15 June 2001

Digging deep to extract the ore

About half the UK tonnage of potash used for fertiliser

manufacture is imported. The other half is extracted from

700 miles of tunnels which start at Boulby, near Whitby

in Cleveland. North-east correspondent Wendy Owen

went down the mine to investigate

AT 1200m (4000ft), Boulby mine is one of the deepest in Europe and extends four miles out under the North Sea. Another series of tunnels runs along a 3.5mile stretch parallel with the coastline. Potash was discovered in England by a company searching for hydrocarbon deposits. It was formed when the seas dried up 230m years ago during the earths evaporation period. Lying above a layer of salt, the potash seam is up to 18m thick and runs under the sea to the German coastline, where another potash mine has been developed.

The mine is owned by Cleveland Potash, which has invested £130m and employs about 800 people. Because the area falls within a national park, there is a strict limit on the number of lorries which can enter and leave the mine. For that reason, the company has its own railway line to transport processed potash and the by-product, salt, to its shipping terminal at nearby Middlesbrough.

Some 3m tonnes of potash ore are extracted each year, producing 1m tonnes of processed potash and 630,000t of salt. Because the mine is so deep, the ambient temperature is a constant 40C (104F) all year round and air is pumped down the mine with forcing fans, emerging through a return airway. The tunnels are much larger than most people imagine at about 8m (26ft) wide and 3.6m (12ft) high.

Mining goes on around the clock, with the workers on eight-hour shifts operating machinery using only headlights and the beam from their miners lamps. They travel around the underground road system in vans which are strictly limited to 25mph.

A geologist works ahead of the miners, probing the rock with a drill containing a tiny camera. This drill reaches 1.5 miles into the rock with a series of detachable rods which identify the seam of potash for excavation. The rods also highlight the presence of mudstone, which is the colour and texture of chocolate and will collapse if any attempt is made to mine it.

7000hp electric motor

Shuttle cars then carry the rock to a crusher, where it is loaded onto a conveyor belt and taken to the lift shaft. Because it is expensive to run the lift, the mine also has its own workshops. A 7000hp electric motor, thought to be the largest in the northern hemisphere, is needed to power the lift and all the operations underground.

The method for extracting the potash involves crushing the ore and mixing it with brine and chemicals which separate the potash from the salt in a kind of giant bubble bath.

Most of the salt is sold to local authorities for de-icing the roads, while about 90% of the extracted potash is sold to fertiliser manufacturers.

"The UK market for potash is about 800,000t," says the companys commercial director, Roger Cockcroft. "In theory, we could supply the whole of the UK requirement, but it all comes down to price – it is cheaper for some companies in the south to bring in shipments from abroad than to transport potash by road."

Some 3m tonnes of potash ore are extracted each year from the Boulby mine.