Dense rock and large capacity
Extensive surveys performed by agencies such as the Norwegian Geotechnical Institute (NGI) have documented that gas-free, stable and dried gypsum residue is a highly suitable material for backfilling and stabilising Norcem’s disused mines.
The mines will become useful once again: They will become a safe disposal site for the end product generated by NOAH’s treatment plant for inorganic hazardous waste on Langøya, outside the coast of Holmestrand.
NGI’s conclusion is that the rock type in the mines, combined with the treatment method, will provide a safe waste disposal facility of the gypsum residue. Detailed simulations also show that, even in the presence of an earthquake, this would have an insignificant effect on the mines and the waste disposed there.
The volume of the mines is also an advantage. The Dalen mines in Brevik has an enormous volume, with a total gallery length of almost 250 kilometers. Two large vehicles can easily pass each other in the galleries. Through the course of its operations, Norcem has built up a great deal of knowledge about the mines and the conditions in the long galleries. The massive size of the mines means that it will be possible to use them as a waste disposal site for almost 30 years.
Suitable for waste disposal
Before NOAH started using fly ash to neutralise the acid from Kronos Titan AS in Fredrikstad, limestone was used in the process. That is the same type of rock which has been mined from Dalen for almost 100 years. The limestone is alkaline, as is the gypsum residue.
The gypsum residue is mainly made up of gypsum, lime, silicate and some iron hydroxides. Heavy metals typically make up 1-3 percent of the waste, and these are stabilised by transforming them into minerals with a low solubility or by binding them to the iron hydroxides. This means that water ingress into the waste disposal site over time will not cause the environmentally hazardous substances to dissolve. Water which leaks into the mines will be pumped up out of the mines and purified before being discharged into the sea.
NOAH has extensive experience with water purification through its operations at Langøya. As the mine is backfilled, water ingress will gradually reduce as the pressures inside and outside the mine equalise.