Process for phosphating asbestos fibers

DETAILED DESCRIPTION OF THE INVENTION The phosphorus chlorides which are used as phosphating agent in the present invention are the phosphorus trichloride, phosphorus pentachloride and phosphorus oxychloride.
Accordingly, whenever the expression `phosphorus chloride` is used it is intended to include said three phosphorus chlorides.
PHOSPHATING STEP The asbestos fibers to be phosphated are heated previously to their entry in the first reaction chamber so as to eliminate as much water as possible.
The preheating is preferably carried out at a temperature which corresponds to that in the first reaction chamber.
Once the preheated, substantially water-free asbestos fibers are introduced in the reaction chamber they are cause to advance by a tumbling action in countercurrent with a gaseous phosphorus chloride supplied to the reaction chamber by means of a carrier gas such as dry air or dry nitrogen.
The amount of water in the asbestos fibers introduced in the first reaction chamber should be less than 2% by weight.
The temperature of the first reaction chamber is maintained between the boiling temperature of the selected phosphorus chloride at atmospheric pressure and 200.
degree.
C.
above said boiling temperature.
The speed of movement of the asbestos fibers and of the selected phosphorus chloride is adjusted so that the contact time between the two varies from 0.
5 to 8 hours with a preferred residence time of about 1 hour.
The amount of phosphorus chloride used in the phosphated step is slightly in excess of the required amount for phosphating purposes.
A large excess of phosphorus chloride should be avoided for many reasons.
First, an excessive amount of phosphorus chloride would tend to agglomerate the asbestos fibers together, thus reducing the technical properties of the fibers such as freeness.
Secondly, the use of an excess of phosphorus chloride will increase the cost of the phosphated asbestos fibers.
Accordingly, the amount of phosphorus chloride should slightly in excess of the amount required for passivation