Process for the dearsenification of polyphosphoric acid

In the process for the continuous dearsenification of polyphosphoric acid, wherein the acid heated to a temperature of more than 80.degree. C. is introduced from above into a gas admission zone, contacted countercurrently therein with circulated gaseous hydrogen sulfide introduced into the gas admission zone from below, the arsenic being precipitated in the form of arsenic sulfide, and the acid being separated from the arsenic sulfide by filtration and also from excess gaseous hydrogen sulfide by means of compressed air, the improvement which comprises:

• (a) dearsenifying the polyphosphoric acid in a gas admission zone subdivided so as to comprise at least two separate liquid matter-receiving zones, each zone being separated from the other by means of a separate gas zone;
• (b) maintaining the liquid in each of the liquid matter-receiving zones at a level 0.1 up to 10 cm high, without admission of gas;
• (c) permitting hydrogen sulfide which remained unreacted in the individual liquid matter-receiving zones to collect in the respective gas zones and finely redispersing it in the liquid matter-receiving zone directly upstream of the respective gas zone by allowing the hydrogen sulfide to flow through a plurality of openings into the acid, the latter being delivered from the respective liquid-matter-receiving zone to the next downstream liquid-matter-receiving zone, and introduced thereinto directly below the liquid matter level therein;
• (d) separating precipitated arsenic sulfide from the acid coming from the lower end of the gas admission zone, and
• (e) freeing the filtrate obtained from excess gaseous hydrogen sulfide.