|Title of Invention||
"A METHOD OF DRYING PRECIPITATED CALCIUM CARBONATE PARTICLES TO PRODUCE AN ULTRA-DRY CALCIUM CARBONATE"
|Abstract||A method of drying precipitated calcium carbonate particles to produce an ultra-dry calcium carbonate having a moisture content of less than 0.1 %H20, wherein the calcium carbonate particles are brought into contact with microwaves, the bed height of the carbonate particles to be dried is of up to 20 cm, the power of the microwaves is of a few kW to upto 100 Kw, the drying is conducted under vacuum, protective gas or air atmosphere, the drying is conducted by means of a continuous belt plant, a discontinuous chamber plant, or a rotary-kiln plant.|
The invention relates to a ultra-dry calcium carbonate. Calcium carbonate is produced by reacting an aqueous calcium hydroxide suspension with CO2 or a gas containing carbon dioxide or by intensive grinding of natural calcium carbonate. The product is dehydrated and dried in known manner.
Calcium carbonate is used e.g. in the production of paper, dyes, sealing compounds, adhesives, polymers, printing inks, rubber etc. It is used as a functional filler with pigment properties.
The range of uses of calcium carbonate is constantly widening owing to its beneficial application properties. The process technology for the production of the calcium carbonate has in the meantime been modified to such an extent that different qualities of calcium carbonate can be produced according to the intended use. Thus for example the structure of the particles can be varied. It is likewise possible to influence the residual moisture content in the end product by varying the drying conditions.
Usually, first of all dehydration takes place by filtration or centrifuging, and then drying takes place with the aid of e.g. belt dryers, fluidised-bed dryers, crusher-dryers etc. The disadvantage of these methods is that the calcium carbonate is initially dried satisfactorily, but absorbs moisture again from the surrounding air during cooling. Depending on the fineness, or, better, depending on the specific surface area, this moisture content may be up to 3% by weight.
The object of the invention is to dry completely and thus prepare for use conventionally produced calcium carbonate by subsequent treatment with microwaves.
According to the invention, the precipitated calcium carbonate which has been dried e.g. by means of a belt dryer, with a residual moisture content of 0.1 to 3%, in special cases up to 80% residual moisture content, is subsequently dried using microwaves.
Microwaves are electromagnetic waves of differing frequencies. Usual frequencies are 915 MHz and 2.45 GHz. In microwave treatment, heat is produced by the direct conversion of electromagnetic energy into kinetic energy of the molecules, i.e. in the moist product itself.
The conversion of electromagnetic energy into thermal energy takes place due to the electromagnetic properties of the materials to be heated. Whether and to what extent a material can be heated or dried by means of microwaves will depend on its molecular
structure. Polar molecules, i.e. molecules with different charge ranges, e.g. water, can be heated effectively with microwaves. The polar molecule is caused to rotate by the high-frequency alternating field of the microwaves and in so doing converts the electromagnetic energy into heat. Since each molecule converts heat and the microwaves can penetrate deeply, depending on the material, the entire volume is heated up. This is an essential advantage over conventional heating or drying, in which the heat can penetrate the body only via the surface of the material.
The microwave energy converted upon complete absorption is
PVeri =2-TT-/-E2£0e'rtan • 5 inW/m3 (1).
The depth of penetration is calculated as:
d = — in cm (2).
/-» I > » '
f frequency in Hz
e0 absolute dielectric constant (DC)
= 8.85x10'12 As/Vm
E amount of field strength of the electric
alternating field in V/m
E = e0 * (£r - j £r), complex DC
tan5 = £r /E'f
5 dielectric loss angle in degrees
AO Wavelength in cm, A0=C//
The temperature profile of the microwave heating is inverse to that of conventional heating. In microwave drying this inverse temperature profile is advantageous, since a high pressure builds up inside the material and forces the water to the surface. This water evaporates at the surface, which keeps it constant moist until the water has been virtually completely removed from the interior. Only after this does the surface also begin to dry.
Since the water owing to its polarity absorbs a large if not the major portion of the microwave energy, a lesser conversion of energy takes place in those regions which are already dry so that the microwaves can penetrate more deeply into the material here. Thus it is possible to reduce very greatly the residual moisture content in the material, so that ultra-dry products can be produced.
