Prospecting the Vulcan Hills for Treasure

NewScientist
Until relatively recently, most used rubber vehicular tyres were buried in landfills and left to slowly rot. But now there are dozens of productive uses to which these rings of vulcanised rubber can be put. A better way of making steel is only one of them:

To conventionally make steel, coke, which is charcoal from coal, and limestone are shoved into a furnace heated to more than 1500º Celsius. The calcium in the limestone scavenges impure elements in the coke, such as silicon and aluminium, creating foamy slag and the product we're after - liquid iron. Just like cappuccino foam, the slag sits atop the liquid iron and insulates it, speeding up the processes of converting coke to iron.

Sahajwalla found that replacing some of the coke with recycled rubber created a more effective slag blanket. The extra heat produced from it reduced energy consumption, and produced more steel. It was the ultimate win-win. Following successful commercial trials in 2007, the technology is now being used in steel mills across Australia and has diverted well over 70,000 tyres from landfill. "It's so satisfying and so exciting," she said. _NS

Another intriguing use for old rubber tyres is to produce carborundum -- silicon carbide -- for tough bits, blades, and tools.

Silicon carbide, known commercially as carborundum, is formed of carbon and silicon atoms arranged in a diamond-like pattern, which results in diamond-like properties. On the Mohs scale of mineral hardness, which has diamond as ten, carborundum scores nine or better. It thus has a wide range of uses, from abrasives and cutting tools to bullet-proof vests and ceramic brakes in sports cars. It is also used as a semiconductor in high-voltage applications.

Normally, silicon carbide is produced by heating sand (which is made of quartz, or silicon dioxide) in an electric furnace with carbon made from oil or coal. The trick used at Tubitak is to get both the carbon and the energy from tyres.

First, the tyres are gasified, a process which is similar to burning but involves less oxygen. This releases a mixture of hydrogen and carbon monoxide, known as syngas, and leaves a residue of amorphous elemental carbon called carbon black. Tyres also contain sulphur, which is added as part of the process of vulcanisation that makes rubber into a suitably resilient material. Gasification liberates this in its elemental form, making it easy to recover. Burning a tyre, by contrast, produces sulphur dioxide, a noxious pollutant.

The carbon black is then mixed with sand and the mixture is heated to between 1,400ºC and 2,100ºC in a syngas-fired oven. The result is high-grade silicon carbide. _Economist

Tyres can also be used to produce valuable oils, carbon black for making more tyres (as reinforcement), and to generate electricity, via pyrolysis.

[Besides re-treading,] Other common uses for scrap tyres include sports and recreational surfaces, landfill engineering, carpet underlay/floor coverings, and road building. Roads manufactured using crumb rubber last longer, have better traction and reduce noise. _Source

Tyres can also be used as structural members when baled, or formed into special block structures.

One can build entire houses and other structures out of used tyres, using the "Earthship" method, the tire bale method, or other creative methods of recycling.

And of course there are always the tyre swing, tyre planters, concrete-filled tyres used as bases for upright poles, rubber sandals made from tyres, and many other commonplace home and hobby uses for worn-out tyres.

The basic point being made is that new and important uses are being found for what was once viewed as trash and environmental hazards. That is where the attention of environmentalists, Greens, activists, and concerned citizens should be focused. Toward innovations which turn the trash into treasure.