AU2015261350A1

AU2015261350A1 - Method for preparing a composite carbon material with a view to the use thereof for manufacturing carbon blocks

Info

Publication number: AU2015261350A1
Application number: AU2015261350A
Authority: AU
Inventors: Benedicte Allard; Florent FIGURA; Jean-Michel ROZ
Original assignee: Carbone Savoie SAS
Current assignee: Carbone Savoie SAS
Priority date: 2014-05-16
Filing date: 2015-05-05
Publication date: 2016-12-22
Anticipated expiration: 2035-05-05
Also published as: FR3021048A1; JP2017519713A; FR3021048B1; WO2015173479A1; EP3142988B1; AU2015261350B2; BR112016026603B8; BR112016026603A2; BR112016026603B1; PL3142988T3; AR100273A1; JP6539726B2; CA2948314C; EP3142988A1; CA2948314A1

Abstract

The invention relates to a method for preparing a composite carbon material. Said method includes at least the following steps: a) preparing a mixture containing at least carbon raw materials, a solvent, and lignin; b) gellifying the mixture resulting from Step a) until a composite carbon material paste is obtained; and c) shaping the composite carbon material paste resulting from Step b) so as to give it a desired shape. The composite carbon material is intended to be used for manufacturing carbon blocks that are used as electrodes in aluminum electrolysis cells or in steel mill furnaces, or more generally as refractory materials.

Description

Process for preparing a composite carbonaceous material in order to use it to manufacture carbon blocks

The present invention relates to a process for preparing a carbonaceous composite material destined to be used to manufacture carbon blocks, for example a prebaked or Soderberg anode, or a cathode in the aluminum industry, or an electrode in the field of steel-making and electrometallurgy, or else refractory carbonaceous blocks. A process for manufacturing carbon blocks implemented in conventional fashion in the aluminum industry comprises the following steps: a) A mixture is prepared comprising carbonaceous raw materials (for example chosen from coke, anthracite and graphite), pitch used as a binder in this mixture, and possibly additives used for example to facilitate shaping and heat treatments. b) The mixture is heated to a temperature of approximately 160°C, so that the pitch becomes liquid and wets the carbonaceous raw materials, and it is kneaded for approximately one hour. c) At the end of kneading, the mixture is cooled by lowering the temperature to approximately 110°C in order to obtain a paste of carbonaceous composite material that is sufficiently cohesive to be shaped, i.e. to be made more dense and modeled into a specific required shape, for example by extrusion, pressing, vibro-compaction, molding. In this way, a "raw" product is obtained. d) The "raw" shaped product is then cooled rapidly so that it can be handled without any risk of being damaged. e) The product obtained in this way is then baked and may also undergo graphitization.

The pitch is a residue from distillation of high molecular weight petroleum or a residue from distillation of coal tar.

Baking consists in making the product conductive by coking the pitch. The baking temperature is approximately 1000°C. Graphitization consists in transforming coke into graphite at around 3000°C.

This manufacturing process may include an impregnation step between baking and graphitization and/or after graphitization.

Baking is usually carried out in a furnace some distance away from the mixing, kneading and shaping equipment, after a period of time that varies according to operational requirements. For this reason, it is essential that at the end of the shaping step (for example an extruding step), the shape of the carbonaceous product must remain exactly 2 the same, and this is particularly the case during storage and transportation of the said product.

To obtain this product characteristic upon termination of the shaping step, and bearing in mind that coke, graphite and anthracite are carbonaceous materials that are not easy to wet, pitch is used as a binder for the mixture of carbonaceous raw materials.

In order to obtain sufficient viscosity of the pitch so that it wets the carbonaceous raw materials, thereby giving rise to a homogeneous mixture during the kneading step, it is necessary to heat the mixture to a temperature of at least 160°C. However, at such a high temperature, pitch gives off highly toxic particles and gases, namely polycyclic aromatic hydrocarbons. In addition, pitch is known to be a carcinogenic compound.

Therefore this process for manufacturing carbon blocks making use of pitch as a binder when preparing the carbonaceous composite material is not completely satisfactory with regard to the toxicity of the volatile materials given off.

In addition, the steps in this manufacturing process must be carried out at high temperatures, namely between approximately 100°C and 160°C, in particular in order to adjust the wettability of the pitch and the rheology of the mixture of carbonaceous raw materials at each manufacturing step (kneading, shaping). In other words, this manufacturing process consumes considerable amounts of energy, and in particular thermal energy.

In this respect, US patent 4 072 599 describes an example of a carbon electrode manufacturing process using pitch as a binder for the carbonaceous raw materials. As set out above, this process is not satisfactory from the point of view of toxicity of the volatile materials given off.

In addition, we are familiar with US patent 4 704 230 which describes an alternative to this carbon electrode manufacturing process. In this American US patent 4 704 230, the binder for the above-mentioned carbonaceous raw materials consists of equal proportions of urea and lignosulfonates which are added to the mixture in solid form. The step in which the carbonaceous raw materials are mixed is carried out at a temperature of at least 132°C, in order to liquefy the urea and maintain it in liquid form in order to fully homogenize the mixture. The mixture is then shaped by means of a conventional technique such as extrusion, in order to obtain a composite carbon material appropriate for manufacturing a carbon electrode.

Therefore, during the process described in US patent 4 704 230, since pitch is replaced by another binder consisting of urea and lignosulfonates, the problems of toxic emissions caused by pitch are minimized. 3

However, we note that this process for preparing a carbonaceous composite material also has to be implemented at high temperatures of at least 132°C due to the melting temperature of urea, which also consumes very large quantities of energy and leads to the emission of volatile organic compounds present in lignosulfonates. 5 In addition, the urea contained in the carbonaceous composite material obtained upon completion of this preparation process has the following disadvantages:

It reduces the carbon efficiency of this carbonaceous composite material. Urea is an organic compound with the following chemical formula: CO(NH2).

It increases the porosity of the carbonaceous composite material, since it is degassed 10 during the baking step in the carbon electrode manufacturing process.

In addition, the temperature increases effected during this manufacturing process may damage the lignosulfonates and thereby lessen their binding properties. It has been observed that lignin tends to deteriorate from 80°C. In other words, the use of lignin in the form of lignosulfonates as a binder in this carbonaceous composite material 15 manufacturing process at such high temperatures (approximately 132°C) is not optimal, and it is even certain that lignin deteriorates during the said process.

Therefore, for all these reasons, the process described in US patent 4 704 230 which constitutes an alternative to the traditional process for preparing composite carbonaceous material in order to manufacture carbon blocks such as carbon electrodes is not fully 20 satisfactory either.

The present invention proposes to remedy the disadvantages detailed above inherent to carbon block manufacturing processes, and more broadly inherent to processes for preparing carbonaceous composite materials in order to manufacture carbon blocks.

The inventors of the present process have developed in quite a surprising manner a new 25 process for preparing a carbonaceous composite material that does not use a compound classified as carcinogenic such as pitch, and which in addition requires much less energy than the preparation processes known to date. At the same time it provides a carbonaceous composite material with physical and chemical properties completely appropriate for use in manufacturing carbon blocks. 30 The preparation process according to the invention also has the following advantages:

It requires less binder than the traditional process using pitch. The advantage of this is that it increases the carbon efficiency of the preparation process and reduces the costs of raw materials.

It can be implemented without any problem in existing installations built to perform the 4 process to prepare carbonaceous composite material for carbon blocks.

Therefore the first subject of the present invention is a process to prepare a carbonaceous composite material that is characterized in that it comprises at least the following steps: a) A mixture is prepared comprising at least carbonaceous raw materials and a binder comprising a solvent and lignin, and as an option at least one binding agent, on the basis of a ratio R defined by the following formula: (mass of lignin) (mass of binder) in which the mass of binder corresponds to the sum of masses of lignin, solvent, and if applicable,at least one binding agent; b) The mixture obtained at the end of step a) is gelled until a paste of carbonaceous composite material is obtained by varying the value of the R ratio in such a way that the value of the R ratio for carbonaceous composite material obtained at the end of step b) is greater than the value of the R ratio of the mixture in step a); c) The paste of carbonaceous composite material obtained at the end of step b) is shaped so as to give the carbonaceous composite material a required shape.

In step a) of the preparation process, the mixture comprises the best selection of carbonaceous materials chosen from among: coke (petroleum coke, pitch coke, metallurgical coke, foundry coke, etc.), anthracite, recycled material from carbonaceous blocks, i.e. pieces of crushed carbonaceous blocks, artificial graphite (obtained by heat treatment of graphitizable carbon at a temperature in the order of 3000°C),

The particle size of the carbonaceous raw materials is preferably less than 50 mm, and even more preferably less than 30 mm. The carbonaceous raw materials and their particle size are chosen by those skilled in the art, and will be determined according to the physical, chemical and mechanical properties that they wish to attribute to the carbonaceous composite material.

