Dual effect of calcium ions on sodium oleate adsorption and spodumene flotation

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The flotation of spodumene with sodium oleate is highly sensitive to calcium concentration, but the underlying interfacial mechanisms remain unclear, especially under inhibitory conditions. To address this, we systematically investigate how calcium ions govern the spodumene-oleate system using micro-flotation, solution chemistry, and nanoscale force measurements. Flotation tests at pH 8.5 reveal a pronounced dual role of calcium ions. Low calcium dosages activate the spodumene surface, increasing the recovery to about 76% at an optimum concentration of 5 × 10 −5 M, whereas concentrations above 9 × 10 −5 M cause a sharp decline, with recovery falling even below that in deionized water. Adsorption and zeta potential measurements indicate that activation arises from a 1:1 exchange of calcium for surface lithium, which generates highly active coordination sites for oleate chemisorption. In contrast, excess calcium promotes the bulk precipitation of calcium dioleate, reducing the concentration of soluble oleate ions and leaving less collector available for adsorption on the spodumene surface. Atomic force microscopy measurements further show that, under inhibitory conditions, the deposited calcium dioleate forms a soft, viscoelastic colloidal coating, characterized by long-range attraction, strong adhesion and an extended adhesive bridge on retraction. This coating physically passivates the spodumene surface and acts as an interfacial barrier to bubble–particle attachment. By linking solution chemistry with nanoscale interfacial mechanics, this work clarifies the dual role of calcium ions in spodumene flotation. Additionally, it provides new insight into mineral systems sensitive to water hardness.