AU2003200307B2 - Method for detoxification of spent potlining - Google Patents

Description

P:\OPER JCdloxificaion omplct.doc-3 I/ 1)13 -1- METHOD FOR THE DETOXIFICATION OF SPENT POTLINING This invention relates to the treatment of waste materials generated during the production of aluminium in an aluminium smelter, and in particular to the treatment of spent potlining (hereinafter referred to as The invention especially relates to a treatment process which removes harmful and toxic constituents from the SPL and converts the remaining SPL to a form which can be safely disposed for example as landfill. Such treatment of a hazardous material is generally referred to as detoxification. The invention further relates to such treatment processes which provide for the extraction of harmful and toxic constituents and their subsequent conversion into commercially useful materials.

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 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 and corrosion by the bath of molten cryolite.

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 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.

SPL usually contains 20 to 40% 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. Therefore, SPL is no longer considered to be a carbon based residue containing inorganic impurities, but rather a complex matrix of inorganic compounds containing large quantities of fluorides and having carbon as one component.

PAOPERMJCW11.1ifi-1i.. -PIO.A.-31A)IMl -2- 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 the harmful and toxic constituents. The basis for such strict environmental controls is as follows: SPL contains free and complex cyanides, fluorides and arsenic; (ii) upon exposure to rainwater, free and complex cyanides, fluorides and arsenic will be leached and enter the environment; (iii) the interaction of sunlight and moisture in air with free and complex cyanides, fluorides and arsenic may result in the release of hydrogen cyanide and hydrogen fluoride; (iv) free and complex cyanides, fluorides and arsenic are corrosive and toxic; 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, fluorides and arsenic; (vi) approximately 50kg of SPL is generated per tonne of aluminium metal produced; and (vii) SPL is generated in large quantities of approximately 800,000-1,000,000 tonnes per year throughout the world.

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 P\OPER\MJiCt=Oifiio. .mpk1d..J-31AOIA)3 -3time for the aluminium metal producer until a 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.

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.

High temperature treatment of SPL destroys cyanide by oxidation, but fluoride emission to the atmosphere is considerable. Furthermore this treatment, which usually produces refractory slags and ash, does not allow potentially valuable components of SPL, such as aluminium compounds or fluoride, to be recovered for further use because these compounds are made chemically refractory as a result of the use of high temperatures in the presence of air.

Most of the wet processes have involved leaching by either acidic or basic solutions in an attempt to extract the inorganic values from the carbonaceous matrix. However, these processes have proved to be inadequate in that they have either failed to completely remove the hazardous constituents or have generated products which are not readily disposable, recyclable or marketable. For example, caustic processes extract aluminium from SPL as a water-soluble aluminate and this is then converted to cryolite. The demand for cryolite is minimal as this solid is formed in excess as an unwanted by-product of the aluminium smelting process. Further, existing aqueous leaching methods produce liquid wastes which are expensive to dispose of and cause environmental problems.

The valuable energy content of the carbon in SPL can be exploited by mixing it with coal to use as fuel for power generation. While this is quite simple and completely destroys the cyanide content of the SPL, it does not deal with the fluoride problem at all. Substantial amounts of fluoride are released in the dangerous gaseous form, which is environmentally unacceptable and ultimately may allow it to enter the food chain. The ash will also contain P\OPER\MJCCtdtoxifiction complcte.doc-3 l)3 -4substantial amounts of leachable fluoride.

High temperature rotary kilns are required for effective clinkering in the production of cement. The extremely high temperatures in the kiln make it possible to use almost any fuel for heat generation. Studies have shown that the fluoride in the SPL is beneficial to the rate of clinkering. Fluoride emissions from the kiln were found to be negligible, possibly due to the large amounts of calcium in the kiln reacting with any airborne fluorides to form Fluorite (CaF2). Additionally, cyanide is completely destroyed at these temperatures. The main disadvantage associated with this process is uncertainty. The long-term effects of the SPL minerals in concrete are not known. Also, it may be impractical to transport the SPL from the aluminium production facility to a cement factory if they are not located close to each 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 limestone and anti-agglomerants. A 120,000 ton/year treatment facility is located at Gum Springs, Arkansas, USA. However, this process has not gained world-wide acceptance as at present for unpublished reasons.

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 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 landfillwaste/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 the level required by this process, the final mixture has a very high alkalinity and is probably not suitable for land fill.

P:OPER\MC'oifi-ition WpIctdoc-31OIAJ3 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 give carbon dioxide, which is known to contribute to the green house effect. Further, the Reynolds process does not allow the valuable fluorides to be recovered in a commercially useful form. 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.

Another 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 matter will be bound and trapped by the molten slag. One of the short-comings of this process is that the 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 corrosion problems and environmental concerns.

Attempt is made to recover some of the volatile fluoride in the off-gas, mainly HF, by adsorption onto a bed of alumina. However, the adsorbed fluoride content is not subsequently isolated in a useful form 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. Again, vast quantities of CO 2 would be produced with the burning of SPL 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 transported to the aluminium smelter. Transportation of either the slag or the SPL are 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 P;\OPER1JCdcdtoir-ion inmplktdocsm-3 IMAlM -6temperatures, consuming significant amounts of heat and causing corrosion of equipment.

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 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.

Problems with such processes are that it does not address the issue of limited solubility fluorides such as cryolite and that a non-selective caustic solution is required for leaching.

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 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, complexed cyanide, sodium fluoride and sodium carbonate and subjecting the aqueous solution and solid residue to separate processes. The solution is first treated with an oxidising agent to destroy free cyanide, then treated with a salt to precipitate complexed cyanide and then treated with a calcium salt to produce a calcium fluoride precipitate. The solid 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 S0 4 or H 2 SiF 6 These acid washing steps allow for the recovery of valuable AIF3 by addition of a source of aluminium cations. A disadvantage of this process is that it involves the use of highly corrosive materials that are difficult to handle, such as HF.

Accordingly a requirement exists for an improved process for treating SPL so that it can be safely disposed of with minimal environmental concern.

Accordingly the present invention provides a process for the treatment of spent potlining comprising the steps of: PA\OPEP.V4JCMdd..'ir-Ii Io pkldm-3 I IAI)I 00

O

-7- Ct S(i) treating the spent potlining with water to form a solid residue containing leachable fluorides and an aqueous solution containing water-soluble inorganic matter; 5 (ii) separating the solid residue from the aqueous solution containing the water-soluble 0 inorganic matter; and (iii) calcining the separated solid residue in the presence of a source of calcium ions and

H

2 0 for a time and under conditions sufficient to convert leachable fluorides into CaF 2 without substantial agglomeration of said residue.

