Vale

Company Advisor: Dr. Vladimir PaserinFaculty Advisor: Dr. Gu Xu

Catherine PereiraAbrar Sidahmed Catherine Silva

• Overview of Process

• Problem Statement

• Engineering Analysis

▫ Optical Microscope

▫ Compositional/EDS Analysis

▫ Distribution of Impurities

▫ EBSD Analysis

• Sustainability & Economic Issues

• Conclusion

• References

2

Outline

MATLS 4Z06 - VALE

Overview of Process

• Nickel carbonyl pellet decomposer

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MATLS 4Z06 - VALEImage from: "Carbonyl Refining of Impure Nickel Metal," in Extractive Metallurgy of Nickel, Cobalt and Platinum-Group Metals.: Elsevier Ltd., 2011, ch. Chapter

22, pp. 269-280.

Fig 1: Pellet decomposer.

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Problem Statement

• Two types of Pellets

▫ Sulphur (S-) Pellets , ~0.0220-0.0300 wt% S

▫ Plating (P-) Pellets, <0.0002 wt% S

• Both types of pellets experience rare

breaking of layer(s) („rattlers‟)

• Broken layers may affect the uniformity of

pellet dissolution

• Detecting trace amounts of impurities within

layers

• Attempt to relate process conditions to layer

formationMATLS 4Z06 - VALE

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0% 100%

Fig2 : Measurements of broken layers in rattler.

Broken Layers

Broken Layers

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- Presence of either one or two broken layer(s)

- Consistent location of layers within pellet

- Outer broken layer observed at the onset of grinding

- Inner broken layer starts to form after the pellet is

ground past half of its original size

Location:

OUTER LAYER AVG: ~ 81% of radius

INNER LAYER AVG: ~ 41% of radius

(0% = centre; 100% outer edge)

MATLS 4Z06 - VALE

• Average thickness of gap: 80-115 microns

• Average thickness of layers: (20-50 microns as stated by

Vale)

▫ Measured and calculated avg. of 30. 7 ± 9.5 microns

▫ Largest thickness 95 microns

• Average number of layers ~ 161 for rattlers

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MATLS 4Z06 - VALE

Fig 3: Micrograph showing broken layer and onion structure of pellet.

Sample Preparation• Cold mounted rattler & normal pellets in epoxy

• Used 320, 400, 600 and 1200 SiC to grind pellets

down to a cross-sectional area

• Polishing: 6 micron, 1 micron diamond polish

followed by colloidal silica

• Etching: 10mL distilled water, 38mL nitric acid, 50mL

acetic acid.

▫ Normal pellets: submerged for 15 sec and 10 sec

▫ Rattler pellets: submerged for 8 sec and 5 sec

8

Sample Preparation

• Cold mounted

rattler under

vacuum

• Etchant was

changed to: 50%

acetic acid, 50%

nitric acid

• Samples etched

for only 5s

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Preliminary Examination - Distribution of layers (thinner near the center, thicker

after the gap)

- Fine grained-structure

-

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MATLS 4Z06 - VALEFig 4 : Micrograph of normal pellet (left) and rattler

(right).

Description of Layers in Rattler

LEGEND:

Yellow – „good

layers‟

White – „bad layers‟

Blue – „bad sub-

layers‟

Red – interfaces

Fig5 : Optical micrograph of rattler layers. x1000

mag.

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MATLS 4Z06 - VALE

• Uneven surface protrusions

• Rough areas may be broken layer pieces that adhered to surface as pellet is recycled through process

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Sectioning of Pellet

MATLS 4Z06 - VALEFig6 : Optical micrograph of surface of inner

centre.

Initial Compositional Analysis

• Carbon/sulfur analysis

• Limit stated by Vale

▫ UK plant standards

• Normal pellets studied meet limit for C for P-

pellet, but exceeds S-content

• Rattlers contain high C/S-content suggesting it

may be S-pellets

Element Normal Rattler (Inner) Rattler (Outer) Rattler Total

C (wt%) 0.0125 0.0167 0.0522 0.0345

S (wt%) 0.0036 0.0361 0.0508 0.0435

Element P-pellet S-pellet

C (wt%) < 0.0150 < 0.0050

S (wt%) < 0.0002 0.0220-0.0300

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MATLS 4Z06 - VALE

Table 2 : Average results from C/S analysis.

Table 1: Compositional limits from Vale.

SEM Analysis

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Structure of Normal Pellet

Normal Pellet: faceted/polygonal grains

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MATLS 4Z06 - VALE

x5200

Mag.

Structure of Rattler

Grain Size/Distribution: bi-modal

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MATLS 4Z06 - VALE

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- Presence of thin interface (bottom

left) and distinct wavy layers (top &

right)

- Wavy layers consistently found after

the broken layer(s) MATLS 4Z06 - VALE

• Cross section analysis across entire diameter• Composition points taken every 200 microns• Slightly higher distribution of minor impurities

(C, Fe, Al, etc.) towards the centre as opposed to the edge of the pellet (not included in the table above)

EDS Analysis: Normal Pellet

Element Wt %

Ni 99.48

O 0.52

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MATLS 4Z06 - VALE

Table 2 : Average EDS results for normal pellet.

