EP0946449A1

EP0946449A1 - Method for the insolubilisation and consolidation of used brasques from hall-heroult electrolyte vessels

Info

Publication number: EP0946449A1
Application number: EP97947133A
Authority: EP
Inventors: Pierre Personnet; Gilbert Bouzat
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 1996-11-26
Filing date: 1997-11-25
Publication date: 1999-10-06
Also published as: FR2756198A1; CA2273004C; CA2273004A1; NO325224B1; FR2756198B1; NO992511D0; AU5229298A; NO992511L; AU726174B2; WO1998023551A1; US5947888A

Abstract

The invention concerns a method for the insolubilisation and consolidation in the form of pellets of used brasques from electrolyte vessels for producing aluminium comprising the grinding of said brasques and mixing them closely with various refractory pulverulent additives before burning characterised in that it consists in: introducing a first additive selected from the group of aluminium silicates for fixing the alkaline metals during burning; introducing a second additive which, during burning, will combine with the saturating fluorine-containing compounds to form new stable and insoluble compounds and by its non-combined supernatant fraction, will ensure the consolidation of the mixture, and this additive is selected from the group of calcium oxides and salts and preferably of lime cements and mortars; carrying out a forming by agglomerating in the form of pellets the close mixture of used ground brasques with said first and second pulverulent additives before burning at a temperature between 700 DEG C and 950 DEG C.

Description

PROCESS FOR THE INSOLUBILIZATION AND CONSOLIDATION OF WASTE BRAKES FROM

HALL-HEROULT ELECTROLYSIS TANKS

TECHNICAL AREA

The invention relates to a process of insolubilization and consolidation in the form of spent pot lining pellets originating from the dismantling of aluminum production tanks by igneous electrolysis according to the Hall-Héroult technique. These spent pot liners are formed by the carbonaceous cathode blocks, the seals and the side linings based on carbonaceous materials but also by all of the refractory and aluminous or silico-aluminous insulators arranged on the lateral walls and the bottom of the metal box forming the electrolysis tank. After use, these pot liners are heavily impregnated with harmful products, such as soluble sodium or sodo-aluminous fluorides and cyanides, which must be insolubilized or destroyed before landfill or possible reuse.

STATE OF THE ART

This field has been the subject of numerous publications. We have already described methods for treating spent pot lining by the wet method, comprising grinding followed by leaching with an alkaline liquor, for example in US patent 4052288 Aluminum Pechiney or US patent 41 13831 Kaiser. These methods are limited to the sole treatment of the carbonaceous parts of the pot lining which must first be separated from the non-carbonized parts formed by the refractories and insulators. They therefore leave without solution the problem of insolubilization before storage or landfill of non carbonaceous solid residues. If the wet treatment method according to US 52451 15

(EP0465388B) of Aluminum Pechiney makes it possible to treat these spent pot liners whatever their composition and in particular their silica and alumina contents, it requires like all wet processes the setting very significant resources if we do not already have a highly developed chemical production facility.

Also known are thermal treatment methods generally operating in a fluidized bed and based either on a pyrohydrolysis at more than 1000 ° C. of the spent linings according to US 4065551 Elkem, or US 41 13832 and US 41 16809 Kaiser, or on a simple combustion. in the air or in an oxidizing atmosphere of carbon elements, at a temperature of about 800 ° C, sufficient to decompose cyanides without causing significant release of volatile fluorinated compounds, according to US 4053375 Reynolds or according to the article by LC Blayden and SG Epstein, Journal of Metals, July 1984 page 24.

In fact, the methods and devices using the thermal route are limited in their possibilities by the nature and the composition of the pot linings to be treated. In fact, due to the fusion of certain eutectic compounds formed during combustion, the pot lining particles have a strong tendency to agglomerate. It becomes impossible to avoid their solidification and consequently to maintain a fluidized bed and a fortiori a dense bed if the combustion is carried out for example in a rotating oven with a long residence time. This already significant agglomeration phenomenon with the fillings of pot lining made up solely of carbonaceous products is greatly accentuated with the fillers containing refractory oxides and in particular silica, the weight content of which must not exceed 3 or 4%, as is apparent from the article by ER Cutshall and LO Daley, Journal of Metals, November 1986, page 37 table II.

