Recycling Tyres

 

Summary

It is very difficult to recycle tyres, ( tires )  so 50% of tyres are burnt as fuel.

However, it takes approximately 130 MJ/KG to produce rubber.

Burning tyres produces 32 MJ/KG.

To convert tyre to crumb rubber takes 2.3 MJ/KG.  (Devulcanising would take more.)

Why 'recover 32 MJ/KG of rubber in cement plants if we can preserve 127 MJ/KG by recycling?

There is a big energy saving if tyres can be recycled.

There are  many ways of recycling tyres, many still in development. We are not choosing the most economic, simply collecting the technologies.

Click on pictures for the source and more details

 

Tyres are made by vulcanising rubber. Natural rubber is soft and sticky. By adding sulfur, the polymer chains are cross-linked to form a more durable substance that can be used for tyres. Disposal is a problem as vulcanised rubber is almost indestructible.

During the average life of a tire, only 15% of the rubber is used.

Over 13 million tyres are dumped each year in Australia. 300 million in USA,, 40 million in UK. 

It works out at about one tyre per person per year. World wide there are about 1 billion tyres manufactured per year.

About 20% are recycled.

 

The options are:

Landfill - one way of achieving carbon capture and storage

Burning for energy

Conversion to liquid and gaseous fuel

Using as crumbs 

De- vulcanising - reversing the chemical vulcanisation process then reusing as rubber feedstock

 

 
 
 
 
 
                                      In the vulcanising process sulfur binds the rubber chains together making them less elastic. It is very difficult to remove this S bond. The process is called "devulcanising".

Burning as fuel

About 50% of tyres are used as a fuel as a 10-25% addition to coal. Half of this in cement kilns and the rest in coal fired power stations attached to paper mills. Cement manufacture allows the sulfur dioxide or trioxide to be captured by the CaO to form CaSO3 or CaSO4

Burning tyres produce the following pollutants:

Sulfur dioxide and trioxide,

volatile organic compounds such as benzene, 

metals such as lead,

polycyclic aromatic hydrocarbons such as benzo(a)pyrene,

synthetic rubber components such as butadiene and styrene.  

The chlorine content in tires leads to the creation of dioxins and furans

If the burning is done under optimum industrial conditions most of these organic pollutants will be burned. However there is much argument over whether this is happening in practice. More..

 

Energy balance

It takes approximately 130 MJ/KG to produce rubber.

Burning tyres produces 32 MJ/KG.

Convert tyre to crumb rubber 2.3 MJ/KG. 

Why 'recover 32 MJ/KG of rubber in cement plants if we can preserve 127 MJ/KG by recycling?

Considering the potential health and environmental hazards of tire burning and the low energy efficiency of incineration relative to recycling scrap tires into rubber products, burning tires in cement kilns is more than irresponsible, it's the worst thing to do with scrap tires.

Ref..

Tyre derived fuels

 

Uncontrolled Tyre Fire

 

 

   

Reduced particle size

The main recycling process at present is the conversion to flakes or crumbs. This can be used in playground surfaces, asphalt, underlay, carpets, shoes, filler in tyres, etc. However there are far too many tyres to be taken up by this use.

Recycling tyres Wikipedia

 

Pyrolysis (Heating without air - destructive distillation)

If the tyre is heated the organic polymers break down into gas and liquid that can be used as fuel. The solids are about 40% of the waste. The steel can be removed leaving mainly carbon black. Usually this is of a low grade and not able to be used in making new tyres. However new processes are recovering the carbon black in a more useful form. It can also be converted to activated carbon by reacting with CO2.

The company is very tight with detail but I think it is pyrolysis under vacuum.
 
Tyremill (Aust) have a process for breaking down the tyres but do not give details of their process. It looks very much like pyrolysis

45% of the tyre is converted to bunker oil.

35% of the tyre turns to carbon black.

13% to steel.

5% of the tyre produces syngas (CO + H2) with 54% of this used to power the process.

 

 

De-vulcanising

There are many processes being developed to reverse the vulcanising process.

  • Steam
  • ​Chemical
  • ​Ultrasound
  • ​Microwave
  • ​Biological
  • Mechanical

​An excellent reference is: http://www.centerplastics.com/img/rd/present_status_of_devulcanization_technology.pdf

Much of the following is quoted directly from this source.

