AU2009230820A1 - Immobilisation of pollutants in spent potlining - Google Patents

Description

Australian Patents Act 1990 - Regulation 3.2 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title "Immobilisation of pollutants in spent potlining" The following statement is a full description of this invention, including the best method of performing it known to me/us: P/00/0 II 00 \nr\DAH\2009\0ctober\40135206 AU complete aDoIn Uni of Melb 29 October.doc - 29/10/09 P:\Opcr\DAH\Provs\40135206 spent potlining doc - 29/10/09 IMMOBILISATION OF POLLUTANTS IN SPENT POTLINING Field of the Invention This invention relates to the treatment of waste materials generated during the 5 production of aluminium in an aluminium smelter, and in particular to the treatment of spent potlining (hereinafter referred to as "SPL"). The invention especially relates to a treatment process which immobilizes harmful and toxic constituents in the SPL and converts the remaining residual SPL to a form which can be safely disposed, for example as landfill. 10 Background of the Invention Aluminium is manufactured using a high temperature process in which alumina is electrolytically reduced in a bath of molten cryolite. This process is conducted in reduction cells, often called pots, and a typical aluminium smelter contains numerous 15 pots connected in series. The pot has a metallic outer structure and an interior bottom lining of refractory brick and a further inner lining of carbon which also extends to cover the side walls. The carbon lining serves as the cathode and also protects the metallic structure of the pot from contact with and corrosion by the bath of molten cryolite. 20 The severe operating conditions experienced within the pot lead to a progressive deterioration of the carbon lining to the extent where either leakage of the molten contents occurs or the aluminium product contains an unacceptably high level of impurities (e.g. iron). At this stage, the pot is decommissioned and the lining 25 completely replaced. The lining, which includes carbon, a mixture of inorganic fluorides and inorganic oxides and refractory brick, is known as spent potlining or SPL. Approximately 50 kg of SPL is generated per tonne of aluminium metal produced. 30 This results in the generation of SPL in large quantities of approximately 800,000 to 1,000,000 tonnes per year throughout the world.

P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 -2 Depending on the method of recovery of the SPL, it may contain 20 to 70% by weight of carbon and significant quantities of refractory brick, cryolite and other aluminium containing compounds in the form of carbides, nitrides, fluorides and oxides. Sodium fluoride, sodium carbonate and calcium fluoride are also present. 5 As SPL contains environmentally harmful and biologically toxic constituents, major restrictions are imposed on its transportation, treatment, storage, handling and disposal. SPL cannot be disposed of in a conventional manner (eg as landfill), without prior processing to remove or immobilise the harmful and toxic constituents. The 10 basis for such strict environmental controls is as follows: (i) SPL contains free and complex cyanides and fluorides, and may also contain arsenic and polyaromatic hydrocarbons (PAHs); 15 (ii) upon exposure to rainwater, free and complex cyanides and fluorides in particular will be leached and enter the environment; (iii) free and complex cyanides, fluorides, arsenic and PAHs are corrosive and toxic; 20 (iv) improper disposal of SPL can result in a substantial hazard to the environment as demonstrated by the migration, mobility and persistence of free and complex cyanides and fluorides in particular. 25 In Australia, raw SPL is either stored in sheds with a concrete base or in secured landfill sites with an impermeable base and a plastic cover. These storage arrangements are costly and there are obvious advantages in developing alternative methods for detoxifying the SPL. Furthermore, storing SPL in sheds can only be a temporary measure to buy some time for the aluminium metal producer until a 30 satisfactory and environmentally acceptable process for detoxifying SPL can be developed. There is an immediate need for the development of a SPL treatment process that is environmentally, economically, and technically feasible.

P:Oper\DAH\Provs\40135206 spent potlining.doc - 29/10/09 -3 In view of these major environment restrictions, numerous processes for treating SPL have been investigated and the majority of these have included either high temperature treatment, wet processes or combinations thereof. 5 Several earlier processes involved high temperature calcination of SPL in air or in oxygen enriched air in order to destroy cyanides. However, this results in the release into the atmosphere of environmentally unacceptable levels of greenhouse gas CO 2 (over 2.5 tonne of CO 2 results from 1 tonne of C) and of corrosive toxic gaseous 10 fluorides such as HF and SiF 4 . These fluorides result from high temperature pyrohydrolysis reactions (reactions of water with fluorides and silicates in the SPL) and must be scrubbed. The ash will also contain substantial amounts of leachable fluorides. A positive aspect is that toxic polyaromatic hydrocarbons are destroyed in the combustion process. 15 It has been proposed that the valuable energy content of the carbon in SPL can be exploited by mixing it with coal to use as fuel for power generation or in industries such as cement or brick manufacture. While this is quite simple and completely destroys the cyanide content of the SPL, it does not deal with the fluoride problem at 20 all. Substantial amounts of fluoride are released in the dangerous gaseous form, which is environmentally unacceptable and ultimately may allow fluoride to enter the food chain. In addition, the long-term effects of the SPL minerals in cement and bricks are not known. It may also be impractical to transport the SPL from the aluminium production facility to a cement factory if they are not located close to each 25 other, because untreated SPL is a hazardous substance. Many processes have been proposed to detoxify SPL. The Reynolds process developed in 1992 aims to destroy the cyanide and stabilise the fluoride by heating the SPL in air to -600-900 *C in the presence of a significant quantity of additives such as 30 limestone and anti-agglomerants. A 120,000 ton/year treatment facility was located at Gum Springs, Arkansas, USA. However, this process has not gained world-wide acceptance at present for unpublished reasons.

