Geology Reference
In-Depth Information
8.8.1 Lead process
Lead is produced and refined by pyro-metallurgical processes, either by sintering/
smelting or by direct smelting. The sintering/smelting of galena requires first a
process, in which a mixture of lead concentrate with fluxes, recycled sinter and other
recycled materials are converted with natural gas burners into a porous material
adequately sized for the blast furnace, which roasts the metal sulphides into oxides
and sulphur dioxide and sinters the material into large lumps for the smelting process
(IPPC, 2009).
The smelting process reduces metallic oxides to their metal in a vertical shaft
blast furnace similar to the one used for producing pig iron (see Sec. 8.2.1). Layers of
sintered ore coke and fluxes are fed into the top whilst air is blown from the bottom,
an action that leads to a partial combustion of coke which in turn produces carbon
monoxide and heat, reducing the lead oxide and melting any metal which flows to
the bottom of the furnace. The product generated is named hot lead bullion and
contains other dissolved elements like gold, silver, copper, tin, antimony, arsenic
and sulphur that need to be recovered stepwise (Kawatra, 2011).
Direct smelting is the most common technique in lead production. It smelts to-
gether primary and recycled lead coming from batteries and consists of a cylindrical
bath furnace where a steel lance is submerged into a bath injected with air/oxygen
rich air. The excess air oxidises the sulphides into sulphur dioxide, which is later
collected to produce sulphuric acid. Coke or natural gas reacts with air and in doing
so supplies the heat needed to melt the charge. This charge is composed of primary
lead concentrates, secondary lead and fluxing agents. The molten bullion is tapped
off at the bottom and the slag is left to float in the bath where it prevents tem-
perature loss 9 . The produced slag, rich in lead, is subsequently recovered. During
this process, some zinc and cadmium oxides escape in the off-gases and need to be
further recovered in an abatement plant.
The pyrometallurgical refining of the lead bullion consists of de-copperising, soft-
ening, de-silverising, de-zincing, bismuth removal and final refining (BCS, 2002b).
Each step is undertaken in a series of heated kettles. To take the first, de-copperising
is achieved by the cooling of lead bullion in a stirred kettle heated to below 550 o C,
where the dissolved metals begin to precipitate as solid dross. The copper content
is first removed as a sulphide.The dross is then further processed for metal recovery.
The next step is lead softening which involves the removal of antimony, arsenic and
tin. In this process the bullion is dissolved with a mixture of sodium hydroxide and
sodium nitrate and the dross is skimmed off. Zinc is then added to the hot lead,
given that it preferentially dissolves gold and silver. At this point the temperature
drops to 325 o C and a zinc crust forms which contains Au and Ag. Zinc is then
separated from silver by vacuum distillation. Bismuth is removed in a similar way
but with a mixture of calcium and magnesium added instead of zinc. In this case
the dross may be oxidised with chlorine gas. The final refining stage consists of
9 This process is the same as that for producing blister copper from copper matte (see Sec. 8.4).
 
Search WWH ::




Custom Search