A method for purifying heavy metal impurities (such as antimony, arsenic, iron, cadmium, and lead) from 28 to 38 weight percent aqueous crude sodium sulfate solution is achieved by adding ferric sulfate to the aqueous crude sodium sulfate solution to form a suspension, progressively neutralizing the suspension from a pH of less than 3.5 up to at least 6, and separating the ferric-sulfate-nucleated precipitate from the suspension. An oxidizing agent such as peroxide is optionally added to the suspension to augment adsorption of the heavy metals.