Field of the Invention

The present invention provides a process for the preparation of a sulphur-aggregate composite.

Background of the Invention

Elemental sulphur or modified sulphur can be used to bind aggregate and filler, thereby providing sulphur- aggregate composites such as sulphur concrete and sulphur asphalt . Sulphur concrete can be used in a variety of pre-cast concrete applications such as marine defences, paving slabs, road barriers and retaining walls. Sulphur asphalt, wherein bitumen and sulphur are used to bind aggregate and filler, is typically used in the road construction and road paving industry.

Sulphur dusts are potentially explosive so processes for the preparation of sulphur-aggregate composites must be carefully controlled to reduce the risk of sulphur dust explosions. The present inventors have sought to provide a process for the preparation of sulphur- aggregate composites having a reduced risk of sulphur dust explosion.

Summary of the Invention

Accordingly, the present invention provides a process for the preparation of a sulphur-aggregate composite comprising the steps of :

(a) mixing sulphur pellets and filler to provide a mixture of sulphur pellets and filler;

(b) heating aggregate to provide hot aggregate; and

(c) mixing the mixture of sulphur pellets and filler with the hot aggregate. The present inventors have found that by using a mixture of sulphur pellets and filler in the process, they can reduce the risk of producing explosible dusts. Dusts made up of sulphur dust and filler are less explosive that dusts made up of sulphur dust alone.

Detailed Description of the Invention

The term "sulphur-aggregate composite" refers to a composite comprising sulphur, filler and aggregate.

Fillers and aggregate are particulate inorganic

materials. Fillers have an average particle size in the range of from 0.1 μπι to 0.1 mm. Fine aggregate has an average particle size in the range of from 0.1 to 5mm. Coarse aggregate has an average particle size in the range of from 5 to 40mm.

In a preferred embodiment, the present invention provides a process for the preparation of sulphur concrete. Sulphur concrete comprises sulphur, filler, coarse aggregate, and optionally fine aggregate.

In another preferred embodiment, the present invention provides a process for the preparation of sulphur mortar. Sulphur mortar comprises sulphur, filler and fine aggregate, but does not comprise coarse aggregate .

In another preferred embodiment, the present invention provides a process for the preparation of sulphur asphalt. Sulphur asphalt comprises sulphur, bitumen, filler and fine and/or coarse aggregate. In this embodiment, the process of the invention comprises the steps of

(a) mixing sulphur pellets and filler to provide a

mixture of sulphur and filler;

(b) heating aggregate to provide hot aggregate; (c) mixing the mixture of sulphur pellets and filler with the hot aggregate;

(d) heating bitumen to provide hot bitumen; and

(e) mixing the hot bitumen with the mixture of sulphur pellets and filler and the hot aggregate.

The mixing steps (c) and (e) may be combined in one step (i.e. the mixture of sulphur pellets and filler, the hot aggregate and the hot bitumen are mixed together in one step) or may be two separate steps (i.e. the mixture of sulphur pellets and filler may be mixed with the hot aggregate, and then may be subsequently mixed with the hot bitumen, or the hot bitumen may be mixed with the hot aggregate and then may be subsequently mixed with the mixture of sulphur pellets and filler, or the mixture of sulphur pellets and filler may be mixed with the hot bitumen and then may be subsequently mixed with the hot aggregate) .

Sulphur is used the form of sulphur pellets.

Reference herein to pellets is to any type of sulphur material that has been cast from the molten state into some kind of regularly sized particle, for example flakes, slates or sphere-shaped sulphur such as prills, granules, nuggets and pastilles or half pea sized sulphur. The sulphur pellets typically comprise from 50 to 100wt% of sulphur, based upon the weight of the sulphur pellets, preferably from 60wt% and most

preferably from 70wt%; and typically to 99wt%, and preferably to 95wt% or to 100wt%. A more preferred range is from 60 to 100wt%. In one embodiment of the

invention, the sulphur pellets are 100wt% sulphur.

The sulphur pellets may contain carbon black and, optionally, other ingredients, such as amyl acetate and wax. Carbon black may be present in amounts up to 5%wt, based on the pellet, preferably up to 2%wt. Suitably, the content of carbon black in the sulphur pellet is at least 0.25%wt. The content of other ingredients, such as amyl acetate and wax, typically does not exceed an amount of 1.0%wt each. When wax is present, it may be in the form of, for example, slack wax or wax derived from a Fischer-Tropsch process. Examples of suitable waxes for use herein are Sasobit (RTM) , a Fischer-Tropsch derived wax commercially available from Sasol, and SX100 wax, a Fischer-Tropsch wax from Shell Malaysia.

