Surface-coated calcium carbonate-containing material and process for the purification of water

Abstract

The present invention relates to at least one surface-reacted calcium carbonate-comprising mineral material and/or a surface-reacted precipitated calcium carbonate that is/are coated with at least one anionic polymer to obtain a surface-coated calcium carbonate-comprising material as well as to a process for the preparation of the surface-coated calcium carbonate-comprising material. Furthermore, the present invention relates to a process for the purification of water and/or dewatering of sludges and/or suspended sediments and to the use of a surface-coated calcium carbonate-comprising material for water purification and/or dewatering of sludges and/or suspended sediments.

Claims

1. A surface-coated calcium carbonate-comprising material comprising at least one surface-reacted calcium carbonate-comprising mineral material and/or a surface-reacted precipitated calcium carbonate, coated with at least one anionic polymer having a negative overall charge density in a range of 1,000 Eq/g (negative charge) to 10,000 Eq/g (negative charge), wherein the surface-reacted calcium carbonate-comprising mineral material is a reaction product obtained by contacting a calcium carbonate-comprising mineral material in an aqueous medium with carbon dioxide and with at least one water soluble acid, wherein the carbon dioxide is formed in situ and/or is supplied from an external source.

2. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the at least one water soluble acid is selected from: i) acids having a pKa value of 0 or less at 20 C. (strong acids) or having a pKa value from 0 to 2.5 at 20 C. (medium strong acids): and/or ii) acids having a pKa of greater than 2.5 and less than or equal to 7 at 20 C. (weak acids), wherein at least one water soluble salt is additionally provided.

3. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the surface-reacted precipitated calcium carbonate is a reaction product obtained by: a) providing precipitated calcium carbonate; b) providing H.sub.3O.sup.+ ions; c) providing at least one anion being capable of forming water insoluble calcium salts, said anion being solubilized in an aqueous medium; and d) contacting the precipitated calcium carbonate of step a) with said H.sub.3O.sup.+ ions of step b) and with said at least one anion of step c) to form a slurry of surface-reacted precipitated calcium carbonate; wherein an excess of solubilized calcium ions is provided during step d); and the surface-reacted precipitated calcium carbonate comprises an insoluble and at least partially crystalline calcium salt of said anion formed on the surface of at least part of the precipitated calcium carbonate provided in step a).

4. The surface-coated calcium carbonate-comprising material according to claim 3, wherein: I) the H.sub.3O.sup.+ ions of step b) are provided by addition of a water soluble acid or acidic salt which simultaneously serves to provide all or part of the excess solubilized calcium ions, wherein the water soluble acid or acid salt is selected from the group consisting of sulfur-comprising acids, sulfuric acid, hydrochloric acid, perchloric acid, formic acid, lactic acid, acetic acid, nitric acid, acidic salts thereof, and water soluble calcium acidic salts thereof: II) the anion of step c) is selected from one or more of the following: phosphate-comprising anions, PO.sub.4.sup.3 and HPO.sub.4.sup.2, oxalate anions (C.sub.2O.sub.4.sup.2), carbonate-comprising anions in the form of CO.sub.3.sup.2, phosphonate anions, succinate anions and fluoride anions; and/or III) the excess of solubilized calcium ions is provided by addition of a water soluble neutral or acidic calcium salt, CaCl.sub.2) or Ca(NO.sub.3).sub.2.

5. The surface-coated calcium carbonate-comprising material according to claim 1, wherein: A) the calcium carbonate-comprising mineral material is selected from the group consisting of marble, chalk, dolomite, limestone, and any mixture thereof; and/or B) the precipitated calcium carbonate is selected from the group consisting of precipitated calcium carbonates having an aragonitic, vateritic or calcitic crystal form, and mixtures thereof.

6. The surface-coated calcium carbonate comprising material according to claim 1, wherein the at least one surface-reacted calcium carbonate-comprising mineral material and/or surface-reacted precipitated calcium carbonate have a volume median particle diameter d.sub.50 value before coating of between 0.01 m and 250 m, and/or a specific surface area before coating of from 1 to 250 m.sup.2/g.

7. The surface-coated calcium carbonate-comprising material according to claim 1, wherein at least 60% of the monomer units of the at least one anionic polymer have an anionic charge.

8. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the at least one anionic polymer is a homopolymer based on monomer units selected from the group consisting of aliphatic unsaturated carboxylic acids having a total amount of 1 to 24 carbon atoms, vinylsulfonic acid, vinylphosphonic acid, esterified acrylates, esterified methacrylates, esterified carbohydrates, acrylic acid and methacrylic acid.

9. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the at least one anionic polymer is a homopolymer based on monomer units of acrylic acid.

10. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the at least one anionic polymer is a copolymer based on monomer units selected from the group consisting of aliphatic unsaturated carboxylic acids having a total amount of 1 to 24 carbon atoms, vinylsulfonic acid, vinylphosphonic acid, esterified acrylates, esterified methacrylates, and esterified carbohydrates, and comonomer units selected from the group consisting of acrylamide, acrylic acid, methacrylic acid, vinylsulfonic acid, vinylpyrrolidone, methacrylamide, N,N-dimethyl acrylamide, styrene, methyl methacrylate, vinyl acetate and mixtures thereof.

11. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the at least one anionic polymer is a copolymer based on monomer units selected from acrylic acid and/or methacrylic acid, and the comonomer units elected from acrylamide and/or diallyldialkyl ammonium salts.

12. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the at least one anionic polymer is a natural homopolymer selected from the group consisting of anionic starch, anionic carboxymethylcellulose, anionic carboxylated cellulose, heparin, anionic dextrane and anionic mannan, or is a natural copolymer based on anionic starch, anionic carboxymethylcellulose, anionic carboxylated cellulose, heparin, anionic dextrane or anionic mannan.

13. The surface-coated calcium carbonate-comprising material according to claim 1, wherein the surface-coated calcium carbonate-comprising material further comprises mineral materials selected from the group consisting of untreated and/or treated ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), surface-reacted calcium carbonate (MCC), kaolin, clay, talc, bentonite, and dolomite.

14. A process for the preparation of the surface-coated calcium carbonate-comprising material according to claim 1, comprising the following steps: a) providing the at least one surface-reacted calcium carbonate-comprising mineral material and/or the surface-reacted precipitated calcium carbonate, b) providing the at least one anionic polymer, and c) contacting the at least one surface-reacted calcium carbonate-comprising mineral material and/or the surface-reacted precipitated calcium carbonate of step a) and the at least one anionic polymer of step b) to obtain the surface-coated calcium carbonate-comprising material.

15. The process according to claim 14, wherein step c) is performed in an aqueous solution.

16. A process for the purification of water and/or dewatering of sludges and/or suspended sediments, comprising the following steps: A) providing water to be purified and/or sludge and/or suspended sediment to be dewatered comprising impurities; B) providing at least one surface-coated calcium carbonate-comprising material according to claim 1, and C) contacting the water and/or sludge and/or suspended sediment of step A) with the at least one surface-coated calcium carbonate-comprising material of step B) to obtain a composite material of surface-coated calcium carbonate-comprising material and impurities.

17. The process according to claim 16, wherein the water and/or sludge and/or suspended sediment of step A) is selected from the group consisting of lake water, river water, water reservoirs, canal water, stream water, brooks water, salty water like brackish water, saline water, brine, estuary water, mining runoff water, mining wash water, sludge, harbour sludge, river sludge, ocean sludge, coastal sludge, suspended sediments, drilling muds, shield wall tunnelling, horizontal directional drilling, micro tunnelling, pipe-jacking, industrial drilling and mining.

18. The process according to claim 16, wherein the water and/or sludge and/or suspended sediment of step A) is brackish water, saline water or brine.

19. The process according to claim 16, wherein the water and/or sludge and/or suspended sediment of step A) is selected from group consisting of drinking water, urban waste water, municipal waste water, industrial waste water, sludge from biogas production or digested sludge, waste water or process water from breweries or beverage industries, waste water or process water in the paper industry, colour-, paints-, or coatings industry, agricultural waste water, slaughterhouse waste water, leather industry waste water and leather tanning industry, process water, waste water, and sludges from on and offshore oil and/or gas industry.

20. The process according to claim 16, wherein the water and/or sludge and/or suspended sediment of step A) is salty water having a conductivity in the range of between 185 S/cm and 350000 S/cm.

