Title of Invention

PROCESS FOR PRODUCING CARBON BLOCKS WITH HIGH THERMAL SHOCK RESISTANCE

Abstract

The invention relates to a simplified production process with which it is possible to obtain carbon blocks, in particular anodes intended for aluminium production using the Hall-Heroult electrolysis process, which have high thermal shock resistance. This process comprises a grain size distribution adjustment operati9n of a carbon aggregate which includes: a) separa,ting an initial ungraded carbon aggregate AC into a first fraction Fl made up of grains whose s1zes are smaller than X, and into a second fraction F2 made up of grains whose sizes are X q, greater., X lying between 0.2 and 2 mm; b) crushing Bl of the first fraction Fl and par PI of the second fraction F2 such as to obtain a powder F comprising a controlled proportion of ultra-fine grains; c) mixing the remaining part P2 of fraction F2 and said powder F in proportions with which it is possible to obtain crude carbon blocks whose so-called "Gr/S" ratio between the proportion PGr of grains larger than 0.3 ΜM and the proportion PS of grains between 30 ,m and 0.3 m in size is at least equal to 4 and baked carbon blocks of high density.

Full Text

PROCESS FOR PRODUCING CARBON BLOCKS WITH HIGH THERMAL SHOCK RESISTANCE Technical field The invention relates to a process for producing carbon blocks with high thermal shock resistance, in particular anodes intended for aluminium production by electrolytic smelting of alumina dissolved in a melted cryolite bath, according to the Hall-Heroult process. Prior art In general, carbon products are obtained by industrial processes comprising a mixing operation of a binder (such as pitch) with carbonaceous material (such as coke), a hot-forming operation of the mixture, and a baking operation of the crude products. The binder and carbon material are mixed in powder form, that is to say using crushed material of determined grain size distribution. In practice, the mixing operation uses several size fractions, so-called "industrial" fractions, of carbon material stored in separate silos, and pitch. In many industrial applications, in particular in electrometallurgy, the baked carbon products must, in addition to their heat and electric conducting properties, offer high resistance to thermal shock. In particular, regarding aluminium production by electrolysis of alumina in a melted cryolite bath, the carbon anodes form a consumable product which is replaced in the electrolytic cell as and when it is consumed by combustion. Typically, a modern electrolysis plant producing 24 0 000 tonnes of aluminium per year for example, simultaneously consumes 150 000 anodes, whose unit weight is in the region of one tonne. However, during replacement operations of spent anodes by new pre-baked anodes, the latter - which are generally at a temperature close to ambient temperature - are added to the melted electrolyte bath which is maintained at a temperature close to 950°C. The thermal shock due to the sudden increase in temperature may cause cracking, even breakage of the anodes. In general, the degradation of anode quality due to thermal shock leads to the onset and development of cracks, and quickly leads to loss of metal production or operating instabilities of the cells due to the breakaway of carbon pieces from the cracked anodes, which fall into the electrolysis bath. The additional operating costs generated by lack of resistance to thermal shock rapidly increase with the percentage of rej ected, cracked anodes due to thermal shock. It therefore becomes of essential importance to ensure good anode resistance to thermal shock before they are placed in service. A rejection rate of less than 1 % is considered acceptable but soon becomes intolerable over and above 2 %. Patent application FR 2 740 771 (equivalent to Canadian patent CA 2 192 707 and to Australian patent AU 12415/97) on behalf of the applicant, describes a method of producing carbon blocks, anodes in particular, having high resistance to thermal shock. This method successively comprises size adjustment of a carbon aggregate by crushing and size gradation, mixing generally at a temperature of between 130°C and 180°C of the crushed aggregate with a predetermined quantity of pitch binder to form a homogeneous paste, densifying by compaction, forming said paste into a carbon block in the crude state, and finally baking the carbon block at a temperature that is generally greater than 900°C. Adjustment of aggregate size is made in accordance with j three fractions: ultra-fine (UF) for grains having a diameter of less than 30 jam, sand (S) for grains with a diameter of between 30 