To get rid of this agglomeration problem but also to completely insolubilize fluorinated compounds US 51641 74 (EP 0542404) Reynolds recommends adding 2 additives to finely ground spent pot lining before heat treatment, on the one hand, calcium carbonate with a view to trapping the fluorine of the fluorinated compounds in the form of CaF ₂ , on the other hand a silicate metallic, preferably calcium disilicate, which acts as an anti-caking agent during heat treatment.

To be completely effective, this process must be completed at the end of the heat treatment carried out at a temperature sufficient to decompose the cyanides, by a final operation of insolubilization of the soluble residual fluorides consisting in spraying milk of lime on the pulverulent mixture leaving the oven. heat treatment. In addition to the fact that this additional operation carried out in a semi-humid environment complicates and increases the cost of the process, the final product thus obtained is difficult to store or handle in the state given the presence of dust and fine particles.

To solve the same problem, US 52451 15 (EP 0465388B) and US 5365012 (EP 0465388B) Aluminum Pechiney recommend the introduction of a single additive based on lime and preferably gypsum, mixed with the finely ground pot lining to insolubilize the fluorinated compounds in the state of CaF ₂ after a heat treatment carried out at a temperature sufficient to decompose the cyanides but of a very short duration to avoid the bonding phenomena. To do this, the pulverulent mixture is injected under gas pressure at the top of a cyclonic reactor to be recovered at its base in the form of inert pulverulent residue. These methods are however limited in capacity due to the relatively reduced flow rates of pulverulent mixture to be treated admissible by the injector. On the other hand, with the spent pot lining lots heavily loaded with sodium fluoride, part of the sodium can recombine with the SO4 ^{2 "} anion of the gypsum in the form of Na ₂ Sθ4, instead of being insolubilized in the form of silicoaluminate. Although non-toxic, the sodium sulphate thus formed being leachable can, beyond a certain leaching rate, make it prohibited to landfill said inert pulverulent residue. PROBLEM

Knowing that an aluminum production plant by electrolysis capacity 240,000 T / year generates about 4000 T / year of spent pot lining and that these used pot lining contains significant quantities of fluorinated derivatives (up to 200 kg of fluorine per tonne ) and sodium products (up to 200 kg of sodium per tonne) as well as significant quantities of cyanides (up to 10 kg / tonne), it has proved necessary, taking into account economic constraints or technical limitations methods of the prior art, to design a new industrial process capable of: a) treating these spent pot lining under acceptable economic conditions whatever their content of silico-aluminous materials; b) ensuring, during the treatment, not only the total decomposition of the cyanides and the insolubilization of the fluorine contained in the fluorinated impregnating compounds but also of all the metals and in particular of the sodium; c) to produce an inert final residue in a sufficiently consolidated form to allow it to be landfilled, stored and transported in the event of new use without risk of pollution of the atmosphere by the formation of dust.

OBJECT OF THE INVENTION

The method according to the invention satisfies this triple condition. It is based on the observation that by adding appropriate additives to the old crushed pot lining, it is possible to effectively insolubilize not only the fluorine of the fluorinated impregnation compounds but also the alkali metals such as sodium, and this while promoting contrary to the usual practices the agglomeration of the constituents of the mixture by suitable shaping before cooking, preferably in the form of pellets whose consolidation continues during cooking then allowing their handling, transport, storage or landfill without formation of dust. More specifically, the invention relates to a process of insolubilization and consolidation in the form of pellets of old pot lining from electrolysis tanks for the production of aluminum comprising the grinding of said pot lining consisting of carbon products and impregnated silico-aluminous products of fluorinated alkaline compounds and cyanides as well as their intimate mixture with various refractory pulverulent additives before heat treatment or cooking, characterized in that after introduction: of a first additive intended during cooking to fix the alkali metals, in particular sodium, coming from fluorinated compounds and which is chosen from the group of silicoaluminates, then from a second additive intended during cooking, on the one hand to combine, with or without fusion, with fluorinated impregnating compounds to form new stable and insoluble compounds such as CaF ₂ , on the other hand, by its uncombined excess fraction , to ensure before but especially during and after cooking, the consolidation of the mixture, said second additive being chosen from the group of calcium oxides and salts and preferably cements and limestone mortars, shaping is carried out by agglomeration in the form of pellets of the intimate mixture of old pot lining crushed with said first and second powdery additives before cooking at a temperature between 700 ° C and

950 ° C sufficient to decompose cyanides but insufficient to cause the start of dissociation of fluorinated compounds.

