Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~09948~
HOECHST AKTIENGESELLSCHAFT HOE 92/F 913 Dr.GL-nu
Werk Gendorf
Process for melting snow and/or ice using an alkali metal
carboxylate
Description
The invention relates to a process for melting snow
and/or ice on traffic surfaces by application of a
specific alkali metal carboxylate.
Snow and/or ice on roads, cycle paths, footpaths,
bridges, sportsgrounds, airports and the like (called
traffic surfaces below) lead to a considerable impairment
in traffic flow and traffic safety. It has therefore
already been known for a long time to apply water-soluble
salts to such surfaces, ice and ænow melting (thawing)
and an aqueous solution of the salt of correspondingly
reduced freezing point being formed.
If salts are employed as de-icing agents, the following
requirements above all should be met: the salt should not
damage or even decompose the material from which the
surfaces mentioned are constructed, for example concrete.
It is furthermore essential that a corrosive action on
metals also is excluded. ~he salt should be acceptable to
humans and animals and have a good biodegradability. ~he
salt employed should cause rapid thawing as substantially
as possible. Finally, from the economic point of view, it
is necessary that only a small amount of de-icing agent
is required, that it costs little and that it is easy to
store and easy to handle.
In view of the requirements mentioned, alkali metal and
alkaline earth metal salts of organic acids are mentioned
above all as advantageous de-icing agents in the prior
art. U.S. Patent 4,855,071 thus describes alkali metal
and/or alkaline earth metal carboxylates having l to 4
.
.
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carbon atoms, prepared by a particular process, as de-
icing agents, an essentially anhydrous calcium/magnesium
acetate being recommended in particular. European Patent
Application 0 483 721 Al describes a liquid de-icing
S agent which essentially comprises water, an alkali metal
acetate and/or alkaline earth metal acetate and specific
corrosion inhibitors.
Although the known de-icing agents based on alkali metal
carboxylates and/or alkaline earth metal carboxylates
meet most of the abovementioned requirements, they leave
something to be desired in particular in the following
respect: the action of a salt as a de-icing agent is
based, as already mentioned above, on the reduction in
the freezing point of water by the salt. Another process
which occurs here is thawing (melting) of the ice and/or
snow, which requires energy and leads to severe, although
short-term, cooling (cold shock) of the immediate
environment, since the ener~y required (heat of fusion)
is withdrawn therefrom. This results not only in a delay
in the thawing process, but also in more or less severe
damage to the traffic surface covering. The water in the
top layer, of for example a concrete covering, in fact
undergoes sudden formation of ice and therefore an
increase in volume because of the cold shock mentioned,
and this can lead to the particular top layer of the
concrete cracking open. The action of the alkali metal
and alkaline earth metal carboxylates hitherto described
as de-icing agents is thus based only on the action of
these salts in reducing the freezing point when dissolved
in water.
The object of the invention accordingly is to propose,
from the large number of de-icing agents in the form of
alkali metal and~or alkaline earth metal carboxylates,
which are advantageous per se, one w~ich has a high
action in respect of reduction in the freezing point and
releases heat in an exothermic reaction during the
process of thawing (melting) of ice and snow, whereby the
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heat of fusion of ice and snow will be more or less
compensated and the cold shock mentioned will therefore
be reduced or even eliminated completely. This salt
furthermore should be particularly ecologically and
physiologically acceptable, have no corrosive action on
metals and the covering of the traffic surfaces, be
readily biodegradable and be easy to handle and store.
Such a salt would be desirable, above all, in all cases
where rapid thawing of ice and/or snow and at the same
time only slight, if any, cooling (low cold shock) of the
environment are important.
It has been found, surprisingly, that a certain alkali
metal carboxylate, and in particular a specific sodium
salt of acetic acid, has all the required properties when
used as a de-icing agent.
The process according to the invention for melting snow
and/or ice on traffic surfaces comprises applying to the
traffic surfaces covered with snow and/or ice an
effective amount of a sodium acetate which is essentially
anhydrous and is in the form of particles having a smooth
surface and a diameter of 0.3 to 10 mm.
A sodium acetate which has a combination of quite parti-
cular features is thus employed in the process according
to the invention. It is a sodium acetate which has been
dried to a very low water content. It should contain less
- than 10% by weight, preferably less than 5% by weight,
and in particular less than 0.5% by weight of water (the
percentages by weight relate to the weight of the sodium
acetate to be employed). Taking into account the
economics of preparation and storage of dry (anhydrous)
sodium acetate, a sodium acetate which contains only
about 0.05 to 10% by weight of water, preferably 0.05 to
5% by weight and in particular about 0.05 to 0.5% by
weight of water will be employed (the percentages by
weight relate to the sodium acetate to be employed). The
sodium acetate proposed according to the invention
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furthermore should have a selected morphology. It should
be in the form of particles (granules), preferably in the
form of grains, beads or prills, in particular in the
form of beads or prills, the particles having a diameter
of about 0.3 to 10 mm, preferably 0.5 to 5 mm and in
particular about 1 to 3 mm. The surface of the particles
should be essentially smooth, that is to say not rough or
porous or the like.
