Introduction

Heavy pressure is on the metallurgical industries to phase out fossil carbon usage. Iron and steelmaking production alone accounts for 7% of total global energy sector carbon dioxide emissions [1]. There are major endeavors in several metallurgical sectors to replace or partially replace fossil carbon by using hydrogen as the reducing agent for example, or by partially or wholly substituting metallurgical coke and coal with biomass or charcoal [2, 3].

Due to its unique properties, carbon is also widely employed as a refractory material (carbon is resistant to both heat and chemical corrosion), as well as an electrical conductor. The fossil carbon materials used as aggregates are calcined petroleum cokes, needle cokes (both petroleum and coal-derived), and anthracites. Coal tar pitch (CTP) is heavily relied upon as a binder in carbon electrodes of all types. Oxide–carbon refractories are used on a large scale in steelmaking, where natural graphite and petroleum-derived resins are the carbonaceous constituents.

The competition for high-quality fossil carbon materials is increasing globally, which could pressure supply for carbon electrode and refractory manufacture. Natural graphite and synthetic graphite derived from petroleum cokes are in increasing demand, largely due to the rapid expansion of the electric vehicle (EV) market and the resulting massive expansion of lithium-ion battery (LIB) production [4]. Natural graphite is a mined resource and is considered a critical mineral by the European Union and the United States Geological Survey [5, 6].

Coal tar pitch (CTP) is a distilled by-product of the production of metallurgical coke from bituminous coal for use as a reductant in blast furnace ironmaking. CTP has carcinogenic constituents and is currently prohibited by the European chemicals regulation REACH for end-use applications, though for now it is exempted for use as an intermediate in for example production of electrodes and electrode paste [7].

Motivation for Research

It is a significant material engineering challenge to develop viable biocarbon-based replacements for fossil carbons in these applications. Carbon materials are broadly classified into two categories: graphitizable carbons and non-graphitizable carbons. These are commonly referred to as “soft” and “hard” carbon materials, respectively. Soft carbon materials can be graphitized by heat treatment up to 3000 °C, and the thermophysical properties of the bulk polycrystalline material approach that of a single crystal of graphite [8, 9]. Most petroleum cokes, anthracites, and coal tar pitches are readily graphitizable. Biogenic carbon materials are hard carbons and will not graphitize to any appreciable extent with heat treatment.

At present, the application of biocarbon-derived materials in these applications to any appreciable extent is infeasible technically, neither as the aggregate component nor as the binder component. Substantial processing and structural modification such as catalytic graphitization will have to be developed on a large scale to render biocarbon materials useable in any sizeable quantities in these applications. This is a review that aims to highlight the current gaps in carbon science and processing technology in order to help advance research and development toward the utilization of biocarbons in electrodes and refractories in the metallurgical industries.

Background

Carbon electrodes of all types (electrodes for Al reduction, Söderberg, prebaked, and graphite) and monolithic carbon refractories are similar in that they are composite materials comprised of a granular carbon aggregate in a carbonaceous binder matrix. Table 1 provides an overview of industrial carbon electrode types, constituents, and annual consumption. Figure 1 is an image of a cross-section of a Söderberg paste briquette taken with CT scanning.

Table 1 Estimated consumption of industrial carbon electrodes in the metallurgical industries and constituents
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Fig. 1
figure 1

Cross-sectional CT scan of a Söderberg paste briquette (courtesy of Elkem Carbon). The aggregate and binder phases can be seen

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The different processes to form carbon electrode materials have many unit operations in common; Fig. 2 is a generalized flowsheet for carbon electrode and paste production.

Fig. 2
figure 2

Generalized flowsheet for carbon electrode and paste production

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The first step is aggregate preparation where raw petroleum cokes, pitch/petroleum needle cokes, or anthracites are calcined (thermally treated) to facilitate carbonization, volumetric stabilization, and partial graphitization [10]. The resulting calcine is then classified into desired size fractions, and oversized material can be milled to form part of a fine fraction. Synthetic graphite can be added at this stage to modify the electrical and thermophysical properties of the final product. Next comes a mixing stage to blend a carbonaceous binder with the carbon aggregate, usually in heated mixers to decrease the binder viscosity and improve wetting of the aggregate by the binder. After that, the next step is usually forming where fluid paste is pressed, molded, or extruded into near-net shapes. Common to carbon anode and cathode, prebaked and graphite electrode production, a following step is baking in large ring furnaces or car-bottom furnaces. Impregnation or coating can be performed with re-baking to reduce porosity in graphite electrode manufacture. As a final stage in graphite electrode production (and cathode production), graphitization is performed in Acheson for lengthwise graphitization (LWG) furnaces. Machining is finally performed to produce finished graphite and prebaked electrodes.

Anodes and Cathodes for Primary Aluminum Smelting

The primary aluminum industry is the largest consumer of calcined petroleum coke worldwide [11]; between 40 and 46 kg of carbon is consumed for every ton of Al produced [12]. Here, alumina is reduced in high-temperature Hall-Héroult electrolytic cells as illustrated in Fig. 3. Carbon electrodes in the cells transfer electrical current for reduction as well as generate Joule heating. The anodes are continuously consumed in the process as oxygen liberated by electrolysis reacts with the anode carbon to generate CO2 gas, while the carbon cathode lasts the lifetime of the cell (> 5 years) [13].

Fig. 3
figure 3

Hall-Héroult reduction cell for production of primary aluminum

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Söderberg anodes were once ubiquitous in aluminum electrolysis, but now prebaked anode is the predominant technology. A typical prebaked anode recipe contains about 65% calcined petroleum (sponge) coke, 20% recycled anode butts, and 15% coal tar pitch binder [14] with softening points between 110 and 115 °C [15]. The operating current density of a typical prebaked anode is around 0.6–1.3 A/cm2 [16].

Carbon cathode blocks are comprised of calcined petroleum coke and/or anthracite, graphite, and coal tar pitch binder. There are different commercial grades ranging from carbon, graphitized, to fully graphitic cathode blocks. The green cathode blocks are baked and the fully graphite grades are heat-treated at very high temperature afterward in graphitization furnaces to 3000 °C [17]. In the construction of the electrolysis cells, carbon ramming paste is commonly used to seal the seams between the cathode blocks [17].