An example of a suitable sulphur pellet for use herein is Thiopave (RTM) pellets commercially available from Shell. Another type of sulphur pellet that is advantageously used in the process of the invention is a pellet prepared by forming solidified sulphur seed particles by a method including intersecting a spray of water droplets through a spray of liquid sulphur droplets to thereby form the solidified sulphur seed particles, and, thereafter, coating the solidified sulphur seed particles with at least one layer of liquid sulphur, wherein each of the at least one layer of liquid sulphur is solidified thereby forming the sulphur particles.

Such pellets and their use is described in WO 2011

149927.

The sulphur pellets provided to the process will inherently contain a certain amount of sulphur dust.

Typically sulphur dust is defined as sulphur particles with longest dimensions smaller than 63μηι. This dust is primarily produced during transportation of the sulphur pellets. The amount of dust will vary from batch to batch, and will even vary within a batch, e.g. there may be more dust at the bottom of a bag of pellets. It is this dust, inherently present within the sulphur pellets, that is primarily responsible for the risk of explosion. Further dust may be produced during the process for the preparation of the sulphur aggregate composite, but it is the dust present within the sulphur pellets that is most problematic. Many suppliers state that their sulphur pellets meet a specification of less than 2wt% sulphur dust (based upon the weight of the supplied sulphur) , but users of the pellets must ensure that their processes are always safe, including on the rare occasions that the sulphur pellets do not meet the specification and the amount of dust is greater than 2wt%.

The filler used in step (a) is typically any available inorganic filler material. Suitably the filler is a material that provides non-explosible dusts. (A test to ensure that a material is non-explosible is outlined in the Examples below.) The filler is suitably chosen from limestone, fly ash, quartz, Portland cement, alumina, titania, carbon black, gypsum, talc, gabbro or mica. The filler is preferably chosen from limestone, fly ash, quartz and Portland cement.

The weight ratio of sulphur pellets to filler in step (a) is suitably between 1:5 and 5:1, preferably between 1:2 and 2:1, most preferably about 1:1. The inventors have found that using a weight ratio of sulphur pellets to filler within this range can provide dusts with reduced explosibility and can also provide sulphur- aggregate composites with desirable properties. If insufficient filler is used, then this may not

significantly reduce the risk of explosion. This is demonstrated further in the examples below, which demonstrate that increasing the relative amount of filler to sulphur dust can provide non-explosible mixtures . If a particular batch of sulphur pellets contains a large proportion of sulphur dust, then it may be desirable to increase the amount of filler in step (a) . The preferred amount of filler will also be determined by the desired properties of the final sulphur-aggregate composite. It is possible to incorporate further filler in the sulphur- aggregate composite, e.g. by heating additional filler with the aggregate in step (b) and mixing this additional filler with the mixture of sulphur pellets and filler and hot aggregate in step (c) .

The amounts of sulphur, filler and aggregate in the sulphur-aggregate composites of the invention can be chosen by the skilled person in view of the proposed application of the sulphur-aggregate composite. The skilled person will seek to ensure that sufficient sulphur is incorporated to bind the filler and aggregate, that sufficient filler and aggregate are incorporated to provide mechanical strength and that the balance of components provides a mixture with suitable workability for the proposed application. In sulphur asphalt, the presence of sulphur can improve the strength and rutting resistance of the paving mixture and it is important to include sufficient sulphur to realise these advantages. However, too much sulphur can decrease the workability of the sulphur asphalt paving mixture . Sulphur mortar preferably comprises from 5 to 40wt% sulphur, from 45 to

90wt% fine aggregate and from 1 to 10wt% filler; more preferably from 5 to 30wt% sulphur, from 55 to 75wt% fine aggregate and from 3 to 8wt% filler. Sulphur concrete preferably comprises from 5 to 40wt% sulphur, from 25 to 50wt% coarse aggregate, from 20 to 40wt% fine aggregate and from 1 to 10wt% filler; more preferably from 5 to 30wt% sulphur, from 30 to 40wt% coarse aggregate, from 25 to 35wt% fine aggregate and from 3 to 8wt% filler. Sulphur asphalt preferably comprises from 0.5 to 8wt% sulphur, from 2 to 10wt% bitumen, from 10 to 70wt% aggregate and from 30 to 90wt% filler; more preferably from 0.8 to 3wt% sulphur, from 3 to 6wt% bitumen, from 20 to 60wt% aggregate and from 40 to 80wt% filler. The weight percentages are based upon the weight of the sulphur-aggregate composite.

