CA1332159C - Long life anode for electrowinning - Google Patents

Abstract

A LONG LIFE LEAD ANODE FOR ELECTROWINNING

ABSTRACT

An anode for electrowinning in an aqueous sulfuric acid-containing electrolytic cell. A plate member has a lead or lead alloy outer surface. The outer surface is rough and contains elevations and depressions. Lead sulfate oxidized from the rough outer surface of the lead plate member to adhere to the elevations and depressions of the rough outer surface for reducing flaking of lead sulfate. A support means suspends the lead plate member in the electrolyte.

Description

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A LONG LIFE ANODE FOR ELECTROWINNING

The present invention relates to the field of anodes for electrowinning. More particularly, it rela~es to an improved lead or '~
alloyed lead anode for electrowinning. ~;

5~ BACKGROUND OF THE ART AND PROB~EM

The~anodes used in the electrowinning of copper are ideally constructed of~materials which are inert or passive in the ;~
el'ectr ~ tic~cell~ Lead anodes are essentially inert in the~
''e1ectrowinning~of~copper. However? in the electrowinning~process, 10~ somé of the lead oxidizes in the~sulfuric acid~electrolyte to form a lead~sulfate layer OD portions of~the anode exposedito'the sulfuric' acid electrolyte.~
The~lead su1fate layer forméd~on the anode is hard,~brittle ~-i~ff~CUit~to~remQVe. Thi8 lead~6ulfate layer adds electrical 15~ re~i~stance~ 1ightly lncreasing the energy required to electrowin ~
copp~r.~ After~tbe~1ead sulfate accumulates on the anode surface for ~' `a~pe'riod Q~ tlme~, the lead sulfate begins to crack and~flake. The '`' '~:
f1alkes~fi~11 off of~ehe~anode surface,~exposing uncovered lead ~ ~' ~ 1~321~9 ~

anode~ which results in an irregular anode current distribution. The irregular anode current distribution reduces the practical operating efficiency by promoting the formation of nodules of copper on the cathode. The nodules on the cathode grow to subtantially mirror image the areas in whi~h the lead sulfate has flaked off the anode, forming on areas across from where the lead sulfate has flaked off.
Additionally, the nodules grow into the ad~acent anodes, short circuiting the electrowinning process. The nodules also entrap any impurities dissolved in the electrolyte such as iron, nickel, cobalt, arsenic, sulfur and lead. ~igh lead levels cause "hot cracking" or "hot shortness" during the rslling of rod for wiremaking. This reduces the applications of electrowon copper.
Thus, it is desirable to produce nodule free electrical grade copper, because electrical grade copper from which wire is formed generally commands a higher market price than lower grade copper.
The lead anode flaking or corrosion has been minimized by -~
~ alloying the lead with 6 percent antimony. However, despite the 6 - ~ percent antimony, the anode con~inues to oxidize and lead sulfate continues to flake from the anode surface, interfering with the -electrowinning process. Another possible suggested solution would be the use of a truly inert anode, such as a refractory metal coated with gold, platinum, iridium, rhodium and ruthenium. These anodes, ~;
however, would be prohibitively expensive.
As far as known, a lead anode which effectively resists the formation of lead sulfate or a cost effectlve substitute for the lead anode has not been discovered.

SUMMARY OF THE INVENTION

The invention comprises an anode and a process for ~
electrowinning in an electrolytic cell having an aqueous sulfuric ~ -acid-containing electrolyte. A lead plate member has a lead or lead alloy outer surface. The outer surface is rough and contains elevations and depressions. The elevations and depressians are effective in causing lead sulfate formed by the oxidation of the rough outer surface of the lead plate member to adhere to the rough outer surface for reducing flsking of the lead sulfate. A support ~ 3~2159 means suspends the lead plate member in the electrolyte.
Preferably, the outer surface of the plate member also cont~ins 6 percent antimony for further resistance to lead sulfate -formation. Ideally, the elevations and depressions are formed by sandblasting. The depressions in the surface preferably are less than 1 mm in depth as measured from ad~acent elevations.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic cross sectional side view of an electrolytic cell illustrating the anodes and cathodes. ~`
Figure 2 is a schematic frontal view of an anode with a portion of the rough outer surface layer of the anode partially broken away. ;~
Figure 3 is an enlarged schematic cross sectional view of a ~;
portion of the plate member of the anode of Figure 2.
Figure 4 is an enlarged schematic cross sectional view of the plate member of the anode of Figure 2 illustrating lead sulfate adheFing to the rough outer surface of the anode. ~`~

