A METHOD AND AN APPARATUS OF PURIFYING A BAYER PROCESS STREAM

FIELD OF INVENTION The invention relates generally to a method and an apparatus for purifying a Bayer process stream.

In particular, although by no means exclusively, the invention relates to a method and an apparatus for purifying a Bayer process stream by using an ionic liquid to remove impurities.

BACKGROUND OF THE INVENTION

The Bayer Process is used for the manufacture of alumina from bauxite ore.

Bauxite ore generally contains organic and inorganic impurities, the amounts of which are specific to the bauxite source.

Part of the process involves purifying aluminate liquor to remove various impurities, both dissolved and undissolved, to form a purified filtrate. Alumina is then precipitated from the filtrate as alumina trihydrate crystals.

The purification step is important because alumina trihydrate containing high levels of organic impurities tend to produce a final product having an undesirably high level of coloration.

The remaining liquid phase or spent liquor may be concentrated to form "strong" liquor. The spent liquor streams are typically returned to the initial digestion step and employed as a digestant of additional ore after being reconstituted with additional caustic.

As the Bayer process is a closed circuit, impurities entering the process stream tend to accumulate with each cycle of the process. These impurities can have a negative impact on the process. Ionic liquids can be utilized to remove impurities from the Bayer process. However, such liquids are typically costly and toxic.

As such, it would be desirable to provide a method and an apparatus of purifying a Bayer process stream that would allow recycling of these ionic liquids.

The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.

SUMMARY OF INVENTION

The invention is a method and an apparatus for purifying a Bayer process stream that uses an ionic liquid as an extractant for impurities in a liquid/liquid extraction method and apparatus to remove the impurities from a Bayer process stream.

A Bayer process stream, in one embodiment, is a liquid stream generated during the Bayer process, and may be one or more of thickener overflow, pregnant liquor, spent liquor and strong liquor streams.

The term "impurities" is understood to mean compounds that may contaminate a Bayer process stream. Impurities include, but are not limited to, organic species and/or inorganic species. A particularly relevant class of impurity is non-oxalate organic compounds (NOOC) generically and empirically represented as Na ₂ C ₅ O ₇ . However, it can be appreciated that tailored selection of the ionic liquid would allow the removal of other classes of impurities from the Bayer process stream.

More particularly, the invention is a method and an apparatus for purifying a Bayer process stream that uses an ionic liquid that can reversibly associate with impurities from the Bayer process stream. The method/apparatus controls the operating parameters so that impurities can associate with the ionic liquid to remove impurities from the Bayer process stream and so that the impurities can dissociate from the ionic liquid to remove the impurities from the ionic liquid and regenerate the ionic liquid for recycling in the method/apparatus. The invention was made following a small-scale pilot plant (herein referred to as the Bench-Top Pilot or BTP) work by the applicant the use of ionic liquids for the method/apparatus. The BTP work was carried out following successful small-scale laboratory experiments on the method/apparatus.

During the BTP work, the applicant encountered a number of problems that were not present in the small-scale laboratory experiments.

One such problem was the impact of particulate matter circulating in an ionic liquid recycling circuit.

More specifically, the applicant discovered that an emulsion, stabilised by the particulate matter, between the ionic liquid and aqueous solutions formed when a threshold concentration of particulate matter in the circuit was reached, which made the recycling circuit inoperable.

This was a surprising discovery because it was previously unknown that particulate matter, particularly, but not limited to, particulate matter originating from the Bayer liquor, would behave in such a detrimental manner when the method/apparatus was scaled up to operate with substantially industrial process flows.

In order to find a solution to this problem, the applicant performed a series of experiments to determine the nature of the particulate matter.

In one experiment, the particulate matter was isolated and analyzed using XRD and it was determined that the particulate matter comprised:

• AI(OH) ₃ (Gibbsite) - major component

• CaCO ₃ - component

• AI(OH) ₃ (Bayerite) - trace

• Na ₃ H(CO ₃ ) ₂ .2H ₂ 0 (trona) - trace

• Na ₂ C ₂ O ₄ (sodium oxalate) - trace

• Ca ₃ AI ₂ (SiO ₄ )(OH) ₈ - trace

• Various other minor components The compositional analysis suggested that the particulate matter originated from the Bayer liquor. However, it can be appreciated that the particulate matter is not confined to this source and can also originate from other sources, such as NaCI brine impurities, or be externally derived particulate matter.

It was also determined that the particulate matter was insoluble in any of the aqueous streams in the method/apparatus and remained locked in the ionic liquid.

Without being bound by theory, it is believed that the particulate matter stabilises emulsions formed between the ionic liquid used and any of the aqueous streams used in the recycle process. These stabilised emulsions are described herein as a “crud”. It is further believed that various ions present in the process aqueous streams being purified combine to form precipitates, which, together with any trapped extraneous particles (such as fugitive dust), are described herein as particulate matter, and which are carried around the circuit in the ionic liquid.

Further analysis revealed that the crud forms at the ionic liquid (organic) phase/aqueous phase interface and subsequently coalesces at this interface, separating the aqueous and ionic liquid phases. The resultant coalesced layer of crud between the ionic liquid and aqueous phases could impede the further separation of fresh incoming mixed phases from the mixer/s, causing a loss of efficiency and a loss of ionic liquid to the aqueous stream exiting the settler/s. This makes separation of the organic ionic liquid phase and the aqueous phase difficult. The crud in the organic phase also accumulates and concentrates within the process. This affects the efficiency of the liquid/liquid extraction occurring at each of the stages. It is noted that a settler can be completely filled with crud, such that no further process flows can be pumped into the settler, rendering the entire recycling circuit inoperable.

During the course of troubleshooting, it was observed that after the equivalent of about 35 - 40 hours of plant run time, settlers used in the BTP were choked with crud.

It was also found that crud could form independently in each of the aqueous and organic phases and crud formation could occur in all stages of the process. It was observed that the crud formation was more pronounced in the EXTRACT stage when the ionic liquid feed stream to that stage interacts with the Bayer process stream.

In some BTP work, it was further observed that once NaCI-impurity derived precipitates reached a threshold concentration, the focal point of crud formation shifted to the

REGEN stage in which the formation of crud increased to an extent that caused entire settlers to be rendered non-operational.

The applicant tested the following methods that the applicant thought would be successful removing the crud from the circuit, with no or limited success.

