Tripolyphosphate made from wet-process phosphoric acid with the use of a rotary kiln

The paper presents results on tripolyphosphate (STPP) preparation with the use of a specially constructed laboratory-scale rotary kiln. The kiln enabled agitation of the material calcined at the temperature range 400-430 degrees C. Wet-process phosphoric acid WPA-1, taken directly from one of the Polish plants, or WPA-2, imported from Finland, was neutralized with Na2CO3 to obtain a phosphate mixture with the molar ratio Na/P = 5/3. To mimic industrial processing with recycling the product, some of the industrial STPP (form-II) was added and the mixture obtained was fed into the rotary kiln through a vibratory feeder. The calcined product was tripolyphosphate in the crystalline form-II, and that did not depend on concentration of the WPA used, or on the amount of the STPP recycled into the kiln. When a similar phosphate mixture WPA + Na2CO3 + STPP-"recycled" was ignited at 400 degrees C in a stationary chamber oven, the product obtained was also STPP in form-II. In contrast, when a phosphate mixture WPA + Na2CO3 without STPP-"recycled" was calcined, the phase composition of the product obtained depended on the sort of WPA (WPA-1 or WPA-2) and, in the case of WPA-1, on its concentration. From concentrated WPA-2 the STPP formed more readily than from concentrated WPA-1. Lower H3PO4 concentration and recycling of the STPP into the phosphate mixture, before its ignition, creates. favorable conditions for the STPP crystallization.