Tyre Thermolysis


Thermolysis is the process of heating materials in a vacuum or anaerobic atmosphere where the material can’t oxidize and burn.  The reactor designed by the Recycle Fuel Technologies (RFT) Inc (“RFT”)  has a designed a reactor that can process about 8,000 tonnes of tyres per year.  The thermolysis is called the Formex Process.  The following materials are produced from the Formex process:

Scrap tyres and technical rubber are depolymerised in a proprietary thermolysis process that converts natural and synthetic rubber polymers into gaseous and liquid alkanes, while recovering the carbon black and steel wires used in the original tyres, thus recycling almost 99 per cent of the original mass into useful products.

The “average” rubber waste available for processing is a mix of rubber from scrapped tyres, and technical grade rubber. In this plan it is assumed that most of the feed will be made of scrap tyres with minor quantities of technical rubber although in Germany there are equal amounts of both available.

Scrap rubber tyres are shredded to reduce tyres to pieces of a diameter of less than 50mm. Some of the tyres’ metal is liberated during this shredding process and discharged via a dedicated conveyor. The scrapped tyres will generally have a bulk density of about 213 Kg/m3 that increases to a bulk density of about 468 Kg/m3 after shredding.

After shredding and classification, the rubber pieces are fed into a sealed thermolysis reactor via a valve capable of admitting the feed while preventing oxygen or atmospheric gases from entering into the sealed thermolysis reactor. This valve design is patented.

This proprietary thermolysis process is characterized by:

(i)      Organic waste products, such as plastics and tyres, are depolymerised at reduced pressure and in oxygen or air free sealed thermolysis chambers;

(ii)     The gaseous organic compounds produced during the thermolysis are cooled and separated into a gaseous and a liquid phase;

(iii)    The thermolysis occurs in baths made up of molten metal or molten salts and a conveyor system with specially designed carrier baskets;

(iv)    The space above the bath in the thermolysis chamber is filled with an inert gas, preferably nitrogen, which covers the thermolysis bath;

(v)     The equilibrium of the thermolysis reaction is shifted towards the synthesis of higher molecular hydrocarbons by:

(a)     Rapid cooling of the product gases thus slowing down the rate of reaction; and

(b)     Removing the gaseous product hydrocarbons from the space above the bath thus shifting the reaction equilibrium towards the products.

This reaction control mechanism can be adjusted to allow the reaction to suit the input material composition within wide limits, thus allowing adjustment of the reaction to achieve the desired composition of the waste materials feeding the process.

A specially designed vacuum valve, allows discharging the rubber waste granules into specially designed wire mesh metal buckets mounted on a conveyor located inside the thermolysis reactor. The bottom half of the conveyor, holding inversed buckets, is fully immersed in the thermolysis bath. As the conveyor inverts the buckets containing the plastic or rubber granules, a mechanism closes the buckets to prevent these lighter materials from floating in the denser thermolysis bath, thus ensuring the materials remain fully immersed during their residence in the reactor’s molten bath.

The shredded rubber feed remains immersed in the molten metal in the reactor, at temperatures between 450 and 550°C, in the absence of oxygen, for the period of time required for the depolymerisation and de-vulcanisation reactions to be completed.

The addition of de-sulphurising agents to the thermolysis bath, such as magnesium metal or similar, has the potential to remove almost all of the sulphur contained in the rubber materials, reducing the sulphur to an inorganic inert sulphide slug posing a very low environmental hazard. However we have found that high sulphur carbon black has superior properties than primary carbon black.

After thermolysis and separation of the reaction products, the solid components of the scrap tyre feed remain in suspension in the thermolytic bath. These solids consist of steel wires, textile fibres and finely divided carbon soot. The solids are sieved to separate the textile fraction and then passed throughout a magnetic separating drum to remove the steel metal. The steel scrap obtained has a direct commercial value without any further processing. The textiles are largely degraded, have no commercial value and from the only inert waste generated by the thermolysis process.

The remaining carbon soot originates from the carbon black fillers used in the original manufacture of the tyres but it differs from commercial carbon black as it also contains some of the inorganic components of the tyre as well as surface deposits of carbon formed during thermolysis. However, the quality can be increased by nominating the thermolysis conditions:

The concentration of the carbon forming hydrocarbons in the gas phase increases with increasing pressure, hence an increase in pressure will increase plating of carbon on the carbon soot surface; and

(b)      An increase of the thermolysis temperature will reduce the amount of hydrocarbons absorbed on the carbon soot surface that are precursors to the thermolytic carbon formation.

An important difference between commercial carbon black and carbon soot is the concentration of inorganic components in the latter. Commercial carbon black usually contains less than 0.2% of ash, whereas the ash concentration in carbon soot can be as high as 13.0%. The most important sources for inorganic components in the carbon soot are usually ZnO and S, which are used as a vulcanisation catalyst and vulcanisation agent respectively, and sometimes mineral fillers such as SiO2 and Al2O3. The carbon soot will be subject to a series of acidic and basic washes that reduces the ash content to about 4% to 6% by weight and eliminates any deposits on the surface of the carbon particles.

The commercial value of the products makes the RFT tyre thermolysis process both ecological friendly and economically attractive.