Minerals and Metals in Ancient India v1.c26.s6.t5: The Charge in the Unfired Retorts
Minerals and Metals in Ancient India v1.c26.s6.t5
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The Charge in the Unfired Retorts
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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.
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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)
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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
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Table 18.6
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Analysis of Zawar Ore, Tail, Zinc Concentrate and Retort Residue, and Relative Proportions of Constituents
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| | 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 |
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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.
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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.
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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.
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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).
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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.
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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.
Cites (1)
- Rasaratnasamuccaya — Vāgbhaṭa (alchemist), Sanskrit