Steam de-vulcanisation

For this kind of processes, heat (often combined with addition of chemicals) is used to break the sulfur bonds and thus to plasticize the rubber. Hall patented in 1858 one of the oldest and most simple processes in the rubber reclaiming industry, the Heater or Pan process (Oil law). In this process, finely ground natural rubber powder is mixed with oils and reclaiming agents and treated with high or medium pressure steam at temperatures varying from 170°C to 200°C. The reclaiming time is long and the homogeneity of the reclaim is low, but this process is able to reclaim a large number of polymers: natural rubber (NR), styrene-butadiene rubber (SBR),chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR) and butyl rubber (IIR) and the equipment is rather inexpensive.

The use of the heater or pan process became less popular after Marks patented the Digester or Alkali process in 1899.  

 More....

 

     Devulcanising pan 1934  - source Museum of Victoria"VICKERS RUWOLT PTY. LTD."

Chemical de-vulcanisation

The following chemicals have been tried with varying degrees of success:„
 thiols; 
„ amines; 
„ disulphides; 
 diallyl disulfide
„ sodium metal in various solvents; 
„ iron oxide/phenyl hydrazine; 
„ cuprous chloride/dibutylamine; 
„ ferrous chloride/phenyl hydrazine; 
„ ureas/dicarboxylic acids (the Lev-Gum process); and  
„ mercaptobenzothiazole/zinc dithiocarbamates (the De-Link process)
 mono- and dithiocarboxylic acids, sulphinates, thiosulphonates, sulphites and 
 phosphites. 

(Hunt & Kovalak, 1999) is based on the use of 2-butanol solvent as a devulcanizing agent for sulfur-cured rubber under high temperature and pressure.

Green Source Energy have U.S. Pat. No. 7,767,722 using turpene or similar in the absence f an alkalai metal. 

One idea is to chemically de-vulcanise the surface of the crumbs, then these can be added to normal rubber for vulcanising again.
 
 

CO2 - Tyromer Devulcanization

Tyromer use supercritical carbon dioxide assisted thermal mechanical extrusion process to convert tyre rubber into tyromer – a Tire-Derived Polymer (TDP).  The conversion is 99% efficient.

They claim the technology can also devulcanize non-tire rubbers such as EPDM and silicone rubber for reuse as a substitute.

The process takes less than one minute.

The energy used is less than 1.5 MJ/KG   (This seems very low, as it takes 3 MJ/Kg to produce crumb rubber.)

The website says the polymer produced can be used to make tyres. A few more details would be useful here.

Tyromer

 

Ozone - VR TEK Global devulcanising

CSIRO and Deakin University researchers have developed a process of de-vulcanising rubber. It has been patented by  VR TEK Global.

The process may use ozone on rubber crumbs while under deformation. (Ozone is responsible for the "perishing" of rubber.)

Their patent is vague on details:

the aggressive medium is chosen from the group comprising ozone, oxygen, halogens, acids, alkalis, oxidising agents or combinations thereof, in gaseous, liquid or solution form.  Patent:

 

Ultrasonic de-vulcanisation

Rubber devulcanization by using ultrasonic energy was first discussed in Okuda and Hatano (1987). It was a batch process in which a small piece of vulcanized rubber was devulcanized using 50 kHz ultrasonic waves after treatment for 20 minutes. The process apparently could break down C-S and S-S bonds, but not carbon-carbon (C-C) bonds. The properties of the revulcanized rubber were found to be very similar to those of the original vulcanizates.  More...
 
The process of ultrasonic devulcanization is very fast, simple, efficient, and it is free of solvents and chemicals. The rate of devulcanization is approximately one second. 

Ref

    The application of ultrasonic waves of certain levels, in the presence of heat and temperature, are able to rapidly break down the three-dimensional structure of the cross-linked rubber.  The devulcanized rubber is soft and re-processable.  The advantages of using ultrasound are that the process is continuous, it occurs within seconds or less, and it does not require the use of any chemicals.

Avraamrubber corp

 
Ultrasonic devulcanization reportedly uses frequencies from 20,000 to 50,000 hert
3-10% of the de-vulcanised rubber can be mixed with new rubber without affecting performance. Normally 20-40% is mixed with new rubber for manufacture of rubber products.

Microwaves

The GRC Hawke 10 process  uses finely tuned microwaves  to turn tyres and other plastics to oil, gas and carbon black.

