Lead anode

Lea.dAnodes. A principal use for lead—calcium—tin alloys is lead anodes for electrowinning. The lead—calcium anodes form a hard, adherent lead dioxide layer during use, resist corrosion, and gready reduce lead contamination of the cathode. Anodes produced from cast lead—calcium (0.03—0.09 wt %) alloys have a tendency to warp owing to low mechanical strength and casting defects. 

Tetravalent lead is obtained when the metal is subjected to strong oxidizing action, such as in the electrolytic oxidation of lead anodes to lead dioxide, Pb02 when bivalent lead compounds are subjected to powerful oxidizing conditions, as in the calcination of lead monoxide to lead tetroxide, Pb O or by wet oxidation of bivalent lead ions to lead dioxide by chlorine water. The inorganic compounds of tetravalent lead are relatively unstable eg, in the presence of water they hydrolyze to give lead dioxide. 

In electrogalvanizing, copper foil, and other oxygen-evolving appHcations, the greatest environmental contribution has been the elimination of lead-contaminated waste streams through replacement of the lead anode. In addition, the dimensionally stable characteristic of the metal anode iatroduces greater consistency and simplification of the process, thus creating a measure of predictabiUty, and a resultant iacreased level of safety. 

In Jiltemative 3 (Fig. 3), the electrolysis may be operated on a semicontinuous basis with the cadmium eventually being stripped completely from the electrolyte, which is then discarded after suitable treatment. Instead of the usual silver—lead anodes, high siUcon-iron anodes, such as Duriron, are commonly used. 

Whenever insoluble anodes are used, the pH of the plating solution decreases along with the metal ion concentration. In some plating baths, a portion of the anodes is replaced with insoluble anodes in order to prevent metal ion buildup or to reduce metal ion concentration. Lead anodes have been used in acid copper sulfate baths, and steel anodes have been used in alkaline plating baths. 

In seawater, lead anodes with 1 or 2% silver may be used for cathodic protection of ships " at current densities of up to 120Am Lead with 6Vo antimony and 1 Vo silver has also been recommended. It is thought that silver might provide small stable nucleation sites for PbOj formation " in a manner similar to the Pb/Pt bi-electrode " (see Section 11.3), which is serviceable at 250 A m . A lead. Wo Ag, 0.5% Bi or 0.5% Te alloy with a platinum micro-electrode will perform well at 500 A m. ... 

The use of small platinum pins to promote the formation of lead dioxide on lead anodes is described in Section 11.3. 

In the case of a lead anode (without a platinum microelectrode), the... 

Electrolyte-sulphuric acid (5% wt.%) plus an inhibitor (0-5kgm ) such as diorthotolyl thiourea, quinoline ethiodide or /3-naphthol quinoline. The temperature should be 75°C, the cathode current density 2000 Am and the time of cathodic polarisation 3 min. The anode should be carbon or lead. If lead anodes are used, lead may deposit on the specimens and cause an error in the weight loss. If the specimen is resistant to nitric acid the lead may be removed by a flash dip in 1 1 nitric acid. Except for this possible source of error, lead is preferred as an anode, as it gives more efficient corrosion product removal. 

The reaction occurs at lead anodes at the cathode metallic magnesium is produced. An ether solution of the Grignard reagent is used to which ethyl chloride is added. The latter reacts with the magnesium that is formed, to regenerate part of the Grignard reagent consumed. The overall reaction in the electrolyzer follows the equation... 

C—Lead Lead anode Lead oxide Secondary Lead—add batteries used for starting, lighting, and ignition (SLI)... 

The reaction is the electrochemical version of the well-known transmetallation with lead salts, and becomes significant when the lead anode oxidizes. The very high current yields (Ca 170-180%) imply that formation of R4Pb by nonelectrodic reactions takes place as well ... 

In certain cases, a single-step electrochemical process can be derived out of a complex sequence of reactions. The electrolysis proposed by Ziegler35 and Lehmkuhl36, in which sodium tetramethylaluminate is electrolyzed between a lead anode and mercury in THF, is an example of complex reactions of very sensitive compounds which are translated into a simplified electrolytic procedure ... 

Heterogeneous route, at the electrode surface. The lead anode is attacked and yields tetraethyllead as the main product. For this stage, several reaction routes are possible, e.g. diethylcadmium may be oxidized on the lead anode to produce ethyl radicals which, in turn, may oxidize metallic lead. Partially alkylated lead compounds thus formed are alkylated to tetraethyllead by ethyl iodide. 

Plates of lead, each coated with lead dioxide, are immersed in fairly concentrated sulphuric acid. Lead is oxidized at the lead anode during discharge ... 

For large scale (200 L capacity plating tank) it proved impossible to apply airborne ultrasound and therefore the ultrasound was introduced into the plating tank (Fig. 6.13). The transducers consisted of two banks of three mounted in dummy tanks and delivered in total 1.4 kW into 135 L making the overall ultrasonic intensity 0.01 W cm The cathode consisted of a steel bar (5 cm diameter and 20 cm length) and 4 large lead anodes (diameter 3.7 cm and length 39 cm) were used. On sonication... 

