Recovery of Gold, Silver, Copper and Niobium from Printed Circuit Boards Using Leaching Column Technique

Transcription

1 Journal of Earth Science and Engineering 2 (2012) D DAVID PUBLISHING Recovery of Gold, Silver, Copper and Niobium from Printed Circuit Boards Using Leaching Column Technique Ricardo Montero, Alicia Guevara and Ernesto de la Torre Faculty of Chemical Engineering and Agro-industry, Extractive Metallurgy Department, National Polytechnic University, Quito , Ecuador Received: Septmber 28, 2012 / Accepted: October 10, 2012 / Published: October 20, Abstract: Nowadays, over 300 tons of Au are used in electronic equipment each year with other precious and strategic metals such as Ag, Pt, Pd, Cu, Nb, Ta, etc.. After the use-phase, the electronic devices become electronic waste (e-waste); consequently it is important to consider e-waste as a secondary supply for the recovery of these metals. This paper presents the recovery of Au, Ag, Cu and Nb from PCBs (printed circuit boards) of discarded computers using leaching column technique. The PCBs were crushed with a hammer mill until reaching a particle size between 3.33 mm to 0.43 mm. Then, it was leached with a sodium cyanide solution in a glass column using the following conditions: sodium cyanide concentration 4 g/l, flux 20 L/d kg PCBs day, ph between 10.5 to 11 and leaching time 15 days. Every day, after leaching, the pregnant solutions passed through a column with activated carbon to complete the closed loop system. The following recoveries were obtained: Au 46.6%, Ag 51.3%, Nb 47.2% and Cu 62.3%. A preliminary technical-economic study shows the feasibility to create a small-scale PCBs recycling plant. The initial investment is on the order of US$155,639, considering the recovered metals from the loaded carbon. The internal rate of return for a 10 years period IRR (internal rate of return) and NPV (net present value) estimated are 27% and US$105,926 respectively. Key words: Printed circuit boards, recovery, precious metals, niobium. 1. Introduction Nowadays, over 300 tons of Au are used in electronic equipment along with other precious and strategic metals such as Ag, Pt, Pd, Nb, Ta, etc. In the last decade, the global consumption of EEE (electrical and electronic equipment) has increased as much as its growth rate. At the end of life of these devices, they become electronic waste (e-waste). E-waste contains precious and strategic metals as well as hazardous compounds like high concentrations of heavy metals (Pb, Zn, Ni, Cu, etc.), brominated flame retardants and other plastic additives [1, 2]. When not recycled the e-waste is incinerated and landfilled. These methods involve not only wasting Corresponding author: Ernesto de la Torre, professor, main research fields: active carbon production and gold recovery. [email protected]. valuable metals, but also creating a potential risk for the environment. There are evidences of various heavy metals leached from landfills [3]. One of the most important components in the e-waste are the PCBs (printed circuit boards) where the precious metals concentrations are ten times higher than in rich precious metals bearing ore [4]. A typical PCB composition is: 30% plastics, 30% refractory oxides and 40% metals. The most abundant metal is copper with a concentration between 10% and 30%. Table 1 shows some of the metals present in PCBs [4-6]. Informal recycling practices applied in developing countries for valuable metals recovery are considered dangerous, because of the toxic emissions and non-technical processes normally used. However, in developed countries e-waste recycling has been shown