DESCRIPTION OF THE INVENTION The Applicant has been able to observe in fact and during numerous tests that the agglomeration of the particles of the mixture which occurs at the start of cooking and can lead, in the absence of anti-caking agent, to solidification of all the charge of the mixture with blocking of the reactions, which it was possible to prevent this solidification by carrying out, before cooking, shaping by agglomeration of the particles constituting the mixture in the presence of judiciously chosen additives favoring the consolidation of agglomerates thus formed in all the subsequent stages of preparation, without detracting from the effectiveness of the insolubilization reactions, therefore to the ability of said additives to combine stably with the fluorine of the fluorinated compounds impregnating the old pot liners and with alkali metals, in particular sodium released by displacement of fluorine.

This consolidation has many advantages by allowing, for example, landfill or storage in a dense and thick bed of the inert final residue without the risk of crushing the pellets which have become sufficiently resistant or even in the case of a new use, the transport and handling without risk of spalling of said pellets with formation of dust and pollution of the atmosphere.

The choice of the most appropriate additives (see Table 2) as well as their optimal conditions of implementation were determined according to their effectiveness after numerous mixing tests, with used pot lining, whose weight composition was quite variable even for the basic constituents, namely:

- carbonaceous materials 30 to 50% - aluminous or silico-aluminous refractories 30 to 40% (silica can represent up to half the weight of silico-aluminous refractory)

these basic constituents being impregnated with cyanides (CN <1% of the total weight), fluorinated compounds, mainly in the state of sodium fluoride, which can represent up to 35% or even 40% of the total weight.

The old, previously crushed pot lining is crushed to obtain particles of dimension less than 5 mm and preferably less than

2.5 mm, but the subsequent insolubilization performance does not improve appreciably below this grinding fineness threshold while the cost on the other hand, industrial exploitation grows rapidly below this same threshold.

The first additive intended mainly during cooking to fix the alkali metals and in particular sodium, chosen from the group of silicoaluminates, is a feldspar or a clay such as for example kaolin or bentonite, some typical compositions of which are indicated in Table 1. below.

TABLE 1

Commercial names.

It should be recalled in this regard that the hot fixing of sodium by a clay to form an insoluble synthetic compound is carried out for example by kaolinite (2Si0 ₂ , Al ₂ 0 ₃ , 2 H2O) with formation of insoluble feldspathoids such as hydroxysodalite depending on the reaction

3 (2 Si0 ₂ , AI2O3, 2 H2O) + 4 Na ₂ 0 → 6 Si0 ₂ , 3 AI2O3, 4 Na ₂ 0, H2O + 5 H2O The second additive, intended during cooking to combine with or without fusion with the fluorinated impregnation compounds, in particular NaF, AIF ₃ and Na ₃ AlFό, while ensuring the consolidation of the agglomerated mixture during its elaboration, can be lime CaO, calcium carbonate CaC0 ₃ but is preferably chosen from the group of cements and calcareous mortars. In this regard, the Applicant has advantageously experimented with Portland type quick-setting cements which, while having good performance in insolubilizing fluorine by combination in the form of CaF ₂ , ensure excellent consolidation of the agglomerates during baking (see table 2).

The choice of additives is not only based on their effectiveness, that is to say their insolubilization and consolidation performance, but also takes into account economic criteria unit cost, availability on the market, quantity to be used relative to weight of pot lining treated.

We have thus determined (Table 4) the optimal ranges of weight compositions of the mixtures which can be processed industrially taking into account fluctuations in the contents of silica and of impregnation products of spent pot lining. Based on a ton of mixture, the composition of a charge varies within the following ranges:

- 400 to 600 kg of crushed spent pot lining <2.5 mm, i.e. 40 to 60% by weight

- 150 to 350 kg of a 1st additive of the kaolinite clay type in powder form, ie 15% to 35% by weight

- 150 to 350 kg of a second cement-type additive in the pulverulent state, ie 15% to 35% by weight.

For proportions by weight of spent pot lining greater than 60%, thus limiting the proportion of additives to 40% at most, there is a deterioration in the insolubilization performance of fluorine and sodium (see table 4), whatever the degree of refinement of pot lining by grinding. For proportions by weight of pot lining less than 40% by weight, and consequently by weight proportions of additives greater than 60%, excellent insolubilization and consolidation performances are recorded, but the process loses its economic interest, the cost of the transformation related to the weight of used spent pot lining becoming prohibitive.

