Caustic recovery from green liquor of agro-based paper mills using electrolysis

Abstract

In the chemical recovery section of pulp and paper mills, the cooking liquor (white liquor, mainly caustic, NaOH) is regenerated by addition of fresh lime (CaO) in green liquor solution (rich in sodium carbonate, Na2CO3), where calcium carbonate (CaCO3) get precipitated leaving NaOH in solution (causticizing process). Subsequently, CaO is regenerated from precipitated CaCO3 following washing/drying/calcination steps which are highly energy intensive and environmentally unsafe. The current investigation reports conversion of green liquor to caustic solution using electrolysis through a cation exchange membrane (CEM) placed between two electrodes. Process parameters e.g. current density, feed concentration, flow velocity, flow promoter, electrode-membrane spacing, role of gravity flow were systematically investigated to find optimum values. Process optimization was performed with Na2CO3 (highest constituent of green liquor) solution to avoid complexity, which were then extended to complex salt mixtures of green liquors (synthetic/ industrial sample). Performance analysis of any batch was carried out based on moles of NaOH produced and associated current efficiency and energy consumption observed. With 600 A·m−2 current density, 1.93 mol·L−1 of NaOH could be produced in 6 h of electrolysis through a cell of 66 × 10−4 m2 of CEM, where flow rates of both feed and catholyte streams were counter current and fixed at 3.472 × 10−3 m·s−1.