AU722834B2 - Carbonaceous block having high resistance to thermal shock - Google Patents

Description

S F Ref: 362190

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

*9 Name and Address of Applicant: Aluminium Pechiney Immeuble Balzac La place des Vosges 92400 Courbevole

FRANCE

Defense Actual Inventor(s): Christian Dreyer and Bernard Samanos Address for Service: Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Invention Title: Carbonaceous Block Having High Resistance to Thermal Shock The following statement Is a full description of this invention, including the best method of performing it known to me/us:- 5845 1 Carbonaceous Block Having High Resistance to Thermal Shock Technical Field The invention relates to a manufacturing process for carbonaceous blocks having a high resistance to thermal shock, in particular for anodes intended for the production of aluminium by way of molten-bath electrolysis of alumina dissolved in a bath of molten cryolite according to the Hall-H6roult process.

Prior Art Among refractory materials, carbonaceous products stand out by their qualities of thermal and electrical conduction. Because of them, they are very much used in electro-metallurgy; in the majority of such applications, however for example as anode and cathode in the cells of molten-bath electrolysis these carbonaceous materials must in addition possess a very high resistance to thermal shock.

Thus in the case of aluminium production by electrolysis of alumina in a bath of molten cryolite, the new, pre-baked anode is introduced into the electrolysis bath at a temperature of close to 10000C, and it is therefore very important to be assured of its good resistance to thermal shock before it is put to use. In fact, the carbonaceous anode is essentially a consumable product which is replaced above the electrolysis cell as it is being consumed by combustion; now, a modern i 20 electrolysis plant producing 240 000t of aluminium a year uses in the process 150 000 anodes each weighing around It. Any degradation in the quality of the anodes, .i brought about by the appearance and development of cracks following thermal shock, quickly leads to a loss of metal production. This leads to process instabilities in the cells due to the fact that pieces of carbon detached from the cracked anodes 25 fall into the electrolysis bath. If an anode waste ratio of I% due to thermal shock cracking is acceptable, the cost per batch of electrolysis rises rapidly when this ratio regularly exceeds 2%.

It is, unfortunately, rather frequent for the person skilled in the art to have to find serious, long-term degradation in the resistance of baked anodes to thermal shock, without however being able to clearly establish their cause, even though the manufacturing conditions and the use of said anodes have not changed. In this respect, it is necessary to recall for better understanding of the present invention that these baked carbonaceous anodes are obtained by mixing, at a temperature generally between 1300C and 1800C, a combination of a carbonaceous aggregate made from crushed coke and a coking binder such as pitch, then shaping the carbonaceous paste so obtained by suitable means such as spinning, vibrocompaction, compaction, and finally baking for a period that can extend to several weeks at a temperature of at least 900'C and generally between 10000C and 12000C.

N:libc\01685 For the operator, the quest for the best possible quality of carbonaceous product is a constant concern, and with regard to anodes for electrolysis it is well known (EP 025 859 US 4 770 826) that achievement of a maximum anode density after baking is beneficial for the regular operation and the energetic yield of electrolysis. At the same time, it translates to a stable, well-spaced and decidedly slower consumption of the anode, thus a decrease in the number of anodes that have to be manufactured for the same output of metal. Nevertheless, this quest for a maximum density of the product after baking does not yield any solution for the problem of resistance to thermal shock for these same carbonaceous products.

To date, in fact, the person skilled in the art does not know of any intrinsic characteristic of the carbonaceous product after baking which would indicate its capacity to withstand thermal shock, so that he is unaware of any manufacturing parameters which he could effectively influence in order to limit, or even suppress, the anode cracks, without by the same token losing the advantages that spring from the achievement of a high density of the baked carbonaceous anode. The person skilled in the art has at his disposal only analogue methods for monitoring the resistance to thermal shock of the carbonaceous products, with the question of just how representative they are for carbonaceous products under practical operating conditions, and thus their reliability, remaining a subject for discussion.

