Comminution of materials

A method of comminuting a solid material comprising calcium carbonate to obtain a product containing at least 60% by weight of particles smaller than 2 microns equivalent spherical diameter comprises forming an aqueous suspension of the solid material which has a solids content in the range of from 5% to 50% by weight of dry solids and contains a dispersing agent, comminuting the solid material in the suspension by agitating the suspension in admixture with a particulate grinding medium, separating from the suspension thereby obtained an aqueous suspension containing comminuted solid material at least 60% by weight of which is smaller than 2 microns equivalent spherical diameter, flocculating the comminuted solid material by means of an electrolyte having a multivalent cation, and dewatering the aqueous suspension containing the comminuted and flocculated solid material.

BACKGROUND OF THE INVENTION 
This invention relates to the comminution of solid materials and, more 
particularly, is concerned with the comminution of solid materials 
comprising calcium carbonate. 
It is known to form an aqueous suspension of a partially comminuted calcium 
carbonate mineral containing a dispersing agent and then to complete 
comminution of the calcium carbonate mineral in the aqueous suspension at 
a high solids content (i.e. at a solids content ranging from about 65 to 
about 80% by weight of solids). Thereafter the calcium carbonate mineral 
is either thermally dried, for example in a spray drier, or stored or 
transported as an aqueous suspension, i.e. in slurry form. 
The energy which must be dissipated in an aqueous suspension of a calcium 
carbonate mineral in order to grind the mineral to a given degree of 
fineness (say, until 70% by weight consists of particles have an 
equivalent spherical diameter smaller than 2 .mu.m) is very much less when 
the grinding is performed using a suspension having a solids content of 
the order of 30% by weight than when the suspension has a solids content 
of the order of 70% by weight, i.e. better utilization of energy is 
obtained when the grinding is performed at a low solids content. 
It is also known to comminute a calcium carbonate mineral in the form of a 
low solids content aqueous suspension (i.e. at a solids content up to 
about 40% by weight of solids) in the absence of a dispersing agent. 
Thereafter the aqueous suspension is dewatered (for example, by 
centrifugal or gravitational sedimentation and/or by filtration) to 
produce a cake of flocculated material having a solids content ranging 
from about 65 to about 80% by weight. This cake may be thermally dried to 
yield a lump or powder product or it may be mixed with a dispersing agent 
to deflocculate the solid and form a slurry which may be stored or 
transported in suitable containers. Unfortunately, filter cakes of calcium 
carbonate minerals which have been ground at a low solids content in the 
absence of a dispersing agent have proved to be very difficult, if not 
impossible, to deflocculate fully on completion of the comminution 
process. 
British Patent Specification No. 1,472,701 describes a method of producing 
an easily pumpable aqueous suspension of a natural calcium carbonate 
mineral which contains at least 60% by weight of particles smaller than 2 
microns equivalent spherical diameter, which method comprises the steps of 
(a) forming a mixture of water, a natural calcium carbonate mineral 
containing at least 60% by weight of particles smaller than 2 microns 
equivalent spherical diameter, and a small amount of calcium hydroxide, 
said small amount being at least 0.1% by weight based on the dry weight of 
the natural calcium carbonate mineral; and thereafter (b) deflocculating 
the natural calcium carbonate mineral with a dispersing agent to produce 
the desired easily pumpable aqueous suspension. The mixture of water, 
natural calcium carbonate mineral containing at least 60% by weight of 
particles smaller than 2 microns equivalent spherical diameter and calcium 
hydroxide formed in step (a) of this method can be obtained by agitating 
an aqueous suspension containing from 10% to 60% by weight of a calcium 
carbonate mineral, calcium hydroxide and a particulate grinding medium. 
Although the method described in British Patent Specification No. 