While generally unnecessary, for particular applications any of the steps may be preceded or followed by an additional water-washing step. The final residue formed after the calcination step may contain some leachable fluoride components which may be removed by a final water-washing step. However, with appropriate operating conditions, only the initial water treatment is generally required to produce a product suitable for disposal. The unwanted end product may be disposed of in a suitable manner, for example, as landfill, as it no longer contains environmentally harmful inorganic matter and, as such, does not 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 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. A typical composition of SPL is shown in Table 1 below: P:OPERV1J~ddmflmUion coinple~oc-31A/) IA -8-

COMPOUNDS

NaF Na 3 AlF 6

C

SiO 2 CaF 2 Al(OH) 3 A1 2 0 3 CaCO 3 NaA1 1 0O 17 NaAlSiO 4 Al metal Na 4 Fe(CN) 6

KF

Fe 2

O

3 MgO TiO 2 Na 2 S04 Na 2

CO

3 A1 4

C

3

AIN

Na metal

H

2 0 As TABLE 1 Composition of SPL

TYPICAL

14 11 26 5 5 6 4 1 8 5 1 0.1 0.005 2.5 0.5 0.3 1.0 6.5 1 1 <0.1 4 0.0005 RANGE 8-16 7-14 20-40 1-7 3-7 5-10 2-7 0-3 5-10 3-7 0.5-3 0.05-0.4 0.00 1-0.1 1.5-3.5 0.2-1.0 0.1-0.7 0.5-4.5 5-10 0.5-3 0.5-1.5 0.005-0.1 3-6 0.0001-0.001 The components of SPL that render it a hazardous waste are the soluble fluoride salts and the cyanide components. The toxicity of cyanide is well known. Fluoride however is less commonly understood. Soluble fluoride can be absorbed by fish, as well as other animals P:\OPER\MJCtdmxOificlio.n omplc. do-31A) I03 -9and enters the food chain. Ingestion of significant amounts results in weakening of bone structure. When fluoride is exposed to acid, highly corrosive HF may be produced.

Cyanide, while only present in small quantities, is highly toxic. Compared with fluoride however, it is relatively easy to remove from the waste. Most of the cyanide present can be removed in a simple water-wash. The remaining cyanide decomposes when raised to a temperature above about 200'C. Thus it is quite easy to ensure the waste is made cyanide free.

The fluoride products however are less easy to deal with, due to their 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. A simple water-wash will safely remove most NaF.

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 0 C. Although this may seem relatively insignificant, over time soluble fluorides will leach into watercourses causing an environmental problem.

The process of the present invention advantageously results in elimination of cyanide, removal of soluble fluorides and the conversion of this fluoride to saleable CaF 2 and the conversion of leachable fluorides, such as cryolite, into unleachable form, CaF 2 The treatment results in an environmentally benign residue of graphite, refractory material and unleachable salts.

As used herein, the term "water-soluble inorganic matter" refers to soluble inorganic material which can be removed from SPL by washing with water or an aqueous treatment solution. Such water-soluble inorganic matter includes soluble fluoride salts, such as NaF, Na 2

CO

3 and soluble cyanide components.

PAOPERU.4Claozoxifilion ouweo.doj-3 1A) 1/03 The term "leachable fluorides" as used herein refers to fluoride salts, minerals or complexes, from which fluoride can be leached from SPL into the environment, and into watercourses, causing environmental problems. An example of such a leachable fluoride material is cryolite.

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 5mm, more preferably 0.5 to 2mm. This grinding/crushing may be achieved using any conventional equipment used to crush minerals, such as a jaw crusher or a roll crusher 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.

Laboratory studies show that the water treatment step is an important step towards the detoxification of the SPL and the formation of a residue suitable for landfill. The raw SPL is highly alkaline in nature due to its chemical composition. The water wash step significantly reduces the alkalinity of the SPL, while minimising the level of soluble fluorides, sodium fluoride, in the sample prior to calcination. In the water wash step, practically all of the water soluble fluoride is removed. This fluoride dissolved in this water wash generally represents about 50% of the total fluoride in the SPL in laboratory tests.

According to the Reynolds process referred to above, when a substantial quantity of Ca(OH) 2 is added to the raw SPL which has not been water washed, the end product is very alkaline and is not suitable for landfill, although some of the fluoride has been successfully fixed/inertized in the calcination process.

In the present process, the quantity of Ca(OH) 2 used is generally only 1/a to of that required in the Reynolds process. Therefore the alkalinity issue is minimized by applying a wash and a significantly reduced level of Ca(OH) 2 P:\OPER JCdtoxifiction comnplctc.doc-.3 I/11A)3 -11- It is important to note that an acidic or alkaline aqueous leaching solution is not preferred because such a leaching solution is not selective in the species being extracted or dissolved. A neutral leaching agent, such as water, facilitates a highly selective extraction step because only NaF, Na 2

CO

3 and ferrocyanides are substantially dissolved. When the carbonate has been removed by neutralisation with an acid and the cyanide has been isolated by precipitation, the solution contains mainly dissolved sodium and fluoride making the recovery of sufficiently pure fluoride compounds for commercial use a very simple operation.

When an acidic or alkaline solution is used, a complex cocktail of dissolved fluorides, cyanides, metal elements, oxides, silicates, etc. will be produced making the recovery of useful chemical compounds very difficult. This waste liquid is also difficult and expensive to dispose.

The water treatment step can be carried out by contacting the (generally crushed or ground) SPL with water or other aqueous treatment solution in any suitable equipment, preferably with stirring, mixing or other means of agitation to ensure good contact between the water and the particles of SPL. The aqueous treatment solution is preferably water, and may be a low grade water with low levels of salinity, or similar liquid. Preferably the aqueous treatment solution is substantially neutral, with a pH in the range of about 6 to 7.

In one embodiment the aqueous treatment solution is the aqueous solution obtained following subsequent treatment of the separated aqueous treatment solution to remove dissolved fluoride and cyanide. These further treatment steps will be described in more detail below.

