Friday 4 October 2013

Three workers killed by gas at German potash mine.

Three workers have died and another four needed to be rescued after a controlled explosion at a Kali & Salz (K+S) GmbH operated mine near Unterbreizbach in Thuringia at about 1.10 pm local time (about 11.10 am GMT) on Tuesday 1 October 2013, which released an undetected pocket of gas, creating a larger than expected explosion and flooding the shaft with carbon dioxide. It is believed the men died as a result of gas inhalation rather than directly due to the explosion; they were equipped with oxygen masks but were apparently overcome by the gas before they had a change to put them on. Two of the men who were rescued are understood to have reached a secure room and phoned the surface for assistance, while remaining two sought shelter in a neighboring shaft. Investigators have not yet been able to re-enter the 700 m deep shaft, which is still flooded with gas. 

The interior of the Unterbreizbach Mine. Mining World.

Potash (potassium salt) is an important mineral in the production of commercial fertilizers. The Potash deposits of Thuringia, as with most central European salt deposits, were laid down during the drying of the Zechstein Sea. The Zechstein was an ancient inland sea in northern Europe, that evaporated away during the Middle to Late Permian, leaving vast mineral deposits that are excavated as far away as northern England. Because different ions have different properties they are precipitated out in a sequence as seawater evaporates, which does not show on a small scale, but which can produce distinct, workable, layered deposits when a body as large as the Zechstein evaporates.

The potash deposits of the Werra Salt Beds around Unterbreizbach are contained between layers of limestone, in an area that has suffered injections of volcanic material since the time of deposition. Heating limestone (CaCO₃), in this case by injecting hot magma into seems running through the limestone beds, causes it to release large amounts of carbon dioxide (CO₂), which in this case has become trapped in pockets within the salt deposits. Miners working these deposits need to take precautions against this, and carefully monitor the levels of carbon dioxide within the salt, though it appears on this occasion they were caught out.

A cross section through the subsurface rocks of northern Thuringia. Wölferbūtt, left of center in the diagram, is about 5 km southeast of Unterbreizbach. Brune et al. (2003).

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