30. Could you describe Dore metals, where from the silver raw materials.
There are two traditional cell designs which, with modern improvements in engineering design and constructional materials, persist today. The Moebius cell uses a vertical electrode arrangement, while the Balbach cell (later improved by Thum) uses horizontal electrodes. Anode slimes collected from the mud of copper refining cells are leached, roasted, fire – refined and cast into anodes, the composition being known as Dore metal (Table 1). In the Moebius cell (Fig 4), the anodes are cast, drilled, then bolted to hander bars. A woven cloth or polymer bag of controlled porosity surrounds the anodes to catch slime. The cathodes are usually stainless-steel plates which facilitate removal of silver crystals by scraping (which may be manual and intermittent or automatic and continuous via reciprocatory wiper blades).
Table 1 Typical composition (% wt) of anode mud and Dore metal obtained from copper electrorefining cells.
Element |
Anode mud |
Dore metal |
Au |
0.8 |
8-9 |
Ag |
8 |
8-92 |
Cu |
30 |
0.5-1.0 |
Pd |
0.0064 |
0.16-0.19 |
Pt |
0.00056 |
0.004-0.009 |
Pb |
2 |
0.02 |
Te |
3 |
0.003 |
Se |
12 |
0.00002 |
As |
2 |
- |
Sb |
0.5 |
- |
Fig. 4. Sectional view of a Moebius cell for electrorefining of silver.
In the latter case, the silver falls to the bottom of the cell from where it is periodically removed. Certain designs incorporate catchment trays to aid product recovery.
A wide variation is found in the detailed design and operating parameters of Moebius-type cells, but the typical range of conditions may be summarized. A “cell” may have between four and twenty cathodes, operating at 100-500 A, and from – 1.5 to – 2.8 V. The cells are usually operated near ambient temperature with a cathode current density of 20-40 mA/m2. The main reaction at the anode: Ago – e =Ag+
At the cathode: Ag+ + e = Ago
While the cathode silver purity is dependent upon anode quality, electrolyte purity and operating conditions, >99.9 wt Ag s usually obtained, with 99.99% wt being realized in some cases.
31.Tell about Moebius cells and its features, parameters, structure of cells.
The first industrially accepted process for silver refining was patented by Moebius in 1884 who used an acidic silver nitrate electrolyte. An industrial plant was commissioned in 1885 and the modern refining of silver still follows this pioncering work quite closely. There are two traditional cell designs which, with modern improvements in engineering design and constructional materials, persist today. The Moebius cell uses a vertical electrode arrangement, while the Balbach cell (later improved by Thum) uses horizontal electrodes. Anode slimes collected from the mud of copper refining cells are leached, roasted, fire – refined and cast into anodes, the composition being known as Dore metal (Table 1). in the Moebius cell (Fig 4), the anodes are cast, drilled, then bolted to hander bars. A woven cloth or polymer bag of controlled porosity surrounds the anodes to catch slime. The cathodes are usually stainless-steel plates which facilitate removal of silver crystals by scraping (which may be manual and intermittent or automatic and continuous via reciprocatory wiper blades).
Fig. 4. Sectional view of a Moebius cell for electrorefining of silver.
In the latter case, the silver falls to the bottom of the cell from where it is periodically removed. Certain designs incorporate catchment trays to aid product recovery.
A wide variation is found in the detailed design and operating parameters of Moebius-type cells, but the typical range of conditions may be summarized. A “cell” may have between four and twenty cathodes, operating at 100-500 A, and from – 1.5 to – 2.8 V. Both cells tend to utilize silver concentrations in the range 30 – 150 g/dm3. The AgNO3 electrolyte often has free HNO3 and CuNO3 is occasionally present to help produce large, well-separated, non-adherent crystallites of silver facilitating removal, washing and drying operations. The recovered silver is normally melted and cast into bars for resale, or transformed into pure silver nitrate or other speciality silver chemicals.The cells are usually operated near ambient temperature with a cathode current density of 20-40 mA/m2. The main reaction at the anode: Ago – e =Ag+;At the cathode: Ag+ + e = Ago. While the cathode silver purity is dependent upon anode quality, electrolyte purity and operating conditions, >99.9 wt Ag s usually obtained, with 99.99% wt being realized in some cases.
32 |
Tell about Balbash - Thum cells and its features, parameters. |
The first industrially accepted process for silver refining was patented by Moebius in 1884 who used an acidic silver nitrate electrolyte. An industrial plant was commissioned in 1885 and the modern refining of silver still follows this pioncering work quite closely. There are two traditional cell designs which, with modern improvements in engineering design and constructional materials, persist today. The Moebius cell uses a vertical electrode arrangement, while the Balbach cell (later improved by Thum) uses horizontal electrodes.
Fig 4.13 A Balbach-Thum cell for electrorefining of silver.
The Balbach – Thum cell (Fig. 4.13) is designed around a rectangular trough containing a thick, solid carbon plate cathode which covers the cell bottom. An anode basket is suspended in the top of the cell; the basket is fabricated so as to support a number of silver anodes. The anodes are usually cast plates, but smaller, more irregular, pieces of silver scrap can be used. Woven cloth or polymer mesh inside the tray serves to contain insoluble anode material. Recovery of silver is usually by manual scraping, following cathode removal. Balbach-Thum cells with their higher anode-cathode gap usually operate at a significantly higher voltage (from – 3.5 to – 5.5 V) than Moebius cells. The main reaction at the anode: Ago – e =Ag+
At the cathode: Ag+ + e = Ago
Both cells tend to utilize silver concentrations in the range 30 – 150 g/dm3. The AgNO3 electrolyte often has free HNO3 and CuNO3 is occasionally present to help produce large, well-separated, non-adherent crystallites of silver facilitating removal, washing and drying operations. The recovered silver is normally melted and cast into bars for resale, or transformed into pure silver nitrate or other speciality silver chemicals.