A Zero-Waste Process for the Treatment of Spent Potliner (SPL) Waste

This work presents a deep analyses of an environmentally friendly process to recover all valuable minerals contained in the spent potliner (SPL) such as graphite carbon and aluminum fluoride (AlF 3 ) and production of sodium sulfate (Na 2 SO 4 ) and gypsum (CaSO 4 ) when H 2 SO 4 is used as the leaching agent. The level of emission of hazardous gases such as HCN (weak acid) and HF are minimized by direct scrubbing of the HCN in aqueous AgNO 3 solution to produce a stable silver cyanide (AgCN) product. The HF can be recovered as a liquid by condensation and used within the process and/or in production of metal fluorides such as the highly-soluble potassium fluoride (KF); a main source of fluoride in industry. Almost pure CO 2 gas is also recovered from the process gas streams.


Introduction
SPL is a hazardous solid waste material produced in the aluminum smelting industry [1]. It is generated when the graphite carbon and the refractory lining of the aluminum electrolytic cell reach the end of their useful life. After about 5 to 8 years of smelter operation, the cathode liner materials deteriorate and affect the aluminum electrolytic cell performance thus need to be replaced. Various factors contribute to cell lining degradation, for example, mechanical stress, electrolyte penetration and side reactions [2]. About 20 to 25 kg of SPL is generated per each ton produced of primary aluminum [3]. Worldwide aluminum production was about 63.6 million tons in 2018, generating about 1.4 million tons of SPL [4], which is a real environmental burden to the aluminum industry, and these figures are subject to increase [5]. In 2018, the United Arab Emirates (UAE) produced 2.64 million tons of aluminum and 29,040 tons of SPL ($11 kg SPL/ton aluminum). This SPL is distributed to the UAE cement industry for use as a feedstock and a fuel alternative [4].
SPL is classified as a hazardous waste by the US Environmental Protection Agency (EPA) since it contains significant amounts of toxic fluoride and cyanide composition [10]. Most of the chemical components of the SPL are direct constituents of the electrolytic bath that infuse the carbon cathode and subsequently the refractory lining. While some of the phases are additives to the electrolytic bath, others are the result of side reactions [11].
Chemical reactions within the cathode [6,8] and their calculated ΔH R and ΔG R at 30°C.

SPL properties
When the linings are removed from the pot they contain substantial amounts of sodium fluoride and sodium aluminum fluoride. In addition, the SPL contains Al metal, Na metal, Aluminum nitride (AlN), Aluminum carbide (Al 4 C 3 ), and sodium cyanide (NaCN) that absorbs and reacts with atmospheric water (humidity) and emits hazardous gases to the atmosphere. Table 4 shows potential gases evolved when the SPL is hydrolyzed, i.e. subjected to humidity, along with their calculated ΔH R and ΔG R at 30°C. However, some authors claim that reactions 19, 23 and 25 (in Table 4) produce Al 2 O 3 . However, it is well known that Al 2 O 3 results from Gibbsite {Al(OH) 3 } only after it is calcined (at temperatures above 400°C) [16].
Other reactions include those of ionic ferro-and ferri-cyanide with water [18]. For example, Note: (ia) is used in the HSC database for aqueous electrolyte (neutral), which is formed from undissociated aqueous species (ions).  Table 3. SPL main elements [14] and their major phases / compounds [15].

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 (AlF 2 OH) is of particular importance among the produced fluorides. It has a high market value and can be converted to aluminum fluoride (AlF 3 ), 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. CaF 2 ) can be used as part of the feed in cement production.

