Compositions: coating or plastic – Coating or plastic compositions – Bituminous material or tarry residue
Patent
1993-02-19
1994-01-11
Klemanski, Helene
Compositions: coating or plastic
Coating or plastic compositions
Bituminous material or tarry residue
1062811, C08L 9500
Patent
active
052777103
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of processing an asphalt mixture which comprises mineral aggregate and/or recycled crushed asphalt and to which bitumen-based binding material has optionally been added, the method comprising heating of mineral aggregate or a mixture of mineral aggregate and crushed asphalt or crushed asphalt as such before mixing the possible preheated bitumen-based binding material with the mixture. In this application crushed asphalt refers to a mixture of bitumen and mineral aggregate.
It is conventional in the processing of an asphalt mixture as described above, i.e. a mixture which not only comprises mineral aggregate but optionally also recycled crushed asphalt, i.e. so-called recycling crush, or only such crush, as a starting material, that the starting material is heated in a drum directly by flame of a burner. Thereby the flame comes into contact with both the mineral aggregate and the crushed asphalt, whereby on the one hand dust is emitted from the mineral aggregate and above all noxious gases are emitted from the crushed asphalt, particularly from the bitumen therein. It is precisely because of the formation of noxious gases that this kind of conventional method is no longer allowed in most countries. However, in substitute methods a mixture comprising recycled crushed asphalt cannot very often be heated to temperatures that are sufficiently high for a successful mixture. Furthermore, several methods used nowadays involve abundant formation of dust when mineral aggregate is heated, whereby these methods require that expensive and efficient filters be used.
A method in which attempts have been made to minimize environmental hazards is known from U.S. Pat. No. 4,519,711. In the method crushed asphalt is heated by heated gases and flame in a drum mixer, and nowhere near complete elimination of dust and noxious gases forming thereby can be achieved by the curtains of water according to the publication.
The object of the present invention is to disclose a new method of processing an asphalt mixture in which the above problems have been successfully avoided. Thus in the method according to the invention even the possible crushed asphalt is successfully heated to a temperature that is sufficiently high for a successful mixture and on the other hand dusting of the mineral aggregate is avoided. This is achieved by the method according to the invention in such a way that mineral aggregate or a mixture of mineral aggregate and crushed asphalt or crushed asphalt as such is heated by leading a heated gas that comprises vapour, preferably superheated water vapour, thereto. Preferably, the temperature of the heated gas is within the range of about 250.degree. to 350.degree. C. By leading the heated gas from below to a mixture disposed in a hopper that is open from above, the mixture at the bottom of the hopper can be very efficiently heated to a temperature desired, whereby the mixture in the hopper functions as a heat insulator with respect to the surroundings. Thereby the coefficient of efficiency of the heating can also be made very high. The hopper is emptied in batches through the bottom thereof as the mixture at the bottom of the hopper reaches the temperature desired.
It is essential to the operation of the method according to the invention that the heated gas contains a quantity of vapour functioning as a heating medium, the vapour being preferably, but not necessarily, superheated water vapour, i.e. the gas is a so-called water gas. On account of this addition of water vapour the heat content of the gas is increased and, additionally, the gas obtains a property that inhibits formation of dust. Preferably, the heated gas is a dry mixed gas of a combustion gas of fuel, such as thin oil, and water vapour.
Preferably, the gas used in the method according to the invention is generated by a gas generator known from U.S. Pat. No. 4,811,723.
In the following the method according to the invention and the apparatus in which it can be applied are described in more detail with reference to the
REFERENCES:
patent: 4519711 (1985-05-01), Gillen et al.
patent: 4811723 (1989-03-01), Vaananen
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