21. The process according to claim 16, wherein the water and/or sludge and/or suspended sediment of step A) is salty water having a conductivity in the range of between 65000 S/cm and 80000 S/cm.

22. The process according to claim 16, wherein the surface-coated calcium carbonate-comprising material is used in a weight ratio of from 1:20000 to 1:30, on a dry weight basis relative to the weight of the dry impurities and/or sludge and/or sediment.

23. A composite material comprising a surface-coated calcium carbonate-comprising material and impurities obtained by the process according to claim 16.

Description

EXAMPLES

(1) The scope and interest of the invention may be better understood on the basis of the following examples which are intended to illustrate embodiments of the present invention. However, they are not to be construed to limit the scope of the claims in any manner whatsoever.

(2) Measurement Processes

(3) The following measurement processes were used to evaluate the parameters given in the examples and claims.

(4) Particle Size Distribution (Mass % Particles with a Diameter<X) and Weight Median Diameter (d.sub.50) of a Particulate Material

(5) Weight median grain diameter and grain diameter mass distribution of a particulate material were determined via the sedimentation process, i.e. an analysis of sedimentation behavior in a gravitational field. The measurement was made with a Sedigraph 5100.

(6) The volume-based median particle diameter of the surface-reacted calcium carbonate-comprising mineral material and/or the surface-reacted precipitated calcium carbonate was determined by using a Malvern Mastersizer 2000.

(7) The processes and instruments are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurements were carried out in an aqueous solution of 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and ultrasound.

(8) BET Specific Surface Area of a Material

(9) The BET specific surface area was measured via the BET process according to ISO 9277 using nitrogen, following conditioning of the sample by heating at 250 C. for a period of 30 minutes. Prior to such measurements, the sample was filtered, rinsed and dried at 110 C. in an oven for at least 12 hours.

(10) pH Measurement

(11) The pH of the water samples is measured by using a standard pH-meter at approximately 25 C.

(12) Conductivity Measurement

(13) The conductivity of salty water is measured at 25 C. using Mettler Toledo Seven Multi instrumentation equipped with the corresponding Mettler Toledo conductivity expansion unit and a Mettler Toledo InLab 741 conductivity probe.

(14) The instrument is first calibrated in the relevant conductivity range using commercially available conductivity calibration solutions from Mettler Toledo. The influence of temperature on conductivity is automatically corrected by the linear correction mode.

(15) Measured conductivities are reported for the reference temperature of 20 C. The reported conductivity values are the endpoint values detected by the instrument (the endpoint is when the measured conductivity differs by less than 0.4% from the average over the last 6 seconds).

(16) The salinity of the salty water is measured with the same equipment under the same conditions and converted into ppt (parts per thousand) or % as defined above.

(17) Charge Density

(18) The charge density of a polymer and of a surface-coated calcium carbonate-comprising material was measured with a particle charge detector (PCD). The used particle charge detector was either a PCD-03 or a PCD-05, both available from Mtek with a measuring cell type 1 (10 to 30 ml).

(19) The measurement of the charge density of a sample was carried out by weighting the sample in the cell as well as 10.0 g of demineralized water. The electrodes inside the cell have to be covered with liquid.

(20) The piston was slowly inserted in the measuring cell and the measurement was started.

(21) The samples were titrated with a 2.5 mmol/1 polyvinylsulfate potassium solution. The solution was prepared by weighting 0.234 g polyvinylsulfate potassium salt in a volumetric flask (500 ml) and dissolving it with approximately 250 ml deionized water. 500 l formaldehyde solution 37% and 100 l benzylalcohol 99% were added and the solution was filled up to 500 ml with deionized water.

(22) The titration solution factor (f) for the 2.5 mmol/1 polyvinylsulfate potassium solution was determined by titrating 10.0 g demineralized water with 1000 ml poly-DADMAC 2.5 mmol/1 solution. The factor f was calculated by the following equitation:
factor (f)=volume theoretical (ml)/volume used (ml)

(23) The charge density of the titrated sample was calculated by the following equation:
charge density=titrant consumption (ml)*2.5 (mol/ml)*factor f/sample weight of the dry sample (g)

(24) TOC (Total Organic Content) Content Measurement

(25) The TOC was measured using the TOC cuvette test from Hach-Lange depending of the TOC range (TOC cuvette test LCK 385 for 3-30 mg/L TOC, LCK 386 for 30-300 mg/L TOC). The samples were prepared as indicated in the operating instructions of the cuvettes. The cuvettes were measured with a spectrophotometer Hach Lange DR2800.

