Hi Saba,  not quite sure what ...

Hi Saba,  not quite sure what the other responders are proposing.  Just to say caustic is highly soluble and therefore rather difficult to extract from your brine.  So as a suggestion you may be able to extract everything else leaving you a caustic solution.  The only way I can think of that may be economically viable is the process of freeze crystalization whereby you super-concentrate your brine by freeze desalination (e.g. hybrid-ice process) leaving behind the most soluble components. Just some thoughts - good luck