Effect of Thermal Pretreatment on the Comminution and Fractional Distribution of Gold in Waste Printed Circuit Boards

Roasting is a standard industrial pretreatment for recycling waste printed circuit boards (WPCBs), used to remove organic combustibles and hazardous bromine (Br) prior to smelting. While effective for detoxification, the impact of high-temperature roasting on mechanical liberation and gold partitioning across particle sizes remains under-investigated. This study evaluates the influence of roasting at 700°C on the mechanical breakdown and gold distribution within WPCBs. Roasted and unroasted samples were comminuted using an Orient Mill, classified into size fractions ranging from +6.7 mm to −2 mm, and subsequently analyzed to determine gold distribution.

Results indicate that thermal pretreatment fundamentally alters WPCB comminution behavior. Roasting significantly improved matrix friability, yielding a 41% mass distribution in the fine (-2 mm) fraction, compared to 23% in unroasted samples. However, roasting also increased coarse particle production (21% at +6.7 mm) relative to the control (2.5%). Crucially, gold distribution depended heavily on the pretreatment state. In roasted samples, gold was present across all sizes but concentrated in the fines (73% in the −2 mm fraction). Conversely, in unroasted samples, gold was localized entirely within −4 mm fractions, with no gold detected in the +4 mm and +6.7 mm ranges.

The presence of gold in the coarsest roasted fractions—despite an original particle size of <100 µm—suggests a potential mechanism of physical entrapment or 'locking' within partially melted components. While roasting improves friability, it may induce gold encapsulation in large, heterogeneous particles, potentially complicating downstream mineral processing recovery.