## 4 Applicability and Effect of Citric Acid-Based Treatment
### The Behavior of Notable Components: Calcium, Iron, and Aluminum
Since gypsum is already concentrated in the fine fraction during selective milling, as reported by [PERSON] et al. [19], it is therefore reasonable to presume that the measured anhydrite in MF010-Min is the fraction from \(\beta\)-CaSO\({}_{4}\). In this case, supported by the results of the water-only washing process in Figures 7 and 8, the solubility limit of \(\beta\)-CaSO\({}_{4}\) is only around 0.02 m (mol\(\cdot\)kg\({}^{-1}\)) at 25 \({}^{\circ}\)C [49]. Given the circumstances in the water-only washing process (300 mL; LS10), this solubility limit will yield a calcium reduction of around 5%. Given the uncertainty range mentioned earlier in the present study, a 5% change is considered constant because it is lower than the minimum 10% to be considered significant. In another case, instead of being dissolved in water [50] and depending on the temperature and concentration, hydrate formation of gypsum from \(\beta\)-CaSO\({}_{4}\) might also be initiated upon contact with water. Aggravated by the slow hydration process [51], the solubility limit of gypsum in water is only up to 0.018 M (mol\(\cdot\)L\({}^{-1}\)) in water at 25 \({}^{\circ}\)C [52]. Since this solubility is lower than that of \(\beta\)-CaSO\({}_{4}\), increasing changes in the calcium concentration cannot be expected. A similar approach will apply even though residual \(\gamma\)-CaSO\({}_{4}\) and hemihydrate are available in MF010-Min. Owing to their metastability, hydration to gypsum will be initiated, yet it will directly crystallize, yielding constant concentration changes in calcium. Adding citric acid to the solvent apparently cannot improve the limitation notably. Neither anhydrite nor gypsum are soluble in a citric acid solution. It has only been reported that citric acid can inhibit gypsum settings [53] and influence its precipitation [54; 55; 56] but is not fully able to dissolve the gypsum, especially at relatively low pH values [57; 58]. In contrast to the CaSO\({}_{4}\) phases, different behaviors of CaCO\({}_{3}\) during the washing process were anticipated, especially during its interaction with citric acid. Despite being known to be insoluble in water [59], the reaction and the resulting products from CaCO\({}_{3}\) strongly depend on the pH values. The fact that CaCO\({}_{3}\) is barely dissolvable in water amplifies the constant calcium concentration in the water-only washing process in Figures 7 and 8. Nonetheless, in the case of CaCO\({}_{3}\) reacting with citric acid, the formation of calcium citrate is expected on the surface of CaCO\({}_{3}\)[60], where a local high pH value might be expected [61]. Furthermore, despite having a solubility limit higher than that of CaCO\({}_{3}\), it was reported by [PERSON] [62] that the solubility of calcium citrate in water at 25 \({}^{\circ}\)C is only approximately 0.002 m, which is a factor of ten lower than the previous solubility limit of \(\beta\)-CaSO\({}_{4}\). The improvement in both the solubility limits of CaCO\({}_{3}\) and calcium citrate can be expected to be significant only if the pH can be maintained at a relatively low value [63; 64; 65], which is the case considered in this study. Therefore, in addition to the calcium originating from calcium-bearing silicates, the concentration change in calcium should also be magnified by the dissolution of CaCO\({}_{3}\). To a certain extent, the precipitation of calcium citrate could explain the results of Figures 5, 6 and 9, where a lower change in calcium was measured if no agitation was involved. In this context, a local high pH value might be initiated, where calcium citrate precipitation is expected and retards further reactions. The subsequent cations observed by [PERSON] et al. [28] would preferably form a complex and dissolve in a citric acid solution after the calcium is composed of iron and aluminum.