PRODUCTIVITY INCREASE IN A PEIRCE-SMITH CONVERTER

USING THE COP KIN AND OPC SYSTEM

Thomas Prietl1, Andreas Filzwieser

2 and Stefan Wallner

3

1Christian Doppler Laboratory for Secondary Metallurgy of the Non-Ferrous Metals

University of Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria 2RHI Non-Ferrous Metals Engineering GmbH

Magnesiststrasse 2, 8700 Leoben, Austria 3RHI Refractories, Business Unit Industrial

Wienerbergstrasse 11, 1100 Vienna, Austria

Key Words: Gas stirring, COPKIN, endpoint, OPC

Abstract

The use of gas stirring systems through the furnace bottom is common for anode and holding

furnaces in the copper industry The first implementation of a gas stirring COP KIN®

system in a

Peirce-Smith converter was performed at the New Boliden smelter, Rönnskar, Sweden. In

addition to other benefits, a decrease in process time and a decrease of the oxygen content in the

blister copper were observed. To determine the effects of the gas stirring system and the process

endpoint, an optical production control ‘Semtech OPC system’ was used. The light emission of

the converter flame as an optical process parameter provides qualitative on-line process

information, and is also used for endpoint determination of the slag making process, on-line

control of iron content in white metal, quality control of slag etc. The results, benefits and risks

of using the COP KIN®

and OPC system for a Peirce-Smith converter are reported.

Introduction

In order to meet stricter product-quality criteria, increasing productivity demands, tighter

energy and environmental constraints, increasing fluctuations in raw-material composition etc.,

pyrometallurgical processes are getting more and more complex and thereby more difficult to

operate and optimize. At the same time, cost-benefit arguments cause the smelting plants and

also the individual processes to become larger with increasing throughput rates, meaning that a

modest efficiency increase in one single process step might have a significant effect on the

plant profitability. Process optimization and control are becoming increasingly important. In

most facilities the operation is guided by static models, based principally on process modeling,

operator experience and the accumulated information on material input and output. The

conversion of copper in a traditional Peirce-Smith (PS) converter might be used to illustrate the

different but complementary nature of this steady-state optimization and dynamic production

control, which can assist in maintaining stable process operation in the face of disturbances.

Examples of disturbances that might enter the conversion process are unforeseen changes in

quality and tonnage of incoming matte and silica, availability and grade of cold charge, operator

interventions, shift changes, timing of cranes etc. In the conversion of copper in a PS converter,

some of the more important entities to optimize are blister-copper quantity, sulfur, oxygen and

impurity contents of blister copper, slag composition and slag temperature. They can all be

controlled by adjustment of the input to the process.

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Converter and Fire Refining Practices

Edited by A. Ross, T. Warner, and K. ScholeyTMS (The Minerals, Metals & Materials Society), 2005

Temperature, for instance, is controlled by the adjustment of air-blow rate, oxygen enrichment

and additions of cooling material. Blister copper quantity and quality are controlled by

optimization of the endpoints of the various blowing steps and the slag quality. The latter, in

turn, is controlled via silica additions. Obviously there are many means for affecting and

controlling the process and thereby the resulting output. On the other hand, there are very few

objective means for finding out the most appropriate action at a given point in time once the

process has started. Or, to be more specific, there are very few means by which to retrieve

objective information on the instantaneous status of the process, for instance as regards to slag

quality or instantaneous oxygen stage. The highly aggressive environment in smelters has

hampered the implementation of sensors for on-line measurements, which is a necessity for true

dynamic control. Consequently, true dynamic production control has developed at a slow pace.

The RHI COP KIN system

The increasing costs of the production process lead to the development of new refining

processes. The gas treatment with different gases (inert and reactive) is one possibility of such a

technology. Removal of unwanted particles from molten melt by flotation is one of the most

useful melt cleaning techniques used by the industry. Increasing the kinetics of chemical

refining reactions between slag and metal is another effect of the gas treatment. Gas injection

through the furnace bottom has been practiced in the steel industry (ladles) for more than 30

years and also in the non-ferrous industry (e.g. anode furnaces, holding furnaces, Peirce-Smith

converter etc.). There are several reasons why the non-ferrous industry does not use stirring

systems to an adequate extent. In the past no supplier would warranty the engineering, hard

ware and start-up know-how for a completely proven gas stirring system and the risk was fully

with the smelter.