and 300 \un, and grain (Gr) for grains having a diameter greater than 300 |nm. The weight ratio of the Gr and S fractions is adjusted so that Gr/S is greater than 4. With this process it is possible to reduce very significantly the rejection rate of carbon blocks which have cracked due to thermal shock. As described in application FR 2 740 771, the three fractions are obtained in practice by mixing several industrial size fractions of dry material derived from separate silos (typically 3 to 5 silos). For example, as illustrated in figure 1, this application describes the use of 4 industrial fractions, namely: - TG (Very Large) formed of grains from recycled matter with a diameter of between 1.5 mm and 15 mm obtained by crushing production-inherent waste such as stubs of spent anodes; - G (Large) formed of coke grains with a diameter of between 1.5 mm and 5 mm and made up of coke grains in the higher size grade after 1.5 mm screening; - M (Medium) formed of coke and recycled grains having a diameter of between 0 and 1.5 mm and made up of size fractions smaller than 1.5 mm of coke and recycled grains; - F(Fines) formed of fines with a diameter of less than 0.2 mm and made up of the fraction of mediums M refined by grinding. This fraction of fines must contain a sufficient proportion of ultra-fines UF with a diameter of less than 0.03 mm. These theoretical and industrial grain size distribution fractions overlap in accordance with a known diagram. Therefore grain Gr is provided by very large TG, large G and part of M; sand S and ultra-fines UF are provided by part of M and. of F. Precise proportions of each fraction (Fl, F2, F3 and F4) must be mixed in order to obtain a final carbon aggregate AF having the desired final grain size distribution. These proportions are adjusted using proportioners Dl, D2, D3, D4 connected to silos SI, S2, S3, S4 containing the different grain size distribution fractions. The powder mixture obtained AF is then generally pre-heated (step PA), mixed with a pitch binder Br in a mixer (step MA), formed (step FB) and baked at high temperature (step CB) . This, relatively complex, process requires strict management of raw material stocks, of dry material flows and of silo reserves, and possibly the addition of an intermediate crushing operation of material derived from some silos. The applicant has sought an industrial production process for carbon blocks of large size (that is to say whose volume is typically greater than 0.2 m3) having high thermal shock resistance, with which it is possible to avoid the disadvantages of the prior art. In particular, the applicant set out to find means for reducing the scale and costs of ■ the required installations and for simplifying the production process, operating modes, stock management and flows of materials. Description of the invention The production process for carbon blocks, anodes in particular, according to the invention comprises the supply , of an. initial carbon aggregate AC, size adjustment of said carbon aggregate AC in order to obtain a final carbon aggregate FA of determined grain size distribution Gf, hot mixing MA of the final carbon aggregate FA with a determined quantity of pitch binder Br to form a homogeneous paste, a forming step FB to form at least one crude carbon block from said paste, and a baking step CB for the crude carbon block or blocks, and is characterized in that said adjustment comprises: a) separating the initial carbon aggregate AC into a first fraction Fl of grain size distribution G-formed of grains whose sizes are less than X, and into a second fraction F2 of grain size distribution G+ made up of grains whose sizes are greater than or equal to X, X lying between 0.2 and 2 mm, and preferably lying between 0.3 and 1 mm, said carbon aggregate AC preferably containing calcined petroleum coke whose percentage of particles smaller than 0.3 mm is no more than 35 %; b) grinding Bl the first fraction Fl and part PI of the second fraction F2, so-called "diverted part PI", in such manner as to obtain a powder F containing a controlled proportion PUF of ultra-fine grains, called UF, that is to say grains whose sizes are less than 30 jam; c) mixing MX the remaining part P2 of fraction F2 and said powder F in proportions enabling carbon blocks to be obtained whose grain size distribution Gf in the crude state is such that the so-called "Gr/S" ratio between the proportion PGr of grains larger than 0.3 mm and the proportion PS of grains between 30 mm and 0.3 mm in size is at least equal to 4, and whose density after baking is high, that is to say preferably greater than 1.55, further preferably greater than 1,6. With the presence