Geology and Chemistry
Perlite is a generic term for naturally occurring siliceous rock. It is a form of natural glass which like pitchstone, obsidian and other similar contains “combined water”. Perlite (containing 1,9% – 4,8% wt. “chemical” or combined water) was formed over a few million years by the chemical weathering of obsidian at the earth’s surface. Perlite deposits that are considered as exploitable for industrial uses, are distinguished in pumiceous (more frothy and least dense), granular and onionskin.
When dispersed in aqueous suspensions results in neutral pH. The main constituent of perlite is amorphous silica ( SiO2=70-76%) while it also contains lower quantities of aluminum (Al2O3 =12-14%), potassium (K2O=4,4-5,6%), sodium (Na2O=2,6-5,1%), calcium (CaO=0,5-1,0%), iron (Fe2O3=0,4-1,5%) and magnesium (MgO up to 0,4%). The major part of the total alkalis (Na2O+K2O=6-10%) is present in the amorphous matrix forming a solid solution. More than 90% of a commercial perlite is amorphous while the rest consists of minor crystal surfaces like feldspars, mica and quartz.
The distinguishing feature, which sets perlite granules apart from other volcanic glasses, is that when heated rapidly it expands from four to twenty times its original apparent volume. Perlite granules’ expansion is caused due to the presence of the “chemically bound” water (i.e. OH-, Silanol groups SiOH, absorbed H2O associated with exchangeable ions, H2O bound through Hydrogen bonding) in the crude perlite rock. When perlite is quickly heated to above 870°C, it enters the “softening area” and the “bound” water exerts forces that push the granules walls from inside to “pop up” in a manner similar to popcorn as the H2O vaporises and creates countless tiny bubbles which account for the light weight (i.e. very low Loose Bulk Density -LBD) and other exceptional physical properties (e.g. low thermal conductivity). These distinguishing characteristics (Table 1) make expanded granular perlite adaptable for numerous traditional and new applications.
In addition, unexpanded, milled, graded perlite is successfully used in various other industrial sectors. In these applications it is not the expandability of perlite but other chemical, physicochemical and physical properties that play a key-role, such as the significant content of amorphous silica (65-75%), which is chemically very reactive against certain solutions and chemical compounds, the rather high alkalis content, the abrasiveness of the perlite granules, etc.
Table 1: Typical physical properties of expanded perlite
|Free Moisture, Maximum||0.5%|
|pH (of water slurry)||6.5 – 8.0|
|Specific Gravity||2.2 – 2.5|
|Loose Bulk Density (LBD)||30-150 kg/m3|
|Size commercially available||6 mm and finer|
|Specific Heat||387 J/kg·K|
|Thermal Conductivity at 24°C||0.038-0.060 W/m·K|
Technology and Commercial Grades
The production of expanded perlite is a two-stage process. The ore is firstly mined, pre-crushed, ground and graded in the processing plant, which is usually located nearby to the mine site or to the loading/shipping point. The output of such plants is a series of co-produced perlite grades (having grain sizes between 3,5 mm and 0,075 mm. Sized perlite grades are then transported either bulk or in big-bags to the expansion plants hundreds or even thousands of kilometers away to expansion plants. There, sized unexpanded perlite grains are fed in vertical furnaces where they are expanded to form lightweight perlite cellular bubbles. This special granular lightweight product is afterwards mechanically (e.g. through dry or wet mixing) or otherwise incorporated into the various end-products.
Typical Applications of Perlite
1. Construction applications (Thermal-acoustical insulation ceiling tiles, Lightweight concrete, plasters and mortars, Loose fill insulation in cavity walls, etc.)
2. Horticulture applications (Hydroponic cultivation, Growing Media mixed with sand and peat (sphagnum moss)
3. Food and chemical industry (Filter-Aids, Phosphoric Acid production for Fertilizers)
4. “Super-insulating” material (Cryogenics and Low Temperature insulation, Alkali Silicate bonded preformed products, Calcium silicate preformed products,
5. There are various other less important applications, like “topping” in Steel and Foundry industries, high performance micro-fillers for coatings, pozzolan and superpozzolan, abrasive in soaps, cleaners, and polishes, in textile processing industry as fabric ageing material, etc. These latter applications account for <5% of the total perlite market by volume and value.