This was the primary reason that 2002 the RHI Non-Ferrous Metals GmbH based in Leoben,

Austria was founded. Today RHI provides a complete gas stirring system called the

“COP KIN (Copper Kinetics) system”. This system can be easily adapted to the needs of each

customer. The gas control station is the main part of the COP KIN system. The gas station is

equipped with a certain number of gas inlets e.g. for nitrogen, air and/or natural gas and also a

certain number of outgoing pipes to the purging plugs. To guarantee a consistent stirring action

the gas pressure must be carefully monitored in real time by the control panel. A minimum

pressure of 6 bar for the inlet gas is required to ensure the gas station has the flexibility to keep

the mass flow at a constant level.

The software controls each plug individually to achieve a constant flow rate. This software is

runs independently of the furnace computer system on site and allows a constant monitoring

and adjustment of the gas flow rates as required in the treatment phases of the process.

Depending on the particular application different gas mixes and flow rates per single plug can

be programmed. In the case of an emergency, several safety devices are installed, e.g. if an inlet

gas line is blocked for some reason the control station will immediately switch to the back-up

second incoming gas line immediately and an alarm will be given. After defining the need and

targets of a gas stirring system the right type, number and position of the plugs have to be

calculated. For this engineering work the CFD software Fluent is used. In Figure 1 the parts of

the COP KIN system are shown [1].

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gas control stationporous plug with well block

control panel

Figure 1: COP KIN system

The purging plugs are produced in various designs as a consequence of the design and

refractory improvements over the years. The lifetime of individual plugs is governed by their

exposure to chemical, operational, mechanical and thermal conditions. The most common

purging plugs in the non-ferrous industry are fired porous plugs, which are characterized by a

high opening rate, an optimal bubble formation and a long in-service lifetime, but other plug

types are also used (Figure 2) [2]. The porous plugs are covered with stainless steel and include

thermocouples, which are connected to the gas control station, to monitor the plug wear. Today

it is known from longstanding experience that the wear of a porous plug using inert gas only

(e.g. nitrogen) is similar to the wear of the surrounding lining. It is also possible to use air or

pure oxygen as a purging gas (reaction gas), but this results in increased plug wear.

For these reasons a changeable plug arrangement was developed. To have the possibility to

change plugs in such a short time and under hot conditions leads to a higher purging

performance because it is now contingent to also use reaction gases. Non-changeable systems

are often taken when changeable systems are not required. In Peirce-Smith (PS) converters a

changeable system is not necessary because the normal tuyere zone repair could also be used to

change the plugs.

179

Figure 2: Purging plug types

Application of the COP KIN system in a Peirce-Smith converter

The dominant copper smelting technology today is the combination of flash furnace, PS

converter and anode furnace, which has been widely used in the copper smelters worldwide. In

2002 the first COP KIN system was installed in one of the PS converters at the copper plant

New Boliden, Rönnskär, Sweden. Initial trials have indicated that the use of an additional

bottom gas stirring system provides metallurgical benefits. The first test trial was carried out

with two purging plugs to determine if the wear of the plugs in the converter cause operational

problems. After a normal operating period of 12 weeks, it was evident that no specific wear had

occurred using the porous plugs. In a second trial, four plugs were installed and the operational

safety and stability was evaluated. In a third and fourth test campaign the advantages of using

the COP KIN system during a whole converter lifetime (time between relining) were carried

out. At Boliden to convert matte with a copper content of approx. 60 % into blister copper (98 –

98.5 % Cu), eight steps need to be performed:

1. Charging

2. First slag blowing

3. First slag tapping

4. Matte blowing

5. Second slag tapping

6. Copper blowing

7. Tapping of the copperoxidic slag

8. Copper tapping

The implementation of the COP KIN system in a PS converter will affect the following

benefits on the various process steps:

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Charging

When the fist ladle of matte is charged into the converter it is not used as a metallurgical reactor

because the tuyere zone is out of the blowing position. To have the possibility to blow air

through the porous plugs to start the first slag blowing, the iron slagging reaction will start

immediately and the converter will be in operation from the very beginning.

First slag blowing and matte blowing

During these two steps the COP KIN system is used to boost the stirring effect in the dead

zone opposite the tuyeres. Different tests with various flow rates of purging gas were used

during these two steps. For the endpoint determination of the two periods the Optical

Production Control (OPC) system was available.