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 (CaF 2 ), sodium aluminum fluorides [e.g. cryolite (Na 3 AlF 6 ) and 5NaF.3AlF 3 complex], aluminum fluoride (AlF 3 ), aluminum hydroxyfluoride (AlF 2 OH) or aluminum hydroxyfluoride hydrate (AlF x (OH) (3-x) .xH 2 O, x = 1 or 2) [19]. The most valuable fluoride among these are AlF 3 and AlF 2 OH. The AlF 3 is constantly needed in aluminum smelters to maintain the cryolite balance [20].  The AlF 2 OH can be easily converted to AlF 3 , for example by its reaction with HF [12]. However, NaF has a low market value since it is not consumed as much as AlF 3 in a typical smelter. The CaF 2 is also of low market value and limited quality. Most of the AlF 3 recovery methods involve very complex and expensive processes mainly because they were not successful in precipitating AlF 3 due to its relatively high solubility in water [21]. Another problem is the AlF 3 meta-stability (200-250 g/L) which can delay its crystallization by several hours [22]. A combination of HF, fluorosilicic acid (H 2 SiF 6 ) and ammonium bi-fluoride (NH 4 HF 2 ) was used to precipitate AlF 3 by [23], however, these acids are highly toxic and/or expensive. In addition, calcination at 500°C to get the final AlF 3 product is required; thus, increasing the energy demand.
Leaching of the SPL CaF 2 and Na 3 AlF 6 by Al(NO 3 ) 3 .9H 2 O or AlCl 3 .6H 2 O was tried and found to be very slow (24 h, at 25°C [24,25]. The SPL fluorides (NaF, CaF 2 and Na 3 AlF 6 ) were leached as fluoride precipitates and the NaF and Na 2 CO 3 were removed from the SPL by water washing [26]. 76-86 mol% of the SPL refractory (Na 3 AlF 6 and CaF 2 ) were extracted by using 0.34 M Al 3+ 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 HNO 3 and 0.36 M Al(NO 3 ) 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 MgF 2 and CaF 2 , respectively.
Bishoy [28] subjected the SPL to NaOH leaching first followed by HNO 3 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 HNO 3 , 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 Na 3 AlF 6 and Al 2 O 3 compounds starting with NaOH leaching. However, they recovered 96.2% of the CaF 2 and NaAl 11 O 17 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 CaF 2 and NaAl 11 O 17 followed by NaOH for leaching of Na 3 AlF 6 and Al 2 O 3 . 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 H 2 SO 4 to recover HF form the SPL. The cake obtained contains graphite powder, aluminum hydroxide {Al(OH) 3 } and alumina (Al 2 O 3 ) while the filtrate contains fluorides and sulfates.
Li et al. [33] employed a two-step leaching process: (1) NaF is leached by water from the imbedded electrolyte, then (2) Na 3 AlF 6 , CaF 2 and NaAl 11 O 17 are leached using acidic anodizing wastewater (H 2 SO 4 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 (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 Na 2 SO 4 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 (NaAlO 2 .xH 2 O) [34,35]. Further, high purity AlF 2 OHÁH 2 O crystals were produced at F:Al ratio of 1.6 and pH of 4.9.
Ntuk et al. [34] used two methods of AlF 2 OH crystallization: (1) partial neutralization-crystallization for the bulk AlF 2 OH and (2) solution evaporationcrystallization for the beneficiation of the very small AlF 2 OH particles (< 30 μm), i.e. those below the acceptable size.
A leachate solution containing (AlF 2 + , Na 2 SO 4 ) was mixed with a controlled amount of NaOH (pH 4.5-5.5) and fed to a crystallizer to selectively produce AlF 2 OH.H 2 O, which was then filtered and separated from the Na 2 SO 4 solution. Around 76-86% of the fluoride was recovered from the SPL. It should also be noted that AlF 2 OH can be easily converted to AlF 3 by its reaction with HF [19].
The main properties of potential leaching acids and the after leaching produced acids are listed in Table 5.

