**1.4 Main products and side products of the SPL treatment**

Fluoride is the main product of the various SPL treatment processes. Fluorides are used as fluoropolymers (e.g. Teflon), which is utilized as a part of an extensive variety of uses such as cosmetic and reconstructive surgeries, paints, cookware, scratching semiconductor gadgets, cleaning, etching glass and aluminum and in evacuating rust. Aluminum hydroxyfluoride (AlF2OH) is of particular importance among the produced fluorides. It has a high market value and can be converted to aluminum fluoride (AlF3), which is one of the important key materials for aluminum metal production and constitutes a major cost in it [19].

Carbon is the main side product recovered during the SPL treatment; over 87% of which is in the form of graphite. Graphite behaves as a non-metal and a metal because it can resist high temperatures and it is a good electrical conductor. Graphite is also good as a refractory material because of its high-temperature stability and chemical inertness thus it is used in the production of refractory bricks. Furthermore, it can be used in production of functional refractories for continuous casting of steel and as lining blocks in iron blast furnaces due to its high thermal conductivity. In high-temperature applications (e.g. arc furnaces), it is used in production of phosphorus and calcium carbide. It can also be used as anode in aqueous electrolytic production of halogens (e.g. chlorine and fluorine), cathode in the aluminum industry, or as a fuel [4]. The other compounds (e.g. CaF2) can be used as part of the feed in cement production.

## **2. Recovery of fluoride values from the chemical leaching of SPL**

The majority of the chemical leaching processes of the SPL targeted fluoride recovery in the form of metal fluorides such as sodium fluoride (Villiaumite, NaF), calcium fluoride (CaF2), sodium aluminum fluorides [e.g. cryolite (Na3AlF6) and 5NaF.3AlF3 complex], aluminum fluoride (AlF3), aluminum hydroxyfluoride (AlF2OH) or aluminum hydroxyfluoride hydrate (AlFx(OH)(3-x).xH2O, x = 1 or 2) [19]. The most valuable fluoride among these are AlF3 and AlF2OH. The AlF3 is constantly needed in aluminum smelters to maintain the cryolite balance [20].

The AlF2OH can be easily converted to AlF3, for example by its reaction with HF [12]. However, NaF has a low market value since it is not consumed as much as AlF3 in a typical smelter. The CaF2 is also of low market value and limited quality.

Most of the AlF3 recovery methods involve very complex and expensive processes mainly because they were not successful in precipitating AlF3 due to its relatively high solubility in water [21]. Another problem is the AlF3 meta-stability (200–250 g/L) which can delay its crystallization by several hours [22]. A combination of HF, fluorosilicic acid (H2SiF6) and ammonium bi-fluoride (NH4HF2) was used to precipitate AlF3 by [23], however, these acids are highly toxic and/or expensive. In addition, calcination at 500°C to get the final AlF3 product is required; thus, increasing the energy demand.

Leaching of the SPL CaF2 and Na3AlF6 by Al(NO3)3.9H2O or AlCl3.6H2O was tried and found to be very slow (24 h, at 25°C [24, 25]. The SPL fluorides (NaF, CaF2 and Na3AlF6) were leached as fluoride precipitates and the NaF and Na2CO3 were removed from the SPL by water washing [26]. 76–86 mol% of the SPL refractory (Na3AlF6 and CaF2) were extracted by using 0.34 M Al3+ solution at 25°C in 24 h.

After an initial water wash to leach NaF, followed by a single-leaching step using 0.5 M HNO3 and 0.36 M Al(NO3)3 at 60°C [27], a total of 96.3% of the remaining fluoride was recovered along with 100% of the Mg and 90% of the Ca originally present in the SPL as MgF2 and CaF2, respectively.

Bishoy [28] subjected the SPL to NaOH leaching first followed by HNO3 leaching at various combinations of temperatures and liquid/solid ratios. The contribution of the alkali and acid concentrations on the leaching process was found to be 51.80% and 2.61%, respectively. The best combination (2.5 M NaOH, 5 M HNO3, 4.5-liter solution/kg SPL (or simply, L/S ratio), and 75°C) resulted in only 50.62% leaching of the SPL compounds.

Shi et al. [29] used a two-step alkaline-acidic leaching process to separate the cryolite from SPL and to purify the graphite carbon. Their results showed a recovery of 65.0% of soluble Na3AlF6 and Al2O3 compounds starting with NaOH leaching. However, they recovered 96.2% of the CaF2 and NaAl11O17 compounds in the following HCl leaching step. By combining the acidic and alkaline leaching solutions, 95.6% of the cryolite precipitates (at pH = 9, T = 70°C, and time = 2 h) with a 96.4% purity.

