Pramāṇa · Search · About · API

Minerals and Metals in Ancient India v1.c26.s6: Discovery of the Ancient Zinc Smelting Outfit

Minerals and Metals in Ancient India v1.c26.s6

Language
English

Contents

Discovery of the Ancient Zinc Smelting Outfit

v1.c26.s6.1

Colonel Tod, the great historian of Rajasthan, wrote in 1829 that as recently as 1760 the Zawar Mines had been in full production but that “the miners are now dead, the mines filled with water”. (Tod, 1829, 1950) The mines were abandoned during the reign of Maharana Bhim Singh (1778-1828) when the State of Mewar was repeatedly plundered by Marāthā invaders. In 1850, Brooke mentioned about the ancient art of zinc distillation and an old man who claimed that he knew the technology. (Brooke, 1850: 212-15)

v1.c26.s6.2

In the neighbouring villages the used brinjal-shaped retorts have been utilised as components of building-walls and this has always been a familiar sight although people forgot how and where these retorts were used. Attention has been drawn to the huge quantity of zinc extraction debris at Zawar (Carsus, 1960; Straczek and Srikantan, 1967) and the deep mine shafts. (Mookherje, 1964: 657-677)

v1.c26.s6.3

Straczek et al. (1967) pointed out that there are two distinct types of smelter residues. One is a sinter or clinker intermixed with fragments of fired clay pots or retorts and contained in unbroken retorts; the retort residue and wall contain substantial amounts of zinc showing that these retorts are possibly zinc smelting crucibles. The other kind of residue is normal glass slag left after the recovery of lead and possibly silver. Straczek et al. (1967) estimated that the quantity of the sinter residues and retorts could be 2.03 to 2.54 lakh tons containing six to seven thousand tons of zinc metal and 400-500 tons of lead. Recently Freestone et al. (1985a) have estimated that the quantity of the extraction debris could be in the neighbourhood of 8 lakh tons.

v1.c26.s6.4

In April 1980, the Hindustan Zinc Limited (HZL) sponsored a three-year research project on recovery of zinc from the ancient slags which was successfully conducted at the Indian Institute of Technology (IIT) Kanpur by the author of this monograph. (Biswas, 1981-84) Some of the results related to phase characterisation of and zinc recovery from the ancient slags would be recorded in this chapter. In 1982, HZL collaborated with British Museum Research Laboratory (P.T. Craddock, Lynn Willies etc.) and the Department of Archaeology, Baroda University (K.T.M. Hegde) on archaeological investigations, and this led to the spectacular discovery of the zinc distillation outfit including furnaces and retorts showing the production strategy. Craddock (1984) described the exciting discovery:

v1.c26.s6.5

On the third day of the excavation (December 1983), one of the Baroda team (Hegde) spotted the corner of a refractory plate sticking out from a heap of spent retorts beside a goat track in a valley on Zawar Mala . . . . . . . With mounting excitement we cleared a small area above and around it to reveal first, the edges of furnace walls, and then the tops of retorts still in situ.

v1.c26.s6.6

Extensive archaeological and archaeo-material investigations followed (Craddock et al., 1983, 1985; Freestone et al., 1985a, 1985b, 1987; Hegde et al., 1986, 1989); we now present the summary of the results of the historical experiments performed in two continents.

Ancient Retorts and Furnaces at Zawar

v1.c26.s6.t1.1

During the 1983 excavations two groups of furnaces were uncovered. A single bank of seven furnaces upon Zawar Mala contained small retorts 20 cm long and 10 cm in diameter. In old Zawar there was a more extensive arrangement of furnaces using larger retorts 30-35 cm long and 10-15 cm diameter. In both groups thirtysix retorts in a six by six arrangement were contained within the truncated pyramid of each furnace. The retorts were supported vertically on perforated bricks through which the condenser tubes passed into the cooler zinc collectors beneath (vide figs. 18.2-18.4).

