Figure shows the ion concentrations for the various acids and their stoichiometric ratio. The concentration of Ca 2+ ions is shown at different times for leaching with 5 % w/w anhydrite. The concentration of Ca 2+ ions increases in the first 30 minutes and then hardly changes. Thereafter, the ion concentration remains constant; consequently, no further Ca 2+ ions are dissolved and the acid leaching is complete. The initial value at 700 mg L -1 is noticeable. This corresponds to the solubility of anhydrite in deionized water at 20°C. According to Lide et al. the solubility product of CaSO4 is 4.93 10 -5 mol 2 L -2 at 20°C, which corresponds with the molar mass of calcium sulfate to 956 mg L -1 in water and is therefore slightly higher than the measured values . No increase in acid concentration can be observed with sulphuric acid. When calcium is dissolved from the anhydrite in water, SO4 2-is released as an anion, which is also the product of the reaction with sulphuric acid. The equilibrium of this reaction is therefore already present due to the saturation of SO4 2-in water, which is why the sulphuric acid cannot dissolve any further ions from the crystal. In this case, the pH value also has no influence on the leaching, as there is no change in the ion concentration even with different acid concentrations in the stoichiometric ratios 𝑆𝑡 between 0.2 and 1. Therefore, this acid is not treated in the further course. In comparison, the dissolved calcium content of hydrochloric acid continues to increase with increasing acid concentration. For example, after 24 h the Ca ion concentration at 𝑆𝑡 1.0 is 3916 mg L -1 , whereas the concentration at 𝑆𝑡 0.5 is 2888 mg L -1 and at 𝑆𝑡 0.2 1717 mg L -1 .