Molten lead copper addition

The hot sinter is screened and mixed with secondary zinc materials and coke pre-heated to 800 degrees centigrade. This mixture is fed into the top of the Imperial Smelting fiimace and the addition of preheated air reduces the lead and zinc oxides to metal. Zinc is produced as vapour, which is then condensed in a spray of molten lead. Liquid zinc separates on cooling and is either sold directly or refined. Lead formed in the furnace collects as bullion in the hearth at the bottom, along with molten slag, and can be separated and tapped. Precious metals and copper report in the lead bullion, which is then partially or completely refined on site. Overall recoveries of refined metals in the most modem ISP plants are 93 per cent or more for zinc and silver, about 70 per cent for copper and effectively 99-100 per cent for lead, if permanently recirculating metal is taken into account. 

The behaviour of iridium is closely analogous to that of rhodium its corrosion diagram is very similar and it is, with rhodium, one of the least corrodible of metals. It is unattacked by alkalis, acids or oxidising agents in aqueous solution, although a fused mixture of caustic potash and potassium nitrate will attack it. The metal has an excellent resistance to fused lead oxide, silicates, molten copper and iron at temperatures up to 1 500°C. Additions of iridium to platinum considerably raise the corrosion resistance of the latter to a very wide range of reagents. 

Finally, paying attention to the outliers of these series, the element lead seems to be added almost at random to all the series of coins, though over time the use of additives seems to have become more consistent, for the most part. There is certainly more lead present than is required to effect lower working temperatures for molten alloys. Other than this variance in lead however, these coins have a notably consistent composition. It appears that the mintmasters and workmen of the foundries had a good understanding and control of copper, lead, and tin in the production of alloys. 

Because of a strong reduction atmosphere in the furnace, some lead metal forms a layer in the bottom of the furnace. The metal is tapped every two or three days through a hole surrounded by a copper block. Molten metal is tapped into kettles through a castable-lined launder. The production rate is 10 -12 tons per each tapping. It is sent to the present anode casting plant to be used as an antimony additive. Typical analysis of the metal is shown in Table III and the minor elements in the metal are ... 

---