CA1101609A

CA1101609A - Mineral fillers

Info

Publication number: CA1101609A
Application number: CA297,483A
Authority: CA
Inventors: Kenneth M. Beazley; William R. Williams
Original assignee: English Clays Lovering Pochin Co Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1977-02-24
Filing date: 1978-02-22
Publication date: 1981-05-26
Also published as: BR7801111A; SE7801998L; ES467282A1; IT7820595A0; NL7802032A; FI780614A; NO780628L; DE2807811A1; FR2381809B1; GB1600862A; FR2381809A1

Abstract

ABSTRACT OF THE DISCLOSURE

A filler for paper or the like comprises particles of a white clay having (a) a particle size distribution such that it contains not more than 18%
by weight of particles smaller than 2 microns equivalent spherical diameter, (b) an abrasion of less than 120 Valley, and (c) a brightness a white clay mineral to a two-stage particle size classification process, in order to obtain a product smaller than 2 microns equivalent spherical diameter, and thereafter subjecting it to a beneficiating process in order to reduce the number of particles having an abrasive character to a level such that the product has an abrasion of less than 120 Valley and removing or bleaching sufficient iron-containing impurities to ensure that the white clay mineral has a brightness of at least 76.

Description

BACKGROUND OF THE INVENTI ON
This invention relates to mineral fillers and, more particularly, is concerned with a white clay filler suita~le for use in the manufacture of paper or the like.
' 5 In the manufacture of paper or the li~e, there is generally incorporated in the fibrous pulp from which the paper or the like is formed a mineral filler the use of which inter alia reduces the cost of the product~ One mineral filler which is used for this purpose is kaolin which is a white clay which also brings about an improvement ; in the opacity and printing properties of the paper. How-ever, the kaolin fillers conventionally used result in a reduction in the strength of a paper or the like product ; containing them.
SUr~ARY OF THE INVE~TION
According to one aspect of the present invention , there is provided a filler for paper or a paper-like product comprising particles of a white clay having (a) a particle size distribution such that it contains not more than 18% by weight of particles smaller than 2 microns equivalent spheri-cal diameter, (b) an abrasion of less than 120 ~7alley, and (c) a brightness (measured as the percentage reflectance to light of wavelength 457 nm) of at least 76.
According to another aspect of the present inven-tion there is a method of preparing a filler for paper or apaper-like product which comprises: su~jecting a white clay mineral to a particle size classification process

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in order to obtain a product containing not more than l~o by weight of particles smaller than 2 microns e~uivalent spherical diameter; if necessary subjecting said white clay mineral to a beneficiation process in order to reduce the number of particles having an abrasive character to a level such that the product has an abrasion of less than 120 Valley; and removing or bleaching suffici.ent iron-containing impurities to ensure that the white clay filler has a.brightness o.f at leas~t 76 (measured as -the percentage reflectance to light of wavelength ~57 nm) According to a further aspec-t of the present ~, pc~pe~
.~ invention there is provided a paper or~thc lil~e product containing a white clay filler, wherein said paper contains a ~uantity of said white clay filler such that said paper or the like product has a burst strength which is at least 60qo of the burst strength of the unfilled paper or the like product and wherein the white clay filler comprises particles of a white clay having (a) a particle size distr.ibution such that the white clay filler contains less than 18% by weight of particles smaller than 2 microns equivalent spherical diameter, (b) an abrasion of less than 120 valley, and (c? a brightness (measured as the percentage reflectance to light of wavelength ~57nm) of at least 76.
In carrying out -the method of the invention the particle size classification process is preferably carried out as a two s-tage gravitational or cen-tri-~ugal sedimentation , ,. . . -, . .
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process. Furthermore, bene~icia-tion of the clay ~iller is advantageously carried out by subjecting it ~ithHr to a ~roth f]otation process, generally using a cationic collector whereby the kaolin particles are caused to float,and the quartz and other abrasive mineral impurities to sink to the bottom of the flotation cell, or to a two phase separation process of the type described in British Patent Specificat1ons Nos. 1,222,508 and 1,~75,881. In order to obt,a,in a whitè clay having'the.required brightness, the iron-containing impurities are preferably removed by a magnetic separation process and/or bleached by a reducing bleaching process.
In one embodiment of the method Qf the invention ' the following steps are carried out:
, (i) a slurry of a raw kaolin is treated to remove grit;
(ii) the degritted kaolin slurry is de-flocculated and subjected to a particle size classification process by gravi.tational or centrifugal sedimentation to produce a fine, paper coating-grade kaolin and a coarse kaolin;
(iii) the coarse kaolin is subjected to at leas-t one further particle size classification process to reduce the percentage by weight o:E particles having an equivalent spheri,cal diameter smaller than 2 ~m in the coarse kaolin to below 18%.
(iv) the coarse kaolin product of step (iii) is subjected to a beneficiation process to reduce the ', ', '' ,' "' ',,,',' " ' ' ' : '' ,' ' '." " ' ;