The sodium acetate to be employed according to the
invention is prepared by mixing acetic acid and aqueous,
sodium hydroxide solution essentially stoichiometrically
and drying the aqueous sodium acetate solution in a spray
granulator to give the particles to be employed according
to the invention. The acetic acid is preferably employed
in the form of a 90 to 99.9% strength by weight aqueous
solution and the sodium hydroxide is employed in the form
of a 30 to 50~ strength by weight aqueous solution
(aqueous sodium hydroxide solution). Acetic acid and the
sodium hydroxide are preferably employed in an amount
such that the resulting sodium acetate solution has a pH
of 7 to 11. Mixing of the aqueous acetic acid and aqueous
sodium hydroxide solution and, if appropriate, additional
water is preferably carried out at a temperature of 40 to
90C, the heat of neutralization being removed by cool-
ing. The resulting aqueous sodium acetate solution as arule comprises 30 to 65% by weight of sodium acetate,
preferably 40 to 60% by weight, the percentages by weight
being based on the weight of the aqueous solution. A true
(unsaturated, lew-viscosity, clear) solution of sodium
acetate in water is thus present (based on the stated
temperature of 40 to 90C). The particles to be employed
according to the invention are obtained from this
specific sodium acetate solution in a customary spray
granulator (spray dryer). For this, the aqueous sodium
acetate solution is sprayed in a granulator with one or
more jets and is dried with the aid of air or nitrogen at
a temperature of preferably 100 to 130C. It has proved
advantageous if sodium acetate particles are already
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present in the fluidized state in the spray-dryer at the
start of spraying of the sodium acetate solution
described, the sodium acetate solution then being sprayed
onto these particles. The particle material (granules)
falling down in and discharged from the granulator in the
form of, preferably, beads or prills has the smooth
surface required according to the invention and the
degree of dryness required according to the invention.
The particle size required according to the invention is
also obtained by separating off (sieving) the small and
large particles in the particle material.
The sodium acetate of low surface area proposed according
to the invention is a particularly advantageous de-icing
agent because of its low water content and its specific
morphology. It meets all the abovementioned requirements,
such as ecological and physiological acceptability, no
corrosive action on metals and traffic surface coverings,
good biodegradability and easy handling and storage (the
sticking together feared as a result of hygroscopy does
not occur). The sodium acetate proposed according to the
invention above all also meets the requirements of a
rapid thawing action and only slight, if any, cooling of
the environment. In particular, it effects not only a
large reduction in the freezing point, but also liberates
heat of solution, which means that the melting of ice and
snow is accelerated and proceeds without the cold shock
mentioned occurring. Furthermore, because of these
special properties, a high preventive effect is also
achieved. A particularly advantageous de-icing agent is
thus available with the sodium acetate proposed according
to the invention.
In the process according to the invention for melting
snow and/or ice on traffic surfaces, an active amount of
the solid de-icing agent described is applied to the
traffic surface to be treated, that is to say an amount
such that the required melting or thawing of ice and/or
snow is achieved. This amount depends above all on the
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external temperature and the amount of ice and/or snow
present, and is in general 10 to 400 g per m2 of ice-
and/or snow-covered surface. The solid de-icing agent can
be applied with the customary gritting vehicles.
The invention will now be illustrated in more detail by
examples and comparison examples.
Example 1
A layer of ice 2 mm thick was produced in a refrigerating,
chamber at a temperature of -5C. 100 g/m2 of sodium
acetate which had the form of prills having a smooth
surface, a diameter of 0.8 to 2 mm and a water content of
9% by weight were applied uniformly to the layer of ice.
After an action time of 30 minutes at the stated -5C,
the ice which was still present and had not melted was
weighed and the weight of this amount of ice was sub-
tracted from the weight of the original amount of ice to
determine the proportion of melted ice (water formed) as
a percentage by weight, based on the original amount of
ice.
Result:
After an action time of 30 minutes at -5C, 26% by weight
of ice had melted.
Example 2
The procedure was as in Example 1, with the difference
that a sodium acetate which had the form of prills having
a smooth surface area, a diameter of 0.8 to 2 mm and a
water content of 4.5% by weight was applied.
Result:
After an action time of 30 minutes at -5C, 30% by weight
of the ice had melted.
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Example 3
The procedure was as in Example 1, with the difference
that a sodium acetate which had the form of prills having
a smooth surface area, a diameter of 0.8 to 2 mm and a
water content of 0.1% by weight was applied.
Result:
After an action time of 30 minutes at -5C, 35~ by weight
of the ice had melted.
Comparison Example 1
The procedure was as in Example 1, with the difference
that a sodium acetate which had the form of prills having
a smooth surface area, a diameter of 0.8 to 2 mm and a
water content of 18% by weight was applied.
Result:
After an action time of 30 minutes at -5C, 20% by weight
of the ice had melted.
Comparison Example 2
The procedure was as in Example 1, with the difference
that a calcium magnesium acetate which had the form of
prills having a smooth surface area, a diameter of 0.8 to
2 mm and a water content of 4.8% by weight was applied.
.
Result:
After an action time of 30 minutes at -5C, 6% by weight
of the ice had melted.
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