A Kraft Bleach Waste
Color Reduction Process Integrated
With the Recovery System
ALBERT J. HERBET, Research Associate
and
HERBERT F. BERGER, Project Leader
National Council for Stream Improvement
Louisiana State University Project
Baton Rouge, Louisiana
INTRODUCTION
The pulping industry has long been plagued with the problem of colored
liquid effluents, especially when such effluents are discharged to clear streams
or to waterways which serve as municipal or industrial water sources. For almost ten years National Council researchers at the Louisiana State University
research project have sought to devise a color removal process that was feasible chemically, mechanically, and economical (1, 2, 3, 4, 5, 6). The
earlier studies were made on total kraft effluent, but the more recent investigations have been undertaken using caustic stage effluent from the bleach
plant since this waste has been shown (5) to contain from 60 to 80 per cent of
the color discharged during the production of bleached kraft pulp.
At the 1957 National Council Annual Meeting, Brown discussed the characteristics and measurement of kraft effluent color. His work has been published as Council Technical Bulletin No. 85 and will not be reviewed here.
The brown color of pulp and paper effluents is due, of course, to the lignins
and tannins dissolved from the wood during digestion and bleaching. The persistence, even increase, of color in passage downstream or through a biological treatment plant is due to lignin's high resistance to microbiological degradation. The fact that a relatively small concentration of organic matter
causes a pronounced color in pulp mill wastes is shown graphically in Figure 1.
The color to organic matter ratio is shown to be about 16 : 1. Note that this
curve does not go through the origin since, as evidenced by a measurable
BOD, caustic extract contains dissolved organics other than lignin.
Lignin can be precipitated by a number of common coagulants, such as
lime, sulfuric acid, and alum. All previous research has led to the conclusion that lime offers the best possibility as a color precipitant in the kraft industry by virtue of its low cost and easy availability and the fact that handling
techniques and equipment are familiar to kraft operating personnel (1,2,3,4).
The initial approaches to the problem sought to precipitate the color
bodies with a minimum lime dose, followed by feeding the partially de-
watered lime-organic sludge to the kiln to destroy the organic matter. The
resulting re-burned lime could then be used in chemical recovery in the normal manner. But this scheme was shown to have major shortcomings, namely:
1. The gelatinous nature of the voluminous lime-organic sludge made
dewatering by filtration, or any other means, extremely difficult.
2. Sludge recarbonation, designed to render the lime-organic sludge
more easily dewatered, resulted in severe foaming, in*addition to which the
sludge dewatering characteristics were not altered significantly by the degree
of carbonation employed.
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