It has been found that calcium carbonate particles which still have a residual moisture content of 0.1 to 3% H2O can be dried further by means of microwaves. However, the suspension obtained from the precipitation or wet grinding or the filter cake obtained therefrom with residual moisture contents of over 80% or over 30% can also be dried. The treatment can be carried out with any initial moisture content. Degrees of dryness of 0 to 0.1% are achieved.
Different configurations of microwave drying plants are known. For large and lumpy materials, continuous belt plants or discontinuously operating chamber plants are used.
Powders or granules are preferably dried in microwave rotary-kiln plants. In this case, the material is passed through the heating zone in a rotating tube and in so doing is heated and dried by the microwaves.
The plant can be operated under vacuum, protective gas or under an air atmosphere. The bed height may be up to 20 cm, according to the construction of the apparatus. A bed height of at most 10 cm has proved advantageous for calcium carbonate. Since only the residual moisture needs to be removed with this apparatus, no very high outputs are required. A few kW are sufficient, but 25 kW to over 100 kW can be used.
The calcium carbonate dried according to the invention can be used as an additive controlling the flow properties e.g. in sealing compounds or adhesives. The ultra-dry calcium carbonate can be used as an additive e.g. in 1-constituent or 2-constituent polyurethane sealing compounds, in silicone sealing compounds or modified silicone sealing compounds, in particular MS polymer sealing compounds
The advantages of microwave drying are:
1. Belt plant drying is static drying, i.e. the product is not subjected to any mechanical stress.
2. A temperature gradient directed towards the surface, i.e. a temperature which is higher in
the interior than on the surface and an associated higher partial pressure, which
transports the liquid to be evaporated to the surface.
3. No drying-out of the surface layer, i.e. it remains permeable.
4. Upon evaporation in the interior, the liquid is guided to the outside by the pore structure.
This results in a higher drying rate.
5. The partial pressure produced in the core by the microwaves speeds up the diffusion
6. Rapid drying of moist products with low thermal conductivity.
7. Short drying times.
The following examples are intended to explain the invention, but not to restrict it.
Predried CaCO3 was dried in a continuous belt plant in a microwave channel (max. output: 6 kW/2450 MHz) with an active length of 2 m.
Examples 1 to 10:
Belt coverage: 15 mm high
CaCO3 with a residual moisture content of 0.37% H2O was used.
Tables 1 and 2 show the drying results under different conditions:
Table 1: (Table Remove)
1. A method of drying precipitated calcium carbonate particles to produce
an ultra-dry calcium carbonate having a moisture content of less than
0.1% H2O, wherein
the calcium carbonate particles are brought into contact with
the bed height of the carbonate particles to be dried is of up to 20
the power of the microwaves is of a few kW to upto 100 Kw,
the drying is conducted under vacuum, protective gas or air
the drying is conducted by means of a continuous belt plant, a
discontinuous chamber plant, or a rotary-kiln plant.
2. A method of drying precipitated calcium carbonate particles as claimed in Claim 1, wherein the calcium carbonate particles with a residual moisture content of 0.1 to 3% H2O are brought into contact with microwaves.
3. A method of drying precipitated calcium carbonate particles as claimed in Claim 1, wherein the calcium carbonate suspension obtained by precipitation or wet grinding or the filter cake obtained therefrom with residual moisture contents of up to >80% H2O are dried with microwaves.
4. A method of drying precipitated calcium carbonate particles as claimed in claim 1, wherein the precipitated calcium carbonate is produced by reacting an aqueous calcium hydroxide suspension with CO2 or a gas containing CO2.
5. Sealing compound or adhesive comprising the ultra-dry calcium carbonate having a moisture content of less than 0.1 % H2O as claimed in claim 1 wherein the sealing compound is a polyurethane sealing compound, in particular a one- or two-component polyurethane sealing compound, a silicone sealing compound, preferably a modified silicone sealing compound.
|Indian Patent Application Number||2524/DELNP/2005|
|PG Journal Number||13/2009|
|Date of Filing||11-Jun-2005|
|Name of Patentee||SOLVAY CHEMICALS GMBH.|
|Applicant Address||HANS-BOCKLER-ALLEE 20, 30173 HANNOVER, GERMANY.|
|PCT International Classification Number||C01F 11/18|
|PCT International Application Number||PCT/EP2003/013663|
|PCT International Filing date||2003-12-04|