Chemistry Reference
In-Depth Information
Phosphoric acid is a low melting solid and it is usually sold as an aque-
ous solution, such as 85% phosphoric acid, which freezes at about 21
∘
C.
More dilute solutions are also sold and they have lower freezing points. For
example, 75% phosphoric acid has a freezing point of
18
∘
C and can be used
when heated storage is not available. Sometimes phosphoric acid is classified
or sold based upon the amount of phosphorous pentoxide present. A solu-
tion of 85% phosphoric acid is 61.5% P
2
O
5
. This can be calculated by taking
into account that it takes two moles of phosphoric acid to make one mole of
phosphorus pentoxide and using the molecular weights.
−
1moleH
3
PO
4
98 g
1moleP
2
O
5
9g
1moleP
2
O
5
=
141
.
85gH
3
PO
4
×
×
2moleH
3
PO
4
×
61
.
5
%
P
2
O
5
Although fertilizer production is the major use and represents almost 90%
of U.S. consumption [13], there are many other applications for phosphoric
acid. For example, it can be used to remove scale from boilers, to treat met-
als for improved corrosion resistance, or as a dilute solution in foods as an
acidulant and flavoring agent.
2.3 LIME
Lime is calcium oxide (CaO). At the end of 2011, there were 72 lime plants
operating in the United States [14]. They manufactured 19 million metric tons.
This compares with a world production of 330 million metric tons. China is
the major world producer with 200 million metric tons. Major markets for
lime are steelmaking, treatment of smokestack emissions, construction, water
treatment, mining, and the paper industry. Lime is often used as a base. It is
used to trap acids, such as in the treatment of smokestack emissions to remove
sulfur dioxide and other acidic gases. It is used in agriculture to adjust the pH
of the soil. It is used to adjust pH levels of industrial waste water. It is also used
for drinking water to remove trace metals by precipitation or flocculation. The
decrease in consumption seen in 2009 is due to the effect of the recession
on such industries as steel and construction. Lime is manufactured by the
calcining of calcium carbonate (CaCO
3
) in high temperature kilns. At the kiln
temperatures, calcium carbonate decomposes to calcium oxide and carbon
dioxide. Calcium carbonate is naturally occurring and found in rocks, coral,
and sea shells. A rock containing at least 50% by weight calcium carbonate
is classified as limestone and limestone mining is the principle source for
calcium oxide manufacture. Limestone is readily available in the earth's crust.
Energy costs for the calcining, done at temperatures of 900 - 1200
∘
C, are a
major factor in the production cost. When lime is treated with water, calcium
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