Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PREPARING AMORPHOUS SII.ICA FROM KIMBERLITE
TAILING
Field of the invention
The present invention relates to a process for the preparation of amorphous
silica from
Kimberlite tailings and more particularly the invention relates to the
utilization of silica in
Kimberlite to prepare soluble alkali metal silicates and subsequently
neutralization of it with
acidifying agents viz. mineral acid in dilute form to obtain amorphous powder.
Background of the invention:
Silica is particularly useful in reinforcing elastomers such as rubbers and in
improving
the resistance of rubbers to abrasion. They are also used as a carrier for
dentally active
substances, which are stored at the site of action and then release the active
substance in
small doses over a relatively long period of time (deposition effect,
controlled release). The
silica thus act as active substance stores which contain the active substance
in absorbed,
adsorbed or chemisorbed form. Any form of silica e.g., precipitated silica or
silica gels or
pyrogenic silica, maybe used. Silica are also used in particular for
catalysis, inks and paper,
in the food industry.
In catalysis, silica is used. as a catalyst support, or as a porous layer
coated or
impregnated on monolithic supports. Due to its optical whiteness and opacity,
silica is used as
an inorganic charge in papers and also specialty papers. Silica is used in
paper because of its
porosity, which facilitates ink absorption. More particularly in the field of
animal feeds, silica
is used as a result of its absorption properties as a feed support,
particularly as a support for
methionine, vitamins, particularly vitamin A and E, for sucroglycerides, etc.
It is remarkable that precipitated silica of such widely different structures
and
properties can be made simply by mixing acid and sodium silicate in different
ways at
suitable times and temperature. It requires knowledge or reaction conditions
in hot solution
that leads to the following processes
1. Nucleation of particles
2. Growth of particles to a desired size
3. Coagulation to form aggregates by control of pH and metal ion concentration
4. Reinforcement of the aggregate structure to the desired degree without
further
nucleation.
US Patent 4,243,428 discloses a process wherein precipitated silica is
obtained by
neutralization of a solution of sodium silicate with an acid under conditions
which influence
the properties of the silica finally obtained. The products obtained have a
specific surface
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area ranging between 100 and 600 m2/g, which do not vary upon drying, due to
the action of
a strong acid on an alkaline silicate. The addition of strong acid to the
alkaline silicate is
interrupted one to three times. According to another embodiment of this prior
art, the strong
acid is added to the silicate solution in two phases wherein the temperature
of the second
phase is marginally higher than the first phase and both the phased were
interrupted by a
specific time factor. Also According to another embodiment of this prior art
the temperature
of the reacting medium is varied during two phases in which strong acid is
successively
added to the silicate, separated by one interruption. Another embodiment of
this prior art is
the speed at which the acid is added. According to a preferred embodiment of
this prior
invention to obtain silica having particle size less than about 20 m, the
acid is added to the
silicate at a rate varying according to a law such that the residual
alkalinity and the
concentration of the total silica decrease linearly as a function of the
reaction time, in
accordance with the following equation in which:
Vi (Ci/62 + Ca/98) Ci/62
D=
T(Ci/62+Ca/98-Cixt/62xT)2
D = flow at any time t; Vi = volume of initial silicate and ; Ci = Na20
concentration of the
initial silicate expressed in g / liter; Ca = concentration of the acid used,
expressed in g / liter;
T= total time of the reaction; t= time.
The drawback of this process is that the addition of acid is intermittently
interrupted
for at least three times and this rate of addition is to be controlled by a
number of variables
shown in the equation above. Besides, as per this equation the rate of
addition of an acid is
continuously increased during the course of the neutralization process which
is critical for
obtaining good quality silica. This makes the process complex. Moreover,
control of
temperature at two different phased at variable rate of addition is difficult.
US Patent 4,495,167 discloses a process to prepare precipitated silica having
specific
surface area higher than 400 m2/g, Dibutyl phthalate number higher than 300
and particle size
having particles < 63 m. The process imbibe simultaneous addition of dilute
sodium silicate
and acid to water kept in the reaction vessel under continuous stirring and at
40 to 42 C and
maintaining the pH between 6 to 7. In this process, both the additions are
stopped after 13'h
minutes and the addition interrupted for 90 minutes, and again resumed till
the completion of
the precipitation. The entire reaction mass is then properly dispersed using a
turbine type
shearing device. Prior to filtration, the reaction mass is aged at room
temperature for a period
in the range of 12 - 17 hours. The drawback of the this process is that (i)
the addition rates
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are to be critically controlled to maintain the pH between 6 to 7; (ii) need
of high shearing
and (iii) aging the mass for 12 - 17 hours make the process economically
unviable.
