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The Effect of Electrolyte Additives on Cathode Surface Quality during Copper Electrorefining

TA Muhlare 26313929

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Presentation Outlined

• Introduction

• Project Background

• Experimental Planning

• Conclusion

• References

Introduction

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• Copper price (LME Cu Prices)-7.082 $/kg

• Electro-refining is used in copper production.

•Typically 99.5% pure refined copper is electro-refined to produce 99.997% pure copper cathode.

• Product/cathode uses-Electrical cables

Introduction

Copper Electrorefining

Anode: Cu0 → Cu+2 + 2e- E0 = -0.34V

Cathode: Cu+2 + 2e- → Cu0 E0 = +0.34V

V1 = A + C + (IR) electrolyte + (IR) contacts

– Reversible potential is zero

Anodic processes

– Less Positive elements disslove

– More Positve elements remain in solution

Cathodic processes

– Noble metals plate

– Less noble remain in solution

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Introduction

Copper Electrorefining at Palabora Mining Company

• Palabora

– 80 000 metric tons of cathode per year

– Current density 260 A/m2

– Current efficiency (CE) 88 – 92%

• Electrolyte composition

– CuSO4 : 45g/l Cu

– H2SO4 : 220g/l

– NiSO4 : 15g/l Ni

– Total Impurities : 25 g/l maximum (incl. Ni)

– Temperature 70C

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Introduction

Electrorefining Copper Cathodes at Palabora

• Dendrites and Nodules• Roping and Striations• Entrapment of anode slimes

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Introduction

Palabora Tankhouse intent

• Copper cathodes with good surface quality

• Electrorefining at high current density to achieve high copper production

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Introduction

In Commercial Practice

• What is a good copper cathode surface quality?

– Smooth, pure, homogenous, pore-less and non nodular or dendritic surfaces

• The nodular or dendritic growth

- Cause short circuit thus reducing current efficiency.

• A rough surface of the cathode

- Cause entrapment of electrolyte, slimes, and suspended particles.

This leads to the contamination of the deposit, resulting in reduced ductility and conductivity

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Introduction

• Metal ion can be reduced in three different regime

– Activation Control

• Smooth deposits

• Low current density

– Diffusion Control

• Powdery

• limiting current density

– Mixed Control Region

• Commercial practice plants operation

• Additives modify the deposit morphology to give fairly smooth deposits.

• Electrode current densities are maximized therefore decreasing the electrode area required for a specific rate of production

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Introduction

• Commercial Electrorefining Practice: Palabora

To prevent the formation of nodules, dendrites, and to obtain smooth deposits, various organic and/or inorganic additives are employed

• Electrolyte Additives at Palabora

– Glue

– Thiourea

– Avitone

– Chloride

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Project Background

Glue

A polarizer and leveling agent that

controls vertical growth to produce

smooth deposit

Mechanism

Glue molecules adsorb to active

negatively charged growth sites on

the cathode.

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O and N terminals of the peptide chains (-CO-NH-) adsorb to the cathode thus increases limiting current density of diffusion and overpotential of a copper cathode

Project Background

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Degradation of glue

• Glue decomposes at high temperature and low pH values after 1.5 to 2 hours.

• Kinetics of glue hydrolysis follow a first order reaction with H2SO4

as a catalyst

Where k’

– (Mn)t and (Mn)0 are average molar mass at time t and 0

– k’ is the degradation rate constant

– [A] is a concentration of the catalyst

Project Background

Disadvantages

– Excess glue concentration lead to rough, striated and brittle deposits

– Too little glue concentration causes a crystalline nodulation on the cathode

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Project Background

Thiourea

– A grain refiner and an accelerator agent that promotes the formation of new nuclei

– Inhibition of crystal growth processes

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Thiourea dithioformamidine

Project Background

• Thiourea molecules adsorb on the cathode and inhibit the crystal growth of processes.

• Copper electrolytes

– Reduces Cu2+ to Cu+

– Thiourea complexes with

Cu+ to produce (Cu-Th)+

Disadvantage

– Increase the sulphur contamination in the cathode if used at high concentration

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Project Background

Avitone

– Hydrocarbon based sodium sulphonate product

– Complement glue and helps in creating a dense smooth deposit on the cathode.

– It also acts as a detergent, washing oily patches on the anodes and cathodes

Disadvantages

– Too little avitone will cause slimes to adhere to the surface of the cathode – slimes entrapment

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Project Background

Chloride

– Improves and brightening cathode deposits when combined with organic additives.

– The chloride ion acts also as a grain refiner agent.

– Co-adsorbs with Thiourea

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Experimental Planning

Hypothesis

• Electrorefining copper cathode quality is affected by additive concentration in electrolyte. An inadequate control of additive concentration in the electrolyte causes the formation of dendrites and nodules on the cathode surface thus affecting the quality of copper cathode.

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Experimental Planning

Factorial Design (Statistics)

• The scientific method we are taught that good scientists hold everything else

constant while they test one factor at a time (OFAT).

• OFAT is an inefficient, inadequate method of experimentation for identifying significant factors.

– requires more experiments than factorial design to test the same number of factors.

– cannot reveal interactions between factors; factorial design can.

