The solution behavior of CaSO4 in saline water and possible precipitation

In the FRIMA salt project in Harlingen (The Netherlands) rock salts of different degrees of purity are produced following the specifications of the clients. The salt plant is fed by solution mining of two caverns, which are located at depths down to 3,050 m. Consequently; they are currently the deepest caverns in the world. The depths of the cavities cause unusual temperature/pressure conditions. There were several factors that in the first phase of sump leaching induced a crystallisation of gypsum in the production annulus of one cavern. To avoid precipitations on the tubing walls it is necessary to distinguish between unchangeable parameters, such as the mineralogical/chemical composition of the salt horizons, and variables, which can be influenced during the leaching process, such as the composition of the solution which is used for the leaching, and the injection pressure which influences the flow rate of the fluids. This paper gives a brief introduction of the technical data of the FRIMA Zoutindustrie project. Afterwards, the processes that induce the gypsum crystallisation are discussed in context with the geochemical fundamentals of the ternary system NaCl-CaSO4-H2O.