Abstract
Representative perlite bulk rock samples from two areas of Milos Island, Greece were collected and the expansion properties of their industrial product were investigated. Coarse crude perlite from Tsigrado exhibits better expansibility, which is assigned to the presence of coarser crystallites in its bulk parent rock. During thermal treatment, the finer crystallites of the coarse crude perlite from Trachilas are entrapped in the groundmass and lead to overheating, which inhibits expansion and eventually results in shrinkage. Geochemical modification of the expanded perlites relative to their crude precursors were investigated, using the isocon method. Volatilisation of crystalline water is the main factor controlling mass reduction of the expanded perlites. Other elements, during the adequate expansion of the Tsigrado perlite, can be classified into three categories. The elements that participate preferentially in crystals decrease in the expanded material at amounts higher than the total mass loss of the rock, due to their escape controlled mainly by the removal of the crystalline phases. The elements equally participating in crystals and the groundmass show losses equivalent to the total mass loss of the rocks, as they escaped in the crystalline phases and airborne particles from the groundmass during thermal treatment. Decrease of highly incompatible elements, which mostly participate in the groundmass, in the expanded products is less than the total mass loss, as they escaped mainly in the airborne particles. The inadequate expansion and burst of the Trachilas perlite did not allow for a similar categorisation, due to random and unpredictable escape of the elements. We propose the application of this method to an artificial system to predict unexpandable mineral phases in bulk perlite, as well as elements that are most likely to participate in the amorphous perlite phase, which cannot be determined from a regular industrial production line. This graphical method may also predict environmental pollution of the atmosphere from the release of volatile compounds and airborne particles during thermal treatment of perlite or other processes of mineral treatment.
Highlights
Perlite is a hydrated natural volcanic glass and is commercially valuable because of its expansion ability from four to 20 times its original volume and its simultaneous reduction in bulk density by up to 90%
Eight perlite samples from Milos Island (Greece) with similar mineralogical and chemical composition, as well as similar physical properties were thermally treated under the same furnace conditions, but they yielded different expansion behaviours
The geochemical modification of perlites after their thermal treatment is investigated as a guide for the prediction of material that escapes in the environment during the expansion process, allowing an assessment of potential environmental consequences
Summary
Perlite is a hydrated natural volcanic glass and is commercially valuable because of its expansion ability from four to 20 times its original volume and its simultaneous reduction in bulk density by up to 90%. Efficient expansion of perlite depends on a complex combination of parameters such as its properties (particle diameter, differences in chemical composition and water content, different sites of water occurrence and surface area per volume ratio of perlite grains) and furnace operating conditions (wall temperature, air feed temperature, and flow rate) [1,2,3]. Eight perlite samples from Milos Island (Greece) with similar mineralogical and chemical composition, as well as similar physical properties were thermally treated under the same furnace conditions, but they yielded different expansion behaviours. The geochemical modification of perlites after their thermal treatment is investigated as a guide for the prediction of material that escapes in the environment during the expansion process, allowing an assessment of potential environmental consequences
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