METHOD FOR TREATING AQUEOUS FEED BY DISSOLVED GAS FLOTATION

Abstract

A method is disclosed for treating aqueous feed by dissolved gas flotation. The aqueous feed originates from industrial processing of fibrous material, where the aqueous feed includes an aqueous phase and solid particulate material suspended in the aqueous phase. The method includes bringing a flocculant in contact with the aqueous feed, flocculating suspended solid particulate material into flocs and contacting formed flocs with gas bubbles and inducing their flotation in a flotation basin. The flocculant includes a polymer composition having a charge density of at the most 1.7 meq/g dry and the polymer composition includes a cationic synthetic first polymer having a charge density at least 1.0 meq/g dry at pH 2.8, and at least one second polymer obtained by polymerization of (meth)acrylamide, the second polymer being polymerized in presence of the cationic first polymer, where the first polymer has a higher charge density than the second polymer.

Claims

1. A method for treating an aqueous feed by dissolved gas flotation, especially by dissolved air flotation (DAF), the aqueous feed originating from an industrial processing of fibrous material of cellulosic origin, such as manufacture of pulp, paper, board, recycled fibre pulp or the like, wherein the aqueous feed comprises an aqueous phase and a solid particulate material suspended in the aqueous phase, wherein the method comprises: bringing a flocculant in contact with the aqueous feed and flocculating the suspended solid particulate material into flocs through interaction of the flocculant and the suspended solid particulate material, contacting formed flocs with gas bubbles and inducing their flotation in a flotation basin, wherein the flocculant, which is used for flocculating the suspended solid particulate material, comprises a polymer composition having a charge density of at the most 1.7 meq/g dry, preferably at the most 1.5 meq/g dry, more preferably at the most 1.1 meq/g dry, the polymer composition comprising: a cationic synthetic first polymer, which has a charge density of at least 1.0 meq/g dry at pH 2.8, at least one second polymer, which is a polymer obtained by polymerization of (meth)acrylamide, the second polymer being polymerized in the presence of the cationic first polymer, wherein the first polymer has a higher charge density than the second polymer.

2. The method according to claim 1, wherein the second polymer is obtained by polymerization of (meth)acrylamide and at least one second monomer, the amount of second monomer being 0.2-19 weight-%, calculated from the total dry polymeric material weight of the polymer composition.

3. The method according to claim 1, wherein the cationic synthetic first polymer has charge density in a range of 1-12 meq/g dry, preferably 1-8 meq/g dry, more preferably 1.3-8 meq/g dry, even more preferably 5-7 meq/g dry, at pH 2.8.

4. The method according to claim 1, wherein the cationic synthetic first polymer is selected from polyamines; homopolymers of cationic first monomer, obtained by radical polymerization; copolymers of acrylamide and a cationic first monomer, obtained by radical polymerization; or any combinations thereof.

5. The method according to claim 4, wherein the cationic synthetic first polymer is polyamine selected from copolymers of epichlorohydrin and dimethylamine, copolymers of epichlorohydrin, dimethylamine and ethylenediamine, and linear or cross-linked polyamidoamines.

6. The method according to claim 4, wherein the cationic synthetic first polymer is a homopolymer of cationic first monomer selected from group consisting of 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzylchloride, 2-(dimethylamino)ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethylsulphate, [3-(acryloylamino)propyl] trimethylammonium chloride (APTAC), [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTAC), and diallyldimethyl-ammonium chloride (DADMAC).

7. The method according to claim 1, wherein the polymer composition comprises cationic synthetic first polymer in amount of 0.5-35 weight-%, preferably 1-15 weight-%, more preferably 2-9 weight-%, calculated from the dry polymeric material weight of the polymer composition.

8. The method according to claim 1, wherein the cationic first polymer has a weight average molecular weight MW<500 000 g/mol, preferably <100 000 g/mol, more preferably <50 000 g/mol, even more preferably <20 000 g/mol.

9. The method according to claim 1, wherein the second polymer is cationic and the second monomer is selected from group consisting of 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-CI), 2-(dimethylamino)ethyl acrylate benzylchloride, 2-(dimethylamino)ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MADAM-CI), 2-dimethylaminoethyl methacrylate dimethylsulphate, [3-(acryloylamino)propyl] trimethylammonium chloride (APTAC), [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTAC), and diallyldimethyl-ammonium chloride (DADMAC).

10. The method according to claim 1, wherein the second polymer is obtained by using adiabatic gel polymerization, preferably at acidic pH<6.

11. The method according to claim 1, wherein the polymer composition has a standard viscosity in a range of 3-6 mPas, preferably 3.6-5.0 mPas.

12. The method according to claim 1, wherein the polymer composition has an intrinsic viscosity in a range of 4-20 dl/g, preferably 7-15 dl/g.

