**9.3.1. Single superphosphate**

Single superphosphate (SSP, ordinary termed as normal or ordinary superphosphate) was the main phosphate fertilizer for more than a century and supplied over 60% of the world's phosphate until as late as 1955. Since then, the importance of SSP has been steadily decreas‐ ing with time. In 1975, it supplied only 20% of the phosphate fertilizers and this amount fell to 17% in 1988. Recent decline in actual tonnage has been small, the single superphosphate is still an important phosphate fertilizer and is likely to remain so even though its importance will decrease [97].

The advantages of single superphosphate fertilizer are [97]:


Despite many advantages, the disadvantage of low analysis (from 16 to 22% P2O5, i.e. from 35 to 48% BLP) and consequent high distribution costs have caused the declining interest in its production because the delivered cost at the farm level is usually higher per unit of P2O5 than that of TSP or ammonium phosphates.

Single superphosphate is an optimal choice in the following situations [97]:


Since the grade of phosphate rock determines the grade of the SSP product, high-grade rocks are desirable. Since less reactive rocks must be ground more finely, the reactivity is also important. It is extremely difficult to produce single superphosphate from some igneous rocks (**Section 7.1.2**). Iron and aluminum compounds can be tolerated up to a certain point, although they decrease the P2O5 water solubility. Silica has no other adverse effect than the decrease in grade. An increase of CaO:P2O5 ratio raises the consumption of sulfuric acid per unit of P2O5 and decreases the grade. High chloride rocks (with the content of up to 0.5% Cl and perhaps higher) can be used without serious disadvantage, since the corrosion is not a serious problem in the production of SSP [88].

The preparation of single superphosphate (**Fig. 18**) via the treatment of finely ground phosphate rocks by sulfuric acid is based on the conversion of insoluble fluorapatite or natural tricalcium phosphate to monocalcium phosphate [97]:

$$\begin{aligned} \text{2 }\text{Ca}\_3\text{(PO}\_4\text{)}\_3\text{F} + 7\text{ H}\_2\text{SO}\_4 + 3\text{ H}\_2\text{O} &\to 7\text{ CaSO}\_4\\ +3\text{ Ca}(\text{H}\_2\text{PO}\_4)\_2\cdot\text{H}\_2\text{O} + 2\text{ HF} \end{aligned} \tag{24}$$

$$\begin{aligned} \text{Ca}\_3\text{(PO}\_4\text{)}\_2 + 2\text{ H}\_2\text{SO}\_4 + 5\text{ H}\_2\text{O} &\to 2\text{ CaSO}\_4 \cdot 2\text{H}\_2\text{O} \\ + \text{Ca}(\text{H}\_2\text{PO}\_4)\_2 \cdot \text{H}\_2\text{O} \end{aligned} \tag{25}$$

It has been generally accepted that this process proceeds in two stages [97]:

**Fig. 18.** Manufacturing of superphosphate [88].

**4.** The fertilizer effectiveness of SSP is unquestioned. In fact, it is a standard for the compar‐

**5.** SSP supplies two secondary elements, sulfur and calcium, which are sometimes defi‐

Despite many advantages, the disadvantage of low analysis (from 16 to 22% P2O5, i.e. from 35 to 48% BLP) and consequent high distribution costs have caused the declining interest in its production because the delivered cost at the farm level is usually higher per unit of P2O5 than

**a.** Where both P2O5 and sulfur are deficient, SSP can be the most economical product to meet

**b.** In small countries or remote regions where the demand is insufficient to justify the economic scale of production of concentrated phosphate fertilizers and where the importation is expensive, SSP can be the most economical means of supplying local needs.

**c.** In many cases, SSP can be an attractive way to use the by-product of sulfuric acid that cannot be used to produce more concentrated product because the quality or the quantity of acid is unsuitable. Likewise, SSP can use the deposits of phosphate rock, which are too

Since the grade of phosphate rock determines the grade of the SSP product, high-grade rocks are desirable. Since less reactive rocks must be ground more finely, the reactivity is also important. It is extremely difficult to produce single superphosphate from some igneous rocks (**Section 7.1.2**). Iron and aluminum compounds can be tolerated up to a certain point, although they decrease the P2O5 water solubility. Silica has no other adverse effect than the decrease in grade. An increase of CaO:P2O5 ratio raises the consumption of sulfuric acid per unit of P2O5 and decreases the grade. High chloride rocks (with the content of up to 0.5% Cl and perhaps higher) can be used without serious disadvantage, since the corrosion is not a