● ● ●

Inner Surface of Outer Shell

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● ● ●

Element Weight %

C 16.40

O 6.97

Fe 2.93

Ni 73.50

Element Weight %

C 13.39

O 0.64

Si 0.43

Ni 85.54

Element Weight%C 9.76S 0.82Ni 89.42

Totals 100.00

Element Weight%

C 29.37O 14.55Al 2.95Si 3.04K 4.39Ni 41.64

Totals 100.00

Element Weight%C 68.08O 11.96Na 1.59Al 0.50S 1.00Cl 2.28K 2.89Ni 11.71

Totals 100.00

Element Weight%

C 25.36O 34.79Al 4.44Si 5.98K 0.53Ni 28.90

Totals 100.00

A B

C D

Outer Surface of Inner Centre Part

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Marker measures A) 50 um; B) 100 um; C) 10 um; D) 50um. MATLS 4Z06 - VALE

● ● ● ●

Rattler Pellet

Average composition of ‘good layer’:

Element Weight %

C 1.76

Ni 98.24

Average composition of ‘good’

interface:Element Weight %

C 0.76

Ni 99.24

●●● ●● ●

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MATLS 4Z06 - VALE

Table 3 : Average EDS results for rattler.

Table 4: Average EDS results rattler.

Rattler PelletAverage composition of ‘wavy layer/

interface’:Element Weight %

C 10.78

O 3.78

Al 0.10

Si 0.13

S 2.41

Cl 0.05

Fe 0.80

Ni 81.95

●

●

●

●

Average composition of ‘bad

layer/interface’ :Element Weight %

C 1.98

O 5.27

Ni 92.76

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MATLS 4Z06 - VALE

Table 5: Average EDS results rattler.

Table 6: Average EDS results rattler.

Distribution of Impurities

Fig 7: Element distribution across layers in rattler.

1) Original SEM image; 2) C distribution; 3) O distribution; 4) S

distribution

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MATLS 4Z06 - VALE

EBSD Analysis

24

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=50 µm; Map1; Step=0.5 µm; Grid386x93

• Other reason for unique etching: difference in grain orientation

• Shows inhomogeneous structure with bands of small and large grains

• Has a higher proportion of Σ3 boundaries

Σ3 Boundary

Σ9 Boundary

Low Angle

Boundary

High Angle

Boundary

Normal/High Sulphur Content Pellet

Results

• Data provided by Valerie Randle, Vale.

MATLS 4Z06 - VALE

2626

Rattler Pellet Results Σ3 Boundary

Σ9 Boundary

Low Angle

Boundary

Grain Boundary

• Grain Boundaries

• Has a higher proportion of Σ3 boundaries, more

specifically after gap

MATLS 4Z06 - VALE

2727

Rattler Pellet Results• Local Misorientation

Misoriented

Grains

MAD: 0.61°

MATLS 4Z06 - VALE

2828

Rattler Pellet Results

MATLS 4Z06 - VALE

• Smaller grains

before gap, and

larger ones after

gap

• Grain size range:

0.2-13 microns

• Grain size avg.

0.8 microns

Role of Impurities

• Preferential etching in rattler not seen with normal pellet:

▫ At „good‟ interface – due to local misorientations

▫ At „bad layer/interface‟ – oxygen impurities distributed within those region

• ‘Wavy layer’ formation

▫ Due to high content of impurities: oxygen and carbon

There is a higher surface roughness and poor adhesion

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MATLS 4Z06 - VALE

Source of the Problem

• Layers break

▫ Impurities affect quality of deposited nickel:

non uniform thickness, weaker adherence

to substrate

▫ Higher proportion of twinning suggests high

internal stresses built in during growth

▫ Combination of above promotes cracking

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MATLS 4Z06 - VALE

Source of the Problem

31

- Thickness of gap is large

(~80 - 115 um) suggests

multiple layers are fracturing

- Occasionally, a full broken

layer does not form

Fig 8: (Right): Semi- broken

layers

- Very dark bands with thin

layers are suggested to be

susceptible to fracture and

breaking

Fig 9: (Left): Dark wavy Ni bands MATLS 4Z06 - VALE

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Source of the Problem

GAPGAP

MATLS 4Z06 - VALE

Evidence of beginning of

broken layer at ‘wavy’

layers

Relation to Process: Sulphur• S-pellet

▫ Evenly distributed sulphur content

▫ Higher sulphur content than

originally expected

▫ Possibly large temperature

gradient induces turbulence on

the substrate

▫ Does not contribute to the

observed dissolution behaviour

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MATLS 4Z06 - VALE

Relation to Process: Oxygen

• Oxidation/contamination of nickel surface as

pellet travels through bucket elevator

• Exposure of oxygen from make-vessel or during

maintenance of reaction chamber

• Occurs at bad interfaces of thinner layers and

becomes preferentially etched

▫ Not present in high amounts

▫ Hinders the deposition/growth of Ni layer

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MATLS 4Z06 - VALE

• Pellet overheated before entering reaction

chamber

▫ Nickel acts as a catalyst

▫ Decomposition of Boudouard Reaction: 2CO = C +

CO2

▫ C adsorbed onto surface, while CO2 is desorbed

• Gas phase precipitation of (Ni(CO)4)

