Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to the purification of
water contaminated with at least coloured substances, and
possibly contaminated with other substances, such as acids,
salts, and residues from an industrial dyeing process.
The contaminated water is hereafter referred to as an aqueous
liquid.
The present invention provides a process for the
purification of effluent dyestuff-containing liquid without
substantially building up the total dissolved solids content
thereof, said effluent liquid being effluent from a dyeing
process and which is contaminated with at least one dye and
heavy metal ions but which is substantially free from dye
complexing retarders, said process comprising the steps of
(a) introducing iron ions into the effluent by
electrolysis from an iron electrode,
(b) oxidizin~ ferrous ions in the effluent to -
ferric ions which precipitate as ferric hydroxide and to
which the dyes and heavy metal ions adhere, and
(c) removing the ferric hydroxide together with
the adhering dyes and heavy metal ions.
The coloured substances may be dyes or pigments.
If the water to be treated is effluent from a textile factory
it may contain fibres and these can be separated in an initial
step by screening or filtration. If dye complexing retarders
from an industrial dyeing process are present, they are at
least partially removed before the treatment with iron. The
removal o~ dye complexing retarders can be carried out by
ion~exchange. The ion-exchange resin to be used will depend
on the retarder charge. For example an anionic dye complex-
ing retarder, such as a sulphonate castor oil can be removedwith a weakly basic anion exchange resin, whereas a cationic
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--- 1049: IL63
dye complexing retarder can be removed with a weakly acidic
cationic exchange resin. The ion exchange treatment can be
by upflow treatment, with the area of exchange resin increas-
ing in the direction of upflow. The upper diameter of the
resin column is such that the upward liquid velocity does
not exceed the stokes velocity resin, resulting in the resin
beads falling back into the exchange resin bed. Such a partial
fluidisation of the bed allows free passage of suspended
material through the bed without excessive pressure increase
or decrease in effluent flow rate.
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The iron ions conveniently are added by elec-
trolysis of iron or scrap iron when effluent is to be re-
cycled, or by the addition of sulphates, chlorides or
nitrates when a build-up of total dissolved solids can be
tolerated.
The iron is present as ferrous or ferric ions,
depending on the oxidation characteristics of the effluent
being treated. If oxidising agents are present in the
effluent, any ferrous ions may be oxidised immediately to
ferric ions. If the action of oxygen in the air is insuffic-
ient, an oxidising agent can be added to ensure the formation
of ferric hydroxide.
The pH can be adjusted to give an optimum precipi-
tation of ferric hydroxide floc, the actual pH depending
preferably on the zeta potential of the colouring material.
For example, if the residual dye or other colouring matter
is anionic, a positive zeta potential on the ferric hydroxide
flocculate would be desirable, and vice versa. Thus, the
pH may be less than 6.6 for treating an aqueous liquid con-
taining an anionic residual dye. Similarly the pH can beadjusted to the optimum value for adsorbing colouring materials
which have a zero zeta potential, or which form complexes
with ferrous or ferric ions, or their hydroxides.
The ferric hydroxide flocculate containing the
dye, pigment or the like adsorbed, absorbed or complexed there-
with may be separated by thickening, clarification, flota-
tion, centrifuging, filtration, or the like, or a combination
of two or more such steps. As the solubility of ferrous
hydroxide is in the region of 5 to 15 mg/litre, it is desir-
` 30 able to oxidise all the iron to the ferric state, thereby
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reducing the solubility to less than 1 mg/litre and ensuring
precipitation of all iron as ferric hydroxide. -
The treated effluent may then be passed through
a colour detection apparatus or system, e.g. a spectrophoto-
meter, photometer, turbudity meter, or the like, or examined
by eye. The treated effluent may then be recycled to be
treated again with iron, or accepted for further use.
The pH of the effluent may be adjusted for further
use, e.g. to a pH suitable for use in a dyehouse. The ion
exchanye resin used to remove retarders may be regenerated
in known manner to yield dye complexing regarder which can
be recycled to the dyehouse. Buffer solutions (e.g. based
on acetic acid, sodium acetate and sodium hydroxide) may be
used for pH adjustment. The ferric hydroxide may be used as
landfill, or may be incinerated and the iron oxides re-
used in the electrolysis step.
The invention is illustrated by reference to the
following non-limiting Example, read with the accompanying
flowsheet.
. .
Example
: The effluent was from a dyehouse and contained:
Acetlc Acid
Sodium acetate
Dye complexing retarder - Sulphonated caster oil
Antifoamer
Cadionic dye (DYE)
Suspended matter such as fibres and synthetic
starches.
The fibres present were first removed by screen-
ing and then the dye complexing retarder was removed by
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upflow ion exchange using a weakly basic anion exchanger
(Amberlite IRA 93).
The reactions which occurred were as follows:
Protonation of resin in acid solution.
RlNH2 + (H) ~ RlNH3
Dissociation of sulphonated castor oil
2 SO3Na~__ ~R2 SO3 + Na+
Ion exchange reaction (Extraction of sulphonate).
RlNH3 + R2SO3 - -~ RlNH3 3 2
As a weak basic anion exchanger was used, only
the strong acids, such as sulphonates and sulphates were
removed from the effluent. The anion exchanger was treated
from time to time to recover the dye complexing retarder
and to regenerate the anion exchange resin. Carboxylic anions
-
such as CH3COO were not removed on passing the effluent
through the resin but would be removed if a strongly basic
anion exchanger were used, resulting in more frequent resin
regenerations. `~
After adding scrap iron, electrolysis of the
effluent from the resin was then effected using a Standard
D.C. power supply and applying Farraday's Laws of Electrolysis
to determine the amount of iron ions liberated in the
effluent.
Ferric hydroxide formation was carried out by
the addition of hydrogen peroxide and using oxygen from air
introduced by airation of effluent. The optimum precipitation
pH for removal of cationic dye was pH = 8. The sodium
acetate and acetic acid acted as a buffer.
Anionic dyes with a negative charge, by comparison,
have an optimum precipitation pH between 5 and 6. The Z.E.T.A. ~ ~
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potential of ferric hydroxide is positive and increasingfor pH values less than 6 and negatively increasing for
pH values greater than 7.
Sludge thickening was achieved using a clarifier,
and ferric hydroxide fines were removed from the overflow
by candle filters and rotary vacuum filters using diatom-
aceous earth.
Colour detection was carried out using Beckman ~
spectrophotometer. Once the colour was acceptable, the pH ~ -
was adjusted to 4,7 with acetic acid, and the treated liquid
was supplied to the dye house and re-used (after adding about
20% of water since some water had been lost on fibre removal
and in thickened sludge drying beds). Rejected effluent
was re-supplied to the electrolysis step. By recycling the
treated liquid, loss of useful soluble substances and out-
side contamination were avoided.
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