WO1992010612A1

WO1992010612A1 - Process for converting old asphaltic concrete into usable new asphaltic concrete

Info

Publication number: WO1992010612A1
Application number: PCT/NL1991/000249
Authority: WO
Inventors: Andreas Goverdina Kemps
Original assignee: Hollandsche Beton Groep NV
Current assignee: Hollandsche Beton Groep NV
Priority date: 1990-12-04
Filing date: 1991-12-04
Publication date: 1992-06-25
Anticipated expiration: 1993-06-04
Also published as: ES2056010T3; EP0560864A1; NL9002659A; NL192958C; NL192958B; EP0560864B1; WO1992010613A1; DE69102771D1; DE69102771T2

Abstract

To convert old asphaltic concrete into usable new asphaltic concrete, coarse pieces of old asphaltic concrete with a minimum size of not less than 5 cm are placed in an autoclave in which the asphaltic constituents are softened in the autoclave by pressurised superheated steam at a temperature of not less than 150 °C being fed into the autoclave.

Description

Title: Process for converting old asphaltic concrete into usable new asphaltic concrete.

The invention relates to a process for converting old asphaltic concrete into usable new asphaltic concrete, in which pieces o old asphaltic concrete are placed in an autoclave and the asphaltic constituents are softened by heating with the aid of steam fed into the autoclave, as disclosed in NL-A-7708800.

It is known to remove a road surface consisting of old asphaltic concrete by milling it or breaking it up, optionally to comminute the coarse pieces in a crusher and to heat it in a rotating drum to a temperature of approximately 160"C with hot gases which have been heated by a burner.

It is also known that old asphaltic concrete is made into ne convertible asphaltic concrete by heating said old asphaltic concrete in a so-called parallel drum to 120" to 150°C and then to bring the entire mixture to a temperature of approximately l60^βC by adding mineral heated to an especially high temperature and bitumen.

The milling and breaking up of old asphaltic concrete and th optionally further crushing is accompanied by considerable noise and dust, with the energy consumption being high. Heating old asphaltic concrete in a rotating drum is a process which is difficult to control, with local overheating of the pieces of old asphaltic concrete being unavoidable. This usually results in damage to the bitumen binder to be regenerated and there are undesirable emissions as a result, inter alia, of the escape of hydrocarbon vapours. In addition, if rotating air furnaces are used there is a danger of blockage. It is known that, after converting a certain quantity of asphaltic concrete, rotating furnaces are no longer usable as a result of encrustation and blockage and then have to be cleaned or replaced.

If, as described in the abovementioned NL-A-7708800, the asphaltic concrete could be heated to the desired temperature in direct contact with steam, the abovementioned disadvantages could be avoided. The steam method described in the Dutch application has hitherto not been put into practice because the temperature of the old asphalt cannot be brought to a sufficiently high temperature above 100°C for long enough, even with relatively high steam temperature; as soon as the steam pressure is lowered, the temperature is found to drop rapidly to approximately 100^βC as a result of condensation. The object of the invention is to avoid this drawback and, for this purpose, the process mentioned in the preamble is characterised in that only coarse pieces of old asphaltic concrete with a minimum size of not less than cm are placed in the autoclave and pressurised superheated steam at a temperature of not less than 150°C is fed into the autoclave.

The invention is based on the insight that, during the steam treatment of relatively small lumps of asphaltic concrete, a relatively large amount of water penetrates the surface and, on evaporation after steaming, said water removes a large amount of heat from those lumps, whereas this phenomenon does not occur, or occurs to a much reduced extent, if coarse lumps of asphaltic concrete are used. In other words, coarse lumps of asphaltic concrete can indeed be brought to a temperature of not less than 120°C (after reducing the steam pressure) and small lumps cannot. With minimum lump dimensions of 8 cm, an asphalt temperature of 130°C can be reached in a short time, and with minimum dimensions of 10 cm, an asphalt temperature of 150^βC can be reached in a short time. This is connected with the accessible hollow space in the asphaltic concrete. In the case of a small lump, a relatively large amount of water penetrates the surface and, on evaporation after the steaming has terminated, said water again removes a large amount of heat from the small lump.