When the sulphur-aggregate composite is sulphur concrete or sulphur mortar, the sulphur-aggregate composite preferably comprises modifier. Sulphur

modifiers are compounds that improve the durability of sulphur concrete and sulphur mortar. The amount of modifier is preferably from 0.001 to lwt%, more

preferably from 0.01 to 0.6wt% and most preferably from 0.01 to 0.4wt%, by weight of the sulphur-aggregate composite. The modifier is typically one of the most expensive components in the concrete, so it is desirable to limit the amount of modifier. Suitable modifiers include olefinic modifiers such as 5 ethylidene-2- norbornene (ENB) , 5 vinyl-2-norbornene (VNB) ,

dicyclopentadiene, limonene or styrene. Alternatively, the modifier may be an organosilane or an organotitanate. Particularly preferred organosilanes are

bis ( 3-triethoxysilylpropyl ) tetrasulphide (TESPT) and bis ( 3-triethoxysilylpropyl ) disulphide . In one embodiment of the invention, the modifier is present in the sulphur pellets. In an alternative embodiment of the invention, the modified is added separately to the process, e.g. during step (c) .

When the sulphur-aggregate composite is sulphur asphalt, the sulphur-aggregate composite comprises bitumen. The bitumen can be selected from a wide range of bituminous compounds . The bitumen that can be employed may be straight run bitumen, thermally cracked residue or precipitation bitumen, e.g. from propane.

Although not necessary, the bitumen may also have been subjected to blowing. The blowing may be carried out by treating the bitumen with an oxygen-containing gas, such as air, oxygen-enriched air, pure oxygen or any other gas that comprises molecular oxygen and an inert gas, such as carbon dioxide or nitrogen. The blowing operation may be conducted at temperatures of 175 to 400°C, preferably from 200 to 350°C. Alternatively, the blowing treatment may be conducted by means of a catalytic process. The bitumen is preferably a paving grade bitumen suitable for road application having a penetration of, for example, from 9 to lOOOdmm, more preferably of from 15 to 450dmm (tested at 25°C according to EN 1426: 1999) and a softening point of from 25 to 100°C, more preferably of from 25 to 60°C (tested according to EN 1427: 1999).

In step (a) of the process of the invention, sulphur pellets are mixed with filler to provide a mixture of sulphur pellets and filler. The sulphur pellets may be mixed with the filler simply by pouring the filler into a weigh bin (or hopper) followed by the sulphur pellets. Then both components are provided simultaneously to step (c) . Alternatively, the sulphur pellets may be mixed with the filler by providing both the sulphur pellets and the filler to a conveyor belt, which then transports these components to step (c) . Step (c) may be carried out at ambient temperature, although it is possible that the filler may be at a higher temperature if it has just been dried.

In step (b) of the process of the invention, aggregate is heated to provide hot aggregate. The aggregate is preferably heated to a temperature of from 125 to 190°C, more preferably from 125 to 150°C. If the temperature is below 125°C, it is possible that the aggregate will not melt the sulphur pellets when they are combined in step (c) . If the temperature too high, then this increases the potential for producing undesirable fumes of hydrogen sulphide and sulphur dioxide. Step (b) suitably takes place in an aggregate dryer drum.

In step (c) of the process of the invention, the mixture of sulphur pellets and filler is mixed with the hot aggregate. This step is suitably carried out at a temperature at which the sulphur is molten, i.e.

typically above 120°C, preferably in the range of from 120 to 150°C, more preferably in the range of from 125 to 140°C. Step (c) suitably takes place in a pug mill.

Alternatively, the process of the invention can be carried out in a drum mix plant and step (c) can take place in a drum or in an after-coater .

When the sulphur-aggregate composite is sulphur asphalt, the process of the invention provides further steps of

(d) heating bitumen to provide hot bitumen; and

(e) mixing the hot bitumen with the mixture of sulphur pellets and filler and the hot aggregate.