1. Mixing or settling out of the ionic liquid, heating of the ionic liquid, dissolution in water and salting out of solution of the ionic liquid. This method was not successful. The salting out process investigated by the applicant involved the addition of a salt solution such as sodium hydroxide to the ionic liquid solution and allowing disengagement of the ionic liquid and the salt solution which allows recovery of the ionic liquid.

2. Ad hoc purging of the crud was not a viable solution due to the rapid accumulation of the crud.

3. Filtering ionic liquid streams from one or more of the EXTRACT, STRIP and REGEN stages to determine whether controlling the concentration of the particulate matter in the regenerated ionic liquid would affect the crud formation. Initial filtering options tested by the applicant were not successful.

Ultimately, further test work on filtering led to the development of the invention.

As a consequence of the further test work, the invention provides an apparatus for purifying a Bayer process stream comprising: an EXTRACT stage comprising at least one contacting/separation device configured to receive and intermix an impurity-containing Bayer process stream and an ionic liquid stream including a quaternary organic cation to form a purified Bayer process stream and an impurity-loaded ionic liquid stream; a STRIP stage comprising at least one contacting/separation device configured to receive and intermix the impurity-loaded ionic liquid stream and a halide-containing salt stream to form a halide-containing ionic liquid stream and an impurity-loaded salt stream; a REGEN stage comprising at least one contacting/separation device configured to receive and intermix the halide-containing ionic liquid stream and a caustic stream to form a regenerated ionic liquid stream and a halide-containing caustic effluent stream, wherein the EXTRACT stage is configured to be in fluid communication with the REGEN stage via a recycle loop to receive at least part of the regenerated ionic liquid stream to form the ionic liquid feed stream, and a FILTER stage comprising at least one filter configured to filter particulate matter from at least one the ionic liquid streams from the EXTRACT, STRIP, and REGEN stages.

Filtering particulate matter from at least one the ionic liquid streams reduces the amounts of circulating particulate matter and reduces the potential for crud formation.

The invention also provides an apparatus for recycling an ionic liquid used to purify a Bayer process stream comprising: a STRIP stage comprising at least one contacting/separation device configured to receive and intermix an impurity-loaded ionic liquid stream and a stripping solution stream to form an impurity-reduced ionic liquid stream and an impurity-loaded stripping solution stream; a REGEN stage comprising at least one contacting/separation device configured to receive and intermix the impurity-reduced ionic liquid stream and a caustic solution stream to form a regenerated ionic liquid stream and a caustic effluent stream; wherein at least one contacting/separation device from the REGEN stage is configured to recycle at least part of the regenerated ionic liquid stream and forming an ionic liquid feed stream to purify a Bayer process stream, and a FILTER stage comprising at least one filter configured to filter particulate matter from at least one the ionic liquid streams from the EXTRACT, STRIP, and REGEN stages.

The term “particulate matter” is understood herein to mean any particulate matter that is in one of the ionic liquid streams. The “particulate matter” may be particulate matter in the Bayer process stream. The “particulate matter” may be any solids that interact with ionic species in the circuit and form a material, such as a “crud” that interferes with a purification or recycling circuit.

The term “filter” is understood herein to be any suitable device for separating particulate material from the ionic liquid feed stream.

The contacting/separation device for the EXTRACT, STRIP, and REGEN stages may be selected from mixer-settlers, columns, centrifuges, static mixers, reactors or other equipment suitable for mixing and separating two at least partially immiscible liquid streams. A preferred contacting/separation device is a mixer-settler, such as is commonly used in solvent extraction circuits.

The ionic liquid may be any suitable ionic liquid.

By way of example, the ionic liquid may comprise an alkyl phosphonium salt that exists in three forms, depending on the molecule bonding with the phosphonium ion. These forms are: stripped (Cl ), regenerated (OH ) and loaded (NOOC). Transitioning between these forms enables the ionic liquid to reversibly associate with impurities.

At least one of the ionic liquid-containing operational units for the EXTRACT, STRIP, and REGEN stages may include a filter of the FILTER stage to remove particulate matter, particularly colloidal solids, from the ionic liquid stream to control the concentration of particulate matter in the regenerated ionic liquid.

In other words, the FILTER stage may be a part of the EXTRACT, STRIP, and REGEN stages. By way of example, the FILTER stage may be a part of a storage tank, a fluid conduit, or the contacting/separation device of the EXTRACT, STRIP, and REGEN stages.

The FILTER stage may also be a separate operational unit to the operational units that form the EXTRACT, STRIP, and REGEN stages.

The FILTER stage may be located after the STRIP stage. In one embodiment, filtration in the FILTER stage is performed on the ionic liquid stream after the STRIP stage. It is emphasized that the invention is not limited to this embodiment.

As is evident from the above, the ionic liquid stream transferred to the FILTER stage typically is an ionic liquid containing a particulate matter with the potential for crud formation.

The solids concentration, i.e. the concentration of the particulate matter, in the ionic liquid stream transferred to the FILTER stage may be up to 0.3g/L solids.

The applicant has tested solids concentrations up to 5.5 g/L and the results of the test work have provided a basis to conclude that the solids concentration may be higher.

The solids concentration may be up to 6 g/L.

The solids concentration may be up to 10 g/L.

Typically, the solids concentration in the ionic liquid stream transferred to the FILTER stage is lower than 3g/L within the ionic liquid stream.

The ionic liquid stream may include a stabilized emulsion which includes an aqueous phase and an ionic liquid phase.

The aqueous phase can be up to 50vol. % of the stabilized emulsion. More suitably, the aqueous phase of the stabilized emulsion is <10vol.%

Typically, filtration in the FILTER stage breaks up the stabilized emulsion into the aqueous and the ionic liquid phases.

The FILTER stage may be performed using any suitable filter that is capable of separating solid material, i.e. particulate matter, from the ionic liquid stream transferred to the FILTER stage.

The filter may be configured to use differential pressure as a driving force for filtration. The filter may be a positive pressure filter.

The filter may be a candle filter. The FILTER stage may include a candle filter with a perlite filtration aid.

Any suitable filter media may be used in the filter.

Suitably, the filter media is compatible with the ionic liquid.

Polypropylene (PP) and polytetrafluoroethylene (PTFE) have been found by the applicant to be suitable on the basis of short-term test work.

The filter media may be of multi- or mono- filament construction.

The filter media may have an air permeability between 3-200 L/dm ² /min.