Nitroso compounds are usually not obtained directly but rather by reoxidation of hydroxylamino compounds or amines. Hydroxylamino compounds are prepared by electrolytic reduction using a lead anode and a copper cathode [573], by zinc in an aqueous solution of ammonium chloride [574 or by aluminum amalgam [147], generally in good yields. 

Baizer, working at the Monsanto Company, showed that good yields of adiponitrile are obtained from aqueous solutions by reduction at mercury or lead in the presence of a high concentration of quaternary ammonium salt [62]. Tetraethyl-ammonium toIuene-4-sulphonate was favoured as electrolyte. The first commercial plant operating the process was commissioned in 1965. It used a divided cell system with a lead cathode and aqueous tetraethylammonium ethylsulphate as electrolyte, with the addition of acid to regulate the pH. A lead anode with an anolyte of dilute sulphuric acid was employed. 

Alternatively, low-grade stibnite ore is converted to its oxide which is then reduced with carbon. Tetrahedrite may be treated with sodium sulfide solution. The solution containing thioantimonate formed is then electrolyzed in a diaphragm cell using a steel cathode and lead anode. The metal is further refined by oxidation or electrorefining process. 

Cadmium also may be recovered from zinc ores and separated from other metals present as impurities by fractional distillation. Alternatively, the cadmium dust obtained from the roasting of zinc ore is mixed with sulfuric acid. Zinc dust is added in small quantities to precipitate out copper and other impurities. The metal impurities are removed by filtration. An excess amount of zinc dust is added to the solution. A spongy cadmium-rich precipitate is formed which may he oxidized and dissolved in dilute sulfuric acid. Cadmium sulfate solution is then electrolyzed using aluminum cathodes and lead anodes. The metal is deposited at the cathode, stripped out regularly, washed and melted in an iron retort in the presence of caustic soda, and drawn into desired shapes. More than half of the world s production of cadmium is obtained by elecrolytic processes. 

Basic lead carbonate also is precipitated by dissolving lead monoxide in lead(II) acetate solution, and treating the solution with carbon dioxide. It also is produced by electrolysis of sodium nitrate or sodium acetate using lead anode and then precipitating out the product by adding sodium carbonate. 

It also is produced by electrolysis of an alkali solution using lead anode. Lead hydroxide is formed on the anode. 

Iron and aluminum precipitate out when treated with ammonia and are removed by filtration. Other metals, such as copper, zinc, lead and arsenic are precipitated and removed as sulfides upon passing hydrogen sufide through the solution. Colloidal particles of metaUic sulfides and sulfur are removed by treatment with iron(ll) sulfide. The purified solution of manganese(ll) sulfate is then electrolyzed in an electrolytic cell using lead anode and HasteUoy or Type 316 stainless steel cathode, both of which are resistant to acid. Manganese is deposited on the cathode as a thin film. 

Zinc also may be produced by electrolysis of zinc sulfate solution. The zinc oxide in the roasted concentrate is leached with sulfuric acid. The oxide is converted to soluble zinc sulfate. Impurity metals, such as iron, copper, cadmium, arsenic, tin, and cobalt are removed by precipitation, floe formation, and other methods. The purified zinc sulfate solution is electrolyzed using aluminum cathodes and lead anodes. Zinc is deposited on the cathode. 

In NaOH solutions, two main lead anodic peaks owing to the formation of PbO and Pb02 have been recorded, followed by oxygen evolution ([153-156] and the references given therein). In the step preceding PbO production, the formation of... 

The lead/acid battery used in conventional gasoline-fueled automobiles consists of six 2.05 V cells connected in series (for a 12 V electrical system). The current collectors are lead grids filled, when in the charged condition, with powdered lead (anode) and a lead/lead(IV) oxide mixture (cathode), and the electrolyte is aqueous sulfuric acid. During discharge, the following reactions occur ... 

LA was first prepd in 1891 by Curtius (Ref 1) by adding Pb acetate to a soln of Na or Amm azide. Curtius Rissom (Ref 2) prepd it by the action of hydrazoic acid on a lead salt. Turrentine (Ref 6) obtd LA during electrolysis of a 3% soln of Na azide on lead anodes. Browne et al (Ref 20) found LA was formed when a soln of Nt Nj, in liq NHS at -67°, was electrolyzed with a Pb anode. Some details of prepg LA have been described by Hyronimus (Ref 3), Stettbacher (Refs 13,85 114), Hodgkinson (Ref 18),... 

Traditionally, anodic oxygen evolution from acid solution—particularly from aqueous electrolytes containing sulfuric acid—has been performed at lead anodes that are passivated and stabilized against corrosion by a selfforming coating of Pb02. 

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