WO1991013701A1

WO1991013701A1 - Spent pot liner treatment process

Info

Publication number: WO1991013701A1
Application number: PCT/AU1991/000087
Authority: WO
Inventors: Ralph Edward Shackleford
Original assignee: Ralph Edward Shackleford
Priority date: 1990-03-12
Filing date: 1991-03-12
Publication date: 1991-09-19

Abstract

A method of treating spent pot liner material arising from the electrolytic production of aluminium is described. The used pot lining is placed in an aqueous electrolyte solution, and an A.C. electrical current is applied to the solution, extracting contaminants from the pot liner material. Preferably the electrolyte solution is dilute hydrochloric acid. In an alternate form of the invention the process is conducted in two stages, the second stage using dilute hydrochloric acid as the electrolyte. In this case, the first stage uses alkaline salts leached from the spent pot liner as the electrolytes. Preferably, hydrogen peroxide is added to the electrolyte solution as an initiator. The A.C. electrical current can be applied continuously or intermittently, preferably at a current density of from 1 to 20 Amp/dm2.

Description

SPENT POT LINER TREATMENT PROCESS

Technical Field

This invention relates to a method of treating spent pot liner material, which arises from the electrolytic production of aluminium. The pot linings, when they are no longer able to be used in the production of aluminium metal, are removed and refined using the process of the invention, so as to remove contaminants and recover purified carbon and other useful material from the pot linings. The treatment process of the invention involves placing the spent pot lining material in an aqueous electrolyte solution, and applying an A.C. electrical current--to the solution, to extract contaminants from the pot liner material. When the contaminants are transported into solution, the solution is removed and the refined pot liner material is recovered. The resulting solution may be treated further to recover the contaminant material, and preferably to regenerate the electrolyte, which can be returned to the process. It is also preferred that hydrogen peroxide is added to the electrolyte solution at the beginning of the process as an initiator of the electrocatalytic process.

Aluminium smelters produce metallic aluminium from alumina by means of a batch electrolysis process where the alumina in molten cryolite is reduced to aluminium in large pots. Each pot comprises a large steel vessel the inside of which has a coating of an insulating material known as "vermiculite". This thermal insulation layer is covered by tiles, which are usually made of aluminium silicate refractories. The outermost layer which forms the cathode is a high density synthetic graphite.

The pot liner materials absorb impurities during their use in refining aluminium metal. After about five years the liner materials must be removed from the steel pot and replaced. The used pot liner material, which contains hazardous impurities must be handled carefully, and is difficult to reprocess. Spent aluminium smelting pot liner material is normally constituted as follows: carbon (40-50%), sodium compounds especially alkalies (10-20%), fluorides (10-20%), aluminium compounds especially nitrides (5-15%), iron (1-3%), silicon compounds (1-4%), a significant amount of aluminium, and lesser amounts of calcium compounds especially alkalies and carbides, sulphur, phosphorus, cyanide compounds, and aluminium carbides. Small amounts of arsenic, antimony, bismuth, magnesium, selenium, tellurium, gallium, zirconium and germanium may also be present. In addition hazardous gases may be released, particularly when pot liner material is stored or buried, because the material tends to be hygroscopic. These gases include acetylene, hydrogen, methane, ammonia, and phosphine. Sometimes hydrogen cyanide, arsine and stibine may be generated from pot liner wastes. These gases are both combustible and highly toxic. In addition, dusts and any leach liquors are also hazardous. There have been a number of plant explosions caused by the emission of combustible gases from spent pot liner wastes.

It can therefore be seen that it would be very useful to develop a process for refining spent pot liner waste, both to remove the hazard, and to recover saleable products for other industrial uses.

One method previously used for disposing of used pot liner materials is by calcining and subsequent encapsulating and storing, frequently as land fill. This is expensive, and degrades the environment.

