Nano-structured silicas and silicates––new materials and their applications in paper
Introduction
Silicas and silicates in their various crystalline macro forms are ubiquitous as minerals and rocks that make up the earth's crust. These are produced by crystallization from silica-containing solutions or melts, or by subsequent geochemical alteration, in accordance with the natural conditions that prevail at the time. Geothermal waters which are cooled at the surface are typically supersaturated in dissolved silica, which progressively precipitates out in an uncontrolled manner, to form amorphous silica sinters. However, by careful control of the precipitation using either a natural (e.g. geothermal water) or a synthetic (e.g. sodium silicate solution) source of silica, it is possible to produce a number of nano-structured silica and silicate materials with specific characteristics, properties and chemical functionality that are not found in nature. This provides the opportunity to produce new functional materials from a commonly available resource [1], [2], [3], [4], [5].
Section snippets
Formation and characterisation
In geothermal water which has a pH = 7–9, the dominate silica species is the silicilate ion H3SiO43− or Si(OH)3O3− which can react with undissociated silicic acid monomers Si(OH)4 or other silicilate ions to produced polymeric hydroxy silica chains which polymerise further to form hydrous silica sol particles of about 4–8 nm diameter.
This process takes place in a number of stages as depicted in Fig. 1. The
Nano-structured calcium silicate
Nano-structured calcium silicate is produced by a proprietary method from geothermal water or a sodium silicate solution. The resulting product is a proprietary material comprising an open framework structure made up of nano-size platelets (Fig. 3), which affords a very large pore volume of about 300–550 g oil. 100 g−1 silicate and a large surface area of about 350–600 m2 g−1 depending upon the synthesis method.
These properties are considerably enhanced over those of the nano-structured silicas.
Conclusion
The unwanted silica deposition in geothermal resource utilisation can be obviated by the controlled precipitation of a nano-structured silica product from the separated geothermal water to provide a product with a network structure that can be used as a filler in newsprint and directory grade paper to reduce print through and enhance the print and optical properties of the paper. A related nano-structured calcium silicate product with an open framework structure can be formed from sodium
References (5)
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- R.T. Harper, J.H. Johnston, S.G. Keyte, T.J. Gresham, The use of geothermal silica to improve newsprint quality at...
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