AU663390B2 - Process for the heat treatment of used brasque linings from Hall-Heroult electrolysis tanks - Google Patents

Description

i _~I~~PI PROCESS FOR THE HEAT TREATMENT OF USED BRASQUE LININGS FRCM HALL-HEROULT ELECTROLYSIS TANKS TECHNICAL FIELD OF THE INVENTION The invention concerns a process for the treatment in a reactor of used brasque linings coming in particular from dismantling of tanks for the production of aluminium by means of electrolysis using the Hall-Heroult process. It should be recalled that a factory for the production of aluminium with a capacity of 240,000 T/year generates about 4,000 T/year of used brasque linings which are formed by carbonaceous cathodic blocks, sealing joints and side linings which are made of carbonaceous paste, but also all of the refractory and insulating materials which are disposed on the side walls and the bottom of the metal casing which forms the electrolysis tank. After use those lining products are heavily impregnated with harmful products such as soluble sodium or sodium-aluminium fluorides and cyanides which have to be rendered insoluble or destroyed before being discharged or re-used.

STATE OF THE ART Processes for the treatment of old brasque linings by a wet method comprising crushing followed by leaching by means of an alkaline liquor have already been described for example in US patent No 4 052 288 to Aluminium Pechiney or U8 patent No 4 113 831 to Kaiser.

Those processes which require the use of substantial items of equipment are limited just to treating the carbonaceous parts of the brasque linings which have to be separated beforehand from the noncarbonaceous parts formed by refractory and insulating materials.

Treatment processes using a thermal procedure are also known, -CI ii I which generally operate in a fluidised bed and which are based either on pyrohydrolysis at more than 1,000°C of the used brasque linings in accordance with US No 4 065 551 to Elkem, or US No 4 113 832 and US No 4 116 809 to Kaiser, or simple combustion in air or in an oxidising atmnosphere of the carbonaceous elements, at a temperature of about 800°C, being sufficient to cause decomposition of the cyanides without giving rise to the substantial production of volatile fluorinated compounds, in accordance with US No 4 053 375 to Reynolds or as disclosed in the article by L.C. Blayden and S.G.

Epstein, Journal of Metals, July 1984, page 24.

In fact, all the processes and apparatuses which use the thermal procedure are limited in terms of what they can do by the nature and composition -f the brasque linings to be treated. Indeed, due to the fusion of certain eutectic compounds which are formed in the course of combustion, the particles of brasque linings have a major tendency to agglomerate. It rapidly becomes impossible to prevent them from solidifying and consequently to maintain a fluidised bed and a fortiori a dense bed if combustion is effected for example in a rotary furnace with a substantial residence time. i That agglomeration phenomenon which is already perceptible with the charges of brasque linings which are formed solely by carbonaceous products is greatly accentuated with charges containing refractory oxides and in particular silica, the proportion by weight of which must not exceed 3 or as can be seen from the article by E.R.

Cutshall and L.O. Daley, Journal of Metals, November 1986, page 37, table II.

The alternative procedure which involves effecting combustion in a reactor with a circulating fluid bed of carbonaceous brasque linings mixed with an additive formed by finely crushed refractory oxides, in accordance with US No 4 763 585 to Ogden makes it possible to limit the risks of surface sticking and then agglomeration of the I r -7 i 3 grains. To do that the additive must remain inert during the combustion procedure, that is to say it must neither react nor melt in contact with the carbonaceous waste in the temperature range which is generally selected to be of the order of 1,00OC. However, in this case also, the problem of agglomeration is overcome only insofar as the procedure involves the treatment of carbonaceous brasque materials or at least brasque materials resulting from linings with a very small proportion of silica, being therefore essentially based on alumina, the service life of which is markedly shorter than that of modern linings of preformed silico-aluminium materials.

A process developed by the applicants and described in a previous patent application (EP-A-O 465 388) makes it possible, without sorting beforehand, to treat all types of brasque lining, in particular those in which the proportion by weight of silica can reach 50%, under conditions which afford total safety in regard to the environment, with total decomposition of the cyanides whose proportion in the used brasque linings may attain 1% and virtually complete insolubilisation of the alkali metal fluorides which can represent up to 20% of fluorine and 20% of sodium by weight.

