**9.3.6. Nitrophosphate and urea nitrate phosphate**

Nitrophosphate (calcium superphosphate nitrate) is prepared by the same way as superphos‐ phate using nitric acid instead of sulfuric acid [78]:

**Fig. 19.** The scheme of a plant for the production of thermal phosphoric acid [101].

$$\text{Ca}\_3\text{(PO}\_4\text{)}\_2 + 4\text{ HNO}\_3 \rightarrow \text{Ca}\left(\text{H}\_2\text{PO}\_4\right)\_2 + 2\text{ Ca}\left(\text{NO}\_3\right)\_2\tag{35}$$

The amount of nitric acid applied for the treatment of phosphate rock affects the ratio between formed monocalcium phosphate and calcium nitrate, as the following equations reveal [101]:

$$\rm Ca\_{3}\left(PO\_{4}\right)\_{3}\rm F+10\ HNO\_{3}\rightarrow \rm 5\ Ca\left(NO\_{3}\right)\_{2}+\rm 3\ H\_{3}PO\_{4}+\rm HF\tag{36}$$

$$\begin{aligned} \text{Ca}\_3\text{(PO}\_4\text{)}\_3\text{F} + 9\text{ HNO}\_3 &\to 0.5\text{ Ca}\text{(H}\_2\text{PO}\_4\text{)}\_2\\ +4.5\text{ Ca}\text{(NO}\_3\text{)}\_2 + 2\text{ H}\_3\text{PO}\_4 + \text{HF} \end{aligned} \tag{37}$$

$$\begin{aligned} \text{Ca}\_{\text{s}} \text{(PO}\_{4}\text{)}\_{\text{3}}\text{F} + 8\text{ HNO}\_{\text{3}} &\rightarrow \text{Ca}(\text{H}\_{2}\text{PO}\_{4})\_{\text{2}} \\ +4\text{ Ca}(\text{NO}\_{3}\text{)}\_{\text{2}} &+ \text{H}\_{3}\text{PO}\_{4} + \text{HF} \end{aligned} \tag{38}$$

$$\begin{aligned} \text{Ca}\_3\text{(PO}\_4\text{)}\_3 &\text{F} + 7 \text{ HNO}\_3 \rightarrow \text{l..5} \text{ Ca}\text{(H}\_2\text{PO}\_4\text{)}\_2\\ + 3.5 \text{ Ca}\text{(NO}\_3\text{)}\_2 &+ \text{HF} \end{aligned} \tag{39}$$

These fertilizers are also the source of nitrogen.16 For the preparation of urea nitrate phos‐ phate, nitrophosphate contains the product of **Eqs. 35** – **39** conveyed to the decanter (**Fig. 19**) to get rid of the insoluble impurities. Urea is added to the solution in the mixer [78],[101].

The following reactions accompany the formation of urea nitrate phosphate [101]:

<sup>16</sup> Fertilizers containing two or more nutrients are termed as compound nutrients (not in the United States). The names complex fertilizers and chemically mixed fertilizers are used in the same meaning in some countries [84].

$$\begin{aligned} \text{Ca}(\text{NO}\_3)\_2 + \text{H}\_3\text{PO}\_4 + 2\text{ CO}(\text{NH}\_2)\_2 &\rightarrow \text{Ca}(\text{H}\_2\text{PO}\_4)(\text{NO}\_3)\text{CO}(\text{NH}\_2)\_2\\ + \text{CO}(\text{NH}\_2)\_2\text{HNO}\_3 \end{aligned} \tag{40}$$

$$\text{Ca(NO}\_3\text{)}\_2 + 4\text{ CO(NH}\_2\text{)}\_2 \rightarrow \text{Ca(NO}\_3\text{)}\_2 \cdot 4\text{CO(NH}\_2\text{)}\_2\tag{41}$$

$$\text{Ca(NO}\_3\text{)}\_2 + \text{H}\_3\text{PO}\_4 \rightarrow \text{Ca(NO}\_3\text{)}\_2 \cdot \text{H}\_3\text{PO}\_4\tag{42}$$

$$\begin{aligned} \text{Ca}(\text{H}\_{2}\text{PO}\_{4})\_{2} + \text{Ca}(\text{NO}\_{3})\_{2} + 2\text{ CO}(\text{NH}\_{2})\_{2} &\to \\ \text{Ca}\_{2}(\text{H}\_{2}\text{PO}\_{4})\_{2}(\text{NO}\_{3})\_{2} &\text{CO(NH}\_{2})\_{2} \end{aligned} \tag{43}$$

$$2\text{ }2\text{ HF} + \text{Ca}(\text{NO}\_3)\_2 + 2\text{ CO}(\text{NH}\_2)\_2 \rightarrow \text{CaF}\_2 + 2\text{CO}(\text{NH}\_2)\_2\cdot\text{HNO}\_3\tag{44}$$

After removing insoluble impurities in the decanter, the reaction product, the slurry is dehydrated and concentrated, granulated and dried.