Graphite Electrodes

The second largest consumer of calcined petroleum coke worldwide is the steel industry [11, 18]. The projected worldwide annual consumption of graphite electrodes in 2023 was 966,000 metric tons [19]. Graphite electrodes are used in extreme conditions where tolerance of high thermal and mechanical shock (high modulus of elasticity) and high current density are required [18]. The primary application is in electric arc furnaces for steel scrap melting. In contrast to the other electrode types, in graphite electrodes the aggregate is comprised of 75–80% calcined needle coke [18, 20]. Needle cokes are highly graphitizable materials derived from petroleum and coal tar with a highly anisotropic micro-texture; these cokes are especially suited for aggregates in graphite electrodes. Coal tar pitch is the predominantly used binder with softening points between 100 and 110 °C. Similar to prebaked electrodes, graphite electrodes are limited in diameter due to size constraints in baking, graphitizing, and machining.

Graphite electrodes are sold in RP (regular power), HP (high power), SHP (super high power), and UHP (ultra-high power) grades. Current densities can reach upwards of 35 A/cm2 [21]. At present, it is extremely difficult to conceive suitable biocarbon substitutes on par with needle coke and coal tar pitch for graphite electrode production without considerable technological development.

Söderberg Electrodes

The patent for the continuous self-baking electrode was originally filed in 1919 by Carl Wilhelm Söderberg from the Elektrokjemisk Industri A/S Fiskaa works in Norway, a company now known as Elkem ASA [22]. Söderberg electrodes are still in prevalent use today in three-phase, alternating-current submerged arc furnaces (SAF) for the production of ferroalloys, platinum, calcium carbide, and copper slag cleaning operations. The role of the electrodes in these electrothermic processes is to supply Joule heat to the furnace. Söderberg electrodes have a clear advantage of lower operating cost compared to prebaked or graphite electrodes [23]. Submerged arc furnaces typically have three electrodes in a delta configuration or up to 6 in-line electrodes. Söderberg electrodes can be up to 2 m in diameter with 150 kA current load in modern furnaces [24, 25]. Electrode tip temperatures in a ferrosilicon production can exceed 2400 °C [26]. The typical current density of a typical Söderberg electrode is around 7 A/cm2 [27].

A Söderberg electrode column comprises a steel casing that is periodically refilled with electrode paste. Figure 4 is an illustration of a Söderberg electrode column. Internal vertical fins in the steel casing increase mechanical rigidity and bonding between the carbon and steel as well as improve electrical contact. Water-cooled contact shoes transfer electrical current to the electrode. Resistive heating from current passing through the casing and pastes, conducted heat from the furnace, and heating fans facilitate softening of the electrode paste which flows to fill the electrode casing. As the electrode column is incrementally slipped downwards, the electrode paste fully bakes into a solid carbonized mass at an isotherm of around 450–475 °C [28]. Solid electrode paste is refilled as the electrode column is consumed downward in the furnace. Volatiles emitted during carbonization escape downwards through pores in the hot baked electrode column, which, in the process, thermally cracks and deposits carbon within the pores [29].

Fig. 4
figure 4

Illustration of a Söderberg electrode column, courtesy of Elkem Carbon. Numbered in the figure: 1. Solid paste cylinders, 2. Melted paste, 3. Baking zone, 4. Baked electrode, 5. Melting zone, 6. Casing with fins, 7. Slipping mechanism, 8. Fan and heating element, 9. Suspension casing, 10. Bus tubes, 11. Heat shield, 12. Contact clamps

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Typical Söderberg electrode paste is composed of 70–80% selected calcined anthracites and petroleum cokes, graphite, and 20–30% coal tar pitch binder [30]. The coal tar pitch binders have softening points ranging anywhere from 65 to 134 °C [31]. At temperatures between the softening point and the baking isotherm, the rheological characteristics of the binder are important as the paste must evenly flow to fill the casing while at the same time segregation is to be avoided [31].

Prebaked Carbon Electrodes

A prebaked carbon electrode is preferred to a Söderberg electrode where iron contamination from a steel casing is undesirable—in silicon smelting and phosphorous production, for example. Baking and machining limitations preclude prebaked electrodes to a maximum diameter of about 55 inches (1400 mm) [32]. Like Söderberg paste, prebaked electrodes are comprised of calcined anthracite or petroleum coke, graphite, and coal tar pitch binder.

Carbon Refractories

Carbon materials are also used extensively in important refractory applications in the metallurgical industries. Carbon blocks and formable monolithic carbon linings are commonly used in constructing furnace linings. Oxide–carbon bricks are used extensively in the iron and steelmaking industry in a variety of applications.

Carbon and Graphite Blocks

Carbon and graphite blocks are employed in blast furnace hearths in ironmaking [32, 36]. Carbon is highly refractory to carbon-saturated iron, and high thermal conductivity is desirable in this application [37].

Monolithic Carbon Refractories

Monolithic carbon linings are prepared by ramming (also called tamping) carbon pastes into shape in successive layers in the furnace lining and then baking to harden and eventually fully carbonize the binder. Baking is often done in situ in a furnace.

In silicon and ferroalloy production, furnace inner lining layers are constructed of either prebaked carbon blocks or rammed carbon paste [38]. Carbon lining pastes are typically 70–80% of calcined anthracite aggregate, proprietary additives, and phenolic formaldehyde (PF) resins binder or proprietary non-hazardous binders.

Cathode ramming pastes are used to fill the seams between cathode blocks and in sidewall when constructing aluminum reduction cells. These pastes are similar in composition to carbon lining pastes. “Cold” ramming pastes that can be worked at ambient temperature are predominant in Western markets.

Oxide–Carbon Refractories

A common carbon-containing basic refractory employed in the steel industry in converters and ladle linings is magnesia carbon (MgO–C). Refractories like MgO–C typically contain 12–20% flake (natural) graphite and 5% petroleum-derived phenolic formaldehyde resin binder [39]. Pitch-bonded varieties include magnesia-carbon refractories that are widely used in basic oxygen furnaces, electric arc furnaces, and in refining ladles for steel. Carbon in these materials is not only refractory but also renders the refractories non-wettable by liquid slag as well as increases the thermal conductivity. Other basic carbon-containing refractories contain CaO and doloma (MgO–CaO).

Other important carbon-containing refractories in steelmaking are alumina-carbon (Al2O3–C), alumina-silicon carbide–carbon, (Al2O3–SiC–C), zirconia-carbon, and magnesia-calcia-carbon [40].

Types and Properties of Fossil-Derived Carbon Aggregates and Binders

Carbonaceous aggregates and binders that are derived from fossil carbons are in widespread use today in many products. An overview of the aggregates and binders primarily used in carbon electrode and refractory manufacture is detailed here. Table 2 details the properties of calcined anthracite and petroleum coke, while Table 3 lists the the properties of typical coal tar pitches utilized in carbon electrodes.

