MEASURED SF₆ EMISSIONS FROM MAGNESIUM DIE CASTING OPERATIONS

This introductory paper is the research content of the paper "MEASURED SF₆ EMISSIONS FROM MAGNESIUM DIE CASTING OPERATIONS" published by TMS(The Minerals, Metals & Materials Society).

1. Overview:

• Title: MEASURED SF₆ EMISSIONS FROM MAGNESIUM DIE CASTING OPERATIONS
• Author: Scott Bartos, Jerry Marks, Ravi Kantamaneni and Curtis Laush
• Publication Year: 2003
• Published Journal/Society: PRESENTED AT THE 132ND TMS ANNUAL MEETING
• Keywords: sulfur hexafluoride (SF₆), magnesium, die casting, emissions, greenhouse gas, melt protection

2. Abstract

Industrial molten magnesium processes primarily utilize sulfur hexafluoride (SF₆) as a cover gas to inhibit surface oxidation, which can result in fires if not controlled. Recent concerns with global warming and subsequent research has determined SF₆ to be a potent and long lived greenhouse gas. Current Intergovernmental Panel on Climate Change (IPCC) Good Practice Guidance for estimating the emissions of SF₆ from industrial magnesium processes assumes that 100 percent of the gas utilized for a given process is emitted directly to the atmosphere. This study examined the SF₆ utilization for a magnesium die-casting process and determined that, for the specific operating conditions, the level of SF₆ destruction was process dependent. Maximum SF₆ destruction was observed to occur during ingot feeding operations, and ranged from 16 to 20 percent. During non-feeding periods the level of destruction decreased to approximately half that observed during feeding periods. While this paper is limited to one facility using a hot-chambered die-casting unit, it does provide a useful step towards defining the true nature of SF₆ emissions from the magnesium industry as a whole.

3. Research Background:

Background of the research topic:

Molten magnesium has violent oxidative properties. The magnesium industry primarily uses sulfur hexafluoride (SF₆) to prevent oxidation and surface burning of the molten metal. SF₆ is used as a source for fluorine species, which are a component in the formation of a dense protective film on the molten magnesium surface (2).

Status of previous research:

• Current Intergovernmental Panel on Climate Change (IPCC) assumes that 100 percent of the SF₆ utilized during melt protection operations is emitted to atmosphere (5).
• Past measurement studies have been conducted to review the protection mechanism provided by SF₆. In 1972, X-ray diffraction identified several potential SF₆ by-products, including sulfur dioxide (SO₂), and magnesium fluoride (MgF₂) (8).
• In 1977, work performed at the Magnesium Research Center at Battelle's Columbus Laboratories. The study reported that the SF₆ concentration at the metal surface was about half that delivered by the metering system, which could indicate a chemical decomposition of SF₆(9).
• Additional work using cover gas mixtures containing SF₆, carbon dioxide (CO₂) and air, identified the presence of both carbon monoxide (CO) and hydrogen fluoride (HF)(10).
• A recent study, It is assumed that the general mechanism for SF₆ melt protection stems from the dissociation of SF₆ to highly reactive fluorine species, such as F or F₂. These species diffuse through the porous magnesium oxide (MgO) layer to form a dense protective film containing MgF₂.(11).

Need for research:

• Expert opinion speculates that during melt protection some form of SF₆ destruction, via chemical reaction, may occur.
• The measurement study described herein provides an initial step towards quantifying the destruction rates of SF₆ during magnesium production and processing operations.
• Since industry opinion on the extent of SF₆ destruction during magnesium production and processing differs significantly, measurements of SF₆ emissions during these operations are required to establish more accurate emission factors.

4. Research purpose and research question:

Research purpose:

• To quantify the destruction rates of SF₆ during magnesium production and processing operations.
• To difine the true nature of SF₆ emissions from the magnesium industry.

Core research:

• To identify the extent of SF₆ destruction occurring at molten magnesium holding furnaces on two hot-chambered die casting machines.

5. Research methodology

• Research Design: Quantitative experimental measurements.
• Data Collection:
  • Continuous and real-time measurements were made using extractive Fourier Transform Infrared (FTIR) spectroscopic systems from MKS Instruments.
  • Gas grab samples were taken from the exhaust stream of the FTIR via one-milliliter syringe pulls, and manually injected into a HP 5890A gas chromatograph (GC).
• Analysis Method:
  • FTIR: Real-time monitoring of gas stream from a Monel sample probe. Instrumental resolution was set to 0.5 cm¯¹ and signal averaging was performed over one or two-minute periods.
  • GC: Used a thermal conductivity detector (TCD) and a HayeSep Q 80/100 column. Helium was used as the carrier gas. The gas chromatograph was calibrated with a multi-point calibration curve prior to use each day.
• Research Scope: Two hot-chambered die casting machines (Machine A and B) located at the Product Technologies Inc. facility in Minnesota. The machines consisted of a holding furnace assembly for the molten magnesium, pneumatic injection piston, and die press apparatus.