1,472,701 has been found to work very well when the natural calcium 
carbonate mineral is chalk, i.e. a calcium carbonate mineral which was 
formed during the cretaceous period of geology from the remains of 
coccoliths or other minute organisms, it has been found that, when the 
calcium carbonate mineral is a harder mineral, such as marble or 
limestone, it is still often very difficult to deflocculate the calcium 
carbonate mineral in the aqueous suspension which is formed in step (a) of 
the method in order to form a suspension which is fluid and rheologically 
stable at high solids content. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a method of 
comminuting a solid material comprising calcium carbonate to obtain a 
comminuted solid material containing at least 60% by weight of particles 
smaller than 2 microns equivelent spherical diameter, which method 
comprises the steps of: 
(a): forming an aqueous suspension of the solid material which has a solids 
content in the range of from 5% to 50% by weight of dry solids, is 
substantially free of large particles, and contains sufficient of a 
dispersing agent to prevent the formation of agglomerates during the 
subsequent comminution of the solid material; 
(b): comminuting the solid material in the suspension formed in step (a) by 
agitating the suspension in admixture with a particulate grinding medium 
which consists of particles not larger than about 10 mm and not smaller 
than about 0.15 mm. 
(c): separating from the product of step (b) an aqueous suspension 
containing comminuted solid material at least 60% by weight of which is 
smaller than 2 microns equivalent spherical diameter; 
(d): flocculating the comminuted solid material in the separated aqueous 
suspension (if it is not already flocculated by the comminution thereof) 
by means of an electrolyte having a multivalent cation; and 
(e): dewatering the aqueous suspension containing the comminuted and 
flocculated solid material. 
The dewatered solid material obtained on carrying out the method of the 
invention can either be thermally dried or be mixed with a small quantity 
of a dispersing agent to form a fluid suspension of good rheological 
stability. 
In step (a) of the method of the invention the solid material in the 
aqueous suspension is preferably substantially free of particles larger 
than 10 mm. Most preferably, the solid material consists of particles 
smaller than 1 mm. A solid material in suitable sizes can be obtained by a 
preliminary comminution which may be performed by means of apparatus such 
as a jaw, cone, or gyratory crusher or a hammer, ball or rod mill or by 
means of a process such that as that described in Belgian Patent 
Specification No. 848,876. The solids content of the aqueous suspension of 
the solid material should be in the range 5% to 50% by weight of dry 
solids. If the aqueous suspension has a solids content below 5% by weight 
the grinding efficiency of step (b) is unacceptably low and the volume of 
the aqueous suspension which has to be handled per unit weight of dry 
solids is unacceptably high. The object of steps (b), (c) and (d) of the 
method of the invention is to produce an aqueous suspension, containing 
comminuted solid material, which is both fluid and flocculated and since a 
flocculated suspension having a solids content greater than 50% by weight 
would be extremely viscous and difficult to dewater, the aqueous 
suspension formed in step (a) should have a solids content not greater 
than 50% by weight. Preferably, the solids content of the aqueous 
suspension formed in step (a) is adjusted to lie in the range of from 20% 
to 45% by weight. The dispersing agent may be, for example, a 
water-soluble salt of a polyphosphoric acid, a water-soluble salt of a 
polysilicic acid, or preferably an organic polymeric dispersing agent such 
as a water-soluble salt of a polyacrylic or a polymethacrylic acid or a 
water-soluble copolymer of the type described in British Patent 
Specification No. 1,414,964. The amount of dispersing agent required is 
that which is just sufficient to confer negative charges on substantially 
all of the new crystal surfaces which are formed by the fracture of 
particles during the comminution step. It is believed that a new surface 
which is formed normally carries both positive and negative charges and 
therefore, in the absence of the dispersing agent, newly fractured 
particles tend to attract one another and form agglomerates which are not 
broken down if the comminuted mineral is subsequently treated with a 
dispersing agent. In the case of the water-soluble salts of polyacrylic or 
polymethacrylic acid or the copolymer dispersing agents disclosed in 
British Patent Specification No. 1,414,964, the amount of dispersing agent 
required is not more than 0.2% by weight based on the weight of dry 
material. Generally in the case of hard calcium carbonate materials, such 
as marble, vein calcite or limestone, the amount of dispersing agent 
required is about 0.15% by weight and in the case of natural chalk the 
amount of dispersing agent required is about 0.1% by weight, based on the 
weight of the dry material. 