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to Figure 1, the electrolytic cell 10 is filled with copper containing aqueous electrolyte 12. Aqueous electrolyte 12, containing sulfuric acid, enters inlet 14 and exits overflow outlet 16. The cell 10 contains anodes 18 and cathodes or blanks 20 suspended in the cell. Tank walls 22 form a rectangularly shaped cell in which the anodes and cathodes are suspended. The anodes 18 are constructed out of an essentially inert or non-dissolving metal such as lead or an antimony lead alloy. Oxygen is produced at the `~ ~ anodes 20 generating 2 bubbles 24. The bubbles 24 interact with a foaming agent to form a protective foam layer 26. The protective ~-foam layer 26 helps prevent sulfuric acid from entering the atmosphere. Skimmer 27 prevents foam 26 from exiting directly ~`~ through overflow outlet 16. The blanks or mandrels 20 are preferably constructed out of titanium.

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The reaction for the electrowinning of copper involves reduction of the cupric ion to metallic copper at the csthode and evolution of gaseous oxygen at the anode.
The lead or lead alloy anode used in the electrowinning procese ideally remains inert. However, ln the presence of sulfuric acid the oxidizing potential of the anode is sufficient to oxidize the outer portion of the lead anode to form a lead sulfate (PbS04) layer on the anode. After about one year of electrowinning copper, -the lead sulfate begins to flake off of a 6 percent antimony lead ~ -anode that lacks the anode surface of the invention.
Referring to Figures 2 and 3, the anode 18 is suspended from support means or crossbar 28. The anode 18 is constructed with ~;~
a plate member 30 having a rough outer surface 32. It has been discovered that a rough outer surface 32 of a lead anode or an `~
antimony lead anode inhibits the flaking off of lead sulfate. The anodes produced for use in electrowinning of copper generally have a ;~
~; smooth outer surface. To inhibit the flaking off of lead sulfate the : smooth surfaces of the anode which are suspended in the electrolyte, are roughened to produce elevations 34 and depressions 36 (elevations 34 and depressions 36 are only shown in Figure 3). For purposes of this specification, an elevation 34 is defined as a surface area above the ayerage height of the rough outer surface 32 and a depression 36 is defined as a surface area below the average height of~the rough outer surface 32.
~ Referring to Figure 4, during use of the blank 18, the lead ~-anode oxitizes to form a~layer of lead sulfate 38. The layer of lead ;~
sulfate adheres to the~elevations 34 and depressions 36, preventing the flaking of lead sulfate from the anode 18. In test, anodes ; j j treated by this method have functioned without flaking for ove~ twoyears. After two years of use, the lead sulfate continues to form on the anode, but the layer of lead sulfate adheres to the anode surface having very little, if any, negatlve effects on the electrowinning process. The smooth anodes by comparison flake after only one year of use disrupting ~he electrowinning process.
The anode of the invention provides the several unique :. ~ .
benefits of: 1) The rough surface anode oxidizes to form a uniform ` ~ layer of lead sulfate which adheres to the roughened lead anode, . ~ . .

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reducing short circuiting of the electrowinning process; 2) The current between the anode and cathode has an efficient uniform current distribution; 3) the uniformly deposited copper has a higher purity, absent the nodules which entrap electrolyte impurities; 4) the market value of the copper is increased; and 5) the invention reduces the maintenance costs of cleaning the lead sulfate from the anode. -The anodes utiliæed in the experiment were cast lead 6 percent antimony tapered anodes. The anodes had a length of 109 cm, (91 cm in electrolyte), and a width of 86 cm. The top thickness of the anode was 2.5 cm and the bottom thickness was 0.8 cm. The anodes -~ -having a weight of 109 kg were suspended by a suppor~ means consisting of 1.3 X 6.4 cm copper cross rods.
The blanks utilized in the experiment were titanium 15 mandrels. The blanks had a length of 114.6 cm, (102 cm in the electrolyte), and a width of 100 cm. The blanks had a thickness of 0.3 cm and a weight of 13 kg. The blanks were suspended by a copper cross rod. Typical electrolyte composition was as follows~