First and second slag tapping

With the use of the COP KIN system it is possible to individually control the flow rate of each

plug. Operating the plugs near the converter endwalls at a maximum flow rate (300 l/min) and

all other plugs at a minimum flow rate (10 l/min) will create a certain slag movement to the

charging/tapping door. This will result in easier slag work combined with improved separation

of slag and white metal. To remove most of the fayalite slag after the matte blowing step is

important because the less fayalite slag remaining in the converter, the more effective the slag

work during the copper blowing will be; and the lower the amount of copperoxidic slag that

will be produced.

Copper blowing

In the copper blowing process the white metal is converted into blister copper (equation 1, 2).

Cu2S + 3/2 O2 Cu2O + SO2 (1)

2 Cu2O + Cu2S 6 Cu + SO2 (2)

While the white metal is still present in the bath, the purging gas will help to minimize the dead

zone in the converter and increase the homogenization in the bath. At the end of the copper

blow a copper oxide rich slag forms. Several effects can be seen as a result of using nitrogen as

the purging gas:

The desulfurization is initiated earlier due the decreased partial pressure of SO2 in

the bath; result of the rising gas bubbles.

The oxygen efficiency increases due to improved agitation

The surface between the slag and the blister copper increases as well as the

interaction rate between both.

The amount of copperoxidic slag decreases

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Copperoxidic slag tapping

As mentioned before with the COP KIN system it is possible to control the flow rate of each

plug separately and so a certain movement of the slag towards the tapping door can be

achieved.

Copper tapping

To discharge a PS converter with 300 tons of blister per cycle, takes at least on hour. With

nitrogen as purging gas the converter can be used as a metallurgical reactor and the sulfur

content can be reduced during the discharging period. If a COP KIN system is also installed in

the anode furnace these two purging systems can work concurrently and further optimize the

process operation.

The aims at Boliden were to achieve better slag tapping, lower sulfur and oxygen contents in

the blister copper, less copperoxidic slag and as a result in consequence a reduction in process

time. Boliden has a specific PS converter process, because there is no possibility in the anode

furnace for an oxidizing step, so the have to reach a sulfur level of less than 100 ppm has to be

reached in the converter. To reach this level it is necessary to over-blow the melt, which

subsequently produces a high amount of copperoxidic slag. To compare the data from converter

1, which was equipped with the COP KIN system, with the values from the other two

converters without a stirring system, the Optical Production Control (OPC) system was used. It

was also important for a specific converter driven by the operating teams and for the endpoint

determination of the different blowing steps at the converters. In the following paragraph the

OPC system is explained [3-7].

The Semtech OPC system

For about 15 years Semtech has developed and industrialized a remote-sensing technology,

based on optics and spectroscopy, for continuous on-line process monitoring and production

control of various pyrometallurgical processes; the Semtech OPC (Optical Production Control)

System. The light emitted by the off-gas flames of a pyrometallurgical process is composed of

heat radiation from particles and droplets and discrete radiation from atoms and molecules in

the vapor phase. The radiation from the latter shows up at well-defined wavelengths, which are

characteristic for each atom and molecule and with intensities which depend on the gas-phase

concentration (e.g., the vapor pressure) of the light-emitting atoms and molecules.

The presence and concentration of a specific atom or molecule in the off-gases is, on the other

hand, determined by the thermodynamics and kinetics inside the furnace, e.g., oxidation stage,

slag composition and temperature. Thus, by analyzing spectroscopically the light emitted by the

off-gases, it is possible to obtain information on what is occurring inside the furnace. The

interest in spectroscopic methods to control smelting processes is triggered by a number of

attractive features inherent in optical measurements:

They can be performed remote, e.g. without introducing any physical sensor into the

furnace,

They can be performed on-line, e.g. sampling is not a prerequisite for the measurement,

They facilitate the detection of short-lived constituents like radicals,

They can provide continuous real-time information,

They are insensitive to electronic noise.

182

In principle the OPC system consists of

One to four light-weight telescopes, which focus light from the off-gas flames into

Optical fibers. The fibers transmit the light to the OPC Server where it enters

A spectrometer. The dispersed light is registered by

A multichannel detector and the spectroscopic information is evaluated in terms of

optical process parameters by

A PC. The optical process parameters are displayed on-line as trend curves (Figure 3)

on color monitors in front of the.

