The Effects of Aluminum Content, Temperature and
Impurities on the Electrical Conductivity of Synthetic
Bayer Liquors
G. R. BROWNE AND C. W. P. FINN
The electrical conductivity of pure sodium aluminate solutions with compositions in the
range 93 to 128 gl-~ free soda and O to 125 gl-~ alumina was investigated over the
temperature range 40 to 70 ~ Under these conditions, the solution electrical conductivity
was found to vary as a linear function of temperature. For a given free soda content, the
temperature coefficient of conductivity varied as an approximate guadratic function of the
alumina content. A single equation was developed relating solution conductivity to free soda
content in the range 105 to 128 gl-~ alumina, 40 to 100 gl-t over the temperature range 40 to
70 ~ The addition of sodium oxalate and sodium succinate at levels up to 20 gl-~ was
found to have no measurable effect on solution conductivity. However, the addition of
sodium carbonate at levels of 40 gl-~ had a significant effect, generally decreasing the
conductivity. The magnitude of the effect is a complex function of solution composition and
temperature. The technique was used to follow the precipitation of alumina from synthetic
Bayer liquor continuously for 4 h. Possible applications to industrial practice, of the
relationship determined, are briefly discussed.
WITH large amounts of capital tied up in Bayer
process streams and with increasing economic pres-
sures, the viability of the process becomes more depen-
dent on improved efficiency. Maintenance of optimum
process conditions generally requires application of
on-line process control techniques. One parameter of
Bayer liquors which lends itself to continuous monitor-
ing is solution electrical conductivity. Bayer process
liquors are highly conductive and changes in the
conductivity of isothermally decomposing solutions are
related to the stoichiometric release of highly conduct-
ing OH - ions ~ according to the equation
2Na A1 (OH)4a q ~ A1203 9 3H20 $+ Na+OH-aq [1]
This concept is not new 2-9 but published data describing
its quantitative application to solutions of industrial
composition are inherently limited in range due to the
requirement to maintain operating conditions and gen-
erally serve as deviation indicators under conditions of
constant temperature and total soda content.
Laboratory application of this technique is demon-
strated in the literature by the authors ~~ and Kelly I~ over
a limited range of temperature, total soda and alumina
contents.
The aim of this investigation was to extended the
range of electrical conductivity data to cover solution
conditions commonly found during the precipitation
stage of the Bayer process and to investigate the effects
of some significant impurities on conductivity.
G. R. BROWNE, formerly Post-Graduate Student, School of
Mining and Mineral Technology, Kalgoorlie, Western Australia, is
presently Metallurgist, Kambalda Nickel Operations, Western Mining
Corporation. C. W. P. FINN, formerly Alcoa Foundation Lecturer in
Thermodynamics, School of Mining and Mineral Technology, Kal-
goorlie, Western Australia, is presently Associate Professor of
Pyrometallurgy, University of the Witwatersrand, Johannesburg,
Republic of South Africa.
Manuscript submitted February 21, 1980.
METALLURGICAL TRANSACTIONS B
EXPERIMENTAL
Preparation of Solutions
Pure sodium aluminate solutions were prepared by
dissolving electrical grade AI wire in analytical grade
NaOH solution. The dissolution reaction is highly
exothermic and was conducted in covered polypro-
pylene beakers, cooled and diluted to 1 liter. Impurities
were added to the prepared pure solutions as analytical
reagent grade sodium carbonate and sodium oxalate
and technical grade sodium succinate. The prepared
solutions were stable with respect to alumina trihydrate
precipitation indefinitely in the absence of seed mate-
rial.
Composition and Temperature Range
Bayer liquors are described in terms of their free soda
content (expressed as Na20) alumina content (expressed
as A1203) and total soda content (expressed as Na20).
The difference between total soda and free soda is a
measure of impurity content (mainly Na2CO3).
In this study, the range of solution compositions and
temperature used maintained consistency with pub-
lished data 1-8 describing the precipitation stage of the
Bayer Process.
Conductivity Measurements
The conductivity of the solutions was measured using
a Philips PW 9501/01 Conductivity Bridge in earthed
mode with an external reference resistance of 1.000~2
(_+ 0.01). The conductivity cell was a Philips PR
9514/10 immersion probe with a cell constant of 1.00
cm-l (_+ 0.01) at a measuring frequency 200 Hz. Con-
ductivity bridge output was monitored on a Metrohm
Labograph chart recorder.
