This invention relates to novel silica spheroids and to their production by a phase separation technique in which an aqueous alkaline stabilized silica sol and an aqueous solution of a non-adsorbing polymer are mixed and acidified to produce the new silica spheroids.

This invention relates to the formation of spheroidal silica aggregates. 
More particularly the invention discloses novel spheroidal silica 
aggregates and a process for the production of such silica aggregates. 
The spheroidal silica aggregates with which this invention is concerned are 
generally rounded particles in the micron size range, for example in the 
range 1 to 400 microns. 
A common route for preparation of silica aggregates is by emulsification of 
silica sols in an oil phase and subsequent aggregation or gelation of the 
silica within the droplets of the emulsion by the addition of acid to form 
the silica spheroids or aggregates. The emulsification route is described, 
for example, in GB No. 2 127 002A (Toyo Soda Manufacturing Company 
Limited) and GB No. 1 452 896 (NRDC). Silica spheroids formed by the 
emulsification route require extensive cleaning procedures to remove oil 
and emulsifier and lead to large volumes of waste solvent. Such cleaning 
is particularly important when these silica spheroids are to be used in 
applications such as analytical chromatographic supports. 
Another route for formation of silica spheroids is described in GB No. 1 
433 242 (E I Du Pont de Nemours & Company). This process comprises mixing 
a silica sol with a polymerisable organic material, i.e. formaldehyde and 
urea or melamine and initiating polymerisation of the organic material to 
cause coacervation of the organic material and colloidal silica into 
microspheres. The organic material is subsequently burned off. According 
to GB No. 1 506 114 and GB No. 1 433 242 (E I Du Pont de Nemours & 
Company) the resulting particles are extremely fragile with a pore volume 
of greater than 50% by volume. 
This invention provides a porous spheroidal silica having a particle size 
in the range 1-400 microns, axial ratios of 1:1 to 1:12, a pore volume in 
the range 26 to 50% and a narrow pore size distribution in which more than 
80% of the porosity is between 50 and 200% of the median pore diameter in 
the pore size range 20 to 1000 .ANG.. 
In a preferred form of the invention the particle size range is between 1 
and 200 microns and the axial ratios from 1:1 to 1:4. 
Accordingly, the present invention provides a process for the preparation 
of the novel silica spheroids comprising mixing together under alkaline 
conditions an aqueous alkali stabilised silica sol and an aqueous solution 
of a non-adsorbing polymer to form a phase separated system comprising 
silica droplets in a polymer rich continuous aqueous phase and acidifying 
the system to aggregate the droplets to form the silica spheroids. 
Normally, in the close-packed structures forming the spheroidal silica 
particles, the median pore diameter is less than the average constituent 
particle diameter. This applies when the silica sol, which generates the 
constituent particles, has a monomodal size distribution. 
Preferably the sol and polymer solutions are mixed at a pH in the range 
8.5-10.5. 
The silica rich dispersed phase units are typically of the order of tenths 
of one micron in size. Surprisingly, however, on addition of acid to 
accelerate aggregation of the silica within the dispersed phase units, 
coalescence of the units into larger dispersed phase units occurs. The 
simultaneous aggregation within the coalescing dispersed phase results in 
the formation of silica spheroids generally of greater than one micron in 
diameter. 
Silica sols useful in this invention are typified by Ludox HS 40 (E I Du 
Pont de Nemours & Co) and the preparation of such sols is described in, 
for example, U.S. Pat. No. 2,801,902. Such sols can have a concentration 
between 1/2% up to 55% by weight silica. In general, sol particle sizes of 
up to about 1000 .ANG. may be used. 
Non-adsorbing polymers useful in this invention are typically organic 
polymers, such as negatively charged polyelectrolytes including, for 
example, sodium dextran sulphate, sodium polyacrylate, sodium 
carboxymethyl cellulose and mixtures of such polyelectrolytes. It is 
essential if phase separation is to achieved that the polymer is not 
adsorbed by silica at the pH of the system. 
A preferred polymer concentration to induce phase separation is in the 
region 0.01-20% by weight in the mixture. The concentration of polymer is 
in addition related to the concentration of silica and the ionic type and 
concentration of electrolytes in the system. For example some electrolytes 
may be present in the original silica sol to control its stability. 
