EP0486410B1 - Wet process treatment of spent pot-linings from Hall-Héroult electrolytic cells - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a process for the treatment, by the wet process, of used pot linings originating notably from the dismantling of tanks for the production of aluminum by electrolysis according to the Hall-Héroult technique. It should be recalled that an aluminum production plant with a capacity of 240,000 t / year generates about 4,000 t / year of spent pot lining, which consists of carbonaceous cathode blocks, seals and side linings made of carbonaceous paste, but also by all of the refractories and insulators arranged on the side walls and the bottom of the metal box forming the electrolysis tank. After use, these packing products are heavily impregnated with harmful products, such as soluble sodium or sodo-aluminous fluorides, and cyanides which must be insolubilized or destroyed before being landfilled or reused.

STATE OF THE ART

There are known 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-A-4,065,551 Elkem, or US-A-4,113,832 and US- A-4 116 809 Kaiser, either on a simple combustion in 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 4,053,375 Reynolds or according to the article by LC Blayden and SG Epstein, Journal of Metals, July 1984 page 24.

In fact, all 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 quickly becomes impossible to avoid their solidification and consequently to maintain a bed fluidized, a fortiori a dense bed, if 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.

There are also known methods for treating old pot lining by the wet method and in particular in an alkaline medium. These treatments consist most of the time in leaching with soda or with soda ash of the carbonaceous mixes previously sorted and ground in order to dissolve the fluorinated compounds in the form of NaF and the aluminous compounds in the form of sodium aluminate either to enhance separately these products (US-A-4,113,831) or to co-precipitate them then in the presence of CO₂, in the state of cryolite (US-A-2,732,283-US-A-3,106,448). These forms of recovery find only an outlet limited to aluminum production sites with, above all, the drawback, like thermal processes, of leaving the problem of insolubilization without solution before storage or landfilling of non-carbonized solid residues .

The insolubilization of fluorinated compounds is more easily obtained by leaching with lime of crushed pot lining with precipitation of CaF₂ and formation of soda which passes into solution as well as certain compounds containing alumina which transforms into recyclable AlO₂Na in a Bayer process. . Thus GB-A-2 056 425 realizes, after a separate grinding from carbon and non-carbon fractions, a separate recovery of the insoluble products after lime leaching, with in particular the recycling of carbonaceous parts as pot lining and the recovery of CaF₂ as source of hydrofluoric acid. If the previous criticism still applies to this type of process which, although more economical in its implementation, remains dependent on the possibilities of recycling leachate products from aluminum production sites only, it should above all be noted that the insolubilization of fluorides by precipitation in the CaF₂ state is never complete as soon as the content of free soda, product of the reaction, becomes sufficiently high to reduce the solubility of alkaline fluorides and in particular NaF in the solution and consequently their yield of transformation into CaF₂. Thus, it was possible to detect, during insolubilization control tests by leaching of insoluble residues, fluorine contents in the form of soluble fluorides which can reach up to 1% of the weight of the residues preventing any landfill of the non-fractions recycled, in particular the non-carbon fraction of these residues.

PROBLEM

• capable de traiter dans des conditions économiques ces brasques usées quelle que soit leur composition et notamment leurs teneurs en silice et alumine.
• offrant toute sécurité pour l'environnement, c'est-à-dire assurant lors du traitement une décomposition totale des cyanures et une insolubilisation quasi complète des fluorures ainsi que des métaux alcalins liés.

Knowing that spent pot lining contains significant quantities of fluorinated derivatives (up to 200 kg of fluorine contained per ton), sodium products (up to 200 kg of Na contained per ton), non-negligible quantities of cyanides (up to at 10 kg per tonne) and that, moreover, these elements harmful to the environment are found both in the carbonaceous part of the internal lining of the electrolysis tank and in the part of the silico-aluminous briquetting constituting the thermal insulating lining of the tanks modern, it became necessary to design an industrial process,

• capable of treating these spent pot lining under economic conditions, whatever their composition and in particular their silica and alumina contents.
• offering all safety for the environment, that is to say ensuring during the treatment a total decomposition of cyanides and an almost complete insolubilization of fluorides as well as bound alkali metals.