Table 2 Typical properties of calcined anthracite and petroleum coke
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Table 3 Properties of typical coal tar pitches used in metallurgical electrodes
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Calcined Anthracites

Anthracite, the highest rank coal, is low in volatiles and ash containing in the range of 92–98% carbon, which exhibits a macromolecular structure of condensed aromatic rings linked by aliphatic and/or ether groups [41]. During calcination, the condensed aromatic rings are rearranged and heteroatoms are removed in the form of volatiles as temperature increases, resulting in a partially graphitized carbonaceous material [42, 43]. Anthracite can be calcined either in a gas-fired kiln or shaft furnaces at < 1300 °C to obtain gas calcined anthracite (GCA) [44], or in a AC or DC electric arc furnaces at higher temperatures up to 3000 °C (in the furnace center) to give rise to electrically calcined anthracite (ECA) [10, 45]. Calcined anthracites have a higher carbon content higher thermal and electrical conductivity than the parent anthracite coal. Both types of calcine are used in the manufacture of carbon electrodes, cathodes, carbon lining pastes, and carbon blocks.

Calcined Petroleum and Pitch Cokes

Petroleum coke (petcoke) is a by-product of crude oil refining. Approximately 90% of petroleum coke is produced by the delayed coking process [44]. Petcoke also must be calcined prior to use in carbon products. Calcined petcoke (CPC) has been used for more than 120 years in the manufacture of carbon anodes used in the Hall-Héroult aluminum smelting process [46]. Gas-calcined petcoke is produced by heat treatment of green petcoke at temperatures above 1200 °C in gas-fired kilns and shaft furnaces. Raw petcoke is obtained directly from the coker unit and can be classified according to its different structural forms [46]:

  • Needle coke has a developed fibrous, highly crystalline structure, a low coefficient of thermal expansion (CTE < 2.0–10–6 K−1), high electrical conductivity, and low heteroatom and sulfur contents; thus, it is widely used in the manufacture of selected electrodes and lithium-ion batteries [47]. Super-premium grade needle coke is used in the production of high-quality ultra-high power graphite electrodes for electric arc furnaces. The quality of needle coke depends both on the content of aromatic hydrocarbons, heteroatoms, and mechanical impurities and on the viscosity of the feed. Needle coke is produced from highly aromatic feedstocks, such as decanted heavy oil produced in a fluid catalytic cracker [48, 49].

  • Sponge coke is the preferred structure for anode production, despite having a slightly higher CTE (3.5–4.8·10–6 K−1), since it has open porosity that allows good pitch penetration during mixing. This is beneficial for its mechanical properties, as an interlocked structure develops after anode baking.

  • Shot coke is characterized by its spherical particle shape, dense and granular texture, high CTE (> 5.5·10–6 K−1), and high sulfur and metal contents, including V and Ni. It is obtained from crude oils rich in resins and asphaltenes, which are high molecular weight constituents.

As mentioned above, petroleum cokes must be heat treated at temperatures above 1200 °C before they can be used to produce carbon electrodes. After calcination, the petroleum coke adopts a more ordered carbon structure, which is more electrically conductive and less reactive to air (oxygen) and CO2. Specifically, the CPC used for anode production has a low ash content (< 0.3 wt%) and a high carbon content (> 97 wt%, depending on sulfur content).

Compared to the conventional petroleum-based needle coke, coal tar-based needle cokes exhibit certain distinctive properties, such as higher thermal stability and lower CTE (intermediate: 3.1–4.0·10–7 K−1; premium: 2.0–3.0·10–7 K−1; super-premium grade: < 2.0·10–7 K−1) [50].

Natural Graphite

Natural graphite is a critical raw material as classified by the European Union and the USGS [5, 6]. The dominant producing country is China [51]. Natural graphite has particular morphological characteristics that make it difficult to replace in certain applications, even with synthetic graphite. At present, there is major demand for natural graphite for the lithium-ion battery industry [52]. There are three basic morphological forms of natural graphite: vein, flake, and amorphous [51].

Coal Tar Pitch

Perhaps the most versatile and widely employed carbonaceous binder is coal tar pitch (CTP). CTP is a thermoplastic material with an extremely complex aromatic structure composed of thousands of polycyclic aromatic hydrocarbons and hybrid compounds containing heteroatoms (e.g., O, N and S) [53, 54]. Due to its unique properties, coal tar pitch has been the anode binder of choice since the early days of the aluminum smelting industry. Coal tar pitch is produced primarily by vacuum flash distillation of coal tar, which in turn is a by-product of the coking process of bituminous coal for use in blast furnace ironmaking [55]. In this process, the tar is first distilled under atmospheric conditions to produce a soft pitch with a softening point of 80–90 °C, which is further distilled under vacuum to produce a pitch with a softening point of approximately 110 °C. Table 2 provides typical physical and chemical properties of pitch that are important for its good performance in the production of quality anodes. Many of these pitch properties depend on the properties of the coal tar from which the pitch is produced [56].

Phenolic Resins

Phenolic resins, also known as phenol–formaldehyde (PF) resins, are thermosetting resins produced from the polymerization of phenol and formaldehyde. The process involves electrophilic aromatic substitution at the ortho- and para-positions of the phenol, followed by crosslinking of the polymer chains [61]. The raw materials for commercial PF resin are petroleum-derived phenol and formaldehyde. Novolacs are a class of PF resins with a formaldehyde-to-phenol ratio of less than one. Polymerization is completed by acid catalysts, and hexamethylenetetramine is a hardener added to facilitate crosslinking. At temperatures above 90 °C, crosslinking occurs by the formation of methylene and dimethylene bridges. Resoles are another class of base-catalyzed PF resins with a formaldehyde-to-phenol ratio of greater than one; these are single-step resins that require no hardener and thermoset above about 120 °C due to crosslinking by methylene and methyl ether bridges [62].

Phenol–formaldehyde resins make versatile binders as they exhibit good mechanical properties, thermal and chemical resistance, electrical insulation, and flame retardancy [63]. Phenolic resin, unlike CTP, is a hard carbon that is not readily graphitizable [64].

