New Technology Spotlight…

Bath Ratio and Temperature Control

Enhancement in the Potroom

By Dr. Marc Dupuis, GeniSim Inc.

ath chemistry and

order to determine the bath ra-

cell operating tem-

tio.¹ When a cryolitic melt is

perature in Hall-

cooled from a liquid state to

Héroult aluminum

a solid state, it goes through

reduction cells must be con-

several phase transformations.

trolled to achieve optimal

Each transformation occurs in

current and energy efficiency.

a specific temperature region,

The conventional way to con-

therefore the bath chemistry

trol the bath ratio and temper-

ratio is proportional to the

ature is to regularly take bath

amount of heat release.

samples for chemistry analysis

Based on this method, the

and measure the bath tempera-

probe tip contains two type K

ture. These measurements are

thermocouples

(Figures

2-3)³

generally performed separate-

that allow these unique cool-

ly. Moreover, bath samples

ing characteristics to be mea-

have to be sent to a laboratory

sured and the bath ratio to be

for analysis, with results avail-

determined. The thermocouple

able in as much as 24 hours

on the left records the cooling

later. Due to the delay in get-

rate of the bath sample, while

ting the bath sample analysis

the thermocouple on the right

results, control decisions have

records the cooling rate of the

to be made primarily relying

metallic mass of the probe.

on old and out of sync infor-

With this probe, the ratio re-

mation. This leads to an un-

Figure 1. STARprobe kit with mobile station and monitoring system

sult can be known after only a

(left) and portable stand showing the probe tip, lance, and head ready

steady feedback control loop,

few steps—insert probe tip into

for measurement (right).³

where the cell is continuously

molten bath to equilibrate with

under or over shooting the tar-

bath temperature in the pot cell,

geted optimum conditions, which causes

exists a high magnetic field, high ambi-

remove probe tip from bath and allow it

sub-optimal cell performance in terms

ent temperature, and a highly dusty en-

to cool, and STARprobe analyzes the

of both current and energy efficiency.^1,2

vironment. This system is mounted on

cooling curve and records the results.

To address this problem, over the last

a cart to make it a mobile station with

10-15 years, Alcoa has worked to de-

spare probe tips and a battery backup to

velop a new device to measure almost

increase the PC tablet autonomy.

instantly the bath chemistry (or excess

Since the probe tip is reusable, a com-

AlF₃) and temperature. The Superheat,

plete measurement cycle requires only a

Temperature, Alumina concentration,

few steps and can take just under 4 min-

and Ratio (STAR) analysis system,^3,4

utes to complete. Furthermore, a single

known as STARprobe™, provides real-

PC tablet can process data from two

time results that allow potroom op-

probe heads simultaneously, allowing

erators to make chemical alumina and

the operator to measure two pot cells in

power adjustments for optimal cell per-

parallel. Considering that a probe tip is

formance. To date, it has been success-

able to take around 100 measurements,

fully deployed in eight Alcoa smelters.

in this way, a trained operator can rou-

tinely measure 64 cells in 4 hours with

Figure 2. Reusable probe tip.

System Overview

an average of 3 minutes and 45 seconds

per measurement.

The STARprobe system (or kit) is

Once the data is available, the STAR-

a portable device that takes real-time

probe system makes use of the well-

measurements of bath properties and

known Differential Thermal Analysis

consists of four major components (Fig-

(DTA) method,⁵ the results of which are

ure 1): a reusable probe tip (Figure 2);

displayed on the tablet screen (Figure

a portable probe stand (lance) that can

3), stored in a file, and transmitted to the

fit in various smelter operations for di-

level 2 control system in the smelter, us-

rect measurement of the bath; electron-

ing an Alcoa QLC cell controller system.

ics to acquire data and perform analy-

sis, which is wirelessly transmitted to

DTA Method

a computer server; and the STARprobe

computer program that integrates with

The STARprobe concept is fairly

a PC tablet.¹ All electronic components

simple and uses the DTA measure-

comply with stringent requirements for

ment method, which analyses cooling

Figure 3. Schematic of the STARprobe tip

operation in the potroom, where there

characteristics of the cryolite melt in

with two type-K thermocouples.

LIGHT METAL AGE, FEBRUARY 2013

Figure 4 presents such a pair of DTA

that could identify the bath composition

one of the Alcoa smelters now using the

curves obtained from a STARprobe

from the shape of each curve measured.

STARprobe to control its bath ratio and

measurement. In this case, the cooling

The high temperature maximum is due

temperature, reported that the technology

rate of the bath sample is slower than

to the solidification of the cryolite,

has proven to be very accurate for ratio

the metallic mass of the probe for two

while the low temperature maximum is

(excess AlF₃) measurements and able

reasons. The first and less significant

mainly due to the solidification of the

to replace traditional bath sampling and

reason is because of the difference of

excess AlF₃. The more AlF₃ in the col-

XRD laboratory analysis.⁸ Alumar also

thermal diffusivity between the bath

lected bath sample, the less intense the

reported the advantages of much faster

sample (liquid and solid) and the me-

high temperature peak will be and the

response time and consequent reaction on

tallic mass of the probe, hence the ini-

more intense the low temperature peak.

chemical additions, faster measurement

tial separation of the two curves be-

Mathematically, this means that the

of bath superheat and alumina concentra-

tween 10 and 18 seconds. Second, at

sample bath ratio (or excess AlF₃ con-

tion, ability to transfer data to pot con-

the bath sample liquidus temperature,

centration) correlates with the ratio of

trollers that are used to modify process

cryolite starts to solidify, which slows

intensity of the two peaks as displayed

control regarding thermal control, and

the bath sample cooling rate down

in the following equation:¹

ability to reject bad results and request

even further. At the cryolite-alumina

a recheck measurement immediately.