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, Na 2 CO 3 , NaCN, and NaAlO 2 are soluble in water but with varying degrees and their solubilities mostly increase with the increase of temperature. Other SPL constituents such as NaAlSiO 4 , Na 3 AlF 6 , CaF 2 , 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, NaAlO 2 and Na 2 CO 3 ) 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, it will produce sodium formate and ammonia gas (for T > 50°C) [36] according to Eq. (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 (ΔG R = -75.3 kJ/mol at 30°C, see Table 4).
When NaCN is dissolved in excess water, hydrated sodium ion [Na(H 2 O) 4 ] + and a CN À ion are produced. However, [Na(H 2 O) 4 ] + is a strong acid conjugate that will not react with water): 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): However, this reaction (Eq. 3) is non-spontaneous (ΔG R = +59.6 kJ/mol, see Table A.5) and is not possible at room temperature, but its reverse reaction is possible (spontaneous, ΔG R = -59.6 kJ/mol) and well known: Compound Name Solubility at 25°C, g/L Solubility at 100°C, g/L NaF dissolves in water to produce hydrated sodium [Na(H 2 O) 4 ] + ion and F À ion: that further reacts with water to form HF(l) and OH À ion (the strongest base): NaAlO 2 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 (ΔG R = -23.8 kJ/mol, see Table A.5): NaAlO 2 is claimed by some authors to react with water at high temperature and with time and produce NaOH and Al(OH) 3 according to.
However, this claim is not true since the reaction is non-spontaneous (ΔG R = +25.6 kJ/mol, see Table A.5) and its spontaneity decreases with temperature (more +ΔG R ) regardless of the retention time. Na 2 CO 3 is also highly soluble in water. The kinds of ions produced are as follows: Again, the claim that Na 2 CO 3 reacts with H 2 O to produce NaOH and CO 2 (g) is also not true because it is non-spontaneous reaction (ΔG R = +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.

Process selection and the decision matrix
Bishoyi [28] made an extensive comparison to find out the best suitable leaching acid among H 2 SO 4 , HCl, HNO 3 , and perchloric acid (HClO 4 ) while fixing the L/S ratio and observed that H 2 SO 4 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: HClO 4 (strongest), HCl, H 2 SO 4 , and HNO 3 (weakest). At 25°C, the dissociation constant (pK a ) of HClO 4 , HCl, H 2 SO 4 , and HNO 3 are -8, -6.3, -3 (pK a,1 ), and -1.4, respectively [38]. The larger the pK a of an acid, the smaller its extent to dissociate at a given pH (i.e. the weaker the acid). Strong acids have pK a values ≤ -2. Note: pK a = pH -log 10  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 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, F i = Factor weight/Σ factor weights. Overall score = Σ F i x Score i . Based on that, the overall score in decreasing order is as follows: H 2 SO 4 (highest), HNO 3 , HCl, and HClO 4 (lowest).
In this work, we have calculated the change in the heat of reaction (ΔH R ) and the change in the Gibbs free energy of reaction (ΔG R ) for the reactions of the individual constituents of the SPL waste.  Table A.5) as well as for the reactions with H 2 SO 4 of potential trace materials that might present in the SPL (see Table A.6).
The operating conditions for these acids are as follows: H 2 SO 4 liquid at room temperature, liquid HNO 3 , HCl gas, and HClO 4 gas. The commercial grades of these acids are usually available at 98 wt% H 2 SO 4 , 68 wt% HNO 3 (pH = 1.2), 34-36 wt% HCl (pH = 1.1), and 70 wt% HClO 4 . 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 CO 2 ), SiO 2 along with the existing graphite carbon. However, H 2 SO 4 produces insoluble gypsum (CaSO 4 ) and soluble sodium sulfate (Na 2 SO 4 ) along with other soluble salts that need to be crystallized and separated (i.e. AlF 2 OH and/or AlF 3 ). However, the other leaching acids produce two soluble salts along with AlF 2 OH and/or AlF 3 that makes separation more difficult. Table 10 shows the generated intermediate and final products when H 2 SO 4 , HNO 3 , HCl, or HClO 4 , are used as the leaching acids. Based on that, the H 2 SO 4 as a leachant seems to have more advantages above the other leaching acids, among which is the production of Na 2 SO 4 ; one of the most profitable sodium salts. Thus, in the next discussion we will concentrate on leaching the SPL constituents by H 2 SO 4 solution. Lastly, it should be noted that the aluminum salts Al 2 (SO 4 ) 3 , Al(NO 3 ) 3 , AlCl 3 , and Al(ClO 4 ) 3 behave as acidic or basic solutions in water. For example, in Al 2 (SO 4 ) 3 , the SO 4 2À anion is neutral while the Al 3+ is not. In the reaction: the produced H 2 SO 4 , which is a strong acid, dissociates in the aqueous phase to form 2H + and SO 4 2À 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 Na 3 AlF 6 and CaF 2 ). This conclusion is used here as a basis for the selection of the SPL acid leaching process.