Figure 3: Semtech OPC trend curves at the end of the copper blowing step

One server can handle the input from up to four telescopes and thereby be used for

simultaneous control of the production in four furnaces or the status at different locations in one

furnace. Figure 4 shows the general layout of the OPC system designed for use at four PS

copper converters.

Figure 4: Schematic layout of Semtech OPC system monitoring the instantaneous process status

in four PS converters

183

The small, robust telescope focuses light from the off-gas flame into an optical fiber. The focal

length of the telescope is very short as compared to the distance to the converter mouth. In this

way the light entering the fiber emanates from various parts of the flame. Thus it represents the

average composition of the gas phase that is the average status of the melt. This is in contrast to,

for instance, conventional sampling via the tuyeres of a PS converter, in which case the analysis

generally yields information on the local conditions in the neighborhood of the tuyeres. For

instance, it generally appears that close to the end of a copper-making step a blister sample via a

tuyere shows higher oxygen content than a spoon sample taken via the converter mouth.

The fiber is connected to a conventional spectrometer equipped with a CCD camera with the

output analyzed on a PC. The spectroscopic information is presented in the form of trend curves

and displayed on color monitors in front of the converter operators and exported to existing data

collection systems. Figure 5 shows an on-line recording from a full converting cycle in a PS

converter. The diagram is a xy-plot of the current values of three different Optical Process

Parameters versus real time. The graph exemplifies what can be seen on a client monitor at the

end of the cycle noting that during the cycle the graph develops gradually. Basically the optical

process parameters included in Figure 5 represent time-resolved registrations of the intensities

of selected emission bands of the PbS (yellow curve) and PbO (green curve) molecules and the

ratio between the intensities of selected emission bands of the CuOH and PbO molecules (red

curve) [8, 9].

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

11.00 12.00 13.00 14.00 15.00 16.00 17.00

Real time

Opt

ical

Pro

cess

Par

amet

ers

(arb

itrar

y un

it)

Figure 5: On-line recording of the optical process parameters PbS, PbO and CuOH/PbO during

a copper converting cycle

The behavior of the Optical Process Parameters PbS and PbO during converting can be seen to

be in qualitative agreement with the thermodynamic prediction of their vapor pressures (Figure

3). As long as the FeS and SiO2 contents of the matte match, that is as long as a fluid slag is

being formed, PbS takes on a high and PbO a low value. When the end of a slag-forming step is

approached PbS starts to decrease and PbO increases, the reason being the increasing oxygen

potential as the matte is being depleted of iron. Thus, close to the end of a step the Optical

Process Parameters gives continuous information on the iron content of the white metal. During

the copper-making step the vapor pressure of PbS and the corresponding Optical Process

Parameter are low while the PbO vapor pressure and the optical parameter PbO are high. The

abrupt change in the value of the Optical Process Parameter CuOH/PbO close to the end of the

step reflects a phase change, namely the disappearance of the white-metal phase and the onset

of the copper-oxide production.

184

The OPC technology has been tested and also permanently implemented at a variety of

metallurgical processes. Naturally the useful spectroscopic information varies between different

processes. However, a common feature of all Optical Process Parameters is that the oxygen

potential or the oxidation stage inside the furnace determines their behavior. Consequences of

using a Semtech OPC System for process optimization are shown in Figures 6, 7 and 8. OPC ok

means that the Semtech OPC system was in operation, not ok the opposite [10, 11, 12].

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0 2 4 6 8 10 12 14 16 18 20 22 24 26

%Cu in slag

Rel

ativ

e fr

eque

ncy

OPC OK OPC not OK

Figure 6: Copper losses in slag

0,00

0,05

0,10

0,15

0,20

0,25

0,30

10 15 20 25 30 35 40 45 50 55 60

% Fe3O4 in slag

Rel

ativ

e fr

eque

ncy

OPC OK OPC not OK

Figure 7: Fe3O4 in slag

185

0,00

0,05

0,10

0,15

0,20

0,25

200 220 240 260 280 300 320 340 360 380 400 420

Total blowing time / cycle (minutes)

Rel

ativ

e fr

eque

ncy

OPC OK OPC not OK

Figure 8: Total blowing time

Results of the tests using the COP KIN and OPC system at a PS converter

The aim at the New Boliden smelter was to make possible a more complete and faster tapping

process of different slags by using eight purging plugs. Special attention was paid to the second

fayalite slag. This has, when it is carried in large amounts into the blister copper blowing a

phase, a disadvantageous effect on the properties of the subsequent copperoxide rich slag.