20 Furthermore, these test methods used on samples taken from batches of carbonaceous products after baking, that is to say, in the final stage of preparation, do not allow anything other than to eliminate, by statistical monitoring of the batches of products presumed to be defective, without any correction being possible upstream in the production since it is not known which are the determining factors. It should also be borne in mind that the most often used method among these analog measurements is based on electrical conductivity and consists in heating a sample of the product by the Joule effect to a temperature considered to correspond to that of its utilisation, then to plunge it abruptly into a fluid of generally ambient temperature.

30 The studies of Sato Sato et al., Carbone 1975, vol.13, pp. 309-316) on the mechanism of damage to the graphite by thermal shock should be noted. This method consists in hewing test pieces from the graphite material to be checked and to heat it abruptly by discharging an electric arc in its centre. A crack is thus produced in the material from the moment when the power of the arc reaches a certain level. Using this principle, FR 263 188 describes a process for testing the resistance to crack formation of carbonaceous products under thermal shock. Even though this achieves a better simulation of thermal shock in the material to be monitored, the methods of abrupt heating by electrical discharge or by induction to not permit to assess with sufficient reliability the future behaviour of the baked N:Iibc\01685 anodes during their immersion, because of the great spread of the results of the measurements for the crack-formation threshold.

To sum up: these analogue monitoring methods are of no help to the person skilled in the art for distinguishing, in carbonaceous blocks and particularly the anodes, an intrinsic characteristic of the material that is truly representative of its capacity to withstand thermal shock, and even less for deducing therefrom that or those parameters on which he may act during manufacture to quickly correct any deviation from that characteristic and do so without simultaneously degrading other characteristics of the anode such as its density.

Problem Posed The manufacture of carbonaceous blocks, in particular of anodes intended for the production of aluminium by molten-bath electrolysis, consistently displaying good resistance to thermal shock as measured by a waste ratio due to crack formation of not more than and without alteration of the other characteristics at that, remains a problem that has not been well solved.

Disclosure of the Invention The invention relates to a manufacturing process for carbonaceous blocks and more particularly of anodes that solves the problem of withstanding the thermal shock the anodes suffer during their immersion in a molten electrolytic bath, without 20 alteration of the other characteristics. The invention is based on the finding that by the appropriate choice of the size grading of the powdery carbonaceous materials, 00 a significant increase in the resistance to thermal shock of the carbonaceous blocks is obtained, surprisingly, monitored in their actual conditions of use and measured by the ratio of waste due to crack formation.

25 More precisely, the invention relates toa manufacturing process for carbonaceous blocks and more particularly for anodes having a high resistance to thermal shock, made up in these successive stages: a) controlling the size grading of a carbonaceous aggregate which constitutes the dry base material, by means of milling and suitable granularity grading; b) mixing the milled aggregate at a temperature generally between 130 0 C and ".-180 0 C with a predetermined amount of pitch based binder so as to form a homogenous paste; c) densification by compaction and shaping of the said paste to make up the carbonaceous block in its green state; d) baking the green carbonaceous block at a temperature generally above 900 0

C,

characterised in that this control of the size grading of the said carbonaceous aggregate by milling and grading is carried out in accordance with three theoretical grain fractions defined as follows: N:libc\01685 Ultra fine, or UF, with a grain diameter of Sand, or S. of a grain diameter ranging from 30~m to 300gm Grain, or Gr, with a grain diameter of >300pm and that the weight ratio of the fractions Gr and S are adjusted in such a way that Gr/S is at least equal to 4 and preferably between 6 and During many trials for controlling the resistance to thermal shock of pre-baked carbonaceous blocks subjected to conditions of industrial use the applicant has been able to establish that the products can show markedly different behaviour even: though their are manufactured and put to use in completely similar lo conditions. While it has not been able to confirm its initial hypothesis aimed at linking these variations in resistance to thermal shock to changes in the sourcing of certain base materials such as the coke the applicant has been able to establish, surprisingly, that by increasing the weight ratio Gr/S it was possible to very significantly reduce the waste ratio of carbonaceous blocks due to crack formation following thermal shock during the immersion of the anode in the bath. Usually this weight ratio Gr/S is consistently below 4 and generally set at between 1 and 3 (Zabreznik, Light Metal No. 24, 26-2-1987, p 527; or also Mannweiler Keller, JOM, February 1994, Fig. 5, p It is in fact known that the powdery dry materials used to prepare the carbonaceous paste by mixing with pitch and subsequently densifying it by compaction are graded in three theoretical granular fractions Gr, S and UF, each having specific and complementary functions during the preparation of the green .i carbonaceous block and which prove to have a determining effect on the properties of the final product.