Leaching of the SPL individual constituents by H 2 SO 4 solution
The leaching process starts with the dissolution of the water-soluble compounds of the SPL (namely, NaF, NaCN, Na 2 CO 3 , and NaAlO 2 ) in the H 2 SO 4 solution rather than leaching in water followed by the acid. However, leaching of these four compounds in water is possible but it is very slow and requires large vessels.
Leaching reactions of the above-mentioned water-soluble compounds with H 2 SO 4 are presented by Eqs. (11) to (14). See reactions R1 to R4 in Table A On the other hand, the graphite present in SPL is the only compound that does not react with acids (e.g. H 2 SO 4 ), alkalis (e.g. NaOH) or acidic Al 3+ solution. However, the reactions of the three other insoluble compounds present in the SPL (namely, NaAlSiO 4 , Na 3 AlF 6 , and CaF 2 ) are explained below. An alternative to this two-step leaching process expressed by Eqs. (16) and (18), the Na 3 AlF 6 can be leached with an acidic Al 3+ solution comprised of Al(OH) 3 and H 2 SO 4 , which was found to be more effective than leaching with an acid only or an alkali only [41,19]. This acidic Al 3+ solution can be prepared according to Eq. (20): and the reaction of Na 3 AlF 6 with the above solution gives However, the Al 2 (SO 4 ) 3 (or acidic Al 3+ ) solution is already produced by Eqs. (14) and (17)  But again, CaF 2 can also react (spontaneously) with the acidic Al 2 (SO 4 ) 3 produced by Eqs. (14) and (17) to give CaSO 4 precipitate and aqueous AlF 3 :

