September 18

Gold colouring baths based on ferrioxalates have been used successfully for decades to colour anodised components for interior and exterior applications. During the colouring process, the iron salts are then incorporated into the porous anodised layer by a simple dipping process. Depending on the amount of salts incorporated, the coloured surface then appears in intensive golden shades. A typical Alufinish product for this type of colouring is called Alficolor Gold 602.

The laboratory experiment involved investigating how efficiently an already used dye bath can be precipitated in the waste water plant by adding sodium hydroxide solution. The following experimental set-up was used for this purpose:

A beaker was filled with a consumed Alficolor Gold 602 colouring bath and 45% sodium hydroxide solution was added drop by drop at ambient temperature (here: 21 – 23°C) while stirring (300 rpm). During the continuous pH measurement, the precipitation behaviour was observed and evaluated.

Start of trial

pH value: 4.42

time: 0 min.

Bath solution looks greenish.

pH value: 6.63

time: 7 min. 43 sec.

The solution turns slightly darker.

pH value: 6.96

time: 9 min. 34 sec.

From approx. pH 7, a clear colour change occurs.

pH value: 7.27

time: 11 min. 9 sec.

The first precipitates become visible, the bath becomes cloudy.

pH value: 7.99

time: 19 min. 18 sec.

The bath turns noticeably darker and the precipitation increases.

pH value: 7.90

time: 25 min.

Without further addition of NaOH, the pH value drops slightly again. The reaction is not yet complete.

pH value: 7.86

time: 32 min. 42 sec.

After switching off the stirrer for a few minutes, turbidity settles at the bottom, but the above solution does not yet clear.

pH value: 8.65

time: 35 min. 45 sec.

Further additions of NaOH.

pH value: 9.21

time: 38 min. 40 sec.

The precipitations increase again. However, the pH value does not drop any more and the precipitation reaction seems to be completed.

pH value: 9.21

time: 41 min. 13 sec.

The pH value remains stable

pH value: 9.22

time: 47 min. 49 sec.

The stirrer is switched off and the solution begins to clear.

pH value: 9.22

time: 65 min. 08 sec.

The turbidity slowly precipitates and the above solution becomes clear.

The sludge-rich phase can be filtered well, the filtrate is clear and almost colourless.

pH value: 9.21

time: 160 min. 21 sec.

Here is the further flocculation behaviour after more than 2 hours reaction time. The above solution is clear. The pH value of the treated solution has not changed further.

In the laboratory test, the used Alficolor Gold 602 colouring bath could be precipitated without any problems by adding sodium hydroxide solution. Only the respective reaction steps have to be observed. To achieve the complete precipitation reaction in the gold colouring bath, a pH value of about 9.2 was required. By using flocculation aids such as Alfiflock 61, the separation process can be additionally accelerated.

It should be considered that when processing gold colouring products containing ammonium ferrioxalate, the addition of caustic soda can lead to the evaporation of ammonia, resulting in a strong ammonia odour.

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