High solids aqueous silica slurry

Described are high solids aqueous slurries of amorphous precipitated hydrated silica, which slurries are of relatively low viscosity. The slurry typically contains from about 40 to about 60 weight percent of hydrated precipitated high structure silica. The viscosity of the high solids slurry is less than about 1000 centipoises. Processes for producing the aqueous high solids silica slurry are disclosed. In one embodiment, a wet cake of amorphous precipitated hydrated silica is liquified and the liquified wet cake charged to a high intensity mill wherein the silica is wet milled until the median agglomerate particle size of the silica is between about 0.3 and about 3 microns. In a further embodiment, finely-divided dry amorphous precipitated silica is slurried in a dispersion mill to the desired solids level and this slurry charged to a high-intensity mill and wet milled therein until the aforesaid agglomerate particle size range is attained. The resulting high solids slurry can be shipped in bulk with only mild agitation to prevent setting. A dispersant may also be added to the slurry prior to milling to retard settling of the silica.

DESCRIPTION OF THE INVENTION 
The present invention relates to aqueous high solids slurries of amorphous 
precipitated hydrated silica as well as to processes for their production. 
Amorphous precipitated hydrated silicas are used in a variety of commercial 
applications such as, for example, in paper coatings, as thickeners and 
flatting agents, and as reinforcing agents or fillers in paper, natural 
and synthetic rubber, elastomers, paints, adhesives, etc. Typically, such 
silicas are supplied in dried, powder form, e.g., in paper bags or as bulk 
shipments in hopper cars. Such packaging requires the customer to handle a 
finely-divided, sometimes dusty product, and in some instances to dispose 
of paper bags in which the silica is packaged. Often, the customer's 
manufacturing process requires that the silica be added in the form of an 
aqueous slurry. This further necessitates preparation of the silica slurry 
by the customer. In the case of bulk shipments, the silica powder may, 
under certain conditions, cake and be difficult to remove from the hopper 
car, i.e., the silica may not readily fluidize or flow freely from the 
hopper car. 
It would be useful if amorphous, precipitated hydrated silica could be 
provided in the form of an aqueous slurry so that such a slurry (after an 
optional adjustment for concentration) could be added directly to the 
customer's manufacturing process, thereby eliminating preparation or the 
slurry by the customer. For example, when silica is used as a filler in 
certain rubber products that are prepared using a rubber latex, such as in 
the production of rubber gloves, it would be desirable to mix an aqueous 
silica slurry with the rubber latex. In addition, product in the form of a 
slurry would avoid handling of a sometimes dusty product, which has the 
usual drawbacks associated with finely-divided dry materials, and further 
would eliminate disposal of the paper bags in which the dry silica is 
usually packaged. 
As the art recognizes, when an aqueous slurry of a conventional inorganic 
pigment, such as amorphous precipitated hydrated silica, is stored or 
shipped in large containers, such as in tank cars or large drums, without 
continuous vigorous agitation or without the addition of a dispersing 
amount of dispersants, the pigment tends to settle to the bottom of the 
container and form a thick, caked deposit. Such deposits may be on the 
order of 1 to 2 feet thick in tank cars. It has been found difficult to 
re-slurry such settled silica so that it may be removed, e.g., by pumping, 
from the container. The problems associated with emptying containers 
having a caked deposit of silica can be significant and have dictated 
against shipping silica in the form of a high solids aqueous slurry. In 
addition, the cost of shipping an aqueous slurry of silica containing a 
relatively low level of solids, e.g., from about 14 to about 25 percent 
solids, over long distances is a further impediment to the shipment of 
silica in the form of an aqueous slurry. 
It has now been surprisingly discovered that an aqueous slurry, 
particularly a high solids aqueous slurry, of amorphous precipitated 
silica that is a pumpable liquid, and that has a relatively low viscosity 
can be readily prepared. In accordance with the present invention, an 
aqueous slurry of amorphous, precipitated hydrated silica having a pH of 
from about 4 to about 9 is introduced into a high speed, fluid shear mill 
and the agglomerates comprising the silica particles reduced in size 
therein to a median agglomerate particle size of between about 0.3 and 
about 3 microns. Usually the silica introduced into such mill has a median 
agglomerate particle size of less than about 30 microns, more particularly 
less than 25 microns, e.g., from above 3 to about 25, preferably from 
about 7 to about 15, microns. In a further embodiment, the milled silica 
slurry may be dewatered to a more preferred higher solids content or 
additional finely-divided silica powder of the same grade or type may be 
added with agitation to the milled slurry until the desired solids level 
is obtained. 
DETAILED DESCRIPTION OF THE INVENTION 
Amorphous precipitated hydrated silicas are typically prepared by 
acidulation of aqueous solutions of alkali metal silicate. Such silicas 
and methods for their preparation are well known in the art. The silicas 
thus produced can be described as agglomerates of ultimate particles, 
which agglomerates have definite structures. As produced, agglomerates of 
ultimate silica particles (which ultimate particles range from about 10 to 
about 100 nanometers in diameter, depending on precipitation conditions) 
are generally between about 15 to 30 microns. These precipitated silicas 
are typically recovered from the acidulation process by filtration. The 
resulting silica filter cake is normally washed to remove residual alkali 
metal salt present as a consequence of the preparative process, the washed 
silica dried and the dried product dry milled. 
Water-soluble alkali metal silicate that may be used to prepare amorphous 
precipitated hydrated silicas may be a commercial or technical grade of 
silicate, e.g., sodium silicate, potassium silicate or lithium silicate. 
