Blasting process for lead anode plates for electro-obtaining zinc

Abstract

This invention deals with a blasting process on lead anode plates for zinc electrowinning.

Description

BLASTING PROCESS FOR LEAD ANODE PLATES FOR ELECTRO-OBTAINING ZINC

[0001] The present application claims priority from Chilean patent application number CL 3625-2018 filed on December 14, 2018. TECHNICAL FIELD

[0002] This invention is devised for maintenance & repair of electrowinning anodes. It deals -specifically- with a blasting process for lead-anode plates for zinc electrowinning. DESCRIPTION OF THE PRIOR ART

[0003] Zinc electrowinning process is carried out in a cell containing an electrolyte made up of a zinc sulfate & sulphuric acid concentrated solution. On the one hand, the anodes making up the positive electrode, are installed. These consist of a lead / lead alloy plate, hanging from a copper conductive bar. On the other hand, the cathodes making up the negative pole are made up of an aluminum plate hanging from a copper conductive bar. Continuous direct current is distributed through these electrodes arrangement, thus causing the zinc contained in the electrolyte to be deposited on the cathodes as metal zinc. On the lead anode gaseous oxygen is discharged, thus completing the two oxide- reduction semi-reactions of the electrolytic process.

[0004] Lead-alloy anodes used in zinc electro-plating -generally- contain between 0.20 % and 1.00 % of silver in its weight. These anodes can be produced both during smelting and by means of lamination. Lead-alloy anodes require a longer time to make up a PbO2 / MnO2 bonding / protecting layer on their surface. This allows O<com:Sub>2< / com:Sub> evolution with no excessive contamination of Pb on the zinc deposit of the cathode. At the initial stage of the anode operation, its even surface does not allow proper bonding of the PbO<com:Sub>2< / com:Sub> / MnO<com:Sub>2< / com:Sub> layer. Such layer easily comes off thus causing subsequent lead contamination of the zinc settled on the cathode. In order to minimize this contamination, during the electrowinning process and before placing the anodes inside the cells, their surface is subject to a mechanical / chemical / electrochemical treatment which makes up a rough surface finishing, which allows a better bonding of the PbO<com:Sub>2< / com:Sub> / MnO<com:Sub>2< / com:Sub> layer, thus –significantly– reducing cathode contamination with lead, at the initial stage of the operation.

[0005] Quality / performance of the anode have a significant impact on the zinc electrowinning process. During the last few years leading edge technology on anodes performance have improved the manufacturing stage of the anode. From a metallurgical point of view, they are nearly exclusively manufactured by laminating lead-silver alloys with a concentration between 0.20 % and 1.00%, of silver.

[0006] Regarding manufacturing optimization, special care has been taken on the coating bonding of the copper conductive bar. That optimization -directly- impacts on both anode conductivity and corrosion resistance of the copper bar. The type of alloy of this coating is also important, because apart from improving conductivity and corrosion resistance, it should not contain harmful components contaminating the electrolyte for zinc electrowinning, thus deteriorating its quality. Similar considerations CA should de done regarding the welding alloy / welding procedure to be used on the joint between the lead-silver laminated plate and the coating of the copper bar.

[0007] A significant part of this fabrication optimization deals with surface treatment of lead anode plates, prior to their use. This is the main topic of this invention.

[0008] Lead contamination on the cathode for zinc is high during the initial period. This situation is caused by the lack of a MnO<com:Sub>2< / com:Sub> protecting layer, which bonds onto the anode surface. This causes a high level of corrosion and, consequently, a high concentration of Pb-PbSO<com:Sub>4< / com:Sub> species on the electrolyte to be later electrochemically / physically embedded on the deposit.

[0009] As the anode ages, its surface gets corroded, due to chemical attacks, thus generating a stable PbO<com:Sub>2< / com:Sub> layer and a thicker MnO<com:Sub>2< / com:Sub> bonding surface layer. Finally, the MnO<com:Sub>2< / com:Sub> starts to come off from the surface as a flake-like material.

[0010] At this stage, the MnO2 layer is stabilized, as the generation speed equalizes the speed the material comes off.

[0011] Formation of more bonding MnO<com:Sub>2< / com:Sub> particles can take up to two weeks. On the other hand, full stabilization of the MnO<com:Sub>2< / com:Sub> layer occurs after five months.

[0012] Prior treatment of the anodes guarantees that newly installed anodes have lead contamination levels on cathodes similar to the oldest ones used in the aisle thus -effectively- eliminating all aforementioned issues. Prior anode treatment methods may be split into three categories, namely, electrochemical / chemical / physical treatments. The electrochemical treatment involves submerging the anodes in a KF-H2SO4 electrolysis bath for several hours, CA at a high temperature, in order to produce a stable / bonding layer of PbO2 onto which MnO<com:Sub>2< / com:Sub> is naturally bonded to. Another chemical treatment method involves submerging the anodes in a H2SO4 – KMnO4 bath, in such that when the anodes are installed a slightly bonding layer of MnO<com:Sub>2< / com:Sub> develops. The aforementioned physical treatment include mechanical brushing, sand blasting and blasting, whether with iron metal particles, smelter's slag or with glass micro balls.