of ultra-fine grains it is possible to increase the density of the carbon blocks. The proportion PUF of ultra-fine grains in powder F is a parameter of the process of the invention which is empirically adjusted so as to reach the highest possible density. The PUF proportion is generally greater than 7 0%, preferably greater than 8 0 %, and further preferably greater than 90 %. The applicant found, in surprising manner, that it is possible to obtain a PGr/PS ratio (also called "Gr/S" ratio) able to assure sufficient resistance to thermal shock without having recourse to the complicated processes set forth in the prior art. In particular, according to the invention, the mixture MX of carbon products is made from only two sources of powdered carbon material, namely P2 and F. With the process of the invention, it is also possible to adjust the Gr/S ratio and anode density easily and simultaneously. The carbon aggregates AC used as raw material for the process of the invention, which are preferably ungraded aggregates, may contain considerable proportions of grains less than 0.3 mm in size. The separating operation of the initial carbon aggregate AC into two fractions and the crushing operation of the first fraction make it possible to increase significantly the Gr/S ratio, even using coke whose percentage of grains smaller than 0.3 mm is as high as 35 %. However, in order to obtain a Gr/S ratio greater than 4, it is preferable to use carbon aggregates whose proportion of grains smaller than 0.3 mm in size is less than 15 %, even less than 10 %. The initial carbon aggregate AC may be formed of carbon aggregates derived from several sources. In particular, the initial carbon aggregate AC may contain a determined proportion of recycled carbon products R. For example, in the aluminium production industry, it is advantageous for economical and technical reasons, to add a proportion of recycled carbon products which may reach as high as 40 % and derived in particular from spent or rejected anodes before or after baking. These products also offer the advantage of containing, in general, a high proportion of very large or large grains in relation to sand or fine grains. Therefore, the initial carbon aggregate AC may typically be made up of a mixture of ungraded calcined petroleum coke C and a minority part of recycled carbon products R. The invention will be better understood with reference to the figures and detailed description given below. Figure 1 illustrates part of the production process of carbon blocks according to the prior art, in which the initial dry material is stored in separate silos according to grain size distribution and composition. Figure 2 illustrates the process of the invention. Figures 3 to 6 illustrate preferred embodiments of the process of the invention. Detailed description of the invention In the process of the invention, such as illustrated in figure 2, the initial carbon aggregate AC is separated, by means of at least one separating means Tl, into a first fraction Fl of grain size distribution G-, and into a second fraction F2 of grain size distribution G+. The separating means are typically a screening device or an air-swept dynamic separator. Said derived part PI is adjusted in relation to the grain size distribution of the initial carbon aggregate AC. Preferably it is such that only a small part of F2 is crushed, that is to say that a small part PI of F2 is diverted to be crushed with Fl. PI is preferably less than 20 %, and typically between 1 % and 10 %. Fraction Fl and diverted part PI of fraction F2 are then crushed in at least one crushing means Bl so as to obtain a powder F containing a controlled proportion PUF of ultra-fine grains, that is to say grains less than 30 fim in size. In order to obtain a given flow DF of powder F, irrespective of the grain size distribution of the initial ■aggregate AC, the diverted part PI may be adjusted so that the flow DF of powder F leaving the crusher Bl is substantially constant. Flow DF is equal to DF2 x PI + DF1, in which DF2 and DF1 are the respective flows of fractions Fl and F2. In practice, parts PI and P2 may be adjusted so as to maintain flow DF substantially constant. This adjustment may be made using the filling levels of buffer silos ST1 andsST2. Bl is typically a ball crusher, optionally fitted with a re-circulation circuit comprising an air-swept dynamic selector with which it is possible to adjust grain size distribution and the PUF fraction. The remaining part P2 of fraction F2 (also called "coarse aggregate" AG) and powder F are then proportioned and combined to re-constitute a carbon aggregate AF (step MX) . Said remaining part