Open access peer-reviewed chapter

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

Written By

Samir I. Abu-Eishah, Manal D.M. Raheem, Fatma A.S. Aljasmi, Fatima M.O. Alameri, Amna G.R. Alblooshi and Intesar F.R. Alnahdi

Submitted: May 10th, 2021 Reviewed: June 24th, 2021 Published: July 31st, 2021

DOI: 10.5772/intechopen.99055

From the Edited Volume

## Waste Material Recycling in the Circular Economy

Edited by Dimitris S. Achilias

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## Abstract

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 (AlF3) and production of sodium sulfate (Na2SO4) and gypsum (CaSO4) when H2SO4 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 AgNO3 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 CO2 gas is also recovered from the process gas streams.

### Keywords

• aluminum production
• spent potliner (SPL)
• leaching by H2SO4
• aluminum fluoride recovery
• graphite carbon recovery
• zero-waste process

## 1. 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 compounds (in addition to a trace amounts of polycyclic aromatic hydrocarbons, PAH), which can have adverse impacts on the environment if not adequately disposed. Cyanides are highly toxic and must be destroyed or removed from the SPL before its disposal or reuse. SPL has a high pH value due to the presence of alkali metals and oxides that make it corrosive.

Some of the SPL constituents react with water and produce flammable, toxic and explosive gases such as H2, NH3 and CH4. Thus, SPL disposal is becoming one of the largest environmental concerns and the SPL stored around the world needs to be safely disposed.

Both the aluminum and fluoride species are very valuable materials and need to be recovered, preferably in the form of aluminum fluoride (AlF3) that can be recycled to the aluminum smelting plant to produce elemental aluminum. The graphite carbon also needs to be recovered and reused at least in manufacturing of cathodes for the aluminum electrolytic cells.

In this work, we are developing an environmentally-friendly process, while properly, safely and effectively disposing the other constituents of the SPL. In this process we aim to recover the aluminum and fluoride species, the graphite carbon, in addition to other side products, that at the end leads to zero-waste. In the discussion below, equations numbering (i) within the text, for i = 1, … n, stands for the final form of the reactions taking place during the leaching process with H2SO4 as well as the equations used in the process analyses.

Also, the numbering appearing in the tables stands for chemical reactions within the cathode (Table 1), potential gases that might evolve from the SPL reactive species when hydrolyzed (Table 4), other potential reactions (Table A.5), and SPL trace constituents’ reactions with H2SO4 (Table A.6).

### 1.1 SPL compounds generated during the aluminum smelting process

The aluminum smelting process involves electrolysis of alumina (Al2O3), dissolved in cryolite (Na3AlF6), in a cell having graphite electrodes and linings used to transmit current from the cathodic collector bar and to contain the molten Aluminum product and the alumina-containing electrolyte.

New lining materials of aluminum electrolytic cells are made from clean and virgin graphite materials. The cathode graphite material is typically 15–25% porous, but it gets penetrated by bath materials after the start of electrolysis [6]. Penetration is initiated by the metallic sodium Na(c), followed by the electrolyte [7]. The chemical reactions within the cathode result in the formation of various carbides, nitrides, cyanides, and others within the pot linings (refractory, cathodes, and sidewalls) [8].

The spent cathode contains a lot of fluoride and cyanide. During the extended operation of the electrolytic cell, fluoride is brought in by AlF3 and Na3AlF6 and is absorbed into the cell linings. Cyanides are produced by the chemical reaction between metallic sodium (from cryolite), atmospheric nitrogen penetrating into the cathode carbon through openings in the potshell and through the cathode carbon itself. Indicative examples of the chemical reactions that take place within the cathode are shown in Table 1 along with their calculated change in the heat of reaction (∆HR) and and change in the Gibbs free energy of reaction (∆GR) (using HSC Chemistry 6.1 software) at 30°C.

#Chemical Reaction∆HR, kJ/mol∆GR, kJ/mol
Na(C), CO, Na2CO3, and NaCN formation reactions:
16NaF + Al → 3Na(c) + Na3AlF6160.1111.8b
2O2(g) + 2C → 2CO(g)−221.2−275.4
33CO(g) + 2Na(c) → Na2CO3 + 2C−814.9−628.7
42Na(c) + 2C + N2(g) → 2NaCN−196.6−155.1
NaAlSiO4 (Nepheline) formation reaction at low SiO2/Al2O3 ratios:
56NaF + 3SiO2 + 2Al2O3 → 3NaAlSiO4 + Na3AlF6−46.9−43.1
NaAlSi3O8 (Albite) formation reaction at high SiO2/Al2O3 ratios:
66NaF + 9SiO2 + 2Al2O3 → 3NaAlSi3O8 + Na3AlF6−80.8−94.4
Reactions that contribute to changes in Na3AlF6 (cryolite) ratio:
7Na3AlF6 + 2CO(g) + 6Na(C) → NaAlO2 + 6NaF + 2C−973.2−827.4
8Na3AlF6 + 2Na2CO3 + 2C → NaAlO2 + 6NaF + 4CO(g)549.4354.9c
92Na3AlF6 + N2(g) + 6Na(c) → 2AlN + 12NaF−742−646.6
10Na3AlF6 + NaCN +2Na(c) → AlN + 6NaF + C(s)−275.9−248.9
Other NaCN consuming reactions:
11a2Al2O3 + NaCN + 2Na(c) → 3NaAlO2 + AlN + C−294.1−247.3
Additional NaAlO2 formation reactions:
12AlN + 2CO(g) + Na(c) → NaAlO2 + 2C + 5 N2(g)−1204.4−1008.1
13aAl2O3 + CO(g) + 2Na(c) → 2 NaAlO2 + C−495.7−412.8
Al4C3 formation reactions:
144Na3AlF6 + 12Na(c) + 3C → Al4C3 + 24NaF−856.1−650.5
15a8Al2O3 + 3C + 12Na(c) → 12NaAlO2 + Al4C3−499.9−343.2
164Al + 3C → Al4C3−215.9−203.3
172Al + N2(g) → 2AlN−636.5−573.3

