**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 (ΔHR) and and change in the Gibbs free energy of reaction (ΔGR) (using HSC Chemistry 6.1 software) at 30°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


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

*a 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 ΔGR 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. b*

*Only -ve at T <sup>&</sup>gt; 700°C. <sup>c</sup> Only -ve at T* ≥ *650°C.*

#### **Table 1.**

*Chemical reactions within the cathode [6, 8] and their calculated ΔHR and ΔGR at 30°C.*

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].

**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].

### *Waste Material Recycling in the Circular Economy - Challenges and Developments*


#### **Table 2.**

*Predominant SPL compounds and their composition ranges [12].*


#### **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].

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

Na4Fe CN ð Þ6ð Þþ ia 6H2O ! 6HCN↑ þ Fe OH ð Þ2↓ þ 4Na<sup>þ</sup> þ 4OH�

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

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


*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).*

#### **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.*
