GB2372251A

GB2372251A - Production of granular mastic

Info

Publication number: GB2372251A
Application number: GB0104090A
Authority: GB
Inventors: Albert Russell
Original assignee: SLP ENGINEERING Ltd
Current assignee: SLP ENGINEERING Ltd
Priority date: 2001-02-20
Filing date: 2001-02-20
Publication date: 2002-08-21
Anticipated expiration: 2021-02-20
Also published as: GB2372251B; GB0104090D0

Abstract

A process for manufacturing a granular mastic compound comprising desired amounts of bitumen, filler and aggregate, the process comprising spraying molten bitumen onto at least a partial volume of the aggregate and an optional partial volume of filler mixed with the aggregate, the aggregate being moistened with a quantity of water in the range 4 to 10% of the total weight of aggregate, to form a partially coated mix product, followed by mixing the partially coated mix product with the remainder or all of the filler and any remaining aggregate, and then storing the resultant granular product for at least 40 hours, turning the product at least once during storage, and until the product has a controlled moisture content in the range 2 to 3%.

Description

GRANULAR MASTIC

BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a process for manufacturing a granular mastic composition, notably a bituminous mastic composition for use in coating welded joints of sub-sea pipelines, and to a granular bituminous composition obtainable by the process.

Underwater pipelines are typically laid by an operation which involves welding together on a surface barge, lengths of concrete-coated steel pipes. After the pipes are welded, they are surrounded by a sleeve and filled with a mastic asphalt which is allowed to cool. Mastic asphalt compositions which are particularly suitable for this use are often referred to as"marine mastics".

Before a marine mastic can be used on a barge, it typically needs to be in particulate or granular form and to be processed by heating and stirring before being poured into the sleeve.

(2) Description of the Prior Art Granular mastics have been known for a number of years. For example it is known to manufacture a mastic asphalt composition as a block or sheet in accordance with British Patent No. 1 494 279, and to produce granular particles of this composition by crushing the blocks or sheets.

It has been proposed, in International Patent Application No. WO 97/24410, to produce a granular mastic by in a first step mixing a partial volume of bitumen at from 140 to 220oC with aggregate at elevated temperatures and filler, the mixing being effected at a temperature above the softening point of the bitumen to form a bitumen coated product, allowing the bitumen coated product to cool, and followed by mixing the bitumen coated product with further bitumen at from 140 to 220oC, the mixing temperature being allowed to fall below the softening point of the bitumen used.

A problem with known granular mastics is that they tend to clump together when stored, particularly in warm conditions.

It has been proposed in WO 00/02962 to produce a granular mastic with reduced tendency to clump together by spraying fluid bitumen onto at least a partial volume of aggregate and an optional partial volume of filler mixed with the aggregate, the aggregate being moistened with a quantity of water in the range 3 to 10% of the total weight of aggregate, to form a partially coated mix product, followed by mixing the partially coated mix product with the remainder or all of the filler and any remaining aggregate. We have found that granular mastic produced by this process initially has low tack, but that after the product has been bagged and stored for some time it loses its low tack and free-flowing properties and clumping can occur.

It is an object of the present invention to provide a process for producing a granular mastic composition which has an improved shelf life.

SUMMARY OF THE INVENTION In accordance with one aspect of the present invention there is provided a process for manufacturing a granular mastic compound comprising desired amounts of bitumen, filler and aggregate, the process comprising spraying molten bitumen onto at least a partial volume of the aggregate and an optional partial volume of filler mixed with the aggregate, the aggregate being moistened with a quantity of water in the range 4 to 10% of the total weight of aggregate, to form a partially coated mix product, followed by mixing the partially coated mix product with the remainder or all of the filler and any remaining aggregate, and then storing the resultant granular product for at least 40 hours, turning the product at least once during storage, and until the product has a controlled moisture content in the range 2 to 3%.

We have surprisingly found that by storing and turning the final product, optionally with air blown through it, a granular mastic results which can be stored in bags for a prolonged period without clumping together. The product has a powder-coated look and feel, and somewhat resembles breadcrumbs.

Turning of the product is preferably carried out at

regular intervals, notably four hour intervals. Any suitable means may be used for turning the granules, preferably mechanical loading shovels or something of a similar nature.

The invention will described for convenience hereinafter with reference to the use of a pugmill for manufacturing the product. However it is to be understood that other suitable vessels could be used instead. The product may be turned as a separate step after it leaves the pugmill.

