Recycling of Scrap Tires

​The recycling of scrap tires may be defined under two different categories: i) using the scrap tires as whole or mechanically modified shapes (in crumps or shredded), and ii) chemical decomposition or separation of scrap tire contents into different materials.

Recycling as-is or after mechanical process has the advantages of directly using scrap tires without major investment. For example, scrap tires can be directly used as boat bumpers at marinas to protect ships from scratching or hitting at the side of wharf (Fig. 3). Similarly, old tires can be placed side by side in half tire shifted pattern for slope stability or under roads for improved stability (Mechanical Concrete®). Ripped tire pieces in large chunks can be directly used as light weight infill material at embankments. Smaller scrap tire pieces can be used as mixture in concrete as gravel substitute to improve tensile capacity or in asphalt paved roads for better traction. Smaller crumbs can be bonded together to generate walking or running mats or soft surfaces for playgrounds. Drainage around building foundations, erosion control for rainwater runoff barriers, wetland establishment, crash barriers at sides of race tracks are other uses of scrap tires without much modification.

Recycling of scrap tires at element level that includes some form of chemical decomposition or transformation is different than the mechanical process. Chemical recycling has additional advantages of obtaining well defined building blocks of a tire separately (such as steel wires, natural gas, oil, carbon black, charcoal etc.). The process in a way reverses the manufacturing process and obtains the elements forming a tire backwards. The materials then can be directly sold or used for energy in factories or diesel cars. Alternatively, burning scrap tires may also be included as a chemical process since long chained carbon based molecules are divided into smaller molecules and carbon forms new bounds with oxygen generating heat and carbon dioxide (CO2). Additionally, hydrogen in the molecules also forms bounds with oxygen forming water (H2O).

Separation of scrap tire contents by thermo-chemical decomposition
Pyrolysis is the common name used for decomposing organic material at elevated temperatures in the absence of oxygen. The oxygen needs to be absent otherwise organic material may burn. Typically the process takes place under pressure and operating temperatures above 430 °C (800 °F). The word is originated from Greek based words “pyr” and “lysis” meaning “fire” and “separating”, respectively.

Initial studies on pyrolysis of scrap tires have shown that tire-derived activated carbon, carbon black, boudouard carbon, and fuel gas are obtained. Considering recycling of scrap tires in the road industry couldn’t pass much beyond 2% of available scrap tire production; therefore, pyrolysis of scrap tires have enough resources to keep the system running. Gas obtained from the decomposition of scrap tires can directly be used in the pyrolysis process itself; therefore, the production can support the process for energy saving and sustainability. Economical evaluation of the pyrolysis have shown that when tipping fee for collecting scrap tires (F), revenue received from sale of products (R), processing cost for operating the facility (C), cost for transportation of tires (T), cost of tire shredding (S), cost of disposal of waste products (D) are considered with the assumption of 35% char, 20% gas, 45% oils, and using 50% of char burn-off during activation, net profit (P) is found to be USD 1.5/tire (1996 prices) with about 6 million USD/year gross income with investment payback of about 3.3 years.

P = F + R – C – T – S – D

Recent evaluation of scrap tires pyrolysis by Rubber Manufacturers Association in 2009 contains some disheartening comments. Even after the increase in oil prices reaching USD 150 per barrel, the market did not support this technology. Carbon black, charcoal, and waste oils demand would determine if the operation is viable. Although methane gas is produced during the process and can be used to operate the pyrolysis facility, the manufactured amount is not large volumes enough to sell economically. The excessive gas is usually flared off. Pyrolysis produces pyrolytic carbon char, often confused as carbon black.

Although pyrolytic carbon char has a high carbon content, it is dissimilar to carbon black, which is a highly engineered product. Pyrolytic carbon char is said to have limited market as a filler in some materials and as a colouring agent for some plastics after extensive refining and cannot be easily sold in carbon black markets where there is a lot of competition. The liquid hydrocarbon material obtained from pyrolysis unfortunately contains some contamination and may not be suitable to be directly used as diesel fuel or in home heating; it should be either used as waste oil or further refined. As a result, pyrolysis technology today could not reach its intended target yet. If it comes to the choice between either dumping the scrap tires to large storage areas as housing to rodents and mosquitoes every often catching fire and polluting air, soil, water or pyrolysis to melt down the scrap tire stocks while obtaining less than perfect charcoal, gas, and oil to be further refined is a relatively easy choice. It would be easier if the process can become environmentally friendly and profitable without government subvention.

