What is Perlite?

Perlite is not a trade name but a generic term for naturally occurring siliceous volcanic rock. The distinguishing feature which sets perlite apart from other volcanic glasses is that when heated to a suitable point in its softening range, it expands four to twenty times its original volume.

This expansion is due to the presence of two to six percent combined water in the crude perlite rock. When quickly heated to above 1600° F (870 C) the crude rock pops in a manner similar to popcorn as the combined water vaporizes and creates countless tiny bubbles in the heat softened glassy particles. It is these tiny glass-sealed bubbles which account for the excellent insulating properties and lightweight of expanded perlite.

Expanded perlite can be manufactured to weigh from 2 lb/ft³ (32 kg/m³) to 15 lb/ft³ (240 kg/m³) making it especially suitable for use in insulation applications. Perlite is used in the manufacture of high temperature and cryogenic insulation, lightweight perlite insulating concrete, insulating board, insulating plasters, masonry wall insulation and as underfloor insulation.

SERVICE TO 2000 ⁰F (1100⁰)C

High temperature performance of expanded perlite insulation has been well documented over a period of many years. The data in Figure 1 (not yet on web page) details the thermal conductivity for various density perlite insulations up to 1800°F (980°C). Although thermal conductivity increases appreciably above 1800°F (980°C), expanded perlite has been used at service temperatures as high as 2000°F (1100°C).

Perlite in ladle topping applications allows easy removal of slag and helps maintain the temperature of the molten metal in the ladle.

HIGH TEMPERATURE APPLICATIONS

Perlite insulation is used in high temperature applications in the steel and foundry industries such as ladle topping, hot topping and risering, in topping compounds, in exothermic and insulating shapes, as a cushioning agent, in molding sand and in the manufacture of refractory blocks and bricks.

LADLE TOPPING

In ladle topping applications, expanded or un-expanded granular ore forms of perlite are added to molten metal pouring ladles. Most often the ore form is used which reacts with slag in the ladle to allow easy removal of the slag layer. In cases where it is desirable to maintain a metal temperature in a ladle, the expanded perlite layer forms an efficient in-sulating blanket.

When perlite is introduced during ladle filling, a gentle rolling motion is produced which cleanses the metal by coagulating impurities. In addition, ladle-lining life is increased by virtue of a vitreous residual coating left on the refractory by the perlite. An added advantage of perlite in this application is that no objectionable smoke or noxious fumes are produced.

HOT TOPPING AND RISERING

Perlite is often mixed with exothermic powders and used in hot tops and risers to prevent shrinkage cavities in ingots and castings. Perlite is the most commonly used lightweight filler used for these applications because of its availability, cost and superior properties. Normal additions of perlite to insulating and exothermic compounds are in the range of 3 to 20% by weight.

Formed shapes are often employed which take the form of hollow cylindrical sleeves for risers and panels for hot tops. These shapes perform essentially the same function as hot topping and risering powders and compounds.

REFRACTORIES

Perlite is used in the manufacture of refractories where the average temperature does not exceed approximately 2000°F (1 100°C). Because of its excellent insulating properties, there is substantial usage of perlite in refractory castables, bricks and blocks. In higher temperature applications, perlite refractories are often used as back-up insulating layers for higher duty refractories.

FOUNDRY CORE AND MOLDING SAND

Perlite is added to foundry core and molding sand mixtures as a cushioning agent to com-pensate for the expansion of crystalline silica as it goes through phase changes at tempera-tures in excess of 1000°F (540°C). Casting defects such as buckles, veining, fissuring and penetration are minimized and cleaning room costs are reduced. In addition, perlite improves permeability of core sands thus re-ducing defects attributable to poor venting

For more information about these uses of perlite and vermiculite, please call or contact us at:

The Schundler Company
10 Central Street
Nahant, MA 01908
(ph)732-287-2244 (fax) 732-287-4185
www.schundler.com
email: info@schundler.com