Overview of rubber tire recycling
Each year globally, 1.5 billion tires go to waste.
Finding circular and cost-effective tire recycling solutions is a major challenge because of the severe environmental impact of end-of-life tires. Synthetic rubber, the main component in tires, is very difficult to recycle.
Each year globally, 1.5 billion tires go to waste. These volumes explain why the need to fix tire recycling is urgent.
In this article, you will learn all about rubber tire recycling.
- What is rubber tire recycling
- Methods to recycle car tires
- About tire pyrolysis
- The sustainable development of new tire recycling technologies
What is rubber tire recycling?
Rubber tire recycling is the process of converting end-of-life tires, which can’t be used anymore due to damage or wear, into reusable material.
Tires have a limited lifespan, as they suffer damage through regular use. Tread depth decreases through normal wear and tear, and tires become unsafe with tread depths of less than 1.6 cm. Hot summer temperatures, improper alignment, and other factors can also damage tires, further restricting their lifespan.
Technological advances helped tire producers to increase the average mileage for light vehicle tires from 45,000 km in 1981 to 69,000 km in 2001. But, people are driving 14500 to 19000 km each year. And with more cars produced and driven globally, numbers of end-of-life tires (ELTs) keep increasing every year.
Instead of treating ELTs as waste, tire recycling treats them as resources for recovering materials. Material components recovered from ELTs are 45 per cent natural and synthetic rubber, 28 per cent carbon black, 13 per cent steel, and 14 per cent textiles and other additives for passenger car tires.
(Image credits: https://doi.org/10.1016/j.resconrec.2021.105745 )
How to recycle car tires
The EU’s DIRECTIVE 2008/98/EC defines recycling as an operation to create products or materials from waste for the original or other purposes. It doesn’t cover energy recovery or the use of materials for backfilling. Rubber tire recycling involves collecting and treating ELTs to recover the materials in the tire, which prevents sending the tire waste to landfills.
Collection
In the EU, there are three systems of tire collection:
- Extended producer Responsibility: In most EU countries, producers collect ELTs under Extended Producer Responsibility systems. Producers then recycle the ELTs or collaborate with specialised organisations.
- Free market Systems: All stakeholders in the waste recycling chain work under free-market conditions but comply with the required legislation.
- Tax system: The government manages tire waste collection and recycling and levies a tax on tire products.
Next, collecting organisations sort the tires for energy recovery and different recycling pathways. Material recycling, entire tire recycling, and recovery through pyrolysis are three rubber tire recycling possibilities.
Material Recycling
This is the most common rubber tire recycling method. There are two techniques- grinding to produce granules and devulcanisation to make rubber regenerates.
Grinding: This requires special machines and several steps:
1. Steel rims and textile cord separation take place before grinding begins. Steel requires melting before reuse. Textiles undergo cleaning before use in energy recovery or as insulation material.
2. There are two main methods for grinding the scrap rubber– ambient temperature grinding and cryogenic grinding.
- Ambient temperature grinding relies on mechanical grinding with shredders, mills, and knives. Repeated processing produces crumb rubber of the required size above 0.3 mm, with rough edges. Cooling is necessary to prevent combustion as the process generates heat.
- Cryogenic grinding uses liquid nitrogen to freeze tire shreds to temperatures below -80oC. Hammer mills crush the brittle rubber to give small, uniform-sized particles of 75 µm, with smooth surfaces and clean, sharp edges. Electromagnets remove steel bits and other processes remove fabrics. This rubber is purer but more expensive than ambient temperature ground rubber.
- Several processes like wet grinding, the Berstoff’s method, and cracker and hyperboloidal cutting mills improve ambient temperature grinding steps to produce fine-sized rubber dust.
3. Rubber screening of the granulates ensures there is no steel wire and other tire parts. Sorting according to granulate size follows this stage. Granulates of various sizes and types are useful for varying purposes.
4. Cleaning the granulates is the last process before packing the granules.
Devulcanisation: This process decomposes natural rubber by breaking down the cross-linking bonds formed during vulcanisation. Thermochemical, physical, and biological means of devulcanisation exist. But the process degrades rubber polymers leading to a loss of many rubber properties.
Entire rubber tire recycling
Civil engineering uses entire tires because of their shape, size, elasticity, stability, and ability to dampen noise and shock vibrations. Tire shreds (50-300 mm), and tire chips (10-50 mm) also have applications in civil engineering, such as the production of paving blocks/tiles, athletic tracks, and absorbing mats for stables. Find out more about Contec’s 30 mm tire chips!
What are tire pyrolysis technologies?
The third way of recycling ELTs to recover materials is more recent. Pyrolysis is an old thermochemical method, but its use for tire recycling has just begun. The word pyrolysis consists of two words: Pyro = heat, lysis = breakdown into parts.
Synthetic rubber in tires has plastic polymers or long chains of hydrocarbons. Pyrolysis heats shredded tires in an oxygen-less atmosphere under controlled conditions at high temperatures between 400-700°C, in special reactors. In the absence of oxygen, the waste tire cannot burn but decomposes.