### Table 1.

Chemical reactions within the cathode [6, 8] and their calculated ∆HR and ∆GR at 30°C.

The alumina data are for α-Al2O3 since the data for the actual similar β-Al2O3 (Na2O·11Al2O3) compound is not available. The Na (l) data was used in the equations that require Na(c) data, which means that the actual ∆GR is slightly more negative when Na(c) is on the right side of the equation and slightly more positive when Na(c) is on the left side of the equation.

Only -ve at T > 700°C.

Only -ve at T ≥ 650°C.

### 1.2 SPL composition

The SPL composition varies from one plant (or from one cell) to another [9]. Various factors contribute to this variation, some of which include the cell design, cathode materials, side reactions, operation time, shutdown time and electrolyte 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].

Typical composition ranges of the SPL constituents are shown in Table 2, from which the SPL contains about 6.2 wt% Al, 17.5 wt% F, 39 wt% C (as graphite), and 21 wt% Na [12].

Compoundwt% (low)wt% (high)
NaF8.016.0
Na2CO36.56.5
NaCN0.12.0
NaAlO25.010.0
C20.040.0
Na3AlF67.014.0
CaF23.07.0
NaAlSiO43.07.0

### Table 2.

Predominant SPL compounds and their composition ranges [12].

Table 3 shows the main elemental composition of the SPL along with the major phases or compounds of these elements. For example, the major forms of cyanides are identified as sodium cyanide (NaCN), sodium ferrocyanide Na4Fe(CN)6 and sodium ferricyanide Na3Fe(CN)6. Fluorides are mostly found in the form of sodium fluoride (NaF). Other reported forms of fluoride include sodium aluminum fluoride (Na3AlF6) and calcium fluoride (CaF2) [15].

ElementComposition range, wt%Major phases/compounds
C9.6–50Graphite carbon
Na7.0–20NaF, Na3AlF6
Al4.7–22.1Al metal, α-Al2O3, others
F9.7–18.9NaF, Na3AlF6, CaF2
Ca1.1–2.9CaF2
Li0.3–1.1LiF, Li3AlF6
Mg0.3–0.9MgF2
Si0.0–2.3Refractory SiO2, NaAlSiO4
Fe0.3–2.1Fe2O3
S0.1–0.3Gypsum (CaSO4)
Ti0.15–0.24TiB2
CN0.02–0.44NaCN, Na4Fe(CN)6, Na3Fe(CN)6

### Table 3.

SPL main elements [13] and their major phases / compounds [14].

### 1.3 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 (Al4C3), 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 ΔHR and ΔGR at 30°C. However, some authors claim that reactions 19, 23 and 25 (in Table 4) produce Al2O3. However, it is well known that Al2O3 results from Gibbsite {Al(OH)3} only after it is calcined (at temperatures above 400°C) [17].

#Chemical Reactionsa∆HR, kJ/mol∆GR, kJ/mol
18Al + 3H2O → Al(OH)3(s) + 1.5H2(g)−419.5−427.7
192Al + 3H2O → Al2O3 + 3H2(g)−819.3−872.3
20Al + NaOH + H2O → NaAlO2 + 1.5H2(g)−422.1−453.3
212Na(c) + 2H2O → 2NaOH + H2(g)−295.6−279.7
22AlN + 3H2O → Al(OH)3 + NH3(g)−147.3−157.0
232AlN + 3H2O → Al2O3 + 2NH3(g)−275.0−330.8
24Al4C3 + 12H2O → 4Al(OH)3 + 3CH4(g)−1686.4−1658.0
25Al4C3 + 6H2O → 2Al2O3 + 3CH4(g)−1647.1−1691.8
26NaCN +2H2O ⇋ HCOONa(ia) + NH3(g)−49.9−75.3

### Table 4.

Potential gases that might evolve from the SPL reactive species when hydrolyzed [7, 16] and their calculated ΔHR and ΔGR at 30°C.

