Method of treatment of the circulation water of a paper mill

A method for removing dissolved and colloidal material from the circulation water of a paper mill with chemical pulp fibers wherein fiber sludge which has been recovered from the paper manufacturing process and acidified to a pH of 1.5-2.5, is mixed with the circulation water of a paper mill whereby the dissolved and colloidal substances are precipitated.

The present invention relates to a method of minimizing the problems caused 
by dissolved and colloidal substances in the circulation water of a paper 
mill. 
Dissolved and colloidal organic substances in the circulation water of a 
paper mill comprise partly carbon hydrates of wood, lignin and digested 
materials. Further, various anionic and cationic additives, such as 
dispersion agents, slime- and foam-fighting materials, starch and dry and 
wet strength adhesives are used in the manufacture of paper. Also coating 
chemicals get into the circulation water. Circulation water often contains 
also very fine-grained fillers. 
The concentration of dissolved and colloidal substances in the circulation 
water of a paper machine can be so high that it severely disturbs the 
control and the economy of the process. 
Some of the problems caused by dissolved and colloidal substances in the 
circulation water are the corrosion, the difficulties in cleanliness and 
in the dewatering. Further there are problems with bleaching and the 
retention of fines and fillers is reduced and the strength properties and 
the brightness of the paper is poor. The problems and their severity vary 
depending on the paper grade to be manufactured and on the machinery used 
in the manufacture. 
There have been attempts to avoid the problems mostly by having an 
appropriately open water circulating system. The volume of water removed 
from the system varies mostly between 10 and 50 m.sup.3 per ton of paper. 
Further, the additives used in paper making such as aluminium sulfate, 
sodium aluminate and organic polymers precipitate dissolved substances to 
the paper web. Also rather large amounts of bentonite, in most cases 4 to 
10 kg per to of paper, and organic polymer have sometimes been added to 
remove dissolved and colloidal substances. 
The object of this invention is to provide an efficient method of reducing 
the volume of dissolved and colloidal substances in the circulation water 
by which method the volume of dissolved organic material is reduces by 
30-50%. The capital and operation costs of the method are easily kept low. 
The method of the present invention is characterized by the feature that 
part of the fiber used for making paper is, after grinding, acidified and 
mixed with a part of the circulation water. After the pH has been 
regulated and possibly also cationic polymers added, the fiber-water 
mixture is added to the paper pulp. If a large volume of circulation water 
is to be treated, the pulp to be added to the circulation water must be 
precipitated before it is added to the pulp to be ground. Precipitation 
can be carried out e.g. in a filter for the circulation water into which 
the acidified fiber is fed to form so-called surface pulp. From the filter 
the fiber is returned to the paper making process with the recovered solid 
material. 
Laboratory tests have shown that addition of 2 g acidified fiber per liter 
circulating water of a paper mill reduces the turbidity of the effluent 
and the amount of dissolved organic substance by 90% expressed in numeric 
form. Precipitation is intensified if cationic polymer is added to the 
fiber-water mixture.

The invention is described further in detail by way of the following 
examples. 
EXAMPLE 1 
The effluent from a fine paper mill, the water comsumption of which was 
approx. 50-60 m.sup.3 per ton of paper, was treated with acidified 
chemical sulfate pulp fibers and with acidified fibers from the fine paper 
mill itself. 
The chemical pulp fibers were acidified with sulfuric acid to a pH of 
approx. 1.8 and they were added to the effluent to be treated to a content 
of 4 to 6 g per liter effluent. 
The pH of the mixture of effluent and chemical pulp fibers was regulated 
with sodium hydroxide to 5-6 and 1-2 g cationic polymer per m.sup.3 of 
effluent was added. The sludge was separated by sedimention. 
The same procedure was used when effluent was treated with the sludge 
received from the fiber recovering unit of the fine paper mill. 
As a result of the treatment the solids content of the effluent reduced by 
approx. 60% from what it had been after a simple sedimention of the same 
duration. A corresponding comparision showed a reduction of the turbidity 
of approx. 90% and the COD.sub.Cr reading had decreased by approx. 20%. A 
test with the sludge from the fiber recovering unit gave corresponding 
results. 
The reduction of turbidity in particular is obviously caused by 
precipitation of the starch in circulation water. 
EXAMPLE 2 
The tests have resulted in the process chart illustrated in the appended 
schematic drawing, by way of example. 
Chemical pulp a, dilution water b and sulfuric acid c is added to a vessel 
indicated by reference numeral 1 in the chart. The pH of the mixture is 
regulated to be approx. 1.5-2.0. The water-fiber mixture is then led to a 
mixing vessel 2 into which the process water d to be treated also is fed. 
After mixing, sodium hydroxide e is added to the water, which is fed to a 
fiber recovering unit 3, to obtain a pH of 5-6. After this in most cases 
1-2 mg of cationic polymer f per liter of mixture is added. The polymer is 
preferably the same substance as is used in the paper machine. 
From the fiber recovering unit the fibers and the dissolved and colloidal 
substance precipitated in the fibers are returned to the process for use 
in the paper manufacturing process. 
Also, the water is recycled to the paper machine. 
The chemical pulp fiber used in the precipitation is ordinary chemical pulp 
used in paper manufacturing. The volume of dissolved material which has 
adhered to the pulp is in the order of 0.1-1.0 kg per m.sup.3 of treated 
effluent. 
EXAMPLE 3 
The main dimensions of a proposed process for removing the dissolved and 
colloidal substances from the circulation water of a fine paper mill which 
produces 200 tons of paper per day and the circulation water volume of 
which is approx. 25 m.sup.3 per ton of paper, are as follows: 
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effluent volume to be treated 
m.sup.3 /h 
200 
required amount of chemical pulp 
kg/h 400 
amount of polymer kg/h 0.2-0.4 
pulp mixing vessel m.sup.3 
3 
(vessel 1 in example 2) 
mixing vessel for effluent and 
m.sup.3 
5 
chemical pulp 
(vessel 2 in example 2) 
precipitating dissolved and 
kg/h 80-200 
colloidal substance 
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When this cleaining process is used the need to purify the circulation 
water system is reduced. If desired, the volume of water introduced to the 
system can be reduced which reduces the energy consumption and the cost of 
purifying the effluent. 
The invention is not intended to be limited to the embodiments described 
here as examples only but several modifications and alternative 
applications of it are possible within the scope of protection defined by 
the appended patent claims.