Comparison of the CO₂ Balance in Electroslag Reduction of Cadmium with Pyrometallurgical and Hydrometallurgical Recovery Methods

Abstract

This study presents a carbon footprint assessment of a novel electroslag method for cadmium (Cd) recovery from spent nickel–cadmium (Ni-Cd) batteries in comparison with the carbon footprints of pyrometallurgical and hydrometallurgical cadmium recovery methods. A comparison of CO₂ emissions in three types of technological processes during the recovery of 1 kg of cadmium is carried out. Energy inputs and CO₂ emissions are calculated for the electroslag process and compared to conventional methods, such as pyrometallurgical and hydrometallurgical reduction methods. The electroslag process eliminates cadmium vaporization by using molten KCl–NaCl flux and carbon under electromagnetic stirring. Cadmium reduction occurs under a layer of flux, which prevents the contact of the reduced cadmium with the atmosphere. The electroslag process temperature is limited to 700 °C, which is lower than the boiling point of cadmium (767 °C). The electroslag remelting process uses molten KCl–NaCl flux and carbon as a reductant under electrovortex flow stirring. The pyrometallurgical method for extracting cadmium from nickel–cadmium batteries is based on the reduction of cadmium with carbon at high temperatures. In the pyrometallurgical process, coal (anthracite) is used as the carbonaceous material, which can extract 99.92% of cadmium at 900 °C. Cadmium is separated using a vacuum at temperatures ranging from 800 °C to 950 °C for several hours. Hydrometallurgy is a metal extraction process involving chemical reactions that occur in organic or aqueous solutions at low temperatures. The hydrometallurgical process involves a series of acid or alkaline leaches, followed by separation and purification methods such as absorption, cementation, ion exchange, and solvent extraction to separate and concentrate metals from leach solutions.

1. Introduction

The objective of this article is to compare the CO₂ emissions from the recovery of 1 kg of cadmium using three processes: electroslag, pyrometallurgical, and hydrometallurgical.

Cadmium recovery from spent Ni-Cd batteries is environmentally critical due to cadmium’s toxicity. Conventional methods involve significant thermal or chemical input, often resulting in material loss or high CO₂ emissions.

This article examines three methods for recovering cadmium from cadmium hydroxide and cadmium oxide: electroslag, pyrometallurgical, and hydrometallurgical processes. This article presents a CO₂ balance based on realistic continuous operation parameters.

A recently proposed electroslag-based process eliminates cadmium vaporization by using molten KCl–NaCl flux and carbon under electromagnetic stirring [1]. Cadmium reduction occurs under a layer of flux, which prevents contact of the reduced cadmium with the atmosphere. The electroslag process temperature is limited to 700 °C, which is lower than the boiling point of cadmium (767 °C).

There are three main pyrometallurgical methods for recycling nickel–cadmium batteries: open-furnace heat treatment of cadmium oxide followed by condensation to form cadmium oxide powder; closed-furnace cadmium reduction with distillation of the reduced cadmium; and battery chlorination in a chlorine gas or hydrochloric acid atmosphere at 960 °C. This article discusses the process used in practice: closed-furnace distillation. This method is performed by companies such as SNAM-SAVAM (Saint-Quentin-Fallavier, France), SAB-NIFE (Oskarshamn, Sweden), and INMETCO (Ellwood City, PA, USA).

Unlike electroslag reduction and pyrometallurgy, the hydrometallurgical process involves mechanical crushing of batteries followed by physical separation of the structural elements, as well as dissolution and separation of valuable metals at temperatures not exceeding 100 °C. Hydrometallurgical technologies are more complex and require more stages than electroslag reduction and pyrometallurgy. Hydrometallurgy ensures selectivity in metal recovery.

2. Materials and Methods

2.1. Emissions Assessment Methodology

The assessment of CO₂ emissions will take into account the enthalpy ΔH of the chemical reaction of the reacting substances used to extract cadmium from cadmium batteries. Since the amount of cadmium compounds in different types of batteries is different, the calculation of CO₂ emissions will be made per kilogram of recovered cadmium. If the method does not provide the complete recovery of cadmium, but stops at an intermediate stage (obtaining cadmium oxide), then the energy and, accordingly, CO₂ emissions will be calculated based on the recovery of cadmium from cadmium oxide with carbon. This will be specified in each specific case. Crushing and extraction of cadmium-containing substances from the battery case will not be taken into account, since this procedure is similar for all types of recycling.

The following restrictions were imposed on the study of CO₂ emissions: Disassembly of spent cadmium batteries was not included in the calculation for comparing CO₂ emissions, since disassembly was similar for all processes. It also did not take into account the carbon footprint from the recycling of nickel–cadmium battery cases.

The calculation begins with obtaining a mass separated by magnetic separation, which consists only of Cd(OH)₂, Ni(OH)₂, Cd, NiOOH, and H₂O. The calculations assume that the batteries are completely discharged, so the electrode mass consists only of cadmium hydroxide Cd(OH)₂.

Three EU countries—Latvia, Germany, and France—are selected for CO₂ emission comparison. Latvia is chosen because the article’s authors are from that country. Germany is the driving force of the EU. France uses nuclear energy more than any other country.

2.2. Comparable Cadmium Reduction Processes

2.2.1. Electroslag Reduction Method: 700 °C, KCl-NaCl Slag, Carbon, and No Cd Evaporation

The essence of the electroslag reduction process of cadmium is that carbon (anthracite) is used as a reducing agent to recover cadmium from cadmium oxide. The density of the carbon is higher than that of the molten flux. The reactor body is a graphite crucible, in the lower part of which the reducing agent—carbon—is placed and covered with molten flux. A graphite electrode is immersed in the molten flux. The alternating current passing through the molten flux generates electromagnetic forces within the melt, inducing an electrovortex flow that stirs the flux.

When cadmium oxide is added onto the surface of the molten flux, it sinks due to its higher density. The electrovortex flow ensures continuous transport of cadmium oxide toward the carbon reducing agent, thereby accelerating the reduction reaction as a result of intensive mixing. During the reduction reaction, metallic cadmium is formed. Because the density of cadmium is much higher than that of the molten flux, cadmium accumulates beneath the flux layer.

The boiling point of cadmium is 767 °C, while the reduction reaction occurs at temperatures not exceeding 700 °C. This prevents cadmium vaporization and eliminates environmental contamination by cadmium emissions. The recovered cadmium is periodically tapped from the crucible. After tapping, fresh reducing agent (carbon) and cadmium oxide are added to the graphite crucible, ensuring continuous operation of the reduction process.

