Effect of the structure of the phosphorus-containing anion in the phosphate binder on the inhibition of thermal oxidation of graphite powders

Abstract

At the present time, carbonaceous ramming bodies are being widely used for lining the metallurgical units working under reducing atmospheres. The bodies of the high-alumina and corundum compositions can contain orthophosphoric acid as a binder and the bodies of the periclase composition can contain commercial-grade sodium polyphosphate binder [1][2][3].The inhibiting effect of the phosphate binders on the thermal oxidation of the graphite powders introduced into the ramming bodies was demonstrated recently [4]. This paper establishes the relationship between the inhibiting effect and the structure of the phosphorus-containing anion of the binder. Figure la shows the generally accepted structure of the phosphate-anion entering the composition of the molecules of orthophosphoric acid. Each PO, 3-ion is capable of forming three bonds.Commercial-grade sodium polyphosphate is a linear polymer having a chainlike structure and consists of a mixture of different polyphosphates having an average polymerization index n in the 8-12 range [5]. Its structure is shown in Fig. lb. Sodium polyphosphate consists of PO43-tetrahedra that are mutually bonded oxygen atoms. Each phosphorus atom in the chain consumes one sodium atom. Besides this, a sodium atom is present at each end of the chain. In aqueous solutions, the end (terminal) groups dissociate to form two Na + ions and a chain-anion that is capable of establishing two bonds.