In energy calculations: Na(l) is used instead of Na(c). (ia) is used in the HSC database for aqueous electrolyte (neutral), which is formed from undissociated aqueous species (ions).

Other reactions include those of ionic ferro- and ferri-cyanide with water [18]. For example,

Na4FeCN6ia+6H2O6HCN+FeOH2+4Na++4OH

Note: (ia) is used in the HSC database for aqueous electrolyte (neutral), which is formed from undissociated aqueous species (ions).

### 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.

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## 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 (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 AlF2OH·H2O 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 evaporation-crystallization 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.

NameMW, kg/kmolBoiling point, °CDensity @ 25°C, kg/m3
Leaching Acid
HCl(g)Hydrochloric36.5−85.11.49
35–37 wt% HCl1200
100wt% HNO3Nitric63.083.01510
68 wt% HNO31410
100wt% HClO4Perchloric100.5203.01768
70 wt% HClO41664
96–98 wt% H2SO4Sulfuric98.1337.01840
Produced Acid
HCN(l)Cyanic27.025.6687.6
HF(l)Fluoric20.119.5990a

### Table 5.

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

Saturated liquid at 19.5°C.

CompoundNameSolubility at 25°C, g/LSolubility at 100°C, g/L
1. SPL main compounds:
NaFSodium fluoride41.550.5
Na2CO3Sodium carbonate170436
NaCNSodium cyanide637480
NaAlO2Sodium aluminateH. solubleH. soluble
NaAlSiO4Sodium aluminosilicateInsolubleInsoluble
Na3AlF6CryoliteInsolubleInsoluble
CaF2Calcium fluoride0.016Insoluble
CGraphiteInsolubleInsoluble
2. Other SPL potential compounds:
NaAlSi2O6Sodium aluminodisilicateInsolubleInsoluble
Na4Fe(CN)6Sodium ferrocyanide(ia)H. solubleH. soluble
Na3Fe(CN)6Sodium ferricyanide(ia)H. solubleH. soluble
TiB2Titanium diborideInsolubleInsoluble
Al2O3Aluminum oxideInsolubleInsoluble
LiFLithium fluorideInsolubleInsoluble
Li3AlF6Lithium aluminum hexafluoride1.12aVery low
MgF2Magnesium fluorideInsolubleInsoluble
TiB2Titanium diborideInsolubleInsoluble
Fe2O3Ferric oxideInsolubleInsoluble

### Table 6.

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

At 20 °C. H. soluble = highly soluble.

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

NaCN+2H2OHCOONaia+NH3gE1

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 (∆GR = ‐75.3 kJ/mol at 30°C, see Table 4).

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

NaCN+4H2ONaH2O4++CNcoldpH>7E2

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):

NaCN+H2ONaOH+HCNgE3

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

HCN+NaOHNaCN+H2OE4

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

NaF+4H2ONaH2O4++FE5

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

Fl+H2OHFl+OHE6

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 (∆GR = -23.8 kJ/mol, see Table A.5):

NaAlO2+2H2ONaAlOH4E7

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.

NaAlO2+2H2ONaOH+AlOH3amorphous.E8

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

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

Na2CO3+H2O2Na++CO32+H3O++OHE9

Again, the claim that Na2CO3 reacts with H2O to produce NaOH and CO2(g) is also not true because it is non-spontaneous reaction (∆GR = +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.

CompoundNameSolubility at 25°C, g/LSolubility at 100°C, g/L
(1) Intermediate products
Al2(SO4)3Aluminum sulfateH. solubleH. soluble
Al(OH)3Aluminum hydroxide0.001Insoluble
SiO2SilicaInsolubleInsoluble
CaSO4GypsumInsolubleInsoluble
(2) Final products
CGraphiteInsolubleInsoluble
Na2SiO3Sodium silicateH. solubleH. soluble
Ca(ClO4)2Calcium perchlorateH. solubleH. soluble
AlFx(OH)(3-x), (x = 1 or 2)Aluminum hydroxyfluorideSolubleLess soluble
AlF3Aluminum fluoride7.317.2
Na2SO4Sodium sulfateH. solubleH. soluble
(3) other products
HFHydrogen fluorideH. solubleH. soluble
KFPotassium fluorideH. solubleH. soluble
HCNHydrogen cyanideH. solubleH. soluble
AgCNSilver cyanideInsolubleInsoluble
CO2Carbon dioxideInsolubleInsoluble

