Car tires contain black carbon, a raw material required for many industrial processes. Up to now, however, recycling has only been possible to a limited extent because mineral admixtures contaminate the carbon. A new process could change that, because it allows mineral ash and black carbon to be separated almost completely. This enables both the carbon black and the mineral content to be reused.
Vehicle tires consist of about half of vulcanized natural rubber or synthetic rubber and also contain fillers, among other things. A frequently used filler is black carbon – a carbon black produced as an industrial base material. Around three kilograms of this carbon black are found in a standard car tire. The problem with this is that about 1.5 tons of fossil raw materials and large amounts of water are needed to produce one tonne of black carbon. In addition, up to three tons of carbon dioxide are produced.
Admixtures interfere with recycling
So far, the majority of used tires have ended up in landfills after their useful life: around four billion carbon-containing tires have already accumulated, and around 1.8 billion tires are added every year. When tires are recycled, the oils they contain are primarily used, which in turn generate energy for industrial processes or which are used as raw materials in refineries. The black carbon, on the other hand, remains largely unused. After the old tires have been heated and pyrolysed, it contains up to 20 percent mineral ash, which consists of the additives used in tire manufacture – mainly silicate compounds and zinc components. These impurities have hitherto restricted the reuse of industrial soot from old tires.
A team of researchers from the Fraunhofer Institute for Building Physics in Valley has now developed a demineralization process by means of which the soot can be separated from the mineral ash. “We use a wet chemical process for this,” explains Fraunhofer researcher Christian Kaiser. “To put it simply: We put the black carbon-ash mixture together with various additives in a reactor, mix it with fluid and run a defined pressure and temperature curve. The individual substances are selectively extracted from the mixture.
The difficulty here is to set the parameters and additives so that only one specific mineral is released from the mixture as pure as possible. In addition, the temperature and pressure must remain so moderate that the process can also be implemented on an industrial scale without major restrictions. The scientists were also not allowed to add too many additives so that the process would not become too expensive. They therefore recover some of the additives so that they can be reused.
Demineralization makes recycled carbon black usable
The result of the research is a demineralization process through which the used tire carbon can be largely purified and recovered. “The recovered black carbon treated in this way is almost free of mineral residues. For example, it can be used 100 percent for the side parts of tires – without adding primary black carbon,” describes Kaiser’s colleague Severin Seifert. “It can thus completely replace the original industrial materials.” The black carbon recycled in this way represents a first serious and sustainable solution for the replacement of technical carbon black, according to the team.
In addition to being used in tires, the recycled black carbon with its high degree of purity can also be used for other rubber products and as a colorant for plastic applications, silicates, which can be used in the building materials industry or for paints, and zinc salts for a wide variety of applications. Another advantage of the new process is that not only high-quality carbon black is produced after demineralization. The minerals are also recovered with great purity and can be reused industrially, according to the research team.
A pilot plant for this demineralization with a reactor volume of 200 liters already exists. In order to transfer the process to an industrial scale, a larger plant is already under construction: the production hall has already been built, the reactor volume for a production line should be 4,000 liters each. A production line of this plant will remove 400 kilograms of recycled black carbon from ash per hour – that is 2500 tons per year. In the final expansion stage, the entire plant should have a capacity of almost 30,000 tons per year.
Source: Fraunhofer Society