The type of plant substance from which brown coal is formed and the conditions in which it forms largely influence its characteristics and properties. On account of a lower degree of coalification and calorific value compared to hard coal and a high content of humic substances, brown coal is becoming a valuable fertiliser component. Humic acids contained in brown coal are also found in the soil’s humus substances, and their beneficial sorptive and complexing properties are a factor speaking in favour of their use both in agriculture and in other branches of the economy. The article presents the manner of preparing a sample of brown coal in order to determine the content of humic acid intended in particular for use in making chemical fertilisers and for other applications, particularly in agriculture. The optimum conditions in which the amount of humic acid in the sample is the highest were determined. The influence of the proportions of water, KOH, and brown coal, temperature, and mixing time on the solubility of the sample of brown coal was analysed. The solutions were mixed in mixers built especially for this purpose. The solubility of brown coal in the prepared solutions was controlled UV-ViS spectrophotometrically.
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