Recovery of Valuable Elements from Spent Printed Circuit Boards by Chlorination

Many elements have been used as raw materials for electronic components, wiring, and plating on printed circuit boards. Such boards contain several hundred to a few thousand ppm of rare metals and precious metals, while containing base metals in the order of a few percent. To develop an efficient low-energy recovery process for such valuable elements from spent products, the dynamic behavior of ten elements was investigated during chlorination of samples prepared in three different ways: by incinerating and pyrolyzing the spent printed circuit boards, and by mixing the incinerated sample with a solid carbon reducing agent. When an incinerated sample was heated, volatilization was observed to start at 600 degrees C for Sn, Cr, Fe, and Zn; at 800 degrees C for Cu and Ni; and at 1,000 degrees C for Pb, Ta, Co, and Au. The volatilization temperature was confirmed to be lower for the pyrolyzed sample than for the incinerated sample. The volatilization of all elements was accelerated when solid carbon was added to the incinerated sample and the starting temperature of volatilization was found to decrease. A non-isothermal kinetic analysis was conducted for the release behavior of elements during the chlorination of the incinerated, pyrolyzed, and carbon-added samples, assuming that the release rate could be expressed as first-order in terms of the content of the solid phase. The addition of carbon to the incinerated sample resulted in a reduction of the activation energy for the volatilization reaction to half or less for all elements except Cr. Since the pyrolyzed sample contained 12.7 wt% carbon, it is presumed that this carbon promoted the chlorination reaction at low temperatures in comparison to the incinerated samples. The change in the extent to which elements were released decreased when the release extents of Sn, Fe, and Zn reached 0.85, 0.70, and 0.65, respectively; however, it was found that the volatilization rate could be restored by crushing and mixing the samples.