(26) Weight Solids (% by Weight) or Solids Content of a Material in Suspension

(27) The weight of solids is determined by dividing the weight of the solid material by the total weight of the aqueous suspension. The solids content of a suspension given in wt.-% in the meaning of the present invention can be determined using a Moisture Analyzer HR73 from Mettler-Toledo (T=120 C., automatic switch off 3, standard drying) with a sample size of 5 to 20 g.

Example 1Preparation of the Surface-Reacted Calcium Carbonate-Comprising Mineral Material (SRCC1)

(28) In a mixing vessel, 1900 liters of an aqueous suspension of ground calcium carbonate was prepared by adjusting the solids content of a ground marble calcium carbonate from Omya Inc. Vermont, USA having a weight based median particle size of 0.7 m, as determined by sedimentation, such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, was obtained.

(29) Whilst rapidly mixing the suspension, 145 kg phosphoric acid in form of an aqueous solution containing 30 wt.-% phosphoric acid, based on the total weight of the aqueous solution, was added to said suspension over a period of 10 minutes at a temperature of 70 C. Simultaneous to the addition of phosphoric acid, 303 kg of sodium silicate in the form of an aqueous solution containing 5 wt.-% sodium silicate was added to said suspension over a period of 10 minutes. After the addition of the acid, the slurry was stirred for additional 5 minutes, before removing it from the vessel and drying. During acid treatment, carbon dioxide was formed in situ in the aqueous suspension.

(30) The resulting surface-reacted calcium carbonate-comprising mineral material SRCC1 had a volume median grain diameter (d.sub.50) of 2.3 m and a d.sub.98 of 5.5 m as measured by laser diffraction and a specific surface area of 38 m.sup.2/g.

Example 2Preparation of the Surface-Coated Calcium Carbonate-Comprising Mineral Material (SCCC1)

(31) The surface-coated calcium carbonate-comprising material (SRCC1) was coated with an anionic polymer.

(32) The used anionic polymer was a sodium polyacrylate available from Coatex Arkema Group

(33) A slurry of the obtained surface-reacted calcium carbonate-comprising mineral material SRCC1 having a solids content of 61 wt.-% was provided. The surface-reacted calcium carbonate-comprising mineral material was coated with 1.2 wt.-% of the anionic polymer, based on the total weight of the surface-reacted calcium carbonate-comprising mineral material. The obtained slurry of the surface-coated calcium carbonate-comprising material was diluted with water to a solids content of 10 wt.-%, based on the total weight of the aqueous slurry. The obtained slurry was vigorously agitated to obtain a homogenous slurry and to avoid settling. The charge of the surface-coated calcium carbonate-comprising material SCCC1 is 61.5 Eq/g (negative charge).

Example 3Application Trials with the Surface-Coated Calcium Carbonate-Comprising Material (SCCC1)

(34) The waste water that has been treated was obtained from a mining containing 2000 ppm of dissolved iron. The waste water was vigorously agitated and the pH of the waste water was adjusted to pH 10 with NaOH solution.

(35) Different dosages of 10, 20, 30, 40 and 50 ppm (vol/vol) of the surface-coated calcium carbonate comprising material (SCCC1) in form of the above described slurry were added simultaneously to the waste water samples. The samples were agitated for a 2 minutes using a standard jar test equipment. After mixing, the flocculation of the iron hydroxide flocs and the surface-coated calcium carbonate comprising material was observed.

(36) In all samples sedimentation could be observed and a clear supernatant was obtained. The iron content of the obtained supernatant was measured with an iron cuvette test from Hach-Lange with photometric detection from Hach Lange DR2800 and was below 0.15 ppm in all samples. This data shows that waste water treatment with the inventive surface-coated calcium carbonate-comprising material is possible, especially the treatment of waste water comprising cationic impurities. With the inventive surface-coated calcium carbonate-comprising material it is possible to reduce the amount of cationic impurities, especially cationic inorganic impurities to nearly zero in the treated waste water samples.

