622
D.A. Wilms, F.F. Notebaert and A.A. Van Haute
a series of experiments with various polyelectrolytes that the best results were obtained
with Praestol 2700. Fig. 6 shows the rate of decantation in 100 ml cylinders respectively
for NaOH and Ca(OH) 2 as neutralisation agent, and for various polymer doses. When
comparing both graphs it is striking that the sludge formed by neutralisation with
Ca(OH) 2 settles considerably faster than the lighter and more aqueous sludge obtained by
neutralisation with NaOH. Although increasing amounts of flocculant considerably hasten
the settlement during the first minutes, the influence of higher doses over a long period is
rather small, so that in this case it is not economical to add more than 0.6 ppm Praestol.
The influence of the flocculant on the adsorption has been examined in an experiment
with CrVI = 3.35 mmol/1, FeM/CrVI = 4 6 5 a n d brought to pH = 6 with Ca(OH) 2 . After
5 minutes decantation a sample of the clear supernatant liquid was withdrawn and the
residual ΟΓΎΙ measured. Without flocculant the residual CrVI w a s \4Q mmol/1 or 36.4 %;
with a flocculant it was 1.334 mmol/1 Cr^I 0 r 34.7 %. The same samples were filtered
and both gave 1.327 mmol/1 Cr
VI or 34.5%. From this follows that the flocculant has
no adverse effect on the adsorption, but on the contrary they favour complete separation
of the floes from the liquid.
CONCLUSION
It appears from the experiments on the adsorption of Cr^I on floes of Fe(OH) 3 that
this adsorption is optimum at a pH = 6, at a high ratio of FeM/CrVI (though not higher
than 20) and at the highest possible CrVI concentration. Depending on the pH and the
iron content of the acid effluents, one will have to mix part of them with the alkaline
chromate wastewaters until the desired pH of about 6 is reached, or until the desired
FeHI/CrVI ratio is reached (eventually by adding Ca(OH) 2 ). After an adequate contact
time, the precipitate is separated using a flocculant. The remaining CrVI j n the
supernatant liquid is then reduced at pH = 6 with an adequate amount of FeS0 4 and
afterwards brought to a pH = 8 with Ca(OH) 2 in order to precipitate the Cr^I
completely. This secondary sludge of Fe(OH) 3 + Cr(OH) 3 is separated by decantation,
and can be used again for a further adsorption of CrVI j n the first stage of the treatment.
Such a procedure has the advantage that for the removal of chromates a minute quantity
of neutralizing agent is required and also that there is a minimum production of waste
sludge, because only a limited amount of extra FeS0 4 has to be added.
REFERENCES
1. KASCHKE, W. "Behandlung von Abwδssern in der metallbearbeitenden Industrie". Wasser, Luft
und Betrieb 12, 557-9, 622-5 (1968).
2. OEHME, F. "Die Entgiftung von Chromaten im alkalischen Milieu bei mit Cyanid gemischten
Abwδssern". Galvanotechnik 56, 545-8 (1965).
3. KRAJEWSKI, S. & KIESZKOWSKI, M. 'Traitement des eaux cyanuroes contenant du chrome
hexavalent et des quantity importantes de fer". La technique de l'eau n° 261, 23-32 (1968).
4. KOLLAU, K. & REIDT, M. "Methods for the determination of total-, hexavalent- and trivalent
D.A. Wilms, F.F. Notebaert and A.A. Van Haute
a series of experiments with various polyelectrolytes that the best results were obtained
with Praestol 2700. Fig. 6 shows the rate of decantation in 100 ml cylinders respectively
for NaOH and Ca(OH) 2 as neutralisation agent, and for various polymer doses. When
comparing both graphs it is striking that the sludge formed by neutralisation with
Ca(OH) 2 settles considerably faster than the lighter and more aqueous sludge obtained by
neutralisation with NaOH. Although increasing amounts of flocculant considerably hasten
the settlement during the first minutes, the influence of higher doses over a long period is
rather small, so that in this case it is not economical to add more than 0.6 ppm Praestol.
The influence of the flocculant on the adsorption has been examined in an experiment
with CrVI = 3.35 mmol/1, FeM/CrVI = 4 6 5 a n d brought to pH = 6 with Ca(OH) 2 . After
5 minutes decantation a sample of the clear supernatant liquid was withdrawn and the
residual ΟΓΎΙ measured. Without flocculant the residual CrVI w a s \4Q mmol/1 or 36.4 %;
with a flocculant it was 1.334 mmol/1 Cr^I 0 r 34.7 %. The same samples were filtered
and both gave 1.327 mmol/1 Cr
VI or 34.5%. From this follows that the flocculant has
no adverse effect on the adsorption, but on the contrary they favour complete separation
of the floes from the liquid.
CONCLUSION
It appears from the experiments on the adsorption of Cr^I on floes of Fe(OH) 3 that
this adsorption is optimum at a pH = 6, at a high ratio of FeM/CrVI (though not higher
than 20) and at the highest possible CrVI concentration. Depending on the pH and the
iron content of the acid effluents, one will have to mix part of them with the alkaline
chromate wastewaters until the desired pH of about 6 is reached, or until the desired
FeHI/CrVI ratio is reached (eventually by adding Ca(OH) 2 ). After an adequate contact
time, the precipitate is separated using a flocculant. The remaining CrVI j n the
supernatant liquid is then reduced at pH = 6 with an adequate amount of FeS0 4 and
afterwards brought to a pH = 8 with Ca(OH) 2 in order to precipitate the Cr^I
completely. This secondary sludge of Fe(OH) 3 + Cr(OH) 3 is separated by decantation,
and can be used again for a further adsorption of CrVI j n the first stage of the treatment.
Such a procedure has the advantage that for the removal of chromates a minute quantity
of neutralizing agent is required and also that there is a minimum production of waste
sludge, because only a limited amount of extra FeS0 4 has to be added.
REFERENCES
1. KASCHKE, W. "Behandlung von Abwδssern in der metallbearbeitenden Industrie". Wasser, Luft
und Betrieb 12, 557-9, 622-5 (1968).
2. OEHME, F. "Die Entgiftung von Chromaten im alkalischen Milieu bei mit Cyanid gemischten
Abwδssern". Galvanotechnik 56, 545-8 (1965).
3. KRAJEWSKI, S. & KIESZKOWSKI, M. 'Traitement des eaux cyanuroes contenant du chrome
hexavalent et des quantity importantes de fer". La technique de l'eau n° 261, 23-32 (1968).
4. KOLLAU, K. & REIDT, M. "Methods for the determination of total-, hexavalent- and trivalent