A method for producing a concentrated aqueous sodium hydroxide solution from a purge stream deriving from a sodium carbonate, or sesquicarbonate, 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.

A treatment method for reducing carbon dioxide emission of combustion exhaust gas includes: a caustic soda synthesis step; a treatment step of reducing carbon dioxide emission of combustion exhaust gas; and a recycling step. In the caustic soda synthesis step, a natural sodium carbonate aqueous solution (Na.sub.2CO.sub.3) prepared by dissolving natural sodium carbonate ore powder composed of Na.sub.2CO.sub.3 and NaHCO.sub.3 in a caustic soda aqueous solution is used to generate a caustic soda aqueous solution and calcium carbonate precipitate by a causticization reaction with slaked lime, and solid-liquid separation is performed to obtain a synthetic caustic soda aqueous solution. In the treatment step, the synthetic caustic soda aqueous solution and purified combustion exhaust gas are brought into gas-liquid countercurrent contact so that carbon dioxide in the exhaust gas is absorbed by the synthetic caustic soda aqueous solution and immobilized as sodium carbonate.

A treatment method for reducing carbon dioxide emission of combustion exhaust gas includes: a caustic soda synthesis step; a treatment step of reducing carbon dioxide emission of combustion exhaust gas; and a recycling step. In the caustic soda synthesis step, a natural sodium carbonate aqueous solution (Na.sub.2CO.sub.3) prepared by dissolving natural sodium carbonate ore powder composed of Na.sub.2CO.sub.3 and NaHCO.sub.3 in a caustic soda aqueous solution is used to generate a caustic soda aqueous solution and calcium carbonate precipitate by a causticization reaction with slaked lime, and solid-liquid separation is performed to obtain a synthetic caustic soda aqueous solution. In the treatment step, the synthetic caustic soda aqueous solution and purified combustion exhaust gas are brought into gas-liquid countercurrent contact so that carbon dioxide in the exhaust gas is absorbed by the synthetic caustic soda aqueous solution and immobilized as sodium carbonate.

A method for recovery of aluminum hydroxide Al(OH).sub.3 from an aluminum enriched water/wastewater treatment sludge is disclosed. The method includes the steps of: adding a hydrated lime slurry to the aluminum enriched water/wastewater treatment sludge to form an alkaline sludge; adding sodium carbonate Na.sub.2CO.sub.3 to the alkaline sludge to form a Na.sub.2CO.sub.3 treated sludge; forming a first supernatant from the Na.sub.2CO.sub.3 treated sludge of step b) containing NaAl(OH).sub.4; introducing CO.sub.2 to the first supernatant to form a precipitate of Al(OH).sub.3 and a second supernatant containing NaHCO.sub.3; and recycling at least a portion of the NaHCO.sub.3 from the second supernatant back to the alkaline sludge of step a).

A method for recovery of aluminum hydroxide Al(OH).sub.3 from an aluminum enriched water/wastewater treatment sludge is disclosed. The method includes the steps of: adding a hydrated lime slurry to the aluminum enriched water/wastewater treatment sludge to form an alkaline sludge; adding sodium carbonate Na.sub.2CO.sub.3 to the alkaline sludge to form a Na.sub.2CO.sub.3 treated sludge; forming a first supernatant from the Na.sub.2CO.sub.3 treated sludge of step b) containing NaAl(OH).sub.4; introducing CO.sub.2 to the first supernatant to form a precipitate of Al(OH).sub.3 and a second supernatant containing NaHCO.sub.3; and recycling at least a portion of the NaHCO.sub.3 from the second supernatant back to the alkaline sludge of step a).

Techniques for converting a portion of a carbonate to hydroxide include receiving an alkaline carbonate solution that includes between 0.1M (moles per liter of solution) to 4.0M hydroxide and between 0.1M to 4.1M carbonate; reacting, in a slaking process, quicklime (CaO) and a low carbonate content fluid to yield a slurry of primarily slaked lime (Ca(OH).sub.2); and reacting the Ca(OH).sub.2 slurry and the alkaline carbonate solution to grow calcium carbonate (CaCO.sub.3) crystal aggregates of 0.0005 mm.sup.3 to 5 mm.sup.3 in volume in a fluidized-bed reactive crystallizer.

Techniques for converting a portion of a carbonate to hydroxide include receiving an alkaline carbonate solution that includes between 0.1M (moles per liter of solution) to 4.0M hydroxide and between 0.1M to 4.1M carbonate; reacting, in a slaking process, quicklime (CaO) and a low carbonate content fluid to yield a slurry of primarily slaked lime (Ca(OH).sub.2); and reacting the Ca(OH).sub.2 slurry and the alkaline carbonate solution to grow calcium carbonate (CaCO.sub.3) crystal aggregates of 0.0005 mm.sup.3 to 5 mm.sup.3 in volume in a fluidized-bed reactive crystallizer.

Concentrated aqueous sodium hydroxide solution comprising: at least 25% by weight of NaOH, sodium chloride (NaCl) and/or sodium sulfate (Na.sub.2SO.sub.4), and one soluble impurity from a sodium carbonate or bicarbonate ore deposit, said soluble impurity being selected among: As, Ba, B, Ca, Co, K, Li, Mg, Mo, P, Pb, Se, Si, Sr, Te, Tl, Ti, V, W, and the soluble impurity being in specific concentrations ranges. And process for producing such concentrated aqueous sodium hydroxide solution by treating a purge stream comprising sodium carbonate or bicarbonate.

Concentrated aqueous sodium hydroxide solution comprising: at least 25% by weight of NaOH, sodium chloride (NaCl) and/or sodium sulfate (Na.sub.2SO.sub.4), and one soluble impurity from a sodium carbonate or bicarbonate ore deposit, said soluble impurity being selected among: As, Ba, B, Ca, Co, K, Li, Mg, Mo, P, Pb, Se, Si, Sr, Te, Tl, Ti, V, W, and the soluble impurity being in specific concentrations ranges. And process for producing such concentrated aqueous sodium hydroxide solution by treating a purge stream comprising sodium carbonate or bicarbonate.

A method for treating a purge stream derived from a sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or sodium bicarbonate and at least 1% by weight of sodium chloride and/or sodium sulfate, the method comprising: causticizing at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate into a caustic solution and into a calcium carbonate mud with lime and water, separating the calcium carbonate 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, separating the crystallized solid from the concentrated caustic solution, said crystallized solid to be disposed of or to be further valorized, recycling part of the concentrated caustic solution to the sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer.