PROCESS FOR PRODUCING A CARBON PASTE FOR THE MANUFACTURE OF HIGH-DENSITY ELECTRODES

Abstract

The disclosed method proposes, for the mixing of a solid and dry granular mixture and a liquid bituminous binder, the solid and dry granular mixture being a mixture of two fractions of constituents, one of which is a coarse fraction including aggregates, the other fraction a fraction of submillimetre fines, that the mixing includes at least two consecutive mixing steps: an aggregate-preimpregnation step, consisting of mixing the coarse fraction of the granular mixture with a part of the bituminous binder, in a quantity at least sufficient for coating the aggregates and at least partially filling the open pore spaces of the aggregates; and a covering mixing step, which consists of mixing the preimpregnated coarse fraction of bituminous binder and originating from the first mixing step, with the balance of the bituminous binder, required for the manufacture of the electrodes, and with the fraction of fines of the granular mixture.

Claims

1-7. (canceled)

8. Method for preparing a carbonaceous paste (5) for the manufacture of high-density electrodes (6), by mixing a solid and dry granular mixture (1a, 1b) and a liquid bituminous binder (3a, 3b), in a quantity required for the manufacture of the electrodes (6), the solid and dry granular mixture being a mixture of two fractions of constituents, one of which is a so-called coarse fraction (1a; 2a), including aggregates, and the other fraction is a fraction of submillimetre fines (1b, 2b), comprising at least two consecutive mixing steps, a first mixing step (102a), of which a so-called aggregate-preimpregnation step consists of mixing said coarse fraction (2a) of the granular mixture with a part (3a) of said bituminous binder, in a quantity at least sufficient for coating the aggregates and at least partially filling the open pore spaces of said aggregates, and is followed by a second mixing step (102b), called a covering step, which consists of mixing said preimpregnated coarse fraction (4a) of bituminous binder and originating from the first mixing step (102a), with the balance (3b) of said bituminous binder, required for the manufacture of the electrodes (6), and with said fraction of fines (2b) of said granular mixture, wherein the bituminous binder (3a) used in the first mixing step (102a) is of a grade different from that of the bituminous binder (3b) used in the second mixing step (102b), and wherein, during the first mixing step (102a), a bituminous binder (3a) is used, with a lower softening point than the softening point of the bituminous binder (3b) used during the second mixing step (102b).

9. Method according to claim 8, wherein, in the second mixing step (102b), the introduction of the balance (3b) of the bituminous binder occurs simultaneously with or after the introduction of the preimpregnated coarse fraction (4a) originating from the first mixing step (102a), and wherein the introduction of the fraction of fines (2b) occurs simultaneously with or after the introduction of the balance (3b) of the bituminous binder.

10. Method according to claim 8, wherein the fines (2b) of the dry granular mixture are premixed (105) with all or part of said balance (3b) of the bituminous binder, then the premixture (4b) thus obtained is introduced into the second mixing step (102b).

11. Method according to claim 8, wherein the grades of the two bituminous binders (3a, 3b) with different softening points are obtained by fractional distillation operations then fluxing from a single grade of bituminous binder provided for preparing the paste (5).

12. Installation for the implementation of the method according to claim 8, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and in that it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

13. Installation according to claim 12, further comprising, in addition, a device (101a, 101b) for the separate preheating of the fraction of aggregates (1a) and the fraction of fines (1b) of the dry granular mixture, as well as separate items of handling equipment, one of which is capable of ensuring the supply of the fraction of aggregates in the first mixer step (102a) and the other the supply of the fraction of fines in the second mixer step (102b).

14. Installation according to claim 12, further comprising, in addition, at least one fractional distillation unit, capable of producing two bituminous binders (3a, 3b) of different grades from the same bituminous binder of intermediate grade.

15. Method according to claim 9, wherein the fines (2b) of the dry granular mixture are premixed (105) with all or part of said balance (3b) of the bituminous binder, then the premixture (4b) thus obtained is introduced into the second mixing step (102b).

16. Method according to claim 9, wherein the grades of the two bituminous binders (3a, 3b) with different softening points are obtained by fractional distillation operations then fluxing from a single grade of bituminous binder provided for preparing the paste (5).

17. Method according to claim 10, wherein the grades of the two bituminous binders (3a, 3b) with different softening points are obtained by fractional distillation operations then fluxing from a single grade of bituminous binder provided for preparing the paste (5).

