CENTRIFUGAL SEPARATION EFFICIENCY IN THE TREATMENT OF WASTE EMULSIFIED OILS A. CAMBIELLA, J. M. BENITO, C. PAZOS and J. COCA  Department of Chemical and Environmental Engineering, University of Oviedo, Oviedo, Spain O il-in-water (O/W) emulsions used in machining processes were treated in a laboratory bottle centrifuge and in a pilot-scale disc-stack centrifuge. Small amounts of a coagulant salt (CaCl 2 , in the concentration range 0.01–0.5 M) were added to promote emulsion destabilization and to improve centrifugation. Grade efficiency curves were obtained from droplet size distribution measurements of the emulsion after centrifu- gation. The effect of the amount of coagulant salt on emulsion breakdown was studied by zeta potential and turbidity measurements. The effect of several parameters, such as angular velocity or centrifugation time on the critical diameter and grade efficiency was also investi- gated. Oil removal efficiencies of 92– 96% were obtained for all centrifugation experiments. Keywords: oil-in-water emulsion; centrifugation; droplet size distribution; critical diameter; grade efficiency. INTRODUCTION Oil-in-water (O/W) emulsions have wide industrial appli- cations in the metalworking industry in operations such as machining, forming, rolling or alkaline degreasing. These emulsions, used mainly for friction reduction and heat dissipation purposes, have similar composition with a 5–10 wt% of organic content in an aqueous medium. The organic phase contains, besides a mineral oil, a wide variety of additives: emulsifiers, corrosion inhibitors, extreme pressure agents, biocides, antifoam compounds, and so on (Mahdi and Sko ¨ld, 1990; Zimmerman et al., 2003; John et al., 2004). Furthermore, these fluids may undergo chemical changes over time and become contami- nated by tramp oils and solids after being used, loosing their lubricant properties and decreasing their performance (Eppert et al., 2003; Bataller et al., 2004; Greeley and Rajagopalan, 2004; Zimmerman et al., 2004). At a certain point the emulsion has to be disposed of. Current environmental regulations classify these fluids as hazardous wastes, so they cannot be discharged into the environment without previous extensive treatment. Moreover, O/W emulsions disposal has increasingly become an expensive challenge to industry and several techniques have been proposed for its treatment (Benito et al., 1998). In practice, a combination of processes is needed (Benito et al., 2002): . Primary treatment: tramp oils and solid particles are removed by skimming, gravity separation or filtration. . Secondary treatment: emulsion breakdown takes place and most of the free oily phase is removed. Chemical coagulation followed by gravity settling is the most broadly used process. . Tertiary treatment: membrane, adsorption or biological treatment processes are applied to reduce the chemical oxygen demand (COD) content of the aqueous effluent, which is sent afterwards to conventional wastewater treatment. The secondary treatment can be considered a key stage to remove the oily phase, where most additives are dissolved, accounting for 90% of pollutant content of the waste emul- sion. This stage implies the emulsion breakdown, i.e., the coalescence of oil droplets, which can be achieved by phys- ical, thermal or chemical methods. Coagulant salts and flocculant polymers are added to carry out the emulsion breakdown (Tansel and Regula, 2000). Most of the O/W emulsions are stabilized by means of electrostatic repulsion between similarly charged droplets (Verbich et al., 1997; Deluhery and Rajagopalan, 2005). Coagulants increase the ionic strength of the solution, compress the electric double layer around the oil droplets and overcome the elec- trostatic repulsion among them (Rı ´os et al., 1998a, b). Furthermore, flocculants are adsorbed onto the droplet surface, bridging and agglomerating droplets into larger flocs. Once the oil droplets have reached a size large enough to make the emulsion unstable, the oily phase can be removed using several processes, mainly sedimentation or centrifugation. Centrifugation is an attractive and efficient method as secondary treatment, compared to gravity settling. Its main advantages are that less space and operation time are required, being especially suitable for small workshops.  Correspondence to: Professor Jose ´ Coca, Department of Chemical and Environmental Engineering, University of Oviedo, C/Julia ´n Claverı ´a 8, 33006 Oviedo, Spain. E-mail: jcp@uniovi.es 69 0263–8762/06/$30.00+0.00 # 2006 Institution of Chemical Engineers www.icheme.org/journals Trans IChemE, Part A, January 2006 doi: 10.1205/cherd.05130 Chemical Engineering Research and Design, 84(A1): 69–76