6. Key research results:

Key research results and presented data analysis:

• The overall average SF₆ destruction, based on the specific operating conditions and neglecting observed process variations, was determined to be on the order of 10 percent.
• For Machine A, SF₆ destruction ranged from 6.5 to 16 percent, while Machine B ranged from 12.5 to 20 percent.
• Maximum SF₆ destruction was observed to occur during the moments following ingot feeding. During this period the destruction of SF₆, for both machines, was observed to be approximately 16-20 percent.
• During non-feeding periods, SF₆ destruction was on average 6.5 and 12.5 percent for Machines A and B, respectively.
• When the casting machine was not operating, the overall destruction of SF₆ decreased to minimal levels, approximately 0.6 to 1.4 percent.
• FTIR sampling identified two gaseous reaction byproducts, HF and SO₂.

List of figure names:

• Figure 1. Cover Gas CO₂, H₂O and SF₆ Concentrations: Machine A
• Figure 2. Percent Cover Gas CO₂ and SF₆ Reduction: Machine A
• Figure 3. SF₆ Dilution and Destruction: Machine A
• Figure 4. SF₆ Dilution and Destruction: Machine B

7. Conclusion:

Summary of key findings:

The level of SF₆ destruction was strongly correlated to die casting process operations. During ingot feeding operations, 6 to 8 kg ingots are added to the melt furnace and, consequently, break the in-situ SF₆ protective film. During this heightened melt surface turbulence, SF₆ destruction reached a maximum(16-20 percent). Since the level of SF₆ destruction dropped to approximately 1 percent when the casting machine was not operating, it can be concluded that a minimum level of SF₆ destruction occurs during casting operations. This average minimum level was observed to be 6.5 and 12.5 percent for Machines A and B, respectively.

Summary of research results. Academic significance of the research, practical implications of the research

• This study provides the specific operating conditions, the level of SF₆ destruction was process dependent.
• It is assumed that all SF₆ decomposition occurs at the melt surface, without the release of gaseous byproducts.
• This paper is a useful step toward defining the true nature of SF₆ emissions from the magnesium industry.

8. References:

1. Roskill Consulting Group, The Economics of Magnesium Metal, 2001.
2. Tranell, G., et al., “A Systematic Approach for Identifying Replacements to SF6/SO2 in the Magnesium Industry – An IMA / SINTEF-NTNU Cooperative Project," Proceedings of the 57th Annual International Magnesium Association Conference, May 2001.
3. Hanawalt, J.D., “Practical Protective Atmospheres for Molten Magnesium," Metals Engineering Quarterly, November 1972. pp. 6.
4. IPCC, Climate Change 2001: The Scientific Basis, Intergovernmental Panel on Climate Change, Third Assessment Report, 2001.
5. IPCC, Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories, Section 3.4, SF 6 Emissions from Magnesium Production, http://www.ipcc-nggip.iges.or.jp/public/gp/pdf/3_Industry.pdf
6. Bartos, S., "Building a Bridge for Climate Protection: U.S. EPA and the Magnesium Industry,” Proceedings of the 59th Annual International Magnesium Association Conference, May 2003.
7. U.S. EPA. Inventory of Greenhouse Gas Emissions and Sinks: 1990-2000, ΕΡΑ 236-R-01-001, April 2002.
8. Hanawalt, J. D., "Practical Protective Atmospheres for Molten Magnesium," Metals Engineering Quarterly, pp 6 – 10, November 1972.
9. Couling, S., Bennett, F., Leontis, T., "Fluxless Melting of Magnesium," Light Metals, pp 545 – 560, 1977.
10. Couling, S., Leontis, T., “Improved Protection of Molten Magnesium with Air/CO2/SF6 Gas Mixtures," Light Metals, pp 997-1009, 1980
11. Tranell, G., G. Pettersen, K. Aastad, T. A. Engh, I. Solheim, M. Syvertsen, B. Oye, “A Systematic Approach for Identifying Replacements to SF 6/SO2 in the Magnesium Industry - An IMA / SINTEF-NTNU Cooperative Project," 57th Anuual IMA Conference Proceedings, May 2001.

9. Copyright:

• This material is a paper by "Scott Bartos, Jerry Marks, Ravi Kantamaneni and Curtis Laush": Based on "MEASURED SF₆ EMISSIONS FROM MAGNESIUM DIE CASTING OPERATIONS".
• Source of paper: Not provided in the OCR text.

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