v1.c26.s6.t1.2

The furnaces are in two parts consisting of a zinc vapour condensation chamber at the bottom and a furnace chamber at the top. The two chambers are separated by a perforated terracotta plate. The condensation chamber measures 65 cm by 65 cm and 20 cm in height. The perforated terracotta plate that separates the two chambers is a composite unit made up of four equal segments of 35 cm². It is 4 cm thick, well-baked, and sturdy. Its perforations include circular holes of two sizes: larger ones of 4 cm diameter, each of which are surrounded by a number of smaller holes of 2.5 cm diameter. Within the furnace the composite terracotta plate was found to be supported on a ledge in the furnace walls on all four sides and a single solid terracotta pillar placed below the junction of its four segments. (Hegde et al., 1986; Hegde, 1989)

v1.c26.s6.t1.3

Up above the perforated terracotta plate is the furnace chamber, in which 36 charged retorts (figs. 18.2-18.3) were arranged, inverted vertically; it may be presumed that 36 vessels were placed, one underneath each retort, to collect the condensed zinc vapour. This arrangement of downward distillation retort with the condensing unit underneath or distillation per descensum, is precisely what had been described in *Rasaratnasamuccaya* text (2.157-166; 9.48-50) to be discussed in chapter 9 of our Vol. II, and what was copied by William Champion in 1748 for his plant in Warmley, England. The brinjal-like retorts in Zawar are also similar to the *vṛntākamūṣā* described in *Rasaratnasamuccaya* (10.22-3).

v1.c26.s6.t1.4

Fig. 18.2 : Midpoint Vertical Section Through a Zinc Reduction — Distillation Furnace at Zawar Mala

v1.c26.s6.t1.5

Fig. 18.3 : Horizontal Section through the Furnace in Fig. 18.2 at Perforated Plate Level

v1.c26.s6.t1.6

Fig. 18.4 : Section Through Small Charged Zinc Reduction — Distillation Retort with Stick Still in Place

v1.c26.s6.t1.7

The furnace structures unearthed at Zawar did not include their roof part. However, curvature of their extant brick walls (*fig. 18.2*) suggests that the furnaces probably had domed roofs with a chimney-like opening at the top, similar to those of the vertical up-draught pottery-baking kilns of Mohenjo-Daro found by Mackay (1938). The opening would have served two purposes: (1) It would have acted as an outlet for the flow of hot gases generated within the furnace, and (2) it also facilitated continuous feeding of the furnace with fuel. The use of thin cow-dung cakes could have enabled quick attainment of a high temperature with a natural induced draught of free air coming through the front opening in the condensation chamber and the smaller holes in the perforated terracotta plate separating the condensation chamber from the furnace chamber. It is significant that the banks of furnaces at Zawar face south-west, probably to receive the natural draught during April to September. The dome-shaped superstructure of the Zawar furnace was possibly dismantled after the completion of each distillation process and rebuilt again for the next batch.

v1.c26.s6.t1.8

A very few undistorted retorts were found showing the original shape of the larger types to be cylindrical, ending in a cone. The retort walls are about 1 cm thick and of clay, containing large fragments of quartzite and phyllite, both local rocks. (Mookherjee 1964) The retorts were prepared in two parts: a condenser was luted onto the open main body of the retort after it had been filled with the charge. Of the condensers, only the heads survive, usually embedded in the retorts (*fig. 18.4*).

v1.c26.s6.t1.9

These heads appear to be of at least two types: a relatively large, thin-walled tulip-shaped variety and a thicker, more robust cone which was formed in a polygonal mould. Droplets of zinc-rich material are found just inside the broken, tapered ends of the heads. (Freestone *et al.*, 1985a : 231 and 243) The heads appear to have tapered into tubes which led the condensed zinc vapour to the collecting vessel underneath.

v1.c26.s6.t1.10

It appears that a cylindrical reed of 1.5 cm diameter was inserted into the retort after it was charged and the funnel part was luted on it (*fig. 18.4*); this is evident from the central hole which is preserved in many retort residues. (Freestone *et al.*, 1985b: 4) The reed helped to keep the charge within the retort when it was inverted and placed in the furnace. When ther furnace was fired, the reed burnt away leaving behind a cylindrical flow channel for the zinc vapour to flow freely out of the retort.