It has also previously been suggested that used pot linings could be combusted, using the carbon content as "fuel". However, the synthetic graphite is extremely difficult to combust, and the processes which have been developed required complex techniques, such as those discussed in Australian Patent 599263 (Application. 21948/88), where a process for combusting pot linings is described which requires grinding the pot linings to a relatively small particle size, and then mixing these ground pot linings with a powdered additive, and burning the pot linings in a combuster at a temperature of 1400°F to 2200°F, with the additive preventing the ground pot linings from agglomerating in the combustion zone.

It would therefore be useful if a simpler and less complex process could be developed for treating spent pot lining material, from which it is possible to recover useful products.

A process has now been discovered for treating spent pot liner material which involves immersing the pot liner material in an aqueous solution of an electrolyte, preferably comprising a dilute acid, or else dilute alkali salts leached from the material, and applying an A.C. electric current to the solution which electrocatalytically leaches the contaminants from the pot . liner materials into solution. The purified pot liner material can then be recovered, and the contaminants can be removed with the solution. The electrolyte solution such as the dilute acid may be regenerated, and any valuable contaminants recovered for further processing.

The use of A.C. current electrolysis is not well known, and has previously been used in only a small number of specific applications, although very many direct current electrolysis applications exist. Background Art

U.S. Patent 4131526 (Moeglich) concerns a process for removing oxidizible contaminants from aqueous liquors, but requires a catalyst which is an oxidation catalyst such as Mn0₂, Cr^O^, Bi₂0₃, or Pb0₂, which is coated onto particles of a carrier such as alumina. An A.C. current is applied, and the amount of certain impurities can be reduced, particularly phenol in waste water such as paper mill waste, or sewage. It is also known to use A.C. current to assist with flocculating fine suspensions, such as described in U.S. Patent 4238304 (Zucker), Japanese Patent 47-21958 (Ota), and U.S. Patent 4094755 (Moeglich). Frequently D.C. current can be used in place of A.C. current, in these applications.

U.S. Patent 3616356 (Roy) describes an electrolytic - - - process for treating water containing dissolved salts, with the aid of particulate carbon packing. However, mostly the described process involves the use of D.C. current, and A.C. current is only used with amphoteric metals such as chromium. The process may involve the known ability of A.C. current to assist in flocculation, as the described process -precipitates metallic impurities as metal oxides, hydroxides, or sulphides which are recovered by filtration from the cell effluent.

Australian Patent 239036 (Application 49368/59) describes the use of A.C. current electrolysis of sea water to prevent marine fouling.

U.S. Patent 3799851 (Milla ) discloses the use of A.C. electrolysis to reduce the amount of organo-lead compounds in aqueous solutions to produce water-insoluble precipitates.

It is generally believed that A.C. electrolysis is unlikely to have any effect other than in certain special instances, where it is believed the major effect is in flocculating suspended metallic compounds. The other main known effect of A.C. current is that it is believed to oxidize dissolved metals to produce non-soluble precipitates. Disclosure of Invention

The process of the present invention is described as "electrocatalytic refining" but it is not entirely clear what chemical processes are at work. If the electrolyte solution is dilute acid, such as hydrochloric acid, then the process is similar to hot acid leaching. However, in the absence of A.C. current, hot acid leaching does not extract very much of the contaminants present in spent pot lining material. Also, hot acid leaching requires the material to be ground very finely, whereas the present invention does not require fine grinding, and in fact, only the minimum amount of grinding need be used, only to the extent of the particle sizes required in the final product. While the process occurring in the present invention are not yet fully understood, it is believed that some of the reasons why the A.C. current enhances the leaching process are that the A.C. current promotes decomposition of the electrolyte at the surface of the spent pot liner material, and that there may be a mechanical pumping action caused by the A.C. current which promotes electrolyte contact and pore penetration. Also, in the case that a chloride containing electrolyte is used, then active chlorine may be generated, which reacts immediately with spent pot liner material to form various soluble chlorides and oxychlorides.

With the electrocatalytic process of the present invention the leaching of the pot liner material is markedly enhanced. For example, with A.C. current of only about 10 watts per tonne hour, the leaching is completed in hours, instead of what could be months with conventional hot acid leaching, and even when the electrolyte is contaminant saturated, the leaching process continues. This has an important advantage of reducing the operating capital costs of the processing plant. Also, if the electrolyte is regenerated, then there may be no plant effluent from the process of the invention, which has important environmental advantages.

