GB2056425A - Treatment of wastes containing water-leachable fluorides - Google Patents

Abstract

Carbon-bearing materials which contain water-leachable fluoride compounds, particularly waste lining materials from electrolytic reduction cells for the production of aluminium metal, are treated by coarsely grinding the material, leaching the coarsely ground material with lime, and recovering separately a coarsely ground carbon-bearing fraction, a finely divided solid fluoride-containing fraction and an aqueous alkaline liquor. In a preferred process, the feed may comprise a coarsely ground carbon-bearing fraction and a more finely ground alumina fraction.

Description

SPECIFICATION Process for treatment of wastes containing water-leachable fluorides The present invention relates to a process for treating materials which contain water-leachable fluoride compounds to render them disposable from an ecological standpoint and to recover useful materials from them. The invention is directed primarily, but not exclusively, to the treatment of waste lining materials from electrolytic reduction cells for the production of aluminium metal. Such reduction cells are lined with an outer, thermal insulation layer composed of high-alumina bricks or packed alumina and an inner layer of carbon blocks, which also form the cathode of the cell. In the course of time the carbon structure becomes disrupted and distorted, so that its effectiveness as a cathode is reduced and periodically the cell requires to be relined.It is also necessary to dispose of the materials constituting the spent lining, which during service have absorbed a considerable amount of the electrolytic bath, which consists essentially of sodium cryolite in admixture with small quantities of other alkali metal- or alkaline earth metal fluorides.

Many different treatments have been put forward for disposal of spent cell linings. Some of these have been concerned with disposal of the waste materials. Others have been primarily concerned with recovery of usable chemical substances from the spent cell linings.

In one former method of the spent cell linings were dumped in waste pits, where the material was exposed to the weather. Since the spent linings contain substantial proportions of water-soluble components, such method is no longer acceptable on ecological grounds. Substantial quantities of ecologically harmful salts, particularly sodium fluoride and cyanides, are leached out of the spent cell linings by the action of rain water.

The chemical compositon of scrap cell lining varies within wide limits, depending upon the proportion of insulating brick-alumina material in it. Thus a typical sample for treatment might contain the following components in the stated ranges: NaF 812% 3NaF.AlF3 (cryolite) 1216% Na2CO3 3~7% NaOH 3~5% C 2550% Al2O3 (including Al metal) 525% CaF2 2-4% SiO2 1-10% along with small concentrations of carbides, nitrides and cyanides.

It is already known to treat such spent cell lining materials with dilute caustic soda, NaOH, to produce a liquor in which the main part of the sodium and fluorine values are dissolved, together with a substantial proportion of the aluminium values. This liquor was then filtered to remove insolubles such as carbon and causticized with lime to precipitate the fluorine values as CaF2 Because of the relatively low solubiity of NaF the volume of liquor employed is large in reiation to the material to be treated.

It has already been proposed to render cell lining wastes ecologically disposable by digesting them in slurry form with lime under pressure and at high temperature to convert the fluoride content into CaF2. The filtered wastes could then be dumped.

The present invention approaches the problem in a different way. In the method of the present invention the spent cell lining material is treated with lime in such a way as to separate the solid residues from the fine insoluble CaF2 resulting from lime digestion. At the same time sodium values from the wastes are converted to sodium hydroxide, some of which reacts with caustic-soluble alumina in the waste to form sodium aluminate. The resultant caustic/sodium aluminate liquor may be employed in any manner known in the art.

The main reaction involved in the digestion of a slurry of the spent cell lining with lime may be represented as [Na2CO3 + NaF + 3NaF.AIF3] + Ca (OH)2e NaOH + NaA102 + CaF2 + CaCO3.

It follows that both NaF and AIF3 values are converted to insoluble calcium fluoride. The insoluble calcium fluoride in the known procedure deposits on the solid residues of the lime digestion and is discharged with it.

The object of this prior proposal was to convert the cell wastes to an ecologically acceptable material to discharge to waste. The object of the present invention, on the contrary, is to produce a reusable carbon fraction, acceptably reduced in fluoride, and/or to produce a calcium fluoride fraction having a largely reduced content of other insolubles, present in the original waste, and essentially free from sodium values.

The present invention is based on the observation that the particle size of the calcium fluoride deposited in the lime digestion stage is very fine. It follows that if the spent cell lining material is ground rather coarsely, the insoluble residues of the spent cell lining may be separated to a large extent from the calcium fluoride by a screening procedure, preferably a wet screening operation, after digestion with lime. This leaves a relatively coarse fraction on the screen which is principally composed of carbon and brick/aluminainsulating material in proportions dependent upon the relative proportions of carbon cathode lining and insulating material in the charge to the process, if these were not separated prior to the leaching operation.However, it is preferred to carry out a preliminary separation of carbon from insulating material so as to produce a reusable carbon fraction.