US Patent 5,094,829 discloses a process to prepare reinforced precipitated
silica
having BET specific surface in the range of 220 to 340 m2/g with pore diameter
of ca. 9 to 20
nm. The process involves the neutralization of alkali metal silicate solution
with acid in
multiple stages, viz. (i) initial neutralization of 60% of the total alkali at
ca. 30 to 40 C; (ii)
simultaneous addition of alkali metal silicate solution and acid to neutralize
60% of
equivalent alkali at 80 to 85 C for minimum period of 115 minutes; (iii)
adding acid to lower
the pH below 7 for two times (iv) aging the reaction mixture for at least
three times in the
entire process. The drawback associated with this process is that the multiple
stages of
neutralization, increasing the pH, lowering the pH with acid make the whole
process
complex. Besides, the entire process involves aging of the reaction mixture at
three different
stages for a pretty long time. This makes the process uneconomical.
US Patents'5,034,207 and 5,123,964 disclose processes to prepared silica with
BET
specific surface in the range of 150 to 350 m2/g; bulk density between 60 to
120 g / 1; and
particles where at least 70% particles are from 1 to 6 m. In -this process,
prior to
neutralization of silicate solution, the later is heated to 70 to 80 C and
acid addition was
continued till 50% of alkali present was neutralized and addition of the acid
was interrupted
for 30 to 120 minutes. Remaining acid was added till pH of suspension was
around 3ØThe
entire reaction mass is sheared at high speed for uniform dispersion. The
silica suspension is
optionally diluted with water and the coarse silica was separated by hydro-
cyclone. The
drawback of the process is that the addition of acid is interrupted for a very
long period and
reaction mass requires high shear which may adversely affect economics of the
process.
US Patent 6180076 describes a process to prepare precipitated silica having
BET
specific surface in the range of 120 - 200 m2/g, Dibutyl phthalate index 120 -
300 and some
of the particles are of less than 1 m after degradation of the particles by
ultra-sonication.
Here silica is obtained by the reaction of alkali metal silicate with mineral
acid at temperature
65 to 95 C at pH of 7.0 to 11.0 with continuous stirring, the reaction is
continued up to
solids concentration of 40g/1- 110 g / 1, the final pH is adjusted to a value
between 3 and 5.
The over all reaction is completed in two steps: 1) Addition of water glass
and acid for 15 to
25 minutes followed by interruption of the addition for 30 to 90 minutes. 2)
Addition of water
glass and acid for 50 to 70 minutes. Total reaction time is 130 to 140
minutes. The drawback
associated with this process is the controlled rate of addition to maintain
the pH of the
reaction mixture and the overall reaction time is 2 to 2.5 hours make the
process unattractive.
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US Patent 5,342,598 describes a process wherein the silica particulates are
prepared
by simultaneous addition of sodium silicate and a diluted acid into a
dispersion of colloidal
silica, under continuous agitation. At the completion of the reaction the pH
of the suspension
is between 3 and 7. In this process, colloidal silica is used as a nucleating
agent and is
separately prepared by heating a sodium silicate solution at around 75 C and
adding acid
until the final pH is around 9.5. The drawback of the process is that both the
solutions are to
be added simultaneously and their rates are to be controlled very critically.
Moreover, dilute
colloidal silica is needed for precipitation of silica.
US Patent 5,851,502 discloses that precipitated silica is prepared by is
prepared by
introducing water into a precipitation vessel, adding water glass until an
alkali value is
between 5-15 and simultaneously adding water glass solution and sulfuric acid
until the pH of
the reaction mass is around 8.5, interrupting precipitation for some time and
then further
continuing the neutralization with acid until the pH of the slurry is ca. 4.
The drawback of the
process is that both the solutions are to be added simultaneously and their
rates are to be
controlled very critically and spray drying is required for obtaining desired
product, which
required high energy input.
US Patent 6,214,912 describes a process for the preparation of silica
including the
reaction of a silicate of an alkali metal M with an acidifying agent, whereby
a suspension of
precipitated silica is obtained, and then the separation and the drying of
this suspension, the
said process being characterized in that the precipitation is carried out in
the following
manner: (i) an initial stock is formed comprising a part of the total quantity
of the silicate of
an alkali metal M involved in the reaction, the silicate concentration,
expressed as Si02, in
the said stock being lower than 20 g / 1, (ii) the acidifying agent is added
to the said initial
stock until at least 5% of the quantity of M20 present in the said initial
stock is neutralized;
(iii) acidifying agent is added to the reaction mixture simultaneously with
the remaining
quantity of alkali metal silicate such that the consolidation ratio, that is
to say the ratio of
quantity of silicate added'(expressed as SiO2)/quantity of silicate
present in the initial
stock (expressed as Si02) is between 12 and 100. Throughout the stage (iii)
the added
quantity of acidifying agent is preferably such that 80 to 99%, of the added
quantity of M20
is neutralized. Aging of the reaction mixture for 1 to 60 minutes, after the
above mentioned
simultaneous addition of silicate solution and acid, is carried out. Finally,
after the
precipitation, in a subsequent stage, an additional quantity of acidifying
agent is added to the
reaction mixture. This addition is generally made until a pH value of the
reaction mixture of
between 3 and 6.5, obtained. The drawback of this process is that during
simultaneous
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addition of silicate and acid in stage (iii) in such a way that neutralization
of the M20 is
between 80 to 99 % of the total M20, which makes the process complicated.