v1.c26.s6.t1.11

Petrography of the ceramic retort bodies and furnace walls shows that these were made from local (Tiri river bank) clays. (Freestone et al., 1985a) Analysis of the ceramic matrices, using Energy-Dispersive Analysis X-ray spectrometer (EDAX) attached to Cambridge S 600 Scanning Electron Microscope (SEM), revealed that these contain around fifteen weight per cent fluxes (FeO, MgO, CaO, Na₂O, K₂O). Thus the ceramics are closer to house bricks than to modern refractories in composition. (vide Table 18.1 for a complete analysis)

v1.c26.s6.t1.12

The temperature to which a ceramic has been subjected may be estimated by the examination of its micro-structure in the SEM. The methodology has been worked out by Tite et al. (1982). As we move from the inside of the retort wall to the outside in direct contact with the furnace flame, we find from the BSE micrographs of the polished thin sections, that the degree of inter-connection of the particles increases and the porosity decreases (indicating temperature 1000-50°C). Finally the pores are isolated and the trapped gases expand, causing the formation of rounded vesicles (T 1100-50°C) and at higher temperatures (1200-50°C) macroscopic bloating. (Freestone et al., 1987: 22)

v1.c26.s6.t1.13

Examination of the vitrification textures of the Zawar materials and comparison with refired examples shows that temperatures of C. 1200-50°C were attained during smelting. The furnace walls shows a gradient in vitrification structure from the high temperature, highly vitrified interior to the feebly vitrified exterior. The temperature gradient depends on the duration of the smelt as well as the temperature achieved in the furnace. Comparing the micromorphologies of the furnace wall at series of points with those of a series of refired examples, and using a simple heat-conduction equation, the time for which a furnace was at high temperature may be estimated. For the zinc smelting process at Zawar, duration of around 5 hr has been estimated. (Tite et al., 1982, 1985; Freestone et al., 1987)

v1.c26.s6.t1.14

In order to obtain some idea of the bulk composition of the retort, a thin slice throguh the body or the wall was crushed and analysed by atomic absorption spectrometry. (AAS: Hughes et al., 1976) The data (Table 18.1) were compared with the electron probe micro-analysis (EPMA; Freestone 1982) data for the clay matrix. The higher SiO₂ content in the retort body (62.49%) compared to that in the clay matrix (53.82%; vide Table 18.1) may be explained in terms of inclusion of large quartzite grains, as earlier observed by Mookherjee (1964), which is naturally reflected in the AAS analysis. Brooke reported in 1850 that the Rana's experiments in the previous year for smelting zinc had resulted in breaking of the crucibles because, according to an old man, the quartz gangue had not been removed from the ore prior to smelting. (Brooke, 1850: 214) We feel however, that the crucible was not properly made and strengthened by inclusion of quartzite and phyllite, the local rocks, as used to be done in the earlier centuries, and hence the failure of the Rana’s experiment.

v1.c26.s6.t1.15

Zawar Zinc Retort Wall: Analysis of Ceramic Materials

v1.c26.s6.t1.16

| | (1) | (2) | (3) | (4) |
| SiO₂ | 62.49 | 53.82 | 64.85 | 57.96 |
| TiO₂ | 0.71 | 0.27 | 0.44 | 0.59 |
| Al₂O₃ | 13.90 | 17.49 | 13.75 | 13.97 |
| FeO | 2.48 | 4.16 | 7.49 | 15.39 |
| MnO | 0.04 | b.d | b.d. | b.d. |
| MgO | 1.44 | 1.29 | 0.93 | 1.35 |
| CaO | 0.64 | 0.34 | 0.39 | 0.70 |
| Na₂O | 0.94 | 1.81 | 0.67 | 0.55 |
| K₂O | 3.08 | 3.11 | 9.90 | 7.54 |
| P₂O₅ | n.a. | 0.50 | 0.28 | b.d. |
| Cl | n.a. | 0.16 | b.d. | b.d. |
| S | n.a. | b.d. | b.d. | b.d. |
| ZnO | 2.18 | 3.22 | 0.61 | 0.61 |
| TOTAL | 87.90 | 86.17 | 99.11 | 98.66 |

v1.c26.s6.t1.17

(1) Bulk analysis of retort body (crushed slice) by atomic absorption spectrometry; (2) Clay analysis by EPMA or electron-probe micro-analysis; (3) and (4) EPMA analysis of partial melt (glass) from outer surface of retort.

