**9.2.7. Phosphorus slag and ferrophosphorus**

device. If complete oxidation to P2O5 is assured, the waste gases must contain 4 to 5% of oxygen. The nebulous P2O5 is conducted from the combustion chamber in a tower like so-called hydrator. The gas entering at the bottom is washed with water or diluted phosphoric acid

Small amount of pure phosphoric acid can be prepared by heating white phosphor with diluted nitric acid (using concentrated HNO3 turns the course of reaction to explosive) or by the

on platinum dish (thickening) leads to the viscous (syrupy) liquid from which concentrated

Phosphogypsum (PG) is a by-product produced by phosphate fertilizer industry during the production of phosphoric acid (**Eq. 19**) from phosphate rocks (**Section 9.2.4**). About 4.5 – 5 kg of phosphogypsum are produced for every kilogram of P2O5 manufactured [68]. PG is mainly composed of gypsum, but it also contains high level of impurities, which include naturally occurring radionuclides, metals and other trace elements, the quantity varying with element and the production process. Major PG reuse includes the production of cement, china and crystallite glass as well as soil amendments in agriculture (PG appears to be good source of S and Ca for crops [69]) without a consideration of element recovery, but even these latter reuses are limited due to the radioactivity within PG. Presently, PG is mainly stockpiled without any treatment. It can, however, be discharged into aquatic environments and pose a radioactive

Potential utilization of phosphogypsum (**Fig. 14**), the by-product from fertilizer industries, as a bitumen modifier for paving industry was reported by CUADRI et al [68]. It was found that when activated with small quantity of sulfuric acid (0.5 wt.%), the addition of 10 wt.% phosphogypsum leads to a notable improvement in the rheological response of resulting material at high temperatures. On the contrary, poor level of modification was noticed when in such formulation phosphogypsum was substituted by the same amount of commercial

P 3HNO H PO NO 2 NO + ® ++ 3 34 <sup>2</sup> (23)

). The evaporation of solution

sprayed in through several rows of nozzles [11].

H3PO4 precipitates [55].

**9.2.6. Phosphogypsum**

gypsum.

oxidation of red phosphor by concentrated nitric acid:

The oxidative reaction is catalyzed by the trace of iodine anions (*I*<sup>−</sup>

432 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

threat to ecosystems [51],[57],[70],[71][72],[73],[74],[75], [76],[77].

Phosphorus slag8 and ferrophosphorus are the by-products of the production of elemental phosphorus. Phosphorus slag has applications similar to those of PG, but more than 80% of phosphorus slag is stored near the factory instead of being recycled. At present, the recy‐ cling of phosphorus slag appears not being considered for its element recovery potential. Ferrophosphorus containing roughly 75% Fe and 25% P is usually sold as an additive for the steel industry after being crushed and screened [51],[78]. Ferrophosphorus is used in the steel industry to prevent the steel plates from sticking together during the pack annealing [79]. Ferrophosphorus contains FeP, Fe2P and perhaps little amount of Fe3P [2]. The utilization of ferrophosphorus as a heavyweight additive for API oil well cements was investigated by BENSTED [80]. Ferrophosphorus is used for steelmaking, providing a convenient source for phosphorus addition to alloy steels [13].
