Method for recovering alkali metal salt and apparatus for recovering alkali metal salt

A method for recovering an alkali metal salt from a solution X containing an alkali metal ion and at least one of a compound HA having a pKa of 3.0 or more and 5.0 or less and an anion A forming the compound HA, the method comprising the following steps (1) to (3),

• (1) a pH adjustment step of adjusting a pH of the solution X to 0 or more and 4.0 or less when the pH is more than 4.0;
• (2) an aluminum salt addition step of adding an aluminum salt to the solution X such that a molar concentration (mol/L) of aluminum is 0.25 times or more a total anion A molar concentration (mol/L), which is a sum of a molar concentration (mol/L) of the compound HA in the solution X and a molar concentration (mol/L) of the anion A present as an ion in the solution X;
• and
• (3) a nanofiltration step of separating, from the solution X after the step (1) and the step (2), the alkali metal ion to a permeation side by using a nanofiltration membrane.