NO341751B1 - Method and device for recycling of dust accumulations in connection with electrolytic production of aluminium - Google Patents

Abstract

The present invention relates to a method for recovering fines in a pot line for aluminum production, wherein the finely divided material comprises aluminum cryolite, the pot line comprising electrolytic cells (6), and the method com prizing a step A1, the step A1 comprises separating (9) the fines into at least a first (11) and at least a second (10) size fraction, and wherein the at least first size fraction (11) comprises particles having substantially smaller particle sizes than the at least second size fraction (10), characterized in that step A1 involves a concentration of alumina in the at least first (11) size fraction and a concentration of the cryolite in the at least second (10) size fraction, and the method further comprises a step B1, Each step B1 includes repatriate (13) the at least first size fraction (11) from step A1 to the pot lines electrolytic cells (6) and a further step B2, which step B2 comprises to repatriate the at least second size fraction (10) in the pot lines supply system for covering materials (ACM) / cryolite.

Description

METHOD AND DEVICE FOR RECYCLING OF DUST ACCUMULATIONS IN CONNECTION WITH ELECTROLYTIC PRODUCTION OF ALUMINIUM

Field of invention

The present invention relates to a method to ensure internal or external recycling of dust accumulations in an aluminium electrolysis pot line and its underlying basement by separating this dust in discrete size fractions and which achieves different material properties. This allows for full recovery of the raw materials contained here by feeding these fractions in the existing raw material supply systems, significantly reducing the dust loads in the smelter without providing major inconvenience for the smelter operations, and thus fully utilizes raw materials that are astray.

Background of the invention

An electrolysis hall, pot line, consists of many electrolytic cells or pots, connected in series. In electrolytic cells cryolite, Na3AlF6, is used as solvent for the applied raw material alumina. Cryolite is therefore the main component of the electrolytic cell electrolyte, i.e. the bath.

When cryolite cools at the cell edges, a so-called side-crust is formed that insulates and protects the anode and cathode for the aggressive liquid cryolite melt. To maintain this desired side-crust, a regularly fed anode covering material (ACM) is supplied to the cells, a mixture of solidified bath material and alumina. Typically, during anode change one will experience that anodecovering material is pushed out of the cathode pot and down on the floor, and then fall down in the pot line cellar. Annually, this could constitute several thousand tons of material, which is thus collected in the pot line basement.

In connection with the primary production of aluminium, it is inevitable to avoid a certain degree of dust load into the pot line hall. During production with prebaked anodes, dust is stirred up when the cover lids on the anode superstructure is removed in connection with routine work on the electrolytic cells. In particular, the dusting increases when anodes are lifted up and out, to be replaced with new, or when the electrolytic cells are opened to be applied ACM (anode cover material). When work is conducted on the cell, the pot suction will in practice be punctured, resulting in hot dust-containing exhaust gases to be emitted out to the pot line hall, were the dust content are emitted to air over the roof or is settled on horizontal surfaces indoors. A considerable amount of such fines falls to the basement below the electrolysis cells. Another dominant dust source in a pot line is the continuous leakage of dust emitting caused by poor anode cover lids in what initially was a sealed anode superstructure. Over time the effective cell suction, and thus the negative pressure in the electrolytic cell, will be reduced which in turn leads to increased dust loads to the pot line hall.

"Basement Materials" in this context is defined as dust heaps, encompassing crushed bath material and aluminium oxide, which falls into a pot line basement as a consequence of activities at the cells above.

Examples of sources which use the term "Basement Materials" are given in the following webpages:

http://www.alcoa.com/global/en/environment/msds_view.asp?LoadMSDS=1844 96 http://www.alcoa.com/global/en/environment/msds_view.asp?LoadMSDS=1823 22 http://www.alcoa.com/global/en/environment/msds_view.asp?LoadMSDS=1626 86

It must therefore be assumed that the term "Basement Material" is used in the technical field of producing aluminium by electrolysis and thus are well known and clearly understood by a skilled person.

Similarly the term "fines" as used in the claims, this is separated smallest size fraction of basement materials, where basement material is defined as deposition of ACM and alumina (aluminium oxide), and a minor amount of second deposition materials that are typically present in a pot line for the production of primary aluminium.

Basement materials can over time be collected in large quantities in the basement under the pot line hall. Some pot lines have several thousand tons of basement materials stored in such a way. Besides that, this constitutes resources astray. It gives rise to dust loads in that outer wind loads may lift the fines up into the pot line hall, and that the stored dust heaps may pose a security risk at the time of an uncontrolled metal leakage from above electrolytic cells.