In a particularly preferred form of the invention, the carbon and the insulating material are each ground separately. The carbon material is ground rather coarsely, and the non-carbon material is ground more finely. The two ground materials are combined to provide a composition that may conveniently be equivalent to the overall average of the waste materials, and the resulting composition leached with lime as noted above.

A first screening operation using a coarse screen separates the unreacted:carbon fraction for reuse, after which the fine calcium fluoride precipitate and other insoluble fines may be separated by filtration from the liquor, which contains principally caustic soda, some sodium aluminates, and other soluble sodium salts. The filter cake may be discarded as a waste but is more preferably treated with sulphuric acid to regenerate hydrogen fluoride for conversion to AIF3 for supply to the cell electrolyte of the reduction cells. Where the CaF2 precipitate is intended for such treatment for regeneration of hydrogen fluoride, it is preferred that it should be rather low in fine insolubles from the spent cell lining, such as carbon tailings and brick/alumina dust.

Alternatively, the fine fluoride material may be mixed with siliceous sand and subjected to hydrolysis at about 1 0000C to recover the fluorine values and leave an inert solid residue suitable for landfill.

The rate at which fluorides can be extracted from the coarsely ground carbonaceous cell lining material will depend, inter alia, on the size of the particles. The particle size is maintained as large as possible to ease separation of the calcium fluoride precipitate by screening and to reduce the power requirements of the crushing operation, while on the other hand the particle size of the ground waste cell lining is kept as low as possible, consistent with foregoing requirements, so that the digestion time required to reduce the alkali metal fluoride content of the solid residue to an acceptably low level is kept low. The acceptable alkali metal fluoride level of the residue will depend on whether the residue is to be dumped after screening to remove calcium fluoride precipitate or whether it is to be recycled on account of its carbon content.

Although somewhat coarser grinding of the spent cell lining is sometimes preferable, particularly if the lime digestion is to be performed at higher temperature and at superatmospheric pressure, the spent cell lining is preferably ground to pass through a 14 mesh B.S.S. screen and is leached with lime in a slurry at a temperature somewhat below boiling point, such as 80--95"C so as to avoid the use of expensive pressure vessels. In carrying out the extraction of F and conversion to CaF2, the weight of lime charged is preferably 1.2-1.6 times the fluoride content (calculated as F) of the spent cell lining material.In experimental examination of the procedure it has been found possible to extract a major proportion, up to 90% or even higher, of the spent cell lining within 3-4 hours and then to separate the extracted fluoride in very finely divided form from the solid residues by wet-screening on a relatively fine screen, preferably 65 or 80 B.S.S. mesh, although coarser screens such as 40 mesh or 50 mesh may be employed when a larger portion of the fines from the spent cell lining is acceptable in the calcium fluoride fraction.

As compared with prior art procedures in which the spent cell lining is treated with sodium hydroxide, it is an advantage of the lime digestion procedure that it can be carried out with a high solids content in the slurry and consequently with much more compact digestion apparatus. Advantageously the slurry contains 200-400 gms/litre of the spent cell lining materials and an appropriate quantity of lime. The digestion may be carried out at temperatures in the range of 30 1 500 C. Below 300C digestion becomes unsatisfactorily slow except in the presence of an undesirably high excess of lime.It is entirely unnecessary to employ heavy pressure vessels involved in digestion at temperatures above 1 50 C, and, as already indicated, an operating temperature of 80-950C is preferred. However, a temperature of about 1 500C would be required if it were necessary to decompose thermally the cyanide impurity in the resulting caustic/aluminate leach liquor.

In preferred operation when treating spent cell lining with Na and F contents of the order noted above the lime charge may be 1 215% of the weight of the lining material. 1520% CaO based on the weight of the lining material provides a satisfactory excess of lime for converting a very large proportion of the Na content of NaOH (or sodium aluminate) and for converting the F content to CaF2.

In the accompany drawings: Figure 1 is a diagrammatic representation of apparatus for treating a single coarsely ground carbonaceous material; and Figure 2 is a diagrammatic representation of apparatus for treating a mixture of coarsely ground carbon-bearing material and more finely ground non-carbon-containing material.

The apparatus of Figure 1 is intended for treatment of spent cell lining to recover cathode carbon.

The spent carbon is first separated from brick or alumina by hand.

