Numerical simulation of vacuum dezincing of lead alloy

Removing zinc by distillation can leave the lead bullion virtually free of zinc and also produces pure zinc crystals. Batch distillation is considered in a hemispherical kettle with water-cooled lid, under high vacuum (50 Pa or less). Sufficient zinc concentration at the evaporating surface is achieved by means of a mechanical stirrer. The numerical model is based on the multiphysics simulation package PHYSICA. The fluid flow module of the code is used to simulate the action of the stirring impeller and to determine the temperature and concentration fields throughout the liquid volume including the evaporating surface. The rate of zinc evaporation and condensation is then modelled using Langmuir’s equations. Diffusion of the zinc vapour through the residual air in the vacuum gap is also taken into account. Computed results show that the mixing is sufficient and the rate-limiting step of the process is the surface evaporation driven by the difference of the equilibrium vapour pressure and the actual partial pressure of zinc vapour. However, at higher zinc concentrations, the heat transfer through the growing zinc crystal crust towards the cold steel lid may become the limiting factor because the crystallization front may reach the melting point. The computational model can be very useful in optimising the process within its safe limits.

Recovery of Bacillus thuringiensis in vegetative form from the phylloplane of clover (Trifolium hybridum) during a growing season

Two media were developed which specifically allow the cultivation of Bacillus thuringiensis while it is in the vegetative as opposed to the spore form. Using these media B. thuringiensis was shown conclusively for the first time to exist in an active form on the phylloplane. The profile of its appearance in vegetative and spore form was followed over a growing season on clover (Trifolium hybridum) in the field. Three simultaneous and sudden rises and declines of both spore and vegetative cell densities were observed. The most common other spore-former on these leaves was Bacillus cereus but the fluctuations in appearance of these two very closely related species were not co-incident. Using specific PCR primers a considerable diversity of cry toxin gene types was found in isolates that had been recovered in vegetative form ([`]vegetative isolates') with the majority possessing multiple [delta]-endotoxin genes while some had only one of those tested. Bioassays against a lepidopteran insect of purified [delta]-endotoxins showed that they were no more potent than those from a laboratory-adapted strain. PCR primers for an internal region of the vip3A gene produced amplification in 70% of the vegetative isolates compared to 25% of the laboratory-adapted strains tested.