LensPhosphates
Figure 1: LIST single-shaft batch reaction system for multi-step syntheses.

Large-volume LIST KneaderReactors designed for high torques are successfully used for stoichiometric or near-stoichiometric multi-step syntheses, such as the production of food-grade phosphates and pyrophosphates (Figure 1). The phosphate process begins with the reaction between concentrated phosphoric acid (H3PO4) and sodium carbonate (soda ash, Na2CO3) to produce monosodium phosphate (NaH2PO4), carbon dioxide and water. This step demands excellent mixing of the reactants.

In a second step, the monosodium phosphate is dried and heated, eliminating water to produce sodium acid pyrophosphate (disodium pyrophosphate, Na2P2H2O7), which is an ingredient in baking powder, along with sodium bicarbonate. The pyrophosphate process is characterized by strong caking, extensive crust formation and poor flow characteristics, making precise temperature control, effective self-cleaning, and the avoidance of dead spots crucial to good conversion and high product quality. “Dry Processing” using LIST equipment does exactly this. Compared to conventional processing, it also cuts operating costs by eliminating the need to remove large quantities of water and to treat the resulting effluent.