ISSN 0360-2141/81/0911-0487500.75/0
9 1981 AMERICAN SOCIETY FOR METALS AND VOLUME 12B, SEPTEMBER 1981--487
THE METALLURGICAL SOCIETY OF AIME
Table I. Range of Variables Used in Conductivity Measurements
Variable Units Range
Free soda gNa20/1 93-128
Alumina gAl203/1 0-125
Sodium carbonate gNa2CO3/1 40
Sodium oxalate
gNa2C20~/1
20
Sodium succinate gNa2C4H404/1 20
Total soda gNa20/1 93-175
Temperature ~ 40-70
Temperature Measurement and Control
Solution temperature was measured with a copper
vs
constantan thermocouple in a stainless steel sheath im-
mersed in the solution. Thermocouple output was
monitored with a Metrohm Labograph chart recorder.
The thermocouple was calibrated against a standard-
ized 76 mm immersion mercury-in-glass thermometer
accurate to ___ 0.05 ~
Solution temperature was maintained within __+ 0.1 ~
with a Grant SX forced circulation water bath. The
bath surface was covered with polypropylene beads to
minimize evaporation.
Solution Containment
On the basis of published corrosion data '2 all equip-
ment in contact with the test solutions was fabricated
from Type 304 stainless steel. This choice of material
provided:
a) a high degree of resistance to corrosion which
prevented solution contamination,
b) excellent heat transfer between the test solutions
and thermostated water bath,
c) electrical earthing of the test solutions.
Solution Agitation
Solution conductivity values increased with stirrer
speed up to a critical value beyond which it became
constant. All reported values were taken at speeds
exceeding this minimum value. Vortex breakers were
installed in the solution container to reduce the stirrer
speed required to reach this plateau region.
Solution Decomposition Test
To test the usefulness of this technique under con-
ditions in which alumina trihydrate is precipitating, a
solution decomposition test of 4 h duration was con-
ducted under the following conditions:
Free Soda (Na20) = 116 g1-1
Initial Alumina (A1203) = 122 gl-
Temperature = 69.7 ~ + 0.1
Initial volume = 1.000 1 ___ 0.005
Prior to the seed addition of 100 g of sandy alumina
trihydrate, solution temperature and conductivity were
stabilized and a 5 ml sample taken for analysis by the
modified Bushey titration method./TM
After seed addition, the stirrer was stopped for 1 min
at half hour intervals, 10 ml samples were withdrawn,
filtered through dry Whatman No. 540 paper to remove
precipitate, and 5 ml of clear solution were analyzed by
the modified Bushey method. ]3,~4
Conductivity and temperature were continuously reg-
istered on a chart recorder. Instrument calibration was
checked at 15 min intervals with a calibrated potentio-
meter. No instrument drift was observed. On comple-
tion of the test, the alumina trihydrate produced was
recovered by filtration, washed with distilled water,
dried at 105 ~ and weighed. The conductivity probe
was checked using an optical microscope at 1000 times
magnification. No evidence of contamination by pre-
cipitate was observed. Electrical calibration against 1.0
molar KCI at 25 ~ before and after the test showed
that the cell constant did not vary.
RESULTS
Pure Sodium Aluminate Solutions
Conductivity
vs
temperature and composition for
pure sodium aluminate solutions are shown in Fig. 1.
Data from Glastonbury 9 at 25 ~ is included in Fig. 1 for
comparison. Conductivity is a function of temperature,
free soda and alumina content of the solution. In
general, conductivity increases with temperature and
free soda content and decreases with alumina content
with other parameters held constant.
.60
"50
U
~ .40-
v
> .30-
~d
C
0
U
o E .20
CO
"10"
SOLUTION
COMPOSI TI ON
(~pD
2
3 105 0
4 93,0
O
5
1~ 41.0
19 O 25.5
I 93.17 38.2
11 llS 63,5
105 60.7
r~ 93.0
57.0
i
it, i 128
100
15 i 105 77J
I
t6
128
t25
17 116 97.5
15 I 93.0 81.2
tg) 116
12~
20i 105 103
0
Data at 25~
after GLASTONBU~Y (10)
~6 20 3b z,b 50 6b 70
T(~
Fig. 1--Solution conductivity
vs
temperature for pure sodium alu-
minate solutions.
