Characterization-and Energy Enhancement of Dawuro Coal Using Froth Flotation and Alkali/Acid Leaching
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Date
2023-10
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Addis Ababa University
Abstract
Ethiopia has huge deposits of low-rank coal, which is characterized by calorific-value (an_ average_4,500_kcal/kg), high_ash_content (up to 50%), high moisture content (up to 35%) and high volatile matter. Thus, the country_is_still dependent-on partial import of better quality-coal,_and so, significant amount of money in foreign-exchange is spent to meet the demand. Therefore, this study was carried out on domestic coal beneficiation by applying the froth-flotation and alkali-acid leaching, sequentially, for_enhancing_the_quality_of Dawuro Coal, in Southwestern-Region_of_Ethiopia. The Design-Expert-(version-13) software was_used for the experimental_design_and the RSM_with_BBD were applied for modeling_the_influence of some factors on the_performance_of flotation and_alkali-acid leaching experiments. Flotation experiments were conducted by considering particle_size, collector-and_frother-dosages as independent variables, and_then, alkali- and acid-leaching experiments were carried out, sequentially, both by using leaching concentration, temperature and time as independent variables. For each, ash-reduction was defined as process-response (dependent variable). The test_results revealed that the resulted ash-reduction models were found to_be statistically_significant and also the_predicted_values were in_good_agreement with_the_experimental results – with R2 value of 0.9801, 0.9858, and 0.9711 for ash-reduction in flotation, alkali and acid-leaching, respectively. Results revealed that particle-size for flotation and leaching concentration followed by temperature for both alkali and acid leaching play important role in ash-reduction. The maximum ash reduction of 73.84%, 71.88% and 49.40% were obtained at the working conditions: (125-63 μm particle-size, 55 g/ton diesel-oil/collector, and 370 g/ton n-octanol/frother), (1 mol/dm3 NaOH, 220 oC and 60 minutes), and (1 mol/dm3 HCl, 80 oC and 3 hours) for flotation, alkali and acid leaching, respectively. The result from proximate-analysis shows that the raw-samples of coal contain 10.67% moisture, 33.73% ash, 30.34% volatile matter, 25.26% fixed-carbon, and 0.67% sulfur with 3,643.08 kcal/kg calorific value. The results for the treated coal sample show that the ash content was 8.92%, 2.51% and 1.27%; and sulfur percentage was 0.45%, 0.38% and 0.37% with 4,996.20 kcal/kg, 6,195.29 kcal/kg and 7,124.58 kcal/kg of calorific values for flotation-concentrate, alkali-leached and acid-leached coal samples, respectively. The results of this study confirm that the treated-samples of coal resulted in higher-heating-values and fixed-carbon, and lower-ash and sulfur-contents as_compared to the_raw_sample. Hence, with the determined optimum conditions, applying froth-flotation followed by sequential alkali/acid leaching is effective to increase the heating-values and upgrade the energy/quality of the sample coal, being as suitable energy source for industries like cement_and_steel.
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Keywords
Froth-flotation, Alkali-acid leaching, Box–Behnken design, Response surface methodology