Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2002-07-01
2004-01-06
Fortuna, Jose A (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S158000, C162S181200, C162S164100, C162S175000, C162S168300
Reexamination Certificate
active
06673208
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to silicate- and silica-based materials. More particularly, the invention relates to polysilicate microgels, their preparation and use in paper making and water purification. The invention further relates to silica-based materials, their preparation and use as precursors for polysilicate microgels.
BACKGROUND OF THE INVENTION
Polysilicate microgels, optionally aluminated, are known in the art as drainage and retention aids in the manufacture of paper and similar cellulosic products. The microgels comprise solutions or dispersions of very small primary silica-based particles, usually with a size of from 1 to 2 nm in diameter and having a high specific surface area, typically at least about 1000 m
2
/g, which are linked together into individual chains to form three-dimensional network structures.
The preparation of polysilicate and aluminated polysilicate microgels generally comprises acidifying a dilute aqueous solution of alkali metal silicate by using an acid or acid ion-exchanger, ageing the acidified mixture and then further diluting the aged mixture to a silica concentration of not greater than 2% by weight. The silica-based microgels normally have poor stability and the high dilution is normally necessary to avoid gelation of the microgels. Because of the stability problems associated with these products, and the prohibitive cost of shipping stable, but extremely dilute, solutions containing about 0.5% by weight or less of silica, the polysilicate microgels are preferably prepared at the location of intended use, for example at the paper mill. Production units or generators for continuously preparing polysilicate microgels that are installable at the paper mill are known in the art. Hereby the microgels obtained can be prepared and continuously introduced into the stock containing cellulosic fibres and filler to be drained. However, any disturbance in the production unit, for example variations in quality and/or quantity of the microgel produced, will change the drainage and retention performance of the product which may adversely affect the paper making process, thereby producing cellulosic products of uneven quality.
SUMMARY OF THE INVENTION
The present invention generally relates to polysilicate microgels and their use in paper making and water purification. The invention also relates to a process for the preparation of aqueous polysilicate microgels which comprises bringing into contact an ion A with a protonic acid comprising an ion B in the presence of an aqueous silicate or polysilicate solution, whereby the ions A and B represent ions of a precipitable salt AB having a pK
S
value of at least 4, measured at 20° C. in water.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention it has been found that polysilicate microgels can be prepared in a very advantageous manner by bringing into contact particles of a salt AB with an aqueous silicate or polysilicate solution, or ions of A with ions of B in the presence of an aqueous silicate or polysilicate solution, the salt AB representing a precipitable salt, whereby AB can be formed and precipitated in the aqueous phase and the contacting of the salt AB or ions A and B with the silicate or polysilicate solution favours polysilicate microgel formation. The process can be carried out by mixing the ions A and B and the silicate or polysilicate solution as three separate components; it is also possible to carry out the process by incorporating the ion A into the silicate or polysilicate solution and mixing the solution with the ion B, or by incorporating the ion B into the silicate or polysilicate solution and mixing the solution with the ion A, whereby the salt AB can be formed and precipitated in the aqueous phase and favours polysilicate microgel formation. More specifically, this invention relates to a process for preparing polysilicate microgels which comprises bringing into contact particles of a salt AB with an aqueous silicate or polysilicate solution, or an ion A with an ion B in the presence of an aqueous silicate or polysilicate solution, whereby the salt AB is precipitating in the aqueous phase, and/or the ions A and B represent ions of a precipitable salt AB having a pK
S
value of at least 4, measured at 20° C. in water. The invention thus relates to a process for preparing polysilicate microgels, polysilicate microgels per se and their use, as further defined in the claims.
According to the present invention there is provided a process for preparing polysilicate microgels employing high-concentration silicate and polysilicate solutions, i.e. solutions containing SiO
2
. According to the invention there is also provided high-concentration silicate and polysilicate solutions containing an ion of a precipitable salt. The high-concentration silicate and polysilicate solutions act as precursors for polysilicate microgels. The silicate and polysilicate solutions, optionally containing an ion of a precipitable salt, i.e. polysilicate microgel precursors, of this invention exhibit high stability and can be easily prepared and shipped at considerably higher SiO
2
concentrations compared to previously known polysilicate microgels. The high-concentration silica-based solutions of this invention can be prepared under controlled conditions in a plant intended for such production and shipped as a concentrated product to the paper mill in an economically attractive manner. By simply mixing particles of AB or ions A and B of a precipitable salt AB with the silicate or polysilicate solution, or by adding the counter-ion of the precipitable salt, B, for example in the form of an aqueous solution, to the high-concentration product containing the ion A, or vice versa, polysilicate microgels can be formed which are ready for use, for example as drainage and retention aids in papermaking. In this application, a storage tank for the high-concentration polysilicate microgel precursor can be installed at the location of intended use, which is more attractive economically than installing a complete polysilicate microgel production unit or generator. Hereby the present invention offers substantial technical and economic benefits. In addition, the high-concentration product containing the ion A of this invention can be added to a suspension containing the desired counter-ion and cellulosic fibres, for example white water to be recirculated in the paper making process, where polysilicate microgels can be formed while recirculating the white water to the fibre suspension to be drained. This application using the high-concentration silica-based material of this invention as a precursor for in situ formation of polysilicate microgels represents a significant progress in the art.
The aqueous solution of silicate or polysilicate to be used in the process of this invention can be selected from any water-soluble silicate salt such as sodium or potassium silicate, or sodium or potassium water glass. These are available with varying molar ratios of SiO
2
to Na
2
O or K
2
O and the molar ratio is usually within the range of from 1.5:1 to 4.5:1, most often from about 2.5:1 to 3.9:1. The alkali metal silicate preferably is a sodium silicate. Aqueous solutions of alkali metal silicate are alkaline and usually have a pH of about 13 or above 13. The alkali metal silicate solution usually has a SiO
2
content of at least 1% by weight and usually the SiO
2
content is within the range of from 5 to 35% by weight, suitably above 10% and preferably within the range of from 15 to 30% by weight.
The aqueous solution of silicate or polysilicate to be used in the process of this invention can also be selected from any water-soluble polysilicate such as sodium or potassium polysilicate, preferably a sodium polysilicate. Water-soluble polysilicates are known in the art. The polysilicate can have a high molar ratio SiO
2
:Na
2
O, for example within the range of from 3:1 to 50:1, suitably from 4:1 to 30:1 and preferably from 4.5:1 or from 5:1 to 20:1. The polysilicate can have
Persson Michael
Sikkar Rein
Tokarz Marek Tadeusz
Akzo Nobel N.V.
Burke Michelle J.
Fortuna José A
Parker Lainie E.
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