P \Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 -4 One of the difficulties which can be anticipated in the operation of this process is that the normal calcination temperature of this process causes the solids to sinter. Hence, agglomeration of particles occurs leading to incomplete stabilisation of the fluoride in 5 the solid sample. Anti-agglomerants such as sand and metal silicates are added to the SPL to reduce the extent of sintering. As a result of the added inert material, the mass of the final residue is about 3 times that of the original SPL mass (2.8kg of landfill waste/kg of raw SPL). Further, the raw SPL is quite alkaline due to its chemical composition. When SPL is mixed with substantial quantities of Ca(OH) 2 at the level 10 required by this process, the final mixture has a very high alkalinity and is probably not suitable for land fill. Another disadvantage of the Reynolds process is that carbonaceous material is treated in air at elevated temperatures and a significant amount of the carbon is oxidised to 15 give carbon dioxide, which is known to contribute to the green house effect. Some of the fluorides in SPL such as NaF are quite soluble in water and other aqueous solutions. Therefore, unless the SPL is properly stabilized, these soluble compounds can readily be leached on exposure to surface water and can cause serious contamination problems. 20 Another high temperature process that has gained some attention is the Ausmelt Process. This process basically allows the carbon component of the SPL to be burned or oxidised in the presence of molten slag, for example from a steel smelter. The aim is that some of the fluoride released as a result of the combustion of the carbonaceous 25 matter will be bound and trapped by the molten slag. One of the short-comings of this process is that the chemical binding effect of the slag is not very efficient due to the big difference in density between the gaseous combustion products containing fluorides and the molten slag. As a result, a significant amount of the fluoride escapes in off-gases from the reactor without being properly stabilized by the slag causing 30 corrosion problems and environmental concerns. Recovery of the volatile fluorides in the off-gases, such as HF and SiF 4 , is attempted by adsorption onto a bed of alumina. However, the adsorbed fluoride content is not subsequently isolated in a useful form P \Oper\DAH\Provs\40135206 spent potlining doc - 29110109 -5 and removed from the smelting system, but is returned to the reduction cell along with the alumina. This recycling of excess fluoride eventually overloads the cell and the excess fluoride is usually removed from the cell as "excess cryolite bath", a hazardous waste with not much commercial value. More importantly, vast quantities of CO 2 5 would also be produced with the burning of SPL, and SiF 4 captured from the off-gases would introduce silica into the smelter. Another disadvantage of the Ausmelt process is that either the toxic SPL has to be transported to the steel smelting site or a substantial quantity of heavy slag has to be 10 transported to the aluminium smelter. Transportation of either the slag or the SPL is costly. Other disadvantages include the increased volume of waste material to be disposed of due to addition of slag, and the fact that the process operates at extremely high temperatures, consuming significant amounts of heat and causing corrosion by fluorides of equipment. 15 In contrast, some wet processes have involved treatment of SPL, or a derivative thereof, with aqueous solutions of inorganic species such as alkaline compounds, mineral acids or corrosive acids such as aqueous hydrogen fluoride and fluorosilicic acids. All of these processes are multi-step, and in some instances high reaction 20 temperatures and pressures are required. None of the solution processes deal with the destruction of PAHs. U.S. Patent No. 5,470,559 discloses a procedure that leaches the SPL in a caustic solution. The SPL is first ground to 28 mesh, then mixed with a solution of between 25 10 and 60 g/L of NaOH. The solid and liquid phases are separated and the liquid containing most of the cyanide and fluoride values is heated at pressure to between 160*C and 220*C to destroy the cyanide and produce a cyanide free solution. A major problem with such a process is that it does not address the issue of limited 30 solubility of fluorides, such as cryolite, in water. Additionally the high-pressure treatment would be expensive on an industrial scale. The waste liquid contains environmentally unacceptable levels of fluorides and cyanide and other metal P \Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 -6 elements making the disposal a difficult and costly step. U.S. Patent No. 5,939,035 describes a process for treating SPL which involves an initial water-wash to dissolve soluble inorganic matter, including free cyanide, 5 complexed cyanide, sodium fluoride and sodium carbonate and subjecting the aqueous solution and solid residue to separate processes. The solution is neutralised and then treated with an oxidising agent to destroy free cyanide, and then treated with a salt to precipitate complexed cyanide. Following this, the resulting solution is treated with a calcium salt and a flocculant to produce a calcium fluoride precipitate. The solid 10 residue is subjected to two acid washing steps, one involving HF (or a source of HF such as H 2 SiF 6 or NH 4

HF

2 ) and the other involving a strong acid such as H 2

SO

4 or

H

2 SiF 6 . These acid washing steps allow for the recovery of valuable AIF 3 by addition of a source of aluminium cations. This process involves many chemical, filtering and washing steps and the use of highly corrosive materials, such as HF and H 2 SiF 6 which 15 require special materials of plant construction and which present serious industrial hazards. AU 2003200307 describes a process for treating SPL which involves an initial water wash to dissolve soluble inorganic matter. The solution, which contains water soluble 20 inorganic matter such as cyanides and fluorides, is then separated from the solid residue, which contains leachable fluorides. The solid residue is calcined to convert water-leachable fluorides into CaF 2 , and cyanide and fluoride is removed from the solution separately. However, the number of steps required to prepare both the aqueous solution and the solid residue for safe disposal is time consuming. 25 A requirement therefore exists for an improved process for treating SPL so that it can be safely disposed of with minimal environmental concern. Summary of the Invention 30 Accordingly, the present invention provides a process for the treatment of spent potlining comprising the steps of: (i) treating the spent potlining with water in the presence of a source of P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 -7 calcium ions and a source of iron ions to form a) an aqueous solution and b) a solid residue comprising precipitated fluorides and precipitated cyanides; (ii) separating the solid residue from the aqueous solution; and 5 (iii) calcining the separated solid residue to produce an aggregate without substantial agglomeration. In the process according to the present invention, soluble fluorides are precipitated with a source of calcium ions and soluble cyanides are precipitated with a source of 10 iron ions from the aqueous solution in step (i). The separated solid residue, comprising these precipitates and the washed spent potlining, is subsequently calcined to produce an aggregate without substantial agglomeration for safe disposal. Prior to calcination the solid residue may be dried. 15 While generally unnecessary, for particular applications any of the steps may be preceded or followed by an additional water washing or rinsing step. However, under appropriate operating conditions, only the initial water treatment is generally required to produce a product suitable for disposal. Accordingly, in one embodiment the process does not comprise any additional water washing steps. In another 20 embodiment of the invention, following step (ii) the separated solid residue is rinsed with an aqueous solution prior to calcination. The resultant product may be disposed of in a suitable manner, for example, as landfill, as it no longer contains environmentally harmful matter, and as such, does not 25 pose a human health hazard. When a sample of the calcined residue is leached in a standard solution prescribed for a leaching test, results from experimental work show that the level of fluoride detected in the leachate is indeed very low. The process of the invention is useful in treating SPL obtained from the electrolytic 30 reduction cells used in aluminium smelting, whether it is delined using a dry method or a wet method. The process can be used for treating first cut and second cut SPL.

