Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/46—Purification of aluminium oxide, aluminium hydroxide or aluminates
  • C01F7/47—Purification of aluminium oxide, aluminium hydroxide or aluminates of aluminates, e.g. removal of compounds of Si, Fe, Ga or of organic compounds from Bayer process liquors
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/14—Aluminium oxide or hydroxide from alkali metal aluminates
  • C01F7/144—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/46—Purification of aluminium oxide, aluminium hydroxide or aluminates
  • C01F7/47—Purification of aluminium oxide, aluminium hydroxide or aluminates of aluminates, e.g. removal of compounds of Si, Fe, Ga or of organic compounds from Bayer process liquors
  • C01F7/473—Removal of organic compounds, e.g. sodium oxalate

Definitions

• the present invention relates to a method for controlling the precipitation of alumina from a Bayer process solution.
• the Bayer process is widely used for the production of alumina from alumina- containing ores such as bauxite.
• the process involves contacting alumina- containing ores with recycled caustic aluminate solutions at elevated temperatures in a process commonly referred to as digestion. Solids are removed from the resulting slurry and the solution cooled to induce a state of supersaturation.
• Alumina is added to the solution as seed to induce precipitation of further aluminium hydroxide therefrom.
• the precipitated alumina is separated from the caustic aluminate solution (known as spent liquor), with a portion of alumina being recycled to be used as seed and the remainder recovered as product.
• the remaining caustic aluminate solution is recycled for further digestion of alumina- containing ore.
• the precipitation reaction can be generally represented by the following chemical equation with reference to the precipitation of aluminium hydroxide.
• a similar equation may be prepared for the precipitation of aluminium oxyhydroxide:
• Liquor carbonation is a technique used in the alumina industry to convert hydroxide to carbonate, and has been used to increase the precipitation yield of alumina.
• liquor carbonation necessitates the excessive purchase cost of lime which is required to regenerate caustic from sodium carbonate. Further, the recausticisation step is inefficient and does not result in complete regeneration of the caustic.
• the hydroxide extraction technology is based on cation exchange principles and involves the exchange of a proton from an acidic lipophilic reagent (HA) dissolved in a non- aqueous solvent, for an aqueous sodium ion at high pH in accordance with the following equation:
• US6322702 indicates that the alumina industry could use the invention disclosed therein to recover NaOH from waste streams. They assert that a secondary advantage of the invention is that removal of NaOH will reduce the pH of the stream below 12.5 which can then more easily be disposed of.
• US6322702 clearly is referring to the disposal of Bayer processing waste streams, such as "red mud", which is the highly alkaline bauxite residue separated after alumina extraction with caustic.
• the teachings of the patent are not directed at Bayer liquor, which is continuously recycled within the plant between the digestion and precipitation circuits.
• the document does not teach or infer use of the invention to drive the precipitation reaction to increase alumina precipitation.
• a method for controlling the precipitation of alumina from a Bayer process solution comprising the steps of:
• alumina shall be taken to include, without limitation, any form of aluminium hydroxide, aluminium oxyhydroxide or aluminium oxide.
• the extraction of a metal cation into the substantially water- immiscible solution leads to a concomitant transfer of a cation from the substantially water-immiscible solution to the Bayer process solution.
• the cation neutralises hydroxide ions present thereby enhancing alumina precipitation.
• the present invention offers distinct advantages over methods employing carbonation to reduce hydroxide concentrations in Bayer process solutions, as carbonation reduces TC without affecting TA, but the present invention reduces both the TC and TA of the Bayer process solution.
• the method comprises the further step of:
• Alumina is more soluble in alkaline solutions than in water and advantageously, the reduction of hydroxide concentration in the Bayer process solution can increase precipitation of alumina.
• the method of the present invention may be utilised to control the form of precipitated alumina and influence the formation of forms such as boehmite, gibbsite, bayerite, doyleite and nordstrandite.
• the alumina may be a mixture of any of the preceding forms.
• the method comprises the further step of:
• the optimal seeding rate will depend on many factors, including the seed and liquor properties and the design of the precipitation circuit, and may be greater than 50 gL "1 and preferably, in the range of 50 to 1300 gl_ '1 .
