Effect of crystallization temperature, stirring rate, and pH on the crystallization of ammonium sulfate in the presence of calcium sulfate

This article investigated the effects of crystallization temperature, stirring rate, and pH on the crystallization of ammonium sulfate (NH₄)₂SO₄) mother liquor containing calcium sulfate (CaSO₄). This study measured the metastable zone width, average particle size, and coefficient of variation of (NH₄)₂SO₄ after CaSO₄ was added. CaSO₄ was found to cover the active nucleation sites of (NH₄)₂SO₄, leading to a widening of the metastable zone. The increase in crystallization temperature increased the rate of molecular thermal movement, resulting in a narrowing of the metastable zone. The increase in stirring rate accelerated solute diffusion; thus, the metastable zone width became narrower. When the stirring rate was 250 rpm, crystals with a large average size were obtained, which was conducive to solid-liquid separation. The decrease in pH caused the solution viscosity to increase; thus, the metastable zone width decreased. When the pH was 4.5, crystallization could be controlled within a better metastable zone width, and burst nucleation did not occur easily, which resulted in the formation of regular-shaped crystals.