In this study, the energy consumption for the initial melting of the KCl-NaCl flux is not taken into account in the calculations for the electroslag reduction method of cadmium, as the flux is effective as a non-consumable transport and protective medium. The reduction of cadmium by the electroslag reduction method occurs as a continuous process with the periodic addition of cadmium oxide to the reactor and periodic draining of reduced cadmium; thus the corresponding energy consumption for melting the flux during a long-term continuous process of electroslag reduction of cadmium tends to zero exponentially.

Calculation of CO₂ emissions for electroslag reduction of cadmium with carbon (700 °C + two-component flux (KN)):

• Mass of CdO: 128 g (1 mol);
• Mass of carbon (C): 500 g (41.7 mol);
• Reaction:

Equation (1) describes the overall charge and discharge reactions in a Ni-Cd battery, which can be described as a cumulative reaction [2]:

$${{\mathrm{C}\mathrm{d}\left(\mathrm{O}\mathrm{H}\right)}_2+2{\mathrm{N}\mathrm{i}\left(\mathrm{O}\mathrm{H}\right)}_2}_{\overrightarrow{ charge}}^{\stackrel{discharge}{\leftarrow }}\mathrm{C}\mathrm{d}+2\mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}+2{\mathrm{H}}_2\mathrm{O}$$

(1)

For the subsequent production of cadmium from Cd(OH)₂ using the technology described in this study, calcination at 400 °C was employed. At a calcination temperature of 300 °C, the monoclinic γ-Cd(OH)₂ phase and the cubic CdO phase were formed. A pure cubic CdO crystalline phase was obtained at 400 °C [3]. Since boiling of the formed cadmium, which occurs at a temperature 767 °C [4], was undesirable, it was proposed to limit the temperature of the cadmium reduction reaction to 700 °C to prevent contamination of the production and the environment with cadmium vapor. The reaction of the decomposition of cadmium hydroxide into cadmium oxide and water corresponds to Equation (2).

$$\mathrm{C}\mathrm{d}(\mathrm{O}{\mathrm{H})}_2\to \mathrm{C}\mathrm{d}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(2)

The complete recycling cycle was achieved through the reduction of Cd, as described by Volynsky et al. [5], via the cumulative reaction occurring in the temperature range of 650–1100 °C, as shown in Equation (3):

$$2\mathrm{C}\mathrm{d}\mathrm{O}+\mathrm{C}\to 2\mathrm{C}\mathrm{d}+{\mathrm{C}\mathrm{O}}_2$$

(3)

The oxidation of carbon to carbon monoxide (CO), followed by the subsequent reaction of CO with CdO, also results in the reduction of Cd, as represented by Equation (4) and Equation (5), respectively:

$$\mathrm{C}\mathrm{d}\mathrm{O}+\mathrm{C}\to \mathrm{C}\mathrm{d}+\mathrm{C}\mathrm{O}$$

(4)

$$\mathrm{C}\mathrm{d}\mathrm{O}+\mathrm{C}\mathrm{O}\to \mathrm{C}\mathrm{d}+{\mathrm{C}\mathrm{O}}_2$$

(5)

In carrying out this study, a molar ratio for CdO:C of 1:41.7 was chosen, significantly exceeding the minimum ratio of 1:0.5 required for the reaction in Equation (3) to proceed, given that the reaction in Equation (4) proceeds completely, and the reaction in Equation (5) proceeds only partially. Such an excess amount of carbon prevents the oxidation of the obtained Cd on the slag surface [6].

We write Equation (3) in molar form:

$$2\times 128.4\left(\mathrm{C}\mathrm{d}\mathrm{O}\right)+1\times 12\left(\mathrm{C}\right)=2\times 112.4\left(\mathrm{C}\mathrm{d}\right)+1\times 44\left({\mathrm{C}\mathrm{O}}_2\right)$$

In Equation (3), during the reduction reaction of cadmium oxide with carbon, the reaction occurs with 2 moles of CdO and 1 mole of C. As a result, we obtain 2 moles of Cd and 1 mole of CO₂. As a result, when the reduction reaction occurs for 1 h with 1 mole of CdO, 0.5 mole of CO₂ is obtained, i.e., 22 g of CO₂.

A total of 1 kg of cadmium is 8.9 moles of cadmium. Since during the reduction reaction, 1 mole of CO₂ is produced per 2 moles of cadmium, then 4.45 moles of CO₂ are produced per 8.9 moles of reduced cadmium.

When 1000 g of cadmium (8.9 mol) is reduced, 4.45 × 44 = 195.8 g of CO₂ is released.

The thermal effects (enthalpy changes) of chemical reactions can be determined from the standard enthalpies of formation of the reactants and products in accordance with Hess’s law. The data for the calculations were taken from the NIST Chemistry WebBook online database of the chemical and physical properties of substances [7].

The standard enthalpy change in a reaction, ${\mathsf{\Delta }\mathrm{H}}_p^0$, is calculated as the difference between the sum of the standard enthalpies of the formation of the products and those of the reactants, shown in Equation (6), with stoichiometric coefficients duly taken into account.

$${\mathsf{\Delta }\mathrm{H}}_{\mathrm{p}}^0=\sum {\mathrm{H}}^0\left(\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}\right)-\sum {\mathrm{H}}^0\left(\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{s}\right)$$

(6)

The reaction involving cadmium reduction by carbon is shown next—Equation (3).

Table 1. Thermal effect (enthalpy change) of the reaction involving cadmium reduction by carbon.

	2CdO	C	2Cd	CO2
n, mol	2	1	2	1
${\mathit{H}}_{\mathbf{298}}^{\mathbf{0}}$, kJ/mol	−258.35	0	0	393.51

shows the thermal effect (enthalpy change) of the reaction involving cadmium reduction by carbon.

$${\mathsf{\Delta }\mathrm{H}}_{\mathrm{p}}^0=\left[\left(2\times 0\right)+\left(1\times 393.51\right)\right]-[\left(2\times \left(-258.35\right)\right)+(1\times 0\left)\right]$$

$${\mathsf{\Delta }\mathrm{H}}_p^0=911.17$$

The cadmium reduction reaction is endothermic.

Next, the reaction involving the decomposition of cadmium hydroxide into cadmium oxide and water is considered—Equation (2).

The workshop temperature of 25 °C was used as the reaction start temperature.