v1.c26.s6.t1.18

n.a. = not analysed; b.d. = below detection limits

v1.c26.s6.t1.19

The glassy layer on the external surface of the used retort contained higher proportion of FeO and K₂O due to a partial melting of the ceramic which filled the pores and prevented the escape of vapours containing zinc. Some amount of zinc was retained in the retort body, a much smaller proportion appearing in the outer glassy layer (vide Table 18.1). The liquidus temperatures of the K₂O — FeO — Al₂O₃ — SiO₂ glasses on the exterior of the retort body are likely to be of the order of 1100-1125°C (Freestone and Powell, 1983) which are just sufficient for reduction of zinc oxide to the metal. (Smith, 1918; Freestone et al., 1985a : 238)

Thermodynamics of Smelting

v1.c26.s6.t2.1

Before we present the composition of the residue in the ancient retorts (left after recovery of zinc by distillation) let us briefly discuss the physico-chemical route possibly adopted in the ancient zinc-smelting process at Zawar.

v1.c26.s6.t2.2

In the Zawar mines, zinc occurs mainly as sphalerite, ZnS, although there are other zinc minerals such as smithsonite, ZnCO₃ etc. in minor proportions. The host rock is dolomitic and arkosic (quartz and feldspar) containing a variety of sulphide minerals such as sphalerite, galena PbS, pyrite FeS₂, pyrrhotite Fe₁₋ₓS, chalcopyrite CuFeS₂, argentite Ag₂S, silver etc. (Straczek et al., 1967)

v1.c26.s6.t2.3

It is necessary to roast sphalerite before it can be smelted, because sphalerite is not decomposed in a reduction reaction. Roasting sphalerite to zinc oxide 2 ZnS + 3 O₂ → 2 ZnO + 2 SO₂ is a partly endothermic reaction requiring a temperature of the order of 900°C. As soon as the oxidation reaction sets in, sulphur in the ore supports further combustion. However, to produce a dead roast, free from sulphur, and also to decompose any zinc sulphate ZnSO₄ formed in the reaction, a higher temperature of the order of 1000°C is necessary. We detected in the retort residue not only a little sulphur (0.35%) but also some ZnSO₄, indicating that zinc ore had been roasted, but not to a dead heat, prior to reduction-smelting in the retort. (Biswas et al., 1984) Barnes (1973) described a medieval method practised in Persia in which the ore used to be roasted at temperature in excess of 1000°C to obtain pure zinc oxide sublimate, free from Ag, Fe and Pb etc., depositing on iron or ceramic frame above the fire. The zinc oxide sublimate was known as tutia in the Persian language and the metal zinc obtained from tutia as tutenaga. The high Ag, Fe and Pb contents as well as presence of sulphur and ZnSO₄ in the retort residue indicate that the Zawar process was one of simple roasting and not the Persian method.

v1.c26.s6.t2.4

Zinc melts at 432°C and under one atmospheric pressure boils at 913°C, which is below the temperature required to reduce the oxide to metal in the presence of carbon. Though the initiation of reduction of zinc oxide to zinc by carbon begins at 950°C, the minimum temperature necessary for successful extraction of the metal in a retort is of the order of 1150-1200°C. The reaction ZnO + C → Zn + CO Δ H = - 57 kcal. is highly endothermic. Therefore, before the advent of modern high-pressure reduction technology or electrolytic extraction processes, zinc had to be produced inevitably as vapour around 1200°C and then condensed as liquid much below 913°C.

v1.c26.s6.t2.5

Furthermore, to overcome re-oxidation of zinc vapour, the partial pressure of oxygen had to be kept very low. The obvious method of doing this was to use excess of charcoal or other suitable carbonaceous matter in the retort to maintain a strongly reducing atmosphere and excess of carbon monoxide. Temperature-pO₂ relation *Fig.* 18.5 shows that the production of metallic zinc requires extremely reducing conditions below the pO₂ required to reduce wüstite (FeO) to metallic iron. Some metallic iron has indeed been detected in the spent charge. Taking mole fractions of 0.05 FeO and 0.10 ZnO in the glassy portion of the spent charge and making simple thermodynamic assumptions, it has been found (Freestone 1985a: 238) that the FeO-Fe and ZnO-Zn reactions are displaced to the dashed curves (*fig.* 18.5). Both the FeO and ZnO contents of the glassy portion of the residue are consistent with operation of the process 1-2 log units of pO₂ below the Zn-ZnO reaction curve, within the temperature range of the interest.