### Table 7.

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

### 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 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.

ParameterH2SO4HNO3HClHClO4
Reactions spontaneity. See Tables A.1 to A.4 in Appendix AAll -veAll -ve5 -ve
2 +ve
5 -ve
2 +ve
Acid molarity (M) [31, 32]55107.5
pKa or degree of corrosivity at 25 °C [38]−3.0−1.4−6.3−8.0
Acid cost, $/kg (2019 prices)0.2–0.350.2–0.250.15–0.354.0–4.5 L/S ratio [31, 32]2.54.54.54.5 Optimum temp., oC [31, 32]5075100100 ### Table 8. Values of the decision parameters sought for various leachant acids. 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). FactorFactor weightFiAcid Individual Score, Scorei H2SO4HNO3HClHClO4 Spontaneity200.221001007080 Acid molarity100.1180805050 Degree of corrosivity100.1150603020 Acid cost100.1175809040 L/S ratio200.2270507050 Optimum temperature200.2280502525 Overall Score =901.0078.368.955.646.7 ### Table 9. Decision matrix: Factor, factor weight, fraction (Fi), individual and overall scores sought for the leaching acids. In this work, we have calculated the change in the heat of reaction (∆HR) and the change in the Gibbs free energy of reaction (∆GR) for the reactions of the individual constituents of the SPL waste. Table A.1 to A.4 in Appendix A show the calculated ∆HR and ∆GR 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 (-∆HR) and spontaneous (-∆GR). We have also calculated ∆HR and ∆GR 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. ProductLeaching Acid H2SO4HNO3HCl(g)HClO4(g) IntermediateAl2(SO4)3Al(NO3)3(ia)AlCl3Al(ClO4)3(ia) Soluble or aqueousAlF2OH, AlF3, Na2SO4AlF2OH, AlF3, NaNO3, Ca(NO3)2AlF2OH, AlF3, NaCl, CaCl2AlF2OH, AlF3, NaClO4, Ca(ClO4)2 InsolubleC, SiO2, CaSO4C, SiO2C, SiO2C, SiO2 GasCO2, HF, HCNCO2, HF, HCNCO2, HF, HCNCO2, HF, HCN ### 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 SO42− anion is neutral while the Al3+ is not. In the reaction: Al2SO43+6H2O2AlOH3+6H++3SO42E10 the produced H2SO4, which is a strong acid, dissociates in the aqueous phase to form 2H+ and SO42− 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. Advertisement ## 3. Leaching of the SPL individual constituents by H2SO4 solution The leaching process starts with the dissolution of the water-soluble compounds of the SPL (namely, NaF, NaCN, Na2CO3, and NaAlO2) in the H2SO4 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 H2SO4 are presented by Eqs. (11) to (14). See reactions R1 to R4 in Table A.1. 2NaFs+H2SO4Na2SO4+2HFgE11 Na2CO3s+H2SO4Na2SO4+H2O+CO2gE12 2NaCNs+H2SO4Na2SO4+2HCNgE13 2NaAlO2s+4H2SO4Al2SO43+Na2SO4+4H2OE14 On the other hand, the graphite present in SPL is the only compound that does not react with acids (e.g. H2SO4), alkalis (e.g. NaOH) or acidic Al3+ solution. However, the reactions of the three other insoluble compounds present in the SPL (namely, NaAlSiO4, Na3AlF6, and CaF2) are explained below. 1. The NaAlSiO4 dissolves in aqueous H2SO4 solution and produces the intermediate product NaAl3(SO4)2(OH)6 according to Eq. (15) 3NaAlSiO4+3H2SO43SiO2s+Na2SO4+NaAl3SO42OH6E15 NaAl3(SO4)2(OH)6 dissolves in excess H2SO4 [39] according to Eq. (16). 2NaAl3SO42OH6+6H2SO43Al2SO43+Na2SO4+12H2OE16 By multiplying Eq. (10) by 2, adding it to Eq. (15), and dividing the result by 3 gives the net result presented by Eq. (17) (similar to that reported by [40]: 2NaAlSiO4+4H2SO4Al2SO43+Na2SO4+2SiO2+4H2OE17 2. The cryolite (Na3AlF6) does not react with H2SO4 spontaneously; it has a high +∆GR. However, it reacts spontaneously with concentrated NaOH solution to produce NaF and the intermediate product NaAl(OH)4 according to Eq. (18): Na3AlF6s+4NaOHNaAlOH4+6NaFaqE18 However, both resulting products (NaF and NaAl(OH)4) need to be leached with or neutralized by H2SO4 according to Eq. (11) (for NaF) and according to Eq. (19) for NaAl(OH)4: 2NaAlOH4+4H2SO4Al2SO43+Na2SO4+8H2OE19 An alternative to this two-step leaching process expressed by Eqs. (16) and (18), the Na3AlF6 can be leached with an acidic Al3+ solution comprised of Al(OH)3 and H2SO4, which was found to be more effective than leaching with an acid only or an alkali only [41, 19]. This acidic Al3+ solution can be prepared according to Eq. (20): 2AlOH3s+3H2SO4Al2SO43aq+6H2OE20 and the reaction of Na3AlF6 with the above solution gives 2Na3AlF6s+2AlOH3s+3H2SO43Na2SO4+4AlF3+6H2OE21 However, the Al2(SO4)3 (or acidic Al3+) solution is already produced by Eqs. (14) and (17) presented above. Here, the Al2(SO4)3 has an amphotericcharacter, i.e. it can both act as an acidic and a basic solution in the aqueous phase. Thus, the Na3AlF6 reacts (spontaneously) with the present acidic Al2(SO4)3 solution to give Na2SO4 and AlF3 according to Eq. (22): 2Na3AlF6s+Al2SO43aq3Na2SO4aq+4AlF3aqE22 3. The reaction of CaF2 with H2SO4 is less spontaneous (very small -∆GR that decreases with temperature) and gives CaSO4 and HF according to Eq. (23), (which is not required at this stage of leaching): CaF2s+H2SO4aqCaSO4s+2HFaqE23 However, CaF2 can react (spontaneously) with the solution presented by Eq. (19) according to Eq. (24). 3CaF2s+2AlOH3s+3H2SO43CaSO4aq+2AlF3aq+6H2OE24 But again, CaF2 can also react (spontaneously) with the acidic Al2(SO4)3 produced by Eqs. (14) and (17) to give CaSO4 precipitate and aqueous AlF3: 3CaF2s+Al2SO43aq3CaSO4s+2AlF3aqE25 Advertisement ## 4. Process description In this work, we propose a process for leaching of the main constitutes of the SPL waste by H2SO4 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 (-ΔHR) except those appearing in bold numbers in the ΔHR 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 pre-prepared H2SO4 solution. To ensure that all the SPL particles are sufficiently exposed to the solution, a 2.5 M H2SO4 (with 5 wt% excess) is used along with a recommended L/S ratio of 2.52 liters of H2SO4 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. CompoundMW, kg/kmolConcentration, wt%Mass Flow rate, kg/hMolar Flow rate, kmol/h NaF41.9915.61925.7022.05 Na2CO3105.996.34375.973.55 NaCN49.011.95115.642.36 NaAlO281.979.76578.787.06 Na3AlF6209.9413.66810.063.86 NaAlSiO4202.146.83405.032.00 CaF278.076.83405.035.19 C12.0039.022313.94192.83 Total =100.005930.14238.89 ### Table 11. Normalized composition of the SPL main constituents used in this work. The products generated during processing are classified into three categories or streams: (1) gaseous stream (HCN, HF and CO2), (2) insoluble products stream (graphite, gypsum and SiO2), and (3) soluble products stream (aluminum fluorides and sodium salts, mainly, Na2SO4). 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 CO2) leaves reactor R-101, cooled (not shown on the PFD) and then sent to a gas emission-control scrubber (T-101) where the HCN gas is scrubbed by its reaction with a silver nitrate (AgNO3) 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. HCNg+Ag(NO3aqAgCNs+HNO3aqE26 The AgCN is insoluble in water, but it is slightly soluble in aqueous HNO3. 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 LD50 oral (rat) of 123 mg/kg. Note: It should be mentioned that no reaction will take place between aqueous AgNO3 used in Eq. (26) and HF(l), HF(g) or CO2, since these reactions are non-spontaneous at temperatures ≤90°C. The HF can be recovered as a liquid from the HF-CO2 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 CO2 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 AlF3 (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 AgNO3 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 AgNO3 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 NaNO3 (very cheap). But still a reactor and a separator are needed. In either case, the resulting gas stream needs to be sent to the CO2 recovery unit. 2. After completion of the leaching step, the slurry mixture is sent to filter F-101 where the insoluble solids (SiO2, graphite and gypsum) are separated from the aqueous solution containing soluble intermediate and final products (Na2SO4, AlF3 (and/or AlF2OH), remaining Al2(SO4)3, unreacted H2SO4, and water). The insoluble solids stream is sent to reactor R-103 where the SiO2 is reacted with aqueous NaOH to produce soluble sodium silicate (Na2SiO3) according to reaction (27). See reaction R9 in Table A.1. SiO2s+2NaOHaqNa2SiO3aq+H2OE27 which is then separated from the graphite-gypsum solid mixture via filter F-102. See Figure 2. The Na2SiO3 in the aqueous solution can then be saturated by evaporation and precipitated as Na2SiO3 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 Al2(SO4)3 is converted to AlF3 (and/or AlF2OH) by its reaction with part of the recovered HF liquid, according to the relatively high spontaneous Eq. (28) (∆GR = -196.65 kJ/mol). at 30°C. See reaction R10 in Table A.1. Al2SO43aq+6HFl2AlF3aq+3H2SO4E28 Due to the presence of fluoride ions in R-102, the dominant crystal species will be AlF3. However, the reaction between Na2SO4 and HF(l) is much less competent than Eq. (28) since it is much less spontaneous (∆GR = -32.7 kJ/mol). See reaction R1 in Table A.1. In order to recover the AlF3 crystals, the contents of reactor R-102 are pumped through P-102 to the reactor-crystallizer RC-101, where the conditions required for AlF3 crystallization have to be established. A controlled amount of NaOH has to be added to neutralize most of the remaining H2SO4 according to Eq. (29). See reaction R11 in Table A.1. H2SO4+2NaOHNa2SO4aq+2H2OE29 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 AlF3 [19], noting that the solubility of AlF2OH (and AlF3) decreases with the increase of the pH. Any AlF2OH produced can be easily converted to AlF3 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. AlFxOH3x+xHFlAlF3+xH2Ox=1or2E30 or, AlFOH2+2HFlAlF3+2H2Oforx=1E31 and AlF2OH+HFlAlF3+H2Oforx=2E32 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 AlF3 cake. To remove the impurities from the AlF3, the stream needs to be washed with fresh water. The AlF3 is then dried, cooled and stored. The filtrate leaving filter F-104 is sent to the evaporator-crystallizer EC-101, where the Na2SO4 solution is saturated by flash evaporation under vacuum and Na2SO4 is crystallized and separated via filter F-105. See Figure 3. The Na2SO4 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. Advertisement ## 5. 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, FCIL, to be 27.32 M$. Raw MaterialsAmount, kg/hValue,$/kgProductsAmount, kg/hValue, $/kg SPL5930.10.12aAlF31156.51.6 H2SO43663.80.086HF284.70.9 NaOH686.90.692Graphite C2213.90.9 Na2SO44567.30.27 CaSO4706.30.4 HNO3134.00.243 Na2SiO3348.00.811 CO2156.10.0 Input water150416.7x10−5Output water15716.76.7x10−5 ### Table 12. Amounts of raw materials and products and their average prices [44]. Estimated cost for crushing, grinding and handling of the SPL. The number of operators per job was estimated based on Eq. (33): NOL=6.29+31.7P2+0.23Nnp0.5E33 where P stand for particulate (solid) and Nnp for non-particulate (fluid) handling equipment (P = 1 for FF-101, Nnp = 15). The total number of operators required over the year = 4.47 NOL. The salary per operator was assumed to be$49000.

