Method of producing filler

Abstract

The invention provides a method of producing a filler comprising calcium carbonate (PCC), preferably to be used in paper or paper board production or in fibre based composites. The method of the invention comprises the steps of; —providing fly ash generated in paper or paper board production; —fractionating said fly ash in at least one step, whereby a coarser fraction is separated from a finer fraction; —forming a suspension of said coarser fraction; —adding carbon dioxide to said suspension to form precipitated calcium carbonate. The method of the invention avoids problems with high amounts of arsenic and heavy metals in the production of filler comprising PCC, when using ash generated in paper or paper board production as a raw material. It has been shown that harmful elements, such as arsenic and heavy metals, are primarily accumulated in the finer fractions of the fly ash. Thus, by using the coarser fraction in the step of carbonation, the amount of arsenic and heavy metals in the final product is reduced.

Claims

1. A method of producing filler, comprising precipitated calcium carbonate (PCC), said method comprising the steps of, providing fly ash generated in paper or paper board production, the fly ash comprising calcium oxide; fractionating said fly ash in at least one step, whereby a first fraction is separated from a second fraction, the first fraction comprising particles that are coarser compared with particles in the second fraction wherein the first fraction has an average particle size greater than 50 μm and less than 100 μm, and the second fraction has an average particle size of 50 μm or less; grinding said first fraction prior to the step of forming a suspension, wherein said first fraction is ground to exhibit an average particle size of between 10 to 20 μm; forming the suspension of only the grounded first fraction having the average particle size of between 10 to 20 μm, wherein the calcium oxide of the fly ash forms calcium hydroxide, wherein the suspension of the grounded first fraction further comprise a furnish that includes cellulose fibers; and, adding carbon dioxide to said suspension of grounded first fraction comprising the furnish to form precipitated calcium carbonate in the presence of the cellulose fibers.

2. The method according to claim 1, wherein the fly ash includes magnetic material that is removed from said first fraction by magnetic separation.

3. The method according to claim 1, wherein the fly ash is derived from incineration of waste materials generated in paper or paper board production.

4. The method according to claim 1, wherein the fly ash comprises at least 10% by weight of calcium oxide, said percentage calculated on the total solid content of the fly ash.

5. The method according to claim 1, wherein the fly ash further comprises Al.sub.2O.sub.3, SiO.sub.2 and/or TiO.sub.2.

6. The method according to claim 3, wherein extra calcium carbonate or calcium oxide is added prior to or during the incineration of the waste materials.

7. The method according to claim 1, wherein dispersion agents are added to the suspension of the first fraction prior to the addition of carbon dioxide.

8. The method of claim 1, wherein the fly ash contains at least 30% by weight of calcium oxide, calculated on the total solids content of the fly ash.

Description

DETAILED DESCRIPTION

(1) The invention will be described further with reference to the accompanying schematic drawing, wherein:

(2) FIG. 1 shows a schematic illustration of a process in accordance with the invention.

(3) In a first step (1), fly ash generated in paper or paperboard production is provided. The fly ash may, e.g. be generated by incineration of deinking sludge. In a second step (2), said fly ash is fractionated into at least two fractions (2a, 2b), one of which is relatively coarse (2a) and the other relatively fine (2b). Said relatively coarse fraction (2a) contains ash particles from about 50 μm to about 100 μm width and/or thickness and said relatively fine fraction (2b) contains particles from less than about 50 μm. The fractionation may be accomplished by use of cyclones and/or by use of screens of predetermined mesh sizes. The relatively fine fraction (2b) is further treated as waste material, whiles the coarse fraction (2a), most advantageously, is grinded in a step (3) to form particles from less than 10 μm width and/or thickness. Preferably, the coarse fraction is grinded in dry state, since this gives better control of the particle sizes. However, the wet grinding could also be used. The grinded ash particles (3a) are thereafter dispersed in hot water to form a suspension in a fourth step (4). The ash particles comprise calcium oxide, which form calcium hydroxide in the suspension. Additives, such as dispersion agents and/or wetting agents, may be added to said suspension. Thereafter, carbon dioxide (5) is added to the suspension. Preferably, a gas stream containing carbon dioxide (5) is bubbled directly into the suspension, whereby the carbon dioxide reacts with calcium hydroxide present in the suspension and PCC is formed.

(4) The therby produced precipitated calcium carbonate may be filtered, added to a pulp furnish in a pulp chest prior to being supplied to the head box of a paper machine. The furnish is thereafter applied to a wire and subsequently formed and dewatered in conventional manners to form paper or paperboard.

EXAMPLE 1

(5) Fly ash from incineration of deinking sludge and recovered wood (energy ratio 25/75%) was collected for further treatment in accordance with this example. The incineration was performed in a Bubbling Fluidized Bed (BFB) boiler, with a bed temperature of 750-850° C. and a gas temperature before superheating of 950-1000° C.

(6) Said ash was fractionated and thereafter grinded in a ball mill system. The ash was fed to a cyclone/classifier wherein the air flow, regulated by rotation speed of the fan, was used to separate a fine fraction from a coarse fraction. In a first classifier test, the rotation speed was chosen to separate a fraction with an average particles size of less than 50 μm (fine fraction) from a coarse fraction. In this way, 35% of the fly ash was separated as fine fraction, and 65% as coarse fraction. In a second classifier test, the rotation speed was chosen to separate a fraction with an average particle size of less than 35 μm from a coarse fraction. In this way, 30% of the fly ash was separated as fine fraction. The coarser fractions were thereafter grinded in the ball mill to average particle sizes of less than 10 μm. A small amount of an anti-agglomeration additive was added to the ash in the grinding step.

