Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2001-07-31
2004-04-06
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S339000
Reexamination Certificate
active
06716408
ABSTRACT:
This invention concerns a procedure for preparing amorphous silica from calcium silicate.
From the literature, many methods are known for obtaining amorphous silica from various materials. The starting materials used are generally silicates of alkaline metals, preferably sodium, as described for example in W096/30301.
A particularly interesting starting material is calcium silicate.
Natural or artificial calcium silicates are low-cost materials which by means of the reaction of CO
2
in water, allow solid mixtures of SiO
2
and CaCO
3
to be obtained, products with a high added value.
However, the use of calcium silicates instead of silicates of alkaline metals, or particularly instead of sodium silicate, presents considerable problems due to the poor solubility of calcium silicate in water, in comparison with sodium silicate.
An example of a procedure for obtaining silica and compounds of silica and calcium carbonate from calcium silicates is described in patent CA 1,122,779. That patent describes a procedure for obtaining silica, in which the calcium silicate crystals are placed in contact with CO
2
in the presence of water and converted into silica, having the same configuration as crystals of silicate, and into particles of calcium carbonate attached to the particles of amorphous silica. The SiO
2
-CaCO
3
mixture is not separable and so, to recover the silica, a treatment with mineral acids is carried out. The acid decomposes the calcium carbonate into CO
2
and calcium salts. The acid may be HCl, for example, thus obtaining CO
2
and calcium chloride. It is then washed with water to eliminate the calcium salt, thus obtaining an aqueous suspension containing silica. However, with a procedure of this type the quantity of silica obtained is extremely low in comparison with the volumes of liquid involved. There is therefore a low production rate per unit volume, which considerably penalises the process described even though it allows particularly pure silica to be obtained.
Moreover this procedure requires the use of an inorganic acid and it is not possible to recover the CaCO
3
.
The aim of this invention is therefore to develop a procedure for obtaining amorphous silica from calcium silicate which presents high yield and a high production rate, along with high purity of the product.
A further aim of this invention is to obtain a process with a low degree of environmental pollution, since instead of mineral acids it uses CO
2
alone as the acid agent. A further aim of this invention is to recover precipitate calcium carbonate in mild temperature and pressure conditions.
The aim of this invention is therefore a procedure for preparing amorphous silica comprising the following phases:
a) reaction of a calcium silicate with CO
2
in an aqueous environment with the formation of a suspension 1 of agglomerated particles of SiO
2
and CaCO
3
;
b) treatment of the suspension 1 with a compound of aluminium, boron or zinc or mixtures of the same in a neutral or basic environment, and formation of a solid phase 2 in a solution 3 containing particles of SiO
2
with nanometric dimensions;
c) separation of the solid phase 2 from the solution 3; and
d) treatment of the solution 3 according to one of the following methods;
e) precipitation or drying;
f) gelation.
In the description and the claims of this invention the expression “amorphous silica” means non crystalline silica which may be obtained in the form of a gel or in the form of a precipitate.
The reaction of phase a) of the process is carried out in an autoclave at a pressure between 0.3 MPa and 3 MPa and at a temperature between 10° C. and 100° C. More particularly the phase a) is carried out at a pressure between 1.0 and 2.5 MPa and at a temperature between 15 and 40° C. Preferably the pressure is equal to 2 MPa and the temperature is equal to 20° C. or 30° C.
The pH is a function of the CO
2
pressure and, around a pressure of 2 MPa, it stabilises around the value 5.2. The water/calcium silicate weight ratio may have any value, preferably between 16 and 6.4.
In these conditions of temperature, reagent concentration, pressure and pH, the reaction proceeds with the dissolution of the calcium silicate and concludes with the formation of an aqueous suspension 1 of agglomerated particles of SiO
2
and CaCO
3
.
The simultaneous precipitation of SiO
2
and CaCO
3
could be avoided by carrying out the reaction with low loads of calcium silicate (water/calcium silicate weight ratio greater than 16), obtaining a precipitate composed prevalently of CaCO
3
in the presence of a solution containing nanometric particles of silica. In this system, however, the quantity of silica in the solution would be too low and therefore not sufficient for an industrial realisation of the process.
Even if the load of silica were increased, the concentration of silica in the solution would not exceed 10 kg/m
3
(±4); moreover in these experimental conditions this solution is a metastable phase which tends to gel quickly, preventing the separation of the silica from the precipitated solid composed of CaCO
3
.
It is known from the literature that the stabilising of a solution containing amorphous silica in particles is a function of the increase of thee pH up to alkaline values≧9.5 or of the reduction of the concentrations of alkaline ions (R. K. Iler, The Chemistry of Silica, 1979). Alkaline pH values may be easily obtained by adding, for example, NaOH, while the problem of reaching higher concentrations of silica in the aqueous phase is more difficult to solve.
In order to solve this problem, the procedure according to this invention is characterised by the phase b), that is the reaction of the aqueous suspension 1, coming from the acid dissolution, with a compound of aluminium, boron or zinc or mixtures of the same in a neutral or basic environment.
The neutral or basic environment (pH≧7) is preferably realised using a solution of hydroxides or salts of alkaline metals or alkaline earth metals. In particular, the solution is an aqueous solution of hydroxides of alkaline metals or alkaline earth metals and, even more particularly, the solution is an aqueous solution of sodium hydroxide.
The compound of aluminium, boron or zinc is preferably a salt chosen among aluminates, borates or zincates. In particular, the salt is an alkaline aluminate or an alkaline earth aluminate and, even more preferably, the salt is a sodium aluminate.
The treatment in phase b) is decisive for the development of the entire process, being essential for the separation, of silica from carbonate.
This reaction allows the modification of the silica surface and the increase of its concentration in the solution 3 at least up to 50-60 kg/m
3
.
The solution 3 which forms in phase b) contains particles of silica of with nanometric dimensions defined as “primary particles”. In particular the dimensions of the primary particles are between 1 and 100 nanometers.
The third phase c) of the process includes the separation of the solid phase 2 rich in calcium carbonate from the solution 3 containing silica.
The separation phase c) is carried out by means of any of the known methods for phase separations, preferably it is done by centrifugation; if the separated solid mixture still contains silica aggregated with carbonate, it may be recycled and again subjected to the treatment with aluminate.
The fourth phase d) of the process contemplates treatment of the solution 3 containing silica, according to one of the following methods:
according to the first method, indicated as e), the treatment may be carried out preferaby by drying or precipitation, obtaining precipitated silica.
Precipitation may come about by means of variation of the chemical-physical parameters, with or without the addition of precipitating agents.
In particular, precipitation may come about with the addition of CO
2
, at environment pressure and temperature. Afterwards there is the separation of the phases that formed during the precipitation phase. In particular, this separation may be achieved by means of filtrat
Cassar Luigi
De Marco Tiziana
Gronchi Paolo
Hedman & Costigan ,P.C.
Italcementi S.p.A.
Johnson Edward M.
Silverman Stanley S.
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