Method for dewatering of sludge from a pulp, paper or board making process

Abstract

A method is disclosed for dewatering of sludge from a pulp, paper or board making process, such as deinking sludge, including obtaining of an aqueous sludge including an aqueous phase and a fibre material suspended in the aqueous phase. The sludge is subjected to a pre-thickening step, where a first part of the aqueous phase is removed from the sludge and to a pressing step, where a second part of the aqueous phase is removed from the sludge, thereby obtaining a dry pressed sludge. Before or at the pre-thickening step a polymer composition having a charge density of at the most 1.7 meq/g, preferably at most 1.5 meq/g, more preferably at most 1.1 meq/g, is added to the sludge.

Claims

1. A method for dewatering of sludge from a pulp, paper or board making process, the method comprising: obtaining an aqueous sludge comprising an aqueous phase and a fibre material suspended in the aqueous phase, subjecting the sludge to a pre-thickening step, wherein a first part of the aqueous phase is removed from the sludge, subjecting the sludge to a pressing step, wherein a second part of the aqueous phase is removed from the sludge and obtaining a dry pressed sludge, and adding to the aqueous sludge before or at the pre-thickening step a polymer composition having a charge density of at most 1.7 meq/g, measured at a pH of 7, and comprising: a cationic synthetic first polymer, which has a charge density of at least 1.0 meq/g at a pH of 2.8, and a cationic second polymer, which is a copolymer obtained by polymerization of (meth)acrylamide and at least one cationic second monomer, the amount of the cationic monomers being 2-19 weight-%, calculated from a total amount of the monomers, the second polymer being polymerized in 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 cationic first polymer has a charge density of 1-12 meq/g, at a pH of 2.8.

3. The method according to claim 2, wherein the cationic first polymer has a charge density of 1.3-8 meq/g, at a pH of 2.8.

4. The method according to claim 1, wherein the cationic first polymer is selected from polyamines, copolymers of epichlorohydrin, dimethylamine and ethylenediamine, linear or cross-linked polyamidoamines, polyvinylamine or at least partially hydrolyzed poly(N-vinylformamide).

5. The method according to claim 1, wherein the polymer composition comprises the cationic first polymer in an amount of 0.5-10 weight-%, calculated from a total dry polymeric material weight of the polymer composition.

6. The method according to claim 5, wherein the polymer composition comprises the cationic first polymer in an amount of 1.5-8 weight-%, calculated from the total dry polymeric material weight of the polymer composition.

7. The method according to claim 1, wherein the cationic first polymer is a copolymer of acrylamide and a cationic first monomer, which is diallyldimethyl-ammonium chloride (DADMAC), obtained by radical polymerization.

8. The method according to claim 1, wherein the cationic first polymer is obtained by radical polymerization and is a homopolymer of a cationic first monomer selected from 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 diallyldimethylammonium chloride (DADMAC).

9. The method according to claim 7, wherein an amount of the cationic first monomers is 0.5-10 weight-%, calculated from a total dry polymeric material weight of the polymer composition.

10. The method according to claim 9, wherein the amount of the cationic first monomers is 1.5-8 weight-%, calculated from the total dry polymeric material weight of the polymer composition.

11. The method according to claim 1, wherein the cationic first polymer has a weight average molecular weight MW<150,000 g/mol.

12. The method according to claim 1, wherein the cationic second monomer is selected from a group comprising 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 diallyldimethylammonium chloride (DADMAC).

13. The method according to claim 1, wherein the composition has a standard viscosity in a range of 3-6 mPas measured at 0.1 weight-% solids content in an aqueous NaCl solution (1 M) at 25 C. using a Brookfield DVII T viscometer with a UL adapter.

14. The method according to claim 1, wherein the sludge comprises a long fibre material in an amount of 2-50 weight-%, calculated from a dry weight of the sludge.