The FILTER stage may include the use of a filter aid such as ceramic material, tricalcium aluminate hexahydrate (TCA), and a flocculant.

The filter aid may be selected for higher throughput and longevity of the filtration media.

The filter aid material may be any suitable chemical resistant material that does not react with the ionic liquid.

The filter aid material may be an expanded perlite.

The filter aid material may be a median particle size between 10-100μm. Suitably, the filter aid median particle size is between 50-85μm. More suitably, a filter aid with a median particle size of 68μm is used for the filtration

The filter aid may be introduced to the ionic liquid stream transferred to the FILTER stage by direct addition to the ionic liquid stream. More suitably, the filter aid is pre-batched with a water solution of a temperature between 20-80°C before being added to the ionic liquid stream. Most suitably, the filter aid is batched with a solution of water at a temperature of 55°C. The pre-batching solution of filter aid may be prepared to a solids concentration between 10-200g/L. More suitably, the solids concentration is between 40-100g/L.

Most suitably, the solids concentration is 60g/L.

The filter aid may be added to the ionic liquid stream transferred to the FILTER stage in a solids ratio between 1:0.1, and 1:5 (Solids in Feed stabilized emulsion: Filter Aid Solids). More suitably, the addition ratio is between 1:1 to 1:2.

The FILTER stage may be configured to operate within a temperature range of 20- 70°C. Most suitably, the FILTER stage operates at temperature range of 55-60°C.

When operating with pressure filters, the FILTER stage may operate within a pressure differential range between 2-8 bar. More suitably, the pressure is between 4-6 bar.

Most suitably, the pressure of operation is 6 bar. The FILTER stage may be configured to operate with a filtration flux rate between IQ-

700 L/m ² /hr. More suitably, the filtration process operates at a flux rate between 50-200 L/m ² /hr. Most suitably, the filtration process operates at a flux rate between 100- 150L/m ² /hr. The FILTER stage may include a cleaning process for removing entrapped ionic liquid within the filter aid and a solids filter cake.

The cleaning process may include filter cake washing steps, typically with return of recovered ionic liquid to the method.

The subsequent washed and cleaned cake may be discharged as a slurry, or as dry cake for disposal.

The filter may be a filter with a pore size ranging from 5-100 μm. Suitably, the filter has a pore size ranging from 10-0 μm is used for the filtering step. More suitably, a filter having a 10-30 μm pore size is used for the filtration. The filter may be any device capable of solid-liquid separation, such as BOchner funnel or a centrifuge.

A positive pressure filtration system, a vacuum filtration system, or a centrifuge can be used to facilitate the filtration.

The apparatus may include a controller to control the parameters of the caustic stream such as flow rate and concentration to account for the dilution of the halide-containing ionic liquid.

The apparatus may comprise at least three mixer settlers. Suitably, the apparatus comprises ten mixer-settlers.

In the EXTRACT stage, at least one mixer-settler may be configured to receive and intermix an impurity-containing Bayer process stream and an ionic liquid feed stream to form a purified Bayer process stream and an impurity-loaded ionic liquid stream, respectively. Suitably, the EXTRACT stage comprises three mixer-settlers arranged in series.

In the STRIP stage, at least one mixer-settler may be configured to receive and intermix the impurity-loaded ionic liquid stream with a halide-containing salt stream to form a halide-containing ionic liquid stream and an impurity-loaded salt stream, respectively. Suitably, the STRIP stage comprises three mixer-settlers arranged in series.

In the REGEN stage, at least one mixer-settler may be configured to receive and intermix the halide-containing ionic liquid stream with a caustic stream to form a regenerated ionic liquid stream and a halide-containing caustic effluent stream, respectively. Suitably, the REGEN stage comprises four mixer-settlers arranged in series.

The first mixer settler in the EXTRACT stage may be configured to be in fluid communication with the end mixer settler in the REGEN stage via a recycle loop to receive at least part of the regenerated ionic liquid stream to form the ionic liquid feed stream. In one embodiment, in use, within the EXTRACT stage mixer-settlers, hydroxyl ions within the ionic liquid feed stream are substituted with NOOC ions in the impurity- containing Bayer process stream to form the purified Bayer process stream and the impurity-loaded ionic liquid stream (NOOC- form). The impurity-loaded ionic liquid stream exiting the third EXTRACT stage mixer-settler then flows into the first mixer- settler of the STRIP stage.

In one embodiment, in use, within the STRIP stage mixer-settlers, NOOC ions are transferred from the impurity-loaded ionic liquid stream to the halide-containing salt stream. NOOC ions in the impurity-loaded ionic liquid stream is substituted with chloride ions from the halide-containing salt stream to form a halide-containing ionic liquid stream (Cl- form) and an impurity-loaded salt stream. The mass transfer is driven by a high concentration of chloride ions in the aqueous phase.

The stripped ionic liquid exiting the third STRIP stage mixer-settler then flows into the first mixer-settler of the REGEN stage.

In one embodiment, in use, within the REGEN stage mixer-settlers, stripped ionic liquid is converted to regenerated ionic liquid (OH- form) on contact with a caustic stream to form a regenerated ionic liquid stream and a halide-containing caustic effluent stream. The regenerated ionic liquid stream exiting the fourth REGEN stage mixer-settler is then recycled to the first mixer-settler of the EXTRACT stage. The halide-containing caustic effluent stream is pumped to a salt separation unit.

The apparatus may be installed after the alumina trihydrate precipitation stage to treat the spent Bayer liquor.

The FILTER stage may be configured such that the concentration of particulate matter in the regenerated ionic liquid stream is below a predetermined threshold concentration before the regenerated ionic liquid is returned to the EXTRACT stage as at least a part of the ionic liquid feed stream. The predetermined threshold concentration may be any suitable concentration having regard to any one or more of the following factors: the Bayer process liquor being processed, the particular ionic liquid, and the processing conditions. Suitably, the filtrate solids are less than 3 g/L particulate matter. More suitably the filtrate solids are less than 0.5 g/L particulate matter. Most suitably the filtrate solids are less than 0.1 g/L particulate matter.

The filter stage may provide multi-stage filtration.

