Molten lead

A more efficient route is based on use of 2iac vapor. Ziac vapor is absorbed ia molten lead to form a 20—30% solutioa, which is coatacted with steam at 300—500°C and 10—20 MPa (100—200 atm). The H2 product is withdrawn, and the ZnO removed from the lead is reduced and recycled (180). 

Copper is frequently a main impurity ia blast-furnace charges, and its limited solubiUty ia molten lead as copper sulfide requires that the excess be removed by chemical reaction with components of the charge. For this reason enough sulfur is left ia the siater to form a copper sulfide matte layer having a specific gravity of 5.2. 

Blast air, preheated to 650°C, is deflvered by centrifugal blowers through a refractory-lined busde main to the furnace. Zinc vapor from the reduced sinter is carried out with the furnace gases to a condenser fitted with mechanical rotors that are partly immersed in a shallow pool of molten lead. The lead flows countercurrenfly to the gas and is vigorously agitated by the rotors to create an intense shower of lead droplets throughout the condenser. 

FoUowing the removal of the enriched dross, the required quantities of calcium, as a lead—calcium alloy and magnesium in the form of metal ingots, are added. The bath is stirred about 30 min to incorporate the reagents and hasten the reaction. The molten lead is cooled gradually to 380°C to permit the precipitate to grow and soHdify. The dross is skimmed for use with the next lot of lead to be treated. 

The lead contains residual calcium and magnesium that must be removed by chlorination or treatment with caustic and niter. The molten lead is pumped or laundered to the casting kettles in which it is again treated with caustic and niter prior to mol ding, After a final drossing, the refined lead is cast into 45-kg pigs or 1- and 2-t blocks. 

Lead—Calcium—Aluminum Alloys. Lead—calcium alloys can be protected against loss of calcium by addition of aluminum. Aluminum provides a protective oxide skin on molten lead—calcium alloys. Even when scrap is remelted, calcium content is maintained by the presence of 0.02 wt % aluminum. Alloys without aluminum rapidly lose calcium, whereas those that contain 0.03 wt % aluminum exhibit negligible calcium losses, as shown in Figure 8 (10). Even with less than optimum aluminum levels, the rate of oxidation is lower than that of aluminum-free alloys. 

Lead—antimony or lead—arsenic ahoys must not be mixed with lead—calcium (aluminum) ahoys in the molten state. Addition of lead—calcium—aluminum ahoys to lead—antimony ahoys results in reaction of calcium or aluminum with the antimony and arsenic to form arsenides and antimonides. The dross containing the arsenides and antimonides floats to the surface of the molten lead ahoy and may generate poisonous arsine or stibine if it becomes wet. Care must be taken to prevent mixing of calcium and antimony ahoys and to ensure proper handling of drosses. 

If the temperature of a molten lead—calcium (tin)—aluminum ahoy is not kept sufficiently high, finely divided aluminum particles may precipitate and float to the top of the melt. These may become mixed with oxides of lead in the dross. The finely divided aluminum particles can react violently with the oxides in the dross if ignited. Ignition can occur if attempts are made to melt or bum the dross away from areas of buildup with a torch. The oxides in the dross can supply oxygen for the combustion of aluminum once ignited. 

Lead Monoxide. Lead monoxide (litharge), PbO, occurs as a reddish alpha form, which is stable up to 489°C where it transforms to a yellow beta form (massicot). The latter is stable at high temperatures. The solubihty of a-PbO ia water is 0.0504 g/L at 25°C the solubihty of the p-PbO is 0.1065 g/L at 25°C (40). Lead monoxide is amphoteric and dissolves ia both acids and alkahes. In alkahes, it forms the plumbite ion PbO - The monoxide is produced commercially by the reaction of molten lead with air or oxygen ia a furnace. Black or gray oxide is manufactured by the Barton process, by the oxidation of atomized molten lead ia air, as well as by the ball mill process, ia which metallic lead balls of high purity are tumbled ia the mill to form partially oxidized lead particles. 

Silicon (3), which resembles metals in its chemical behavior, generally has a valence of +4. In a few compounds it exhibits a +2 valence, and in silicides it exists as a negative ion and largely violates the normal valency rules. Silicon, carbon, germanium, tin, and lead comprise the Group 14 (IVA) elements. Silicon and carbon form the carbide, SiC (see Carbides). Silicon and germanium are isomorphous and thus mutually soluble in all proportions. Neither tin nor lead reacts with silicon. Molten silicon is immiscible in both molten tin and molten lead. 

It was an adaptation of the Castner cell to sodium chloride for fused caustic electrolysis. A mixture of sodium chloride and other chlorides, molten at 620°C, was electroly2ed ia rectangular or oval cells heated only by the current. Several cells have been patented for the electrolysis of fused salt ia cells with molten lead cathodes (65). However, it is difficult to separate the lead from the sodium (see Electrochemical processing). 

In the Parkes desilvering process, 1—2% zinc is added to molten lead where it reacts with any gold, silver, and copper to form intermetaUic compounds which float as cmsts or dross that is skimmed (see Lead and lead alloys). 

Lead alloys containing small amounts of calcium aie formed by plunging a basket containing a 77 or 75% calcium—23—25% A1 alloy into a molten lead bath or by stirring the Ca—A1 alloy into a vortex created by a mixing impeUor (19). 

The removal of silver from lead is accomplished by die addition of zinc to the molten lead, and slowly cooling to a temperature just above the melting point of lead (600 K). A crust of zinc containing the silver can be separated from the liquid, and the zinc can be removed from tlris product by distillation. The residual zinc in the lead can be removed eitlrer by distillation of the zinc, or by pumping chlorine tluough the metal to form a zinc-lead chloride slag. 

Abzug, m. hood, fume cupboard scum, dross, specif, a scum on molten lead, sharp slag outlet, discharge, drainage, drain vent drawing off deduction proof sheet, proof py (Photog.) print. 

Lead Although subject to slight penetration at 980°C it shows no detrimental effects in stress rupture tests when tested in molten lead at this temperature or at 815°C . It is highly resistant to mass transfer in liquid lead as indicated by data obtained in tests at 800°C with a thermal gradient of 300°C . ... 

Lead Tantalum is highly resistant to liquid lead at temperatures up to 1000°C with a rate of attack of less than 0.025 mm/y. It exhibits no detrimental effects when stress rupture tests are conducted in molten lead at 815°C . 

With molten lead or tin, limited use of high-alloy steels is possible. In the case of containers for lead baths, it is important to avoid the combination... 

Table 7.36 Results in thermal convection loop tests of material in contact with molten lead...

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