Typically, basement material contains ca.40% alumina and 60% bath material (cryolite). Basement material is often recovered through electrolysis plant process line for ACM/cryolite, but since basement materials mainly are fines swirled in the air, this easily give rise to undesirable operational effects on the aluminium electrolysis process. Such an addition to the regular ACM will create a higher dust load when handled and reduce its permeability. Thus, the fines have a negative effect to the operation in that heat is kept inside the pots under the ACM instead of the heat optimally ventilates via the pot suction. Increased content of fines in the cover material will also cause increased dust loads in suction ducts. Over time, such mix of basement material in the ACM will contribute to clog the ventilation faster than otherwise.

Many of the world's aluminium smelters based on Soderberg technology was in the early 2000s phased out due to efficiency and environmental standards. Today's dominant technology with prebaked anodes means that plants are net producers of bath materials in regular operation. The bath surplus has in the past often been consumed in the Soderberg pot lines, but since these now are being closed, it is not that easy to get rid of the bath surplus. Clean bath material/cryolite is regarded as a product and an international commodity.

Experience shows it is significantly worse for the owners of a smelter to sell fines of basement materials containing a large fraction of alumina as well.

Basement materials has therefore been packed, stored and to some degree been sold as start-up material for new pot lines. Basement material is also to some extent recovered internally, by taking it out from the basement and mixing it into the regular ACM, and thereby to return it to the top of the pots, with the obvious negative effects increased proportion of fines provides on work atmosphere, environmental conditions, and operational excellence. Long-term storage in Big Bags in anticipation of an eventual sale or equivalent also provides by experience some quality challenges over time, for example moisture penetration. This can in some cases make landfilling to be considered a realistic alternative.

Basement material is a mix of aluminium oxide (Al2O3) and bath materials (cryolite) and origins mainly from the ACM that covers the side crust and the anodes in the pots. When it is time for anode change parts of the ACM tends to follow the remains of the spent anode. In the following crushing process of the ACM, it is attempted to maintain certain particle coarseness so that it can function optimally as ACM when it is to be put back to the pot. The amount of fines generated in the crushing process is strongly dependent on the selected crushing technology.

Untreated basement material cannot be supplied in the delivery system for alumina, as this could give rise to clogging of supply systems and feed nozzles, as well as provide for a too high mechanical load on the filter bags in the dry scrubbers. Pot lines with fluidised flow systems for alumina feeding may also experience unwanted operational challenges by adding basement materials directly to the alumina delivery system. Fines of basement material being admixed with ACM may also reinforce the familiar silo storage problem of segregation in the cover materials. Segregation challenge of ACM is attempted solved in Norwegian patent NO 330583. This patent possesses features which are substantially different to the present invention.

EP 0615786 A1 discloses an apparatus for separating non-fluidizable contaminants from primary oxide being delivered to an aluminium smelter. Primary oxide is pure oxide being delivered from the alumina producer. Primary oxide as swept up from floor/covers can be treated in the apparatus described in this publication, so as to obtain purified oxide, free of non-fluidising foreign elements such as bolts, tools, and other debris, so that the oxide may go into production in a regular manner.

The present application gives treatment to "Basement Materials" that is a mixture of down-fall of added ACM, that is a mixture of cryolite and secondary oxide, and fallouts of secondary oxide charged directly to the pots as feed material to the electrolysis process. Secondary oxide is contaminated primary oxide used as the cleansing medium in a preceding dry-scrubber process for the gases emitted from the electrolytic cell. In modern pot lines all primary oxide are processed through the dry-scrubber plant before fed to the electrolytic cells. Upon delivery to the cells, ACM and secondary oxide cause a polluting dusting in the pot line hall as this dust falls to floor and cellar. During mechanical work on the cells, steel pieces, welding-rods, bolts etc. in addition to aluminium spills tend to fall to the basement. All this constitutes basement material, where ACM and secondary oxide constitutes the main volume.

Fines of ACM and secondary oxide can both be fluidizing documenting that the invention of EP 0615786 A1 is unsuitable for the current application, since it only separates fluidizing and non-fluidizing material. An object of the present invention is that by sieving basement material a significant concentration increase of Al2O3 in the fines fraction occur enabling recovery of the fines in the basement material through a controlled admixing with the ordinary alumina supply system. Since all these are fine fractions and potentially also fluidizing, it is not necessarily a requirement for the recovery of it.