18. Installation for the implementation of the method according to claim 9, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and wherein it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

19. Installation for the implementation of the method according to claim 10, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and wherein it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

20. Installation for the implementation of the method according to claim 11, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and wherein it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

21. Installation according to claim 13, further comprising, in addition, at least one fractional distillation unit, capable of producing two bituminous binders (3a, 3b) of different grades from the same bituminous binder of intermediate grade.

22. Method according to claim 15, wherein the grades of the two bituminous binders (3a, 3b) with different softening points are obtained by fractional distillation operations then fluxing from a single grade of bituminous binder provided for preparing the paste (5).

23. Installation for the implementation of the method according to claim 15, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and wherein it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

24. Installation for the implementation of the method according to claim 16, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and wherein it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

25. Installation for the implementation of the method according to claim 17, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and wherein it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

26. Installation for the implementation of the method according to claim 22, further comprising two mixer steps, in continuous or discontinuous mode, the first mixer step of which (102a), intended for the implementation of the first step of the method, has at least one inlet capable of receiving a solid and dry granular flow of aggregates (2a) and a flow of liquid bituminous binder (3a), as well as an outlet capable of providing a pasty mixture (4a) resulting from the first mixing step, and the second mixer step of which (102b), intended for the implementation of the second, so-called covering mixing step, comprises an inlet capable of receiving said pasty mixture (4a) originating from the first mixer step (102a), as well as at least one other inlet capable of receiving a solid and dry granular flow of fines (2b) and a make-up flow (3b) of bituminous binder, the second mixer step (102b) also having an outlet capable of delivering the prepared paste (5), suitable for the forming of the electrodes (6), and wherein it is capable of storing two binders of different grades each in one respectively of two devices with a storage tank and loop for supplying the first mixer step (102a) and the second mixer step (102b) respectively, as well as with a device for batching each of the two grades of bituminous binder (3a, 3b) supplying the first mixer step (102a) and the second mixer step (102b). respectively.

Description

5. PRESENTATION OF THE FIGURES

[0044] Other characteristics and advantages of the invention will become apparent from the description given below, non-limitatively, of example embodiments with reference to the attached drawings in which the same reference numbers denote identical or analogous and corresponding elements. In these drawings:

[0045] FIGS. 1a and 1b show diagrammatically and respectively two methods for preparing carbonaceous paste known from the prior art, already described above in the general presentation of the state of the art;

[0046] FIG. 2a, already described above, diagrammatically shows an aggregate coated with binder and fines, as obtained by the implementation of the methods of the prior art;

[0047] FIG. 2b is a representation analogous to FIG. 2a, of an aggregate with bituminous binder for precoating and penetrating into the open pore spaces of the aggregate, following the first mixing step, as well as the bituminous binder for covering, following the second mixing step, according to the method of the invention;

[0048] FIGS. 3a and 3b are two diagrammatic views, analogous to FIGS. 1a and 1b, and showing respectively two embodiments of the method according to the invention and corresponding installations.

6. IMPLEMENTATION OF THE INVENTION

[0049] The present invention is based on the observation that the great majority of the open pore spaces accessible to the binder are concentrated in the coarse fraction of the dry granulometric mixture, the fines having far fewer and smaller-sized pore spaces due to the fact that these fines essentially originate from phenomena of attrition or grinding in steps upstream of the method of the invention.

[0050] According to the invention, it is proposed to break down the mixing step of the state of the art into two steps specific to the invention, as illustrated in FIG. 3a.

[0051] A first, so-called “preimpregnation” mixing step, relates only to the coarse granulometric fraction of the dry granular mixture, with the exception of the fines. This coarse fraction 1a is firstly supplied into a preheater 101a, from where it leaves in the preheated state 2a, in order to be supplied into a first mixing step 102a, in which this preheated coarse fraction 2a is mixed with only a part 3a of the bituminous binder required for the manufacture of the electrodes, and also supplied into the first mixing step 102a. This first mixing step aims to promote the access and penetration of the bituminous binder into the pore spaces of the coarse fraction of the dry granular mixture, in the absence of the fines of the dry granular mixture, which facilitates the distribution of the binder around the aggregates and improves the homogeneous coating thereof. This first step is supplied according to the same principle as the mixers of the prior art, namely that the part 3a of bituminous binder is introduced into the first mixing step 102a simultaneously with or after the preheated coarse granular fraction 2a, so as to avoid lining the walls of the mixer with the binder.

