The present invention concerns the technical field of drying granular materials.

The subject matter of the present invention is more particularly a unit for drying granular materials of any kind and a method of drying such granular materials.

Such granular materials may for example comprise sand used for the fabrication of hot-mix bituminous surfacing materials, concrete or dry mortar. Such granular materials may equally comprise mineral additives such as natural pozzola- na or certain kinds of fly ash for use in cementmaking .

Finally, such granular materials may equally comprise seeds for agriculture-food use.

In all these industrial applications, it is important to obtain a granular material that is as dry as possible.

Desiccation of the seeds in the agriculture-food industry improves the shelf-life of the seeds.

The drying of the mineral additives in the cement industry makes it possible to limit the hydration of. the hydraulic binder on adding them to the hydraulic binder.

The drying of the sand in the fabrication of hot-mix bituminous surfacing materials is particularly important because it makes it possible to ensure that the sand is coat ^¬ ed with the bituminous binder. In fact, as is known, the fabrication of bituminous surfacing materials consists in heating and drying granular materials, for example sand, and then mixing them with filler and a hydrocarbon-based binder, for example bitumen.

These two steps may be carried out. in a power station drum mixer in which the granular materials introduced into the drum are dried and heated after which the dried and heated granular materials are coated with bitumen introduced into the drum in liquid form at a temperature between 150 °C and 180°C.

The granular materials used for the fabrication of the bituminous surfacing materials include sand (the particles of which generally have a maximum size (Dmax) less than or equal to 4 mm) , and gravel (the particles of which generally have a minimum size (Dmin) greater than 2 mm and preferably have a Dmax less than or equal to 20 mm or more) .

These granular materials include, siliceous or siliceous- calcareous materials.

The granular materials are generally heated to a temperature of the same order as that of the liquid bitumen in order to encourage the adhesion of the bitumen around the granular materials.

This heating also leads to the drying of the granular materials .

This drying is necessary in order to eliminate water trapped in the pores of the granular materials. Indeed, liquid bitumen is hydrophobic and the water trapped in the pores of the granular material can prevent the liquid bitumen from adhering correctly to the granular materials .

This defective adhesion can lead to separation of granular materials in a bituminous surface coating and thereby accelerate its deterioration.

It is therefore essential in the method of fabricating hot- mix bituminous surfacing materials to dry the granular materials in order to eliminate the water present.

Note that a standard sand may have a moisture content of approximately 5 to 15% and that a drum mixer is generally fed with fossil fuels such as fuel oil or gas.

Consequently, given that the quantity of energy and therefore of fossil fuel necessary for drying the granular materials is directly linked to the quantity of water to be eliminated, it appears advantageous to introduce into the drum mixer granular materials having a limited moisture content, notably less than 5%.

However, it appears that drying the granular materials on the upstream side of the drum mixer is viable only if the cost of the energy used for this upstream drying added to the cost of production of the surfacing material from the previously dried material is less than the overall cost of using a dryer drum without drying the materials beforehand.

An object of the present invention is to remove some or all of the drawbacks referred to above, notably by providing a cost-effective solution offering high performance in the drying of granular materials on the upstream side of their introduction into a drum mixer. The drying mechanisms operating in the method are absorption of solar radiation, convection of air and, possibly, draining by gravity, i.e. principles that do not consume energy from fossil fuels.

To this end, the present invention consists in a unit for drying granular materials including:

- a support structure,

- a drying surface arranged on the support structure and intended to receive a layer of granular materials to be dried,

and wherein the drying surface covers all or part of the surface of a turning cone in which the value of the angle of the generatrices relative to the horizontal is between 25° and 40° so as to ensure the stability of the layer of granular materials on the drying surface.

This angle value is close to the angle of the natural slope of the granular material and makes it possible to create a layer of material of uniform thickness by simple gravity flow from the top of the cone.

The turning cone shape conferred by the support structure on the drying surface makes it possible to expose the meridian areas of the layer of granular materials to solar radiation and to the prevailing wind successively during the day.

The combination of these features makes it possible to re ^¬ duce the moisture content of the layer of granular materials to a value of a few percent. This residual water can be eliminated subsequently in the drum mixer whilst limiting its energy expenditure.

In accordance with one aspect of the invention, the drying surface is exposed to solar radiation.

This arrangement enables the granular materials to absorb solar radiation.

Consequently, the drying surface is not disposed in a closed enclosure sheltered from solar radiation.