The raw spent carbon cell lining first passes to a crusher 1 which may be of the jaw, cone or hammer type. From the crusher it passes to a metal eliminator 2 to remove the larger pieces of free aluminium metal which may accidentally be present in the spent cell lining and then to a roll crusher 3, which is set to grind the spent cell lining material coarsely to approximately14 mesh size. The ground material is then passed through a screen 4 (in this case a 14 mesh B.S.S. screen), the oversize particles being returned to the roll crusher 3.Although a further screening stage to seive out fines may optionally be employed, in the present instance the -14 mesh particles pass through screen 4 into a rotary leaching apparatus 5 into which finely ground hydrated lime is fed at appropriate rate to provide approximately 1 5 parts CaO to 100 parts ground spent cell lining and caustic solution is recirculated from a later stage in the process, at a rate sufficient to maintain a slurry having a solids content of 1 5-30%.

The slurry from the rotary leacher 5 passes to a rotating screen washer 6, in which the solids above 65 mesh are screened out and passed to a steam dryer 7. In the present instance this material has a major content of carbon, better than 80%. Water, in sufficient quantity to make good process loss, is supplied in the washer 6 to assist in screening out the -65 mesh particles, which are carried with the liquor to a continuous belt filter 8, in which the very fine CaF2 particles, together with other fine particles, are filtered out from the liquor.

The moist filter cake, which typically includes up to 50% carbon fines and 50% CaF2, is then discharged from the belt of the filter 8 at 40% moisture and is passed to storage after optional drying.

A part of the liquor from filter 8 is recirculated to the rotary leacher 5 and a part is led out of the system to remove typically 0.18 ton NaOH, 0.0275 ton Al203 (in solution) and 0.004 ton F per ton of carbon potlining processed. The caustic soda/aluminate liquor circulated in the process preferably contains about 50 g/l of caustic soda, calculated as Na2CO3.

The filter cake from the belt filter 8 may be dumped or may be used for regeneration of hydrogen fluoride or sold as a low grade metallurgical spar, as economic considerations dictate.

Where the aluminium smelter also includes a Bayer process alumina plant, the withdrawn part of the caustic soda-aluminate liquor can be forwarded thereto. Where this method of disposal is not available the valuable caustic soda-aluminate liquor can be subjected to other processing and in such case it is preferred to treat it for the removal of cyanide, such treatment being unnecessary for liquor forwarded to a Bayer process plant.

One convenient route for disposal of the liquor is to employ it in the production of Dawsonite, Na Al (OH)2 CO3, by reaction with aluminium-containing dross, such as is available in every aluminium smelter.

Dawsonite is a valuable product employed in filter aids, fire-retardant compositions, absorption agents etc.

The concentration of CN in potlining leach liquor (based on 300 gpl potlining charge) is about 200 ppm.

If the liquor is to be employed in the production of Dawsonite, the CN can be removed chemically by precipitation either with minute amounts of CuSO4.5H2O or FeCI3 followed by 1 5-minute gassing with Cl2.

In both cases, the residual CN- concentration comes down to below 2 ppm.

After being freed from cyanide the caustic soda-aluminate solution is conveniently reacted with Al-containing dross or alumina trihydrate to increase its AL/Na weight ratio to a desired value within the range of 0.65-to 0.8. The dissolved alumina is then tied up in solution by reacting the alumina-enriched liquor with sodium bicarbonate, which is added as a strong solution or as a slurry (or even as a dry solid).

The sodium bicarbonate is mixed with the caustic soda-aluminate liquor by gassing with carbon dioxide, the gassing being continued until the pH value of the liquor decreases about pH 9. The precipitate, which consists of Dawsonite or a Dawsonite/sodium bicarbonate mixture, is then filtered off. The filtrate, a relatively weak sodium bicarbonate solution, is then concentrated to 80-100 g/l sodium bicarbonate and is recirculated for reaction with the alumina-enriched caustic soda-aluminate liquor.

The following tables illustrate how the rate of extraction of F varies with the particle size of the spent cell lining, lime digestion being performed at atmospheric pressure.

TABLE I Experimental Results on Leaching Carbon Block Fraction of Scrap Potlining with CaO Basis: (I) 100 g Potlining (carbon fraction) crusched to -4+10 mesh, -10+14, or -14 mesh.

(II) CaO Charge: 15 g to 500 cc of leach slurry containing 100 g Potlining.

(III) Re@ction time: 21/2 hours at boiling point.