%xs AlF3 = f1[ΔHAlF3/Na5Al3F14 /

phase diagram bath eutectic tempera-

Test groups are pointing to a big poten-

(ΔHAlF3/Na5Al3F14+ ΔHNa3AlF6)]

ture, the alumina starts to solidify as

tial in terms of voltage reduction as well

well. Finally, at a much lower temper-

= f2[S2/(S1+S2)]

as reduced aluminum fluoride additions.

ature (at the cryolite-AlF₃ phase bath

Also, test groups have shown good levels

= f3[DT2/(DT1+DT2]

eutectic temperature), the excess AlF₃

of superheat with a decreasing trend after

finally solidifies.

The method used by the correlations

starting the STARprobe measurements.

algorithm to calculate DT1 and DT2

is not described in Alcoa’s published

Conclusion

research, as it remains a trade secret.

However, even without the latent heat

The new STARprobe bath properties

released during the solidification of the

measurement device is offering smelters

bath sample, the reference temperature

a significant potential to improve pro-

drifts apart from the sample temperature

cess efficiency. While Alcoa is not cur-

(Figure 4 illustrates a possible definition

rently licensing its QLC or its active pot

of DT1 and DT2 by the author that may

control logic, the company has selected

or may not be close to Alcoa’s correla-

STAS as worldwide distributor for its

tion algorithm research). For sure, Al-

revolutionary STARprobe technology.

coa’s correlation algorithm is fast and

For more information, visit: www.stas.

Figure 4. Recorded cooling rate of bath sam-

the calculated results are comparable to

com/en/starprobe/html.

ple and the metallic mass of the probe, which

XRD analysis and have been indepen-

act as reference temperatures in the DTA.

dently verified on many occasions in

References

demonstrations preformed in smelters

The difference of temperature be-

around the world.⁶

1. Wang, Xiangwen, Bob Hosler, and

tween the two curves is computed and

Gary Tarcy,

“Alcoa STARprobe^TM,”

presented on a second graph (Figure 5).

Process Control Improvements

Light Metals 2011, pp. 483-489.

In this case, the sample temperature is

Achieved by Alcoa

2. Dupuis, M., “Excess AlF₃ concen-

selected as an X coordinate. The shape

tration in bath control logic,” National

of that curve is independent of the cool-

In parallel with the development of

Conference on Advancements in Alu-

ing rate, so the bath sample analysis re-

the STARprobe, Alcoa developed a new

minium Electrolysis, Indian Institute of

sults will not be affected by fluctuation

cell controller called QLC that takes full

Metals, Angul Chapter, 2006.

of the ambient conditions.¹ In fact, the

advantage of its bath properties mea-

3. Hosler, Bob, Xiangwen Wang, Jay

shape of the curve depends only on two

surement technology. QLC automati-

Bruggeman, and Patrick O’Connor,

things, the design of the probe tip and

cally acquires the results of STARprobe

“Molten Cryolytic Bath Probe,” U.S.

the composition of the bath sample. This

measurements in real time and takes

Patent No. 2005/0069018 A1, 2005.

means that for a given probe tip design,

the measured cell superheat in consid-

4. Wang, Xiangwen, Bob Hosler,

the shape of the curve uniquely depends

eration in its new STARprobe-based

and Gary Tarcy, “Systems and Meth-

on the composition of the bath sample.

active pot control logic.^1,7 The gains re-

ods Useful in Controlling Operation of

This is the reason Alcoa was able to

ported by Alcoa are 0.5% improved cur-

Metal Electrolysis Cells,” U.S. Patent

come up with a correlation algorithm

rent efficiency, 35 mV voltage savings,

No. 2007/0295615 A1, 2007.

5% AlF₃ savings, a one time capital cost

5. Mackenzie, R.C., Differential Ther-

saving (X-ray equipment), labor savings

mal Analysis, Academic Press, London,

for sampling/analysis, and improved

1970.

understanding by operators, as well as

6. Dupuis, M., P. Bouchard, and J.-P.

a potential of 100-150 day potlife im-

Gagné, “Measuring bath properties us-

provement (still being tested).

ing the STARprobe™,” 19^th Internation-

The potential for improvement for

al ICSOBA Symposium, 2012.

a given smelter depends on its current

7. Wang, X., G. Tarcy, E. Batista, and

level of process efficiency. For example

G. Wood, “Active pot control using Al-

the two cases of current efficiency (CE)

coa STARprobe™,” Light Metals 2011,

improvement reported by Alcoa were

pp. 491-496.

from about 94% moving up to about

8. Silva, Ari F., et al., “Implementa-

94.5%;⁷ clearly, a smelter already operat-

tion of STARprobe™ Measurements

Figure 5. Differential temperature curve and

one possible way to perform the DTA analy-

ing at 95.5% CE should not expect the

and Integrated Pot Control at Alumar,”

sis.

same level of improvement. Alumar,

ICSOBA Symposium, 2012.

LIGHT METAL AGE, FEBRUARY 2013