Critical Properties

Reactivity

An important characteristic of carbon materials in electrodes and refractories is sufficiently low reactivity rates with air (O2), CO2, and H2O(g) in a furnace atmosphere. Carbon in the electrode oxidizes according to chemical reactions:

$${\text{C + O}}_{{2}} {\text{(g) = CO}}_{{2}} {\text{(g)}}$$
(1)
$${\text{C + CO}}_{{2}} {\text{(g) = CO(g)}}$$
(2)
$${\text{C + H}}_{{2}} {\text{O(g) = CO(g) + H}}_{{2}} {\text{(g)}}$$
(3)

Reactions 2 and 3 are the well-described Boudouard and water–gas shift reactions. These are surface reactions and as such available surface area plays a role, but also “active” sites such as edges, dislocations, oxygen functional groups, and inorganic impurities facilitate in chemisorption of gaseous species. Inorganic components in the carbon (alkali metals for example) are known to catalyze the oxidation of carbon [8] (Table 3).

Rheological Properties

Coal tar pitch is a non-Newtonian fluid [58, 66], i.e., the viscosity is not independent of the shear stress, and more precisely been described as a shear thinning fluid [30] where viscosity decreases under shear strain. This has implications for how Söderberg electrode paste flows when filling the electrode casing.

Wettability

The degree of wetting is of great importance in understanding how the aggregate and binder interact with each other during mixing, which is a crucial step in the preparation of all types of carbon electrodes. The wettability of the pitch on an aggregate determines the quality of the binding between the two materials [67]. The interaction between the pitch and the substrate depends on the physical characteristics of both binder (softening point, surface tension, viscosity, etc.) and substrate (particle size, texture, porosity, etc.), but more importantly on the presence of functional groups on the surface of the substrate that can form chemical bonds with the pitch [68].

The contact angle is a measure of the ability of a liquid to spread over the solid surface. The sessile drop technique consists of monitoring the change of the contact angle with time under an inert gas atmosphere at a certain temperature. A smaller contact angle implies better wettability of the substrate by the pitch. If there is an increase in heteroatoms and heteroatom-containing functional groups, a decrease in the contact angle will be observed. The presence of heteroatoms enhances hydrogen bonds and condensation reactions, thus improving the wettability of the substrate by the pitch. In addition, the presence of C=C bonds, indicative of aromatic content, can enhance wetting through electrostatic interactions [69].

Graphitizability

Graphitizability is an extremely important property in these applications. Coal tar pitches and petroleum cokes are readily graphitizable. This is due to the formation of a “mesophase” stage during the heating [70]. At temperatures above approximately 400 °C, aromatic hydrocarbons go through an intermediate liquid or plastic stage, commonly referred to as mesophase. So-called mesophase pitches are aromatic anisotropic pitches characterized by a highly oriented structure, and high purity resulting in a high coke yield. Due to their excellent electrical, graphitizing, mechanical, and thermal properties, they are used in a wide variety of applications, such as carbon fibers, graphite electrodes, needle cokes, and high-quality binder pitches [71]. Most anthracites are partially graphitizable with simple heat treatment, though some of the organic material is hard carbon and will not graphitize even at 2900 °C [43].

Graphitization Phenomena

Graphitization is the transformation of a disordered (turbostratic) carbon material into a polycrystalline structure. The graphitization of soft carbon materials is very energy intensive and requires thermal processing up to 3000 °C for complete transformation. In very simplistic terms thermodynamically, the transformation of amorphous carbon to crystalline graphite is an exothermic, spontaneous process [72]. Kinetically, graphitization is considered a growth process with a single-valued activation energy, in the order of 962 ± 60 kJ/mol in the temperature range of 2300–2900 °C, with the rate-controlling mechanism being vacancy diffusion [73]. As a result, very high processing temperatures are required to achieve a sufficiently rapid rate of graphitization.

A landmark contribution by Biscoe and Warren studied carbon black structure by X-ray diffraction [74]. They introduced use of the term “turbostratic” to describe the structure of carbon where graphene layers are “stacked together roughly parallel and equidistant, but with each layer having a completely random orientation about the layer normal.” In other words, the basal graphene planes are out of alignment in a turbostratic carbon material [75].

Seminal work by Rosalind Franklin in 1951 [75] studied graphitization with X-ray diffraction and was the first to categorize “graphitizing” and “non-graphitizing” carbon materials. In graphitizing carbons, “neighboring crystallites have a strong tendency to lie in nearly parallel orientation.” Non-graphitizing carbons are characterized by random orientation of the crystallites (isotropic) with strong crosslinking and fine-structure porosity. Franklin concluded that crystallite growth takes place by the displacement of graphene layers or groups of layers, so the original orientation of the crystallites impacts this growth. In non-graphitizing carbons, the random orientation and crosslinking impede further growth [75].

Mesophase Formation

As mentioned above, coal tar pitch and petroleum coke graphitize easily since they undergo an observable “mesophase” stage transformation during thermal treatment. Figure 5 is a polarized light micrograph of coal tar pitch at 50 micrometer scale showing mesophase spherules. Above approximately 400 °C, aromatic hydrocarbons form an intermediate liquid or plastic stage, commonly referred to as mesophase. The term “mesophase” was firstly used in the work of Brooks and Taylors in 1965 [76]. In this stage, the carbonaceous material behaves like a nematic liquid crystal, which is characterized by molecules being disk-shaped and arranged in layers with their long sides aligned in parallel lines [77]. Aromatic hydrocarbons condense into polyaromatic molecules of high molecular weight (approximately 1000 g/mol), and organize into spherulites, gradually increasing in size and aligning due to mutual Van der Waals forces, which eventually leads to the formation of “green cokes.” The green cokes contain volatiles ranging from 6 to 20 wt.%, which are released upon heating to above 600 °C [77].

Fig. 5
figure 5

Polarized light micrograph of CTP mesophase spheres (courtesy of Elkem Carbon)

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Elucidation of Graphitic Structure

X-ray diffraction is the tool that has been most widely used to elucidate the structure of carbon materials. Crystallinity in carbon is indicated by the development of 002, 004, and 101 peaks (Fig. 6).

Fig. 6
figure 6

Schematic of diffraction patterns of hexagonal graphite, adapted from [78] and [79]

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Crystallite size is quantified by X-ray diffraction from the span of the (110) and (002) lines; this is a relative value useful in determining degree of graphitization. The interplanar spacing (d002) is also another metric of graphitization degree.

Raman spectroscopy can also be employed to determine the graphitic structure of hard carbon materials. There are two main bands in the graphite spectrum: the G-(graphitic) and the D-band. The G-band is a resonant band observed in both graphene and graphite at ~ 1580 cm−1, representing the planar configuration of symmetrically sp2-bonded carbon in graphene. It is often used to determine the thickness of graphene layers. In contrast, the D-band is a resonant band found at ~ 1350 cm−1, indicating defects in the carbon material. Unlike the G-band, which is independent of the excitation laser frequency, the D-band shows dispersive behavior, and its shape and position may vary with different excitation laser frequencies [80]. The degree of disorder in hard carbon materials can be estimated by the ratio of the integrated values of the D-band and G-band (ID/IG) [81].