In accordance with one aspect of the invention, the support structure is connected to a support resting on the ground, notably to a mast.

In accordance with one aspect of the invention, the drying unit includes mechanical, means for rotating the support structure relative to the ground around the support.

The drying surface then constitutes .a complete turning cone .

This makes it possible to ensure that all the meridian sectors forming the layer of granular materials are oriented transversely to the sun's rays successively once or more times in each day.

The moisture content is therefore reduced to a value of the order of a few percent throughout the volume of the exposed granular materials. In accordance with one aspect of the invention, the support structure is immobilized against rotation relative to the ground, the drying surface being oriented toward the south.

The drying surface may therefore comprise only a portion of a cone, the portion exposed to the north being unable to benefit from solar radiation. .

Moreover, this arrangement is simpler to produce and reduces the cost of the installation.

In accordance with one aspect of the invention, the drying unit includes means for evacuation of the layer of granular materials intended to be received by the drying surface.

This makes it possible to recover the dried granular materials by simple gravity flow in order to store them and to free up space on the drying surface so as to be able to receive a new layer of granular materials to be dried.

In accordance with one aspect of the invention, the evacuation means are adapted to move the support structure between a first position in which the value of the angle of the generatrices of the cone relative to the horizontal is a minimum so as to retain the layer of granular materials on the drying surface and a second position in which the value of the angle of the generatrices of the cone relative to the horizontal is a maximum so as to evacuate the layer of granular materials disposed on the drying surface.

This makes it possible to evacuate the layer of granular materials whilst limiting the expenditure on energy used for this purpose. In accordance with one aspect of the invention, the drying surface includes an overlap area the area of which is adjustable .

This makes it possible to adjust the value of the angle of the generatrices of the cone relative to the horizontal as a function of the nature of the layer of granular materials to be dried.

In accordance with one aspect of the invention, the drying unit includes a transparent wall forming a double roof disposed over the drying surface. This wall may be flexible or rigid.

This makes it possible to generate a greenhouse effect and/or a chimney effect and to protect the exposed material from rain.

In accordance with one aspect of the invention, the support structure or the drying surface includes wedges arranged in the lower part of the drying surface to retain all or part of the layer of granular materials on the drying surface.

In accordance with one aspect of the invention, the drying surface supporting the layer of granular materials is per ^¬ meable to water, the drying unit further including water capture means disposed under the drying surface.

This makes it possible to isolate the recovered water and the dried granular materials evacuated after drying them.

In accordance with one aspect of the invention, the drying unit includes a hopper disposed under the drying surface. This makes it possible to recover the dried granular materials.

The present invention also consists in a method of drying granular materials including a step consisting in tipping a predefined quantity of granular materials onto a drying surface arranged on a support structure, the drying surface covering all or part of the surface of a turning cone in which the value of the angle of the generatrices relative to the horizontal is between 25° and 40°.

In accordance with one embodiment of the method, the method further includes a step consisting in causing the support structure to turn relative to the ground.

In accordance with one embodiment of the method, the method includes a step of evacuating the layer of granular materials intended to be received by the drying surface in which the support structure is caused to move between a first position in which the value of the angle of the generatrices of the cone relative to the horizontal is a minimum so as to retain the layer of granular materials on the drying surface and a second position in which the value of the angle of the generatrices of the cone relative to the horizontal is a maximum so as to evacuate the layer of granular materials disposed on the drying surface.

In any event, the invention will be clearly understood with the aid of the following description referring to the appended diagrammatic drawings representing a drying unit in accordance with the invention by way of nonlimiting example . Figure 1 is a diagrammatic general view of one embodiment of a drying unit in accordance with the invention.

Figure 2 shows part of a preferred embodiment of a drying unit in accordance with the invention.

Figure 3 shows the movement of the support structure of the drying unit shown in Figure 2.

Figure 4 shows another embodiment of the drying unit.

As shown in these various figures, the drying unit 1 in accordance with the invention includes a support structure 10 and a drying surface 20 disposed on the support structure 10 and intended to receive a layer of granular materials G to be dried.

In the examples shown, the support structure 10 rests on a support connected to the ground S, notably a mast extending substantially vertically from the ground S.

Of course, the present invention is in no way limited to the presence of this support or to a particular type of support .

In fact, for some embodiments of the invention, this support 2 could equally be provided by a wall or by a naturally sloping terrain.

In the example shown in Figure 2, the mast 2 is fixed and comprises three parts 2a, 2b, 2c each having a hollow cylindrical general shape.

