Note: Descriptions are shown in the official language in which they were submitted.
~L~52943 ~:
PC-2118
Electrorefining of copper is an important step in
the recovery of high purity copper from less pure copper
materials used as anodes for the process.
Electrorefining is a power-intensive operation.
This factor impels managers of copper refineries continually
to be on the lookout for means of reducing power requirements
in the recovery of copper. It has long been recognized that
the electrochemical activity of the anode material employed
in the electrorefining operation bears a significant relation
to power requirements for the process. Thus the anode mate-
rial should be active so that the electrorefining can be
conducted at bigh current density. In addition, it is desir-
able that the voltage drop across the electrorefining cell
be kept as low as possible. The electrorefining operation
lS has been studied closely over the years and particularly in
relation to the various reactions taking place at the anode
surface. Thus, in a paper entitled "Anode Passivation in
Copper Refining" by S. Abe, B.W. Burrows and V.A. Ettel, it
was found that passivation of copper anodes could occur due
to precipitation of copper sulfate on the anode surface. In
addition, it was established that the slime layer created on
the anode face as a result of anodic corrosion was a factor
bearing heavily on passivation of the anode. It is considered
that the slime layer acts as a diffusion barrier which inter-
feres with the transport of the copper ions away from the
anode surface into the solution. The composition of anodes
from different copper refineries is found to vary considerably
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depending upon the type of ore and the type of processing employed in
smelting. For example, a survey of the composition of anode specimens from
nine different copper refineries indicated a range of impurity contents of
from 610 to 3900 parts per million of oxygen, from 149 to 9600 parts per
million of nickel, from 41 to 1982 parts per million of lead, from 39 to 746
p æts per million of æsenic, from 3 to 250 parts per million of antimony,
from 46 to 544 p æts per million of selenium, from 2 to 212 parts per million
of tellurium, from 5 to 46 parts per million of sulfur, from 4 to 33 parts
per million of iron, from 3 to 134 parts per million of bismuth, from 4 to
55 parts per million of tin, from 1 to 66 parts per million of gold and from
197 to 5500 p æ ts per million of silver. The passivating tendencies of the
various anode materials were also found to vary greatly. However, the var-
iations in passivating tendency activity have not been explainable in terms
of the composition of the unrefined anode.
SUMM~Y OF TRE INVENTION
The present invention is based on the discovery that a simple heat
treatment, preferably involving a slow cooling fmm temperatures of about
500C down to about 150C, preferably to about 175C, strongly activates
copper anodes which otherwise w~uld tend to become passive in the absence
~0 of such a heat treatment. This heat treatment should last at least 10 hours.
BRIEF DESCRI~TlON OF THE DR~WING
The figure of the drawing attached hereto depicts temperature-tIme
curves labelled "Q" and "S" which are illustrative heat treatmRnts con-
templated in accordance with the invention.
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DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, copper anodes to
be electrorefined are subjected to a heat treatment compris-
ing a slow cooling in the temperature range of about 500C
down to about 300C such that the residence time of the anodes
in the aforesaid temperature range is at least 10 hours.
The heating may be an incident of the formation of the anodes
initially. Thus, the anodes may be removed from the casting
wheel at a temperature in the range of possibly 700C to
1000C and placed in apparatus for affecting slow cooling
through the range from 500C down to 300C. Alternatively,
the anodes may be produced on the standard casting wheel in
accordance to usual practice and then reheated such that the
anodes are heated through at a temperature of at least about
700C and then are 810wly cooled in the temperature range of
500C down to 300C. Preferably the slow cooling is continued
down to temperatures on the order of about 150C. The slow
cooling provided in accordance to the invention has been
found dramatically to improve the activity of the anode sub-
jected thereto. As an example, passivation times of 1 to 3
minutes in a standard test have been increased to more than
80 minutes by a heat treatment in accordance to the invention.
Other advantages stemming from the improved anodic activity
contributed in accordance with the invention, include reduced
2S electrical short circuiting which results in substantial
labor effort in the tankhouse, reduced energy requirement
and reduced cost of processing anode slimes because the
amount of anode slime is reduced.
Some examples will now be given.
