**8.7. Flotation**

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

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‐

**iv. Traveling grate-kilns, rotary kilns systems** [36],[37]: use low strength, somewhat

ance of fluidization of the material bed, minimal dusting, etc.

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

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

calciner (2, **Fig. 16**) and cooler (3).

**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

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.

individual zones.

Flotation is a selective separation process that consists of attaching hydrophobic particles to rising air bubbles to form a particle-rich froth on the suspension surface, which flows over the lip of the cell. Hydrophilic particles do not attach to the bubbles and settle at the bottom to be discharged. Flotation has been the workhorse of mineral industry for over 100 years and has been expanded into many other areas, including deinking of wastepaper for recycling, water treatment and separation of plastics, crude oils, effluents, microorganisms and proteins [40].

The beneficiation of phosphate ores using froth flotation method has been practiced for at least 65 years. Extensive research work has been carried out in the last 25 years on various phosphate-containing ores. Despite extensive research and industrial experience, there are some challenges remaining in particular in beneficiation of siliceous-, calcite- and heavy mineral-containing phosphate ores [41].

Despite the fact that the flotation of apatite is difficult due to its physicochemical properties being similar to other minerals present in phosphate ores [42],[43], the froth flotation is widely used in mineral processing technologies to separate finely ground valuable minerals from a mixture with gangue minerals initially present in a pulp. The technique involves the contact of air bubbles with the solids [44]. Flotation technology is also used to remove suspended impurities during the treatment of wastewater, water purification, recovery of bacteria, cereal cleaning, recovery of metal and colloidal matters and recovery of ions and surfactants from the solution [45].

**Fig. 17.** Mechanical flotation cell (a) and flotation column (b) [4].

Currently, more than half of the world's marketable phosphates are concentrated by the flotation process [46]. Two types of flotation machines are available [4]:


The comparison of both systems is shown in **Table 2** [4]. Phosphoric tailings are fine-grained rock produced from the flotation processes [47].


**Table 2.** Comparison of mechanical flotation cell and column flotation [4].

Flotation cells are usually designed to perform several functions simultaneously, some of them are [45]:


In most flotation plants, the cells are interconnected in batteries, and the first flotation stage, called roughing, permits quick rejection of most of the gangue and achieves high recoveries with low grades. A schematic circuit, which includes roughing, cleaning, re-cleaning and scavenging stages, is given in **Fig. 18**.

**Fig. 18.** Schematic representation of a flotation circuit [45]

**Fig. 17.** Mechanical flotation cell (a) and flotation column (b) [4].

**1.** Mechanical flotation cell (**Fig. 17**(**a**));

rock produced from the flotation processes [47].

Air induced or injected through the impeller to generate

Convectional plant operation history and knowledge base on

**Table 2.** Comparison of mechanical flotation cell and column flotation [4].

bubbles

mechanical cells

**Mechanical Flotation Cells Column Flotation**

Cell sizes ranging from ~0.1 to 350 m3 Available up to 4 m in diameter

Well known to operators and easier to operate Newer and less known by operators

**2.** Colum flotation cell (**Fig. 17**(**b**)).

Currently, more than half of the world's marketable phosphates are concentrated by the

The comparison of both systems is shown in **Table 2** [4]. Phosphoric tailings are fine-grained

Bubble-particle interaction through mixing by impeller Bubble-particle interaction through the countercurrent

Less favorable for the bubble-particle attachment Generally considered more favorable for the bubble-

Typical heights around 9 – 15 m

Produces smaller bubbles

particle attachment

No moving parts

Harder to operate

Internal or external spargers generate air bubbles

action-descending slurry and rising bubbles

Better metallurgical performance (grade and recovery) Axial mixing can significantly reduce the overall performance (especially in larger-diameter columns)

flotation process [46]. Two types of flotation machines are available [4]:

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

Flotation is a dynamic process [48]. A whole range of variables can affect the performance of flotation systems (**Fig. 19**), such as their operating variables, particle size, reagents, ore composition and also the presence of ionic species in water [42],[46],[49]. The suspension of soluble minerals such as apatite is bonding large amounts of ions that interact with the mineral surface and affects the flotation performance. The interactions of dissolved anions from minerals in a pulp with a collector can form insoluble surfactant salts, which can precipitate non-selectively on mineral surface. These ions are so called potential-determining ions of fluorapatite such as Ca2+, CaOH+ , PO4 3−, HPO4 2−, H2PO4 − , F<sup>−</sup> , H+ and OH<sup>−</sup> [46],[50]. The effect of water quality on the flotation process was described by LIU et al [51].

