Hollow silica-alumina microspheres (cenospheres) are recovered from fly ashes formed at coal-fired thermoelectric power stations (TEPS). The properties of microspheres recovered from fly ashes are similar to microspheres industrially produced from melts. It is of great importance that hollow cenospheres recovered from TEPS ashes are much (several fold) cheaper than industrially produced ones. The shape of cenospheres is almost spherical, their surface is smooth, and their diameter is from 5 to 500 mcm. They contain gaseous phase including mostly nitrogen, oxygen, and carbon oxide. Cenospheres are characterized by a combination of unique properties: low density, spherical shape, high melting point and hardness, and are chemically inert, which considerably improves workability of cenospheres and their use in a wide variety of modern industrial applications.
The spherical shape means that it takes less resin, binder, water etc to wet out the surface of cenospheres than any other shaped filler. This results in low resin or binder demand which in turn allows high solids formulations, lower shrinkage and often lower cost.
Spherical fillers exhibit excellent flow and are easy to spray, pump, trowel etc. Spherical fillers also reduce shrinkage not only because they allow lower binder contents but directly as a result of their shape. At high concentrations, the spheres are packed together but find it difficult to get closer together in the way that irregular shaped fillers can during shrinkage of the binder, evaporation of solvent or water. Spheres therefore help maintain the volume of the original product and as such are excellent fillers for crack and joint compounds, sealants etc.
The advantages of low density are quite obvious and clear. At 0.7 g/cc cenospheres are approximately 25 % of the density of other mineral fillers but are also strong enough to withstand most mixing, compounding and processing activities.
The building industry all over the world is moving more and more to lightweight products. Low density results in ease of application and use, easier mixing, lower transport costs, low slump and sag, reduced worker fatigue, easier sanding, easier cutting and drilling.
Silica-alumina spheres inherently have very low reactivity. Their chemical composition makes them very resistant to acids and alkalis. They are neutral in pH and do not interfere in the chemistry or reaction of the products they are used in.
The free flowing nature of cenospheres means that they are easy to handle in a factory environment. They can be easily gravity fed without blockage and can be pumped or pneumatically transported in a dry form.
Cenospheres have a low thermal conductivity of 0.11 W*m-1K-1. As a result they have found applications in insulating refractories, oil pipeline insulation, geothermal cements, external wall insulation stuccos and many other applications where good thermal insulation is required.
Cenospheres have a high melting point of 1200 - 1600 °C, which is considerably higher than synthetic glass microspheres. They are therefore excellent for use in high temperature insulating refractories and fire resistant coatings and panels.
The hard surface of cenospheres provides excellent erosion resistance. The glassy shell of the cenospheres is totally impermeable to liquids and gases.
Oil Manufacturing: oil well cements, drilling muds, grinding materials, explosives
Construction: specialty cements, mortars, grouts, stucco, roofing materials, acoustical panels, coatings, shotcrete, gunite
Ceramics: fire-proof materials, fire bricks, coatings, insulating materials
Plastics: nylon, polyethylene, polypropylene and other materials of diffirent densities
Automotive: composites, engine parts, sound proofing materials, undercoatings