OBJECT OF THE INVENTION

The method according to the invention relates to a treatment for insolubilization by the wet method of used soldering with a view to their storage or their landfill. These mixtures of variable composition consist of carbonaceous products (30% to 50% by weight) but also of refractory elements, generally of the silico-aluminous type, impregnated with cyanides (CN ≦ 1% by weight) of fluorinated compounds (F ≦ 20%) generally alkaline, sodium (Na ≦ 20%) mainly as NaF.

• on effectue un broyage, de préférence en milieu aqueux, des brasquages usés "tout venant" et préalablement concassés, à une granulométrie comprise entre 0 et 250 mm en présence de chaux en quantité supérieure de 5% à 25% en poids à la quantité stoéchiométrique pour précipiter la totalité du fluor contenu, et l'on forme une suspension en milieu aqueux qui est maintenue sous agitation pendant un temps suffisant pour obtenir la précipitation sous forme de CaF₂ des fluorures alcalins avec libération de soude ;
• on chauffe ensuite entre 10 minutes et 4 heures la suspension à une température T > 140°C en présence d'argile en quantité suffisante pour fixer la soude libre par formation d'un composé de synthèse insoluble, puis l'on refroidit la dite duspension ;
• on sépare le résidu solide, constitué par les éléments de brasquage non attaqués, le précipité de CaF₂ et le composé de synthèse insoluble, des eaux-mères et après lavage à l'eau le résidu est mis en décharge, alors que les eaux-mères et les eaux de lavage sont réunies pour être recyclées dans les deux premières étapes du procédé.

More precisely, the method according to the invention is a wet treatment of spent pot linings, previously ground, by calcium attack with a view in particular to insolubilize in the state of CaF₂ the fluorine of the impregnation compounds of these pot linings characterized by the 3 following main steps:

• a grinding is carried out, preferably in an aqueous medium, of the used "all comings" and previously crushed pot linings, at a particle size between 0 and 250 mm in the presence of lime in an amount greater than 5% to 25% by weight than the stoichiometric amount to precipitate all of the fluorine contained, and a suspension is formed in an aqueous medium which is maintained under stirring for a time sufficient to obtain the precipitation in the form of CaF₂ of the alkaline fluorides with release of soda;
• the suspension is then heated between 10 minutes and 4 hours to a temperature T> 140 ° C in the presence of clay in sufficient quantity to fix the free sodium hydroxide by formation of an insoluble synthetic compound, then the said suspension is cooled ;
• the solid residue, consisting of the non-etched brazing elements, the CaF de precipitate and the insoluble synthetic compound, is separated from the mother liquors and after washing with water the residue is landfilled, while the mother liquors and the washing water is combined to be recycled in the first two stages of the process.

DESCRIPTION OF THE INVENTION

The process according to the invention is based on the observation that it is possible to recombine, with inexpensive and readily available additives, almost all of the fluorine of the fluorinated compounds impregnating the spent pot lining as well as the free soda produced from the causticization reaction, to form insoluble compounds which are easily filterable, while the remaining soluble compounds such as cyanides, generally in the state of sodium cyanide, are dissociated under the temperature and pressure conditions chosen to carry out these insolubilization reactions.

To achieve these results, a certain number of essential operating conditions must therefore be observed.

Il est nécessaire toutefois pour obtenir une précipitation quasi complète des ions fluor en CaF₂ d'utiliser un excès de chaux de 5 % à 25 % en poids par rapport aux quantités stoechiométriques.Thus the reactions of lime with the main fluorinated compounds NaF, AlF₃ and Na₃AlF₆ systematically lead to the precipitation of CaF₂, for example:

It is necessary, however, to obtain an almost complete precipitation of the fluorine ions into CaF₂, to use an excess of lime of 5% to 25% by weight relative to the stoichiometric quantities.

Furthermore, the soda released by the calcium attack on alkaline fluorides tends to react hot with certain non-soluble aluminous compounds to form sodium aluminate. It is therefore preferable to carry out lime leaching at a temperature t <80 ° C. and in a sufficiently diluted medium, preferably with a dry matter concentration of less than 500 g per liter of suspension.