• the present invention can negate the need to reduce the temperature of Bayer process solutions to encourage supersaturation. It is known that precipitation rates decrease with temperature. In a gibbsite precipitation circuit, precipitation commences at about 90 0 C and ends at about 60 0 C at the completion of the precipitation phase. Without being limited by theory, it is believed that the method of the present invention may permit precipitation of alumina at temperatures as high as the boiling point of the liquor at that pressure.
• the present invention may be utilised to increase precipitation yields beyond current limits without initially increasing TC in digestion. It may provide means of inducing supersaturation without appreciable liquor cooling.
• a sodium/aluminate ion pair exists on or near the surface of precipitated alumina and hinders further deposition of alumina onto the surface.
• removing sodium from the Bayer process solution is believed to increase alumina precipitation.
• the present invention does not advocate a measurable reduction in solution pH as is specified in US6322702.
• Bayer liquor pH is above measurable limits (>14) and it has been discovered that a significant increase in precipitation yield can be obtained by instigating a decrease in caustic concentration by solvent extraction, whereby liquor pH is still kept well above a value of 14.
• the extractant is provided in the form of a weak acid.
• the extraction of the metal ion into the substantially water-immiscible solution will be accompanied by the transfer of a proton from the substantially water-immiscible solution into the Bayer process solution.
• the metal cation is provided in the form of a sodium ion.
• the weak acid extractant comprises at least one polar group with an ionisable proton with a pKa of between about 9 and about 13.
• the extractant is preferably a straight chain, branched chain or cyclic hydrocarbon, a halogenated hydrocarbon, an aliphatic or aromatic ether or alcohol with more than 6 carbon atoms.
• the extractant comprises an alcohol or phenol functional group.
• Suitable extractants include 1 H,1H-perfluorononanol, 1 H,1 H,9H-hexadecafluorononanol, 1 ,1 ,1-trifluoro-3-(4-terf-octylphenoxy)-2- propanol, 1 ,1 ,1-trifluoro-2-(p-tolyl)/sopropanol, 1-(p-tolyl)-2,2,2-trifluoroethanol, hexafluoro-2-(p-tolyl)isopropanol, 2-(methyl)-2-(dodecyl)tetradecanoic acid, 3-(perfluorohexyl)propenol and 1-(1 ,1 ,2,2-tetrafluoroethoxy)-3-(4-te/Y- octylphenoxy)-2-propanol, te/t-octylphenyl, para-nonylphenol,
• substantially water-immiscible solution may form the extractant.
• the acidic form of the extractant is substantially insoluble in water.
• the deprotonated form of the extractant is substantially insoluble in water.
• the extractant concentration will depend on a number of factors including the intended amount of induced supersaturation which in turn will be influenced by the temperature at which precipitation will be initiated.
• the degree of deprotonation in the extraction step will depend on the acidity of the ionisable proton (as well as the pH and salt content of the Bayer process solution).
• phase transfer catalysts may enhance extractions rates.
• Suitable phase transfer catalysts may be selected from lipophilic quaternary ammonium or phosphonium salts or organic macrocycles such as crown ethers, calixarenes, calixarene-crown ethers, spherands and cryptands.
• Specific complexing ligands may be added to the organic mixtures, to either synergistically enhance sodium ion extraction, and/or to additionally extract impurities from Bayer liquor and enhance precipitation in a secondary manner.
• the step of contacting the Bayer process solution with a substantially water- immiscible solution comprising an extractant may be performed in a process side stream.
• the substantially water-immiscible solution is an organic liquid, a combination of organic liquids or an ionic liquid.
• the organic liquid is substantially non-polar.
• the organic liquid is a high boiling organic liquid with a low vapour pressure at Bayer process temperatures.
• the organic liquid is alkaline stable.
• the organic liquid is preferably a straight chain, branched chain or cyclic hydrocarbon, a halogenated hydrocarbon, an aliphatic or aromatic ether or alcohol with more than 4 carbon atoms.
• Suitable solvents include benzene, toluene, xylene, stilbene, 1-octanol, 2-octanoI, 1-decanol, /so-octyl alcohol (such as that commercially available as Exxal 8 from ExxonMobil), iso-nonylalcohol (such as that commercially available as Exxal 9 from ExxonMobil), /so-decanol, iso- tridecanol, 2-ethyl-1-hexanol, kerosene and other hydrocarbons commercially available under the names Escaid 100, Escaid 110, Escaid 240, Escaid 300, lsopar L, lsopar M, Solvesso 150 from ExxonMo
• the organic liquid solvates the extractant in both its acid and sodium salt forms.