The FCIL along with the estimated annual costs of labor COL, raw materials CRM, utilities CUT, and waste treatment CWT (given in Table 13) were used to calculate the cost of manufacturing excluding depreciation, COMd, according to Eq. (34):

Cost ItemM$/year Operating labor cost, COL1.421 Raw materials cost, CRM10.14 Utilities cost, CUT0.38 Waste treatment cost, CWT0.0 Cost of manufacturing excluding depreciation, COMd21.73 ### Table 13. Estimated individual operating costs and COMd. COMd=0.18FCIL+2.73COL+1.23CRM+CUT+CWTE34 The calculated COMd = 21.73 M$/year.

Now, assuming priceless produced HNO3, Na2SiO3, CO2 and output water, the income from main sales (revenue, R) was found to be 38.09 M$/year. Also, since AgNO3 and AgCN are very expensive and sharply affect the profitably of the process, this option has been excluded in the economic analysis. The input data used for generating the cumulative cash flow analysis are presented in Table 14. The discounted cumulative cash flow diagram for the above process analysis is shown in Figure 4. Following [43] economic analyses and using the data presented above, and assuming an interest rate of 10%, a tax rate of 20%, the calculated net present value, NPV = 42.24 M$, the discounted payback period, DPBP = 2.38 years, and the discounted cash flow rate of return, DCFROR = 31.73%.

ItemValueUnits
FCIL27.32M$Land cost, L = 0.1 FCIL2.732M$
Working capital, WC = 0.2 FCIL5.464M$Salvage value, S = 0.1 FCIL2.732M$
Construction period2years
Project life, n10years
Depreciation period, nd5years

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## Acknowledgments

The authors are grateful for the United Arab Emirates University (Office of the Deputy Vice Chancellor for Research & Graduate Studies) who funded this work under Fund number G00003084, 2019/2020.

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## Appendix A

Note that reactions R8, R8*, R9, and R12 presented in Table A.1 (for H2SO4) are common in all acid-leaching processes using HNO3 (Table A.2), HCl (Table A.3), and HClO4 (Table A.4)

#Reaction∆HR, kJ/mol∆GR, kJ/mol
R12NaF + H2SO4 → Na2SO4 + 2HF(l)−20.3−32.7
R1a2NaF + H2SO4 → Na2SO4 + 2HF(g)32.5−39.4
R2Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2(g)−122.6−164.3
R32NaCN + H2SO4 → Na2SO4 + 2HCN(l)−181.8−176.1
R3a2NaCN + H2SO4 → Na2SO4 + 2HCN(g)−122.6−170.5
R42NaAlO2 + 4H2SO4 → Al2(SO4)3 + Na2SO4 + 4H2O−450.0−419.7
R52NaAlSiO4 + 4H2SO4 → Al2(SO4)3 + Na2SO4 + 2SiO2 + 4H2O−348.7−310.8
R62Na3AlF6 + Al2(SO4)3 → 3Na2SO4 + 4AlF3−119.9−119.5
R73CaF2 + Al2(SO4)3 → 3CaSO4 + 2AlF3−209.1−211.6
R8HCN(l) + AgNO3 → AgCN + HNO3−13.0−15.8
R8aHCN(g) + AgNO3 → AgCN + HNO3−10.2−4.4
R9SiO2 + 2NaOH → Na2SiO3 + H2O−84.7−88.9
R10Al2(SO4)3 + 6HF(l) → 2AlF3 + 3H2SO4−223.3−196.7
R11H2SO4 + 2NaOH → Na2SO4 + 2H2O−294.1295.3
R12-aAl(OH)2F(g) + 2HF(l) → AlF3 + 2H2O−155.6−140.7
R12-bAl(OH)F2(g) + HF(l) → AlF3 + H2O−140.7−106.1

### Table A.1.

Calculated ∆HR and ∆GR at 30°C for the reactions of the main SPL constituents when leached with H2SO4 solution.

