Dezincing

The dezincing chamber is set first in the drossed lead bath, then water connections are immediately made in order to prevent the formation of steam within the water jacket. While the temperature is being raised, the vacuum pump is placed in operation and the agitator started. The temperature is then raised to 600°C and held throughout the operation. 

In the Betterton-KroU process the dezinced lead is pumped to the debismuthizing kettie, in which special care is taken to remove drosses that wastefuUy consume the calcium and magnesium. The skimmed blocks from the previous debismuthizing kettie are added to the bath at 420°C and stirred for a short time to enrich the dross with the bismuth being extracted from the new charge. This enriched dross is skimmed to blocks and sent to the bismuth recovery plant. 

Zinc brasses containing phosphorus, antimony, and/or arsenic dezinc-ify much less readily than brasses free of these elements. Brasses containing less than 15% zinc are virtually immune to attack, and those containing more than 32% zinc are readily dealloyed. 

In the tests described by Tracy, a high-tensile brass suffered severe dezinc-ification (Table 4.11). The loss in tensile strength for this material was 100% and for a non-arsenical 70/30 brass 54% no other material lost more than 23% during 20 years exposure. In Mattsson and Holm s tests the highest corrosion rates were shown by some of the brasses. Dezincification caused losses of tensile strength of up to 32% for a P brass and up to 12% for some of the a-P brasses no other materials lost more than 5% in 7 years. Dezinc-ification, but to a lesser degree, occurred also in the a brasses tested, even in a material with as high a copper content as 92%. Incorporation of arsenic in the a brasses consistently prevented dezincification only in marine atmospheres. 

Dezincing, lead refining by, 14 754-755 DGEBA-aromatic polyamine adduct system, 10 416, 417. See also Diglycidyl ether of bisphenol A (DGEBA)... 

Chlorine is used in the manufacture of many products, as a bleaching compound for residential and commercial use, and as a biocide for municipal water and waste treatment (i.e., purifying and disinfecting water, detinning and dezincing iron) (Budavari 1989). It also was used as chemical-warfare agent in World War I (Withers and Lees 1987). 

Grades purer than the Prime Western product have been made by vacuum dezincing the Hquid alloy to recover 99.9% zinc at 0.02% lead. Distillation (New Jersey Zinc process) produces a purer product at a considerable cost in energy. 

Less critically, galvanizing prolongs the life of iron or steel products which are exposed to corrosive environments, but adds a difficulty by requiring dezincing before recycling [46, 47]. 

Method Vacuum Dezlndrig Vacuum Dezincing Vacuum Dezincina > KUum Dezincing Vacuum Dezincing... 

P.R.M. Van Negen and P. Weertes, Vacuum Dezincing of Parkes Triple Alloy in an Induction Furnace , Electric Furnace Techniques in Metallurgy. Bad Salzdetfurth, 19-20 March, 1987,169-176. 

Total Cycle Time Dezincing Time Dezincing Temperature Vacuum Final Zinc Zinc R overy Fuel Consumption... 

It is often important in the study of a process to determine whether the limitations are governed by equilibrium or kinetic considerations. Sometimes it is possible to devise a design of the process or the equipment so that the apparent limitations set by equilibrium considerations are overcome. In this paper, detailed consideration is given to the lead refining processes of sulftir-decoppering and of vacuum dezincing to illustrate this, and possible implications for other refining processes are indicated. 

When apparently only one reaction is to occur, it may still be of advantage to cause a second reaction to take place, so as to produce a more favorable outcome. This is not necessarily done by introducing additional reagents, but can possibly be done by suitable design of the reaction vessel. This is the case for the removal and recovery of zinc finm lead after the desilverising operation. The bperation is performed under vacuum, so that it will proceed at reasonable speed at relatively low temperatures, and is known as vacuum dezincing. 

Figure S - Batch Vacuum Dezincing Plant, as at St. Joe, Herculaneum, See Reference (34) The Distance Between the Evaporating and Condensing Surfaces is About 30 cm A Mechanical Vacuum Pump Produces a Residual Pressure of About 30 Microns Hg A Lead Temperature of 600°C and Stirring Time of 5 hours Reduced Zinc rom 0.56% to 0.05% Zn in the Dezinced Lead.

Figure 6(b) - Continuous Vacuum Dezincing Plant With Liquid Zinc Condenser,... 

Figure 6(a) - (Semi)-Continuous Vacuum Dezincing Plant at B.H.A.S. Port Pirie,... 

There have been many theoretical determinations of the rates of distillation under vacuum, but none that appeared to be applicable to vacuum dezincing when its development was commenced at Port Pirie in 1946. Thus it was felt to be desirable to develop the theoretical side (37) simultaneously with the practical development, as a guide to understanding and possible later application to optimising of the process. Figure 7 illustrates the concept of the distillation process which was developed. It was necessary to discard some faulty ideas or misconceptions, which derived fix>m implicit notions associated with equilibrium, but not kinetic conditions. Carman (38), for example, assumed that the partial pressure of the vapour of the condensing species is equal to its partial pressure in the condenser. It is not, unless the condensation rate is zero. Richardson (6) assumed that the measured vacuum is equal to the distilling species, which it is not, but is instead the partial pressure of the inert atmosphere. Warner (40) assumed the partial pressure of zinc to be constant across the distillation space, which is not correct unless the distillation rate is zero. 

Evaluations of Equations (IS) and (16), flying values of the constants which would be appropriate for the vacuiun dezincing of lead in practice, showed that there was no significant difference between the two. It also showed that the exponential term in Equation (15) could be neglected, without introducing an error of more than 1%, so that Equation (15) simplifies to ... 

N.A. Warner, Kinetics of Vacuum Dezincing , Symposium on Advances in Extractive Metallurgy. London, Instn. of Min. and Met., London, 1967,317-332. 

C.F. Harris, J.F. Castle and J. McNish, Vacuum Dezincing , I.S.P. Conference 1967. Imperial Smelting Corporation, London, 1967,852-887. 

Remaining copper and other impurities were originally removed by the addition of zinc, and the removal of zinc was by chlorine to form a zinc chloride dross or by drossing with caustic soda. Vacuum dezincing of lead was developed as a practical technique in 1946. 

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