and said powder F are mixed in respective proportions PG + and PF with which it is possible to obtain crude carbon blocks having a grain size distribution GF such that the ratio between the proportion PGr of grains larger than 0.3 mm and the proportion PS of grains between 30 mm and 0.3 mm in size is at least equal to 4, and to obtain baked 'carbon blocks of high density, that is to say a density of preferably greater than 1.55, and further preferably greater than 1.60. The ratio Gr/S is at least equal to 4, preferably greater than 10, and further preferably greater than 15. In order to facilitate the mixture of the final aggregate with the binder, the final aggregate is generally pre-heated (step PA). In practice, it is advantageous to perform the mixing MX and preheating PA in one same device which may typically be a preheating screw heated with heat transfer fluid. Proportioning of the intermediate aggregates AG and F may be made using proportioners Dl and D2 typically installed at the inlet of the device. With these proportioners which are typically screw or strip dosimeters, it is possible to adjust the flow of intermediate aggregates and consequently the proportions PG+ and PF of these aggregates. The proportion PF determines the proportion of ultrafine grains PUF which, to a large extent, determines the density of the baked carbon blocks. The final aggregate is mixed with a pitch binder Br and hot mixed to form a homogeneous paste (step MA). Typically, this step is conducted in a mixer, such as a mixer of Buss® or Eirich® type. Mixing temperature preferably ranges from 130°C to 220°C, and typically from 170°C to 190°C. Mixing is followed by a forming step FB to form said paste into one or more carbon blocks in the crude state. This forming may be conducted in a vibrocompactor or a press. This last step advantageously comprises a densifying operation by compacting said paste. The crude carbon blocks are then baked at a temperature that is generally higher than 900°C in a furnace, generally a rotating furnace (step CB) . It is advantageous to store the remaining part P2 and powder F in buffer reserves, such as silos, respectively denoted ST2 and ST1. This buffer storage allows for greater control over the flow of carbon materials. In one preferred embodiment of the invention, illustrated in figure 3, the initial carbon aggregate AC undergoes prior treatment D intended to remove the grains whose size is greater than a determined value (or tolerance threshold) Y, which is preferably 20 mm, and further preferably 30 mm. By means of this operation it is possible in particular to avoid damaging certain types of mixers downstream from the process and/or to better control the flow of materials on belt conveyors, screws, vibrators, launders used to transport the grain materials. As illustrated in figure 4a to 4c, prior treatment D preferably comprises crushing B2 and optionally a separating operation T2 of the initial carbon aggregate into a fraction Fll only containing grains smaller in size than the tolerance threshold Y and into a fraction F12 made up of grains at and over the threshold size. The separating operation T2, which therefore concerns the grains whose size is greater than Y, makes it possible to increase the efficiency and productivity of crushing operation B2. Crusher B2 is advantageously a grinder for material bed grinding under pressure, such as a Rhodax® cone vibrating crusher made by FCB, or a roll crusher which avoids having recourse to the separating operation T2 (figure 4a). In the variant illustrated in figure 4b, the carbon aggregate A derived from step D, which is treated in the subsequent steps of the process of the invention, is made up of the crushed aggregate F13, derived from crushing operation B2, and optionally also of fraction Fll. The prior step D with no separating operation T2 (figure 4a) may for example be conducted using an open-circuit compression crusher or an open-circuit material bed under-pressure grinder which is able to crush the very large grains efficiently without producing substantial quantities of sand, fines or ultra-fines, that is to say without changing the ratio between the proportion of grains Gr and grains S in unacceptable manner. In the variant illustrated in figure 4c, the aggregate F13 derived from crusher B2 is re-added to the separating means T2. According to the embodiment of the invention illustrated in figure 5, the process also comprises a prior separating step T2 of the initial carbon aggregate AC, which allows removal of those grains whose size is greater than Y (flow F3), and the mixing of these grains with fraction Fl before or during said crushing