Barton process

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. 

One technical process involves blowing air above the surface of molten lead. (cf. The Barton process in Sec. 4.2.1), but also, at room temperature, reaction (1) soon covers any piece of lead exposed to air with a dull gray layer of lead oxide (cf. The milling process in Sec. 4.2.1). 

Another process, the Barton process, is based on molten lead. The core of such a device is the "Barton reactor", a heated pot that is partly filled with molten lead. It is continuously refilled by a fine stream of molten lead. Fine droplets of lead are produced by a fast rotating paddle that is partly immersed under the surface of the molten lead within the "Barton reactor". The surface of each droplet is transformed by oxidation into a shell of PbO by an airstream that simultaneously carries away the oxidized particles if they are small enough otherwise, they fall back into the melt and the process is repeated. Thus the airstream acts as a classifier for particle size. 

A short description of both processes is given in Ref. [16]. Nowadays, the Barton process is preferred for a number of reasons it can more easily be installed in small units and it can be controlled faster [17]. (In a mill it takes a number of hours for the material to run through the drum, so controlling actions are slow). 

Barton process - [LEADCOMPOUNDS - LEAD SALTS] (Vol 15) -for battery oxide [BATTERIES - SECONDARYCELLS - LEAD-ACID] (Vol 3)... 

The crystal modification of the lead oxide depends on the conditions of its manufacture. At temperatures of oxidation below 470 °C (as is the case with ball mill PbO), the tetragonal polymorph is formed. If oxidation is performed in the temperature range of 460—470 °C through the Barton process, 60—70 wt% of the lead oxide formed is tetragonal (a-PbO), 15—20 wt% is orthorhombic (p-PbO) and the remaining balance is amorphous lead oxide. 

Leady oxide consists of lead oxide and free metallic lead. When produced by the ball mill method, the degree of oxidation of 65—75% is achieved, while the Barton process yields leady oxide powder with 70—80 wt% PbO content. When the leady oxide contains more than 30 wt% free lead, the material becomes very reactive. It is readily oxidized in humid air and the heat produced by this exothermic reaction may lead to combustion and damage the production equipment. Hence, effective control of the percent content of imoxidized lead in the leady oxide is very important, both during the production process and during the subsequent handling and storage of the leady oxide. 

Both litharge and black oxide are prepared in processes involving the oxidation of metallic lead. The properties of the exhaust gases from production of black oxide by the ball mill or Barton process are shown in Table 5.7. A fabric filter (or baghouse) is an essential part of plant design and the quoted emission factor is determined after passage through the filter. 

The oxide exiting either the Barton or ball mill reactor is conveyed by an air stream to separating equipment, ie, settling tank, cyclone, and baghouse, after which it is stored in large hoppers or dmmmed for use in paste mixing. Purity of the lead feed stock is extremely critical because minute quantities of some impurities can either accelerate or slow the oxidation reaction markedly. Detailed discussions of the oxide-making process and product are contained in references 55—57. 

Barton, J. A. and Nolan, P. F. Incidents in the Chemical Industry due to Thermal-runaway Chemical Reactions, Hazards X Process... 

Barton, G.W. and Perkins, J.D., 1988. Experiences with SPEEDUP in the mineral processing industries. Chemical Engineering Research and Design, 66, 408. 

Barton, H., Ivadizing Ion Vapour Deposition of Aluminium , in Ion Assisted Surface Treatments, Techniques and Processes, The Metal Society, London, pp. 1-6 (1982)... 

Carbon-centered radicals generated by Barton s thiohydroxamate method can also participate in ring-forming reactions (see Scheme 26).52b,s3 For example, irradiation of 129 results in the formation of compound 130 (82% yield). The outcome of this transformation is reminiscent of Stork s elegant radical cyclization/trapping processes (see Schemes 7 and 8), in that/botn alkene carbon atoms have become functionalized. / I... 

Organic chemists have been aware of reversible addition-fragmentation involving xanthate esters in organic chemistry for some time. It is the basis of the Barton-McCombie process for deoxygenation of alcohols (Scheme 9.37).402 404... 

As for the salt formation and single-electron transfer, thermodynamics for simple redox processes may be applied to predict their selectivity. As a first approximation, a cation with red lower and higher than 0.2 V would give a salt and a radical pair, respectively, when combined with [2 ]. In practice, the cations which were found to give salts with [2 ] have red values more negative than —0.8 V. On the other hand, quantitative single-electron transfer has been observed from [2 ] to the heptaphenyltropylium ion which is relatively unstable p/fR+ —0.54 in methanol (Battiste and Barton, 1968) and E ed —0.30 V vs. Ag/Ag in acetonitrile (Kitagawa et al., 1992). 

Barton, P. I., The modeling and simulation of combined discrete/continuous processes. Ph.D. Thesis, University of London (1992). 

Alexander, B., Barton, G., Petrie, J., Romagnoli, J. (2000) Process Synthesis and Optimisation Tools for Environmental Design Methodology and Structure. Computers in Chemical Engineering, 24(2-7), 1195-1200. 

Barton, P.l. and Pantelides, C.C., 1991, Int. Sympos. on Process Engng Systems, Montebello, Canada. 

The Barton-McCombie process is an important synthetic tool for the generation of radical species. Me3SiO-(SiHMeO)n-SiMe3 works as a stoichiometric reductant in the tin-catalyzed reaction since Bu3Sn(OPh) is reduced to Bu3SnH as shown in Scheme 31 [71]. 

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