Content 2015/S5_1... · Part I: Phase-sensitive eddy-current method Coil system generating an...

1

Analyses of the chemical and structural integrity of

gold bullions by eddy current probes

LBMA 2015, Bochud-Jodry-Sutter

Daniel Sutter, Helmut Fischer AG Switzerland

Quentin Bochud, Metalor SA Switzerland

LBMA Assaying and Refining Conference, London, March 2015

Content

Introduction: Basic idea of using eddy-current probes to study the propertiesof precious metal bullions

Part I: method and instrumentation

Part II: applications on bullions

Conclusions

___

Abbreviation: electrical conductivity, s (Sigma)

Page 2LBMA 2015, Bochud-Jodry-Sutter

LBMA Assaying & Refining Conference 2015 8-10 March 2015

Session 5 - Bochud & Sutter

http://www.helmut-fischer.ch/de/ch/startseite/




2

Introduction

Fake gold bars:

Gold coated tungsten bars (>)

Gold bars with tungsten rod inclusion (>)

Bars with defects

Pores, bubbles, inclusions

Hammerings (>)

Bars with poor quality in other aspects

Instrumentation available to identifyboth fakes or low quality bars?


Part I: Methods and instrumentation

Element composition: Fire-assay, WD-XRF, ED-XRF, ICP-OES,Spark-OES, etc.*

Structural integrity: Ultrasonic methods, transmission X-raymethods, eddy-current flaw detection technologies, radio wavetransmission, density analysis, metallography, etc.**


ED-XRF*

Ultrasonic**

Eddy-current

Analytical integrity, composition

Inspection of bullions







3

Part I: Metals and electrical conductivity

Electrical conductivity, s , units MS/m

Gold: 45, Silver: 63, Copper: 60 MS/m

Materials properties affecting s

Microstructure: perfect crystal lattice → maximal sAny defect reducing s (impurities, alloying, inclusions, precipitations, pores,stress&strain, grain boundaries, surfaces, etc.)

Temperature: T s


Ref: ESA Edu site

s

s

Electrical conductivity, s , units MS/m

Gold: 45, Silver: 63, Copper: 60 MS/m

Materials properties affecting s

Microstructure: perfect crystal lattice → maximal sAny defect reducing s (impurities, alloying, inclusions, precipitations, pores,stress&strain, grain boundaries, surfaces, etc.)

Temperature: T s

Part I: Metals and electrical conductivity


Parameter Implication

Alloying elements of interest

Inclusions, precipitations, pores of interest

Grain boundaries avoid influence

Stress&strain avoid influence

Temperature avoid influence

Surfaces avoid influence







4

Part I: Phase-sensitive eddy-current method

Following ISO 2360 for coating thickness measurements and DINEN 2004-1 / ASTM E1004 for conductivity measurements

Simple and industry approved instrumentation


Sigmascope®

by Helmut Fischer


Coil system generating an alternating magnetic field which isinducing eddy-currents in el. conducting materials. Eddy-currentsreduce the total magnetic flux, change of flux is detected by theprobe

Evaluation:amplitude > distance from material+thicknessphase > conductivity of material+thickness

Calibration:phase vs. s of certified referencematerials








5


Coil system generating an alternating magnetic field which isinducing eddy-currents in el. conducting materials. Eddy-currentsreduce the total magnetic flux, change of flux is detected by theprobe

Evaluation:amplitude > distance from material+thicknessphase > conductivity of material+thickness

Calibration:phase vs. s of certified referencematerials



Performance (Sigmascope Gold B, FS14 probe at 1 kHz)Repeatability: 0.03 MS/mAccuracy: 0.3 MS/mMeasuring time: 2 sec

Robust againstContact quality: can be operated in non-contact modeSurface quality: neither grease nor roughness a problem

Sensitive againstThickness: frequency dependent probing depthTemperature: to be monitored and corrected (sensors available)Stamping: has minor influence








6


PerformanceRepeatability:Accuracy:Measuring time:




Metals can be analyzed through plastic foils, no coupling medium needed

0

1

2

3

4

5

0 500 1000 1500rela

tive

devi

atio

n [%

]

polymer foil thickness [um]






Probe frequency for different bar types (it’s the only parameter!)

In doubt stack two bars…

s

0

1

fdepth







7






s changes ~ 5% per 10°C! Software applies corrections to measured s with input of material and temperature.