One further aspect with controlled admixing is that the amount of mixing of fine basement materials must be balanced against the so-called scaling formation in the walls of the supply systems. Scaling is hard unwanted growing surface layer that could potentially clog the transport systems. Scaling is formed as a function of fines, moisture, volume flow, pressure drop and content of AlF3. It is therefore up to each pot line to decide how large a fines fraction ratio of basement material there is to be fed to the alumina delivery system since a too high proportion of AlF3 may be undesirable in relation to scaling, among others. In principle, 100% admixing may be possible according to the present application, although realistically it would involve lower percentages. Below 2% admixing would in practice not lead to scaling and/or much of any other pollution from fines. EP 0615786 A1 specifies nor any mix regulation, since it is all about separating a clean homogeneous material such as primary oxide, and then to mix it with the same type of material. Admixing ratio is therefore not an issue.

100% may be appropriate in a crisis situation in case ordinary supply is completely stopped.

EP 0615786 A1 describes a process for separating fluidized primary oxide from non-fluidizing materials so that the cleaned and fluidizing primary oxide can be fed to the ordinary alumina supply lines. Separated impurities described in the referred publication would not be appropriate or suitable to add in a process supply line to the pot line. The present invention is however directed to highly contaminated basement materials (containing ACM and secondary oxide) and a mechanical sieve is utilized with a specified sieve cloth in opposition to EP 0615786 A1 that uses a permeable fluidizing membrane for separation.

The publication CN102268696A deals only with alumina or primary oxide and cleaning in connection with loading facilities in connection with rail transport. This has little to no relevance to the present process, and the results that we obtain with an up-concentration of cryolite in the coarse fraction and of Al2O3 in the fines fraction enabling the fractions to be recycled through a controlled admixing trough the regular feed systems of a primary aluminium plant.

Alcoa Case Studies, Brazil - 2011 describes the sweepings of material from the floor in a pot line hall can be sieved and packed, and thus be sold or returned to the cells. This is a well-known established procedure that is not suitable.

According to the present invention, the material is sieved to obtain an upconcentration of cryolite and Al2O3 in the respective fractions, so that it becomes possible with a controlled admixing and return via the regular feed lines in the pot line.

US 4,113,832 A discloses a recycling process, but describes several materials that together with carbon materials undergoes a pyro-hydrolysis process, and from this is produced a clinker which Al2O3 can be recovered from the basis of the Bayer process. Besides the goal of recovery, this publication seems not to have any relevance to the present invention.

US 3,077,266 A discloses a screen for separating materials of different size fractions.

NO 164665 B describes a process for recovering aluminium from waste material to return this to the electrolytic pot. The scrap is divided into a coarse fraction containing pure metal and a fine fraction, which mainly contains oxide. The method further comprises hot water leaching and a subsequent acidic slurry processing. The dissolved aluminium with acid slurry is recovered by precipitation, drying, and calcination.

GB2067596 describes a process for separation and re-use of alumina and cryolite, which are present in dust in electrolysis gases. The process comprises producing a foam containing the impurities and a slurry comprising cryolite and alumina. This can be dried and filtrated and be returned to the electrolysis bath.

US 3635408 relates to a process for recovering carbon, fluorides and alumina for the carbon lining in an electrolytic cell. The carbon lining is crushed to a moderate size, a hydration is performed and thereafter the material is filtered to sort according to size. This publication mentions that different materials can be found in the different size fractions and that the finer size fraction is suitable for return to the electrolytic bath.

CN 103088365A describes a process for recovering materials from an electrolysis cell being replaced. It is described treatment of both anode materials and cathode materials and waste fractions from these.

According to the present invention, "Basement Material" is treated and which is a mixture of downfall of added anode cover material (ACM). By processing this dust and separating it into separate size fractions, it has surprisingly been found that the finest fraction is enriched with alumina and the coarsest fractions are enriched with cryolite. The discovery that the enrichment obtained by fractioning also opens for the related invention that full recovery thus may be obtained through the pot lines regular raw material feed lines, where bath enriched fractions are recycled through the ACM line and alumina enriched fractions are recycled through the regular Aluminium Oxide feed lines. Smaller amounts of other content such as aluminium and steel are also separated in the processing of the basement material and such constituents can thus be readily recycled as well.