The heat catalyses chemical reactions, which break down the large vulcanised molecules into smaller compounds to produce Carbon Black, gas, oil, and other chemicals. Most of the vaporised gases, when cooled, liquify to produce oils rich in aromatic hydrocarbons.
The remaining gas is an excellent fuel that can replace natural gas. Steel bits are removed before heating. The burnt portion at the end is Carbon Black, an important reinforcement material for tires.
Technology leaders like Contec have improved the pyrolysis process with proprietary innovations to achieve >85 per cent material recovery from ELTs. Moreover, the innovations have made the process safer, enhanced product quality, and reduced their environmental impact. Learn more about Contec’s process.
Sustainable development of rubber tire recycling technologies
The adoption of novel technologies like pyrolysis for tire recycling is not widespread.
Tire recycling remains a problem on a global scale. ELTs make up 2 per cent of solid waste, and currently, 75 per cent of ELTs end up in landfills.
Though ELTs are categorised as non-hazardous waste, they produce leachates that cause land and water pollution.
ELTs also pose a fire hazard. For example, in 2016, a vast illegal dump of 90000 ELTs near Sesena in Spain caught fire and burnt for 20 days. The burning tires released toxic compounds like sulphur oxides, polycyclic aromatic hydrocarbons (PAHs), and fine particulate dust. These airborne pollutants also got indoors and increased cancer risks for people living nearby.
The initial attempts at ELT management, such as open burning and use as fuel for cement kilns, had the same negative impact on the environment and people.
More recent rubber tire recycling products also have environmental issues. Ground rubber in artificial turfs and sports fields leads to microplastic pollution. So do tire shreds used for civil engineering.
Efficient tire recycling through pyrolysis can prevent these disasters and pollution.
EU Tire collection regulations in tire recycling and pyrolysis industry
Globally, tire companies are leveraging pyrolysis technology to join the circular economy. This is partly to follow EU regulations on waste management.
- The Landfill Directive (EC Directive 1999/31) aims to reduce the amount of waste dumped in landfills. It also encouraged nations to set up laws to improve recycling and recover materials and energy to protect natural resources.
- The EU Waste Directive 2008/98/EC defines wastes, material recovery, recycling, and energy recovery to guide industries. However, the definition of ELTs has resulted in additional costs and work in terms of collection and transportation for the tire industry.
- This hurdle remains since the recent Revision Directive (EU) 2018/851 has not yet tackled the issue. However, this EU directive has renewed its emphasis on extended producer responsibility schemes. Here producers are responsible and have to pay for the disposal of end-of-life products.
As a result, tire manufacturers are aiming for circularity. They want to get secondary raw materials and use renewable resources. The tire industry relies on the 7Rs hierarchy from production to post-consumption stages to guide it.
New operations for tire creation will prioritise: Reduce, Reuse, Recycle, Redesign, Renew, Repair, and Recover!
Pyrolysis fits well in this hierarchy. This tire recycling process can help tire manufacturers close the loop for many tire components.
At Contec, we call this the “tire-to-tire” model.
What are the value-added products and applications of recycled rubber materials?
Conventional recycling products like rubber crumbs and powder are no longer profitable in developed countries due to market saturation. This has led to a great interest in secondary products from pyrolysis, all of which are in demand and many are lucrative.
- Carbon Black: Recovered Carbon Black is the most attractive. It accounts for 33 per cent of the pyrolysis output. High-quality recovered Carbon Black can replace 25 per cent of the virgin Carbon Black produced from fossil fuels in tire manufacturing. Recovered Carbon Black is also useful in paints, inks, industrial, and consumer rubber goods like cables, wires, etc.
- Recovered tire pyrolysis oil: The Contec process produces 40 per cent of oil of high calorific value. It requires desulphurisation and refining before further use. It can serve as fuel for vehicles, engines, power plants, and alone or mixed with other petrochemicals.
- Recovered steel: Though not as pricey as the other products, there is a high demand for the 15 per cent steel recovered from the Contec process.
- Gas: Part of the 12 per cent of recovered gas currently fuels the Contec plant. The rest will go towards electricity generation for the local community.
Some new applications of conventional recycled products are also profitable:
- Rubber crumbs and powder can be used to make moulded rubber products like dustbins, urban furniture, wheelbarrows, railroad ties, etc.
- Devulcanisation: Rubber regenerates from devulcanisation can be used to make rubber mixtures for manufacturing footwear, washers, cables, rubber slabs, mats, etc.
For any recycling method to work, various stakeholders must work together, like tire users, public institutions, private companies, and treatment facilities, to apply these applications.
This is especially true for circular options like pyrolysis, where producers and recycling units can work together to plan and develop new products that are more sustainable. For more information about tire recycling, subscribe to our LinkedIn newsletter to receive industry-related information about the circular economy in manufacturing.
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