Degree of Graphitization

Graphitization is not simply the ordering of perfect graphite layers within a 3D structure. The three-dimensional ordering of carbon occurs only after the graphene layers have reached a near perfect two-dimensional structure. The controlling mechanism of the graphitization process is annealing out of defects. Perfecting of defects within the graphene layers comprises the removal of heteroatoms (i.e., oxygen, sulfur, and nitrogen), vacancies (Schottky defects), crosslinking functional groups, and interstitial carbon atoms [8] (Fig. 7).

Fig. 7
figure 7

Turbostratic stacks of graphene layers, La and Lc are the crystallite width and height [8]

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The degree of graphitization can be quantitatively characterized for turbostratic carbons and graphite using the following equation:

$${\text{Degree of graphitization }}\left( \% \right) = \frac{{0.3440 - d_{002} }}{0.3440 - 0.3354} \cdot 100$$
(4)

In this equation:

  • 0.3440 nm (or 344 pm) represents the lowest possible value of d002 spacing in a turbostratic carbon.

  • 0.3354 nm (or 335.4 pm) represents the d002 spacing of graphite.

Catalytic Graphitization

Catalytic graphitization could be a potential process route to transform hard carbon materials into highly graphitic materials. The phenomenon of catalytic graphitization of carbon has been researched since at least the early 1950s. In the past few years, there has been renewed interest worldwide in this subject, due to electric vehicle (EV) market demand and lithium-ion battery (LIB) production [82]. There are many known catalysts that have been demonstrated to significantly increase the rate of graphitization of both soft and hard carbon materials.

There is a comprehensive review article from 1982 by Ōya and Marsh that still stands as a valuable source, covering the catalytic graphitization by the entire known array of catalytic elements and compounds [83]. According to these authors, the phenomenon of catalytic graphitization has been known since the advent of the Acheson process in 1900. A large body of research has been contributed by Japanese and German investigators over several decades. They cited several earlier reviews of catalytic graphitization back to 1968–1971. They defined catalytic graphitization as “the enhancement of the crystallinity of the carbon by the formation of graphite material involving a chemical reaction between the ungraphitized carbon and the metal or inorganic compound which constitutes the graphitization catalyst.” Ōya and Marsh categorized the phenomena of catalytic graphitization into the four categories or processes: the G-effect, the Ts-effect, the A-effect, and the Tn-effect.

The G-effect

This effect involves the formation of graphitic carbon within a less-crystalline parent carbon when heated in the presence of a graphitization catalyst in appreciable quantity (particle size > 100 nm). The resulting material contains both a broad profile from the parent carbon and a sharp profile from the graphitic component (2θ = 26.5°.

The T s-effect

When a finely divided catalyst (particle size of ~ 20 nm) is heated in the presence of a non-graphitizing parent carbon, the turbostratic Ts-component is formed (2θ = 26.0°. This component has a more ordered structure than the parent carbon but lacks the three-dimensionally ordered graphitic structure.

The A-effect

A more homogeneous catalytic graphitization occurs when the parent carbon is heated in the presence of a very finely divided catalyst, such as vaporized metal or elemental substitution in the carbon crystallite. This process results in a single sharpened profile of diffraction and a more homogeneous structure.

The T n-effect

In this effect, a non-graphitizing carbon heated to graphitization temperatures produces a complicated X-ray diffraction profile with two small but sharp peaks, the Tn-component and the G-component (2θ = 26.0° and 26.5°), respectively. The Tn-component becomes more pronounced when a non-graphitizing carbon is heated in the presence of a suitable catalyst, such as charcoal with calcium vapor.

Conditions of Catalytic Graphitization

A catalytic graphitization process (Fig. 9) consists of several steps. Raw carbon material is first prepared by physical and chemical pre-treatment. A carbonization step at lower temperature prior to catalyst incorporation is an optional step recommended by some researchers [84, 85]. Large quantities of catalyst (upwards of 11–33 wt% [86]) is incorporated to pre-treated or carbonized material by impregnation (e.g., solutions prepared from metal salts) or physical mixing (e.g., metal oxides). Graphitization at elevated temperature is the main stage of the process. The presence of catalyst helps to reduce the required heat treatment temperature. After graphitic carbon is attained, catalyst residues need to be removed, as they may hinder graphite properties in future applications. Acid washing is the most widespread method of catalyst removal on smaller scales, but other methods should be considered as it involves the use of large volumes of environmentally unfriendly solvents on larger scales.

Graphitization can be performed with different carbon precursors, using different catalyst compositions, at different temperatures and with the addition of activating agent and/or pre-treatment [88].

Lignocellulosic biomass mainly consists of three biopolymers: lignin, cellulose, and hemicellulose. As lignin shows a high concentration of aromatic hydrocarbons, carbon precursors with higher lignin and lower cellulose fractions, and lower oxygen and higher nitrogen contents, result in higher yields of more graphitizable and conductive carbon materials with reduced number of defects.

The addition of catalyst accelerates the graphitization by reducing the activation energy during the conversion of both graphitizing and non-graphitizing carbons into graphitic carbon. Increasing the catalyst loading on a carbon enhances the degree of graphitization. In other words, crystallite size increases with concentration of catalyst. According to Ōya and Marsh [83], certain catalysts can form A-, Ts- and G-components depending on their concentration. For example, the addition of less than 1 wt% of boron to a non-graphitizing carbon yields the A-component, while 1 to 5 wt% produces the Ts-component, and 10 wt% leads to the formation of the G-component. The use of more finely dispersed catalysts gives rise to the formation of Ts-component graphite. The Ts-component is observed in carbons containing nickel particles with particle size of about 20 nm. In contrast, when the particle size exceeds 80 nm, only the G-component is formed [83].

As heat treatment temperature (HTT) increases, carbon structure becomes more ordered and a higher degree of graphitization is obtained. Catalyst particles nucleate and grow into larger ones with the increase in temperature, which facilitates the formation of shell-like graphitic nanostructure that will eventually evolve into graphite. For example, when an iron catalyst is heated with a non-graphitizing carbon, the amount of G-component graphite increases with heat treatment temperature [83].

