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

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

Ya Xiao 1,2,3, Wenli Zhou 4, Linheng Mo 2, Jianli Chen 5, Meiying Li 2,3 and Shaojun Liu 1,3,*

1 State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China; [email protected] 2 Cultural Relics and Archaeology Institute of Hunan, Changsha 410083, China; [email protected] (L.M.); [email protected] (M.L.) 3 Hunan Key Laboratory for Scientiﬁc Archaeology and Conservation Science, Changsha 410083, China 4 Institute for the History of Natural Sciences, Chinese Academy of Sciences, Beijing 100190, China; [email protected] 5 School of Archaeology and Museology, Peking University, Beijing 100871, China; [email protected] * Correspondence: [email protected]

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), Citation: Xiao, Y.; Zhou, W.; Mo, L.; and for the first time provides us with critical archeological evidence to gain an insight into the Chen, J.; Li, M.; Liu, S. Microstructure, roasting technology in ancient zinc metallurgy technology. In this paper, the microstructure and Mineralogical Characterization and mineralogical features of the zinc calcine relics found at the Doulingxia site were characterized by the Metallurgical Process X-ray diffraction (XRD) and scanning electron microscopy coupled with X-ray energy dispersive Reconstruction of the Zinc Calcine Relics from the Zinc Smelting Site spectrometer (SEM-EDS). To reconstruct the metallurgical process, the original roasting temperature (Qing Dynasty). Materials 2021, 14, of the unearthed zinc calcine was estimated by thermogravimetric analysis and differential thermal 2087. https://doi.org/10.3390/ analysis (TG-DTA), combined with reheating experiments and phase composition analysis as well as ma14082087 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 Academic Editor: Hideki Hosoda 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 Received: 3 March 2021 roasting products mainly include tiny porous particles of ZnFe2O4, Fe2O3, PbSO4, and ZnO. These Accepted: 16 April 2021 findings are helpful to reconstruct the ancient zinc metallurgy technology of zinc sulfide ores. Published: 20 April 2021

Keywords: archeological evidence; zinc metallurgy; zinc calcine; roasting; characterization; Publisher’s Note: MDPI stays neutral experimental reconstruction with regard to jurisdictional claims in published maps and institutional afﬁl- iations.

1. Introduction The smelting of zinc is considered as one of the most challenging technologies in ancient civilization since the reduction temperature of zinc oxide ores is very close to Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the boiling point of metallic zinc [1–3]. Although zinc ores are widely distributed, often This article is an open access article associated with other metals, such as lead and silver, zinc is one of the last major metals to distributed under the terms and be produced [4]. Before the establishment of modern zinc smelting technology in Europe, conditions of the Creative Commons only India and China could produce metallic zinc on a large scale [1]. Attribution (CC BY) license (https:// Ancient India is the earliest country recorded by literature in which metallic zinc was creativecommons.org/licenses/by/ produced [1,5]. It has been revealed by archeological evidence that the metallic zinc was 4.0/). distilled and condensed by using specially designed retorts ~1000 years ago in Zawar,

Materials 2021, 14, 2087. https://doi.org/10.3390/ma14082087 https://www.mdpi.com/journal/materials Materials 2021, 14, 2087 2 of 16

Ancient India is the earliest country recorded by literature in which metallic zinc was produced [1,5]. It has been revealed by archeological evidence that the metallic zinc was Materials 2021, 14, 2087 2 of 15 distilled and condensed by using specially designed retorts ~1000 years ago in Zawar, northwest India [5]. Primary sources for zinc smelting in Zawar mainly come from the sulfidic ore deposits of the Aravalli Hills, mainly constituted by sphalerite (ZnS), galena (PbS),northwest and pyrite India [(FeS5]. Primary2) [6]. However, sources the for zincsmelting smelting residues in Zawar show mainlythat the comemetal from was ob- the tainedsulfidic from ore depositssmelting ofof theoxidized Aravalli zinc Hills, ores mainly [7,8]. The constituted ore is likely by sphaleriteto have been (ZnS), roasted galena at 900–950(PbS), and °C to pyrite convert (FeS the2)[ starting6]. However, materials the to smelting oxides [9]. residues However, show neither that remains the metal of wasany kilnsobtained or ovens from suitable smelting for of oxidizedthe oxidizing zinc oresstage [ 7nor,8]. any The roasting ore is likely products to have were been found roasted in ◦ Zawarat 900–950 [2]. C to convert the starting materials to oxides [9]. However, neither remains of any kilnsIn contrast, or ovens the suitable ancient for Chinese the oxidizing zinc pyrometallurgical stage nor any roasting technology products is weremainly found based in onZawar the smelting [2]. of non-sulfide zinc ores, which include smithsonite (ZnCO3) and hemi- morphiteIn contrast, [Zn4Si2O the7(OH) ancient2 (H2 ChineseO)] [4,10]. zinc The pyrometallurgical archaeological evidence technology [11,12] is mainlyand scientific based analyseson the smelting [13–15] ofshowed non-sulfide that the zinc ores ores, distilled which includein Chongqing, smithsonite southwest (ZnCO China,3) and in hemi- the Mingmorphite (CE [Zn1368–1644)4Si2O7(OH) and2(H Qing2O)] (CE [4,10 1636–1912)]. The archaeological dynasties, evidence were mainly [11,12 ]oxidized and scientific ores. Moreover,analyses [13 the–15 investigations] showed that of the traditional ores distilled Chinese in Chongqing, zinc smelting southwest technology China, indicate in the Ming that, until(CE 1368–1644) the late 20th and century, Qing (CE zinc 1636–1912) metallurgy dynasties, in Western were Central mainly China, oxidized including ores. Moreover,the prov- incesthe investigations of Yunnan [16], of Guizhou traditional [17], Chinese and Sichuan zinc smelting [18,19], technologywas still limited indicate to the that, non-sulfide until the ores.late 20th century, zinc metallurgy in Western Central China, including the provinces of YunnanIn recent [16], Guizhou years, several [17], and zinc Sichuan smelting [18 si,tes19], have was been still limited found in to Guiyang the non-sulfide County, ores. Hu- nan ProvinceIn recent in years, central several south China zinc smelting [20]. Amon sitesg these have sites, been the found most in representative Guiyang County, ones areHunan the Tongmuling Province in centralsite and south the Doulingxia China [20]. site Among, dated these back sites, to the the Qing most dynasty. representative Signifi- cantlyones are distinguished the Tongmuling from sitethe andzinc thesmelting Doulingxia sites found site, dated in Chongqing back to the [11–15], Qing dynasty.a large numberSignificantly of cylindrical distinguished hearths from (Figure the zinc1a) were smelting found sites to be found distributed in Chongqing on the edge [11– of15 ],the a large number of cylindrical hearths (Figure1a) were found to be distributed on the edge smelting area in the sites [20]. Our previous study reveals that in these cylindrical hearths, of the smelting area in the sites [20]. Our previous study reveals that in these cylindrical a lengthy roasting process was conducted to oxidize the zinc sulfide ores at a lower tem- hearths, a lengthy roasting process was conducted to oxidize the zinc sulfide ores at a lower perature before distilling [21]. In addition, a basket of well-preserved reddish-brown pow- temperature before distilling [21]. In addition, a basket of well-preserved reddish-brown ders confirmed as the products of the roasted zinc sulfide ores has been found at one of powders confirmed as the products of the roasted zinc sulfide ores has been found at the house foundations in the Doulingxia site (Figure 1b). The distilling retorts and tech- one of the house foundations in the Doulingxia site (Figure1b). The distilling retorts nological smelting process of zinc sulfide ores in Guiyang County have been recon- and technological smelting process of zinc sulfide ores in Guiyang County have been structed [21]. However, the roasting technology, including the roasting conditions and the reconstructed [21]. However, the roasting technology, including the roasting conditions metallurgical processes during roasting are still unclear. and the metallurgical processes during roasting are still unclear.

