Separation and Recovery of ABS and HIPS from Mixed Plastics via Froth Flotation

Every day, obsolete appliances, consumer electronics, and cars make their way into landfills. These no-longer-wanted items contain something valuable—plastics that have the potential to be recycled. Although current technologies enable the separation of some plastics, they do not yet offer cost-effective purity and yields. Additionally, these methods do not effectively separate plastics that have the same density.

Argonne and Appliance Recycling Centers of America (ARCA) undertook a project to develop a process to effectively separate and recover high-quality acrylonitrile butadiene styrene (ABS)—a plastic used to produce lightweight, tough, rigid products—from the mixed-plastics wastes generated in ARCA’s appliance-recycling operation.

In this collaborative research and development agreement (CRADA), the key challenge was to separate the ABS from high-impact polystyrene (HIPS). Of the mixed-plastics waste generated from obsolete appliances, about 70% is ABS, 20% is HIPS, and 10% is other plastics. The ABS and the HIPS are readily separable from the other plastics, but because they have the same densities and physical properties, they are not easily separable from each other.

Argonne and ARCA pursued a froth flotation technology to separate the ABS from the HIPS. In froth flotation, water-based solutions are used to separate particles based on their relative hydrophobic (“water-fearing”) or hydrophilic (“water-loving”) properties. Research identified a set of conditions for the solution (including density, pH, and surface tension) that allowed for an air bubble to attach to the hydrophilic HIPS, thereby decreasing its apparent density relative to the ABS. The HIPS floated in the solution, while the ABS sank. The separation of the plastics was thus achieved.

At ARCA’s facility in Minneapolis, a pilot plant was built to test the viability of the separation method. Processing 1000 lbs/hour, the plant recovered ABS at purities greater than 99% and at yields of more than 80%. The ABS recovered was successfully used to make injection-molded automotive parts, confirming the feasibility of using obsolete post-consumer plastics—in some cases, plastics more than 15 years old—to meet the performance requirements of parts produced for this industry today. Economic analysis of the process indicates that the payback is about 2 years.

Argonne's 1000 lb/hr plastics separation pilot plant.

A patent has been granted for the basic process for separating and recovering ABS from HIPS. Argonne has since adapted the basic process to separations of plastics from other mixed-waste streams, including the separation and recovery of ABS/polycarbonates from the mixed plastics found in consumer electronics and the separation of polyolefins and styrenics found in auto shredder residues.

For its process to recover plastics from mixed-plastics waste, Argonne was a finalist in the Discover Award competition in 2000. The process is currently undergoing evaluation and demonstration for the recovery of plastics from shredder residue as part of a CRADA between Argonne, USCAR’s Vehicle Recycling Partnership (representing Chrysler, Ford, and General Motors), and the American Chemistry Council-Plastics Division.

May 2008