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Copyright (c) 2013 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected].

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/TIA.2013.2290841, IEEE Transactions on Industry Applications

BMC: A Modulating Bar for Copper Electrowining

Designed for Heavy Duty and High Reliability

Eduardo P. Wiechmann, Senior Member, IEEE, Pablo E. Aqueveque, Member, IEEE,

Jorge Henriquez, Student Member, IEEE, Luis Muoz, Student Member, IEEE, and Anibal Morales.

AbstractThis works presents the research,

development and design of a BMC intercell bar for copper

electrowining following heavy duty and high-reliability

guidelines. As a result, the configuration ensures a low-

maximum temperature with high-temperature rated

connectors. These properties guarantee the hardness and

integrity of the contacts and consequently its lifetime.

Some key performance indicators include: control and

balance of the cathodic current densities, short circuit

protection and bypass of weak contacts. Further, it has

the unique property of hot-swapability for replacing

impaired connectors. Thus, the Availability of the intercell

bar under typical electrowining operating conditions is

ensured.

I. NOMENCLATURE

Symbol Description Unit

Surface normal vector

Current density 2

Inward current

Electrical conductivity

Length of the contact layer

Voltage difference

Density 2

Heat capacity (3 )

Temperature

Thermal conductivity ( )

Domain heat

Surface heat

II. INTRODUCTION

Modern copper electrowining (Cu-EW) facilities in

operation worldwide are sized for a rated production from

120,000 to 200,000 Tons per year. The process uses 26,000

cathodes divided into 4 electrical independent circuits of 100

concrete polymeric cells each, with corresponding

electrolyte's hydraulic feeding/recycling system, diesel based

heat transfer plant, solvent extraction plant, exhaust fans for

acid mist control and other Cu-EW service's installations.

Each EW circuit is supplied by a high-current rectifier

rated up to 50 kA. Thus, the average cathode current density

at EW plants ranges from 200 to 450 A/m2 depending on the

number of cathodes per cell and the electrolyte's copper

concentration. It should be pointed out that the latest Cu-EW

plants put into service, present fully automatic harvesting

operation. This leaves room only for new technologies which

can provide high availability and high efficiency in order to

ensure KPIs (Key Performance Indicators) of production,

cathode quality and energy. Any solution should offer low

standard deviation of current densities and low specific

energy consumption. Therefore, metallurgical short-circuits

and heat losses should be overcome.

In 2011 the first operational Current Source (CS) intercell

bar connection was installed in Zaldivar mining company, a

Barrick's facility located at 170 km southeast from

Antofagasta, Chile. Results accomplished with the

technology were quite satisfactory [1], reducing cathode

current density deviation by 40% and specific energy

consumption by 4% while increasing current efficiency by

0.6% and physical cathode quality by 4% (keeping chemical

quality). The solution has received industrial and academic

recognition.

The success of the CS configuration is based on its

equivalent electric circuit having a reasonably short-circuit

resistance and less sensitivity to parameter variations.

However, the CS exhibited an impairing weakness. It

happens in the event of an unexpected phenomenon (not

previously seen or anticipated) producing high temperatures

in electrode contacts. It appears if a weak contact is

established on intercell bar connectors. With CS the current

source pushes the current to flow throughout the weak

contact high resistance producing heat and energy losses. The

problem is not produced by open circuits where the connector

current is zero, and consequently power and heat dissipation

are also zero (open circuits are closed through the

electrolyte).

The electric model of the current modulating bar BMC

(by its acronym in Spanish) uses interconnector resistances in

the intercell bar. It is a hybrid configuration that lies between

equipotential Voltage Source (VS, Walker based intercell bar



Fig.1. Electric model superimposed over physical diagram of the process: a) VS, b) CS and c) BMC.

[2]) and Current Source (CS, Optibar intercell bar [3]) taking

the advantages from both (See Fig. 1).

III. HEAVY-DUTY AND HIGH-RELIABILITY DESIGN

Enhancing Availability (A) requires extending the Mean

Time Between Failure (MTBF) while reducing the Mean

Time to Repair (MTTR):

=1

=

1

(1)

= (2)

=

+ (3)

Pursuing this goal, the definition of the following design

guidelines imposes strict boundaries to the research and

development of the BMC. Five requirements should be

fulfilled; the first four are related with MTBF and the last and

most strict with MTTR. Therefore, the intercell bar should be

designed to work with:

1. High rated temperature of contacts.

2. High yield strength of contacts.

3. High chemical resistance.

4. Operation free of Hot-Spots.

5. Hot-Swapability.

To increase the rated temperature of contacts copper must

be replaced. This decision is a turning point for intercell bars.

