In the iron tailings, calcite is the main source of calcium and therefore the target product. However, it is bound to the other mineral structures and cannot be used in this form, so leaching is a necessary step. As with copper tailings and anhydrite, a large proportion of the steady-state value, 70 %, is released within the first minute. Overall, the leaching of calcite from iron tailings is slower than the leaching of gypsum from copper tailings. However, with 640 mg L -1 , 1.78 times more calcium is leached in absolute terms, and with 𝐶 𝐶𝑎,𝐼𝑟𝑜𝑛 * = 0.47, 10 % more is leached compared to copper tailings.