The Retort Residue or the Spent Charge: Results from the British Museum Research Laboratory

v1.c26.s6.t3.1

The retorts were found filled with somewhat less that 1 kg of spent charge most of which could be tipped out but some of which was fused to the walls, more towards the centre of the retorts than at the rear or front. The material was retrieved typically in the form of cindery pellets one cm³, most representative of the charge, and a fine dust fraction, which is essentially a mixture of the pellets and material that has spalled off the retort walls. This is evident from the analysis of the different components of the retort residue (Tables 18.2 and 18.3). The analysis of residue from different parts of the retort and also ash heap are provided in Table 18.4.

v1.c26.s6.t3.2

X-ray diffraction and polarised light microscopy of polished thin sections (Freestone *et al.*, 1985a: 236) show the pellets to be composed primarily of carbonates (of Ca and Mg) but their state of aggregation and rigidity is imparted by a skeleton of silicate glass, in which are embedded numerous crystals of diopside. Fine grains of metallic iron were also seen. The EPMA or Electron-Probe Micro-analysis data (columns 1 and 2 in Table 18.2) show association of diopside CaMgSi₂O₆ crystal with some Fe, Mn and Zn. The glass is rich in zinc, containing up to fifteen weight per cent ZnO, and also contains higher proportions of Al, Mn, Na, K, Cl and P₂O₅.

v1.c26.s6.t3.3

Fig. 18.5 Plot of Partial Oxygen Pressure versus Temperature for Selected Reactions. The Stippled Area represents the likely conditions within the zinc Reduction-Distillation Retorts.

v1.c26.s6.t3.4

Zinc is markedly depleted at the closed end or top of each retort (vide Table 18.4), reflecting the movement of zinc vapour during the smelting process. In contrast lead does not appear to have been very mobile during smelting (Tables 18.3 and 18.4). Substantial amount of ZnO (2.18 %) in the retort body or wall (Tables 18.1 and 18.3) is due to the entry of zinc vapour and zinc-rich glass into the pores. The pore volume decreased in the outer and hotter surface of the retort and that is why the zinc content in the outer surface is much less (columns 3 and 4 in Table 18.1).

v1.c26.s6.t3.5

Sodium in the spent charge could have been derived from gangue such as albite NaAlSi₃O₈, but high chlorine content (0.53 %, vide column 5 in Table 18.2) is less easy to explain. The fourteenth-century AD text Rasaratnasamuccaya (2.157) recommended the inclusion of salt in the zinc-smelting charge, an idea which was incorporated by William Champion in his AD 1748 British patent. Common salt would have served the zinc distillation process in two ways. Firstly, the soda vapour would help to sinter calcium and magnesium oxides and hence keep the charge within the retorts in a porous open texture. This Analysis of the Spent Charge from Zawar Zinc-Smelting Retorts

v1.c26.s6.t3.6

| | (1) | (2) | (3) | (4) | (5) |
| SiO₂ | 54.36 | 50.50 | 20.82 | 40.48 | 42.48 |
| TiO₂ | b.d. | 0.28 | 0.15 | 0.27 | 0.17 |
| Al₂O₃ | 0.45 | 8.97 | 3.61 | 6.42 | 5.42 |
| FeO | 4.42 | 5.72 | 8.30 | 8.97 | 8.12 (Fe₂O₃) |
| MnO | 1.93 | 3.11 | 1.60 | 0.96 | 1.64 (MnO₂) |
| MgO | 14.10 | 0.28 | 10.16 | 5.94 | 11.16 |
| CaO | 21.85 | 6.43 | 24.54 | 15.25 | 24.54 |
| Na₂O | 0.34 | 7.13 | 0.34 | 0.80 | 1.28 |
| K₂O | b.d | 5.07 | 0.51 | 1.06 | 0.31 |
| P₂O₅ | b.d | 0.27 | n.a | n.a | 0.27 |
| Cl | b.d | 0.70 | n.a | n.a | 0.53 |
| S | b.d | 0.43 | n.a | n.a | 0.23 |
| ZnO | 3.44 | 12.45 | 0.61 | 1.22 | 2.28 |
| PbO | — | — | — | — | 1.72 |
| TOTAL | 100.89 | 101.34 | 70.65* | 81.37* | 100.15 |

