Chemistry of inorganic compounds – Phosphorus or compound thereof
Patent
1993-02-11
1996-11-12
Langel, Wayne
Chemistry of inorganic compounds
Phosphorus or compound thereof
422231, 422239, 422284, 422290, 422305, C01B 2500, B01J 1600
Patent
active
055737404
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns the generation of phosphine and safety systems for phosphine generators. More particularly it concerns the controlled production of phosphine (preferably using a phosphine generator which incorporates a novel safety system) for use in, for example, the fumigation of grain and similar products stored in bulk. However, the present invention is not restricted in its use to the fumigation of grain (and it should be noted that in the context of this specification, the term "grain" will be regarded as encompassing grains of all types and pulses, and similar materials). Other uses of the phosphine generators of this invention include the fumigation of greenhouses, herbariums and rabbit warrens.
BACKGROUND TO THE INVENTION
Phosphine (PH.sub.3) is a preferred gaseous fumigant for stored grain because any residue of the fumigant will be lost or oxidised to a harmless phosphate when the grain is processed to produce a food. The phosphine is normally produced by the action of water or water vapour on a metal phosphide (typically aluminium phosphide or magnesium phosphide).
The conventional technique for generating phosphine for disinfesting a grain silo or the like involves the use of a probe to position formulations of aluminium phosphide or magnesium phosphide within the grain mass, where moisture from the grain, or in the air circulating through the grain, reacts with the phosphide to generate phosphine. A modification of this technique involves the placement of pellets of a metallic phosphide in the head-space of a grain silo and allowing the phosphine produced when the phosphide reacts with moisture in the atmosphere above the grain to be circulated through the stored grain by the natural convection currents that are present in the silo. These techniques, however, cannot be guaranteed to supply phosphine to all regions of the bulk stored grain to eradicate weevils and other unwanted grain pests. In addition, when these techniques are used, there is no control over the concentration of phosphine in the grain after the phosphide formulation has been added, and there is no possibility of regulating the concentration/time regime in the bulk storage to be most effective. The use of a probe to insert pellets into the grain mass also requires a significant labour input.
An alternative fumigation technique requires the placement of a quantity of a metallic phosphide (in granular or pellet form) in the head space above the grain, then actively recirculating air from the head space through the grain using recirculation ducts. This technique also has the disadvantage of lack of control over the concentration of phosphine in the grain after the phosphide formulation has been placed in the head space. The same disadvantage occurs when (in a modified form of this technique) the phosphide formulation is placed in a recirculation duct instead of being placed in the head space.
It has also been proposed that phosphine from cylinders of the gas (usually mixed with carbon dioxide) should be mixed with air externally of the stored grain and the resultant gas mixture should then be pumped through the grain. This disinfestation of the grain should be carried out periodically, or whenever it is perceived that fumigation of the grain is necessary or desirable. A major difficulty with this approach to the fumigation of grain is the need to guarantee the provision of expensive cylinders of the gas mixture (usually containing only 3 per cent phosphine) to the treatment site.
Clearly it would be advantageous to have a method of generating phosphine in a controlled manner at the treatment site using the readily available commercial phosphide formulations or similar light and easily handled feedstock. One such on-site phosphine generator is described in the specification of European patent application No 88119701.6, which is European publication No A-0318040.
There is one major problem with the use of phosphine, namely its flammability and explosivity in air at certain concentrations. The generally
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Banks Henry J.
Waterford Colin J.
Commonwealth Scientific and Industrial Research Organization
Langel Wayne
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