P:\Oper\DAH\Provs\40135206 spent potlining.doc - 29/10/09 -8 An analysis of the total amount of various components of two separate samples of first cut wet delined SPL (supplied by Tomago Aluminium Company Pty Ltd NSW Australia, crushed to less than 1 mm) is displayed in Table 1. These two samples provided average values of 64.2% for carbon and 67.9% for "loss on ignition". This 5 indicates that while about 95% of polluting effluent is carbon based, there is a very significant quantity of other gaseous effluent which would contain toxic and corrosive fluorides. TABLE 1 Units Sample 1 Sample 2 Moisture % 0.4 0.4 Ash Yield % 31.9 32.3 Loss on Ignition % 68.1 67.7 (LOI) C % 64.4 63.9 Si % 0.6 0.5 Al % 4.4 4.4 Fe % 0.3 0.3 Na % 9.0 9.5 Ca % 1.0 1.1 Total Fluoride % 11.1 10.9 Water Leachable % 3.1 3.0 Fluoride Cyanide (CN) mg/kg 131 130 Water Leachable mg/kg 119 128 Cyanide (CN) Arsenic mg/kg 6.2 6.5 10 Two components of SPL that render it a hazardous waste are the soluble fluoride salts and the cyanide components (minor components such as arsenic and PAHs may also be present). The toxicity of cyanide is well known. Fluoride however is less P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 -9 commonly understood. Soluble fluoride can be absorbed by fish, as well as other animals and enters the food chain. Ingestion of significant amounts results in weakening of bone structure. 5 The fluoride products in SPL have varying levels of solubility. CaF 2 is almost completely insoluble in water, and as such is not dangerous. NaF is highly soluble in water and will rapidly leach into watercourses. The NaF however is not difficult to extract from the SPL because of its high solubility. The extracted NaF can be 10 immobilised as CaF 2 . Cryolite however is more difficult to treat. Cryolite is most usually referred to as insoluble or sparingly soluble in water. Its solubility in water is 0.042 g/L at 25 *C. Although this may seem relatively insignificant, over time sparingly soluble fluorides 15 will leach into watercourses causing an environmental problem. In the present invention fluoride, as soluble and sparingly soluble fluorides, is precipitated from the aqueous solution with a source of calcium ions. Virtually all of this fluoride is precipitated on the SPL. 20 Cyanide, while only present in small quantities, is highly toxic. Most, if not all, of the cyanide present is immobilised by precipitation with a source of iron ions. If oxygen is present during calcination, any remaining cyanide decomposes when raised to a temperature above about 200 *C. 25 Another hazardous component of SPL is arsenic. Arsenic compounds (such as arsenides) may also be immobilised through the use of a source of calcium ions and a source of iron ions in the present invention. 30 Raw spent potlining also contains polyaromatic hydrocarbons (PAHs), which have a boiling point in the range of 200-450 'C. These may be removed by volatilisation at a calcination temperature of 600 *C and can be destroyed subsequently by high P:\OPER\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 10 temperature atmospheric oxidation. In the process of the present invention, PAHs may be reduced by a factor of at least 100. Polyaromatic hydrocarbons present in the raw SPL may include one or more of naphthalene, acenaphthlene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benza(a)anthracene, 5 chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1.2.3.cd)pyrene, dibenz(a.h)anthracene and benzo(g.h.i)perylene. Following calcination, these polyaromatic hydrocarbons may be reduced to a concentration of less than 0.005 mg/L. 10 Prior to (or during) the treatment of the SPL with water it is generally necessary to crush or grind the SPL so that the particles of SPL are a suitable size. The average particle size will generally be from 0.1 to 10mm, more preferably 0.1 to 5mm, more preferably 0.3 to 5mm, and most preferably from 0.5 to 2mm. This grinding/crushing may be achieved using any conventional equipment used to crush minerals, for 15 example, but not limited to a jaw crusher or a roll crusher. Such jaw crushers or roll crushers may be used in a crushing/screening circuit. After grinding or crushing the SPL may be treated magnetically to remove iron and/or iron oxides which may be present, although the presence of this material in the SPL to be treated has not been shown to cause any difficulties. 20 Laboratory studies show that the water treatment step is an important step towards the detoxification of the SPL and the immobilisation of pollutants with the formation of a residue suitable for landfill. The raw SPL is highly alkaline in nature due to the presence of Na 2

CO

3 . The water treatment step minimises the level of soluble 25 fluorides, such as sodium fluoride, in the residue prior to calcination. In the water treatment step, virtually all of the water soluble fluoride is precipitated with a source of calcium ions, as well as cyanide by reaction with a source of iron ions. The amount of water used will depend on a number of factors, including the chemical 30 nature of the SPL, the particle size and available volume in treatment vessels. With less water, a longer water treatment may be required than if a larger volume of water is used. However, the larger the volume of water used, the larger amount of aqueous P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 11 solution for disposal after separation. Generally the amount of water used will be between 3, 4 and 10 times the weight of SPL being treated, more preferably between 5 and 7 times. 5 In one embodiment of the invention, in step (i) the crushed spent potlining is treated by contacting the spent potlining with water prior to the addition of the source of calcium ions and the source of iron ions. The spent potlining may be treated with water for 0 to 6 hours prior to the addition of the source of calcium ions and the source of iron ions, preferably 0.25 to 4 hours, more preferably 0.5 to 2 hours. In one 10 embodiment, the spent potlining is treated with water for approximately 1 hour prior to the addition of the source of calcium ions and the source of iron ions. While the initial treatment with water may be performed at, for example from 10 to 60 *C, it is generally only necessary to perform the initial treatment with water at ambient 15 temperature and pressure. The water used to treat the spent potlining in this initial step may be low grade water with low levels of salinity, or similar liquid. Plain water, river water, waste water, or sea water may be used. Additional salts or minerals may also be dissolved in the 20 water solution if necessary. In one embodiment, the water used is plain water. After the initial addition of water (prior to the addition of the source of calcium ions and the source of iron ions), the pH of the aqueous solution is generally alkaline. Therefore in one embodiment of the present invention, after the initial addition of 25 water, the aqueous solution is acidified prior to the addition of a source of calcium ions and a source of iron ions. In this acidification step the pH of the aqueous solution may be adjusted to from 1 to 6, preferably from 2 to 5.5, and most preferably from 3 to 5. 30 The acidification step may be performed with a wide range of acids. In one embodiment, inorganic acids are used, such as hydrochloric acid, sulphuric acid, nitric acid, methanesulphonic acid or phosphoric acid. In one embodiment, hydrochloric P:\Opcr\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 12 acid is used. When hydrochloric acid is used, a benign effluent may be advantageously produced. The initial treatment with water leaches soluble fluorides such as NaF, complex 5 cyanides such as Na4[Fe(CN) 6 ] and free cyanide into the water solution. Carbonates, such as Na 2