Graphitization Catalysts

Transition metals are the most studied graphitization catalysts due to their ability to form stable carbides. Among them, Fe, Co, and Ni stand out due to their high catalytic activity. Iron in particular shows better graphitizing ability than Co and Ni due to its lower number of vacant electrons in the d-shell orbitals, which allows it to accept more electrons from carbon so that it can form covalent bonds and be dissolved by the metal [89]. In addition, Fe exhibits the maximum solubility of carbon at high temperatures among these three metals when it is in its austenite phase [90]. Conversely, other metallic elements that are able to form stable carbides and are demonstrated graphitization catalysts include Pt, Ti, Zr, V, Ta, Cr, Mo, W, Mn, and Re. Possible mechanisms have been proposed in the literature [89]:

Dissolution–Precipitation Mechanism

Non-graphitizable carbons are mixed with transition metal catalysts and subjected to high-temperature heat treatment. These carbons decomposed into a disordered carbon matrix and interact with metals in their reduced form or in the form of metal carbides. These metal particles and/or carbides are homogeneously distributed over the disordered carbon matrix and by heat treatment, the carbon diffuses and dissolves in the metal particles and/or carbides until saturation at a given temperature and period of time. When this temperature is reduced, the metal is supersaturated and the carbon precipitates forming graphite crystallites. Therefore, the solubility of the carbon in the metal is a key factor in this mechanism. Copper, for example, exhibits low carbon solubility compared to nickel and cobalt [91]. On the other hand, the solubility of carbon in iron depends on which phase (α, γ, δ) is formed, with the austenite phase (γ-Fe) showing the highest solubility.

Metal Carbide Formation-Decomposition Mechanism

During heat treatment, transition metals react with non-graphitizable carbons giving rise to metal carbides, which eventually decompose into reduced metal and graphite with increasing temperature. In this mechanism, metal carbides do not act as reaction products, but as intermediates. Graphite is formed as a result of the decomposition of these metal carbides. Several properties must be taken into consideration, such as the strength of the metal–carbon bond in metal carbides, the carbide thermal stability, and the carbon content in the carbides. For instance, Fe and Mo form more stable carbides at lower reaction temperatures than Ni and Co [91].

$${\text{M }} + {\text{ C }} \to {\text{ MC }}\left( {{\text{carbide}}} \right) \, \to {\text{ M }} + {\text{ Graphitizable carbon }} \to {\text{ M }} + {\text{ Graphite}}$$
(5)

In a fairly recent study, Gómez Martin et al. [92] pyrolyzed medium density fiberboard (MDF) wood pieces impregnated with ferric chloride ranging at different temperatures ranging from 850 to 1600 °C in order to describe the different states of iron during the carbonization-graphitization process (scheme 1). At temperatures below 700 °C, Fe from ferric chloride undergoes different oxidation states. Reduction process starts from Fe2O3 to Fe3O4 until finally forming FeO at around 700 °C. Above 700 °C, FeO is completely reduced to Fe0, and immediately Fe3C is formed and graphitization begins. Decomposition of Fe3C starts above 750 °C and the austenite (γ-Fe) phase is formed. Upon cooling, formation of the ferrite (α-Fe) phase may occur giving rise to additional graphitic regions. The greatest degree of structural development takes place in the temperature range from 1000 to 1600 °C, with an increase in the average size of the graphitic domains and with flattening and stacking of the graphene sheets. The use of large particle size catalysts favors the reduction of sheet curvature and a better stacking of graphene sheets with less turbostratic disorder.

$${\text{FeCl}}_{3} \to {\text{ Fe}}_{2} {\text{O}}_{3} \to {\text{ Fe}}_{2} {\text{O}}_{4} \to {\text{ FeO }} \to {\text{ Fe}}^{0} \to {\text{ Fe}}_{3} {\text{C }} \to \, \gamma - {\text{Fe}}$$
(6)

Boron in different forms has been demonstrated to be an effective catalyst. In 1979, Ōya et al. [93] published a scientific article studying the graphitizing properties of elemental boron in both soft and hard carbons under different heating treatment temperatures and catalyst concentrations. Since then, numerous articles have been published on this topic, exploring the use of boron in its various forms: elemental boron, boric acid, boron salts, boron oxides, etc. In the work of Talabi et al. [94], three mechanisms have been proposed for the catalytic graphitization of phenolic resin hard carbons using boron oxide and boric acid as catalysts:

  1. i.

    Dissolution–precipitation: Liquid boron oxide with a melting point of 450 °C is obtained by the oxidation of boron compounds and acts as solvent for carbon dissolution. The formation and break of B–O–C at approximately 800 °C bond promote graphitization. This is the more accepted reaction pathway.

  2. ii.

    Carbide formation-decomposition: Boron carbide (B4C) is formed when temperature of carbonization exceeds 500 °C. The cleavage of B–C bond gives rise to graphitic carbon.

  3. iii.

    Incorporation of boron atoms via substitution: The carbon structure becomes more ordered during carbonization and boron slips within the carbon lamellae by occupying interstitial sites of the pre-existing carbon.

In addition to its graphitizing properties, boron is of interest in the aluminum industry, as it is used in coatings for protecting carbon anodes. Carbon anodes are sensitive to air and CO2 at the high temperatures observed in the aluminum smelting process. The inherent reactivity of carbon can be reduced by introducing boron into the anodes, since it has been found to inhibit the oxidation of graphitic carbons. However, it is necessary to find the optimum concentration of boron in carbon anodes so that the quality of the metals produced and the catalytic properties of boron are not compromised. Boron is often not desirable in end products, which could preclude its use.

Novel catalysts, such as graphene oxide, have been studied in the catalytic graphitization of sucrose [95], furan [96], and phenolic resins [81]. Zhang et al. [81] prepared slurries with different concentrations of graphene oxide and phenolic resin, which were cured at 150 °C and then carbonized at 1400 °C, and graphitic carbons with improved electrical properties were successfully obtained. The proposed catalytic graphitization mechanism is as follows (Fig. 8):

  • Stage I (< 200 °C): Phenolic resin molecules undergo polymerization, giving rise to intermediate crosslinked macromolecules rich in hydroxyl functional groups.

  • Stage II (< 500 °C): The hydroxyl groups of the phenolic resin and its intermediates interact with the oxygen-containing functional groups on the graphene oxide layer. During the pyrolysis process, volatile molecules such as water, CO, and CO2 are released through the graphene layers without generating porosity.