In step (b) of the method of the invention, the particulate grinding medium 
preferably consists of particles not larger than about 7 mm and not 
smaller than about 0.25 mm. The grinding medium may be formed of a harder 
material than the solid material to be ground; for example, it may consist 
of approximately spherical particles of silica sand. Alternatively, the 
particulate grinding medium may be formed of the same material as or a 
similar material to the solid material to be ground; for example it may 
consist of particles of vein calcite, limestone or marble. 
In step (c) of the method of the invention separation from the product of 
step (b) of comminuted solid material at least 60% by weight of which is 
smaller than 2 microns equivalent spherical diameter can be effected by 
elutriation or by means of a sieve. 
Thus steps (b) and (c) of the method of the invention can be conveniently 
carried out by agitating the aqueous suspension of the solid material and 
the particulate grinding medium in a vessel which is provided with an 
impeller mounted on a central, vertical shaft and with a sieve in the side 
of the vessel which allows particles to pass which have been sufficiently 
finely ground but retains coarser particles in the vessel. The aperture 
size of this sieve is conveniently about one half of the diameter of the 
smallest particles which are present in the particulate grinding medium. 
For example if the smallest particles of the particulate grinding medium 
have a diameter of 0.5 mm the sieve will have an aperture size of 0.25 mm. 
In step (d) of the method of the invention it is preferred not to add a 
flocculant and, if the quantity of the dispersing agent used in step (a) 
has been correctly determined, the comminuted solid material will be in a 
weakly flocculated state on completion of comminution because the new 
surfaces formed by the fracture of particles during comminution will have 
completely absorbed all the dispersing agent which is present. If, 
however, after comminution the solid material is still in a deflocculated 
state and a flocculant has to be added, the preferred flocculants are 
electrolytes containing calcium or aluminium ions, for example calcium 
chloride, aluminium sulphate or calcium hydroxide. The flocculant used 
should be one which has a reversible effect. Polymeric flocculants, such 
as polyacrylamide and its derivatives have been found not to be suitable 
for use in the present invention because they flocculate the ground 
material too strongly. The amount of flocculant used should be the minimum 
which will bring about flocculation. If the amount of dispersing agent 
added in step (a) is judged correctly so that the comminuted solid 
material is weakly flocculated at the completion of comminution, not only 
is the cost of the flocculant avoided but also the quantity of dispersing 
agent necessary to form a deflocculated suspension of the materials at a 
high solids content after the dewatering operation is greatly reduced. 
In step (e) of the method of the invention the suspension containing the 
flocculated solid material is preferably dewatered by filtration, but 
other methods such as sedimentation can be used. 
If a fluid suspension at high solids content is required the cake of 
dewatered material formed in step (e) is mixed with a small quantity of a 
dispersing agent. The dispersing agent used can be the same as that used 
in step (a). The dispersing agent is generally added as an aqueous 
solution. It is possible to add it in powder form but most dispersing 
agents are so hygroscopic that they are not easily metered in powdered 
form. Additional water is not necessary in order to form a fluid 
suspension. For example, a powdered dispersing agent can be mixed with a 
filterpress cake and a fluid suspension obtained with the water present in 
the filterpress cake as soon as adequate mixing has taken place. Any 
liquid-solid mixer can be used; and a relatively slow speed mixer such as 
a blunger is suitable for this purpose. Even if the comminuted solid 
material is left in dewatered form the customer will generally wish to use 
it in the form of a deflocculated suspension at a high solids content, for 
example as a paper coating pigment or as a pigment or extender for 
emulsion paints, and it is therefore important that the comminuted 
material should have good rheological properties as a deflocculated 
aqueous suspension. In the case of the polyacrylate, polymethacrylate and 
copolymeric dispersing agents mentioned hereinabove the quantity of 
additional dispersing agent is usually such that the total amount of 
dispersing agent used in step (a) and in this operation does not exceed 
about 0.4% by weight based on the weight of dry material. 
