# Patent

## Title
Method for removing lignin from water

## Abstract

The invention relates to a method for removing lignin from an aqueous solution. It is accomplished by adding a calcium compound and an aluminium compound to said solution. Said calcium compound is preferably calcium chloride and said aluminium compound is preferably a polymeric aluminium hydroxide salt.

## Background
The invention relates to a method for removing lignin from an aqueous solution by adding a calcium compound and an aluminium compound thereto.
The bleaching sequence of chemical pulp often comprises one or several peroxide steps. The bleaching conditions with respect to pH are alkaline, also being relatively hot (>60° C.). Under these conditions, the lignin contained in the pulp is partly dissolved and partly becomes less coloured on the fiber surface. After bleaching, the pulp is washed and the drained water is normally passed to a water treatment unit. Lignin dissolving in water results in a yellowish brown colour thereof. Lignin is considered as a sparingly biodegradable substance, thus having a great influence on the properties of purified waste water. Therefore it would be desirable to be able to remove lignin from the waste water prior to biological treatment thereof.
It is known that calcium precipitates lignin. Several references, even recent ones, to this issue may be found in the literature. However, the precipitating power thereof is rather limited. See Table 2 below.
U.S. Pat. No. 4,420,369 discloses the addition of an acid liquid containing calcium and aluminium to an aqueous solution containing lignin, thus resulting in neutralization of the liquid mixture for lignin precipitation. The result of this precipitation is, however, unsatisfactory.
Accordingly, the object of the invention is to provide a more effective method for removing lignin from an aqueous solution such as waste water from bleaching of chemical pulp. The object has now been achieved by adding a calcium compound and an aluminium compound to said aqueous solution for lignin removal. It was found that the precipitation result could be improved by avoiding the acidic step of said US-publication.
As is mentioned, the invention closely relates to lignin removal from bleaching water of chemical pulp. Normally, this bleaching water is basic having a pH of about 10. In this case, it is preferable for the invention to adjust the pH value of the aqueous solution between 5 and 6. This may for instance be carried out by adding an inorganic acid e.g. sulphuric acid. In case the aluminium compound used causes an acidic reaction in water, the pH may also otherwise reach the desired level.
It has also been found that the order of addition of the calcium and aluminium compounds is important when removing lignin according to the invention. In the method, it is preferable to add the calcium compound to alkaline water containing lignin, and allow the reaction to proceed for a period of time, followed by adjusting the pH to a neutral or slightly acidic value, and finally adding the aluminium compound. Thus, an ample precipitate is formed accompanied by the almost total disappearance of the yellowish colour of lignin. Sedimentation of the precipitate may be enhanced with a flocculant.
A parameter illustrating the lignin content is the UV absorbance at the wavelength of 280 nm. In best cases, the absorbance was reduced from a level of 3.7 to a level of 1.8.
Preferably an amount from 10 to 10,000 mg/l, more preferably from 50 to 1000 is mg/l of the calcium compound is added to the aqueous solution, calculated as calcium. Preferably an amount from 100 to 10,000 mg/l, more preferably from 300 to 1000 mg/l of the aluminium compound is added to the aqueous solution.
The calcium compound used in the invention is preferably selected from the group consisting of calcium oxide, calcium hydroxide, and inorganic calcium salts. More preferably, the calcium compound is calcium chloride or calcium sulphate, preferably calcium chloride.
The aluminium compound is preferably a polymeric aluminium hydroxide, or a polymeric aluminium hydroxide salt. The polymeric structure is obtained from a compound with hydroxylic groups building oxo bridges. Even more preferably, the aluminium compound is a polymeric aluminium hydroxide sulphate, polymeric aluminium hydroxide chloride, or a polymeric aluminium hydroxide diformate. Most preferably, the aluminium compound is aluminium hydroxide chloride having a ratio of OH to Al between 1:10 and 9:10.
In the following, the invention is illustrated by means of working and reference examples only meant for illustration.


EXAMPLES
Background
Waste waters from bleaching of straw contain dissolved lignin. The removal thereof with biological treatment methods is assumed to be difficult. In this work, precipitation of lignin using various chemicals e.g. polyvalent cations and organic polymers was studied.
Summary
In this work, the influence of Ca, Fe, and Al salts on turbidity, COD, and TOC of waste waters, as well as on its UV absorbance at a wavelenght of 280 nm, reflecting the lignin content thereof was studied.
Concurrent use of calcium and a polymeric aluminium salt (Kempac 18) was clearly shown as the most efficient combination. In Table below, calcium is added as an aqueous CaCl2 solution.



