US Patent Application No. 20030118500 discloses a process for preparation of
precipitated silica, comprising the reaction of a silicate with an acidifying
agent, whereby a
suspension of precipitated silica is obtained, followed by separation and
drying of this
suspension, characterized in that: the precipitation is carried out in the
following way: (i) an
initial stock solution is formed containing at least some of the total amount
of the silicate
used in the reaction and at least one electrolyte, the concentration of
silicate (expressed as
Si02) in the said initial stock solution being between 50 and 60 g/l, (ii) the
acidifying agent is
added to the said stock solution until a pH of between 7 and 8.5 for the
reaction medium is
obtained, (iii) the acidifying agent is added to the reaction medium along
with, where
appropriate, simultaneously, the remaining amount of the silicate, an
additional amount of
acidifying agent is added to the reaction medium, preferably until a pH of
between 4 and 6 is
obtained in the reaction medium. The separation comprises a filtration and
washing operation
using a filter equipped with a means of compacting, a suspension having a
solids content of
less than 17% by weight is dried by spraying. The drawback of the process is
that both the
solutions are to be added simultaneously and their rates are to be controlled
very critically
and spray drying is required for obtaining desired product, which required
high energy input.
Indian Patent No. 176707 discloses a process for preparation of precipitated
silica.
Precipitated silica is prepared at ambient temperature (using hydrochloric
acid) by preparing
aqueous solution of sodium silicate having 1 to 1.3 N Na+ ion concentration,
adding 14-16%
hydrochloric acid to the solution at constant rate over a period of 10-60 min
under continuous
stirring to bring down pH of resultant mixture to around 10.8, continuing
addition of the same
acid for 3-6 hour to bring down pH between 3-4 to obtain the precipitated
silica, separating,
washing, drying and pulverizing the said separated silica by known methods.
The drawback
of this process is that for complete neutralization of alkali metal at ambient
temperature takes
very long time which makes process uneconomical. Besides the process has a
limitation that
of all the mineral acids, the process is feasible only with hydrochloric acid.
US Patent 6,702,888 describes a process for preparation of precipitated silica
containing aluminum for their application in the reinforcement of the
elastomers. The process
involves addition of an acidifying agent, to a preheated sodium silicate
solution containing
100 g Si02 / liter and less than 17g electrolyte / liter while maintaining
temperature ca. 80 C,
until the pH of the slurry is between 7.0 and 8Ø At this pH value, the
remaining sodium
silicate solution and the acidifying agent are added simultaneously at
controlled rate while
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critically controlling the pH between 7.0 to 8.0 To this slurry diluted acid
is added to bring
the pH less than 7. The silica so obtained is separated by filtration and
washed and filter cake
is re-dispersed in water and to this either organic or inorganic salt of
aluminum (Aluminum
compound A) and a base is added simultaneously or an aluminate of alkali metal
is added and
pH of the reaction mixture is adjusted between 7.2 to 8.6. To this slurry
again an acidifying
agent is added to bring down the pH of the slurry between 3.4 and 4.5 and the
product is
separated by filtration and wet cake is dried to obtain a product. The
drawback of the process
is that silicate and acid solutions are added simultaneously and their rates
of addition are to be
controlled very critically. Moreover, the addition of aluminum compounds after
filtering,
washing and re-dispersing make the process overall complex and complicated.
US Patent 6,468,493 describes a process for the preparation of precipitated
silica. The
preparation is carried out in following way: i) an initial stock solution is
prepared containing
some of the total amount of silicate used in the reaction and at least one
electrolyte. The
silicate concentration in terms of Si02 in the said initial stock solution is
in the range of 40 to
330 g/l and electrolyte concentration is in the range of 12 to 20 g/l.
Acidifying agent is added
to the stock solution until pH value of between 7 and 8.5 is obtained. To this
solution an
acidifying agent and remaining silicate solution is simultaneously added in
such a way that
the pH of the reaction mixture does not change. Subsequently an additional
amount of
acidifying agent is added to the reaction mixture to bring down the pH of the
reaction mixture
is kept in the range of 4.5 to 5.5. The temperature of the reaction medium is
kept between 68
C to 98 C. The drawback of the process is that initial addition of acid to
bring down pH to
below 8.5 is critical as it leads to probability of gel formation. Besides, in
the simultaneous
addition phase the rate are to be critically controlled to maintain the pH.
