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Removal of Dissolved Impurity Elements from Molten Aluminium

–Future Trends

Norges teknisk-naturvitenskapelige universitet

PhD Trial Lecture by Mark William Kennedy

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Learning Objectives• Understand the types, sources, concentrations and

analysis methods of dissolved impurities in liquid aluminium.

• Achieve a basic understanding of the process chemistry and kinetics of refining of dissolved impurities.

• Become more familiar with commercial refining processes for liquid aluminium, and some of the equipment types.

• Be aware of some of the probable future challenges and possible changes in the field of liquid aluminium refining.

3Typical Hall-Héroult Primary Aluminium Cell

4Metal Quality Requirements by Application

Waite (2002)

Today’s talk is focused only on dissolved impurities. Of primary concern in refining are:•Dissolved H2 (0.1-0.3 ppm 1 mL/100g Al ≡ 1.12 ppm)•Na (30-150 ppm), •Ca (2-5 ppm),•Li (0-20 ppm),•Mg (from secondary sources).•‘Noble’ metals are controlled by controlling the purity of feed material and preventing contamination.

5Sources of Impurities in Liquid Aluminium

Metal comes from several sources, each of which has its own sources of impurities and therefore ‘quality’:•Primary metal from electrolysis

• Smelter Grade Alumina (SGA)• Secondary alumina (from scrubbing)• Cryolite (Na3AlF6) bath and AlF3 make-up• Anode carbon• Steel tools and current collectors

•Secondary ‘re-melt’ of previously solidified primary metal –primary impurities, plus processing (e.g. H2, TiB2) and alloying elements•Tertiary metal (recycled scrap, new or old)

6Impurities in Primary Electrolysis Aluminium

Knut Arne Paulson (2010)

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Impurities Originating in Electrolysis Bath Feed

Al2O3 Na3AlF6 AlF3

Impurity (wt%) (wt%) (wt%)SiO2 0.007-0.020 0.12-0.13 0.10-0.15

Fe2O3 0.008-0.022 0.04-0.11 0.01-0.02TiO2 0.002-0.008 0-0.001 0.008-0.0012CaO 0.003-0.035 0.06-0.10ZnO 0.001-0.011V2O5 0.0012-0.004 0.001-0.005 0.0002-0.0003P2O5 0.0004-0.0011 0.015-0.02Cr2O3 0.002Ga2O3 0.007-0.008Na2O 0.3-0.45 0.10-0.15K2O 0.01-0.08H2O 0.17-0.3 0.2-0.3Ca 0-0.08 0.009-0.070

SO4 0.54-0.69 0.9-1.5Thonstad et al. (2001)

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Petroleum Coke Used for Anodes

Thonstad et al. (2001)

Impurity PPM Impurity PPM

Si 50‐250 B 1Fe 50‐400 Na 30‐120Ti 2‐50 Mg 100Zn 2‐50 Ca 20‐100V 30‐350 Mn 4Cr 1‐2 Ga 14Ni 50‐220 Pb 3Cu 1‐3 Al 50‐250S 5000‐35000 Ash 1000‐2000

9Impact of Impurity Metals on Aluminium Electrical Conductivity

100% IACS = 58 MS/m, High purity Al 65%, Electrical grade >61%

Cooper and Kearns (1996)

10Aluminium Melt Processing Before Solidification

Impurities are both added and removed during refining operations:•Dissolved H2 by contact with atmospheric H2O•Impurities from re-melt and scrap•Impurities from grain refiner additions•Solid inclusions from refractories and other contamination

Waite (2002)

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Typical Impurities in Electrolysis and Re-melt Aluminium

Waite (2002)

12Aluminium Alloy Major Series

- a major challenge for secondary/tertiary materials

http://www.aluminum.org/Content/NavigationMenu/TheIndustry/IndustryStandards/Teal_Sheets.pdf

Uncontrolled blending of old scrap of different alloy families will lead to unacceptable ‘impurity’ levels.

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Impurity Removal by Refining Step

Taylor (2003)

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Removal of Alkali and Alkali Earth Metals by Fluoride Fluxing

15Treatment of Aluminium in a Crucible (with AlF3)

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TAC Efficiency for Alkaline Metals Removal

Dubé and Newburry (1990)

More intense mixing leads to higher reaction rates and greater removal.