The present invention also allow for separating fines from sieve or suction system in the pot line bath/ACM crushing line, and recycle this through the ordinary supply lines to the pot line. In this way, one can customize the ACM to a non-dusting material when fed to the side-crust. The fine fraction is instead fed through the feed holes inside the enclosed pot, thus not contributing negatively with regard to dusting in the pot line hall.

Summary of the invention

The objective of the invention is to remedy or reduce at least one of the disadvantages of the prior art, or to provide an alternative to prior art.

The object is achieved according to the invention by a method for recovering fines in a pot line for aluminium production, wherein the finely divided material comprises alumina and cryolite, wherein the pot line comprising electrolytic cells, and wherein the method comprising a step A1, wherein the step A1 comprises separating the fines in least a first and at least a second ( size fraction, and wherein the at least first size fraction comprises particles with substantially smaller particle size than the at least second size fraction wherein step A1 involves a concentration of alumina in the least first size fraction and a concentration of the cryolite in the at least second size fraction, and the method further comprises a step B1, wherein step B1 includes to repatriate the at least first size fraction from step A1 to the pot lines electrolytic cells and a further step B2, wherein step B2 comprises to repatriate the at least second size fraction in the pot lines supply system for covering materials (ACM)/cryolite.

The at least first size fraction preferably comprises finely divided material with a particle size of less than 1 mm, more preferably less than 0.5 mm and most preferably less than 0.2 mm.

The method preferably includes a step A0 before step A1, where step A0 comprises coarse screening of the finely divided material. Step A0 preferably comprises coarse screening the fines with a sieve with a mesh size of less than 100 mm, more preferably less than 70 mm, and most preferably less than 40 mm.

According to a one embodiment of the method according to the invention, step B1 preferably comprises to repatriate the at least the first size fraction to the pot line electrolytic cells in a weight ratio of less than or equal to 100% of the total supply to the pot lines electrolytic cells, more preferably less than 50%, and most preferably less than 2%.

The finely divided material preferably comprises basement material and/or finely divided material from covering materials (ACM).

The basic inventive concept is to utilize the surprising discovery that there are concentration differences of cryolite in comparison to alumina in various size fractions of basement material. Exploitation of knowledge about these differences in concentration allows almost complete recovery of such Basement Materials through the regular supply systems for raw materials in aluminium pot lines.

Alumina used in aluminium production has mainly a particle size less than 200 microns, in which the main mass proportion is about 50 to 100 microns. ACM may vary from the finest dust to coarse spheres of 20 mm diameter. ACM size is dependent on the technology that is in use at each Plant, the selection of operating mode, and other practical matters.

Basement material consists mainly of cover material (AMC) which is composed of aluminium oxide (Al2O3) and bath materials (cryolite). In ACM constitutes bath material typically 60% and oxide constitute the remaining 40%. By separating basement material in various size fractions, the analysis surprising show alumina content increases in the finest fractions and bath content increases in the coarser fractions.

By allowing finely divided material of basement materials, in a size fraction resembling alumina, to be added in a smaller percentage to the regular alumina supply line, it will then be fed into the electrolytic pot as normal alumina/secondary oxide. The advantage of this is that it completely avoids the formation of undesired segregation and clogging of supply systems. Meanwhile, the alumina containing the finely divided fraction of basement materials is fed with the alumina directly into the open feed-hole straight down into the enclosed pot without causing any adverse operating condition or significantly increased dust load in the pot lines inner or outer environment.

Likewise, the coarser fractions of basement materials will be free of the finest dust and thus it is favourable to admixing this in to the pot lines regular flow of ACM material.

Dependant the basement material properties around 90% of all alumina in the basement material can be recovered through the regular alumina supply line when the upper size limit is set at 200 microns. Increased bath surplus from the pot line as a result of such a recovery will normally provide increased amounts of bath materials from the electrolytic process, thus opening up a commercial opportunity to the pot line since tapped bath/cryolite can be traded on commercial terms.

Besides recovery of own produce, the invention also cause less dust load in the pot line and give less dust load in the extraction system. The solution will also give better permeability of the ACM and thereby contribute to optimize the electrolytic pots temperature balance.

The invention also permit for removal of fines from ACM processing line etc. so that such fines are extracted from the cover material, but instead are supplied to the pot with the fine fraction of the basement material. The solution is not limited to recycling at the same smelter, but also allows pot lines that adopt such a recovery to receive basement materials and the like from other works.