13. The method according to claim 1, wherein the aqueous feed comprises suspended solid particulate material in total amount of less than 6000 mg/l 50-5000 mg/l, preferably 150-4000 mg/l.

14. The method according to claim 1, wherein the aqueous feed has a conductivity in a range of 0.2-10 mS/cm, preferably 0.5-5.0 mS/cm, more preferably 1.0-4.0 mS/cm.

15. The method according to claim 1, wherein the aqueous feed has a cationic demand value in a range of 20-3000 eq/l, preferably 200-3000 eq/l, more preferably 100-2000 eq/l, even more preferably 400-1500 eq/l, such as 500-1500 eq/l.

16. The method according to claim 1, wherein the pH of the aqueous feed is in a range of 4-9.5, preferably 4-8.

17. The method according to claim 1, wherein the suspended solid particulate material comprises inorganic mineral particles and cellulosic fibres and/or fibrils.

18. The method according to claim 1, wherein the flocculant comprising the polymer composition is used in amount of 0.1-10 mg/l, preferably 0.2-3 mg/l, more preferably 0.2-1.0 mg/l, given as dry polymer composition per aqueous feed volume.

19. The method according to claim 1, wherein a coagulant is introduced to the aqueous feed before the aqueous feed is brought in contact with the flocculant.

Description

EXPERIMENTAL

[0075] Some embodiments of the invention are described in the following non-limiting examples.

Preparation of Polymer Compositions Used in the Examples

[0076] The cationic first polymer was a condensation copolymer of epichlorohydrin and dimethylamine. The amount of cationic first polymer in all final polymer compositions was 6 weight-%, based on dry polymeric material weight of the polymer composition.

[0077] The second polymer was copolymer of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-CI). Before the polymerisation of the second polymer the used monomers, the first polymer, pH adjustment agents (e.g. adipic acid, citric acid), chain transfer agent, chelating agent, redox initiators and thermal initiators in aqueous solutions were degassed with nitrogen. Acrylamide and ADAM-Cl monomers were added to a solution of the first polymer, i.e. host polymer. The mol-% of the used monomers are given in Table 1.

[0078] The obtained reaction solution was cooled down at 3 C., a redox initiator added, and free radical polymerisation reaction started. The polymerisation was done in a batch reactor and it was adiabatic. After the polymerisation reaction was finished, the obtained polymer gel was processed with mince meat processor and dried in the oven overnight. After drying the polymer was ground to obtain a powder having a dry content about 90-93 weight-%.

[0079] The amount of second polymer in all final polymer compositions was 94 weight-%, based on dry polymeric material weight of the polymer composition.

[0080] Reference polymer composition REF-5 was a copolymer of acrylamide (95 mol-%) and ADAM-Cl (5 mol-%), standard viscosity 4.8 mPas, charge density about 0.6 meq/g dry.

[0081] The standard viscosities and charge densities of the used polymer compositions are given in Table 1.

TABLE-US-00001 TABLE 1 Standard viscosities, intrinsic viscosities and charge densities of the used polymer compositions Amount Amount Standard Intrinsic Charge Acrylamide ADAM-Cl Viscosity Viscosity Density Polymer [mol-%] [mol-%] [mPas] [dl/g] [meq/g dry] C-1.5 98.5 1.5 3.9 11 0.94 C-3 97 3 4.1 10 1.15 C-5 95 5 4.2 12 1.33

[0082] All polymers were dissolved to 25 C. deionised water at 0.4 weight-% concentration and further diluted to 0.02 weight-% concentration prior to use.

Dissolved Air Flotation Tests

[0083] Test apparatus was constructed from pressurised aeration vessel having 0.4 dm.sup.3 volume and from Kemira Flocculator mixing unit (Kemira Oyj, Finland) attached to 1000 ml glass beaker. Aeration vessel had valve for filling with water, valve to add pressurised air and valve to empty dispersion water. Aeration vessel bottom was connected with pipe of 3 mm inside diameter to flocculator unit bottom in order to add dispersion water. Pressurised air was adjusted to 4 bar pressure. 60 ml of clear filtrate at 25 C. was added to aeration vessel. Pressurised air was added by opening and closing the valve and aeration vessel was mixed to dissolve air. Air addition and mixing was repeated 3 times to achieve equilibrium in dissolved air amount. 600 ml of cloudy filtrate at 50 C. was added to the beaker.

[0084] Sequence of the dissolved air flotation test with flocculator is given in Table 2.