v1.c26.s6.t3.7

Silicate fractions: (1) diopside, from carrier material, EPMA electron-probe micro-analysis; (2) glass, from carrier material, EPMA; (3) example of carrier fragment, AAS atomic absorption spectrometry; (4) fine fraction from within retort, AAS; (5) overall analysis of the spent charge (Hegde, 1989: 92).
* Most of CaO and MgO present as carbonates, accounting for low totals.

v1.c26.s6.t3.8

would minimize the chance of zinc vapour getting trapped in any one part of the retort and forcing that part to burst open. Little bit of the glassy phase helped in sintering by acting as skeletal adhesive; 0.7 per cent chlorine in the residue glass, with 7.13 per cent Na₂O (column 2 in Table 18.2) is probably close to saturation. (Freestone *et al.*, 1985a: 237) Secondly, soda vapour would react with the silica and alumina of the clay in the interior surface of the retort wall to produce a thin film of glaze. This glaze would prevent the loss of zinc vapour through the micropores in the thin clay wall of the retort. (Hegde, 1989: 92) The low level of lead oxide (1.72 %, *vide* column 5 in Table 18.2, also 18.3 and 18.4)

v1.c26.s6.t3.9

Silver, Lead and Zinc Contents of Zawar Retort Residue Materials

v1.c26.s6.t3.10

| | | %Ag | %ZnO | %PbO |
| Coarse fraction of carrier | (a) | 0.023 | 0.61 | 0.36 |
| | (b) | 0.029 | 0.65 | 0.03 |
| | (c) | 0.012 | 0.30 | 0.04 |
| Fine fraction of carrier | | 0.010 | 1.22 | 0.26 |
| Retort Body | | — | 2.19 | 0.03 |

v1.c26.s6.t3.11

Table 18.4

v1.c26.s6.t3.12

Analysis of Residues from Different Parts of a Zawar Retort and of Ash Heap

v1.c26.s6.t3.13

| Sample No. | Pb(%) | Zn (%) | Fe (%) | Ag (ppm) | Cd (ppm) | Comment |
| 1 | 1.17 | 4.50 | 7.7 | 124 | 23 | Retort residue |
| 2 | 0.82 | 5.95 | 7.6 | 29 | 10 | Retort residue |
| 3 | 2.9 | 2.83 | 7.2 | 126 | 12 | " |
| 4. | 1.85 | 5.05 | 9.8 | 92 | 22 | " |
| 5 | 0.17 | 1.15 | 13.1 | 88 | 12 | " |
| 6 | 2.50 | 4.45 | 11.5 | 121 | 12 | " |
| 7 | 1.44 | 3.00 | 3.6 | 90 | 18 | " |
| 8. | 2.24 | 6.42 | 7.2 | 92 | NA | Retort bottom |
| 9 | 2.58 | 0.12 | 8.1 | 119 | NA | Retort centre |
| 10 | 2.40 | ND | 9.4 | 264 | NA | Retort top |
| 11 | 0.50 | 0.12 | 11.0 | 57 | NA | Retort top |
| 12 | 0.85 | 2.62 | 12.2 | 58 | NA | Retort bottom |
| 13 | 0.21 | 1.31 | 1.5 | 10 | 133 | Ash heap |
| 14 | 0.61 | 2.75 | 2.7 | 22 | 214 | Ash heap |
| 15 | 0.66 | 3.34 | 3.8 | 24 | 167 | Ash heap |
| 16 | 0.14 | 1.55 | 1.6 | 12 | 198 | Ash heap |

v1.c26.s6.t3.14

in retort residue suggests that much care was taken in beneficiating the zinc ore.

