Method for producing pigment for use in sizing compositions

Methods for producing pigment for use in sizing agents and filler agents for coating paper, cardboard and the like are disclosed. The method includes providing calcium sulphate dihydrate as the pigment and beating the calcium sulphate dihydrate in the presence of a dispersing agent comprising a polysaccharide substituted with carboxyl groups, preferably carboxymethyl cellulose.

FIELD OF INVENTION 
The present invention relates to a method of making a sizing agent and/or 
filler agent, especially for paper, cardboard and the like. More 
particularly, the present invention relates to such method wherein the 
sizing or filler agent includes calcium sulphate as pigment. 
BACKGROUND OF THE INVENTION 
The printability properties of sized paper are largely determined by the 
pigment or pigments applied during sizing. The properties of a ready 
coating slip depend upon the preparation of the individual components of 
the coating. Thus, the slip must not include agglomerating pigments, 
unsolved binding agent particles or other impurities, which may cause 
scratches in the coated sheet. In addition, the slip must have a viscosity 
which permits the slip to be pumped all the way to the machine and to flow 
out on the base paper. 
The main constituents in a sizing composition which is to be used for 
sizing paper and cardboard or as paint, are pigment, binding agent and a 
liquid phase. It is generally also necessary to add a dispersing agent in 
order to obtain good distribution of the pigment. The viscosity of the 
composition can thereby often simultaneously be lowered so that a higher 
dry content can be obtained without a concomitant deterioration in 
coatability. 
For coating paper, cardboard or the like, kaolin, titanium dioxide or chalk 
have conventionally been employed as pigment. It has also been proposed, 
however, to use gypsum. Gypsum has not been used in practice, however, 
probably because it has been considered that gypsum requires a 
substantially greater amount of dispersing agent. 
Conventional binding agents used for sizing compositions include 
water-soluble substances such as starch and casein, or dispersions of 
different polymers in water, such as, for example, polyacrylate, 
polymethacrylate and polyvinyl alcohol. It is also known, however, to use 
certain cellulose derivatives, such as methyl cellulose, ethyl cellulose, 
hydroxyethyl cellulose and carboxymethyl cellulose. A great number of 
materials are available for use as dispersing agents. The most commonly 
used dispersing agents, beyond all comparison, are different types of 
phosphates, particularly sodium hexametaphosphate. This dispersing agent, 
however, like several others, has certain disadvantages. For example, the 
binding force of the binding agent used is often reduced. Furthermore, 
when these dispersing agents are to be used for the coating of paper which 
is to be offset printed, they can, in certain cases, give rise to 
emulsification of the printing ink in the offset water, and an undesired 
toning of the print may thus result. In view thereof in accordance with 
the method of the present invention, a dispersing agent is utilized which 
itself acts substantially as a binding agent, viz. a polysaccharide 
substituted with carboxyl groups. 
Calcium sulphate or gypsum is a natural material found in several 
countries, for example in the Mediterranean countries. Its usual form is 
the dihydrate, having the composition CaSO.sub.4.2 H.sub.2 O. Gypsum, 
however, also exists as a waste product from the manufacture of phosphoric 
acid, where it often is called "chemical gypsum". The term "calcium 
sulphate" in connection with the present invention is to be understood as 
calcium sulphate dihydrate. Due to the abundant supply of gypsum at a 
relatively low price, it has long been desired to be able to utilize the 
gypsum as pigment for coating slips. The desire to utilize gypsum for this 
purpose has not been successfully realized in practice, however, 
apparently because the special prerequisites which must prevail for 
obtaining a good result have not been understood. 
SUMMARY OF THE INVENTION 
In accordance with the present invention these problems have been solved, 
and pigment can now be produced for use as a sizing or filler agent by 
providing calcium sulphate (gypsum) and beating the gypsum in the presence 
of a dispersing agent comprising a polysaccharide substituted with 
carboxyl groups, and preferably carboxymethyl groups. 
In one embodiment of the present invention, the beating step is carried out 
in the presence of water. In accordance with a preferred embodiment of 
this invention it has been discovered through various experiments carried 
out in connection with the development of this invention, that especially 
good results can be obtained by beating in the presence of carboxymethyl 
cellulose, usually called CMC, and particularly with CMC having a mean 
molecular weight of from about 10,000 to 500,000, preferably from about 
20,000 to 150,000. When CMC is thus used, it is added in amounts ranging 
from about 0.5 to 3 percent by weight, calculated on the amount of dry 
calcium sulphate. 
