This question is way beyond my area of expertise. However, I do know that in the electric furnace process for production of phosphoric acid from phosphate rock, the electric furnace combustion chambers are often made of water-cooled graphite blocks cemented together with resin-based cement. Type 316 stainless steel has also been used successfully for the combustion chamber and hydrator when it can be cooled by water and possible chloride stress corrosion cracking issues on the water side have been addressed. I suggest you consult the many good technical reference books that deal with high temperature corrosion and/or production of phosphoric acid as a starting point for more information. One example is the book written by Ralph Ross entitled "Materials Selector Series for Hazardous Chemicals - MS-7: Phosphoric Acid," (Materials Technology Institute, 2004).