The term "binder" refers to the lignin, solvent and binding agents if any, taken as a whole. The mass of lignin is understood to be the mass of lignin dry matter. 5

Modification of the R ratio during step b) of the process makes it possible in particular to achieve satisfactory wettability of the carbonaceous raw materials in step a) by the binder, and gelling of the mixture through gelling of the binder in step b).

Gelling of the mixture is taken to mean that the viscosity of the mixture changes (increases with the R value) until a paste of composite carbonaceous material is obtained that is sufficiently cohesive to be shaped, due to the change in binder viscosity when the R ratio is made to vary.

Shaping is taken to mean that the density of the paste of carbonaceous composite material is increased and that the paste is modeled into a particular required shape, for example by extrusion, pressing, vibro-compaction or molding.

Preferably the value of the R ratio should be multiplied by an S factor at least equal to 1.2, and more preferably at least equal to 1.5 during the gelling step b).

Tests have shown that such a variation in the R ratio causes a change in the mixture's rheology, making it possible to consecutively obtain optimization of wettability of carbonaceous raw materials by lignin in step a) and cohesive properties of the carbonaceous composite material appropriate for shaping on completion of step b).

This S factor should however remain preferably lower than 5, and more preferably lower than 3, to prevent the paste of carbonaceous composite material becoming too dry and/or to limit the energy necessary to withdraw too great a quantity of solvent from the binder.

In one embodiment of the invention, the solvent in the mixture at step a) is an aqueous solution.

In one embodiment of the invention, in step a) of the preparation process the pH of the mixture is basic, i.e. greater than 7, and preferably between 8 and 12. This avoids any risk of corrosion, and facilitates dissolving of lignin, in particular if it has been added to the mixture in powder form.

In addition, a basic pH makes it possible to increase and accelerate wettability of the mixture's carbonaceous raw materials, even at ambient temperature.

So for all these reasons, a basic pH can be advantageous at step a) of the preparation process according to the invention.

To achieve this, it is better if the mixture comprises at least one base chosen from soda, ammoniac or weaker bases such as potassium hydroxide (KOH, i.e. caustic potash), magnesium hydroxide (Mg(OH)2), and calcium oxide (CaO, i.e. quick lime). 6

Therefore the solvent for step a) is advantageously chosen from among the aqueous solutions of soda, ammoniac, or weaker bases such as potassium hydroxide, magnesium hydroxide and calcium oxide.

If an aqueous solution of soda is used as the solvent, the carbonaceous composite product obtained on completion of the preparation process according to the invention may comprise a level of sodium that is prejudicial for certain applications of the said carbonaceous composite product (for example for manufacturing anodes for aluminum electrolysis). Therefore, for these applications of the carbonaceous composite material, other solvents will be used instead, such as aqueous solutions of ammoniac. The advantage of ammoniac is that it evaporates easily at around 60°C and therefore will not be found in the final product, which is the carbonaceous composite material.

The mass concentration of the base in the aqueous solution is preferably less than 30%, and even more preferably between 1 and 10%.

In another embodiment of the invention, the solvent is an organic solvent, preferably an organic solvent in which Kraft lignin is soluble. Kraft lignin is described below in greater detail.

The organic solvent is preferably chosen from among dimethylformamide, dimethylsulfoxide, dioxane, 2 methoxyethanol, acetone and ethanol. The advantage of these organic solvents is that they dissolve Kraft lignin.

Use of an organic solvent, preferably such as those detailed above, can be given priority when the carbonaceous composite material obtained according to the preparation process according to the invention is destined for manufacture of carbon blocks (for example an anode for aluminum electrolysis) the industrial application of which requires a low or even zero level of sodium in these carbon blocks.

The solvent can also be a binary mixture of aqueous and organic solvent.

Lignin is a three dimensional amorphous polymer composed of methoxylated phenylpropane structures. It is a thermoplastic polymer. Lignin can be derived from polymerization of p-coumarylic coniferylic and sinapylic alcohols.

The lignin in the mixture for step a) is preferably derived from a paper-making process, for example a process for manufacturing pulp, during which it is produced as a residue and every effort is made to eliminate as far as possible the undesirable components of wood so that only cellulose fibers are left.

During pulp manufacturing processes, black liquors containing lignin are recovered as a residue. They may consist of: 7

Kraft lignin which is the lignin found in the black liquor produced during the sulfate process (also known as the "Kraft process). Nearly 90% of chemical pulps produced in the world are the result of this sulfate process, based on soda and sodium sulfate. Kraft lignin is a mixture of thiolignin and phenolates. It is a hydrophobic product, but is soluble in an alkaline medium. It has been observed that lignin resulting from the Kraft process dissolves well in basic aqueous solutions.