The preparation of single superphosphate (**Fig. 18**) via the treatment of finely ground phosphate rocks by sulfuric acid is based on the conversion of insoluble fluorapatite or natural

5 4 24 2 3 4

+ +®

+ ×+ (24)

+ × (25)

2 Ca PO F 7 H SO 3 H O 7 CaSO

3 4 24 2 2 4 2

+ +® ×

Ca PO 2 H SO 5 H O 2 CaSO 2H O

Single superphosphate is an optimal choice in the following situations [97]:

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

ison of other phosphate fertilizers.

that of TSP or ammonium phosphates.

small to supply more extensive plant.

serious problem in the production of SSP [88].

tricalcium phosphate to monocalcium phosphate [97]:

( ) ( )

( ) ( )

24 2 2

Ca H PO H O

24 2 2

3 Ca H PO H O 2 HF

It has been generally accepted that this process proceeds in two stages [97]:

cient in the soil.

these needs.


These two reactions proceed concurrently, but the first one is fast, while the second one continues for several days or weeks.

#### **9.3.2. Triple superphosphate**

Triple superphosphate (TSP) was prepared by the reaction of phosphoric acid with phos‐ phate rock or bone ash [78]:

$$\text{Ca}\_3\text{(PO}\_4\text{)}\_3\text{F} + 7\text{ H}\_3\text{PO}\_4 + 5\text{ H}\_2\text{O} \rightarrow \text{5 Ca}\text{(H}\_2\text{PO}\_4\text{)}\_2 \cdot \text{H}\_2\text{O} + \text{HF} \tag{26}$$

$$\text{Ca}\_3\text{(PO}\_4\text{)}\_2 + 4\text{ H}\_3\text{PO}\_4 \rightarrow \text{3 Ca}\left(\text{H}\_2\text{PO}\_4\right)\_2\tag{27}$$

Triple phosphate is a concentrated form of single superphosphate and it contains 46 – 48% P2O5, i.e. the content of P2O5 is three times higher than that of single superphosphate.

### **9.3.3. Enriched superphosphate**

Enriched superphosphate is essentially a mixture of SSP and TSP, usually made by the acidification of phosphate rock with a mixture of sulfuric and phosphoric acid. Theoretically, any grade between SSP and TSP can be produced, but the usual range is 25 – 35% P2O5. The processes and used equipment are the same as for SSP. Enriched superphosphate may be useful product for the applications in sulfur-deficient areas where SSP would supply more sulfur than necessary. One advantage is that mixed acid of proper concentration can be obtained by mixing concentrated sulfuric acid (93 – 98% H2SO4) with diluted phosphoric acid (30% P2O5), thereby avoiding the need for concentrating the latter [88].

## **9.3.4. Thomas slag**

Thomas, basic or phosphatic slag15 is actually obtained as a by-product in iron industries. This results from the presence of small amounts of phosphorus in iron ores. When the iron ore is burnt with limestone in the presence of air, calcium phosphate and calcium silicate appear as slag. Thus, the mixture of calcium phosphate and calcium silicate is known as Thomas (basic or phosphatic) slag and contains 14 – 18% of P2O5 and 40% of lime. The following reactions accompany the formation of Thomas slag [78],[99],[100]:

$$\text{4 P} + \text{5 O}\_2 \rightarrow \text{2 P}\_2\text{O}\_3 \tag{28}$$

$$\text{C}\ \text{CaO} + \text{P}\_2\text{O}\_3 \rightarrow \text{Ca}\_3\text{(PO}\_4\text{)}\_2\tag{29}$$

$$\text{CaO} + \text{SiO}\_2 \rightarrow \text{CaSiO}\_3 \tag{30}$$

The composition of Thomas slag is shown in **Fig. 20**.

#### **9.3.5. Ammonium phosphate**

Ammonium phosphate (ammoniated superphosphate) was prepared from phosphate ore treated by ammonium sulfate and sulfuric acid [78]:

$$\text{Ca}\_3\text{(PO}\_4\text{)}\_2 + \text{(NH}\_4\text{)}\_2\text{SO}\_4 + 2\text{ H}\_2\text{SO}\_4 \rightarrow 2\text{ NH}\_4\text{H}\_2\text{PO}\_4 + 3\text{ CaSO}\_4\tag{31}$$

This product contains 61 – 73% P2O5.