Table 2. Thermal effect (enthalpy change) in the decomposition reaction of cadmium hydroxide.

	Cd(OH)2	CdO	Н2О
n, mol	1	1	1
${\mathit{H}}_{\mathbf{298}}^{\mathbf{0}}$, kJ/mol	−563	−258.35	−241.83

shows the thermal effect (enthalpy change) in the reaction involving the decomposition of cadmium hydroxide.

$${\mathsf{\Delta }\mathrm{H}}_p^0=\left[\left(1\times (-258.35)\right)+\left(1\times (-241.83\right)\right]-\left[\left(1\times \left(-563\right)\right)\right]$$

$${\mathsf{\Delta }\mathrm{H}}_p^0=62.82$$

The decomposition reaction of cadmium hydroxide is endothermic.

The amount of cadmium hydroxide required to produce 1 kg of cadmium in a two-stage process was determined:

• Decomposition of cadmium hydroxide to obtain cadmium oxide.
• Reduction reaction of cadmium from cadmium oxide with carbon.

From Equation (3), written in molar form, the amount of cadmium oxide required to obtain 1 kg of cadmium was found.

$$2\times 128.4\left(\mathrm{C}\mathrm{d}\mathrm{O}\right)+12\left(\mathrm{C}\right)=2\times 112.4\left(\mathrm{C}\mathrm{d}\right)+44\left(\mathrm{C}{\mathrm{O}}_2\right)$$

From the stoichiometric equation written in molar form, the amount of cadmium oxide required to produce 1 kg of cadmium was determined by Equation (3).

From the molar masses, it follows that 2 mol of CdO (2 × 128.4 g) yields 2 mol of Cd (2 × 112.4 g).

Hence, the following proportion can be written:

$${\displaystyle \frac{128.4 \left(\mathrm{CdO}\right)}{112.4 \left(\mathrm{Cd}\right)}}={\displaystyle \frac{\mathrm{X} \left(\mathrm{CdO}\right)}{1000 \left(\mathrm{Cd}\right)}}$$

Solving the proportion gives:

$$\mathrm{X}={\displaystyle \frac{128.4\times 1000}{112.4}}=1142.3 \mathrm{g} \mathrm{CdO}$$

Therefore, 1 kg of cadmium requires 1142.3 g of cadmium oxide according to the stoichiometric relationship.

Next is Equation (2), written in molar form:

$$1\times 146.4\left(\mathrm{C}\mathrm{d}\right(\mathrm{O}{\mathrm{H})}_2)\to 1\times 128.4(\mathrm{C}\mathrm{d}\mathrm{O})+1\times 18({\mathrm{H}}_2\mathrm{O})$$

From the stoichiometric equation written in molar form, the amount of cadmium hydroxide required to produce 1142.3 g of cadmium oxide is determined.

From the molar masses, it follows that 1 mol of Cd(OH)₂ (1 ×146.4 g) yields 1 mol of CdO (1 × 128.4 g).

Hence, the following proportion can be written:

$${\displaystyle \frac{146.4\left(\mathrm{C}\mathrm{d}\right(\mathrm{O}{\mathrm{H})}_2)}{128.4 \left(\mathrm{Cd}\mathrm{O}\right)}}={\displaystyle \frac{\mathrm{X} \left(\mathrm{C}\mathrm{d}\right(\mathrm{O}{\mathrm{H})}_2)}{1142.3 \left(\mathrm{Cd}\mathrm{O}\right)}}$$

Solving the proportion gives:

$$\mathrm{X}={\displaystyle \frac{146.4\times 1142.3}{128.4}}=1302.44 \mathrm{g} \mathrm{CdO}$$

Therefore, 1142.3 g of cadmium oxide requires 1302.44 g of cadmium hydroxide according to the stoichiometric relationship.

Next, we determine the amount of energy required to heat the reagents Cd(OH)₂ and CdO to a given temperature.

To obtain 1 kg of cadmium in the final stage of the second stage, 1302.44 g of cadmium hydroxide Cd(OH)₂ is required in the first stage. According to the formula:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}({\mathrm{O}\mathrm{H})}_2}=\mathrm{C}\mathrm{M}({\mathrm{t}}_2-{\mathrm{t}}_1)$$

(7)

Next, we determine the amount of energy required to heat 1302.44 g of Cd(OH)₂ to a temperature of 300 °C.

Molar heat capacity of Cd(OH)₂ C=95 J/(mol $\times$ °C).

Mass of Cd(OH)₂: 1302.44 g or 8.9 mol.

Initial temperature t₁ = 25 °C.

Final temperature t₂ = 300 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}({\mathrm{O}\mathrm{H})}_2}={\displaystyle \frac{95\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (300 °\mathrm{C}-25 °\mathrm{C})$$

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}({\mathrm{O}\mathrm{H})}_2}=232.51 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required to heat 1142.3 g of CdO to a temperature of 700 °C.

Molar heat capacity of CdO C = 43.64 J/(mol × °C).

Mass of CdO: 1142.3 g or 8.9 mol.

Initial temperature t₁ = 300 °C.

Final temperature t₂ = 700 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}\mathrm{O}}={\displaystyle \frac{43.64 \mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (700 °\mathrm{C}-300 °\mathrm{C})$$

$$Q_{\mathrm{C}\mathrm{d}\mathrm{O}}=155.38 \mathrm{k}\mathrm{J}$$

Total energy expended in the two-stage process of the reduction of 1 kg of cadmium from cadmium hydroxide:

$$\sum \mathrm{Q}=232.51 \mathrm{k}\mathrm{J}+155.38 \mathrm{k}\mathrm{J}=387.89 \mathrm{k}\mathrm{J}$$

Forced cooling of reduced cadmium is not used in the electroslag process. Reduced cadmium is poured into uncooled molds. The reduced cadmium cools naturally under a flux layer to a workshop temperature of 25 °C, eliminating energy costs for cooling and ensuring an environmentally friendly process.

According to the power designation 1 J = 1 W × s, we obtain that the energy Q = 387.89 kJ spent on the reduction of 1 kg of cadmium corresponds to 0.11 kWh.

The energy expenditure corresponds to the emissions of a certain amount of CO₂.

Table 3. CO₂ emissions from different electricity sources (per 1 kWh).