> more on this in Part II






Surface features with significant conductivity have an impact on how fields propagate. Deep stampings affect the measurements (especially at high frequencies).







8

Part II: Applications on bullions


Part II: Analyses of bars 999.9 ‰ and defects


Bullion 1 Bullion 2 Bullion 3 Bullion 4 Bullion 5 Bullion 6

Mean 44.56 44.40 44.32 44.50 44.43 33.52

Std.dev. 0.08 0.10 0.13 0.13 0.10 0.68

What’s the problem?







9

Part II: Analyses of bars 999.9 ‰ and defects


Bullion 1 Bullion 2 Bullion 3 Bullion 4 Bullion 5 Bullion 6

Mean 44.56 44.40 44.32 44.50 44.43 33.52

Std.dev. 0.08 0.10 0.13 0.13 0.10 0.68

What’s the problem?

Eddy Currents allow the detection of bullions with defects:

Bubbles

Inclusions

Part II: Integrity of gold bullions Analysis


Pure gold Gold with tungstenrods

45.15 31.46

45.22 33.50

45.20 30.93

45.26 34.41

45.25 33.56

Mean 45.22 32.77

Std.dev. 0.04 1.50

Difference of response between pure and fake bullions

Worse repeatability on fake bar (not homogeneous)

Response is similar to that of bar with defects







10


Pure gold Gold with tungstenrods

45.15 31.46

45.22 33.50

45.20 30.93

45.26 34.41

45.25 33.56

Mean 45.22 32.77

Std.dev. 0.04 1.50

Difference of response between pure and fake bullions

Worse repeatability on fake bar (not homogeneous)

Response is similar to that of bar with defects

A Significant correlation R2 > 0.6

But an important variation without a specific rang

Influence of temperature Influence of minor elements Influence of surface

>100 bars with titles in995 - 999.9 ‰ were tested

Part II: Gold between 995 and 999.9‰


y = 1.1778x - 1133.7R² = 0.6253

3637383940414243444546

995 996 997 998 999 1000

Con

duct

ivity

[MS

/m]

Au title [‰]







11

Influence of temperature

Without control (>)

under control (with a stabilization of sample and equipment)at fixed temperature: 20°C at variable temperature

(corrected by Sigmascope)

Part II: Gold between 995 and 999.9‰


43.0

43.5

44.0

44.5

45.0

45.5

15 20 25 30

cond

uctiv

ity [M

S/m

]

temperature [°C]

Au5N Au 997.5 Au 995

Average 46.64 44.36 41.28

SD 0.15 0.17 0.12

Au5N Au 997.5 Au 995

Average 46.69 44.38 41.33

SD 0.11 0.18 0.09

Possibility to do a calibration (in range of 995-999.9 ‰)

But it works only with Binary alloys

Or to perform QC on home-made alloys

Part II: Binary alloys analysis


y = 1.1007x - 1053.9R² = 0.9951

40.0

41.0

42.0

43.0

44.0

45.0

46.0

47.0

48.0

994 995 996 997 998 999 1000 1001

cond

uctiv

ity [M

S/m

]

Gold title [‰]







12

Why only with binary alloys ?

Conductivities are different withdifferent minor elements!

Exactly the same purity of gold (995 ‰)

But the conductivity of gold with copper is lower than that of gold with silver

Part II: Binary alloys analysis


Au 995 ‰Ag 5 ‰

Au 995 ‰Cu 5 ‰

41.45 36.41

41.42 36.51

41.41 36.53

41.39 36.40

41.35 36.49

Mean 41.40 36.47

Std.dev. 0.04 0.06

Conclusions


Refiner

Banks / Industry

Assay officesTrade

Recycler

Monitoring of production

Check integrity of bar

Judgments based on simple criteria! s > 44 MS/m: 999.5 grade bar 44 > s > 35 MS/m: LBMA grade

bar s < 35 MS/m: suspicious bar →

further investigation

Quality control information Title Pores Inclusions Other defects…

Gold

Tolerance limits display







13

Conclusions


Questions….

Thank you for your attention.

___

Additional Informationwww.goldanalysis.comwww.helmutfischer.com

ContactsDaniel Sutter, [email protected] Bochud, [email protected]





Content 2015/S5_1... · Part I: Phase-sensitive eddy-current method Coil system generating an...

Documents

Transcript of Content 2015/S5_1... · Part I: Phase-sensitive eddy-current method Coil system generating an...