15. The method according to claim 1, wherein the sludge comprises inorganic mineral particles and has an ash content in a range of 20-90%, measured by using Standard ISO 1762, temperature 525 C.

16. The method according to claim 1, wherein the sludge has solids content in a range of 1-5 weight-%.

17. The method according to claim 1, wherein the polymer composition is added to the sludge in an amount of 0.5-3 kg, given per ton dry sludge.

18. The method according to claim 1, wherein the dry pressed sludge has a solids content at least 45 weight-%.

19. The method according to claim 1, wherein the aqueous phase of the sludge has a cationic demand in a range of 200-2000 eq/l and/or conductivity in a range of 1-5 mS/cm.

20. The method according to claim 1, wherein the polymer composition has a charge density of at most 1.5 meq/g, at a pH of 7.

Description

EXPERIMENTAL

(1) Some embodiments of the invention are described in the following non-limiting examples.

(2) The apparatuses and methods used in the following examples are given in Table 1.

(3) Usable, i.e. long, fibre content was determined by measuring 100 g of sludge to a 150 m wire, where the distance between the wire threads is 150 m, i.e. 100 mesh screen. The sludge was washed with running water until all other material except the fibres was washed off. After this the fibres were collected from the wire and dried in oven at 105 C. overnight. The dry fibres were weighed. Usable fibre content (150 m wire) was calculated by using equation (1):

(4) $\begin{array}{cc}\mathrm{Usable}\mathrm{fibers}\left(150\mathrm{.Math.m}\mathrm{wire}\right)=\frac{\mathrm{mass}\mathrm{of}\mathrm{dry}\mathrm{fibers}}{\mathrm{sludge}\mathrm{dry}\mathrm{solids}*\mathrm{mass}\mathrm{of}\mathrm{sludge}\mathrm{sample}}& \left(1\right)\end{array}$

(5) Gravity dewaterability of sludge was tested with Polytest. The sludge samples were filtered with Polytest cylinder of 10 cm diameter using in the bottom a wire cloth having air permeability of 5400 m.sup.3/m.sup.2 h. Treads/cm was 13.0/5.9. The sample amount was 200-400 g, but always identical between samples compared. Mixing of polymer composition was done with motor stirrer in baffled mixing vessel. Mixing speed was 600 rpm and mixing time was 10 seconds.

(6) TABLE-US-00001 TABLE 1 Characterisation apparatuses and methods used in the examples. Property Apparatus/Standard pH Knick Portamess 911 pH Charge density Mtek Conductivity Knick Portamess 911 Cond Dry solids SFS 3008 Suspended solids SFS 3008 Ash (525 C.) ISO 1762 Turbidity HACH 2100AN IS Turbidimeter// ISO 7027

(7) Preparation of Polymer Compositions Used in the Examples

(8) The cationic first polymer was polyamine (CAS #25988-97-0 or CAS #42751-79-1) or copolymer of acrylamide and diallyldimethylammonium chloride (CAS #26590-05-6).

(9) The second polymer was copolymer of acrylamide and [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl). 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 host polymer. 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-%.

(10) Reference compositions R1, R2 and R3 were copolymers of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-Cl).

(11) Properties of the polymer compositions and reference compositions were classified as given in Table 2.

(12) TABLE-US-00002 TABLE 2 Classification of standard viscosity and charge density ranges for polymer compositions and reference compositions. CLASSIFICATION POLYMER PROPERTY Very low Low Medium High Standard Viscosity, mPas 3-4 3.5-4.5 4-5 4.5-5.5 Charge Density, meq/g 0.1-0.4 0.5-0.8 0.9-1.4

(13) The polymer compositions and reference compositions are defined in Table 3.