The most preferred concentration cannot be expressed simply, since this is 
a complex function of the polymer type, charge and flexibility, the 
electrolyte type and concentration and the silica sol type and 
concentration. The most preferred concentration, however, can be obtained 
by the following procedure which, in addition, serves as a test procedure 
for selecting suitable polymers for the invention. 
A polymer type and concentration is selected by preparing an aqueous 
polymer solution in the region of up to 20% w/w and adding this slowly to 
a silica sol in the concentration region of 20% w/w at an approximate pH 
9, until the mixture just becomes appreciably more turbid. The mixture is 
allowed to mix thoroughly. If the turbidity decreases appreciably, more 
polymer is added until the mixture remains turbid after thorough mixing. 
The polymer concentration in the mixture would be in the region of 0.1 to 
10% w/w and typically in the region 1 to 5% w/w based on the mixture. The 
phase separation state is confirmed by addition of an equal volume of 
water to the mixture which results in a dramatic decrease in turbidity, 
back to that of the order of the original silica sol. 
Higher polymer concentrations can be used, as long as reversibility is 
observed on dilution to below the above-determined concentration of 
polymer. 
Unsuitable polymer types, levels or molecular weights will not result in 
the above reversible phase separation. For example, no apparent increase 
in turbidity would be observed with silica sol compatible polymers such as 
nonionic dextran. 
Undesirable irreversible turbidity would be due to aggregation rather than 
phase separation and may be caused by: 
(i) excessive polyelectrolyte or electrolyte; 
(ii) adsorbing rather than non-adsorbing polymers, e.g. cationic 
polyelectrolytes. 
The above process of phase separation is discussed in "Polymeric 
Stabilisation of Colloidal Dispersions" by Donald H Napper, Academic Press 
1983, and is understood to occur by a depletion flocculation mechanism. 
Phase separation is, therefore, more readily achieved with polymers of 
high molecular weight and high anionic charge density. 
Certain anionic polymers such as xanthan gum, of very high molecular 
weight, will induce phase separation but because of their gelling 
characteristics are difficult to handle and are, therefore, less desirable 
than, for example, sodium dextran sulphate. 
The process of aggregation or gelation of the silica droplets is initiated 
by the addition to the system of a mineral acid such as sulphuric or 
hydrochloric acid, although other acid may also be used. The amount of 
acid required to induce the gelation or aggregation is that required to 
reduce the pH of the mixture to below that of the stable silica sol 
generally to within the range pH 4-8.5, preferably in the range pH 5-8. A 
further reduction in pH may be desirable to enhance the recovery of the 
spheroids from the polymer solution to as low as pH 2. The various pH 
reductions required in the process affect the final state of the silica 
spheroids and, as will be seen in the examples, significant time spans are 
involved and stepwise additions of acid are preferred. 
The silica spheroids prepared according to the present invention generally 
have the size in the range 1-400 microns, preferably in the range 1-200 
microns. Sub-micron aggregates may be present as unwanted material at the 
end of the process. 
It will be appreciated that the silica spheroids formed according to the 
present invention avoid the problem of oil contamination inherent in the 
earlier processes and the plant and equipment used in their preparation 
avoids the use of inflammable oil phases, hence can be more simple and not 
essentially flameproof.

EXAMPLE 1 
An aqueous dispersion containing 15% w/w SiO.sub.2 (Ludox HS 40) and 2% w/w 
sodium polyacrylate (MW 230,000) at pH 9.6 was prepared and maintained 
gently stirring for three days. The sample appeared turbid and under an 
optical microscope contained sub-micron agglomerates. On dilution with 
water, the sub-micron units disappeared with a noticeable decrease in 
turbidity. 
On reduction of the pH by addition of concentrated hydrochloric acid to pH 
8.8 and maintaining stirring for one day there was no apparent difference 
in the above behaviour. 
However, on further addition of acid to pH 7.0, markedly larger spheroids 
of up to 20 microns in size were formed in the dispersion. On dilution, 
the spheroids did not disrupt and there was no noticeable decrease in 
turbidity. 
Examination of the silica by scanning electron microscopy showed the 
presence of smooth surface silica spheroids. Examination by transmission 
electron microscopy showed the material to be composed of densely packed 
colloidal silica. 