The invention also provides a method of purifying a Bayer process stream comprising: providing an ionic liquid feed stream including a quaternary organic cation, wherein the ionic liquid feed stream is at least partially immiscible with the Bayer process stream; intermixing the Bayer process stream with the ionic liquid feed stream and forming an aqueous phase comprising purified Bayer process liquor and an organic phase comprising impurity-loaded ionic liquid, wherein the intermixing reduces the concentration of impurities in the Bayer process liquor; at least partially separating the aqueous phase from the organic phase and forming a purified Bayer process stream and an impurity-loaded ionic liquid stream; intermixing the impurity-loaded ionic liquid stream and a halide-containing salt stream to form an aqueous phase comprising an impurity-loaded salt and an organic phase comprising a halide-containing ionic liquid, wherein the intermixing reduces the concentration of impurities in the impurity-loaded ionic liquid stream; at least partially separating the aqueous phase from the organic phase and forming a halide-containing ionic liquid stream and an impurity-loaded salt stream; intermixing the halide-containing ionic liquid stream and a caustic solution and forming an aqueous phase comprising a halide-containing caustic solution and an organic phase comprising a regenerated ionic liquid, wherein the intermixing substitutes at least some of the halide groups in the halide-containing ionic liquid with hydroxyl groups from the caustic solution; at least partially separating the aqueous phase from the organic phase and forming a halide-containing salt stream and a regenerated ionic liquid steam; filtering at least one of the ionic liquid streams from the EXTRACT, STRIP, and REGEN stages and removing particulate matter; and recycling at least part of the regenerated ionic liquid stream and forming at least a part of the ionic liquid feed stream.

The invention also provides a method of recycling an ionic liquid used to purify a Bayer process stream, comprising: intermixing an impurity-loaded ionic liquid stream with a stripping solution stream and forming an aqueous phase comprising an impurity-loaded stripping solution and an organic phase comprising an impurity-reduced ionic liquid; at least partially separating the aqueous phase from the organic phase and forming an impurity-reduced ionic liquid stream and an impurity-loaded stripping solution stream; intermixing the impurity-reduced ionic liquid stream and a caustic solution stream and forming an aqueous phase comprising a spent caustic solution and an organic phase comprising a regenerated ionic liquid; filtering at least one of the ionic liquid streams from the EXTRACT, STRIP, and REGEN stages and removing particulate matter; and recycling at least part of the regenerated ionic liquid stream to form the ionic liquid feed stream.

The filtering step may include removing particulate matter such that the concentration of particulate matter in the regenerated ionic liquid stream is below a predetermined threshold concentration.

In one embodiment, the ionic liquid comprises an alkyl phosphonium salt that exists in three forms, depending on the molecule bonding with the phosphonium ion. These forms are: stripped (CI-), regenerated (OH-) and loaded (NOOC-). Transitioning between these forms enables the ionic liquid to reversibly associate with the impurity.

The method may include providing an ionic liquid feed stream that is at least partially immiscible with the Bayer process stream. The ionic liquid feed stream may include an ionic liquid comprising a quaternary organic cation.

Australian patent 2010337293 in the name of Cytec Technology Corp. discloses ionic liquids comprising a quaternary organic cation and methods and compositions for the removal of impurities from impurity-loaded ionic liquids. The reference herein to the Cytec Australian patent is not an admission that the disclosure in the patent is part of the common general knowledge in Australia or elsewhere.

The following description of the quaternary organic cation is based closely on description in the Cytec Australian patent.

As reported in the Cytec Australian patent, the quaternary organic cation may be selected from the group consisting of phosphonium, ammonium, sulfonium, pyridinium, pyridazinium, pyrimidinium, pyrazinium, pyrazolium, imidazolium, thiazolium, oxazolium, pyrrolidinium, quinolinium, isoquinolinium, guanidinium, piperidinium and methylmorpholinium. Suitably, the quaternary organic cation is selected from the group consisting of: wherein R _a , R _b , R _c , R _d , R _e , R _f may each be independently selected from hydrogen, or a substituted C ₁ -C ₅₀ alkyl group, where the substituents include one or more selected from alkyl, cycloalkyl, alkenyl, cycloalkynyl alkynyl, alkoxy, alkoxyalkyl, aldehyde, ester, ether, ketone, carboxylic acid, alcohol, carboxylate, hydroxyl, nitro, silyl, aryl and halide functionalities.

R _a through R _f individually may comprise from about 1 to about 50 carbon atoms. It can be appreciated that two or more of R _a through R _f may form a ring structure.

R ₁ through R7 may each independently be selected from hydrogen, halogen, or a substituted C ₁ -C ₅₀ alkyl group, where the substituents include one or more selected from alkyl, cyclocalkyl, alkenyl, cycloalkynyl, alkynyl, alkoxy, alkoxyalkyl, aldehyde, ester, ether, ketone, carboxylic acid, alcohol, carboxylate, hydroxyl, nitro, silyl, aryl and halide functionalities. R ₁ through R7 may individually comprise from about 1 to about 50 carbon atoms. It will be appreciated that two or more of R ₁ through R7 may form a ring structure.

Examples of quaternary organic cations include, but are not limited to tributyloctylphosphonium, tributyl(methyl)phosphonium, tributyl-8- hydroxyoctylphosphonium, tetrabutylphosphonium, tetrapentylphosphonium, tetrahexylphosphonium, tetraoctylphosphonium, octyl(tributyl)phosphonium, tetradecyl(tributyl)phosphonium, tetradecyl(trihexyl)phosphonium, tributyl(methyl)ammonium, tetrabutylammonium, tetrapentylammonium, tetrahexylammonium, tetraoctylammonium, tetradecyl(tributyl)ammonium, tetradecyl(trihexyl)ammonium, dimethyl dicoco quaternary ammonium, stearamidopropyldimethyl-2-hydroxyethylammonium, ethyl(tetradecyldiundecyl)ammonium, tallowalkyltrimethyl ammonium, N,N,N-trimethyl- 1-dodecanamonium, benzyldimethylcocoalkylammonium, N,N-dimethyl- Ndodecylglycine, butylmethylpyrrolidinium, l-octyl-2,3-dimethylimidazolium, l-butyl-3- methylimidazolium, sulfonium and guanidinium. The term "coco" refers to the alkyl group derived from the mixture of fatty acids found in coconut oil, which are generally saturated fats with about 12 carbon atoms. A preferred quaternary organic cation is tributyloctylphosphonium. The quaternary organic cation is typically associated with an anionic counterion or anion. The anion may be a chaotropic anion or a kosmotropic anion.