Solid materials which may be treated by the method of the invention 
include: minerals which are formed from calcium carbonate, for example 
limestone, vein calcite, calcite marble, or chalk; minerals which are 
formed from calcium carbonate and a substantial proportion of other 
materials, for example dolomite; mixtures of calcium carbonate minerals 
and other minerals such as clay and talc; and other materials comprising 
calcium carbonate, for example oyster or other marine shells rich in 
calcium carbonate.

The invention is illustrated by the following Examples. 
EXAMPLE 1 
Step (a) 
There were formed six aqueous suspensions containing partially-comminuted 
samples of (i) chalk, (ii) limestone or (iii) marble. Each suspension 
contained 40% by weight of dry solids and either 0.1% by weight or 0.15% 
by weight, based on the weight of dry solids, of a sodium polyacrylate 
dispersing agent which had a number average molecular weight of 1650. 
The partially-comminuted chalk (i) was conventionally refined chalk from 
Wiltshire, England having a particle size distribution such that 35% by 
weight consisted of particles having an equivalent spherical diameter 
smaller than 2 .mu.m. 
The partially-comminuted limestone (ii) was prepared by crushing raw 
limestone from Landelise in Belgium and then passing the crushed material 
through a roll mill until substantially all particles passed through a No. 
10 mesh B.S. sieve (nominal aperture 1.68 mm). The milled material was 
then mixed with sufficient water to form a suspension containing 40% by 
weight of dry solids and was further ground for 2 hours in a pebble mill 
with 11/2 inch (38 mm) diameter flint pebbles. The ground material was 
screened through a No. 300 mesh B.S. sieve (nominal aperture 53 .mu.m), 
filtered and dried. 
The partially-comminuted marble (iii) was prepared by treating raw marble 
from Carrara, Italy in the same manner as the limestone. 
Step (b) 
Each of the six aqueous suspensions was agitated in a sand grinding vessel 
with Leighton Buzzard sand which consisted of approximately spherical 
particles of silica sand having diameters in the range of from 0.5 to 1 
mm. The amount of sand used was such that its volume was equal to the 
volume of the suspension, and agitation of the contents of the sand 
grinding vessel was continued until the particle size distribution of the 
comminuted material was such that approximately 90% by weight consisted of 
particles having an equivalent spherical diameter smaller than 2 .mu.m. 
Steps (c) and (d) 
After each suspension had been ground by agitation with sand as described 
above, the ground material (which was found to be flocculated) was in each 
case screened through a No. 300 mesh B.S. sieve). 
Step (e) 
The screened suspensions were dewatered by filtration. 
Each of the filter cakes was then mixed with a quantity of water containing 
the minimum quantity of the sodium polyacrylate dispersing agent which 
would just deflocculate the material and form a fluid suspension. In each 
case the filter cake was first mixed by hand with the solution of 
dispersing agent to form a rough slurry which was then subjected to 
vigorous mixing for 30 seconds in a laboratory homogeniser. The viscosity 
of each of the suspensions was then measured by means of a Brookfield 
viscometer using Spindle No. 4 at a speed of 100 rpm. Further small 
additions of the dispersing agent were then made and the viscosity of each 
of the suspensions was measured after each additional quantity of 
dispersing agent had been thoroughly mixed with the suspension. The 
viscosity of each of the suspensions was then plotted against the total 
quantity of dispersing agent present (i.e. including the initial 0.1% or 
0.15% by weight) and the minimum viscosity and the quantity of dispersing 
agent required to achieve that minimum were recorded. The results obtained 
are set forth in Table 1 below. 
