Chapter 24 - Roasting of Gold Ore in the Circulating Fluidized-Bed Technology

Abstract

Gold roasting is a process, applicable to refractory ores and concentrates that contain sulfide sulfur, organic carbon, and potentially arsenic. Removal of these compounds is necessary to achieve high gold recovery (>90%) during cyanide leaching. Gold roasting is typically a metallurgical fluidized bed process. Process and equipment design have been influenced by other fluidized bed metallurgical processes (e.g., alumina calcination), which are also analyzed here. Two types of fluidized beds have been widely used industrially: the circulating fluidized bed (CFB) and the bubbling fluidized bed (BFB). The fluid-dynamic characteristics of these units are analyzed, as well as the scaling methodology, which includes experiments and modeling. The importance of the characteristic Froude number hereto is analyzed. To date, application of these units within the roasting flow sheet essentially depends on the concentration of arsenic within the feed. In the case where the feed arsenic content is <1% (w/w), then a one-stage CFB roaster is used for sulfide sulfur and organic carbon oxidation. The CFB has with time replaced the BFB unit for this case based on several inherent advantages, which include (1) higher sulfur and carbon removal; (2) higher specific throughput per roaster unit; (3) tighter temperature control within 10°C of the set-point, which ensures that particle pores will not be sintered; and (4) less need for calcium sorbent for in situ SO2 capture. The flow-sheets of several existing roasters are analyzed here in detail (i.e., Kalgoorlie, Placer dome—Cortez, Newmont, and Syama). On the other hand, should the arsenic content in the feed be significant, that is, >1% (w/w), the two-stage roasting process is necessitated. This process is currently realized with use of two BFB roasters connected in series in terms of solid flow. The first stage is a partial roasting operation where partial oxidation of the sulfur provides process heat for arsenic compound volatilization (dealt with in the gas cleaning section) while oxygen is absent from the process gas. Such unit operation is also found in dearsenifying copper roasting. The second stage is carried out in oxygen excess and aims at the complete oxidation of sulfide sulfur and organic carbon. Hence, the second unit represents a “dead” roasting operation. The flow-sheet of the Tongguan roaster plant is representative of the two-stage roasting process and is also analyzed here. All in all, the article aims to provide the main aspects of the experience of Outotec GmbH & Co. KG, which has been attained during more than 50years through designing hundreds of fluidized bed plants for a multitude of process applications.