The mast 2 therefore has an axis A of symmetry. The first part 2a is fixed to the ground and has a first determined diameter.

The second part 2b is disposed immediately above the first part 2a and has a second determined diameter less than the first determined diameter of the first part 2a.

The third part 2c has a third determined diameter less than the second determined diameter of ^' the second part 2b because it is not greatly loaded mechanically.

The total height of the mast 2 defined by these three parts 2a, 2b, 2c can be as much as 25 m.

A hopper 3 is connected to the top of the mast 2 in the vicinity of the free end of the third part 2c.

Similarly, a distributor cone 6 is connected to the third part 2c of the mast 2 between the hopper 3 and the support structure 10.

The granular materials G to be dried by the drying unit 1 are therefore first tipped into the hopper 3 and then distributed uniformly over the drying surface 20 using the distributor cone 6.

As shown in Figure 2, the mast 2 may be stabilized by stays 4, preferably three stays regularly distributed around the mast 2 and connecting the hopper 3 to the ground S.

In the example shown, the support structure 10 extends around all of the circumference of the mast 2. Of course, this support structure 10 could extend around only one part of the circumference of the mast 2.

In the example shown, this support structure 10 comprises a series of upper ribs 11 and a series of lower ribs 12 regularly distributed around the mast 2.

Each of the upper ribs 11 is connected to a lower rib 12 by an articulation 13.

The upper ribs 11 are connected to the mast 2 by pivot connections and an annular ring 14 mobile in rotation about the top of the second part 2b of the mast 2. The lower ribs 12 are connected to the mast 2 by pivot connections and an annular ring 15.

The annular ring 15 is mobile in rotation about the top of the first part 2a of the mast 2 and mobile in translation along the axis A of symmetry of the mast 2 so as to be able to slide around a portion of the first part 2a of the mast 2.

Also, the upper ribs 11 and the lower ribs 12 each include reinforcing elements 16 adapted to limit their flexibility.

The drying surface 20, for example a canvas, is disposed on the upper ribs 11 of the support structure 10. This drying surface 20 is exposed to solar radiation so that when granular materials G are disposed on the drying surface 20 then those granular materials G absorb solar radiation . Also, the drying surface 20 is placed in an open space so as also to benefit from the effects of convection of the surrounding air and above all the wind.

The drying surface 20 may be made from a technical textile material, for example polyester coated with a thermoplastic polymer such as polyvinyl chloride.

By virtue of such an arrangement, the drying surface 20 covers all or part of the surface of a turning cone in which the value of the angle of the generatrices relative to the horizontal is between 25° and 40°. The drying surface 20 is sized according to the requirements of use: the length of the generatrices is therefore defined accordingly.

In the proposed example, the drying surface 20 extends around all of the circumference of the mast 2 and therefore covers all of the surface of a turning cone.

Also, to limit the formation of cups on portions of the drying surface 20 caused notably by the action of the weight of the granular materials G to be dried, the support structure 10 may also include cables (not shown) connecting each of the upper ribs 11 to the two upper ribs 11 adjacent the upper rib 11 concerned.

These cables may also be distributed over a plurality of different heights of the upper ribs 11 so as further to limit the formation of these cups.

Also, the support structure 10 and/or the drying surface 20 may have wedges (not shown) arranged to retain all or part of the layer of granular materials G on the drying surface 20 so as to prevent it sliding down the slope of the drying surface 20.

In the proposed example, the drying unit 1 includes mechanical means 5 for rotating the support structure 10 relative to the ground around the mast 2.

Such means comprise, for example, a motor disposed at the top of the first part 2a of the mast 2.

In the example shown, a drying unit 1 in accordance with the invention also includes water capture means 50 and means 30 for evacuating the layer of granular materials G from the drying surface 20.

The capture means 50 are disposed under the drying surface 20 and their function is to collect percolating water able to flow by gravity through the drying surface 20, assuming that the latter were permeable.

These capture means 50 take the general form of a funnel and may be made from any impermeable materials resistant to water, notably resistant to oxidation.

The evacuation means 30 may be manoeuvred by mechanical, hydraulic, electrical or other means.

In the example shown, the evacuation means 30 comprise the various articulated connections specific to the support structure 10 and the various connections between the support structure 10 and the mast 2.

In particular, in addition to being able to turn about a portion of the first part 2a of the mast 2, the mobile an- nular ring 15 is adapted to slide along a wider portion of the first part 2a of the mast 2.