Electricity Source	Emission Factor (kg CO2/kWh)	Source
Latvia (Nowtricity)	0.17	https://www.nowtricity.com/country/latvia/ (accessed on 31 July 2025) Average 2024 year
Germany (Climatiq)	0.33	Climatiq Germany
Germany (UBA 2023)	0.38	UBA Germany
France (LCA)	0.004	https://www.sfen.org/rgn/les-emissions-carbone-du-nucleaire-francais-37g-de-co2-le-kwh/ (accessed on 31 July 2025)
Nuclear (LCA ADEME)	0.006	ADEME France
Solar (UNECE) EU28	0.011–0.037	UNECE LCA 2021
Natural gas, EU28	0.43	UNECE LCA 2021

shows CO₂ emissions from different electricity sources in Latvia, Germany, and France (per 1 kWh).

CO₂ emissions were calculated for the organization of the process of the recovery of 1 kg of cadmium using the electroslag method for three countries—Latvia, Germany, and France. CO₂ emissions in the process of the chemical reaction reducing cadmium with carbon are at a constant value and are equal to 0.1958 kg of CO₂ per 1 kg of cadmium.

Table 4. CO₂ emissions for the reduction of 1 kg of cadmium with carbon using the electroslag reduction method, in kg.

Country	CO2 Emissions for the Reduction of 1 kg of Cadmium with Carbon Using the Electroslag Reduction Method, in kg
	During a Chemical Reaction, the Value is Constant	The Costs of Organizing a Technical Process	Total
Latvia	0.1958 kg	0.17 kg CO2/kWh × 0.11 kWh = 0.0187 kg	0.2145 kg
Germany (UBA 2023)	0.1958 kg	0.38 kg CO2/kWh × 0.11 kWh = 0.0418 kg	0.2376 kg
France, Nuclear Energy (LCA ADEME)	0.1958 kg	0.004 kg CO2/kWh × 0.11 kWh = 0.0004 kg	0.1962 kg

shows CO₂ emissions for the reduction of 1 kg of cadmium with carbon using the electroslag reduction method in Latvia, Germany, and France (per 1 kWh).

Figure 1 shows the CO₂ emissions from the reduction of 1 kg of cadmium with carbon in electroslag reduction, in kg. CO₂ emissions during electroslag reduction comprise CO₂ emissions from the chemical reaction of cadmium reduction (shown in blue) and CO₂ emissions due to the costs of organizing the technological process that ensures this chemical reaction occurs (shown in orange). CO₂ emissions from the chemical reaction of cadmium reduction using the electroslag process are the same everywhere, since the cadmium reduction reaction proceeds in the same way everywhere.

CO₂ emissions due to the costs of organizing the technological process that ensures the chemical reaction occurs vary depending on the method of electricity generation. Each country has a different mix of electricity generation methods: green energy (solar and wind), hydroelectric power plants, gas or coal-fired power plants, and nuclear power. This is clearly illustrated by the countries selected for comparison: Latvia, Germany, and France.

2.2.2. Pyrometallurgy (Distillation)

The conventional method for recovering cadmium from nickel–cadmium (Ni–Cd) batteries is carbothermal reduction. In this process, anthracite coal is employed as the carbonaceous reductant, enabling the extraction of 99.92% Cd at 900 °C, with a Ni–Co alloy obtained as a by-product. Process efficiency can be enhanced by applying vacuum conditions at 800 °C for 2.5 h [8].

A more recent pyrometallurgical approach for cadmium extraction from Ni–Cd batteries is based on distillation at elevated environmental temperatures [9,10,11,12,13,14]. In urban mining practice, three principal pyrometallurgical recycling techniques are employed:

• Heat treatment of cadmium oxide in an open furnace, followed by condensation to produce cadmium oxide powder;
• Distillation in a closed furnace atmosphere, yielding metallic cadmium powder and an Fe–Ni alloy;
• Chlorination of batteries under a chlorine gas atmosphere or in hydrochloric acid at 960 °C to form cadmium chloride.

Three major industrial-scale pyrometallurgical processes for closed-furnace cadmium distillation [13,15] have been implemented: “SNAM–SAVAM” (France), “SAB–NIFE” (Sweden) [16], and “INMETCO” (United States) [17]. In the SNAM–SAVAM and SAB–NIFE processes, distillation is typically conducted at 850–900 °C, producing metallic cadmium of 99.95% purity, suitable for reuse in manufacturing new Ni–Cd batteries. In the next-generation INMETCO facility (commissioned in 1995), cadmium oxide reduction to metallic Cd is achieved using carbon within a high-temperature reactor, followed by evaporation and condensation [11,18,19,20].

The difference from electroslag reduction is that the temperature of cadmium reduction with carbon increases and exceeds the boiling point of cadmium for subsequent evaporation and precipitation. Different furnaces from different companies use different temperatures. The minimum temperature is 850 °C (“SNAM—SAVAM” (France) [21]; “SAB—NIFE” (Sweden) [16]).

Since cadmium is reduced with carbon in pyrometallurgy, the calculations of CO₂ emissions during cadmium reduction with carbon, which are given in Section 2.2.1., are identical. According to Equations (2)–(5), during the reduction of 1000 g of cadmium (8.9 mol), 4.45 × 44 = 195.8 g of CO₂ is released.

The amount of energy required to heat 1302.44 g of Cd(OH)₂ to a temperature of 300 °C is identical to electroslag reduction.

Molar heat capacity of Cd(OH)₂ C = 95 J/(mol × °C).

Mass of Cd(OH)₂: 1302.44 g or 8.9 mol.

Initial temperature t₁ = 25 °C.

Final temperature t₂ = 300 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}(\mathrm{O}\mathrm{H}{)}_2}={\displaystyle \frac{95 \mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (300 °\mathrm{C}-25 °\mathrm{C})$$

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}(\mathrm{O}\mathrm{H}{)}_2}=232.51 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required to heat 1142.3 g of CdO to the boiling point of cadmium (767 °C). It was assumed that all the CdO was reduced by carbon to the boiling point of cadmium.

Molar heat capacity of CdO C = 43.64 J/(mol $\times$°C).

Mass of CdO: 1142.3 g or 8.9 mol.

Initial temperature t₁ = 300 °C.

Final temperature t₂ = 767 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}\mathrm{O}}={\displaystyle \frac{43.64 \mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (767 °\mathrm{C}-300 °\mathrm{C})$$

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}\mathrm{O}}=181.38 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required for boiling reduced cadmium.

The boiling point of cadmium is 767 °C.

The molar heat of the evaporation of cadmium is L_ev = 99.6 kJ/mol.