The amount of water used will depend on a number of factors, including the chemical 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 solution produced which needs to be treated to remove soluble fluoride and cyanide. Generally the PAOPERM1ACddoxiiioll cornplc.doc311IA)3 -12amount of water used will be between 3, 4 and 10 times the weight of SPL being treated, more preferably between 5 and 7 times. This water-washing step dissolves the water soluble sodium salts, such as sodium fluoride and sodium carbonate without dissolving a significant amount of cryolite. When using a pH neutral solution the treatment is very highly selective in leaching those components from the SPL. Accordingly, the resulting aqueous solution contains dissolved materials, such as fluoride, in a form suitable for conversion into useful products, such as, for example, calcium fluoride.

Following treatment with water, the 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. Preferably the washed residue will be allowed to dry, or be dried using direct heat rotary tumble drier with mixing-ribbon spirals or lifting baffles to remove excess water. It is important to remove most of the water from the residue as the presence of too much water during the subsequent calcination step can lead to unwanted agglomeration.

The SPL 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 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. With careful control of the amount of 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 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.

P:\OPER\MJOdeloxilicaloncoinplctc.doc-31illnO1 -13- The source of calcium ions and water in the calcination reactor may be a hydrated calcium oxide, or a hydrated calcium salt. Examples of suitable materials include Ca(OH) 2 (or lime) or CaSO 4

.H

2 0 or CaSO 4 1 /2H 2 0. Alternatively the calcium ion source can be calcium oxide or a calcium salt with a separate H 2 0 source, such as CaO or CaC12. In one embodiment the H 2 0 is (or includes) the water left in the SPL after the initial water treatment. In another embodiment the water or moisture is added externally, before or during calcination. Preferably the source of calcium ions and H 2 0 is Ca(OH) 2 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 ensure intimate mixing of the SPL and the calcium material.

The calcination conditions will depend on a number of factors, including the source of the SPL, the amount of leachable fluoride material in the SPL, the type of reactor, and the nature of the calcium containing material. Preferably the amount of calcium material combined with the SPL is sufficient to convert substantially all reactive/leachable fluorides present in the SPL into CaF 2 When Ca(OH) 2 is used as the source of calcium and water it is preferably combined with the SPL in a Ca(OH) 2 :SPL weight ratio of from 20:80 to 60:40, more preferably from 25:75 to 50:50. While it may be possible to obtain complete conversion of the reactive fluorides into CaF 2 using larger amounts of Ca(OH) 2 the addition of excess Ca(OH) 2 will merely add to the weight of SPL residue for disposal and increase its alkalinity. Accordingly it is advantageous to minimise the amount of calcium source added to the SPL.

The calcination temperature is preferably between 550 and 1000 0 C, more preferably 580 to 800 0 C and most preferably about 600 0 C. Higher temperatures can lead to increased sintering and consequential agglomeration of the SPL residue.

The calcination time will depend on the ratio of calcium material to SPL, the temperature of calcination and the nature of the reactor. Generally the calcination time will be between P:OPER\MJC\dctoxificaion completc.doc-31)I/)3 -14- 0.25 to 5 hours, more generally between 0.5 and 3 hours, and most preferably about 2 hours.

The solid residue from the water-washing step contains mainly carbon and sparingly soluble fluoride compounds, such as AlF 3 Na 3 AlF 6 and NaAlF 4 Without wishing to be limited by theory, it is believed that the calcination step involves a combination of chemical reactions, or a series of known reactions which together provide a novel effect.

When Ca(OH) 2 is heated to temperatures above 580°C, the combined water is released.

The dissociation of water from CaO may be written as: Ca(OH) 2 CaO The water produced from the dissociation reaction can readily react with the fluoride containing compounds in pyrohydrolysis reactions: 2 AlF 3 3 H 2 0 A1 2 0 3 6 HF 2 Na 3 AlF 6 6 H20 3 Na 2 0 A1 2 0 3 12 HF Na 2 0 would be expected to react with A1 2 0 3 to form refractory sodium aluminates. Since the HF is formed in the solid bed and is in close proximity to CaO produced, CaF 2 is formed according to the following reaction: 2 HF CaO CaF 2

H

2 0 It is to be noted that H20 is regenerated in the fluoridation of CaO and is available to pyrohydrolyse other fluoride containing compounds. It is important to realise that the HF produced in the pyrohydrolysis reactions reacts readily with CaO present in the immediate neighbourhood. This is not unexpected since the reactions involving AlF 3 and Na 3 AlF 6 are relatively fast reactions at the elevated temperatures used in the calcination step. At this reaction temperature, the reactions may be considered to be virtually instantaneous compared to the mass transport rate. The overall novel effect in the calcinator is the P:\OPER\MJC'dcloxifision complctc.do-3 ]A 1A)3 breaking down of complex fluoride-containing compounds and the regeneration of the water and the stabilisation of the fluoride compounds to produce CaF 2 XRD analysis of SPL samples before and after the calcination step show that the concentration of the cryolite in the sample is indeed significantly reduced, and the concentration of CaF 2 in the sample is significantly increased.

Since water is recycled, only a very small amount of water is needed. Given sufficient calcination time, substantially complete stabilisation of the fluoro-compounds is achieved.

Introduction of excess steam generating a saturated environment may not be desirable as condensable moisture can cause agglomeration of the particles. It can be shown that in a well designed reactor, the quantity of Ca(OH) 2 needed in an optimized process to produce the required stabilisation effect will be significantly less than the equivalent amount used in the Reynolds Process.

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.

It is believed that the mechanisms which operate when other sources of calcium and water are used will be analogous to those described above.

While the calcination step should be capable of converting substantially all leachable fluorides into CaF 2 without substantial agglomeration, it is possible to subject the calcined product to a further aqueous washing step prior to disposal if necessary. Fluoride ions present in the washed solution can be converted to CaF 2 by the addition of a suitable calcium source, such as a calcium salt to produce a calcium fluoride precipitate.

The leachate from the water treatment step can be treated as described in U.S. Patent No.

5,939,035. This solution will contain sodium fluoride, sodium carbonate, cyanide, as well P:\OPER\MJC\dtoifiClion omplto.doe-3 I )IA)3 -16as other species. Cyanide is present as both free cyanide and complex cyanide and must be removed prior to the recovery of wanted products from the solution. The free cyanide can be destroyed, in situ, by the addition of a suitable oxidising agent, such as, for example, hydrogen peroxide or sodium hypochlorite.

The complex cyanide is present as the soluble ferrocyanide anion and, unlike free cyanide, it is resistant to oxidation in solution. However, after neutralisation of a solution with a mineral acid, such as HCl or H 2 S0 4 the complex cyanide can be selectively precipitated from the solution by the addition to the solution of a salt containing a suitable counter 3+ 2+ cation, such as, for example Fe 3 or Zn Preferably, zinc sulphate, zinc acetate or ferric sulfate is added to the aqueous solution to give a precipitate of zinc or iron ferrocyanide, Zn 2 Fe(CN) 6 or Fe4[Fe(CN) 6 3 which is then separated from the solution by any suitable technique, such as, for example, filtration.