(14) TABLE-US-00003 TABLE 3 Compositions and properties of polymer compositions and reference compositions. Composition, 1.sup.st Polymer 2.sup.nd Polymer, Standard Polymer Amount, Charge, Charge, Charge Density, Viscosity, Composition Type* weight-% mol-% mol-% meq/g dry mPas R1 n.a. 0 (n.a.) n.a. 1.5 0.2 6.1 (reference) R2 n.a. 0 (n.a.) n.a. 5 0.6 4.9 (reference) R3 n.a. 0 (n.a.) n.a. 10 1.2 4.0 (reference) R4 n.a. 0 (n.a.) n.a. 5 0.6 4.2 (reference) C1 E 6 100% E 1.5 0.5 5.3 C2 E 4 100% E 1.5 0.5 4.8 C3 E 8 100% E 1.5 0.5 4.3 C4 E 6 100% E 5 0.9 5.6 C5 E 6 100% E 10 1.5 4.9 D1 F 4 15 1.5 0.3 4.2 D2 F 6 15 1.5 0.3 4.3 D3 F 6 15 10 1.3 5.3 D4 F 6 15 5 0.7 4.8 D5 F 8 15 1.5 0.3 4.0 *E = polyamine, (CAS# 25988-97-0 or CAS# 42751-79-1), F = copolymer of acrylamide and diallyldimethylammonium chloride
Used Sludges

(15) Properties of deinking pulp (DIP) sludges used in the Examples 1-4 are given in Table 4. Properties of mixed sludges used in Example 5 are also given in Table 5.

(16) TABLE-US-00004 TABLE 4 Properties of sludges in the Examples. Ex. 5, Ex. 5, Property Ex. 1 Ex. 2 Ex. 3 Ex. 4 Test (a) Test (b) pH 7.7 7.53 7.51 7.4 7.7 7.9 Charge density, eq/l 410 270 1385 1562 780 1080 Conductivity, mS/cm 3.92 3.69 3.20 3.22 1.5 2.0 Dry solids; % 2.95 2.52 2.74 2.58 1.5 1.6 Suspended solids, % 2.3 2.23 2.01 1.4 1.5 Usable fibres (150 m wire), % 11.35 9.47 9.89 12.82 8.3 5.5 Ash (525 C.), % 63.55 65.63 61.55 58.67 43 37

Example 1

(17) Deinking pulp (DIP) sludge denotes sludge that is generated in processing and pulping of recycled paper. This example simulates dewatering process of newsprint DIP sludge. Measured sludge properties are presented in Table 4.

(18) Polymer compositions were diluted to 0.1% concentration before dosing to the sludge. Dewatering rate was tested with Polytest as described above. Polymer composition doses were 1.0 and 1.5 kg/t dry sludge. Sludge sample amount was 400 g. Amount of drained water was measured after 10 and 25 seconds. After Polytest the sludge was pressed with Afmitec Friesland B.V. minipress for 60 seconds with 5 bar pressure. Dry solids content of the sludge was measured after the pressing. Results from these experiments are presented in Table 5.

(19) From Table 5 it is seen that a polymer composition according to the present invention and having a medium charge density and high standard viscosity provides a better performance than conventional copolymer of acrylamide having medium charge density and high standard viscosity. The polymer composition according to the invention achieved faster dewatering, drainage after 10 and 25 seconds, as well as higher dry solids content after pressing than the reference polymer. All of these factors are advantageous for effective sludge dewatering in industrial scale.

(20) TABLE-US-00005 TABLE 5 Results for drainage and dry solids after pressing. Dry solids Polymer Polymer dose Drainage 10 s Drainage 25 s after pressing, Composition [kg/t DS] [g] [g] [%] R3 (reference) 1.0 225.5 288.8 58.8 C5 1.0 255.2 305.8 60.2 R3 (reference) 1.5 293.8 315.5 58.6 C5 1.5 306.5 318.5 60.7

Example 2

(21) This example simulates dewatering process of newsprint DIP sludge. Measured sludge properties are presented in Table 4.