• volume of substantially water-immiscible solution relative to the volume of the Bayer process solution may vary according to the manner in which both the Bayer process solution and the substantially water- immiscible solution are contacted and the loading of the extractant in the substantially water-immiscible solution.
• the present invention permits the precipitation of alumina at higher temperatures than conventional precipitation techniques.
• supersaturation is only created by lowering the temperature to below about 90 0 C. It is known to use a series of precipitators where each subsequent precipitator operates at a lower temperature, and where the last tank out of, for example, seven in series is at 60-70 0 C. Lowering the temperature simultaneously reduces precipitation kinetics.
• the contact time between the Bayer process solution and the organic phase should be sufficient for reaction to occur between the extractant and the metal cations to form a metal cation-depleted aqueous phase and a proton-depleted organic phase.
• Said contact time will be influenced by many factors including the pKa of the ionisable proton on the extractant, the pH of the aqueous phase, the volumes of the aqueous and organic phases, the temperature, the concentration of the extractant and sodium ions, the total alkalinity, the total caustic concentration, the extent of agitation and the presence of other species in the liquor.
• the volumes of the Bayer process solution and the substantially water-immiscible solution need not be the same. It should be appreciated that where the method is performed as a countercurrent flow or continuous processing, volumes of the phases are less critical than with batch methods.
• the method comprises the further step of:
• the step of separating the Bayer process solutions and the substantially water-immiscible solution may be performed by any method known in the art including centrifugation.
• the method comprises the further steps of:
• the stripping solution may be provided in form of water or a Bayer process liquor including condensate or lake water.
• the stripping solution has a pH of at least 5.
• the method comprises the further steps of: separating the stripping solution and the substantially water-immiscible solution.
• the stripping solution, after contact with the substantially water- immiscible solution can be re-used in subsequent steps in the Bayer process or in subsequent stripping steps.
• the aqueous solution of sodium hydroxide may be re-used in other stages of the Bayer circuit. Depending on the concentration of sodium hydroxide, the aqueous solution may need to be pre-treated prior to subsequent use.
• the step of stripping the sodium ions and subsequent regeneration of hydroxide may require no further chemicals for recausticisation.
• the method comprises the further step of:
• an organic solvent comprising an extractant in the form of a weak acid for controlling the precipitation of alumina from Bayer process solutions, wherein the precipitation of alumina comprises the steps of:
• a method for controlling the precipitation of alumina from a Bayer process solution comprising the steps of:
• the bauxite may be provided in the form of gibbsitic bauxite, boehmitic bauxite, diasporic bauxite or any combination thereof.
• Figure 1 a is a schematic flow sheet of a Bayer Process circuit
• Figure 1 b is a schematic flow sheet showing how a method in accordance with a first embodiment of the present invention may be utilised in a Bayer Process circuit
• Figure 1c is a schematic flow sheet showing how a method in accordance with a second embodiment of the present invention may be utilised in a Bayer Process circuit
• Figure 1d is a schematic flow sheet showing how a method in accordance with a third embodiment of the present invention may be utilised in a Bayer Process circuit
• Figure 2 is a graph showing the effect of seed loading on the precipitation of alumina from Bayer liquor
• Figure 3 is a graph showing the effect of extractant concentration on alumina yield
• Figure 4 is a graph showing the effect of temperature on the precipitation yield of alumina for extraction using ferf-octylphenol and hexadecafluorononanol in 1-octanol;
• Figure 5 is a graph showing the extraction of caustic from plant liquor as a function of time using fe/f-octylphenol/1 -octanol and hexadecafluorononanol/1- octanol mixtures;
• Figure 6 is a graph showing the stripping of soda from solutions of tert- octylphenol/1 -octanol and hexadecafluorononanol/1-octanol into the aqueous phase as a function of time;
• Figure 7 is a graph showing the recovery of soda from terf-octylphenol/1 - octanol as a function of the number of volume contacts with the organic phase
• Figure 8 is a graph showing the recovery of soda from hexadecafluorononanol/1-octanol as a function of the number of volume contacts with the organic phase;
• Figure 9 is a graph showing the extraction of caustic and the precipitation of hydrate from plant liquor after treatment with varying concentrations of para- nonylphenol in 1-octanol;
• Figure 10 is a graph showing the extraction of caustic and the precipitation of hydrate from plant liquor after treatment with varying concentrations of tert- octylphenol in 1-octanol;
• Figure 11 is a graph comparing the extraction capability of para-nonylphenol and ferf-octylphenol in 1-octanol as a function of extractant concentration
• Figure 12 is a graph comparing the stripping efficiency of para-nonylphenol/1- octanol and terf-octylphenol/1-octanol as a function of temperature.
