A Novel Composite Nanostructured Calcium Silicate – Calcium Carbonate Material Produced from Geothermal Water and its Environmental Remediation and Industry Applications

J.H. Johnston, T. Borrmann
Victoria University of Wellington,
New Zealand

Keywords: nanostructured calcium silicate - calcium carbonate, composite material, silica, geothermal, filler, environment, absorbent, plastics, concrete, mine water, remediation

Summary:

We have developed a novel nanostructured calcium silicate – calcium carbonate composite material with excellent whiteness, chemical reactivity and physical absorbent properties, from the problematic silica contained in separated geothermal water. It is an environmentally attractive material produced from a natural silica source with a very low manufacturing energy footprint. Potential applications of this naturally-sourced composite material which we are developing include its use: as a functional filler in tyres, plastics, paper, concrete and thermal insulation; in the recovery of dissolved phosphate from waterways and lakes, and base metals from mine waters; as a general absorbent and soil conditioning agent. Geothermal energy is a significant renewable energy resource worldwide. In geothermal electricity generation, sub-surface superheated geothermal water at about 250-350 oC, saturated in dissolved silica, is piped to the surface where the pressure is reduced and some 30 % of the water is flashed into steam which drives a turbine to generate electricity. The remaining 70 % of water containing the dissolved silica, cools to 120-130 oC and flows through the heat exchangers of a binary cycle turbine to extract further heat energy and generate additional electricity. The cooler water is then reinjected to recharge the reservoir. On cooling, this water becomes supersaturated in silica which precipitates out and blocks pipework, heat exchangers and reinjection wells (Fig. 1). This is a major problem which limits energy recovery and generates high maintenance and operating costs. We have turned this problem into an opportunity by producing a useful nanostructured calcium silicate – calcium carbonate composite material, thereby removing the unwanted silica and enabling more energy to be recovered. Collectively, our approach, technology and novel product offer significant commercial opportunities. We react the supersaturated silica in the water with calcium entities to form a nano-structured calcium silicate material. As geothermal water also contains HCO3-, a nano-structured calcium silicate – calcium carbonate composite product is formed. The calcium silicate comprises nano-size platelets stacked together in a unique open framework structure forming discrete particles of 1-5 µm in size (Fig. 2), giving it a high surface area and high liquid absorbency. These particles do not stick together or onto metal surfaces as silica particles do, but remain suspended in the geothermal water. The useful nanostructured calcium silicate – calcium carbonate composite material is then separated continuously before reinjection. A typical geothermal field can yield some 10,000+ tpy of product. The whiteness, high surface area, high oil absorbency, low solids bulk density, the overall negative surface charge on the particles, and the surface calcium ions, make this composite and attractive material in a variety of functional filler, environment remediation and absorbency uses, as noted above. Its natural source and low energy production footprint are attractive attributes. We are developing the technology at pilot scale operation at a New Zealand geothermal field, and optimising the product characteristics for these uses. We are actively seeking partner companies to further develop and commercialise this novel nanostructured calcium silicate – calcium carbonate material.