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}={\mathrm{L}}_{\mathrm{e}\mathrm{v}}\times \mathrm{M}$$

(8)

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}=99.6{\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}=886.44 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required to heat 1 kg of boiling cadmium from 767 °C to 850 °C.

Molar heat capacity of Cd C = 26 J/(mol $\times$ °C).

Mass of Cd: 1000 g or 8.9 mol.

Initial temperature t₁ = 767 °C.

Final temperature t₂ = 850 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}={\displaystyle \frac{26}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (850 °\mathrm{C}-767 °\mathrm{C})$$

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}=19.21 \mathrm{k}\mathrm{J}$$

Total energy expended in the two-stage process of recovery of 1 kg of cadmium from cadmium hydroxide and its evaporation:

$$\sum \mathrm{Q}=232.51 \mathrm{k}\mathrm{J}+181.38 \mathrm{k}\mathrm{J}+886.44 \mathrm{k}\mathrm{J}+19.21 \mathrm{k}\mathrm{J}=1319.54 \mathrm{k}\mathrm{J}$$

According to Formula (1), J = 1 W $\times$ s, we determine that the energy Q = 1319.54 kJ spent on the reduction of 1 kg of cadmium by the pyrometallurgical method corresponds to 0.37 kWh.

Next, we determine the amount of energy required for cooling reduced cadmium, heated to a temperature of 850 °C, to a temperature of 25 °C. During the cooling process, energy is consumed in the following five stages: cooling gaseous cadmium to a boiling point of 767 °C; the transition of cadmium from the gaseous phase to the liquid phase; cooling liquid cadmium to a melting point of 321 °C; the transition of cadmium from the liquid phase to the solid phase; and cooling solid cadmium to a temperature of 25 °C.

Next, we determine the amount of energy required for cooling reduced cadmium, heated to a temperature of 850 °C, to a temperature of 767 °C.

Molar heat capacity of Cd C = 26 J/(mol $\times$°C).

Mass of Cd: 1000 g or 8.9 mol.

Initial temperature t₁ = 850 °C.

Final temperature t₂ = 767 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}={\displaystyle \frac{26}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (850 °\mathrm{C}-767 °\mathrm{C})$$

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}=19.21 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required to condense the reduced gaseous cadmium to a liquid state — Equation (8).

The boiling point of cadmium is 767 °C.

The molar heat of evaporation of cadmium is L_ev = 99.6 kJ/mol.

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}=99.6{\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}=886.44 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required for cooling 1 kg of liquid cadmium, heated to a temperature of 767 °C, to a temperature of 321 °C.

Molar heat capacity of Cd C = 26 J/(mol $\times$°C).

Mass of Cd: 1000 g or 8.9 mol.

Initial temperature t₁ = 767 °C.

Final temperature t₂ = 321 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}={\displaystyle \frac{26}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (767 °\mathrm{C}-321 °\mathrm{C})$$

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}=103.2 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required for the phase transition of cadmium from the liquid state to the solid state—Equation (8).

The melting point of cadmium is 321 °C.

The molar heat of melting of cadmium is L_m = 6.23 kJ/mol.

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}=6.23{\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}=55.45 \mathrm{k}\mathrm{J}$$

Next, we determined the amount of energy required to cool 1 kg of solid cadmium with a temperature of 321 °C to a temperature of 25 °C.

Molar heat capacity of Cd C = 26 J/(mol $\times$°C).

Mass of Cd: 1000 g or 8.9 mol.

Initial temperature t₁ = 321 °C.

Final temperature t₂ = 25 °C.

As a result of the calculations, the following was obtained:

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}={\displaystyle \frac{26}{\mathrm{m}\mathrm{o}\mathrm{l}\times °\mathrm{C}}}\times 8.9 \mathrm{m}\mathrm{o}\mathrm{l}\times (321 °\mathrm{C}-25 °\mathrm{C})$$

$${\mathrm{Q}}_{\mathrm{C}\mathrm{d}}=68.49 \mathrm{k}\mathrm{J}$$

Total energy costs for cooling 1 kg of cadmium, heated to a temperature of 850 °C, to a temperature of 25 °C:

$$\sum \mathrm{Q}=19.21 \mathrm{k}\mathrm{J}+886.44 \mathrm{k}\mathrm{J}+103.2 \mathrm{k}\mathrm{J}+\mathrm{55,45} \mathrm{k}\mathrm{J}+68.49 \mathrm{k}\mathrm{J}=1132.79 \mathrm{k}\mathrm{J}$$

According to Formula (1), J = 1 W $\times$ s, we obtain that the energy Q = 1132.79 kJ spent on the reduction of 1 kg of cadmium by the pyrometallurgical method corresponds to 0.31 kWh.

Total energy costs for the two-stage process of reducing 1 kg of cadmium from cadmium hydroxide and evaporating it, and for cooling 1 kg of cadmium, heated to a temperature of 850 °C, to a temperature of 25 °C:

$$\sum \mathrm{Q}=1319.54 \mathrm{k}\mathrm{J}+1132.79 \mathrm{k}\mathrm{J}=2452.33 \mathrm{k}\mathrm{J}$$

According to Formula (1), J = 1 W∙s, we obtain that the energy Q = 2452.33 kJ spent on the reduction of 1 kg of cadmium by the pyrometallurgical method corresponds to 0.68 kWh.

CO₂ emissions were calculated for the processes of recovery of 1 kg of cadmium using the pyrometallurgical method for three countries—Latvia, Germany, and France. CO₂ emissions in the process of the chemical reaction reducing cadmium with carbon are at a constant value and are equal to 0.1958 kg of CO₂ per 1 kg of cadmium.

Table 5. CO₂ emissions for the recovery of 1 kg of cadmium with carbon using the pyrometallurgical method, in kg.

Country	CO2 Emissions During Pyrometallurgical Reduction of 1 kg of Cadmium, in kg
	During a Chemical Reaction, the Value Is Constant	The Costs of Organizing a Technical Process	Total
Latvia	0.1958 kg	0.17 kg CO2/kWh × 0.68 kWh = 0.1156 kg	0.3114 kg
Germany (UBA 2023)	0.1958 kg	0.38 kg CO2/kWh × 0.68 kWh = 0.2584 kg	0.4542 kg
France, Nuclear Energy (LCA ADEME)	0.1958 kg	0.004 kg CO2/kWh × 0.68 kWh = 0.0027 kg	0.1985 kg

shows CO₂ emissions for the recovery of 1 kg of cadmium with carbon using the pyrometallurgical method in Latvia, Germany, and France (per 1 kWh).

