Note: Descriptions are shown in the official language in which they were submitted.
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
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.~ 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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~50 mljmin and the weighted block is set into motion and
allowed to continue until 6000 cycles have been completed.
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
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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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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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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
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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~
59
57 49
49 39
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- 15 It will be seen that the filler in accordance
.
with the invention gives a higher strength for a given
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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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