US Patent 6,800,267 describes a preparation of precipitated silica involving
following
steps: a) heating a mixture of water and sodium silicate at a temperature of
from 70 to 86 C.
and adding sulfuric acid until half of the sodium silicate is neutralized; b)
aging the mixture
for a time of from 30 to 120 minutes; c) adjusting the pH of the mixture with
sulfuric acid to
a range of from 3.0 to 7.0, thereby precipitating the aluminum-doped silica;
d) filtering the
aluminum-doped silica from the mixture to form a filter cake and washing the
filter cake; e) '
drying and/or grinding the washed filter cake, wherein an aluminum salt
solution is metered
into the mixture at step a) and/or step c), the precipitated aluminum-doped
silica has a BET
surface of more than 300 m2 /g, an Al2 O3 content of from 0.05
to 0.25% by
weight, and the aluminum is distributed uniformly in the aluminum-doped
silica. Wherein at
least one or more of steps a), b), and c) are carried out with shearing. The
drawback of the
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process is that it requires shearing, which involves high energy input and
reaction time is
longer than that of present invention which, adversely affect the economy of
the process.
Objects of the invention
The main objective of the invention is to provide a. process for the
preparation of
amorphous silica from Kimberlite tailings which, obviates the drawbacks as
detailed above.
Another object of the invention is to utilize the in-built silica present in
Kimberlite
tailing for the preparation of soluble silicates.
Still another objective of the invention is to use soluble silicate so
prepared in the
preparation of amorphous silica.
Yet another object of the invention is to prepare amorphous silica that is
compatible
for use in rubber, toothpaste, paint and paper industries.
Yet another object of the invention is prepare amorphous silica having bulk
density
from 0.09 to 0.16 gram per cc; average particle size of 10 micrometer; and
surface area of
about 200 to 400 square meter per gram.
Summary of the invention
The aim of the present invention is directed to provide a process for
preparing a free
flowing hydrated amorphous silica having low bulk density; fine particle size
and silica
content greater than 99 % on anhydrous basis. The sodium silicate, prepared by
reacting the
silica present in acid treated Kimberlite with caustic soda, was used as a
source of silica. The
dilute silicate solution was neutralized with an acidifying agent viz. mineral
acid under
controlled conditions of concentration of reactants, temperature, time and
mode of addition.
During the neutralization process the reaction mass was continuously agitated
to avoid the
coagulation of particles. The present process can be carried in an open vessel
and the product
can be easily filtered by conventional techniques, dried and pulverized to
obtain fine powder.
Accordingly, the present invention provides a process for preparing amorphous
silica
from Kimberlite tailings by neutralization of alkali metal present in a sodium
silicate solution
by mineral acid, the sodium silicate being prepared by dissolving silica
present in Kimberlite
tailing, the process comprising:
(i) treating Kimberlite tailings with a mineral acid and washing the acid
treated tailings;
(ii) reacting the acid treated and washed Kimberlite tailings with caustic
soda solution to
obtain a reaction mass;
(iii) filtering reaction mass obtained in step (ii) to obtain sodium silicate
solution as filtrate;
(iv) diluting the sodium silicate solution obtained in step (iii) to obtain a
concentration of
silica in the range of 40 to 125 grams per liter;
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(v) diluting mineral acid to a concentration range of 2 to 15 N for use as an
acidifying
agent;
(vi) adjusting Na+ ion concentration in sodium silicate solution to a range of
0.5 to 1.5
moles per liter;
(vii) heating the diluted sodium silicate solution;
(viii) adding the diluted acid of step (v) to the heated sodium silicate
solution of step (vii) to
partially neutralize alkali in the silicate solution;
(ix) intermittently aging the partially neutralized silicate solution obtained
in step (viii) for a
time ranging from 5 to 20 minutes under stirring; .
(x) adding the remaining acid to complete neutralization of alkali present in
sodium silicate
solution;
(xi) bringing final pH of solution to range of 2 to 6 to completely
precipitate silica;
(xii) filtering slurry the silica precipitate out and washing the obtained
cake till it is free from
sulfate ions, drying and pulverizing the dried cake to obtain fine silica
powder.
In one embodiment of the invention the Kimberlite tailing used has the
following
chemical constituents 30-32% silica; 2-5% A1203; 5-8% Ti02; 8-10% CaO; 20-24%
MgO; 5-
11% Fe203 and loss on ignition in the range of 13-15%.
In another embodiment of the invention, the mineral acid used is selected from
the
group consisting of HCl and HN03 and the acid treatment in step (i) is
effected at a
temperature in the range of 75 to 100 C, in acid concentration range of 1 to
10 N and period
of reaction in the range of 1 to 6 hours.
In yet another embodiment of the invention, the acid treated Kimberlite
tailing is
treated with the sodium hydroxide solution in a concentration range of 6 to
10% at either
boiling temperature in an open vessel or at higher temperature in the range of
95 to 200 C in
a closed vessel and for a period in the range of 1 to 6 hours.
In another embodiment of the invention the alkali treated reaction mass is
filtered to
obtain sodium silicate solution and the residue is disposed off.
In another embodiment of the invention, the concentration of silica in sodium
silicate
solution is varied in the range of 40 to 125 grams per liter.