Figure 2 shows the CO₂ emissions from the reduction of 1 kg of cadmium with carbon during pyrometallurgical reduction, in kg. CO₂ emissions during pyrometallurgical reduction comprise CO₂ emissions from the chemical reaction of cadmium reduction (shown in blue) and CO₂ emissions due to the costs of organizing the technological process that ensures this chemical reaction (shown in orange).

2.2.3. Hydrometallurgy

The hydrometallurgical process for nickel–cadmium (Ni–Cd) battery recycling involves mechanical crushing of the batteries, followed by physical separation of structural components, dissolution of valuable metals, and subsequent separation and purification. Compared with pyrometallurgical methods, hydrometallurgical technologies are typically more complex and require additional stages; however, they offer greater efficiency, flexibility, cost-effectiveness, and selectivity in metal extraction. This versatility enables the simultaneous processing of various waste types with similar compositions [22].

In contrast to pyrometallurgy, hydrometallurgy operates at low temperatures, with metal recovery achieved through chemical reactions in aqueous or organic solutions [23,24,25]. The process generally involves acidic (HCl, HNO₃, H₂SO₄) or alkaline leaching, followed by purification and concentration techniques such as adsorption (activated carbon), cementation, ion exchange, and solvent extraction to concentrate and separate metals from the leaching solutions.

Industrial-scale hydrometallurgical processes for Ni–Cd battery recycling include the TNO (Netherlands) and BATENUS (Germany) methods [26]. The TNO process entails crushing and magnetic separation of battery materials into two fractions, followed by separate leaching in 6 N HCl at 30–60 °C. Cadmium is recovered from the leachate via solvent extraction using a mixture of 75% tributyl phosphate (TBP) and 25% cyclohexane-2-methylpropyl acetate (ShellSol R), then re-extracted with dilute HCl and electrodeposited [27].

The BATENUS process operates in a closed reagent cycle, combining electro-chemical and membrane technologies [28]. Nickel and cadmium are extracted from the leachate using ion-exchange resins, eluted with dilute sulfuric acid, and finally recovered by electrolysis [29].

Extensive research has focused on both optimizing individual hydrometallurgical stages and developing innovative full-scale processes for recovering valuable metals from spent Ni–Cd batteries, with portable batteries being the primary study objects [20]. Leaching is the key step, enabling nearly all metallic components to dissolve into solution, from which metals can be recovered by deposition, solvent extraction, ion exchange, or electrolysis, yielding either pure metals or their compounds (oxides, hydroxides, or salts).

Sulfuric acid leaching is the most widely applied technique [9,13,26,28,30,31,32,33]. Studies have shown that up to 99.5% Cd and 96% Ni can be recovered from spent Ni–Cd battery powder (69% Ni, 15% Cd, and 0.94% Fe) by leaching with 5.86 vol.% H₂SO₄ at 328 K [28]. The addition of hydrogen peroxide increases Ni leaching efficiency due to in situ formation of strong oxidants—peroxymonosulfuric (H₂SO₅) and peroxydisulfuric (H₂S₂O₈) acids. Another study [34] demonstrated that cobalt and cadmium hydroxides can be leached in 5.86 vol.% H₂SO₄ for 15 min at ~323 K; Ni leaching efficiencies reached ~73% and 93% from anode and cathode materials, respectively, at 358 K.

Optimization of temperature, acid concentration, and liquid-to-solid ratio (L/S) can yield >95% recovery of Ni, Cd, and Co under the following conditions: ~100 °C, C(H₂SO₄) = 2.3–2.7 M, and L/S = 8–10 L·kg⁻¹ [30].

A modified hydrometallurgical process [26]—comprising hot H₂SO₄ leaching with H₂O₂ addition, Cd electrodeposition, Fe precipitation as Fe(OH)₃, and Ni electrodeposition—produces high-purity metals.

Leaching in HCl-based systems can dissolve all metallic components of Ni–Cd battery scrap [33,35,36,37,38], with better performance compared to other acids. However, H₂SO₄ remains the preferred reagent due to its higher overall process efficiency and regeneration capability [20,39].

The hydrometallurgical process of cadmium recovery includes leaching of Cd(OH)₂ using a solution of sulfuric acid and hydrogen peroxide, subsequent extraction of cadmium with organic solvents and its electrodeposition, as well as ion exchange and precipitation of impurities.

Based on past publications [27,28,29], the scheme and calculation of reagents and energy required to obtain 1 kg of cadmium using the BATENUS process and the TNO process can be presented.

Hydrometallurgical Process Using the BATENUS Method

The following is a CO₂ emission calculation for the BATENUS process, which uses the following reagents (quantities are given for the recovery of 1 kg of cadmium): 0.6058 kg H₂SO₄ (5.86% solution, 10.3 L at L/S = 10); 0.412 kg H₂O₂ (4% additive); ~0.2 kg organic solvents (TBP and ShellSol R); and ~0.05 kg ion-exchange resin.

The CO₂-equivalent emission factor for sulfuric acid (H₂SO₄) is approximately 0.14 kg CO₂-eq per kg of H₂SO₄.

Reference: This value comes from a source titled “Emission Factors in kg CO₂-Equivalent per Unit City of Winnipeg/Winnipeg, Canada, WSTP South End Plant Process Selection Report, Appendix H, 2012”, updated in 2023 [40].

Additional consistent values include 0.12 kg CO₂-eq/kg from Germany’s BAFA Emission Factors (2025) and 0.14 kg CO₂-eq/kg reported by CarbonCloud for manufacturers worldwide (2021). Reference: Climatiq—API for Carbon Footprint Calculations www.climatiq.io (accessed on 15 September 2025) [41].

Thus, the consensus for cradle-to-gate CO₂ equivalent emissions for sulfuric acid is about 0.12 to 0.14 kg CO₂-eq per kg.

The CO₂ equivalent (CO₂-eq) for hydrogen peroxide (H₂O₂) is approximately 1.13 kg CO₂-eq per kg of H₂O₂. This value represents the greenhouse gas emissions in terms of carbon dioxide equivalent per kilogram of hydrogen peroxide produced or used, typically from cradle-to-gate life cycle assessments.

Reference: Climatiq Emission Factor database states CO₂-eq 1.13 kg/kg H₂O₂. Other sources indicate industrial H₂O₂ production emits around 3 kg CO₂ per kg but the normalized CO₂ equivalent for direct comparisons is about 1.13 kg/kg [41].