CO

3 also leach into the aqueous solution from the spent potlining during this initial treatment step, making the solution alkaline. Fluoride and cyanide are precipitated from the aqueous solution through reactions 10 such as: Ca 2+ + 2F - CaF 2 4Fe 3 + + 3[Fe(CN) 6

]

4 ~ -> Fe 4 [Fe(CN) 6

]

3 15 Fe 3 + + 6CN- -+ [Fe(CN)6]3 + Fe 3 + -> Fe[Fe(CN) 6 ] The suspension of the SPL residue, any precipitates formed, and the source of calcium ions is stirred so that sparingly soluble fluorides in the SPL such as cryolite, Na 3

AIF

6 , 20 are gradually converted to insoluble CaF 2 because of the slightly smaller solubility of CaF 2 compared with that of Na 3 AlF 6 and because of the mass action effect of the excess Ca2+. CaF 2 normally precipitates from aqueous solution as a gel, and to assist in filtration the use of a flocculant may be advantageous. In these reactions, CaF 2 also adsorbs onto the granular SPL, assisting in filtration. 25 As used herein, unless otherwise specified, the terms "source of calcium ions" and "source of iron ions" are intended to refer to a single source of the ions, as well as multiple sources of the ions. It is also to be understood that at least a portion, and in some cases the major portion, of the source of ions may be present in the water 30 treatment step in undissolved form, for example in the case of sparingly soluble CaCO 3

.

P \Oper\DAH\Provs\40135206 spent potlinng doc - 29/10/09 - 13 In one embodiment, the source of calcium ions comprises a calcium halide. This may be CaCl 2 . This source of calcium ions is preferably soluble in water. When CaCl 2 is used to immobilise fluoride as CaF 2 , fluoride levels are provided in the final residue which are generally less than 5% of the levels required by the Victorian 5 Environmental Protection Agency (EPA) for land fill disposal. Calcium nitrate, or any other soluble calcium source, may also be used to immobilise fluoride. In one embodiment, the source of calcium ions comprises a calcium halide, a calcium nitrate, or a mixture thereof. CaCl 2 , if used, presents a benign anion in effluents as illustrated in the following reaction: 10 CaCl 2 + 2NaF - CaF 2 + 2NaCI The source of calcium ions should be sufficiently soluble to provide sufficient quantities of calcium ions in solution to precipitate fluorides. This source of calcium 15 ions may accordingly be fully soluble, or sparingly soluble in water. The calcium ions in the water treatment step may derive from a single source, or from two or more sources. 20 In an embodiment of the invention, two or more sources of calcium ions are used. In one embodiment, these additional calcium ions are present in a sparingly soluble calcium compounds, such as calcium carbonate. In an embodiment of the invention, the additional source of calcium ions may predominantly provide a different functional purpose to the first source of calcium ions, for example in the case of CaCl 2 25 and CaCO 3 . Experimental results indicate that the use of CaCO 3 allows the solid products to be filtered more readily. The presence of CaCO 3 also allows any residual fluoride, if present, to be immobilised as CaF 2 during calcination. Other suitable sources of calcium ions that may be used include Ca(OH) 2 , CaSO 4 , 30 limestone, CaO, and mixtures thereof, including mixtures with CaCl 2 and/or CaCO 3 . In one embodiment the source of calcium ions comprises CaCl 2 and CaCO 3

.

P:\Opcr\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 14 It is believed that the use of Ca(OH) 2 results in the aggregate after calcination possessing higher pH values than when sources of calcium such as CaCO 3 are used. CaSO 4 may also be used, but this results in higher levels of sulphate in the leached residue and residual sulphate in the filtrate and effluents. This accordingly may 5 present problems in effluent treatment and disposal that would not be encountered when chlorides or carbonates are used throughout the process. Crushed limestone is also suitable. However, the microcrystalline nature of the crushed mineral and its resultant reduced rate of chemical reactivity would lead to 10 longer reaction times in the immobilisation of fluoride by Ca ions. Advantageously, the use of crushed limestone results in waste disposal products that are more compatible with the land mass, due to a more neutral pH, than when precipitated CaCO 3 is used. 15 A person skilled in the art would appreciate that the amount of the source of calcium ions required will vary depending on the amount of fluoride, and other components, in the spent potlining. The amount of the source of calcium ions required will also vary depending on the water solubility of the calcium ion compound. 20 The amount of the source of calcium ions required may be calculated by a person skilled in the art. For example, a sample of spent potlining could be tested to determine the amount of fluoride present. The amount of calcium ions required in solution to immobilise this fluoride could be calculated on the basis that one mole of calcium ions will immobilise two moles of fluoride. 25 In one embodiment of the invention, from 3 to 20 mol of a source of calcium ions is used per kilogram of SPL; preferably from 4 to 17 mol of a source of calcium ions is used per kilogram of SPL; more preferably from 5 to 15 mol of a source of calcium ions is used per kilogram of SPL; and most preferably from 6 to 13 mol of a source of 30 calcium ions is used per kilogram of SPL. In another embodiment of the invention, from 3 to 10 mol of calcium ions in solution P:\Oper\DAH\Provs\40135206 spcnt pothning doc - 29/10/09 - 15 is present in water treatment step (i) per kilogram of SPL; preferably from 4 to 9 mol of calcium ions in solution is present in water treatment step (i) per kilogram of SPL; more preferably from 5 to 8 mol of calcium ions in solution is present in water treatment step (i) per kilogram of SPL; and most preferably from 6 to 7 mol of 5 calcium ions in solution is present in water treatment step (i) per kilogram of SPL. A feature of the invention is that a source of iron ions is used to precipitate cyanide, resulting in cyanide levels in the treated residue which are extremely small. The source of iron ions should be sufficiently soluble to provide sufficient quantities of 10 iron ions in solution to precipitate cyanide. This source of iron ions may accordingly be very soluble, or sparingly soluble in water. In one embodiment, the source of iron ions used in the present invention is soluble in water. In another embodiment of the present invention, the source of iron ions is or includes an iron halide, an iron nitrate, an iron sulphate, or a mixture thereof. In another embodiment, the source of iron ions 15 is FeCI 3 . If FeCl 3 is used then the following reactions may occur: 4FeCI 3 + 3Na4[Fe(CN) 6 ] -+ Fe 4 [Fe(CN) 6

