GB2056422A - Processing of Spent Cell Lining Materials from Electrolytic Reduction Cells for Production of Aluminium - Google Patents

Abstract

Spent cell lining material from electrolytic reduction cells for the production of aluminium are treated by grinding such material to a fine particle size, mixing the ground material with a large excess of strong sulphuric acid over the stoichiometric amount for release of F therefrom, holding the mix at elevated temperature for a time sufficient to release some 85-95% of the F content as gaseous compounds, absorbing the generated gaseous F compounds in known manner, subjecting the highly sulphated residues to reduction pyrolysis at a temperature in the range 1200- 1300 DEG C in the presence of water vapour until substantially the whole S content has been converted to SO2, and leaching out sodium aluminate from the solid residue.

Description

SPECIFICATION Processing of Spent Cell Lining Materials from Electrolytic Reduction Cells for Production of Aluminium Reduction cells for the production of aluminium are lined with an outer, thermal insulation composed of high-alumina bricks and an inner layer of carbon blocks, which also form the cathode of the cell.

The cell lining becomes progressively contaminated with constituents of the electrolytic bath which leads to the distortion and disruption of the carbon structure, reducing its effectiveness as a cathode. Periodically the cell requires to be relined. It is then necessary to dispose of the materials constituting the spent lining.

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 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, are leached out of the spent cell linings by the action of rain water.

A typical spent cell lining includes the following substances in the ranges set out NaF 812% 3NaF.AiF3 (cryolite) 1216% Na2CO3 37% NaOH 35% total Awl203 (as both metal and oxide) 1 5--3 5% CaF2 35% SiO2 312% C 2550% along with small concentrations of carbides, nitrides and cyanides.

The disposal of spent cell linings has become a serious problem in the aluminium industry and many aluminium producers have accumulated large tonnages of such material. It is an object of the present invention to provide a process applicable to large scale treatment of spent cell linings and resulting in the substantially complete elimination of ecologically undesirable components from the solid residues.

It is already known that fluorine values may be released from fluorides such as NaF, AIF3, CaF2 by treatment at elevated temperatures. Such fluorine values are released as HF and SiF4 (if silica is present).

According to the present invention a process for the treatment of spent cell lining material from electrolytic reduction cells for the production of aluminium comprises grinding such material to a fine particle size, mixing the ground material with a large excess of strong sulphuric acid over the stoichiometric amount for release of F therefrom. holding the mix at elevated temperature for a time sufficient to release some 8595% of the F content as gaseous compounds, absorbing the generated gaseous F compounds in known manner, subjecting the highly sulphated residues to reduction pyrolysis at a temperature in the range of 1200--13000C in the presence of water vapour until substantially the whole S content has been converted to SO2 and leaching out sodium aluminate from the solid residue.The residue of this final leaching operation is ecologically harmless and may be dumped.

The effect of the pyrolysis of the sulphated cell-lining residues is to convert the sodium sulphate and aluminium sulphate content to sodium aluminate which can conveniently be recovered by aqueous leaching. After filtration it may be returned to a Bayer process circuit or gassed with CO2 to precipitate alumina values.

The pyrolysis reaction may be represented as Na2SO4+AI2(SO4)3+2C+H20e2NaAI02+4SO2+2CO2+H20 It will be seen that the water vapour has only a catalytic effect.

In a large scale plant for the treatment of spent pot lining SO2 is continuously converted back to sulphuric acid which is recycled to the input end of the process.

The accompanying Figure 1 shows schematicaily a complete system for the treatment of one-ton lots of spent cell lining material.

On this scale the mixing of the spent cell lining (pot lining) with concentrated sulphuric acid presents considerable difficulty to provide a material which can conveniently be fed to a reactor in which HF and SiF4 are evolved at a satisfactory rate. For this purpose the pot lining-sulphuric acid mixture should be supplied to the reactor in the form of small friable granules. The reactor is conveniently of the type employed in plants for the generation of HF by reaction of CaF2 concentrates with concentrated H2SO4, in the production of AIF3.

The pot lining-sulphuric acid mix employs 3 to 4 times the stoichiometric amount of H2SO4, calculated on the total F content of the pot lining, in order to effectively attack a difficult-to-decompose NaF and Al203 in the following over-all reaction: [NaF+AIF3+Na2CO3+AI203+SiO2]+H2SO4(excess)

Heat ) (HF+SiF4) gas +CO2+nNa2SO4.AI2(SO4)3mH20.

Mix If the pot lining and sulphuric acid are mixed before introduction to the reactor (in which the mixture is heated at 200--3000C for 30-60 minutes) there is a rapid reaction between the sulphuric acid with sodium compounds and reactive alumina, which results in the formation of sodium alum at a very early stage. This tends to crystallize in a highly hydrated form, converting the mix into a tough asphalt-like paste, which is very difficult to handle and leads to severe scaling of the reactor and mixing equipment. We have now found that this difficulty can be overcome by preheating the reactants before mixing.Thus the reactants should be preheated to a level in which the resultant mix has a temperature of 1400--1700C or higher in a typical operation, when treating 1 part of pot lining (ground to -65 mesh and containing between 13 and 1 6%F) with around 1.7 parts by weight of 9698% H2SO4 in a rapid mixing operation lasting 2 4 minutes or more.

Since the premixing operation should be carried out under relatively gentle free "lift-fall" conditions, an ideal type of apparatus to bring about this sort of mixing action is either a single shaft ribbon mixer or a long paddle, double-shaft, self-cleaning pug mixer, as shown in Figure 2.

Due to the fact that the required retention time is of an extremely short duration, some 2-4 minutes, a relatively small-size machine would be capable of giving very high throughputs. This feature is very desirable since the mixers in question would have to be made from expensive materials resistant to hot sulphuric acid in the presence of HF.

The acid should be introduced preferably as a coarse spray at several points, starting from the inlet of the potlining solids.

The preheating of potlining solids was found to be a relatively easy operation. This could be done either in an externally heated tube screw-conveyor, or in an air slide-type heating apparatus, employing hot flue gases.

Referring further to Figure 2, the preheated solids are fed via a feed screw 1 to a mixer 2, provided with a heating jacket 3. Preheated sulphuric acid is introduced via a manifold 4 and sprayed into the mixer at various positions along its length. The mixer 2 is preferably of the single-shaft ribbon or double-shaft paddle type and is driven relatively rapidly relatively to a second-stage mixer 5, provided with a heating jacket 6. At the discharge end of the second stage mixer the friable pot lining/sulphuric acid mix is discharged via a feed conduit 7 to a reactor 8 of the known type, which is provided with either internal or external heating arrangement for holding the mix at a temperature in the range of 200-3000C to evolve fluorine containing gases and discharge them through the conduit 9 from whence they are led to the scrubber in the diagram of Figure 1.

Claims (1)

1. Claims

   1. A process for the treatment of spent cell lining material from electrolytic reduction cells for the production of aluminium, comprising grinding such material to a fine particle size, mixing the ground material with a large excess of strong sulphuric acid over the stoichiometric amount for release of F therefrom, holding the mix at elevated temperature for a time sufficient to release some 8595% of the F content as gaseous compounds, absorbing the generated gaseous F compounds in known manner, subjecting the highly sulphated residues to reduction pyrolysis at a temperature in the range 1200-1 3000C in the presence of water vapour until substantially the whole S content has been converted to SO2 and leaching out sodium aluminate from the solid residue.