Heat treatment of expansible materials to form lightweight aggregate

A method for heat treating expansible material to form lightweight aggregate in a rotary kiln having a heated interior space of a length, breadth and cross-sectional area defined by a kiln lining, and which is inclined downwardly from a charge end to a discharge end, comprising the steps of:

• a) providing a kiln having a maximum ratio of said length to said breadth of 5:1;
• b) heating said interior space by means of at least one burner directed into said interior space from at least one end of said kiln, said interior space being heated thereby to a substantially constant elevated process temperature, said temperature being substantially constant along said length;
• c) passing expansible material from the charge end to the discharge end along the length of said heated interior space, said material occupying a maximum of 10% of the cross-sectional area at the charge end and being subjected almost immediately to said process temperature upon entering the interior space, with heat being transferred to the material as it moves along said length primarily by radiation from the heated interior space and the kiln lining and by direct contact with the kiln lining,
• whereby said material is heated to incipient fusion in passing from the charge end to the discharge end, producing gas which is trapped internally, to form said thereby said lightweight aggregate.