CA1152943A - Activity of unrefined copper anodes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improved method of electrorefining copper is disclosed. The anode activity of unrefined copper anodes in copper electrorefining electro-lyte is improved by holding the anodes in a temperature range between 500°C
and 300°C for at least about 10 hours. Preferably, the anodes are held in a protective atmosphere during the treatment described in the preceding sentence.

Description

~L~52943 ~:

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 .q~ , ~. , , .. ~

~52943 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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1~529~3 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.

' . , ~ . . ~ ' ~1529~3 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 '` , , :

l~SZ943 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 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 ~2943 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 ~152943 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 ~15Z943 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.

, . . . .

~1~i2943 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 _9_ ~5~43 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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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method for improving the anode activity of unrefined copper anodes in copper electrorefining electrolyte which comprises holding said anodes in the temperature regime between 500°C and 150°C for at least about 10 hours.

2. The method in accordance with claim 1 wherein the anodes are held in a protective atmosphere whilst being held in said temperature regime.

3. The method in accordance with claim 1 wherein said anodes are cooled at a rate not exceeding about 20°C per hour from about 500°C to about 150°C.

4. The method in accordance with claim 1 wherein said anodes are annealed at a temperature of about 700°C to about 1050°C prior to being held in said temperature regime.

5. A method in accordance with claim 1, or 2, or 3, wherein the temp-erature regime is between 500°C and 300°C.