Examples of suitable anions include, but are not limited to, halide (e.g. fluoride, chloride, bromide, iodide), hydroxyl, alkylsulfate (e.g., methylsulfate, ethylsulfate, octylsulfate), dialkylphosphate, sulfate, nitrate, phosphate, sulfite, phosphite, nitrite, hypochlorite, chlorite, chlorate, perchlorate, carbonate, bicarbonate, carboxylate (e.g. formate, acetate, propionate, butyrate, hexanoate, fumarate, maleate, lactate, oxalate, pyruvate), bis(trifluoromethyl)sulfonylimide ([NTF2]), tetrafluoroborate, hexafluorophosphate, CN-, SCN-, and OCN.

The group of halides and halogen-containing compounds may include, but are not limited to: F-, Cl-, Br, l-, BF ₄ -, CIO ₃ -, CIO ₄ -, BrO ₃ - , BrO ₄ -, IO ₃ -, IO ₄ -. PF ₆ -, AICI ₄ -, AI ₂ Cl-, AI ₃ CI ₁₀ -, AIBr ₄ -, FeCI ₄ -, BCI ₄ -, SbF ₆ -, AsF ₆ -, ZnCI ₃ -, SnCI ₃ -, CuCI ₂ -, CF ₃ SO ₃ -, (CF ₃ SO ₃ ) ₂ N-, CF ₃ CO ₂ - and CCI ₃ CO ₂ -.

A preferred class of anions is halides. A preferred anion is chloride.

The ionic liquid may include any pairing of any of the quaternary organic cations and anions.

The ionic liquid may be selected from the group consisting of tributyloctylphosphonium chloride, trihexyltetradecylphosphonium chloride, tetrabutylphosphonium chloride, tetradecyl(tributyl)phosphonium chloride, tributyl(8-hydroxyoctyl)phosphonium chloride and octyl(tributyl)phosphonium chloride.

The ionic liquid may be selected from the group consisting of tetrabutylammonium hydroxide, tetrabutylammonium chloride, stearamidopropyldimethyl-2- hydroxyethylammonium nitrate, ethyltetradecyldiundecyl ammonium chloride, tetrahexylammonium bromide, dodecyltrimethyl ammonium chloride, benzyldimethylcoco ammonium chloride, N,N-dimethyl-N-dodecylglycine betaine, Adogen 462®, Aliquat® HTA-1, and tallowalkyltrimethyl ammonium chloride. The ionic liquid may be a phosphonium salt which exists in three forms, depending on the molecule bonding with the phosphonium ion. These forms are: stripped (CI-), regenerated (OH-) and loaded (NOOC-).

The ionic liquid may be a tributyloctyl phosphonium salt which exists in the following three forms: stripped (CI-), regenerated (OH-) and loaded (NOOC-).

The ionic liquid feed stream may include a diluent. The diluent may be alcohols (e.g., isopropanol), polyols and/or polyethyleneoxide. Such diluents may facilitate phase separation.

The ionic liquid feed stream may include at least 1 wt.% of ionic liquid. Suitably, the ionic liquid feed stream includes at least about 10% by weight of ionic liquid. More suitably, the ionic liquid feed stream includes at least about 50% by weight of ionic liquid. Even more suitably, the ionic liquid feed stream comprises about 70% by weight of ionic liquid.

The method includes mixing the ionic liquid feed stream with the Bayer process stream.

Although the ionic liquid stream and the Bayer process stream may be mutually soluble to some extent, typically the two phases are at least partially immiscible with one another to form a liquid/liquid mixture in an EXTRACT stage.

The ionic liquid entering the EXTRACT stage may be in its regenerated form. Suitably, the ionic liquid entering the EXTRACT stage is tributyloctyl phosphonium hydroxide.

In the EXTRACT stage, the external O/A ratio may range 0.5 to 2. Suitably, the external O/A ratio ranges from 0.67 to 1: Most suitably, the external O/A ratio ranges from 0.75 to 0.85.

The external O/A ratio defines the O/A ratio of each stage. This is in contrast to the internal O/A ratio which defines the O/A ratio of each mixer-settler within each stage. During the intermixing, the impurity, for example NOOC, is extracted from the spent liquor and transferred to the ionic liquid. This reduces the concentration of impurities in the Bayer process stream and forms an aqueous phase comprising purified Bayer process liquor and an organic phase comprising impurity-loaded ionic liquid.

The aqueous and organic phases are then at least partially separated to form a purified Bayer process stream and an impurity-loaded ionic liquid stream.

The purified Bayer process stream from the EXTRACT stage is sent off for further processing, for example, to a refinery, while the impurity-loaded ionic liquid stream is directed to a STRIP stage.

The ionic liquid entering the STRIP stage may be in its loaded form. Suitably, the ionic liquid entering the EXTRACT stage is NOOC-associated tributyloctyl phosphonium salt.

In the STRIP stage, the external O/A ratio may range 0.5 to 3. Suitably, the external O/A ratio ranges from 0.67 to 2.5: Most suitably, the external O/A ratio is 1.89.

The internal O/A ratio may range from 0.5 to 2. Suitably, the internal O/A ratio ranges from 0.8 to 1.2.

In the STRIP stage, the impurity-loaded ionic liquid stream is intermixed with a stripping solution. Suitably, the stripping solution is a halide-containing salt stream. More suitably, the halide-containing salt stream is a brine solution (sodium chloride).

During the intermixing, ion exchange occurs between the ionic liquid and halide- containing salt in which the anionic impurity from the ionic liquid is swapped with the halide group from the salt to reduce the concentration of impurities in the impurity- loaded ionic liquid stream. This forms a mixture comprising an aqueous phase comprising an impurity-loaded salt and an organic phase comprising a halide- containing ionic liquid.

The aqueous and organic phases are then at least partially separated to form an impurity-loaded salt stream and a halide-containing ionic liquid stream. The impurity-loaded salt stream is processed to remove residual ionic liquid before discharge into the environment. A certain level of ionic liquid entrainment is expected in the impurity-loaded salt stream by nature of the mixing/settling process, typically ranging from 300-400 ppm. As such, the impurity-loaded salt stream is passed through a coalescer designed to agglomerate and collect the residual ionic liquid for recovery back into the circuit.

The impurity-loaded salt stream may also be passed through at least one activated carbon column to further reduce the final ionic liquid concentration before being discharged into the environment. At least part of the salt from the stream may be recycled back to the STRIP stage.

The final ionic liquid concentration in the salt stream may be <1 ppm.

The halide-containing ionic liquid stream is directed to a REGEN stage.