Figure 1. ArchaeologicalArchaeological evidence found in the Doulingxia site. ( a)) Cylindrical roasting hearths; (b) (theb) the unearthed unearthed zinc zinc calcine. calcine.

ItIt should should be be mentioned mentioned that that the the shape shape and and arrangement arrangement of the roasting hearths found inin the the zinc smelting sites sites of of Guiyang are are similar to those used in the Songbai Traditional Zinc Smelting Plant in Hunan province. The The pl plantant started to carry out zinc smelting in 1905 by technicians recruited from Guiyang. The roasting method in Guiyang in the Qing dynasty period could likely be consistent with that in the Songbai zinc smelting plant. It was recordedrecorded thatthat the the entire entire roasting roasting process process in thein the Songbai Songbai Traditional Traditional Zinc SmeltingZinc Smelting Plant in Hunan province usually takes 21 days, requiring repeated calcination and crushing three times [21,22]. It can be seen that the roasting process is very time-consuming. In contrast, in present-day industrial zinc smelting, zinc sulﬁde ores are usually roasted in the roasting ◦ furnace at temperatures exceeding 900 C for several hours [23,24]. For example, roasting in the ﬂuid-bed roaster at approximately 950 ◦C for 6 h can convert up to 94% of sulfur contained in the ore to sulfur dioxide [25]. Materials 2021, 14, 2087 3 of 15

The zinc calcine remains found in the Doulingxia site, for the first time, provide us with critical archeological evidence to have an insight into the roasting technology of zinc sulfide ores. The present investigation aims to clarify the metallurgical process involving the roasting technology in pre-modern zinc production using sulfide zinc ores. The main issues discussed include the mineralogical and microstructural characterization of the unearthed calcine, the estimation of the roasting temperature, and the reconstruction of the chemical reactions that occur during the roasting process.

2. Materials and Methods 2.1. Materials The unearthed zinc calcines from the Doulingxia site are reddish-brown powders that have been carefully roasted and crushed. The ores used for the reconstruction experiments come from the Huangshaping Lead-Zinc Mine, which is a large polymetallic deposit that was mined for an extended period in the Qing Dynasty [26,27], about 5 km away from the Doulingxia site. The ores used for the experiments were selected according to the analytical results of the unearthed calcines and the ores found in the Tongmuling site [21]. To avoid the inhomogeneity of the mineral content in the ores affecting the results of the experiments, and to shorten the retention time, the ores were crushed to particles with average diameters in a range from 2 mm to 1 cm.

2.2. Characterization of the Unearthed Zinc Calcine This research is based on the thoroughly comprehensive characterization of the unearthed zinc calcine. The mineralogical and microstructural features of the unearthed calcine particles were ﬁrstly characterized by SEM-EDS, combined with XRD. To provide basic information for the later simulation experiments, the analysis focused on the mineral composition of the ores before roasting, the metallurgical reactions that occurred during roasting, and the microstructure and composition of the ﬁnally roasted products.

2.3. Estimate of the Roasting Temperatures In this investigation, the method of reheating combined with XRD phase analysis was used to confirm the original roasting temperature of the calcine. Ignoring the affection of the weathering product during burial, when the unearthed calcine is reheated to a higher temperature than the original roasting temperature, the phase composition, and microstructure of the calcine will change significantly. The reheating temperatures were determined by the TG-DTA analysis results. The specific method was: insertion of 100 g of unearthed ore sand samples in a muffle furnace; reheating at the pre-set temperature for 3 h; mineralogical and textural analysis of the cooled samples by XRD and SEM-EDS.

2.4. Experimental Reconstruction of the Roasting Process Experimental reconstruction was used in this research to decipher complex ancient roasting technologies. All experiments were conducted in a muffle furnace. Corundum crucibles 12 cm long and 6 cm wide were used as reaction containers. The air flow and redox conditions within the furnace could not be controlled, although presumably oxidizing conditions were maintained during the experimental runs as determined by the analytical results of the unearthed zinc calcine. The experimental temperatures were determined by the above preliminary estimation results, and the retention time was adjusted according to the actual roasting situation. The simulated roasting products were characterized by WDXRF, XRD, and SEM-EDS. It should be noted that the entire roasting process in the Songbai Traditional Zinc Smelting Plant in Hunan province requires repeated calcination and crushing for three cycles, which takes 21 days in total [28]. The final state of the experimental products should be consistent with the unearthed zinc calcine. By comparing the chemical composition, phase composition, and microscopic morphology of the experimental roasting products and the Materials 2021, 14, 2087 4 of 15

unearthed zinc calcine, the original roasting temperature was further conﬁrmed. Besides, the main metallurgical reactions that occurred during the roasting process were discussed. The roasting degree is quantiﬁed by the degree of desulfurization of the simulation zinc calcine. The degree of desulfurization (Ds) is given by the equation below:

Ds = S0 − S/S0 × 100 (1)

where S0 is the sulfur content of the zinc ores for reconstruction experiments before roasting and S is the sulfur content of the ﬁnal experimental roasting products.

