History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t5: (V.4) Physicochemical Changes Occurring Inside the Retort

History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t5

Language: English

(V.4) Physicochemical Changes Occurring Inside the Retort

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As the temperature of the furnace was raised the refractory of the retort got sintered and graphitised and at the same time the following chemical reactions took place inside the retort. At temperature below 1000°C PbO and Ag₂O present in the ore got reduced by carbon producing liquid Pb-Ag alloy and CO gas. At the same time the reed plug got charred and burnt releasing CO gas inside the condenser chamber where it expelled out the air from this chamber and created a reducing atmosphere. At the same time the liquid alloy of Pb-Ag trickled down through the hole and got collected inside the condenser vessel. This must have been followed by the reduction of ZnO to Zn vapour and CO gas at 1200- 1300°C, which were [Figure: APERTURE FOR OUT FLOW OF HOT GASES AND FEEDING FUEL
1. FURNACE CHAMBER 2. PERFORATED PLATE
3. TERRACOTTA PILLAR 4. METAL CONDENSING VESSEL
5. RETORT]

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*Fig. 44. Sketch of the complete zinc retort furnace, showing retorts and condenser vessel in position*

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forced through the small hole in the retort. As this mixture of Zn vapour and CO was forced through the condenser tube liquid Zn metal must have been produced below 580 °C by the following triple action.

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(a) Cooling of the gas to the jet action of the vapour i.e. sudden volume expansion leading to lowering of vapour temperature.

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(b) Chilling action of the Pb-Ag alloy already collected in the condenser at low temperature.

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At the same time Borax and salt added to the mixtures must have helped in low temperature sintering of the charge as well as slag formation. The studies carried out by Gangopadhyaya et al. (1984) on the retorts and debris of the Zawar area have shown that the material remaining after the distillation was both as fused slag as well as sintered mass and their analysis was as given in Table 11 with respect to Zn, Pb, Fe and S. This table also gives the metallic content of the retort wall, and from the study of this table it is very obvious that major part of the Zn was extracted leaving only 3% Zn in the residue, and the density of the retort wall was good Table 10 — Mineralogical Analysis of the Ceramic Material of the Zinc Retort Wall.

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| | (1) | (2) | (3) | (4) |
| SiO2 | 62.49 | 53.82 | 57.96 | 64.85 |
| 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 |

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Note :- (1) Bulk analysis ‘crushed slice’ AAS
(2) Clay Analysis EPMA
(3) Partial melt (glass on surface) EPMA
(4) Same as (3)

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Table 11 — Metallic Content of Zawar Charge Residue and Retort Wall.

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| | | % Ag | % ZnO | % Pbo | % Fe | Reference |
| 1. | Coarse fraction of carrier (residue) | 0.023 to 0.012 | 0.61 to 0.3 | 0.36 to 0.04 | — | Freestone et al. 1985 |
| 2. | Fine fraction carrier (residue) | 0.01 | 0.122 | 0.26 | — | “ |
| 3. | Charge residue loose | — | 3.04% Zn | Traces | 7.60 | Gangopadhaya et al. 1984 |
| 4. | Slag inside the report | — | 2.74% Zn | Traces | 9.52 | “ |
| 5. | (a) Retort Body | — | 2.19 | 0.3 | — | Freestone et al, 1985 |
| | (b) Retort Body | — | Traces | Traces | 4.37 | Gangopadhaya et al, 1984 |

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enough to prevent the diffusion or escape of Zn vapour through it. The lower Pb and Ag content of the residue shows that either the mineral of these metals were completely separated by beneficiation prior to the preparation of the charge pellet or these metals got Co-extracted as described above.

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In the author's opinion the Co-production of Pb-Ag alloy and Zn-Ag alloy is more probable because Pb and Zn have very restricted solubility and silver can alloy with both these metals. Hence, these alloys will be solidified in the condenser in two separate layers which could be easily separated. Afterwards Pb-Ag alloy could be refined by cupellation technique to produce Ag and PbO, and PbO may be treated in a separate furnace to produce lead. The surrounding area of Zawar has been also known for the production of these metals ever since pre- Harappan period. The process has been so successful that in 18th century Champion designed his vertical report on similar principles as described by Craddock(1987). It is also surmised that this as well as the Belgian report furnace (19th century A.D.) were designed after studying the Indian distillation system.