It has thus also been discovered that the particle size of the gypsum is 
essential. When, for example, a coating composition is to be prepared for 
use in preparing a mat paper, the gypsum must have a particle size 
distribution such that about 30 percent by weight have a size smaller than 
about 2.mu.. For coating compositions to be used in preparing a glazed 
paper, 95 percent by weight of the material should have a particle size 
smaller than about 2.mu.. When, however, pigment for filler agents is 
involved, the particle size of the calcium sulphate should be such that 
about 50 percent by weight of the material has a particle size smaller 
than about 5.mu.. The particle size distributions discussed above are 
determined by means of an Andreasen sedimentation apparatus which is 
marketed in Sweden by the enterprise Kebo Grave under the reference number 
111,904. Test samples are prepared by weighing about 10 g. gypsum slip, to 
which 2 ml of a 1-% CMC solution is added, whereafter the slip is mixed 
and stirred with about 40 ml water. The sample is transferred to the 
sedimentation apparatus, and 10 ml amounts are taken out with a pipette 
according to the following schedule: 
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1st sample after 0 minutes 
2nd sample after 60 minutes 
3rd sample after 120 minutes 
4th sample after 1080 minutes 
5th sample after 1560 minutes 
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According to another preferred embodiment of the method of the present 
invention, a binding agent such as latex or starch is employed. Addition 
in such case is preferably made subsequent to the beating step. As 
mentioned above, a high dry content if of great importance for the 
preparation of an effective and economic coating agent. By selecting the 
binding agent/dispersing agent selected according to this invention, dry 
contents for the coating agent as high as about 60 to 80 percent by weight 
can be achieved in connection with the preparation of mat paper. For such 
coating agents used to produce glazed paper, dry contents of from about 55 
to 80 percent by weight are obtained.

DETAILED DESCRIPTION 
As is noted above, in accordance with this invention, calcium sulphate 
dihydrate is employed as the pigment therein. When the so-called chemical 
gypsum is employed, for example, the gypsum is preferably first 
purified/bleached, by treating the gypsum with an oxidizing bleaching 
agent, such as chlorine, hypochlorous acid, hypochlorite, chlorine dioxide 
and/or peroxides. Such bleaching preferably is carried out with the 
calcium sulphate being in the form of a suspension in water, preferably at 
dry contents of, at maximum, 60 percent by weight, and preferably, at 
maximum, 50 percent by weight. It has also been found that when bleaching 
with chlorine, hypochlorous acid or hypochlorite it is essential that the 
pH values in the slurry be held to from about 1.5 to 6, preferably from 
about 2 to 4, and most preferably 3. The bleaching can be carried out for 
a period of from about 5 minutes to 1 hour, preferably from about 20 
minutes to 30 minutes. When the oxidizing bleaching agent is chlorine or 
chlorine compounds, these should be added in amounts of from about 0.1 to 
about 1%, and preferably from about 0.3 to 0.5% active chlorine, 
calculated on the amount of calcium sulphate. The calcium sulphate can be 
purified by fractionation, preferably in a cyclone, either prior or 
subsequent to the bleaching step, in order to remove undesired particles. 
After the bleaching, the calcium sulphate is washed with water, for 
example on a filter or in a column, in order to remove chlorine residue, 
chlorides and oxidizing compounds. The invention is described in greater 
detail in the following example thereof. 
EXAMPLE 
(a) Bleaching/Purification of Calcium Sulphate 
In this example waste gypsum from the manufacture of phosphoric acid was 
used as the raw material. The raw material is suspended in water in a 
container, 1, to a dry content of about 15 percent by weight, and the pH 
value of the slurry is adjusted to a value below 3. This value is a 
critical one, because at higher pH values discoloring impurities will 
adhere to the gypsum particles. Gypsum of the type used in this embodiment 
includes as impurities, in addition to organic material, heavily 
discolored gypsum, calcium fluoride and silicates from previous processes. 
When the pH value for the slurry exceeds 3, acidification is preferably 
carried out with sulphuric acid. The resultant slurry is then fractionated 
in a hydrocyclone, 2, whereby a fraction having a particle size of from 
about 25 to 100.mu. was recovered. If the particle size is thus below 
25.mu., it has been found that dewatering in a subsequent process step 
would be rendered substantially more difficult. The fraction including 
particles with a size above 100.mu. includes dark-colored particles, 
partially consisting of tourmaline. However, the rock phosphate which 
constitutes the starting material for the manufacture of phosphoric acid 
exists in many different qualities, and the impurities therein can 
therefore be of various types. 
