Process for making hollow carbon microspheres

A batch process for manufacturing hollow carbon microspheres from hollow microballoons made of heat carbonizable material and having microporous walls, including the steps of:

• (a) heating a first chamber to a temperature which is above the carbonizing temperature of said microballoon material and within the range of 2000 to 3000 degrees F., and continuously maintaining the temperature;
• (b) enclosing a batch of microballoons within a protective graphite boat which substantially surrounds the microballoons except for small openings sufficient to pass gases through the boat;
• (c) confining and heating the boat and batch in the heated first chamber for about four hours and until the batch is carbonized to form microspheres while flushing the chamber with an inert gas, the enclosing of the batch in the carbon of the boat and the heat insulating properties of the enclosing boat being sufficient to slow the rate of heat transfer from the first chamber to the batch and prolong it over an interval of time sufficient to allow the expanding gases within the microballoons to diffuse through their micropores without rupturing the microballoons;
• (d) then removing the heated batch from the first chamber and immediately confining it in a second unheated chamber, and flushing the second chamber with an inert gas until the batch cools below its self-ignition temperature in air;
• and
• (e) confining in the first chamber another batch of microballoons within another protective boat according to step (b), and repeating the heating and cooling steps (c) and (d) for said other batch.