Recycled Rubber
|
|
| AN ESTIMATED 273 million tires are discarded every year in the U.S. alone. This recycling plant in Toronto, Canada uses a new batch belt press made of nickel stainless steel to recycle 200 tonnes of rubber a year. |
 |
Nickel stainless steel belts help to improve the economics of recycling
rubber
BY VIRGINIA HEFFERNAN
Nickel magazine, Dec. 01 -- Canada's NRI Industries has transformed its rubber recycling plant by replacing aging, labour-intensive batch presses with the type of continuous press more commonly seen in hardboard manufacturing plants.
The continuous press, which spans half a city block (45 metres), has boosted NRI's sheet rubber manufacturing capacity by 77% to 20 million kilograms and improved working conditions in the company's Toronto plant. And because the press requires only three operators per shift compared to 12 per shift previously, labour costs have dropped significantly.
Two stainless steel belts, manufactured by Sweden's Sandvik Process Systems, a division of Sandvik, are integral components of the NRI press. The Sandvik 1650SM belts are made of low-carbon, martensitic stainless steel characterized by excellent static and fatigue strength. Their chemical make-up includes 14% chromium, 7% nickel, 1.5% silicon, 1% manganese, 0.8% molybdenum, 0.7% copper and 0.3% titanium, a composition very similar to that of S15700.
NRI recently purchased two new 1.72-tonne belts at a cost of about C$400,000. The belts will rejuvenate the second-hand press, which NRI purchased from a company that used it to make conveyor belts for the British coal mining industry.
NRI uses the press to convert tires and industrial rubber scrap into products primarily for the automotive industry. About 70% of the estimated 273 million tires that are discarded annually in the United States are reused or recycled, according to the Scrap Tire Management Council. The other main uses are tire-derived fuel, civil engineering uses and new products.
The scrap rubber is crumbled to a particle size of 2 to 6 millimetres (mm), mixed with curing agents and other chemicals and sheared between two rolls moving at different speeds to form a hot sticky mass, then rolled to the desired dimensions. The rolls are cured by a combination of heat (about 200°C) and pressure (230 bar) between the steel belts.
The 2-mm-thick belts require extraordinary strength in order to twist 180 degrees, on average, every two minutes while the press is operating. "The belts have to flex an awful lot," says Ted Pattenden, president of NRI. "But they are remarkably resistant to this constant flexing."
At 200°C, the belts have a yield strength of 1,350 megapascals (MPa), a tensile strength of 1,360 MPa and elongation of 5%. They resist corrosion and wear and are relatively easy to repair. The 44-metre-long, 2.5-metre-wide belts move at a rate of about 7 metres per minute.
Theoretically, the belts should last forever, though they can fail prematurely if parts of the machine (i.e., bolts, nuts, etc.) or tools fall on to them and proceed through the press. Sandvik calls this "catastrophic failure." According to Robbin Wood in Sandvik's Canadian office, an old belt can be sent to a scrap yard for remelting (and will eventually end up there), although it is usually kept on-site as an emergency backup belt until a new one can be delivered, or as a source for patches in case the operating belt is torn.
NRI is the first rubber recycler in the world to use a continuous press. The market for continuous presses (and therefore Sandvik belts) is growing, but the applications are mostly flooring and hardboard manufacturing. Sandvik is not aware of any other company using belts for rubber recycling.
Recyclers have traditionally preferred batch presses because, in order for the curing process to be effective, rubber must be subjected to a specific heat and pressure for a minimum amount of time. But by controlling the speed of the belts to allow just enough time in the desired heat and temperature zone, NRI has been able to boost output. At the same time, gauge uniformity has improved, resulting in a better-quality product.
NRI's main product, called Symar masticated rubber, is a mixture of rubber and fibre, and consists of 95% recycled material. Recycled rubber is suitable for reuse as tires only in small quantities. The current thinking is that new tires can be manufactured with as much as 10% recycled content (that is, old tires) without sacrificing tire durability or driving performance. Symar is used instead as rubber seals on the inside of cars and trucks. Other applications include molded sheets for die cutting, mudflaps for trucks, dock bumpers, and walls and flooring for horse trailers.
Virginia Heffernan is a Toronto-based science writer
Photo: Tom Skudra/NiDI
|
Rosemarie Lamprecht |
