Characterization of Soda and Kraft Black Liquor for Sugarcane Bagasse

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Published on International Journal of Engineering & Industry
Publication Date: April 21, 2019

Medhanit Mamaye, Zebene Kiflie, Sisay Feleke & Abubeker Yimam
School of Chemical and Bio Engineering, Addis Ababa Institute of Technology, Addis Ababa University
Ethiopian Agricultural Research Council Secretariat

Journal Full Text PDF: Characterization of Soda and Kraft Black Liquor for Ethiopian Sugarcane Bagasse.

This paper investigates the physio-chemical characterizations and comparison of the black liquor generated from untreated and physical fractionation pretreated (PFP) Ethiopian sugarcane bagasse (ESCB) soda and Kraft pulping processes. The lignin, COD, BOD and TDS content are found to be (28.45 gm/L, 50000 mg/mL, 328 mg/mL, 145.83 mg/L), (23.29 gm/L, 74100 mg/mL, 192 mg/mL, 157.49 gm/L), (40.53 gm/L, 54300 mg/mL, 356 mg/mL and 157.88 gm/L) and (36.51 gm/mL, 77400 mg/mL, 200 mg/mL and 164.86 gm/L) for PSBL, PKBL, USBL and UKBL, respectively. Based on the results, USBL and UKBL present higher contents of lignin, COD, BOD and TDS than PSBL and PKBL. By comparing results obtained for the both black liquors, untreated and physical fractionation pretreated Ethiopian sugarcane bagasse, the lignin, COD, BOD and TDS contents of untreated SCB is greater that of pretreated one. As a result, physical fractionation pretreatment is vital to decrease the pollution load generated during pulp production from sugarcane bagasse.

Keywords: Black Liquor, Pretreatment, Pollution, Lignin, COD & BOD.

1. Introduction
Pulp and paper industry consume large amount of fresh water in various processes including raw material preparation, pulp washing and paper manufacturing, pulp bleaching and fiber recycling, hence it is water intensive process (Ashra et al., 2015; Merayo et al., 2013). The amount of water used depends on the raw material and the pulping method used (Kesalkar et al., 2012). At the beginning of the 20th century 500 to1000 m3 of water was consumed in the production of one ton of pulp (Toczylowska, 2017: Kamali & Khodaparast, 2015). In modern pulp mills the water consumption decreased 10 to 50 m3 per tons of pulp. This is due to the modern pulp mills used wet process throughout the pulping and papermaking process (Toczylowska, 2017). The properties and volume of wastewater generated in each process depends on the raw material used, pulping method, bleaching method and number of bleaching stages, chemicals used, quantity and quality of pulp produced (Ashra et al., 2015; Kamali & Khodaparast, 2015). Approximately 7 tons of black liquor are generated in the manufacturing of 1 ton of pulp in a chemical pulping process (Kim et al., 2017). Kraft (or sulfate) and soda are the two major alkaline processes to produce chemical pulps. In this pulping process, the products resulted from the digester reactions are the cellulose pulp and the wastewater (Cardoso et al., 2009). These processes generate large volume of wastewater, which has a significant impact on the environment (Ashra et al., 2015; Merayo et al., 2013).
The wastewater generated from pulp and paper industry has a black or dark brown color is known as black liquor (Kamali & khodaparast, 2015; Zhu & Theliander, 2015; Domínguez et al., 2017; Irfan et al., 2017; Hidayati et al., 2018). The chemical composition of black liquor depends on the type of the raw materials processes (softwood such as pine, hard wood such as eucalyptus and non-wood such as bamboo and sugarcane bagasse), the operational conditions used, type of chemicals used in pulping and bleaching (El-mekkawi et al., 2011; Cardoso et al., 2009). The black liquor can be considered as a complex aqueous solution of organic materials from wood and non-wood resources (lignin, polysaccharides and resins compounds) and inorganic compounds (mainly soluble salt ions such as sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), sodium sulfide (Na2S) and sodium thiosulfate (Na2S2O3) (Andreuccetti et al., 2011; Kesalkar et al., 2012; Ashra et al., 2015; Kim et al., 2017; Hidayati et al., 2018).
The environmental pollution load of black liquor is evaluated by the amount of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Dissolved Solid (TDS), organic and inorganic materials and color of effluent (Kamali et al., 2016; Kamali & Khodaparast, 2015). The pulp and paper industry effluent are characterized by high BOD load of 10-50 kg/ton of pulp and COD of about 20-200 kg/ton of pulp. The COD of black liquor varies from 10,000 to 120,000 mg/L and the pH range from 10 to 13 (Kaur et al., 2017; Hu et al., 2015). The high COD value of black liquor is due to dissolved inorganic and organic chemicals like dyes, heavy metals, detergents, starch, turbidity and toxic substances (Zhu & Theliander, 2015). The BOD gives information about the readily biodegradable fraction of the organic load in the wastewater. It indicates the amount of oxygen taken up through the respiratory activity of microorganism growing on the organic compounds present in the sample (Jouanneau et al., 2013). The biodegradability index (BI) is expressed as a ratio of BOD5: COD. It is a measure the extent to which a waste is amenable to biodegradation (Kamali & Khadaparast, 2015; Tembhekar et al., 2015).
In this work, the chemical composition and physical properties of untreated and pretreated sugarcane bagasse (SCB) soda and Kraft black liquor was determined, the color of the black liquor was analyzed to correlate the pretreatment and pulping methods impact on sugarcane bagasse black liquor.