The lignosulfates found in the black liquor from the sulfite acid process (also called "bisulfite process"). These products are soluble in water but their sulfur content may reach 6%. This is the family of lignin that is implemented in the process described in the above-mentioned US patent 4 704 230.

Therefore, in this embodiment of the invention, the advantage of the preparation process is that it recycles the lignin produced as a residue in a paper-making process (for example pulp manufacture). Lignin represents the second natural polymer (after cellulose) and has the advantage of being available in large quantities.

In step a) of the preparation process, the lignin in the mixture is preferably Kraft lignin. Kraft lignin has the following advantages compared to lignosulfonates:

It contains less sulfur. The presence of sulfur can be prejudicial from an environmental point of view during the aluminum electrolysis process or during baking or during graphitization of carbonaceous materials.

It dissolves well in a basic medium, as explained above. The basic aqueous solution for the mixture in step a) makes it possible to neutralize possible interactions between functions, in particular acids, lignin molecules and to facilitate dissolving of lignin.

In one embodiment of the invention, lignin was incorporated into the mixture in step a) in the form of a black liquor resulting from the Kraft process. The solvent in the mixture for step a) then comprises at least the black liquor in which the Kraft lignin is dissolved.

But the quality of the black liquor may vary. This is likely to result in risks of variability of the physical, chemical and mechanical properties of the carbonaceous products obtained.

For this reason, in another embodiment of the invention, the lignin incorporated into the mixture in step a) is presented in the form of a lignin powder. The lignin has preferably been obtained from a process for extracting and purifying a black liquor produced during the Kraft process. The advantage of this lignin powder is that it is free of impurities.

The process for extracting and purifying the lignin contained in the black liquor can consist of an acid, alkaline or heat treatment in order to obtain a lignin compound in the form of a powder. Lignins are precipitated from the black liquors in the Kraft process by means of 8 acidification of the carbon dioxide and sulfuric acid. In this respect, the US patent application 2010/0325947 A1 describes a process to extract lignin in powder form from black liquor resulting from a pulp manufacturing process that makes it possible to obtain lignin free of impurities. This process is known by the commercial name "lignoboost®". 5 The process to extract and purify lignin originating from black liquors produced during the paper-making industry Kraft process is well within the capacity of those skilled in the art.

In the embodiment of the invention in which lignin is present in the form of a powder, this lignin powder may have been dissolved, prior to step a) of the preparation process, in at least part of the solvent which is preferably a basic solution, for example an aqueous 10 solution with a pH greater than 7, and more preferably with a pH between 8 and 12. The advantages of this preferred pH range are that it combines effective dissolving of lignin and easy industrial implementation of preparation of the mixture. Highly basic solutions pose many handling difficulties at the industrial level. Subsequently the basic aqueous solution comprising lignin obtained in this way is incorporated into the mixture in step a) 15 comprising the carbonaceous raw materials.

Dissolving of the lignin powder can be performed at ambient temperature, or facilitated by increasing the temperature of the lignin and solvent up to approximately 80°C.

In another embodiment of the invention, the mixture for step a) is prepared by incorporating into a mixer, in addition to the above-mentioned carbonaceous raw 20 materials, lignin in powder form, and the solvent which is preferably a basic solution (for example a solution of soda, ammoniac, potassium hydroxide, magnesium hydroxide Mg(OH)2 or calcium oxide) as detailed above.

In another embodiment of the invention, prior to step a), a pre-mix is prepared by first mixing together all the dry matter comprised in the mixture in step a), i.e. at least the 25 carbonaceous raw materials and the lignin in powder form. Then the solvent is added to this pre-mix.

In one embodiment of the invention, the mixture in step a) comprises the following mass percentages of the total mass of dry matter in the said mixture: 80 % to 97% of carbonaceous raw materials, and preferably 90 % to 97%; 30 - 3 % to 20% of lignin, and preferably 3% to 10%.

The proportions of carbonaceous raw materials and lignin can be adjusted according to the required physical and chemical properties of the carbonaceous composite material obtained on completion of the preparation process according to the invention. This 9 adjustment of components in the mixture for step a) is well within the capacities of those skilled in the art.

Advantageously the mixture in step a) comprises at least one binding agent. This binding agent helps to improve the binding properties of lignin in the said mixture.

The binding agent can be chosen from among polysaccharides and carbohydrates. In particular they increase the carbon efficiency of the carbonaceous composite material obtained according to the preparation process according to the invention. They are known to interact strongly with lignins.

In the mixture in step a) , the mass of the binding agents can represent up to 20% of the mass of lignin.

