Everything about Lead totally explained
Lead is a transitional
element with a symbol
Pb . Lead has the
atomic number 82. Lead is a soft and
heavy toxic and
malleable poor metal. Lead has bluish white color when freshly cut, but tarnishes to dull grayish color when it's exposed to air and is a shiny chrome silver when melted into a liquid. Lead is used in building construction,
lead-acid batteries,
bullets and
shot, weights, and is part of
solder,
pewter, and fusible
alloys. Lead has the highest
atomic number of all
stable elements, although the next element,
bismuth, has a half-life so long (longer than the estimated age of the universe) it can be considered stable. Like
mercury, another heavy metal, lead is a potent
neurotoxin that accumulates in soft tissues and bone over time.
Notable characteristics
Lead has a dull luster and is a
dense,
ductile, very soft, highly malleable, bluish-white metal that has poor
electrical conductivity. This true metal is highly resistant to
corrosion, and because of this property, it's used to contain corrosive liquids (for example
sulfuric acid). Because lead is very malleable and resistant to corrosion it's extensively used in building construction for example external coverings of roofing joints. Lead can be toughened by adding a small amount of
antimony or other metals to it. It is a common misconception that lead has a zero
Thomson effect. All lead, except
204Pb, is the end product of a complex radioactive decay (see isotopes of lead below). Lead is also
poisonous.
History
Lead has been commonly used for thousands of years because it's widespread, easy to extract and easy to work with. It is highly malleable and ductile as well as easy to
smelt. In the early
Bronze Age, lead was used with
antimony and
arsenic. Lead was mentioned in the
Book of Exodus (15:10). In
alchemy, lead was thought to be the oldest metal and was associated with the planet
Saturn. Lead pipes that bear the insignia of Roman emperors are still in service and many Roman "pigs" (ingots) of lead figure in
Derbyshire lead mining history and in the history of the industry in other English centres. The Romans also used lead in molten form to secure iron pins that held together large
limestone blocks in certain monumental buildings. Lead's symbol Pb is an abbreviation of its
Latin name
plumbum for soft metals; originally it was
plumbum nigrum, where
plumbum candidum was
tin. The English words "plumbing" and "
plumb-bob" also derive from this Latin root.
Lead also refers collectively to the organic and inorganic compounds of lead, which are toxic.
Lead poisoning was documented in ancient Rome, Greece, and China. In the 20th century, the use of lead in paint
pigments was sharply reduced because of the danger of lead poisoning, especially to children. By the mid-1980s, a significant shift in lead end-use patterns had taken place. Much of this shift was a result of the U.S. lead consumers' compliance with environmental regulations that significantly reduced or eliminated the use of lead in non-battery products, including
gasoline, paints, solders, and water systems. Lead use is being further curtailed by the European Union's
RoHS directive. Lead may still be found in harmful quantities in stoneware, vinyl (such as that used for tubing and the insulation of electrical cords), and brass manufactured in China. Between 2006 and 2007 many children's toys made in China were recalled, primarily due to lead in paint used to color the product.
Occurrence
Metallic lead does occur in nature, but it's rare. Lead is usually found in ore with
zinc,
silver and (most abundantly)
copper, and is extracted together with these metals. The main lead
mineral is
galena (PbS), which contains 86.6% lead. Other common varieties are
cerussite (PbCO
3) and
anglesite (PbSO
4).
Sources of Lead
Top lead producing countries (2008) are Australia, China, USA, Peru, Canada, Mexico, Sweden, Morocco, South Africa and North Korea.
Lead mining in Wales
Lead ore (galena) is found commonly in northeast Wales. The Northeast Wales Orefield was by far the most important source of lead and zinc in Wales and second in national importance only to the North Pennine Orefield. Lead mining dates back to at least Roman times and continued until well into the 20th century. Galena is present in steeply dipping fissure veins and in pipes and is in
Mississippi Valley-type lead-zinc-fluorite and copper-dolomite associations. The mineralisation occurs in the upper parts of the Loggerheads and Cefn Mawr Formations of the Carboniferous Limestone.
Processing of metal from ore
The principal ores of lead are
galena (PbS),
anglesite and
cerussite (PbCO
3). Most ores contain less than 10% lead, and ores containing as little as 3% lead can be economically exploited. Ores are crushed and concentrated by
froth flotation typically to 70% or more.