Thus the grains Gr, by their compact stacking, form a skeleton displaying only weak thermal contraction.

S The ultra-fines UF form a non-running plastic mixture with the liquid pitch when leaving the mixing process.

As regards the sand S, it plays an intermediary role.

These points having been established, it follows that the person skilled in the art has no reason to set these weight proportions outside the limits commonly prescribed since nothing, in prior art, leads to suppose that there is any correlation whatsoever between this ratio Gr/S and the ratio of waste due to crack formation of the carbonaceous blocks and in particular of the anodes.

Furthermore, any marked modification of the relative weight proportions can alter their complementary relationships and create heterogeneities in the carbonaceous paste and subsequently in the block, even a degradation of the green and post-baking densities, even though the overall weight ratio of dry material (Gr+S+UF) to pitch remains unchanged in the 5 carbonaceous paste.

N:libc\01685 ii And finally, the person skilled in the art is limited in his initiatives for changing the weight proportions of the three granulometric fractions Gr, S and UF by economic operating constraints which shall be examined further on.

Nevertheless, by deliberately stepping outside of the common boundaries of using powdery carbonaceous base materials, the applicant has proved (see Table 1 below) that for a ratio Gr/S<4 the ratio of waste due to crack formation of the anodes did not exceed while for the values 6<Gr/S<15 the reject rate was zero, and that without degradation in the density characteristics. By contrast, for a noticeable decrease in both the green and post-baking densities is recorded.

Table 1 (Real-life immersion test on 50 anodes for each Gr/S) Gr/S anode rejects rejects green density density after baking 0.002 3 6 1.640 1.590 2 4 1.640 1.595 4 1 2 1.640 1.595 6 0 0 1.641 1.600 o 0 1.642 1.595 0 0 1.640 1 590 0 0 1.626 1 578 These new settings of Gr/S in unusual granulometric ranges have furthermore been achieved without cost penalties for the industrial process, that is to say, taking 15 into account the inherent constraints of the basic process of anode manufacture.

These constraints, well-known to the person skilled in the art, have to do with the need to employ, from the outset, four industrial granulometric fractions as powdery dry material, to wit: TG (very thick) 1.5mm<recirculate grains<15mm obtained by grinding of the 20 waste inherent in the manufacturing process, such as the fag ends of used anodes.

S G (thick) 1.5mm<grains of coke<5mm made up of the upper granulometric portion of grains of coke after screening to M (medium) 0<grains of coke and recirculates<1.5mm made up of the granulometric fractions of grains of coke and recirculate that are smaller than 25 F (fine) <0.2mm made up from the fraction of the medium M further refined by grinding. This fraction of fines must include a sufficient proportion of ultra-fines UF <0.03mm (after the ternary theoretical formulation previously defined). The theoretical and industrial granulometric fractions thus overlap in accordance with the known diagram of Figure 1.

Thus the grain GR is supplied by TG, G and a portion of M. The sand S is supplied by a portion of M and F. and the same is true of the UF.

N:libc\01685 The increase of Gr/S with a view to improve the resistance of the anodes to thermal shock obviously leads to increasing the fraction G and/or to reducing the fraction S. but the new setting of the formulation of the dry material apart from the need for preserving a certain percentage of ultra-fines must respect a certain number of constraints, which are as follows: S The four industrial fractions TG/G/M/F exist and are inherent in the processes of manufacturing anodes for electrolysis (recycling of the fag ends of anodes).

For obvious economic reasons, all of these four industrial fractions must be used up following their re-grading according to the three theoretical granulometric fractions Gr/S/UF and the adjustment of the respective weight proportions.