This process involves heating the used brasque linings violently and for a very short period of time to a temperature sufficient to permit decomposition of the cyanides, and preferably in the presence of a reactive powdery additive, that is to say which is capable of chemically combining with the fluorinated products which 25 impregnate the brasque linings, to form stable insoluble compounds such as CaF 2 binary, ternary or quaternary compounds of NaF, CaF 2 CaO, SiO, Al 203 CaSO 4 Na2SO 4 of the nepheline, hauyne or other type.

More precisely the mixture of crushed brasque 'linings with 30 the powdery additive, preferably kaolin, anhydrous or hydrated CaSO 4 CaO, or a mixture thereof, is injected in the upper part of a reactor

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4 in a gas flow flowing at a tempera"ure of between 1100°C and 1800°C, and the contact time of the solid particles of the mixture with the circulating gas flow is regulated in such a way that the particles attain a temperature of at least 750°C before being extracted at the base of the reactor with the gas flow. In a steady-state operating condition, the temperature of the gas flow as measured at a point in the reactor is regulated to a reference value by adjustment of the flow rate by weight of powdery mixture injected at the top of the reactor. At the outlet at the base of the reactor, after cooling, the gas flow is separated from the solid particles constituting the stabilised residue, that is to say the residue whose harmful compounds have been insolubilised or destroyed.

PROBLEM SET In the course of carrying out that process on an industrial scale, the applic its were confronted with two fresh difficulties of such a nature as to compromise the economic attraction of that heat treatment: on the one hand, the appearance in steady-state operating conditions of an adherent deposit of increasing thickness of fine particles on the inside wall of the reactor when their temperature exceeds 1200°C, thereby necessitating frequent stoppages for cleaning purposes, which are therefore incompatible with the operating procedure initially envisaged; and on the other hand, the presence in the cooled gaseous phase after separation of the solid particles of a certain amount of HF, requiring the addition of an expensive and bulky piece of equipment for trapping the HF vapours in the gaseous phase, by a wet process, before they are discharged into the atmosphere.

SUBJECT OF THE INVENTION Analysis of those two problems which are apparently unrelated C' sf r s r Ij I showed in actual fact that they both resulted from a single cause: an excessively high temperature of the gas flow in the. upper part of the reactor where, while nonetheless being cooled, the inside wall can locally reach 1200'C. That high temperature causes on the one hand sticking of the fine particles entrained in the gas flow displaced with a i 5 turbulent movement and remaining in prolonged contact with the wall at more than 11001C, and on the other hand the begimnnings of pyrohydrolysis of the alkali metal fluorides as hydrofluoric acid by virtue of the water vapour originating from the residual moisture of the used brasque linings, the proportion of which can attain 2% by weight, depending on the storage conditions and time. That pyrohydrolysis reaction at 1100 0

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may occur immediately at the outlet from the reactor when the gas flow comes into contact at a temperature of more than 1000'C with the saturated atmosphere generated by the water retention tank placed beneath the reactor.

The solution provided by the applicants involved as a matter of priority not acting directly on the temperature of the combustion gases at the location of the injector but increasing the flow rate by weight of the powder mixture feeding the injector in order to reduce to less than 1100"C the temperature of the gas flow circulating in the reaction chamber.

Against every expectation, in spite of the substantial increase in the amounts of cyanides and fluorides introduced into the reactor by virtue of the flow rate by weight of powder mixture being virtually doubled, the excellent levels of pollution reduction recorded with the basic process have been not only preserved but improved in the process claimed herein.

