Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2003-02-06
2004-02-17
Ford, John M. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06693191
ABSTRACT:
The present invention relates to a process for the preparation of a particulate tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product by combining a first aqueous solution comprising methylammonium N-methyldithiocarbamate with a second aqueous solution comprising formaldehyde, followed by separation and drying of the resulting solid.
Tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione, which is also known under the common name dazomet, is employed in agriculture and horticulture for soil sterilization, i.e. for controlling nematodes, germinating plants and cell fungi (cf. U.S. Pat. No. 2,838,389). The action is based on the slow release of methyl isothiocyanate in the soil by hydrolytic and/or enzymatic breakdown of tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione.
To avoid the formation of aerosols, which are a potential health hazard, during packaging, handling and/or applying the active ingredient, a particulate tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product is desirable whose fines, with particle sizes of less than 100 &mgr;m, amount to as little as possible. On the other hand, the particulate product should also not contain a substantial proportion of coarse particles with particle sizes of over 400 &mgr;m in order to guarantee sufficiently rapid decomposition of the active ingredient in the soil. The known preparation processes of tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione only allow the preparation of particulate tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione products with an inadequately homogeneous particle size distribution.
WO 93/13085 describes a process for the preparation of tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione granules by reacting the methylammonium salt of N-methyldithiocarbamic acid with formaldehyde in the presence of a diaminoalkylene. Under the reaction conditions, the diaminoalkylene leads to the formation of products which act as crystallization inhibitors and which, together with the crystallites of the active ingredient, form a random conglomerate. WO 93/13085 recommends adding the methylammonium N-methyldithiocarbamate solution to an aqueous formaldehyde solution. It has emerged that reproducible particle size distributions can only be obtained by the known process when a large number of parameters, including the rate at which the reactants are added, the intensity of mixing the reactants, the mixing time and the like, are observed accurately. The preparation of a product with constant properties and of flexible response to varying demands is thus made difficult.
It is an object of the present invention to provide a process for the preparation of a particulate tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product which, despite simple process control, yields products with a narrow particle size distribution, in particular with a reduced content of fines of a particle size of less than 100 &mgr;m.
This object is achieved according to the invention by a process for the preparation of a particulate tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product by combining a first aqueous solution comprising methylammonium N-methyldithiocarbamate with a second aqueous solution comprising formaldehyde, followed by separation and drying of the resulting solid, which comprises combining the first and the second aqueous solutions in such a way that the ratio between the concentrations of dithiocarbamate functions and of formaldehyde is essentially constant in the reaction mixture over time.
The invention furthermore relates to a particulate agrotechnical product with a tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione content of at least 95% by weight and such a particle size distribution that less than 7% by weight, preferably less than 3% by weight, of the particles have a particle diameter of less than 100 &mgr;m, over 50% by weight of the particles have a particle diameter of less than 200 &mgr;m, over 90% by weight of the particles have a particle diameter of less than 300 &mgr;m, and over 95% by weight of the particles have a particle diameter of less than 400 &mgr;m.
The particulate product obtained in accordance with the invention preferably has a bulk density of 0.4 to 0.8 kg/l, in particular 0.6 to 0.7 kg/l.
It has been found that narrow particle size distributions can be obtained when the first and the second aqueous solutions are combined in such a way that the ratio between the molar concentration of dithiocarbamate function and formaldehyde in the reaction mixture is essentially constant over the duration of the reaction.
The process according to the invention thus differs essentially from the known processes, in which a reactant, as a rule the aqueous formaldehyde solution, is introduced into the reaction vessel and the other reactant is metered in over a period of time. It can be seen that, in the known processes, the reactant which has initially been introduced is present in a multiple molar excess at the beginning of the metering-in operation. The ratio between the reactant which has initially been introduced and the reactant which is metered in then decreases constantly over the duration of the metering-in operation.
The process according to the invention can be carried out semicontinuously or continuously. To this end, it is expedient to introduce essentially stoichiometrically equivalent amounts of the first aqueous solution, calculated as dithiocarbamate functions, and of the second aqueous solution, calculated as formaldehyde, i.e. essentially twice the molar amount of formaldehyde, into a reaction space per unit time. An “essentially stoichiometrically equivalent” amount is such an amount which is within 20%, preferably within 10%, of the stoichiometrically required amount of the reactant in question. It is also possible to introduce an amount greater than the stoichiometrically required amount of a reactant if the accumulation, in the reaction mixture, of the reactant employed in excess is prevented by suitable measures. For example, the accumulation can be prevented by continuously removing the excess, for example by continuously discarding some of the mother liquor which is obtained when the product is removed from the reaction mixture, as is illustrated in further detail hereinbelow.
Reactors which are suitable for carrying out the reaction procedure continuously are customary reactors such as, in particular, a continuous stirred-vessel reactor or a stirred-vessel cascade. It is expedient to ensure good mixing of the reactants in the reaction space. The introduction of the first and/or second aqueous reaction solution can be carried out for example in such a way that some of the reaction mixture is continuously removed from the reaction space, mixed with the first and/or second aqueous reaction solution and recirculated into the reaction space. Removal, mixing and recirculating are effected for example by pumping the reaction mixture via a metering and mixing section into which the first and/or second solution are fed. Instead of stirred-vessel reactors or stirred-vessel cascades, tubular reactors which are optionally provided with elements like static mixers may also be used.
Sparingly soluble tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione is formed as aqueous suspension when the aqueous solutions are combined. When the process according to the invention is carried out continuously, suspension is continuously withdrawn from the reaction space or, when using a reactor cascade, from the last reactor of the cascade. The solid is separated from the resulting suspension by customary processes, for example by filtration or centrifugation. Suitable devices, such as pressure filters, vacuum filter belts, rotary drum type filters and centrifuges, are known to those skilled in the art. All or some of the mother liquor which remains can be recirculated or eliminated from the process. An accumulation of contaminants or of reactants employed in excess can be prevented in the system by discharging at least some of the mother liquor.
The solid which has been separated off from the mother liquor
Burkart Kirsten
Drögemüller Michael
Franke Dirk
Isak Heinz
Munzinger Manfred
BASF - Aktiengesellschaft
Ford John M.
Keil & Weinkauf
LandOfFree
Method for the production of a particle-containing... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for the production of a particle-containing..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for the production of a particle-containing... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3352372