The value of the R ratio in the mixture in step a) is preferably lower than 0.25 and more preferably between 0.07 and 0.25 and more preferably still between 0.15 and 0.20.

An R ratio defined as above in the mixture in step a) is particularly appropriate to ensure: satisfactory dissolving of the lignin, satisfactory wettability of carbonaceous raw materials, and easy mixing in step a) of the preparation process according to the invention.

This makes it possible to obtain a homogeneous mixture on completion of step a). In addition, it is useful to minimize the quantity of solvent in the mixture in step a) as far as possible, since it may subsequently be necessary, during the preparation process to remove at least part of it.

The mixture in step a) can in addition contain additives known to those skilled in the art that are used to facilitate shaping and heat treatments of the carbonaceous composite material that is obtained on completion of the preparation process.

The mixture in step a) is advantageously mixed at a temperature between 5°C and 50°C, preferably between 10°C and 40°C, and more preferably still between 15 and 25°C. Therefore step a) of the preparation process can be implemented at ambient temperature. This is particularly advantageous from the point of view of the energy consumption of the preparation process according to the invention.

As has been explained above, it is possible to increase the temperature of the mixture to approximately 80°C, at the start of step a) of the preparation process, in order to facilitate dissolving of the lignin in the solvent, if for example it has been added to the mixture as a powder. 10

In order to obtain a homogeneous mixture in step a) of the preparation process, we can for example use a kneading machine, a concrete mixer, or any other type of mixer appropriate for mixing the carbonaceous raw materials described above.

In step a), it is important for the mixture to be mixed in such a way that the carbonaceous 5 raw materials are completely wetted by the solvent and the lignin, such that the lignin can act as a binder of the said carbonaceous raw materials.

In step b) of the process the mixture obtained at the end of step a) is gelled until a paste of carbonaceous composite material is obtained.

It is important to obtain the carbonaceous composite material paste in step b), in order to 10 obtain the physical and chemical properties of the said material (in other words the "raw product") that are effective and comparable to those of a carbonaceous composite material obtained using the state of the art preparation processes described above.

As was explained above, during the gelling step b), the value of the R ratio as defined above increases in relation to the value of the R ratio for the mixture in step a). 15 Surprisingly, it has been discovered that increasing this R ratio as defined above leads to spontaneous gelling of the mixture obtained upon completion of step a), such that a carbonaceous composite material paste is obtained presenting sufficient cohesion to be shaped in step c), and perfectly suitable for handling in accordance with the industrial applications detailed above for the carbonaceous composite material obtained according 20 to the invention's preparation process.

Advantageously, the value of the R ratio for the carbonaceous composite material paste obtained on completion of step b) is greater than 0.25, more preferably greater than 0.30 and more preferably still between 0.30 and 0.70. However, the disadvantage of too high an R ratio value, i.e. greater than 0.70, may be that the carbonaceous composite material 25 paste obtained on completion of step b) may crumble.

Advantageously, the step b) gelling step is carried out at a temperature between 5°C and 150°C.

In one embodiment of the invention, step b) is carried out by evaporating at least part of the solvent. For example, half the solvent that was present in the mixture in step a) is 30 evaporated, so that the R ratio value as defined above changes from a value of 0.20 in the mixture in step a) to a value of 0.40 on completion of step b).

For example, evaporation of the solvent consists in subjecting the mixture in step a) to drying by air suction or convection, at a temperature between 25 and 150°C, preferably between 40 and 80°C to avoid deterioration of the lignin, and more preferably still at a 11 temperature in the order of 50°C. This evaporation is advantageously carried out for a period of between 15 minutes and one hour, and preferably for approximately 30 minutes.

The solvent evaporation apparatus may include an evaporation system using heating, suction, air convection in a mixer, a kneading machine, a concrete mixer, a rotating drum, 5 a freeze-dryer, a heat chamber, a tunnel dryer, an oven for example a micro-wave or high frequency induction oven. Of course the solvent evaporation apparatus is well within the capacities of those skilled in the art.

Advantageously, the solvent is evaporated by means of convection, combining air sweeping with agitation of the carbonaceous composite material paste. 10 In another embodiment of the invention, step b) is carried out by adding lignin to the mixture obtained upon completion of step a), preferably in a gradual manner, while ensuring the mixture remains homogeneous, and preferably until the value of the R ratio as defined above is greater than 0.25, more preferably greater than 0.30 and more preferably still between 0.30 and 0.70. 15 In a very advantageous manner, in step b) lignin powder is added, preferably Kraft lignin powder, to the mixture obtained on completion of step a).