TABLE 1 
__________________________________________________________________________ 
Initial quantity of dispersing agent (% by wt.) 
0.1% 0.15% 
total total 
quantity of solids 
quantity of solids 
dispersing minimum 
content of 
dispersing 
minimum 
content of 
agent viscosity 
suspension 
agent viscosity 
suspension 
(% by wt) (centipoise) 
(% by wt) 
(% by wt) 
centipoise 
(% by wt) 
__________________________________________________________________________ 
Chalk 0.18 100 69.9 0.24* 
87* 69.6* 
Limestone 
0.21 117 69.2 0.22 70 70.0 
Marble 
0.20 88 71.4 0.22 72 71.9 
__________________________________________________________________________ 
*The chalk to which 0.15% by weight of dispersing agent had been initiall 
added was found still to be deflocculated on completion of the sand 
grinding operation. The minimum quantity of calcium chloride which would 
just flocculate the ground material was therefore added before the 
suspension was filtered. 
These results show that a good combination of low viscosity and low 
dispersing agent consumption are obtained with limestone and marble with 
an initial dispersing agent dose of 0.15% by weight, and with chalk with 
an initial dispersing agent dose of 0.1% by weight. 
EXAMPLE 2 
Steps (a) and (b) 
Aqueous suspensions of partially comminuted limestone and marble (identical 
to that used in Example 1) were subjected to a sand grinding operation 
under the same conditions as described in Example 1 except that the 
initial quantity of dispersing agent used was 0.2% by weight. At the end 
of the sand grinding operation the limestone contained 86% by weight of 
particles having an equivalent spherical diameter smaller than 2 microns, 
and the marble contained 83% by weight of particles having an equivalent 
spherical diameter smaller than 2 microns. 
Step (c) 
The suspension of the comminuted minerals (which were found to be still 
deflocculated at the completion of the sand grinding operation) were each 
screened through a No. 300 mesh B.S. sieve. 
Step (d) 
The screened suspensions were then treated with the minimum quantity of 
calcium chloride required to flocculate the comminuted solid material. 
Step (e) 
The suspension of screened and flocculated solid material was then 
dewatered by filtration. 
The minimum viscosity and the corresponding total quantity of dispersing 
agent were determined as described in Example 1; and the results obtained 
are set forth in Table 2 below. 
TABLE 2 
______________________________________ 
Total minimum solids content 
of dispersing viscosity of suspension 
agent (% by wt) (centipoise) 
(% by wt) 
______________________________________ 
Limestone 
0.28 67 73.0 
Marble 0.30 123 74.2 
______________________________________ 
These results show that a low viscosity suspension at about 70% by weight 
of solids is still obtainable if the initial quantity of dispersing agent 
is 0.2% by weight, but at the expense of an increased total consumption of 
dispersing agent and the added cost of a flocculant. 
EXAMPLE 3 
Steps (a) and (b) 
Three samples (A, B and C) of partially comminuted limestone (each 
identical to that used in Example 1) were each formed into aqueous 
suspensions and each was deflocculated with 0.2% by weight of the same 
sodium polyacrylate dispersing agent, and then subjected to sand grinding 
under the same conditions, as described in Example 1. At the completion of 
the sand grinding operation each of the suspensions was found to be 
deflocculated and to contain 87% by weight (A), 84% by weight (B) and 96% 
by weight (C), respectively, of particles smaller than 2 microns. 
Step (c) 
The suspensions were screened through a No. 300 mesh B.S. sieve. 
Step (d) 
Sample A was flocculated with the minimum quantity of calcium chloride; 
sample B was flocculated with the minimum quantity of aluminium sulphate; 
and sample C was flocculated with the minimum quantity of calcium 
hydroxide. 
Step (e) 
The suspensions were then each dewatered by filtration. 
