Optimization of conventional hydroxamic acid for cassiterite flotation: Application of structural modification under principle of isomerism

In this paper, through the structural modification of N-phenyl formohydroxamic acid (NPFHA), an isomer of benzohydroxamic acid (BHA), we designed and synthesized a series of N-phenyl hydroxamic acids (NPHA), including N-phenyl acetohydroxamic (NPHA-2), N-phenyl butyrohydroxamic acid (NPHA-4), N-phenyl hexanohydroxamic acid (NPHA-6) and N-phenyl octanohydroxamic acid (NPHA-8), and first introduced them as flotation collectors for selectively separating cassiterite from quartz and feldspar. Micro-flotation tests indicated that with increase of non-polar carbon chain length, the collecting ability to cassiterite improved dramatically. More to the point, compared to BHA, NPHA-6 and NPHA-8 possessed superior flotation performances to cassiterite and enabled the separation of cassiterite and quartz/feldspar over a wide pH range, with no application of frother or activator, providing a new insight into the development of cassiterite collector. Zeta potential experiments indicated that NPHA-8 chemisorbed onto cassiterite surfaces, with no significant adsorption to quartz or feldspar detected. DFT calculations, FT-IR and XPS analyses further demonstrated that NPHA-8 chemisorbed on cassiterite surfaces by forming C-O-Sn and N-O-Sn bonds, accompanied by proposing two interaction geometries, first with the same Sn atom of cassiterite surface, leading to a five-membered chelating ring, second with two different Sn atoms, resulting in constituting an irregular complex.