The ionic liquid entering the REGEN stage may be in its stripped form. Suitably, the ionic liquid entering the REGEN stage is tributyloctyl phosphonium chloride.

In this stage, the external O/A ratio may range 0.5 to 2. Suitably, the external O/A ratio ranges from 0.67 to 1.5: Most suitably, the external O/A ratio ranges is 1.13 The internal O/A ratio may range from 0.5 to 2. Suitably, the internal O/A ratio ranges from 0.8 to 1.2.

In the REGEN stage, the halide-containing ionic liquid stream is intermixed with a caustic stream.

The caustic concentration may be less than 50wt%. Suitably, the caustic concentration is less than 30wt%. More suitably, the caustic concentration is less than 20wt%. Most suitably the caustic concentration is 10wt%.

The caustic stream may comprise sodium hydroxide.

During the intermixing, the halide group from the halide-containing ionic liquid is substituted with the hydroxyl group from the caustic stream. This forms a liquid/liquid mixture comprises an aqueous phase comprising regenerated ionic liquid and a halide- containing caustic solution.

The aqueous and organic phases are then at least partially separated to form a regenerated ionic liquid stream and a halide-containing caustic stream.

The halide-containing caustic stream is processed for discharge into the environment.

At least part of the regenerated ionic liquid stream is to be recycled to the EXTRACT stage to form the ionic liquid feed stream.

The ionic liquid inventory is gradually degraded or lost in the circuit, particularly in the EXTRACT and REGEN stages which have high concentration of caustic. To maintain the concentration of ionic liquid in the circuit, fresh ionic liquid may be added to the process, suitably at the REGEN stage. Suitably, the fresh ionic liquid is in its stripped form. More suitably, the fresh ionic liquid is tributyloctyl phosphonium chloride.

Suitably, either or both incoming and exiting REGEN organic ionic liquid are filtered.

Because the viscosity of the ionic liquid at ambient temperature is too high to enable the filtration of neat ionic liquid efficiently, the ionic liquid may be diluted before being filtered to facilitate the filtration process. Suitably, the ionic liquid is diluted with water at a ratio ranging from 0.5 (1:2) to 2 (2:1). Suitably, the ionic liquid to water ratio is 1 (1:1). The parameters of the caustic solution stream such as flow rate and concentration may be adjusted to account for the dilution of the ionic liquid.

Dilution of the ionic liquid may occur on the stream entering the REGEN stage.

Suitably, water is added to the stream entering the REGEN stage to dilute the ionic liquid stream.

Once the colloidal solids are filtered out, at least part of the resulting filtered stream may be recycled to the ionic liquid feed stream.

The filtered stream may be intermixed with a metal halide salt and/or a caustic solution to recover the ionic liquid. Suitably, the recovered ionic liquid is recycled back into the circuit. The recovered ionic liquid may be decanted before being recycled back into the circuit.

The regenerated ionic liquid stream may be diluted with water before the filtering step.

In each of the EXTRACT, STRIP and REGEN stages, the aqueous stream may flow counter-current to the organic ionic-liquid containing stream. Operating under counter- current mode enhances the transfer of impurities from the impurity-containing stream to the extractant stream by maintaining a nearly constant concentration gradient between the two streams over their entire length of contact.

In each of the EXTRACT, STRIP and REGEN stages, the flowrate of each stream may range from 2-44m ³ /hr. The organic ionic liquid stream in the EXTRACT stage may have a flowrate ranging from 10-26m ³ /hr.

The aqueous stream in the EXTRACT stage may be any suitable flowrate. The aqueous stream in the STRIP stage may have a flowrate ranging from 4-14m ³ /hr. The aqueous stream in the REGEN stage may have a flowrate ranging from 6-23m ³ /hr.

In each of the EXTRACT, STRIP and REGEN stages, the intermixing step may be performed in various ways including by batch, semi-continuous or continuous methods.

Suitably, the intermixing step is a continuous process.

Each intermixing step may involve feeding the organic and aqueous streams into any suitable apparatus that can be used for mixing and phase separation or settling. Examples of mixing and phase separation or settling apparatus that may be suitable include, but is not limited to, continuous mixer/settler units, static mixers, in-line mixers, columns, centrifuges, and hydrocyclones. A preferred apparatus is a mixer-settler. In each of the EXTRACT, STRIP and REGEN stages, the operating temperature may be up to 100°C. The operating temperature may be varied to control the speed of phase separation.

The operating temperature of each of the EXTRACT, STRIP and REGEN stages may range from 50-80°C, suitably 65-75°C, more suitably 60-65°C.

The method may include controlling the internal and/or external O/A ratios of each of the EXTRACT, STRIP and REGEN stages. The O/A ratios may range 0.001 (1:1000) to 100 (100:1). In one embodiment, the O/A ratios range from 0.01 to 100. In another embodiment, the ratios range from 0.1 to 10. In a further embodiment, the ratios range from 0.25 to 6.67. In yet another embodiment, the ratios range from 0.25 to 2.

BRIEF DESCRIPTION OF DRAWINGS

An embodiment of the invention is hereinafter described by way of example only with reference to the accompanying drawings, wherein:

Figure 1 is flow diagram of an embodiment of a method of recycling an ionic liquid according to the invention; and

Figure 2 is a diagram of an embodiment of an apparatus configured to recycle an ionic liquid according to the invention.

DETAILED DESCRIPTION

The embodiments described herein are embodiments of a method and apparatus for purifying a Bayer process stream using an ionic liquid to remove impurities from the Bayer process stream in accordance with the invention.

The examples described therein focus on applying the invention to removing impurities from a spent Bayer liquor stream. As noted above, a particularly relevant class of impurity in spent Bayer liquor stream is NOOC.

The embodiment of the method of purifying a Bayer process stream in accordance with the invention is marked as 10 in Figure 1. The embodiment of the apparatus for purifying a Bayer process stream in accordance with the invention is marked as 110 in Figure 2.

With reference to the Figures, the basic unit operations for the apparatus are an EXTRACT stage 16, a STRIP stage 18, and a REGEN stage 20.

The EXTRACT stage 16 comprises three mixer-settlers comprising mixers 48A-C and settlers 46A-C (Figure 2). The EXTRACT mixer-settlers are arranged in series with first EXTRACT mixer-settler E1 comprising mixer 48A and settler 46A and the end EXTRACT mixer-settler E3 comprising mixer 48C and settler 46C.

