Minerals and Metals in Ancient India v1.c26.s6.t4: Phase Studies on Ancient Zawar Residue at IIT, Kanpur
Minerals and Metals in Ancient India v1.c26.s6.t4
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Phase Studies on Ancient Zawar Residue at IIT, Kanpur
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We have referred earlier to the research project at the Indian Institute of Technology (IIT), Kanpur, sponsored by the Hindustan Zinc Limited. (Biswas, 1981-84) The characterisation of Pb-Zn bearing retort residues and slags found at Zawar was carried out using Philips EM 301 transmission electron microscope, Philips PSEM 500 scanning electron microscope fitted with an EDAX or X-ray microanalyser, GEC XRD-6 X-ray diffractometer and a MOM 3427 derivatograph for differential thermal analysis (DTA) and thermogravimetry (TGA).
v1.c26.s6.t4.2
The phases identified by X-ray and electron diffraction studies (Biswas et al., 1984: 236) are summarised in Table 18.5. The results show that in the roasting operation prior to retort distillation, a small part of sphalerite was not converted into oxide and remained in the retort as ZnS and ZnSO₄. The presence of goslarite or ZnSO₄, 7H₂O (hydrated at a later stage) was confirmed by the endothermic DTA peaks apart from X-ray and electron diffraction studies. Some ZnO might have remained unconverted in the retort to undergo atmospheric conversion to basic carbonate at a later stage. While most of the ZnO was reduced to metal, a part of it must have been converted at high temperature to phosphate and silicates from which the metal could not be recovered. Biswas et al. (1984) detected a number of zinc-, calcium-zinc, lead-zinc, calcium-lead-zinc and magnesium-aluminium silicates. Later, Freestone et al. reported (1985b: 7) that lead slag contained iron-, calcium-iron, calcium-zinc, magnesium and calcium-magnesium silicates (vide chapter 17). The only silicate phase that Freestone et al. could report in the retort residue was CaMgSi₂O₆ or diopside.
v1.c26.s6.t4.3
Biswas et al. (1984: 237-41) obtained secondary electron images of retort residue samples and found that the particles have a variety of morphology: plates, needles and spheres. Scanning electron microscopy and X-ray micro-analysis showed characteristic peaks of many elements, the most prominent being those of silicon and calcium. Scanning was done by a fine electron probe on the zinc-containing particles ranging in size between 0.5-40 μm.
v1.c26.s6.t4.4
Several of the small particles (0.5-8.0 μm) show approximately constant values for the ratio of intensities corresponding to the elements Si, Ca, Mg, Fe,
v1.c26.s6.t4.5
Table 18.5 Phases Identified in Zawar Retort Residue (Content), Wall and (Lead) Slag by X-ray and Electron Diffraction Studies
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| Sample | Phases identified | |
| | *X-ray diffraction* | *Electron diffraction* |
| Retort content | Zn₂P₂O₇, 5H₂O, goslarite ZnSO₄, 7H₂O, hemimorphite Zn₄(OH)₂Si₂O₇, H₂O, esperite Ca₃Pb(ZnSiO₄)₄, Sphalerite ZnS, Chalcopyrite CuFeS₂, larsenite, PbZnSiO₄ | Zn₂P₂O₇, 5H₂O, goslarite, hemimorphite, esperite, quartz, Mg₂Al₄Si₅O₁₈, hardistonite Ca₂ZnSi₂O₇, hydrozincite or aurichalcite Zn₅(CO₃)₂(OH)₆ |
| Retort wall | Goslarite, hardistonite, hemimorphite Zn₂P₂O₇, 5H₂O, aurichalcite, quartz, sphalerite | Goslarite, hemimorphite, willemite Zn₂SiO₄, hardistonite |
| Slag | Quartz, goslarite, Zn₂P₂O₇, 5H₂O, hemimorphite, aurichalcite, sphalerite, chalcopyrite hardistonite. | Hemimorphite, goslarite, hydrozincite, quartz, sphalerite |
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Al, Zn, Pb, Mn, Na, K, S (in decreasing order of occurrence) indicating that these particles containing a few of the above elements are homogeneous in nature. The larger particles show wide variation in intensity ratios and hence variation in the chemical composition of the grains from point to point. Thus the approximate size of the zinc-containing and other homogeneous grains in the retort residue is in the range of 0.5 to 8.0 µm.
v1.c26.s6.t4.8
The size of the particles in the lead slag sample was found to be lower than that in the retort residue. The size of the zinc-containing grains was estimated to be in the range 0.5-6.0 µm, by carrying out point to point analysis. The X-ray spectrum showed the presence of titanium apart from the other elements noted in the retort residue.
v1.c26.s6.t4.9
The work at IIT, Kanpur was primarily directed towards the recovery of zinc from the siliceous retort residue and slag at Zawar. We found (Biswas and Kumar, 1986) that the non-silicate phase such as hydrozincite Zn₅(CO₃)₂(OH)₆ could be easily leached by acid. A special ‘fast leaching’ technique, making use of the water-starved nature of the silicate-sulphuric acid system, could recover more than 80 per cent of the zinc contained in the silicate phases such as hemimorphite Zn₄(OH)₂Si₂O₇, H₂O, willemite Zn₂SiO₄ etc.