New Products from Coal Combustion Ash: Selective Extraction of Particles with Density < 2

Spherical and hollow fly ash particles having density < 1g/cm are known as cenospheres. Their concentration in coal combustion ash is typically << 1%; as a consequence, their value is relatively high (~$0.70/kg). However, cenospheres are really particles containing gas bubbles independent of their density. Hence, cenosphere densities can vary between 0.6 g/cm to near or greater than 2.0 g/cm. They can constitute up to 80% of some fly ashes, but there is no economic way of selectively extracting them. We report on the selective extraction of cenospheres, defined as particles having bulk densities < 2 gm/cm, from combustion fly ashes by the use of a specially-designed, pneumatic transport, triboelectric separation system. Processing at feed rates up to 20 kg/hr, the concentration of low density products were measured by float-sink analysis plus centrifugation. The float-sink media included distilled water and lithium metatungstate, the densities of which were varied between 1.0 gm/cm-to-2.0 gm/cm. Pneumatic transport, triboelectric separation tests to optimize product selectivity are described. The products were analyzed by optical and scanning electron microscopy, He pycnometry and laser granulometry. Depending on the ash, an overall product density of near 1.6 gm/cm could be obtained. They would be useful for creating lighter-weight ceramics and building materials, and for specialized coatings. Introduction Although dependent on the size of the coal injected into the combustor, the type of combustor and coal-firing technology, the mineral matter content of the coal and other operation and coal parameters, typical fly ash particle diameters are between 0-300 ìm (1) These particles contain a myriad of constituents which, if efficiently and economically extracted, may increase significantly the commercial applications of combustion ash. Cenospheres have high intrinsic values and many known applications (2-3). Cenospheres are formed from the ash when it is in a molten state. In this state, a spherical shape is formed because it minimizes surface tension. Flowing with the combustion gas stream, their temperature is rapidly quenched, thereby ‘freezing in’ the spherical shape. Any gas bubbles within the molten particles are also trapped inside the spheres. These bubbles cause the production of cenospheres; bubbles may occur in multiple forms within the ‘frozen’ particles, or as single, concentric forms that are nearly as great as the diameter of the particles. The thickness of cenosphere walls may be very small eg < 10% of the particle diameter and, if so, the resultant bulk densities are less than 1 gm/cm. These particles float on water within wet ash impoundments and their harvesting can be accomplished by pond skimming. Traditionally, the word cenosphere has been identified with these <1 gm/cm particles. However, the real meaning of cenosphere entails particles that have gas bubbles incorporated within their structure. In general, it is known that the relative amount of <1 gm/cm cenospheres in combustion fly ashes is around 1%. However, there is actually considerably higher concentrations of cenospheres in combustion ash. It has been determined (1) that, for fly ash produced during the combustion of Kentucky No. 9 coal, the concentration of cenospheres having bulk densities less than 2 gm/cm is about 9%; for fly ash from San Miguel coal, the concentration of cenospheres with densities less than 2 gm/cm was almost 87%. Of course, these values depend on parameters other than just the type of coal combusted. These data suggest significant opportunities for technology application if selective extraction of cenospheres could be accomplished efficiently.