A new roasting process slated for commercialization Edited by Gerald Ondrey

Outotec Oyj (Espoo, Finland; www.outotec.com) has introduced a new partial roasting process to purify copper and gold concentrates that are contaminated with arsenic, antimony and carbon. The process is a pretreatment stage for Cu- and Au-extraction plants, and enables the extraction of metals from ores that had previously been unattractive because the impurities create a “huge problem” of handling dust laden with As and other intermediate products formed in conventional Cu or Au smelters, says Lars Hedstrom, head of Outotec’s Roasting Competence Center in Skelleftea, Sweden.

In the new process (flowsheet), ore concentrate is fed to the top of a fluidized-bed roaster. Preheated air is added through a large numbers of tuyeres at the furnace bottom. The air acts as fluidization media and supplies oxygen to the roasting reactions. Heat is generated by exothermic combustion reactions whereby iron sulfide in the concentrate is oxidized to magnetite and a SO2-rich process gas.

Impurities, such as As and Sb, are vaporized at the high reaction temperature, and thus removed from the solid calcine that contains the valuable metals (normally Cu, Au and Ag). Most of the calcine is pneumatically transported out from the roaster and separated from process gas in cyclones; the more-coarse fraction of calcine leaves the roaster through the bed outlet. The calcine is cooled to stop further reactions between calcine and air. This cooled calcine is now a raw material to conventional copper smelters. It contains enough sulfur to be mixed with conventional copper concentrates to make a suitable feed for oxygen based processes.

Meanwhile, the process gas contains combustible sulfur compounds, such as arsenic sulfide and elemental sulfur; these compounds are oxidized in the post-combustion chamber using preheated air. The process gas, now only containing oxide compounds, is cooled in a cooling tower and then cleaned of its dust content in a hot electrostatic precipitator (ESP). Arsenic oxide passes through the ESP and is wash solution when the gas is quenched in a washing tower. The SO2-rich process gas is further cleaned and converted into sulfuric acid. Arsenic is converted into stable compounds by separate conversion processes, which are also supplied by Outotec.

The process was first demonstrated about 30 years ago in a 45 ton/h plant, which is still operated by Boliden in Sweden. Outotec is now operating a 25 kg/h pilot plant in Frankfurt, Germany.

The company is also currently building the world’s largest As-removing roasting furnace at Codelco Mina Minstra Hales mine near Calama, northern Chile. The new plant will treat up to 550,000 metric tons (m.t.) of Cu concentrate per year and it will produce approximately 250,000 m.t./yr of H2SO4. More than 90% of the As contained in the concentrate will be removed, says the company.

Fuente: Chemical Engineering - www.che.com - March 2012