operation Bl of fraction Fl. In practice, a separating means T2 may be associated with separating means Tl in order to remove the large-size grains from the initial carbon aggregate and add them to crusher Bl. This variant offers the advantage of removing grains larger than Y at least cost, since in this case only a single separating means and a single crusher are required. According to the variant of the invention shown in figure 6, the process also comprises an additional separating step T2 of the initial carbon aggregate AC which allows removal of grains larger than Y (flow F3), their crushing B3, and mixing of the crushed product obtained F13 with said second part P2 in such manner as to limit the proportion of these grains in part P2 of grain size distribution G+. Crusher B3 is preferably an material bed under-pressure grinder. This variant has the advantage of using a low-capacity crusher since the quantity of grains larger than Y remains low in AC. This variant also offers the advantage of making it possible to group together the separating means Tl and T2. The process of the invention is intended in particular for the production of carbon anodes used for the production of aluminium based on the Hall-Heroult process, and enables the complete process to be simplified and to reduce production costs. The carbon anodes obtained with the process of the invention may advantageously be used for a primary aluminium production process based on the Hall-Heroult electrolysis process on account of the high thermal shock resistance of the anodes.. Examples Tests were conducted using the process of the invention according to the embodiment shown in figures ■ 3 and 4,a. Crusher B2 was a Rhodax® crusher. The carbon aggregate was made up of an ungraded mixture of 7 0 % coke and 30 % recycled material. The recycled material was derived from spent electrolysis cell anodes. The average grain size distribution of the carbon aggregate and its constituents, obtained over a one-week production period of aluminium electrolysis anodes (corresponding to the production of approximately 300 0 anodes) is given in Table 1. The grain size distribution of the pre-crushed aggregate A derived from prior treatment step D is also given in Table 1. It shows that this step enables the efficient removal of grains larger than 30 mm in size. The pre-crushed aggregate A was then separated into two fractions Fl and F2 by means of a screen fitted with square-mesh sheeting. The sorting threshold X between these two fractions was 1 mm. On average, 88 % of the pre-crushed aggregate A passed to side F2 and 12 % passed to side Fl. PI was equal to 5 % which means that fraction Fl and 5 % of F2 were then entirely crushed in Bl which was a ball crusher, in order to obtain a powder of fines F. An aggregate was then reconstituted by mixing part P2 (grain size distribution G+) and fines F in the following proportions: PG+ =84 % of P2, (corresponding to 88 % of A less the 5 % of F2 diverted towards crusher Bl) and PF = 16 % of F (corresponding to 12 % of A plus the 5 % of F2) . The grain size distribution of fractions F2 and Fl, of fines F and of the reconstituted aggregate at level Ml is given in Table 2. The Gr/S ratio of the carbon aggregate entering the paste unit, that is to say upstream from the process, was therefore much lower than the Gr/S ratio of the reconstituted aggregate (namely 8.3 compared with 21.4). The prior treatment step D (pre-crushing) made it possible to remove the large grains while only reducing the Gr/S very slightly (namely 8.3 on entering the crusher, and 7.3 after pre-crushing). The reconstituted carbon aggregate was mixed with petroleum pitch, then hot mixed using a mixer so as to form a homogeneous paste. This paste was then formed into crude carbon anodes which were subsequently baked in a ring furnace. The average geometrical density, after baking, of the anodes obtained according to the process of the invention was 1.580 with a standard deviation of 0.015. The anodes obtained were used in electrolysis cells for the production of aluminium using the Hall-Heroult process. None of the anodes produced broke under thermal shock in the electrolysis cells in which they were used. Advantages of the invention Through the simplicity of the process it is possible to limit investment and maintenance costs, and also the risk of breakdowns since less electromechanical and mechanical equipment is required. In particular, the reduced number of proportioners substantially limits the risks of equipment breakdown. 