Improvement chemistry of pressure oxidation and cyanidation leaching for copper and lead removal from Merrill-Crowe precipitates in acidic media to produce 98% pure Dore

Resumo

At the start of 2025, newly imposed tariffs—particularly those threatened by the U.S.—have significantly impacted global gold and silver mining production. In Mexico, extractive metallurgy plays a vital role in recovering these and other metals (including copper, lead, and cadmium) from ores, with applications ranging from magnetic materials to high-tech industries. The electronics sector is the largest consumer of gold and silver, using approximately 80% of gold in microchips and semiconductors. Beyond electronics, gold serves as an efficient catalyst in fuel cells and solar panels, enhancing energy conversion. In conventional Merrill-Crowe operations, precious metal precipitates often suffer from low quality due to co-precipitation of impurities like arsenic, copper, and especially lead, which increases zinc dust consumption, flux requirements during smelting, and shortens crucible lifespan. To address this, the present study proposes a hermetically sealed leaching process using sulfuric acid to selectively remove lead and copper. Following leaching, a barren solution undergoes gravity-driven nucleated precipitation in a serpentine reactor, employing sodium sulfide as a reducing/precipitating agent and sulfuric acid for pH control. Experimental results indicate that pH critically influences the removal efficiency of zinc and copper cyanide complexes. Optimal removal occurs at pH 7–9.5 for zinc and pH 2–4 for copper, achieving up to 99% elimination of these cyanide species. Furthermore, over 90% of cyanide bound in copper, lead, zinc, and iron complexes was recovered. Using natural hydroxyapatite to treat the pregnant cyanide solution effectively removed residual copper and lead ions, yielding a high-purity doré product with 99% precious metal content.