Bleaching stilbene yellow dyed wood pulp

Chlorine dioxide is a specific bleaching agent for stilbene yellow dye which has been used to dye paper stock. Chlorine dioxide may be used in amounts as low as 0.25% based on the oven dry weight of the pulp fiber.

CROSS REFERENCE TO RELATED APPLICATION 
This application contains material that is common to my copending patent 
application Ser. No. 8,460 filed Feb. 1, 1979. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
Bleaching stilbene yellow. 
2. Summary of the Invention 
At the present time it is considered environmentally sound to repulp paper 
for reuse. One type of paper that may be reused is the yellow pages of 
telephone books. One of the stilbene yellow dyes is the dye that is used 
to color these pages. At the present time the sole reuse of these pages is 
as a furnish for new yellow pages since it has not been possible to remove 
the yellow color. 
One of the reasons for this is that the exact composition of the stilbene 
yellow dyes is considered to be unknown or undetermined. This is the 
statement given in the Chemline Databank. 
It has now been found that chlorine dioxide is a specific bleaching agent 
for stilbene yellow dye. Concentrations of chlorine dioxide as low as 
0.25% up to 4% based on the oven dry weight of the paper fiber are enough 
to bleach the stilbene yellow. Other bleaching agents have been tried and 
do not bleach the dye.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Paper dyed with a stilbene yellow dye was placed in a beater, water was 
added and the material beaten until it was repulped. It had a consistency 
of 36.3%. The consistency is the amount of fiber in solution and is given 
as a percent of the weight of the fiber-water slurry. Water was added to 
reduce the consistency of the slurry to 10%. 
Samples were then treated with chlorine dioxide. Each of the samples 
contained 5 grams of fiber on an oven-dry basis. The treatment was for an 
hour at 70.degree. C. The pH of the material was not adjusted. The table 
gives the amount of chlorine dioxide added as a percent of the oven-dry 
weight of the fiber, in grams, and in milliliters of solution based on a 
chlorine dioxide weight of 8.7 grams/liter. The table also gives the GE 
brightness of the bleached pulp. 
TABLE I 
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C10.sub.2 GE 
Ex. % 9m ml Brightness 
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1 0.25 0.0125 1.4 63.2 
2 0.5 0.025 2.8 65.7 
3 1.0 0.05 5.6 67.8 
4 2.0 0.10 11.2 65.5 
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Other bleaching agents such as ozone, oxygen, hydrogen peroxide, chlorine 
and hypochlorite were tried and did not bleach the stilbene yellow. From 
this it was concluded that chlorine dioxide was a specific bleaching agent 
for stilbene yellow. 
The usual conditions for chlorine dioxide bleaching would be used. These 
would include the in situ formation of chlorine dioxide as, for example, 
by the reaction of sodium chlorite with a suitable acid, or in the 
presence of hypochlorite.