• the invention focuses on the control of alumina precipitation in the Bayer process by extraction of sodium ions from a Bayer process solution into a water immiscible solvent. By careful manipulation of the extraction reaction, the precipitation of alumina from aluminate solutions may be controlled.
• Seeded precipitation experiments have shown clearly that use of the present invention during alumina precipitation leads to an increase in precipitation yield compared to identical experiments performed without solvent extraction. Without being limited by theory, it is believed that seeding is advantageous for the control of precipitation properties such as particle sizes and precipitation rates.
• Figure 1a shows a schematic flow sheet of the Bayer process circuit for a refinery using a single digestion circuit comprising the steps of:
• the liquor 24 is contacted in a solvent extraction apparatus 26 with a solution of an organic solvent 25 comprising an extractant.
• the aqueous layer 28 and the organic layer 30 are separated and the aqueous layer 28 seeded to induce alumina precipitation 32.
• the organic layer 30 is contacted 34 with an aqueous solution 36 to back extract sodium ions from the organic layer 30 to the aqueous solution 36.
• the aqueous solution of increased causticity 38 may then be used in the causticisation of further bauxite or in other places in the circuit as appropriate such as, for example, as a pre-treatment step in the washing of bauxite before digestion to remove impurities or in the washing of seed or oxalate.
• Back extraction of the organic layer 30 results in regeneration of the protonated form of the extractant.
• the extractant may then be re-used in further extraction steps.
• the liquor 20 is contacted in a solvent extraction apparatus 26 with a solution of an organic solvent 25 comprising an extractant.
• the aqueous layer 28 and the organic layer 30 are separated and the aqueous layer 28 seeded to induce alumina precipitation 32.
• the organic layer 30 is contacted 34 with an aqueous solution 36 to back extract sodium ions from the organic layer 30 to the aqueous solution 36.
• the aqueous solution of increased causticity 38 may then be used in the causticisation of further bauxite or in other places in the circuit as appropriate such as, for example, as a pre-treatment step in the washing of bauxite before digestion to remove impurities or in the washing of seed or oxalate.
• Back extraction of the organic layer 30 results in regeneration of the protonated form of the extractant.
• the extractant may then be re-used in further extraction steps.
• the liquor 20 is seeded 21 and contacted in a solvent extraction apparatus 26 with a solution of an organic solvent 25 comprising an extractant.
• the precipitated alumina 27 is removed and the aqueous layer and the organic layer 30 separated.
• the organic layer 30 is contacted 34 with an aqueous solution 36 to back extract sodium ions from the organic layer 30 to the aqueous solution 36.
• the aqueous solution of increased causticity 38 may then be used in the causticisation of further bauxite or in other places in the circuit as appropriate such as, for example, as a pre-treatment step in the washing of bauxite before digestion to remove impurities or in the washing of seed or oxalate.
• Back extraction of the organic layer 30 results in regeneration of the protonated form of the extractant.
• the extractant may then be re-used in further extraction steps.
• Gibbsite was used as seed for all experiments involving precipitation and these were conducted as batches using polypropylene bottles of 250 ml_ capacity positioned in a rotating water bath. Unless stated otherwise, 10 g of seed was used per 100 mL of liquor for precipitation experiments.
• reaction temperature was maintained at 70 0 C for all experiments unless stated otherwise. Both the aqueous and organic solutions were preheated to the reaction temperature separately prior to contact.
• precipitated alumina was collected by filtration, washed with hot water, dried in an oven at 105 °C and weighed.