This means for every kilogram of hydrogen peroxide, around 1.13 kg of CO₂-equivalent greenhouse gases are emitted.

The CO₂ equivalent (CO₂-eq) for ion-exchange resin is approximately 2.0 to 4.0 kg CO₂-eq per kg cyclohexane-2-methylpropyl acetate [42].

The specific CO₂ equivalent (kg CO₂-eq per kg) for ion-exchange resin production is not straightforwardly given in the search results, but one relevant reference indicates an emission factor of approximately 3200 kg CO₂-eq per cubic meter of ion-exchange resin. Since resin density typically ranges around 1.1 to 1.2 kg/L, this roughly translates to about 3.2 to 3.5 kg CO₂-eq per kg of resin. This factor was noted as an average found in the literature for ion-exchange resin in an industrial carbon footprint assessment document [43].

For a precise source, the document titled “Assessing the carbon footprint of industrial equipment” provides this emission factor reference for ion-exchange resins, but it reports primarily per volume (m³) rather than direct mass-based CO₂-eq, which can be converted as above. If a more exact or experimentally measured life cycle assessment number is needed, specialized LCA reports or pilot plant studies on ion-exchange resin production could be consulted.

In summary, a reasonable reference CO₂-equivalent emission factor for ion-exchange resin production is about 3.2 to 3.5 kg CO₂-eq per kg of resin, based on available industrial emission factor reports.

Table 6. CO₂ emissions from reagent production for the BATENUS process.

Reagent	Mass (kg)	Specific Emissions, kg CO2/kg	CO2, kg
H2SO4	0.6058	0.14	0.0848
H2O2	0.412	1.13	0.466
TBP + ShellSol R	~0.20	4.00	0.80
Ion-exchange resin	~0.05	3.50	0.175
Total	1.2678	-	1.5258

shows the CO₂ emissions from reagent production for the BATENUS process.

Energy consumption per 1 kg Cd:

Heating the solution to 55 °C: 0.36 kWh;

Electrodeposition of Cd: 0.22 kWh;

Other processes (filtration, pumps): 0.10 kWh;

Total: 0.68 kWh.

Table 7. CO₂ emissions from electricity for the BATENUS process.

Country	CO2 Emissions During Hydrometallurgical Reduction of 1 kg of Cadmium for the BATENUS Process, in kg
	During a Chemical Reaction, the Value Is Constant	The Costs of Organizing a Technical Process	Total
Latvia	1.5258 kg	0.17 kg CO2/kWh × 0.68 kWh = 0.1156 kg	1.6414 kg
Germany (UBA 2023)	1.5258 kg	0.38 kg CO2/kWh × 0.68 kWh = 0.2584 kg	1.7842 kg
France, Nuclear Energy (LCA ADEME)	1.5258 kg	0.004 kg CO2/kWh × 0.68 kWh = 0.0027 kg	1.5285 kg

shows the CO₂ emissions from electricity for the BATENUS process.

Figure 3 shows the CO₂ emissions from the reduction of 1 kg of cadmium with carbon during hydrometallurgical reduction for the BATENUS process, in kg. CO₂ emissions during hydrometallurgical reduction for the BATENUS process comprise CO₂ emissions from the chemical reaction of cadmium reduction (shown in blue) and CO₂ emissions due to the costs of organizing the technological process that ensures this chemical reaction occurs (shown in orange).

The hydrometallurgical method of cadmium recovery using the BATENUS process is characterized by high CO₂ emissions, primarily due to the reagents (especially H₂O₂ and organic solvents) rather than the electricity generated. CO₂ emissions from the process that ensures the chemical reaction of cadmium reduction and removal occurs depend on the electricity generation method. France has an advantage with its use of nuclear power plants, which ensures minimal CO₂ emissions.

Hydrometallurgical Process Using the TNO Method

Below is a calculation of CO₂ emissions using the TNO method, which uses the following reagents (the quantities are given for the recovery of 1 kg of cadmium): 2 kg HCl; 0.05 kg TBP extractant; and 0.1 kg alkali for pH correction (Na₂CO₃ or NaOH).

Hydrochloric acid (HCl): Approximately 1.06 kg CO₂-eq per kg of HCl produced. This is based on life cycle assessment data including raw material and energy inputs [44].

Tributyl phosphate (TBP): No direct CO₂-eq emission factor was readily found in public LCA databases, but it is an organophosphorus chemical produced by reacting phosphoryl chloride with n-butanol. Its carbon footprint is likely to be influenced by the precursor chemicals and energy intensity of production, but specific CO₂-eq values are not publicly reported in common databases. The tributyl phosphate production volume is relatively small (3000–5000 tons worldwide).

Sodium carbonate (Na₂CO₃): Around 0.46 to 0.52 kg CO₂-eq per kg of sodium carbonate production, depending on the source and production method [45].

Sodium hydroxide (NaOH): Estimates range between about 0.50 to 1.12 kg CO₂-eq per kg of NaOH produced. The value depends on production technology and region, but 1.12 kg CO₂-eq/kg is a widely reported figure.

Table 8. CO₂ emissions from the production reagents using the TNO method.

Reagent	Mass (kg)	Specific Emissions, kg CO2/kg	CO2, kg
HCl	2	1.06	2.12
TBP	0.05	No data	-
Na2CO3	0.1	0.52	0.052
Total	2.15	-	2.172

shows CO₂ emissions from the production reagents using the TNO method.

Energy consumption per 1 kg Cd:

Heating the solution to 60 °C: 0.4 kWh;

Electrodeposition of Cd: 0.22 kWh;

Other processes (filtration, pumps): 0.10 kWh;

Total: 0.72 kWh.

Table 9. CO₂ emissions during hydrometallurgical reduction of 1 kg of cadmium for the TNO method, in kg.

Country	CO2 Emissions During Hydrometallurgical Reduction of 1 kg of Cadmium for the TNO Method, in kg
	During a Chemical Reaction, the Value Is Constant	The Costs of Organizing a Technical Process	Total
Latvia	2.172 kg	0.17 kg CO2/kWh × 0.72 kWh = 0.1224 kg	2.2944 kg
Germany (UBA 2023)	2.172 kg	0.38 kg CO2/kWh × 0.72 kWh = 0.2736 kg	2.4456 kg
France, Nuclear Energy (LCA ADEME)	2.172 kg	0.004 kg CO2/kWh × 0.72 kWh = 0.0029 kg	2.1749 kg

shows CO₂ emissions during hydrometallurgical reduction of 1 kg of cadmium for the TNO method, in kg.

