『Abstract
Minjingu phosphate rock (MPR) in northern Tanzania with an estimated
reserve of 7 million tonnes consists of two types including hard
MPR and soft MPR. Both types seem promising for direct application
as phosphorus (P) fertilizers but unproved knowledge about their
reactivity is needed. Since mineral reactivity depends on the
chemical and mineralogical composition and crystal size, these
properties were investigated on different MPR samples by means
of combined chemical, X-ray diffraction (XRD), Fourier Transform
Infrared spectroscopy (FT-IR) and scanning electron microscopy
(SEM)-microprobe analyses and compared with similar properties
of five well-known sedimentary and igneous PR references. As a
direct test on reactivity, the solubility of MPR and reference
samples in neutral ammonium citrate (NAC) was determined. The
chemical composition indicated that the apatites in both MPR types
are fluorine deficient francolites. For hard MPR this is in agreement
with the mineralogical investigation, while XRD suggests that
soft MPR belongs to excess fluorine francolites with low carbonate
substitution, which generally have low reactivity. However, the
observed very small crystal size together with high substitution
rates of carbonate for phosphate and of strontium for calcium
can explain the high reactivity of soft MPR. In agreement with
the rather than NAC solubility, i.e. 4.2％ and 6.2％ P2O5 for hard and soft MPR, respectively, these properties
place hard MPR and soft MPR among the medium to highly reactive
PR, respectively. Consequently, both hard and soft MPR may be
considered suitable for direct application as P fertilizers. This
is in good accordance with the results of previously performed
field and pot trials showing that direct application of hard MPR
or soft MPR resulted in maize yields comparable to the yields
obtained using water-soluble phosphate (triplesuperphosphate)
on acid soils low in available P and Ca in sub-humid to humid
Tanzania.
Keywords: Phosphate reactivity; Direct application; Apatite; Francolite;
Collophane; Guano』
1. Introduction
2. Materials and methods
2.1. The Minjingu deposit
2.2. Chemical and mineralogical analyses
3. Results and discussion
3.1. Composition of Minjingu phosphate rocks
3.2. Reactivity of Minjingu phosphate rocks
4. Conclusions
Acknowledgements
References