Furthermore, a better tapping of the fayalite slag would reduce the amount of copper oxide-rich

slag. At Boliden, because of a lack of an exhaust system for the SO2-containing waste gas, it is

not possible to further lower the sulfur content by oxidation in the anode furnace. For this

reason the sulfur content in the blister copper must be adjusted to about 100 ppm in the

converters already and in order to achieve such a low sulfur content, there must be an over

oxidation of the melt. The oxygen contained in the copper, which causes problems when the

anodes are cast, must then be removed at great effort via a reduction process in the anode

furnace. The excessive oxygen content causes a high by the large number of Cu2O precipitates

in the solidified metal so that the ears of the anodes break off.

By carrying out several experiments with various rates of purging with nitrogen or air, it was

investigated whether the time needed to reach a sulfur content of 100 ppm in blister copper

could be reduced. Furthermore it was investigated whether the desired sulfur content could be

achieved with a lower oxygen demand by having the purging bubbles lower the SO2-partial

pressure in the melt. The results with the developed optimal purging program are shown in the

following Figures. In Figure 9 the position of the porous purging plugs in the PS converter is

detailed.

Figure 9: Position of the plugs in the PS converter

186

The optimal purging rates during the various process steps were carried out in several different

experiments and the result, the performed purging program, is apparent in Figure 10. The whole

purging program consists of divers sub-programs.

Figure 10: Performed purging program

With the performed purging program it was possible to lower the sulfur content and the amount

of dissolved oxygen in the blister copper by increasing oxygen efficiency (Figure 11).

51,0

70,0

67,1

40

45

50

55

60

65

70

75

sulf

ur

con

ten

t [p

pm

]

conv 1 with purging plugs conv 2 all conv 3 all

3

2

1

Figure 11: Sulfur content in the blister copper; converter one was equipped with the

purging plugs

The oxygen in the blister copper in converter one was more than 1000 ppm lower than the other

converters without the COP KIN system. Also, the time to reach the endpoint after the copper

turn (the white metal phase is gone) could be abbreviated (Figure 12).

187

22,9

27,3

29,1

20

21

22

23

24

25

26

27

28

29

30

[min

] for

300

tons

and

700

Nm

3/m

in

conv 1 with purging plugs conv 2 all conv 3 all

3

2

1

Figure 12: Time needed to reach the endpoint after copper turn

With the sub-programs used during the slag tapping steps a forced movement of the slag to the

charging/tapping door was seen. This is reflected in a better tapping of the fayalite slag and a

lower amount of copperoxidic slag at the end of the conversion process (Figure 13).

18,6

26,0

21,8

17

18

19

20

21

22

23

24

25

26

27

Konvertoren

ton

s co

pp

ero

xid

ic s

lag

conv 1 with purging plugs conv 2 all conv 3 all

2

3

1

Figure 13: Produced amounts of copperoxidic slag

It was possible to achieve good results with the use of the COP KIN and Semtech OPC system

during this pilot project. Total savings have been calculated from the theoretical and

experimental data for 350 production days. By reducing the copper-oxide rich slag by one ton,

there is a timesavings of 0.5 minutes. Furthermore, 2.6 minutes for the copper blowing can be

reduced (according to New Boliden) by an increase in the oxygen utilisation by 1%. Besides

this there is an average savings of 5 minutes during the final copper blowing period (experiment

results). By using the plugs, the copper-oxide rich slag could be reduced by approx. 5 tons, i.e.,

2.5 minutes can be saved. In total, a savings of approx. 10 minutes could be saved during one

blowing cycle. If both systems are installed in all of the three converters, at New Boliden, you

will get approximately 22 blowing cycles (6600 tons of blister in a 300 tons converter) more per

year. For 350 production days and an average price for the blister copper of € 300, a benefit of

1.98 million €/year occurs.

188

Another important factor is the oxygen content in the blister copper. The consumption of

ammonia in the anode furnace (at New Boliden) depends on that amount. With the amounts of

oxygen in the blister copper using the COP KIN and Semtech OPC system, savings in process

time in the anode furnace operation are also possible. In the future the New Boliden copper

plant is going to install the COP KIN system in all converters.

Conclusion

Using the COP KIN and Semtech OPC system in the converter production control i. e. shorter

blowing times, less copper losses in slag a.s.o. can be achieved. For both systems the working

reliability and economy were sufficiently examined.

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