Brief description of the drawings

In what follows is described an example of a preferred method and embodiment illustrated in the accompanying drawings, wherein:

Fig. 1 is a flow chart showing normal alumina supply and recovery of basement material;

Fig. 2 is a schematic diagram showing an electrolytic cell with side-crust; and

FIG. 3 is a schematic view showing a sieve arrangement for separating coarse and fine fractions of basement material.

Detailed description of the invention

Figure 1 is a flow chart showing normal alumina feed in a pot line 1-6, specifies processing, and supply of basement materials 7 to 13, where reference numeral 7 indicates basement material in the pot line basement ready for removal for further processing. The processing may include a simple extraction of coarse substances before if desired is transferred to a temporary storage silo 8 prior sieving 9. The sieving process need to give at least two fractions where the fineness of the screen will distinguish fine and coarse material. The coarse fraction goes if desired to an intermediate storage silo 10 and further processing, for if desired to be recovered as ACM in the pot line. The fine fraction goes in a similar way to an intermediate storage silo 11 before it through a regulating feed system 12 is supplied as a percentage into the line for alumina/ secondary alumina feed 13 according to the invention.

In Figure 2 there is shown a sectional view of an aluminium electrolysis pot. The structure of such a pot will be known to one skilled in the art. The pot comprises in known manner an anode 14, a cathode 17, molten aluminium 18, molten cryolite 16 and side-crust/ACM/alumina 15.

Figure 3 is a principle drawing which schematically shows a sieve process for basement material. The added basement material 19 is sieved though successively finer sieves 20-22 and sieve fractions are collected in bins 23-26. Bin 23 collects the fine fraction, bins 24-25 collects coarse fractions and bin 26 collects the overflow fraction.

Exemplary embodiment

Basement material 7 consisting of 60% bath materials and 40% alumina was screened through a 50 mm sieve to remove iron bolts, concrete pieces, gloves and other foreign materials before further processing. Basement material 7 was then sieved 9 into two fractions with a size separation of 0.2 mm. The finest 11 fraction constituted 70% after sieving and the coarser 10 fraction amounted to 30%.

After sieving, a simple specific gravity analysis showed that the finest fraction consisted of 50% alumina and 50% bath material. This result is quite surprising since this document that 89% of the available alumina contained in Basement Material after processing is concentrated in the finest proportion.

The finest fraction may typically be supplied through the regular alumina supply line 1-6 with a continuous admixing of 1.0%, which in practice is substantially unproblematic to handle for the supply lines, abatement plants and cell point feeders in an aluminium smelter.

Although in the foregoing description and in the appended claims is stated that the various fractions is recycled to the same pot line, it shall be understood that one or more fractions can be used in another pot line.

Claims (7)

P a t e n t c l a i m s

1.

A method for recovering fines in a pot line for aluminium production, wherein the finely divided material comprises alumina and cryolite, wherein the pot line comprising electrolytic cells (6), and wherein the method comprising a step A1, wherein the step A1 comprises separating (9) the fines in least a first (11) and at least a second (10) size fraction, and wherein the at least first size fraction (11) comprises particles with substantially smaller particle size than the at least second size fraction (10), c h a r a c t e r i s e d in that step A1 involves a concentration of alumina in the least first (11) size fraction and a concentration of the cryolite in the at least second (10) size fraction, and the method further comprises a step B1, wherein step B1 includes to repatriate (13) the at least first size fraction (11) from step A1 to the pot lines electrolytic cells (6) and a further step B2, wherein step B2 comprises to repatriate the at least second size fraction (10) in the pot lines supply system for covering materials (ACM)/cryolite.

2.

A method according to claim 1, wherein the at least first size fraction (11) comprises a finely divided material (7) with a particle size of less than 1 mm, more preferably less than 0.5 mm, and most preferably less than 0.2 mm.

3.

A method according to claim 1, wherein the method comprises a step A0 before step A1, where step A0 comprises coarse screening of the finely divided material (7).

4.

A method according to claim 3, wherein step A0 comprises coarse screening the finely divided material (7) with a sieve with a mesh size of less than 100 mm, more preferably less than 70 mm, and most preferably less than 40 mm.

5.

A method according to claim 1, wherein the step B1 comprises to repatriate in least the first size fraction (11) to the pot line electrolytic cells in a weight percentage of less than or equal to 100% of the total supply to the pot lines electrolytic cells, more preferably less than 50%, and most preferably less than 2%.

6.

A method according to any one of the preceding claims, wherein the finely divided material (7) comprises basement material.

7.

A method according to any one of the preceding claims, wherein the finely divided material (7) comprises finely divided material from covering materials (ACM)