~- Temperature - cell inlet - 60C
20 Temperature - cell outlet - 65-70C u Composition - cell inlet - Cu 50-65 gpl H S04 180-200 gpl Fe 7-10 gpl ;~
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Ni 8-12 gpl Co 6-10 gpl As 2-2.5 gpl Se~ 0.005 gpl Te c0-005 gpl Copper Concentration - cell outlet - Cu 40-50 gpl H S0 190 210 1 `~
.. ~, - - .-The flow rate was about 40 liters per minute (lpm) through ; a cell having 67 anodes and 66 titanium cathodes.
To create the rough outer surface the anodes were sandblasted to form surface elevations and depressions. The surface "~
of the antimony lead anodes was prepared by sandblasting with silica i3~?,~,~9 sand. The silica sand was sorted with a screen having approximately 12.6 openings per cm (No. 32 mesh size). The anode was sprayed with a silica sand at about 7.5atmospheres gage (110 psig) through a venturi type nozzle. The nozle had an intemal diameter of approximately 1.1 cm (7/16 in). The depressions created in the S relatively soft lead were less than 1 mm in depth as measured from adjacent elevations.
Both previously used anodes having flaking lead sulfate on the surface of the plate member and new cast or rolled anodes were roughened by sandblasting togreatly improve the anodes useful life. To produce a rough surface on an anode covered or partially covered with lead sulfate, the lead sulfate must first be removed. The flaking lead sulfate proved difficult to remove. A wire brush was ineffective at removing the lead sulfate. However, when anodes havir~g a lead sulfate layer are sandblasted, the lead sulfate is easily stripped away from the anode, leaving an anode having a rough and relatively clean surface free of lead sulfate.
lS The surface of a new anode may be produced by several methods, such as rolling with a rough surface, pressing with a rough surface, casting in a roughened surface mold, machining the surface or by any other known methods of producing a rough surface.
The depressions and elevations of the invention proved ef~ective in preventing the flsking of lead sulfate. I'~ivo years after the introduction of the roughened test anodes to the cell, there was a slight lead sulfate accumulation on the sandblasted anodes which adhered tightly to the anode. This layer of soft lead sulfate which formed on the anode surface had the texture of fine sandpaper and had no tendency to peel or flake off.
It was also discovered that the rough outer surface anodes produced more 2 bubbles when f~rst introduced into the cell. As the lead sulfate formed,larger but fewer bubbles formed adjacent to the anodes. The bubble size effects the formation of the fosm layer on the electrolyte surface which provides acid mist protection. To prevent disparities in foam production it is recommended that allanodes utilize similar surface roughness when a standard amount of foaming agentis used.

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While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention, those skilled in the art will understand that changes may : :
be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

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Claims (13)

1. An anode for electrowinning in an electrolytic cell having an aqueous sulfuric acid-containing electrolyte, the anode comprising a lead-containing plate member having a lead or lead alloy outer surface, the outer surface being rough, the rough outer surface having elevations and depressions effective to cause lead sulfate oxidized from the rough outer surface of the lead plate member to adhere to the rough outer surface for reducing flaking of the lead sulfate, and a support means for suspending the lead plate member in the electrolyte.

2. The anode of claim 1 wherein the outer surface of the plate member additionally contains antimony.

3. The anode of claim 1 wherein the outer surface of the plate member additionally includes about 6 percent antimony.

4. The anode of claim 1 wherein the support means comprises a cross bar attached to the top of the plate member for engagement on opposing walls of the electrolytic cell.

5. The anode of claim 1 wherein the elevations and depressions are formed by sandblasting.

6. The anode of claim 1 wherein the depressions in the surface are less than one millimeter in depth as measured from adjacent elevations.

7. An electrolytic cell for electrowinning, the cell having a tank holding an aqueous sulfuric acid-containing electrolyte, a plurality of anodes and cathodes, support means for suspending the anodes and cathodes in the electrolyte, each anode comprising a lead-containing plate member having a lead or lead alloy outer surface, the outer surface being rough, the rough outer surface having elevations and depressions effective to cause lead sulfate oxidized from the rough outer surface of the lead plate member to adhere to the rough outer surface for reducing flaking of the lead sulfate.

8, The cell of claim 7 wherein the outer surface of each plate member additionally contains antimony.

9. The cell of claim 7 wherein the outer surface of each plate member additionally includes about 6 percent antimony.

10. The cell of claim 7 wherein the elevations and depressions in each plate member are formed by sandblasting.

11. The cell of claim 7, wherein the depressions in the surface of each plate member are less than one millimeter in depth as measured from adjacent elevations.

12. The cell of claim 7 wherein the support means includes for each anode a cross bar attached to the top of the plate member, the cross bars being supported by opposing walls of the tank.

13. The cell of any one of claims 7 to 12 wherein the electrolyte additionally includes copper.