Copper has been always selected based on its high

conductivity. However, it becomes soft with fairly low

temperatures (160C-sustained) and losses hardness (See Fig.

2). In turn this leads to deteriorated contacts. After reviewing

the state of art in copper high conductive alloys, the decision

was to select copper-chromium alloy C18200. It has 80%

IACS (electrical conductivity of commercially pure annealed

copper according to International Annealed Copper Standard)

securing very good conductivity and stands sustained

temperatures over 500C (design requirement #1). The

hardness of this standard alloy is 3X better than pure

annealed copper (design requirement #2). See table I.

Moreover, copper-chromium exhibits better chemical

resistance to sulfuric acid (aerated or submerged), surpassing

copper (design requirement #3) [4]. Finally, the slight

reduction in conductivity of intercell bar connectors is

irrelevant because its resistance is negligible next to the

contact resistances under normal operating conditions. See

table I.

Removing hot-spots of a CS intercell bar was a challenge.

The goal is to overcome the heat produced in the event of

weak contacts. This was accomplished by upgrading the

electric circuit configuration of the CS configuration. The

solution was achieved using interconnector electric

resistances in the BMC intercell bar. These resistances

provide secondary current paths to bypass defective contacts

while keeping short circuit protection. Moreover, for thermal

diffusion, connectors are thermally interconnected with super

duplex stainless steel 2507 pieces (see Table II, Fig. 3). This

steel resists the sulfuric acid and thermal environment of the

process. With these innovations the maximum working

temperature was reduced from 160C to 85C (design



requirement #4). Also, lower operation temperatures should

reduce chemical corrosion (aerated) of the connectors. See

Figure 3.

These improvements should mean a +1% addition to

energy efficiency (from 94% to 95%) operating at a current

density of 330 A/m2. For higher current densities, the

percentage increase in efficiency will be higher as well.

Currently, MTBF data for intercell bars is not available.

However, the field experience allow to set a low value for the

MTBF with conventional VS and CS bars while the

improvements aforementioned allow to predict a high MTBF

for BMC.

The last requirement is to reduce MTTR. Replacing a

defective intercell bar with any existing technology requires

the use of a 10 Ton short-circuit frame to bypass the process

current of three cells (affecting four cells in fact). Then,

personnel of plant operations proceed to drain the electrolyte

of the cells. Next, the corresponding 393 electrodes are lifted

away and the intercell bar becomes free and can be replaced.

This operation takes up to 12 hours. The complexity and the

time required for this procedure means that only in case of

extreme malfunction (major impairment of specific energy or

physical cathode quality), triangular bars are rotated or

replaced. Therefore, intercell bars normally operate with

defective connectors impairing productivity. This under

performance goes until a full housekeeping of the cell is

done. This operation is regularly scheduled every 3 months.

At this time, if a triangular bar presents many defective

contacts is rotated 120. Being triangular after two rotations

the entire bar must be replaced.

BMC offers hot-swapping of contacts and connectors

(design requirement #5). This means high Availability free of

defective connectors and very low maintenance cost. The last

is explained because instead of replacing a bar, only 1 out of

65 connectors requires to be rotated or replaced at once,

without disturbing the process. By using a pentagonal cross

section each connector possesses 5 contacts. Therefore, when

a contact becomes worn the pentagonal segment can be

rotated to the next contact position. Furthermore, pentagonal

connectors may be replaced with spare parts if required. The

final MTTR depends on inspection and detection of a

defective connector (the rotation or replacement time is

estimated to last less than 5 minutes). From industrial

practice, 8 hours is a conservative time to diagnose an

impaired contact and rotate or replace a connector. Fig. 4

shows a pentagonal connector. Finally, the lifetime of the

entire bar is greatly improved because the number of

available contact positions is boosted from 3 to 325 (5 x 65).

Also, in case of failure, only one connector requires to be

replaced instead of a complete intercell bar. Therefore,

maintenance costs are lower.

