Adsorption mechanism of SDBS collector in flotation separation of K2SO4/[Kx(NH4)(1-x)]2SO4 and NH4Cl generated via double decomposition

Abstract

Potassium sulfate (K₂SO₄) is an essential potassium fertilizer, and its production entailing the double decomposition reaction of KCl and (NH₄)₂SO₄ is a crucial industrial process. However, the subsequent separation of K₂SO₄ and NH₄Cl from the mixed products via crystallization necessitates high amounts of energy. In this study, flotation was introduced to achieve an efficient separation of K₂SO₄ and NH₄Cl at a lower cost, and sodium dodecyl benzene sulfonate (SDBS) was used as the collector for K₂SO₄ flotation. After one flotation, the K₂O yield reached 81.20 %; however, the K₂O and Cl^- contents of the obtained product were only 37.12 % and 10.18 %, respectively, which did not meet fertilizer product requirements. This resulted from the inevitable presence of the [K_x(NH₄)_(1-x)]₂SO₄ complex salt, produced in the economically acceptable concentration range during the double decomposition reaction. Molecular dynamics (MD) simulations confirmed that the interaction between SDBS and the surfaces of K₂SO₄ and [K_x(NH₄)_(1-x)]₂SO₄ (x = 0.75) was stronger than that of their ion hydration layer, allowing it to be adsorbed onto their surfaces, whereas its adsorption onto the surface of NH₄Cl was impeded, which enabled the flotation separation of K₂SO₄/[K_0.75(NH₄)_0.25]₂SO₄ and NH₄Cl. However, the adsorption of SDBS onto the surfaces of K₂SO₄ and [K_x(NH₄)_(1-x)]₂SO₄ occurred simultaneously without significant differences, which hindered their effective separation and resulted in a low K₂O content in the product. Based on this mechanistic understanding, a secondary decomposition reaction-flotation process was introduced, enabling a final K₂SO₄ yield of 91.78 %, with an increased K₂O content of 42.06 %–44.29 %, and reduced Cl^- content of 2.97 %–4.05 %, meeting the requirements of compound fertilizer products.