The electrostatic separation has proved to be an effective and environment-friendly treatment for the recovery of crushed waste printed circuit boards (PCBs). In this paper a more sophisticated response surface methodology was applied to build quadratic models and optimize the three main factors on a roll-type electrostatic separator. The sample of granular mixture got from crushed PCB wastes (size 0.3-0.45 mm, containing 25% metal and 75% nonmetal). According to the analysis of the experiment data, some quadratic effects were found, and two quadratic models for conductor production (C) and middling production (M) were established. The results indicated that the high voltage level and roll speed are the most important factors for the process. Because of the existence of the quadratic effect, the highest efficiency can also be achieved at a lower voltage level (27-29 kV) compared with a maximum voltage level. (C) 2009 Elsevier B.V. All rights reserved.