The present invention therefore involves a method of treating spent pot liner material arising from aluminium production to remove contaminants, which comprises immersing the spent pot liner material in an aqueous solution of an electrolyte and applying an A.C. electric current to the solution to remove the contaminants from the pot liner material into solution, and then separating the treated material from the solution which contains the contaminants.

Preferably the pot liner material is crushed before processing in this manner. It is also preferred that the pot liner material is separated into its various constituent parts, such as graphite, the aluminium silicate tiles, and vermiculite, and that each of these components is treated separately. The various types of pot liner material can be treated separately because their processing times vary according to the structure and make-up of the material.

Preferably the process is a batch process, although a continuous process is also in accordance with the present invention.

It is also preferred that the electrolyte is a chloride containing electrolyte, and most preferred the electrolyte is dilute hydrochloric acid. Alternately, the electrolyte can be alkaline salts, such as of those of sodium, calcium or potassium. In this case, these salts can be obtained from an initial leach of the pot liner waste, this leachate possibly becoming the electrolyte solution for use with a further batch of waste. Different electrolyte solutions can be used, at different stages in a multi-stage leaching process.

A further preferred embodiment of the invention involves the addition of hydrogen peroxide (H^O^) as an initiator at the beginning of the process.

It is also a preferred feature of the invention that after the used electrolyte solution containing the contaminants is removed from the processed pot liner material, the contaminants are then removed from the electrolyte solution, which is then reused. In the case of dilute hydrochloric acid being the electrolyte, the removed solution can be burnt, and the gaseous substituents recovered and reconstituted as hydrochloric acid to be returned to the electrocatalytic process, and other solids and gaseous components recovered separately for further processing.

Any A.C. current is appropriate to the process, but an A.C. current of 50-60 Hertz is preferred. It is also a useful feature of the present invention when the electrolyte solution is recycled continuously through the pot liner batches to monitor the current density, and to determine when the contaminants are substantially leached from the pot liner material, by determining when, at a given voltage the current density becomes substantially stable due to the little or no change in the concentration - 7 - or purity of impurities and electrolytes in the electrolyte solution.

The A.C. current can be applied continuously or else pulsed at regular intervals. Considerable power savings can also be obtained by pulsing the current, and preferably the current is pulsed at 20 to 60 minute intervals. If higher current densities are used, for periods of 5 minutes or more, and the current pulsed at 20 to 60 minute intervals, then the same results can be obtained as if the current was applied continuously at low current density for the same amount of time. In this case, by pulsing the current power savings of up to 30% or 40% can be obtained. For example, by pulsing the A.C. current., for periods of 5 minutes or more,, at regular intervals, the same leach rate is obtained as for running the current continuously for one hour.

Another aspect of the invention concerns leaching the pot liner waste in two stages. Firstly, the waste is leached in water containing alkali salts extracted from the leaching process of another batch, for example, until no further alkali salts are being extracted. This leachate is then removed, by pumping to a storage vessel, for instance, and replaced with dilute HC1. The leaching is then continued as before. The sodium, calcium and potassium alkali salts that are leached from the pot line waste are sufficiently soluble to carry the A.C. current in the first stage of this process. This initical leaching can remove much of the alkali salt in the waste, and by conducting the leaching in these two stages the consumption of dilute HC1, which is used in the second stage, is substantially reduced, sometimes by as much as 60% to 80% compared with carrying out the leaching only with dilute HCl, or another acid, as the electrolyte solution. The alkaline leachate can be reused, after processing to remove the major part of the electrolyte salts, or with further dilution, in another batch of waste, or else in a counter-current continuous process, for example. In a preferred form, the invention may proceed as follows. The spent pot liner is removed from the pot, and divided into discrete components, for example, separating the tile, etc from the cathode graphite. The separated materials are then crushed and sized to customer specifications for the treated product. The crushed material is then fed to a leach tank, and covered with the leach liquor which may be, for example, dilute (about 10%), hydrochloric acid. Hydrogen peroxide is preferably added as an initiator, and an A.C. current of, for example, 50 Hertz, is applied. The effect of the hydrogen peroxide initiator may be to prevent an initial passive film, of hydrogen gas, forming. Hydrogen peroxide is produced in any case once the A.C. current is applied, and its initial addition acts to speed up the process.