Stands for reactions involving HF(g) or HCN(g).

#ReactionΔHR, kJ/molΔGR, kJ/mol
R1NaF + HNO3(l) → NaNO3 + HF(l)−16.8−12.2
R1aNaF + HNO3(l) → NaNO3 + HF(g)9.6−15.5
R2Na2CO3 + 2HNO3(l) → 2NaNO3 + H2O + CO2(g)−135.9−155.9
R3NaCN + HNO3(l) → NaNO3 + HCN(l)−97.6−83.9
R3aNaCN + HNO3(l) → NaNO3 + HCN(g)−68.0−81.1
R4NaAlO2 + 4HNO3(l) → NaNO3 + Al(NO3)3(ia) + 2H2O−367.4−268.2
R5NaAlSiO4 + 4HNO3(l) → SiO2 + Al(NO3)3(ia) + NaNO3 + 2H2O−316.8−213.8
R6-aNa3AlF6 + Al(NO3)3(ia) → 3NaNO3 + 2AlF355.815.3
R6-bNa3AlF6 + Al(NO3)3.6H2O → 3NaNO3 + 2AlF3 + 6H2O32.5−28.05c
R6bNa3AlF6 + 3HNO3(l) + Al(OH)3 → 3NaNO3 + 2AlF3 + 3H2O−160.5−135.0
R71.5CaF2 + Al(NO3)3(ia) → 1.5Ca(NO3)2 + AlF383.640.3
R7b1.5CaF2 + Al(OH)3 + 3HNO3(l) → 1.5Ca(NO3)2 + AlF3 + 3H2O−132.6−110.03d
R10Al(NO3)3(ia) + 3HF(l) → AlF3 + 3HNO324.0−35.9
R11HNO3 + NaOH → NaNO3 + H2O−153.6−143.4

### Table A.2.

SPL reactions with HNO3 and their ∆HR and ∆GR at 30°C.

Stands for reactions involving HF(g) or HCN(g).

Stands for alternative spontaneous reaction.

∆GR at T > 100°C.

∆GR at T > 180°C.

#ReactionΔHR, kJ/molΔGR, kJ/mol
R1NaF + HCl(g) → NaCl + HF(l)−41.9−14.5
R1aNaF + HCl(g) → NaCl + HF(g)−15.5−17.9
R2Na2CO3 + 2HCl(g) → 2NaCl + H2O + CO2(g)−186.1−160.7
R3NaCN + HCl(g) → NaCl + HCN(l)−122.6−86.2
R3aNaCN + HCl(g) → NaCl + HCN(g)−93.1−83.5
R4NaAlO2 + 4HCl(g) → NaCl + AlCl3 + 2H2O−185.6−35.4
R5NaAlSiO4 + 4HCl(g) → SiO2 + AlCl3 + NaCl +2H2O−134.919.0
R6Na3AlF6 + AlCl3 → 3NaCl + 2AlF3−226.2−226.7
R73CaF2 + 2AlCl3 → 3CaCl2 + 2AlF3−311.8−322.1
R10AlCl3 + 3HF(l) → AlF3 + 3HCl−432.3−363.9
R11HCl(a) + NaOH → NaCl + H2O−95.5−114.7

### Table A.3.

SPL reactions with HCl(g) and their ∆HR and ∆GR at 30°C.

Stands for reactions involving HF(g) or HCN(g).

#ReactionΔHR, kJ/molΔGR, kJ/mol
R1NaF + HClO4(g) → NaClO4 + HF(l)−111.0−71.9
R1aNaF + HClO4(g) → NaClO4 + HF(g)−84.6−75.2
R1cNaF + HClO4(ia) → NaClO4 + HF(g)50.424.8d
R2Na2CO3 + 2HClO4(g) → 2NaClO4 + H2O(l) + CO2(g)−324.4−275.3
R3NaCN + HClO4(g) → NaClO4 + HCN(l)−191.8−143.6
R3aNaCN + HClO4(g) → NaClO4 + HCN(g)−162.1−140.7
R4NaAlO2 + 4HClO4(g) → NaClO4 + Al(ClO4)3(ia) + 2H2O−743.6−523.7
R5NaAlSiO4 + 4HClO4(g) → SiO2 + Al(ClO4)3(ia) + NaClO4 + 2H2O−693.0−469.3
R6Na3AlF6 + Al(ClO4)3(ia) → 3NaClO4 + 2AlF355.232.1e
R6bNa3AlF6 + 3HClO4(g) + Al(OH)3 → 3NaClO4 + 2AlF3 + 3H2O−443.1−314.1
R71.5CaF2 + Al(ClO4)3(ia) → 1.5Ca(ClO4)2 + AlF3141.732.1f
R7b1.5CaF2 + Al(OH)3 + 3HClO4(g) → 1.5Ca(ClO4)2 + AlF3 + 3H2O−356.5−224.9
R10Al(ClO4)3(ia) + 3HF(l) → AlF3 + 3HClO4(g)305.8159.8g
R10aAl(OH)3 + 3HClO4(g) → Al(ClO4)3(ia) + 3H2O−498.3−346.1
R11HClO4(g) + NaOH → NaClO4 + H2O−247.7−203.0