(22) Polymer compositions were diluted to 0.1% concentration before dosing to the sludge. Dewatering rate was tested with Polytest as described above. Polymer doses were 0.9 kg/t dry sludge. Size of the sludge samples were 400 g. Amount of drained water was measured after 10 and 25 seconds. Suspended solids content was measured from the drained water. Results from these experiments are presented in Table 6.

(23) TABLE-US-00006 TABLE 6 Results for drainage and reject water suspended solids. Polymer Drainage 10 s Drainage 25 s Reject water SS Composition [g] [g] [mg/l] R2 (reference) 270.7 324.6 784.3 D4 298.0 329.5 689.6

(24) From Table 6 it is seen that a polymer composition according to the present invention provides a better performance than conventional copolymer of acrylamide having a low charge density and medium standard viscosity. The polymer composition according to the invention achieved faster dewatering as well as better reject water quality than the reference polymer. All of these factors are advantageous for effective sludge dewatering in industrial scale.

Example 3

(25) This example simulates dewatering process of newsprint DIP sludge. Measured sludge properties are presented in Table 4.

(26) Polymer compositions were diluted to 0.1% concentration before dosing to the sludge. Dewatering rate was tested with Polytest as described above. Polymer doses were 1.0 and 1.5 kg/t dry sludge. Size of the sludge samples were 400 g. Amount of drained water was measured after 10 and 25 seconds. Suspended solids content was measured from the drained water. After Polytest the sludge was pressed with Amfitec minipress for 60 seconds with 5 bar pressure. Dry solids content of the sludge was measured after the pressing. Results from these experiments are presented in Table 7.

(27) TABLE-US-00007 TABLE 7 Results for drainage, reject water suspended solids and dry solids content after pressing. Polymer Drainage Drainage Reject DS after Polymer dose 10 s 25 s water SS pressing Comp. [kg/t DS] [g] [g] [mg/l] [%] R1 1.0 259 324 952 56.6 (reference) C1 1.0 279 332 769 60.1 R1 1.5 305 338 756 n.a. (reference) C1 1.5 324 341 535 58.9

(28) From Table 7 it is seen that a polymer composition according to the present invention and having a very low charge density and low standard viscosity provides a better performance than conventional copolymer of acrylamide having very low charge density and medium standard viscosity. The polymer composition according to the invention achieved faster dewatering better reject water quality as well as higher dry solids content after pressing than the reference polymer. All of these factors are advantageous for effective sludge dewatering in industrial scale.

Example 4

(29) This example simulates dewatering process of newsprint DIP sludge. Measured sludge properties are presented in Table 4.

(30) Example 4 demonstrates the difference obtainable when a polymer composition is used according to the present invention and when a blend of corresponding individual polymers are used. The blend comprised 94 weight-% of polymer R1 and 6 weight-% of polyamine (CAS #25988-97-0 or CAS #42751-79-1). The blend was prepared by mechanical mixing of the two polymer composition solutions.

(31) Polymer compositions were diluted to 0.1% concentration before dosing to the sludge. Dewatering rate was tested with Polytest as described above. Polymer doses were 0.75 and 1.0 kg/t dry sludge. Size of the sludge samples were 200 g. Amount of drained water was measured after 5 seconds. Turbidity and suspended solids content was measured from the drained water. Results from these experiments are presented in Table 8.

(32) TABLE-US-00008 TABLE 8 Results for drainage, reject water suspended solids and turbidity. Polymer Drainage Reject water Reject dose 5 s turbidity, water Polymer Composition [kg/t DS] [g] [NTU] SS [mg/l] Blend of R1 + 0.75 135 5245 1949 polyamine, as defined above (reference) C1 0.75 141 4300 1731 Blend of R1 + 1.0 144 3105 1392 polyamine, as defined above (reference) C1 1.0 148 1536 1004

(33) From Table 8 it is seen that a polymer composition according to the present invention and having a very low charge density and low standard viscosity provides a better performance than a blend of individual polymers corresponding the components of the composition. The polymer composition according to the invention achieved faster dewatering and better filtrate quality, which is advantageous for effective sludge dewatering in industrial scale. Example 4 demonstrates that the use of the polymer composition provides unexpected benefits over the use of a blend comprising similar individual components.