In another embodiment of the invention, the acidifying agent is a mineral acid
selected from the group consisting of HCl and HNO3 and the concentration
thereof is varied
in the range of 2 to 15 N.
In another embodiment of the invention, the Na+ concentration in the sodium
silicate
solution is varied in the range of 0.5 to 1.5 moles per liter.
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In another embodiment of the invention, the total volume of sodium silicate
solution
is taken in a reaction vessel and heated to a temperature in the range of 50
to 90 C under
continuous stirring.
In another embodiment of the invention, the alkali present in sodium silicate
solution
is partially neutralized in the range of 10 to 40% by addition of diluted acid
at a constant rate
for a time in the range of 5 to 15 minutes at elevated temperature and under
constant stirring.
In another embodiment of the invention, the remaining alkali present in sodium
silicate solution is neutralized by addition of dilute acid for a time in the
range of 20 to 60
minutes, at the same temperature and under constant stirring till pH of final
slurry is in the
rangeof2to6.
In another embodiment of the invention, the aqueous slurry containing
amorphous
silica is filtered, washed with tap water till free from sulphate ions, dried
in air circulated
drier and pulverized to obtain fine powder.
In another embodiment of the invention, the final pH of the slurry is brought
to the
range of 3 to 5 to completely precipitate the silica.
Detailed description of the Invention
In the process of the invention, Kimberlite tailing is treated with mineral
acid at about
100 C to remove acid soluble matter and thereby Kimberlite enriched with
silica is obtained.
This acid treated Kimberlite is digested with caustic soda at 100 C to 190 C
in stainless steel
jacketed reactor with continuous stirring for desired time 3 to 6 hours. The
liquid sodium
silicate is separated by filtration. Sodium silicate thus obtained is diluted
with water to adjust
desired Si02 concentration is the range of 40 to 125 g/1 and desired Na+ ion
concentration in
the solution is adjusted by addition of an electrolyte in the range of 0.5 to
1.5 moles per liter.
This solution is heated to desired temperature in the range of 50 C to 90 C
and neutralized
with 2 to 15 N mineral acid in two steps. In the first step 10 to 40% of the
total alkali present
in the solution is neutralize in 5 to 15 minutes, after this partial
neutralization acid addition is
interrupted for 5 to 20 minutes. In the second stage acid addition is resumed
and final pH of
the solution is adjusted to 2 to 6. Precipitated silica thus obtained is
separated by filtration
and filter cake is washed thoroughly till free from sulfate ions. The product
is dried and
pulverized to obtain fine powder.
Methods of characterization of product
The finished product, precipitated silica prepared using Sulfuric acid and
sodium
silicate is characterized for bulk density, pH, Oil absorption, Water
absorption, specific
surface area and particle size analysis.
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Determination of bulk density
The method prescribed by the Indian Standard Specification (10) for 'Light
Basic
Magnesium Carbonate for Rubber Industry' was adopted to determine the bulk
density. The
apparatus consists of wooden stand, a measuring cylinder with rubber cork. The
distance
between zero and 250 ml graduation mark is 200mm. The height of the free fall
of the
cylinder was 25 mm. A pre-weighed quantity (W) of the sieve and dried sample
is gently
slipped in to cylinder. The cylinder with the rubber cork is then assembled in
to the wooden
stand. The cylinder is gently raised to a height of 25 mm in such a way that
it did not dash
with the upper stop and then released smoothly. The timing of rising and
releasing were
adjusted so that one tap is given at every two seconds. Total 50 taps are
given. The volume
(V) is noted and bulk density calculated as under;
Bulk density (g/cc) = Weight of powder (W)/Volume of Powder (V)
Determination of pH
Exactly 5.0 g of thoroughly mixed, oven-dried sample is taken in a 150m1
capacity
beaker. To this 100 ml of distilled water free from dissolved carbon dioxide,
cations and
anions is added. The powder is uniformly dispersed by means of glass rod. The
pH meter is
standardized with standard buffer solution and pH of the silica slurry
prepared is measured.
Determination of Oil absorption
The apparatus consists of a glass plate, 6 inches by 6 inches fixed in a
wooden frame
and sharp edged stainless steel spatula (2 inches width) with handle.
Accurately weighed 5.Og of sample is placed on the plate. A burette is filled
with
double boiled linseed oil. Oil is added drop by drop into the powder and mixed
with powder
by means of a spatula. Test is completed when oil mixed with powder produces a
stiff, putty
like paste, which does not break or separate. Volume of oil added is noted.
From the density
of oil, the weight of oil is calculated. From this, the percentage oil
absorption is calculated.
Determination of Water absorption
Known weight of sample (- 5.0g) is taken in clean dried 250.0 cm3 beaker. To
this
distilled water from burette is added drop wise and mixed with powder by means
of spatula.
Test is completed when water mixed with powder produces, a stiff, putty like
paste, which
does not break or separate. The volume of water added is noted. From the
density of water,
the weight of water is calculated. From this, the percentage water absorption
is calculated.
