Research on cationic surfactant adsorption performance on different density lignite particles by XPS nitrogen analysis

Lignite is well known for its hard-to-float property. Many surfactants, including anionic, cationic and non-ionic have been applied to improve the floatation efficiency. In this work, cationic dodecylamine was applied to pretreat the lignite samples with different densities, which are −1.45 g/cm3, 1.45-1.80 g/cm3 and +1.80 g/cm3. The surface charged properties of the raw samples were determined by a zeta potential analyzer. The adsorption performance was characterized by XPS and UV/VIS spectrophotometer methods. From the surface zeta-potential results, three curves of zeta-potential figures versus pH value are similar to each other qualitatively in the pH range 5-8, which indicates that the organic structure has the similar ionization/ protonation ability with inorganic minerals at neutral pH. The negatively charged surfaces attract the cationic dodecylamine molecules at neutral pH for the three density fractions, which is shown by the XPS narrow sweep of heteroatom nitrogen before and after the pretreatment process. On the lignite surface, nitrogen was mainly existed in two forms of pyrrolic and quaternary. After the pretreatment, the quaternary-N peak was increased on the lignite surface due to the adsorption of dodecylamine molecules, especially on the surface of the highest density fraction. Correspondingly, UV/VIS spectrophotometer results showed that the adsorption amounts of dodecylamine molecules were 11.4, 18.7 and 20.0 mg/g on the surface of the samples from low to high density. On the other hand, the flotation result showed that more high ash particles and some low ash ones were both floated with the increase of dodecylamine dosage. Hence, if dodecylamine is to be used in lignite flotation, reagents should be added to selectively depress the adsorption process of cationic dodecylamine on the low or high ash particles.