In another preferred embodiment, the crushed material is covered with a leach liquour containing alkali salts from another or previous leach, and pulsed A.C. current applied at high current densities until no additional alkali salts are being extracted. This leach liquour is removed and replaced with dilute HC1, and pulsed A.C. current applied to complete the extraction process.

Any frequency of alternating current can be used, for example from 0.5 to 800 Hz. Preferably A.C. current in a range of 25 to 200 Hz, and most preferably the normal frequency of 50 or 60 Hz may be used. The current density chosen depends on the material being treated, and for example, vermiculite requires a lower current density than graphite, for a similar result. The greater the current density the faster the leaching process occurs. A choice can be made whether the pot lining material is treated for a short time using high current density, in small cells, or if electric power is expensive, then the materials can be treated in large cells, at low current density, for a longer time period. While the appropriate current density can be chosen according to the actual circumstances, a preferred current density is from 1-10 Amp/dm , if continuous current is applied. If pulsed A.C. current is applied then current densities of up to 20

2 Amp/dm , or higher, are preferred.

In continuous mode, where the electrolyte is continuously being regenerated and cycled through the cells containing pot liner material, the purity of the electrolyte improves as less of the impurity, or "tramp" material is removed from the cell. The conductivity of the electrolyte will therefore decrease over time, as does the current density for a given voltage. When the current density remains substantially constant for a period, this is a good indication that the refining process is complete. This also enables the process to be automated. In addition, if desired, the relationship between current density_^and the purity of the final product can be determined, and products of pre-determined purity can be obtained as a result.

Any electrolyte can be used. The electrolyte may be a dilute acid, such as hydrochloric, sulphuric or acetic acids, or a dilute salt solution. For example, dilute sodium chloride solution may be used, or sea water may also be used. In the multi-stage process the initial stage can use alkali salts as the electrolyte. It is preferably that in at least one stage of the process, the electrolyte contains chloride, and most preferred that the electrolyte should be dilute hydrochloric acid, in this stage, which is normally the final stage.

Once the leaching process is substantially or sufficiently complete, or between stages in a multi-stage process, then the electrolyte solution containing the contaminants is removed from the cell. The purified pot liner material is then preferably washed using water and A.C. current a number of times. The first washing may be used as electrolyte make-up, such as in the first alkaline electrolyte stage, and the other washings kept to wash subsequent batches.

If dilute HC1 is used to least the pot liner waste, then the electrolyte can be regenerated by combustion. This technology is known, and a number of commercial HC1 regenerators exist. This involves spraying the electrolyte HC1 solution in a burning gas stream. The gases generated at the time the pot liner is added to the electrolyte solution and during the electrocatalytic refining process may also be burnt in the gas stream. These gases, described previously, are usually combustible, and would be dangerous unless burnt. The HC1 is sprayed in a burning gas stream of natural gas, for example, and the gaseous products resulting are then condensed. The dilute HC1 electrolyte solution is regenerated, and the other products which are normally obtained include hydrofluoric acid and various salts. These products are valuable and will help make the invention economically viable.

The leaching process does not need to be conducted at elevated temperatures, but it is convenient to operate the HC1 leaching stage at about 60^*C. The regenerated HC1 can be returned at this temperature and the A.C. current assists with maintaining the temperature of the leaching process.

The solid material, after washing, is dryed in a rotary dryer, for example, and packed. The material recovered from the process include purified carbon, as carbon black and carbon lining, having a purity at least as good, or even better, than the pure synthetic graphite which initially lined the alumina electrolysic cell. The aluminium and impurity salts recovered are also valuable, and can be sold profitably. Hydrofluoric acid is also recoverable, and a valuable industrial product. It is also possible to use a present invention as a method of purifying graphite or other carbon materials.

