Originally Posted by
NyFirefighter357
Most of that lead was closer to the surface as well as a byproduct of mining gold & silver.
"The most important lead ore is galena (PbS). Other important ores, such as cerrusite (PbCO3) and anglesite (PbSO4), may be regarded as weathered products of galena and are usually found nearer the surface.
The primary manufacture of lead involves three stages:
a) ore concentration
b) smelting
c) refining Lead and zinc ores (usually sphalerite, ZnS) often occur together and may also contain silver, copper and gold. The ore has first to be separated from clays and other silicates ('gangue') after which the lead ore and the zinc ore are separated.
The process used is froth flotation, a succession of stages each delivering a greater concentration of the lead ore. The ore, containing gangue (which typically contains 3-8% lead) is ground with water to a particle size similar to that of fine sand (<0.25 mm). This is then mixed with water and a frothing agent (a detergent) and is violently agitated by air to form a fine suspension with a froth of bubbles on the top. The process is carried out in a series of tanks. As the lead and zinc minerals are less readily wetted than the gangue, they adhere to the air bubbles which are carried to the surface. The rock particles sink and the lead and zinc ores are skimmed off.
Then the lead ore is separated from the zinc ore. A chemical called a depressant is added which is soluble in water (for example, zinc sulfate) and the zinc ore sinks and the lead ore is skimmed off. Later, a chemical such as copper(II) sulfate is added and the zinc ore now floats and is skimmed off.
The lead ore concentrate from the flotation tanks now contains about 50% lead and ca 0.1% of silver, a relatively small but valuable amount.
(b) Smelting
Smelting is usually a two-stage process as described here, although single-stage methods with lower energy use and emissions are also used.
After mixing with limestone the filtered concentrated ore is roasted in air or oxygen-enriched air on a moving belt. Most of the sulfide is converted to lead(II) oxide:
Sulfur dioxide gas can then be cleaned and used to make sulfuric acid.
The lead(II) oxide is heated and made into lumps, a process known as sintering. The lumps (the sinter) are crushed and sorted to a suitable size for subsequent treatment in a blast furnace, similar in construction to those for making iron but smaller.
The graded sinter (each lump about the size of a fist) is mixed with approximately 7% of its mass in coke and limestone. The coke is added for two purposes, one as the reducing agent and the other as a source of heat when it reacts with the air which, as in the manufacture of iron is pumped into the furnace. The limestone provides material for the flux containing the impurities, the slag. The mixture is fed into the top of the blast furnace, and the lead(ll) oxide is reduced to molten lead. Carbon and carbon monoxide, produced from the coke, are the reducing agents:
The molten lead is tapped off from the base of the furnace and either cast into, typically, 4 tonne ingots or put into a 'holding kettle' which keeps the metal molten for the refining process.
The product contains about 99.5% lead, the remaining 0.5% being mostly antimony and silver with smaller amounts of other metals, including gold. Because it contains silver and gold at this stage, the lead is referred to as bullion lead.
The fluxes form a molten slag of metal oxides and silicates which floats on top. The slag contains much of the zinc remaining in the ore and is treated later to produce metallic zinc.
Smelting can also be done using a lance through which oil and oxygen-enriched air pass at high speed forming turbulent conditions. There are several such processes operating under names such as Isasmelt, Ausmelt and Sirosmelt. The one stage, Isasmelt, process is discussed at the end of this unit.
(c) Refining
The bullion is heated to just above its melting point. Solid copper and copper sulfide rise to the surface and are skimmed off.
Arsenic is then removed by stirring the lead with an air blast and skimming off the resulting slag which contains arsenic oxides.
Silver is removed by a technique known as the Parkes process. Approximately 2% zinc is added to the lead and a silver-rich zinc crust forms and is removed. More zinc is then added at 740 K (its mp is 693 K) and the bath cooled to just above its melting point. During cooling a solid silver/zinc crust separates, rises to the surface and is removed continuously.
The Parkes process depends on the following:
lead and zinc are almost immiscible just above their melting points
silver is much more soluble in zinc than in lead
silver/zinc alloys have higher melting points than pure zinc.
At 863 K zinc is removed by transferring the lead to a 'dezincing kettle'. At this stage the desilvered lead contains about 0.6% zinc and 0.0004% silver. Dezincing is carried out by vacuum distillation at 860 K when the zinc vaporises.
Finally, all traces of antimony and zinc are removed by mixing sodium hydroxide into molten lead at 760 K (its melting point is 600 K). The resulting sodium zincate and antimonate form a skin on the surface of the molten lead which is skimmed off. The refined lead is of 99.99% purity."
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