2. Materials and Methods
2.1 Materials
The raw material sugarcane bagasse (SCB) was obtained from Wonji-Shao sugar factory, Ethiopia. Black liquor was produced from untreated and pretreated sugarcane bagasse (physical fractionation pretreatment (PFP) using 1mm size sieve) using Kraft and soda pulping, at a temperature of 130oC for 60 min, with 10% sodium hydroxide concentration and a solid: liquid ratio of 1:10 kg/L, for Kraft pulping process the sodium sulfide concentration of 20% and sodium hydroxide to sodium sulfide ratio of 3:1. After reaction, the separation of the pulp (solid fractions) and black liquor (liquid) was carried out using 150 mesh (0.11 mm) stainless steel sieve and the resultant black liquor was characterized. Black liquors samples are called as PSBL, USBL, PKBL and UKBL. PSBL and PKBL refers to pretreated soda and Kraft black liquor respectively, and USBL and UKBL refers to untreated soda and Kraft black liquor respectively.

2.2 Methods
2.2.1 Density and pH measurement
The pH of black liquor was determined by using pH meter (HI2020 edge ® Multiparameter pH meter). Density was determined following the pycnometer method by measuring the mass of black liquor of a known volume of black liquor at room temperature.

2.2.3 Biochemical oxygen demand determination
The black liquor Biochemical oxygen demand (BOD) measurement was performed in a closed bottle test following the American Public Health Association (APHA method 5210 B) method. The black liquor sample was pipetted into a BOD bottle containing aerated dilution water. The dissolved oxygen content was determined in a sealed and incubated at 20oC for five days bottle in a dark room. After an incubation period of 5 days, the final dissolved oxygen was measured (Hang, 2010; Jouanneau et al., 2013). Then the BOD value was calculated using the following equation:
Where: DOB = initial oxygen in diluted sample (mg/L)
DOD = oxygen in diluted sample after 5-day incubation (mg/L)
P= ml sample*100/capacity of bottle

2.2.4 Chemical oxygen demand determination
The COD of the effluent was assigned by dichromate open reflux technique, American Public Health Association (APHA method 5520 B) method. Fifty milliliters of black liquor sample were pipetted into refluxing flask, 1 gm mercuric sulfate (HgSO4) and 5 mL sulfuric acid (H2SO4) reagents were added to dissolve HgSO4. Then 25 mL potassium dichromate (K2Cr2O7) was added to the solution and finally adjusted by adding 70 mL H2SO4 reagent and refluxed for 2 h. At the end the mixture was cooled and diluted to about twice its volume with distilled water. The cooled mixture to room temperature was titrated for the excess of K2Cr2O7 with ammonium ferrous sulfate ((NH4)2Fe(SO4)2) solution (Hang, 2010). Then the COD value was calculated using the following equation:
Where: A= Amount of ferrous ammonium sulfate, mL used for blank
B= Amount of ferrous ammonium sulfate used for sample, mL
M= molarity of ferrous ammonium sulfate
D= Amount of sample used, mL
8000= milliequivalent weight of oxygen *1000 ml/L

2.2.5 Lignin content determination
Klason lignin (acid insoluble) of the black liquor was determined by hydrolysis treatment with 72% sulfuric acid (H2SO4). The short method is as follows, in 5ml of black liquor 3ml of 72% H2SO4 was added and incubated for 15 min at room temperature and held in thermostat condition of 30oC in a water bath. Then the mixture was diluted to 3% by adding distilled water and autoclaved at 120oC for an hour for hydrolysis of the polysaccharides. At the end of the hydrolysis the sample was cooled to room temperature filtrated by filter paper and the insoluble solid residue referred to Klason lignin was measured gravimetrically according to Technical Association of Pulp and Paper Industry (TAPPI) T222 cm-00 (Westman et al., 2014; Zhu & Theliander, 2015).

2.2.6 Total dissolved solid, inorganic and organic matter determination
Total dissolved solid (TDS) was determined using TAPPI T264 cm-97. Inorganic matter was determined after combustion of the sample at 525oC in a furnace for 2 h using TAPPI T211 om-93 procedure. The organic matter is determined by the difference between total dissolved solids and inorganic matter (Garcia et al., 2009; Sharari et al., 2011). Each experimental run was done in triplicate.

2.2.7 Color measurement
The color of the black liquor was evaluated using spectrophotometer CM-600d (KONICA MINOLTA, INC., JAPAN). Color measurement is expressed as lightness (L*), redness (a*) and yellowness (b*). Lightness (L*) was scaled from 0 (black) to 100 (white); redness (a*) from -60 (green) to 60 (red) and yellowness (b*) from -60 (blue) to 60 (yellow). Three measurements were made on each sample.

2.3 Statistical analysis
SPSS software (Statistical Package for Social Sciences, version 20) was used for statistical analysis, to determine the significant differences between the untreated and pretreated SCB Kraft and soda black liquor chemical composition, physical properties and color. This method allows a direct visual comparison of means for the analysis within each sample and provides a more accurate picture of standard error.

4. Results and Discussion
The results of the physicochemical characteristics of untreated and pretreated ESCB soda and Kraft black liquor are summarized in Table 1. In SCB soda and Kraft pulping around 900 mL concentrated black liquor was generated and it consumes an average of 14970 mL fresh water was used in pulp washing for 49.93 gm pulp production. Totally around 15970 mL of black liquor was generated.