Figure 4 shows the CO₂ emissions from the reduction of 1 kg of cadmium with carbon in the hydrometallurgical reduction process for the TNO method. CO₂ emissions during hydrometallurgical reduction for the TNO method comprise CO₂ emissions from the chemical reaction of cadmium reduction (shown in blue) and CO₂ emissions due to the costs of organizing the technological process that ensures this chemical reaction occurs (shown in orange).

The hydrometallurgical method of cadmium recovery using the TNO method is characterized by high CO₂ emissions, primarily due to the reagents (especially HCl) rather than the electricity. CO₂ emissions from the technical process that ensures the chemical reaction of cadmium reduction and recovery occurs depend on the electricity generation method. France has an advantage with its use of nuclear power plants, which ensures minimal CO₂ emissions.

3. Results

A comparison of CO₂ emission calculations for cadmium reduction using various technological processes and various energy sources revealed that the primary influence on CO₂ emissions is the choice of technological process, not the method of energy generation.

The hydrometallurgical method of cadmium reduction demonstrated the highest CO₂ emissions due to CO₂ emissions during reagent production.

The electroslag reduction method of cadmium and the pyrometallurgical method offer advantages due to the use of a single consumable reagent—natural carbon—as a reducing agent.

The KCl-NaCl cover flux and carbon used in electroslag reduction are of natural origin. They are synthesized by nature itself, meaning they have zero CO₂ emissions. Furthermore, they do not participate in the reduction reaction and are not consumed during the cadmium reduction process. Electroslag reduction of cadmium is characterized by lower CO₂ emissions due to the lower reaction temperature and the absence of vacuum and cooling for cadmium condensation compared to the pyrometallurgical process. In electroslag reduction, cadmium cooling occurs naturally, eliminating the energy inputs required in the pyrometallurgical process.

Table 10. CO₂ emissions during the recovery of 1 kg of cadmium in various processes.

Country		Latvia	Germany (UBA 2023)	France, Nuclear Energy (LCA ADEME)
		CO2 Emissions, kg
Electroslag reduction method	In the process of a chemical reaction, constant	0.1958	0.1958	0.1958
	The costs of organizing a technical process that ensures a chemical reaction occurs	0.0187	0.0418	0.0004
	Total	0.2145	0.2376	0.1962
Pyrometallurgical method	In the process of a chemical reaction, constant	0.1958	0.1958	0.1958
	The costs of organizing a technical process that ensures a chemical reaction occurs	0.1156	0.2584	0.0027
	Total	0.3114	0.4542	0.1985
Hydrometallurgical method BANETUS	In the process of a chemical reaction, constant	1.5258	1.5258	1.5258
	The costs of organizing a technical process that ensures a chemical reaction occurs	0.116	0.258	0.0027
	Total	1.6418	1.7838	1.5285
Hydrometallurgical method TNO	In the process of a chemical reaction, constant	2.172	2.172	2.172
	The costs of organizing a technical process that ensures a chemical reaction occurs	0.1224	0.2736	0.0029
	Total	2.2944	2.4456	2.1749

compares CO₂ emissions for electroslag reduction of cadmium, the pyrometallurgical process, and two types of hydrometallurgical processes—the BANETUS method and the TNO method.

Figure 5 clearly shows the CO₂ emissions from the recovery of 1 kg of cadmium in various processes. The primary emissions depend on the choice of process. To a lesser extent, CO₂ emissions depend on the energy costs of organizing the technical process that enables the chemical reaction to occur.

4. Discussion

This study reveals significant differences in CO₂ emissions across cadmium recovery technologies:

• Electroslag reduction demonstrates the lowest total CO₂ emissions. This method benefits from low operating temperatures (≤700 °C) and a minimal flux role in emissions. Electroslag reduction technology for cadmium prevents cadmium evaporation, which also helps to reduce energy consumption and emissions.
• Pyrometallurgical recovery, though similarly reliant on carbon as a reductant, operates at higher temperatures (>850 °C) and requires additional energy for cadmium evaporation and condensation. While the chemical CO₂ emissions remain identical to electroslag, the higher energy input increases total emissions.
• Hydrometallurgical recovery generates the highest CO₂ emissions, not from energy usage but from reagent production. It is less environmentally favorable in terms of greenhouse gas emission.

In all cases, electricity source is a less important variable. The use of nuclear or renewable energy reduces total emissions, demonstrating the importance of energy mix in evaluating green technologies.

5. Conclusions

This study quantitatively assessed the CO₂ emissions associated with cadmium recovery from spent Ni-Cd batteries using three technological approaches: electroslag reduction, pyrometallurgical distillation, and hydrometallurgical leaching. The comparative analysis revealed the following key findings:

• Electroslag reduction demonstrated the lowest total CO₂ emissions per kilogram of recovered cadmium (0.196–0.241 kg CO₂), primarily due to its moderate process temperature (700 °C), controlled environment preventing cadmium evaporation, and low energy demand. Its environmental performance is further enhanced when powered by low-carbon electricity sources.
• Pyrometallurgical methods exhibited slightly higher emissions (0.199–0.454 kg CO₂/kg Cd), attributable to elevated operating temperatures (850–900 °C) and additional energy required for cadmium vaporization and condensation. Despite high recovery efficiency, the thermal intensity of this method presents a notable environmental drawback.
• Hydrometallurgical recovery showed significantly higher emissions (1.529–2.446 kg CO₂/kg Cd), dominated by upstream emissions from the production of chemical reagents. Although advantageous for selective metal recovery and operation at lower temperatures, this route remains less favorable in terms of CO₂ balance.

Comparing CO₂ emissions in different technological processes and taking into account different sources of electricity (Figure 5), we see that the main influence on CO₂ emissions is not the method of generating electricity, but the choice of technological processes.

Overall, the electroslag process offers a promising alternative for sustainable cadmium recovery, combining low carbon intensity with operational simplicity and process continuity. The electroslag reduction utilizes primary process reagents, such as carbon and potassium and sodium chlorides, which do not require special synthesis, unlike the reagents used in hydrometallurgy.

This study compared the environmental safety of three cadmium reduction methods: electroslag reduction, pyrometallurgical, and hydrometallurgical methods. The study demonstrated the high environmental safety of the electroslag reduction method for cadmium by creating conditions that eliminate cadmium evaporation.

The results highlight the environmental and technological advantages as well as the high potential for the application of the electroslag reduction method for cadmium.