A method for producing a concentrated aqueous sodium hydroxide solution from a purge stream deriving from a sodium carbonate, or sesqui-carbonate, or wegsheiderite crystallizer, or sodium bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or bicarbonate, and at least 1% of sodium chloride or sodium sulfate and a soluble impurity from an ore deposit comprising at least one of the following elements: As, Ba, B, Ca, Co, K, Li, Mo, P, Pb, Se, Sn, Sr, Te, Tl, Ti, V, and W, to be purified, the method comprising: causticizing at least 50 mol. % of the sodium carbonate into a caustic solution and into a calcium carbonate mud with lime and water; separating the mud from the caustic solution; concentrating the caustic solution by removing part of the water to obtain a concentrated caustic solution comprising at least 25% NaOH, and a crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate; and separating the crystallized solid from the concentrated caustic solution, said crystallized solid to be disposed of or to be further valorized.