Figure 7. The effect of Solid-to-Liquid ratio (S/L) on copper elution efficiency. Elutants: (×), 0.1 M CaCl₂ (pH 2.5); (*), 0.1 M CaCl₂ (pH 3.0); (○), 0.1 M CaCl₂ (pH 3.5); (+), 0.1 M CaCl₂ (pH 4.0); (●), 0.1 M CaCl₂ (pH 6.0);(◊), 0.1 M HCl; (Δ), 0.1 M HNO₃; (□), 0.1 M H₂SO₄.