Size and shape characterisation of dye flocs during chemical
coagulation of Reactive Black 5 dye
Chair of Mineral Processing, Brandenburg University of Technology
03046 Cottbus, Germ
In the present paper, flocs derived from chemical coagulation were characterized in terms of
size distribution and shape by concurrent methods. A simple model encompassing
commercially used textile disazo dye CI “Reactive Black 5” produced by Ciba coagulated with
synthetic primary coagulants from CIBA has been chosen. Size and shape characteristics were
determined by Galai_Ankersmid CIS
100 particle counting system and a
used in LALS mode. An image analysis of the flocculated dye
sludge has been carried out by a
Quantimet 600 system giving Roundness as a shape factor. The pros and cons of each
measurement technique used have been summarized for the stud
ied case. Parallel to the floc
characterization, surface charge measurement based on streaming current was carried out for
the coagulated dye sludge by means of a streaming current particle charge detector. A standard
Jar Test has been used for the coagula
Different solid/liquid separation methods (paper filtration, sedimentation and centrifugation)
have been studied with the purpose to find out a hypothesis about the predominant mechanism
of color removal.
comparable role in color removal
although centrifugation presented a better removal than sedimentation. The best result by
100% was achieved with coagulant 7102 at dose 40mg/l. The best
removal of 100% was reached in the case
of 7103 at a dose of 40 mg/l, where
the fractal dimension was characterized by 2,74 and the mean diameter
m. It was
established that by varying the operating conditions, the removal could be governed either by
charge neutralization or by bridging me
chanism. This assumption is supported by surface
charge sign/value progression and by the different characteristics of the respective flocs as well.
In cases of all coagulants, a good correlation between the surface charge and color removal
was seen. Surf
ace charge of coagulated dye
flocs has increased parallel to coagulant dose
increase and it has reached zero point and became positive what was manifested in color
removal decreasing. The best color removal corresponded to Point of Zero Charge and it has
oincided with an optimal dose 50mg/l in case of coagulant 7101 and 40mg/l with 7102 &7103.
This correlation between the color removal and surface charge implies that the adsorption of
coagulant species onto dye surface is leading to the dye
further aggregation into larger flocs.
some relation between color removal and fractal dimension, a parameter derived
from LALS technique, which is used to characterize the compactness of the aggregate
structure, has been a
lso drawn. In cases of two coagulants, Fractal dimension presents the
same trend and it increases parallel to coagulant dose increases and to the removal by
sedimentation increases, to the dose in which the color removal is the best (50 and 40mg/l
vely), reaches the values about 3,0 what implies a high compactness of the flocs.
According to the Image Analysis, a relation between the shape factor of the coagulated dye
flocks and settling ability was found. The best color removal obtained by sediment
corresponds to flocs Roundness about 2, which indicates a dense structure of the flocks.
flocs have consisted of a large irregular flocks and a finer one what has
implied a bi
modal size distribution and has been distinctly seen at th
e pictures taken.
In the case of CI “Reactive Blue 5”, surface charge neutralization seems to be a predominant
mechanism for color removal by chemical coagulation using synthetic coagulants. The evident
relationship between the surface charge progression f
or coagulated dye sludge and color
removal achievable under different solid/liquid separation methods provides a motivation for
further investigation of the mechanism of dye coagulation.
: chemical coagulation, textile dyes, floc characterisation