The effect of crosslinking additives and molecular weight of starch depressants on pyrrhotite and pentlandite floatabilities

Abstract

• Synthesized starches (paste starches and crosslinked starch) can depress hexagonal pyrrhotite and pentlandite at pH 9.2. • Paste starch with higher molecular weight and crosslinked starch showed a certain selectivity in a binary mineral system. • The presence of metal hydroxylate species on both minerals allowed their chemical interaction with starch groups according to surface analysis. • Crosslinking additives within starch structure promoted a mixed interaction with mineral surfaces via chemisorption and electrostatic attraction. Pentlandite is very important to the mining industry since it is the main source of metallic nickel. However, its separation from hexagonal pyrrhotite by froth flotation is challenging due to their similar floatabilities, and inefficiencies in this process generate environmental issues caused by emissions of sulfur oxide gases during the pyrometallurgy of nickel concentrates. Although successful, the current pyrrhotite depressant (diethylenetriamine - DETA) is also harmful to the environment which compels the research and development of non-toxic and biodegradable reagents such as polysaccharides. Therefore, three starch depressants for hexagonal pyrrhotite were used in this study. They include a paste starch with higher molecular weight (MW), a paste starch with lower molecular weight, and a crosslinked starch formed by mixing the lower MW paste starch and copper ions. We hypothesize that the presence of crosslinking additives such as copper provide a “driving” effect towards the gangue mineral. The performance of depressants was investigated in single and binary mineral systems. For single minerals, all depressants impacted hexagonal pyrrhotite and pentlandite flotation recovery. For the binary mineral systems, the higher MW paste starch and crosslinked starch showed an improved Ni separation efficiency of 90.5% and 85.6%, respectively, at an intermediate PIBX dosage (9.66x10 −6 M) compared to the lower MW paste starch (58.6%), demonstrating a certain selectivity. The choice of the most suitable starch depressant proved to be complex since both minerals were able to chemically interact with the starch depressants as verified via X-ray photoelectron spectroscopy (XPS). Hydroxyl groups present on the α-D-glucose units bound with surface metal (Fe and/or Ni) hydroxylate species while the crosslinked starch also interacts with the mineral surfaces by electrostatic attraction. These findings will contribute to the advancement of knowledge related to the use of polysaccharides as depressants for complex sulfide mineral systems.