▫ Enhanced thermal convection/overheating of pellet

▫ Black metallic precipitates deposit on the substrate

• Pellets could also be contaminated by grease

when entering the bucket elevator

▫ Grease - organic contaminant

Relation to Process: Carbon

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MATLS 4Z06 - VALE

Sustainability Issues• Nickel carbonyl very toxic compound

▫ Make-vessel purged with inert gas to ensure

no carbonyl gas escapes

▫ Recycling of CO = efficiency

Economic Issues• Pellets used in electroplating market

▫ Broken layer has an effect on uniform

dissolution of pellet

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MATLS 4Z06 - VALE

Conclusion

• Layer breaks as a result of high content of

impurities

• Future Recommendation:

▫ Use of SAM to detect exact amount of

impurities within layers

▫ Study pellets from the same process/batch to

determine precise process conditions have

effect on pellet formation

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MATLS 4Z06 - VALE

References

[1] M. D. Head, V. A. Englesakis, B. C. Pearson, and D. H. Wilkinso, "Nickel Refining by the TBRC smelting and pressure carbonyl route," in 105th AIME Annual Meeting, Las Vegas, Nevada, 1976.

[2] "Carbonyl Refining of Impure Nickel Metal," in Extractive Metallurgy of Nickel, Cobalt and Platinum-Group Metals.: Elsevier Ltd., 2011, ch. Chapter 22, pp. 269-280.

[3] C. Chen, "Experimental study of the microstructural evolution of chemical vapour deposited (CVD) nickel upon annealing," University of Toronto, Toronto, Masters Thesis 2011.

[4] M. Skibo and F.A. Greulich, "Characterization of Chemically Vapor Deposited Ni-(0.05-0.20wt%)B Alloys," Thin Solid Films – Preparation and Characterization, vol. volume, (issue number), pp.225-234, January 13 1984.

[5] Valerie Randle. “Electron Backscatter Diffraction (EBSD) of Sectioned Nickel Pellets to reveal core grain boundary structure”. [Vale]

[6] B. Mellor, Surface Coatings for Protection Against Wear, Cambridge, England: WoodheadPublishing Limited, 2006.

[7] P. B. Bansa, “Property Characterization of CVD Nickel,” Toronto, 1998-2001.[8] L. Mond, L. Carl and Q. Friedrich, “Action of Carbon Monoxide on Nickel,” Journal of the

Chemical Society Transactions, vol. 57, no. 0, pp. 749-753, 1890.[9] S. D. Jackson, N. Hussain and S. Munro, “High temperature adsorption of carbon

monoxide and hydrocarbon gases over nickel and platinum catalysts,” Journal of the Chemical Society, Faraday Transactions, vol. 94, pp. 955-961, 1998.

[10] C. V. S. Lim, “Length Scale Effect on the Microstructural Evolution of Cu Layer in a Roll-bonded,” ProQuest LLC, Ann Arbor, MI, 2008.

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MATLS 4Z06 - VALE

Thank you.

Any questions or comments?

39

MATLS 4Z06 - VALE

Effect of Temperature

• Heat transfer across bed of pellets• Carbonyl gas enters chamber at 70C

(http://books.google.ca/books?id=6aP3te2hGuQC&pg=PA372&lpg=PA372&dq=nickel+carbon+monoxide+in+bucket+elevator&source=bl&ots=wPZIEPR7Zc&sig=IFcrwkDYak0iFkQt6pkcOUrjUL8&hl=en&sa=X&ei=EBNwT4vYNcz5ggel46Rr&ved=0CDkQ6AEwAw#v=onepage&q=nickel%20carbon%20monoxide%20in%20bucket%20elevator&f=false)

• Carbonyl content is max 15wt% to prevent pellet coalescence by high rate of nickel deposition

• Enter preheater at 180C and gets heated to 220 (US decomposer patent)

• Presence of baffle located before rxn chamber to slow down the passage of pellets

• Flow of pellets laminar (patent)• High diffusion rate of S on the 111 plane; S –poisoning of Ni catalyst• CVD diffusion controlled; boundary layer = mass transfer

▫ Inc. Thermal conv removes CO faster from surface = faster reaction

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