2.5. Characterization The unearthed zinc calcines, the ores for the experiments, and the calcines produced by reconstruction experiments were characterized by the following analytical methods and techniques.

2.5.1. Scanning Electron Microscopy Coupled with X-ray Energy Dispersive Spectrometer (SEM-EDS) The microstructure and chemistry were characterized by the scanning electronic mi- croscope (Zeiss EVO MA 10, Jena, Germany) and X-ray energy dispersive spectrometer (Brooke Xﬂash 6130, Berlin, Germany), respectively. The powdered samples of the unearthed zinc calcine and the simulated zinc calcine, were cold-inlaid with epoxy resin. Then, they were polished after solidiﬁcation. The surface of the samples were sprayed with gold after cleaning and drying. The acceleration voltage applied to all analyses was 20 kV and the working distance was about 10.5 mm. The EDS results were assessed by semiquantitative analysis.

2.5.2. Thermogravimetric Analysis and Differential Thermal Analysis (TG-DTA) Thermal transformation of the unearthed zinc calcine was recorded with TG-DTA (SDT Q600, New Castle, TA, USA). The temperature range was 25–1010 ◦C with a heating ◦ rate of 10 C/min in argon. The reference material was α-Al2O3.

2.5.3. X-ray Diffraction (XRD) Analysis The mineralogical components of the samples were determined by an X-ray diffrac- tometer (Rigaku D/max 2500 PC, Tokyo, Japan) equipped with Cu Kα radiation and secondary graphite monochromator using dry and ﬁnely ground powders. The tube voltage and current were 40 kV and 40 mA, respectively. The scanning angle range (2θ) was in the range from 5 to 75◦ with a scanning speed of 8◦/min.

2.5.4. Wavelength Dispersive X-ray Fluorescence Measurement (WDXRF) The chemical content of the samples was analyzed by a WDXRF spectrometer (Axios- Advanced, Malvern Analytical, Almelo, The Netherlands). The samples for WDXRF characterization were prepared from ﬁnely ground powders, which were further pressed into sheets by mixing a certain amount of boric acid.

3. Results and Discussion 3.1. Microstructure and Mineralogical Characterization of the Unearthed Zinc Calcine The mineralogical composition of the unearthed zinc calcine includes major amounts of franklinite (ZnFe2O4), cerussite (PbCO3), and willemite (Zn2SiO4), as well as mi- nor amounts of hemimorphite [Zn4Si2O7(OH)2(H2O)], anglesite (PbSO4), ﬂuorite (CaF2), quartz (SiO2), and hematite (Fe2O3)[21]. Figure2 shows the general morphology and microstructure of the unearthed zinc calcine. There are no-table differences in composition and morphology between different particles. The calcine particles can be divided into three categories according to their morphologies: compact angular-shaped particles, cracked particles, and porous particles. Materials 2021, 14, 2087 5 of 16

3. Results and Discussion 3.1. Microstructure and Mineralogical Characterization of the Unearthed Zinc Calcine The mineralogical composition of the unearthed zinc calcine includes major amounts of franklinite (ZnFe2O4), cerussite (PbCO3), and willemite (Zn2SiO4), as well as minor amounts of hemimorphite [Zn4Si2O7(OH)2(H2O)], anglesite (PbSO4), fluorite (CaF2), quartz (SiO2), and hematite (Fe2O3) [21]. Figure 2 shows the general morphology and microstructure of the unearthed zinc calcine. There are no-table differences in composition and mor- Materials 2021, 14, 2087 phology between different particles. The calcine particles can be divided into three5 ofcate- 15 gories according to their morphologies: compact angular-shaped particles, cracked particles, and porous particles.

Figure 2. Backscattered electron images showing the morphology of the zinc calcine found at the Figure 2. Backscattered electron images showing the morphology of the zinc calcine found at the Doulingxia site. (a) Low magnification, (b) tiny particles. Doulingxia site. (a) Low magniﬁcation, (b) tiny particles.