Example 4Preparation of the Surface-Reacted Calcium Carbonate-Comprising Material (SRCC2)

(37) The feed used for preparing the surface-reacted calcium carbonate comprising mineral material was an aqueous suspension of ground calcium carbonate from Omya Hustadmarmor, Norway, having a weight median grain diameter d.sub.50 of 8 m and a solids content of 40 wt.-%, based on the total weight of the aqueous suspension.

(38) The feed was ground in a DynoMill MultiLab (W. Bachofen AG) using Verac grinding media with a diameter of 0.7-1.4 mm in form of an aqueous suspension in order to obtain a finer calcium carbonate. The obtained aqueous suspension had a weight median grain diameter d.sub.50 of 1 m and a solids content of 18 wt.-%, based on the total weight of the aqueous suspension.

(39) The obtained ground feed suspension was placed in a mixing vessel and while rapidly mixing the suspension, phosphoric acid was added to that suspension in an amount of 9 to 12 wt.-% of active phosphoric acid, based on the dry weight of the ground calcium carbonate. After the addition of the acid, the slurry was stirred for additional 5 minutes, before removing it from the vessel, dewatering it mechanically and drying the resulting filter cake. During acid treatment, carbon dioxide was formed in situ in the aqueous suspension.

(40) The resulting surface-reacted calcium carbonate-comprising mineral material SRCC2 was in the form of a dry powder and had a volume median grain diameter (d.sub.50) of 5.25 m and a d.sub.98 of 16 m as measured by laser diffraction and a specific surface area of 39.3 m.sup.2/g.

Example 5Preparation of the Surface-Coated Calcium Carbonate-Comprising Material (SCCC2)

(41) The surface-coated calcium carbonate-comprising mineral material (SRCC2) was coated with an anionic polymer.

(42) The used anionic polymer was an anionic sodium polyacrylate polymer, commercially sold under the brandname Nerolan AG 580, which is commercially available from Nerolan Wassertechnik GmbH, Germany. The sodium polyacrylate polymer had a charge density of 7840 Eq/g (negative charge).

(43) The obtained surface-reacted calcium carbonate-comprising material SRCC2 consists of a dry powder that was coated with the anionic polymer by mixing the dry SRCC2 with the dry anionic polymer in an amount of 2 wt.-%, based on the total weight of the surface-reacted calcium carbonate-comprising mineral material. Afterwards the surface-coated calcium carbonate comprising material (SCCC2) was mixed with water in order to obtain a homogenous slurry at a solids content of 10 wt.-%, based on the total weight of the aqueous suspension. The charge density of the surface-coated calcium carbonate-comprising material SCCC2 was 65.7 Eq/g (negative charge). The obtained slurry was vigorously agitated to obtain a homogenous slurry and to avoid settling.

Example 6Application Trials with the Surface-Coated Calcium Carbonate-Comprising Material (SCCC2)

(44) The waste water that has been treated was a brine water sample (for the composition of the brine water sample see table 1) from the regeneration of a sorption media (resin) in an ion exchange (IC) water treatment process. That IC brine waste concentrate contains organic impurities in an amount of 992 mg/l TOC. TOC is the total organic carbon in the sample. The conductivity of the brine water sample was 70.4 mS/cm.

(45) TABLE-US-00001 TABLE 1 max. min. avg. NO.sub.3 (mg/l) 391 228 302 PO.sub.4 (mg/l) 9.5 1.3 3.9 Na (g/l) 18 10 15 Ca (mg/l) 30.5 14.6 25.9 Mg (g/l) 0.07 0.07 0.07 K (g/l) 0.07 0.07 0.07 Cl (g/l) 20 10 14 HCO.sub.3 (g/l) 41.3 4.7 10.2 CO.sub.3 (g/l) 1.6 1.6 1.6 SO.sub.4 (g/l) 12.7 7.1 10

(46) Different dosages of the surface-coated calcium carbonate comprising material (SCCC2) in form of the above described slurry were added to the waste water samples together with different non-polymeric flocculation aids. The used flocculation aids were aluminium sulphate (Al.sub.2(SO.sub.4).sub.3) available from Sigma Aldrich, iron chloride (FeCl.sub.3) available from Sigma Aldrich and powder activated carbon (PAC) available from Norit AC under the brand name SAE Super 8008.3. The samples were agitated for 2 minutes with a magnetic stirrer. After mixing, flocculation was observed combined with significant colour removal from the supernatant.