The green and post-baking densities must remain as high as possible, all the while preferably leaving the ratio of pitch to dry material unchanged, since the total quantity of dry matter Gr+S+UF remains constant.

The size grading of the coke, being the green material, is imposed by the constraints of the process used by the suppliers.

The fixed percentage of TG must be sufficient to allow recycling of all of the rejects inherent in the manufacture, especially the fag ends of anodes. It must therefore by higher than 10%, and preferably be between 20 and 30% of the total quantity of dry carbonaceous materials used.

20 With these details given, the range of the respective weight proportions of the three theoretical granulometric fractions Gr, S and UF that permit increasing the .o:i Gr/S without degradation of the density characteristics are as follows: o. 60%<Gr90% including particularly 20 to 30% of TG in the form of recycled grains 0.5%<S<15% 5%<UF_25% As far as the practical aspects are concerned, the invention will be better understood by the detailed description of its implementation, that is to say by the description of the means by which the ratio Gr/S is increased to at least 4 and preferably to between 6 and 15 for a precise formulation of the dry material, with the help of Fig. 1 which represents the granulometric distributions of the four ***.industrial fractions TG, G. M and F and the three reconstituted theoretical fractions Gr, S and UF.

Best Mode or Modes of Carrying Out the Invention Gr is first maximised, then S is minimised.

a) To maximise Gr the size grading of the two thickest industrial fractions of the carbonaceous aggregate, TG and G needs to be preserved. Thus, for a classic formulation in which the entirety of the recycled grains, for example 26%, are used in the form of TG, typical size grading from the different N:libc\01685 industrial fractions such as those indicated in the Table 2, with TG=26%- G=21%-M+F=53% will be obtained (knowing that the fines F will ultimately be obtained by the specific milling of at least an evenly divisible portion of M.

Table 2 TG 26% G 21% M+F 53% Calc.

Total Cumul.

total a Tyler lx% 2x% lx% 2x% lx% 2x% Gr/S 6.75 Mesh RT3/8 45.0 11.7 5.9 1.2 12.9 RT4 30.9 8 26.2 5.5 13.5 26.4 RT 10 18.7 4.9 48.1 10.1 4.2 2.2 17.2 43.6 Gr 87.1 RT 20 3.7 1.0 11.7 2.5 28.5 15.1 18.6 62.2 RT 48 0.8 0.2 5.8 1.2 44.3 23.5 24.9 87.1 RT 100 0.2 0.05 1.5 0.3 15.4 8.2 8.55 95.65 RT 200 0.2 0.05 0.5 0.1 4.9 2.6 2.75 98.4 S 12.9 0.5 0.1 0.3 0.1 2.7 1.6 100.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 100.0 UF 0.0 Total 100 26.0 100 21 100 53.0 100 1x percentage of sieving residue per 100g, expressed in mesh 2x percentage of sieving residue, expressed as the granulometric fraction.

By the other course of action, consisting in minimising S, the ratio of Gr/S can absolutely be regulated in the desired range and produce the amount of ultra-fines needed for obtaining a high anode density.

10 b) To minimise S. two methods can be used that conform to the constraint of using up all of each of the four industrial fractions. These two methods are described in the following examples.

Example 1 It consists in increasing the UF fraction at the expense of the S fraction by 15 intensively milling at least an evenly divisible portion F of the industrial fraction M.

Ordinarily, this evenly divisible milled portion F is intended to enrich the two ternary fractions S and UF. According to the invention, the UF fraction is augmented while decreasing the S fraction by way of a more pushed" milling in a ball mill. Thus in its usual operation a ball mill produces around 35% of ultra-fines UF, and therefore by extension around 65% of S. Finer milling therefore consists in reducing S.

Table 3 below permits to highlight the influence of the degree of fineness of the industrial fraction M after milling on Gr, S and thus on the ratio Gr/S, and does so for different percentages of ultra-fines UF in the final reconstituted formulation of the mixture Gr, S and UF, percentages that must obviously remain in the range from 5 to 25% so as not to ultimately alter other properties of the final product.