488--VOLUME 12B, SEPTEMBER 1981 METALLURGICAL TRANSACTIONS B
Sodium Aluminate Solutions
Containing Impurities
The conductivity characteristics of sodium aluminate
solutions with either sodium oxalate or sodium suc-
cinate additions show no significant deviation from the
pure solution data up to addition of 20 gl- l which
exceeds the normal equilibrium values in industrial
solutionsJ
However, the effect of 40 gl- l of sodium carbonate
was very significant as shown in Fig. 2. Generally, the
effect of sodium carbonate addition was a reduction in
solution conductivity at a given level of temperature,
free soda and alumina, this effect being most pro-
nounced at low temperatures and alumina contents.
Solution Decomposition Test
Figure 3 shows the variation of conductivity with
time during the precipitation test as well as the dis-
solved alumina content remaining in solution deter-
mined from both conductivity and titration. The equi-
librium alumina content of 68 gl- ~ was calculated from
the data of Pearson. l
The final alumina trihydrate product weighted 178 g
of m1203 9 3H20 of which 100g g was the original seed.
Thus 78 g of trihydrate equivalent to 51 g of A1203
precipitated during the test. Since the original solution
.60-
.50-
'E
u
~
.40-
2
v
.30
21
r-
0
U
o ~ '20-
O
U3
.10-
50LUTION
COMPOSITION
(gpt)
Line 5 ~'-
NO
105
2 116 0
3 93.0
4 128
5 105
10
106
128
128
116
05
14
15
16
17
li~
19
20
-- r
41.0
~/
38.2
60.5
63,5
60.7 ~A~
57.0
77.1 ~
100
125
97.5
81.2
125
103
lb 2b 3'0 4'0 5'0 6b 7b
T(~
Fig.
2--Solution conductivity
vs
temperature for sodium aluminate
solutions containing 40 gl -l of sodium carbonate.
130-
120
~-110-
gloo
E
O
u 90-
_
E 80-
<
f~ 7o-
5
m 60-
50-
40-
-- by titfimetric OnQlysis
EquJ[JbrJum A[umJno. Content
1 2_ 3 4
T!me
(hrs)
Fig. 3--Solution alumina content
vs
time during precipitation of
AI203 9 H20.
contained 122 g of
A120 3
then 71 g remains per liter of
solution which compares favorably with the concen-
tration determined by both conductivity and titration.
No correction was made for the small amount of
precipitate removed during sampling as after the stirrer
was stopped for sampling the solids settled rapidly.
DISCUSSION
i) Pure Solutions
Least squares regression analysis of the conductivity
vs
temperature data for the pure solutions produced a
series of linear equations of the form:
K = m T + b
[2]
whereK = solution electrical conductivity (s cm-t),
m = (OK/OT)S,A
= temperature coefficient of con-
ductivity (s ~ cm- ~~ t), and b = apparent conduc-
tivity at 0 ~ (s cm-l).
The values of these regression coefficients are con-
tained in Table II. The uniqueness of each line was
evaluated by considering the difference between either
one or both of the regression coefficients m and b for
adjacent lines. In the majority of cases, the difference
was significant at the 99 pct confidence level. The
minimum degree of significance obtained was at the 90
pct confidence level.
The value of the temperature coefficient of electrical
conductivity (m) varies with both the solution-free soda
(S) and alumina (A) contents, but the value of the
METALLURGICAL TRANSACTIONS B VOLUME 12B, SEPTEMBER 1981--489
Table II. Pure Solutions
Free Soda 93 gl-
Alumina, gl- ) 0 25.5 38.2 57.0 81.2
m
(x
I03) 5.00 4.66 4,70 3.70 3.71
b 0.185 0.125 0.104 0.098 0.024
r 0.99 0.99 0.99 0.99 0.99
Free Soda 105 gl -
AI203, gl-' 0 41 60.7 77.1 103
m (x 103) 5.32 5.07 4.73 4.12 3.42
b 0.191 0.126 0.065 0.041 (-)0.097
r 0.99
Free Soda 116 gl-
AlzO3, gl -I 0 51 63.5 97.5 125
m (x 103) 5.41 5.40 5.24 3.96 3.86
b 0.203 0.098 0.062 0.018 ( - )0,011
r 0.99
Free Soda 128 gl- i
A1203, gl -I 0 41 60.7 100 125
m (• 103) 5.81 5.70 5,55 4.78 4.43
b 0.194 0.136 0.036 0.013 0.0023
r 0.99
ordinate mtercept (b) is relatively independent of S over
most of the range of A.