Determination of Specific Surface area
Specific surface area is determined using Surface area analyzer by Brunauer,
Emmet
and Teller method described in Journal of American Chemical Society, Volume
60, page 309
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(1938). The method involves measurement of nitrogen adsorption at liquid
nitrogen
temperature at different partial pressure of the nitrogen and using BET
equation monolayer
capacity is calculated and from that specific surface area is calculated.
Determination of Particle size
Particle size of the product is determined by laser -diffraction technique.
The dry
dispersion method was employed for the determination of particle size using
Mastersizer
2000 (Malvern Instrument Ltd., Malvern UK).
Product Specifications
Properties Invention Product Sipernat 22 LS
De ssa
BET surface area (m2/g) 195 190
Bulk density (g/cc) 0.09 0.08
Moisture (%) (Drying loss 2h at 4.2 6.0
105 C
Loss on ignition (2h at 1000 C (%) 5.2 5.0
Oil absorption value 100 230
DBP absorption value 100 - 270
pH value (5% aqueous dispersion) 6.3 6.3
Free flowing hydrated amorphous silica in accordance with the invention, can
be
prepared from silicon dioxide (SiO2) in-built in Kimberlite tailings, which is
waste in
diamond mining. Prior to alkali (caustic soda) treatment, Kimberlite - tailing
is treated with
mineral acid to dissolve acid soluble salts. Soluble alkali metal silicate so
formed is filtered to
remove insoluble impurities. Alkali (Na20) in diluted metal silicate solution
is neutralized
with a mineral acid under a wide range of variables as described in the text.
Silica so prepared
is filtered, washed, dried and pulverized by conventional techniques. The
prior art does not
divulge nor teach how silica can be prepared from Kimberlite - tailing. It is
reported for the
first time in the present invention how the in-built silicon dioxide from
Kimberlite - tailing,
can be extracted and utilized for preparing amorphous silica. The invented
steps adopted in
the present invention are (i) the simple neutralization steps dispense the
multi stage
neutralization; (ii) the pH of the reaction mass is maintained on. alkaline
side until 95% of
alkali is neutralized; (iii) the process of neutralization is interrupted for
a short period. This
obviates the need of very long interruption time; (iv) the complete
neutralization of alkali is
achieved in ca. 65 to 70 minutes. Long reaction time is adversely affect the
economics of the
process; (v) the time of adding of,mineral acid only to be controlled and
regulated. In the
simultaneous addition of acid and silicate solution, the rate of addition of
both the reactant
solutions and pH of the reaction mass are critically controlled.
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The following examples are given by way of illustration and therefore should
not be
constructed to limit the scope of the present invention.
Example-I
2.7 kg of Kimberlite was ti-eated with 1:1 hydrochloric acid: water mixture at
95-
100 C under continuous stirring keeping solid to liquid ratio 1:4 for 3 hrs,
then the slurry was
filtered and washed with tap water till wet solid cake becomes free from acid.
The cake was
dried in oven at 100-110 C and is used for preparation of sodium silicate. For
preparation of
sodium silicate 0.64 kg of an alkali was dissolved in 8 lit of water and taken
in to stainless
steel autoclave. To this 2 kg of acid treated Kimberlite was added under
continuous stirring to
prepare homogenous slurry. Reactor was heated for 1 to 3.5 hours at boiling
temperature.
After completion of reaction, slurry was filtered with vacuum filter. Filtrate
was analyzed for
SiO2 and Na20. SiO2/NazO mole ratio was 3- 3.6 depending on time of digestion
with alkali.
Examples 2
199g of a of sodium silicate containing Si02 =17.66%, Na20 = 5.64%; SiO2/Na2O
with modulus of 3.24, was introduced into a stainless steel reactor equipped
with a pitch
blade stirrer and diluted to 640 ml to adjust the silica concentration of 55g
per liter. The Na+
ion concentration of this solution was adjusted to 22.3g per liter by adding
soluble sodium
salt. Prior to initial neutralization process, solution of sodium silicate was
heated and
temperature was raised to 70 C and 2.3 N sulfuric acid was added at controlled
rate of 6.2m1
per minute for the period of 5 minutes. After 5 minutes, acid addition was
interrupted for 10
minutes. Then remaining acid of same concentration was added at rate of 3.6
ml/minute till
pH of reaction mixture is 4. The slurry is then agitated for another 5
minutes, filtered, washed
till free from adhering anions, dried in air circulated oven and comminuted to
fine size. The
properties of the silica are (i) average particle size = 10.25 M; (ii) BET
Specific surface area
= 380 m2/g; (iii) bulk density = 0.1 g/cc; (iv) oil absorption capacity = 225
g/100g; (iv)
moisture = 4.0 %; (v) loss on ignition = 6.0 % and (vi) pH (5% aqueous
suspension) = 7.3.
