Optimization of ammonium sulfate crystallization under sodium fluoride based on response surface method

In order to investigate the crystallization mechanism of (NH₄)₂SO₄ under the action of NaF, this paper adopts the laser method to determine the solubility and the width of the metastable zone of (NH₄)₂SO₄ under different NaF addition amount, and then designs the experiments through the response surface method to study the mechanism of the influence of NaF addition amount, pH value and stirring rate and their interactions on the amount of crystallization and the average size of (NH₄)₂SO₄ and to set up their regression models. It is shown that the salting-out effect and hydrogen bonding lead to a decrease in the solubility of (NH₄)₂SO₄ and the occupation of active growth sites by NaF, which leads to an increase in the crystal nucleation the (NH₄)₂SO₄ metastable zone. Three factors have a significant effect on both the crystallization amount and the average particle size of (NH₄)₂SO₄. On the crystallization amount the influence order is: stirring rate > pH value > NaF addition amount, and on the average particle size the influence order is: stirring rate > NaF addition amount > pH value. Because of the collision of solutes with the stirring paddle and crystallizer during the stirring process, secondary nucleation occurs, so the stirring rate significantly affects the amount of crystallization and the average particle size. Through the optimization, the NaF addition of 0.4%, pH value of 5.71 and stirring rate of 329.82 rmp were obtained, and the error between the theoretical and experimental validation values under these conditions was small, which indicated that the model had high reliability.