This movement of the annular ring 15 in translation may be transmitted by the same mechanical rotation means 5, such as a motor, coupled to a clutch device.

In fact, as shown in Figure 3 in particular, the various pivot connections between the upper ribs 11 and the lower ribs 12, between the upper ribs 11 and the annular ring 14 fixed against movement in translation along the axis A, and between the lower ribs 12 and the annular ring 15 mobile in translation along the axis A make it possible to render the support structure 10 mobile in an axial direction.

The support structure 10 is therefore mobile between a first position Pi in which the value of the angle a of the generatrices of the cone relative to the horizontal is a minimum so as to make it possible to retain the granular materials G on the drying surface 20 and a second position P2 in which the value of the angle a of the generatrices of the current relative to the horizontal is a maximum so as to allow the granular materials G to slide off the drying surface 20.

The support structure 10 may be moved from its first position Pi to its second position P2 by independent driving means, by the same driving means 5 and/or by the simple effect of gravity, and returned to its first position PI by return means, for example one or more springs.

The drying surface 20 is preferably relatively taut whe the support structure 10 is in the first position Pi. The range of values for the angle a of the generatrices of the cone relative to the horizontal corresponds to an adjustment of the inclination of the drying surface 20 as a function · of the slope adopted by the natural slope of the type of granular materials G intended to be tipped onto the drying surface 20.

The drying surface 20, which retains the possibility of sliding freely on the support structure 10, may also include an overlap area R the area of which is adjustable.

This overlap area R is seen in the embodiment shown in Figure 4. mU -! _ 1 „„„„ Γ) 4 4- ,-, .ρ^,^,ν, ^ -P ^^^ -^ ^ -p 4- -, ^ c. i■, v- _ face of the turning cone covered by the drying surface 20.

If the area of this overlap area R increases, then the rest of the surface of the turning cone decreases and the drying surface 20 exerts a force on the support structure 10 that tends to increase the value of the angle a.

If the area of this overlap area R decreases then the rest of the surface of the turning cone increases and the force exerted by the drying surface 20 on the support structure 10 decreases, which tends to reduce the value of the angle a .

The variation of the area of this overlap area R therefore makes it possible to adjust further the value of the angle a of the generatrices of the cone relative to the horizontal as a function of the nature of the layer of granular materials to be dried. Indeed, the natural slope of a given type of granular material G may be different from that of another type of granular material G. The drying unit 1 can therefore be adapted to a particular ^' type of granular material by refining the adjustment of the value of the angle a in the first position PI of the support structure 10. For example, the area of this overlap area R may be varied by the application of two opposite tangential forces to two ends El, E2 sufficiently close to the contour C of the drying surface 20 to enable the two portions of the drying surface 20 forming the overlap area R to slide one on the other.

This contour C may also be a spiral in order to facilitate this sliding. In its first position PI, the apparent diameter measured at the base of the turning cone shape formed by the drying surface 20 can reach 28 m.

In the second position P2 of the support structure 10, the angle a may have a value between 50° and 75°, this value being limited by the overall size of the upper ribs 11.

It may therefore be necessary in the second position P2 to define a maximum angle _. a of the cone exceeding which would lead to contact of the support structure 10 with a stop so as to prevent contact of the upper ribs 11 with the mast 2 on going from the first position Pi to the second position P2. Other evacuation means 30 may be envisaged instead of or. in addition to a mobile and articulated support structure 10.

For example, these evacuation means 30 may include means for vibrating the support structure 10.

Moreover, in the proposed example the means 30 for evacuating the granular materials G include a first hopper 31 and a second hopper 32.

The first hopper 31 is disposed coaxially with the mast 2 and below the support structure 10.

The first hopper 31 occupies an apparent area greater than the apparent area of the drying surface 20 when the support structure 10 is in its first position Pi and for a projection onto a plane transverse to the axis A of symmetry of the mast 2.

In a preferred embodiment, the first hopper 31 is made from a flexible fabric and is connected to the lower ribs 12.

This first hopper 31 is made from the same material as the drying surface 20, for example.

The first hopper 10 is therefore mobile and synchronized with the movements of the support structure 10.

The first hopper 31 includes a passage 32 to which converge the granular materials G tipped into the first hopper 31.

This passage 32 therefore produces a downward movement along the first part 2a of the mast 2 when the mobile structure 10 moves from its first position PI to its second position P2 and an upward movement along the first part 2a of the mast 2 when the mobile structure 10 moves from its second position P2 to its first position PI.

