Very few people have even heard of rhenium, but the rare metal with an ultra-high melting point is becoming a vital part of the aerospace industry and one of the most expensive commodities in the world.

New demand for rhenium is also likely to come from technology that converts gas to liquid.

The silvery white metal is used in refinery catalysts to make lead free gasoline.

This application in the early 1980s helped push prices to above $3,300 a kg, but prices slipped as Russia sold off its stock of the metal.

"Rhenium's use as a catalyst is as important now as it was in the 1970s," Lipmann said.

Analysts say billions of cubic feet of gas in Africa, Russia, Asia and the Middle East are lost annually because there is no economic way to transport it.   

Rising demand for faster and more environmentally friendly aeroplanes has meant demand for rhenium, which helps engines reach higher temperatures and consume less fuel, is rapidly outstripping supply and producing a global deficit.

This deficit is very unlikely to be reversed and rhenium prices, which have surged 700 percent to around $8,000 per kg in just 18 months, will stay sky high.

 "In aero engines, rhenium is only a tiny constituent, but it is an essential one," said Anthony Lipmann owner of trading company Lipmann Walton.

"If you can raise the temperature it means you burn fuel more efficiently, save money and reduce the emissions in the atmosphere."

The metal -- discovered more than 80 years ago -- has been a boon to those seeking to reduce carbon emissions from energy use and demand has jumped.

Prices started to climb in 2005 when the deficit started to grow as demand from aero engine makers accelerated.

Some traders think prices will hit $10,000 a kg in the coming months.

Rhenium came to attention as early as the 1980s when engine makers discovered that nickel-base alloys containing 3 to 6 per cent rhenium were able to withstand more heat and so they designed aero engine to improve performance.

Major users include aero engine makers such as UK's Rolls Royce and US-based General Electric and Pratt Whitney, a division of United Technologies.

Rapid expansion in the airline industry and higher spending on military aircraft in the US could widen the deficit and fuel further price rises over coming years.

The civil aircraft sector expects orders for more than 20,000 aeroplanes over the next 20 years, while the US Joint Strike Fighter project is expected to involve more than 3,000 aircraft.

Rhenium has the second highest melting point -- 3,186 degrees Celsius -- of any metal after tungsten, allowing alloys to endure temperatures of up to 1,500 Celsius.

The metal's use in aeroplane engines and land based power turbines has grown along with the need to cut carbon emissions and cut costs as jet fuel prices have surged by around 300 percent since early 2002 to around $730 a tonne.

"Nickel-base superalloys for the aerospace and industrial gas turbine are the largest market for rhenium metal, accounting for about 60 per cent of world demand for the metal," said Steven Munnoch, managing director at recycler and refiner Avon Metals.

Demand for rhenium last year was estimated at between 60 and 69 tonnes against supplies of between 58 and 64 tonnes. Supplies included 15 tonnes of stockpile from one of the world's top producers, Kazakhstan.

"Supply of rhenium is very much limited and I would expect there to continue to be a supply deficit as it will be difficult to significantly increase production," said Niel Cahill, deputy head of sales and logistics at copper miner Kazakhmys.

The deficit last year on the surface looks quite low at around five tonnes, but it was probably larger as the Kazakhstan stockpile was sold to a merchant in the US, who appears to have rationed sales, traders said.<