To improve the kinetics of the reactions which therefore take place at relatively low temperature, it is advisable to obtain by grinding a large state of division of the potashings previously crushed in the form of blocks of dimension less than 250 mm; so that the mixture of pot lining and lime particles, once ground, reaches a particle size between 0 and 1 mm. This grinding is preferably carried out in a humid environment consisting of the recycled liquor from the mixture of mother liquors and washing water, so that CaF₂ precipitated on the surface of grains of crushed potash is eliminated as it is formed. thus avoiding any slowing or blocking of the reaction at the heart of the grains.

In the following stage, the hot fixation of the soda with a clay, to form an insoluble synthetic compound, is carried out for example by kaolinite (2SiO₂, Al₂O₃, 2H₂O) in the natural state of kaolin, with formation of insoluble feldspathoids such as hydroxysodalite, depending on the reaction:



3 (2SiO₂, Al₂O₃, 2H₂O) + 8NaOH ---> 3Al₂O₃, 6SiO₂, 4Na₂O, H₂O + 9H₂O



which requires the use of at least 2.42 kg of kaolinite per kg of free sodium hydroxide.

The amount of free sodium hydroxide obviously varies according to the chemical composition of the pot lining and the proportion of pot lining crushed in the suspension. In practice, the addition of kaolinite is regulated between 0.12 and 0.20 kg per kg of suspension, the weight composition of which, for sodium contents in the used pot lining generally between 10% and 15% by weight, varies in the following proportions: Crushed solderings <5 mm 20 to 30% Slaked lime 4 to 7% Recycled water or liquor 63 to 76%

In addition to kaolinite, the main clays usable include illite and smectites such as montmorillonite, bentonite, which by fixing the free sodium hydroxide lead to the formation of a mixture of insoluble zeolites (analcime and zeolite Na-Pt). More generally, the mixtures thus formed are synthetic feldspathoids or zeolites.

Also note in this sodium insolubilization step the importance of the reaction times between 10 minutes and 4 hours and preferably between 30 minutes and 3 hours, but especially the reaction temperature which must be above 140 ° C. and preferably between 180 ° C and 250 ° C. The increase in temperature promotes, on the one hand the reaction in the direction of the transformation of the clay into insoluble synthetic compound by fixing of free soda, on the other hand the decomposition of the cyanides by hydrolysis according to the basic reaction



CN⁻ + 2H₂O ---> NH₃ + HCOO⁻



The increase in temperature finally promotes the formation of precipitates with a "permeable" texture, that is to say easily filterable and washable.

In fact, in the final stage of separation by filtration of the solid residue from the mother liquors, the excellent permeability of the solid cake which can be effectively washed with a relatively small volume of water is observed since the "wash ratio" is that is to say the volume of water necessary for washing relative to the volume of water impregnating the wet cake after spinning, is substantially close to 1. After filtration and washing, the mother liquors and the washing waters are combined to form a weakly alkaline liquor which is recycled in the first two stages of the process for the preparation of the pot lining and lime suspensions on the one hand and kaolin on the other hand.

The fact that the process does not generate any liquid discharge, since it loses a volume of impregnating liquid in the spun cake equivalent to the volume of washing water injected, makes it possible to overcome the problem of storage and specific treatment of the water excess wash containing soluble impurities. In the present case, the weakly alkaline liquor resulting from the union of the mother liquors and the washing waters is therefore entirely recycled.

As for the wet residue, it is subjected to a leaching test according to French standard X 31-210 to determine the levels of cyanide, sodium and fluorine; the CN content by weight must remain below 0.01% and the fluorine content must remain below a limit which varies, according to the national standards in force, between 0.05% and 0.3%.

Table 1 below summarizes the most significant characteristics of comparative tests of autoclave treatments, for 3 hours, at different temperatures 145 ° C, 180 ° C and 220 ° C of 3 samples of 2 kg of a suspension of "all comings" crushed pot lining whose fluorine and sodium contents were 10.1% and 14.9% respectively. The weight composition of the 2 kg samples was 0.47 kg of pot lining, 0.1 kg of slaked lime and 1.43 kg of water. The proportion of Kaolin then added was 0.15 kg per kg of suspension. The suspension after autoclave treatment was cooled to below 100 ° C and filtered under a vacuum of 500 mm Hg. TABLE 1 Test No. 1 2 3 T ° C autoclaving 145 180 220 Filtration speed m / h 0.49 1.16 1.77 Wash ratio 1.9 1.5 1.4 Washing speed m / h 0.58 1.1 1.43 Filtrate analysis Na2O g / l 22.1 11.4 5.3 Mother Liquor F g / l 1.42 0.925 0.72 Filtrate analysis Na2 O g / l 11.6 7.6 3.6 Wash water F g / l 1.04 0.77 0.54 Ton dry residue per m² per hour 0.13 0.21 0.29

