**9.2.4. Wet process**

Another applications of phosphoric acid include the treatment of surface of metals (**Sec‐ tion 10.8**), the utilization in dentistry [28],[29],[30],[31],[37],[38] (described in **Section 10.1.2**), phosphate binders [32], geopolymers [33], phosphoric acid fuel cells [34],[35],[36], gel-based electrolytes [39] and solid membranes [40],[41],[42] for fuel cells, the activation of carbon adsorbents [43],[44] and catalysts [45],[46], the modification of zeolites [47], catalytic decom‐ position of H2O2 [48] and organic syntheses, e.g. esterification [49],[50]. Direct applications of

Fertilizers

Metal cleaning

Rustproofing

Refractory bonding

Dental cements

Building blocks

Anodizing and chemical

polishing

electrodeposition of amorphous

Electroplating and

alloys Phosphorus chemicals

Clay soil stabilization

Catalysts **H3PO4**

Fuel cell electrolytes

Medicines

**Fig. 8.** Direct applications of orthophosphoric acid [2].

Animal feeds

NMR reference standard

Soft drinks and foodstuffs

**Fig. 7.** Sonoluminescence occurs as the bubble collapse under some specific conditions including very low vapor pres‐

Collapsing shockwave

Hot opaque plasma

Emissive shell

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

Outer bubble

phosphoric acid are shown in **Fig. 8**.

sure liquids [25].

Several companies began developing the hemihydrate technologies during the 1960s and fullscale plants began to appear during the 1970s [53]. The wet process (**Fig. 10**) is named as wet because the concentrated solution of sulfuric acid (93%) was used to digest the apatite ore [5], [26],[58]:

$$\begin{aligned} \text{Ca}\_{10} \text{(PO}\_4\text{)}\_6 \text{(OH)}\_2 + 10 &\text{H}\_2\text{SO}\_4 \rightarrow 10 &\text{CaSO}\_4 \text{(s)}\\ +6 &\text{H}\_3\text{PO}\_4 + 2 &\text{H}\_2\text{O} \end{aligned} \tag{17}$$

Since the ore is invariably contaminated by fluorapatite (Ca10(PO4)6F2) and calcium carbo‐ nate (CaCO3), hydrofluoric acid is formed during the dissolution process:

**Fig. 10.** The flow sheet for the production of orthophosphoric acid via the fission hemihydrate process (a) and the dihy‐ drate process (b) [12].

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{F}\_2 + 10\text{ H}\_2\text{SO}\_4 \rightarrow 10\text{ CaSO}\_4\text{(s)} + 6\text{ H}\_3\text{PO}\_4 + 2\text{ HF}\tag{18}$$

or

$$\begin{aligned} \text{Ca}\_{10} \text{(PO}\_4\text{)}\_6\text{F}\_2 + 10 &\text{H}\_2\text{SO}\_4 + 20 &\text{H}\_2\text{O} \rightarrow 10 &\text{CaSO}\_4 \cdot 2\text{H}\_2\text{O} \text{(s)}\\ +6 &\text{H}\_3\text{PO}\_4 + 2 &\text{HF} \end{aligned} \tag{19}$$

$$\rm CaCO\_3 + H\_2SO\_4 \rightarrow CaSO\_4 + CO\_2(g) + H\_2O \tag{20}$$

The dissolution of silicate minerals in the ore by HF leads to the formation of fluorosilicates, including volatile silicon tetrafluoride (SiF4) and hexafluorsilicic acid (H2SiF6) or its salts (Na2SiF6).

According to the kind of produced hydrate of calcium sulfate (**Fig. 11**), the wet process is further divided to:

**Fig. 11.** The influence of reaction conditions on the crystallization of calcium sulfate [53].

Ca PO F 10 H SO 10 CaSO s 6 H PO 2 HF 10 4 2 ( )<sup>6</sup> + ® ++ 2 4 <sup>4</sup> ( ) 3 4 (18)

Multistage countercurrent washing

CaSO4·2H2O filtration, multistage washing

Filtrate

CaSO4·0.5H2O conversion CaSO4·2H2O

Vacuum evaporation and cooler

CaSO4·2H2O to disposal or utilization

Filtrate form 1st stage

Sulfuric acid

to scrubbing tower

Water

+ + (19)

CaCO H SO CaSO CO g H O 3 24 4 2 2 +®+ + ( ) (20)

10 4 2 ( ) 2 4 <sup>2</sup> 4 2 ( ) <sup>6</sup>

+ +® ×

The dissolution of silicate minerals in the ore by HF leads to the formation of fluorosilicates, including volatile silicon tetrafluoride (SiF4) and hexafluorsilicic acid (H2SiF6) or its salts

According to the kind of produced hydrate of calcium sulfate (**Fig. 11**), the wet process is

Ca PO F 10 H SO 20 H O 10 CaSO 2H O s

3 4

6 H PO 2 HF

or

Apatite

**(a)**

Sulfuric acid cooling air

**(b)**

drate process (b) [12].

Reactor Reactor

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

Washing solution

> Product acid 54% P2O5

CaSO4·0.5H2O filtration

CaSO4·2H2O to disposal or utilization

Apatite Filtrate (dilute phosphoric acid)

Reactor Filter recipient

Product acid 28 to 30 % P2O5

**Fig. 10.** The flow sheet for the production of orthophosphoric acid via the fission hemihydrate process (a) and the dihy‐

CaSO4·2H2O filtration

To scrubbing tower

Sulfuric acid

(Na2SiF6).

further divided to:


Calcium sulfate and other insoluble impurities such as silica were filtered out and the vacuum distillation process was used to increase the concentration of phosphoric acid7 to 56%. The major part (about 85%) of phosphorous fertilizers is produced by wet process [26]. Since phosphate ores are a potential resource of rare-earth elements (REE) as well, the recovery of rare earths during the wet processing of phosphoric acid is investigated [59],[60],[61],[62], [63].

The crucial step in the decomposition of apatite is the formation of calcium sulfate. Its properties, in particular, its ability to be filtered, are very important, e.g. for the throughput optimization. The incorporation of phosphate in the crystal lattice of calcium sulfate reduces the phosphate yield and can render the calcium sulfate unusable in the building industry [12].

<sup>7</sup> The Karl Fisher titration was used to determine the amount of water in phosphoric acid [64].

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

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‐ fully [53].

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

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 was investigated.