(7) The ash was analyzed with regard to the mineral contents such as silicate and oxide minerals, and harmful metals, as shown in table 1 and table 2.

(8) As can be seen in table 1, the calcium oxide content was higher than 50% in all fractions. As can be seen in table 2, the amount of harmful elements, such as arsenic, cadmium and lead, were remarkably reduced in both the first and second classifier tests.

(9) Thereafter, the coarse fractions were dispersed in hot water to form a suspension of about 25-30%. Carbon dioxide was bubbled to the suspension whereby precipitated calcium carbonate was formed. The process was controlled by pH measurements, which optimally cannot decrease below 8.3-8.5.

(10) The PCC achieved by said process is of high quality, it comprises a low content of harmful elements and show a high brightness.

(11) TABLE-US-00001 TABLE 1 First Second First classifier Second classifier classifier test-grinded classifier test-grinded Original test-fine coarse test-fine coarse ELEMENTS unit ash material material material material Dry Solid % 100 100 100 100 100 Content (DS) GR % of DS 99.2 98.6 SiO2 % DS 18.5 19 20.1 17 19 Al2O3 % DS 8.54 8.88 8.45 8.09 8.62 CaO % DS 57.2 53.6 56.5 55.1 59.8 Fe2O3 % DS 0.882 1.65 0.881 0.78 0.78 K2O % DS 0.537 0.699 0.402 0.541 0.348 MgO % DS 3.23 3.21 3.31 3.4 3.17 MnO % DS 0.0693 0.0798 0.0585 0.0797 0.0551 Na2O % DS 0.51 0.486 0.399 0.516 0.403 P2O5 % DS 0.169 0.176 0.141 0.175 0.143 TiO2 % DS 0.491 0.522 0.473 0.45 0.433 Total Sum % DS 90.1 88.3 90.7 86.1 92.8 LOI 1000° C. % DS 7.2 3.7 3 6.5 6.3

(12) TABLE-US-00002 TABLE 2 Reduction of heavy Reduction of First metals in Second heavy metals classifier coarse classifier in coarse First test- material Second test- material classifier grinded first classifier grinded second Original test-fine coarse classifier test-fine coarse classifier ELEMENTS unit ash material material test, % material material test, % As mg/kg DS 67.9 84.8 37.2 45.2 111 44.2 34.9 Ba mg/kg DS 520 628 438 15.8 623 452 13.1 Be mg/kg DS 0.983 1.03 1.08 −9.9 1.18 0.642 34.7 Cd mg/kg DS 2.82 3.49 1.29 54.3 4.01 1.3 53.9 Co mg/kg DS 7.7 9.08 5.95 22.7 8.92 6.48 15.8 Cr mg/kg DS 148 189 104 29.7 189 99.2 33.0 Cu mg/kg DS 595 752 407 31.6 815 402 32.4 Hg mg/kg DS 0.186 0.2 0.0409 78.0 0.288 0.0204 89.0 Mo mg/kg DS 4.92 5.39 3.26 33.7 4.76 3.3 32.9 Nb mg/kg DS 6.29 6.33 6.27 0.3 5.17 6.13 2.5 Ni mg/kg DS 59 60.5 51.8 12.2 63.1 54.2 8.1 Pb mg/kg DS 287 343 126 56.1 346 112 61.0 S mg/kg DS 4620 6150 2270 50.9 7060 2430 47.4 Sc mg/kg DS 2.78 2.1 2.04 26.6 2.54 2.49 10.4 Sn mg/kg DS 9.99 10.6 7.25 27.4 12.1 7.64 23.5 Sr mg/kg DS 796 811 844 −6.0 825 778 2.3 V mg/kg DS 19.4 28.5 18.4 5.2 18.8 16.4 15.5 Y mg/kg DS 9.54 11.3 9.74 −2.1 8.92 8.96 6.1 Zn mg/kg DS 1550 2370 906 41.5 2560 774 50.1 Zr mg/kg DS 122 101 98.8 19.0 114 118 3.3

EXAMPLE 2

(13) A second trial was performed to study the effect of magnetic separation on the brightness of the formed PCC. In this trial, fractionated and grinded fly ash was slaked at 90° C. for 5 hours followed by precipitation at 20° C. with CO.sub.2 feed 0.5 l/min, 2 wt % ash. After precipitation, the material was filtered and dried.

(14) A reference sample (Ref Sample), produced in accordance with the above described process, was compared with a sample (Sample 1), also produced in accordance with the above described process but with the additional step of magnetic separation of magnetic materials prior to the step of precipitation. The magnetic separation was performed using a magnetic field of 3T. The brightness (D65) of the ref. Sample and Sample 1 is shown in table 3 below. As can be seen in the table, the brightness of the PCC produced in a method including magnetic separation prior to precipitation was considerable higher than the reference.

(15) TABLE-US-00003 TABLE 3 Brightness Sample (D65) [%] Ref. Sample 67.0 Sample 1 73.7