EXAMPLE 2 
A 20% w/w silica dispersion was prepared by dilution of Ludox HS 40 with 
distilled water. This was added to an equal volume of 4% w/w solution of 
sodium polyacrylate at pH 9.6 prepared by addition of concentrated sodium 
hydroxide to a solution of polyacrylic acid (approx. M.W. 230,000). The 
dispersion was rapidly stirred and the pH rapidly adjusted to 7.4 by 
addition of concentrated sulphuric acid. Large silica spheroids of up to 
approximately 30 microns were formed. Several particles were elongated in 
shape which was interpreted as due to the rapid shear in the system. 
EXAMPLE 3 
Equal volumes of a 30% silica sol (Ludox HS 40) and a 4% sodium 
polyacrylate (MW 230,000) solution pH 9.6 was stirred gently and the pH 
lowered rapidly to pH 5.6 by addition of concentrated hydrochloric acid. 
While large aggregates of the order of 10 microns were formed, the 
aggregates were not as effectively coalesced as those of Example 1 and 2. 
This is probably a consequence of the lower pH greatly accelerating 
aggregation of the sub-micron units so that coalescence is somewhat 
impaired. On repeating the Example, but adding a lower concentration of 
hydrochloric acid more slowly to pH 6.8, coalesced silica spheroids were 
produced. 
EXAMPLE 4 
A 53 g mixture at pH 9.8 containing 3.8% w/w sodium dextran sulphate 
(Pharmacia M.sub.w 5.times.10.sup.5) and 20.9% w/w SiO.sub.2 (Ludox HS40 E 
I Du Pont de Nemours) was thoroughly mixed on a Silverson mixer on maximum 
power for one minute. Examination of the turbid mixture by optical 
microscopy showed the presence of indistinguishable sub-micron units. The 
pH of the mixture was reduced to pH 7.8 with concentrated hydrochloric 
acid and left stirring (magnetic stirrer) for 16 hours, then reduced to pH 
6.9 and left stirring for 72 hours. Optical microscopy showed the presence 
of polydisperse spheroids up to approximately 50 .mu.m. 
EXAMPLE 5 
An approximate repeat of Example 4 in the presence of 0.6% w/w NaCl was 
carried out as follows. 
A 50 g mixture at pH 9.6 containing 3.8% w/w sodium dextran sulphate, 20.9% 
w/w SiO.sub.2 and 0.6% w/w NaCl, based on the total weight of the mixture 
was mixed on a Silverson mixer as above. The sample was more translucent 
than that for Example 4. The sample was stirred at pH 9.6 for 11/2 hours, 
reduced to pH 7.8 and stirred for 24 hours. The mixture was now turbid. 
The pH was dropped to pH 6.9 and left stirring for 24 hours to produce a 
turbid dispersion containing silica spheroids up to approximately 40 
.mu.m. 
EXAMPLE 6 
An approximate repeat of Example 5 in the presence of 5.0% w/w polymer was 
carried out as follows. 
A 50 g mixture at pH 9.6 containing 5.0 w/w sodium dextran sulphate, 20.9% 
w/w SiO.sub.2 and 0.6% w/w NaCl, based on the total volume of the mixture, 
was mixed on a Silverson mixer for one minute followed by stirring for 
11/2 hours. 
The resulting dispersion remained turbid. The pH was reduced to pH 7.7 and 
the mixture stirred for 24 hours. The pH was then reduced to pH 6.9 and 
left stirring for a further 24 hours to produce a dispersion containing 
silica spheroids up to approximately 100 .mu.m in size. 
EXAMPLE 7 
A 900 cm.sup.3 dispersion containing 2.6% w/w sodium polyacrylate (prepared 
by neutralising a 2.0% w/w polyacrylic acid (MW 230,000) solution with 
concentrated sodium hydroxide) and 15.8% w/w SiO.sub.2 (Ludox HS40) at pH 
9.8, was mixed on a Silverson mixer for 5 minutes then left undisturbed 
for a period of two weeks. 
The pH of the mixture was reduced to pH 7.4 and gently stirred at 120 rpm 
(stirrer motor with paddle blade stirrer) for one day then reduced to pH 
7.1 and continued stirring for a further day to allow the silica spheroids 
to harden. 