Sodium silicate is readily available commercially and is the least 
expensive of the aforedescribed silicates and hence is the alkali metal 
silicate of choice. The alkali metal silicate may be represented by the 
molecular formula, M.sub.2 O(SiO.sub.2).sub.x, wherein M is the alkali 
metal sodium, potassium or lithium, and x is a number from 1 to 5. More 
commonly, x is a number from 2 to 4, such as between 3.0 and 3.4, e.g., 
3.2 or 3.3. The aqueous alkali metal silicate reactant solution 
concentration can vary widely. For example, sodium silicate solutions may 
be used having an Na.sub.2 O concentration of from about 18.75 grams per 
liter to about 90 grams per liter. 
Acidifying agents generally used in the process of preparing amorphous 
precipitated hydrated silicas are inorganic acids, such as carbonic acid, 
hydrochloric acid or sulfuric acid. A sufficient amount of acidification 
agent is used in the acidulation step so that the recovered dried silica 
product will exhibit a pH between about 4.0 and about 9.0, e.g., from 
about 4 to about 8.5. The desired value for the pH of the recovered silica 
will depend on the ultimate application of the milled silica product. The 
pH of the silica is determined by measuring the pH of a 5 weight percent 
aqueous suspension of the particular silica at 25.degree. C. 
The acidulated aqueous alkali metal silicate reaction slurry, i.e., the 
silica product slurry, is filtered and the filter cake washed with water 
to reduce the amount of by-product alkali metal salt, i.e., the alkali 
metal salt of the acidification agent, to commercially acceptable levels, 
e.g., from about 0.5 to 2.0 weight percent, usually 1.0 to 2.0 weight 
percent. This filter cake is then dried using conventional drying means, 
e.g., spray or rotary driers, and the dried product dry milled or ground 
to obtain a silica of the desired degree of fineness. 
The filter cake, although having the appearance of a moist solid, contains 
a relatively large amount of water. The water associated with the silica 
content of such filter cake has been referred to as structural water 
because it occupies the available space between the silica agglomerates 
and also the space inside the silica agglomerates. See, for example, U.S. 
Pat. No. 4,157,920. When precipitated silicas hold a high percentage of 
water, i.e., from about 70 to about 85 weight percent, they have been 
referred to as high structure silicas. Precipitated silicas holding less 
than 70 weight percent water, e.,g., from about 50 to about 70 weight 
percent, have been referred to as low structure silicas. 
The solids content of the filter cake obtained by filtering the reaction 
slurry of an amorphous, precipitated high structure silica may vary, 
depending on the type of precipitated silica produced, from about 9 to 
about 28 or 30 weight percent. Attempts to produce a slurry of greater 
than 35 weight percent solids from such a filter cake, e.g., by the use of 
a mechanical liquifier, colloid or dispersion mill, such as a Cowles mill, 
results in the formation of a non-liquid, non-flowable, non-pumpable solid 
even though the amount of water in the solid is greater than 50 weight 
percent. 
The upper limit for the solids content for a readily flowable and pumpable 
aqueous slurry of high structure silicas prepared from a filter cake using 
conventional liquifiers or dispersion mills is from about 0 to about 6 
weight percent less than the solids content of the filter cake for that 
particular silica. Pumpable, aqueous slurries prepared by re-wetting dried 
silica using such mills may contain up to about 30 to 35 weight percent 
silica, but at higher solids levels, the viscosity of the slurry becomes 
too high to be pumpable. Transporting aqueous silica slurries having these 
relatively low levels of solids is economically unattractive. Moreover, a 
financial penalty is paid to first dry the filter cake and then re-slurry 
the dried silica. In addition, many applications for aqueous silica 
slurries require a significantly higher level of solids than 30-35 
percent. 
Chemically, dried amorphous precipitated hydrated high structure silica 
commonly contains at least 85, usually at least 88, weight percent 
SiO.sub.2 on an anhydrous basis, i.e., not including free water (water 
removed by heating at 105.degree. C. for 24 hours). 
The BET surface area of precipitated silicas typically varies from about 30 
to about 300 square meters per gram. The surface area of the precipitated 
silicas product may be varied within that range by varying the conditions 
of precipitation, techniques which are known to those skilled in the art. 
More recently, amorphous precipitated silica having a BET surface area of 
up to about 700 m.sup.2 /gram has been described. See U.S. Pat. No. 
4,495,167. The BET method for measuring surface area is that described in 
J. Am. Chem. Soc. 60, 309 (1938) by Brunauer, Emmett and Teller. BET 
surface areas reported herein were obtained using nitrogen as the gas 
adsorbed. 
The oil absorption of such precipitated silicas may vary from about 80 to 
about 350 milliliters of oil, e.g., dibutyl phthalate, per 100 grams of 
silica, usually between about 120 and 280 milliliters of oil per 100 grams 
of silica. 
In accordance with an embodiment of the present invention, an aqueous 
slurry containing less than about 50 or 55 weight percent, preferably at 
least 40 weight percent, of amorphous, precipitated hydrated high 
structure silica solids is charged to a high intensity mill, e.g., a high 
speed fluid shear wet mill, and the silica solids so charged to the high 
intensity mill are milled therein for a time sufficient to reduce the 
median agglomerate particle size of the silica to between about 0.3 and 3 
microns, e.g., between about 0.5 and 2.0 microns, preferably between about 
1.0 and about 1.5 microns, as measured by a Coulter counter. The silica 
charged to the high intensity mill will customarily have a pH of from 
about 4 to about 8.5, more preferably from about 4 to about 8, and still 
more preferably from about 5.5 to about 7.5, and having a median 
agglomerate size of less than about 30 microns, e.g., from above 3 to 
about 25 microns. 