TABLE-US-00002 TABLE 2 Procedure for dissolved air flotation test Time Action 20 s Cloudy filtrate addition to beaker, Flocculator started with 300 rpm 10 s Polymer feeding 10 . . . 0 s Dispersion water feed by opening valve from aeration vessel 0 s Flocculator speed changed to 20 rpm 0 . . . 100 s Rising time determination visually, time recorded when 90% flocks have arrived to surface 75 s Measurement of surface sludge layer thickness with ruler 90 . . . 120 s Filtrate sampling with 100 ml glass pipette two times from 300 ml beaker height 120 s Stop of the flocculator mixing 150 s Filtrate turbidity measurement

[0085] Suspended solids measurement for the filtrate was done by using tared white ribbon Munktell filter paper in Bhner vacuum funnel. 200 ml of filtrate was filtered.

[0086] Filter papers with the filtration pad were dried in 110 C. for 4 hours, cooled in exicator and weighted.

Other Apparatuses and Methods

[0087] Other apparatuses and methods used in the following examples are given in Table 3.

TABLE-US-00003 TABLE 3 Characterisation of apparatuses and methods used in the examples. Property Apparatus/Standard pH Knick Portamess 911 pH Cationic Demand Mtek Particle Charge Detector Conductivity Knick Portamess 911 Cond Suspended solids SFS 3008 Ash (525 C.) ISO 1762 Turbidity HACH 2100AN IS Turbidimeter// ISO 7027

Example 1

[0088] Water samples, which were treated, were obtained from a second loop of old newsprint deinking process.

[0089] The deinking plant second loop disc filter cloudy filtrate, used for feed, had following characteristics:

[0090] 500 mg/l of suspended solids, pH 7.5, ash content 55%, conductivity 2 mS/cm, cationic demand 300 eq/l.

[0091] Used dispersion water had following characteristics: 150 mg/l of suspended solids, pH 7.5, ash content 55%, conductivity 2 mS/cm, cationic demand 300 eq/l.

[0092] The results in Table 4 show that the use of the flocculants comprising polymer compositions enable faster flock rising time, lower filtrate turbidity and lower suspended solids in the clear filtrate in comparison with the reference composition, at corresponding dosage, given as ppm (solid polymer to feed flow). Alternatively, it is possible to achieve a good performance by using a lower dosage of polymer composition. An additional benefit may be increased feed capacity of the dissolved gas flotation unit, as a result of a faster rising time.

TABLE-US-00004 TABLE 4 Dissolved air flotation test program and results. Polymer dosage added at 10 s Rising Suspended Test [ppm] time Turbidity solids # C-3 REF-5 C-1.5 C-5 [s] [NTU] [mg/l] 0 0 0 0 0 100 1520 390 1 2 28 580 120 2 1 700 94 3 0.5 930 200 4 0.3 980 170 9 2 34 800 120 10 1 1020 170 11 0.5 1180 270 13 2 14 460 38 14 1 590 41 15 0.5 700 43 16 0.3 783 58 17 2 26 740 120 18 1 900 150 19 0.5 1070 190

Example 2

[0093] Water samples, which were treated, were obtained from a first loop of tissue washing deinking process for mixed office waste.

[0094] The washer filtrate, used for feed, had following characteristics: 1950 mg/l of suspended solids, pH 7.7, ash content 34%, conductivity 0.64 mS/cm, cationic demand 250 eq/l.

[0095] Used dispersion water had following characteristics:

[0096] 50 mg/l of suspended solids, pH 7.5, ash content 15%, conductivity 0.62 mS/cm, cationic demand 250 eq/l.

[0097] The results in Table 5 show that the use of the flocculants comprising polymer compositions according to invention enable faster flock rising time, lower filtrate turbidity and lower suspended solids in the clear filtrate in comparison with the reference composition, at corresponding dosage, given as ppm (solid polymer to feed flow). Alternatively, it is possible to achieve a good performance by using a lower dosage of polymer composition. An additional benefit may be increased feed capacity of the dissolved gas flotation unit, as a result of a faster rising time. Further, a reduction in thickness of the surface sludge thickness indicates that the surface sludge can be separated at higher consistency, which is beneficial for the successive sludge treatment steps.

TABLE-US-00005 TABLE 5 Dissolved air flotation test program and results. Polymer dosage added at 10 s Sludge Rising Suspended Test [ppm] thickness time Turbidity solids # REF-5 C-1.5 C-3 C-5 [mm] [s] [NTU] [mg/l] 2-0 0 19 90 650 310 2-1 2 14 45 230 80 2-2 1 14 50 230 110 2-3 0.3 18 60 350 130 2-4 2 10 15 110 30 2-5 1 11 26 100 30 2-6 0.3 15 38 130 22 2-7 2 11 25 91 3 2-8 1 16 37 110 6 2-9 0.3 17 43 160 36 2-10 2 11 20 100 14 2-11 1 14 32 120 28 2-12 0.3 16 35 210 84

[0098] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.