Optimising Salt Recovery—Four-Year Operational Insights into Na2SO4 Recovery from Saline Waters Using Pipe Freeze-Crystallization

Abstract

Managing high-salinity industrial wastewater poses environmental and operational challenges, particularly in recovering valuable salts like Na2SO4. Traditional methods such as evaporation and distillation are energy-intensive (2200 kJ/kg) and environmentally unsustainable. Addressing these limitations, this study investigates the application and optimisation of pipe freeze-crystallization (PFC), an innovative, energy efficient technology operating at 330 kJ/kg, to achieve zero-waste treatment objectives. This research used OLI ESP software to model the crystallization dynamics, accurately predicting Na2SO4 recovery and reductions in sulphate concentrations from 74.3 g/L to 6.9 g/L at temperatures below −2 °C. The recovered Na2SO4 was analysed using X-ray diffraction with its purity increasing over the years from 50% to 84.9%. Over a four-year operational period at a demonstration plant in Olifantsfontein, South Africa, modifications including extending pipe length from 90 m to 120 m and increasing pipe diameter from 20 mm to 25 mm improved salt recovery rates from 3.5 t/month to 9.1 t/month. Enhanced chiller performance sustained sub-zero temperatures, achieving a cooling capacity of 7 kW while enabling consistent salt and ice recovery. Results showed that feedwater composition substantially influenced crystallization dynamics, with high NaCl concentrations delaying Na2SO4 crystallization. The plant’s adaptability to diverse feedwaters and scalability for broader industrial applications highlights its potential as a cost-effective solution. These findings establish PFC as a transformative technology for sustainable saline wastewater treatment, offering industry compliance with environmental regulations, and economic benefits through resource recovery.