[0052] A second, so-called “covering” mixing step, is a step in which the balance of the dry granular mixture, i.e. the fraction of fines (fines or ultrafines), as well as the additional binder required for the manufacture of the electrodes are added to the product leaving the first step. To this end, the coarse fraction 4a precoated with bituminous binder and leaving the first mixing step 102a, is supplied into a second mixing step 102b, also supplied, on the one hand, with the preheated fine fraction 2b leaving a second preheater 101b, supplied with the fine fraction 1b, and, on the other hand, with the balance of the bituminous binder required 3b. This second mixing step essentially aims to homogeneously coat all of the granular mixture, and in particular the fines which develop the largest specific surface area, and to distribute these fines, coated as homogeneously as possible between the preimpregnated and precoated aggregates, in order to fill the intergranular voids in the best possible way, as shown diagrammatically in FIG. 2b. In this FIG. 2b, an aggregate 201 is shown coated with precoating bituminous binder 203a, having, in addition, filled the open pore spaces of the aggregate 201, as shown for the pore space 213, by the effect of the first mixing step, and this layer of precoating binder 203a is itself covered with bituminous covering binder 203b, supplied to the second mixing step 102b, and in which the fines 202, also supplied to this second mixing step 102b, are distributed substantially homogeneously.

[0053] In this second mixing step, in order to avoid the “drying out” of the paste 4a originating from the first step at the time of the addition of the preheated fines 2b, with the peak power and the inherent risks of clogging the mixer, it is ensured that the balance of the bituminous binder 3b required is introduced simultaneously with or after the paste 4a originating from the first mixing step, but simultaneously with or before the balance of the dry granular mixture, i.e. the preheated fine fraction 2b leaving the second preheater 101b of a device for the separate preheating of the coarse and fine fractions, which also comprises the first preheater 101a of the coarse fraction, which is novel with respect to the prior art on several points.

[0054] Then, the carbonaceous paste 5 leaving the second mixing step 102b is ready to supply the forming, which is carried out, as previously, in a press or a vibrating compactor 104, from where the electrode blocks 6 leave, ready for baking.

[0055] The distribution of the binder 3a, 3b to be injected at each of the two mixing steps can be calculated: [0056] either so that the product leaving each step has the same content by weight of binder as that required in the final paste 5 for producing the raw electrode; [0057] or, as a minimum, by injecting in the first step the quantity of binder 3a that is just necessary for coating the aggregates and filling their open pore spaces, and by injecting the remainder of the binder 3b in the second step so as to obtain the content by weight required in the final paste 5 for producing the raw electrode; the fraction of binder 3a required for coating the aggregates in the first step is assessed on a prorata basis with respect to the surface area of the aggregates to be coated in relation to the extended surface of all of the dry granular mixture; [0058] or also any other distribution intermediate to the two preceding ones and as chosen arbitrarily.

[0059] An embodiment of the invention is described below, based on the following basic data: [0060] Production rate: 50 tonnes per hour (t/h) of paste [0061] Content of binder in the raw electrode: 15% by weight, i.e. 7.5 t/h of binder and 42.5 t/h of dry granular mixture [0062] Composition of the dry granular mixture: [0063] Blaine specific surface area of the dry granular mixture: 3500 cm2/g [0064] 80% coarse fraction i.e. 34 t/h [0065] 20% fine fraction i.e. 8.5 t/h, [0066] Characteristics of the coarse fraction: [0067] Blaine specific surface area of the coarse fraction alone: approximately 350 cm2/g [0068] Density: 2.0 kg/dm3, [0069] % by volume of open pore spaces: estimated at 10%, [0070] Density of the binder: 1.3 kg/dm3

Results:

[0071] For an equal content by weight of binder leaving the two steps: [0072] flow rate of paste leaving the first step: 42.5×0.80/(1−0.15)=40 t/h [0073] flow rate of binder to be injected in the first step: 40×0.15=6.0 t/h [0074] flow rate of binder to be injected in the second step: 7.5−6.0=1.5 t/h [0075] As a minimum: [0076] Flow rate of binder to be injected in the first step for coating the coarse fraction: 7.5×(350×40)/(3500×42.5)=0.7 t/h [0077] Flow rate of binder required in the first step for filling the open pore spaces: (42.5×0.80/2.0)×0.10×1.3=2.2 t/h [0078] Total flow rate of binder to be injected in the first step: 0.7+2.2=2.9 t/h [0079] Flow rate of binder to be injected in the second step: 7.5−2.9=4.6 t/h [0080] Intermediate choice: [0081] Example: 50/50 equal distribution between the two steps [0082] Flow rate of binder to be injected in each step: 7.5/2=3.75 t/h

[0083] The addition of the fraction of fines 2b and the make-up of bituminous binder 3b in the second step can be carried out in two different ways: either for each of them, independently and directly into the mixing apparatus ensuring the implementation of the second mixing step 102a (see FIG. 3a), or, by contrast, simultaneously in a single flow, after a prior premixing step which has the objective of improving the dispersion of the fines 202 in the covering binder 203b, and subsequently of ensuring their more homogeneous distribution around the preimpregnated aggregates 201 originating from the first mixing step 102a, as shown in FIG. 3b.

