Evaluación de la eliminación de sulfato de sodio mediante precipitación Química y Nanofiltracion para incrementar la recuperación de efluentes salobres en una planta cloro-soda

Abstract

CONDITION FOR PROJECT PUBLICATION. In membrane-based chlor-alkali processes, stream recirculation is crucial for improving operational sustainability. However, this practice tends to promote the accumulation of sodium sulfate (Na₂SO₄), an impurity present in the feed brine, reaching concentrations above 10.0 g L⁻¹ that hinder chlorine production. This study evaluates a treatment scheme integrating chemical precipitation and nanofiltration (NF) to remove Na₂SO₄ and enhance effluent reuse in a chlor-alkali plant located in Guayaquil, Ecuador. Laboratory-scale precipitation experiments assessed the effects of calcium chloride (CaCl₂) dosage, temperature, and pH on Na₂SO₄ removal from real industrial effluent. The highest removal efficiency (49.3 ± 4.2 %) was achieved at 70.0 °C using 45.0 mL L⁻¹ of CaCl₂ at pH 9.0–11.0 with a mixing time of 20.0 minutes, whereas performance declined below 30.0 % at 40.0–55.0 °C, even at higher reagent dosages (50.0 mL L⁻¹). Parallel trials conducted on an industrial-scale NF unit demonstrated a Na₂SO₄ rejection of 75.7 % at 25.0 bar with a recovery of 69.7 %. The supernatant from the precipitation step and the NF permeate can be successfully reincorporated into the brine circuit. Mass balance analysis indicated that the integrated process recovers up to 81.3 % of the effluent, equivalent to 6,828.0 m³ year⁻¹ of water and 1,393.0 tons year⁻¹ of NaCl, while avoiding saline discharges. The estimated capital investment for the treatment system (USD 117,021.0) offsets the annual raw material losses in the purge stream (USD 93,964.0 year⁻¹), resulting in an investment depreciation period of approximately 9.0 years. Overall, this case demonstrates that coupling technologies such as chemical precipitation and nanofiltration enhances circularity by reducing freshwater demand and mitigating saline discharges in chlor-alkali operations, without incurring excessive costs. Keywords: Electrolyzer, sodium sulfate, industrial effluent treatment, economic savings