TABLE I. CHEMICAL, MECHANICAL AND PHYSICAL PROPERTIES OF ANNEALED

ETP C11000 COPPER AND COPPER-CHROMIUM ALLOY C18200 @ 20C [4]

Mechanical Properties

Hardness (Rockwell B) 47 70

Tensile Strength (KSI) 26 70

Yield Strength (KSI) 6 55

Elongation (% in 2") 20 21

Physical Properties

Thermal Conductivity

(BTU/ft2-hr) 226 187

Thermal Expansion

(per F) 0.0000098 0.0000098

Density (lb per inch3) 0.323 0.321

Electrical Conductivity

(%IACS) 100 80

Modulus of Elasticity

(KSI) 17,000 17,000

TABLE II. Chemical, Mechanical and Physical Properties of Super Duplex

Stainless Steel 2507@ 20C [6]

Mechanical Properties

Hardness 32

Tensile Strength 116

Yield Strength 85

Elongation 15

Physical Properties

Thermal Conductivity

(BTU/ft2-hr) 8.7

Thermal Expansion

(per F) 0.0000072

Density (lb per inch3) 0.28

Electrical Conductivity

(-inch) 31.5

Modulus of Elasticity (KSI) 29,000

IV. CU-EW PROCESS MODELING & BMC DESIGN

To evaluate the performance of the BMC electrical

configuration two different models of the Cu-EW process

were employed. First, an electrical model of concentrated

parameters with proven accuracy was used to predict the

cathode current density distribution of EW cells using VS,

CS and BMC configurations. The results obtained help to

validate the key performance indicators of the BMC

connection: control and balance of the cathode current

densities, short-circuit limiting capability and bypass of weak

contacts. Next, a 3D Finite Element model with distributed

parameters of the Cu-EW cells was developed and used to

predict the electrical



Fig. 2. Progressive deterioration of the hardness with temperature for

Copper ETP C11000 and Copper-Chromium C18200[5]

Fig.3. Corrosion resistance of super duplex stainless steel 2507 against other

commercial standard steels [6].

behavior and the temperature distribution among the intercell

bar contacts of the BMC.

A. Concentrated Parameter Electrical Model

Due to the slow dynamics of the copper electrometallurgical

process, a steady-state concentrated parameter electrical

model (CPM) can represent with enough accuracy the current

distributions [7]. The model of a Cu-EW cell is shown in

Figure 1.

The model includes the electrolyte resistance, the voltage

drop on the electrodes contacts, represented by a cathode and

anode contact resistance, the polarization voltage of the redox

reaction, and cathode and anode overpotentials. Electrolyte

Fig. 4. Pentagonal connector. (a) Upper view of two adjacent EW cells (b)

Close view of intercell connectors (c) Pentagon connector removed.

resistance dependents on the distance between electrodes,

copper concentration, sulfuric acid concentration and the

electrolyte temperature. The electrode overpotentials depend

mainly on the cathode current density. These variables are

summarized in a stochastic model that was adjusted to real

data measured on-site at Zaldivar EW plant.

For simulation purposes the same resistance values

(contacts and electrolyte resistances) were used for VS, CS

and BMC configurations. The simulations were performed

using the nodes method for solving matrix resistance

networks. The inputs to solve the cathode current distribution

of the EW cells are the total cell DC current (INET) and the

admittance matrix (YNET). The system (see equation 4) is

solved for the node voltages (VNET). Next, the cathode

150

140

130

120

110

100

90

80

70

60

400100 200 600300 500

CHROMIUM COPPER

ZIRCONUIM COPPER

SILVER COPPER

TEMPERATURE (C)

HA

RD

NE

SS H

V



currents can be founded by using each node voltage and

branch resistance in the network. The interconnector

resistance of BMC was set to 800 and is generated in the

model with low resistance paths between connectors (see

Fig.1). Bolts were used in the same material as the connectors

(copper-chromium alloy C18200).

= ()1 (4)

The outputs of this model are the electrolyte and the

cathode current densities. Theses outputs are introduced into

the 3D Finite Element Model as initial conditions.