From this series of equations a single equation was
developed (Appendix). It predicts the alumina content
(A) of a pure sodium aluminate solution from its
electrical conductivity, temperature and free soda con-
tent.
K
-
(M4T
+
B4)
S 2 +
(MsT + B 5) S
+
(M6T
+
B6)
A =
(M1T + B1)S 2
+(M2 T +
B2)S +(M3T+ B3)
[31
where
M t = 7.89 X 10 -s
M 2 = --1.78 X 10 -s
M 3 = 9.76 x 10 -4
M 4 =
-5.27 X l0 -6
M s = 1.23 10 -3
M s = -6.50 X 10 -2
B] = 3.50 x 10 -6
B 2 = 8.17 x 10 -4
B 3 = -4.93 X 10 -2
B4 = 2,75 x 10 -4
B 5 = -6.34 10 -2
B~ = 3.83
The mean absolute prediction error was 1.5 pct, max-
imum absolute error 4.7 pct and error variance 2.8 pct.
Sodium Aluminate Solutions Containing
40 gl-t of Sodium Carbonate
For all but the highest concentration of free soda (128
gl-~) the addition of 40 gl-~ of sodium carbonate to
pure sodium aluminate solutions lowered their electrical
conductivity, in some cases by over 20 pct as shown in
Fig. 4 to 7. Least squares regression analysis of con-
ductivity
vs
temperature data in Fig. 1 gave a series of
linear equations of the form given in Eq. [2]. The values
of the regression coefficient are given in Table III. In all
cases, the change in conductivity from the pure solution
values was significant at the 99 pct confidence level with
respect to either or both the regression coefficients.
Quantitative description of the effect of Na2CO 3
24
I>Z 20
"5
O
U
.~ 12-
"5
~ -
g- a-
-8-
0
Free 5cx:Io = 93 Ogpl
folal Soda = 116gpl
20 46 66 8C) 1()0 120 1/*0
Solution Alumina Content (gpl)
Fig. 4--Decrease in solution conductivity
vs
alumina content for 40
g/- ~ of Na~ CO 3 and 93 g/- ] free soda.
24-
Fr6~, ~ ~ 105gt~
To~ol Soda : 12B~
0
(..9
g 12-
"5 .
tn 8-
~ T:~~
._~
I1J
-8-
' '(3 ' 8b ' '
I).0
0 "20 4 60 100 120
Solution Alumina Content (gpl)
Fig. 5--Decrease in solution conductivity
vs
a]umina content for 40
gl-~ of Na2CO 3 and 105 gl- ' free soda.
490--VOLUME 12B, SEPTEMBER 1981 METALLURGICAL TRANSACTIONS B
addition at the 40 gl- l level over all ranges of free soda,
alumina and temperature was attempted with limited
Success.
An equation for the decrease in solution conductivity
24"
>,
20,
u
~
16.
0
t-
O
12
8.
~- 4
.E
io
b
~4-
~ -
--8-
Free 5oda, = 116gpl
Tct(zl 5oda = 139aj:~
-
o 2'0 16o
Solution Alumina Content (gp0
Fig. 6---Decrease in solution conductivity vs alumina content for 40
gl-I
of Na2CO3 and 116 gl-1 free soda.
~ 20-
"5
~
16-
0
L)
o c
12-
r-
80-
~ T=60~ Free Sod~
(5)
= 128c1~
Sodo
= 151gp[
o 2'o 46 6'o 16o
Solution Alumina Content (gpl)
Fig. 7--Decrease in solution conductivity
vs
alumina content for 40
gl -I of Na2CO3 and 128 glmin ~ free soda.