### Table A.4.

SPL reactions with HClO4(g) and their ∆HR and ∆GR at 30°C.

Stands for reactions involving HF(g) or HCN(g).

Stands for alternative spontaneous reaction.

Stands for HClO4(ia).

∆GR at T > 240°C.

∆GR at T > 225°C.

∆GR at T > 550°C.

∆GR at T > 350°C.

#ReactionΔHR, kJ/molΔGR, kJ/mol
27NaCN +4H2O → [Na(H2O)4]+ + CN
28NaCN + H2O → NaOH + HCN56.159.6
29HCN + NaOH → NaCN + H2O−56.1−59.6
30NaF + 4H2O → [Na(H2O)4]+ + Faa
31F(l) + H2O → HF(l) + OH
32NaAlO2 + 2H2O → Na(Al(OH)4)−26.1−23.8
33NaAlO2 + 2H2O → NaOH + Al(OH)32.625.6
34Na2CO3 + H2O → 2Na+ + (CO3)2− + H3O+ + (OH)
35Al2(SO4)3 + 6H2O ⇋ 2Al(OH)3 + 6H++ 3SO42−
363NaAlSiO4 + 3H2SO4 → 3SiO2 + Na2SO4 + NaAl3(SO4)2(OH)6−527.5−451.4
372NaAl3(SO4)2(OH)6 + 6H2SO4 → 3Al2(SO4)3 + Na2SO4 + 12H2O8.9−29.7
38Na3AlF6 + 4NaOH → NaAl(OH)4 + 6NaF−164.8−169.3
392NaAl(OH)4 + 4H2SO4 → Al2(SO4)3 + Na2SO4 + 8H2O−397.8−372.1
402Al(OH)3 + 3H2SO4 → Al2(SO4)3 + 6H2O−161.1−175.6
412Na3AlF6 + 2Al(OH)3 + 3H2SO4 → 3Na2SO4 + 4AlF3 + 6H2O−140.5−147.6
42CaF2 + H2SO4 → CaSO4 + 2HF(l)+57.6−11.6
433CaF2 + 2Al(OH)3 + 3H2SO4 → 3CaSO4 + 2AlF3 + 6H2O−185.1−193.6

### Table A.5.

Calculated ∆HR and ∆GR at 30°C for other potential reactions taking place during the SPL leaching process.

Ionic reactions have no specific ΔHR or ΔGR.

#Reaction∆HR, kJ/mol∆GR, kJ/mol
44Li3AlF6 + Al(OH)3 + 1.5H2SO4 → 1.5Li2SO4 + 2AlF3 + 3H2O−152.1−158.3
452LiF + H2SO4 → Li2SO4 + 2HF(g)64.6−5.9
462Na3Fe(CN)6(ia) + 6H2SO4 → 3Na2SO4 + Fe2(SO4)3 + 12HCN(l)−279.4−365.3
47Na4Fe(CN)6(ia) + 3H2SO4 → 2Na2SO4 + FeSO4 + 6HCN(l)−123.3−214.7
481.5MgF2 + Al(OH)3 + 1.5H2SO4 → 1.5MgSO4 + AlF3 + 3H2O−76.9−83.7
49Al2O3(s) + 3H2SO4 → Al2(SO4)3 + 3H2O−180.8−158.7
50Al4C3 + 6H2SO4 → 2Al2(SO4)3 + 3C + 6H2(g)−1784.2−1858.8
51Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O−178.0−164.9
52SiO2 + 4HF → SiF4(g) + 2H2O−77.6−101.7
53SiO2 + 6HF → H2SiF6(ia) + 2H2O−260.7−190.8

### Table A.6.

Calculated ∆HR and ∆GR at 30°C for the reactions of the SPL trace constituents when subjected to H2SO4 leaching.

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Written By

Samir I. Abu-Eishah, Manal D.M. Raheem, Fatma A.S. Aljasmi, Fatima M.O. Alameri, Amna G.R. Alblooshi and Intesar F.R. Alnahdi

Submitted: May 10th, 2021 Reviewed: June 24th, 2021 Published: July 31st, 2021