Examination by optical microscopy showed the particles to be predominantly 
in the region of 5 to 10 .mu.m with no visible fine material. The 
particles were, however, irregularly shaped. This shape difference is 
believed to be due to the slow aggregation at pH 9.8 in the absence of a 
shear field. 
An additional 700 g of silica sol (Ludox HS40 42% w/w silica) was added 
slowly over a twenty day period to the above dispersion with continuous 
stirring at 120 rpm and maintaining the pH at approximately pH 7.7. 
The pH was finally adjusted to 7.0 for a further 24 hours then adjusted to 
pH 4.0 and allowed to settle for 48 hours. Examination of the particles by 
electron microscopy showed that the particles had grown to a smoother, 
more spheroidal geometry with larger particles up to approximately 50 
.mu.m together with finer material in the region of down to approximately 
1 .mu.m. 
The supernatant was removed and the sediment redispersed into water and 
allowed to settle for a period of four days. This operation was repeated 
five times and then the sediment was freeze-dried. 
Examination of the spheroids by mercury porosimetry showed a porosity of 
0.38 cm.sup.3 g.sup.-1 between 30 and 1000 .ANG. with the majority, 0.3 
cm.sup.3 g.sup.-1, between 30 and 100 .ANG.. 
EXAMPLE 8 
Two 4 liter silica sol and sodium polyacrylate dispersions were mixed in 
the same proportions as in Example 7. One sample (a), was stirred at 300 
rpm whereas a second sample (b) was mixed on a Silverson mixer and then 
allowed to stir at 300 rpm in a 5 liter round bottom flask. 
After 2 days at pH 9.7 sample (a) contained smooth spheroidal entities up 
to approximately 100 .mu.m whereas sample (b) contained indistinguishable 
sub-micron entities. No change occurred on continuous stirring for 16 
days. 
The pH of the dispersions was reduced to pH 7.8 for one day then to pH 7.2 
for a further day while maintaining stirring. 
Examination by optical microscopy showed smooth spheroids up to 
approximately 200 .mu.m for sample (a) and up to approximately 5 .mu.m for 
sample (b). 
Sample (a) was allowed to settle and was redispersed several times in 
distilled water then the sediment was freeze-dried. The material was 
heated to 555.degree. C. for two hours then examined by mercury 
porosimetry. The material had a porosity of 0.32 cm.sup.3 g.sup.-1 with 
the majority, 0.28 cm.sup.3 g.sup.-1, between 30 and 100 .ANG.. 
EXAMPLE 9 
A 4 liter dispersion was mixed in the same proportions as in Example 7. The 
dispersion was mixed on a Silverson mixer at maximum power for three 
minutes then transferred to a 5 liter round bottom flask and stirred for 4 
hours at 400 rpm at pH 9.8. 
The pH was reduced to 7.85 and stirring continued for 24 hours. Examination 
by optical microscopy showed the appearance of spheroidal entities in the 
3 .mu.m region with a few larger, approximately 10 .mu.m, entities. 
The dispersion was stirred for 2 days then the pH reduced to pH 7 and 
stirred for a further one day. 
The pH of the dispersion was reduced to pH 2 in order to reduce the 
viscosity of the continuous phase, allowed to settle for 4 days, decanted 
and the sediment redispersed into water at pH 2. This was repeated a 
further five times, the final three redispersions being into water at 
neutral pH. The sediment was then freeze-dried. 
A total of 320 g material was recovered. The sample was fractionated using 
an air classifier into a fraction 70% between 3.8 and 6.2 .mu.m. The 
material was heated at 500.degree. C. for two hours and examined by 
mercury porosimetry. The results shown in FIG. 1 show the interparticle 
voids in the region of 2 .mu.m and the particle porosity of 0.3 cm.sup.3 
g.sup.-1 at less than 1000 .ANG. with a mean pore diameter of 
approximately 40 .ANG.. SEM examination of the material showed the smooth 
surface character as shown in FIGS. 2 and 3. 