Parhi & Rath [30] adopted a similar two-step leaching process to recover carbon and cryolite fractions from the SPL. They used HCl for leaching of CaF2 and NaAl11O17 followed by NaOH for leaching of Na3AlF6 and Al2O3. A maximum leaching efficiency of 86.01% was achieved at (10 M HCl, 1.5 M NaOH, 4.5 L/S ratio and 100°C). The carbon recovery increased from 42.19% to 76.85% after treatment.

Zhao (2012) [31] presented a leaching process using water and H2SO4 to recover HF form the SPL. The cake obtained contains graphite powder, aluminum hydroxide {Al(OH)3} and alumina (Al2O3) while the filtrate contains fluorides and sulfates.

Cao et al. [32] recovered fluoride and carbon from the SPL by a water washing followed by leaching with aluminum sulfate {Al2(SO4)3.18H2O} solution at 25°C for 24 h. The carbon recovery achieved was 88%. Al2[(OH)0.46F0.54].6H2O and 5NaF.3AlF3 precipitated at (90°C, pH 5.5, 3 h) with a maximum fluoride recovery of 99.7%. The main products after calcination were AlF3 and 5NaF.3AlF3.

Li et al. [33] employed a two-step leaching process: (1) NaF is leached by water from the imbedded electrolyte, then (2) Na3AlF6, CaF2 and NaAl11O17 are leached using acidic anodizing wastewater (H2SO4 solution). Then the electrolyte components are precipitated from the mixed filtrates of steps (1) and (2). Most of the NaF in the SPL was dissolved in step (1); the residual electrolyte was mainly cryolite (with 0.95% NaF). The purity of the carbon recovered was about 95.5% under

*A Zero-Waste Process for the Treatment of Spent Potliner (SPL) Waste DOI: http://dx.doi.org/10.5772/intechopen.99055*

(80°C; L/S = 8 L/kg; 300 rpm; 3 h). The cryolite recovery from the mixed filtrate at (75°C; 4 h; pH 9; F/Al ratio of 6:1) was 98.4% while the Na2SO4 crystals purity was 92.0%.

The solubility of aluminum hydroxyfluoride at 30–70°C and its precipitation from synthetic solutions was studied by [34]. Their results suggest that when NaOH is used for the pH adjustment, a high F:Al ratio as well as higher pH were problematic because of the competitive co-precipitation of sodium fluoroaluminates hydrates (NaAlO2.xH2O) [34, 35]. Further, high purity AlF2OHH2O crystals were produced at F:Al ratio of 1.6 and pH of 4.9.

Ntuk et al. [34] used two methods of AlF2OH crystallization: (1) partial neutralization-crystallization for the bulk AlF2OH and (2) solution evaporationcrystallization for the beneficiation of the very small AlF2OH particles (< 30 μm), i.e. those below the acceptable size.

A leachate solution containing (AlF2 + , Na2SO4) was mixed with a controlled amount of NaOH (pH 4.5–5.5) and fed to a crystallizer to selectively produce AlF2OH.H2O, which was then filtered and separated from the Na2SO4 solution. Around 76–86% of the fluoride was recovered from the SPL. It should also be noted that AlF2OH can be easily converted to AlF3 by its reaction with HF [19].

The main properties of potential leaching acids and the after leaching produced acids are listed in **Table 5**.

#### **2.1 Solubility of SPL constituents in water**

Water leaching is a process that can extract a substance by its dissolution in water. Some of the SPL constituents such as NaF, Na2CO3, NaCN, and NaAlO2 are soluble in water but with varying degrees and their solubilities mostly increase with the increase of temperature. Other SPL constituents such as NaAlSiO4, Na3AlF6, CaF2, and C are insoluble in water even at high temperatures (say, 100°C). **Table 6** shows the SPL individual constituents'solubilities in water at 25 and 100°C.

The hydrolysis of some of the SPL individual constituents (namely, NaCN, NaF, NaAlO2 and Na2CO3) is discussed below.

NaCN when mixed with water or come in contact with aquatic species, the results will be detrimental to the health of that species. When NaCN is hydrolyzed,


#### **Table 5.**

*Some properties of mineral acids (sought for SPL leaching) and after-leaching produced acids.*


#### **Table 6.**

*Solubility of the SPL individual constituents in water at 25 and 100°C.*

it will produce sodium formate and ammonia gas (for T > 50°C) [36] according to Eq. (1):

$$\text{NaCN} + 2\text{H}\_2\text{O} \leftrightharpoons \text{HCOONa} (\text{ia}) + \text{NH}\_3(\text{g}) \tag{1}$$

where (ia) refers to aqueous electrolyte (neutral) formed from undissociated aqueous species. However, the above reaction (Eq. 1) is very slow [37] although it is spontaneous (ΔGR <sup>=</sup> ‐75.3 kJ/mol at 30°C, see **Table 4**).