We note the influence of the increase in the autoclaving temperature, both on the lowering of the free soda content expressed as Na₂O resulting from a higher conversion rate into hydroxysodalite, both on the increase in filtration and washing speeds and ultimately on the tonnage of residue in the dry state which can be treated per m² of filtration surface and per hour.

In Table 2 below, the results of the checks on the distribution of pollutants F and Na in solid residues and in recycled liquors (mother liquors + washing water) are collected for the same three tests. , on the other hand, the results of the leaching tests on dry residues according to standard F.X31-210. TABLE 2 TEST NO. 1 2 3 TEMPERATURE ° C 145 180 220 Residue F% 93.07 95.55 96.94 Liqueur F% 6.93 4.45 3.06 Residue Na% 57.28 77.55 90.02 Na% liquor 42.72 22.45 9.98 Dry residue leaching Leachable content F% 0.11 0.09 0.1 N / A % 0.17 0.18 0.18 CN% <0.01 <0.01 <0.01

It is confirmed that for autoclaving temperatures higher than 200 ° C. the fluorine and especially sodium contents in the insoluble residue increase whereas on the contrary they decrease in the liquor due in particular to a better conversion rate in hydroxysodalite. As for the leaching tests on dry residues, they are satisfactory in especially when it comes to cyanides.

IMPLEMENTATION OF THE PROCESS

The invention will be better understood by the description of its industrial implementation according to the example below and with reference to the process diagram in FIG. 1.

Three tonnes of old pot lining from the dry de-bashing of aluminum electrolysis tanks constituting an all-in-one mixture 1 (silico-aluminous refractory + carbon blocks) of grain size 0-700mm are crushed giving a product 2 of grain size 0-250mm whose fluorine, sodium, silicon and cyanide contents are 9.2%, 14.6%, 11.1% and 0.12% respectively.

Then an autogenous grinder is fed simultaneously with the crushed product 2 at a rate of 1 t / h, quicklime 3 (grain size 0-10 mm) at a rate of 0.25 t / h, recycled liquor 4 at a rate of 4, 0m³ / h. The autogenous grinder operates in a closed circuit on a 1 mm mesh screen. The suspension obtained 5 is stored in a stirred tank for an average period of 1 hour.

• un échangeur monotubulaire de réchauffage 11 où la suspension est portée à 220°C.
• Un autoclave de temps de séjour (1 heure) non chauffé et calorifugé 12.
• Un échangeur monotubulaire de refroidissement 13 où la suspension est refroidie à 100°C.

At the same time, a suspension is prepared at 500 g / l of crude kaolin 7 in recycled liquor 8. A high pressure pump discharges under 35 bars (3.5 MPa) the mixture of the suspensions of old pot liners crushed with lime, ie 4, 6 m³ / h and kaolin 9, i.e. 1.9 m³ / h in a pressure treatment installation which includes:

• a monotube reheating exchanger 11 where the suspension is brought to 220 ° C.
• An unheated and insulated residence time autoclave (1 hour) 12.
• A monotube cooling exchanger 13 where the suspension is cooled to 100 ° C.

A heat transfer fluid rotates in a closed circuit between the two exchangers and a boiler where an additional thermal energy is made 21, 22 and 23.
The cooled suspension 13 is then filtered and the cake formed is washed on belt filter. The mother liquors 15 and the washing waters 17 representing a total of 5.5 m³ / h are combined in a storage tank to form the recycled liquor 4 and 6. The washing water 16 is 3.1 m³ / hour compensates for the loss of water by impregnating the residue.

$\text{volume horaire sortant : EM et Eaux Lavage+résidu humide=5,5+4,1 =9,6 m³/h.}$

The balance of liquids and solids circulating at the washing filtration station can be summarized as follows:

$\text{incoming hourly volume: suspension + washing water = 6.5 + 3.1 = 9.6 m³ / h}$

$\text{hourly outgoing volume: EM and Wash Water + wet residue = 5.5 + 4.1 = 9.6 m³ / h.}$

Note that the wet residue has a total weight of 5.5 tonnes, including 3.1 tonnes of water.

