Oil recovery from waste cutting fluid via the combination of suspension crystallization and freeze-thaw processes

Abstract

A set of experiments was conducted so as to detect the influences of concerned factors including freezing temperature, stirring speed and pH adjustment on combined method of suspension crystallization and freeze-thaw technique for waste cutting fluid treatment and oil recovery. The optimal operating conditions of freezing temperature and stirring speed were confirmed by the experiment result to be −8 °C and 300 rpm respectively, while the chemical oxygen demand (COD) removal efficiency was above 90% without pH adjustment. The concentrates from the suspension crystallization were further treated by freeze-thaw method, and the oil recovery rate could reach to 95%. The micrograms of concentrates from suspension crystallization showed that the size of oil droplets in concentrates was about 148 μm in diameter, which became significantly larger after the suspension crystallization process. As a consequence, higher possibility for oil droplets to gather and separate was provided by their expanded size. The results of infrared spectroscopy studies indicated that the oil recovered from waste cutting fluid precipitates was mainly composed of esters. Up to 2.85 kg fat was recovered from 1 m3 of waste cutting fluid, while the required power of the combined freezing process is theoretically 42 kW h. Based on the results, a freezing/concentration concept for oil recovery from waste cutting fluid is proposed.