**9.3. Production of phosphatic fertilizers**

Phosphorus is essential to plant and animal life.9 It provides the matter for the skeletal bone structure in animals and for the cell membranes in both plants and animals. Phosphorus is also

<sup>8</sup> The by-product known as Thomas (basic or phosphatic) slag is obtained from the iron industry [78].

<sup>9</sup> Three primary (major) plant nutrients are N, P and K. The elements Ca, Mg and S are the secondary nutrients and Fe, Cu, Zn, Mn, B, Mo and Cl are the micronutrients [84]. Primary and secondary nutrients together with C, H and O (available from air and water) are termed as macronutrients [85].

the second most abundant element in the human body after calcium. It is essential for healthy formation of bones and teeth and energy storage system and helps maintain the blood sugar level. The largest and least expensive source of phosphorus is obtained by mining and beneficiation of phosphate rocks from numerous phosphate deposits throughout the world (**Section 7.2**). The principal use of these phosphate rocks is the manufacture of fertilizers (**Fig. 15**) [81],[82]. Phosphorus is important because it is an essential component of energy metabolism of all forms of life. Together with N and K, phosphorus presents the three macronutrients needed by all crops [83].

**Fig. 15.** The utilization of phosphate ore [82].

Most virgin soils in temperate climates contain enough phosphorus to support good crop production, but many soils in tropical climates are naturally deficient in phosphorus be‐ cause it is leached out during advanced stages of chemical weathering. Phosphorus can be added to deficient soils in the form of natural or artificial fertilizers,10 and for sustainably good yields, it must be added to all soils when they are cropped heavily for long periods. Phos‐ phate fertilizers are used extensively now in the developed countries, but in many less developed countries their use must be strongly increased to bring the crop production to acceptable levels [86].

Incidental phosphorus fertilization in the form of manures, such as plant and animal bio‐ mass, and other natural materials, such as bones, has probably been practiced since the agriculture began. Although the specific nitrification benefits were unknown, ARTHUR YOUNG described in the *Annals of Agriculture* in the mid-19th century the experiment evaluating a wide range of products including the poultry dung, the gunpowder, the charcoal, ashes and various salts. The results showed positive crop responses to certain materials. Benefiting from the achievements in chemistry by ANTOINE LAVOISIER and others, THEODORE DE SAUSSURE was perhaps the first to develop the concept that plants absorb specific mineral elements from the soil. The science of plant nutrition advanced considerably in the 19th century, owing to the contribu‐

<sup>10</sup> A fertilizer is any material, organic or inorganic, natural or synthetic, that supplies the plants with one or more chemical elements necessary for normal growth [85].

tion by CARL SPRENGEL, A.F. WIEGMANN, JEAN-BAPTISTE BOUSSINGAULT and JUSTUS VON LIEBIG. Based on the ubiquitous presence of phosphorus in soil and plant materials and the crop response to phosphorus-containing products, it became apparent that phosphorus is essential for the plant growth [87].

An important concept in the nitrification of plant was developed by J. VON LIEBIG in about 1840. It is known as "The Law of the Minimum" (**Fig. 16**). According to this concept, the plant growth is limited by particular growth factors that are in the shortest supply to the plant. Different factors could potentially control the rate of plant growth at different times during the crop growth. For example, the temperature might limit early spring growth, the moisture affects the growth during a droughty period and nitrogen supply may influence the growth even in cases when the previous factors are inactive [88].

**Fig. 16.** Illustration of the Liebig's law of the minimum [88].

the second most abundant element in the human body after calcium. It is essential for healthy formation of bones and teeth and energy storage system and helps maintain the blood sugar level. The largest and least expensive source of phosphorus is obtained by mining and beneficiation of phosphate rocks from numerous phosphate deposits throughout the world (**Section 7.2**). The principal use of these phosphate rocks is the manufacture of fertilizers (**Fig. 15**) [81],[82]. Phosphorus is important because it is an essential component of energy metabolism of all forms of life. Together with N and K, phosphorus presents the three

> Phosphoric acid

added to deficient soils in the form of natural or artificial fertilizers,10

Ammonization

Most virgin soils in temperate climates contain enough phosphorus to support good crop production, but many soils in tropical climates are naturally deficient in phosphorus be‐ cause it is leached out during advanced stages of chemical weathering. Phosphorus can be

yields, it must be added to all soils when they are cropped heavily for long periods. Phos‐ phate fertilizers are used extensively now in the developed countries, but in many less developed countries their use must be strongly increased to bring the crop production to

Incidental phosphorus fertilization in the form of manures, such as plant and animal bio‐ mass, and other natural materials, such as bones, has probably been practiced since the agriculture began. Although the specific nitrification benefits were unknown, ARTHUR YOUNG described in the *Annals of Agriculture* in the mid-19th century the experiment evaluating a wide range of products including the poultry dung, the gunpowder, the charcoal, ashes and various salts. The results showed positive crop responses to certain materials. Benefiting from the achievements in chemistry by ANTOINE LAVOISIER and others, THEODORE DE SAUSSURE was perhaps the first to develop the concept that plants absorb specific mineral elements from the soil. The science of plant nutrition advanced considerably in the 19th century, owing to the contribu‐

10 A fertilizer is any material, organic or inorganic, natural or synthetic, that supplies the plants with one or more chemical

Fertilizers

Normal superphosphate

Triple superphosphate

Ammonium phosphate

**Industrial Chemicals**

and for sustainably good

macronutrients needed by all crops [83].

Thermal reduction

**Fig. 15.** The utilization of phosphate ore [82].

**Elemental phosphorus**

acceptable levels [86].

elements necessary for normal growth [85].

