TREATMENT OF BLACK LIQUOR USING THERMOCHEMICAL PRECIPITATION FOLLOWED BY WET OXIDATION

Abstract

Wet oxidation (WO) is a very effective treatment method for waste waters containing non-biodegradable and refractory matters. It involves the liquid phase oxidation of soluble or suspended oxidizable organic and inorganic components at elevated temperature (170-320 °C) and pressure (2 - 20 MPa) using a source of oxygen (usually air). Use of heterogeneous and homogeneous catalysts can reduce the severity of oxidation conditions. In the present study, the treatment of black liquor emanating from a local kraft pulp and paper mill has been studied using coagulation/ thermal treatment and catalytic wet air oxidation either alone or in combination. Black liquor contains lignin, cellulose, phenolic compounds, and low BOD/ COD ratio etc. Heterogeneous catalysts (5%CuO/ 95%activated carbon), (60%CuO/ 40%CeO2), and (60%CuO/ 40%MnO2) as well as homogeneous catalyst, CUSO4, were used in wet oxidation. The coagulants used include commercial alum, ammonium alum, aluminium sulfate, polyaluminium chloride and ferrous sulfate. CUSO4 was also used as a coagulant/ catalyst in the pretreatment step. Heterogeneous catalysts were prepared by coprecipitation method using the metal salts in the desired concentration. The wet oxidation studies were conducted in a 0.5 1 stainless steel (S.S. 316) high pressure reactor (SSR) having necessary arrangements for controlling temperature and pressure, agitation of reaction mixture and liquid sampling during the run. The coagulation/ thermal treatment studies were carried out in an atmospheric glass reactor (AGR) (capacity 0.5 1) having similar arrangements as in high pressure reactor. COD of the sample was measured by open reflux method. Standard methods were used to determine various parameters. The black liquor obtained from a pulp and paper mill (COD = 7,00,000 mg l"1) was diluted with tap water to have a synthetic waste water with its COD equal to 7000 mg l"1. The treatment of this diluted black liquor was carried out in two steps, namely, (i) thermal pretreatment, followed by (ii) wet oxidation (WO) of the filtrate obtained from the first step. In the presence of CUSO4 catalyst, the maximum COD reduction was found to be 61.4 % at atmospheric pressure and 95 °C. The catalyst mass loading used was 5 kg m"3. The studies at elevated pressures were conducted in ii the temperature range of 110 - 170 °C and a total pressure of0.85 MPa for 4 hat a pH of 8.0. Maximum COD reduction was found to be 88.9 %during WO step using 5%CuO/ 95%activated carbon (catalyst mass loading = 8 kg m"3) at 170 °C and 0.85 MPa total pressure. The overall COD reduction was found to be 95.7 %(including thermal pretreatment step): from an initial value of 7000 mg l"1 to 300 mg 1' . The pH ofthe solution during the experimental run decreases initially due to the formation of carboxylic acid and then increases due to the decomposition of acids. The kinetics of the CWAO reaction was studied with 5%CuO/ 95%activated carbon and CuS04 catalysts. With 5%CuO/ 95%activated carbon, the reaction occurred in three zones: first slow step, fast second step and the third slow step. The activation energy and Arrhenius constant for the fast reaction steps were found to be 82.5 kJ mol"1 and 1.03 x 108 min'1, respectively. For CuS04, the reaction occurred in two distinct zones. The activation energy and Arrhenius constant for the first fast step were found to be 57.0 kJ mol'1 and 3.51 x 104 min"1, respectively. Thewet air oxidation (WAO) of the synthetic waste water was also carried out at moderate temperature and pressure conditions using a heterogeneous catalyst (60%CuO/ 40%CeO2) with air as an oxidant. TheWAO studies were conducted in the temperature range of 115 - 150 °C and a total pressure of 0.6 MPafor 5 h. The effect of temperature, pressure and initial COD was studied at an optimum pH0 of 3.0. Maximum COD reduction was found to be 77.3 %(final COD of1585 mg l'1 from an initial value of 7000 mg l'1) at 150 °C temperature and 0.6 MPa total pressure for a catalyst concentration of8 kg m'3. The reduction in COD may be due to the thermal or oxidative degradation during the heating period. The