Example - 3
201 g of a of sodium silicate containing Si02 =13.9 %, Na20 = 4.78 %; Si02
/Na20
having modulus of 3.2, was introduced into a stainless steel reactor equipped
with a pitch
blade stirrer and diluted to 466 ml to adjust the silica concentration of 60
g/liter. The Na+ ion
concentration of this solution was adjusted to 22.7 g/liter by adding soluble
sodium salt. Prior
to initial neutralization process, solution of sodium silicate was heated and
temperature was
raised to 70 C and 2.39 N sulfuric acid was added at controlled rate of 6.5
ml per minute for
a period of 5 minutes. After 5 min acid addition was interrupted for 10
minutes. Then
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remaining acid of same concentration was added at the rate of 2.8 ml/minute
till pH of
reaction mixture is 4. Slurry is then agitated for another 5 minutes,
filtered, washed till free
from adhering anions, dried in air circulated oven and comminuted to fine
size. Properties of
the silica are (i) bulk density = 0.13 g/cc; (ii) oil absorption capacity =
251 g/100g; (iii)
moisture = 7.0 %; (iv) loss on ignition = 5.0 % and (v) pH (5% aqueous
suspension) = 7Ø
Example - 4
25kg of sodium silicate containing Si02=14.63%, Na20 = 4.77%; Si02/Na2O
modulus
of 3.2, was introduced into stainless steel reactor equipped with pitch blade
stirrer and diluted
to 81.3 liters to adjust silica concentration to 45 g/liter. Na+ ion
concentration of this solution
was adjusted to 20.0 g/liter by adding soluble sodium salt. Prior to initial
neutralization
process, solution of sodium silicate was heated and temperature was raised to
70 C and 2.47
N sulfuric acid was added at controlled rate of 550 ml/minute for a period of
5 minutes.
After 5 min acid addition was interrupted for 10 minutes. Then remaining acid
of the same
concentration was added at the rate of 360 ml per minute until the pH of the
reaction mixture
is 4. The slurry is then agitated for another 5 minutes, filtered, washed till
free from adhering
anions, dried in air circulated oven and comminuted to fine size. The
properties of the silica
are (i) BET Specific surface area = 290 m2/g; (ii) bulk density = 0.14 g/cc;
(iii) oil absorption
capacity = 190 g/100g; (iv) pH (5% aqueous suspension) = 7.5 (v) moisture =
5.7
Example - 5
2.84 kg of a of sodium silicate containing Si02 =12.92 %, Na20 = 3.7 %; Si02
/Na20
having modulus of 3.6, was introduced into a stainless steel reactor equipped
with a pitch
blade stirrer and diluted to 3.5 liter to adjust the silica concentration of
105 g per liter. The
Na+ ion concentration of this solution was adjusted to 22.3 g per liter with
the addition of
soluble sodium salt. Prior to initial neutralization process, the solution of
sodium silicate was
heated and temperature was raised to 80 C and 5.1 N sulfuric acid was added
at controlled
rate of 11.6 ml per minute for the period of 10 minutes. After 10 min acid
addition was
interrupted for 8 minutes. Then remaining acid of the same concentration was
added at the
rate of 20.4 ml per minute until the pH of the reaction mixture is 4. The
slurry is then agitated
for another 5 minutes, filtered, washed till free from adhering anions, dried
in air circulated
oven and comminuted to fine size. The product having 0.11 cc/g of bulk density
and 195
g/100 g of oil absorption capacity was obtained.
Example - 6
71 kg of a of sodium silicate containing Si02 =9.35 %, Na20 = 3.27 %; Si02
/Na20
having modulus of 3.6, was introduced into a stainless steel reactor equipped
with a pitch
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blade stirrer and diluted to 70 liter to adjust the silica concentration of 95
g per liter. The Na+
ion concentration of this solution was adjusted to 24.6 g per liter with the
addition of soluble
sodium salt. Prior to initial neutralization process, the solution of sodium
silicate was heated
and temperature was raised to 78 C and 5.1 N sulfuric acid was added at
controlled rate of
440 ml per minute for the period of 10 minutes. After 10 min acid addition was
interrupted
for 10 minutes. Then remaining acid of the same concentration was added at the
rate of 456
ml per minute until the pH of the reaction mixture is 4. The slurry is then
agitated for another
5 minutes, filtered, washed till free from adhering anions, dried in air
circulated oven and
comminuted to fine size. The properties of the silica are (i) bulk density =
0.1 g/cc; (ii) oil
absorption capacity = 225 g/100g; (iii) moisture =5. 4%; and (iv) loss on
ignition = 6.5 %.