The control of the distribution of fluorine and sodium in the residue and the liquor (mother liquors + washing water) indicated: Residue F% 97.2 Residue Na% 91.2 Liqueur F% 2.8 Na% liquor 8.8

In addition, the leaching test on samples of residues washed and dried in the oven gave the following results: F% 0.09 N / A % 0.17 CN% <0.01

These results confirm the effectiveness of this process of insolubilization and trapping of pollutants, which moreover makes it possible to industrially treat significant amounts (1.5 T / hour) of spent pot lining under entirely favorable economic conditions taking into account the possibility of avoiding specific treatment of washing water which is fully recycled, and the use of inexpensive reagents, readily available, such as kaolin and lime.

Claims (13)

1. A process for the wet treatment of previously ground spent pot linings by calcic attack, particularly with a view to insolubilising in the CaF₂ state the fluorine of the impregnation compounds, characterised by the following three main stages:

   - grinding takes place of the miscellaneous spent pot linings which have been previously crushed to a grain size of between 0 and 250 mm, in the presence of lime which in quantity is more than 5% to 25% by weight than the stoichiometric quantity, in order to precipitate all the fluorine contained, and a suspension is formed in an aqueous medium which is kept in an agitated state for a sufficient time to bring about the precipitation in CaF₂ form of the alkali metal fluorides, with the release of soda;

   - the suspension is then heated for between 10 minutes and 4 hours to a temperature T > 140°C in the presence of an adequate quantity of clay to fix the free soda by the formation of an insoluble synthetic compound and said suspension is then cooled;

   - the solid residue, constituted by the unattacked pot lining elements, the CaF₂ precipitate and the insoluble synthetic compound, is separated from the mother liquors, and after washing with water the residue is discharged, whereas the mother liquors and the washing waters are combined for recycling to the first two stages of the process.
2. A treatment process according to Claim 1, characterised in that the contents by weight of cyanide, fluorine and sodium in the spent pot linings are respectively CN ≦ 1%, F ≦ 20%, Na ≦ 20%.
3. A treatment process according to Claim 1, characterised in that the mixture of particles of spent pot linings and lime, after grinding, has a grain size of between 0 and 1 mm.
4. A treatment process according to Claim 1, characterised in that the grinding of the previously crushed, spent pot linings is carried out in aqueous medium.
5. A treatment process according to one of Claims 1 or 4, characterised in that the suspension is usually constituted, in proportions by weight, by 20 to 30% ground pot linings. 4 to 7% lime and 63 to 76% water.
6. A treatment process according to Claim 1, characterised in that the temperature of the suspension, maintained agitated after grinding, is below 80°C.
7. A treatment process according to any one of Claims 1, 5 or 6, characterised in that the clay added to the suspension is selected from the group of illite, kaolinite and smectites.
8. A treatment process according to Claim 7, characterised in that the kaolinite (2SiO₂, Al₂O₃, 2H₂O) in the natural kaolin state is added to the suspension in a proportion, by weight, of between 0.12 and 0.20 kg per kg of kaolinite per kg of suspension.
9. A treatment process according to Claim 7 or Claim 8, characterised in that the quantity of kaolinite used is at least 2.42 kg per kg of free soda in the suspension.
10. A treatment process according to any one of Claims 1 and 7 to 9, characterised in that the suspension to which kaolin is added is heated to a temperature T, preferably between 180°C and 250°C.
11. A treatment process according to any one of Claims 1 and 7 to 10, characterised in that the suspension to which kaolin is added is heated to the temperature T for between 30 minutes and 3 hours.
12. A treatment process according to Claim 1, characterised in that the solid residue is washed with a volume of water which is equivalent to the volume of impregnation water retained in said solid residue.
13. A treatment process according to Claim 1 or Claim 11, characterised in that the mother liquors and washing waters are combined to form a slightly alkaline liquor which is recycled to the first two stages of the process for forming suspensions of pot linings and lime, on the one hand, and kaolin, on the other hand.

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