Table III. Carbonate Solutions,
40 g1-1
Free Soda, 93 gl-' Total Soda, 116 gl-
Alumina, gl- ~ 0 25.5 38.2 57.0 81.2
m (• 103) 4.94 4.80 4.92 4.21 4.16
b 0.150 0.086 0.066 0.063 -0.009
9 0.99
Free Soda, 93 gl- ~ Total Soda, 128 gl- t
Alumina, gl- k 0
41 60.7 77.1 103
m (• 103) 4.24 4.40 4.32 3.99 3.59
b 0.230 0.144 0.071 0.034 - 0.030
9 0.99
Free Soda, 93 gl -t Total Soda, 139 gl-
Alumina, gl- i 0 51.0 63.5 97.5 125
m (• 103) 5.52 5.61 5.37 4.31 4.40
b 0.163 0.065 0.035 - 0.009 - 0.049
9 0.99
Free Soda, 93 gl -I Total Soda, 151 gl 1
Alumina, gl- i 0 41.0 60.7 100 125
m (• 103) 10.2 8.74 7.69 5.12 3.75
b -0.078 -0.055 -0.049 -0.017 +0.032
r 0.99
Table IV. Solution Decomposition Test
Conductivity AI203 Content, gl- i
Time, h s - ~ cm- t
Conductivity Titration
0 0.260 123 122
0.5 0.286 102 102
1.0 0.349 83.5 87.0
2.0 0.370 75.0 77.5
3.0 0.390 73.0 76.2
4.0 0.398 71.0 73.0
in terms of S 2, S, A 2, A and T was obtained 15 but the
prediction was poor with deviations between predicted
and actual of between 5 and 50 pct.
APPLICABILITY
The laboratory application of technique of monitor-
ing solution electrical conductivity as a means of
following temporal variations in the composition of a
pure decomposing sodium aluminate solution is well
illustrated by Fig. 3. One important factor in this test
was that the measured conductivity was not affected by
the presence of seed particles.
The development of nonintrusive, corrosion resistant
conductivity probes, as reported by several
authors4,6,~6increases the applicability of the process to
industrial conditons. An inherent feature of Bayer
Process streams, particularly at the precipitation stage,
is that the composition changes only slowly, so it is
important that optimum conditions are strictly main-
tained. If the electrical conductivity is characterised
METALLURGICAL TRANSACTIONS B VOLUME 12B, SEPTEMBER 1981--491
over the narrow range of solution compositions likely to
be encountered, and free and total soda are essentially
constant, measurement of conductivity and tempera-
tures will give directly the solution alumina content.
CONCLUSIONS
1) Solution electrical conductivity of pure synthetic
Bayer process liquors is a predictable function of
temperature, free soda and alumina contents.
2) Additions of sodium oxalate or sodium succinate
at concentrations up to 20 gl- l have no measurable
effect on solution conductivity.
3) Addition of sodium carbonate at a concentration
of 40 gl- ~ has a significant effect, generally reducing
solution conductivity. No satisfactory mathematical
prediction equation was found.
4) Solution conductivity can be used to follow the
isothermal decomposition of pure sodium aluminate
solution in the laboratory with a high degree of
accuracy.
APPENDIX
Development of Equation 2
The lines in Fig. 1 each fit equations of the form
K -- mT + b
[1]
Least squares linear regression of m against A at each
value of S and of b against A at each value of S yield
equations of the form
(
O~--)S A
m = +mo
and
= + bo
with a high degree of correlation. This yielded coef-
ficients at each level of S.
It was found that regression of these coefficients
against S give the best fit when a quadratic least
squares regression was used. This yielded equations of
the form
~m
~A = M~S2 + M2S + M3
m o = M4 S2+MSS+M 6
Ob
O-A = B ISZ + B2S + B3
bo = B4S: + BsS
+ B 6
492--VOLUME
12B, SEPTEMBER 1981
Then on substitution of [4] and [5] into [2] and [6] and
[71
into
[31
followed by substitution of
I21
and
~31
into
[11
yields,
K = (M~S 2 + M2S + M3)A + (M,S 2 + MsS + M6)T
+ B~S 2 + B2S + B3)A +
(B4S 2 +
BsS
+
B6)
[8]
which on solving for A becomes Eq. [3] in the text.
SYMBOLS
A Solution alumina content (gl- t of A1203)
b Intercept in
K vs T
regression equation (~2-~ cm-t)
Bi Prediction equation coefficient
K Solution electrical conductivity (~2-~ cm- i)
m Temperature coefficient of conductivity (~2-
cm-,oc-I)
Mi Prediction equation coefficients
r Correlation coefficient for
K vs
Tregression
S Solution free soda content (gl -~ of Na:O)
T Solution temperature (~
ACKNOWLEDGMENTS
The authors express their appreciation to Western
Mining Corporation for technical assistance and finan-
cial support of one of the authors (GRB), to Alcoa of
Australia Ltd for technical advice and to the staff of the
Western Australian School of Mines, especially Dr. T.
Pyle and Mr. L. Stonehouse for their advice.
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no. 4, pp. 185-88. Translated by T. D. Gedeon for
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METALLURGICAL TRANSACTIONS B