Milling times in the high intensity mill will vary; but, generally will be 
for a time sufficient to reduce the silica agglomerate size to within the 
range desired. Milling times may range between about 2 and about 60 
minutes, more usually between about 3 and about 25 minutes. If a 
significant quantity of the silica agglomerates charged to the high 
intensity mill are substantially larger than 25 microns, the mill will not 
effect a significant reduction in the size of the large agglomerates 
because the grinding media in such mills is too small, vis a vis, the size 
of the silica agglomerates. Consequently, it may be necessary to first 
reduce the median agglomerate size of that quantity of large silica 
agglomerates to less than about 25 microns before charging the slurry to 
the high intensity mill. This first stage reduction may be accomplished in 
conventional dispersion mills and/or moderate intensity mills, as 
described herein. 
The silica slurry charged to the high intensity mill can be obtained by 
liquifying filter cake obtained from the recovery of precipitated silica 
in the aforedescribed process for preparing amorphous precipitated silica. 
In the liquification step, the wet filter cake, which may contain between 
about 9 and about 30 weight percent silica solids, is liquified with 
mechanical agitation and, if needed, the addition of small amounts of 
water, e.g., in a conventional dispersion or colloid mill such as a 
Cowles, Colloid, Premier or Kotthoff mill. In addition, previously dried 
silica, i.e., silica obtained by drying the filter cake (and optionally 
dry-milling the dried silica) may be re-slurried to prepare the slurry 
charged to the high intensity mill. In such latter embodiment, dry 
amorphous precipitated silica is charged to a conventional dispersion, 
colloid or moderate intensity mill and wet milled therein to obtain the 
desired silica slurry. Such slurry will customarily have a solids content 
of up to 30-35 weight percent. Water may be added to the mill 
simultaneously with the silica, prior to or subsequent to the silica 
charged to the mill. Preferably, the water is added to the mill prior to 
the silica charge. The amount of silica and water used are adjusted to 
produce a slurry of the desired solids content. Moderate intensity mills 
permit the preparation of slurries having a higher solids content than 
conventional dispersion or colloid mills, and may be used, for example, to 
prepare slurries of from 35 to 50 or 55 percent solids. 
To increase the solids content of the slurry introduced to the high 
intensity mill from that obtained from the conventional dispersion or 
colloid mill, additional dry silica of the same type or grade may be added 
to the slurry while subjecting the slurry to moderately intensive milling, 
e.g., in a Kady mill. Such milling (in a moderate intensity mill) will 
reduce the size of the silica agglomerates, e.g., to a median agglomerate 
size in the range of about 7 to about 25 microns, and allow the 
preparation of relatively fluid, pumpable slurries of moderate viscosity 
having a solids content of from about 35 to about 45 percent. It is 
contemplated that pumpable slurries from about 35 to 50 or 55 percent 
solids also may be prepared using the moderate intensity mill, e.g., a 
Kady mill. Care should be observed to prevent the discharge product from 
the moderate intensity mill from settling as it will do so in the absence 
of continued agitation or without the use of dispersants. More 
particularly, the lower the solids content of the discharge from the 
moderate intensity mill, the more fluid will be the slurry and the less 
energy required to maintain the solids in suspension. Slurries having 35 
to 40 percent solids will require mild agitation to maintain the 
dispersion, whereas slurries having 40 to 45 or 50 percent solids will 
require more vigorous agitation, i.e., the higher the solids content, the 
higher the energy input required to keep the solids dispersed and vice 
versa. Moderately intense milling will reduce, if necessary, the silica 
median agglomerate particle size to that more suitable for the high 
intensity mill, e.g., from about 7 to 25 microns, but more particularly, 
will reduce the number of oversized agglomerates, i.e., those greater than 
25 to 30 microns, to a particle size within the aforesaid range. 
In another embodiment, it is contemplated that a liquified silica filter 
cake or silica slurry prepared from dried silica will be charged to a 
moderate intensity mill and the resulting milled product dewatered to the 
desired solids content, e.g., from 35 to 50 or 55 percent, and this 
dewatered slurry charged to the high intensity mill. The silica filter 
cake or silica slurry can, of course, be liquified or prepared 
respectively in a moderate intensity mill rather than have such slurries 
prepared in a conventional dispersion mill and that product charged to the 
moderate intensity mill. 
In a preferred embodiment, the solids level of the aqueous slurry charged 
to the high intensity mill is at substantially the desired level for the 
product slurry so that de-watering of the product slurry is not necessary. 
The highest solids level that can be achieved for the slurry feed to the 
high intensity mill is conditioned on the particular silica being used, 
the median agglomerate particle size of the silica in the feed, which may 
be a function of the type of milling to which the silica is subjected 
prior to high intensity milling, the source of the feed, i.e., liquified 
wet cake or re-slurried dried silica, the pH of the silica, and any 
dewatering or other treatment of the slurry used to increase its solid 
content. 
Any commercially available high speed, high intensity fluid shear mill 
capable of reducing conventional precipitated silica agglomerates having a 
median agglomerate size of less than 30 microns, e.g., from above 3 to 
less than 30 microns, such as to about 25 microns, to a median agglomerate 
size of less than about 3 microns may be used to wet mill the 
aforedescribed silica slurry feed. Examples of such high speed fluid shear 
mills are the Morehouse mill, which is a high speed disk type of mill 
manufactured by Morehouse-Cowles, Inc., and the Premier high intensity 
mill, which is manufactured by the Premier Mill Corp. 