1. Process for producing carbon blocks, in particular anodes, comprising the supply of an initial carbon , aggregate AC, a grain size distribution adjustment operation of said carbon aggregate AC such as to obtain a final carbon aggregate AF of determined grain size distribution Gf, hot mixing MA of the carbon aggregate with a determined quantity of pitch binder Br to form a homogeneous paste, a forming step FB to form at least one crude carbon block from said paste, and a baking step CB of the crude carbon block (s) , and characterized in that said adjustment comprises: a) separating said initial carbon aggregate AC into a first fraction Fl, of grain size distribution G- made up of grains whose size is smaller than X, and into a second fraction F2, of grain size distribution G+, made up of grains whose size is greater than or equal to X, X lying between 0.2 and 2 mm; b) crushing Bl the first fraction Fl and part PI of the second fraction F2, so-called "diverted part PI' in order to obtain a powder F containing a controlled proportion PUF of ultra-fine grains, called UF, that is to say grains smaller in size than 30 jam; c) mixing MX the remaining part P2 of fraction F2 and said powder F in proportions allowing carbon blocks to be obtained whose grain size distribution Gf in the crude state is such that the so-called "Gr/S" ratio between the proportion PGr of grains larger than 0.3 mm and the proportion PS of grains between 30 mm and 0.3 mm in size is at least equal to 4, and whose density after baking is greater than 1.55. 2. Process according to claim 1, characterized in that X lies between 0.3 and 1 mm. 3. Process according to claim 1 or 2, characterized in that said initial carbon aggregate is ungraded aggregate. 4. Process according to any of claims 1 to 3, characterized in that parts PI and P2 are adjusted so as to maintain a substantially constant flow DF of powder F leaving the crushing operation Bl. 5. Process according to any of claims 1 to 4, characterized in that the initial aggregate AC contains calcined petroleum coke whose percentage TS of grains smaller than 0.3 mm is no more than 35 %. 6. Process according to claim 5, characterized in that the TS percentage is 10 % or less. 7. Process according to any of claims 1 to 6, characterized in that said initial aggregate AC contains up to 40 % of recycled carbon products. 8. Process according to any of claims 1 to 7, characterized in that the crushing step Bl is conducted by means.of a ball crusher optionally fitted with a recirculation circuit comprising an air swept dynamic separator enabling adjustment of grain size distribution and of the PUF fraction. 9. Process according to any of claims 1 to 8, characterized in that the part Pi is less than 20 %. 10. Process according to any of claims 1 to 9, characterized in that said PUF proportion is greater than 70 %. 11. Process according to any of claims 1 to 9, characterized in that the ratio Gr/S is greater than 10. 12. Process according to any of claims 1 to 11, characterized in that it also comprises a prior treatment operation of the initial aggregate AC which enables removal of grains greater in size than a determined value Y. 13. Process according to claim 12, characterized in that said prior treatment comprises a crushing operation B2 by crushing the material into layers for example using a cone vibrating crusher. 14. Process according to claim 13, characterized in that said prior treatment also comprises a separating operation T2 of the initial carbon aggregate AC with which it is possible to remove grains greater in size than a determined value Y, and in that said crushing B2 concerns these grains. 15. Process according to claim 12, characterized in that the prior treatment operation comprises a separating step T2 of the initial carbon aggregate AC with which it is possible to remove the grains greater in size than a determined value Y, and mixing of these grains with fraction Fl before or during said crushing operation Bl of fraction Fl. 16. Process according to claim 12, characterized in that the prior treatment operation comprises a separating step T2 of the initial carbon aggregate AC with which it is possible to remove the grains greater in size than a determined value Y, crushing B3 of these very large grains, and mixing the crushed product obtained F13 with said second part P2. 17. Process according to any of claims 12 to 16, characterized in that said determined value Y is 20 mm. 18. Use of a process according to any of claims 1 to 17 for the production of carbon anodes intended for tne production of aluminium based on the Hall-Heroult . process. 19. A process for producing carbon blocks, substantially as herein described with reference to the accompanying drawings.

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