Kairbekov, Z., Z. Eshova, D. Akbayeva, and BashirbayevаR. “Оptimization of Humic Acids Extraction from Coal of Oy-Karagay Field”. Chemical Bulletin of Kazakh National University, Vol. 68, no. 4, Dec. 2012, pp. 79-83, doi:https://doi.org/10.15328/chemb_2012_479-83.