2 Recovery of Gold, Silver, Copper and Niobium from Printed Circuit Boards 591 Table 1 Representative metal composition of printed circuit boards [6]. Metals Composition Cu (%) Pb (%) Zn (%) Au 80-1,000 (g/t) Au 110-3,301 (g/t) Pt (g/t) Pd (g/t) to be a profitable business, because these are suitable technologies to get high yields [7]. In Latin America, there are few e-waste recycling companies. The typical steps in recycling processes are: collection, disassembly, sorting and exporting. Nowadays, there are no refining plants to recover metals from e-waste in Latin America [8]. PCBs recycling processes provide many environmental benefits such as energy savings; reduction of metals obtained from traditional extractive metallurgical processes and reduced CO 2 emissions. The energy savings from using recycled metals compared to virgin material for copper is 85%, steel 74%, lead 65% and zinc 60% [3]. The precious metals in PCBs represent more than 80% of the total intrinsic value. Table 2 shows the composition and the intrinsic value of PCBs metals. In the last decades, the research of the hydrometallurgical processes for the recovery of valuable metals from PCBs has increased. These processes are more controllable and predictable than Table 2 Metal composition and the intrinsic value of PCBs [9]. Metal Wt.% Value PCBs intrinsic value (L/d kg) (L/d kg PCBs) (%) Au Pd Ag Cu Al Fe Sn Pb Ni Zn the hydrometallurgical ones. They also required lower investments and can be carried out at small scale [10]. The most used leaching agents for metals recovering from PCBs are: aqua regia (HCl, HNO 3 ), ammonia, sulfuric acid and sodium cyanide [5, 10, 11]. Previous studies carried out in an agitated batch leaching reported recoveries over 80% of Au and Ag from mobile phone PCBs, using a 4 g/l sodium cyanide solution within 24 h, 500 rpm and a particle size of about 1.0 mm. In a leaching column using crushed computer processors, with particle size range from 3.33 mm to 0.42 mm, 4 g/l sodium cyanide solution and flux 20 L/d kg PCBs, at ph 11 for 15 days recoveries of 53.7% Au, 9.1% Ag, 74.7% Cu and 51.3% Nb were obtained [12, 13]. This study aims to the recovery of Au, Ag, Cu and Nb from PCB and to determine the process flow diagram for a small-scale recycling plant of PCBs using leaching column technique and adsorption with activated carbon to complete the closed loop system. 2. Experiment Through a manual separation process the heat sinks (aluminium scrap) are removed from PCBs and the rest is crushed using a hammer mill (CONDUX D6431) and sieved with a screener. The precious metal content of PCBs was analyzed using AAS (atomic absorption spectrophotometer) (Perkin-Elmer, AAnalyst 300) previously digested in nitric acid and aqua regia. Column leaching experiments were carried out using 10 L of cyanide solution (4 g NaCN/L), 0.5 kg of PCBs (particle size range from 3.33 mm to 0.42 mm), flux 20 L/d kg PCBs, at ph 11 for 15 days. Every day after leaching, the pregnant solutions were passed through a column with activated carbon to complete the closed loop system under the following conditions: flux 20 L/d kg PCBs and active carbon to PCBs mass ratio 3:1. Every day after leaching and the adsorption process, the concentrations of Au, Ag and Cu in the solution were measured by AAS. Finally, the leached PCBs were washed and dried.

3 592 Recovery of Gold, Silver, Copper and Niobium from Printed Circuit Boards The Au and Ag concentrations were determined by fire assay. The Nb content was analyzed by Scanning Electron Microscope (Tescan Vega LMU with Bruker X-ray analyzer) in the fire assay core. 3. Results and Discussion The amount of Au, Ag, Nb and Cu found in used PCBs were: Au 613 g/t, Ag 1,515 g/t, Nb 36 g/t and Cu 23.4%. The precious metals distribution in different PCBs crushed fractions is shown in Fig. 1. The 80% of the precious metals in the PCBs are contained in the particle size range from 3.33 mm to 0.43 mm. The precious metals concentration in the < 0.43 mm particle size is 13%. This fraction is not used in the leaching process because it reduces the material permeability in the column. On the other hand, the > 5.66 mm fraction was neither used because it has a very small surface area. The precious metals concentrations were obtained by fire assay. The column leaching results show that the Au dissolution rate is higher than those of the Ag and Cu during the first 10 days of the process ( Fig. 2). From day 11, there is a reduction in the Au and Ag recovery rates because of a reduction of the precious metals contact area, due to copper oxide and copper hydroxide layers on the material surface. Furthermore, the copper recovery has a significant increase in the dissolution rate from day 7, because of competitive reduction reactions involving Au and Ag. These results indicate that the leaching of copper dominates the processing time since the 11th day. The niobium leaching rates in aqueous solution were not included in the Fig. 2 because the Nb aqueous concentrations were lower than detection limit for the AAS. There was an Au-Nb-Ni alloy in the PCBs (21: 1 Au-Nb mass ratio in the alloy). The Au in the alloy is leached with cyanide. It is assumed that niobium particles are also dissolved although there are not studies of niobium solubility in cyanide solutions. The Nb recovery in aqueous solution was determined by the difference between the starting concentration (unleached PCBs core bead) and the Fig. 1 Precious metals distribution in the PCBs crushing fractions. Fig. 2 Au, Ag, Cu leaching rate. Conditions: [NaCN] = 4 g/l; particle size range ( mm); leaching solution volume =10 L; PCBs mass = 0.5 kg; flux = 20 L/d kg PCBs.