B. Finite Element Model

A 3D Finite Element model (FEM) with distributed

parameters of the Cu-EW cells was used to generate the

thermal profile of the three intercell bar configurations. The

FEM was implemented and simulated in Comsol

MultiphysicsTM 4.2a environment using a stationary analysis

with Electric Current & Heat Transfer coupling (see Fig. 5).

Giving the size of an electrolytic tankhouse (a volume of 1.4

m x 1.5 m x 6 m per cell), the finite element simulation was

limited only to the intercell bar (the focus of the thermal and

electrical behavior analysis) using as input the data collected

from the concentrated parameter model of the EW cell.

All domains, equivalent to each element included in the

model (hanger bars, intercell connectors, base-boards, bolts,

steel interconnectors) and interface boundaries (between the

following elements of the model: hanger bars - intercell

connectors, intercell connectors - copper bolts, copper bolts -

steel interconnectors, described elements - cell environment

surroundings), were defined in the software considering the

specific physical properties of each material as stated in Table

I and II (copper-chromium alloy C18200, super duplex

stainless steel 2507, ETP copper C11000). For the tankhouse

environment a convective forced air flow at 20C was

considered.

For simplicity, the ground potential in the FEM model

was assigned to the end of the anode hangerbars. The inward

currents () was assign to the opposite end on the cathode

hangerbars, according to the equation 5.

= 0

(5)

The contact resistance for anodes and cathodes was

modeled by using an interface resistance layer, according to

equation 6. The contact's conductivity () in the FEM model

was tuned using the same admittance matrix defined to the

CPM. The interface layer length (ds) for each contact was

assumed 0.1 mm.

=

(6)

For the thermal model (heat transfer), boundary

conditions were used on the extremes of anodes and cathodes

hangerbars. The temperatures at these boundaries were

established close to the electrolyte operating temperature to

40C. To represent the heat losses and the temperature (T)

produced by the electrical currents flowing through the

elements, a heat source (see equation 7) representing the total

electrical power dissipation was included on every domain.

= 2 + (7)

Equation 7 includes the electrical heat source () and a

thermal dissipation term. Since a steady-state analysis must

be done, the time-dependent term is not included. Due the

voltage drop between the hangerbar and the intercell bar

connectors, power is dissipated in the interface. A boundary

heat source must be added to represent accurately this effect

(see equation 8).

() = (8)

V. RESULTS

For comparison purposes, BMC results are measured

against VS and CS solutions.

Fig 5. 3D FEM model mesh for a Cu-EW intercell bar.



A. Operational Thermal Profile

A 3D finite element model to research the thermal

behavior exhibited by VS, Cs and BMC technologies was

used. Fig. 5 depicts the results of the normal operation and

the two most demanding scenarios for the technologies; weak

contact and short-circuit.

Weak contacts are produced by physical deformation of

contacts either on the intercell bar or in the hanger bar of the

electrode (the last beyond the scope of this paper). Also, a

weak contact may be produced by chemical corrosion or

deficient housekeeping practices (i.e. poor contact cleaning).

When a deficient contact is present, the contact resistance

increases up to 10 times the normal resistance.

In a VS intercell bar, the current is free to find lower

resistance paths and the current flowing through the weak

contact high resistance is low. So, the VS bar is immune to

weak contacts [8]. In contrast, CS pushes the current to flow

through the deficient contact increasing the heat dissipated in

the contact. As shown (See Fig. 6.e), CS exhibits high

temperature in the contact, reaching up to 160C. BMC

works like VS thanks to the secondary current paths avoiding

excessive heat in the defective contact. Specifically, under

weak contact the temperature in VS and BMC bars reach

only 77C.

Facing a short circuit anomaly, VS exhibits high over-

currents through the electrodes and contacts involved. This

also increases the temperature up to 200 C in the intercell

bar's contacts. By opposite, CS bars and BMC limit the

occurrence and magnitude of short-circuits. The thermal

profile of contacts for both reach only 88C under short-

circuit occurrence.

Fig.6, show the heat reduction of BMC facing weak

contacts and short-circuits. BMC ensures an operation free of

hot-spots under typical operating conditions.

The contact normal temperature for BMC was estimated

in 55C (see Fig.6). This value depends on several EW cells

parameters: busbar shape, segment size, cathode current

density, the use of capping- or base-boards, physical

properties of the materials used to implement the intercell

bars, process environment conditions and electrolyte

temperature among others.

