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Main Index > Raw Material > Water Content
Water Content in Raw (Unexpanded) Perlite
by: Arthur D. Anderson
Water in raw (unexpanded) perlite occurs two main forms,
free water and combined water.
Free water is simply wetness on the surface of the rock. It does little to effect the
expansion process except to make handling more difficult and to consume energy otherwise
needed for expansion.
It is the existance of combined water that gives perlite its ability to expand and become
what some have termed "the most versatile mineral in the world". The water has
two effects: it lowers the softening point of the mineral, and it acts as the blowing
agent which causes the molten rock to expand.
The presence of water is the result of a natural process. Perlite is found at the selvedge
of lava flows, near the original surface, where the lava was able to chill quickly to form
obsidian. In the subsequent years, the action of meteoric water permeating through the
obsideian caused hydration to occur. The amount of water in the hydrated obsidian
(perlite) can vary but is typically less than 4% in most commercial grades.
Experimental work seems to show that there are several different types of bond between the
perlite and the combined water with varying amounts of water being released with different
levels of energy.
Expansion of perlite requires very carefull delivery of heat and then removal of the
particle from the heat zone. The particle must be heated quickly enough so that it becomes
soft enough to expand before the water needed for expansion is driven off. This is most
efficiently accomplished in specially designed furnaces which carry out the process in two
or more stages and which include energy saving recuperation equipment.
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