]

3 + 12NaCl FeCl 3 + 6NaCN - Na 3 [Fe(CN) 6 ] + 3NaCl 20 Na 3 [Fe(CN) 6 ] + FeCl 3 -+ Fe[Fe(CN) 6 ] + 3NaCl Small amounts of Fe 4 [Fe(CN) 6

]

3 are notoriously difficult to filter from normal aqueous solutions, as Fe 4 [Fe(CN) 6

]

3 is colloidal under some conditions. In the present 25 invention, Fe 4 [Fe(CN) 6

]

3 is precipitated onto granular SPL, leading to easy filtration. A person skilled in the art would appreciate that the amount of the source of iron ions required will vary depending on the amount of cyanide, and other components, in the spent potlining. The amount of the source of iron ions required will also vary 30 depending on the water solubility of the iron ion compound. The amount of the source of iron ions required may be calculated by a person skilled P:\Oper\DAHf\Provs\40135206 spent potlining doc - 29/10/09 - 16 in the art. For example, a sample of spent potlining could be tested to determine the amount of cyanide present and the amount of iron ions required to immobilise this cyanide could be calculated in light of the equations above. 5 In one embodiment of the invention, from 0.01 to 0.1 mol of a source of iron ions is used per kilogram of SPL; preferably from 0.02 to 0.07 mol of a source of iron ions is used per kilogram of SPL; and more preferably from 0.03 to 0.05 mol of a source of iron ions is used per kilogram of SPL. 10 In another embodiment of the invention, from 0.01 to 0.1 mol of iron ions in solution is present in water treatment step (i) per kilogram of SPL; preferably from 0.02 to 0.07 mol of iron ions in solution is present in water treatment step (i) per kilogram of SPL; and more preferably from 0.03 to 0.05 mol of iron ions in solution is present in water treatment step (i) per kilogram of SPL. 15 In one embodiment, the solid residue formed in step (i) further comprises precipitated arsenic compounds. For example, a source of calcium ions, such as CaCl 2 , may immobilise arsenic (V) as calcium hydrogen arsenate, CaHAsO 4 . Moreover, a source of iron ions, such as FeC 3 may isolate arsenic (III) as ferric arsenite, FeAsO 3 . 20 Therefore, while arsenic levels may be low (6 to 6.5 mg/kg) in the SPL being treated, arsenic levels may be reduced to about 1% of the EPA limit when using the process of the present invention. As described herein, any reference to a chemical includes their hydrates and solvates. 25 For example, a reference to CaCl 2 includes CaCl 2 .2H 2 0, and a reference to FeC 3 includes FeCl 3 .6H 2 0. In one embodiment, the solid residue formed in step (i) comprises precipitated fluorides, precipitated cyanides and residual spent potlining. In another embodiment, 30 the solid residue formed in step (i) comprises precipitated fluorides, precipitated cyanides, residual spent potlining, and precipitated arsenic compounds. In a further embodiment, the solid residue formed in step (i) comprises precipitated fluorides, P:\Oper\DAH\Provs\40135206 spent potlining.doc - 29/10/09 - 17 precipitated cyanides, residual spent potlining, precipitated arsenic compounds and polyaromatic hydrocarbons (PAHs). In one embodiment, the solid residue formed in step (i) comprises precipitated 5 fluorides, precipitated cyanides, residual spent potlining and solid CaCO 3 . In another embodiment, the solid residue formed in step (i) comprises precipitated fluorides, precipitated cyanides, residual spent potlining, precipitated arsenic compounds and solid CaCO 3 . In a further embodiment, the solid residue formed in step (i) comprises precipitated fluorides, precipitated cyanides, residual spent potlining, precipitated 10 arsenic compounds, polyaromatic hydrocarbons (PAHs) and solid CaCO 3 . The water treatment step can be carried out by contacting the (generally crushed or ground) SPL with water in the presence of a source of calcium ions and a source of iron ions in any suitable equipment, preferably with stirring, mixing or other means of 15 agitation to ensure good contact between the water and the particles of SPL. In one embodiment the water used to treat the spent potlining in step (i) comprises the aqueous solution separated in step (ii) from a previous treatment of SPL. Following treatment with water and the precipitation of fluorides and cyanides, the 20 solid residue is separated from the aqueous solution using any suitable technique. Generally the residue will be separated from the aqueous solution using standard filtering equipment such as continuous vacuum drum filter or continuous vacuum belt filter. 25 After separation of the solid residue, the solid residue may be washed with water. In one embodiment, the solid residue from step (ii) is subjected to a drying step prior to calcination. The solid residue may be allowed to dry, or may be dried using direct heat rotary tumble drier with mixing-ribbon spirals or lifting baffles to remove excess water. The solid residue may be dried at a temperature between 90 *C and 160 *C, 30 preferably 100 *C to 120 *C, and most preferably about 110 *C. It is important to remove most of the water, if not virtually all of the "free" water, from the residue as the presence of too much water during the subsequent calcination step can lead to P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 18 pyrohydrolysis reactions which would lead to release of HF during calcination. The solid residue is then subjected to calcination in a suitable reactor. The reactor, for example, may be a stationary reactor or may be a rotary or tumbling reactor. In a 5 preferred embodiment the reactor is heated by an external heating source, such as an electric furnace. Care must be taken in calcining the solid SPL residue in the reactor as the presence of oxygen in the reactor can cause combustion and heat generation in the furnace, raising the temperature higher than desired. It can also generate unwanted greenhouse gases, such as CO 2 . With careful control of the amount of 10 oxygen in a calcination reactor, the heat generated by combustion of carbon in the reactor can be used to provide the heat necessary for calcination. If an excess quantity of oxygen is present in concentrated form in the gas phase, it can cause localised combustion of carbon and localised overheating of a fraction of the solid mixture. In a preferred embodiment the reaction is conducted in an oxygen free or inert 15 environment, for example using an inert gas such as nitrogen or argon, with the heat being provided from an external source. Preferably the inert gas is nitrogen. In another embodiment, CO 2 may be used. In a further embodiment, a mixture of N 2 and

