GB1081891A - Improvements relating to the production of phosphoric acids - Google Patents

Abstract

Phosphoric acid of a P2O5 content greater than 68% w/w (IFB) is produced by continuously feeding wet-process phosphoric acid to a heating zone, e.g. in a submerged combustion evaporator, and withdrawing the more concentrated product acid. In the zone, the acid is maintained at its boiling-point by releasing hot combustion products into the acid, e.g. through a dip-tube extending vertically into the acid so as to form a stable bubble on the annular surface around the exhaust orifice at the lower end of dip-tube. The gaseous effluent from this zone is passed to a scrubbing zone, flowing therein at 15-100 f.p.s., and passed through apertures in a barrier obturating this scrubbing zone at 100-400 f.p.s., preferably 300-350 f.p.s., effecting a pressure drop across the barrier of not less than 15m w.g., preferably 35-40m w.g. A spray of water and/or hot weak phosphoric acid containing not more than 50% P2O5 (e.g. 30-50% at 70-120 DEG C.) is sprayed on to the upstream face of the barrier to cause large droplets of the acid to break into smaller droplets such that they mix intimately with the gaseous effluent. The acid is then separated from the effluent and thereafter, the effluent is treated with cold water. The product acid has a low content of insoluble polyphosphate and metaphosphate compounds. The apparatus is also claimed. Preferably, the annular surface surrounding the exhaust orifice of the dip tube has a radial extent, L, given by the formula L = kR(Qa -1) wherein R is the radius of the external orifice, Q is the heat-release rate of the burner in millions of B.Thu/hour, and k, a are constants such that k is \sF0.6, preferably \sF0.9, and a is 0.29-0.32, preferably 0.3.ALSO:Phosphoric acid of a P2O5 content greater than 68% W/W (IFB) are produced by continuously feeding wet-process phosphoric acid to a heating-zone e.g. in a submerged combustion evaporator, and withdrawing the more concentrated product acid. In the zone, the acid is maintained at its boiling-point by releasing hot combustion products into the acid e.g. through a dip-tube extending vertically into the acid so as to form a stable bubble on the annular surface around the exhaust orifice at the lower end of the dip tube. The gaseous effluent from this zone is passed to a scrubbing zone, flowing therein at 15-100 fps, and passed through apertures in a barrier obturating this scrubbing zone at 100-400 fps, preferably 300-350 fps, effecting a pressure drop across the barrier of not less than 15" w.g., preferably 35 - 40" w.g. A spray of water and/or hot, weak phosphoric acid containing not more than 50% P2O5 (e.g. 30-50%) at 70 - 100 DEG C is sprayed onto the upstream face of the barrier to cause large droplets of the acid to break into smaller droplets such that they mix intimately with the gaseous effluent. The acid is then separated from the effluent and thereafter, the effluent is treated with cold water. The product acid has a low content of insoluble polyphosphate and metaphosphate compounds. Preferably, the annular surface around the exhaust orifice of the dip tube has a radius, L. given by the formula:- <FORM:1081891/B1-B2/1> wherein R is the radius of the external orifice, Q is the heat release rate of the burner in millions of B.Th.U/ per hour, and a, k are constants, k being greater than or equal to 0.6, preferably greater than or equal to 0.9, and a is 0.29 - 0.32, preferably 0.3.