The resulting solution, which has been neutralised with a mineral acid prior to cyanide precipitation, now contains mainly sodium fluoride. This solution can be further processed to recover calcium fluoride by precipitation on the addition of a suitable calcium salt. It is preferred that if the neutralising acid is HCI, then the calcium salt is calcium chloride, such that the aqueous liquid after precipitation of fluoride contains dissolved sodium chloride which is benign. Alternatively, if the neutralising acid is H 2 S0 4 then the calcium salt is either calcium sulphate or calcium sulphate dihydrate.

Preferably, sulphuric acid is used with calcium sulphate dihydrate as this gives a precipitate of calcium fluoride and a solution of sodium sulphate. The calcium fluoride can be separated from the solution and used as a raw material for HF production, and the aqueous sodium sulphate is a saleable commodity. Alternatively, the solution can be recycled to treat more fresh SPL. It has been shown in batchwise laboratory experiments that this solution can be recycled twice before becoming saturated with dissolved sodium sulphate.

P:COPER\MJWdctoxificntion complcc.doc-3 IMAI13 -17- Since Na 2

SO

4 and NaCI have very high solubilities in water, their presence at low level may not significantly affect the solubility of ferrocyanide and fluoride in this solution.

Ultimately, some of this process liquor will need to be purged. The disposal of NaCI solution would not pose significant environmental concerns. If the spent liquor is a Na 2

SO

4 solution, it may be reacted with CaCl 2 to give CaSO 4 crystals and a disposable liquid, a solution of NaC1.

The aqueous leaching of SPL and the precipitation of cyanide and fluoride are preferably carried out at ambient temperatures. It has been shown at laboratory and demonstration plant scales that the precipitation steps are fast reactions. High levels of fluoride and cyanide extraction and recovery can be achieved, and the procedures can be optimised to obtain the maximum level of extraction and recovery.

According to a preferred process according to the invention, the aqueous solution containing water-soluble inorganic matter is subjected to the following steps: treatment with an oxidising agent to destroy free cyanide; (ii) neutralising the solution with a mineral acid; (iii) treating the neutralised solution with a salt containing a counterion to precipitate complexed cyanide; (iv) separating the precipitated complexed cyanide from the treated neutralised solution to provide a substantially cyanide-free solution; and treating the substantially cyanide-free solution with a calcium salt to precipitate dissolved fluoride ions as calcium fluoride.

It should be noted that steps and (ii) can be performed in any order, although preferably step precedes step It is also possible to destroy the cyanide in the precipitated P:\OPER\MJOddtoxiicalion omniplct.doc-3 A 1/3 18complexed cyanide by treatment with an oxidising agent, such as sodium hypochlorite, and thereby recycle the counterion.

It is evident that the process according to the present invention possesses several advantages, both commercially and environmentally. Some of these advantages are as follows: 1. A significant level of the cyanide present in the SPL is easily isolated and destroyed.

2. A significant level of the fluoride in the raw SPL is extracted in the leaching step and can be subsequently recovered as good quality CaF 2 3. Calcination of SPL is performed at moderate temperatures, avoiding problems commonly seen in other high temperature processes, such as agglomeration of the particles and corrosion of the reactor.

4. Use of inert atmosphere 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 is low compared to other proposed processes.

6. The level of leachable fluoride in the final residue is very low such that the final solid is suitable for landfill.

7. Compared with the Reynolds process, a lower operating cost is expected due to the low temperature of calcination and the significantly lower demand for Ca(OH) 2 8. Unlike the Ausmelt Process, the level of HF discharged from the process in the gas phase according to the invention is expected to be negligible.

P:\OPER\MIJCdcloxificaion complc.doc-3 111 1/A)3 -19- 9. The aqueous liquid leaving the system is benign, containing mainly NaCl. Since CaF 2 is sparingly soluble in water, the level of fluoride in the spent liquor is very low.

The aqueous liquid can be recycled a few times before becoming saturated with sodium salt.

The invention will now be described with reference to a number of examples and drawings which illustrate some preferred aspects of the invention. However it is to be understood that the particularity of the following description of the invention is not to supersede the generality of the preceding description of the invention.

Referring to the drawings: Figure 1 is a schematic diagram of a tubular stationary reactor for calcining water-washed SPL according to the invention.

0 Figure 2 is a schematic diagram of a stainless steel packed bed reactor for calcining waterwashed SPL according to the invention.

Figure 3 is a schematic diagram of a stainless steel tumbling reactor for calcining waterwashed SPL according to the invention.

Figure 4 is a schematic diagram of a glass rotary reactor for calcining water-washed SPL according to the invention.

P:\OPERMJC\dctoxification complcte.doc-31/01/O 3

EXAMPLES

Example 1 The SPL sample and Pretreatment Raw SPL was obtained from three aluminium smelters. The SPL consisted of a mixture of carbon lining, inorganic chemical components and refractory brick. The SPL was removed from the pot using a wet-delining technique in some cases and a dry-delining technique in the others. The SPL was first crushed to give particles with a fixed maximum diameter.

Analysis of the various types of SPL using ICP Atomic Emission Spectroscopy gave the following elemental concentrations as Shown in Tables 2, 3 and 4.

Table 2 Composition of Raw SPL from Tomago Aluminium Company Pty Limited, NSW (Australia) Size of sample delivered 200 kg Particle diameter after crushing <1 mm Wet-delined Concentration of fluoride in raw SPL 11.9 W/W% Concentration of cyanide in raw SPL 2.46 g/kg or 0.246W/W% Elements W/W Sample Na Al Ca Si Fe K Mg Ti 1 14.9 10.3 2.8 5.8 0.77 0.4 0.12 0.13 2 16.2 11.0 3.1 6.7 0.70 0.4 0.12 0.15 P: OPER\MOJdcloxication conplcc. doc-31/01A)3 -21 Table 3 Composition of Raw SPL from Capral Aluminium Limited, NSW (Australia) Size of sample delivered 6.4 kg Particle diameter after crushing <2 mm Wet-delined Concentration of fluoride in raw SPL 9.66 W/W% Concentration of cyanide in raw SPL 0.05 W/W% Elements W/W Sample Na Al Ca Si Fe K Mg Ti 1 10.7 16.8 1.54 3.6 4.8 0.12 0.17 0.12 2 11.2 14.8 1.53 3.4 4.4 0.13 0.15 0.10 Table 4 Composition of Raw SPL from Columbia Aluminium Corporation (USA) Size of sample delivered -10 kg Particle diameter after crushing <1.2 mm Dry-delined Concentration of fluoride in raw SPL 16.6 W/W% Concentration of cyanide in raw SPL 0.0083 W/W% Elements W/W Sample Na Al Ca Si Fe K Mg Ti 1 14.7 11.2 2.37 4.3 0.80 0.3 0.12 0.15 2 13.2 10.0 2.09 3.4 0.57 0.2 0.11 0.11 The analysis show that the SPL is not a homogeneous material and elemental concentrations can vary significantly within the same batch.