The STRIP stage 18 comprises three mixer-settlers comprising mixers 52A-C and settlers 50A-C (Figure 2). The STRIP mixer-settlers are arranged in series with the first STRIP mixer-settler S1 comprising mixer 52A and settler 50A and the end STRIP mixer-settler S3 comprising mixer 52C and settler 50C.

The REGEN stage 20 comprises four mixer-settlers comprising mixers 56A-C and settlers 54A-C (Figure 2) arranged in series.

The method and apparatus of the invention is characterised by a FILTER stage comprising at least one filter configured to filter particulate matter from at least one the ionic liquid streams from the EXTRACT, STRIP, and REGEN stages 16, 18, 20.

An ionic liquid feed stream 12 comprising 70% by weight of tributyloctylphosphonium hydroxide and 30% by weight of water, and a spent Bayer liquor stream 14 containing NOOC at a concentration of 22.5g/L as carbon are fed into the EXTRACT stage 16 (Figure 1) in countercurrent at an external O/A ratio ranging from 0.67 to 1.0 and an internal O/A ratio ranging from 0.8 to 1.2 to intermix. The spent Bayer liquor stream 14 enters via end EXTRACT mixer-settler E3 while the ionic liquid feed stream 12 enters via first EXTRACT mixer-settler E1.

The temperature of the spent Bayer liquor stream 14 ranges from 60-80°C while the temperature of the ionic liquid feed stream ranges from 20-30°C, preferably from 20- An operating temperature ranging from 50-80°C, preferably 60-65°C is maintained in the EXTRACT stage.

As noted above, the EXTRACT stage 16 mixer-settlers comprise mixers 48A-C and settlers 46A-C (Figure 2).

The first mixer settler E1 is in fluid communication with the end mixer settler R4 comprising mixer 56C and settler 54C in the REGEN stage 20 via a recycle loop. E1 receives regenerated ionic liquid from the REGEN stage to form the ionic liquid feed stream 12 and the end EXTRACT mixer-settler E3 is in fluid communication with the first mixer-settler S1 comprising mixer 52A and settler 50A of the STRIP stage 18.

During intermixing in the mixers 48A-C, NOOC is extracted from the spent Bayer liquor stream 14 and is transferred to the ionic liquid feed stream 12. This reduces the concentration of NOOC in the spent Bayer liquor stream 14 and forms an aqueous purified spent Bayer liquor and an organic NOOC-loaded ionic liquid.

The aqueous and organic phases are then separated to form a purified spent Bayer process stream 24 having a lower NOOC concentration than the spent Bayer liquor stream 14, by way of example 6.9g/L or lower, and a NOOC-loaded ionic liquid stream 26 that exits the EXTRACT stage.

The purified spent Bayer liquor stream 24 (exiting via first EXTRACT mixer-settler E1) is transferred for further processing in a refinery, while the NOOC-loaded ionic liquid stream 26 is transferred to the STRIP stage 18.

As noted above, the STRIP stage 18 comprises three mixer-settlers comprising mixers 52A-C and settlers 50A-C (Figure 2) arranged in series.

The first STRIP mixer-settler S1 is in fluid communication with the end mixer-settler E3 comprising mixer 48C and settler 46C of the EXTRACT stage 16 and the end STRIP mixer-settler S3 is in fluid communication with the first mixer-settler R1 comprising mixer 56A and settler 54A of the REGEN stage 20. In the STRIP stage 18, the NOOC-loaded ionic liquid stream 26 flows counter-current to a brine (sodium chloride) stream 28 at an external O/A ratio ranging from 0.67 to 2.5 and an internal O/A ratio ranging from 0.8 to 1.2 to intermix. The brine (sodium chloride) stream 28 is fed into end STRIP mixer-settler S3 while the NOOC-loaded ionic liquid stream 26 enters via first STRIP mixer-settler S1.

An operating temperature ranging from 60-80°C, preferably 70-75°C is maintained in the STRIP stage.

During intermixing in the 52A-C, ion exchange occurs between the ionic liquid and brine in which the anionic NOOC is swapped with the chloride group in the brine. This forms a mixture comprising an aqueous phase comprising NOOC-loaded brine and an organic phase comprising chloride-containing ionic liquid.

The aqueous and organic phases are then separated to form an NOOC-loaded brine stream 30 having a NOOC concentration typically at least 20g/L and a chloride- containing ionic liquid stream 32 that exits the STRIP stage.

The NOOC-loaded brine stream 30 (exiting via first STRIP mixer-settler S1) is transferred for further processing before discharge into the environment.

The further processing includes passing the NOOC-loaded brine stream 30 through a coalescer to agglomerate and collect any entrained ionic liquid, typically ranging from 300-500 ppm, for recovery back into the circuit.

The further processing also includes passing the NOOC-loaded brine stream 30 through a series of activated carbon columns to reduce the final ionic liquid concentration to less <1 ppm before the cleaned brine stream is discharged into the environment.

Part of the cleaned brine stream is recycled to the REGEN stage.

The chloride-containing ionic liquid stream 32 is directed to the REGEN stage 20 which comprises four mixer-settlers. As noted above, Figure 2 shows the four REGEN mixer-settlers as mixers 56A-C and settlers 54A-C arranged in series.

The first mixer-settler R1 of the REGEN stage 20 is in fluid communication with the end mixer-settler S3 of the STRIP stage 18 and the end mixer-settler R4 of the REGEN stage 20 is in fluid communication with the first mixer-settler E1 of the EXTRACT stage 16 to transfer regenerated ionic liquid to the EXTRACT stage 16.

In the REGEN stage 20, the chloride-containing ionic liquid stream 32 flows counter- current to a caustic (sodium hydroxide) stream 35 having a concentration typically having a concentration of at least 10wt% at an external O/A ratio ranging from 0.67 to 1.5 and an internal O/A ratio ranging from 0.8 to 1.2 to intermix. The caustic (sodium hydroxide) stream 35 is fed into end REGEN mixer-settler R4 while the chloride- containing ionic liquid stream 32 is fed into first REGEN mixer-settler R1.

The temperature of the caustic stream 35 ranges from 10-30°C, preferably 25°C.

An operating temperature ranging from 50-80°C, preferably 60-65°C is maintained in the REGEN stage.