CO

2 is used. 20 Advantageously, when an inert gas is used, it is believed that there is no net release of

CO

2 or release of toxic gases during calcination. In some cases, such as when CaCO 3 is used, the calcined residue has a significantly lower pH when calcination is performed in the presence of carbon dioxide. This 25 indicates that the use of small partial pressure of CO 2 (for example less than 0.1 atmosphere) in the stream of an inert gas may inhibit decomposition of CaCO 3 to the strongly basic CaO at hot spots in the calcination kiln. In the present invention, a source of calcium ions is generally present during the 30 calcination step. The source of calcium ions may be present by virtue of their addition during step (i) of the process according to the present invention. For example, an excess of calcium ions may be present due to the addition of a sparingly soluble P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 19 calcium compound, such as CaCO 3 , in step (i). In another embodiment, the solid residue from step (ii) may be calcined in the presence of an additional source of calcium ions. 5 If an additional source of calcium ions is added to the washed SPL for calcination, the washed SPL can be combined and mixed with the calcium containing material prior to charging the reactor, or the components may be added separately to the reactor. Where the components are added separately to the reactor, it is preferable to use a rotary or tumbling type reactor to intimately mix the SPL and the calcium material. 10 The calcination conditions will depend on a number of factors, including the source of the SPL, the conditions of water treatment step (i), the type of reactor, and the nature of the calcium containing material present. 15 The calcination temperature is preferably between 550 and 1000 *C, more preferably 580 to 800 *C and most preferably about 600 *C. Higher temperatures can lead to increased sintering and consequential agglomeration of the SPL residue. The calcination time will depend on the solid residue to be calcined, the temperature 20 of calcination and the nature of the reactor. Generally the calcination time will be between 0.25 to 5 hours, more generally between 0.5 and 4 hours, and most preferably about 2 hours. Calcination of the separated solid residue may reduce levels of polyaromatic 25 hydrocarbons (PAHs) by a factor of 100. The solid residue from the water-washing step contains mainly carbon and sparingly soluble fluoride compounds, such as CaF 2 , AlF 3 , Na 3 AlF 6 and NaAIF 4 , and sparing soluble cyanide compounds. 30 If water is present during calcination, compounds such as AIF 3 and Na 3

AIF

6 may produce HF. Accordingly, in one embodiment of the invention, the separated solid P \Oper\DAH\Provs\40135206 spent potlning doc - 29/10/09 - 20 residue prior to calcination is substantially free of water. However, even if no water is present in this separated solid residue at the time of calcination, it is possible that traces of water may develop from this residue during calcination. 5 For example, water may develop from the separated solid residue during calcination through the following reactions, in which FeCl 3 is illustratively used as the source of iron ions. During the water treatment step (i), FeCl 3 could be hydrolysed to form FeO.OH as 10 follows: FeC 3 + 2H 2 0 -> FeO.OH + 2HCl At calcination, FeO.OH could lose water: 4FeO.OH -+ 2Fe 2

O

3 + 2H 2 0 15 This water could cause pyrohydrolysis of sparingly soluble fluoride compounds, such as Na 3

AIF

6 : 2Na 3 AlF 6 + 6H 2 0 -+ 3Na 2 O + A1 2 0 3 + 12HF (Na 2 0 could react with A1 2 0 3 to form sodium aluminates and other oxides) 20 The presence of a source of calcium ions, such as CaCO 3 , during the calcination step allows substantially complete stabilisation of such HF. This can be achieved by reactions such as: CaCO 3 + 2HF -> CaF 2 + CO 2 + H 2 0 25 Experimental results indicated that when calcination is conducted at a temperature of 600 *C, then CaF 2 cannot undergo pyrohydrolysis. It can be shown that in a well designed reactor, the quantity of calcium ions needed in 30 an optimized process to produce the required stabilisation effect will be significantly less than the equivalent amount of solids used in the Reynolds Process.

