Paul Williams and William Hall noted that pyrolysis has been well studied for the recycling of polymers, where the products are gases, oils and chars which are useful as chemical feedstocks and fuels. In addition, glass fibre from reinforced polymers is easily separated out.

The duo collected circuit boards from televisions, mobile phones and computers/monitors and cut them into small pieces (1.5-2 cm2) to fit in a lab-scale fixed-bed reactor. After purging with nitrogen to remove all traces of oxygen, the materials were heated to 800 °C at 10 °C/min then held at this temperature for 135 min. The pyrolysis gases and oils were trapped in a series of condensers for subsequent analysis.

The organic gases were analysed by GC-FID and nitrogen, carbon monoxide, carbon dioxide, oxygen and hydrogen, the so-called permanent gases, were analysed by GC-TCD. Compounds present in the pyrolysis oils were identified by GC/MS with electron ionisation and quantified by GC-FID. Phosphorus-containing compounds in the oils were identified and quantified by GC-ECD. The chars remaining in the reaction vessel were analysed by scanning electron microscopy, energy dispersive X-ray analysis and ICPMS to determine the metals content. The proportion of residue, oil and gas (wt.%) was 60-82, 15-28.5 and 2.3-6.5, respectively, for circuit boards from computers, televisions and cell phones. The gases comprised mainly carbon monoxide and carbon dioxide and hydrogen, together making up about 76-87% of the total gases. Apart from very small amounts of chlorine and bromine, the remainder comprised 7 C1-C4 alkanes and alkenes. These organic gases could by siphoned off to be used as a fuel for the pyrolysis process.

The pyrolysis oils consisted of a high proportion of phenol. 4-Isopropylphenol, 4-hydroxyphenol and bisphenol A were also found in significant amounts and many other phenols were also present, deriving from epoxy resins on the boards. The researchers argued that these compounds could be recovered for reuse as raw chemicals.

The oils also contained flame retardants such as tetrabromobisphenol A (TBBA) and one of the newer, alternative phosphorus-based flame retardants triphenyl phosphate (TPP). Williams and Hall believe that these too could be recovered for reuse.

The solid char residue was easily and quickly separated by hand into glass fibre, an organic fraction and a metallic fraction. The major metal in the ash was copper (33% of the ash) with smaller but significant proportions of calcium, iron, nickel, zinc, aluminium, lead and silver. Significant amounts of toxic barium were also found, along with very small amounts of gold and the toxic mercury and cadmium. These metals could be recovered from the ash and individually separated but further processing would be required.

European legislation, as well as a lot of legislation outside the EU, demands that printed circuit boards have to be recycled. Pyrolysis presents one possible way forward, separating the components as it does and being far more manageable than physically extracting the components directly from intact boards.