During intermixing in the mixers 56A-C, the chloride group from the ionic liquid is substituted with the hydroxyl group from the caustic solution. This forms an aqueous phase comprising a regenerated ionic liquid and a chloride-containing caustic solution. The aqueous and organic phases are then separated to form a regenerated ionic liquid stream 12 and a chloride-containing caustic stream 36 that exits the REGEN stage 20 via first REGEN mixer-settler R1.

The chloride-containing caustic stream 36 is further processed in a salt separation unit 40. The processed caustic stream generates a sodium chloride stream 42 that is fed at least in part to the brine stream 28 or generates a sodium hydroxide stream 44 that is fed at least in part to the caustic stream 35.

The ionic liquid inventory is gradually degraded or lost in the circuit. To maintain the ionic liquid concentration, fresh ionic liquid 37 in the form of tributyloctyl phosphonium chloride is added to the REGEN stage 20.

As previously discussed, one problem encountered during the course of BTP work was that once a threshold concentration of particulate matter in an ionic liquid recycling circuit was reached, an emulsion (referred to above as a “crud”), stabilised by the particulate matter, between the ionic liquid used and aqueous solutions formed and made the purification or recycling circuit inoperable. To mitigate the formation of the crud, the exiting STRIP organic ionic liquid stream is passed through a suitably sized filtration device to maintain a solids concentration, i.e. particulate matter, below a threshold concentration in the filtered stream before it is transferred to the REGEN stage as the chloride-containing ionic liquid stream 32. In another embodiment, to mitigate the formation of the crud, the exiting REGEN organic ionic liquid stream is passed through a suitably sized filtration device to maintain a solids concentration, i.e. particulate matter, below a threshold concentration in the filtered stream before it is recycled as ionic liquid feed stream 12. Once the colloidal solids are filtered out, the resulting stream may be salted out using sodium chloride and/or caustic to recover the ionic liquid, before the ionic liquid is decanted and recycled back into the circuit.

As noted above, the invention extends to filtering the organic ionic liquid stream before or after any one or more than one of the EXTRACT, STRIP, and REGEN stages 16,

18, 20.

In order to evaluate the invention, the above-mentioned Bench-Top Pilot (BTP) was operated using a similar method and apparatus to that described above using the operating parameters outlined in Table 1. Table 1: Parameters for BTP

The measured operating parameters are listed in the “Actual” column while the set parameters are listed in the “Standard” column.

Table 2 provides a summary of the analytic results of the BTP.

Table 2: Summary of analytic results of the BTP Triplicate samples were taken from each of the three feed aqueous stock tanks (EXTRACT - DSL, STRIP - SA, REGEN - RA), as well as from every settler aqueous overflow weir for a total of 39 samples. Every sample was individually analysed for its phosphorus, chloride and total organic carbon content. Once the sample results were received, the median of each set of three results was taken and is presented in Table 2 above.

Phosphorus was selected as a representative species for the ionic liquid, since the P background was expected, and has proven, to be low enough to not interfere with the analysis, and because the ionic liquid contains phosphorus.

All values less than 20 ppm were reported as “<20 ppm”. In the table above, where this “<20 ppm” was reported for SA and RA, the value was entered in the table above as 0, since this is a pure solution prior to contact with any ionic liquor. In all other instances where a “<20 ppm” value was reported, a value of 10 ppm was entered instead as a working estimate of the P content of the sample.

The following observations were made based on the results in Table 2:

• Purification of the spent Bayer liquor stream 14 containing NOOC occurred in the EXTRACT stage. This is evidenced by a decrease in TOC content of

22,900mg/L in the spent Bayer liquor stream 14 fed into E3 to 6,910mg/L in the purified spent Bayer process stream 24 exiting E1.

• NOOC was predominantly removed in the STRIP stage. This is evidenced by an increase in TOC content from the brine (S3; SA) stream 28 supplied into S3 of 6,880mg/L to 21,700mg/L in the NOOC-loaded brine stream (S1; SSA) 30 exiting S1.

• Regeneration of ionic liquid in the REGEN stage is evidenced by an increase in Cl concentration of the caustic (R4; RA) stream 35 supplied into R4 of 14.4ppm to 23,900ppm in the chloride-containing caustic (R1; SRA) stream 36 exiting R1.

• A small amount of ionic liquid was removed by the chloride-containing caustic stream (R1; SRA) 36 exiting the REGEN stage 20. This is evidenced by an increase in P concentration from the caustic (R4; RA) stream 35 from Omg/L supplied into R4 to 10mg/L in the chloride-containing caustic (SRA) stream 36 exiting R1. The effectiveness of filtering the organic ionic liquid streams is illustrated by the following filtration experiments summarized in Examples 1-3 below.

Example 1

- Filtration device: DrM TSD Filter in 316L stainless steel with 32mm diameter candle filter.

- No filter aid or dilution

- Temperature: 51 °C

- G11 M080/30 Filter cloth (polypropylene)

- Feed solids 1.7 g/L

- Maximum filtration pressure: 4 bar

- Filtrate solids after filter cake development, 0.43 g/kg.

- Overall flux 48 L/m ² /hr after 240 minutes

Example 2

- Filtration device: DrM TSD Filter in 316L stainless steel with 32mmm diameter candle filter.

- Coarse perlite AP 70, no dilution of feed with water

- Temperature: 57°C

- Filter Aid body feed ratio 1 : 1

- G11 M080/30 Filter cloth (polypropylene)

- Feed solids 4.3 g/L

- Maximum filtration pressure: 3 bar

- Overall flux 89 L/m ² /hr after 117 minutes

- No visible filtrate solids observed (indicative of <0.1 g/L based on correlation of visual to measurements of previous tests)

Example 3

- Filtration device: DrM TSD Filter in 316L stainless steel with 32mmm diameter candle filter.

- coarse perlite AP 70, no dilution

- Temperature: 58°C

- Filter Aid body feed ratio 2:1

- G11 M080/30 Filter cloth (polypropylene)

- Feed solids 0.7 g/L

- Maximum filtration pressure: 3 bar - Overall flux 378 L/m ² /hr after 23 minutes

- No visible filtrate solids observed (indicative of <0.1 g/L based on correlation of visual to measurements of previous tests)

The above observations show that the filter(s) installed in the apparatus effectively removes circulating particulate matter to prevent crud formation and allow the apparatus to function.

Many modifications may be made to the embodiments of the invention described above without departing from the spirit and scope of the invention.

By way of example, mixers and settlers in the EXTRACT, STRIP and REGEN stages 16, 18, 20 may be any suitable mixers and settlers.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

GLOSSARY