Removal of color from paper mill waste waters

Color is removed from paper mill waste water by a precipitation method using relatively small amounts of polyethylene imine as a complexing agent and precipitant for color bodies in such waste waters.

BACKGROUND 
It is generally recognized that the amount of organic material in the form 
of BOD (biological oxygen demand), COD (chemical oxygen demand), and color 
bodies currently being discharged into public water supplies from pulp and 
paper mills is a problem of increasing importance in water pollution. 
In the manufacture of paper, cellulose from both hard woods and soft woods 
is converted to pulp by various processes such as the sulfite process and 
the sulfate process and the pulp is subjected to various treatments 
particularly bleaching which involve the use of chemical reagents such as 
chlorine, chlorine dioxide, sodium hypochlorite and caustic alkalis. As a 
result of these treatments, the paper making process produces highly 
colored effluents. The bleaching of the pulp, in particular the caustic 
extract effluent, is a major contributor to pollution problems. 
Lignin is a primary source of color in the pulp. Chlorination of the pulp 
during the bleaching operation results in the formation of color bodies 
which are leached from the pulp by caustic alkali solutions. Thus, the 
caustic extract effluent contains a major proportion of the color bodies 
and other organic materials which have to be disposed of in waste water 
treatment. 
Several processes are now used or advocated for use in removing color from 
paper mill waste. One of these is the so-called "massive lime process". 
This process utilizes lime in quantities ranging from 5,000 to 20,000 
parts per million (ppm) to remove by coagulation and sedimentation 
techniques the organic colored bodies from paper mill wastes. 
Another process which is disclosed in U.S. Pat. No. 3,412,018 involves the 
use of certain types of water insoluble amines in combination with a 
diluting agent which is non-miscible in water and which has a dielectric 
constant, measured at 20.degree. C., of less than 4.8 in order to form 
substantially water insoluble organophilic amine complexes which are 
extracted by the organic phase. The organic phase is thereafter decanted 
and separated from the aqueous phase, the amine complex is precipitated 
from the organic phase and the amine and non-miscible diluting agent are 
recovered from the organic phase and recycled for further use in the 
process. This process involves the use of a very substantial amount of the 
water insoluble amine as well as substantial amounts of organic solvent. 
Thus, as reported by the patent, the amount of amine employed should be at 
least 0.5% by volume which would be in excess of 5,000 ppm with respect to 
the total amount of water treated. The amount of organic solvent is at 
least 5% by volume with respect to the amount of water treated which would 
normally correspond to in excess of 50,000 ppm. 
The foregoing amine-solvent extraction process has been investigated 
extensively and is reported to have a number of disadvantages including 
residual odors from the organic solvents and the tendency for emulsion 
formation during the treatment and regenerating steps of the process 
causing excessive loss of amine from the process in the formation of 
emulsions in the various intermediate steps of the process. 
OBJECTS 
One of the objects of the present invention is to provide a precipitation 
process for removal of color bodies from paper mill waste waters and 
bleachery effluents wherein relatively small amounts of an organic 
compound are employed and no organic solvent is required. 
A further object of the invention is to provide a process of the type 
described which can be carried out with readily available raw material. 
Other objects will appear hereinafter. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the invention it has been found that color can be 
removed from paper mill waste waters by adding to such waste waters, 
preferably at a pH within the range of about 2 to about 7, and in most 
instances within the range of 2 to 5, a relatively small amount of a 
polyethylene imine having a molecular weight of at least 300, the amount 
used being sufficient to form a complex solid of the imine with the color 
bodies in the waste waters which can be separated by settling or 
filtration. The amount of the imine can be as low as 10 ppm with respect 
to the waste water or bleachery effluent and, in most cases, will not 
exceed 2,000 ppm, depending upon the particular imine employed. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention is particularly applicable to pulp mill bleachery waste 
waters which are obtained as a result of bleaching processes applied to 
the pulp wherein chlorine, chlorine dioxide or sodium hypochlorite are 
employed and the pulp is treated with caustic alkali solutions (i.e., 
sodium hydroxide solutions) which extract the color bodies and produce a 
highly colored effluent. It is essential for the purpose of the process 
that the pH of this effluent be reduced to a pH within the range of 2 to 7 
and preferably around 2.5 to 5.0. In a typical paper mill plant the waste 
waters from the whole mill might total 25,000,000 to 30,000,000 gallons 
per day and the bleachery waste waters would constitute a smaller 
proportion of the total, say, 1,000,000 to 6,000,000 gallons per day. The 
present invention can be used to treat all of the waste waters but it is 
usually more practical and more essential to treat the bleachery waste 
waters in order to remove or to reduce to a minimum the color bodies 
therein. 
The present invention is based on the discovery that relatively small 
amounts of imines of the type previously mentioned will combine with the 
color bodies in the aforementioned waste waters under certain pH 
conditions to form a water insoluble solid and that the resultant 
imine-color body solids can be removed by settling or filtration. 
It appears to be essential for the purpose of the present invention that 
the imines employed have an average molecular weight of at least 300. The 
polyethylene imines are available commercially as PEI 3, PEI 6, and PEI 18 
with average molecular weights of 300, 600 and 1800, respectively. They 
are normally water soluble and can be used as aqueous solutions. Higher 
molecular weight imines can be used. Inasmuch as the reaction with the 
color body apparently occurs at the imino 
##STR1## 
group, it is deemed to be immaterial whether the imine itself is water 
soluble or water insoluble. 
In carrying out the process the pH can be achieved by blending acidic 
chlorination effluent from the bleachery process with caustic extraction 
effluent or the caustic extraction effluent can be acidified with sulfuric 
acid or waste liquor containing sulfuric acid, or hydrochloric acid. 
The invention has been evaluated by treating the waste waters with the 
imine, removing the resultant solids and comparing the clarified effluent 
or filtrate with the waste water prior to treatment. This comparison was 
carried out in two ways, (1) using a Gardner Holt colorimeter and 
observing the readings, and (2) obtaining color readings in terms of APHA 
units. The treated samples were centrifuged or filtered through No. 40 
filter paper to remove floc and the filtrates or centrifugates were 
adjusted to pH 7 before color readings were taken. The raw effluents used 
in the test were adjusted to pH 7 and then filtered through a millipore 
filter before color readings were made to get a base line reading on 
original color. 
The treatment of the waste water with the imine can be supplemented by a 
subsequent addition of a water insoluble amine containing at least eight 
carbon atoms, preferably 8 to 24 carbon atoms, in the hydrocarbon group 
linked to the amino group, e.g., oleyl amine.