It has been surprisingly found that the wet milled silica product removed 
from the high intensity mill retains essentially the same performance 
properties as the product recovered from the reaction slurry, i.e., the 
physical performance properties of the milled silica in the ultimate 
application, e.g., in paper, are the same as the non-high intensity milled 
washed filter cake product or the dried and dry milled silica product 
obtained from said washed filter cake. Thus, the high intensity wet milled 
silica product retains its high structure properties. The BET surface area 
of the high intensity milled silica remains substantially the same as the 
silica charged to the mill. The viscosity of the milled silica slurry 
removed from the high intensity mill is generally less than about 1000 
centipoises. Usually the smaller the median agglomerate size of the wet 
milled silica, i.e., below 3 microns, the lower will be the viscosity of 
the high intensity milled silica slurry for an equivalent solids content. 
Viscosities of the high intensity wet milled silica slurry may be less 
than about 500 centipoises, sometimes less than about 250, e.g., from 
about 50 to about 150 centipoises, as measured by a Brookfield viscometer. 
A slurry of less than about 1000 centipoises is a very flowable liquid and 
is readily pumpable. Slurries of less than 200 centipoises are milk-like 
in character. By pumpable, is meant that the slurry can be pumped by any 
pump designed to transfer slurries. Liquid slurries of less than 1000 
centipoises are easily subject to being transferred by pumps, e.g., 
centrifugal pumps. 
In addition, it has been surprisingly discovered also that when the high 
intensity milled silica slurry is added to liquid paper coating 
compositions, it does not increase the viscosity of such compositions as 
much as the conventional dry milled dried powdered form of a corresponding 
silica used in that application. Moreover, it has been discovered further 
that a high intensity milled silica slurry that is maintained under mild 
agitation does not show an increase in viscosity with time, e.g., during 
storage, as is characteristic of conventional silica slurries. 
The silica product removed from the high intensity mill will customarily 
have the same solids content as the feed to the mill, e.g., between about 
15 and about 50 or 55 weight percent solids, preferably between about 30 
or 40 and 50 or 55 weight percent solids, will be a flowable liquid, and 
will be pumpable. This product may be dewatered, if needed, by standard 
evaporative techniques such as vacuum evaporation, cross-flow filtration, 
continuous centrifugation, expression filtration, etc. The wet cake 
resulting from such liquid-solid separation techniques can be readily 
reslurried by liquefaction for transportation as a slurry, or for use by 
the customer. Dewatering, if practiced, is continued until the solids of 
the dewatered product reaches the desired level, e.g., between about 40 
and about 60 weight percent solids, preferably from about 45 to 55 weight 
percent solids. 
Thus, in accordance with the present invention, there is provided a 
pumpable aqueous solids slurry comprising from about 40 to 60 weight 
percent of amorphous precipitated silica, and water as the dispersing 
medium, the silica having a median agglomerate particle size of less than 
3 microns, and the slurry having a viscosity of less than about 1000 
centipoises. As used herein, the percent solids in the slurry is the value 
obtained by subtracting from 100 the amount of water in the slurry, e.g., 
as determined by an Ohaus moisture balance. 
In another contemplated embodiment, the aqueous dispersion of wet milled 
silica discharged from the high intensity mill may be mixed with similar 
and compatible finely-ground, dry amorphous precipitated silica until the 
resultant mixture reaches the desired solids level, e.g., of between about 
40 and about 60 weight percent. 
The high solids aqueous dispersion of amorphous precipitated silica 
obtained from the high intensity mill may be shipped in bulk, e.g., by 
tank truck or railroad tank car, with only mild agitation to prevent 
settling. Such agitation may be provided by bubbling air through the 
dispersion, e.g., by means of a series of internal spargers installed near 
the bottom of the cargo container through which compressed air is passed, 
or by the rocking motion of the moving tank truck or tank car. Some 
settling of the silica may still occur; but, the settled silica may be 
readily re-dispersed by subjecting it to agitation with agitating means 
known to those skilled in the art. Alternatively, a dispersant may be 
added to the silica before, during or after the high intensity milling to 
retard settling of the milled silica. 
Dispersants that may be used with the milled silica slurry are 
finely-divided solids that are chemically and physically compatible with 
the silica and the product application in which the milled silica is to be 
used. Such solids include pigments used in the paper and rubber industry 
such as clays, titanium dioxide, calcium carbonate, magnesium carbonate, 
talc, zinc oxide, sodium polyphosphate, hydrated aluminas and insoluble 
inorganic salts such as barium sulfate. Mixtures of such materials may 
also be used. The solids may be either smaller or larger than the milled 
silica since their function is to keep the silica particles separated. 
Preferably, the size distribution of such solid dispersants is unimodal 
rather than bimodal. Water soluble dispersants for pigments such as 
polyacrylates, e.g., low molecular weight sodium polyacrylate, polyacrylic 
acids and/or partially neutralized polyacrylic acids may also be used. 
Typically, the amount of water-soluble dispersant used will be that amount 
sufficient to maintain the high intensity wet milled silica in a dispersed 
state, e.g., a dispersing amount, which typically is less than about 15 
weight percent, based on the amount of silica in the milled slurry. For 
example, from about 0.1 to about 15 weight percent; more particularly, 
from about 0.15 to about 5, e.g., 0.2 to 1.2, weight percent of water 
soluble dispersants may be used. 