The cakes obtained by dewatering were dried at 80.degree. C. The dry 
material in each case was redispersed in water to form a suspension 
containing about 70% by weight of solids, and sufficient of the dispersing 
agent was added just to form a fluid suspension. The minimum viscosity and 
the corresponding total quantity of dispersing agent were determined as 
described in Example 1, and the results obtained are set forth in Table 3 
below. 
TABLE 3 
______________________________________ 
Total quantity 
Minimum solids content 
of dispersing 
viscosity of suspension 
Flocculant agent (% by wt) 
(centipoise) 
(% by wt) 
______________________________________ 
Calcium chloride 
0.27 47 70.2 
Aluminium 
sulphate 0.34 53 68.9 
Calcium 
hydroxide 0.32 82 70.2 
______________________________________ 
These results show that, if it is necessary to use a flocculant, of the 
three flocculants tested calcium chloride gives the best combination of 
low viscosity and small total quantity of dispersing agent. 
EXAMPLE 4 (Comparison) 
A sample of Carrara marble chips was crushed to pass through a No. 10 mesh 
B.S. sieve (nominal aperture 1.68 mm), and the crushed material was then 
mixed with water to form a suspension containing 40% by weight of dry 
solids which were then ground for 5 hours in a pebble mill using 38 mm 
flint pebbles. The partially-comminuted product was then screened through 
a No. 300 mesh B.S. sieve, dewatered by filtration and dried at 80.degree. 
C. 
The dried product was then mixed with sufficient water to form a suspension 
containing 30% by weight of dry solids, and with 0.3% by weight, based on 
the weight of dry solids, of calcium hydroxide but no dispersing agent. 
The resultant suspension was then subjected to attrition grinding, 
following the process described in British Patent Specification No. 
1,472,701 using Leighton Buzzard sand comprising particles having sizes in 
the range 0.5-1 mm, the quantity of sand used being such that the volume 
ratio of sand to suspension was 1.08:1. Sand grinding was continued until 
the particle size distribution of the ground marble was such that about 
90% by weight of the particles had an equivalent spherical diameter 
smaller than 2 .mu.m. 
The suspension of ground material was screened through a No. 300 mesh B.S. 
sieve and dewatered by filtration to form a cake containing 70.6% by 
weight of solids. The cake was thoroughly mixed with 0.15% by weight, 
based on the weight of dry solids, of the sodium polyacrylate dispersing 
agent used in Example 1, and the viscosity of the resultant suspension was 
measured with a Brookfield Viscometer using Spindle No. 3 at 50 rpm. 
Further quantities of the dispersing agent were then thoroughly mixed into 
the suspension and the viscosity measured after each addition. The results 
are set forth in Table 4 below. 
TABLE 4 
______________________________________ 
Total quantity Solids content 
of dispersing Viscosity of suspension 
agent (% by wt) 
(centipoise) (% by wt.) 
______________________________________ 
0.15 1967 70.5 
0.30 801 70.4 
0.50 789 70.3 
______________________________________ 
It will be seen that the suspensions were very much more viscous than those 
prepared by the method of the invention even when large amounts of 
dispersing agent were used. 
A further batch of the partially comminuted marble was subjected to sand 
grinding as described above but in the absence of the calcium hydroxide. 
After screening and dewatering the ground material, it was found to be 
impossible to deflocculate the cake to form a fluid suspension even when 
0.6% by weight of dispersing agent, based on the weight of dry solids, was 
used. 
EXAMPLE 5 
Step (a) 
A sample of dolomite (which had been ground in a dry fluid-energy mill 
until it had a particle size distribution such that there were no 
particles coarser than 1 mm., 48% by weight consisted of particles having 
an equivalent spherical diameter larger than 10 .mu.m, and 17% by weight 
consisted of particles having an equivalent spherical diameter smaller 
than 2 .mu.m) was formed into an aqueous suspension containing 40% by 
weight of the dry, ground dolomite and 0.15% by weight, based on the 
weight of dry dolomite, of the same sodium polyacrylate dispersing agent 
as was used in Example 1. 