[0084] The method according to FIG. 3b differs from that according to FIG. 3a in that the preheated fine fraction 2b leaving the preheater 101b, and the balance of the bituminous binder 3b required directly supply a premixer 105, and in that the paste 4b resulting from this premixing, leaving the premixer 105, is supplied, like the paste 4a leaving the first mixing step 102a, into the second mixing step 102b, leaving which the carbonaceous paste 5 supplying the forming 104 is obtained, as under the preceding conditions, in order to obtain moulded electrode blocks 6.

[0085] A proposed development of this basic method consists of differentiating between the grade of the binders added at each of the steps in order to take into account the specific objectives of each of them: [0086] Thus, in the first step or preimpregnation step, the objective is essentially to ensure good coating of the aggregates 201 and good filling of their open pore spaces 213. The aim is therefore to use the most fluid binder possible, but also to remain if possible close to the temperature for forming the electrode, in order to avoid an additional step of cooling the paste which generates emissions of light aromatic compounds. For this, a binder with a low SP is therefore chosen, the SP of which is typically at least 50° C., preferably 80° C., below the forming temperature (example: for a forming temperature of 160 to 170° C., a binder with an SP of 80 to 90° C. will be suitable). [0087] In the second step or covering step, the objective is to coat the aggregates 201 and fill the intergranular voids with the fines 202, which are themselves correctly coated with the covering binder 203b and distributed homogeneously, but without there being any need for impregnation of open pore spaces, since the aggregates 201 are already preimpregnated and the fines 202 are by nature not very porous. As indicated previously, this step requires little specific power once the temperature is at least 30° C. above the SP of the binder. It is therefore possible to choose a binder with a higher SP which fulfils this condition close to the forming temperature (example: for a forming temperature of 160 to 170° C., a binder with an SP of 130 to 140° C. will be suitable). The benefit of choosing a binder with a high SP for this second step resides in the need to compensate for the choice of a binder with a low SP for the first, so-called preimpregnation step, in order so far as possible to maintain the coking value of the mixture of the binders and, finally, not to lose the density gain obtained by better impregnation of the aggregates 201, during the baking of the electrodes 6.

[0088] In FIG. 2b, the bituminous binder 203a for precoating and penetrating into the open pore spaces 213 is the binder with a low SP, if two different grades of binder are used, or in the case of two binders with different SPs, whereas the bituminous covering binder 203b is the binder with a high SP.

[0089] At this stage, it is therefore assumed that the operator is provided with two different grades of binder, with dedicated storage infrastructures, which can represent a major obstacle. Also, in order to overcome this obstacle, another development proposed for this method allows the operators to implement the invention without modifying the current supply logistics and limiting the impact on the storage infrastructures just by adding a second day tank: it is in fact a matter of inserting, between the main store and the day tanks, a small fractional distillation unit capable of producing a binder with a high SP 203b from a fraction of the crude binder with an average SP. The light oils recovered during this operation are then fluxed with the remaining fraction of the raw binder with an average SP in order to produce the binder with a low SP 203a. Thus, for example, from a binder with an SP of 110° C., and assuming a 50/50 distribution of the binder between the two steps, it is possible to envisage obtaining a binder with an SP of 130° C. 203b and a binder with an SP of 90° C. 203a in this way. This operation makes it possible to obtain two grades of binder 203a, 203b for the implementation of the invention, without creating any new supply constraint for the operator, and without any residual flow to be re-treated.

