**8.5. Removing of carbonates**

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 environmental hazards and they can be recycled [1].

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

$$\text{CaCO}\_3 + 2\text{ CH}\_3\text{COOH} \rightarrow \text{Ca(CH}\_3\text{OO)}\_2 + \text{CO}\_2 + \text{H}\_2\text{O} \tag{9}$$

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

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:

$$\begin{aligned} \text{Ca(CH}\_3\text{OO)}\_2 + \text{H}\_2\text{SO}\_4 + 2 \text{ H}\_2\text{O} &\to 2 \text{ CH}\_3\text{COOH} \\ + \text{CaSO}\_4 \cdot 2\text{H}\_2\text{O} \end{aligned} \tag{10}$$

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

**Fig. 11.** Proposed flow sheet for leaching of phosphate rock/ores using formic acid [16].

$$\begin{aligned} \text{CaCO}\_3 + 2\text{ CH}\_3\text{CH}(\text{OH})\text{COOH} &\rightarrow \text{Ca}\{\text{CH}\_3\text{CH}(\text{OH})\text{OO}\}\_2\\ +\text{CO}\_2 + \text{H}\_2\text{O} \end{aligned} \tag{11}$$

The main factors investigated by the researchers were: leaching reagent, acid concentration, reaction time, liquid/solid ratio (pulp solid percent), temperature, particle size distribution, stirring speed and type and nature of ore [16].
