Iron recovery from water clarification sludge as ferric chloride

Abstract

The circular economy (CE) promotes strategies aimed at waste minimization, treatment, and resource recovery, integrating industrial development with environmental and socioeconomic priorities. In this context, the present study investigates the recovery of iron from an oil refinery sludge generated during the water clarification stage of a steam production plant. This residue exhibits a high iron content (37 wt.%), highlighting its potential as a secondary raw material and alternative iron source. Iron recovery was carried out by hydrochloric acid (HCl) leaching under different operating conditions, including HCl concentration, reaction time, temperature, and solid/liquid ratio (S/L). These parameters were investigated to optimize iron dissolution and obtain ferric chloride, a coagulant widely used in water treatment processes. Optimal leaching conditions (37 wt.% HCl, room temperature, 24 h of agitation, S/L = 1/3.5) produced leachates containing 13.4 wt.% Fe, a value comparable to that of commercial ferric chloride solutions (≈ 14 wt.%). The recovered ferric chloride was subsequently evaluated as a coagulant in three types of water (irrigation, tap, and distilled water). Removal efficiencies of up to 98.7% for Fe, 95.2% for Cr, 99.8% for Zn, 82.4% for Cu, and 82.6% for As were achieved. These efficiencies are comparable to or approaching those obtained using a commercial reagent. The results demonstrate the technical feasibility of valorizing oil refinery sludge into a marketable coagulant, providing environmental benefits while supporting circular economy principles.