B. Key Performance Indicators

The BMC bar retains CS advantageous features and

resolves its main weakness. The simulation of a circuit of 15

cells with 50 pairs of electrodes predicts a (+1%) further

improvement in energy efficiency. This solution offers good

standard deviation, short circuit immunity, better energy

consumption and lower maximum-operation temperature.

The price paid is a slight increase in the short circuit current.

Table III shows comparative results.

TABLE III

Key Performance Indicators of BMC connection against VS and CS

Process Parameter VS CS BMC

Current Density Dispersion (%) 15.2 10.1 10.5

Short-circuits per cell per cycle 0.42 0.13 0.10

Short-circuit Overcurrent (%) 141% 40% 54%

Specific Energy (kWh/Ton) 1970 1910 1865

Maximum Current Density (A/m2) 330 360 400

Average Normal Operating Temp (C) 55 55 55

Max. Weak Contact Temp (C) 75 160 77

Max. Short-Circuit Tem (C) 200 85 88

C. BMC Reliability Improvements

A summary is shown in Table IV. The efficiency

increment for CS was measured during the implementation of

the technology. The BMC efficiency was estimated, using the

previously detailed models.

TABLE IV

BMC hybrid technology improvements

Parameter VS CS BMC

Efficiency Increment - 4% 5%

Contact Rated

Temperature 160C 160C 500C

Yield strength @ 20C



Fig. 6. Thermal profile of VS, CS and BMC: a) VS Normal, b) CS Normal, c) BMC Normal, d) VS Weak Contact, e) CS Weak Contact, f) BMC Weak

Contact, g) VS Short Circuit, h) CS Short Circuit, and i) BMC Short Circuit.

VI. CONCLUSION

The BMC hybrid solution takes the advantages of the

voltage source and current source intercell bars. The synergy

result is better efficiency and full capability of handling

short-circuits and weak contacts. It reduces its counterparts

maximum temperatures from 200C to 88C. Further, it

improves the MTBF with better yield strength 10X, better

rated temperature from 160C to 500C and, better resistance

to chemical corrosion by sulphuric acid. Finally, it

incorporates connectors hot-swappability improving the

availability of the electrowining cells under typical operating

conditions. Finally, with better MTBF and MTTR the

Availability with high energy efficiency is improved.

VII. REFERENCES

[1] E.P. Wiechmann, A.S. Morales, P.E. Aqueveque, and R.A. Mayne-

Nicholls, Reducing Specific Energy to Shrink the Carbon Footprint

in a Copper Electrowinning Facility, IEEE Transactions on Industry

Applications, Vol. 47, No 3, pp. 11751179, 2011.

URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arn

umber=5729335&isnumber=5768120

[2] E.P. Wiechmann, G.A. Vidal, A.J. Pagliero and J.A. Gonzalez,

Copper Electrowinning using Segmented Intercell Bars for Improved

Current Distribution, Canadian Metallurgical Quarterly, Vol 41, No 4,

pp. 425-432, 2002.

[3] A. Walker, Plant for the electrodeposition of metals, U.S. Patent 687

800, Dec. 3, 1901.

[4] J.R. Davis, "Copper and Copper Alloys",ASM Specialty Handbook,

USA, 2001.

[5] S. Heikkinen, "Fatige of Metals - Copper Alloys", 2001.

[6] Sand Meyer Steel website: http://www.sandmeyersteel.com/

[7] E.P. Wiechmann, G.A. Vidal, and A.J. Pagliero, Current-source

connection of electrolytic cell electrodes: an improvement for

electrowinning and electrorefinery, IEEE Transactions on Industry

Applications, Vol 42, No 3, pp.851855, 2006.

URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1634

691&isnumber=34279

[8] E.P. Wiechmann, L.G. Muoz, P.E. Aqueveque, G.A. Vidal and J.A.

Henrquez, "Introducing a Bypass-Backup Connection System for

Current mode Copper Electrowinning Intercell Bars", Industry

Applications Society Annual Meeting (IAS), 2012 IEEE, pp. 1-6, 7-11

Oct. 2012.
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5729335&isnumber=5768120http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5729335&isnumber=5768120http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1634691&isnumber=34279http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1634691&isnumber=34279

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