Electroless Displacement Deposition of Noble Metal on Silicon Powder for Recovering from Urban Mines

Noble metal is not only precious but also crucial for such industries as electronics, catalysts, and jewelry. Recycling noble metals from scrap electronics and industrial wastes, which are called urban mines, is important to preserve natural resources and the environment. In the conventional recycling process, scraps and wastes are dissolved in an aqueous solution and refined chemically. To recover noble metals from a low concentration solution, an ion-exchange resin, activated carbon, electrolysis, or cementation is used. We have been studying electroless displacement deposition of noble metal nanoparticles on silicon (Si) substrates (1) for efficient solar cell production (2) and metal film formation (3). This deposition is a local galvanic reaction expressed by followings:

Mⁿ⁺ → M + nh⁺              (h⁺: hole)

Si + 4HF₂^- + 2h⁺ → SiF₆^2- + 2HF + H₂

and/or       Si + 6F^- + 4h⁺ → SiF₆^2-.

Recently, we applied this deposition for recovering noble metals from aqueous solutions (4). Figure 1 shows outline of this recovering process. The first step of the process is addition of hydrofluoric acid (HF) and Si powder into an aqueous solution of noble metal ions. Noble metal particles are deposited on Si powder. The second step of the process is addition of oxidizing agent, such as nitric acid into the solution. Only Si is dissolved into the solution, and only noble metal particles remain in the solid-state. This method is classified into cementation. Features of this recovering are simplicity of process, high efficiency of recovering noble metal, and availability of using waste Si powder and HF provided by electronics industries. Furthermore, this recovering can selectively recover noble metal and copper unlike conventional cementation. In this study, we have investigated recover of noble metal using sawdust Si powder, and replacement of toxic HF by non-toxic agent such as ammonium fluoride (NH₄F).

                   0.2 M (M: mol dm^-3) Si powder was added in a mixture solution of metals and 0.15 M HF. The initial concentration of each metal salt in the solution was 1.0 mM. After stirring at 200 rpm at a solution temperature of 298 K, the solution was filtered. The recovery was determined by measurement of change in metal ion concentration of the solution with inductively coupled plasma-atomic emission spectrometry (ICP-AES).  Figure 2 shows the recovery plots of noble metals versus treatment time. These recovering uses 350 mesh Si powder or slicing slurry including sawdust Si after using for industrial wefering. Mean diameter of 350 mesh Si powder is 25 μm, and of slicing slurry is 10 μm. Slicing slurry is result in higher efficiency than Si powder. This efficiency of recovery is explained by specific surface area of Si.

                   Figure 3 shows recovery plots of noble metals using 350 mesh Si powder and 0.15 M NH₄F instead of HF. Au and Pt were recovered using non-toxic agent under lower rate than that for using HF.

ACKNOWLEDGEMENTS

The authors thank Dong Rong Electronics Co., Ltd. for supplying used silicon slurry. The present work was partly supported by Exploratory Research of A-STEP from JST, Grant-in-Aid from Kawanishi Memorial ShinMaywa Education Foundation, and JSPS KAKENHI Grant Number 23560875.

REFERENCES

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