The invention pertains to a method for processing MgCl2 solutions comprising the steps of - providing an aqueous solution comprising 5-25 wt.% of MgCl2 and optionally organic contaminants to an evaporation step, wherein water and, where present, organic components are evaporated, - withdrawing an aqueous solution with a MgCl2 concentration of 25-35 wt.% from an evaporation step and providing it to a preconcentrator where it is contacted with a HCl containing gas stream with a temperature of at least 300°C, - providing an aqueous solution with a MgCl2 concentration of 35-45 wt.% resulting from the preconcentrator to a thermohydrolysis reactor, the reactor being at a temperature of at least 300°C, - withdrawing MgO from the thermohydrolysis reactor in solid form, and withdrawing a HCl containing gas stream from the thermohydrolysis reactor, said HCl-containing gas stream having a temperature of at least 300°C, - providing the HCl-containing gas stream with a temperature of at least 300°C to the preconcentrator, - withdrawing a HCl-containing gas stream with a temperature of at most 150°C from the preconcentrator. It has been found that the process according to the invention can be integrated into a fermentation process in such a manner that a stable and efficient process is obtained.