P:\OPERMJCdetoxiflion wompct dc.31A)IA)3 -22- Example 2 Initial Water-wash 1 part in mass of SPL was agitated in 5 part in volume of water 5 ml/gm of SPL) at room temperature for three hours. The solid carbonaceous residue was separated from the liquid phase by filtration, dried in air at 40°C, and weighed. The volume of the filtrate was also measured. The conditions applied in the water-wash and the change in mass of the solid phase are given in Table Evaporation of a portion of the liquid phase produced a solid crystalline residue, which was further dried at 110 0 C to remove free moisture. The mass of this residue was measured. The adjusted mass of the crystalline residue based on the-entire volume of the solution was determined. The elemental composition of the crystalline material was determined and is shown in Table 6. XRD analysis showed that the crystalline contains mainly NaF and Na 2

CO

3 and the mass ratio of NaF:Na 2

CO

3 is 3:1.

Table 5 Experimental Conditions and Results of Water-washing of SPL Sample Source of Vol. of Initial Final dried of Mass of Normalized No. SPL Water SPL SPL mass Initial Crystalline Mass of mass SPL Mass Residue Crystalline Extracted Residue (gm/kg) 1 Tomago 5 Litres 1 kg 790.15 gm 20.88 172.82 gm 172.82 2 Tomago 500 96.6 75.01 kg 22.35 13.89 kg 147.99 Litres kg 3 Capral 250 ml 50 gm 46.36 gm 7.28 4 Capral 250 ml 50 gm 45.91 gm 8.38 Columbia 250 ml 50 gm 45.24 gm 9.52 4.84 gm 96.80 6 Columbia 250 ml 50 gm 45.19 gm 9.62 4.78 gm 95.60 P:\OPER\MJCdctoxii-alio omplctc.doc-3 IA0ll) -23- Table 6 Composition of Crystalline Residue from Drying of Filtrate Elements W/W Sample No. Source of SPL Na Si Fe K 1 Tomago 51 0.57 0.47 0.3 3 Tomago 51 0.74 0.4 0.3 4 Capral Capral 6 Columbia 48 0.38 0.65 <0.1 7 Columbia 48 0.43 0.50 <0.1 Example 3 Calcination of Water-washed Residue in Ca(OH) 2 using a Tubular Static Reactor Capral SPL was used for this series of experiments.

A sample of Ca(OH) 2 was dried overnight in an oven at 110 0 C. To a wide mouth polypropylene bottle with a diameter of -90 mm and a capacity of 1.5 litres, fixed and carefully weighed masses of dried Ca(OH) 2 and dried water-washed SPL totalling -600 g were added. The bottle was sealed and placed on a rotary tumble mixer and the content was mixed for 24 hours. A sample of this SPL/Ca(OH) 2 mixture was taken and calcined in a nitrogen environment at a constant temperature for a fixed period of time in a quartz tubular static reactor. A schematic diagram of the reaction system is shown in Fig. 1.

The reactor is made up of a horizontal tubular sample chamber la and gas inlet and outlet pipes. The gas inlet tube is connected to a nitrogen supply unit 2 via a rotameter 3. The gas outlet tube is connected to a gas scrubber 4 containing a sodium hydroxide solution.

The reactor la is heated externally by an electrical furnace 5 which is fitted with an automatic temperature monitoring and controlling system 6. In a typical run, a 10 g sample of the SPL/Ca(OH) 2 mixture 7 was taken and placed in the quartz reactor.

Nitrogen was passed into the reactor 20 minutes prior to the start of the calcination reaction P:\OPER\MJO loxiicalion conplctc.doc-31I/ln3 -24and throughout the reaction period. The sample was heated to the required calcination temperature for a pre-set period of reaction time. After the reaction time had elapsed, the sample was allowed to return to room temperature before the flow of nitrogen was stopped.

The residue was recovered from the reactor after the run. A 5 g sample of the calcined SPL was leached in a solution containing 34 ml of glacial acetic acid and 66 ml of distilled water with stirring for 2 hours at room temperature. The undissolved solid was separated from the liquid by filtration. The concentration of the fluoride in the acetic acid solution was determined using a fluoride selective electrode. The acetic acid leaching was performed as a simulation of the Standard Toxic Characteristic Leaching Procedure (TCLP) test which requires a similar leaching procedure except operated at a larger scale.

The reaction conditions and the results of this series of experiment are shown in Table 7.

Table 7 Temperature SPL:Ca(OH) 2 Initial Final mass Concentration of Concentration of mass Fluoride in Fluoride in Acetic Scrubber Solution Acid Filtrate (mg/1) (mg/1) 300 50:50 10 8.75 <1 6.31 400 25:75 10 8.18 <1 6.85 400 50:50 10 8.63 <1 6.32 400 75:25 10 9.18 <1 9.49 500 25:75 10 8.05 <1 6.81 500 50:50 10 8.62 <1 5.42 500 75:25 10 9.10 <1 17.35 600 50:50 10 8.45 <1 6.50 800 50:50 10 8.32 <1 10.20 900 50:50 10 7.48 <1 9.53 P:\OPERlCXl etoxfiion complr.doc-3 I/01/03 25 Example 4 Calcination of Water-washed Residue in Ca(OH) 2 Using a Packed Bed Reactor Columbia SPL was used for this series of experiments.

A sample of Ca(OH) 2 was dried overnight in an oven at 110 0 C. To a wide mouth polypropylene bottle with a diameter of -90 mm and a capacity of 1.5 litres, fixed and carefully weighed masses of dried Ca(OH) 2 and dried water-washed SPL were added. The bottle was sealed and placed on a rotary tumble mixer and the content was allowed to be mixed for 24 hours. A sample of this SPL/Ca(OH) 2 mixture was taken and calcined in a nitrogen environment at a constant temperature for a fixed period of time in a stainless steel packed bed reactor. A schematic diagram of the reaction system is shown in Fig. 2.

