Multistage Crystallization Strategy for Simultaneous Recovery of NH₄SCN and (NH₄)₂SO₄ from Hazardous Desulfurization Residues: Process Optimization and Techno-Economic Assessment

The coking industry generates hazardous desulfurization waste liquid (DWL), which contains abundant sulfur-containing salts such as ammonium thiocyanate (AT), ammonium sulfate (AS), and ammonium thiosulfate. However, improper handling of DWL poses severe environmental risks. Conventional treatment methods, including high-temperature pyrolysis, chemical precipitation, and evaporation crystallization, suffer from critical limitations: low sulfur recovery rates (typically 40%–70%), high operational costs, and insufficient product purity. To address these challenges, this study innovatively proposes an evaporation-cooling pretreatment process guided by the NH₄SCN-AS-H₂O ternary phase equilibrium system. For the first time, this method achieves ultra-high-purity products: ammonium thiocyanate (AT) at 98.54% purity and ammonium sulfate (AS) at 99.92% purity, with a total sulfur recovery rate of 90.92%—40% higher than traditional approaches. Additionally, the mixed salt (MS) byproduct undergoes solvent crystallization to yield secondary products with purity exceeding 99.96%, eliminating the need for toxic reagents. By integrating phase diagram-guided optimization with streamlined operational procedures, this process significantly reduces costs while ensuring environmental sustainability, thereby establishing a novel pathway for resource recovery from DWL.