Insoluble inorganic salts may be used also as dispersants in dispersing 
amounts, e.g., less than 15 weight percent, such as from 0.5 to 10 or 15 
weight percent. Pigmentary material described hereinabove may be used in 
dispersing amounts to avoid hard settling, e.g., 0.5 to 10 or 15 weight 
percent, or in amounts that will modify the basic characteristics of the 
silica slurry and influence the performance of the resulting blend. For 
example, aqueous dispersions of a blend of silica and titanium dioxide or 
clay are contemplated. The amount of pigmentary materials thus may vary 
from as low as about 0.5 weight percent to as high as about 70 weight 
percent.

The present invention is more particularly described in the following 
examples which are intended as illustrative only, since numerous 
modifications and variations therein will be apparent to those skilled in 
the art. 
EXAMPLE 1 
10,000 grams of a washed, wet cake of amorphous, precipitated hydrated high 
structure silica prepared by acidifying an aqueous sodium silicate 
solution with sulfuric acid was charged to a 5-gallon pail. The wet cake 
comprised about 31 percent silica solids. Dried silica obtained from such 
silica wet cake typically has a pH of about 8.5, an oil absorption of 
about 150 milliliters/100 grams, a BET surface area of about 35 square 
meters per gram (m.sup.2 /g ), a hydrated silica content (dry basis) of 
about 88 weight percent, about 1.5 weight percent of by-product sodium 
sulfate salt and a median agglomerated particle size of about 12 microns. 
Such precipitated silica is available as SAN-SIL.TM. CG 102 from SANTEK, a 
business unit of PPG Industries, Inc. 
The 5-gallon pail containing the silica wet cake was placed under the blade 
of a Premier Mill Lab Dispersator. The Dispersator blade was turned slowly 
for 3 to 5 minutes while 2400 milliliters of water were added to the pail. 
When all of the water had been added, the rate of rotation of the blade 
was increased until a vortex developed. The resulting fluid slurry had a 
silica solids content of 25 percent. The median particle size (Coulter 
Counter) of the solids in the slurry was 19 microns. 
All of the fluid slurry prepared using the Premier Dispersator was charged 
to a HM 1.5 Premier mill and milled therein. The calculated residence time 
in the grinding chamber was about 21/2 minutes. The grinding configuration 
of the mill was a urethane coated disk straight Delrin spacer having a 
disk peripheral speed of 2700 FPM (feet per minute) (13.7 meters per 
second) and a grinding chamber 85 percent filled with 1.25-1.60 millimeter 
(mm) zirconium silicate spheres. The grinding conditions were: shell inlet 
pressure--zero PSIG (pounds per square inch); slurry inlet 
temperature--64.degree. F. (17.8.degree. C.); slurry outlet 
temperature--76.degree. F. (24.4.degree. C.); slurry flow rate--3.2 GPH 
(gallons per hour) (0.012 m.sup.3 /hr); and power consumption--3.5 
amperes. The product discharged from the mill was a flowable liquid having 
a Brookfield viscosity of 100 centipoises (cps) (measured with No. 2 
spindle, 100 rpm at room temperature). The Coulter Counter median particle 
size of the silica in the product was 1.55 microns; but the slurry was 
also microscopically observed to contain a small fraction of oversized 
(&gt;25-30 micron) particles. This indicated that the slurry charged to the 
high intensity mill contained particles to large to be significantly 
reduced by the mill. The percent solids of the liquid product was 25 
percent. 
EXAMPLE 2 
A silica slurry containing 25 percent solids, which was prepared in the 
manner described in Example 1 using the Premier Mill Lab Dispersator, was 
charged to a Colloid mill and recirculated therein for 5 minutes. The 
Coulter Counter median particle size of the Colloid mill discharge was 
14.5 microns. The Colloid mill discharge was forwarded to the HM 1.5 
Premier high intensity mill described in Example 1 and milled using the 
same grinding conditions described in Example 1. The liquid product slurry 
of the high intensity mill had a Brookfield viscosity of about 100 
centipoises and a solids level of 25 percent. The particles in the product 
slurry had a Coulter Counter median particle size of 1.50 microns. 
Microscopic observation of the product slurry did not reveal any oversized 
particles. 
EXAMPLE 3 
A slurry of amorphous precipitated hydrated high structure silica 
containing 25 weight percent solids was prepared in the manner described 
in Example 1 using the Premier Dispersator and silica of the type 
described in Example 1. A portion of this slurry was forwarded to a HM 15 
Premier mill and milled therein. The calculated residence time in the 
grinding chamber was about 21/2 minutes. The grinding configuration of the 
mill was a urethane coated disk straight Delrin spacer having a disk 
peripheral speed of 2700 FPM (13.7 meter per second) and a grinding 
chamber 85 percent filled with 1.25-1.60 mm zirconium silicate spheres. 
The grinding conditions were: shell inlet pressure--3 PSIG (20.7 kPA); 
slurry inlet temperature--66.degree. F. (18.9.degree. C.); slurry outlet 
temperature--90.degree. F. (32.2.degree. C.); slurry flow rate--32 GPH 
(0.12 m.sup.3 /hr); power consumption--24 amperes. The product discharged 
from the mill was a flowable liquid having a Brookfield viscosity of 100 
centipoises. The Coulter Counter median particle size of the silica in the 
high intensity milled product was 1.39 microns. The percent solids of the 
liquid product was 25 percent. 