Thus for four increasing degrees of fineness Fl, F2, F3, F4, measured by the percentage of ultra-fines UF (<30pm) based on the ratio of throughput through the 200 mesh (<75pm) screen as follows: N:libc\01685 F1=54%, F2=64%, F3=74%, F4=100%, it is found, for three distinct grades of ultra-fines, for example 9% and 12%, in the final reconstituted formulation of the mix Gr, S. UF deduced from a granulometric grading table not shown but analogous to Table 2 that for each of the four degrees of fineness: the fact of finer milling F2 in relation to Fl, F3 in relation to F2 and F4 in relation to F3, permits to increase Gr/S only in the case of a milled powder with 100% of UF (F4) when Gr/S increases also with the ratio of ultra-fines is the region of the final mixture Gr, S. UF entered.

Table 3 Degree of fineness F1 F2 F3 F4 UF in PT200 54 64 74 100 6% UF in reconst. formulation Gr 73.7 77.7 79.9 82.5 S 20.3 16.3 14.1 11.5 Gr/S 3.63 4.77 5.67 7.17 9% UF in reconst. formulation Gr 67.0 73.0 76.3 80.2 S 24.0 18.0 14.7 10.8 Gr/S 2.79 4.06 5.19 7.43 12% UF in reconst. formulation Gr 60.4 68.3 72.8 77.9 S 27.6 19.7 15.2 10.1 Gr/S 2.19 3.47 4.78 7.71 Milled M fraction after sieving to Example 2 The S fraction is reduced while avoiding passing on to the milling the upper 15 granulometric range of M extending from 0.3mm to 1.5mm, which, as can be seen from Fig. 1, constitutes a fraction of Gr. This comes down to reducing the granulometric spread of the average industrial fraction M by making it very close to that of the sand fraction S. To this end it is sufficient to replace the screen of by a screen of a smaller mesh as close as possible to 0.3mm and to carry out an intensive milling of at least an evenly divisible portion of, but preferably all of, the said industrial fraction M which passes through the new screen. If the aimed-for quantity of ultra-fines aimed for in the final reconstituted formulation of the mixture Gr, S and UF ranging between 5 and 25% is not obtained from the milled industrial fraction M alone, a complementary input will be employed based on the milled industrial fractions of grains G or TG.

Table 4 below permits to highlight, as before, the influence on the ratio Gr/S of the degree of fineness of the new industrial fraction M screened to 0.3mm then milled for different percentages of ultra-fines F1=54%, F2=64%, F3=74% and F4=100%; and this for three different percentages of ultra-fines UF in the final -0reconstituted formulation of the mixture Gr, S and UF, to wit 9% and 12%.

N:libc\01685 Table 4 Degree of fineness F1 F2 F3 F4 UF in P 200 54 64 74 100 6% UF in reconst. formulation Gr82.7** 87.1 87.1 87.1 _S 11.3 6.9 6.9 6.9 Gr/S 7.32 12.62 12.62 12.62 9% UF in reconst. formulation Gr 73.9** 81.7** 86.0** 87.1 S 17.1 9.3 5 3.9 Gr/S 4.32 8.78 17.2 22.33 12% UF in reconst. formulation Gr 65.3** 75.7** 81.4** 87.1 S 22.7 12.3 6.6 .9 Gr/S 2.88 6.15 12.33 96.7 Milled M fraction after screening to 0.3mm.

means it was necessary to mill thick grains G to arrive at the desired ratio of ultra-fines.

It is found that, apart from one exception, values above 4 are obtained, with every possibility of clearly exceeding the preferred range of 6 to 15, particularly in the case of a powder of 100% UF, where once again the Gr/S increases with the content of ultra-fines, contrary to what is observed with the other powders Fl, F2 and F3.

10 The person skilled in the art therefore has at his disposal two methods for minimising S and adjusting Gr/S. The first method is preferred if suitable milling means is available; otherwise the second method, which only requires some extra 0: screening equipment due to the shifting of the cutoff threshold of M toward the fines will be employed; but this obviously does not preclude a combination of the 15 two methods being used.