100 KG of round-up tyres produces 50 litres of diesel oil, 150 cubic metres of gas, 11 kg of steel and 37 kg of carbon black.

Another report claims 100 KG of tyres produces 25 KG of diesel.

The microwaves can be tuned to vibrate selected molecules, or parts of molecules, causing them to break apart or to separate.

As well as tyres, the shredding of old cars produces about 30% of "autofluff", plastic with metal etc in it. The microwave process gasifies the plastic and makes it easy to recover the metals.

GRC - Global resource Corp Wikipedia

Patent

 

Thermo mechanical de-vulcanisation

Lancaster-Banbury process

The thermo-mechanical reclaiming processes make use of shearing forces to plasticize the
rubber. Energy is introduced into the materials, resulting in a significant temperature increase, high enough to cause thermal degradation. The Lancaster-Banbury process is one of the oldest processes. Fiber-free coarse ground rubber scrap is mixed with reclaiming agents and sheared in a high speed, high-pressure internal mixer. When a continuously working, multiscrew devulcanizer is used instead of the internal mixer, the process is called the Ficker reclaiming process. More...

De-Link process

Another mechanical reclaiming process is the De-Link process. In this process finely ground rubber powder is mixed with the De-Link masterbatch (DeVulc) : a zinc salt of dimethyldithiocarbamate and mercaptobenzothiazole in a molar ratio of 1:1 to 1:12, dispersed in thiols and activated by stearic acid, zinc oxide and sulfur. Advantages of the process are its simplicity and the fact that standard rubber equipment is used.  No evidence is available to demonstrate that the De-Link process is used beyond laboratory or pilot scale. More...

Toyota process

The Toyota process is another development of mechanical reclaiming. In this process a mixture of ground rubber, virgin rubber, oils and a devulcanization aid is masticated on a two-roll mill or in an extruder. Mechanical devulcanization is achieved through the repeated deformation of rubber particles under specific conditions of temperature and pressure.The result is a devulcanized rubber, ready for further processing. 
 
Toyota developed another continuous process, Toyota Gosei (TG) combining pulverization,reclaiming and deodorization. The rubber waste has to be ground to a particle size of 5-10 mm before it can be fed into a “modular screw-type reactor” with a pulverization zone and a reaction zone. The operating temperature is in the range of 100-300°C and 100-900 rpm screw speeds are applied, the process requires about 100 Kwh to process 200 to 300 kg (kilograms)/hr of rubber, or approximately 0.4 kWh/kg
 
 

Mechano - chemical de-vulcanisation

Mixing of the rubber powder with a peptizer (chemicals used to reduce the viscosity of NR) and a reclaiming agent prior to the mechanical breakdown of the material improves the reclaiming process. The devulcanization aid is supposed to selectively break the sulfur crosslinks in the rubber network. This chemical breakdown is combined with input of thermal and/or mechanical energy, as the rate of this process is sufficiently high only at higher temperatures. The most common devulcanization aids are disulfides, e.g. aryl disulfides or diphenyl sulfides, thiophenols and their zinc salts and mercaptanes. These chemical compounds are radical scavengers: they react with the radicals generated by chain- or crosslink scission and prevent recombination of the molecules. Typical concentrations for the reclaiming agents are 0.5 to 4 wt%. Suitable peptizers are aromatic and naphthenic oils with a high boiling point More..

 

Levgum

Levgum mechanical-chemical process is carried out at room temperature using 3% ureas/dicarboxylic acids to break the sulfur bonds..

The cost of recycled rubber is less than half that of virgin rubber.

It can be used 10-20% with new rubber to make tyres.

 

 

Microbial de-vulcanisation

Thiobacillus-bacteria are able to oxidise the sulfur in polysulfonic bonds to sulphate. This reaction is limited to a surface layer of the rubber with a thickness of less than 1 μm and the oxidation takes several weeks. The thiophilic bacteria Sulfolobus Acidocaldarius is able to split carbonsulfur bonds in a stepwise oxidation reaction of the carbon-bound sulfur into a sulfoxide, a sulfone and finally to a sulphate.

. The disadvantage of these processes is the low devulcanization rate. More..
 

Costs of de-vulcanisation

Very roughly the costs are estimated to be:

Mechanical   $1.80 /KG

Chemical      $3.00 /KG

Ultrasonic     $2.50 /KG

Ref..