Alternatively, or additionally, turning of the granules may take place in the pugmill. We have found that the drying process can be accelerated by opening a side of the pugmill to permit more air to enter and more steam to leave, or by forcing air through the pugmill by blowing or suction. In a preferred embodiment, the product leaves the pugmill at a temperature in the range 45 to 65OC, preferably about 55OC, and is then turned for 40 to 55 hours, notably 48 hours, before being bagged.

All percentages are given by weight, unless otherwise indicated.

The quantities of aggregate, filler, and bitumen are preferably the same as for a conventional marine mastic material. However, it will be understood that by selecting suitable ingredients and proportions, well known to those skilled in the art, granular mastics may be manufactured for uses other than as marine mastics.

In making conventional marine mastic, the control of

temperature during mixing is not controlled, the period of time for mixing is not controlled, and blow drying is not applied. We have found that the process of mixing is enhanced when the temperature and the time period is controlled and when the bitumen is molten. The bitumen must be sufficiently fluid to be applied by spraying. The optimum temperature range for the molten bitumen will vary for the different bitumen grades. Preferred grades are R115/15, R85/25, and polymer modified bitumen (PMB).

R115/15 bitumen is preferably maintained at a temperature immediately prior to spraying in the range 170 to 190oC, notably 175 to 185OC. R85/25 is fluid enough to spray at lower temperatures (about 120oC or above). We have also found that best results are obtained by mixing in a pugmill which has at least one open side or a pugmill that is provided with an extraction/ventilation unit to dissipate moisture to the atmosphere. Once the granular mastic has been mixed under the controlled process, we have also found that blow drying of the granular mastic for a controlled period of time enhances the product such that it forms a powdery consistency.

Without limiting the invention in any way, it is thought that if filler on the outer surface of the granules has too high a moisture content, over time adjacent particles may tend to become "cemented" together, thereby causing clumping. Turning the granules while reducing the total moisture content to 2 to 3% is thought to prevent or sufficiently reduce this problem.

All of the aggregate may be mixed with the bitumen, or a

partial volume may be mixed with the bitumen to form the partially coated mix product. Preferably the volume of aggregate which is used is 85 to 100% of the total volume, notably about 90%. For convenience hereinafter, the invention will be described with reference to the use of a partial volume of aggregate in the partially coated mix product, but it is to be understood that the invention is not limited to this embodiment.

Prior to mixing the aggregate is typically fed through a screen, for screening purposes only. There is generally no need to dry the aggregate since moisture is necessary to the process. However, drying could be carried out if the moisture content was excessive so that when water is subsequently sprayed the relative quantities of filler and aggregate and water are more precisely known. After screening, the aggregate may be stored at different sizes for different applications.

During the mixing process water is preferably added by spraying to mix the aggregate which will bind the materials together, at an approximate rate of 1.5 kg to 1.7 kg per second for a period of 24 to 26 seconds.

Preferably the quantity of water moisturising the aggregate is 4% to 5% by weight of the partial volume of aggregate. In a preferred embodiment, the moist aggregate is at a temperature of about 40 C immediately before being sprayed with the bitumen.

The first mix of the sprayed aggregate (for example from 85% to 95% of the total weight of aggregate) may be mixed

in a pneumatically or otherwise driven reciprocating paddle mixing apparatus, for example an open sided or vented pugmill. It is preferred that immediately on completion of spraying the aggregate with water, the liquid bitumen is sprayed into the mixer over a controlled period of about 23 to 27 seconds, notably about 25 seconds at a controlled rate, for example at 5.4 kg to 7.4 kg per second, preferably about 6.4 kg per second.

The final product preferably has approximately the following solids composition expressed as percentages of total solids by weight: Bitumen 14-17% Aggregate 73-80% Filler 6-10%.

DETAILED DESCRIPTION Embodiments of the invention will now be described, by way of example. In both examples, the mixing was carried out in a pugmill having approximate dimensions 6 x 3 metres, and 2 metres deep. On one of the long sides, a portion was removed, approximately 5 x 0.5 metres in size, to facilitate escape of steam.