Sulfide ores are
roasted, producing primarily lead oxide and a mixture of
sulfates and
silicates of lead and other metals contained in the ore.
Lead oxide from the roasting process is reduced in a coke-fired blast furnace. This converts most of the lead to its metallic form. Three additional layers separate in the process and float to the top of the metallic lead. These are
slag (silicates containing 1.5% lead),
matte (sulfides containing 15% lead), and
speiss (arsenides of iron and copper). These wastes contain concentrations of copper, zinc, cadmium, and bismuth that can be recovered economically, as can their content of unreduced lead.
Most lead ores contain significant concentrations of
silver, resulting in the smelted metal also containing silver as a contaminant. Metallic silver as well as gold is removed and recovered economically by means of the
Parkes process.
- Annual Metal Production (2006): 7918 Thousand tonnes
- Annual Mine Production (2006): 3442 Thousand tonnes (lead content)
At current use rates, the supply of lead is estimated to run out in 42 years. Environmental analyst,
Lester Brown, however, has suggested lead could run out within 18 years based on an extrapolation of 2% growth per year. This may need to be reviewed to take account of renewed interest in
recycling, and rapid progress in
fuel cell technology.
Isotopes
Lead has seven
isotopes in total (3 stable, 3 unstable 1 radiogenic). The 3 stable isotopes are
206Pb,
207Pb, &
208Pb, and the 3 unstable isotopes are
204Pb,
205Pb, &
210Pb, along with one common
radiogenic isotope 202Pb with a
half-life of ~53,000 years.
Health effects
Lead is a poisonous metal that can damage nervous connections (especially in young children) and cause blood and brain disorders. Long term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO
2) can cause
nephropathy, and
colic-like abdominal pains. The concern about lead's role in cognitive deficits in children has brought about widespread reduction in its use (lead exposure has been linked to
schizophrenia). Most cases of adult elevated blood lead levels are workplace-related. High blood levels are associated with delayed puberty in girls.
Older houses may still contain substantial amounts of
lead paint. White lead paint has been withdrawn from sale in industrialized countries, but the yellow
lead chromate is still in use; for example,
Holland Colours Holcolan Yellow. Old paint shouldn't be stripped by sanding, as this produces inhalable dust.
Lead salts used in pottery glazes have on occasion caused poisoning, when acid drinks, such as fruit juices, have leached lead ions out of the glaze. It has been suggested that what was known as "
Devon colic" arose from the use of lead-lined presses to extract apple juice in the manufacture of
cider. Lead is considered to be particularly harmful for women's ability to reproduce. For that reason, many universities don't hand out lead-containing samples to women for instructional laboratory analyses.
Lead acetate (also known as
sugar of lead) was used by the
Roman Empire as a sweetener for wine, and some consider this to be the cause of the
dementia that affected many of the Roman Emperors.
Lead as a
soil contaminant is a widespread issue, since lead is present in natural deposits and may also enter soil through (leaded) gasoline leaks from
underground storage tanks or through a wastestream of lead paint or lead grindings from certain industrial operations.
Biochemistry of lead poisoning
In medicine, lead inhibits α-aminolevulinate (
ALA) dehydratase and ferrochelatase, preventing both
porphobilinogen formation and the incorporation of
iron into
protoporphyrin IX, the final step in
heme synthesis. This causes ineffective heme synthesis and subsequent
microcytic anemia.
Leaching of lead from metal surfaces
Lead(II) forms a series of complexes with chloride, the formation of which alters the corrosion chemistry of the lead. This will tend to limit the solubility of lead in saline media.
» {|
|
| Equilibrium constants for aqueous lead chloride complexes at 25 °C |
| Pb2+ + Cl– → PbCl+ |
|
K1 = 12.59 |
| PbCl+ + Cl− → PbCl20 |
|
K2 = 14.45 |
| PbCl20 + Cl− → PbCl3− |
|
K3 = 3.98 ×10−1 |
| PbCl3− + Cl− → PbCl42− |
|
K4 = 8.92 × 10−2 |
Further Information
Get more info on 'Lead'.
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