Chemistry Reference
In-Depth Information
Sedimentary phosphate rocks, such as those of North Africa and the Middle
East, are the dominant sources, and igneous phosphate rocks are also mined.
In all of these, the most important phosphate mineral is apatite,
Ca
5
(PO
4
)
3
(OH); the OH site can contain carbonate (sedimentary sources) or
fluoride (igneous sources). Chemical processing during manufacture varies
from nil, when phosphate rock is simply sold as a crushed and ground material
suitable for direct application, to chemical manufacturing processes that
produce phosphoric acid. This is then used as a raw material both for the
manufacture of fertilisers and other industrial chemical feedstocks and
materials, such as detergents. When used directly as a fertiliser, without
chemical processing, phosphate rock is a slow-release source of P, in which the
availability of phosphate depends on the dissolution of apatite in the soil
solution:
d
n
1
r
2
n
g
|
3
Þ
zH
z
~5Ca
2z
z3PO
4
3{
zH
2
O
Ca
5
PO
4
ð
Þ
3
OH
ð
ð
3
Þ
Potassium fertilisers differ from chemically processed N and P fertilisers in
that they are mined as a readily soluble salt. Geological deposits of potassium
salts, generally termed 'potash', occur within rocks known as evaporites,
produced by the evaporation of saline brines in very specific geological
processes; these are mimicked in the artificial production of salts from natural
brines, for example from the Dead Sea brines. Examples of potash minerals are
given in Table 1. Once mined, typically they are either used directly, with
limited processing, or processed to separate specific components. They may be
blended with other fertiliser products to give a compound fertiliser.
Table 1
Potash minerals. (Note that it is conventional to express the potash
content in terms of the equivalent amount of K
2
O).
Mineral name
Chemical formula
Potash content (%K
2
O)
Sylvite
KCl
63%
Carnallite
KCl.MgCl
2
.6H
2
O
19%
Langbeinite
K
2
SO
4
.2MgSO
4
23%
Kainite
4KCl.4MgSO
4
.11H
2
O
19%
Polyhalite
K
2
SO
4
.2CaSO
4
.MgSO
4
.2H
2
O
16%
1.2 Minerals in Plants
Chemically, plants are complex systems. Their composition is dominated by
water and carbon compounds, and they also contain inorganic components,
commonly described as the mineral content of a plant, a crop or a food. This is
the residue that is left behind as an ash following complete combustion of plant
material.