EXAMPLE 4 
The remainder of the Premier Dispersator 25 percent solids slurry prepared 
in Example 3 was divided into four equal portions and identified as 
Samples A, B, C and D. Sample A was the control. To Sample B was added 
0.0775 pounds of Colloid 211 sodium polyacrylate solution as a dispersant 
for each pound of original wet cake and the mixture stirred for 5 minutes. 
This mixture was charged to the HM 1.5 Premier mill and ground using the 
grinding configuration and conditions described in Example 1. Sample C was 
mixed with 0.194 pounds of Colloid 211 dispersant, and Sample D was mixed 
with 0.388 pounds of Colloid 211 dispersant. Samples A, C and D were also 
milled in the HM 1.5 Premier Mill and ground using the grinding 
configuration and grinding conditions of Example 1. The Coulter Counter 
median particle sizes for the slurries milled with the HM 1.5 Premier mill 
were: Sample A--1.85 microns; Sample B--1.55 microns; Sample C--1.55 
microns; Sample D--1.53 microns. 
A portion of milled samples A, B, C and D were stored at ambient 
temperature in quart cans for five months. Only the silica of Sample A was 
observed to have hard settling. Samples, B, C and D could be reslurried by 
shaking the cans in which the samples were stored by hand. This shows that 
hard settling can be prevented by the use of suitable dispersants. The 
Coulter Counter median particle size of the stored Sample C was 
redetermined and found to be 1.48 microns, which showed that no growth in 
median particle size of the silica had occurred during the 5 month aging 
of the sample. 
EXAMPLE 5 
Sufficient product of the HM 15 Premier milled 25 percent solids slurry 
from Example 3 was filtered through a Buchner funnel using Whatman No. 42 
filter paper and aspirator vacuum to obtain enough wet cake for evaluation 
as a paper coating pigment. The Coulter Counter median particle size of 
the resulting wet cake was 1.9 microns. The percent solids in the wet 
filter cake was 54.5 percent. A Waring blender was charged with 50 
milliliters of deionized water and, with the blender agitator rotating 
(Variac setting of 50), 201.9 grams of the 54.4 percent solids wet cake 
were added slowly to the mixer. The resulting fluid slurry had a 
Brookfield viscosity of 78 cps (No. 2 spindle, at 100 RPM, room 
temperature) and a Brookfield viscosity of 70 cps (No. 2 spindle, 20 RPM, 
room temperature). The percent solids of the fluid slurry was 45 percent 
(Ohaus moisture balance) and the Coulter Counter median particle size of 
the silica in the fluid slurry was 1.6 microns. 
EXAMPLE 6 
The following pigments were added in the order listed to water in a 
suitable container and stirred using a Cowles.RTM. mixer. The amounts 
indicated are on a dry basis. Enough water was used to prepare a pigment 
slurry having 66.7 percent solids. 
______________________________________ 
Ingredient Amount, grams 
______________________________________ 
Hydrafine .TM. Clay 
160.0 
Fluid Slurry of Example 5 
30.0 
(45 percent slurry) 
Ti-Pure .RTM. titanium dioxide* 
10.0 
______________________________________ 
* Added as 13.3 grams of predispersed TiPure .RTM. TiO.sub.2 
After all of the pigments had been added, mixing continued for 5 minutes. 
To this slurry was added in the order listed the following ingredients: 
______________________________________ 
Ingredient Amount, grams 
______________________________________ 
Penford Gum 280 starch (30 percent solids) 
10.0 
Dow Latex CP 620 NA (50 percent solids) 
20.0 
______________________________________ 
The final slurry had a solids content of 61.7 percent. This slurry was used 
to coat a wood-free paper base sheet (basis weight=55 pounds/3300 sq. ft. 
(25 kg/306 m.sup.2)) at levels around 5 pounds (2.3 kg) and 12 pounds (5.5 
kg) coating/3300 sq. ft. (306 m.sup.2) using a hand blade coater. 
The coated sheet was dried for 3 to 5 minutes at 218.degree. F. 
(103.degree. C.) and placed in a 50 percent relative humidity/68.degree. 
F. (20.degree. C.) sample conditioning room overnight to equilibrate the 
sheet. The coat weight was determined by TAPPI procedure T-410 om-83. The 
brightness, opacity, gloss and ink receptivity were obtained using TAPPI 
procedures T-452 om-87, T-425 om-86 and T-480 om-85 respectively. The 
performance results for the coated sheet are reported as Run 1 in Table I 
for a calculated coat weight of 8 pounds/3300 ft.sup.2 (3.6 kg/306 
m.sup.2). 
The foregoing recipe and procedure were repeated with the same ingredients 
except that silica obtained by drying a washed wet cake equivalent to the 
wet cake of Examples 1 and 3 was used instead of the fluid slurry of 
Example 5. The performance results for the coated sheet are reported as 
Run 2 in Table I. 
EXAMPLE 7 
The procedure of Example 6 was followed to prepare a further paper coating 
pigment formulation using the following ingredients. The coating was used 
to coat a paper sheet as described in Example 6. The amounts indicated are 
on a dry basis. Enough water was used to prepare a pigment slurry having 
62.6 percent solids. 
______________________________________ 
Ingredient Amount, grams 
______________________________________ 
Hydrafine .TM. Clay 150.0 
Fluid Slurry of Example 5 
46.0 
(45 percent slurry) 
Ti-Pure .RTM. titanium dioxide* 
4.0 
Penford Gum 280 starch (30 percent solids) 
10.0 
Dow Latex CP 620 NA (50 percent solids) 
20.0 
______________________________________ 
*Added as 5.3 grams of predispersed TiPure .RTM. TiO.sub.2 
The final slurry had a solids content of 58.5 percent. Performance results 
for the coated sheet are reported as Run 3 in Table I for a calculated 
coat weight of 8 pounds/3300 ft.sup.2 (3.6 kg/306 m.sup.2). 
