Amidoximation of cross-linked polyacrylonitrile fiber and its highly selective gallium recovery from Bayer liquor

Fibrous adsorbent functionalized with amidoxime group has been recognized as the most potentially adsorbing material for recovering gallium from Bayer liquors, but the practical process has not been so attractive owing to the performance decreasing in strongly alkaline conditions. The purpose of this research aims to enhance the Ga/V selectivity and efficiency of extraction and retain the mechanical stability of the adsorbent by cross-linking the polyacrylonitrile fiber before amidoximation. In this study, cross-linking amidoxime-functionalized PAN fiber (C-PAO) was prepared by cross-linking polyacrylonitrile fiber through triethylene tetramine. Subsequently, the cross-linked PAN fiber was reacted with hydroxylamine at 70 °C and the C-PAO fiber was obtained. C-PAN and C-PAO were characterized with Fourier transmission infrared spectrometer, scanning electron microscope and single-fiber electronic tensile strength tester, respectively. The performance of C-PAO for gallium adsorption was measured by batch and multilevel adsorption experiments. Experimental data showed that the maximum static adsorption capacity of Ga in simulating Bayer liquor was 26.9 mg/g, and the equilibrium adsorption time was 60 min which indicated that the shorter exposure time equated to the less degradation of the amidoxime groups and the reduction of mechanical performance. The extraction efficiency of gallium was more than 85% using hydrochloric acid (1 mol/L) as stripping agent. The maximum static adsorption capacity in Bayer liquor was 17.6 mg/g, and the saturated adsorption time was 140 min, which are both better than that of commodity gallium-recycled resin. In addition, after four-stage adsorption by C-PAO, gallium was entirely adsorbed while a reduction of vanadium is no more than 20%, which showed a good selective adsorption of gallium. In conclusion, the C-PAO fiber is a highly effective adsorbent for enrichment and separation of gallium in the presence of competitive ions such as V⁵⁺ and Al³⁺ in strongly alkaline Bayer liquid.