REMOVAL OF COLLOIDAL TITANIUM DIOXIDE PIGMENT
FROM AN INDUSTRIAL WASTE EFFLUENT
Michael D. Walters, Graduate Student
Irvine W. Wei, Assistant Professor
Environmental Engineering Program
Northeastern University
Boston, Massachusetts 02115
INTRODUCTION
This paper is concerned with the removal of titanium dioxide pigment (Ti02) from
aqueous industrial plant effluents.   Removal of the pigment is a two-step physicochem-
ical procedure.   First, the pigment is usually found in the waste stream as a stable colloid
which must be coagulated, flocculated and settled.   Then the resultant Ti02 sludge must
be removed from the waste effluent.
Initially, the coagulation (colloid destabilization) of three common Ti02 colloidal
dispersions are closely examined using alum and a cationic polymer as coagulants.   The
turbidity of the waste effluent is investigated to determine its relationship to the Ti02
pigment concentration.   A relationship of turbidity reduction to alum dose is developed
and presented as an important coagulant dose parameter.   The settling characteristics of
the coagulated Ti02 particles are determined for each of the waste effluents studied.
The second portion of the procedure concerns the physical removal of the sludge
from the waste effluent.   A Trommel Rotary Vacuum Drum Filter, with a continuous
newsprint paper filtering sheet, is employed to remove and dewater the coagulated Ti02
sludge.   Design criteria are presented on the various parameters of this filter.
The paper ends by developing a general treatment scheme using the various parameters
obtained in this study.   A treatment system is presented as a process flow diagram with
a brief design and operating description.
Titanium Dioxide Pigment
Titanium dioxide pigment is used by a number of industries to obtain a bright white
color.   The paint, paper and plastics industries are major consumers of this pigment [ 1 ].
The plant effluents studied in this paper are from industries which use Ti02 pigment as
a raw material.   The colloidal suspensions of Ti02 enter the waste stream from washing
and cleaning of various processing equipment at the manufacturing locations.   Titanium
dioxide pigment is an inert, nontoxic, insoluble particle in most aqueous solutions [ 1 ].
The small size of its crystalline structure (0.2 to 0.25m) provides an extremely large specific
surface area, which can be utilized to form a fairly stable colloidal dispersion.   Because
of the pigment's high density (> 4.0 specific gravity), a substantial surface charge must
be created on each particle to insure colloidal stability and offset the natural settling of
the pigment [2].   Titanium dioxide particles form a highly charged electronegative colloid within the stable range (having a zeta potential greater than -35 mV), when mixed
in water [3,4].    The physical-colloidal characteristics of this pigment parallel that of
many other inorganic colloids such as silica, barium sulfate, zinc oxide and some natural
clays [3].    The use of this pigment usually requires the creating of a stable colloidal
dispersion to insure even spreading and distribution of the pigment in the final product.
Surfactants are often employed to further stabilize the pigment dispersion.    The addition of surfactants and the adjustment of the solution pH can greatly increase the surface charge (zeta potential) on the pigment particles [5].    The two industrial waste
effluents studied in this paper represent typical Ti02 dispersions.   These waste effluents,
a latex paint and a paper coating solution, along with a pure pigment-water mix
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