Step (b) 
The suspension formed in step (a) was introduced into a sand grinding 
vessel containing Leighton Buzzard sane consisting of particles with 
diameters in the range of from 0.5 mm to 1 mm, the amount of said used 
being such that its volume was equal to the volume of the suspension, and 
agitation was continued until a given amount of energy had been dissipated 
in the suspension. The particle size distribution pf the finely ground 
dolomite was then determined. The experiment was then repeated two more 
times with different amounts of energy being dissipated in the suspension. 
The results are set forth in Table 5 below. 
TABLE 5 
______________________________________ 
Energy dissipated in the 
suspension per unit wt. of 
% by wt. of particles 
dry dolomite larger than 
smaller than 
kilojoules per kilogram 
10 .mu.m 2 .mu.m 
______________________________________ 
0 48.0 17 
529 1.3 68 
727 0.3 82 
1190 0.2 97 
______________________________________ 
Steps (c) and (d) 
After each suspension had been ground by agitation with sand, the ground 
material (which was found to be flocculated) was in each case screened 
through a No. 300 mesh B.S. sieve. 
Step (e) 
Each of the screened suspensions was dewatered in a filterpress. 
Each of the filter cakes was redispersed in a blunger to which was added, 
in aqueous solution, a further small quantity of the same sodium 
polyacrylate dispersing agent. In each case a fluid suspension was formed 
which had good rheological stability. 
EXAMPLE 6 
Step (a) 
Samples of partly refined kaolin (having a particle size distribution such 
that 17% by weight consisted of particles having an equivalent spherical 
diameter larger than 10 .mu.m and 46% by weight consisted of particles 
having an equivalent spherical diameter smaller than 2 .mu.m) and samples 
of a partially comminuted marble (having a particle size distribution such 
that 53% by weight consisted of particles having an equivalent spherical 
diameter larger than 10 .mu.m and 11% by weight consisted of particles 
having an equivalent spherical diameter smaller than 2 .mu.m) were mixed 
in different proportions and the resultant mixtures formed into aqueous 
suspensions containing 40% by weight of dry solids. No particles larger 
than 1 mm were present and each suspension also contained 0.15% by weight, 
based on the weight of total dry solids, of the same sodium polyacrylate 
dispersing agent as was used in Example 1. 
Step (b) 
Each suspension was introduced into a sand grinding bessel containing 
Leighton Buzzard sand consisting of particles with diameters in the range 
of from 0.5 mm to 1 mm, the amount of sand in each case being such that 
its volume was equal to the volume of the suspension, and agitation was 
continued until the proportion by weight of particles in the kaolin/marble 
mixture having an equivalent spherical diameter smaller than 2 .mu.m was 
about 80%. The composition of each mixture, the energy dissipated and the 
particle size parameters of the ground mixtures are set forth in Table 6 
below. 
TABLE 6 
______________________________________ 
Wt ratio 
Energy dissipated in 
of suspension per unit wt. 
% by wt of particles 
Kaolin: 
of dry solids larger than 
smaller than 
marble kilojoules per kilogram 
10 .mu.m 2 .mu.m 
______________________________________ 
80:20 330 2 78 
60:40 344 3 80 
40:60 317 4 78 
______________________________________ 
Steps (c) and (d) 
After each suspension had been ground, as described above, the ground 
material (which was found to be flocculated) was in each case screened 
through a No. 300 mesh B.S. sieve. 
Step (e) 
Each suspension was dewatered in a filterpress. 
Each of the filter cakes was redispersed in a blunger to which was added in 
aqueous solution, a further small quantity of the same sodium polyacrylate 
dispersing agent. In each case a fluid suspension was formed which had 
good rheological stability.