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ExamPle 1
An unrefined copper anode containing in ppm, 1700
2~ 4638 nickel, 83 lead, 45 arsenic, 371 selenium and 352
ailver was 6elected. Portions of the anode were remelted
and the oxygen contents of individual melts were either raised
or lowered by, for example, treatment with a graphite rod to
reduce the oxygen content or by adding cuprous oxide (Cu2O)
to raise the oxygen content. Anode coupons made from the
original material and from the remelted portions measuring 4
x 10 x 1 centimeter were thermally processed in an atmosphere
of argon according to the schedule shown in curve S in the
accompanying Figure 1. The heat treated anode coupons were
then pre-anodized at a current density of 200 amperes per
square meters for 20 hours in a copper electrolyte containing
40 grams per liter copper, 20 grams per liter nickel and 200
grams per liter sulfuric acid at 50C. Each processed anode
coupon was then subjected to an activity test at a current
density of 400 amperes per square meter in this copper elec-
¦ trolyte. In the activity test, values of passivation time
(tp) were measured to provide an indication of the ease or
difficulty for the respective anodes to passivate. The results
of the passivation tests on the original material and the
heat treated material are given in the following Table 1.
TABLE I
; 25 Passivation Time, min
Anode _2 Content (Ppm) Untreated Treated
G-0 (as received) 1700 1 >200
G-l 450 14 >200
G-2 750 7 >200
G-3 5000 1 >200
G-4 7200 1 160
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A dramatic increase in the activity of all the copper samples
is shown.
ExamPle II
Small coupons measuring 4 x 10 x 1 centimeters
were cut from copper anodes received from a number of differ-
ent sources. The analyses of the anodes are shown in the
following Table II.
TABLE II
(ppm)
Anode 2 Ni Pb As Sb Se Te S Bi Sn Aq
A 2300 14731320 67 160 46 2 31 1228 197
E 610 149 45 39 17 420 23 10 17 4 288
F 1600 4565 50 47 4 356 94 18 16 5 350
G 1700 4638 83 45 5 371 49 10 2325 352
H 1500 9321174 57 150 75 9 17 1552 227
K 3100 8261982 46 150 48 9 52 1055 222
The samples were subjected to the thermal treatment
described by curve S in the accompanying Figure and the
passivation times of these samples were determined in the
~ 20 same manner employed in Example 1 with the following
: results.
TABLE III
Anode Passivation Time, min
Untreated Treated
A 12 >200
E 94 >200
F 3 >200
G 1 >200
H 17 >200
K 11 >200
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Example III
Coupons measuring 1.3 X 3 x 1 centimeter were cut
from a copper anode containing in ppm 1190 2~ 8000 nickel,
51 lead, 50 arsenic and 350 silver. The coupons were thermally
processed in a protective atmosphere according to the heat
treatment schedule shown in the following Table IV. Again
passivation times for the untreated material and for the
material treated according to the heat treatment schedule of
Table IV are also set forth in the table.
TABLE IV
Annealing Temperature Cooling Rate* tp
Anode _ and Time_ (C/h) (min)
OO~untreated)
AA 700C for <1 min 340 5
BB 700C ~or 1 min 56.7 7
CC 700C for 1 hr 340 22
DD 700C for 1 hr 56.7 34
EE 700C for 1 hr 10.0 46
FF 700C for 5 hr 56.7 66
GG 700C for 22 hr 56.7 72
HH 1050C for 5 hr 57.2 >200
* from annealing temperature to about 200C.
The results indicate that higher temperatures and longer
treating times in annealing as well as slower cooling rates
from the annealing temperature appear to provide improved
results in activating the copper anodes.
Example IV
Samples of a copper anode (A) containing, in ppm,
2000 oxygen, 3750 nickel, 54 lead, 50 arsenic, 380 selenium,
66 tellurium, 10 sulfur and 395 silver, were heat treated in
a protective atmosphere as shown in the following Table V.
The slow cooling referred in the table was at a rate of about
200C per hour in the temperature regime from the maximum
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i.e., either 700C or 950C to about 500C and the cooling
rate from about 500C to about 150C was approximately 10C
per hour. The original material and heat treated material
were subjected to the passivation test described in Example
I with the following results.
TABLE V
Passivation Time
Treatment tP, (min)
Anode A Anode B
1) as-received 7 22
2) annealed at 700C for 1 h
+ slow cooling 82>300
3) annealed at 950C for 1 h
+ slow cooling 8>300
The result of the 950C heating plus slow cooling
can be ascribed to the high nickel and oxygen content of the
Anode A material. A similar heat treatment performed upon a
different unrefined copper Anode (B) containing only 1000
ppm oxygen and 244 ppm nickel, on the other hand, provided
a tp exceeding of 300 minutes in the passivation test as
shown in the Table.