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Impurity Removal by Refining Step

Taylor (2003)

18Removal of Dissolved Metals by Reaction with Chlorine Gas

Chlorine gas should not be used to refine high Mg alloys.

19Rotary Gasing/Fluxing Equipment for In-Furnace Treatment

Snif PHD 50 with flux injector STAS Brochure RGI/RFI

STAS Brochure RGI/RFI

20In-Furnace Treatment with Salt and Gas Injection

Frank (2005)

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Removal of Alkaline Metals by Cl2/Ar

Using a PHD-50

Williams (2001)

22HD 2000 with Synthetic Carnalite Salt Flux K3Mg2Cl7

Ohno (2010)

Na Reduction (%) for 5000 Series Alloy HD 2000

The obvious addition of dissolved Mg may not be suitable depending on the alloy ultimately being produced.

23Reduction of Chlorine Use During Aluminium Refining

Waite (2002) Use of salt injection can eliminate Cl2, but not HCl gas or particulate emissions.

e.g.:

MgCl2 + H2O(g) = MgOHCl + HCl(g)

Future trend = continued reductions in Chlorine.

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Emissions from Chlorine and Salt

25Aluminium for Electrical Applications- Use of the melting furnace to refine by precipitation and settling of dissolved impurities as solid inclusions

Refining agent addition:Boron as either AlB2 or AlB12

Cooper and Kearns (1996)

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Impurity Removal by Refining Step

Taylor (2003)

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Alcan Compact in-line Degasser (ACD)

Robichaud (2011)

Taylor (2003)

28In-Line Degassingwith Snif System

Frank (2005)

29Hydrogen In Liquid Aluminium Alloys

• Moisture in the atmosphere leads to the dissolution of hydrogen in aluminium and the formation of oxide inclusions.

• Scrap Melts (0.5 mL/100 g Melt)• For Wrought Alloys (0.18 mL/100 g Melt)• For Cast Alloys (0.4 mL/100 g Melt)

30Reduction in Hydrogen Solubility At the Point of Solidification

• Hydrogen content, solid particulate or bifilm nuclei, cooling rate and ambient pressure determine: number, peak size and distribution of hydrogen porosity.

• <0.03 mL/100g there is negligible porosity.

Tan et al. (2011)

0.62 mL/100 g

0.035 mL/100 g

31Impact of Dissolved Hydrogen on Metal Quality - Porosity

• Hydrogen <0.18 mL/100g desired for good quality castings.

(2010)

1 Bar 80 mBar

32Impact of Cooling Rate and H2 Content on Pore Diameter

Kaufman and Rooy (2004)

1= 0.25 mL/100g no grain refiner2= 0.31 mL/100g grain refiner3= 0.25 mL/100g grain refiner4= 0.11 mL/100g grain refiner and modified5= 0.31 mL/100g grain refiner and modified

Cooling Rate, oC/s

Casting No.

100

10

10.1 1 10 100

12345

5213

4

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Impact of H2 on PorosityLiao et al. (2012)

34Kinetics of Hydrogen Removal by Different ‘Stripping’ Techniques

Friedrich and Kräutlein (2006)

[H]=10(-2692/T+2.726) PH2½, mL/100g

Future Trend from Less to More Intensive Mixing/Gas Dispersion

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http://www.pyrotek.info/documents/techpapers/2005-11--MQW3--SNIF_In-line_Refining--Robert_Frank.pdf

Removal of Hydrogen Gas By Inert Gas ‘Stripping’

Presence of Cl2in the bubble can produce HCl.

36Mass Transport Modelling for Hydrogen Removal

Complicated looking formula but actually very simple.

Power per unit volume and superficial gas velocity produce interfacial gas-liquid surface area that promotes ‘stripping’ of the dissolved hydrogen gas.

Frank (2005)

37Rate of Refining is Usually Mass-Transport Limited

Friedrich and Kräutlein (2006)

Increased kLafor constant gas injection rate.