  • Stage III (> 1000 °C): The aromatic rings adjacent to the graphene layer exhibit a strong tendency to align due to their similar structures and the local graphitization of hard carbon materials is induced during the high-temperature carbonization, resulting in lower specific surface areas, lower amount of trapped defects, and improved electrochemical properties.

Fig. 8
figure 8

Catalytic graphitization process. Adapted from [88]

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Bio-binders

Technical Lignins

Lignocellulosic biomass in general is comprised of the main components cellulose, hemicellulose, and lignin. Lignin content in woody biomass ranges from 15 to 40%, and only 1–2% of the total 50–70 million tons of lignin generated annually is transformed into value-added materials [97].

Technical lignins, or native lignins, are by-products generated in enormous quantities mainly from wood pulping processes for the production of paper products. There are Kraft and soda lignins, lignosulfonates, organosolv, and hydrolytic lignins.

Kraft Lignins

Kraft lignin is derived from black liquor from the Kraft pulping (sulfate) process and stands for 85% of the total world lignin production [98]. Here, wood chips are cooked with hot water, sodium hydroxide, and sodium sulfite (white liquor) in pressurized digesters to break down bonds between cellulose, hemicellulose, and lignin.

Lignosulfonates

Lignosulfonates are recovered from the spent pulping liquor from sulfite pulping [56]. These are water-soluble bio-binders that already find wide application as binders in well paper, particle board, linoleum flooring, and charcoal briquetting.

Pyrolysis Oil

Pyrolysis between 450 and 600 °C at low oxygen partial pressure at a very rapid heating rate results in the generation of solid, liquid, and gaseous products [99]. The liquid condensate from the rapid cooling of the hot gas is called bio-oil or pyrolysis oil. The solid phase is bio-char. Fast pyrolysis maximizes the yield of liquid product, whereas slow pyrolysis (carbonization) aims to maximize the yield of solid product. The starting biomass feedstock has a large impact on the characteristics of the resulting pyrolysis oil.

Pyrolysis oil basically consists of lignin fragments emulsified in a water soluble, carbohydrate-rich phase [99]. The oil is characterized by high water content and oxygen-bearing monomers and oligomers. Oxygen contents in pyrolysis oil can be upwards of 20 wt%. Acidic solid catalysts can be employed in situ or ex situ to reduce the oxygen content.

Pyrolysis oil, to a limited degree, can be used directly as a binding agent, for example, in the briquetting of bio-char [100].

Derivatives of Pyrolysis Oil

Further refining of pyrolysis oil yields by aqueous treatment yields a water-insoluble pyrolytic lignin (PL) fraction and a hydrophilic pyrolytic sugar fraction [99, 101]. These two fractionates have very different characteristics.

Pyrolytic Lignin

High-purity pyrolytic lignin results from treatment with a high water-to-oil ratios, and the resulting PL is in the form of a precipitated fine powder [99, 102]. The softening point of solid PL can range from 90 to 100 °C and carbonization begins at 200 °C. Due to its high content of phenolics, pyrolytic lignins are also suitable feedstocks to produce phenol–formaldehyde resins [102].

Pyrolytic Sugars

Aqueous washing of pyrolysis oil yields 60–70 wt% pyrolytic sugars on a wet basis. Pyrolytic sugars are cellulose-derived anhydrosugars, primarily levoglucosan, pentoses, organic acids, and furans [99]. It could be possible to utilize liquid-phase pyrolytic sugars directly as binders in some applications.

Bio-pitches

Simple thermal distillation of pyrolysis oil yields so-called bio-pitches. Water and low-molecular mass molecules are vaporized, while polymerization starts which converts the oil into a thermoplastic pitch-like material with a defined softening point.

Rocha et al. in 2002 produced bio-pitches from eucalyptus pyrolysis oil by heat-treating in a stainless steel reactor at 250–270 °C and reduced pressure [103]. The resulting bio-pitch was black and solid at room temperature. The softening points ranged from 56 to 108 °C. The bio-pitches contained low ash contents (0.5–0.15 wt%), favorable carbon contents, though high oxygen contents from 21.9 to 23.1%. Carbon “electrode” samples consisting of bio-char aggregate and bio-pitch binder were prepared and graphitized, though no description was given of the final electrical properties.

More recently, research has been conducted on generation of bio-pitch for application in carbon anodes for aluminum electrolysis [104,105,106]. In these studies, bio-pitch was produced by thermal treatment of pyrolysis oil to 160, 180, and 200 °C and at atmospheric pressure at varying heating rates. Pyrolysis oil was derived from fast pyrolysis of mixed softwood. A main focus of the research was on the resulting thermophysical properties of the bio-pitch and the practical implications in prebaked carbon anode production such as softening points, viscosity, and wetting properties. Functioning carbon anodes made with bio-pitch binder were demonstrated on the laboratory scale [107]. Though it has been demonstrated that bio-pitch can be produced with favorable softening points and rheology for anode carbon electrodes, key difficulties that remain to overcome are high oxygen content, low coking value (~ 34.9 wt%), and low graphitizability [105, 107].

Bio-derived PF Resins

Research is being carried out to develop phenolic resins from lignins [108,109,110,111]. Challenges with lignin-derived phenol are low reactivity with formaldehyde (to synthesize phenolic resin), high molecular weight, and steric hindering [109].

Mixed Binders

A pragmatic approach could be to blend bio-derived and fossil-derived binders, for example, blending coal tar pitch and biomass pitch [54] or phenolic resins [112].

Biocarbon Aggregates

Bio-chars

Bio-char, or charcoal, is a solid residue of pyrolysis. Charcoal is a low-density hard carbon such that its electrical conductivity remains low even after thermal treatment, which currently precludes its use in most electrodes in any sizeable percentage without considerable processing. Catalytic graphitization of charcoal could be one approach to transform charcoal into a material suitable for electrode and refractory applications [83], though there are considerable technical challenges to overcome such as efficient catalyst separation. For prebaked anodes for aluminum reduction, charcoal as a direct substitute for the fossil carbon aggregate in carbon electrodes is not technically feasible to more than ten percent [113]. Additionally, alkali and alkaline earth metals present in the ash catalyze the reactivity with oxygen and CO2 [114]. What is more, high porosity and low density of bio-char materials limit application as carbon aggregate materials, though this could possibly be overcome by comminution and agglomeration [115].

Carbonized Lignin

Catalytic graphitization of lignin by-products could be another approach to generating solid carbon aggregate [116]. Approximately 50 million tons of Kraft lignin is produced worldwide, yet only a fraction (less than 2%) is utilized for other uses than fuel [117].