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proportion of abrasive particl.es;
~v) the bene~iciated coarse kaolin is subjected , in the form of an aqueous slurry to a process such that discolouring iron-containing impurities are removed or rendered lighter in colour; and.
(vi) the product of step (v).is dewatered in such a way as to minimise the formation of par-ticles having an equivalent spherical diameter smaller than 1 ~m and to produce a white cl,ay filler containing less than 25% by weight water.
Pri.or to step (i) of the method o~ the invention, the slurry of raw kaolin is usually thickened to a specific ~:
gravity in the range 1.050 to 1.100.
In step (i) of the method of the inve'ntion, particles coarser than 53 microns are removed.
In step (ii) of the method of the lnvention the degritted kaolin slurry may be de-~locculated with, for example, a water-soluble condensed phosphate salt, a water.soluble salt of apolysilici~c acid, or an organic 20 - polymeric disp,ersing agent, such as a.water-soluble salt of a polyacrylic acid having a number average molecular weight not greater than lOjO00 or a water-soluble copolymer deflocculant of the type disclosed in British Patent Specification No. 1,414,964.
In step (iii) of the method of the invention the particle size classif'ication process is conveniently performed by gravitational or centrifugal sedimentation : ~-.
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and generally under eonditions such that, in theory, all particles having an equivalent spherical diameter larger than d~um, where d is selected to be in the range from to 12, should report to the coarse fraction. I-t is necessary to carry out a least one further particle size classification p~ocess because, when using a sedimentation process in practice, many particles having an equivalent spherical dlameter considerably smaller than d ~ are carried into tlle c.oarse ~raction with the larger particles.
Only one further particle size classification process is generally required as this is usuallly sufficient to reduce the percentage by weight of particles having an equivalent spherical diameter smaller than 2~um to below ~%. Preferably, the particle size classification process is carried out in a manner reducing the percentage by weight of particles having an equivalent spherical diameter smaller than 1 ~m to 10% or less.
The mineral beneficiation process of step (iv) of -the method of the invention is convenlently a froth flotation process or a two liqui.d phase separation process.
In either case a cationic collector reagent is used to render the kaolinite particles hydrophobic so that in the ease of frotli flotatlon -they report -to the froth, and in the case of -the two liquid phase separation -to the interf`ace between the aqueous medium an~l-the non-polar organic liquid, whi].e the abrasive i.mpurity par-ticles remain in -the aqueous medium. The abrasion o~ the beneficiated :: : ; . . :: .. ,: , :;.:
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kaolin as measured by the Vall.ey abrasion test should be less than 120, and preferably less than 100. The Valley abrasion test as used herein is carried ou-t in the following manner: The apparatus used comprises a machine which rubs a slurry of the test materi&l over the surface of a rectangular piece of paper rnachine "wire.'' (i.e.
wire mesh), thus causing the wire to wear. Essential components of the apparatus are a flat perforated support plate for the piece of wire and a frame to clamp the wire firmly in place, a circula-ting system to supply the slurry of the test material at a cons-tant rate to the upper surface of the wire, a weighted brass block with a perforated base made of a synthetic plastics materia]. to distribute the slurry over the surface of the wire, and a suitable motor arrangement to operate -the circulating pump and to move the wei~hted block back and f.orth across the surface of the : wire. It is the sliding action of the weighted block on the wire, with the slurry of the test material at -the interface, which causes the wire to wear. The paper machine wire used in the test is 60 mesh x 60 mesh plain weave phosphor bronze wirecloth with a wire diameter of 0.17 mm and each piece is cut to the dimensions 9~ inches x 4~ inches (241mm x 121mm). The wei~hted block has a total weight, not including the driving arm, of 19.5 lb (8.85k~) and the perfora-ted base is formed from a Linen-reinforced phenol-formaldehyde resin materi.aL whlch is marketed under the Trade Mark "TUFNOL,", the area in contact with the wire .