38Bubbling Regimes in Stirred Tanks

Middleton (1985)

Increasing gas-liquid interfacial area and enhanced mass transport

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Impact of Shear (Mixing Energy) on Bubble Size (Interfacial Area)

200 rpm16 L/minPower =X

600 rpm16 L/minPower~27X

Kennedy (1996)

1000 rpm16 L/minPower~125X

40Alspek Hydrogen Sensor

• Modern on-line/real time metal quality tool – “Key Enabling Technology”

Foseco Brochure

Pascual (2009)

41AlScan On-line Industrial Hydrogen Analysis

• Continuous/optimized high efficiency operation is now possible due to continuous monitoring of residual hydrogen concentration.

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Impurity Removal by Refining Step

Taylor (2003)

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Aluminium Carbide

• Marginally soluble in primary metal.

• Insoluble at casting temperatures.

• Future development to required to effectively remove carbides.

Qiu and Metselaar (1994)

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AlF3 Active Grain Filtration

Görner et al. (2007)

45Developments in Refining Technology –Last 20 Years (and Future)

• Reducing alkaline metal (Na, Li, etc.) content to <3 PPM (chemically active filter material).

• Reducing inclusion levels to PPT (further reduction in conc. and improved efficiency for smaller sizes, i.e. <20 μm).

• Reducing dissolved hydrogen levels to <0.2 mL/100g (improved process control by continuous monitoring).

• Reducing processing time.• Continuous instead of batch processing or semi-continuous.• Decreasing or eliminating use of Cl2 gas (elimination of Cl2

and reduction in HCl and particulate emissions).

Schlesinger (2007)

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Possible Future Challenges

• Changes to aluminium electrolysis technology, like inert anodes (with the objective of elimination of CO2 releases), but can contaminate the metal with increased levels of ‘heavy’ metals.

• Tighter environmental restrictions.• Higher impurity levels due to greater scrap percentages,

particularly of ‘old’ scrap.

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Possible Future Developments• Improved quality control (less scrap

and off-spec) by the use of continuous on-line analytical equipment: Ultrasonic, LiMCA, AlScan, Alspek H, XRF, IR, LIBS, etc.

• Active filter media filters producing no Cl2, HCl or particulate emissions.

• Advanced scrap sorting, e.g. prevent melt contamination, avoid the need to de-Mag and re-Mag high magnesium alloys, using LIBS, XRF, automated colour sorting, etc.

48Increased Use of On-line ‘Continuous’ Measurements

• Will allow future process engineers to see cause and effect.

• Lead to improved process understanding/models and ultimately to improved equipment and process performance.

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Summary• Dissolved impurities in liquid aluminium consist

of both ‘noble’ metals, dissolved gases (H2) and reactive metals.

• Chemically stable metals are best controlled by the purity of the feed materials to electrolysis and the sorting of secondary and tertiary metal feedstock.

• Industrial refining currently focuses on the removal of hydrogen gas and alkaline metals (Na, Li, and Ca) and achieved by the injection of inert gases (N2 or Ar), Chlorine and salt fluxes (AlF3, MgCl2, synthetic Carnalite).

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Summary• Refining reactions are in general first order

dependent on the concentration of the dissolved impurities and the kinetics of removal are generally liquid phase mass transport limited.

• Industrial developments have focused on improved rates of removal, e.g. greater mixing and the generation of interfacial area and on-line chemical analysis.

• Future developments will be driven by on-line analytical tools and the improved process understanding that these provide.

51Homework Assignment

• Perform a small literature search on the impact of impurities, hydrogen, reactive (alkali) and transition metals on aluminium physical/mechanical properties and be prepared to discuss next week.

• Calculate the treatment time required to achieve 50% removal of 0.5 mL H2/100 g of Aluminium in a ladle containing 10 mt of liquid metal at 700oC, assuming complete saturation of the injected argon (equilibrium achieved at all times) in a batch process, using Ar at 100 normal L/min.

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Future ExpectationsIn the Future it can be expected that processes for the removal of dissolved impurities will be More: •Continuous•‘Intensive’•Optimized (less reagents and other consumables like graphite rotors)•Monitored (continuous on-line)•Efficient (with higher metal quality)•Environmentally friendly

53Removal of Calcium and Inclusions by HD 2000

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Laser Induced Breakdown Spectroscopy

• Ability to monitor metal quality in real time.

• Optimized processing (minimum time, reagents, maximum quality control).

• Powerful tool for process development.

http://www.er-co.com/libs-melt.html

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Removal of Dissolved Metals by Reaction with Chloride Salts