In step (d) , bitumen is preferably heated at a temperature of from 60°C to 200°C, preferably from 80 to

150°C, more preferably from 100°C to 145°C, and even more preferably from 125°C to 145°C. Step (d) suitably takes place in a bitumen tank.

The mixing steps (c) and (e) may be combined in one step (i.e. the mixture of sulphur pellets and filler, the hot aggregate and the hot bitumen are mixed together in one step) or may be two separate steps (i.e. the mixture of sulphur pellets and filler may be mixed with the hot aggregate, and then may be subsequently mixed with the hot bitumen, or the hot bitumen may be mixed with the hot aggregate and then may be subsequently mixed with the mixture of sulphur pellets and filler, or the mixture of sulphur pellets and filler may be mixed with the hot bitumen and then may be subsequently mixed with the hot aggregate) . Preferably the hot bitumen is mixed with the hot aggregate, and then there is subsequent mixing with the mixture of sulphur pellets and filler as this may provide reduced fuming.

When the sulphur-aggregate composite is sulphur mortar or sulphur concrete, the process of the invention suitably provides further steps of

(f) pouring the product of step (c) into a mould; and (g) cooling.

After cooling, the sulphur-aggregate composite can be demoulded .

Other components may be incorporated into the sulphur-aggregate composite. For example, for sulphur asphalt it may be desirable to include polymers in to the composition. These polymers may be pre-blended with the bitumen or may be added in one of the mixing steps, i.e. in step (c) or step (e) . One preferred type of polymer is a copolymer comprising one or more vinyl aromatic compounds and one or more conjugated dienes, such as styrene-butadiene-styrene . This is desirably pre-blended with the bitumen. Another preferred type of polymer is a copolymer formed from monomers including ethylene and glycidyl methacrylate or glycidyl acrylate. For sulphur concrete or sulphur mortar it may be desirable to incorporate pigments and it is preferable to incorporate modifiers as described above.

The invention further provides a process for preparing an asphalt pavement, wherein sulphur asphalt is prepared by a process according to the invention, and further comprising steps of :

(h) spreading the product of step (e) into a layer; and (i) compacting the layer.

Examples

The invention is further illustrated by means of the following non-limiting examples.

Tests were carried out to determine the limiting sulphur concentration, i.e. the maximum quantity (%wt.) of sulphur dust mixed with the filler dust that is non- explosible across a wide range of dust loadings. The tests were carried out using a 20-litre sphere and conformed to the principles of BS EN 14034-1: and BS EN

14034-2 Appendix C.

The standard test (Test Protocol 1) involved charging a known quantity of dust into the charge port of the sphere, which is then pressurised to 20 barg. The sphere was evacuated to -600 mbarg and the dust was injected into it from the charge port via a fast acting valve. The combination of vacuum in the sphere and pressure in the charge port resulted in net pressure of 0 barg when the dust was dispersed in the sphere.

Dispersion of the dust cloud was achieved by the use of a rebound nozzle. An attempt was made to ignite the dust cloud using Sobbe chemical igniters and two Kistler piezo resistive pressure transducers detected the resulting overpressure. The results were recorded and analysed within the proprietary Kiihner software. The ignition energies were achieved by attaching two Sobbe chemical igniters in parallel, 2 x lkJ and 2 x 250J for the 2kJ and 500J ignition energies respectively. It is known that the explosion severity of a dust cloud is markedly increased in the presence of a

flammable gas or vapour even at low concentrations (i.e. less than the Lower Explosive Limit of the gas alone) . These mixtures of dust and gas/ vapour are called hybrid admixtures. The enhanced explosivity of the dust hybrid admixtures could also lead to otherwise non-explosible dusts becoming explosible.

Tests involving hybrid admixtures (Test Protocol 2) were all carried out in a nominal 10% Lower Explosive

Limit (LEL) ethanol atmosphere. This was created using the partial pressure method as follows:

i. The sphere was evacuated to -900 mbarg.

ii. After evacuation the ethanol was introduced to the sphere using a precision glass HPLC syringe via a

PTFE lined septum on the front of the vessel.

