Process for treating a sodium carbonate purge

A method for treating a purge stream derived from an anhydrous sodium carbonate crystallizer, or a sodium carbonate monohydrate crystallizer, or a sodium carbonate decahydrate crystallizer, or a sodium sesquicarbonate crystallizer, or a wegsheiderite crystallizer, or a sodium bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or sodium bicarbonate and at least 1% by weight of a sodium salt selected among sodium chloride, sodium sulfate and mixtures thereof,

• the method comprising the following steps:
• f) adding lime to the purge stream, in presence of water,
• to causticize at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate,
• into an aqueous sodium hydroxide solution and
• into a calcium carbonate mud;
• g) separating the calcium carbonate mud from the aqueous sodium hydroxide solution;
• h) concentrating the aqueous sodium hydroxide solution by removing part of the water to obtain:
• a concentrated aqueous sodium hydroxide solution comprising at least 25% NaOH, and
• to crystallize a solid comprising sodium carbonate and comprising sodium chloride and/or sodium sulfate,
• wherein lime added at step f) and water removed at step h) are controlled so that the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 2,
• i) separating the crystallized solid comprising sodium carbonate and sodium chloride and/or sodium sulfate from the concentrated aqueous sodium hydroxide solution, said crystallized solid to be disposed of or to be further valorized,
• j) valorizing the concentrated aqueous sodium hydroxide solution as a salable sodium hydroxide solution or recycling at least one part of the concentrated aqueous sodium hydroxide solution to the anhydrous sodium carbonate crystallizer, or to the sodium carbonate monohydrate crystallizer, or to the sodium carbonate decahydrate crystallizer, or to the sodium sesquicarbonate crystallizer, or to the bicarbonate crystallizer, or to processes upstream of the anhydrous sodium carbonate crystallizer, or of the sodium carbonate monohydrate crystallizer, or of the sodium carbonate decahydrate crystallizer, or of the sodium sesquicarbonate crystallizer, or of the bicarbonate crystallizer.