Another essential characteristic of the process is the shaping by agglomeration of the crushed spent pot lining mixture / additives before cooking. This shaping of the powder mixture by agglomeration can be obtained by any means for shaping known powder materials such as compression or extrusion of the mixture dry or slightly humidified. This gives pellets or granules which can then be enlarged and rounded in the form of pellets by granulation, but even more simply by direct granulation of the pre-humidified mixture at a humidity level preferably set between 15 % and 25% of the weight of the dry mixture, pellets with excellent cohesion are obtained as will be described later in the detailed implementation of the process.

Cooking or heat treatment of the pellets, the diameter of which can be adjusted between 6 and 20 mm, in particular depending on the heat treatment conditions, must be carried out at a temperature of at least 700 ° C to decompose the cyanides, but without exceeding 950 ° C so as not to risk the onset of dissociation of the already stabilized fluorides. The temperature is preferably set between 800 ° C and 900 ° C (see table 3). The cooking time can vary from 1 hour to 6 hours depending on the nature of the additives and the weight composition of the mixture, but especially depending on the cooking method adopted and the average size of the pellets. For identical meatballs, the optimal cooking time, generally 4 hours in a static bed, is no more than 3 hours in a pass-through oven with cradles and 1 to 2 hours in a rotary oven or in a fluidized bed oven, the latter being particularly suitable for the process according to the invention taking into account the excellent cohesion of the dumplings during cooking in spite of the shocks and the effects of attrition between dumplings (see table 2). Note that if the carbon content of spent pot lining is very high and makes it possible to reach, after mixing, a carbon content of at least 25% in raw pellets, these can be used as auxiliary fuel in an oven incineration.

After cooking, the inert residue in the form of pellets can either be landfilled, stored or reused in a new application, for example as a fill for backfill.

The checks carried out on the meatballs before, during or after cooking are as follows:

Leaching test on cooked pellets crushed to 4 mm according to standard NF-X31210; the leachable fractions of fluorine and sodium are dosed and the weights of F and Na relative to the weight of the residue in the form of pellets. The concentration of fluoride ion F- in the leaching juices is also indicated.

Gaseous fluoride emissions during cooking

Sampling of gases under controlled flow rate by a capture device at the outlet of the oven chimney and determination of the fluorine expressed as a percentage by weight relative to the fluorine contained in the raw pelletized mixture.

Mechanical strength

3 types of test are carried out on raw dumplings and on cooked dumplings:

a) Cohesion test carried out with TURBULA ^® device on 40 gr of pellets placed in a 250 ml cylindrical jar and subjected for 5 min to turbulence along 3 successive axes of rotation: . if after the test the percentage of fines collected is less than 1% for cooked meatballs and 5% for raw meatballs the result is good. Cohesion is considered bad or insufficient if the percentage of fines is more than 4% for cooked meatballs and 15% for raw meatballs. In the intermediate ranges, cohesion is considered to be medium or weak.

b) Resilience test consisting of dropping the pellets 2 meters high on a steel support. The test consists of 5 successive releases of 5 dumplings and the result is expressed as a percentage of dumplings remaining intact, and it is therefore 100% if no dumpling breaks. This test makes it possible in particular to check the resilience of the pellets during the bucketing.

c) Crush resistance test consisting of dropping a steel ball of 11 grams at a height of 65 cm onto a dumpling; this in order to check the resistance of the pellets during loading and unloading.

As before, the result is expressed as a percentage of pellets that have remained intact.

These details being given, the most significant results of the various tests carried out are collated in Tables 2 to 4 below.

In Table 2 more precisely are grouped the results of insolubilization and consolidation tests carried out with various Kaolin additives, feldspar on the one hand, cements on the other hand in mixture in the respective weight proportions of 25% and 25% with 50% spent pot lining all crushed between 0 and 2.5 mm and this for standard cooking conditions of 1 hour / at 900 ° C. TABLE 2

It is found that in the absence of cement and despite excellent insolubilization performance resulting in a very low leaching rate of Na and F as well as a completely satisfactory mechanical strength after firing (at least according to 2 of the 3 tests performed), the dumplings prepared from the only brazed / kaolin binary mixture (test n ° 4), show very poor raw performance, considerably limiting the advantage of the process for this formulation.