Vaporisation of the ethanol was facilitated by the reduced pressure within the vessel and corresponded to a rise in the pressure within the vessel.

iii. The rise in pressure was measured using a digital precision pressure gauge accurate to 0.1 mbar (a pressure rise of 1 mbar is equivalent to 0.1% vol) thus allowing accurate measurement of the quantity of ethanol introduced.

iv. Following the introduction of the ethanol the

pressure within the sphere was adjusted to -600 mbarg by addition of air and the standard test procedure for the introduction of the dust was then followed .

The LEL of ethanol was taken to be 3.3% vol. and the nominal volume of ethanol vapour required for the tests was 10% of the LEL, 0.33% vol. This corresponded to a pressure rise of 3.3 mbar when injecting the ethanol into the sphere. The ethanol was injected into the sphere using a precision HPLC syringe. A nominal volume of 0.08ml ethanol was required to generate the required vapour pressure. However, the suction from the sphere pulled the ethanol from the syringe, making control of the actual injected volume difficult. For the range of tests the volume injected into the sphere varied giving pressure rises between 2.8 mbar and 4.0 mbar, which equate to 0.28% vol. (8.5% of LEL) and 0.40% vol. (12.1% of LEL) .

The evaluation of whether a dust is explosible or not is carried out automatically by the Kuhner software. Any value of P _m [bar] ≥ 0.2 bar is classed as explosible. Any value of P _m [bar] < 0.2 bar is classed as non- explosible. P _m is the corrected explosion overpressure.

Mixtures of powdered elemental sulphur and filler materials were prepared. The filler materials were limestone, flyash, quartz and Portland cement.

Table 1 shows the composition of the mixtures, the Test Protocol used, the Ignition Energy and whether the mixture was explosible or non-explosible .

Table 1

Composition (wt%) Test Ignition Explosibility

Protocol Energy

Limestone Flyash Quartz Cement Sulphur

[kJ]

93 - - - 7 1 2 Non-explosible

93 - - - 7 2 2 Explosible

94 - - - 6 2 2 Non-explosible

90 - - - 10 2 0.5 Non-explosible

- 90 - - 10 2 2 Explosible

- 91 - - 9 2 2 Explosible

- 92 - - 8 2 2 Explosible

- 93 - - 7 2 2 Non-explosible

- 94 - - 6 2 2 Non-explosible

- - 90 - 10 2 2 Explosible

- - 92 - 8 2 2 Explosible

- - 93 - 7 2 2 Explosible

- - 94 - 6 2 2 Non-explosible

- - - 90 10 1 2 Explosible

- - - 93 7 1 2 Explosible

- - - 94 6 1 2 Non-explosible

The presence of a 10% LEL ethanol atmosphere (as used in Test Protocol 2) has an effect on the

explosibility of limestone and sulphur mixtures. A mixture of limestone with 7% sulphur is not explosible under Test Protocol 1, but is explosible in the hybrid atmosphere. The limiting sulphur concentration is reduced to 6% sulphur by wt . in the hybrid atmosphere.

The ignition energy has a significant effect on the explosibility of the limestone and sulphur mixtures .

Mixtures of limestone containing 10% ES produced an average explosion overpressure of 1.1 bar when tested using a 2kJ ignition energy. Whereas, mixtures of limestone with 10% sulphur tested using a 500J ignition energy produced far lower explosion overpressures of ≤ 0.1 bar even in the presence of a 10% LEL ethanol hybrid atmosphere .

The limiting sulphur concentration of mixtures of sulphur with flyash in a 10% LEL ethanol hybrid

atmosphere is 7% sulphur by wt .

The limiting sulphur concentration of mixtures of sulphur with quartz in a 10% LEL ethanol hybrid

atmosphere is 6% sulphur by wt .

The limiting sulphur concentration of mixtures of sulphur with Portland cement is 6% sulphur by wt .

These results show that mixtures of sulphur dust with filler (limestone, fly ash, quartz or Portland cement) may be non-explosible, provided there is

sufficient filler. In the present invention, sulphur pellets are provided in combination with filler and the resulting dusts will contain both sulphur dust and filler. These dusts will have a lower risk of explosion when compared to dusts containing sulphur alone. These dusts may not always be non-explosible (depending on the relative amounts of sulphur dust and filler, and

depending on the variability in the amount of sulphur dust) so in many cases other risk management features will be necessary to mitigate the risk of explosion.

However, the present invention has provided the inventors with an additional means of reducing the risk of sulphur dust explosion.