Lead-acid batteries from automotive applications normally have a shorter service life than the car itself. After their service life has elapsed, they are no longer suitable for use. Because of their high lead content, lead-acid batteries have always been eagerly snapped up by secondary lead smelters. Via the car dealer and garage network, lead-acid batteries are collected in large quantities and then transported to secondary lead smelters. The logistics system is geared to lead recycling. The first battery reprocessing step yields not only lead but also PP in a form of the casing fragments. Accordingly, the polymer is available without additional cost. As the casing makes up a substantial part of the total battery, the quantities of polypropylene obtained are sufficient to warrant the operation of a plastics recycling plant (Figure 2). The secondary lead smelter
Figure 2. Parts of an original 12V 44Ah lead-acid battery with a casing made of polypropylene.
processes up to 60,000 tpa of used batteries corresponding to about 3000 tpa of polypropylene. For this material stream, a recycling plant was developed, built and put into operation in 1986.
Crushing and separation
In the first step, the batteries are processed through a crushing and separation
system operating on the TONOLLI principle (Figure 3) which has been successfully employed in various battery recycling plants in Europe and North America. Next, the heavy fractions (lead, lattice metal) and Ebonite are separated from the light fractions (polypropylene and impurities). At this stage, the polypropylene has a purity of 97 %, which is still insufficient for its further processing. It is therefore routed to an upgrading stage, where it is further reduced in size in a wet-type rotary grinder and subsequently separated from water by sedimentation. After having passed through two series-connected driers and a cyclone separator, the polypropylene is available as so-called regrind with a purity of 99.5 %. The regrind consists of various types of polypropylene differing in their formulation, molecular composition and stabilizer content, having a broad spectrum of characteristics. Suitable mixing yields an intermediate product with a narrowed range of statistically uniform product characteristics.
Further processing to polypropylene granulate
As a next step, the regrind is routed to compounding (Figure 4). By controlled addition of additives, polymers and fillers, the feed mix can be adjusted to suit the specific customer requirements. This feed mix is then gravimetrically me-tered into a special twin-screw kneader where it is molten under the dual action of an external heater and internal shear forces to obtain a homogeneous compound. Volatile matter is extracted and impurities resulting from unmolten components are filtered out. Subsequently, the melt is pelletized in a melt granulator. The resulting granulate is quenched in a water bath, centrifuged and finally processed through a hammer mill to break up lumps. The end product is a packagable granulate suitable for injection molding.
This granulate is a secondary raw material which not only meets customer specifications but is also manufactured under a quality assurance system.
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