characteristics of the solid residue formed after the reaction were also studied. The power law model with a first order reaction with respect to COD adequately described the reaction. The Arrhenius constant and activation energy were found to be 0.64 min'1 and 20.22 Id mole"1, respectively. The heating value of the solid residue formed as a result of the reaction was found to be comparable to that of Indian coal (heating value of Indian coal = 20.90 MJ kg'1). The thermo-gravimetric analysis ofthe solid residue was also carried out in air atmosphere. The data obtained from TGA - DTA were fitted with the different available kinetic models. The power law model fitted with the experimental data with a very high correlation coefficient, giving the reaction order of 2.58 and the activation energy of46.07 kJ mol'1 for overall degradation under air atmosphere. in In another study, the synthetic waste water having an initial COD of 7000 mg l'1, was thermally treated using CuS04, 5% CuO/ 95%activated carbon, 60%CuO/ 40%CeO2 and activated carbon at atmospheric pressure and 95 °C temperature. Q1SO4 showed the best activity amongst the above catalysts. A maximum COD reduction of 63.3 % was obtained at an optimum pHo 5.0 with CUSO4 catalyst. For CUSO4, the temperature of the reaction was varied from ambient (i.e. 20 °C) to 95 °C. The colour removal was 88 % at pH 8 in comparison to 80 % at a pH of 5.0. The settling characteristics as well as filterability of the effluent improved with an increase in temperature, though temperature doest not affect the COD reduction efficiency. The cake resistance and the resistance of filter medium were found to be 102.8 mkg'1 and 0.692 x 105 m'1, respectively at 95 °C in comparison to 272.2 mkg"1 and 4.84 x 105 m"1 at 20 °C. The "One way transport diffusion model" was found to be the most suitable model, showing first order reaction kinetics (r2 - 0.980) and activation energy of73.7 kJ mol'1, to describe the thermal degradation ofthe sludge. The effect of different parameters, such as initial pH, temperature and coagulant dose on the COD and colour removal of paper mill effluent (COD - 7000 mg l"1) using different coagulants was also investigated. The coagulants used for the reaction were commercial alum, iron sulfate, aluminium sulfate, poly aluminium chloride (PAC) and ammonium alum. The separation of dissolved solids and decolourization of the waste water by lowering the pH using H2SO4 was also studied. It was found that the temperature has no effect on COD reduction. A pHo 5.0 was found to be the optimum for coagulation with commercial alum. The colour removal was found to be 90 %at pH 5 for an alum dose of 5 kg m'3. The precipitation using H2SO4 was tested in a pHo range of 0.5 to 5.0. COD reduction decreases with the increase in pHo, although very mildly (61% at pHo 1.5 and 64 % COD reduction at pHo 0.5). The settling and filtration characteristics were better for alum treated effluent than that treated with H2SO4. The TGA data of the solid residue obtained from the alum treated effluent could be best correlated with the Agarwal and Sivasubramanian kinetic equation showing the order of reaction to be 2.04 (r = 0.957) and activation energy of 113. 9 kJ mol'1. The settling and filterability characteristics were found to be poorer than those found with CUSO4 catalyst. The study also suggests a scheme for the treatment of pulp and paper mill effluent. This scheme includes the coagulation followed by wet oxidation at moderate conditions. CUSO4 was found to work as a coagulant as well as a catalyst in WAO. iv The scheme does away with the need of biological oxidation. The main advantage of CuS04 is in the use of the residual Cu++ ions in the WO reaction as a catalyst. The residual copper after the WO reaction can be eliminated by pH adjustment. Future studies should focus on the effect ofcatalyst particle size and the effect ofpH for all the catalysts. The identification ofthe products formed as intermediate during the WO reactions can be done, so that some useful chemicals can be recovered from the effluent. The settling and filtration characteristics should be improved by adding some other coagulants and treatment at enhanced temperatures.