P:\Oper\DAH\Provs\40135206 spent potining doc - 29/10/09 - 21 It has been observed that the quantity of HF being carried out of the reactor during a laboratory run is negligible. This is an important observation which lends support to the proposed reaction mechanisms and to the environmentally friendly nature of the calcination step. 5 It is believed that the mechanisms which operate when other sources of calcium and water are used will be analogous to those described above. The aggregate obtained after calcination is suitable for transferring to landfill because 10 it may be aggregated, friable, with a pH value less than that of the products of many previously reported processes. In one embodiment, the aggregate is substantially dust-free. The process according to the present invention may be used to produce a well aggregated substantially dust-free dried residue that easily meets EPA land fill requirements for cyanide, fluoride, arsenic and PAH levels and should also meet 15 Occupational Health and Safety requirements for handling and transport. The procedures outlined above are expected to be applicable to second cut SPL because the presence of refractory material and other chemically inactive components would have no effect on the basic chemical immobilisation reactions. The proposed 20 process is believed to be applicable to a wide range of SPL materials of differing active chemical content. All that would be necessary to apply the process to other SPL materials would be to analyse the material for total fluoride and cyanide contents, as well as arsenic compounds, and use appropriate excess concentrations of a source of calcium ions and a source of iron ions to precipitate fluorides, cyanides and arsenic 25 compounds. The simplicity and adaptability of the process means that site specific plants could be constructed to deal locally with various grades of SPL with the additional advantage of elimination of transportation of hazardous material to a central plant. 30 It is evident that the process according to the present invention possesses several advantages, both commercially and environmentally. Some of these advantages may P \Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 22 include the following: * Calcination of SPL may be performed at moderate temperatures, avoiding problems commonly seen in other high temperature processes, such as agglomeration of the particles and corrosion of the reactor. 5 * An inert atmosphere may be used for the calcination means that minimal amounts of CO 2 are produced such that there is virtually no greenhouse problem. * The quantity of landfill waste relative to raw SPL generated by the process may be low compared to other proposed processes 10 0 Plain water, which can be recycled, or waste or river water or possibly even sea water may be used in the single aqueous operation. Other solution based processes have used strong and corrosive acids and strongly alkaline solutions and high temperatures and pressures. e Reagent solutions and wash liquours can be recycled with adequate analytical 15 control, reagent topping up and purging. 0 Chemicals utilised in the process may be selected to provide safer effluent disposal than is the case for many other solution processes. * Chlorides when used as precipitants and occurring as the reaction product NaCl have the advantage of being particularly benign in recycling and in 20 effluent treatment. Certain embodiments of the invention will now be described with reference to the following examples which are intended for the purpose of illustration only and are not intended to limit the scope of the generality hereinbefore described. 25 Examples Example 1 50.22 g of crushed SPL was stirred at ambient temperature with 500mL H 2 0 for 1 30 hour. The pH of the solution was then adjusted from 12 to 4 using HCl. 30.10g CaCO 3 , 50.22g CaCl 2 .2H 2 0 and 0.56g FeCl 3 .6H 2 0 were next added and the mixture P:\Oper\DAH\Provs\40135206 spent potfining.doc - 29/10/09 - 23 stirred for 3 hours. After filtration the solid was washed and dried at I10 *C and weighed 49.55g. 9.01g of the dried residue was calcined under flowing N 2 at 600 *C for 3 hours. The dried residue consisted of friable aggregated particles with a low degree of dustiness. A 5% leachate of the residue had a pH value of 12.2. 5 The following results (expressed as mg/l) were obtained for Australian standard leaching procedure (ASLP) tests on the calcined residue (all analyses and ASLP tests were conducted by HRL Technology Pty Ltd Mulgrave Victoria Australia). CN F As Ca Fe <0.005 6.0 0.007 820 <0.1 10 Two samples of raw crushed Tomago SPL were analysed for PAH content. Each gave identical levels for the polyaromatic hydrocarbons (PAHs) naphthalene, acenaphthlene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, 15 pyrene, benza(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1.2.3.cd)pyrene, dibenz(a.h)anthracene and benzo(g.h.i)perylene at <0.5 mg/kg. These PAHs have boiling points in the range 200-450 *C, and are removed by volatilisation at the calcination temperature of 600 *C and can be destroyed subsequently by high temperature atmospheric oxidation. 20 ASLP analyses of the calcined residues from two samples which had been subjected to the present detoxification process gave identical levels of <0.005mg/L for each of the above PAHs. 25 Example 2 50.08g of crushed SPL was stirred at ambient temperature with 500mL of H 2 0 for 1 hour and the pH was adjusted to about 4 using HCl. 50.06g CaCl 2 .2H 2 0 and 0.53g FeCl 3 .6H 2 0 were added and the mixture stirred for 3 hours. No CaCO 3 was added. After filtration, the solid was washed, dried at 110 'C and weighed 33.37g. 9.80g of 30 the dried solid was calcined under flowing N 2 for 3 hours. The calcined solid was of a powdery consistency with a medium level of dustiness and gave the following values P:\Oper\DAH\Provs\40135206 spent pollining doc - 29/10/09 - 24 (expressed as mg/L) for ASLP tests: CN F Ca SO 4 0.026 5.6 900 7.9 Example 3 5 50.23 g SPL was treated as in Example 2, except that 30.09g CaSO 4 .2H 2 0 was added in addition to CaCl 2 .2H 2 0 and FeC1 3 .6H 2 0. Following calcination, the solid obtained was of a powdery consistency with a medium level of dustiness and had a pH of 11.9. The solid provided the following values (expressed as mg/L) for ASLP results: CN F Ca Fe 0.026 240 <0.1 7.9 10 Example 4 100.2g of crushed SPL was stirred with 500mL of H 2 0 at ambient temperature for 1 hour and the pH of the suspension was adjusted from 12 to 5 using HCL. 60.18g 15 limestone, 100.05g CaCl 2 .2H 2 0 and 1.51g FeC1 3 .6H 2 0 were added and the mixture stirred for 3 hours. The resulting solid was filtered, washed twice with 500mL of H 2 0 and dried at 110 *C. After drying the solid weighed 123.76g and had a pH of 9.7. This dried solid was then calcined and the calcined solid provided the following values (expressed as mg/L) for ASLP results: 20 CN F Ca Cl Fe 0.07 110 9.4 8.5 <0.001 Comparative Example 1 A quantity of crushed SPL was treated in an analogous fashion to the method 25 disclosed in AU 2003200307. 500.6g of crushed SPL was stirred at ambient temperature with 2.5L of H 2 0 for 3 hours. The solid was filtered, washed with IL of H 2 0 and dried at 110 *C. The residue weighed 458.66g. The pH of the filtrate from the water washing was adjusted P \Opcr\DAH\Provs\40135206 spent potlining.doc - 29/10/09 - 25 to 4 using HCL. 2.08g of the flocculant MagnaFloc 1011 (as an 0.1% aqueous solution) was added followed by 69.66g of CaCl 2 .2H 2 0. This yielded a precipitate of CaF 2 which after washing and drying weighed 24.67g. To the filtrate, after CaF 2 separation, was added 1.55g FeCl 3 .6H 2 0 which yielded a precipitate of 0.39g of 5 Fe 4 [Fe(CN) 6