TheThe compact compact angular-shaped angular-shaped particles particles are are homogeneous, homogeneous, mainly mainly including including ﬂuorite fluorite (particle(particle 1), 1), quartz quartz (particle (particle 2), 2), and and aluminosilicate aluminosilicate (particle (particle 3) 3) particles particles (Figure (Figure2a). 2a). It It can can bebe inferred inferred that that these these particles, particles, which which have have excellent excellent thermal thermal stability, stability, are are directly directly derived derived fromfrom the the ores ores and and do do not not react react during during the the roasting roasting process. process. These These dense, dense, unreacted unreacted particles parti- occurcles occur as tiny as particlestiny particles (~200 (~200µm) (Figureμm) (Figure2b), indicating 2b), indicating that the that ore the has ore been has been crushed. crushed. TheThe cracked cracked particles particles are are heterogeneous heterogeneous and and partially partially roasted, roasted, since since they they still still retain retain thethe angular-shaped angular-shaped morphologymorphology of of the the original original particle. particle. A Apartially partially reacted reacted sphalerite sphalerite par- particleticle occurs occurs at atthe the lower lower middle middle of of Figure Figure 22aa (particle(particle 4).4). TheThe particleparticle is is slightly slightly oxidized oxidized andand divided divided into into several several parts parts by by the the cracks. cracks. The The partially partially reacted reacted phase phase contains contains a a high high amountamount of of Fe, Fe, as as well well as minoras minor amounts amounts of Pb of andPb and Mn; theMn; average the average analysis analysis gives 51.2gives wt.% 51.2 Zn,wt.% 32.4 Zn, wt.% 32.4 Fe, wt.% 4.0 wt.%Fe, 4.0 S, wt.% 3.5 wt.% S, 3.5 Pb, wt and.% 3.4Pb, wt.% and Mn.3.4 wt.% The surfaceMn. The and surface the portions and the alongportions the cracksalong the have cracks been convertedhave been toconvert a (Fe,ed Zn) to oxide a (Fe, which Zn) oxide contains which small contains amounts small of Znamounts (~3.0 wt.%). of Zn Another(~3.0 wt.%). cracked Another particle cracked is composed particle is of composed Zn2SiO4, (Fe,of Zn Zn)2SiO O,4, Pb-Zn-As(Fe, Zn) O, oxide,Pb-Zn-As and Zn-Si-Aloxide, and oxide Zn-Si-Al phases oxide (Figure phases2a, particle (Figure 5).2a, particle 5). TheThe porous porous particles particles can can be be considered considered as as the the ﬁnal final products products of of the the zinc zinc calcine calcine after after completecomplete reaction.reaction. AsAs shownshown inin FigureFigure2 a,2a, a a large large porous porous particle particle (~1 (~1 mm) mm) (particle (particle 6) 6) mainlymainly consistsconsists of of tiny tiny grains grains of of ZnFe ZnFe2O2O4,4, minor minor amounts amounts of of Zn Zn2SiO2SiO4,4, asaswell well as as trace trace amountsamounts of of Fe-Zn-Pb Fe-Zn-Pb oxide, oxide, PbSO PbSO4 4or or PbCO PbCO3 3particles. particles. TheThe crudelycrudely spherical spherical shapes, shapes, the the porousporous texture texture and and the the heterogeneous heterogeneous composition composition imply imply that that these these large large porous porous particles particles areare formed formed by by agglomeration agglomeration of of the the tiny tiny particles particles [ 29[29].]. In In addition, addition, porous porous particles particles can can alsoalso bebe foundfound in the the tiny tiny zinc zinc calcine calcine particles. particles. These These tiny tiny porous porous particles particles are mainly are mainly com- composedposed of Fe of2O Fe3,2 ZnFeO3, ZnFe2O4, Zn2O24SiO, Zn4,2 andSiO4 PbSO, and4 PbSO or PbCO4 or3 PbCO. It is noted3. It is that noted these that tiny these particles tiny particlesstill retain still the retain angular the angularshaped morphology shaped morphology of the original of the originalparticles particles (Figure (Figure2b). 2b). Occasionally,Occasionally, partially partially reacted reacted sphalerite sphalerite particles particles are are detected detected in thein the calcine. calcine.Figure Figure3 shows3 shows the the morphology morphology and and the the composition composition as well as we asll the as distributionthe distribution of O, of S, O, Zn, S, Fe,Zn, Pb, Fe, andPb, Siand in Si one in one of these of these particles. particles. As As shown shown in in Figure Figure3a, 3a, the the particle particle shows shows a core-shella core-shell morphology.morphology. The The core core of of the the particle particle is is compact, compact, while while the the shell shell is is much much more more porous. porous. As As shownshown in in Figure Figure3 b,3b, the the core core (zone (zone A) A) of of the the particle particle is is slightly slightly reacted reacted sphalerite, sphalerite, which which containscontains 61.061.0 wt.%wt.% Zn,Zn, 31.731.7 wt.%wt.% FeFe and and 4.5 4.5 wt.% wt.% S. S. The The brighter brighter rim rim (zone (zone B), B), which which wrapped the core and divided the core into two parts, is Fe-Pb-Zn-Si-O phase and contains approximately 17.3 wt.% Pb with a trace amount of Cu (~1.6 wt.%). The adjacent dark grey rim (zone C) is a compact Fe-Zn-Si-O phase, with more Zn and less Pb than the brghter rim. The next grey rim (zone D) and the outer porous portion (zone E) separated from the core are a mixture of ZnFe2O4 and minor Zn2SiO4. Materials 2021, 14, 2087 6 of 16

wrapped the core and divided the core into two parts, is Fe-Pb-Zn-Si-O phase and contains approximately 17.3 wt.% Pb with a trace amount of Cu (~1.6 wt.%). The adjacent Materials 2021, 14, 2087 dark grey rim (zone C) is a compact Fe-Zn-Si-O phase, with more Zn and less Pb than6 ofthe 15 brghter rim. The next grey rim (zone D) and the outer porous portion (zone E) separated from the core are a mixture of ZnFe2O4 and minor Zn2SiO4.

Figure 3. X-ray maps showing the distributions of O, S, Zn, Fe, Pb,Pb, and Si in a partially reacted sphalerite particle. (a) The secondary electron (SE) image shows the morphology of the grain; ( b) Different grey levels in the backscattered electron (BSE) image imply different compositions. A– lightly reacted sphalerite (core), B– Fe-Pb-Zn-Si-O phase (brighter rim), C– (BSE) image imply different compositions. A—lightly reacted sphalerite (core), B—Fe-Pb-Zn-Si-O phase (brighter rim), Fe-Zn-Si-O phase (dark grey rim), D– a mixture of ZnFe2O4 and Zn2SiO4 (grey rim), E– a mixture of ZnFe2O4 and Zn2SiO4 C—Fe-Zn-Si-O phase (dark grey rim), D—a mixture of ZnFe O and Zn SiO (grey rim), E—a mixture of ZnFe O and (porous portion); (c) O Kα map; (d) S Kα map; (e) Zn Kα map;2 (f4) Fe Kα map;2 4(g) Pb Lα map; and (h) Si Kα map. 2 4 Zn2SiO4 (porous portion); (c)OKα map; (d)SKα map; (e) Zn Kα map; (f) Fe Kα map; (g) Pb Lα map; and (h) Si Kα map. The distributions of O, S, Zn, Fe, Pb, and Si in the partially reacted sphalerite particle The distributions of O, S, Zn, Fe, Pb, and Si in the partially reacted sphalerite particle could provide some helpful Information about the oxidation mechanism of the zinc sul- could provide some helpful Information about the oxidation mechanism of the zinc sulfide fide ores during roasting. It can be observed that during the roasting process, the sulfur ores during roasting. It can be observed that during the roasting process, the sulfur in the insphalerite the sphalerite particle particle is gradually is gradually replaced replaced by oxygen by oxygen (Figure (Figure3c), and 3c), the and sulfur the sulfur originally orig- inallypresent present in the in reacted the reacted portion portion has disappeared has disappeared (Figure (Figure3d). The 3d). zinc The remainszinc remains in-situ in-situ and andreacts reacts with with oxygen oxygen and and iron iron to form to form a (Zn, a (Zn, Fe)O Fe)O phase phase (Figure (Figure3e). As3e). the As iron the graduallyiron gradually migrates to the periphery of the particle (Figure 3f), the ZnFe2O4 phase is finally migrates to the periphery of the particle (Figure3f), the ZnFe 2O4 phase is finally formed formedon the outeron the reacted outer reacted portion portion [25]. Additionally, [25]. Additionally, a trace a amounttrace amount of lead of (Figurelead (Figure3g) and 3g) andsilicon silicon (Figure (Figure3h) are3h) detected are detected in specific in specific points points of the of particle.the particle. The The lead lead is detected is detected as asFe-Pb-Zn-Si-O Fe-Pb-Zn-Si-O phase phase enveloped enveloped by by a thin a thin shell shell of Fe-Zn-Si-Oof Fe-Zn-Si-O phase. phase. Combined Combined with with the theabove above results results in Figure in Figure2a (particle 2a (particle 4), the 4), lead the andlead silicium and silicium are more are likelymore likely to be presentto be pre- in sentthe original in the original sphalerite sphalerite particle particle instead instead of the oxide of the fume oxide deposited fume deposited on the particle. on the particle. During 2 4 Duringroasting, roasting, the Zn2 SiOthe 4Znis formedSiO is byformed the solid by the reaction solid betweenreaction between ZnO and ZnO silicates and [ 30silicates], and [30],a trace and amount a trace ofamount Pb-Zn of silicate Pb-Zn is silicate formed is informed the portion in the portion where Pb where is distributed. Pb is distributed. By the Bydifferences the differences in the densitiesin the densities of the of sphalerite the sphalerite and its and reaction its reaction products products and the and evolution the evo- lutionof SO2 ofgas SO during2 gas during roasting, roasting, the reacted the reacted portion port ision gradually is gradually decomposed decomposed and turnsand turns into intoporous porous or tiny or tiny particles particles [25]. [25].