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having the dimensions 3 1/16 inches x 3 3/~ inches (78mm x 95mm). The weighted block is reciprocated by means of a driving arm sliding in an elongated bearing -to ensure accurate re~ilinear motion. The driving arm is actuated through a connecting rod by a driving wheel which is rotated in a horizontal plane by the motor arrangement.
Thes~ed of reciprocation of the weighted block is 95 cycles pèr minute and the stroke is ~ inches (102mm) so that the total wearing area on the piece of wire is 3 1/16 inches x 7 3/4 inches (78 mm x 197 mm).
The slurry of the test material is prepared by disperslng 75g of the test material in cold water, screening the suspension through a No 150 mesh B.S. Sieve (nominal aperture lO~um), making the volume of the screened suspension up to 2400 ml with cold water and correcting the p~l to 5.0 with dilute acid or alkali as necessary.
To carry out the Valley abrasion test, a rectangular piece of wire of the dimensions and type described above is washed in cold water and any loose pieces are removed. The wire is then dried in an oven at 80C for 30 minutes and allowed to cool in a ~esiccator for 10 minutes. The weight of the piece of wire is then determined to the nearest milligram. The piece of wire is laid on the perforated support plate and clamped firrnly in place with the frame. The slurry is circulated through the perforated base of the weigh-ted block, the piece of wire and the perforated support plate at a s-teady flow rate o~

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The motor is`then switched off and the wire removed, washed and dried as before, and the weight determined to the nearest milligram. The loss in weight in milligrams of the piece o~ wire gives a measure of the abrasiveness of the test material, but in order to allow for possible variations in the properties of the pieces or wire, the loss in weight of a second piece of wire which is as nearly as possible identical to the first piece is determined under the same conditions but using a slurry containing a standard material of known Valley abrasion. The valley abrasion of the test material is then calculated by means of the formula.
At = As W-t s - Where At is the Valley abrasion of the test material As is the Valley abrasion of the standard material Wt is the loss of weight of -the wire using the .test material Ws is the loss of weight of the wire using the standard material In s-tep (v) of the method of the invention, the undesired effect of discolouring iron-containing impurities is preferably ameliorated by means of either magnetic separation or chemical solution. In the case oL magnetic separation, in order to achieve a good combination O:e high extraction of discolouring impuri-ties and a good ,-_ g _ ~ , : ': . ;., :' ' ` :~: :, ` ` ; ~ ` `, ': `
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throughput rate, the magnetic Eield intensity is pre Eerably at least lO,ObO gauss. In the case o~ chemical solution oE
the iron-containing impurities this is preferably efEected by trea-ting an aqueous suspension oE the beneficiated kaolin obta.ined at the end o:~ step (iv) with a reducing agent such as sodium or zinc dithionite, which reduces the iron to the Eerrous state and renders it so].uble in water.
IE necessary a combination of magne-tic separation and chemical solution may be used,.The reflectance to light of 457 nm wavelength of the product of step (v) should be at least 76.0 and preEerably at least 78.0 as measured, by an ELREPHO brightness meter, in accordance with ISO
standards Nos. 2469, 2470 and 2471. The word ''ELREPH0"
is a Trade Mark.
In step (vi) oi the method o~ the invention the aqueous slurry obtained at the end of s-tep (v) is pre.Eerably dewatered by pressure ~i.ltration at a pressure in excess of 150 psig to give a dewatered product containing less than 25%,.~nd preferably less -than 20%, by weight oE water. A
tube pressure fil-ter such as is described in British Patent Specii`ication ~o. 1,240,465 is very suitable Eor carrying out this step. A thermal drying step may be used aiter the pressure Eiltration step provided that very little mechanical work is performed on the filter cake and the sur~'ace temperature oE the material is not allowed to exceed 120 C. ~ suitable drier would be a hand drier in which a ~il.ter cake is deposited without :rurther : . .- . . -. ~,,.. ~ :
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mechanical treatmen-t, such as extrusion or pelletising, on a moving wire mesh belt and passed through a.heated zone in which the temperature is such that the surface temperature o~ the material does no-~ exceed 120C.
The produc-t of -the method o~ the invention when incorporated in a stock of paper fibres should give a paper which has a burst strength which is at least 60% of the burst strength of un~llled paper formed from the same fibres.at a loadin.g of about 1~7% by weight. or less o~ the ~ filler.
The inventi.on is illustrated by the following Example.
E~AMPLE
A white clay filler for paper was prepared in the following manner:
~n aqueous slurry oi` raw kaolin, con-taining mica, quartz and feldspar as impurities, was thickened to a specific gravi-ty in the range 1.050 and 1.100 and then degri.tted to remove substantlally all particles having a diameter greater than 53~m. The thickened slurry of degritted kaolin was deflocculated with a sodium poly-acrylate dispersing agent having a number average molecular weight of 1,650 and the resulting def`locculated slurry subjected to a particle si.ze classification process in a scroll-type centrifuge under conditions such that, in theory, a].l particles having an equivalent spherical diameter larger than 5~um should report to the coarse .. , ,. . . .~ ;

