Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1997-01-07
2004-07-13
Hendrickson, Stuart (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
Reexamination Certificate
active
06761867
ABSTRACT:
The present invention relates to an aqueous suspension of precipitated silica.
Aqueous silica suspensions are employed in very varied fields, and especially the papermaking industry or that of concrete.
For these diverse applications it in advantageous to have suspensions which have a high solids content. However, such suspensions exhibit very high viscosities, and this results in difficulties in pumping and therefore reduces the possibilities of industrial utilization.
Moreover, these suspensions tend to settle or to gel and, as a result, they exhibit an instability which makes then difficult to transport or store. Very frequently, after transportation or after a more or less long period of storage, the formation of a gel is actually observed or the deposition of a hard layer above which there is a slurry which is fluid but low in solids. It is then often impossible to resuspend the silica or to obtain a slurry exhibiting a viscosity which is sufficiently low to make it pumpable and therefore usable industrially.
The aim of the present invention is therefore to provide aqueous silica suspensions of high solids content, exhibiting a low viscosity and a good stability in time.
Thus, more precisely, the subject of the invention is an aqueous suspension of precipitated silica, characterized in that its solids content is between 10 and 40%, its viscosity is lower than 4×10
−2
Pa.s at a shear rate of 50 s
−1
and the quantity of silica present in the supernatant obtained after centrifuging the said suspension at 7500 revolutions per minute for 30 minutes represents more than 50% of the weight of the silica present in the suspension.
The solids content of the said suspension is preferably between 15 and 35% by weight. The viscosity of the said suspension is advantageously lower than 2×10
−2
Pa.s at a shear rate of 50 s
−1
.
The suspension according to the invention is very stable and its stability can be assessed by virtue of a settling test which consists in centrifuging the said suspension at 7500 rev/min for 30 minutes. The quantity of silica present in the supernatant obtained at the end of this centrifuging, measured after drying the supernatant at 160° C. until a constant weight of material is obtained, represents more than 50%, preferably more than 60% of the weight of the silica present in the suspension.
The quantity of silica present in the supernatant obtained after centrifuging advantageously represents more than 70%, in particular more than 90% of the weight of the silica present in the suspension.
Another characteristic of the suspension according to the invention relates to the particle size of the silica particles in suspension.
In fact, concentrated silica suspensions known hitherto, besides their high viscosity, have the disadvantage of comprising large-sized agglomerates in suspension, in particular making injection into a porous medium difficult.
The particle size distribution of the suspended matter can be defined by means of the median diameter D
50
, which is the particle diameter such that 50% of the population of particles in suspension have a smaller diameter.
Similarly, D
95
represents the particle diameter such that 95% of the population of particles in suspension have a smaller diameter.
Another characteristic quantity of the suspensions is the deagglomeration factor F
D
. This factor, which is proportionally higher the more the silical suspension is deagglomerated, is an indication of the fines content, that is to say the content of particles smaller than 0.1 &mgr;m in size, which are not detected by a particle size analyser.
F
D
is measured by introducing into a particle size analyser a known volume V of suspension which is diluted so as to obtain a silica content of 4% by weight, and is equal to the ratio (10×V in ml/(optical concentration detected by the particle size analyser).
The silica agglomerates present in the suspension according to the invention are small in size.
The particle size distribution of the agglomerates in suspension is preferably such that their median diameter D
50
is smaller than 5 &mgr;m and the deagglomeration factor F
D
is greater than 3 ml.
The diameter D
50
is advantageously smaller than 2 &mgr;m, the factor F
D
being greater than 13 ml and, furthermore, the diameter D
95
being smaller than 20 &mgr;m.
The suspension according to the invention preferably includes a filter cake originating from a precipitation reaction (referred to in what follows as “precipitation cake”), which is washed, if need be, and then crumbled.
According to an embodiment of the invention the crumbling is carried out, inter alia, by a chemical route, with the result that the suspension includes aluminium in a quantity such that the Al/SiO
2
weight ratio is between 1000 and 3300 ppm.
The said suspension can be prepared by various processes which are described below and which are also subjects of the present invention.
The synthesis of the silica is performed by a precipitation reaction (A) making use of an alkali metal silicate SiO
2
M
2
O, n being the molar ratio of the silicate, and an acidifying agent. Next, in a stage (B), the precipitation cake in separated from the reaction mixture and then, in a stage (C), is converted into a suspension which has the desired properties.
Any common form of silicates can be employed as silicate for the precipitation reaction, such as meta-silicates, disilicates and advantageously a silicate of an alkali metal M in which M is sodium or potassium.
In the case where sodium silicate is employed, the latter advantageously has an SiO
2
/Na
2
O molar ratio of between 2 and 4, more particularly between 3.0 and 3.7.
The choice of the acidifying agent and of the silicate is made in a way which is well-known per se. It may be recalled that the acidifying agent generally employed is a strong mineral acid such as sulphuric acid, phosphoric acid, nitric acid or hydrochloric acid, or an organic acid such as acetic acid, formic acid or carbonic acid.
In general it is preferred to employ sulphuric acid as acidifying agent and sodium silicate as silicate.
In a first embodiment of the invention the precipitation (A) in carried out in the following manner:
(i) an initial base stock is formed, comprising absorsportion of the total quantity of the alkali metal (M) silicate introduced into the reaction, the silicate concentration expressed as SiO
2
in the said base stock being lower 20 g/l,
(ii) the acidifying agent is added to the said initial base stock until at least 5% of the quantity of M
2
O present in the said initial base stock is neutralized,
(iii) acidifying agent is added to the reaction mixture simultaneously with the remaining quantity of alkali metal silicate such that the ratio (quantity of silica added)/(quantity of silica present in the initial base stock) is between 10 and 100. This latter ratio is called a consolidation ratio.
It has thus been found that a very low silicate concentration, expressed as SiO
2
, in the initial base stock and an appropriate consolidation ratio during the simultaneous addition stage constitute important conditions for endowing the products obtained with their outstanding properties.
In this embodiment the operation is preferably carried out as follows:
a base stock which includes silicate is formed first of all. The quantity of silicate present in this initial base stock advantageously represents only a proportion of the total quantity of silicate introduced into the reaction.
According to an essential characteristic of the process of preparation according to the invention, the silicate concentration in the initial base stock is lower than 20 g of SiO
2
per liter. This concentration is preferably not more than 11 g/l and, possibly, not more than 8 g/l.
The conditions imposed on the silicate concentration in the initial base stock partially determine the characteristics of the silicas obtained.
The initial base stock may include an electrolyte. Nevertheless, preferably, no electrolyte in employed during the process of preparation acc
Frouin Lâürent
Prat Evelyne
Hendrickson Stuart
Rhone-Poulenc Chimie
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