Ca
Kempac18

Turbidity
UV abs
COD
TOC



mg/l
mg/l
pH
NTU
280 nm
mg/l
mg/l























0
—
—
10.2
66
3.673
2326
803


1
1000
1000
5.3
5
1.861
1276
375


2
100
1000
5.8
3
2.316
1374
393









CaCl2 may be replaced with more sparingly soluble calcium oxide or gypsum. Precipitation result is slightly weaker.
Neither the precipitation, floc formation, nor sedimentation was made more efficient by the use of organic flocculants.
Results
The sample was designated as a mixture of raw and wash waters of WS, 21st of Apr., 2004. Following properties were determined:







TABLE 1







Original water










Not centrifuged
Centrifuged
















pH

10.2
10.2



Turbidity
NTU

66.4



COD
mg/l
2170
2326



TOC
mg/l
797.3
802.9



Cat.need
meq/l
2.259
2.342



UV abs
280 nm

3.673



SS
mg/l
52










Tables 2-5 show experimental results in small groups. In these experiments, calcium is added as CaCl2 (the amount shown in the Table is calculated as Ca). Advantages of the concurrent use of calcium, and a polymeric aluminium salt are clearly shown by the results. Also the need for pH adjustment is substantial.
It is to be noted that the UV absorption is not reduced at all by only lowering the pH to a value of 3.
It was attempted to improve the efficiency of precipitation and flocculation by using polymers (Fennopols A321, K1384, K3450, and K506). The effects of these agents were very minimal, and often the addition of the polymer even impaired the flocculation. The polymer may, however, still work under practical conditions.
Table 6 presents the so-called confirmation test for a sample of 1 litre. Clarification of the precipitation was observed visually. Higher Ca doses clearly improved the sedimentation and compacted the precipitate.
In Table 7, the influence of different calcium salts on precipitation is compared. Reduction of the dose to a value below 1 g/l has an immediate effect on UV absorbance. Calcium oxide and gypsum are almost equivalent, gypsum being perhaps slightly more efficient. The reduction of the amount of Kempac to half immediately made both of the Ca salts inefficient.
Procedure
The testing was carried out with samples of 50-100 ml. The calcium salt was added first, mixed for about 2 minutes, or 30 minutes in cases the salt was solid. This was followed by the adjustment of the pH to 7, and the addition of the other coagulant. A mixing for a few minutes followed. Finally, the water was centrifuged and the particular properties were determined from the clarified sample.














TABLE 2










Tur-






Test
Coagulant

bidity
UV abs
COD
TOC















no.

mg/l
pH
NTU
280 nm
mg/l
mg/l
Note





















10.2
66
3.673
2326
803
added


1
Ca
10
10.1
52



as


2
Ca
100
10.0
32



a solid


3
Ca
1000
9.7
13
3.645
1564
555


4
Ca
5000
9.4
3



Ca
5000
9.3
2


5
Fennoferri
100
10.1
53


6
Fennoferri
500
9.8
63
4.000
2202
801


7
Fennoferri
1000
8.8
77


8
Kempac18
100
10.1
56


9
Kempac18
500
9.7
64
3.831
2260
803


10
Kempac18
1000
8.3
83


11
Alum
10
10.2
54


12
Alum
100
10.1
59


13
Alum
500
9.2
59


14
Alum
1000
7.0
74
3.588
2232
788























TABLE 3









1M

Tur-





Test
Coagulant
H2SO4

bidity
UV abs
COD
TOC















no.

mg/l
μl/50 ml
pH
NTU
280 nm
mg/l
mg/l


















15
—
—
675
3
146
3.854
2240
699


16
Ca
1000
225
6.8
1
3.591
1660
537


17
Fennoferri
500
150
7.0
63
3.700
2354
754


18
Kempac18
500
125
6.9
60
3.529
2406
747


19
Alum
500
100
6.9
67
3.822
2378
897























TABLE 4












UV








Tur-
abs


Test
Coagulant
Coagulant

bidity
280
COD
TOC
















no.

mg/l

mg/l
pH
NTU
nm
mg/l
mg/l



















20
Ca
1000
Kempac18
100
6.4
18
3.523
1590
530.2


21
Ca
1000
Kempac18
500
5.4
3
1.795
1286
405.9


22
Ca
1000
Kempac18
1000
5.3
2
1.605
1264
396.5


23
Ca
1000
Fennoferi
500
6.1
39
3.528
1492
473.3


24
Ca
1000
PEO
5
7.0
5
3.492


25
Ca
1000
PEO
10
7.0
5
3.531


30
Ca
100
Kempac18
1000
5.8
1
1.919


31
Ca
500
Kempac18
1000
5.7
3
2.169


34


FF240
400
7.2
335
3.800


35


FF240
800
7.4
7
3.370


36


ORC2003
400
6.8
124
4.000


37


Orc2003
800
6.4
155
3.790





















TABLE 5







Test
Coagulant
Coagulant

Turbidity
UV abs














no.