Preferably, the carbonaceous composite material paste is kneaded during step b) so that gelling occurs homogeneously throughout the carbonaceous composite material that is being formed. 20 Step b) of the preparation process is important in order to obtain a carbonaceous composite material paste that is perfectly appropriate for subsequent shaping, using a conventional shaping technique used in the technical domain of the aluminum or steelmaking industry.

During step c) of the preparation process, the carbonaceous composite material paste 25 obtained on completion of step b) is shaped, in order to give the carbonaceous composite material a required shape.

The shaping step can be performed by extrusion, pressing, vibro-compaction or molding.

The shaping step c) is advantageously performed at a temperature between 15 and 50°C, or at ambient temperature. This temperature is much lower than the temperature required 30 during the state of the art processes for preparation of carbonaceous composite material that were detailed above.

Optionally, on completion of the shaping step c), the preparation process according to the invention also comprises a step to solidify the shaped carbonaceous composite material in order to increase its mechanical strength so that it can be handled and stored more easily. 12

This solidification step preferably consists of a heat treatment, for example drying carried out a temperature between 30°C and 100°C and/or application of air convection. More precisely, during this solidification step, a crust forms on the surface of the carbonaceous composite material that improves its cohesion and mechanical strength. A particularly advantageous embodiment of the carbonaceous composite material preparation process according to the invention comprises the following steps: A basic aqueous solution of lignin is prepared by dissolving Kraft lignin powder, advantageously purified Kraft lignin, preferably at approximately 80°C, (it should be noted that in this Kraft lignin, the sulfur and sodium levels and the ash rate are minimal), in an aqueous soda solution, for example with a mass concentration of 500g/L and pH 10. A mixture is prepared by mixing carbonaceous raw materials (for example those detailed above) with the said basic aqueous solution of lignin thereby obtained, advantageously for approximately 10 minutes, at ambient temperature (approximately 25°C).

Preferably in an air suction mixer set at a temperature of 50°C, the aqueous soda solution is evaporated, advantageously for approximately 30 minutes, so that the mixture thereby obtained in the previous step spontaneously gels, and a paste of carbonaceous composite material is obtained.

The paste of carbonaceous composite material is shaped, for example by extrusion, at ambient temperature (i.e. approximately 25°C).

The carbonaceous composite material obtained on completion of the preparation process according to the invention has a mass percentage of dry binder matter (i.e. essentially the mass of lignin, in certain cases the binding agents added to the mixture at step a) and dry solvent residue), advantageously lower than 10%, and preferably lower than 8%. The said mass percentage of dry binder matter is calculated in relation to the total mass of carbonaceous composite material dry matter obtained on completion of the process according to the invention.

We note that the carbonaceous composite material obtained using the preparation process in the present invention may comprise 50% less mass of dry binder matter compared to a carbonaceous composite material obtained using pitch as the binder. Indeed the mass percentage of pitch compared to the dry mass of such a carbonaceous composite material is usually between 10 and 20%. The preparation process according to the invention therefore presents very high carbon efficiency, and also a lower implementation cost due to the fact that it uses less binder. 13

In addition, the reduction of mass percentage of binder in the carbonaceous composite material makes it possible to significantly reduce the production of volatile matter emitted during baking, which is quicker.

Therefore the process for preparation of a carbonaceous composite material according to the invention has the following advantages: replacement of pitch by a non carcinogenic product which is lignin, and in addition with recycling of the lignin produced as a residue of paper-making processes; implementation of the mixing, gelling and shaping steps of the process in existing installations, and in particular in installations for preparation of carbonaceous composite material which use pitch as a binder, a very considerable reduction in (thermal) energy consumption by the process, while obtaining a carbonaceous composite material possessing optimal physical and chemical properties, equivalent to those of state of the art carbonaceous composite materials.

Indeed, since the steps in the preparation procedure according to the invention are carried out at lower temperatures (or even without any need for heating), than those required for state of the art process steps, the preparation process according to the invention requires total energy consumption (particularly thermal consumption) that is much lower (more than 80% lower) than that of state of the art processes. This represents a very considerable advantage for the invention.

With regard to the mixing step, it is important to note that the preparation process according to the invention does not require this step to be implemented at such high temperatures as the prior art preparation processes, that for example use pitch or a mixture of urea and lignin as a binder. Indeed, the mixing step in step a) of the preparation process according to the invention can be performed at ambient temperature.

Finally, with the preparation process according to the invention, productivity is improved by at least 50% compared to the productivity of the state of the art processes, since the mixing step for wetting of carbonaceous raw materials is faster than with pitch or a mixture of urea and lignin, and heating of the materials to high temperatures (i.e. in the order of 160°C) is no longer necessary.