The pollution of the environment by volatile substances such as hydrocarbons and PCAs is a thing of the past. Afterburning of these substances is no longer necessary to meet the maximum permissible emissions. Filters are also no longer necessary within this framework in order to trap the dust which is produced during the entire process. An additional big advantage is that no overheating of the bitumen takes place because very hot gases are absent, with the result that the reduction in binder quality no longer occurs and no additional asphaltic constituents have to be added.

If a particular milling technique or breaking-up method presents finer pieces of old asphaltic concrete, these small pieces of asphaltic concrete can be preheated with steam to approximately 100°C and compressed into lumps whose minimum size is not less than 5 cm. This intermediate can then be treated in the manner described above.

The result of the invention is a plastic paste which can readily be used for a new road surface without adding new constituents and without reduction in quality. Should it be found that heating coarse pieces of asphaltic concrete with superheated steam takes too long or results to an insufficient extent in a usable plastic paste (for example because the old asphaltic concrete has relatively few open spaces), pieces of the old asphaltic concrete preheated and softened in the autoclave can be poured onto a belt which is passed through a microwave furnace in which at least the rock portion of the asphaltic concrete is heated to a temperature of not less than l6θ°C in a second heating phase.

This two-phase method is extremely expedient in relation to the thorough heating of the asphaltic concrete, the saving of energy, the maintenance of the quality of the binder and the minimum pollution of the environment. In the first phase, during the heating to not less than 120°C, no volatile substances such as, for example, hydrocarbon escape from the bitumen of the asphaltic concrete and other optional admixtures. In the second phase it is chiefly only the rock which is heated by the microwaves, said rock heating the bitumen and other substances indirectly, with the result that the bitumen and other admixtures will virtually not degenerate in relation to their properties. Virtually no volatile vapours will escape during this last phase either. The microwaves used preferably have a frequency of 9l6 MHz in order to obtain an economically effective heating.

It is pointed out that US-A-4,856,202 discloses a process for converting old asphaltic concrete into new asphaltic concrete, in which the asphaltic constituent of the old asphaltic concrete is heated in a first phase to a temperature of approximately 16 ^βC by hot air and the rock portion is heated in a second phase to a temperature of approximately l65"C by means of microwaves. The air in the first phase is heated by means of burners in a chamber in which impellers are installed to feed the hot air via pipes onto finely divided broken pieces of asphaltic concrete. It has been found that the temperature of the air blown onto the asphaltic concrete varies considerably and is locally much higher than l65^βC, as a result of which volatile substances are released from the asphaltic concrete. These disappear and produce a serious pollution of the environment. In order to ensure the desired quality of the new asphaltic concrete, additional substances will have to be added. In addition, the heat transfer from air to solids is relatively poor, with the result that this method consumes a large amount of energy.

In regard to this known method, it is new and essential in the present invention that coarse lumps of asphaltic concrete having a minimum size of not less than 5 cm are exposed to superheated steam at a temperature of not less than 150°C in an autoclave. In this process, an efficient heating of the asphaltic concrete is achieved with relatively little energy and without volatile substances being released. The process according to the invention results in old asphaltic concrete being heated without the emission of harmful substances in such a way that softening occurs and it can disintegrate into smaller pieces which can readily be reused.

Claims

1. Process for converting old asphaltic concrete into usable new asphaltic concrete, in which pieces of old asphaltic concrete are placed in an autoclave and the asphaltic constituents are softened with the aid of steam fed into the autoclave, characterised in that only coarse pieces of old asphaltic concrete with a minimum size of not less than 5 cm are placed in the autoclave and pressurised superheated steam at a temperature of not less than 150°C is fed into the autoclave.

2. Process according to Claim 1, characterised in that the pieces of old asphaltic concrete have a minimum dimension of not less than 8 cm.

3. Process according to Claim 1 or 2, characterised in that before feeding the asphaltic concrete into the autoclave, the small pieces of asphaltic concrete are preheated to approximately 100°C with steam and are compressed into lumps with a minimum size of not less than 5 cm.

*•*.. Process according to one of the preceding claims, characterised in that old asphaltic concrete heated and softened in the autoclave is poured onto a belt which is passed through a microwave furnace in which at least the rock portion of the asphaltic concrete is heated to a temperature of not less than l6θ^βC in a second heating phase.