The two types of Phosphates

PHOSPHATE DEPOSITS

The known phosphate deposits of the world are estimated at approximately 900 000 million tons of phosphate ore containing the mineral apatite Ca10(PO4)6F2. It is also estimated that the continental shelves (in the sea) could produce a further 300 000 million tons.  Approximately 5 000 million tons of the former volume are located in South Africa.  As the present world consumption of rock phosphate is approximately 90 million tons per annum, there are enough deposits for several millenniums.  At present, approximately 1.5 million tons per annum are used in South Africa, while the mining of rock phosphate for domestic use as well as for export is between 2.9 and 315 million tons per annum.

Phosphate fertilizers are manufactured by the processing of rock phosphate, which could involve one or more of the following processes:  beneficiation of the raw phosphate ore, heat treatment, or treatment with acids. The phosphorous is in the form of orthophosphate, PO4, which may be mono-, di- or tribasic. (The corresponding calcium salts are water soluble, citric acid soluble and strong-acid soluble, respectively).

PHOSPHATE CONCENTRATES

After milling, the mined ore is beneficiated in a flotation process (the apatite is concentrated by removing the unwanted minerals such as magnetite, phlogopite and vermiculite from the ore) into phosphate concentrates (sometimes erroneously referred to as ‘rock phosphate’).  The concentrate still contains impurities, e.g. CaCO3.  There are two types of ore from which concentrates are beneficiated:

(i) Igneous rocks
These are solidified magma occurring as intrusions in other rock formations or as vertical pipes. They contain a strong crystalline apatite mineral that is hard and practically insoluble in water and weak acids.  As such, it is unsuitable as a fertilizer and must first be treated with strong acids.
Examples:  Kola (former USSR), pyroxenite and foskorite (Phalaborwa, South Africa).

(ii) Sedimentary rocks
These are deposits of biological origin (particularly marine fauna) which is formed after the chemical breakdown of bones, animal manure and beneficiated rocks.  This apatite (sometimes called phosphorite) is less crystalline and practically amorphous and therefore more soluble than beneficiated rocks – it dissolves partially in diluted weak acids such as citric acid, and in beneficiated and finely milled form it can be used as a phosphate fertilizer, especially in acid soils.
Examples:  Morocco, Taiba, Togo, Gafsa (Tunis), Tennessee and Florida (USA), island deposits and ‘Langfos’ (Langebaan Road, South Africa).