Research on heat induced water liberation from rock salt with polyhalite in situ. Temperature test 5 at Asse salt mine

The temperature Test 5, which was begun in April 1982 to investigate the geological boundary conditions at heating of rock salt formations, was shut down according to schedule in February 1983 at termination of the 5th heating step. The test was carried out in the rock salt with polyhalite seams Na2P at the 775 m level of the Asse salt mine. Polyhalite K2Ca2Mg(SO4).42H2O contains crystalline water and may also be found in small quantities in the Older Halite Na2β, which is considered to be a suitable disposal medium. In order to investigate the temperature dependent start of the liberation of crystalline water, i.e., the decomposition of polyhalite in situ, the rock salt was heated stepwise to 1000C, 1500C, 2000C, 2300C and 2700C. The heaters were placed in a 7 m deep horizontal borehole. Apart from the initially high liberation rates in heating step 1, the highest liberation rates were registered in heating step 5 due to the original liberation of adsorptive water bound to the borehole wall. This was found to be a reference to the beginning of crystalline water liberation above 2300C. Extensive chemical/mineralogical analyses on heated rock salt were carried out in 1984. The immediate surroundings of the heaters, showed partly decomposed polyhalite crystals. A detailed temperature analysis showed that such decomposed crystals were only found in salt samples taken from areas which had temperatures higher than 2300C. Hereby it could be proved that the decomposition of polyhalite is negligible at 2000C