**9.2.5. Thermal process**

The **thermal (dry) process** or the electric furnace process is based on the oxidation of phos‐ phorus in the furnace heated to temperatures from 1800 to 3000 K (**Eq. 21**). If wet (undried) air is used for the oxidation, the gaseous product of this reaction is led directly through the hydration tower, where P4O10 gas is adsorbed and hydrated by water to phosphoric acid (**Eq. 22**) [55]:

$$\text{P}\_4\left(\ell\right) + \text{S} \text{ O}\_2\left(\text{g}\right) \to \text{P}\_4\text{O}\_{10}\left(\text{g}\right) \tag{21}$$

$$\text{P}\_4\text{O}\_{10}\text{(g)} + 6\text{ H}\_2\text{O}\text{(}\ell\text{)} \to 4\text{ H}\_3\text{PO}\_4\text{(aq)}\tag{22}$$

The product of phosphorus oxidized by dry air, P2O5, was condensed as white powder and next hydrated to phosphoric acid.

**Fig. 13.** The scheme of the plant for the production of thermal phosphoric acid [11].

Yara, as Fisons Fertilizers, started its research and development to find an alternative to the traditional dihydrate process route (**Fig. 12**) during the late 1950s and early 1960s at its R&D establishment at Levington, UK. The comprehensive laboratory-scale work established that the hemihydrate process route was a feasible alternative. Subsequent pilot plant testing was employed at the Fisons (JAMES FISON) Kings Lynn factory to develop the process on a larger scale and to confirm earlier findings that the hemihydrate process could be adopted success‐

Sulfuric acid

Process Water

Gypsum

Flash Cooler

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

HH Filter

HH Reactors Phosphoric Acid DH Reactors

When the Kings Lynn factory closed, all research and development was concentrated at the R&D center in Levington, UK. A new pilot plant facility was constructed, this time using reactors made from high-grade stainless steel. The size of the equipment was somewhat smaller because all raw materials had to be specially delivered and produced acid and the byproduct phosphogypsum dispatched to the nearest production unit for disposal. During the 1980s and 1990s, the research facilities were subsequently based in Sweden, the Netherlands and Norway. Currently, all phosphoric acid research and development is carried out at Yara's

However, the organic matter (OM) contained in acid may interact with organic solvents to form stable foams, preventing the phase settling, or simply by forming cross-layers and organic phases and denaturing part of the solvent. Hence, the removal of these organics seems to be an important step for the production of decontaminated phosphoric acid. The utilization of activated bentonite [65],[66], activated carbon and coal [67] for the adsorption of organic matter

The **thermal (dry) process** or the electric furnace process is based on the oxidation of phos‐ phorus in the furnace heated to temperatures from 1800 to 3000 K (**Eq. 21**). If wet (undried) air is used for the oxidation, the gaseous product of this reaction is led directly through the hydration tower, where P4O10 gas is adsorbed and hydrated by water to phosphoric acid

fully [53].

Sulfuric acid

Phosphate Rock

**Fig. 12.** Yara HDH process [53].

was investigated.

(**Eq. 22**) [55]:

**9.2.5. Thermal process**

phosphate R&D center in Siilinjarvi, Finland [53].

These two operations can be carried out in the apparatus connected in series (**Fig. 13**), but the hydration takes place in large extent directly in the combustion tower. The combustion nozzle is arranged centrally at the head of the tower and directed downwards. It is a nozzle for two components. Phosphorus is atomized by compressed air or vapor into fine particles, which spontaneously ignite immediately upon leaving the nozzle. Phosphorus is fed in the liquid state through pipes by means of pumps or pressure vessels. The combustion aid is either forced through the apparatus by means of suction fan arranged at the outlet of final purification 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 sprayed in through several rows of nozzles [11].

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 oxidation of red phosphor by concentrated nitric acid:

$$\text{P} + \text{3HNO}\_3 \rightarrow \text{H}\_3\text{PO}\_4 + \text{NO} + \text{2 NO}\_2 \tag{23}$$

The oxidative reaction is catalyzed by the trace of iodine anions (*I*<sup>−</sup> ). The evaporation of solution on platinum dish (thickening) leads to the viscous (syrupy) liquid from which concentrated H3PO4 precipitates [55].

#### **9.2.6. Phosphogypsum**

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

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 gypsum.

**Fig. 14.** Main environmental concerns due to the storage and agricultural use of phosphogypsum [77].

The reduction in the concentration of radionuclides content from PG was based on leaching of 226Ra, 210Pb, 238U and 40K using tributyl phosphate (TBP) and trioctyl phosphine oxide (TOPO) in kerosene [70].
