Decontamination of Process Streams Through Electrohydraulic Discharge

Discharging a high-energy spark underwater creates a shock wave which dissipates through an acoustic field. The hydroxyl radicals created by the field are able to mildly oxidize the surfaces of ink and toner particles suspended in water, thereby decreasing their zeta potential. This decrease inhibits the tendency of the particles to reattach to fiber, which is negatively charged. Pilot studies showed that sparking a slurry of recycled newspaper and magazine fur­ nish decreased residual ink on fiber by 21%, increased brightness by 1%, and reduced fiber loss by 1% when the treated furnish was subsequently subjected to flotation deinking. No benefit was realized if the furnish was sparked during deinking, possibly because the shock wave changes the bubble size distribution. Introduction Flotation deinking is commonly used in paper recycling to remove ink from fiber (1-4). Atmospheric pressure cells are most effective in removing particles in the 15-50 μm range. Pres­ surized deinking modules generate bubbles of smaller size and are able to remove smaller parti­ cles. Ink removal is inevitably accompanied by fiber loss (5), and a compromise is usually struck based on product specifications. We have previously shown that firing a high-intensity spark underwater creates a spheri­ cal shock wave that generates an acoustic field, which, in turn, breaks water into hydroxyl radi­ cals (6). The surfaces of acrylate particles suspended in water are mildly oxidized upon expo­ sure, and their zeta potential and tack drops. These contaminants are commonly found in recycle paper process streams, and a reduction in tack minimizes deposits and other operational prob­ lems (7, 8). In this paper we demonstrate that the zeta potentials of ink particles can be similarly reduced upon sparking, and that this reduction can serve as the basis of a new technique that promotes ink removal, while simultaneously decreasing fiber loss during flotation deinking. Experimental The sparking unit used was obtained from Pulse Power Inc., Stoney Creek, Ontario, Can­ ada. Laboratory experiments were conducted in a 10-L tank constructed from a section of pipe with integrated 1.5-cm diameter steel electrodes with a 4-mm gap. Pilot work was done with a submersible electrode unit, which was placed inside the flotation cell. The approximately 0.1 msec discharge was of 15 kA and 4 kV. Zeta potential was measured with a Zetasizer (Malvern Instruments Ltd., UK). These measurements were made on laser printer toner, type PX Black (containing styreneacrylate co­ polymer, carbon black, polypropylene and charge control agents), supplied by Southern Cross Systems Corporation. The toner was cured at 400C for 30 seconds; it fuses in less than one sec­ ond at 400C in a printer. The fused toner (0.63 g) was ground to a fine powder and suspended in 7.2 L of DI water. The pH was adjusted to 6.6 with 50% NaOH. The suspension was sub­