3.2. Estimate Estimate of of the the Roasting Roasting Temperature To determine the original roasting temperature, a comprehensivecomprehensive thermal analysis (TG-DTA) was conducted on thethe unearthedunearthed zinczinc calcinecalcine (DZC-1).(DZC-1). The thermal analysis ◦ curve (Figure (Figure 44)) shows a a noticeable noticeable weight weight loss loss between between 256 256 and and 357 357 °C accompaniedC accompanied by twoby two endothermic endothermic peaks. peaks. In contrast, In contrast, another another distinct distinct weight weight loss lossappears appears at temperatures at tempera- ◦ highertures higher than 847 than °C. 847 SinceC. the Since unearthed the unearthed calcines calcines have been have buried been buriedfor an extended for an extended period afterperiod roasting, after roasting, they may they have may been have impacted been impacted by the ambient by the ambientenvironment. environment. One possible One consequencepossible consequence is that phase is that transformation phase transformation in the calcines in the took calcines place during took place the long during burial the process.long burial process.

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FigureFigure 4. 4. TG-DTATG-DTA analysis analysis result result of of the the zinc zinc calcine calcine (DZC-1). (DZC-1).

CombinedCombined with with the the TG-DTA TG-DTA analysis analysis result results,s, the the reheating reheating temperature temperature was was selected selected as 200, 300, 500, 700, and 900 ◦C, respectively. As it can be seen in Figure5, the phase as 200, 300, 500, 700, and 900 °C, respectively. As it can be seen in Figure◦ 5, the phase compositioncomposition of of the the zinc zinc calcine calcine begins begins to to change change after after reheating reheating at at 300 300 °C.C. On On one one hand, hand, hemimorphite (Zn4Si2O7(OH)2(H2O)) loses its water of crystallization, turning into dehy- hemimorphite (Zn4Si2O7(OH)2(H2O)) loses its water of crystallization, turning into dehy- drated hemimorphite (Zn4Si2O7(OH)2). On the other hand, cerussite (PbCO3) is completely drated hemimorphite (Zn4Si2O7(OH)2). On the other hand, cerussite (PbCO3) is completely decomposed and combines with water molecules to form lead oxide hydrate (PbO·xH2O). decomposed and combines◦ with water molecules to form lead oxide hydrate (PbO·xH2O). After heating at 500 C, PbO·xH2O loses water of crystallization to turn into PbO, and After heating at 500 °C, PbO·xH2O loses water of◦ crystallization to turn into PbO, and partially oxidizes to Pb3O4. After heating at 700 C, the phase compositions of the zinc partially oxidizes to Pb3O4. After heating at 700 °C, the phase compositions of the zinc calcine change significantly. Among them, Zn4Si2O7(OH)2 loses structural water [31] and is calcine change significantly. Among them, Zn4Si2O7(OH)2 loses structural water [31] and transformed into Zn2SiO4. The PbO and Pb3O4 phases disappear. In addition, new phases is transformed into Zn2SiO4. The PbO and Pb3O4 phases disappear. In addition, new like larsenite (PbZnSiO4) and lead oxysulfate (Pb2SO5) appear. However, the particle phases like larsenite (PbZnSiO4) and lead oxysulfate (Pb2SO5) appear. However, the par- morphology of the reheated calcine is still consistent with that of the unearthed zinc calcine ticle morphology of the reheated calcine is still consistent with that of the unearthed zinc before heating, keeping a certain angular shaped morphology of the original particle. After calcine before heating,◦ keeping a certain angular shaped morphology of the original par- heating at 900 C, the PbSO4 grains in the zinc calcine completely disappear and transform ticle. After heating at 900 °C, the PbSO4 grains in the zinc calcine completely disappear into lanarkite (Pb2O(SO4)). In addition, fluorite (CaF2) in the zinc calcine partially reacts and transform into lanarkite (Pb2O(SO4)). In addition, fluorite (CaF2) in the zinc calcine to form a large amount of lead fluoride silicate sulfate (Pb10(SiO4)3(SO4)3F2) and a small partially reacts to form a large amount of lead fluoride silicate sulfate (Pb10(SiO4)3(SO4)3F2) amount of calcium sulfate (CaSO4). At this temperature step, the microstructure of the zinc andcalcine a small has alsoamount undergone of calcium significant sulfate (CaSO changes4). At (Figure this temperature6). The lead-containing step, the microstruc- particles tureappear of the to bezinc homogenized. calcine has also Moreover, undergone the significant angular-shaped changes morphology (Figure 6). The of the lead-con- calcine tainingparticles particles becomes appear crudely to sphericalbe homogenized. with increased Moreover, porosity. the angular-shaped morphology of the calcine particles becomes crudely spherical with increased porosity.

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Figure 5. XRD analysis results showing the phase transformation of the zinc calcine (DZC-1) re- Figure 5. XRD analysis results showing the phase transformation of the zinc calcine (DZC-1) reheated Figureheated 5. at XRD different analysis temperatures. results showing the phase transformation of the zinc calcine (DZC-1) re- at different temperatures. heated at different temperatures.