The crushed vermiculite and tiling also have a number of commercial uses, and their purity, in terms of having the contaminants leaching out, is normally higher than the starting materials used for their manufacture.

The purity of the end product can be varied according to the length of treatment of the pot liner material. If a low purity product is desired, then the leaching process does not need to be carried out as long, and/or at such high current densities, as when very pure product is required.

The physical construction of the leaching cells can vary substantially. However, one possible design is to use a number of cylindrical baskets, each of which contain the crushed pot liner material. These baskets are placed in the electrolyte cell so they are distributed evenly throughout the cell, but allowing the electrolyte to circulate within the cell. The electrolyte solution is pumped in and out of the cell as appropriate, and after washing, the baskets are each removed, emptied into the dryer, and then refilled to begin the process again.

In _^addition, a hood is preferably lowered over the top of the cell before electrolyte solution is allowed to enter the cell, so that dangerous gases can be removed, which may subsequently be burnt in the regenerator as fuel. However, certain of the gases will dissolve in the electrolyte solution.

The advantage of using dilute hydrochloric acid is at the spent electrolyte solution can be readily regenerated, and there is therefore no refinery discard, as all the products are reusable and saleable.

The electrolyte composition used in the invention is not critical, as the composition of it changes from moment to moment- during the refining process. For example, the pot liner material is generally alkaline in nature, and in the two-stage process, if alkaline electrolyte is initially used, becomes more alkaline as the leaching progresses. When the dilute HC1 is added, the pH initially falls, but during the electrolysis refining process, increases again. If the electrolyte is reused continuously, then varying electrolyte concentrations could be used in the process depending on the differences in feed material make-up, and electrolyte losses. Modes for carrying out the invention

The invention is now described as reference to several examples, which are not limiting on the scope of the invention.

Example 1

A cell was prepared containing baskets of crushed pot liner material. The cell had a capacity of 30 kg, and was charged with 20 kg of pot liner material. The experiment was repeated for different types of pot liner material, namely for cathode, carbon liner, tile, insulation, and miscellaneous waste which was mainly spent carbon amode and cryolite.

The cell was filled with sea water pumped from the ocean in one series of experiments, and with 10% V/V HC1 in another series of experiments.

An A.C. current of 50 Hz was applied in a series of experiments, a current of from 5 to 100 amps was used, the current being increased in limits of 5 amps in each trial. In the other experiments a current of 50 amps was used.

When the current was initially applied there was an initial rise in it, and for example, the current will rise from a starting value of 25 amps to about 100 amps during the first 10 to 20 minutes. Once the rise in current ceases, the A.C. eletrolysis was generally- continued for about 2 hours.

The electrolyte solution was then removed, and the sample material reused with fresh water, with A.C. current applied, for about 15 minutes.

The sample was then dried and analysed.

This procedure was repeated for each of the various experiments described above.

In some experiments 0.1% V/V H₂0₂ was added at the start.

From the tests conducted with and without the addition of hydrogen peroxide, it was found that the use of hydrogen peroxide was equivalent to 30 minutes of electrolyte conditioning, or in other words addition of hydrogen peroxide had the effect of reducing the processing time by approximately 30 minutes. The results obtained from 30 runs of the process, using dilute HCl at an initial current density of 50 Hz are as follows:

S ent Pot Line

(in %)

Cathode 32.7 0.95

Sideliner 35.9 2.32

Tile 17.0 0.41

Vermiculate 8.8 0.23

The variation over 30 runs was within + 5% of those values

It generally was found that generally after 1 hour most of the finer material was refined. After eight hours all of the material, including the coarsest fractions, were refined, to a level of purity consistent with the purchased material specifications. Therefore, after this period, all the impurities had been removed from the material. Some tests were also conducted on a continuous basis by pumping sea water through the material. It was found that the batch process had slightly better results, and the continuous process also had the disadvantage that discarded sea water contained high levels of pot liner contamination, even after processing.