Conversely, if we record perfectly acceptable raw strength of the pellets obtained from ternary brazing / kaolin / cement mixtures (tests 1, 2 and 3), we can consider their insolubilization performance as very average. less for the first 2 formulations whose leaching rates remain around 0.45% for Na and around 0.25% for F.

It is ultimately the ternary brazing / feldspar / cement mixtures, which prove to be the most effective in all aspects since the pellets prepared from these mixtures (tests 5, 6 and 7) exhibit excellent mechanical strength at the same time before and after cooking, (whatever the test used), as well as very good insolubilization performance, in particular with regard to tests 6 and 7.

The influence of the cooking conditions on the insolubilization performance of various mixtures is studied in Table 3 below, which groups together the leaching characteristics obtained on pellets prepared from the preceding binary mixture (pot lining / kaolin) as well as 3 ternary mixes (pot lining / kaolin / cement) (pot lining / feldspar / cement) (pot lining / kaolin / clay) and cooked under different conditions of temperature and duration. TABLE 3

) n ° of the corresponding test in table 2

It is noted (test No. 1 7) that the elimination of cyanides is almost complete after 1 hour of cooking at 700 ° C. of the brazed binary / kaolin mixture but that the insolubilization of the fluorinated compounds is completely insufficient. If after the calcination / _ at 900 ° C the leaching results are excellent (test no. 18), on the other hand, there is an aggravation of the losses of fluorine in gaseous form linked to the absence of a 2nd additive (cement , clay, CaO or CaC0 ₃ ) capable of consolidating the residue pellets but also of trapping fluorine originating from the start of dissociation of certain fluorinated compounds.

The tests relating to the ternary brazing / kaolin / cement (n ° 8, 9 and 10) and brazing / kaolin / clay (n ° 14, 15 and 16) mixtures show that from 3 hours of cooking at 835 ° C obtains leaching rates for Na and F quite comparable to those obtained after 1 h ¹ / "cooking at 900 ° C. These leaching rates, representative of the degree of insolubilization of the residues in the form of pellets after cooking, are further improved if the cooking time is extended to 6 hours, but in this case the cost of treatment is very significantly increased. and can become prohibitive in terms of industrial exploitation.

In table 4 below are recorded the results of the leaching and mechanical strength tests on ternary mixtures of used pot lining / FHB 200 feldspar and Portland cement with rapid setting where, on the one hand, the rate of used pot lining was varied (tests n ° 19 to 21). TABLE 4

() n ° of the corresponding test in Tables 2 and 3.

From a rate of spent pot lining of 60%, a significant degradation of the leaching performance and therefore of insolubilization is observed, and to a lesser degree the mechanical strength after baking and therefore consolidation.

As previously indicated, for spent pot lining rates of less than 40%, the method according to the invention applies all the better in terms of performance but, on the other hand, its operating cost becomes prohibitive.

IMPLEMENTATION OF THE INVENTION

The method according to the invention will be better understood from the detailed description of its implementation based on FIG. 1 which is the schematic representation of the successive stages of the method. One tonne of spent pot lining 1 coming from the dry unmasking of aluminum production electrolysis tanks and constituting a complete mixture of silico-aluminous refractories and carbon blocks with a particle size of 0-700 mm is crushed and then ground A to obtain a powder product 2 with a particle size of 0-2.5 mm, the weight contents of carbon, fluorine, sodium, silicon and cyanide being 38.4%, 9.2%, 14.6%, 1 1, 1% and 0 respectively , 12%.

The pulverulent product 2 is mixed B with on the one hand 500 kg of a first pulverulent additive 3 consisting of feldspar FHB200 (sodium feldspar albite type, a typical analysis of which is given in table 1) and on the other hand 500 kg of a second powder additive 4 consisting of Portland cement with rapid setting, for example Lafarge CEM cement 52, 5.

The resulting powder mixture 5 is moistened by spraying water 6 at a rate of 15% to 25% of the dry mix weight before being C-shaped in ^the state of pellets in a pelletizer constituted by an inclined circular table and in rotation allowing the formation of small nodules. These nodules grow by coating in contact with the humidified pulverulent mixture 5 which is continuously recycled, to form increasingly large pellets which are removed from the device when they reach the desired size, generally a diameter of between 6 mm and 20 mm and fixed in this case 10 mm.

