Rock Salt Production
The rock salt we use for gritting roads comes from mines of ancient underground salt deposits. In the UK, mines are situated in Cleveland, County Antrim and below the Cheshire town of Winsford.
There are many hundreds of salt beds or domes (as they are sometimes known) across the world. Mines vary in depth from 100 metres or so, to a mile or more. Within the mines, there are networks of pathways formed by the areas from which salt has already been extracted. These are used as roads for mining vehicles to move from one part of the mine to another, or for personnel to travel from the shafts to the working face. The UK’s salt mines have some 140 miles of tunnels – that’s almost as long as the M5 motorway!
In Britain, rock salt is mined using two techniques:
Cut and Blast Mining
A slot is cut at the base of the rock face using a machine called an undercutter, with a jib carrying a series of tungsten-carbide picks. The face is then drilled with a series of carefully sited holes, using an electro-hydraulic rotary drill. The holes are charged with explosives and the face is blasted, yielding about 1,400 tonnes of broken rock salt.
The rock is crushed into pieces about the size of a football, using a feeder-breaker. It is then carried on a conveyor belt to the main crusher which breaks it down into smaller pieces, passing through a type of sieve or screen to ensure that it has reached the correct size for use in road de-icing. The salt is then hoisted to the surface in skips.
This type of mining produces smaller lumps of rock than the cut and blast technique. A boring machine, similar to a pneumatic drill used in digging up roads, is used. It has a rotating head carrying tungsten-carbide tips, which bores into the salt. The lumps are then taken directly to a crushing and screening plant, without the need to be crushed by a feeder-breaker first.
Under either technique, care must be taken to ensure that the mine is stable by leaving substantial ‘pillars of salt’ to support the mine roof. This mine layout is called ‘Room and Pillar’ mining, and mine engineers use the principles of rock mechanics to calculate the optimum size of the pillars, for safety and stability.
Before storage, the salt is treated with an anti-caking agent to stop the pieces coagulating. This ensures that it can be held in local storage depots, ready for use on the roads as soon as frost is forecast.