When NaCN is dissolved in excess water, hydrated sodium ion [Na(H2O)4] <sup>+</sup> and a CN� ion are produced. However, [Na(H2O)4] <sup>+</sup> is a strong acid conjugate that will not react with water):

$$\text{NaCN} + 4\text{H}\_2\text{O} \rightarrow \left[\text{Na}(\text{H}\_2\text{O})\_4\right]^+ + \text{CN}^- \text{ (cold, pH} > 7) \tag{2}$$

According to [36], it was stated that when NaCN is mixed with water at room temperature, it can undergo the reaction given by Eq. (3):

$$\text{NaCN} + \text{H}\_2\text{O} \rightarrow \text{NaOH} + \text{HCN}(\text{g}) \tag{3}$$

However, this reaction (Eq. 3) is non-spontaneous (ΔGR = +59.6 kJ/mol, see **Table A.5**) and is not possible at room temperature, but its reverse reaction is possible (spontaneous, ΔGR = -59.6 kJ/mol) and well known:

*A Zero-Waste Process for the Treatment of Spent Potliner (SPL) Waste DOI: http://dx.doi.org/10.5772/intechopen.99055*

$$\text{HCN} + \text{NaOH} \rightarrow \text{NaCN} + \text{H}\_2\text{O} \tag{4}$$

NaF dissolves in water to produce hydrated sodium [Na(H2O)4] <sup>+</sup> ion and F� ion:

$$\text{NaF} + 4\text{H}\_2\text{O} \rightarrow \left[\text{Na}(\text{H}\_2\text{O})\_4\right]^+ + \text{F}^- \tag{5}$$

that further reacts with water to form HF(l) and OH� ion (the strongest base):

$$\text{H}^-\text{(l)} + \text{H}\_2\text{O} \rightarrow \text{HF(l)} + \text{OH}^-\tag{6}$$

NaAlO2 is highly soluble in water and decomposes completely in highly alkaline solutions and turns to sodium tetra-hydroxy aluminate Na[Al(OH)4] or its ionic forms (ΔGR = -23.8 kJ/mol, see **Table A.5**):

$$\text{NaAlO}\_2 + 2\text{H}\_2\text{O} \to \text{Na} \left[ \text{Al}(\text{OH})\_4 \right] \tag{7}$$

NaAlO2 is claimed by some authors to react with water at high temperature and with time and produce NaOH and Al(OH)3 according to.

$$\text{NaAlO}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{NaOH} + \text{Al}(\text{OH})\_3 \downarrow (\text{amorphous}).\tag{8}$$

However, this claim is not true since the reaction is non-spontaneous (ΔGR = +25.6 kJ/mol, see **Table A.5**) and its spontaneity decreases with temperature (more +ΔGR) regardless of the retention time.

Na2CO3 is also highly soluble in water. The kinds of ions produced are as follows:

$$\text{Na}\_2\text{CO}\_3 + \text{H}\_2\text{O} \rightarrow 2\text{Na}^+ + \text{(CO}\_3\text{)}^{2-} + \text{H}\_3\text{O}^+ + \text{(OH)}^- \tag{9}$$

Again, the claim that Na2CO3 reacts with H2O to produce NaOH and CO2(g) is also not true because it is non-spontaneous reaction (ΔGR = +131 kJ/mol, see **Table A.5**).

On the other hand, **Table 7** shows the solubilities of the compounds produced after SPL acid leaching and/or during processing. These information are very helpful in devising the separation techniques of these products as discussed below in process description.

#### **2.2 Process selection and the decision matrix**

Bishoyi [28] made an extensive comparison to find out the best suitable leaching acid among H2SO4, HCl, HNO3, and perchloric acid (HClO4) while fixing the L/S ratio and observed that H2SO4 gave maximum leaching efficiency at 25°C. But as the temperature is increased from 25–100 °C, all of these acids gave rise to almost the same leaching percentage. However, all of the acids undergo complete ionization in water.

