A method for producing a concentrated aqueous sodium hydroxide solution from a purge stream, said purge stream deriving 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 wegscheiderite crystallizer, or a sodium bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or sodium bicarbonate from a sodium carbonate or bicarbonate ore deposit, and said purge stream comprising at least 1 % by weight of a sodium salt selected among sodium chloride, sodium sulfate and mixtures thereof, and said concentrated aqueous sodium hydroxide solution comprising : - at least 25 % by weight of NaOH, - and at least one soluble salt, said soluble salt comprising at least one of the following elements: As, Ba, B, Ca, Co, K, Li, Mg, Mo, P, Pb, Se, Si, Sr, Te, Tl, Ti, V, W, and said method comprising the following steps : - f) adding lime to the purge stream, in presence of water, so that 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 in order to obtain : the concentrated aqueous sodium hydroxide solution comprising at least 25 % by weight of NaOH, and to crystallize a solid comprising sodium carbonate and comprising sodium chloride and/or 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 preferably at most 2, - i) separating the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate from the concentrated aqueous sodium hydroxide solution. - at least 25 % by weight of NaOH, - and at least one soluble salt, said soluble salt comprising at least one of the following elements: As, Ba, B, Ca, Co, K, Li, Mg, Mo, P, Pb, Se, Si, Sr, Te, Tl, Ti, V, W, - f) adding lime to the purge stream, in presence of water, so that 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 in order to obtain : the concentrated aqueous sodium hydroxide solution comprising at least 25 % by weight of NaOH, and to crystallize a solid comprising sodium carbonate and comprising sodium chloride and/or 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 preferably at most 2, the concentrated aqueous sodium hydroxide solution comprising at least 25 % by weight of NaOH, and to crystallize a solid comprising sodium carbonate and comprising sodium chloride and/or 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 preferably at most 2, - i) separating the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate from the concentrated aqueous sodium hydroxide solution. <EOS>