Agriculture Reference
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It also depended on factors that influenced the uranium partitioning, such as redox
potential, digestion temperature, sorption of humic substances and clay, and coprecipitation
with fluorides (Hull et al., 1996).
There are other methods of wet processing phosphate rocks using HCl and HNO
3
. When
using HCl, the by-products generated are CaF
2
and CaCl
2
. The CaF
2
is insoluble and
precipitates as sludges, CaCl
2
while is soluble. As
226
Ra is chemically analogue to calcium,
226
RaF
2
and
226
RaCl
2
are also formed as by products, being the first one insoluble and the
latter soluble (Paradiens et al., 2001; IAEA, 2003). Using HNO
3
, produces Ca(NO
3
)
2
and
H
3
PO
4
in solution, which can be used to obtain a complex fertilizer, avoiding the generation
of phosphogypsum (Righi et al., 2005). Calcium oxide is the main waste stream and uranium
and radium is expected to be found in them. Further research is needed in order to assess the
fluxes of naturally occurring radionuclides in these processes (IAEA, 2003).
The production of elemental phosphorus can also be achieved by means of thermal
processing. In this process, phosphate rock is melted in a furnace at high temperature
(~1400 ºC) with silica (sand), coke and iron compounds. Elemental phosphorus and CO
2
are
driven off as gases. The
210
Pb and
210
Po content of the original phosphate ore are released into
the gas stream (Papastefanou et al., 2001; IAEA, 2003).
The gas effluent is filtered through electrostatic dust filters, and the
210
Pb-
210
Po content in
the collected dust is about 1000 Bq·kg
-1
. The solid residue, known as slag, consists mainly in
ferrophosphorus and calcium silicate, and its quantity is about 85% of the phosphate ore. The
majority of the uranium and radium in the phosphate ore is attached to the slag. The naturally
occurring radionuclides in the furnace slag ranged within 407-1517 Bq·kg
-1
for
226
Ra, 444-
2072 Bq·kg
-1
for
238
U and 9-41 Bq·kg
-1
for
232
Th (IAEA, 2003).
Radionuclide Content of Fertilizers
Table 4 shows the range of naturally occurring radionuclide content in several fertilizer
types reported worldwide. The phosphate based fertilizers (NPK, TSP, SSP, MAP, DAP, and
Nitrophos) presented a higher range of uranium content, which reflected its range reported for
phosphate rock due to differences in their origin. Their radium content is usually lower than
the uranium because of the different chemical behaviour in the wet production processes. The
NP or P fertilizers presented lower
40
K content than NPK fertilizers or potash, in which the
potassium content is significant. Urea, an N fertilizer, and other fertilizer not based on
phosphate presented low content of naturally occurring radionuclides.
Environmental Impact of Residues
Phosphogypsum is the main residue of the industrial production of fertilizers, because the
acid leaching of phosphate rocks with sulphuric acid is the most used process. The
management of this waste was carried in different ways. Most of them were accumulated in
stacks, although waste discharges into rivers and sea also occurred until they were banned
(OSPAR, 1996). The accumulation of phosphogypsum in stacks can pose radiological
hazards mainly due to the external exposure of γ rays (mainly
226
Ra,
232
Th, and
40
K), the dust
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