Sulfate removal in anodization waste water

Introduction

In aluminium anodization, the sulfuric acid process is most commonly used. The waste water flow, produced by this process, will contain lots of sulfates (typically 5.000...6.000 mg/l). These remain in the waste water, even after physical-chemical treatment. Even when during the waste water treatment a double stoichiometric amount of calcium (whether as chloride or as hydroxide) is added, there will remain more than 3.000 mg/l SO42- in the effluent. Because in Europe, the effluent limit is 2.000 mg/l SO42-, a problem arises.

In the literature, the following ways are mentioned in order to deal with this problem:

If these methods were to be applied in an actual anodizing plant, some practical problems could arise.

Conclusion

From what has been discussed above, we learn that -with regard to the initial investment and maintenance cost- the most economical and reliable way to solve the SO42- problem in anodization waste water, consists in a controlled dosing of Ba2+ . This requires an agreement of the environmental authorities with regard to the use of Ba2+ salts.

BaCl2 dosing practically

Overdosing of barium is to avoid at all cost. Therefor, a certain quantity of free sulfate should remain in the waste water after treatment. Because the effluent limit usually is 2.000 mg/l, there shouldn't be any problem there. Furthermore, removing all sulfates would require more barium and thus cost more.

In order to prevent an overdosing of barium, the following conditions ought to be met:

Sulfate analyzer

A dedicated sulfate analyzer, to be used to control the dosing of barium chloride solution in anodizing plant waste water treatment has been developed.
It's main features are:

measuring range500...5.000 mg/l SO42-
accuracy+/- 200 mg/l SO42-
repeatability200 mg/l SO42-
principlesequencing batch measuring
sampling pumps2 built-in so as to sample 2 different water flows simultaneously (for instance influent and effluent)
measuring frequency1 sample every 15 min
analog outputs 2 "sample and hold", for influent and effluent sulfate concentration
controlembedded microprocessor
screen6" touch screen
analog input for flow meter
analog output for dosing pump
connections
  • 1/2" water supply (for rinsing purposes).
    Water should be supplied at 3 bars at least and contain less than 50 mg/l sulfates
  • 2 x 8 mm hose pillar for influent and effluent sampling.
    Maximum length of sample lines: 5 m
  • drain to sewer (or waste water treatment)
power supply230VAC, 50Hz, 10A