[0013] With these methods, a quick PbO<com:Sub>2< / com:Sub> bonding layer is generated, onto which an initial MnO<com:Sub>2< / com:Sub> bonding / shiny layer is initially formed. This layer gets thick and eventually comes off after a few months.

[0014] Sand blasting / blasting advantages, blasting above all, compared with other prior treatments is that it also hardens the alloy, which improves mechanical properties of the anode and fosters quick generation of a bonding corrosion layer. Sand blasting used in these cases was widely used during the last few decades, but now it has a big environmental / health disadvantage, as it has been forbidden all around the world, due to environmental dust contamination in the surrounding areas to the site. It is also forbidden, because it's the cause of a severe lung disease in exposed workers, named as silicosis. Besides, as these are angled particles, they got deeply encrusted onto the lead substrate of the anode and the hardness increase on the surface was rather low.

[0015] Because of this situation various blasting media have been tested, whether with glass micro balls, gray iron, steel or smelter's slag. The glass micro balls produce a shallow blasting, because of their low hardness. Most of them get broken, thus making reuse has a higher cost than other methods. CA

[0016] Regarding metal balls these are made from various types of iron / steel and smelter's slag, which is widely used. They cause severe encrusting problems on the lead surface of the anode, which is very smooth, and are submerged along with the anodes into the electrolytic bath, where they eventually are dissolved by the electrochemical corrosion existing in such bath, thus contaminating the electrolyte with non desired components. SUMMARY OF THE INVENTION

[0017] This invention describes a blasting process for anodes plates for zinc electrowinning which improves the bonding of a PbO2 / MnO2 layer, it increases the anode surface hardness, thus improving its mechanical features and avoiding encrustation of harmful components contaminating the electrolyte during the electrowinning. The blasting process is carried out on both plate faces and uses highly pure metal zinc blasting, with a diameter between 0.1 mm to 5 mm. This is obtained by blasting processes with molten metal, with cut zinc thread or by cooling zinc spray. Encrustations on the anode plate, caused by the zinc blasting process are dissolved in the electrolyte by means of electrolytic corrosion of the anode. The surface of both plate faces has an average final roughness between 0.05 mm to 1 mm, preferably between 0.4 mm to 0.5 mm. BRIEF DESCRIPTION OF THE FIGURES

[0018] Figure 1 describes an anode for zinc recovery. (prior art) COMPREHENSIVE DESCRIPTION OF THE INVENTION

[0019] This document describes a blasting process for both plate faces of a lead anode used in the zinc electrowinning process, which improves initial bonding of a PbO2 / MnO2 layer and increasing surface hardness of the anode, thus improving its mechanical features. The blasting process is made by using highly-pure metal zinc blasting with a diameter between 0.1 to 5.0 mm. It is obtained based on a molten zinc blasting process, with cut zinc thread or by cooling zinc spray. The blasting process causes metal zinc encrustations on both plate faces, when the blasting impacts on the plate. Such encrustations will be dissolved by the electrochemical corrosion of the anode. During the electrowinning process only the cation Zn 2<com:Sup>+< / com:Sup> is added to the electrolyte, in very small amounts compared with the total concentration of Zn 2+, in the existing electrolyte. Therefore, it does not add any harmful substance for the production of highly pure metal zinc.

[0020] This document, additionally, describes an anode obtained from blasting a lead anode plate for zinc electrowinning. This process improves the bonding of a PbO2 / MnO2 surface layer, thus increasing the anode surface hardness and improving its mechanical features. The objective is to prevent encrustation of harmful components which contaminate the electrolyte during the electrowinning process, where the surface of both plate faces have an average final roughness between 0.05 mm to 1 mm, preferably between 0.4 mm to 0.5 mm. CA

Claims

1. A blasting process of a lead anode plate for zinc electrowinning which improves bonding of a PbO 2 / MnO 2 surface layer for increasing the anode surface hardness and improving the mechanical features of the lead anode plate aimed to avoid encrustation of harmful components which contaminate the electrolyte during the electrowinning process, comprising performing highly-pure metal zinc blasting of the lead anode plate on both plate faces of the lead anode plate.

2. The process according to Claim 1, wherein the metal zinc blasting material used is between 0.1 mm to 5 mm in diameter.

3. The process according to Claim 1 or Claim 2, wherein the zinc blasting is obtained by one of: blasting molten metal; cutting zinc thread; and cooling zinc spray.

4. The process according to any one of Claims 1 to 3, wherein encrustations on the anode plate faces, caused by the zinc blasting, are dissolved in the electrolyte, due to electrolytic corrosion of the anode.

5. An anode obtained by a blasting process of a lead anode plate for zinc electrowinning that improves the bonding of a PbO2 / MnO2 surface layer, increasing the anode surface hardness, and that improves the mechanical features of the lead anode plate aimed to avoid encrustation of harmful components which contaminate the electrolyte during the electrowinning process, wherein a surface of each of the two plate faces of the anode have an average final roughness between 0.05 mm to 1 mm.

6. The anode according to Claim 5, wherein the surface of each of the two plate faces of the anode have an average final roughness between 0.4 mm to 0.5 mm. •