The foregoing procedure was repeated with the same ingredients except that 
silica obtained by drying a washed wet cake equivalent to the wet cake of 
Examples 1 and 3 was used instead of the fluid slurry of Example 5. The 
performance results for the coated sheet are reported as Run 4 in Table I. 
TABLE I.sup.a. 
______________________________________ 
INK 
RUN BRIGHTNESS, OITY, RECEP- 
NO. % % TIVITY* GLOSS 
______________________________________ 
1 81.3 93.1 27.9 49.4 
2 81.4 92.1 26.7 40.7 
3 82.0 92.2 31.1 46.5 
4 81.0 92.2 31.8 33.3 
BASE 78.6 86.0 42.1 9.0 
SHEET 
______________________________________ 
*Ink Receptivity reported as % decrease. 
.sup.a. Data reported for a calculated coat weight of 8 pounds/3300 
ft.sup.2 (3.6 kg/306 m.sup.2). 
The data of Table I shows that the reported paper properties were not 
effected by the high energy milling of the silica except that the smaller 
mean particle size enhanced the gloss of the coated sheet. 
EXAMPLE 8 
The following pigments were added in the order listed to water charged to a 
suitable container and stirred using a Cowles.RTM. mixer. Sufficient water 
was used to prepare a pigment slurry having 67.9 percent solids. Amounts 
indicated are on a dry basis. 
______________________________________ 
Ingredient Amount, grams 
______________________________________ 
Hydrafine .TM. Clay 
190.0 
Ti-Pure .RTM. titanium dioxide* 
10.0 
______________________________________ 
*Added as 13.3 grams of predispersed TiPure .RTM. TiO.sub.2 
After all of the pigments had been added, mixing continued for 5 minutes. 
To this slurry was added in the order listed the following ingredients: 
______________________________________ 
Ingredient Amount, grams 
______________________________________ 
Penford Gum 280 starch (30 percent solids) 
10.0 
Dow Latex CP 620 NA (50 percent solids) 
20.0 
______________________________________ 
The final slurry had a solids content of 62.5 percent. This slurry was used 
to coat a mechanical pulp base stock sheet (basis weight=28 pounds/3300 
sq. ft.) (12.7 kg/306 m.sup.2) at levels around 5 pounds (2.3 kg) and 12 
pounds (5.5 kg) coating/3300 sq. ft. (306 m.sup.2) using a hand blade 
coater. The coated sheet was dried for 3-5 minutes at 218.degree. F. 
(103.degree. C.) and placed in a 50 percent relative humidity 168.degree. 
F. (20.degree. C.) sample conditioning room overnight. The brightness, 
opacity, gloss and ink receptivity were obtained using the TAPPI 
procedures described in Example 6. The results are listed as Run No. 1 in 
Table II and is the control run for the series described in this Example 
for a calculated coat weight of 8 pounds/3300 ft.sup.2 (3.6 kg/306 
m.sup.2). 
The foregoing procedure was repeated except that the 10.0 grams of 
Ti-Pure.RTM. titanium dioxide were replaced with 4.0 grams of Ti-Pure.RTM. 
titanium dioxide and 6.0 grams of SAN-SIL.TM. CG 102 silica. The pigment 
slurry had a solids content of 68.5 percent. The final slurry (after 
adding the starch and latex) had a solids content of 62.5 percent. Results 
are tabulated in Table II as Run No. 2. 
The procedure of Run No. 2 was repeated except that the 6.0 grams of 
SAN-SIL.TM. CG 102 silica was replaced with a equivalent amount of wet 
cake from Example No. 5 that was reslurried to a solids level of 45 
percent using water and a Cowles.RTM. mixer. Results are tabulated in 
Table II as Run No. 3. 
The procedure of Run No. 1 was repeated except that the Ti-Pure.RTM. 
titanium dioxide was replaced with 10.0 grams of SAN-SIL.TM. CG 102 silica 
of Run 2. Results are tabulated in Table II as Run No. 4. 
The procedure of Run No. 4 was repeated, except that the 10.0 grams of 
silica was replaced with an equivalent amount of wet cake from Example 5 
that was reslurried to a solids level of 45 percent using water and a 
Cowles.RTM. mixer. Results are tabulated in Table II as Run No. 5. 
TABLE II.sup.a. 
______________________________________ 
INK 
RUN BRIGHTNESS, OITY, RECEP- 
NO. % % TIVITY* GLOSS 
______________________________________ 
1 74.1 92.6 20.9 42.0 
2 73.7 91.8 22.1 40.1 
3 73.7 92.8 24.3 43.0 
4 73.6 92.0 23.8 40.2 
5 73.7 91.9 24.5 43.8 
BASE 68.3 85.2 41.0 9.6 
SHEET 
______________________________________ 
*Ink Receptivity reported as % decrease. 
.sup.a. Data reported for a calculated coat weight of 8 pounds/3300 
ft.sup.2 (3.6 kg/306 m.sup.2) 
The data of Table II show that brightness and opacity are equivalent to the 
control, but ink receptivity is significantly increased for Runs 2-5, with 
better results for the high intensity milled silica used in Runs 3 and 5. 
Further, the gloss for Runs 3 and 5 is significantly better than that 
obtained with unmilled silica, i.e., Runs 2 and 4, and marginally better 
than the control, i.e., Run 1. 