Example 2

The process generally described in Example 1 was repeated using as electrolytes: (i) sodium chloride solution, (ii) dilute hydrochloric acid, and (iii) dilute sulphuric acid solutions, as well as combinations of these.

The concentration of the electrolyte varied, for acids from 30% v/v down to 1% v/v. The electrolyte was also used up to its saturation level, and was found to remain effective at this level. The current density

2 varied from 1 to 10 A/dm in batch mode or in continuous mode from 1 to 100 A/dm 2. Above 10 A/dm2, in each of these examples, the process time savings became marginal, and much of the current was wasted as heat. It is also clear that while electrolyte conditioning, without the benefit of H₂0₂, can be used, there are clear and obvious benefits of adding a 0.1% v/v solution of H₂0₉ to the process at its beginning, to initiate the leaching. For example, at 100 A/dm , with average particle sizes of 100 micron, this feed stock is refined in one minute with a 0.1% H₂0₂ addition, but this takes almost two minutes without the addition of H„0₂. However, it is expensive to operate the process at these high current densities, and in practice lower densities would be used. However this experiment shows the effect of the addition of hydrogen peroxide to the process.

When the spent pot liner is initially charged, the soluble salt content of the electrolyte will usually increase dramatically for the first 15 to 30 minutes. The soluble salt content levels off for about 10 minutes, in the examples referred to above, and then gradually declines. When the material is refined, no further soluble ions are produced, and the conductivity drops. The current density becomes stable, and this is itself an indication that the batch can be washed and removed.

Example 3

The process generally described in Example 2 was repeated. The pot liner waste was initially leached using water containing alkali from the previous batch, diluted

2 if necessary, using pulsed A.C. currents at 20 Amp/dm until no further alkali was leached. This leachate was pumped to a storage vessel, and replaced with dilute HCl, and again leached with pulsed A.C. current, as previously described. Less power was required from the same leach result, and less dilute HCl was needed.

The process was conducted as follows:

Four samples of material were taken from a recently failed pot. The samples were approximately 5kg each and duplicate 5kg samples were put aside for comparative analysis on a untreated basis. One of the samples was kept as a solid 5kg lump of carbon. The remaining samples were crushed to approximately 12 - 15 mm before treatment.

The leaching was conducted as described above for about 15 minutes each. Afterwards, the treated samples were prepared and subjected to the standard solid waste leach test with the resulting leachate being analysed for

Fluoride and Cyanide.

Results Leachate F mg/L Leachate CN~ mg/1 Before After Before After

The invention has been described in connection with various preferments and examples, but is not to be limited to these, because persons skilled in the art can use variations from those described without departing from the spirit or scope of the invention.

Claims

THE CLAIMS

1. A process for treating spent pot liner material arising from aluminium production to remove contaminants, which comprises immersing the spent pot liner material in an aqueous electrolyte solution and applying an A.C. current to the solution to extract the containments from the pot liner material into solution, and then separating the treated material from the solution containing the contaminants.

2. The process of claim 1, wherein the electrolyte solutin is a chloride solution.

3. The process of claim 2, wherein the electrolyte solution is dilute hydrochloric acid.

4. The process of claim 1, which is conducted in two stages, wherein the first stage uses an electrolyte solution comprising dilute alkaline salts leachable from the pot liner material, and the second stage uses an electrolyte solution comprising dilute hydrochloric acid.

5. The process of claim 1 wherein hydrogen peroxide is added to the electrolyte solution as a initiator.

6. The process of claim 1 wherein 50 to 60 Hz A.C. current is applied continuously or intermittently at a

2 current density of from 1 to 20 Amp/dm .

7. The process of claim 6 wherein the current is applied intermittently for periods of at least 5 minutes at intervals of from 20 to 60 minutes.

8. The process of claim 1 which comprises an initial step of separating the spent pot liner material into its different constituents, which are then processed separately.

9. The process of claim 1 wherein the solution containing the contaminants resulting from the process is further processed to recover the electrolyte solution.

10. The process of claim 1 wherein current density is monitored to determine when the containments have been substantially extracted.