Ezample - 7
201 g of a of sodium silicate containing Si02 =13.9 %, Na20 = 4.78 %; Si02
/Na2O
having modulus of 3.2, was introduced into a stainless steel reactor equipped
with a pitch
blade stirrer and diluted to 622 ml to adjust the silica concentration of 45 g
per liter. The Na+
ion concentration of this solution was adjusted to 20.0 g per liter with the
addition of soluble
sodium salt. Prior to initial neutralization process, the solution of sodium
silicate was heated
and temperature was raised to 90 C and 2.39 N sulfuric acid was added at
controlled rate of
5.7 ml per minute for the period of 5 minutes. After 5 min acid addition was
interrupted for
10 minutes. Then remaining acid of the same concentration was added at the
rate of 2.8 ml
per minute until the pH of the reaction mixture is 4. The slurry is then
agitated for another 5
minutes, filtered, washed till free from adhering anions, dried in air
circulated oven and
comminuted to fine size. The properties of the silica are (i) bulk density =
0.1 g/cc, (ii) oil
absorption capacity = 240 g/lOOg and pH of 5% aqueous suspension = 7.3.
Example - 8
2.84 kg of a of sodium silicate containing SiO2 =12.92 %, Na20 = 3.7 %; Si02
/Na20
having modulus of 3.6, was introduced into a stainless steel reactor equipped
with a pitch
blade stirrer and diluted to 3.5 liter to adjust the silica concentration of
105 g per liter. The
Na+ ion concentration of this solution was adjusted to 22.3 g per liter with
the addition of
soluble sodium salt. Prior to initial neutralization process, the solution of
sodium silicate was
heated and temperature was raised to 80 C and 5.1 N sulfuric acid was added
at controlled
rate of 16 ml per minute for the period of 10 minutes. After 10 min acid
addition was
interrupted for 5 minutes. Then remaining acid of the same concentration was
added at the
rate of 19 ml per minute until the pH of the reaction mixture is 4. The slurry
is then agitated
for another 5 minutes, filtered, washed till free from adhering anions, dried
in air circulated
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oven and comminuted to fine size. The properties of the product are (i) bulk
density = 0.15
g/cc and oil absorption capacity = 230 g/l00g
Example - 9
2.84 kg of a of sodium silicate containing Si02 =12.92 %, Na20 = 3.7 %; Si02
/Na2O
having modulus of 3.6, was introduced into a stainless steel reactor equipped
with a pitch
blade stirrer and diluted to 3.5 liter to adjust the silica concentration of
105 g per liter. The
Na+ ion concentration of this solution was adjusted to 22.3 g per liter with
the addition of
soluble sodium salt. Prior to initial neutralization process, the solution of
sodium silicate was
heated and temperature was raised to 80 C and 5.1 N sulfuric acid was added
at controlled
rate of 9.3 ml per minute for the period of 15 minutes. After 15 min acid
addition was
interrupted for 8 minutes. Then remaining acid of the same concentration was
added at the
rate of 20 ml per minute until the pH of the reaction mixture is 4. The slurry
is then agitated
for another 5 minutes, filtered, washed till free from adhering anions, dried
in air circulated
oven and comminuted to fine size. The properties of the product are (i) bulk
density = 0.13
g/cc and oil absorption capacity = 186 g/100g.
Example - 10
3.2 kg of a of sodium silicate containing Si02 =9.35 %, Na20 = 3.27 %; Si02
/Na20
having modulus of 3.6, was introduced into a stainless steel reactor equipped
with a pitch
blade stirrer and diluted to 3.15 liter to adjust the silica concentration of
95 g per liter. The
Na+ ion concentration of this solution was adjusted to 24.6 g per liter with
the addition of
soluble sodium salt. Prior to initial neutralization process, the solution of
sodium silicate was
heated and temperature was raised to 78 C and 5.35 N sulfuric acid was added
at controlled
rate of 19.4 ml per minute for the period of 10 minutes. After 10 min acid
addition was
interrupted for 10 minutes. Then remaining acid of the same concentration was
added at the
rate of 20.0 ml per minute until the pH of the reaction mixture is 4. The
slurry is then agitated
for another 5 minutes, filtered, washed till free from adhering anions, dried
in air circulated
oven and comminuted to fine size. The properties of the silica are (i) BET
Specific surface
area = 195 m2/g (ii) bulk density = 0.09 g/cc; (iii) oil absorption capacity =
230 g/100g; (iv)
moisture = 4.2 %; and (v) loss on ignition = 5.2 % (vi) pH=6.3.
Advantages of the present invention
1. Kimberlite tailings - a waste of diamond mining is utilized for the
preparation of
amorphous silica. It alleviates the disposal problem of Kiniberlite.
2. The in-built silica in Kimberlite can be easily made soluble in alkali
solution at boiling
temperature and atmospheric pressure.
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WO 2006/070400 PCT/IN2004/000437
3. The invention alleviates the precise control of pH of the reaction mass
during the
simultaneous addition of acid and alkali metal silicate solutions.
4. Simple neutralization procedure is adopted wherein the rate of addition of
diluted acid is
only to be controlled.
5. The process dispenses the need of linger aging period.
6. Neutralization of alkali present in alkali metal silicate solution with
mineral acid can be
completed in short duration.
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