Method for removing humic substances from an aqueous alkaline solution

Abstract

A method for removing humic substances comprising lignin, other lignin type compounds and their disintegration products from an aqueous alkaline waste water from a bleaching of chemical pulp, which method comprises obtaining an aqueous alkaline waste water comprising humic substances such as dissolved lignin, and adding a high cationic starch having a charge density value of at least 1.8 meq/g dry matter of starch derivates determined at pH 7-7.5 to the alkaline waste water to precipitate humic substances.

Claims

1. A method for removing humic substances comprising lignin, other lignin type compounds and their disintegration products from an aqueous alkaline waste water from a bleaching of chemical pulp, said method comprising: obtaining an aqueous alkaline waste water from a bleaching of chemical pulp comprising humic substances, being dissolved lignin, adding a high cationic starch having a charge density value of at least 1.8 meq/g dry matter of starch derivates determined at pH 7-7.5 and a viscosity of over 20 mPas, wherein said viscosity is measured in a 3% starch solution in water with addition of NaCl in amount of five times that of starch, to the aqueous alkaline waste water to precipitate humic substances, and separating precipitated humic substances from the waste water.

2. The method according to claim 1, wherein the high cationic starch has a charge density value of preferably at least 2 meq/g, at least 2.5 meq/g or at least 3 meq/g dry matter of starch derivates determined at pH 7-7.5.

3. The method according to claim 1, wherein high cationic starch has a charge density in a range of 1.8-4.5 meq/g dry matter of starch derivates determined by titrating at pH 7-7.5.

4. The method according to claim 1, wherein the viscosity of the cationic starch is over 40 mPas measured in a 3% starch solution in water with addition of NaCl in amount of five times that of starch.

5. The method according to claim 1, wherein the aqueous alkaline waste water has a pH over 8, over 9 or in a range of 10 to 12.

6. The method according to claim 1, wherein the high cationic starch is added to the waste water in an amount from 0.5 to 5 g/g C of humic substances, or from 1 to 3 g/g C of humic substances.

7. The method according to claim 1, further comprising: conveying a sludge comprising the precipitated humic substances to black liquor incinerator.

8. The method according to claim 1, further comprising: adding a flocculating agent to the alkaline waste water before the separation of the precipitated humic substances.

9. The method according to claim 8, wherein the flocculating agent is selected from polymer flocculants, being modified polyacrylamides.

10. The method according to claim 1, wherein the alkaline waste water is a filtrate from the bleaching of chemical pulp.

11. The method according to claim 1, wherein the COD of the untreated alkaline waste water or process flow is over 1000 g/m.sup.3, or over 2000 g/m.sup.3.

Description

EXPERIMENTAL

(1) A better understanding of the present invention may be obtained through the following example which is set worth to illustrate, but is not to be construed as the limit of the present invention.

(2) Waste waters from bleaching contain humic substances such as dissolved lignin. The removal thereof with biological treatment methods is assumed to be difficult. In this work, precipitation of lignin using high cationic starch according to the invention was studied.

(3) Coagulation and flocculation tests for COD removal were carried out in Mets Fibre nekoski mill laboratory. Tests were carried out with fresh wastewater sample from alkaline bleaching filtrate line. Wastewater sample temperature was 65 C.

(4) Batch size was 500 ml in mini flocculator. The flocculator was operated as follows: 1) fast mixing (350 rpm), cationic starch coagulant addition (2000 ppm) in the beginning and flocculating agent dosing (2 ppm) in the end, 2) slow mixing (40 rpm), and 3) sedimentation 10 minutes.

(5) The coagulant used in the test procedure was 1% cationised starch, which has the following properties: viscosity 471 mPas measured from 3% solution in de-ionised water, viscosity 47 mPas measured from 3% solution in water with addition of NaCl in amount of five times that of the starch, charge density 4.0 meq/g dry matter of starch derivates determined by titrating at pH 7-7.5.

(6) The flocculating agent used in the test was 0.1% polymer N7980 solution. The polymer N7980 is a non-ionic polyacrylamide, with standard viscosity (SV) about 4 mPas. SV was measured from a 0.1% polymer in 1M NaCl solution.

(7) Viscosity of cationised starch and polymer solution were measured using Small Sample Adapter of Brookfield viscometer with 13R chamber and spindle #18 at 25 C. The rotation speed used in the measurement is 60 rpm or lower, when needed.

(8) Analyses from the supernatant in the mill laboratory were pH, turbidity, filtered (0.45 m) UV-abs (254 nm) and COD. Dissolved organic carbon fractions of filtered sample were later analyzed with LC-OCD in Kemira R&D laboratory at Espoo. Experimental results of treated and untreated samples are shown in Tables 1 and 2.

(9) TABLE-US-00001 TABLE 1 Test records of treated and untreated sample in the mill. 0.45 m filtrate t Total UV abs COD Sample name pH [ C.] Turbidity 254 nm [mg/l] Alkaline untreated 10.2 65 40.2 4.2 2596 Treated with starch 10.3 262 3.65 1264

(10) TABLE-US-00002 TABLE 2 Test records of DOC fractions of treated and untreated sample. DOC ppm Humic Building Bio- Subst. Blocks Neutrals Acids Polymers Sample ~1000 300-500 <350 <350 >>20 000 Total Alkaline 606 142 110 89 5 947 untreated Treated with 167 113 95 62 18 438 starch

(11) Concentrations are shown as initial sample volume, test chemical dilution is excluded. The chemical oxygen demand (COD) value of the treated water stream is reduced about 49% from the COD value of the alkaline water stream before the addition of cationised starch, and especially the dissolved organic carbon (DOC) value of humic substances has reduced significantly.

(12) The invention is not restricted to the examples of the above description, but it can be modified within the scope of the inventive idea presented in the claims.