### **9.3.7. Dicalcium phosphate**

Ca PO 4 HNO Ca H PO 2 Ca NO 3 4 ( )<sup>2</sup> +® + 3 24 ( ) () 2 2 <sup>3</sup> (35)

Fine

Granulator Dryer

Product

Mill

Ca PO F 10 HNO 5 Ca NO 3 H PO HF 5 4 ( )3 2 + ® ++ <sup>3</sup> ( ) 3 34 (36)

+ ++ (37)

+ ++ (38)

+ + (39)

For the preparation of urea nitrate phos‐

The amount of nitric acid applied for the treatment of phosphate rock affects the ratio between formed monocalcium phosphate and calcium nitrate, as the following equations reveal [101]:

( ) ( )

5 4 3 2 3 2 4 3 34 2 Ca PO F 9 HNO 0.5 Ca H PO 4.5 Ca NO 2 H PO HF + ®

( ) ( )

+ ®

5 4 3 2 3 24

( ) ( )

5 4 3 2 3 2 4

phate, nitrophosphate contains the product of **Eqs. 35** – **39** conveyed to the decanter (**Fig. 19**) to get rid of the insoluble impurities. Urea is added to the solution in the mixer [78],[101].

<sup>16</sup> Fertilizers containing two or more nutrients are termed as compound nutrients (not in the United States). The names

3 34 2 Ca PO F 8 HNO Ca H PO 4 Ca NO H PO HF

Ca PO F 7 HNO 1.5 Ca H PO

+ ®

The following reactions accompany the formation of urea nitrate phosphate [101]:

complex fertilizers and chemically mixed fertilizers are used in the same meaning in some countries [84].

( )

( )

( )

3.5 Ca NO HF

These fertilizers are also the source of nitrogen.16

Insoluble materials are removed Urea

Nitric Acid

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

Phosphate Rock

Digester

Decanter

Mixer

Concentrator

**Fig. 19.** The scheme of a plant for the production of thermal phosphoric acid [101].

3 2

Dicalcium phosphate is a citrate soluble fertilizer. Phosphate rock is first converted to orthophosphoric acid via the treatment by HCl. Orthophosphoric acid reacting with lime gives dicalcium phosphate [78]:

$$\rm H\_3PO\_4 + CaCO \to CaHPO\_4 + H\_2O \tag{45}$$

#### **9.3.8. Thermophosphates**

Thermophosphates (rhenania phosphates, thermal phosphate) are manufactured by the reaction of rock phosphates with soda and quartz [96],[102]:

$$\begin{aligned} \text{Ca}\_{\text{s}} \text{(PO}\_{4}\text{)}\_{\text{3}}\text{F} + 2\text{ Na}\_{2}\text{CO}\_{3} + \text{SiO}\_{2} &\rightarrow 3\text{ NaCaPO}\_{4} + \\ +\text{Ca}\_{2}\text{SiO}\_{4} + \text{NaF} + 2\text{ CO}\_{2} &\text{(g)} \end{aligned} \tag{46}$$

Rhenania phosphate is prepared by the calcination of mixture of phosphate rock, sodium carbonate and silica in a rotary kiln at 1250°C. The fertilizer contains 28 – 30% P2O5 and has an alkaline effect and hence is more efficient in acid soils. In neutral or basic soils, it reacts more slowly.

A somewhat similar product, Roechling phosphate uses soda slag, which is a by-product from the steel industry, as the source of sodium. Also, naturally occurring source of minerals such as trona (Na3(HCO3)(CO3)·2H2O [103]) or natron (Na2CO3·10H2O [104]) can be applied. A similar product can be also prepared by sintering potassium carbonate with phosphate rocks and silica where the formation of CaKPO4 is supposed [85]:

$$\begin{aligned} \text{Ca}\_s(\text{PO}\_4)\_3 &\text{F} + 2 \text{ K}\_2\text{CO}\_3 + \text{SiO}\_2 \rightarrow 3 \text{ K}\text{CaPO}\_4 + \text{Ca}\_2\text{SiO}\_4\\ + \text{KF} + 2 \text{ CO}\_2(\text{g}) \end{aligned} \tag{47}$$

The origin of thermal phosphate can be derived from Thomas slag (**Fig. 20**). Since Thomas slag became popular, numerous attempts have been made to produce fertilizers by thermal treatment of phosphate rock with additives. The most of these attempts are not successful, except for a few that attained commercial production of fertilizers such as rhenania phos‐ phate, fused magnesium phosphate (FMP) and calcined defluorinated phosphate [84].

