The present invention relates to a method for manufacturing granular mastic asphalt, in which filler and heated aggregate, in a first stage, are mixed with a partial amount of bitumen preheated into a liquid state, and the mixture allowed to cool to ambient temperature to form a granular semifinished product, whereupon the said semifinished product, in a second stage, is mixed with a remaining amount of bitumen heated into a liquid state, to form a granular mastic asphalt end product. Such mastic granular is primarily intended for use as a protective coating over welded seams between pipe sections in subsea pipelines, particularly for conveying hydrocarbons. However, it is suitable also in other applications where there is a need for transporting mastic asphalt in a cold state over longer distances.

The word"filler"is the technical term used to denote rock flour or stone dust, while "aggregate"is the technical term denoting mineral materials in the form of crushed rock, sand, etc.

GB patent 334 588 discloses a method for preparing mastic asphalt in blocs, intended for use as an ordinary paving asphalt. Here, firstly a coarse aggregate fraction is mixed with bitumen, then a fine aggregate fraction is added, optionally together with a remaining amount of bitumen, and finally filler material. The entire operation is performed in one stage without any intermediate cooling, and the resulting product is in the form of blocks-not granular. GB patent 1 494 279, like the above GB 334 588, discloses a method of fabricating mastic asphalt in the form of solid blocks, where the essential aspect is the choice of the proportional amounts of the various components, bitumen, filler and aggregate, and the particle size of the latter.

A method of the introductorily stated kind, for the manufacture of granular mastic asphalt, is known from PCT patent application, publication no. WO 97/24410. The mastic asphalt product resulting from this prior method is an improvement of the mastic asphalt product described in GB 1 494 279, the improvement being that the product according to the PCT application appears in the form of a granular which, compared to the bloc-shaped mastic asphalt, requires less consumption of energy in manufacture and is more easily handled in use, such as described in the latter document. A drawback of this prior method, however, is a non-uniform composition of the mastic asphalt granular, tending to result in the formation of lumps.

It has now surprisingly been found that the above drawback of the technique according to the PCT application is practically eliminated by retaining, in the first mixing stage, not only a partial amount of the total amount of bitumen, but also a partial amount of the total amount of filler to be contained in the end product, as stated more fully in the appending claim 1. Thereby the undesirable lumping tendency is obviated and the ease of handling is correspondingly improved. More particularly, the method according to the present invention possesses the following advantages over the prior art according to the above PCT application: -Better coating of the material of the semifinished product, with less filler in the aggregate.

-Better distribution of the filler in the granular, because the suspension acts as a binder and is uniformly distributed on the material of the semifinished product.

-There will be no pure binder in the surface of the mastic asphalt granular, whereby the stickiness of the granular is considerably reduced and its storing capacity correspondingly improved. This is a significant advantage since the granular is normally supplied in 800 kg sacks stacked above each other.

In an embodiment of the method according to the invention, using a bitumen partial amount of 15-25% by weight of the total amount of bitumen in the first mixing stage, a filler partial amount of 15-30% by weight of the total amount of filler is used.

According to an advantageous feature of the method according to the invention, PBS ("Phonix Bitumen Stabilizer", a synthetic amid polymer) is added to the bitumen/filler suspension in the proportion 2-5, preferably about 3% of the total amount of bitumen, raising the softening point of the bitumen from 85 to 110°C, and lowering the processing temperature from 200 to 180°C. This results in less consumption of energy for heating purposes and quicker setting after grouting of the mastic asphalt.

Preferably the mixing is performed in the second stage as a continuous process in which bitumen/filler suspension is sprayed onto a stream of semi-finished product.

Preferably water is added to the semifinished product in the second stage, in an amount of 0.5-1.5% by weight of the semifinished product. This results firstly in a foaming effect owing to the evaporation of the water impinging the hot bitumen thereby improving the dispersion of the semifinished product granules in the bitumen suspension and thus an improved coating of bitumen on the granules, and secondly a cooling effect lowering the temperature during the mixing process to below the bitumen softening point, with the result that individual granules in the granular end product are less sticky, with less tendency to lump formation.

The invention also comprises the semifinished product per se to be used in the manufacture of granular mastic asphalt, consisting of 90-95% by weight of aggregate, 2-5% by weight of filler and 2-4% by weight of bitumen.

The invention will now be described in more detail.

Generally the procedure when manufacturing the granular mastic asphalt according to the invention is as follows.

A semifinished product is prepared consisting of 90-95% aggregate, 2-5% filler and 2-4% bitumen, all percentages (also in the preceding as well as the succeeding description) being based on weight.

The preparation of the semifinished product is performed in a conventional manner by heating the aggregate including the filler and mixing it with hot, liquid bitumen.

The mixture is then allowed to cool to ambient temperature (air temperature).

After cooling the mixture, owing to its relatively low content of bitumen, will appear in the form of a homogenuous granular.

The end product, i. e. mastic asphalt in the form of granules, is then prepared by mixing the semifinished product with a suspension of hot bitumen and filler in a weight ratio of about 1: 1 (50/50%). The proportional amount of semifinished product and suspension is adjusted to provide a mixture containing a total of about 69% aggregate, 16% filler and 15% bitumen.

Preferably, PBS is added to the suspension in an amount of about 3% of the total amount of bitumen. It will then suffice to keep the temperature of the suspension at about 180°C, in contrast to 200°C without PBS.

The mixture of semifinished product and bitumen/filler suspension is preferably carried out by dispensing the granular semifinished product, optionally with water added to it, as a continuous stream into a mixer in which a hot bitumen/filler suspension is sprayed onto the granular stream.

A such continuous mixing process, i. e. spraying the remaining proportion of bitumen/filler in the form of a suspension onto the continuous stream of granular semifinished product, further assists in securing a homogenuous granular semifinished product.

It is generally well-known technique within the art of mixing, to mix particle and liquid substances in a continuous process as described above, and therefore there is no need for a detailed description of the mixer per se. Any suitable mixer could be used. Also belonging to well-known mixing technique that need not need any further description herein, is the proportional dispensing of semifinished product and bitumen/filler suspension in a such continuous process for achieving the above stated percentage weight proportion of aggregate, filler and bitumen.

Although the mixing of semifinished product and bitumen/filler suspension preferably is carried out as a continuous process as described above, it is within the purview of the invention to mix the ingredients i. e. semifinished product and suspension, in any other way, such as by a continuous or batch mixing process using a stirrer or the like.

Example 92.5% aggregate in the form of Ansit with a particle size of 0.075-5mm is heated to between 140°C and 160°C, e. g. about 150°C, and is mixed in an asphalt plant with 4% filler in the form of Ansit of particle size 0-75 pm and 3.5% bitumen at about 90°C. The mixture is then dropped on the ground to be cooled while turned around by means of a grab.

A suspension is prepared consisting of bitumen and filler in the proportion 1: 1, mixed in a vessel by means of a stirrer effectively maintaining the mixture in a homogenuous state. 3% PBS based on the total weight of bitumen is added to the suspension. The mixing temperature of the suspension is about 180°C.

Semifinished product at air temperature, together with about 1% by weight of water, is dosed as a continuous stream into a mixing device in which bitumen/filler suspension at about 180°C is sprayed onto the granular flowing through the mixing device.

The end product exiting the mixing device as a continuous stream, is in the form of a granular containing about 69% by weight of aggregate, about 16% by weight of filler and about 15% by weight of bitumen.