Example V
Four different copper anodes containing widely
j varying oxygen and nickel contents were heat treated in an
! 25 atmosphere of argon according to the respective schedules in
curves "S" and "Q" in Figure 1. These anodes were then elec-
trolyzed at current densities of 200 and 300 amperes per
square meter in the copper electrolyte described in Example
I. The change in cell voltage was monitored in each test
for up to four days. The results obtained at a current
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density of 300 A/m2 are summarized in the following Table
VI.
TABLE VI
Heat Cell Volta~e at different time of electrolYsis: (V~
Anode Treatment* ~0 1 daY2 daYs 3 davs 4 days
I 0.265 0.2850.305 0.297 0.315
B II 0.185 0.1920.220 0.181 0.203
III 0.201 0.2170.205 0.255 0.278
I 0.274 0.2910.316 0.321 0.5
F II 0.176 0.1970.170 0.228 0.217
III 0.205 0.2200.233 0.252 0.259
I 0.286 ~0.5 ~
L II 0.180 0.1970.233 0.215 0.214
lS III 0.20 0.2080.220 0.205 0.255
I 0.288 >0.5
G7 II 0.180 0.1980.195 0.202 0.211
III 0.~5 0.2370.260 0.286 0.280
note: I; as recieved
IIt treatment "S"
III; treatment "Q"
Example VI
Unrefined anode samples as-cast and after heat
treatments shown respectively as "Q" and "S" in the Figure
were corroded in a copper electrolyte as described in Example
I at 200 amperes per square meter, 50C for 2 to 4 days.
The slime fall after treatment "Q" was 20% less than that of
the untreated anode, while the slime fall after treatment
ns" was 40% less than that of the untreated anode.
It appears that heat treatments in accordance with
the invention effectively provide a phase conversion of the
Cu2O content of the unrefined copper anode to CuO. This
result is demonstrated by analyzing for the presence of Cu2O
phase on the copper surface.
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This phase can be seen if the IF curve gives a
peak at -0.42 volts. Heat treatment S as set forth in the
drawing caused disappearance of a curve deflection represent-
ing Cu2O in cathodic polarization testing. The anode tested
contained 7200 parts per million of oxygen.
ExamPle VII
Samples of copper anodes A and B prepared as
described in Example IV (at 950C) were subjected to electro-
refining in copper electrolyte containing, in g/Q, 40 Cu,
200 H2SO4 and 2 Ni at 60C, at 200 A/m2 for 6 to 8 days.
The electrolyte was recirculated gently (at a rate of ~5Q/h)
through a by-pass provided outside the electrolytic cell,
and suspended solid particles formed during electrolysis
were collected by means of a filter paper placed mid-way of
the by-pa8s, water washed, dried and then weighed. The
results of the amounts of suspended slimes created on the
copper anodes are shown in Table VI.
TABLE VI
SusPended Slimes Formed
Effect of Heat Treatment
Suspended slimes (mg/~)
Anode Untreated Slow cooled
(A) 18 12
(B) 11 4
The reduction of the suspended slimes formation
after a heat treatment in accordance with the invention is
apparent.
The invention not only effects phase conversion of
Cu2O to CuO, but can also produce recrystallized grains 10
to 20 times larger than the original grains. Compounds of
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various impurities become more crystalline in the anode which
makes the slime layer less colloidal as it forms.
It appears that heat treatment in accordance with
the invention effects a substantial change in the nature of
the slime film which forms on the anode during anodic corros-
ion. Prior work on the phenomenon of copper anode passiva~
tion has indicated that the nature of the slime film is
extremely important. The fact that the slime film is indeed
important appears to create a requirement that in heat
treating copper anodes in accordance with the invention a
surface protective treatment either by means of a controlled
atmosphere composition or by covering the anode with a mate-
rial preventing access of atmospheric air is needed. Thus
in normal production of copper anodes on casting wheels using
water-cooled copper molds, the anode is cooled as rapidly as
may be done ~e.g., by use of water sprays) considering the
size and weight of the material involved. The fact that the
time at temperature is limited in normal production in most
cases prevents substantial oxidation of the as-cast anode
surface. Since the invention contemplates holding the anode
at temperature for a time longer than the normal experience
the need for preventing undue oxidation of the surface of
the anode is evident.
Although the present invention has been described
in conjunction with preferred embodiments, it is to be under-
stood that modifications and variations may be resorted to
without departing from the spirit and scope of the inven-
iton, as those skilled in the art will readily understand.
Such modifications and variations are considered to be within
the purview and scope of the invention and appended claims.
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