RADIOLOGICAL IMPACT OF FERTILIZERS: PRODUCTION AND USE - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 285

Table 2. Naturally occurring radionuclides in phosphate rocks, expressed from Bq·kg -1 ,
from sedimentary and volcanic origin. Data from (Guimond et al., 1989; Makweba et
al., 1993; Papastefanou et al., 2001; IAEA, 2003; Falk et al., 2006)

Type

238 U 226 Ra 232 Th

Sedimentary 60 - 11000 200 - 5800 7 - 1100

Volcanic 70 - 200 200 100 - 400

Radionuclide Fluxes in the Production Processes

There are different industrial processes to manufacture phosphate fertilizer from raw
phosphate ore. The most commonly process used, ~ 90 %, is the wet process (Rutherford et
al., 1994), in which the phosphate ore is acid leached, usually by means of H 2 SO 4 , in order to
produce phosphoric acid. The general chemical reaction for fluorapatite is shown in
equation 1.

 

Ca F PO  10 H SO  20 H O   10 CaSO · H O  6 H PO  2 HF (

10 2 4 6 2 4 2 4 2 3 4

In this process, a great quantity of phosphogypsum (CaSO 4 ·xH 2 O) is generated,
approximately 4-5 T per ton of P 2 O 5 produced (IAEA, 2003). The annual production of
phosphogypsum is estimated within the range (3-280)·10 6 T (Rutherford et al., 1994; Bolívar
et al., 1995; Burnett et al., 2001). The degree of hydration of the phosphogypsum depended
on the acid concentrations and the temperature of the operating procedure, being ·2H 2 O the
most common (Rutherford et al., 1994).
This production method based on the chemical attack disrupts the decay chain scheme
within the phosphate ore. Each naturally occurring radionuclide goes along with a given
product according to their chemical properties. The majority of uranium and thorium goes
mainly with the phosphoric acid produced (Rutherford et al., 1994, Erdem et al., 1996,
Bolívar et al., 2009). In fact, the 238 U content showed a positive correlation with the purity of
the P 2 O 5 obtained in different steps (Righi et al., 2005; Bolívar et al., 2009). The association
of the naturally occurring radionuclides with the P 2 O 5 produced decreased in the following
order (Poole et al., 1995; Bolívar et al., 2009):
238 U ≈ 234 U > 230 Th > 210 Pb ≈ 210 Po > 226 Ra

The association of these naturally occurring radionuclides with the phospogypsum
follows the same order, but in reverse. About 60-100 % of the 226 Ra and 90-100% of the 210 Po
was associated mainly with the phosphogypsum (Rutherford et al., 1994, Hull et al., 1996).
Table 3 shows the range of these radionuclides reported in phosphogypsum produced in
different countries worldwide. It can be observed that the phosphogypsum presented higher
contents of 226 Ra, 210 Pb, and 210 Po than uranium and thorium. In some occasions a significant
percentage of the uranium may be associated with phosphogypsum. Its content in
phosphogypsum was mainly due to unreacted phosphate rock (Hull et al., 1996, Bolívar et al.,
2009). The uranium content was also observed to decrease with successive washings of the
phosphogypsum, which partially removed that unreacted material (Bolívar et al., 2009).