EXAMPLE 10 
A 25 g dispersion containing 2.4% w/w Courlose F75P (MW approx 100,000 
SCMC) and 8.4% w/w SiO.sub.2 (Ludox HS40) was mixed at pH 9.8 for two 
minutes on a Silverson mixer followed by two minutes on a soniprobe. The 
pH was reduced to 7.8 for two hours then to pH 7.2 for one hour with 
continuous stirring using a magnetic stirrer. The pH was then reduced to 
pH 6.9 and left stirring for 24 hours. On dilution with water the silica 
spheroids were still somewhat reversible, swelling and dispersing on 
examination by optical microscopy. After a further 120 hours, irreversible 
spheroids up to approximately 70 .mu.m were formed. 
EXAMPLE 11 
A 62.7 g dispersion was prepared as in Example 10 except the pH was 
immediately reduced to pH 5.1 and left stirring for six days. Spheroidal 
silica up to 30 .mu.m with a large number of particles less than 2 .mu.m 
were formed which did not redisperse on dilution with water. 
EXAMPLE 12 
A 75.5 g dispersion at pH 9.6 was prepared containing 2.4% w/w Courlose F8P 
(MW approx 30,000 SCMC) and 8.3% w/w SiO.sub.2 (Ludox HS40). The 
dispersion was mixed on a Silverson mixer for 2 minutes followed by 
sonication for 4 minutes. 
The pH was dropped to 7.8 for 2 hours then to 7.2 for 2 hours with 
continuous stirring on a magnetic stirrer. 
The pH was then reduced to 7.0 ad allowed to stir for 4 days. Spheroidal 
particles up to approximately 20 .mu.m were produced together with a 
background of sub-micron entities. The aggregation was irreversible on 
dilution with water. 
EXAMPLE 13 
A dispersion (500 g) was prepared containing 4.0% w/w sodium dextran 
sulphate (BDH, 500,000 mol wt) and 20.6% w/w wilica (Ludox HS40) at pH 9.4 
and maintained dispersed by stirring. After 4 hours the pH was reduced to 
7.9 and after a further 72 the pH was reduced to 7.0. After a further 16 
hours stirring was stopped and the dispersion allowed to settle for 24 
hours. The sediment was recovered, washed (three times) by dispersing in 
distilled water and leaving to settle for 24 hours and then freeze dried. 
Microscopic examination of the freeze dried solid shows it to consist of 
prolate ellipsoids of size up to 70 microns with axial ratios ranging from 
1:1 to 1:6. 
The freeze dried material was divided into three portions which were 
calcined at 400.degree., 600.degree. and 800.degree. C. respectively for 2 
hours. The samples were then examined by mercury porosimetry. 
Calcining at 400.degree. C. produced a material with a porosity of 0.39 
cm.sup.3 g.sup.-1 between 40 and 1000 .ANG., the majority, 0.34 cm.sup.-1, 
between 40 and 100 .ANG.. 
Calcining at 600.degree. C. produced a material with a porosity of 0.37 
cm.sup.3 g.sup.-1, between 40 and 1000 .ANG., with the majority, 0.34 
cm.sup.3 g.sup.-1, between 40 and 100 .ANG.. 
Calcining at 800.degree. C. produced a material with a porosity of 0.33 
cm.sup.3 g.sup.-1, between 40 and 1000 .ANG. with the majority, 0.32 
cm.sup.3 g.sup.-1, between 40 and 250 .ANG.. 
EXAMPLE 14 
A dispersion (426 g) was prepared containing 5.8% w/w sodium dextran 
sulphate (BDH, 500,000 mol wt) and 20% w/w silica (Ludox HS40) at pH 9.6 
and maintained dispersed by stirring. After 4 hours the pH was reduced to 
7.9, and after a further 48 hours readjusted to 7.8. After a further 72 
hours the pH was reduced to 6.8 and after a further 72 hours the stirring 
was stopped and the dispersion was allowed to settle for 24 hours. The 
sediment was recovered, washed (three times) by dispersing in distilled 
water and leaving to settle for 24 hours and freeze dried. 
The resulting solid was sieved and the fraction &gt;106 .mu.m was collected, 
calcined (585.degree. C., 2 hours) and examined by mercury porosimetry. 
The result shows that the material has a porosity of 0.30 cm.sup.3 
g.sup.-1 between 40 and 1000 .ANG. with the majority, 0.27 cm.sup.3 
g.sup.-1, between 40 and 100 .ANG.. 
