Production of phosphorus pentoxide with utilization of reaction heat

In the process for making phosphorus pentoxide by subjecting elementary phosphorus to a combustion reaction with the aid of dried air inside a combustion chamber of which the metallic walls are arranged so as to form a cooling system with cavities therein;

• circulating a cooling medium through the cooling system for taking up the reaction heat, the improvement which comprises subjecting the phosphorus to combustion with dried air containing, per m.sup.3, 5 g down to 0.01 g of water;
• circulating a liquid or liquid/steam-mixture through the cooling system under pressures within the range of 1 to 150 bars, the liquid or mixture assuming a temperature of more than 150.degree. C. up to 500.degree. C.;
• steam which is being formed in the cooling system being continuously taken therefrom and utilized for the production of energy;
• an equivalent proportion of fresh liquid being introduced into the cooling system;
• and hot P.sub.2 O.sub.5 issuing in vapor form from the combustion chamber being condensed or worked up into desirable final product.