The raw dumplings 7 calibrated with a diameter of 10 mm and whose specific volume before cooking is between 1 and 1, 2 g / cm ³ are then baked for 1 hour at 900 ° CD in a fluidized bed reactor by adjusting appropriately the fluidization air flow. The fluorine content of the gases leaving the reactor at constant flow rate is checked by taking a gas sample every 15 minutes. The cooked pellets 8 representing 1600 kg of residue are then subjected to the various insolubilization control tests (according to standard NF-X31210) and mechanical strength. The following results were obtained:

Leachable fluorine% = 0.1

Na leachable% = 0.1 7 mechanical strength: a) cohesion test (Turbula): good (<1% of fines) b) resilience test: 100% c) crush resistance test: 84%.

Note that on this same batch of cooked dumplings 8 leaching tests were carried out, according to 2 other standards (E.P.A in the United States and M.E.N.V.I.Q in Canada.). The results recorded are also within the authorized limits with respectively:

- according to E.P.A. Leachable fluorine% = 0.07

Na leachable% = 0.65 - according to M.E.N.V.I.Q. Leachable fluorine% = 0.006

Na leachable% = 0.55.

The low leaching rates recorded, much lower than the standards currently in force (F leachable <0.5%), as well as the excellent mechanical strength of the pellets authorized the landfill 8a of lot 8 of 1600 kg of inert residue in the form of pellets of which a fraction 8b was taken to constitute a make-up charge in an embankment.

Claims

. Insolubilization and consolidation process in the form of spent pot lining pellets from electrolytic cells for the production of aluminum comprising the grinding of said pot lining consisting of carbonaceous products and silico-aluminous products impregnated with fluorinated alkaline compounds and cyanides as well as their intimate mixing with various refractory pulverulent additives before heat treatment or cooking, characterized in that after introduction: - of a first additive intended during cooking to fix the alkali metals, in particular sodium, coming from fluorinated compounds and which is chosen in the group of silicoaluminates, then of a second additive intended during cooking, on the one hand to combine, with or without fusion, with the fluorinated compounds of impregnation to form new stable and insoluble compounds such as CaF2, on the other hand, by its non-combined excess fraction, to be insured before, but especially during and after c uisson, the consolidation of the mixture, said second additive being chosen from the group of calcium oxides and salts and preferably cements and calcareous mortars, an intimate shaping is carried out in the form of pellets of the intimate mixture of old pot liners crushed with said first and second powder additives before baking at a temperature between 700 ° C and 950 ° C.

2. Method according to claim 1 characterized in that the old pot liners are ground to a particle size of less than 5 mm and preferably less than 2.5 mm.

3. Method according to claims 1 or 2 characterized in that said first additive chosen from the group of silicoaluminates is a clay or a feldspar.

4. Method according to any one of claims 1 to 3 characterized in that said second additive chosen from the group of oxides of calcium salts is CaO or CaC0 ₃ .

5. Method according to any one of claims 1 to 3 characterized in that said second additive chosen from the group of limestone cements and mortars is Portland cement.

6. Method according to any one of claims 1 to 5 characterized in that the weight proportion of old pot lining crushed in said intimate mixture is between 40% and 60%.

7. Method according to any one of claims 1 to 6 characterized in that the weight proportion of the first additive is between 15% and 35%.

8. Method according to any one of claims 1 to 7 characterized in that the weight proportions of the second additive are between 15% and 35%.

9. Method according to any one of claims 1 to 8 characterized in that the shaping by agglomeration in the form of pellets is carried out by compression or extrusion of the prehumidified mixture followed by granulation.

10. Method according to any one of claims 1 to 8 characterized in that the shaping by agglomeration in the form of pellets is carried out by direct granulation of the prehumidified mixture at a humidity level set between 15% and 25% by weight dry mix and up to a diameter of pellets between 6 mm and 20mm.

1 1. Process according to any one of Claims 1 to 10, characterized in that the temperature for cooking the pellets is preferably set between 800 ° C and 900 ° C.

12. Method according to any one of claims 1 to 1 1 characterized in that the cooking time of the pellets is between 1 hour and 6 hours.

13. Method according to claim 12 characterized in that the cooking time of the pellets is between 1 hour and 2 hours.

14. The method of claim 13 characterized in that the cooking of the pellets is carried out in a rotary oven or in a fluidized bed oven.

15. Method according to claim 14 characterized in that the cooking of the raw pellets whose carbon content is at least 25% is carried out in an incineration oven where they act as auxiliary fuel.

1 6. Application of the pellets from the process according to claims 1 to 14 for the manufacture of embankments.