**Fig. 20.** Ternary diagram of thermal phosphates: calcined defluorinated phosphate tested in the United States (A), Thomas slag (B), calcined defluorinated phosphates produced in Japan, United States, etc. (C), silicophosphate tested in England (E) and Rhenania phosphate in Germany (F) [84].

The composition of some systems is shown in the ternary diagram in **Fig. 20**. Rhenanite (R, CaNaPO4) has two forms (β- and α-rhenanite) with β → α transition temperature of 670°C. Both phases are highly soluble in 2% citric acid and ammonium citrate. β-Rhenanite is the major constituent of Rhenania phosphate [84].

#### **9.3.9. Environmental demand on phosphate fertilizers**

Fertilizers are essential to provide adequate nutrients for the crop growth and to ensure successful harvests. Continuing exponential growth in human population and increasing demand for biofuels point to ever-increasing demand for fertilizers. Despite the apparent success of current agricultural production systems, the overuse of fertilizers has caused severe environmental problems and increasing number of health concerns. Overall, environmental and human health concerns associated with the overuse of fertilizer result in two main problems [90],[105],[106],[107],[108]:

similar product can be also prepared by sintering potassium carbonate with phosphate rocks

5 4 23 2 3 4 24

+ +® +

The origin of thermal phosphate can be derived from Thomas slag (**Fig. 20**). Since Thomas slag became popular, numerous attempts have been made to produce fertilizers by thermal treatment of phosphate rock with additives. The most of these attempts are not successful, except for a few that attained commercial production of fertilizers such as rhenania phos‐ phate, fused magnesium phosphate (FMP) and calcined defluorinated phosphate [84].

> Ca3(PO4)2 T

> > A

B

Ca2SiO4 CaNaPO4

**Fig. 20.** Ternary diagram of thermal phosphates: calcined defluorinated phosphate tested in the United States (A), Thomas slag (B), calcined defluorinated phosphates produced in Japan, United States, etc. (C), silicophosphate tested

The composition of some systems is shown in the ternary diagram in **Fig. 20**. Rhenanite (R, CaNaPO4) has two forms (β- and α-rhenanite) with β → α transition temperature of 670°C. Both phases are highly soluble in 2% citric acid and ammonium citrate. β-Rhenanite is the

Fertilizers are essential to provide adequate nutrients for the crop growth and to ensure successful harvests. Continuing exponential growth in human population and increasing demand for biofuels point to ever-increasing demand for fertilizers. Despite the apparent

D R

Weigh [%]

E C

F

+ + (47)

Ca PO F 2 K CO SiO 3 KCaPO Ca SiO

and silica where the formation of CaKPO4 is supposed [85]:

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

( )

2

( )

in England (E) and Rhenania phosphate in Germany (F) [84].

major constituent of Rhenania phosphate [84].

**9.3.9. Environmental demand on phosphate fertilizers**

KF 2 CO g


Moreover, fertilizers can be adulterated products containing the raw material sometimes from unknown and/or questionable sources. Besides certified nutritional ingredients for plants, they may contain, most notably, the trace element contaminants that can be inadvertently intro‐ duced into soils. The fertilizer applications are by far one of the most consistent sources of trace elements to accumulate in cropland soils. Based on the analysis of existing data from litera‐ ture and of results from model simulations, we have concluded that a long-term use of phosphorus fertilizers and micronutrients could cause the As, Cd and Pb contents of the cropland soils to rise if the products used contained high levels of these elements [92],[105], [106].

Since naturally occurring nuclides 238U, 232Th and 40K have strong association with phosphate ore, which is the major raw material for the production of phosphate fertilizers, radon 222 (222Rn) (the most significant natural isotope of radioactive element radon) was formed as a decay product of 238U. Radon is colorless, odorless poisonous gas, and sustained exposure of humans to its increased level can lead to lung cancer. Radon is a noble gas and does not undergo chemical reaction. When concentrated in enclosed environment, it can only diminish by diffusion, advection and radioactive decay [70],[109],[110],[111],[112].

Phosphate fertilizers are being enriched with 238U during their production from phosphate rocks. The activity of 238U is higher in phosphate-rich fertilizers like TSP and SSP. The application of phosphate fertilizer significantly increases the radioactivity level of cultivated soil as compared to soil from barren land [113],[114].

Current waste of phosphorus fertilizers causes a great deal of environmental problems, and it is questionable if it is a good idea to extract all the phosphate rock reserves if it would still end up in lakes, streams and sea. As more and more phosphorus has been added to the ecosys‐ tem, many lakes and coasts have seen an increased algae growth, which in some cases have led to serious eutrophication and dead zones due to lack of oxygen [91]. CARPENTER and BENNETT [115] even consider that the planetary limits for the eutrophication of freshwater due to phosphorus have already been exceeded. It is possible to recycle phosphorus from different sources (human excreta, manure, different types of waste products, etc.) and improve the efficiency in the production and usage in order to postpone the potential production peak.
