Direct extraction of Ni(II) from Fe(III), Al(III), Ca(II),
Mg(II),
and Mn(II) is a bottleneck issue in the hydrometallurgy processes
of laterite-nickel ore, spent batteries, and other secondary sources.
One of the biggest challenges in achieving direct extraction of Ni(II)
is the lack of stable and efficient extractants. Hence, a novel bipyridine
extractant MSL211 was designed and synthesized in this paper. With
its strong steric resistance and electrostatic interaction, MSL211
can form a stable eight-membered chelating ring with Ni(II), thereby
having a high bonding ability toward Ni(II). Therefore, based on the
high bonding ability of MSL211 toward Ni(II) and the low extraction
capacity of DNNSA for Fe(III), an efficient synergistic extraction
system of DNNSA-MSL211 was proposed. The synergistic extraction system
of DNNSA-MSL211 realizes direct extraction of Ni(II) from Fe(III),
Al(III), Ca(II), Mg(II), and Mn(II) without neutralization, precipitation,
and solid/liquid separation processes, which are inevitable but problematic
processes in the current hydrometallurgy processes for removing Fe(III),
Al(III), and Mn(II). The effects of the molar ratios of DNNSA:MSL211,
shaking time, temperature, O/A ratios, and equilibrium pH on the extraction
behavior of Ni(II) were investigated. Under optimum conditions, the
extraction efficiency of Ni(II) was as high as 97.62%, and the separation
coefficient of Ni(II) over all impurities was greater than 400. To
elucidate the mechanism of the high selectivity of DNNSA-MSL211 toward
Ni(II), DFT calculations were carried out to calculate the binding
energy (ΔE) and Gibbs free energy (ΔG(ext)) of the extraction process. Furthermore, this paper
investigated the entire ″extraction-stripping″ process,
demonstrating that the synergistic extraction system of DNNSA-MSL211
avoids neutralization, precipitation, and solid/liquid separation
processes, minimizes hazardous waste generation, and shortens process
flow while meeting the objectives of sustainable and clean production.