Development of a Black Liquor Electrolyzer for Lignin Extraction
Authors: Belo Inês C. , Oliveira Raisa Costa Paes , Mateus Margarida , Bordado João C.M. , Pinto Paula , Santos Diogo M.F. 
Published: February 2020

Nowadays, the pulp and paper industry is looking for different biorefinery and bioproduct market areas, in order to turn the production more sustainable and improve the economic feedback from its biomass and secondary products [1].

One of the significant challenges for pulp mills is the development of technologies for separation and conversion of lignin from black liquor [2]. The black liquor is a by-product produced during the Kraft pulping, which consists in a complex mixture of inorganic and organic constituents [1,3]. Considering the organics, lignin is one of the most interesting biopolymers from wood [4], making up 30-45 wt.% of the total solid composition [5]. This compound has wide applications, namely as a substitute of phenol in phenolic resins, as adjunct crosslinker for epichlorohydrin reaction (used in epoxy resins production), as binder in construction systems and as biodegradable plastic additive [6].

Millions of tons of black liquor, and consequently of lignin, are produced every year in pulp mills. A ton of bleached Kraft pulp generates 400-500 kg of lignin (dissolved in black liquor) and only 2% of this amount is commercialized [5]. The pulping process begins with the cooking of the wood chips in a digester, containing NaOH and Na2S, in order to obtain a fiber suspension [1,7]. The raw pulp is then washed with water, generating pulp and black liquor. The latter is concentrated and then burned in a recovery furnace for energy generation and chemicals recovery. Therefore, the production capacity of pulp mills is limited by the recovery furnace capacity [6,8].

The present work deals with the use of electrolysis as an alternative for reducing the volume of black liquor sent to the recovery furnace. It should establish the fraction of lignin that an electrolyzer can remove from the liquor stream before it returns to the process. The electrolysis of black liquor leads to lignin recovery by anodic electrodeposition and to hydrogen generation at the cathode [7].

Herein, we first determine the physicochemical properties of Kraft black liquor samples from Eucalyptus globulus, such as conductivity, dry solids content, organic/inorganic ratio and lignin content. Then, electrolysis experiments using a laboratory electrolyzer are performed, in order to extract the lignin from the black liquor. We assess the effect of several cell operation parameters, such as the electrolysis time, the effect of the electrode surface treatment, the volume of black liquor, and working temperature. Different materials, such as nickel, stainless steel, iron and cadmium are tested as anodes/cathodes, in order to select the best electrodes. For each distinct electrodes’ combination, the individual cathode and anode potentials are measured versus a saturated calomel electrode, to ensure that the working potentials are enough to attain the lignin oxidation and the hydrogen evolution reaction processes with high kinetics. This procedure enables optimizing the cell operation and helps developing the most cost-effective and energy-efficient electrolyzer. Polarization curves are drawn for each experiment. The obtained lignin is analyzed by optical microscopy, SEM-EDS, TGA and FTIR, and compared with lyophilized lignin obtained from black liquor acidification and with lignin from LignoBoost process [9]. The results show that the proposed black liquor electrolyzer allows extracting the lignin contained in Kraft black liquor, enabling the pulping processes to be more sustainable.

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