Figure 6. Backscattered electron image showing the morphology of the zinc calcine reheated at 900 °C. 1–PbZnSiO 4 (light), Figure2–ZnFe 6.2 OBackscattered4 (grey), 3–Zn electron2SiO4 (dark image grey), showing 4–pore the (black), morphology 5–Pb10 (SiOof the4)3 (SOzinc4 )calcine3F2 (light), reheated 6–CaSO at 9004 (dark).◦ °C. 1–PbZnSiO 4 (light), Figure 6. Backscattered electron image showing the morphology of the zinc calcine reheated at 900 C. 1—PbZnSiO4 (light), 2–ZnFe2O4 (grey), 3–Zn2SiO4 (dark grey), 4–pore (black), 5–Pb10(SiO4)3(SO4)3F2 (light), 6–CaSO4 (dark). 2—ZnFe O (grey), 3—Zn SiO (dark grey), 4—pore (black), 5—Pb (SiO ) (SO ) F (light), 6—CaSO (dark). 2 4 2 4 Combined with the TG-DTA analysis10 4 3 result4 3 (Figure2 4) and the4 XRD analysis results (FigureCombined 5), the thermalwith the weightTG-DTA loss analysis between result 256 (Figureand 357 4) °C and should the XRD be mainly analysis due results to the (Figure 5), the thermal weight loss between 256 and 357 °C should be mainly due to the

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Combined with the TG-DTA analysis result (Figure4) and the XRD analysis results (Figure5), the thermal weight loss between 256 and 357 ◦C should be mainly due to the crystal water loss of hemimorphite and the decomposition of cerussite. We would like to mention that the formation temperature of franklinite (ZnFe2O4) is approximately 600 ◦C [23]. Given the weathering effect during the burial process, it is reasonable to con- sider that both cerussite and hemimorphite could be weathering products [21] rather than phases formed after roasting. The main reason for the obvious weight loss at around 847 ◦C on the TG-DTA curve may be the decomposition of PbSO4 and the volatilization of PbO. Therefore, it can be inferred that the original roasting temperature of the unearthed calcines should be above the formation temperature of ZnFe2O4 and lower than the apparent weight loss temperature on the TG-DTA curve at about 850 ◦C.

3.3. Experimental Reconstruction of the Roasting Process The WDXRF and XRD analysis results of the unearthed zinc calcine, the ore used for the simulation experiments, and the samples obtained under different simulated conditions are shown in Table1. According to the analytical results, the ores used in the simulation experiments and the zinc calcine unearthed from the Doulingxia site contains a large amount of Fe, Zn, and Pb. The main mineralogy of the ore used for the simulation experiments includes sphalerite ((Zn, Fe)S), galena (PbS) and siderite (Fe(CO3)), as well as minor amounts of pyrrhotite (Fe7S8), pyrite (FeS2), and marcasite (FeS2). It is noted that the sulfur content in the unearthed calcine is quite low (0.33 wt.%), which indicates that the roasting process was carried out thoroughly. Materials 2021, 14, 2087 10 of 15

Table 1. The WDXRF and XRD analysis results of the unearthed zinc calcine, the ore used for the experiments, and the samples obtained under different experimental conditions.

Chemical Contents (wt.%) Phases Composition No. O S Si Fe Zn Pb Major Phases Minor Phases

hemimorphite(Zn4Si2O7(OH)2(H2O), anglesite (PbSO4), DZC-1 14.48 0.33 2.19 29.01 26.97 16.90 franklinite (ZnFe2O4), cerussite (PbCO3), willemite (Zn2SiO4) ﬂuorite (CaF2), quartz (SiO2), hematite (Fe2O3) Hsp-1 8.42 10.64 1.12 26.85 24.67 23.40 sphalerite ((Zn,Fe)S), galena (PbS), siderite (Fe(CO3)) pyrrhotite (Fe7S8), pyrite (FeS2), marcasite (FeS2) 650-1 h 12.53 8.55 1.30 39.76 17.45 13.57 sphalerite, galena pyrrhotite, hematite, orthoclase (KAlSi3O8) 650-6 h 13.66 6.14 1.22 38.08 18.63 15.58 sphalerite, galena, hematite anglesite, pyrrhotite, sanidine (KAlSi3O8) 650-12 h 14.85 2.84 1.24 35.50 19.00 20.90 sphalerite, galena, hematite anglesite, pyrrhotite 650-24 h 15.64 2.15 0.84 38.39 13.71 23.46 sphalerite, galena, hematite, anglesite zincite (ZnO), franklinite 650-48 h 15.40 2.13 0.83 39.43 13.83 22.58 sphalerite, galena, hematite, anglesite, franklinite zincite, ﬂuorite 650-96 h 16.53 1.95 0.93 37.16 15.79 21.99 anglesite, hematite, franklinite, galena, zincite zinc oxide sulfate (Zn3O(SO4)) 650-192 h 15.97 2.22 1.33 35.93 13.93 24.61 anglesite, hematite, franklinite, galena zincite, zinc oxide sulfate 550-48 h 14.51 4.81 0.61 37.67 18.04 19.34 sphalerite, galena, hematite anglesite 750-48 h 15.29 1.49 0.48 28.03 23.54 26.83 anglesite, hematite, franklinite, zincite, galena 850-48 h 13.06 1.06 0.50 32.29 24.95 24.06 franklinite, zincite, hematite, anglesite, lanarkite (Pb2OSO4) 950-48 h 12.22 0.38 0.41 38.21 27.41 16.55 franklinite, zincite, magnetoplumbite (PbFe12O19) hematite, willemite 650-48 h-2 15.21 2.04 0.61 36.15 12.80 27.44 anglesite, hematite, franklinite zincite, galena, zinc oxide sulfate 750-48 h-2 14.92 0.73 1.75 42.49 19.91 13.79 hematite, franklinite, zincite, anglesite orthoclase 850-48 h-2 13.45 0.61 1.02 37.51 22.89 17.38 franklinite, zincite, hematite, lanarkite willemite, anglesite 650-48 h-3 15.17 1.89 0.77 37.79 16.63 22.36 anglesite, hematite, franklinite, galena, zincite zinc oxide sulfate 750-48 h-3 14.81 0.81 0.99 38.67 21.37 17.93 hematite, franklinite, zincite, anglesite orthoclase 850-48 h-3 13.84 0.43 0.58 39.63 22.21 17.06 franklinite, zincite, hematite lanarkite, willemite The WDXRF results were normalized to 100 wt.%. DZC-1 refers to the zinc calcine unearthed in the Doulingxia site. Hsp-1 refers to the ore used for reconstruction experiments. 650-1 h means roasting at 650 ◦C for 1 h. 650-48 h-2 means roasting at 650 ◦C for 48 h twice. The numbering methods of the other samples can be similarly deduced. Materials 2021, 14, 2087 11 of 16