Processing to Remove Catalysts

As described above, effectively demonstrated catalysts for catalytic graphitization are transition metals such as iron, nickel, or cobalt. In general, large quantities of finely divided and highly dispersed catalyst must be added to achieve an effect. Ōya and Marsh [83] pointed out that industrial catalytic graphitization has not taken off due to the “undesirable characteristics of graphites containing significant amounts of residual catalyst material.”

If catalytic graphitization is ever to have a chance to be implemented on an industrial process scale at thousands or millions of tons of material generation per year, a practical and sustainable method to efficiently remove the large quantities of catalyst removal needs to be developed. Demonstrated laboratory process routes for iron removal have typically been variations of acid washing. Scale-up of such a process could be fraught with environmental challenges due to generating large amounts of acidic effluent with dissolved heavy metals.

Thermal Purification

Another method of catalyst removal could be the thermal treatment of catalyzed bio-graphite at very high temperature in excess of 2500 °C. This may at first sound extreme, but it is pointed out that conventional routes for electrode production already involve such unit operations as electrical calciners or Acheson furnaces for calcination or graphitization. The boiling point of iron is 2862 °C and could be driven off in the vapor phase at very high temperature off in the vapor phase. This has also been described by [118, 119] for purification of natural graphite from lithium-ion battery (LIB) scrap and natural graphite. Of course, such thermal processing is very energy intensive and has technical challenges of its own such as effectively collecting large amounts of condensed vapor produced in the process.

Chloridizing Roasting

Another viable processing alternative that has precedence in the graphite industry is chloridizing roasting; this is already an established process route for purification of natural and synthetic graphite for ultra-purity applications [41]. Roasting involves selectively forming metal chloride species with high vapor pressures at moderate temperatures to extract them out of the system in the vapor phase.

Iron catalyst added to the biocarbon to effect graphite formation is present in the carbon is in the form of iron carbide; thus the chloridization reaction involves forming iron chloride vapor by the reaction:

$${\text{Fe}}_{{3}} {\text{C + Cl}}_{{2}} {\text{(g) = 2FeCl}}_{2} {\text{(g)}}$$
(7)

In the presence of carbon, this is often described as carbo-chlorination as carbon oxidation maintains a very low oxygen potential. A phase stability diagram for the system Fe–Cl–C at 1000 °C (Fig. 9) shows that formation of FeCl2(g) vapor species is possible already at a low chlorine partial pressure of 10–8 atm.

Fig. 9
figure 9

Phase stability diagram for the Fe–C–Cl system at 1000 °C calculated by FactSage [87]

Full size image

A technically feasible and highly scalable operation could be to perform chloridizing roasting of bio-graphite in a circulating fluidized bed, as described by [120].

Summary and Conclusions

As outlined in detail above, fossil carbon materials find special applications in electrodes and refractories. These fossil carbon materials—aggregates of petroleum coke, anthracite, and natural flake graphite, and binders such as coal tar pitch have unique characteristics. These are so-called soft carbon materials that are in general highly graphitizable with favorable thermophysical properties such as high density, low porosity, and low reactivity with air and CO2.

Potential in Primary Aluminum Anodes and Cathodes

Partial substitution with bio-based aggregate in carbon anodes for aluminum reduction has already been demonstrated up to 10 wt% [113], though beyond that will require substantial modification of the material. Substitution of coal tar pitch binder with a suitable bio-binder could be less challenging from a rheological standpoint since prebaked electrodes are formed and baked statically. Challenges with low coking value, shrinkage, reactivity, and low graphitizability still have to be overcome.

Direct substitution of biogenic carbon in cathode production, unfortunately, does not appear feasible without extensive catalytic processing since these products typically require a high degree of graphitization.

Potential in Söderberg Electrodes

A challenging application for biogenic carbon would be in Söderberg electrode paste. Firstly, the carbon aggregate must have high density, low reactivity, and sufficiently low electrical resistivity. Secondly, the rheological properties of the binder have to be such that the flow of the paste after softening fills the electrode casing. The binder should then carbonize with low volumetric change and graphitize with increasing temperature. Replacement of coal tar pitch with an equivalent bio-binder is a material engineering challenge; tailoring the thermo-rheological properties of a bio-binder to approximate of coal tar pitch in a critical temperature interval up to carbonization could be possible.

Potential in Graphite Electrodes

Perhaps the most daunting challenge would be to develop entirely bio-based graphite electrodes. This is an application that requires electrodes that can operate under extreme conditions with high current density, and tolerance to thermal shock and mechanical stresses. Here, both the aggregate and binder used in the manufacture must be highly graphitized in the final product. High density, low porosity, and near complete graphitization are extremely challenging to achieve with biogenic precursor materials without considerable technological innovation.

Areas for Further Research and Technological Development

In summary, considerable additional development is still required from the laboratory scale and upwards to make possible biocarbon materials with the required properties for these particularly demanding applications in the massive quantities that are demanded by the metallurgical industries. Further development of processing techniques are needed to overcome the issues of the following:

  • Low graphitizability

  • High oxygen, alkali and alkaline earth metal contents

  • Low coking value

  • High porosity and low density

A clearly identified research area that could make a rapid environmental impact is the development of feasible bio-derived binders for electrode and refractory materials. The replacement of fossil-derived binders by bio-binders alone in carbon electrode and refractory materials would substantially reduce the carbon footprints of these products. Pyrolytic lignin and bio-pitches derived from pyrolysis oils with engineered properties already show promise to be utilized in carbon anodes and even in Söderberg electrode paste provided the rheological properties and graphitization degree can be modified. Bio-derived phenolic resins could find ready application in lining and ramming pastes as well as in oxide–carbon refractories.

Considering the favorable properties and large available tonnages, technical lignins could conceivably be processed into both aggregate carbon and binder. Graphitizable bio-binders and biogenic aggregates could be possible with efficient catalytic graphitization processing and subsequent catalyst removal. Transition metals such as iron have been demonstrated to be effective catalysts on the laboratory scale, though large amounts of finely dispersed catalyst are necessary. Catalyzation with graphene oxide could prove to be a feasible route that would not require catalyst removal.

Effective removal of large amounts of such heterogeneous catalyst such as iron has been demonstrated using acid washing, though this technique would certainly pose an environmental challenge if scaled up to the huge quantities required for manufacturing these products. This is identified as a major obstacle to increasing the technological readiness level of catalytic graphitization with iron for example. Perhaps somewhat less environmentally egregious routes to catalyst removal could be thermal purification or chloridizing roasting.