(47) In all samples sedimentation could be observed and a relative clear supernatant was obtained. The total organic content (TOC) of the obtained supernatant was measured.

(48) TABLE-US-00002 TABLE 2 Sample number 6.1 6.2 6.3 6.4 SCCC2 + + + Flocculation aid Al.sub.2(SO.sub.4).sub.3 FeCl.sub.3 PAC Sample dosage (g) 50 30 15 50 Flocculation aid 12 ml 15 ml 2 g dosage (5 wt.-% (5 wt.-% solution) solution) Flocculation 18 ml 15 ml 2 ml dosage (10 wt.-% (10 wt.-% (10 wt.-% slurry) slurry) slurry) TOC (mg/l) 992 256 618 315 Color of Red brown yellowish clear Slightly supernatant yellowish

(49) This data shows that waste water treatment of salty water, namely brine, is possible with the inventive surface-coated calcium carbonate comprising material. Furthermore, it is possible to use an additionally flocculation aid, for example an inorganic flocculation aid, in combination with the inventive surface-coated calcium carbonate comprising material. The use of such a flocculation aid alone in said brine does not provide an appropriate settling of the sludge. By the combination of the surface-coated calcium carbonate comprising material and the additionally flocculation aid it is possible to drastically reduce the amount of organic impurities in the obtained supernatant and to reach a significant colour removal in the obtained supernatant.

Example 7Application Trials with the Surface-Coated Calcium Carbonate Comprising Material (SCCC2)

(50) The sludge that has been treated was a polishing sludge from limestone plates having a solids content of 81.7%. The sludge comprises 96.0% CaCO.sub.3, 1.6% MgCO.sub.3 and 1.5% SiO.sub.2. The sludge has been diluted to a solid content of 10 wt.-% based on the total solids amount in the sludge. The sludge sample was vigorously agitated to obtain a homogeneous sludge suspension and to avoid settling of the sludge sample.

(51) Different dosages of 0 kg.sub.dry SCCC/t.sub.dry sludge to 20 kg.sub.dry SCCC/t.sub.dry sludge (table 2) of the surface-coated calcium carbonate-comprising material (SCCC2) in form of the above described slurry were added to the sludge samples. For example, 0.5 mL of the surface-coated calcium carbonate-comprising mineral material (SCCC2) having a solids content of 10 wt.-% were added to a volume of 50 ml of sludge having a solids content of 10 wt.-%, corresponding to 10 kg.sub.dry SCCC/t.sub.dry sludge. The samples were agitated for 2 minutes with a magnetic stirrer. After mixing, the sample was filtrated on a paper filter and after 5 minutes the amount of filtrate volume was measured.

(52) TABLE-US-00003 TABLE 3 SCCC2 (kg.sub.dry SCCC/t.sub.dry sludge) Dewatering (ml/min) 0 3.6 0.2 4.2 0.5 4.3 1 4.2 2 4.2 5 5.3 10 6.2 20 6.6

(53) In all samples flocculation could be observed. 3.6 ml clear solvent of the blank sludge sample could be filtrated when no inventive surface-coated calcium carbonate comprising material (SCCC2) was added. By the addition of 0.2 kg.sub.dry SCCC/t.sub.dry sludge 4.2 ml clear solvent could be filtrated. This represents an increase of 16%. By the addition of 20 kg.sub.dry SCCC/t.sub.dry sludge 6.6 ml clear solvent could be filtrated. This represents an increase of 83%.

(54) This data shows that sludge treatment like polishing sludge is possible with the inventive surface-coated calcium carbonate comprising material. Furthermore, it can be seen that the filtration of the sludge is facilitated by addition of the inventive surface-coated calcium carbonate-comprising material and, therefore, larger quantities can be filtrated in shorter time periods, which leads to a reduction in the treatment time of sludge. A reduced treatment time leads indirectly to decreased treatment costs.