Process description
In this work, we propose a process for leaching of the main constitutes of the SPL waste by H 2 SO 4 solution. The combination of Figures 1, 2 and 3 constitute the process flow diagram (PFD) of the proposed leaching process. Note: The numbers in red color beside the stream numbers on these figures, are the stream input temperature (30°C) or the calculated temperature using heat of mixing and reaction thermochemical data along with the energy balance equations. Most of the acid leaching reactions are exothermic (-ΔH R ) except those appearing in bold numbers in the ΔH R column of Table A.1 in particular.
The collected SPL waste first passes through crushing and grinding steps. The resulting SPL fines are fed to an agitated semi-batch reactor filled with a preprepared H 2 SO 4 solution. To ensure that all the SPL particles are sufficiently exposed to the solution, a 2.5 M H 2 SO 4 (with 5 wt% excess) is used along with a recommended L/S ratio of 2.52 liters of H 2 SO 4 acid solution per kg of SPL [19]. The reactor contents should be kept under agitation for 2-4 h. A 40,000 tons of SPL is assumed to be processed annually (or 5930 kg/h based on a stream factor of 0.77). However, a total of 220 working days per year (batch-wise operation, 22 working days per month, and allowing 2 months for shutdown and maintenance, i.e. stream factor = 0.6) is suggested elsewhere [19].
Considering the composition ranges of the SPL main constituents reported in [12] and presented in Table 2, the composition, the mass and molar flow rates based on the SPL upper composition limit are given in Table 11.
The products generated during processing are classified into three categories or streams: (1) gaseous stream (HCN, HF and CO 2 ), (2) insoluble products stream (graphite, gypsum and SiO 2 ), and (3) soluble products stream (aluminum fluorides and sodium salts, mainly, Na 2 SO 4 ). Details on processing of each of these streams are given below and demonstrated in Figures 1, 2 and 3 generated by the authors. 1. During the leaching step, a gas stream (mainly, HCN, HF and CO 2 ) leaves reactor R-101, cooled (not shown on the PFD) and then sent to a gas emissioncontrol scrubber (T-101) where the HCN gas is scrubbed by its reaction with a silver nitrate (AgNO 3 ) solution sprayed at the top. See Figure 1. This reaction is spontaneous and exothermic. As a result, silver cyanide (AgCN) is produced according to Eq. (26). See reaction R8 in Table A.1.
The AgCN is insoluble in water, but it is slightly soluble in aqueous HNO 3 . The AgCN, is separated from the aqueous solution via filter F-103. The AgCN salt is stable at ambient conditions and is very valuable in gold extraction. However, it is highly toxic by ingestion and its contact with skin and eyes can cause severe irritation. It has a LD 50 oral (rat) of 123 mg/kg.
Note: It should be mentioned that no reaction will take place between aqueous AgNO 3 used in Eq. (26) and HF(l), HF(g) or CO 2 , since these reactions are non-spontaneous at temperatures ≤90°C.
The HF can be recovered as a liquid from the HF-CO 2 gas mixture by cooling/ condensation in E-101 to below its condensation temperature (at its partial pressure in the gas stream). The remaining gas from E-101 is sent to a CO 2 recovery unit. The recovered HF liquid is pumped (P-101) where part of it is used within the process to ensure that all the remaining aluminum sulfate is converted to AlF 3 (as explained below). The remaining part of the HF liquid can be sold as is or converted to potassium fluoride (KF); an important source of fluoride in many industries.
On the other hand, the normal boiling points of HF and HCN are 25.6°C and 19.5°C, respectively. Thus, one much better option (and much cheaper than scrubbing by AgNO 3 solution) is the condensation of the HF gas followed by the condensation of HCN gas at their partial pressures in the gas phase stream leaving reactor R-101. This option avoids using the very expensive AgNO 3 salt, but in this case, the condensed HCN must be destroyed by direct oxidation or it can be converted to a stable NaCN (soluble) salt by reacting HCN liquid with NaNO 3 (very cheap). But still a reactor and a separator are needed. In either case, the resulting gas stream needs to be sent to the CO 2 recovery unit.
2. After completion of the leaching step, the slurry mixture is sent to filter F-101 where the insoluble solids (SiO 2 , graphite and gypsum) are separated from the aqueous solution containing soluble intermediate and final products (Na 2 SO 4 , AlF 3 (and/or AlF 2 OH), remaining Al 2 (SO 4 ) 3 , unreacted H 2 SO 4 , and water).
The insoluble solids stream is sent to reactor R-103 where the SiO 2 is reacted with aqueous NaOH to produce soluble sodium silicate (Na 2 SiO 3 ) according to reaction (27). See reaction R9 in Table A.1.
which is then separated from the graphite-gypsum solid mixture via filter F-102. See Figure 2. The Na 2 SiO 3 in the aqueous solution can then be saturated by evaporation and precipitated as Na 2 SiO 3 crystals (not shown on the PFD).
The graphite and gypsum can be then separated from each other in a froth flotation unit (FF-101) where an oil (e.g. 1-10 wt% kerosene) in water is used, along with air bubbling and slow agitation. See Figure 3. The recommended particle size for froth flotation lies between +25 and 75 μm [42]. The hydrophobic graphite along with kerosene floats up as a froth while the hydrophilic gypsum along with water settles to the bottom of the unit. The graphite-kerosene stream is sent to filter F-106 to recover the graphite and recycle the kerosene back to the froth flotation unit. Similarly, the gypsum-water stream is sent to filter F-107 to recover the gypsum and recycle the water back to the froth flotation unit.
It should be mentioned that we have experimentally separated the graphite carbon from gypsum (using a kerosene/water volumetric ratio = 0.1 along with air bubbling at room temperature).
3. The aqueous phase from filter F-101 is cooled in E-102 and then sent to reactor R-102, where the remaining Al 2 (SO 4 ) 3 is converted to AlF 3 (and/or AlF 2 OH) by its reaction with part of the recovered HF liquid, according to the relatively high spontaneous Eq. (28) (ΔG R = -196.65 kJ/mol). at 30°C. See reaction R10 in  Table 11. Normalized composition of the SPL main constituents used in this work.
Due to the presence of fluoride ions in R-102, the dominant crystal species will be AlF 3 . However, the reaction between Na 2 SO 4 and HF(l) is much less competent than Eq. (28) since it is much less spontaneous (ΔG R = -32.7 kJ/mol). See reaction R1 in and at the same time to maintain the solution in RC-101 at a pH of 4.5-5.5; required to saturate and precipitate AlF 3 [19], noting that the solubility of AlF 2 OH (and AlF 3 ) decreases with the increase of the pH.
Any AlF 2 OH produced can be easily converted to AlF 3 by its reaction with some of the HF liquid recovered earlier, according to the spontaneous presented by Eq. (30). See reaction R12 in Table A.1. or, and Thus, the reaction presented by Eq. (30) can be carried out before the addition of the NaOH solution.
The crystals produced in the reactor-crystallizer RC-101 are separated via filter F-104 as AlF 3 cake. To remove the impurities from the AlF 3 , the stream needs to be washed with fresh water. The AlF 3 is then dried, cooled and stored.
The filtrate leaving filter F-104 is sent to the evaporator-crystallizer EC-101, where the Na 2 SO 4 solution is saturated by flash evaporation under vacuum and Na 2 SO 4 is crystallized and separated via filter F-105. See Figure 3. The Na 2 SO 4 crystals can be further dehydrated and dried before being stored.
Lastly, the water vapor leaving EC-101 is condensed in E-103 and collected for reuse within the process, along with other recovered water from the various streams of the above described process.