[0090] The implementation of the method of the invention, with a single grade of binder or two different grades of binder, can easily be carried out by combining 2 conventional mixer steps, in both discontinuous mode and continuous mode, and is similar in this case to the state of the art with a mixer-cooler 103 (see FIG. 1b). The few operating differences between these devices and the state of the art require only minor modifications: [0091] The first mixer stage 102a, intended for the implementation of the first so-called preimpregnation step, operates either close to the mixing temperature of the current state of the art in the case where a single grade of binder is used, or close to the forming temperature (and not up to 30° C. above) in the case where two different grades of binder are used and where the binder with a low SP 203a is injected in 3a into this first mixing stage 102a. Moreover this first stage 102a receives, at the inlet, a granular flow and a binder liquid flow as in the current state of the art, in order to produce, at the outlet, a more grainy and therefore less adhesive paste than in the conventional method: rather, all of the operating conditions develop in a very favourable direction. [0092] The second mixer stage 102b, intended for the implementation of the second so-called covering step, operates close to the forming temperature, in the same way as a mixer-cooler of the state of the art, but receives at the inlet, on the one hand, the granular paste 4a discharged from the first stage 102a, and, on the other hand, the fines 2b and the make-up binder 3b, which is the binder with a high SP if two grades of binder are used, preferably mixed together beforehand in 105, instead of the injection of cooling water into the abovementioned mixer-cooler of the state of the art, in order to finally produce a paste 5 which is denser but at virtually the same temperature as the paste according to the state of the art.

[0093] Alternatively, the implementation of the method of the invention can be envisaged with a single item of mixing equipment, with the obvious advantage of this solution in terms of installation and capital expenditure cost:

either by discontinuous mixing, with the sequential introduction of the materials as described above and specific to the invention, so as to have a succession, in the cycle of the device, of a phase of preimpregnation (102a) of the aggregates (2a) with the binder with a low SP (3a), followed by a phase of covering (102b) these preimpregnated aggregates (4a) by the addition of the binder with a high SP (3b) then fines (2b), or alternatively by the addition of the fines and of the binder with a high SP (3b) which are premixed (in 105);
or by continuous mixing, which incorporates the sequential introduction of the materials into the mixer as described above and specific to the invention, so as to create within the mixer, an upstream, so-called preimpregnation zone of the aggregates (2a) with the binder with a low SP (3a), followed by a downstream so-called covering zone of these aggregates preimpregnated with the binder with a high SP (3b) then fines (2b), or alternatively by the addition of the premixed fines (2b) and binder with a high SP (3b). [0094] Thus, for example, it is possible to envisage introducing the aggregates (2a) at the upstream end of a horizontal tubular mixer, followed immediately afterwards by the introduction of the binder with a low SP (3a), then, after a length sufficient for the preimpregnation of the aggregates, typically half-way along the mixing zone, introducing the binder with a high SP (3b) followed immediately by the introduction of the fines (2b), the carbonaceous final paste (5) being discharged at the downstream end of the horizontal mixer after a length sufficient for the homogeneous covering of the aggregates with the mixture of fines (2b) and binder with a high SP (3b). Alternatively, it is possible to directly introduce, half-way along the mixer, the mixture of fines (2b) and binder with a high SP (3b). [0095] Similarly it is possible to envisage, for example, the use of a continuous mixer, as described in patent document WO2011/064461, in which the upstream zone is modified in order to be divided into two zones: upstream, a preimpregnation zone supplied with the aggregates (2a) and the binder with a low SP (3a), then a device for supplying, in succession, the binder with a high SP (3b) then fines (2b), or, alternatively, a mixture of both of these, followed by a downstream covering zone, the zone for cooling the paste as described in patent document WO2011/064461 being dispensed with. In this configuration, the pressure roller as well as the deagglomeration device and the distributing flap equip the entire useful length of the device. [0096] Or also, any other configuration of mixer, which allows for the sequential introduction of the materials as described in the invention, so as to provide a zone for preimpregnating the aggregates followed by a covering zone.

[0097] In any event, the implementation of the invention either with a single mixer or with two mixers in series, also requires modification of the upstream part of the conventional treatment installation in order to incorporate therein the additional equipment required for the supply of the new method, namely: [0098] A separate preheating (101a, 101b) of the aggregates (1a) and of the fines (1b) in the dry granular mixture, as well as separate handling equipment up to their point of injection into one or other of the mixers; [0099] Optionally, for the use of binders of different grades in the preimpregnation zone and step and in the covering zone and step: [0100] either, in the case of separate supplies of the two grades of binder, a second storage tank for receiving the deliveries of the second grade of binder, with its own reheating, loading and unloading devices; [0101] or, in the case where a single supply of an intermediate grade of binder is maintained, a unit for the production of the two grades of binder based on the existing single storage unit; [0102] in both cases, a second day storage tank, with a loop for supplying and batching the second grade of binder; [0103] Optionally, a stirred tank for premixing (105) the fines (2b) and the binder with a high SP (3b) with a specific extraction/batching system, for directly supplying this premixture to the covering zone inlet.