The invention also concerns use of the carbonaceous composite material obtained by means of the preparation process according to the invention, for manufacture of carbon blocks, used for example as electrodes in aluminum electrolysis vessels or in steelmaking furnaces, or else as refractory products. 14

The invention also concerns a process for manufacturing a carbon block, which comprises at least the following steps: a) A carbonaceous composite material is prepared using the preparation process as described above. b) The carbonaceous composite material obtained on completion of step a) is cooked until a solid baked carbonaceous composite product is obtained.

In one embodiment of the invention, the process for manufacturing a carbon block comprises in addition a graphitization step of the product obtained upon completion of step b).

The usual baking and graphitization steps are completely within the capacities of those skilled in the art. The parameters for these steps are the technical parameters used conventionally in the technical domain of electrode manufacture, in particular for aluminum or steel-making, or else manufacture of carbonaceous refractory materials.

The carbon block is preferably a pre-baked or Soderberg anode, an aluminum industry cathode, an electrode in the steel-making or electro-metallurgical field or else a refractory carbonaceous block.

In addition, the process for manufacturing a carbon block according to the invention can be very easily implemented in installations that perform the existing processes for manufacturing carbon blocks, and in particular in installations that use pitch as a binder when preparing the carbonaceous composite material.

Indeed, the mixing apparatus in such installations can be used very easily for step a) of the process for preparing carbonaceous composite material according to the invention, without making any technical modifications to the apparatus.

In addition, step b) in which gelling takes place by evaporation of solvent in the invention's preparation process can be performed in a mixer or else in a cooling apparatus using air convection combined with agitation. These are the types of apparatus usually used in the state of the art carbon block manufacturing process described above.

Description of the figures:

Figure 1 is a diagram of isoresponses from the contact angle on carbonaceous raw materials comprising petroleum coke;

Figure 2 is a diagram of isoresponses from the contact angle on carbonaceous raw materials comprising graphite; 15

Figure 3 shows curves representing the change in viscosity as a function of pH according to the mass concentration of lignin in a basic aqueous solution.

Experimental part:

Analyses were performed to determine the wettability on carbonaceous raw materials of 5 aqueous solutions of soda in which Kraft lignin powder had been dissolved. More specifically on: a) carbonaceous raw materials comprising petroleum coke (figure 1); b) carbonaceous raw materials comprising graphite (figure 2);

To do this, the lignin concentration and the pH of the aqueous solutions of soda were 10 varied, and the contact angle on these solid carbonaceous products was measured, at an ambient temperature of approximately 25°C. The contact angle represents the surface interactions between the lignin in solution and the carbonaceous raw materials, therefore the wettability.

Figures 1 and 2 show the diagrams of isoresponses from the contact angle of the lignin in 15 solution on carbonaceous raw materials as a function of the mass concentration of lignin and the pH.

More precisely, the mass concentration of lignin is defined by the following formula (1): Mass concentration of lignin = (mass of lignin) QQ (mass of binder) 20 In other words, the formula (1) corresponds to the R ratio defined above, multiplied by 100.

The binder mass corresponds to the sum of masses of lignin and soda aqueous solution.

From the graphs in figures 1 and 2, we can observe that a basic aqueous solution containing lignin is particularly suitable for wetting carbonaceous raw materials chosen 25 from petroleum and graphite, because the wettability is greater when the contact angle is small.

In addition, we measured the viscosity expressed in Pa.s of basic aqueous solutions in which Kraft lignin powder had been dissolved, at different mass concentrations of lignin determined according to formula (1) detailed above. 30 Therefore, the curves in figure 3 represent the change in viscosity as a function of the pH of aqueous solutions of soda in which Kraft lignin powder has been dissolved, at different mass concentrations of lignin. 16

From figure 3, we therefore observe that the more the mass concentration of lignin increases (the more the R ratio increases), the more the viscosity of the basic aqueous solution containing lignin increases.

Therefore at a pH of 8.5, if we vary the mass concentration of lignin (i.e. the R ratio as 5 defined above), we observe a substantial change in the viscosity of the basic aqueous solution containing lignin.

This phenomenon consisting in a change in viscosity observed in the experiment described above is the reason for the spontaneous gelling that occurs in the mixture during the step b) detailed above in the process for preparing a carbonaceous composite 10 material according to the invention.