Preliminary economic analysis
A preliminary economic analysis has been made on the above proposed process (assuming a theoretical 100% conversion and/or recovery) following the guidelines of ref. [43]. The amounts and costs of raw materials used as well as the amounts and market prices of the materials produced are listed in Table 12. The annual cost or price of a given material = amount (kg/h) x unit cost or price ($/kg) x 6475.2 (h/year). The 6475.2 factor comes from 0.77 x 24 x 365. We made a preliminary design for the process equipment and estimated the fixed capital cost of the plant excluding land, FCI L , to be 27.32 M$.
The number of operators per job was estimated based on Eq. (33): N OL ¼ 6:29 þ 31:7 P 2 þ 0:23 * N np À Á 0:5 (33) where P stand for particulate (solid) and N np for non-particulate (fluid) handling equipment (P = 1 for FF-101, N np = 15). The total number of operators required over the year = 4.47 N OL . The salary per operator was assumed to be $49000.
The FCI L along with the estimated annual costs of labor C OL , raw materials C RM , utilities C UT , and waste treatment C WT (given in Table 13) were used to calculate the cost of manufacturing excluding depreciation, COM d , according to Eq. (34): The calculated COM d = 21.73 M$/year. Now, assuming priceless produced HNO 3 , Na 2 SiO 3 , CO 2 and output water, the income from main sales (revenue, R) was found to be 38.09 M$/year. Also, since AgNO 3 and AgCN are very expensive and sharply affect the profitably of the process, this option has been excluded in the economic analysis.  The input data used for generating the cumulative cash flow analysis are presented in Table 14. The discounted cumulative cash flow diagram for the above process analysis is shown in Figure 4. Following [43] economic analyses and using the data presented above, and assuming an interest rate of 10%, a tax rate of 20%, the calculated net present value, NPV = 42.24 M$, the discounted payback period, DPBP = 2.38 years, and the discounted cash flow rate of return, DCFROR = 31.73%.

Conclusions
In this work an environmentally friendly process to recover the valuable elements contained in the SPL is presented and deeply analyzed. The decision to use H 2 SO 4 as a leachant was justified through deep analysis. The proposed process along with the process flow diagram and complete material balance results have been explained and included. Interest rate, i 10 % Table 14.
Input data for discounted cumulative cash flow analysis. The recovered materials include graphite carbon, aluminum fluoride (AlF 3 ), sodium sulfate (Na 2 SO 4 ), and others when H 2 SO 4 is used as the leaching agent. The level of emission of the hazardous gases such as HCN and HF are minimized. The recovered HF liquid is partially used within the process. The remaining HF can be used in production of potassium fluoride (KF). Also, CO 2 gas can also be recovered from the process gas streams.
The economic analyses indicate that the process will be profitable under the conditions stated in this work. The process net present value, NPV = 42.24 M$, the discounted payback period, DPBP = 2.38 years, and the discounted cash flow rate of return, DCFROR = 31.73%.