Integrated system for recovery of CaCO3, NaCl and MgSO4·7H2O from nanofiltration retentate

An integrated membrane system has been developed in order to recover dissolved salts that are present, at low concentration, in typical feed streams to desalination plants. The experimental work aimed to obtain CaCO3, NaCl and MgSO4·7H2O as solid products from nanofiltration retentate. Since gypsum scale causes reduction of SO42- content in the solution and drastically limits the recovery of epsomite, Ca2+ ions have been almost quantitatively precipitated by reaction with NaHCO3/Na2CO3. Sodium (bi)carbonate solutions have been produced by reactive absorption of CO2 into sodium hydroxide solutions carried out by membrane contactors technology. The flow sheet has been completed with a membrane crystallization stage that allows the generation of supersaturation for salts crystallization.