The order of decreasing strength of the four acids under investigation is as follows: HClO4 (strongest), HCl, H2SO4, and HNO3 (weakest). At 25°C, the dissociation constant (pKa) of HClO4, HCl, H2SO4, and HNO3 are -8, -6.3, -3 (pKa,1), and -1.4, respectively [38]. The larger the pKa of an acid, the smaller its extent to dissociate at a given pH (i.e. the weaker the acid). Strong acids have pKa values ≤ -2. Note: pKa = pH - log10[A�]/[HA], [HA] and [A�] are the molar equilibrium concentrations (mol/L) of the acid and its anionic part, respectively.

On the other hand, the corrosivity of an acid depends on its level of dissociation, its concentration and phase. A vapor phase acid is more corrosive than a liquid


#### **Table 7.**

*Solubility of the after-leaching SPL products at 25 and 100°C.*


#### **Table 8.**

*Values of the decision parameters sought for various leachant acids.*

phase acid. In addition, the corrosivity of an acid increases as temperature is increased. **Table 8** shows the values of the parameters used in process selection among the four leachant acids mentioned above.

**Table 9** shows the factors affecting process selection (decision matrix), factors weight and fraction among the sought leachant acids. In **Table 9**, Fi = Factor weight/Σ factor weights. Overall score = Σ Fi x Score i. Based on that, the overall score in decreasing order is as follows: H2SO4 (highest), HNO3, HCl, and HClO4 (lowest).

In this work, we have calculated the change in the heat of reaction (ΔHR) and the change in the Gibbs free energy of reaction (ΔGR) for the reactions of the


*A Zero-Waste Process for the Treatment of Spent Potliner (SPL) Waste DOI: http://dx.doi.org/10.5772/intechopen.99055*

**Table 9.**

*Decision matrix: Factor, factor weight, fraction (Fi), individual and overall scores sought for the leaching acids.*

individual constituents of the SPL waste. **Table A.1** to **A.4** in Appendix A show the calculated ΔHR and ΔGR at 30°C for the reactions with H2SO4, HNO3, HCl, and HClO4, respectively. Inspection of these values shows that most of these reactions are exothermic (-ΔHR) and spontaneous (-ΔGR). We have also calculated ΔHR and ΔGR for all other potential reactions of the SPL constituents with H2SO4 (see **Table A.5**) as well as for the reactions with H2SO4 of potential trace materials that might present in the SPL (see **Table A.6**).

The operating conditions for these acids are as follows: H2SO4 liquid at room temperature, liquid HNO3, HCl gas, and HClO4 gas. The commercial grades of these acids are usually available at 98 wt% H2SO4, 68 wt% HNO3 (pH = 1.2), 34–36 wt% HCl (pH = 1.1), and 70 wt% HClO4. Because of this, the higher the concentration of the acid available for use, the lower the molarity is required for leaching. However, in all cases, an alkali leachant (e.g. NaOH) needs to be used either before or after the acid leaching step. But in this work, we have decided to add NaOH after the acid leaching step.

All of these leaching acids produce the same acid gases (namely, HCN, HF and CO2), SiO2 along with the existing graphite carbon. However, H2SO4 produces insoluble gypsum (CaSO4) and soluble sodium sulfate (Na2SO4) along with other soluble salts that need to be crystallized and separated (i.e. AlF2OH and/or AlF3). However, the other leaching acids produce two soluble salts along with AlF2OH and/or AlF3 that makes separation more difficult. **Table 10** shows the generated intermediate and final products when H2SO4, HNO3, HCl, or HClO4, are used as the leaching acids. Based on that, the H2SO4 as a leachant seems to have more advantages above the other leaching acids, among which is the production of Na2SO4; one of the most profitable sodium salts. Thus, in the next discussion we will concentrate on leaching the SPL constituents by H2SO4 solution.


#### **Table 10.**

*Products resulting from SPL treatment as a function of leachant acid.*

Lastly, it should be noted that the aluminum salts Al2(SO4)3, Al(NO3)3, AlCl3, and Al(ClO4)3 behave as acidic or basic solutions in water. For example, in Al2(SO4)3, the SO4 <sup>2</sup>� anion is neutral while the Al3+ is not. In the reaction:

$$\text{Al}\_2(\text{SO}\_4)\_3 + \text{6H}\_2\text{O} \leftrightharpoons 2\text{Al}(\text{OH})\_3 + \text{6H}^+ + \text{3SO}\_4^{2-} \tag{10}$$

the produced H2SO4, which is a strong acid, dissociates in the aqueous phase to form 2H+ and SO4 <sup>2</sup>� ions, and as a result, the solution is considered acidic. For this reason, any of the above-mentioned aluminum salts, if present in the aqueous solution, can behave as acidic leachants for some of the SPL constituents (such as Na3AlF6 and CaF2). This conclusion is used here as a basis for the selection of the SPL acid leaching process.