More precisely the invention concerns a process for the heat treatment after crushing of used brasque linings comprising carbonaceous products and silico-aluminous products impregnated with fluorinated mineral compounds, crushed to a particle size of less than 5mm, said process comprising: I forming a mixture of said brasque linings and 0 to 50% by weight, based on said mixture, of a powdery mineral additive capable of combining in the hot condition, with or without fusion, with said fluorinated mineral compounds to form stable, insoluble compounds; j injecting said mixture into a gas flow circulating at a speed of at least im/sec in the upper part of a reactor so as to maintain a temperature T of between 7001C and 11001C; regulating the contact time of the solid particles of the mixture with the circulating gas flow to between 0.3 and 3 second whereby said particles attain a temperature t of between 400'C and 7501C prior to their extraction at the base of the reactor with said gas flow; maintaining the temperature of said gas flow as measured at the base of the reactor at a reference value T o such that T T o t; separating said particles from said gas flow at the reactor outlet after cooling; and [N:\LIBZ]00424:EAR II I I 6 removing dust from said gas flow before said gas flow is discharged to the atmosphere.

The essential characteristic of the process according to the invention is based on the fact that, without solidification of the charge of used brasque linings consequent upon possible fusion of the particles, it is possible to effect pollution removal reactions comprising destroying the cyanides and immobilising the impregnation fluorinated compounds by chemical recombination in particular with the additive by violently bringing into contact for a very short period of time, of the order of from 0.3 to 3 seconds, the solid particles of the mixture and a gas flow at a temperature T wbich is between 700 and 1100°C and preferably between 800°C and 1000°C. The hot gas flow is the product of combustion of a fuel gas such as [N:\LIBZ]00424:EAR 1 i i -L8uur: ij methane, butane or natural gas, in the presence of air, oxygen or a mixture thereof. The thermal shock which is generated in that way permits decomposition of the cyanides and recombination of the impregnation fluorinated agents virtually instantaneously whereas the temperature in the solid particles remains lower than that of the gas flow and therefore between 400°C and 750°C, preferably between 450°C and 7000C. Possible softening of the solid particles by partial fusion, which is linked in particular to the formation of fusible eutectic compounds, is not troublesome insofar as any prolonged contact and therefore any sticking as between particles is prevented by producing sufficient agitation in the reaction chamber. That agitation effect is produced by regulating the speed of the gas flow at a sufficient value, at least 1 m/second at the treatment temperature in question.

In addition to the temperature T and the speed of the gas flow which are governed by regulation of the flow rates of fuel and ccmbustion-suppoiting gases, it is also appropriate to fix the mean residence time of the particles in the reaction chamber. The range of from 0.3 to 3 seconds adopted is the best compromise for effective treatment of brasque linings of greatly different compositions, within the limit of the maximum proportions acceptable (Na 20% and Si necessitating heating of the particles of the mixture to a temperature t which is preferably bftween 4500C and 700°C to ensure an adequate level of kinetics of the recombination reactions without causing downright fusion of the particles with the risk of solidification. The particle residence time depends not only on the speed of the circulating gas flow but also more technological parameters such as the dimensions of the reaction chamber wlichi is generally of a conical or cylindrical-conical shape as well as the trajectory of the circulating gas flow, which is linked to the method of injection selected and therefore the characteristics of the injector.

1 i Adaptation of the characteristics of the mixture, in particular, its composition and its granulcmetry, is also essential for carrying the process into effect. Thus the used brasque linings are crushed to produce particles of a size of less than 5 mm and preferably less than 3 mn. In addition the powder mineral additive which is mixed with the crushed brasque linings in a proportion by weight of from O to 50% of the final mixture and which is capable of chemically combining with the impregnation products, in particular with the fluorides, to form stable insoluble compounds, is preferably kaolin, anhydrous or hydrated CaSO4, lime or a mixture thereof, being inexpensive reactive materials which are readily available. In order to increase the reactivity of the powder additive of a granulcmetry of between 0 and 1 mm, it is appropriate for it to be intimately mixed with the crushed brasque linings, and an alternative procedure in terms of preparation of the mixture may consist of introducing the additive int the coarsel' crushed brasque linings and terminating the crushing operation on the mixture until the granulcmetry required for the brasque lining particles is achieved.