Claims

1. A process for preparing a carbonaceous composite material, characterized by the fact that it comprises at least the following steps: a) A mixture is prepared comprising at least carbonaceous raw materials and a binder comprising a solvent and lignin, and as an option at least one binding agent, on the basis of a ratio R defined by the following formula:

in which the mass of binder corresponds to the sum of masses of lignin, solvent, and if applicable,at least one binding agent, b) The mixture obtained at the end of step a) is gelled until a paste of carbonaceous composite material is obtained by varying the value of the R ratio in such a way that the value of the R ratio for carbonaceous composite material obtained at the end of step b) is greater than the value of the R ratio of the mixture in step a). c) The paste of carbonaceous composite material obtained at the end of step b) is shaped so as to give the carbonaceous composite material a required shape.

2. A preparation process according to claim 1, characterized in that in step a), the mixture has a pH greater than 7, preferably between 8 and 12.

3. A preparation process according to claim 2, characterized in that the solvent is a basic aqueous solution chosen from among aqueous solutions of soda, ammoniac, potassium hydroxide, magnesium hydroxide and calcium oxide.

4. A preparation process according to claim 1, characterized in that the solvent is an organic solvent.

5. A preparation process according to claim 4, characterized in that the solvent is an organic solvent is chosen from among dimethylformamide, dimethylsulfoxide, dioxane, 2 methoxyethanol, acetone and ethanol.

6. A preparation process according to one of claims 1 to 5, characterized in that the lignin is Kraft lignin.

7. A preparation process according to any one of claims 1 to 6, characterized in that the lignin incorporated into the mixture in step a) is in powder form.

8. A preparation process according to claim 7, characterized in that the lignin has been dissolved in at least part of the solvent prior to step a).

9. A preparation process according to claim 7, characterized in that the mixture in step a) is prepared by incorporating into a mixer the carbonaceous raw materials, the lignin in powder form and the solvent.

10. A preparation process according to claim 7, characterized in that prior to step a), a pre-mix is prepared by first mixing together all the dry matter comprised in the mixture in step a).

11. A preparation process according to any one of claims 1 to 10, characterized in that the mixture in step a) comprises at least one binding agent.

12. A preparation process according to claim 11, characterized in that the binding agent is chosen from among polysaccharides and carbohydrates.

13. A preparation process according to one of claims 1 to 12, characterized in that the value of the R ratio in the mixture in step a) is less than 0.25, preferably between 0.07 and 0.25, and even more preferably between 0.15 and 0.20.

14. A preparation process according to one of claims 1 to 13, characterized in that the value of the R ratio for the carbonaceous composite material paste obtained on completion of step b) is greater than 0.25, more preferably greater than 0.30 and more preferably still between 0.30 and 0.70.

15. A preparation process according to one of claims 1 to 14, characterized in that the value of the R ratio is multiplied by an S factor at least equal to 1.2, and more preferably at least equal to 1.5 during the gelling step b).

16. A preparation process according to claim 15, characterized in that the S factor is less than 5, and preferably less than 3.

17. A preparation process according to any one of claims 1 to 16, characterized in that the mixture in step a) is mixed at a temperature between 5°C and 50°C.

18. A preparation process according to any one of claims 1 to 17, characterized in that step b) is performed by evaporating at least part of the solvent.

19. A preparation process according to any one of claims 1 to 18, characterized in that step b) is performed by adding lignin to the mixture obtained on completion of step a).

20. A preparation process according to any one of claims 1 to 19, characterized in that shaping in step c) is performed by extrusion, pressing, vibro-compaction or molding.

21. A preparation process according to any one of claims 1 to 20, characterized in that it comprises in addition, upon completion of step c), a step in which the shaped carbonaceous composite material is solidified.

22. A preparation process according to claim 21, characterized in that the solidification step consists in drying carried out a temperature between 30°C and 100°C and/or application of air convection.

23. A preparation process according to any one of claims 1 to 22, characterized in that it comprises at least the following steps: - A basic aqueous solution of lignin is prepared by dissolving Kraft lignin powder in an aqueous solution of soda. - A mixture is prepared by mixing carbonaceous raw materials with the said basic aqueous solution of lignin. - The aqueous soda solution is evaporated so that the mixture thereby obtained in the previous step spontaneously gels, and we obtain a paste of carbonaceous composite material. - The paste of carbonaceous composite material is shaped.

24. A process for manufacturing a carbon block, which comprises at least the following steps: a) A carbonaceous composite material is prepared using the preparation process according to any one of the claims 1 to 23. b) The carbonaceous composite material obtained on completion of step a) is cooked in order to obtain a solid baked carbonaceous composite product.

25. A process to manufacture a carbon block according to claim 24, characterized in that it comprises in addition a graphitization step of the product obtained on completion of step b).