# How Insoluble Inclusions and Intersecting Layers Affect the Leaching Process within Potash Seams
## 3 Results
### Leaching Zone Evolution for Low Dissolution Rates (Da \(\approx\) 1)  
To evaluate the shift from reaction- (Da \(<\) 1) to transport-dominated (Da \(>\) 1) systems in the case of \(k_{max}\) = 5 \(\times\) 10\({}^{-6}\) cm/s, the distribution of Mg\({}^{2+}\) after different time periods is shown in Figure 9. The Mg\({}^{2+}\) concentration is a useful indicator for fluid density and saturation: an absence of Mg\({}^{2+}\) usually represents a NaCl solution with a density of 1200 kg/m\({}^{3}\), whereas a solution with \(>\)85 g/1 Mg\({}^{2+}\) (at 25 \({}^{\circ}\)C) has a density of \(>\)1270 kg/m\({}^{3}\) and is fully saturated with respect to halite, sylvie and carnallite. In cases of homogeneous potash seams (Figure 9), the solution is highly undersaturated with regard to sylvie and carnallite along the entire dissolution front during the first two years. Accordingly, both minerals are fully dissolved and only halite remains. In contrast to halitic areas, the rock is fully permeated and porosity is high (\(\approx\)30%) within these zones (further referred to as halite zones). The dissolution front is planar during that time, whereas concentration and density gradients are comparatively low. However, the Mg\({}^{2+}\) concentration at the bottom increases, and after 2 years, when Da = 1 is reached, the dissolution of sylvie and carnallite is significantly reduced at the lower end of the dissolution front. As a result, a sylvinitic zone, consisting of halite and sylvie (Figure 9d,e), is formed next to it and an inclination of the dissolution front occurs. Over time, its upper end moves upwards (Figure 9b,c) and the sylvinitic zone with lower porosity grows. Additionally, a small barrier is formed next to the lower end of the inflow due to the precipitation of halite (Figure 9d). In cases of halite inclusions, the leaching zone evolution is basically similar to the homogeneous case. However, the more often the dissolution front is disturbed by inclusions, the more irregular it becomes (Figure 10a-c). The same applies to the sylvinitic zone within the lower area. Apart from that, sylvinitic zones now also occur above or behind inclusions and are often re-dissolved as soon as the flow regime changes and local Mg\({}^{2+}\) concentrations decrease. A slower growth rate, which is usually associated with a sylvinitic  
Figure 9: Convection cell of a system shifted from a reaction- to a transport-dominated one: Mg\({}^{2+}\) concentration distribution after simulation times of (**a**) 2 years, (**b**) 5 years and (**c**) 13.4 years; (**d**) porosity distribution and (**e**) mineralogical composition after 10 years for a homogeneous potash seam and \(k_{max}\) = 5 \(\times\) 10\({}^{-6}\) cm/s. zone next to the dissolution front, can now be observed within the upper area as well (Figure 10c). Generally, the convection cell is increasingly divided into smaller sub-cells, the broader the inclusions are. However, with regard to the shape and penetration depth of the leaching zone, differences compared to the homogeneous case are negligible. In the case of an intersecting, insoluble halite layer, the system is split up into two convection cells that evolve independently of each other. In the case of a horizontal layer (Figure 10d), both convection cells show nearly the same evolution compared to the homogeneous case (Figure 9). Although the sylvnitic zone is formed slightly later--after about 3 years instead of 2 years--it quickly covers a larger part of the dissolution front: after 10 years, approximately 50% of it, compared to 30% in the homogeneous case (Figures 9 and 10d). However, the penetration depth at the upper end of the dissolution front is approximately the same. In contrast, the convection cells develop differently after some years if the intersecting layers are inclined (Figure 10e,f).