The filler used was fine granite having the following particle size distribution: Sieve/pm % Passing 600 100

300 98 150 89 75 53 The aggregate was granite having the following particle size distribution: Sieve % Passing 6. 3 mm 100 4.75 mm 92 2.36 mm 69 1.18 mm 50

600 urn 30 300 urn 17 150 urn 10 75 pm 5 Example 1 The bitumen used in this example was type R115/15.

The aggregate (765 kg) and filler (76 kg) were mixed together at 40oC, and 90% of this mixture was added to the pugmill. Water (40 kg) was sprayed through a sprinkler system at a rate of 1.6 kg per second for a period of 25 seconds with continuous mixing.

To this wet mixture was added the fluid bitumen (160 kg), at a temperature in the range 175 to 185OC at a rate of 6.4 kg for a period of 25 seconds, with continuous mixing.

The remaining 10% of the aggregate/filler mixture was then added to the mixture in the pugmill for a controlled period of 25 seconds.

The resultant mastic was then removed from the pugmill and taken to a covered area where it was blow dried for a period of 48 hours, with turning every 4 hours. Turning was carried out by means of a mechanical loading shovel.

The granules were lifted and trickled back into a pile so that air had free access to the particles. Blowing was carried out by roof-mounted fans. Other turning and blowing means could of course be used.

The finished product was a granular mastic which had the appearance of being powder-coated and was suitable for packing in bags. It had a moisture content in the range 2 to 3%. The size of the granules was less than 5 mm and they had no noticeable tackiness or tendency to clump together at air temperatures up to 30oC.

Example 2 The method, aggregate and filler, were identical to that of Example 1, except that the bitumen used in this example was type R85/25. The bitumen was maintained at a temperature of about 120oC immediately prior to being sprayed.

The finished product was a granular mastic suitable for packing in bags. It had a moisture content in the range 2 to 3%. The size of the granules was less than 5 mm and they had no noticeable tackiness or tendency to clump together at air temperatures up to 30 C. The granules had the appearance of being powder-coated.

Claims

1. A process for manufacturing a granular mastic compound comprising desired amounts of bitumen, filler and aggregate, the process comprising spraying molten bitumen onto at least a partial volume of the aggregate and an optional partial volume of filler mixed with the aggregate, the aggregate being moistened with a quantity of water in the range 4 to 10% of the total weight of aggregate, to form a partially coated mix product, followed by mixing the partially coated mix product with the remainder or all of the filler and any remaining aggregate, and then storing the resultant granular product for at least 40 hours, turning the product at least once during storage, and until the product has a controlled moisture content in the range 2 to 3%.

2. A process as claimed in claim 1, which is carried out in an open-sided pugmill or a pugmill with an extraction/ventilation unit.

3. A process as claimed in claim 1 or claim 2, wherein the storing of the granular product is carried out for a period in the range 40 to 55 hours.

4. A process as claimed in claim 3, wherein the storing is carried out for about 48 hours.

5. A process as claimed in any one of the preceding claims, wherein the turning is carried out at regular internals.

6. A process as claimed in claim 5, wherein the turning is carried out at four hour intervals.

7. A process as claimed in any one of the preceding claims, wherein the granules are blown for at least some of the time during which they are stored.

8. A process as claimed in any one of the preceding claims, wherein the mixture is at a temperature in the range 45 to 65 C immediately prior to storage.

9. A process as claimed in claim 8, wherein the mixture is at a temperature of about 55 Oc immediately prior to storage.

10. A process as claimed in any one of the preceding claims, wherein the bitumen is sprayed for a period in the range 23 to 27 seconds before the remainder or all of the filler and any remaining aggregate are added.

11. A process as claimed in any one of the preceding claims, wherein the volume of aggregate used to make the partial mix product is 85 to 100% of the total volume of aggregate.

12. A process as claimed in any one of the preceding claims, wherein the moist aggregate is at a temperature of about 40 C immediately before being sprayed with the bitumen.

13. A process as claimed in any one of the preceding

claims, wherein the aggregate is moistened by spraying with water.

14. A process for producing a granular mastic material comprising desired amounts of bitumen, filler and aggregate, and a moisture content in the range 2 to 3% by weight, substantially as herein described with reference to the examples.

15. A process as claimed in any one of the preceding claims, including the step of sealing the product in bags at the end of the storage period.

16. A granular mastic material comprising desired amounts of bitumen, filler and aggregate, and a moisture content in the range 2 to 3% by weight, obtainable by the process of any one of the preceding claims.