The reactor lb is made up of a vertical packed column fitted with gas inlet and outlet pipes. Nitrogen gas 2 at a metered flowrate enters the reactor from the bottom of the reactor and exits from the top of the reactor. The SPL is placed in the packed bed and is supported by a gas distributor 8 in form of a stainless steel fritted disk. The reactor lb is heated externally by an electrical furnace 5which is fitted with an automatic temperature monitoring and controlling system 6. In a typical run, the flow of nitrogen was turned on, and the reactor containing no reagent was heated to the required temperature. When the reaction temperature had been reached, a 50 g sample of the SPL/Ca(OH) 2 mixture was added to the reactor through the top. The reaction was allowed to continue for a fixed period of time. After the reaction time had elapsed, the sample was allowed to return to room temperature before the flow of nitrogen was stopped.

The residue was recovered from the reactor at the end of a run. A 5 g sample of the calcined SPL was leached in a solution containing 34 ml of glacial acetic acid and 66 ml of distilled water with stirring for 2 hours at room temperature. The undissolved solid was separated from the liquid by filtration. The concentration of the fluoride in the acetic acid was determined using a fluoride selective electrode. The acetic acid leaching was performed as a simulation of the TCLP test which requires a similar leaching procedure P:\OPER\MJCMdeoxification comple]tdoc-31/01/3 -26except operated at a larger scale. The results of this series of experiment are shown in Tables 8 and 10. The solid sample recovered at the end of the calcination reaction was tested for the presence of minerals by XRD. The results are shown in Table 9 and 11.

Table 8 Results for calcination of samples at Various temperatures followed by leaching tests Temperature Reaction SPL:Ca(OH) 2 Initial mass Final mass Concentration of Time (hrs) Fluoride in Acetic Acid Filtrate (mg/1) 400 2 50:50 50 46.5 6.06 500 2 50:50 50 44.7 7.93 600 2 50:50 50 43.1 6.40 800 2 50:50 50 42.5 6.83 1000 2 50:50 50 39.4 3.18 Table 9 Summary of XRD analysis for samples calcined at various Temperatures Reaction Temperature NaF Cryolite CaF 2

(C)

No Peak Strong Peak Weak Peak 400 No Peak Weak Peak Weak Peak 600 No Peak No Peak Strong Peak 1000 No Peak No Peak Strong Peak P:\OPER\MJdcloxificaio complete doc-lI/l01ll -27- Table 10 Results for calcination of samples with various SPL:Ca(OH) 2 Ratio followed by leaching tests Temperature Reaction SPL:Ca(OH) 2 Initial Mass Final mass Concentration of Time (hrs) Fluoride in Acetic Acid Filtrate (mg/l) 600 2 50:50 50 43.1 6.4 600 2 75:25 50 44.3 16.6 600 2 90:10 50 45.8 298 600 2 95:5 50 44.3 492 600 2 99:1 50 45.4 279 Table 11 Summary of XRD analysis for samples with various SPL:Ca(OH) 2 Ratio SPL:Ca(OH) 2 NaF Cryolite CaF 2 50:50 No Peak No Peak Strong Peak 75:25 No Peak No Peak Strong Peak 90:10 Medium Peak Weak Peak Strong Peak 95:5 No Peak Medium Peak Strong Peak 99:1 Weak Peak Medium Peak Medium Peak Example Calcination of Water-washed Residue in Ca(OH) 2 Using a Rotary Reactor Columbia SPL was used for this series of experiments.

A sample of Ca(OH) 2 was dried overnight in an oven at 110 0 C. To a wide mouth polypropylene bottle with a diameter of -90 mm and a capacity of 1.5 litres, fixed and carefully weighed masses of dried Ca(OH) 2 and dried water-washed SPL were added. The PAOPERvIJOdctoxificaoion opletdc.-3IA)l13 -28bottle was sealed and placed on a rotary tumble mixer and the contents were mixed for 24 hours. A sample of this SPL/Ca(OH) 2 mixture was taken and calcined in a nitrogen environment at a constant temperature for a fixed period of time in a stainless steel rotary reactor. A schematic diagram of the reaction system is shown in Fig. 3.

The rotary reactor Ic has a horizontal sample chamber with the dimension of diameter X 100 mm long. The rotary action of the reactor is driven by a variable speed motor and the rotation rate is set at about 30 rpm. The reactor is made of stainless and is fitted with gas inlet and outlet pipes. The gas inlet tube is connected to a nitrogen supply unit 2 via a rotameter 3. The gas outlet tube is connected to a gas scrubber 4 containing a sodium hydroxide solution. The reactor is heated externally by an electrical furnace which is fitted with an automatic temperature monitoring and controlling system 6. In a typical run, a 30 g sample of the SPL/Ca(OH) 2 mixture 7 was taken and placed in the rotary reactor. The motor was turned on and set to the required rotating speed. Nitrogen was passed into the reactor 20 minutes prior to the start of the calcination reaction and throughout the reaction period. At the start of a run, the sample was heated to the required temperature for a pre-set period of reaction time. After the reaction time has elapsed, the furnace was turned off and the sample was allowed to return to room temperature before the flow of nitrogen was stopped and the motor turned off.

The residue was recovered from the reactor at the end of a run. A 5 g sample of the calcined SPL was leached in a solution containing 34 ml of glacial acetic acid and 66 ml of distilled water with stirring for 2 hours at room temperature. The undissolved solid was separated from the liquid by filtration. The concentration of the fluoride in the acetic acid was determined using a fluoride selective electrode. The acetic acid leaching was performed as a simulation of the TCLP test which requires a similar leaching procedure except operated at a larger scale. The results of this series of experiment are shown in Tables 12, 13 and 14.

P:\OPER\MJCI ctoxificnlion conplec.doc-31)1/)3 -29- Table 12 Results for Calcination of Samples* for Various Reaction Times, SPL:Ca(OH) 2 75:25 Temperature Reaction SPL:Ca(OH) 2 Initial mass Final mass Concentration Time (hrs) of Fluoride in Acetic Acid Filtrate (mg/l) 600 0.5 75:25 30 28.46 13.00 600 1 75:25 30 22.13 2.30 600 2 75:25 30 17.53 2.12 600 3 75:25 30 27.17 2.57 Columbia SPL was used for these experiments, particle diameter <1.2 mm.