Burning scrap tires for energy
Another chemical process on scrap tires is burning in high temperature ovens for energy. The burning is usually carried out at thermoelectric power plants and cement production in kiln with clinkers. Although burning a tire usually produces a dark heavy smoke, burning at high temperature furnaces with proper chimney filtering achieves a complete burning without similar smoke. Using scrap tires as fuel is referred as TDF (tire derived fuel) by Scrap Tire Management Council, which was established in 1990 by the North American tire manufacturers.

Cement is produced in high temperature kilns as the raw materials are placed in cement kiln and heated to a temperature range of 1455 to 1510 °C (2650 to 2750 °F). At this temperature the formation of tricalcium silicate (ALITE), the principal compound of portland cement clinker, occurs. A flame temperature of 1925ºC (3500ºF) is necessary to arrive at this temperature. Scrap tires (TDF) can be completely destroyed in cement kilns since the temperatures are extremely high along with a positive oxygen atmosphere and relatively long periods of 4 to 12 seconds at the elevated temperatures ensures the complete combustion of the scrap tire; therefore, incomplete combustion (PICs) or black smoke or odors release is prevented.

When tires are burned in the cement production, the production rates may increase in preheater kilns. This is made possible as the preheater calcination rate is increased in the preheater when burning tires compared to the normal calcination rate when burning coal only. Calcination rates were reported to be increased from 45% to 56% when burning tires instead of burning coal. The carbon dioxide transported by the kiln is reduced when scrap tires are burned in kilns; in this way, additional oxygen be used in the kiln, which allows for the burning of additional clinker.

Burning scrap tires raises some concerns from environmental point of view since tires include up to 17 heavy metals (e.g., lead, chromium, cadmium, and mercury) in addition to natural rubber from rubber trees, synthetic rubber made from petrochemical feedstocks, carbon black, extender oils, steel wire, other petrochemicals and chlorine. Synthetic rubber often contains the organic chemicals styrene and butadiene. Styrene, a benzene derivative, is a suspected human carcinogen. Butadiene is known to cause cancer in laboratory animals and is a suspected human carcinogen. Studies show a strong association between leukemia and butadiene. Extender oils contain benzene based compounds which cause cancer in laboratory animals but totally burnt at high temperatures. A coal and tire chlorine content comparison showed that tires may contain as much as 2 to 5 times the chlorine level of coal. The coal averaged a chlorine weight of 0.04% and tires showed a weight range of 0.07% to 0.2%. (CIWMBA, 1992). Most of the mentioned toxic material are in low percentages and remain in the burnt wastes or bound inside the cement. Factories and power plants that burn tires must therefore have proper filtering at chimneys in case the pollutants remain in the ashes or emitted gasses (Page, 1980). The non-condensable gases are filtered (using a demister filter) and are passed through a wet scrubbing system to remove acid components by NaOH (4%) injection (Sharma et. al.).

Strengthening structures using scrap tires, structural engineering applications
In a recent study of using scrap tires as confinement material for concrete columns (Abdulmoula and Saatcioglu, 2009), the tires were used as peripheral material to confine concrete. When concrete is axially loaded, it tends to expand defined by Poisson’s ratio. If the lateral expansion is prevented, axial compressive strength of concrete is significantly increased. In the mentioned study, a series of concrete columns were cast inside scrap tires which were placed on top of each other. The cylindrical shape formed by carefully and centrally alined pile of scrap tires have also formed a natural form to be easily filled by concrete. Following the strength gain of concrete in 28 days, the steel wires and rims inside the scrap tires served as horizontal confinement for the columns. It was shown both experimentally and analytically that steel-belted tires can be used effectively to confine concrete in reinforced concrete columns. The exterior scrap tire also protects the column and steel reinforcement inside the column from corrosion.

AIShred Pre-processing System for Scrap Tires: https://www.aishred.com/application/waste-tire-shredding.html