All those parameters having been examined and fixed by experiment, it was found in carrying the invention into effect that it was possible to control continuously and precisely the heat treatment of substantial amounts of used brasque linings, affording the process a high degree of reliability and a high level of flexibility of operation on an industrial scale. It is in fact possible by means of the gas flow/solid material exchanger system, between the inlet and the outlet of the reactor, to stabilise the operating condition of the reactor by maintaining a reference value To in respect of the temperature of the gas flow measured prior to its discharge from the reaction chamber at the lower part of the reactor. For that purpose it is only necessary to modify in dependence on the variations in tenperature with respect to the r 9 reference temperature To, the flow rate by weight of the powder mixture injected into the reactor or possibly the temperature of the gas flow in the upper part of the reactor by regulating the flow rate of combustible gas. Those methods of continuously monitoring and regulating the heat treatment of the used brasque linings are in fact effective only insofar as the items of equipment used ard in particular the reactor with its injector are properly adapted as Jis the case with the apparatus produced by Ets VICARB, which is useC to carry out the process in its preferred version.

Finally the gas flow charged with solid particles is extrLacted at the base of the reactor where after cooling the gas flow is separated from the solid particles forming an insoluble residue while the gas flow from which dust is removed is directly discharged to the jznmosphere without any trace of fluorinated gas.

The effectiveness of the insolubilisation treatment is monitored by taking samples of the residue and leaching in accordance with the French standard X 31-210. Quantitative determination operations in respect of CN and F are carried out on the leaching liquors, in whidch respect the proportions of solubilised cyanide and fluorine with respect to the weight of residue must remain respectively below 0.001l% and in accordance with the applicable nationa). standards.

CARRYING OUT THE PROCESS The process will be better appreciated from the following description of the way in which it is carried out in its preferred version, referring to the flow chart in Figure 1.

The powdery material containing at least 50% by weight of crushed brasque linings 1 of a granulcnetry of from 0 to 3 min with addition at 2 of anhydrous or hydrated CaSO 4 or kaolin or lime or a mixtre thereof is extracted at the base of a storage hopper by a screw-type metering transporting device of variable speed of rotation. The screw-type metering device is connected at 3 to the upper part of a cylindrical reactor of VICARB type provided with a gas injector, in accordance with a patented design (EP 0 171 316), acting as a generator for generating a gas flow at a temperature T which is preferably selected at between 800°C and 1000°C by initial regulation of combustion of the fuel gas air mixture completed by adjustment of the flow rate of powdery mixture which in the present case can vary by means of the controllable-speed screw at between 50 kg/hour and 600 kg/hour, to form with the gas flow a suspension whcoe concentration is between 0.1 and 3 Kg per Mn and preferably between 0.3 and 2 Kg per M The metered powdery mixture 3 is injected at the centre of vortex formed by the flow of hot gases 6 arriving tangentially at the top of the cylindrical reaction chamber and involving a swirling movement. In order to provide for good dispersion of the particles of the mixture in the gas flow and good agitation effect, the speed cf the flow is regulated at 6m/second and having regard to the dimensions of the reaction chamber (diameter 1 metre and height 3 metres) the corresponding residence tirie is 0.5 second. Depending on the temperature T selected at the inlet of the reactor, which is generally between 800°C and 100 0 and for a mean residence time of the order of 0.5 second for the solid particles, the flow rate by weight of which can vary from 50 to 600 kg/h, a temperature of between 400°C and 750°C and preferably between 450°C and 700°C is attained for those solid particles before issuing from the reactor and cooling. In parallel monitoring of the temperature To of the gas flow as measured in the same region shows that To varies like t between 400°C and 750°C depending on the thermal operating conditions involved and in accordance with the relationship T>To A t. Once the value of To is fixed for example at 550°C, the temperature a- r ~-_1YiilCi 11 differences s, T with respect to that reference value are corrected by adaptation of the flow rate of powder material injected by means of the feed screw with its variable speed of rotat.ion, under the control of a device for measuring and recording variations in temperature of an amplitude of greater than I 50C with respect to the reference temperature To 550C.

To limit the risk of the particles sticking to the inside wall of the reaction chamber of cylindrical shape, the temperature of which can locally exceed IiC 0 C in the event of a regulating incident, it may be advantageous to provide for cooling of those walls, for example by sweeping the outside wall using air.