4 Recovery of Gold, Silver, Copper and Niobium from Printed Circuit Boards 593 final concentration (leached PCBs core bead). The niobium concentration in the core bead from PCBs before cyanidation process is shown in the Fig. 3. The dore bead composition is: 77.5% Ag, 19.2% Au and 3.3% Nb. The fire assay is a reliable method for precious metals analysis (Au, Ag and platinum group element). Indeed, the behavior of niobium is very similar to the precious metals. The niobium concentration in activated carbon was determined by SEM analysis in the fire assay dore bead from loaded activated carbon. After leaching, the adsorption process was done and the results are presented in Table 3. As expected, Table 3 shows that the niobium recovery in the activate carbon is very close to the one of the precious metals. The Au and Ag recoveries on activated carbon are above 97% while for copper the recovery is closed to 81%. The high concentration of copper in the PCB produces concentrations above 2,000 mg/l of copper in the pregnant solution. The difference in the inflow produces variations in the residence time and, subsequently, in the recovery rates. In this study, the high inflow benefited the Au and Ag recoveries. The characteristic adsorption rate in the activated carbon for each metal was other parameter that differentiates their recoveries in this process. Based on the laboratory results obtained a PFD (process flow diagram) and an experimental scheme for recovery of Au, Ag, Cu and Nb of PCBs was developed (Fig. 4). The PFD displays the connection system of pipes and pumps used for a small-scale recycling plant of PCBs. The PFD consists of two stages of size reduction to obtain crushed PCBs. Subsequently, the material passes through a sieve to remove the < 0.42 mm diameter particles, obtaining a particle size range from 3.33 mm to 0.42 mm for the cyanidation column Table 3 Cu, Au, Ag, Nb recoveries from leaching activated carbon adsorption processes. Metal Concentration Aq. Sol. recovery (%) Active carbon recovery (%) Cu 23.4 (%) Au 613 (g/t) Ag 1515 (g/t) Nb 36 (g/t) Fig. 3 Energy-dispersive X-ray spectroscopy point analysis of unleached PCBs core bead.

5 594 Recovery of Gold, Silver, Copper and Niobium from Printed Circuit Boards Fig. 4 Flow sheet for Au, Ag, Cu and Nb recovery from printed circuit board scrap. process. In this process, leaching solution is fed from a storage tank. The solution passes through the crushed PCBs bed obtaining a pregnant solution. This solution is fed to a column filled with activated carbon for the desired metals recovery. Finally, the barren solution is stored in a tank where the concentration of sodium cyanide for the following leaching process is reconditioned. At the end of the process the obtained products are: (1) loaded carbon that contains the recovered metals, and (2) processed PCBs with precious metal contents. For the calculation of economic indicators, a 10 years plant operation time was considered. Also, 10 years of linear depreciation and a repayment rate of 10% was taken into account. The following economic indicators were obtained: 27% IRR (internal rate of return) and US$105,925 NPV (net present value). 4. Conclusions The cyanidation of PCBs presented recoveries of 47.9% of Au, 51.6% Ag, 48.1% Nb and 77.2% Cu in a column leaching using [NaCN], 20 L/d kg PCBs flux and particle size range from 3.33 mm to 0.43 mm. Recoveries yielded 97.3% Au, 99.3% Ag, 98.2% Nb and 80.7% Cu in the activated carbon adsorption process. The behavior of niobium in fire assay and adsorption in activate carbon is very similar to precious metals. The precious metals content in the solid waste obtained at the end of the process were 319 g/t Au, 733 g/t Ag, this material can be sold to foreign markets such as Europe which process plastic-metal mixture after mechanical processing [14, 15]. The economic feasibility analysis for a small-scale recycling plant of printed circuit boards with a monthly supply of 1,000 kg raw material and an operation perspective of 10 years determined a financial yield of 27% and NPV of US$ 105,925. Acknowledgments This research was financed through the PIS09-04 project sponsored by National Polytechnic School, Quito-Ecuador and carried out in the Extractive Metallurgy Department. The authors are grateful for the financial support of Kinross Ecuador for presentation of this paper and Research Unit, E-Scrap Ecuador.

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