mg/l

mg/l
pH
NTU
280 nm

















26
Ca
100
Kempac18
1000
8.4
63
3.596


27
Ca
500
Kempac18
1000
7.8
18
3.654


28
Ca
1000
Kempac18
1000
8.4
6
3.480


29
Ca
1000
Kempac18
5000
9.3
5
3.586


32
FF240
400



272
3.772


33
FF240
800



373
4.000









Without pH Adjustment
















TABLE 6







Ca
Kempac18

Turbidity
UV abs
COD
TOC



mg/l
mg/l
pH
NTU
280 nm
mg/l
mg/l























0
—
—
10.2
66
3.673
2326
803


1
1000
1000
5.3
5
1.861
1276
375


2
100
1000
5.8
3
2.316
1374
393





Rapid mixing at 400 rpm for 10 sec during which Ca was added, slow mixing for 5 min.


pH was adjusted using magnetic stirring


Rapid mixing at 400 rpm for 10 sec during which Kempac was added, slow mixing for 5 min.


Sedimentation over night


In test 1, proportion of precipitate of about 100 ml/500 ml


In test 2, proportion of precipitate of about 200 ml/500 ml


In test 3, sedimentation of the precipitate was slower, the precipitate being lighter and partly floating.













TABLE 7







Forms of addition of calcium















Test
Coagulant
Coagulant
1M H2SO4

Turbidity
UV abs
COD
TOC


















no.


mg/l

mg/l
μl/100 ml
pH
NTU
280 nm
mg/l
mg/l





















0
—
—
—
—
—
—
10.2
66
3.673
2326
803


1
CaO
Ca
1000
Kempac18
1000
560
5.9
1
1.978
1372
391


2
CaSO4
Ca
1000
Kempac18
1000
400
5.8
1
1.928
1356
391


3
Ca(OH)2
Ca
1000
Kempac18
1000
600
5.9
2
2.167
1406
410


5
CaO
Ca

Kempac18
1000

6.2
33
3.016
1552
472


4
CaO
Ca

Kempac18
1000
500
6.8
7
2.469
1478
446


7
CaSO4
Ca

Kempac18
1000

6.1
10
2.274
1434
426


6
CaSO4
Ca

Kempac18
1000

6.3
7
2.387
1420
432


8
CaO
Ca

Kempac18
500

6.5
43
3.595


9
CaO
Ca

Kempac18
100

6.9
64
3.622


10
CaSO4
Ca

Kempac18
500

6.3
24
3.606


11
CaSO4
Ca

Kempac18
100

6.7
22
3.688

## Claims

1. Method for removing lignin from an aqueous solution, comprising:
adding a calcium compound and an aluminium compound to the aqueous solution, characterized in that said aqueous solution is basic waste water from the bleaching of chemical pulp, characterized in that said calcium compound and said aqueous solution are combined to give a solution having a pH from 4.5 to 7, followed by the addition of said aluminium compound to this solution to precipitate lignin,

characterized in that said aluminium compound is a polymeric aluminium hydroxide, or a polymeric aluminium hydroxide salt; and
removing precipitated lignin from said aqueous solution.

2. Method of claim 1, characterized in that the pH of the solution is adjusted to a value from 5 to 6.

3. Method of claim 1, characterized in that an amount from 10 to 10,000 mg/l of said calcium compound is added to said aqueous solution, calculated as calcium.

4. Method of claim 3, characterized in that an amount from 50 to 1000 mg/l of said calcium compound is added to said aqueous solution, calculated as calcium.

5. Method of claim 1, characterized in that an amount from 100 to 10,000 mg/l of said aluminium compound is added to said aqueous solution.

6. Method of claim 5, characterized in that an amount from 300 to 1000 mg/l of said aluminium compound is added to said aqueous solution.

7. Method of claim 6, characterized in that said calcium compound is calcium chloride, or calcium sulphate.

8. Method of claim 1, characterized in that said calcium compound selected from the group consisting of calcium oxide, calcium hydroxide, and inorganic calcium salts.

9. Method of claim 1, characterized in that said aluminium compound is a polymeric aluminium hydroxide sulphate, polymeric aluminium hydroxide chloride, or a polymeric aluminium hydroxide formate.

10. Method of claim 9, characterized in that said aluminium compound is polymeric aluminium hydroxide chloride having a ratio of OH to Al between 1:10 and 9:10.