Original Potlining Leached-Out Residue Weight of CaF2 (%) (%) Leached-Out % Extraction Precipitate Sample Residue (Mesh) F Na2 O Al2O3 CaO F Na2O Al2O3 CaC (g) F Na2O Al2O3 Weight (g) % F 1 (-4+10 mesh) 12.2 11.3 13.1 2.23 7.8 6.8 12.4 2.80 81.6 48.4 50.8 24.8 26.8 23.3 2 (-10+14 ") 12.5 6.4 7.7 2.28 3.7 1.9 6.3 2.95 72.1 79.0 79.1 41.3 23.7 37.1 3 (-14 mesh) 13.7 10.2 11.5 2.30 3.1 2.1 6.4 2.95 36 91.9 92.5 80.0 50.8* 19.4 * N.B. Nearly 50% of this sample is a very fine carbon TABLE II Basis: (I) 100 g of Potlining (carbon fraction) crushed to -3+14 mesh and -14+65 mesh fractions.

(II) CaO Charge: 20 g/100 g of potlining.

(III) Reaction Time: 3 hours at 88 C.

Original Potlining Leached-Out Residue Weight of % Extraction (%) (%) Leached-Out Sample Residue (Mesh) F Na Al Ca F Na Al Ca (g) F Na Al -3+14 13.5 15.8 3.7 2.8 6.8 7.3 2.8 3.1 85.0 57 60 35 -14+85 13.0 14.6 3.5 2.6 2.5 3.0 1.8 3.2 62.0 87 87 68 Although the above tables suggest that spent cell lining should be ground to a maximum particle size of 1 4 mesh, nevertheless economic advantages are probably achievable by coarser grinding, for example to -8 mesh, with a higher caustic soda content, such as 100 gms/litre (as Na2C03) and a shorter digestion process time, such as 12 hours. This would lead to a carbon product with somewhat higher levels of F and Na than appear in the -14 mesh fraction when processed under the conditions of Table 1 or 2 but which would in many cases be acceptable for use in cathode blocks, particularly when blended with fresh carbon and it has the advantage that the CaF2 fraction is less contaminated with carbon fines.

Referring now to Figure 2, fluoride waste materials are divided into two streams, a carbon-rich waste potlining 10 and a brick-rich waste potlining 12. The carbon-rich material is crushed at 14 to -14 mesh and screened at 1 6. The brick-rich material is crushed at 1 8 to -65 mesh and screened at 20. The two crushed streams, 10,12 are re-combined in a blending bin 22 in such proportions that the resulting mix represents an overall average of the waste materials. This mixture is introduced to a leaching reactor 24 and there subjected to leaching with lime, introduced via line 26 under conditions described above.

The leached material is screened at 28 over a 65 mesh screen, and the (mainly carbonaceous) material that does not pass through the screen is removed via line 30. The slurry passing through the screen 28 is filtered at 32, and the precipitate passed via a storage vessel 34 to a blending bin 36 where it is blended with sand supplied from vessel 38. This blend is then aggiomerated at 40 to an appropriately sized aggregate and charged to a hydrolyser vessel 42 which may be of the expanded fluidized bed type. Heat is supplied to this vessel 42 by natural gas, oil or any other suitable means via line 44, and by air via an air compressor 46. The hydrolyser off-gases are removed via line 48 and cleaned in a cyclone 50.The solid feed of the hydrolyser vessel 42 should not be pre-heated by direct contact with the hydrolyser off-gases, since silica in the solid cold feed stream would react with the hydrogen fluoride in the gaseous stream to contaminate it with silicon fluoride. Steam for the hydrolyser 42 is supplied by a boiler 52. The solid residue from the hydrolyser is cooled by heat exchange, first with the incoming air at 54 and second with the incoming steam at 56. The stated order in which the gases are pre-heated is required to minimise the SiF2 impurities in the exit gas stream.

The sold residue from the hydrolyser 42, which is composed of a mix of alumina, calcium silicate and silica, may be discarded as inert land fill or used as raw material or refractories. It may also be used to replace brick and/or alumina as electrolytic cell insulation.

The filtrate from filter 32 is passed to an evaporator cooler 58, where use is made of the sensible heat in the hydrolyser off-gas from 50. This same caustic liquor may be concentrated or further concentrated, using other sources of heat (not shown). The cooled gas, cleaned of any residual solids, may be fed to an existing potroom dry scrubbing system for recovering the hydrogen fluoride as aluminium fluoride.

Claims (1)

1. CLAIM

   1. A process for treating carbon-bearing materials which contain water-leachable fluoride compounds, which process comprises leaching the coarsely ground materials with lime, and recovering separately a coarsely ground carbon-bearing fraction, a finely divided solid fluoride-containing fraction and an aqueous alkaline liquor.