**Fig. 19.** Flotation system with interrelating subsystems (chemistry, equipment and operating components)(a) and so‐ dium oleate in an anionic collector that can be used to render apatite hydrophobic in alkaline environments (b) [4].

The selectivity of froth flotation processes is highly influenced by the specificity of integra‐ tions between minerals and reagents, which are used to control the hydrophobic/hydrophil‐ ic character of mineral/water interfaces [52].

The use of additives is a tool for the control of surface tension of the flotation system. Additives (flotation reagents) used in phosphate flotation are synthetic organic species. They are produced via the ethoxylation of fatty alcohols. Alcohols are obtained from vegetable oils or animal fats. Ethylene oxide comes from the petroleum industry. These reagents may exhibit variable molecular composition and number of carbon atoms in the hydrocarbon chain, as well as the presence of double bonds, different stereochemistry (*cis*-*trans* isomerism) and also several levels of ethoxylation. The additives employed in phosphate ore flotation contain the carbon chains of different lengths, with a predominance of 18 carbon atoms. The ethoxyla‐ tion level is represented by the average number of ethylene oxide groups in the molecule. Best results were achieved with three or four groups. The dosage of additives is 5% with respect to the collector dosage, reaching 10% under special conditions [53].

The organic reagents, such as guar gum, cashew gum, tannins, dextrin, ethyl cellulose and carboxymethylcellulose, are capable of acting as depressor in the flotation of igneous phos‐ phate ores. The performance of corn starches was consistently superior to that of those reagents [53],[54]. The depressing ability of starch and ethyl cellulose appears to be related to steric compatibility between the positions of cations present on the mineral surface and hydroxyl groups within the molecular structure of reagents [52].

The role of surface and porosity was investigated by ZHONG et al [55]. When the samples were not aged prior to the collector (potassium oleate) addition, the floatability was controlled by the dissolution (of calcium) and adsorption (of oleate) behaviors, which, in turn, were governed by the surface area. It appears that the surface constituted by pores had lower influence on the adsorption and dissolution characteristics than the external surface. This was suggested to be due to slow diffusion of calcium through the pores, which resulted in reduced dissolution rate, as well as the non-participation of a substantial portion of pores in the adsorption process. When the samples were aged prior to the oleate addition, the bulk

precipitation of calcium oleate complex was found to play a crucial role. Since the bulk precipitation is not an interfacial process, the effect of surface area was slighter with aged samples.

A critical review of reagents used in the flotation of phosphate ores was performed by SIS and CHANDER [56]. Based on the literature, it was concluded that the usage of surfactant mixtures has certain advantages over single surfactant as the synergistic effects between surfactant mixtures were observed during different experiments such as surface tension, contact angle, adsorption and flotation. The synergism of surfactant mixtures at air/liquid, liquid/oil and liquid/solid interfaces arises from the improvement of froth properties, emulsification of hydrocarbon oil (e.g. fuel oil) and homogenous adsorption of collector on the minerals and protection of the collector from harmful effect of dissolved ions in the presence of auxiliary surfactant.

The activation of apatite particles during dry milling may enhance the adsorption of re‐ agents, which favors the recovery of apatite. However, active defects may serve as the sites for the adsorption of water and some very fine gangue particles on the apatite surfaces, causing apatite particles to be less responsive to flotation. As a result, dry milling did not have much impact on the recovery and flotation kinetics of apatite [42].

The fact that microorganisms, both living and dead, and products derived from the organ‐ isms can function as flotation agents and flocculation agents is abundantly clear. They can modify the surfaces of minerals. They can function as flotation collectors and as flotation depressants and activators. In many cases, they or their products can function as specific flocculation agents [57].

Many strains of bacteria are able to adsorb Ca(II) and Mg(II) ions from aqueous solution and, in some cases, the adsorption can be very specific. For example, *Bacillus subtilis* typically binds Mg(II) much more readily than Ca(II). Bacteria can also adhere to the surfaces of minerals containing calcium and magnesium, either enhancing or depressing the flotation of these minerals. Since *B. subtilis* binds Mg(II) preferentially, it was reasoned that the adhesion to a mineral containing magnesium and calcium (dolomite) might be quite different from the adhesion to a mineral containing only calcium (apatite) and this difference could possibly be utilized in mineral processing. The experiments investigating the binding of Ca(II) and Mg(II) to *B. subtilis* cells were initiated, and anionic collector microflotation of pure dolomite and apatite mineral samples in the presence and absence of these bacteria was performed. Since Ca(II) and Mg(II) also bind to dolomite and apatite, the zeta-potential measurements as a function of pH in the presence and absence of these ions were performed in order to better elucidate the effect this binding may have on the attachment of *B. subtilis* to those two minerals [58].