Example 5

(34) Mixed sludge denotes sludge that is generated in effluent treatment process and it contains at least two different sludges. This example simulates dewatering process of mixed sludge which comprises primary clarifier sludge and biological sludge. Two tests using two different mixed sludges, Test (a) and Test (b), were conducted. Measured sludge properties are presented in Table 4.

(35) Properties of polymer compositions used in Example 5 and their properties are presented in Table 9. In polymer composition C-inv the cationic first polymer was a condensation copolymer of epichlorohydrin and dimethylamine, having charge density about 7 meq/g dry. The first polymer was present in the final polymer composition in amount of 6 weight-%, based on weight of total polymeric material, as dry. The second polymer was copolymer of acrylamide, amount of monomers for the second polymer are shown in Table 9.

(36) Reference polymers C-ref1 and C-ref2 were copolymers of acrylamide and ADAM-Cl.

(37) TABLE-US-00009 TABLE 9 Properties of polymer compositions in Example 5. The amount of cationic first polymer in all final polymer compositions was 6 weight-%, based on weight of total polymeric material, as dry. Polymer Composition Amount Amount Standard Charge Acrylamide ADAM-Cl Viscosity Density Polymer [mol-%] [mol-%] [mPas] [meq/g dry] C-inv 95 5 4.2 1.33 (2.sup.nd polymer) (2.sup.nd polymer) C-ref1 95 5 4.2 0.6 (reference) C-ref2 95 5 4.9 0.6 (reference)

(38) Polymer compositions were diluted to 0.2% concentration before dosing to the sludge. Dewatering rate was tested with Polytest as described above. Polymer dose were 7.0 kg/t dry sludge. Size of the sludge samples were 500 g. Amount of drained water was measured after 10 and 25 seconds. Turbidity was measured from the drained water.

(39) Test (a)

(40) Sludge consisted of 40 volume-% of biological sludge and 60 volume-% of primary clarifier sludge. Mixed sludge properties are given in Table 4. Results are presented in Table 10.

(41) TABLE-US-00010 TABLE 10 Results for drainage and reject water turbidity Polymer Polymer dose Drainage Drainage Reject water Composition [kg/t DS] 10 s [g] 25 s [g] turbidity, [NTU] C-ref1 7.0 229 287 104 (reference) C-inv 7.0 245 321 53

(42) Test (b)

(43) Test (a) was repeated but the sludge was changed and consisted of 60 volume-% of biological sludge and 40 volume-% of primary clarifier sludge. Mixed sludge properties are given in Table 4. Results are presented in Table 11.

(44) TABLE-US-00011 TABLE 11 Results for drainage and reject water turbidity. Polymer Polymer dose Drainage Drainage Reject water Composition [kg/t DS] 10 s [g] 25 s [g] turbidity, [NTU] C-ref1 9.0 108 159 145 (reference) C-ref2 9.0 66 109 155 (reference) C-inv 9.0 313 418 42

(45) From Tables 10 and 11 it is seen that a polymer composition according to the present invention and having a medium charge density and high standard viscosity provides a better performance than conventional copolymer of acrylamide having medium charge density and high standard viscosity. The polymer composition according to the invention achieved faster dewatering, drainage after 10 and 25 seconds, as well as lower reject water turbidity than the reference polymers. Typically dewatering is hard when the amount of biological sludge increases in the mixed sludge. However, it can be seen from the results of Example 5 that the polymer composition according to the present invention and having a medium charge density and high standard viscosity performs significantly better than the reference compositions of conventional copolymer of acrylamide having medium charge density and high standard viscosity, even when amount of biological sludge in the mixed sludge is high. All of these factors are advantageous for effective sludge dewatering in industrial scale.

(46) 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.