or both of claims  1  and  2 , characterised in that the weakly acidic ion-exchanger binds nickel ions more strongly than zinc ions.  
     
     
         4 . Process according to  claim 3 , characterised in that the weakly acidic ion-exchanger bears chelate-forming iminodiacetic-acid groups.  
     
     
         5 . Process according to one or more of  claims 1  to  5 , characterised in that the weakly acidic ion-exchanger is regenerated with a strong acid after exhaustion.  
     
     
         6 . Process according to  claim 5 , characterised in that the strong acid is constituted by phosphoric acid which, where desired, may contain a total of up to 10 mol. % nitric acid, hydrochloric acid and/or hydrofluoric acid, relative to the overall quantity of acid.  
     
     
         7 . Process according to one or both of claims  5  and  6 , characterised in that after the regeneration with a strong acid the ion-exchanger is rinsed with water or with a quantity of lye that corresponds to a maximum of 0.5 bed volumes of 4-% caustic-soda solution for such time until the pH value of the rinsing solution running off from the ion-exchanger lies between 2.1 and 4.5, preferably between 3.0 and 4.1.  
     
     
         8 . Process according to one or both of claims  5  and  6 , characterised in that the regeneration of the ion-exchanger is carried out in such a way that a concentrate fraction is ejected that contains at least 0.5 wt. % nickel ions and said concentrate fraction is re-used immediately after replenishment with active substances for the purpose of replenishing a phosphating solution.  
     
     
         9 . Process according to one or more of  claims 1  to  4 , characterised in that the solution obtained after passing through the weakly acidic ion-exchanger is used as rinsing water for the metal parts to be phosphated after the degreasing thereof.  
     
     
         10 . Process according to  claim 2 , characterised in that a membrane filtration is carried out and in that the retentate of the membrane filtration is recycled into the phosphating solution.