]

3 . Separate 50g portions of the water washed SPL residue were treated with the reagents listed below in (a) to (e) and the resulting solids were removed by filtration, washed and dried at 110 'C and portions of each were then calcined under flowing N 2 at 600 10 *C for 3 hours. (a) 50.01g of SPL residue and 30.05g calcium hydroxide Ca(OH) 2 were stirred with H 2 0 and CO 2 was bubbled through the suspension for 3 hours, giving a filtrate with a final pH value of about 7 and a dried residue weighing 60.37g. 15 (b) 50.14g SPL residue was stirred with 30.11 g CaCO 3 in 500mL of H 2 0 for 3 hours giving a fmal residue of 64.52g. (c) 50.01g of SPL residue, 25.05g CaCO 3 and 20.04g CaCl 2 .2H 2 0 were stirred in H 2 0 for 3 hours. The dried residue weighed 62.95g. (d) 49.98g SPL residue, 24.96g CaCO 3 , 20.08g CaCl 2 .2H 2 0 and 1.50g 20 FeCl 3 .6H 2 0 under the same conditions gave a dried residue of 61.92g. (e) 50.23g SPL residue stirred with 30.07g CaSO 4 .2H 2 0 gave a dried residue of 60.37g. The dried residues from (b), (c) and (d) were all aggregated solids with a low degree 25 of dustiness and the calcined residues had pH values from 11.4 to 12.1. The residue from (a) was a solid mass and that from (e) was powdery with a medium degree of dustiness. The residues from (a) (b) (c) and (d) were ASLP tested and gave levels for CN and F 30 well within EPA limits as listed below (concentrations in mg/L) P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 - 26 CN F Ca <0.5 to 0.18 2.3 to 4.0 615 to 2260 Samples (c) and (d) had Cl levels of <0.1 and sample (a) had a level for Fe of 0.1 mg/L. Sample (e) showed relatively high levels of 1925 mg/L for SO 4 but had similar levels to (b), (e) and (d) for CN, F and Ca. 5 It is significant that five samples of crushed SPL, after precipitation and filtration in an analogous manner to the process disclosed in AU 2003200307 to remove water soluble fluoride, and subsequent treatment with various concentrations of FeCl 3 , CaCl 2 , CaSO 4 and CaCO 3 , followed by calcination under N 2 at 600 *C as in the 10 present invention, gave ASLP values for F and CN in the same range as for Examples I to 4. This indicates that the single step aqueous reaction and simple filtration of the present invention is at least as effective in providing a residue for safe land-fill as the multi-step procedure of AU 2003200307. 15 Throughout this specification and claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers. 20 The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. 25 Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within the spirit and scope. The invention also includes all of the steps, 30 features, compositions and compounds referred to or indicated in this specification, P:\Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 -27 individually or collectively, and any and all combinations of any two or more of said steps or features.

Claims (21)

1. A process for the treatment of spent potlining comprising the steps of: (i) treating the spent potlining with water in the presence of a source of 5 calcium ions and a source of iron ions to form a) an aqueous solution and b) a solid residue comprising precipitated fluorides and precipitated cyanides; (ii) separating the solid residue from the aqueous solution; and (iii) calcining the separated solid residue to produce an aggregate without 10 substantial agglomeration.

2. A process according to claim 1, wherein the solid residue formed in step (i) further comprises precipitated arsenic compounds. 15

3. A process according to claim I or 2 wherein the process does not comprise any additional water washing steps.

4. A process according to any one of claims 1 to 3 wherein the spent potlining is crushed or ground prior to or during the water treatment step. 20

5. A process according to any one of claims 1 to 4 wherein the average particle size of the spent potlining is from 0.1 to 5mm.

6. A process according to claim 5 wherein the crushing or grinding is achieved using 25 a jaw crusher or roll crusher.

7. A process according to any one of claims I to 6 wherein the solid residue from step (ii) is subjected to a drying step prior to calcination. 30

8. A process according to any one of claims I to 7 wherein the calcination step is conducted in an oxygen free or inert environment. P:\Oper\DAH\Provs\40135206 spend potlining doc - 29/10/09 - 29

9. A process according to claim 8 wherein the calcination is conducted under nitrogen.

10. A process according to any one of claims I to 9 wherein the source of calcium 5 ions comprises a calcium halide, a calcium nitrate or a mixture thereof.

11. A process according to claim 10, wherein the source of calcium ions comprises CaCl 2 . 10

12. A process according to claim 10 or 11, wherein the source of calcium ions further comprises calcium carbonate.

13. A process according to any one of claims 1 to 12, wherein the source of iron ions is an iron halide, an iron sulphate, an iron nitrate or a mixture thereof. 15

14. A process according to claim 13, wherein the source of iron ions is FeCl 3 .

15. A process according to any one of claims 1 to 14, wherein in step (i) the spent potlining is treated by contacting the spent potlining with water prior to the 20 addition of the source of calcium ions and the source of iron ions.

16. A process according to claim 15, wherein in step (i) the aqueous solution is acidified prior to the addition of the source of calcium ions and the source of iron ions. 25

17. A process according to any one of claims I to 16 wherein the calcination temperature is between 550 and I 000*C.

18. A process according to claim 17 wherein the calcination temperature is from 580 30 to 800 0 C.

19. A process according to any one of claims 1 to 18, wherein the aggregate produced P \Oper\DAH\Provs\40135206 spent potlining doc - 29/10/09 -30 in step (iii) is substantially dust-free.

20. A process according to any one of claims 1 to 19 wherein the calcined residue is disposed of as landfill. 5

21. A process according to any one of claims 1 to 20 wherein the spent potlining is obtained from the electrolytic reduction cells used in aluminium smelting.