Ind. Eng. Chem. Res. 2010, 49, 9355–9362
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Energy-Efficient Noble Metal Recovery by the Use of Acid-Stable Nanomagnets Michael Rossier,† Fabian M. Koehler,† Evagelos K. Athanassiou,† Robert N. Grass,† Markus Waelle,‡ Karin Birbaum,‡ Detlef Gu¨nther,‡ and Wendelin J. Stark*,† Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland, and Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland
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 m3). 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 m3 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 (95% of the ions initially present were extracted. In a second step, the selective extraction of platinum and gold was investigated in solutions containing non-noble metal ion impurities (Fe, Cu, Zn). The extraction procedure was kept exactly the same, but now the solution additionally contained iron, copper, and zinc (1000 ppm or 1 kg/m3 of each non-noble metal) next to gold and platinum (1 ppm each ≡ 1 g/m3). As depicted in Figure 4, >75% of the noble metals ions were removed after 5 min while the concentration of the other metals (i.e., Fe, Cu, Zn) did not change significantly. Considering the large amount of heavy metal and the small amount of noble metal present before and after the extraction, the affinity of the functionalized particles has to be much larger for noble metal than for heavy metal to achieve such extraction efficiency. Nevertheless, it cannot be excluded that a small amount of heavy metal may also be adsorbed once the solution had been depleted from noble metal. Gold Recovery from Nanomagnets (Reagent Recycling). Since our most recent laboratory-scale study had shown that gold could be removed from “naked” carbon/metal nanomagnets
Figure 2. Comparison of the “naked particles” (no polymer coating, * indicates previous work)25 with functionalized nanomagnets. For gold, >95% present in 10 mL of acidic solution was removed by both particle types (20 mg used). For platinum, the polymer/nanomagnets were very efficient (>95% Pt removed), whereas particles without a polymer shell (“naked”) showed a low removal efficiency. The solutions were initially composed of 100 ppb (mg/m3) and 10 ppm (g/m3) of noble metal. After 5 min, the particles were removed and the concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS).
(no surface modification or chelator) by washing them with acid (mix of 1/3 vol. HNO3, 2/3 vol. HCl),25 a similar procedure was tested with the here utilized, more complex, noble-metalloaded polymer/nanomagnet reagent. The mass balance of gold during these adsorption and desorption (washing) processes could be closed for the functionalized particles within an error of (2.2% (Table 1). The relatively high surface area (36 m2/g) of the utilized particles allowed a considerable gold loading capacity: the maximal amount of gold extracted after 5 min of adsorption corresponded to 121 ( 12 kg/ton of reagent from solutions containing 10 g of dissolved gold per m3. The overall process of adsorption and recovery of noble metal can be seen in Scheme 2. Mini-Pilot-Scale Gold Adsorption Kinetics. The influence of time on gold adsorption was measured in a 20 L batch reactor containing 7 L of gold solution using the most demanding conditions (high acidity, pH ) 2.1; very low gold concentration, 0.035 g/m3). Particles (0.05 g) were dispersed and stirred in the solution for 60 min. Liquid samples (0.01 L each) were
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Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 Scheme 2. Schematic Description of the Overall Process: In the First Part, The Noble Metal Is Extracted from the Solution; During the Second Part, The Loaded Nanomagnets Are Washed in a Highly Acidic Solution, Allowing the Desorption or Recovery of the Noble Metal and the Recycling of the Magnetic Particles
Figure 3. Simultaneous extraction of Au and Pt. More than 95% of the noble metal present in 10 mL of acidic solution was removed using 20 mg of functionalized particles. The solutions were initially composed of 50 ppb (mg/m3) gold and 1000 ppb (mg/m3) platinum (solution 1), 525 ppb gold and 525 ppb platinum (solution 2), and 1000 ppb gold and 50 ppb platinum (solution 3). After 5 min, the particles were removed and the concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS).
sedimenting particles in a process-site tank with 4 m of water height (i.e., typical tank volumes ∼10-1000 m3). This sedimentation separation process has the advantage that almost no energy is required (no bulk liquid moving) and the use of magnetic particles allows their fast and simple collection by the aid of an external magnet field. For this experiment, platinum was used instead of gold (minipilot-scale experiment) to show that these nanomagnets are able
Figure 4. Selectivity of the functionalized particles for noble metals in a concentrated heavy metal solution. Removal of >60% of the gold and 80% of the platinum initially (cinitial ) 1 ppm each) present in a heavy metal (iron, cupper, zinc; cinitial ) 1000 ppm each) acidic solution. Samples of 20 mg functionalized particles were stirred in 10 mL of solution for 5 min and then removed with a permanent magnet. The concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS). Table 1. Mass Balance on Gold (10.3 mg of Particles in 5 mL of 0.1 g/L Au Solution); Particles Were Washed in 10 mL of HCl/HNO3 Concentrated for 15 min Au adsorbed (mg)
Au removed (mg)
error (%)
0.47 ( 0.01
0.48 ( 0.01
2.2
collected during the entire experiment and analyzed with inductively coupled plasma mass spectrometry (ICP-MS). As can be seen in Figure 5a and b, almost 90% of the initial amount of gold (35 ppb ≡ 0.035 g/m3) was removed by the particles after 30 min and ∼95% was removed after 60 min. The remaining gold concentration in the solution was