Phosphate Rock

Acidulation

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

LIEBIG observed that dissolving bones in sulfuric acid enhanced the phosphorus availability to plants. Familiar with Liebig's work, JOHN LAWES and coworkers evaluated several apatitecontaining products as the phosphorus nutritional source for plants. They performed experiments in which they ultimately became the world's most famous agricultural experi‐ ment station — his estate in Rothamsted. Limited supply of bones prompted the develop‐ ments in the utilization of phosphate rocks where LAWES obtained the first patent concerning the utilization of acid-treated phosphate rock in 1842. The first commercial production of rock phosphate began in Suffolk, England, in 1847. Mining of phosphate in the United States began in 1867. Thus, the phosphorous fertilizer industry began [87].

Non-organic fertilizers contain mainly phosphate, nitrate, ammonium and potassium salts [90]. Phosphorus as an essential element for all life on earth and global food production is highly dependent on the use of fertilizers produced from phosphate rock [91]. Globally, about 150 million tones of phosphate rock are extracted each year for the production of

phosphate fertilizers (**Fig. 17**) and the demand for this finite resource is projected to increase to feed growing global population [92]. Due to higher cost per unit of nutrient and availabili‐ ty the bone meal, guano and other natural organic phosphate sources are of only minor commercial importance [89].

**Fig. 17.** The relation between phosphate rock and fertilizers [89].

The content of phosphorus in fertilizers (**Table 1**, the grade of fertilizer11) is usually ex‐ pressed as P2O5 (or as P4O10) and the phosphate rock grade is often listed in the trade publica‐ tions as **BPL,** 12 referring to "**bone phosphate of lime**" [93],[94], the common name for tricalcium phosphate (Ca3(PO4)2). Early workers believed that tricalcium phosphate was the chief constituent of phosphate rock. These commercial terms are widely used and the following conversion factors are included for the reference purposes [51],[85],[88],[89],[95].


<sup>11</sup> The grade of a fertilizer is the nutrient content expressed in weight percentages [85].

<sup>12 1%</sup> P2O5 = 2.1852% BLP [89]. The peculiar compound occurs in bones after the calcination, which gives a gelatinous precipitate when pouring calcium chloride into a solution of rhombic phosphate of soda or when adding ammonia to the solution of any phosphate of in acids [93].

<sup>13</sup> Denotes the triphosphate of lime. This salt is commonly called neutral phosphate and appears as a granular precipitate when rhombic phosphate of soda is added drop by drop to calcium chloride in excess [93].

<sup>14</sup> A product made by treating phosphate rock with sulfuric or phosphoric acid or by a mixture of two acids [85].


**Table 1.** Direct phosphate fertilizers [96].

phosphate fertilizers (**Fig. 17**) and the demand for this finite resource is projected to increase to feed growing global population [92]. Due to higher cost per unit of nutrient and availabili‐ ty the bone meal, guano and other natural organic phosphate sources are of only minor

> +H2SO4 +H2SO4 +HNO3

> > WPA

pressed as P2O5 (or as P4O10) and the phosphate rock grade is often listed in the trade publica‐

tricalcium phosphate (Ca3(PO4)2). Early workers believed that tricalcium phosphate was the chief constituent of phosphate rock. These commercial terms are widely used and the following

12 1% P2O5 = 2.1852% BLP [89]. The peculiar compound occurs in bones after the calcination, which gives a gelatinous precipitate when pouring calcium chloride into a solution of rhombic phosphate of soda or when adding ammonia to the

13 Denotes the triphosphate of lime. This salt is commonly called neutral phosphate and appears as a granular precipitate

14 A product made by treating phosphate rock with sulfuric or phosphoric acid or by a mixture of two acids [85].

**Phosphate Rock**

CaSO4·XH2O

Nitrophosphates

NPKs

) is usually ex‐

+Phosphate Rock

TSP

referring to "**bone phosphate of lime**" [93],[94], the common name for

DAP, MAP

The content of phosphorus in fertilizers (**Table 1**, the grade of fertilizer11

conversion factors are included for the reference purposes [51],[85],[88],[89],[95].

11 The grade of a fertilizer is the nutrient content expressed in weight percentages [85].

when rhombic phosphate of soda is added drop by drop to calcium chloride in excess [93].

+Ammonia

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

commercial importance [89].

SSP

NPKs

tions as **BPL,** <sup>12</sup>

**•** P2O5 = 0.4576·BLP;

**•** BLP = 2.1852·P2O5;

= BLP;

**•** P2O5 = P·2.2914 and P = P2O5·0.4364.

solution of any phosphate of in acids [93].

**•** P = 0.1997·BLP;

**•** TPL13

**Fig. 17.** The relation between phosphate rock and fertilizers [89].

Commercial phosphate rocks vary in grade from about 80 to 60 BLP. Most international trade involves higher-grade phosphate rocks and lower-grade rocks are often used at local process‐ ing facilities [88].

Phosphorus is utilized in fully oxidized and hydrated forms as orthophosphate. Plants typically absorb either H2PO4 − or HPO4 2− depending on the pH of growing medium. However, under certain conditions, plants might absorb soluble organic phosphates, including nucleic acids. A portion of absorbed inorganic phosphorus is quickly combined into organic mole‐ cules upon the entry into the roots or after it is transported into the roots. Total phosphorus in plant tissue ranges from 0.1 to 1%. Typical plant might contain approximately 0.004% P as DNA (deoxyribonucleic acid), 0.04% P as RNA (ribonucleic acid), 0.03% as lipid, 0.02% as ester and 0.13% as inorganics [87].