Similarly, the apparent diameter of the first hopper 31 decreases when the mobile structure 10 moves from its first position PI to its second position P2 and the apparent diameter of the first hopper 31 increases when the mobile structure 10 moves from its second position P2 to its first position PI.

In another embodiment (not shown) , the first hopper 31 can be _. fixed and connected to the first part 2a of the mast 2 without being connected to the mobile support structure 10.

In such an embodiment, this first hopper 31 must however be connected to the first part 2a of the mast 2 so that its position does not impede the movement of the support structure 10.

The passage 32 discharges above a second hopper 33 also forming part of the means 30 for evacuating the granular materials G.

This second hopper 33 is rigid and connected to the first part 2a of the mast 2.

The second hopper 33 includes a passage 34 toward which converge the granular materials G tipped by the first hopper 31 into the second hopper 33.

This passage 34 discharges above conveyor means 35, for example a conveyor belt or a truck intended to convey the dried granular materials G to a place of storage. Finally, in one particular embodiment of the invention, the drying unit 1 includes a transparent and impermeable wall 40 disposed facing it in whole or in part and at a deter- mined distance above the drying surface 20.

This transparent wall 40 may be made of any material enabling more efficient drying of the granular materials G by increasing the temperature or by improving the circulation of air between the wall 40 and the drying surface 20. The circulation of the air is preferably natural.

Moreover, this wall 40 also has a chimney 41 disposed in whole or in part around the third part 2c of the mast 2.

This wall 40 may notably be a canvas made from a translucent technical textile material, for example polyethylene.

The function of this wall 40 is explained by the Figure 1 theoretical diagram explained in detail hereinafter.

A determined quantity of wet granular materials G to be dried is introduced into the hopper 3. These granular materials G then drop onto the distributor cone 6 so as to render the granular materials G uniform over the drying surface 20 in a substantially uniform layer of granular materials G. A method in accordance with the invention of drying granular materials G therefore includes a step consisting in tipping a predefined quantity of granular materials G onto the drying surface 20 disposed on the support structure 10, the drying surface 20 covering all or part of the surface of a turning cone in which the value of the angle a of the generatrices relative to the horizontal is between 25° and 40° . The layer of granular materials G typically has a thickness between 2 and 10 cm.

During the step of tipping the granular materials G onto the drying surface 20, the support structure 10 is in its first position PI.

The stability of the layer of granular materials G on the drying surface 20 is assured firstly by the value of the angle a and may be reinforced by the wedges disposed on the drying surface 20 and/or on the support structure 10,

Some of the water contained in very wet granular materials G percolates through the permeable drying surface 20. This water is recovered by the water capture means 50.

More of the water evaporates because of the action of solar radiation . This solar radiation is distributed over all of the layer of granular materials G to be dried when the support structure 10 is rotated around the mast 2 in the first position Pi of the support structure 10. The rotation speed is relatively low and of the order of one revolution per hour.

Consequently, a method in accordance with the invention of drying granular materials G also includes a step consisting in causing the support structure 10 to turn about the mast 2.

Moreover, the presence of the transparent and impermeable wall 40 generates a greenhouse effect that on the one hand accentuates the evaporation of the water contained in the layer of granular materials G and on the other hand accentuates the natural convection movement of the air circulating between the wall 40 and the layer of granular materials G.

As shown by arrows in Figure 1, air is therefore engulfed laterally between the wall 40 and the layer of granular materials G to be dried and then rises by convection up to the chimne ^w 41 in order to be evacuated there .

During this circulation, the air in contact with the layer of granular materials G entrains the evaporated water toward the chimney 41, which limits the phenomenon of condensation of the water on the wall 40.

Once the layer of granular materials G has been deemed to be dried sufficiently, the evacuation means 30 are actuated .

The support structure 10 is therefore moved to its second position P2.

In particular, the annular ring 50 slides downwards along the first part 2a of the mast 2.

This downward movement entrains the lower ribs 12 and the upper ribs 11 and leads to accentuation of the slope of the turning cone shape of the drying surface 20. This slope does not allow the granular materials G to retain their equilibrium on the drying surface 20. The granular materials G are therefore tipped successively into the first hopper 31 and then into the second hopper 33 in order to be conveyed by conveyor means toward a place of storage or use. This drying unit 1 therefore enables efficient and ecological drying of the granular materials G.

Although the invention has been described in connection with particular embodiments, it is obvious that it is in no way limited thereto and that it encompasses all technical equivalents of the means described and combinations thereof.