Electrolytic Treatment of Black Liquor with Caustic Recovery, 2013 PEERS Conference

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The technical feasibility of electrolytic acidification of black liquor in order to precipitate lignin was demonstrated with our pilot plant. The electrolytic process was tested in the 4.5 – 7.0 pH range. The best energetic performance was obtained at pH 7.0 while converting 63% of the recovered black liquor sodium into a 10 % caustic solution using 3 920 kWh/MT. At pH 4.5, the sodium recovery could reach 70% but at the expense of higher energy consumption partially due to the excessive formation of carbon dioxide. A cyclic caustic rinse of the anode compartment helps to remove the fine lignin particle deposits and restore the cell voltage. The proposed electrochemical option leading to lignin precipitation requires no acidifying agents and generates no additionnal effluent. Moreover, it represents an advantage compare to the chemical approach using CO₂ for kraft mills limited by the recausticizing system.