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fraction, This classi~ication process in fact produced a fine paper coating grade kaolin and a coarse kaolin ha~ing the properties of a conventional paper fi'ller and still containing about 20% by weight o~ particles having an e~uivalent spherical diameter sma:ller than l/um.
The coarse product was subjected to a further similar particle siæe classification step in a second scroll type centrlfuge and the coarse kaolin obtained as a result of this second classificatïon process was subjected to a froth flotation process using octadecylamine acetate as the cationic collector reagent so that the substantially non-abrasive ~aolinite and mica particles were caused to float and the abrasive particles, predominantly quartz and feldspar sank to the bottom of the flotation Icells. The froth product comprislng approximately 50% by weight of the feed to the froth flotation step was sprayed with water to break the froth and the resul-tan-t aqueous suspension was subjected to magnetic separation in a high intensity magne.tic separator which comprised a separating chamber packed with steel woo] and electromagnet coils for establishing in the region of the separating chamber a magnetic field of about 20~000 gauss.
The impurities extracted in the magnetic separation step were predominantly iron-containing mica particles. The non-magnetic product was found to have an ~; acceptable reflectance to violet light but also had a yellowness, as measured by the difference between the .

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g re~lectance to yellow light of wavelength 37~ nm and the reflectance to viole-t light of wavelength 457 nm. This was probably due to very finely divided iron impurities. The non-magnetic product was blended with about 25% of its own weight of the coarse product of the second particle size classification process and the blended suspension was then treated with 3 Kg. of sodium dithionite per tonne of dry material in order to bleach the kaolin.
. .. ~he blended and bleached aqueous slurry was then dewatered in a tube pressure filter at a pressure of 1200 psig to produce a filter cake of the desired filler containing 16% by weight of water.
The foregoing process is represented diagrammatically in the accompanying drawing.
The reflectance to visible light of wavelength 457 nm and 574 nm and the particle size distribution of the material at various stages of the process are set forth in Table 1 below.

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~1~16~9 TABL~ 1 Material Re~lectance to light % by wt. o~
of wavelength particles having an ~.S.D. smaller than 457 nm 574 nm. 2 ~ 1 Coarse product of second classification step 72.5 7g.9 6 3 Froth flotation product 73.8 82.0 7 4 Non-magnetic product78.~ 87.3 10 6 Blended product 75.5 84.1 9 5 , Chemically bleached product 78.5 85.2 9 5 Tube pressure filter cake 78.5 85.2 13 7 The tube pressure filter cake was found to have an abrasion value as measured by the Valley abrasion test described above of 73.
An aqueous suspension containing 2% by weight of fibres obtained by treating and refining a bleached sulphite pulp was mixed in a stirred tank with 1.5% by weight, based on the weight of dry fibres, of a fortified rosin size and

3% by weight, based on the weight of dry fibres, of powdered aluminium sulphate. The resultant suspension of sized fibres was diluted with water to 1% by weight of fibres and a flocculated SUspensiQn of a white clay f'iller according to - the invention and prepared by the method described above was added in various proportions -to give different loadings of the filler on the fibres.
- The various mixtures were fed to the head box of 1 ~ -;. i :.. . .: , .,., : :

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6~9 a Fourdri~ier paper machine on which, :for each loading of clay, a web o~ paper was formed on the wire, dewatered and thermally dried. ~ampl.es of the web for each loading of clay were weighed dry and then incinerated and the weight of ash was used to calcula-te the percentage by weight of filler in the dry paper, aIter allowing for the loss of ignition of the clay.
Other samples of each paper were tested for their burst.strength by .the test prescribed in T.A.P.P.I.
Standard T401 os-74. The burst strength is defined as the hydrostatic pressure in kilonewtons per square metre required to produce rupture of the material when the pressure is increased at a controlled constant rate -th~ough a rubber diaphragm to a circular area 30.5 mm in diameter. The area of the material under test is initially flat and is held rigidly at the circumference but is Iree to bulge during the test.
The burst strengths were divided by the weight per unit~area of the paper to give a burst ratio and the burst ratio for each sheet of filled paper was then expressed as a percentage of the burst ratio for a sheet of paper prepared -~rom the same fibre stock but containing no filler.
The experiments describe~ ahove were then repeated but using a conventional kaolin filler which had a particle size distribution such that 51% by weight consisted of particles having an equivalent spherical diameter smaller than 2/um and 38% by ~veight consisted of particles having an .
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equivalent spherical diameter smaller than 1~. The reflectance to light of 457 nm waveleng-th was 81.0 and to light o~ 574 nm wavelength 86.7. The resul-ts ~re set forth in Table II below.
TABLE II
% by wt. of filler Burst ratio (% of unfllled paper (Based on dry ~iller wt.) burst ratio) ~iller in accordance conventional with invention filler : 87 8~

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- 15 It will be seen that the filler in accordance .
with the invention gives a higher strength for a given : `I
loading in the paper than the conventional filler.
Alternatively a greater amount of the filler in accordance with the lnvention can be incorporated in the paper for a 20 given reduction in strength.
~ The reflectance to light of 457 nm was measured for the samples of paper and it was discovered that, for a given loading of filler, the reflectance of the paper containing the filler in accordance with -the invention was 25 the same as that of the paper containing the conven-tional filler within the limits of expe.rimental accuracy.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A filler for paper or paper-like product comprising particles of a white clay having (a) a particle size distri-bution such that it contains not more than 18% by weight of particles smaller than 2 microns equivalent spherical dia-meter, (b) an abrasion of less than 120 Valley, and (c) a brightness (measured as the percentage reflectance to light of wavelength 457 nm) of at least 76.

2. A filler as claimed in claim 1, wherein said white clay has a particle size distribution such that it contains not more than 10% by weight of particles smaller than 1 micron equivalent spherical diameter.

3. A paper or paper-like product containing a white clay filler, wherein said paper contains a quantity of said white clay filler such that said paper or the like product has a burst strength which is at least 60% of the burst strength the white clay filler comprises particles of a white clay having (a) a particle size distribution such that the white clay filler contains less than 18% by weight of particles smaller than 2 microns equivalent spherical diameter, (b) an abrasion of less than 120 Valley, and (c) a brightness of at least 76 (measured as the percentage reflectance to light of wavelength 457 nm).

4. A paper or paper-like product as claimed in claim 3, wherein said paper contains up to 17% by weight of said white clay filler and wherein said white clay filler has a 17 particle size distribution such that it contains less than 10% of particles smaller than 1 micron equivalent spherical diameter.

5. A method of preparing a filler for a paper-like product which comprises: subjecting a white clay mineral to a particle size classification process in order to obtain a product containing not more than 18% by weight of particles smaller than 2 microns equivalent spherical diameter; if necessary subjecting said white clay mineral to a beneficia-tion process in order to reduce the number of particles having an abrasive character to a level such that the pro-duct has an abrasion of less than 120 Valley; and removing or bleaching sufficient iron-containing impurities to ensure that the white clay mineral has a brightness of at least 76 (measured as the percentage reflectance to light of wavelength 457 nm).

6. A method according to claim 5 wherein the particle size classification process is carried out as a two-stage gravitational or centrifugal sedimentation process.

7. A method according to claim 6 and comprising the following steps:
(i) treating a slurry of a raw kaolin clay to remove grit therefrom;
(ii) deflocculating the degritted kaolin slurry and subjecting it to a particle size classification process by gravitational or centrifugal sedimentation to produce a fine, paper coating-grade kaolin and a coarse kaolin;
(iii) subjecting said coarse kaolin to at least one further particle size classification process to reduce the percentage by weight of particles having an equivalent spherical diameter smaller than 2,um in the coarse kaolin to below 18%;
(iv) beneficiating the coarse kaolin product of step (iii) to reduce the proportion of abrasive particles to a level such that the coarse kaolin has an abrasion of less than 100 Valley;
(v) subjecting the beneficiated coarse kaolin in the form of an aqueous slurry to a process such that discolouring iron-containing impurities are removed or rendered lighter in colour; and (vi) dewatering the product of step (v) in such a way as to minimise the formation of particles having an equivalent spherical diameter smaller than 1,um and to produce a white clay filler containing less than 25% by weight water.

8. A method according to claim 7, wherein prior to step (i) the slurry of raw kaolin clay is thickened to a specific gravity in the range of from 1.050 and 1.100.

9. A method according to claim 7, wherein in step (i) particles coarser than 53 microns are removed.