HIGH TEMPERATURE APPLICATIONS FOR PERLITE IN THE STEEL AND FOUNDRY INDUSTRIES
|INTRODUCTION||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 from 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 degrees F (870 degrees C) the crude rock pops in a manner similar to popcorn as the combined water vaporises 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 expanding Perlite.
SILVALITE expanded Perlite can be manufactured to weigh from 40kg/m3 to 140kg/m3 making it especially suitable for use in insulating applications. SILVALITE 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.
High temperature performance of SILVALITE expanded Perlite insulation has been well documented over a period of many years. The data in fig. 1 details the thermal conductivity for various density Perlite insulations up to 1800 degrees F (980 degrees C). Although thermal conductivity increases appreciable above 1800 degrees F (980 degrees C), expanded Perlite has been used at service temperatures as high as 2000 degrees F (1100 degrees C).
|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 moulding sand and in the manufacture of refractory blocks and bricks.|
|LADLE TOPPINGS||In ladle topping applications SILVALITE expanded Perlite or SILVASLAG Coagulant Perlite ore granular form of Perlite are added to molten metal pouring ladles.
Most often the ore form, SILVASLAG, is used which reacts with slag in the ladle to allow easy removal of the slag layer. In the cases where it is desirable to maintain a metal temperature in a ladle, the expanded Perlite layer forms an efficient insulating 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||SILVALITE expanded Perlite is often mixed with
exothermic powders and used in hot tops and risers to prevent shrinkage cavities in ingots and castings. Perlite shrinkage 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 degrees F (1100 degrees C). Because of it 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-up insulating layers for higher duty refractories.|
|Perlite insulation is added to foundry core and moulding
sand mixtures as a cushioning agent to compensate
for the expansion of crystalline silica as it goes through
phase changes at temperatures in excess of 1000
degrees F (540 degrees C) casting defects such as
buckles, veining, fissuring and penetration are
minimised and cleaning room costs are reduced.
In addition, Perlite improves permeability of core
sands thus reducing defects attributable to poor venting.