ROCK SALT MINING

Being "sent to the salt mines" has always had dark and foreboding connotations which were probably justified when salt mining first started at Wieliczka, Poland in 1,000 A.D. Mining methods have come a long way since that time and being "sent to the salt mines" is now a pleasant experience.

A modern salt mine is a bright and airy place of constant temperature and low humidity. It is made up of large, open rooms and roof-supporting pillars of salt so that the method of mining is known as "room & pillar".

A shaft is sunk through the overlying rock to the salt deposit and the salt is removed leaving large square caverns alternating with large square pillars of salt which are necessary to support the rocks above the deposit. The size of the rooms and pillars is determined by the depth and thickness of the deposit.

Blasting breaks the salt into large but manageable pieces which are conveyed to underground crushers which, in some cases, reduce it to its final sizes; in others it is carried to the surface after the first crushing.

After crushing, the salt, either underground or on the surface or sometimes both, is screened into the various sizes required by the consumer.

The large tonnages possible with rock salt mining make it a fairly economical method of production. The costs of shaft sinking, equipment and bringing a mine into production are, however, quite high so that large tonnages are necessary to justify the investment.

The purity of rock salt, depending more on the purity of the deposit than does evaporated salt, varies considerably. The purity is usually between 92% to 98%, although in some mines it approaches 99.5% and, even in the lower purity mines, the purity can be improved by "beneficiation" which, of course, raises the cost of production.