Anhydrous ammonia passes through triple superphosphate in a rotary drum ammoniator where **reaction 32** takes place to evolve considerable amount of heat to granulate the am‐ moniate superphosphate, which is subsequently dried and bagged [101]:

<sup>15</sup> Also known as Thomas phosphate powder. The utilization as fertilizer was firstly tried as a manure on fields in Germany, and in November 1883, HERREN HOYRMANN and MEYER were able to report to the German Royal Agricultural Society the most excellent effect of its use [99].

$$\text{NH}\_3 + \text{Ca} \left(\text{H}\_2\text{PO}\_4\right)\_2 \rightarrow \text{NH}\_4\cdot\text{H}\_2\text{PO}\_4 + \text{CaHPO}\_4 \tag{32}$$

Orthophosphoric acid neutralized by ammonia yields to monoammonium phosphate (MAP):

$$\rm NH\_3 + H\_3PO\_4 \to NH\_4H\_2PO\_4 \tag{33}$$

When ammonia passes through phosphoric acid to maintain the pH of the resulting solution in the range from 5.8 to 6, diammonium phosphate (diammonium hydrogen phosphate, DAP) is formed:

$$2\text{ }\text{NH}\_3 + \text{H}\_3\text{PO}\_4 \rightarrow \text{(NH}\_4\text{)}\_2\text{HPO}\_4\tag{34}$$

Mono- and diammonium phosphates are manufactured by treating orthophosphoric acid in a preneutralizer where the ratio of NH3:PO4 is adjusted to 0.6 in the case of monoammonium phosphate and to 1.4 in the case of diammonium phosphate. The heat of reaction raises the slurry temperature to the boiling point and some moisture evaporates. Further addition of ammonia, so that the NH3:PO4 ratio increases to 1.0 (monoammonium phosphate) and to 2.0 (diammonium phosphate), generates additional heat to evaporate water during the granula‐ tion [84],[101].

Diammonium phosphate provides the following additional favorable factors [84]:


any grade between SSP and TSP can be produced, but the usual range is 25 – 35% P2O5. The processes and used equipment are the same as for SSP. Enriched superphosphate may be useful product for the applications in sulfur-deficient areas where SSP would supply more sulfur than necessary. One advantage is that mixed acid of proper concentration can be obtained by mixing concentrated sulfuric acid (93 – 98% H2SO4) with diluted phosphoric acid (30% P2O5),

results from the presence of small amounts of phosphorus in iron ores. When the iron ore is burnt with limestone in the presence of air, calcium phosphate and calcium silicate appear as slag. Thus, the mixture of calcium phosphate and calcium silicate is known as Thomas (basic or phosphatic) slag and contains 14 – 18% of P2O5 and 40% of lime. The following reactions

Ammonium phosphate (ammoniated superphosphate) was prepared from phosphate ore

Anhydrous ammonia passes through triple superphosphate in a rotary drum ammoniator where **reaction 32** takes place to evolve considerable amount of heat to granulate the am‐

15 Also known as Thomas phosphate powder. The utilization as fertilizer was firstly tried as a manure on fields in Germany, and in November 1883, HERREN HOYRMANN and MEYER were able to report to the German Royal Agricultural

moniate superphosphate, which is subsequently dried and bagged [101]:

Ca PO NH SO 2 H SO 2 NH H PO 3 CaSO 3 4 ( )( ) 2 2 + +® + 4 4 24 42 4 <sup>4</sup> (31)

is actually obtained as a by-product in iron industries. This

2 25 4 P 5 O 2 PO + ® (28)

25 3 4 ( )<sup>2</sup> 3 CaO P O Ca PO + ® (29)

CaO SiO CaSiO + ®2 3 (30)

thereby avoiding the need for concentrating the latter [88].

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

accompany the formation of Thomas slag [78],[99],[100]:

The composition of Thomas slag is shown in **Fig. 20**.

treated by ammonium sulfate and sulfuric acid [78]:

**9.3.5. Ammonium phosphate**

This product contains 61 – 73% P2O5.

Society the most excellent effect of its use [99].

**9.3.4. Thomas slag**

Thomas, basic or phosphatic slag15

**vi.** Less tendency of product caking in storage.

DAP can favorably be used in irrigation systems and in the production of liquid suspension fertilizers, because it is completely soluble in water [84].