v1.c26.s6.t3.15

The numerous heaps of white ash in the Zawar area seem to represent the spent fuel ash removed from the furnaces (after a smelt) contaminated with some residue from the retort, while dismantling and cleaning of the set-up took place. The ash contains lower proportions of Pb, Zn, Fe and Ag compared to the retort residue, but is substantially enriched in terms of cadmium (Table 18.4).

v1.c26.s6.t3.16

The R.O.M. in Zawar mine contains 3.5-4.4 per cent zinc and 0.014-0.02 per cent cadmium. Cadmium is reduced from its oxide at 600°C and boils at 767°C. Thus cadmium is likely to be depleted from the retort before zinc and not completely condensed. As oxide it is likely to find its way to the cooler parts of the furnace and absorbed in the spent fuel ash. (Freestone 1985b: 10)

v1.c26.s6.t3.17

It may be noted here that we referred to zinc-rich copper ore of Ambaji and Basanthgarh in the previous chapter. The Ambaji ore contains 400 ppm cadmium which exceeds the value for Zawar ore.

v1.c26.s6.t3.18

In addition to zinc smelting, Zawar yielded considerable evidence for the smelting of lead, in the form of at least sixteen deposits of slag, distributed along the valley of the river Tiri. We have mentioned these slags in our previous chapter and provided their compositions in Table 17.5. The glassy slag, not associated with zinc-smelting debris, assayed high in zinc content (7-11%) and contained melilite Ca₂ZnSi₂O₇, apart from olivine Mg₂SiO₄ and magnesium-rich clinopyroxene CaMgSi₂O₆. The crystalline slag was associated with zinc retort distillation debris, and contained lower proportions of zinc (less than 3%) presumably because this metal had been extracted or lead smelting was done after manual separation between sphalerite-rich and galena-rich portions of the rock.

Phase Studies on Ancient Zawar Residue at IIT, Kanpur

v1.c26.s6.t4.1

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

v1.c26.s6.t4.6

| 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 |

v1.c26.s6.t4.7

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.

The Charge in the Unfired Retorts

v1.c26.s6.t5.1

No unfired retort containing the original charge prior to firing has been recovered. Thus the composition of the mixed charge subjected to retort smelting and distillation can only be speculated upon. *Rasaratnasamuccaya* (2.157-164) mentioned a number of carbonaceous ingredients included in the zinc-smelting charge : turmeric powder, soot, lac treacle, mustard, clarified butter etc. These could serve both as reductant as well as binding agents to produce the 1 cm³ pellets. However it is doubtful whether the fancy organic reagents were used in mass-scale production. Hegde suggested (1989: 94) that light, thin cow-dung cakes might have been used as fuel in the furnace. The same material or charcoal (Freestone *et al.*, 1985b: 11) could have been used as the reductant and as a constituent of the pellet inside the retort.

v1.c26.s6.t5.2

A number of ash mounds at the periphery of the smelting site contained stratified layers of ash, charcoal and fragments of calcined dolomite. These indicate that sphalerite along with the dolomitic gangue must have been roasted prior to smelting in the retort. (Hegde, 1989: 89-90)

v1.c26.s6.t5.3

Analysis of Zawar ore, tail, concentrate (in the modern plant) and the ancient residue, together with the computed relative proportions of the constituents (with elemental iron as the base) provide valuable insight (Table 18.6). Freestone

v1.c26.s6.t5.4

Table 18.6

v1.c26.s6.t5.5

Analysis of Zawar Ore, Tail, Zinc Concentrate and Retort Residue, and Relative Proportions of Constituents