Table 13 Results for Calcination of Samples* for Various Reaction Times, SPL:Ca(OH) 2 60:40 Temperature Reaction SPL:Ca(OH) 2 Initial Mass Final mass Concentration of Time Fluoride in Acetic (hrs) Acid Filtrate (mg/l) 600 0.5 60:40 30 25.55 11.45 600 1 60:40 30 24.74 8.65 600 2 60:40 30 27.18 8.65 600 3 60:40 30 25.22 11.70 Columbia SPL was used for these experiments, particle diameter 1.2 mm.

P:OPER\MJOdcoxifcation completcdo-3/01dA3 Table 14 Results for Calcination of Samples* for Various Reaction Times, SPL:Ca(OH) 2 50:50 Temperature Reaction SPL:Ca(OH) 2 Particle Size Initial Final Concentration of Time Mass mass Fluoride in (hrs) Acetic Acid Filtrate (mg/1) 600 0.5 50:50 500tm 30 25.55 8.91 600 1 50:50 500 212m 30 24.74 8.85 600 2 50:50 212- 106m 30 27.18 2.42 600 3 50:50 <106 pm 30 25.22 7.55 Tomago SPL was used for these experiments.

Example 6 Precipitation and Recovery of Cyanide and Fluoride from Water-Wash Liquid Phase The precipitation of cyanide and calcium fluoride was performed in a demonstration plant trial. A sample of 96.59 kg of SPL was agitated and washed in 500 litres of water in a 1200 litres stirred tank reactor for 3 hours at room temperature. When leaching time had elapsed, the stirrer was turned off and the mixture was allowed to settle for 20 hours. The mixture in the reactor separated to give a clear solution on the top and a dark grey slurry containing fine as well as coarse particles in the bottom. The clear solution was first decanted and a sample of -480 litres of clear solution was obtained. The slurry at the bottom of the reactor was separately collected and stored.

Analysis of the untreated SPL showed that it contained 119 g of fluoride and 2.46 g of cyanide per kilogram of raw SPL. Analysis of the clear solution from the water-wash of SPL showed that this solution contained 462 mg/L of cyanide and 11.82 g/L of fluoride.

Simple mass balance calculations showed that 93.3% of the cyanide and 49.6% of the fluoride in the raw SPL has been extracted and transferred to the liquid phase.

P:\OPER\MJCd coxifiO tioncompte.doc-3INtIA)3 -31 The clear liquid was found to be basic with a pH of 12 to 13. This liquid contained mainly Na CO 3 2 and a very small amount of cyanide. The clear liquid was placed in stirred tank reactor and was neutralized with 3.15 litres of concentrated HC1 with mixing to give a final pH of A sample of 600 gm of zinc acetate was added to the neutralized solution with mixing to produce a white precipitate of Zn 2 Fe(CN) 6 Laboratory experience showed that this precipitate was crystalline in nature and could be isolated easily by filtration. In the present plant trial, the precipitate of Zn 2 Fe(CN) 6 was left in the reactor and was removed from the solution only after the precipitation of the fluoride has been completed.

Without first separating the complex cyanide precipitate, 17 kg of CaCl 2 and 8 g of Superfloc N300 (polyacrylamide based flocculant) were added to the mixture and stirred in the tank reactor for 1 hour. A white slurry was produced which upon standing separated to give a second clear solution at the top and a white slurry in the bottom. The second clear liquid was separated by decantation and the remaining slurry was dried at room temperature to give a dried white solid.

After drying the slurry, it was found that 10.34 kg of the white solid had been produced from 96.59 kg of raw SPL. Chemical analysis of this solid showed that it contained 76.1% CaF 2 5.9% NaC1, and 3.1% Zn 2 Fe(CN) 6 The balance is mainly hydrated water. Hence, 7.89 kg of CaF 2 had been produced from 96.59 kg of raw SPL. This result indicated that 8.2 kg of CaF 2 (equivalent to 4 kg of fluoride) could be recovered from each 100 kg of raw SPL as saleable product.

Analysis of the second clear liquid showed that it contained 2.65 ppm of cyanide. This result show that, of the cyanide extracted into the solution in the initial water-wash, 99.42% had been removed from the solution as Zn 2 Fe(CN) 6 Although not attempted, the remaining soluble cyanide, most likely present as free cyanide, could have been destroyed by the addition of sodium hypochlorite to the solution. Further analysis showed that this second clear solution contained mainly dissolved sodium chloride.

P:\OPER IJCdtoxification complt.doc-3 IiO )3 -32- Example 7 Toxicity Characterization Leaching Properties Tests (TCLP) of Calcined SPL Residue A sample of 1 kg of raw Capral SPL was agitated in 5 litres of water at room temperature for 3 hours. The undissolved solid was separated from the liquid phase by filtration and was dried in air at 40 0 C. A sample of Ca(OH) 2 was dried overnight in an oven at 110 0

C.

200 g of dried water-washed SPL and 200 g of dried Ca(OH) 2 were added to a wide mouth bottle, which was sealed and the content was allowed to be mixed for 24 hours using a rotary tumble mixer. A sample of 50 g of this mixture was placed in the glass rotary reactor and was calcined for 2 hours at the required temperature. The residue recovered at the end of the calcination step was used in a TCLP Test based on the US Standard. In performing this test, 110 g of the solid sample was needed. Due to the size of the rotary reactor, it was necessary to produce the required -110 g in three runs.

A schematic diagram of the reaction system is shown in Fig. 4. The rotary reactor ld has a horizontal sample chamber with dimensions of 50mm diameter and 100 mm length. The rotary action of the reactor is provided by a variable speed motor 9 and the rotation rate was set at about 30 rpm. The reactor Id was made of Pyrex glass and was fitted with gas inlet and outlet pipes. The gas inlet tube was connected to a nitrogen supply unit 2 via a rotameter 3. The gas outlet tube was connected to a gas scrubber 4 containing a sodium hydroxide solution. The reactor was heated externally by an electrical furnace 5 which was fitted with an automatic temperature monitoring and controlling system 6. In a typical run, a 50 g sample of the SPL/Ca(OH) 2 mixture 7 was taken and placed in the rotary reactor. The motor was turned on and set to the required rotating speed. Nitrogen was passed into the reactor 20 minutes prior to the start of the calcination reaction and throughout the reaction period. At the start of a run, the sample was heated to the required temperature for a pre-set period of reaction time. After the reaction time had elapsed, the sample was allowed to return to room temperature before the flow of nitrogen was stopped and the motor was turned off. The reaction conditions applied in the tests are given in Table P:OPER\MlOdctoxfilction completedoc-3A1/IA)3 -33- Table 15 Reaction Condition Used for Calcination of Samples Temperature Reaction Time (hrs) SPL:Ca(OH) 2 Initial Mass (g)

W%)

400 2 50:50 500 2 50:50 Capral SPL was used for these experiments.

