Refractories

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 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 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

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 2000oF (1100oC). 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 insulating layers for higher duty refractories.

Core and Moulding Sand

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 1000oF (540oC) 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.