Energy-Efficient Noble Metal Recovery by the Use of Acid-Stable Nanomagnets

The present work investigates the potential use of metal-based carbon-coated magnetic nanoparticles for the efficient extraction of gold and platinum at high dilution (milligram to gram per ton ppb to ppm) at a mini-pilot level (0.1 m(3)). Acid-stable nanomagnets were first prepared by reducing flame synthesis and consisted of graphene-like carbon-coated cobalt metal nanoparticles (20-40 nm diameter) with an onion-like core/shell structure. The use of a metal core affords high saturation magnetization, while carbon shells are highly resistant to most chemical conditions. The nanomagnet surface was further coated with a standard noble metal extraction resin-like polymer (thiourea groups on a poly(ethylene imine)). Extraction runs were tested both at laboratory scale (0.1-10 L; Au and Pt removal > 95%; down to the milligram per ton level) and in a tank model (vertical tank section, 4 m height, 0.1 m(3) volume, Pt removal > 80% at 50 mg/ton of acid water). Delivery of freshly dispersed nanomagnet dispersions onto the top layer of the tank model's water zone (top 0.1 m) resulted in agglomeration and subsequent sedimentation through the tank model's water column while simultaneously adsorbing platinum with an efficiency of 90%. At the bottom of the tank model, the nanomagnets could be efficiently collected through sweeping the tank model's bottom surface with an array of permanent magnets. This process circumvents moving a tank's liquid volume (energy costs for pumping) through conventionally used and time-consuming fixed-bed assemblies. In contrast, the presented process only moves a very small mass (<1 mass %) of the noble metal-containing volume and may therefore become an energy-efficient alternative to adsorption onto fixed beds or usually applied chromatography-type processes.