v1.c26.s6.t5.6

| | MOCHIA ORE¹ | | MOCHIA TAIL¹ | | ZINC CONC.¹ | | RETORT RESIDUE | |
| | per cent | relative | per cent | relative | per cent | relative | per cent | relative |
| Fe | 3.00 | 1.00 | 3.10 | 1.00 | 8.00 | 1.00 | 5.7 | 1.00 |
| Zn | 3.50 | 1.17 | 0.35 | 0.11 | 52.00 | 6.50 | 3.04³ | 0.53 |
| Pb | 1.40 | 0.47 | 0.22 | 0.07 | 1.30 | 0.16 | 1.55 | 0.27 |
| Mn | 0.35 | 0.12 | — | — | 0.035 | 4.4x10⁻³ | 1.03 | 0.18 |
| Ag (ppm) | 40.00 | 10⁻³ | — | — | 140.00 | 1.7x10⁻³ | 114⁴ | 2x10⁻³ |
| Insoluble (Silica) | 30.00 | 10.00 | 33.20 | 10.70 | 1.25 | 0.16 | 42.5 | 7.45 |
| Ca | 13.00 | 4.33 | 14.00 | 4.50 | 1.00 | 0.125 | 17.2 | 3.00 |
| Mg | 7.5 | 2.50 | 8.50 | 2.70 | 1.20 | 0.15 | 6.7 | 1.18 |

v1.c26.s6.t5.7

Notes :
1. The data for the ore, tail and concentrate in the modern plant are taken from Chatterjee and Agarwal (1981 : B.78).
2. Elemental percentages calculated from oxide compositions given by Hegde in column 5 Table 18.2.
3. From Hegde's data, elemental zinc per cent in the retort residue comes out to be low (1.82) whereas Freestone (1985b: 8) gives the figure 3.85. Our estimate (Biswas et al., 1984) is 3.04.
4. Average of the values for silver quoted in Tables 18.3 and 18.4.

v1.c26.s6.t5.8

et al. (1985 b: 8) observed that “Fe/Pb/Ag ratios are closely similar in the retort residues and in the modern concentrates”, and on that basis 500 g of zinc could have been present in the retort charge before smelting, there being 50 g of zinc left in the spent residue.

v1.c26.s6.t5.9

The assumption made by Freestone et al. meant that the retort charge before smelting assayed more than 30 per cent in zinc and metal recovery therefrom was of the order of 90 per cent. We do not know how far this assumption can be justified. There are evidences of careful manual hand-picking and benefication of zinc-rich and lead-rich portions of the rock. (Brooke, 1850: 214) The zinc/lead ratio of the retort charge must have been considerably higher than the average. But it is difficult to believe that the retort charge in the pre-modern period could approach in purity the concentrate (zinc 52 %) now being produced in the modern plant using twentieth century technique of froth floatation. The doubt arises because there are high percentages of insoluble (silica), calcium and magnesium in the retort residue which resembles the ore and the tailing rather than the concentrate in this regard.

v1.c26.s6.t5.10

There is little reason to believe that additional oxides of Si, Ca and Mg were added to the beneficiated zinc ore in retort charge. Freestone et al. (1985b: 11) were “confident that dolomite was added separately and was not gangue unintentionally included”. Hegde et al. (1986: 255) repeated this assertion in view of the ‘high CaO and MgO composition of the spent charge’. We do not agree since the data speak otherwise. The insoluble silica/Ca/Mg ratio is 1 : 0.43 : 0.25 in the ore and tail, and 1 : 0.4 : 0.16 in the retort residue. This contradicts the idea that dolomite was separately added to the charge. During the roasting of sphalerite ore there was some calcination of the dolomitic gangue. The other constituents of the gangue were quartz and feldspar which contributed to the contents of SiO₂ and alkali metals (K and Na) in the retort residue (Table 18.2).

v1.c26.s6.t5.11

Though best attempts were made to obtain zinc-rich portion of the rock by crushing and selective picking (Brooke, 1850: 214), the separation between sphalerite and the gangue minerals of quartz, feldspar and dolomite could not be efficient.

v1.c26.s6.t5.12

On the basis of their assumptions and the observation that the residue in each retort weighed about 1 kg and contained about 50 g of unrecovered zinc metal, Freestone et al. (1985a and b) speculated the 200-500 g of zinc vapour condensate might have been recovered per retort. This range corresponds to 80-90 per cent recovery from the retort charge assaying 20-33 per cent in elemental zinc. While Freestone et al. accepted the upper limit of the range for further calculations, we propose the conservative value or the lower limit of the proposed range.

Linked texts (2)