The invention will be further illustrated but is not limited by the 
following examples in which the quantities are given by weight unless 
otherwise indicated. 
EXAMPLE I 
A soft wood bleachery caustic alkali extract was acidified with 
hydrochloric acid to give a pH of approximately 3 and then treated with 
400 ppm of polyethylene imine having an average molecular weight of 300 
(PEI 3). The Gardner color was reduced from 18 to 12. The temperature used 
in this test was 40.degree. C. Other tests were carried out at ambient 
temperatures of 20.degree.-25.degree. C. with generally similar results in 
color reduction. 
Subsequent additions of 300 ppm of oleyl amine reduced the Gardner color to 
5. 
EXAMPLE II 
Samples of various types of paper mill waste waters were tested using 500 
ml samples to which a polyethylene imine was added at various pH's and in 
various proportions. The samples were stirred at ambient temperatures at 
100-120 rpm for 2 minutes and at 40 rpm for 2 minutes. Color and clarity 
on the APHA color scale on a Hellige instrument was determined. Readings 
were made after cup centrifuging at the treated pH and then again at pH 
7.0 after adjustment. Usually color values go up 20 APHA units at pH 7.0. 
The additives were all added as solutions in water diluted to a 1% 
concentration of the polyethylene imine. 
In one series of tests the bleachery caustic extract had an APHA value of 
500 and a pH of 9.1. The addition of polyethylene imine having an average 
molecular weight of 1800 (PEI-18 Dow) at a dosage of 50 ppm reduced the 
APHA color to 20. At dosages of 30 ppm and 10 ppm there was no reduction 
in the APHA color. 
The same polyethylene imine at dosages of 50 ppm, 30 ppm and 10 ppm and at 
a pH of 5 in each case reduced the APHA color of the sample from 500 to 
20, 30 and 120, respectively. 
The same polyethylene imine at dosages of 50 ppm, 30 ppm and 10 ppm and at 
a pH of 7 reduced the APHA color from 500 to 60, and 200 APHA in the case 
of the 50 ppm dosage and the 30 ppm dosage, respectively. A dosage of 10 
ppm at pH 7 produced no reduction in the APHA color and no floc formation. 
The same polyethylene imine at dosages of 50 ppm, 30 ppm and 10 ppm and at 
a pH of 3 reduced the APHA color from 500 to 40, 50 and 60, respectively. 
EXAMPLE III 
The procedure was the same as in Example II except that polyethylene imine 
having an average molecular weight of 600 was used. At dosages of 50 ppm, 
30 ppm and 10 ppm and a pH of 3, the APHA color was reduced to 40 and the 
floc formation was good except at the 10 ppm dosage where it was fairly 
good. 
In carrying out the process it is also possible to acid-extract the 
precipitated sludge and recover a portion of the imine which can then be 
recycled in the process. Inasmuch as the amounts of imine employed are 
relatively small, this is not always practical. 
It should be understood that the proportions of the various additives may 
vary to some extent depending upon the composition of the particular paper 
mill waste water. The process is particularly useful for removing color 
from softwood and hardwood paper pulp bleachery effluents where the 
content of color bodies is relatively high. It may not always be practical 
from an economical standpoint to use the process on the total effluent. 
The treatment can also be used not only for the purpose of reducing color 
in paper pulp waste water but also for reducing total solids and for 
reducing total suspended solids.