The fractionation was carried out in two steps, 3 and 4, respectively. In 
the first step, 3, the coarser fraction was removed in line 19, and in the 
second step, 4, the finer fraction was removed in line 20, and in each 
case the fractions so removed were combined and carried away in line 21 as 
waste from the process. By carrying out the separation in this manner it 
is possible to operate independently of the operation conditions 
prevailing in the phosphoric acid factory, from which the gypsum was 
collected. In any event, the resultant slurry had a dry content of about 
45 percent by weight. The recovered gypsum fraction was then bleached in a 
continuous bleaching plant, 5, by addition of chlorine water through line 
6. The dry content dropped to about 20 percent by weight as a result of 
this addition of chlorine water. After addition of the chlorine water the 
pH value was adjusted to 3, and the bleaching was then carried out by 
stirring in two tanks connected in series in order to extend the 
stay-time. The amount of added chlorine was 0.5 percent by weight, 
calculated on the basis of dry gypsum. After the bleaching step, the 
slurry was thickened in a sedimentation thickener to a dry content of 
about 35 percent by weight. 
In a subsequent washing vessel, 7, the gypsum was washed in order to remove 
foaming agents, chlorides, organic material and other impurities. The 
washing was carried out continuously with water entering through line 8, 
and flowing countercurrent to the slurry in vertical pipes, and with the 
gypsum being taken out by a screw at the lower portion of the vessel 7. 
During this washing step thickening to about 45 percent by weight dry 
content simultaneously was obtained. Filtering of the slurry resulting 
from the washing step was then carried out on rotary vacuum filters, 9, 
and simultaneously water was sprayed thereon, primarily in order to remove 
residue chlorides. From the filter, 9, a clean, bleached product having a 
dry content of about 76 percent by weight is obtained. The effluents from 
the washing vessel, 7, and the filters, 9, are removed through lines 10 
and 11, respectively, and can then be combined and removed together 
through line 22. 
(b) Preparation of the Sizing Agent 
The clean, bleached product resulting from (a) was supplied to dispersion 
equipment, 12, while 1.6 percent by weight of carboxymethyl cellulose 
(CMC), calculated on the basis of dry gypsum, was simultaneously added to 
the dispersion equipment, 12, through line 13. The CMC was added in the 
form of a 14% solution. Thereafter the pH value of the solution was 
increased to 12 by the addition of sodium hydroxide, because in a case 
such as this one the gypsum can include small amounts of phosphoric acid, 
which is set free during the beating step. In addition, conventional 
glaze-raising agents, antifoaming agents, optical bleaching agents, 
coloring substances and preservatives can be added to this mixture, as may 
be desired. The various components are continuously supplied to the mixing 
vessel so as to obtain a flow of about 4 m.sup.3 /h. Dispersion is 
effected by means of a rapidly rotating stirrer. From the dispersion 
equipment, 12, the dispersion is continuously transferred to a vacuum 
deaerator, 14, for removing entrained air. After deaeration, the 
dispersion was beaten in a pearl beater, 15, at a temperature of about 
50.degree. C., in a single step, cooled in cooling equipment, 16, and then 
screened in a self-cleaning screen, 17. The final product removed through 
line 18 was a storage-durable slurry with a dry content of 71%. The 
product can be pumped and delivered filled in tanks or barrels. Paper 
coating tests were carried out with compositions prepared in accordance 
with the method of this invention. For purposes of comparison, coating was 
carried out under the same conditions with a conventional coating agent 
which included kaolin as pigment. The results are shown in the Table I 
below. 
TABLE I 
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Gypsum 
According to 
Kaolin this invention 
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Slip Data 
Dry content, % 58.7 60.5 
Viscosity, Brookfield 
(100 rpm), cP 1310 1480 
Paper data 
Coat, g/m.sup.2 13.6 13.6 
Substances, g/m.sup.2 
67.6 67.6 
Thickness, .mu.m 75 78 
Density, Kg/m.sup.3 906 868 
Bending force, mN 1,62/0,67 1,52/0,61 
Water absorption 
Cobb CL, g/m.sup.2 33.5 21.8 
Oil Absorption, 
Vanceometer, s &gt;600 &gt;600 
Ink absorption, mm 7.0 6.8 
Color absorption, K&N, % 
24.7 34.4 
Surface strength, IGT, m/s 
0.50 0.51 
roughness, Bendtsen, -1 kg, ml/min. 
26 34 
Parker Print Surf 10S, .mu.m 
1.96 2.22 
Porosity, Bendtsen, ml/min. 
10 14 
Brightness, % 72.7 74.8 
Luminance, % 78.5 79.9 
Diffusion coefficient, 
m.sup.2 /kg 54.5 56.5 
Opacity, % 92.9 92.8 
Glaze, % 43.8 42.7 
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The invention is not restricted to the embodiments described, but can be 
varied within the scope of the inventive concept.