EXAMPLE 15 
A dispersion (300 g) was prepared containing 4.0% w/w sodium dextran 
sulphate (BDH, 500,000 ml wt) and 20.6% silica (Ludox HS40) at pH 9.6 and 
maintained dispersed by stirring. After 2 hours the pH was reduced to 7.0. 
After a further 16 hours the pH was readjusted to 7.0, stirring was 
stopped, and the sample was diluted with distilled water (approx 300 g) 
and allowed to settle for 72 hours. The sediment was recovered, washed 
(three times) by redispersing in distilled water and leaving to settle for 
24 hours and freeze dried. 
The resulting solid was sieved and the fraction &gt;53 .mu.m was calcined 
(585.degree. C., 2 hours) and examined by mercury porosimetry. The results 
show that the material has a porosity of 0.37 cm.sup.3 g.sup.-1 between 40 
and 1000 .ANG., with the majority, 0.34 cm.sup.3 g.sup.-1, between 40 and 
100 .ANG.. 
EXAMPLE 16 
A dispersion (500 g) was prepared containing 2.0% w/w sodium carboxy methyl 
cellulose (Courtaulds, Courlose F8P, approx 30,000 mol wt) and 16.5% w/w 
silica (Ludox HS40) at pH 9.7 and maintained dispersed by stirring. After 
1 hour the pH was reduced to 7.8 and then to 6.8 after a further 2 hours. 
The dispersion was stirred at this pH for 72 hours, then diluted to approx 
1200 cm.sup.3 with distilled water and allowed to settle. After settling 
for 24 hours the sediment was recovered, washed (twice) by dispersing in 
distilled water and settling for 24 hours, and freeze dried. 
Microscopic examination of the material showed it to consist of prolate 
ellipsoids of size up to approx 130 microns with axial ratios of 1:1.1 to 
1:7. 
The resulting solid was sieved and the fraction &gt;53 microns &gt;106 microns 
was calcined (585.degree. C., 2 hours) and examined by mercury 
porosimetry. The results show that the material has a porosity of 0.43 
cm.sup.3 g.sup.-1 between 40 and 1000 .ANG. with the majority, 0.38 
cm.sup.3 g.sup.-1, between 40 and 100 .ANG.. 
EXAMPLE 17 
A dispersion (300 g) was prepared containing 4.3% w/w sodium dextran 
sulphate (BDH, 500,000 mol wt) and 19.6% w/w silica (Ludox SM) at pH 10.1 
and maintained dispersed by gentle stirring. After 3 hours the pH was 
reduced to 7.8 and then to 6.9 after 48 hours. After a further 24 hours 
stirring was stopped and the dispersion diluted with distilled water 
(approx 200 cm.sup.3) and allowed to settle for 72 hours. The sediment was 
then recovered, washed (five times) by dispersing in distilled water and 
settling for a minimum of 24 hours, and freeze dried. 
Microscopic examination of the freeze dried solid showed it to consist of 
prolate ellipsoids of size up to approximately 75 microns with axial 
ratios of 1:1.5 to 1:6. 
The freeze dried solid was calcined (585.degree. C., 2 hours), examined by 
mercury porosimetry and shown to have a porosity of 0.47 cm.sup.3 g.sup.-1 
for pores between 40 and 1000 .ANG., with the majority, 0.42 cm.sup.3 
g.sup.-1, between 40 and 100 .ANG.. 
EXAMPLE 18 
Equal weights (150 g) of a 30.8% w/w silica dispersion (Ludox TM) and a 
4.0% w/w polyacrylic acid solution (mol wt 230,000, pH 9.7) were mixed and 
gently stirred. After 5 hours the pH of the system was reduced from 9.0 to 
7.8, then to 7.5 after 24 hours, 7.2 after 30 hours and 6.8 after 48 
hours. After a further 5 days the pH had dropped to 6.3, the stirring was 
stopped and the dispersion allowed to settle. After 24 hours the sediment 
was recovered, washed (three times) by dispersing in distilled water and 
leaving to settle for 24 hours, and freeze dried. 
The freeze dried solid was calcined (585.degree. C., 2 hours), examined by 
mercury porosimetry and found to have a porosity of 0.26 cm.sup.3 g.sup.-1 
for pores between 40 and 1000 .ANG., with the majority, 0.22 cm.sup.3 
g.sup.-1, between 40 and 100 .ANG..