• Table of Contents
• Introduction
• Plant Data
• Electrolyte Properties
• Concentration Converter
• Cell Voltage
• Alumina Feeding
• Anode Table Layout

• Table of Contents
• Introduction
• How to Use AlPrg
• Plant Data
• Alumina Feeding
• Electrolyte Properties
• Cell Voltage

• Table of Contents
• Introduction
• ATab Version 2
• Layout-View
• Evaluation-View
• 3D-View
• Floating Windows
• Optimization Window
• Examples
• Downloads

• Table of Contents
• Principles Hall-Héroult
• Mass Balance
• Electrolyte Properties
• Cell Voltage
• Energy Balance

• Current Efficiency

• Raw Materials

• Cell Operation

• Measurements

• Environment, Emissions
• Examples, Exercises
• Literature

• News and Events
• Name Index
• Subject Index
• Sitemap
• Recreation

10. Environment, Emissions

10.1. General

Every cell produces hazardous and non hazardous wastes in gaseous, liquid or solid state, which are relevant for the assessments of occupational health and ecological impact. Modern aluminum smelting plants are equipped with efficient cleaning systems (scrubbers) that reduce drastically the emission of toxic and dangerous substances. At present the aluminum smelting industry undertakes considerable efforts to control and reduce the emission of Greenhouse Gases.
To get a full picture of the ecological impact of the aluminum fabrication, it is necessary to take into account also emissions during bauxite mining, alumina refining, electrical power production and anode fabrication.

10.2. Greenhouse Effect

The Greenhouse Effect is caused by the atmosphere containing gases that absorb and emit infrared radiation. Greenhouse gases trap heat within the surface-troposphere system, causing heating at the surface of the earth (Global Warming).

10.1 Greenhouse Effect (figure taken from Wikipedia) . Greenhouse Gases like carbon dioxide (CO2), methane (CH4) or chlorofluorocarbons (CF4 or C2F6, for instance) absorb and emit infrared radiation and heat the atmosphere (Global Warming).

The Global Warming Potential (GWP) estimates how much a given mass of greenhouse gas contributes to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxid.
Other measures are the Equivalent Carbon Dioxide (CO₂e) that is the concentration of CO₂ that would cause the same level of radiative forcing as a given type and concentration of greenhouse gas and the Carbon Dioxide Equivalent (CDE) that is a quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO₂ that would have the same global warming potential (GWP).

10.3. Aluminum Smelting Greenhouse Gases

The electrolysis cell generates several greenhouse gases.

Carbon Dioxide (CO₂, GWP = 1): produced by electrolysis and parasitic reactions like airburn of the anodes (about 1300kg/tAl).

Tetrafluoromethane (CF₄ or PFC-14, GWP = 6500) and

Hexafluoroethane (C₂F₆ or PFC-116, GWP = 9200).

These gases are formed during anode effect (about 0.2 kg/tAl) [Lit.] in fact CF₄ from carbon and cryolite (4Na₃AlF₆) according to the reaction 10.1

from carbon, cryolite (4Na₃AlF₆) and alumina (Al₂O₃) according to the reactions 10.2

and C₂F₆ from cryolite (Na₃AlF₆)

According to [Lit.] the CO₂e ( = GWP) value of anode effects are estimated with:

	(10.4)

The Greenhouse Gases interactive web page of AlWeb uses for its calculation methods and relations discribed in [Lit.]. The algorithm of this publication determines the emission rate of carbontetrafluoride by using an anode effect parameter (number and duration) and a so called slope coefficient.

	(10.5)

[Lit.] reports tetrafluoride slope coefficients for different cell technologies.

Table 10.1: Comparison of Revised PFC Slope Values and Uncertainty with Previous Values.

A second relation uses the anode effect overvoltage:

	(10.6)

with the corresponding values

Table 10.2: Comparison of Revised Overvoltage Coefficient (U_AE).

And finally AlWeb calculates the emission rate of hexafluoroethane using a weight fraction (and not another slope factor):

	(10.7)

using the following ratio values:

Table 10.3: Comparison of Revised Weight Ratios of C₂F₆ to CF₄.

10.4. Carbon Footprint

The Carbon Footprint measures the total greenhouse gas emissions caused directly and indirectly by a person, organization, event or product. Figure 10-2 [Lit., Fig. 3] shows the essential components that contribute to the aluminum smelting carbon footprint namely production of electric energy, number and duration of anode effects, production and consumption of the carbon anodes.

10.2 Carbon Footprint of Aluminum Smelting [Lit., Fig. 3]. The total CO2e value depends very much how the electric energy is produced. Another essential contribution is the number of anode effects.

B. Welch et al. and J. T. Keniry discuss the essential steps to reduce the production of GHG (greenhouse gases) by aluminum electrolysis:

	reduce gross carbon consumption,
	reduce parasitic carbon oxidation reactions,
	increase current efficiency,
	improve feeding and dissolution of alumina (sludge),
	improve detection of underfeed limits,
	reduce number and duration of anode effects.