Furthermore, by setting Gr/S in the preferred range of 6 to 15, any degradation of the other characteristics of the anodes will be avoided, and particularly of the densities in the green and post-baking states, all the while preserving the standard conditions of preparing the anodes during all subsequent stages of manufacture.

These known conditions are: S Mixing in the presence of a predetermined amount of pitch, preferably at around 150°C of the milled coke-based aggregate whose grading range, reconstituted according to the ternary mixture Gr/S/UF, satisfies the relationship G< Gr/S<UF, and formation of a carbonaceous paste.

Densification by compaction, and shaping of the said paste to make up the green anode.

Baking of the green anode, preferably between 1000°C and 1200°C, in a chamber furnace.

N:libc\01685

Claims (12)

1. A process for manufacturing carbonaceous blocks having a high resistance to thermal shock, including the following successive stages: a) controlling the size grading of a carbonaceous aggregate by means of milling and suitable granularity grading; b) mixing the milled aggregate at a temperature generally between 130 0 C and 180"C with a predetermined amount of pitch-based binder so as to form a homogenous paste; c) densification by compaction and shaping of the paste to make up the carbonaceous block in its green state; d) baking the green carbonaceous block at a temperature generally above 900 0 C, characterised in that the size grading of the carbonaceous aggregate is controlled by milling and grading according to three fractions: Ultra fine, or UF, with a grain diameter of Sand, or S. of a grain diameter ranging from 30pm to 300pm Grain, or Gr, with a grain diameter of >300pm and that the weight ratio of the fractions Gr and S are adjusted in such a way that Gr/S is higher than or equal to 4.

2. Process according to claim 1, characterised in that the carbonaceous 20 blocks are anodes.

3. Process according to claim 1 or claim 2, characterised in that the ratio Gr/S is set at between 6 and

4. Process according to any one of claims 1 to 3, characterised in that the respective weight proportions of the three theoretical granulometric fractions Gr, S and UF are: 60%< Gr< 90% S< 15% UF<

5. Process according to any one of claims 1 to 4, characterised in that the value of Gr is maximised while preserving the grading range of the two industrial fractions TG and G. being the thickest of the carbonaceous aggregate: TG is made up of grains recycled after milling and having a size greater than 1.5mm but 30 less than 15mm,'- G is made up of sieved coke grains having a size larger than but less than

6. Process according to any one of claims 1 to 5, characterised in that the value of S is minimised by intensive milling of at least an evenly divisible portion of the industrial fraction M of the carbonaceous aggregate of a grading range between 0 and 1.5mm in such a way as to increase the UF weight fraction at the expense of S in the ternary distribution Gr/S/UF.

7. Process according to Claim 6, characterised in that the degree of fineness of the milling defined as intensive, measured by the percentage of ultra- fines UF in the granulometric fraction that pass the 200 mesh screen, ranges between 54% and 100%. N:libc\01685

8. Process according to any one of claims 1 to 5, characterised in that the value of S is minimised by reducing the grading spread of the average industrial fraction M to bring it very close to that of the sand fraction S, by shifting the usual cutoff threshold of 1.5mm toward 0.3mm and by carrying out an intensive milling on at least an evenly divisible portion of industrial fraction M.

9. Process according to claim 8, characterised in that an intensive milling is carried out on all of industrial fraction M. Process according to claim 8 or claim 9, characterised in that the weight percentage of ultra-fines UF in the industrial fraction M after intensive milling ranges from 50% to 100% of all that passes through a 200 mesh screen.

11. A process for manufacturing carbonaceous blocks having a high resistance to thermal shock, substantially as hereinbefore described with reference to any one of the examples.

12. A process for manufacturing carbonaceous blocks having a high resistance to thermal shock, substantially as hereinbefore described with reference to the accompanying drawings.

13. Carbonaceous blocks having a high resistance to thermal shock manufactured by a process according to any one of claims 1 to 12. Dated 30 January, 1997 20 ALUMINIUM PECHINEY 0 oeS Patent Attorneys for the Applicant/Nominated Person SPRUSON&FERGUSON *a 0***SS N:libc\01685