The gas flow 7 at the outlet from the reactor is separated from the solid particles which are cooled and trapped by falling into a water-type retention tank 8 disposed beneath the reactor in line with thereaction chamber. After extraction franom the tank, a first solid residue 9 is obtained. The gas flow 10 is passed to a device 11 for cooling by the evaporation of water, where it is cooled at 12 to a temperature of about 200cC before passing into a filter 14 in order to be freed of its dusts 13 whi'ch constitute a second solid residue before being discharged to the atmosphere at 15 with amounts of HF and dusts which are respectively less than 1 mg/Nm3 and 2 mg/Nm3.

The two solid residues resulting from the double gas/solid separation operation associated with a double cooling action are mixed and, the final residue is subjected to monitoring in respect of pollution removal, namely monitoring in respect of the contents of CN and F on the basis of the samples of residue which are taken off and leached in accordance with the French standard X 31-210. That check indicates a residual leachable proportion of cyanide which is systematically lower than 0.0005% and a leachable proportion of fluorine which does not exceed 0.2% and which can be reduced, r -7 12 i f depending on the additive used in the mixture, to 0.015%.

i EXAMPLES OF USE: They concern 4 ccmpositions of honogenous mixtures of 1000 Kg of a granulcmetry of from 0 to 1 mm, using crushed brasque linings with the following proportions by weight: Na% 14.0 F% 9.14 Si% 11.0 CN% 0.08 Those brasque materials were successively mixed with the following additives: Example 1 Anhydrous CaSO 4 (anhydrite) Example 2 Lime Example 3 Kaolin anhydrous CaSO 4 Example 4 CaSO 4 2H 2 0 (gypsum) The main operating characteristics and the results of the pollution removal tests are indicated in Table 1 below.

It is found, for the same compositions of mixture as those reccrmnended by the prior art (EP-A-O 465 388) that there is a better level of pollution removal in the residue in particular with mixtures 1, 3 and 4 which respectively use as additives anhydrous CaSO4, the combination of anhydrous CaSO 4 and kaolin, and CaSO 4 2H20, that being in spite of virtually double the feed flow rates of mixture and therefore the amounts of impurities.

It is to be noted that the heat treatment of the brasque materials under the conditions of the invention but in the absence of additive results in a markedly insufficient level of pollution removal in the residue since the level of leachable F is still between 1.5 and that is to say at leRt five times greater than the least severe national standards.

a 1 13 TABLE 1 EXAMPLES No 1 2 3 4 Total initial mixture Kg 1000 100m 100 lc canprising: old brasque materials Kg 600 600 500 600 Additive :anhydrous CaSO4 Kg 400 0 100 0 Kaolin Kg 0 0 400) 0 Lime Kg 0 400 0 0 Gypsumi Kg 0 0 0 40X0 Heat treatment Mixture flow rate Kg/hour 272 428 214 240 Teinperature gas flow 0 High T 900 1000 900 900 LOW To 550 550 550 510 Spedgas flow m/s 6 6 6 6 Mean residence time second 0.5 0.5 0.5 Level of pollution remval in the residue Total leachable CNI 40.0005 <0.000C5 '0.0005 <0.000X5 Total leachable F 0.015 0.20 0.04 0.03

Claims (16)

1. A process for the heat treatment after crushing of used brasque linings comprising carbonaceous products and silico-aluminous products impregnated with fluorinated mineral compounds, crushed to a particle size of less than 5mm, said process comprising: forming a mixture of said brasque linings and 0 to 50% by weight, based on said mixture, of a powdery mineral additive capable of combining in the hot condition, with or without fusion, with said fluorinated mineral compounds to form stable, insoluble compounds; injecting said mixture into a gas flow circulating at a speed of at least Im/sec in the upper part of a reactor so as to maintain a temperature T of between 700°C and 1100°C; regulating the contact time of the solid particles of the mixture with the circulating gas flow to between 0.3 and 3 second whereby said particles attain a temperature t of between 400°C and 750 C prior to their extraction at the base of the reactor with said gas flow; maintaining the temperature of said gas flow as measured at the base of the reactor at a reference value T o such that T T o t; separating said particles from said gas flow at the reactor outlet after cooling; and removing dust from said gas flow before said gas flow is discharged to the atmosphere.

