Microstructure, Mineralogical Characterization and the Metallurgical Process Reconstruction of the Zinc Calcine Relics from the Zinc Smelting Site (Qing Dynasty)

Abstract

The smelting of zinc is considered as one of the most challenging technologies in ancient civilization. Compared with non-sulfide zinc ores, the smelting of zinc sulfide ores is more complicated since they have to be roasted before smelting. The technological smelting process of the ancient zinc metallurgy technology has been studied and partly reconstructed. However, the roasting technology, including the roasting conditions and involved metallurgical processes, is still unclear. The discovery of the zinc smelting site of Doulingxia dates back to the Qing dynasty (CE 1636–1912), and for the first time provides us with critical archeological evidence to gain an insight into the roasting technology in ancient zinc metallurgy technology. In this paper, the microstructure and mineralogical features of the zinc calcine relics found at the Doulingxia site were characterized by X-ray diffraction (XRD) and scanning electron microscopy coupled with X-ray energy dispersive spectrometer (SEM-EDS). To reconstruct the metallurgical process, the original roasting temperature of the unearthed zinc calcine was estimated by thermogravimetric analysis and differential thermal analysis (TG-DTA), combined with reheating experiments and phase composition analysis as well as microstructural analysis. The simulation experiments were conducted to reconstruct the roasting process. The results indicated that the original roasting temperature of the unearthed zinc calcine should be in a range of 650–850 °C, most probably near 750 °C. As long as the retention time is long enough, all sphalerite can be oxidized when the roasting temperature is above 650 °C. The final roasting products mainly include tiny porous particles of ZnFe2O4, Fe2O3, PbSO4, and ZnO. These findings are helpful to reconstruct the ancient zinc metallurgy technology of zinc sulfide ores.

Brass products in the coronet excavated from an M2-numbered Sui-Tang-dynasty tomb situated in Kun Lun Company in Xi'an, Shaanxi

“…When heated, zinc ore was reduced to metallic zinc vapor and diffused into copper, forming brass then. Finally, in the third stage dated back to around the fifteenth century and with the availability of zinc metal, brass could be obtained by smelting pure copper and metallic zinc [17]. Thus, the composition distribution and metallographic structure of the sample were consistent with brass prepared by smelting copper ore [35,36] consisting in melting an appropriate mixture of zinc ore and copper ore over 920 °C [12,22].…”

“…However, based on numerous excavations, the mass manufacturing of brass products and brass coins could have occurred in China not earlier than in the fifteenth century, though According to scientists, Chinese brass technology could have become widespread during the Ming Dynasty (明) epoch [13]. Although the appearance of elemental zinc metal in China is also dated back to the Ming Dynasty period [14,15], brass could be obtained by smelting copper-zinc symbiotic ore, copper-zinc mixed ore or copper with zinc oxide ore earlier [12,16,17]. Various new studies revealed that almost certainly brass was employed much earlier.…”