Chemical Destabilization of Fresh and Spent Cutting Oil Emulsions: Differences between an Ecofriendly and Two Commercial Synthetic Lubricants

The aim of this study was to evaluate if eco-friendly lubricants had an additional advantage over conventional synthetic lubricants in terms of emulsion treatment of metalworking wastewater. To these purpose, two fresh commercial synthetic cutting oil emulsions were compared with an emulsion obtained from a new cutting oil produced from the reuse of slaughtering waste (CADT-605 from Kimya Srl, Bari, Italy). The breaking of the fresh emulsions was carried out with the addition of small amounts of sulfuric acid (2–20 mL/L) followed by pH neutralization by means of calcium hydroxide. During the two-step treatments, COD, BOD5 and non-ionic surfactant (NIS) concentrations were monitored in the aqueous phase. The results demonstrated that the best results were obtained with the emulsion produced with the bio-based lubricant. In addition, this cutting oil was tested in a metalworking plant within 30 days and the resulting spent emulsion was treated with the proposed process. In all the cases, the treatment led to a drastic decrease of the COD, BOD5 and NIS contents that allow the discharge of resulting aqueous phase in sewers or in surface water bodies, together with the other wastewater produced by the plant, in agreement with the Italian regulation.