The residue was recovered from the reactor at the end of a run. Leaching of the residue was next carried out using the standard procedure prescribed by the US Standard for the TCPL Tests. Initially, 5 g of the solid material was added to 96.5 ml of distilled water and stirred for 5 minutes with a magnetic stirrer and the pH recorded. As the pH was less than the solid was leached using Extraction Fluid No. 1 which was made up as follows: 5.7 ml glacial acetic acid, 500 ml distilled water, 64.3 ml of 1 M NaOH, diluted to 1 litre.

The pH of the extraction fluid should be in the range of 4.93±0.05 (measured at 4.89). 100 g sample of the solid was then added to 2000 g of Extraction Fluid No. 1 and sealed in a 3 litre plastic screw cap jar. The container was placed on a tumble mixer and the content was mixed for 18 hours. The mixture was filtered and the pH of the solution recorded.

Finally the solution was analyzed for the presence of various components. The final analysis of the solution was performed by an external laboratory (Water Eco-science Laboratory) and the results are shown in Table 16.

PAOPER\MJC\dcoxifiation complee.doc-31/)IA)3 -34- Table 16. Results for TCLP Tests on Extraction of Calcined SPL Residue Using a Standard Fluid Reaction 400 0 C 500 0

C

Temperature Constituent Concentration of Concentration of TCLP Limits (ppm) Elements (ppm) Elements (ppm) Arsenic <0.001 <0.001 0.06 Antimony 0.2x10-3 0.12 Barium 0.6 0.6 12 Beryllium <0.001 <0.001 0.012 Cadmium <0.001 <0.001 0.06 Chromium <0.01 <0.01 1.2 Lead 0.04 0.03 0.18 Mercury <0.001 <0.001 0.024 Nickel <0.01 <0.01 1.2 Selenium <0.005 <0.005 0.6 Silver 0.4 0.2 0.6 Fluoride 6.0 7.0 48 Cyanide 0.1 0.03 2.4 The results show that each of the elements tested are below the allowable limit prescribed by the US EPA in accordance with the TCLP Test Procedure.

P:\OPERWMJIto&i-aio n-plo.4-31M Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as '"comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

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, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Claims (26)

1. A process for the treatment of spent potlining comprising the steps of: treating the spent potlining with water to form a solid residue containing Sleachable fluorides and an aqueous solution containing water-soluble inorganic matter; (ii) separating the solid residue from the aqueous solution containing the water- soluble inorganic matter; and (iii) calcining the separated solid residue in the presence of a source of calcium ions and H 2 0 for a time and under conditions sufficient to convert leachable fluorides into CaF 2 without substantial agglomeration of said residue.

2. A process according to claim 1 wherein neither of steps or (ii) is preceded or followed by an additional water washing step.

3. A process according to claim 1 or claim 2 wherein none of steps (ii) or (iii) is preceded or followed by an additional water washing step.

4. A process according to any one of claims I to 3 wherein the calcined residue is disposed of as landfill.

5. A process according to any one of claims 1 to 4 wherein the spent potlining is obtained from the electrolytic reduction cells used in aluminium smelting.

6. A process according to any one of claims 1 to 5 wherein the spent potlining is crushed or ground prior to or during the water treatment step.

7. A process according to any one of claims 1 to 6 wherein the average particle size of P:\OPER\MJOdcoxificaioncomplcic.doc-3 IA)11)3 -37- the spent potlining is from 0.1 to

8. A process according to claim 7 wherein the average particle size is from 0.5 to 2mm.

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

A process according to any one of claims 1 to 9 wherein the water treatment step is performed under substantially neutral conditions.

11. A process according to any one of claims 1 to 10 wherein the water used to treat the spent potlining comprises the aqueous solution separated in step (ii).

12. A process according to claim 11 wherein, prior to use in treating the spent potlining, the aqueous solution separated from step (ii) is treated to remove dissolved fluoride and cyanide.

13. A process according to any one of claims 1 to 12 wherein the amount of water used to treat the spent potlining is between 3 and 10 times the weight of the spent potlining.

14. A process according to any one of claims 1 to 13 wherein the solid residue from step (ii) is subjected to a drying step prior to calcination.

A process according to any one of claims 1 to 14 wherein the calcination step is conducted in an oxygen free or inert environment.

16. A process according to claim 15 wherein the calcination is conducted under nitrogen. P;\OPER\MJCOdloxiicaion coiplItc.doc-31Ml3A) -38-

17. A process according to any one of claims 1 to 16 wherein the source of calcium ions is a hydrated calcium oxide or a hydrated calcium salt.

18. A process according to claim 17 wherein the source of calcium ions is Ca(OH) 2

19. A process according to claim 18 wherein the Ca(OH) 2 is combined with the solid residue of spent potlining (SPL) in a Ca(OH) 2 :SPL weight ratio of from 20:80 to 60:40.

20. A process according to claim 19 wherein the ratio is from 25:75 to 50:50.

21. A process according to any one of claims 1 to 20 wherein the calcination temperature is between 550 and 1000°C.

22. A process according to claim 21 wherein the calcination temperature is from 580 to 800 0 C.

23. A process according to claim 22 wherein the calcination temperature is about 600 0 C.

24. A process according to any one of claims 1 to 23 wherein the aqueous solution from step (ii) is treated to remove cyanide and fluoride.

A process according to any one of claims 1 to 23 wherein the aqueous solution from step (ii) is subjected to the following steps: treatment with an oxidising agent to destroy free cyanide; (ii) neutralising the solution with a mineral acid; (iii) treating the neutralised solution with a salt containing a counterion to P:OPER\MJOdldcloxiiction complccdoc-31AIA)3 -39- precipitate complexed cyanide; (iv) separating the precipitated complexed cyanide from the treated neutralised solution to provide a substantially cyanide-free solution; and treating the substantially cyanide-free solution with a calcium salt to precipitate dissolved fluoride ions as calcium fluoride.

26. A process for the treatment of spent potlining substantially as hereinbefore described with reference to any one of the examples.