2. Process according to claim 1, wherein said brasque linings are obtained from Hall-Heroult electrolytic cells.

3. Process according to claim 1 or claim 2 characterised in that said fluorinated mineral compounds are mainly one or both of alkali metal and alkaline-earth fluorides.

4. Process according to claim 3 wherein said alkali metal and alkaline earth fluorides are NaF, CaF 2 or A1F 3 o o, 5. Process according to any one of the preceding claims characterised in that said 8 used brasque linings comprise at most 20wt% fluorine, lwt% cyanide, 25wt% silicon and sodium.

6. Process according to any one of the preceding claims characterised in that the particle size of said used brasque linings is less than 3mm.

7. Process according to any one of the preceding claims characterised in that the particle size of said powdery mineral additive is less than or equal to lmm.

8. Process according to any one of the preceding claims characterised in that said mineral additive is any one or more of kaolin, anhydrous or hydrated CaS0 4 and CaO.

9. Process according to any one of the preceding claims, characterised in that said temperature T is between 800°C and 1000°C. Process according to any one of the preceding claims characterised in that said ,1jL s. temperature t is between 450°C and 700°C. [N:\LIBZ]00424:EAR -i r- -i p. -F

11. Process according to any one of the preceding claims characterised in that the concentration of said mixture in said circulating gas flow is between 0.1 and 3Kg per Nm 3

12. Process according to claim 11 characterised in that the concentratizn of said mixture in said circulating gas flow is between 0.3 and 3Kg per Nm 3

13. Process according to any one of the preceding claims characterised in that said j reference temperature T o is between 450'C and 700'C.

14. Process according to any one of the preceding claims characterised in that said Sreference temperature T o is maintained by adjusting the weight flow rate of said mixture lo injected into said gas flow. Process according to claim 14 characterised in that said weight flow rate of said mixture is adjusted by varying the speed of a feed screw which is controlled by a device for monitoring the temperature difference of said gas flow at the base of the reactor with respect to said temperature T o

16. Process according to any one of claims 1 to 13 characterised in that said reference temperature T o is maintained by adjusting the flow rate of combustible gas which governs the temperature of said gas flow at a constant weight flow rate of said mixture.

17. Process according to any one of the preceding claims characterised in that said mixture is injected at the centre of a vortex formed by said gas flow arriving tangentially in the top of the reactor and which involves a swirling movement.

18. Process according to any one of the preceding claims characterised in that the stabilised residue resulting from the heat treatment of said mixture is obtained after a double gas/solid separation operation associated with a double cooling operation.

19. Process for the heat treatment after crushing of used brasque linings, substantially as hereinbefore described with reference to any one of the Examples. Process for the heat treatment after crushing of used brasque linings, substantially as hereinbefore described with reference to the accompanying drawings. Dated 4 Tugust, 1995 i 30 Aluminium Pechiney Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON i [N:\LBZ100424:EAR I -i Process for the Heat Treatment of Used Brasque Linings from Hall- ii Heroult Electrolysis Tanks Abstract The invention concerns a process for the heat treatment of crushed brasque lilings formed both by carbonaceous products and silico-aluminous products impregnated with fluorinated mineral compounds mixed with a powdery mineral additive capable of combining in the hot condition, with or without fusion, with the fluorinated impregnation compounds to form new stable and insoluble compounds. The powdery mixture is injected in the upper part of a reactor into a gas flow at a temperature not exceeding 1100°C. After having regulated the contact time of the solid particles with the gas flow in such a way that they attain a temperature of between 400°C and 750 C at the base of the reactor prior to their extraction, the temperature of the gas flow at the base of the reactor is maintained at a reference value by adjustment of the flow rate of powdery mixture. At the reactor outlet, after cooling, the gas flow is separated from the particles which then constitute a stabilised residue whose content of leachable cyanide is less than 0.0005% and whose content of leachable fluorine does not exceed 0.2%. iFigure Figure 1 ILibU\0OO083:GSA 2 of 6 i