Materials 2021, 14, 2087 11 of 15 In order to verify the roasting temperatures of the unearthed zinc calcine estimated throughIn order the above to verify reheating the roasting experiments, temperatures the simulated of the unearthed roasting temperature zinc calcine was estimated further throughselected theas 550, above 650, reheating 750, 850, experiments, and 950 °C, respecti the simulatedvely. Firstly, roasting the temperature ores were roasted was further at 650 selected°C for 1, 6, as 12, 550, 24, 650, 48, 96, 750, and 850, 192 and h, respectively, 950 ◦C, respectively. to evaluate Firstly, the dependence the ores were of the roasted roasting at 650time◦ Confor the 1, roasting 6, 12, 24, process. 48, 96, and As 192shown h, respectively, in Figure 7a, to the evaluate degree the of dependencedesulfurization of thein- roastingcreases significantly time on the roasting at 650 °C. process. After As24 h shown of roasting in Figure treatment,7a, the degree the sulfur of desulfurization content in the increasesroasted products signiﬁcantly is reduced at 650 ◦ C.from After 10.6 24 to h of2.15 roasting wt.% treatment,(Table 1). The the sulfurexperiments content clearly in the roastedshow that products the degree is reduced of desulfurization from 10.6 to 2.15 almost wt.% keeps (Table stable1). The after experiments 24 h (Figure clearly 7a). showCom- thatbined the with degree the ofXRD desulfurization results, it is concluded almost keeps that stable the following after 24 h basic (Figure chemical7a). Combined reactions with can thebe proposed. XRD results, it is concluded that the following basic chemical reactions can be proposed.

◦ FigureFigure 7. Degree 7. Degree of desulfurization of desulfurization of the of products the products obtained obtained under under different different conditions. conditions. (a) Products (a) Products roasted roasted at 650 at °C 650 forC different for roastingdifferent times, roasting (b) products times, roasted (b) products for 48 roastedh at different for 48 temperatures. h at different temperatures.

Firstly, in addition to the decomposition of the associated siderite (FeCO ), the pyrite, Firstly, in addition to the decomposition of the associated siderite (FeCO33), the pyrite, marcasite,marcasite, andand pyrrhotitepyrrhotite areare oxidizedoxidized asas describeddescribed inin EquationsEquations (2)–(4)(2)–(4) below.below.

4FeCO4FeCO3(s)(+s)O+O2(g)(g→) →2Fe2Fe2O3O(s(s)) + +4CO 4CO2(g(g)) (2)(2) 4FeCO (s) +O (g) →2Fe O (s) + 4CO (g) (3) 4FeCO3(s) + O2(g) → 2Fe2O3(s) + 4CO2(g) (3)

2ZnS2ZnS(s)(+s)3O+3O2(g)(g→) →2ZnO2ZnO(s()s)++2SO2SO2(g(g))

4Fe4Fe7S8(Ss)(+s)53O+ 53O2(g)(g→) →14Fe 14Fe2O3O(s(s)) + +32SO 32SO2(g()g) (4)(4) WithWith thethe proceeding proceeding of of the the roasting roasting process, process, galena galena and and sphalerite sphalerite are oxidizedare oxidized to lead to sulfatelead sulfate and zinc and oxide,zinc oxide, respectively, respectively, as described as described in Equations in Equations (5) and (5) (6) and below. (6) below.

PbS(s) +2O(g) →PbSO(s) (5) PbS(s) + 2O2(g) → PbSO4(s) (5)

2ZnS(s) +3O(g) →2ZnO(s) +2SO(g) (6) 2ZnS(s) + 3O2(g) → 2ZnO(s) + 2SO2(g) (6) We would like to mention that iron is usually present in sphalerite’s crystal lattice, substitutingWe would zinc. like Therefore, to mention if the that oxidation iron is usually reaction present of the insphalerite sphalerite’s takes crystal place, lattice, sulfur substitutingin the sulfide zinc. can Therefore,be replaced if theby oxygen oxidation to reactionproduce ofzinc the oxide, sphalerite which takes could place, react sulfur with in the sulﬁde can be replaced by oxygen to produce zinc oxide, which could react with coexisting Fe2O3 to produce franklinite through a solid-phase reaction [32,33], as described coexisting Fe O to produce franklinite through a solid-phase reaction [32,33], as described in Equation (7)2 3below. in Equation (7) below. ZnOZnO(s)(s+) Fe+Fe2O3O(s)(s→) →ZnFeZnFe2OO4(s(s)) (7)(7)

However,However, itit isis notednoted inin TableTable1 1that that no no ZnFe ZnFe2O2O44 cancan bebe detecteddetected untiluntil thethe roastingroasting timetime isis 2424 h.h. ItIt isis believedbelieved thatthat thethe formationformation ofof franklinitefranklinite isis aa solid-statesolid-state reactionreaction processprocess betweenbetween zinczinc oxideoxide andand ironiron oxide.oxide. AlthoughAlthough thethe closeclose contactcontact ofof ZnOZnO withwith Fe Fe2O2O33 cancan facilitatefacilitate thethe solid-phasesolid-phase reaction,reaction, thethe diffusiondiffusion ofof thethe solid-statesolid-state particlesparticles isis stillstill slow.slow. InIn particular,particular, thethe producedproduced newnew intermediateintermediate phasephase cancan furtherfurther efﬁcientlyefficiently hinderhinder thethe atomicatomic diffusion process [34,35]. It should be pointed out that there is a large amount of franklinite