EXAMPLE 9 
A Kady mill was charged with 1260 grams of water and 700 grams of 
SAN-SIL.TM. CG 102 silica. The mixture was mixed by hand until the mass 
was wet and then the mill was started. An additional 700 grams of the 
silica were added slowly and the resulting mixture mixed in the mill for 
15 minutes. The percent moisture of the silica in the mill was analyzed 
and found to be about 47 percent. The slurry, which was very viscous, was 
charged to a Premier high intensity vertical laboratory mill. The main 
drive peripheral speed of the mill is 2100 ft/minute (6.1 meters/second) 
and the grinding chamber was 60 percent filled with 1.25-1.6 mm. zirconium 
silicate beads. The resulting mill product was a very fluid slurry having 
a Brookfield viscosity of 70 cps (No. 2 spindle, 20 rpm, room temperature) 
and 78 cps (No. 2 spindle, 100 rpm, room temperature). The Coulter Counter 
median particle size of the final milled product was 1.35 microns. The 
percent solids of the final milled product was 47 percent. 
EXAMPLE 10 
A suitable container was charged with 160 grams of ASTRA PLATE SD 
delaminated clay, 40 grams of No. 2 coating clay (KCS SD)-both available 
from Georgia Kaolin, and enough water to produce a slurry containing 70 
percent solids. This slurry was mixed in a Premier Mill Lab Dispersator at 
85 percent maximum speed for 5 minutes. To this milled slurry was added on 
a dry weight basis 12 grams of Penford Gum 290 starch, 18 grams of Dow 
Latex CP 620 NA, 2.4 grams of Calsan.TM. 50 calcium stearate lubricant, 
0.5 grams of Colloid 211 dispersant and sufficient water to give a final 
solids content of 52 percent. This mixture was stirred for one minute and 
then used to coat a merchant grade wood free basestock having a basis 
weight of 52 pounds/3300 ft.sup.2 (23.6 kg/306 m.sup.2) with a Modern 
Metalcraft Laboratory Coater run at a speed to give a coating weight of 
6.75-7.25 pounds/3300 ft.sup.2 (3-3.3 kg/306 m.sup.2). The dryer drum was 
set at 248.degree. F. (120.degree. C.). The coated sheet was 
supercalendered using a Beloit Wheeler laboratory supercalender (3 passes, 
1500 psi [0.01 MPa], speed of 3, temperature of 150.degree. F. 
[65.6.degree. C.]). The calendered sheet was placed in a 50 percent 
relative humidity 168.degree. F. (20.degree. C.) sample conditioning room 
overnight. Brightness, gloss, opacity and ink receptivity were obtained 
using the TAPPI procedures described in Example 6. Results are reported as 
Run 1 in Table III. 
The above-described procedure of Run 1, was followed to prepare a paper 
coating composition and coated paper except that the pigment slurry used 
153.6 grams of the delaminated clay, 38.4 grams of No. 2 coating clay, 8.0 
grams of SAN-SIL.TM. KU 33 silica and enough water to produce a slurry 
containing 68.5 percent solids. SAN-SIL.TM. KU 33 is an amorphous 
precipitated silica similar to SAN-SIL.TM. CG 102 except that it has a pH 
of 7.0, an agglomerated particle size of 2.5 microns, a BET surface area 
of 70 m.sup.2 /g and an oil absorption of about 135 ml/100 gram as typical 
physical properties. Results are tabulated in Table III as Run No. 2. 
The above-described procedure of Run 2 was followed except that the 
SAN-SIL.TM. KU 33 was replaced by an equivalent amount of the 47 percent 
slurry prepared in Example 9. Results are tabulated in Table III as Run 
No. 3. 
The above-described procedure of Run 2 was followed to prepare a paper 
coating composition and coated paper except that the pigment slurry used 
147.2 grams of the delaminated clay, 36.8 grams of the No. 2 coating clay, 
16.0 grams of SAN-SIL.TM. KU 33 and enough water to give a slurry 
containing 66.9 percent solids. Results are tabulated in Table III as Run 
No. 4. 
The above-described procedure of Run 4 was followed except that the 
SAN-SIL.TM. KU 33 was replaced with an equivalent amount of the 47 percent 
slurry prepared in Example 9. Results are tabulated in Table III as Run 
No. 5. 
TABLE III.sup.a 
______________________________________ 
RUN BRIGHT- OITY, INK 
NO. NESS, % % RECEPTIVITY* 
GLOSS 
______________________________________ 
1 83.6 91.1 91.2 48.6 
2 84.8 91.0 83.9 49.1 
3 85.1 90.8 82.7 50.4 
4 85.3 91.1 84.0 50.8 
5 85.0 91.0 83.7 50.3 
BASE 
SHEET 
______________________________________ 
*Ink Receptivity reported as brightness values. 
.sup.a Data reported for a calculated coat weight of 6.75-7.25 pounds/330 
ft.sup.2 (3-3.3 kg/306 m.sup.2). 
The data of Table III show that all properties measured were significantly 
better than or equal to the control (Run 1) and the optical properties as 
measured in Runs 3 and 5 are equivalent to those reported for Runs 2 and 
4, thereby indicating that high intensity milling of silica as described 
herein does not change its high structure properties. 
Although the present invention has been described with reference to 
specific details of certain embodiments thereof, it is not intended that 
such details should be regarded as limitations upon the scope of the 
invention, except as and to the extent that they may be included in the 
accompanying claims.