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Experimental Planning

Factorial Design

• Two level factorial design

– Identify factors with significant effects on the response– Identify interactions among factors– Identify which factors have the most important effects on the

response

• Four electrolyte additives – 4 variables

• Two levels (-1 and 1)

• 5 center points (0)

• Randomization

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Experimental Planning

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Factorial Design 21 experiments

Experimental Planning

• Material and Equipment

– Rotating Cylinder Hull Cell RCH

– Scanning Electron Microscope SEM

– Optical Microscope

– pH Meter

– Thermometer

– Lab ware - Plastics

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Experimental Planning

Industry Practice and Laboratory Experiment

– Laboratory experiment must represent industry practice

– Similar electrolyte composition and concentration

– Acceptable representation of hydrodynamics

– Same temperature

Traditional Hull cell• Non uniform Current distribution

• Poor mass transport conditions compared to industry

Rotating Cylinder Hull Cell RCH– Achieve acceptable representation of hydrodynamics

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Experimental Planning

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Rotating cathodic sample

Anode mesh

Autolab HT RotaHull RCH

Rotating Cylinder Hull Cell RCH

Experimental Planning

Rotating Cylinder Hull Cell RCH

• Deliberately non-uniform current distribution

– Non-uniform current distribution

along the cathode

– Range of current density can be

investigated at a single experiment

– Geometry of the RCH determines

the current distribution

– Current density maximum at the top end of the cathode and minimum at the bottom end

– Different plating morphology along the cathode

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Experimental Planning

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Rotating Cylinder Hull Cell RCH

RCH (C.T.J. Low, 2005)

Experimental Planning

Rotating Cylinder Hull Cell RCH

• Hydrodynamics – Correlation of industry copper electrorefining and laboratory experiment– Typical diffusion layer thickness was used– Acceptable representation in mass transport conditions of industry

practice and experiment

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Experimental Planning

Calculations

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Experimental Planning

Rotating Cylinder Hull Cell RCH

• RCH calibration method

– Steel as the cathode

– Run RCH at uniform current distribution and plate copper

– Weigh cathode before and after electrolysis

– Measure the thickness of copper platted

– Reverse steel cathode to be the anode

– 100% CE

– Mass, Time and Current density

– RCH current density scale

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Experimental Planning

Experimental results analysis

• Optical analysis– Observe the different morphology on the cathode with naked eye

• Microscope– Surface morphology at a high magnification

• Scanning Electron Microscope SEM– Elemental analysis– Surface roughness

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Experimental Planning

Experimental results analysis

• Large electrolyte operating window

– PMC current density

– Appropriate / Acceptable cathode surface smoothness

• Effects of Additives

– Additives which give significant effects on the response

– Interactions of Additives – factorial design

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Experimental Planning

Safety

• Perform a Risk Assessment– Severity (S),Probability (P),Detectability (D), Risk Priority Number

(RPN)– RPN = S X P X D

• Recommendation of MSDS and General Lab Safety

• Personal Protective Equipment PPE– Safety Glasses– Lab Coat– Closed Shoes– Long Pants– Gloves

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Experimental Planning

Safety Cont.

• Have knowledge of positions emergency devices:

– Fire extinguishers

– Breathing Apparatus

– Fresh water

– Emergency contact number lists

• Know evacuation plans and routes from laboratories

• Decent ventilation must be maintained at all times

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Experimental Planning

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Time Management

• Tasks to be performed

– Solution preparation

– Set up experiment

– Run experiment

– Analyse results

• Sample preparation

• Microscope

• SEM

Experimental Planning

Time Management

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Conclusion

Additives are added to the electrolyte to maintain cathode quality and promote a smooth deposit. These additives absorb on the cathode surface and take part in the electrochemical crystallization process. However these additives also affect the quality of the cathodes when controlled incorrectly.

Project investigates the effect of additives using a Rotating cylinder hull cell in copper electrorefining.

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References

1. C. Madore, D. Landolt, C. Haßenpflug and J.A. Hermann. 1995. Plating & Surface Finishing. Application of the Rotating Cylinder Hull Cell to the measurement of Throwing Power and the monitoring of Copper Plating Baths. p 36 – 41.

2. Box G.E.P. Hunter, J.S. Hunter W.G, 2005. Statistics for Experiments: Fractional Factorial Design, Willey. USA.

3. Ilkhchi, M.O, Yoozbashizadeh H, Safarzadeh M.S., 2006. Chemical Engineering and Processing: The Effect of Additives on Anode Passivation in Electrorefining of Copper. Vol. 46. p 757 – 763.

4. Krzewska, S. Pajdowski, I. Podsiadly, H and Podsiadly, J. 1984. Metallurgical Transactions B: Electrochemical Determination of Thiourea and Glue in the Industrial Copper Electrolyte, Vol 15B, p 451 – 459

5. Andersen, T.N, Budd, R.D and Strachan, R.W. 1976. Metallurgical Transactions B: A Rapid Electrochemical Method for Measuring the Concentration of Active Glue in Copper Refinery Electrolyte Which Contains Thiourea. Vol. 7B. p 333 – 338.

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References

5. Lowa, C.T.J. Roberts E.P.L. Walsh, F.C. 2007. Electrochimica Acta: Numerical simulation of the current, potential and concentration distributions along the cathode of a rotating cylinder Hull cell. Vol. 52. p 3831–3840.

6. Teeratananon, M. Pruksathorn, K. Damronglerd, S. Dupuy, F. Vergnes, H. Fenouilletc, B and Duverneuilc, P. 2004. ScienceAsia: Experimental investigation of the current distribution in Mohler cell and Rotating Cylinder Hull cell. Vol. 30. p 375-381.

7. Moats, M.S. Hiskey, J.B. Collins. D.W. 2000. Hydrometallurgy: The effect of copper, acid, and temperature on the diffusion coefficient of cupric ions in simulated electrorefining electrolytes. Vol 56. p. 255–268.

8. Hiskey, J.B and Cheng, X. 1998. Metallurgical and Materials Transactions B: Fundamental Studies of Copper Anode Passivation during Electrorefining: Part III. The Effect of Thiourea. Vol. 29B. p. 53 – 58.

9. Jaskula, M.J, 1983. Electrochimica Acta. Some Remarks on the Problem of Current Density Optimization in Copper Electrorefining Process, Vol. 28. no. 10. p. 1395 – 1406

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