(320f) Adsorptive Separation of Platinum Group Metals in Fixed-Bed Columns Containing Amine-Immobilized Activated Carbons

The properties of activated carbon pellets (USA-Norit, 0.8) which make them suitable for use as adsorbents are: (i) small average pore diameter (19 ± 2nm) and (ii) a huge surface area (900 ± 50 m²/g) which is characterized by valency electron-deficient sites on the edges of its hexagonal platelet structure and a high proportion of “oxy” surface groups. Nevertheless, native AC pellets exhibit non-selective properties towards molecules or ions with similar ionic radii and charges. In this regard, the art of separating Pt anions (PtCl₃^-4) from metallic cations (Cu²⁺ ,Ni²⁺,Cr²⁺ Fe²⁺) on the surface of native AC pellets is based on the concept that a highly protonated surface of ACs favors electrostatic interaction with anions and repels positively charged ions. Subsequently, adsorbed species of chlorocomplex anions are allowed to diffuse through the AC capillary pores under a chemical potential. However, it is possible to induce selectivity properties on the surface of activated carbons by introducing amide groups. Recent adsorption data from fixed-bed column operations showed that amine-treated ACs adsorbed Pt anions preferentially.

We attempted to separate platinum from a mixed chloride solution containing base metal ions by using fixed bed columns loaded with AC pellets. Subsequently, a multicomponent adsorption model which predicts separation factors and adsorption capacity of Pt was proposed. The feed solution was prepared by dissolving known weights of NiCl₂ , CuCl₂ , CrCl₄ and FeCl₂ the bulk NaCl solution to simulate a typical industrial leach solution. The feed pH was adjusted by adding droplets of 0, 1M, HCl solution.

Extended Langmuir and IAS (ideal adsorbed solution) models were found to be adequate to describe platinum adsorption from a multicomponent solution. The effect of counter ions on the diffusion of platinum anions into the AC pores was elucidated by separation factors although the mechanism of molecular transport is not yet clear. The order of magnitude of separation factors varied as follows: Pt/Cu > Pt/Ni > Pt/Cr > Pt/Fe in the pH range 1.3 – 2.9. A high load of ACs in the column adversely affected adsorption equilibria profiles of platinum. The bed volume of pure HCl solution, which was required to strip platinum completely from the fixed bed column was quantified. The adsorption capacity of platinum in a fixed bed column was comparably smaller to adsorption capacity values obtained from highly sparsed solid-liquid mixers.

KEY WORDS: Process, Extraction, PGMs, Leach Solutions, Activated Carbons (AC)