Magnesium chloride solutions including providing aqueous magnesium chloride solution with magnesium chloride concentration of 10-30 wt. % to concentration step where water is evaporated, resulting in concentrated magnesium chloride solution with magnesium chloride concentration of 30-50 wt. %, wherein concentration step is carried out in one or more stages, wherein at least one of the stages is conducted at elevated pressure, withdrawing concentrated magnesium chloride solution from concentration step, and providing it to thermohydrolysis reactor of at least 300° C., withdrawing MgO from thermohydrolysis reactor in solid form, and withdrawing a HCl containing gas stream of at least 300° C. from thermohydrolysis reactor, providing the HCl-containing gas stream of at least 300° C. to cooling step, where HCl-containing gas stream is contacted with cooling liquid, withdrawing HCl-containing gas stream below 150° C. from cooling step, circulating cooling liquid through heat exchanger where energy is transferred to heating liquid which circulates from heat exchanger to concentration step.