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in the unearthed zinc calcine as well. From this view, it is believed that the unearthed zinc calcine should have been obtained by a lengthy roasting process with a duration not less than 12 h. In the present simulation investigation, the maximum duration time for the roasting experiments was set at 48 h to ensure a fully completed roasting process. As expected, the results show that the degree of desulfurization gradually increases with the increase in the roasting temperature (Figure7b). When the roasting temperature increases to 650 ◦C from 550 ◦C, the degree of desulfurization increases most significantly (Figure7b). In contrast, when the roasting temperature is between 650 and 950 ◦C, the degree of desulfurization increases slowly. Considering the XRD results, at 550 ◦C, even after roasting for 48 h, sphalerite has not been completely oxidized. However, no zinc oxide phases such as ZnO ◦ and ZnFe2O4 can be detected, as seen in Table1. After roasting at 950 C for 48 h, the sulfur content in the roasted product can be as low as 0.38 wt.%. However, it is observed that the product turns black, which is obviously different from the unearthed zinc calcine. It is due to a large amount of magnetoplumbite (PbFe12O19) that is formed in the roasted product and cannot be detected in the unearthed calcine. Overall, when the roasting temperature is in a range of 650–850 ◦C, the roasted products by reconstruction experiments are similar to the zinc calcine unearthed at the Doulingxia site. To make a better comparison with the unearthed zinc calcine, the products roasted for 48 h at 650, 750, and 850 ◦C were crushed and roasted for three cycles in the present reconstruction experiments. According to the results, the degree of desulfurization of the products almost remains constant when roasted again at 650 ◦C. In contrast, roasting for more cycles at 750 and 850 ◦C can further improve the degree of desulfurization of the products (Figure7b). In particular, even after roasting for three cycles at 650 ◦C, a large amount of galena can still be detected by XRD analysis. As shown in Figure8a, SEM images show that most of the final product obtained at 650 ◦C still retains the angular shaped morphology of the original particles. Furthermore, most of the PbSO4 particles show a layered porous structure, sometimes covering the unreacted galena (Figure8a). In comparison, the mineralogical composition and microstructure of the final product at 750 ◦C, as revealed by SEM analyses, is very close to that of the unearthed zinc calcine. Zinc mainly occurs as franklinite and zincite, while lead exists in the form of lead sulfate (Table1 ). The angular structure of the particles in the final product at 750 ◦C becomes less obvious, and several spherical porous particles appear in the image (Figure8b). By contrast, ◦ the final roasting product at 850 C contains a small amount of lanarkite (Pb2OSO4), which has not been detected in the unearthed zinc calcine. Moreover, the microstructure of the final product obtained at 850 ◦C is dominated by spherical porous particles with relatively large pores (Figure8c). Therefore, it is reasonable to deduce that the roasting temperature of the unearthed zinc calcine should be between 650 and 850 ◦C, most probably near 750 ◦C. The final roasting products obtained at 750 ◦C are mainly composed of a major amount of hematite (Fe2O3), franklinite (ZnFe2O4), zincite (ZnO), and anglesite (PbSO4), as shown in Table1. By contrast, major amounts of willemite (Zn 2SiO4) and cerussite (PbCO3), as well as minor amounts of hemimorphite [Zn4Si2O7(OH)2(H2O)] and fluorite (CaF2) can be found in the unearthed zinc calcine. The presence of Zn2SiO4 in the unearthed calcines mainly contributed by the higher content of Si (Table1). Fluorite should come from gangue because of its chemical stability. Combined with the reheating experimental results, it confirms that PbCO3 and Zn4Si2O7(OH)2(H2O) found in the unearthed zinc calcine should have been formed by weathering. Materials 2021, 14, 2087 13 of 15 Materials 2021, 14, 2087 13 of 16

Figure 8. Backscattered electron images showing the morphology of the final products obtained Figure 8. Backscattered electron images showing the morphology of the ﬁnal products obtained under different simulated (a) 650. °C; (b) 750 °C; (c) 850 °C. under different simulated (a) 650 ◦C; (b) 750 ◦C; (c) 850 ◦C.

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It is noted that in the unearthed zinc calcine, zinc mainly exists in the form of ZnFe2O4. The reduction in zinc from the ZnFe2O4 is much more complicated than that from the ZnO. In a reducing atmosphere, ZnFe2O4 can be reductively decomposed into ZnO and Fe2O3 at about 700 ◦C[36]. As can be inferred from the Ellingham Diagram [24], iron oxides were reduced more easily than zinc oxide. A large amount of iron oxide in the calcine will increase the consumption of reducing coal. In addition, the metal iron that appears in the slag could lower its gas permeability. The metal iron can even adhere to the inner wall of the retorts, affecting the slag tapping process and the reutilization of the retorts. According to the analysis results of the slags at the Tongmuling site, slags tipped out from the pots present as loose granular particles, and only a small amount of iron can be found in the slags attached inside the pots [24]. It is likely that sufficient reducing coal had been added in the pots to prevent the formation of molten slag during the distillation processing [19,28]. Therefore, the presence of ZnFe2O4 does not bring too much negative influence on the entire reduction process and the final yield of zinc.

4. Conclusions Combining the microstructure and mineralogical characterization of the unearthed calcine with the experimental reconstruction, the roasting technology in pre-modern zinc metallurgy production from zinc sulfide ores has been clarified and reconstructed. The original roasting temperature of the unearthed zinc calcine should be in a range of 650~850 ◦C, most probably near 750 ◦C. If the retention time is long enough, all sphalerite can be oxidized when the roasting temperature is above 650 ◦C. In the final roasted products, zinc mainly exists in the form of ZnFe2O4, ZnO, and Zn2SiO4 phases, while the lead exists in the form of PbSO4. PbCO3 and Zn4Si2O7(OH)2(H2O) found in the unearthed zinc calcine are considered as the weathering product during its burial process. The final roasting product is mainly tiny porous particles, which is beneficial for further distillation. These findings are helpful to completely reconstruct the ancient zinc metallurgy technology of zinc sulfide ores.

Author Contributions: Methodology, Y.X. and S.L.; formal analysis, Y.X. and W.Z.; investigation, Y.X., W.Z. and M.L.; resources, L.M. and J.C.; data curation, Y.X., S.L. and M.L.; writing—original draft preparation, Y.X. and S.L.; writing—review and editing, Y.X., S.L., W.Z. and J.C.; funding acquisition, L.M. and Y.X. All authors have read and agreed to the published version of the manuscript. Funding: This work is financially supported by the National Social Science Foundation of China (No. 18BKG026), and the National Key R&D Program of China (No. 2019YFC1520100). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Acknowledgments: The authors gratefully acknowledge the help of Xiaomin Liao from the Historical and Cultural Research Center of Guiyang County, who first identified this site, and invited us for an on-site field survey. We also appreciate the help from Shengqiang Luo, from the Cultural Relics Management Office of Chenzhou, and Zhiqiang Zhao, from the Cultural Relics and Archaeology Institute of Hunan, who participate in the investigation and excavation. Conflicts of Interest: The authors declare no conflict of interest.

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