**8.6. Calcination**

**8.5. Removing of carbonates**

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

environmental hazards and they can be recycled [1].

Suggested reaction between acetic acid and carbonates is [1],[16],[29]:

is driven by the surface chemical reaction kinetic model: (1 - (1 - *α*)1/3) [30].

lactic acids (**Eq. 11**) can be used to dissolve carbonate minerals [16],[30]:

4 2

CaSO 2H O

( ) 3 24 2 <sup>2</sup> <sup>3</sup>

Ca CH OO H SO 2 H O 2 CH COOH

++®

The removal of carbonates from phosphate rock has been the focus of significant research efforts. Several countries have large deposits of phosphate rock that contain significant amounts of calcite (CaCO3) and dolomite (CaMg(CO3)2). The calcination of phosphate ores to remove carbonates is expensive because of high costs of energy. Calcination is practiced commercially at several phosphate rock mining operations around the world, mainly to improve final product quality by removing minor amounts of carbonates and organic matter. Calcination is also used to remove carbonates where the cost of natural gas is very low [1],[6].

Calcium and magnesium carbonates are readily dissolvable in both mineral (strong acids) and organic acids (weak acids). In the case of calcareous phosphate ores, although mineral acids dissolve carbonates at high reaction rates, they also attack the phosphorus-bearing minerals and cause losses in the P2O5 content of the ore; hence, they are not appropriate if the inten‐ tion is only to beneficiate the ore not to dissolve phosphates. To avoid this problem, organic acids were studied as carbonate leaching agents, although their reaction rates are low. These organic acids may be expensive and will certainly add to the production cost. On the other hand, they are selective to leaching carbonates, their capital cost is low, they do not cause

The organic acids most commonly used in carbonate leaching are acetic acid, citric acid and formic acid. They are used for some specific advantages (may be the cost, availability, etc.).

The dissolution kinetics of calcareous material with acetic acid solution was found to fit the shrinking core model for the reaction-controlled process. The activation energy was deter‐ mined to be 41.0 kJ·mol−1, which is consistent with a chemically controlled reaction. The process

Acetic acid may be recovered by reversing the above reaction at high CO2 pressure in a separate reactor or by using sulfuric acid to precipitate calcium sulfate and to liberate acetic acid:

It is noted that the by-products such as calcium sulfate (gypsum) could be used and/or sold to lower the costs of acetic acid and its recovery by sulfuric acid (**Eq. 10**). Similarly, formic and

3 3 ( ) 3 22 <sup>2</sup> CaCO 2 CH COOH Ca CH OO CO H O + ® ++ (9)

+ × (10)

More than 10% of the world's marketable phosphates are produced by calcination. Tradition‐ ally, the heat treatment of phosphate ores is defined as heating up the ore to a certain temperature to obtain a product with specific properties. The main processes that take place during the thermal treatment of apatite ore are [1],[3],[31],[32]:


The calcination process of phosphate ore is schematically shown in **Fig. 12**.

**Fig. 12.** Illustration of the defluorination process of phosphate rocks [1].

There are various types of units that can be used for the calcination of phosphate ores, such as [1]:


**Fig. 13.** Schematic diagram of annular-shaft kiln (a) and fluidized-bed calciner (b) [34].

**iii. Rotary kilns**<sup>9</sup> [33],[34],[35]: are extremely versatile incineration systems. They differ greatly in size with respect to their diameter (150 – 390 cm) and length (1800 – 1350 cm). Basic rotary kiln is composed of a cylindrical, refractory-lined steel shell, supported on two or more trunnions. The kiln is gently sloped (usually up to 0.03 m/ m) and rotates slowly (1 – 5 rpm, the rotation rate is usually less than 2 rpm). The kiln may be operated in the co-current (parallel) or countercurrent mode (**Fig. 14**) with respect to the relative direction of gas and solid flow.

**3. Thermal decomposition of carbonates**, i.e. the calcination within the temperature range

There are various types of units that can be used for the calcination of phosphate ores, such

**i. Vertical-shaft kilns** [33],[34]: are the most popular type of kilns, having varying

**ii. Fluidized-bed reactors** (**calciners**) [33],[34]: the hog gases perform two functions: (1)

heights, diameters and constructional details. There are two types, namely mixed- (a) and unmixed-fuel type (b). The construction of a vertical shaft may be cylindrical, conical or a combination of both shapes with varying diameters in different zones

fluidize the particles and (2) transfer the heat to the particles (**Fig. 13**(**b**)). Since the fluidization is a function of particle size, only fine particles can be introduced as the

**4. Removal of fluorine**, i.e. the **defluorination** at temperatures higher than 1350°C.

The calcination process of phosphate ore is schematically shown in **Fig. 12**.

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

**Fig. 12.** Illustration of the defluorination process of phosphate rocks [1].

as [1]:

(**Fig. 13**(**a**)).

feed particles.

from 850 to 1000°C;

**Fig. 14.** Schematic representation of countercurrent flow rotary kiln [34].

The rate of movement of the material through the kiln may be estimated using several relationships, e.g.:

$$
\Theta = \frac{0.19 \text{ L}\_{\text{T}}}{\text{NDS}} \tag{12}
$$

where *Θ* is the residence time [min], *L*T is the length of the kiln [m], *N* is the kiln rotation velocity (rpm), *D* is the kiln internal diameter [m] and *S* is the slope of the

<sup>9</sup> Rotary kilns are synonymous with cement and lime kilns probably because of the history of their evolution and development.

kiln [m/m]. Since the rotary kiln is divided to zones, the relationships should, more appropriately, be used for several reasonably uniform zones along the kiln and the total residence time can be calculated as the sum of the residence times for the individual zones.

**iv. Traveling grate-kilns, rotary kilns systems** [36],[37]: use low strength, somewhat wet pellets. These pellets are placed in a uniform bed upon a traveling grate, hot air being blown upward from below. The dehydration and partial calcination occur on the grate. Pellets are then fed to a short rotary kiln. The example of grate-kiln technology for the thermal treatment of pellets is shown in **Fig. 15**. The main advantages of this system are controlled feed rate, no flushing of materials into the kiln, no segregation of raw material due to different shapes and densities, avoid‐ ance of fluidization of the material bed, minimal dusting, etc.

**Fig. 15.** Thermal treatment of pellet using grate-kiln technology [36].

**v. Flash calciner** [31],[38],[39]: is one of more recent developments in calcination, but it is not really a kiln. There are three main elements including preheater (1), flash calciner (2, **Fig. 16**) and cooler (3).

The unique characteristics of flash calcination are particularly suited to pressing phosphate. Phosphate is a complicated mineral that varies from deposit to deposit with each ore requiring its own special processing consideration. During thermal treatment, it is important not to destroy the delicate crystal structure of phosphate by overheating. Flash calcination rates, very good oxygen contact and rapid cooling, all of these characteristics, are very important in the production of high-quality calcined phosphate. The operating conditions in the range from 800°C to 1000°C are required.

**Fig. 16.** Schematic representation of flash furnace [38].