• the aqueous filtrate was stabilised by the addition of sodium gluconate to prevent gibbsite precipitation from solution upon liquor cooling to room temperature and analysed for TC and alumina content by titration and ICP.
• Plant liquor was adjusted using standard methods, to give an A/TC of 0.5 and a TC of 200.
• Precipitation experiments were performed using 10 g of seed and 0.5 M of the extractants fe/t-octylphenol and hexadecafluorononanol. All other conditions were identical to those of previous experiments.
• solution A/TC ratios should not be relied upon to give an indication of the extent of precipitation. Under normal precipitation conditions with no solvent extraction, the A/TC ratio decreases because alumina leaves the solution and the solution TC increases slightly (the caustic level remains constant but solution volume decreases slightly due to alumina precipitation). However, in the case of sodium ion extraction, the TC and the alumina concentration decrease as a result of precipitation. In effect, the A/TC ratio may not change much but the level of precipitation could still be significant.
• the precipitation yield of gibbsite as a function of time is shown in Table 7.
• the starting A ⁇ O 3 concentration was 99-107 gL '1 and the starting TC was 201-214 gL '1 .
• the A/TC ratio was kept constant at 0.50.
• a bulk organic phase generated by contacting 0.75 M fenf-octylphenol in 1- octanol with plant liquor from ex-precipitation, was used in these experiments.
• the organic phase was analysed for Na content and found to contain 0.623 M of Na expressed as 33.0 gl_ '1 of Na 2 CO 3 .
• the stripping experiments were performed at 70 0 C in a 3 L capacity stainless steel reactor, equipped with a four blade impeller. The impeller was rotated at 450 rpm for 10 min to affect phase contact.
• the organic (initially 600 imL) and aqueous phases were preheated to 70 0 C. Following contact, the phases were separated and a sub-sample of the aqueous phase was withdrawn for analysis. The remaining aqueous phase was contacted with twice the volume of fresh loaded organic and the procedure repeated until a total of six independent contacts, using the same aqueous strip, had been conducted.
• a bulk organic phase generated by contacting 0.50 M hexadecafluorononanol in 1-octanol with plant liquor from ex-precipitation, was used in these experiments.
• the organic phase was analysed and found to contain a sodium content of 13.24 gl_ "1 expressed as Na 2 CO 3 .
• the stripping experiments were conducted in polypropylene bottles using a total volume of 200 ml_ and a time of 24 hr.
• a volume ratio of organic:aqueous of 2:1 consisted of 135 ml_ of organic to 65 ml_ of aqueous phases.
• the temperature was held constant at 90 0 C and the initial soda content of the lake water was 33.6 gl_ '1 expressed as Na 2 CO 3 .
• Trials were conducted using para-nonylphenol under conditions designed to simulate more closely the situation that would occur in the adoption of the present invention at a refinery where spent liquor would be supersaturated by solvent extraction of soda and then fed through a seeded precipitation circuit in the absence of organic solvent.
• plant liquor with an A/TC value of 0.5 was contacted with specified quantities of the extractant in 1-octanol (1:1 ratio of organic phase:aqueous) in the absence of seed. After a contact time of 10 minutes with rapid stirring, the phases were separated and the treated plant liquor was then seeded and allowed to form precipitate for 24 hr at 70 0 C.
• mixtures of para-nonylphenol, which is a liquid at room temperature, in 1-octanol showed much improved phase separation behaviour, and lower viscosity, after mixing with plant liquor than those of fe/ ⁇ -octylphenol and hexadecafluorononanol in 1-octanol.
• These factors enable para-nonylphenol to be used in higher concentrations in 1-octanol than either ferf-octylphenol or hexadecafluorononanol.
• para-nonylphenol appears to be a more versatile extractant than tert- octylphenol as solutions maybe prepared at higher concentrations, it is compatible with more industrial solvents and caustic can be recovered from it more efficiently by stripping into an aqueous phase than it can for ferf-octylphenol.
• the tests were conducted in plastic bottles, positioned in a bottle roller placed in a water bath at 70 0 C for a period of 24 hr.
• the ratio of organic phase to water was 1:1 and the initial concentration of soda in the organic phase was determined by acid stripping.
• the results for the three tests conducted are presented in Table 14 and show that soda can be stripped with high efficiency from the Exxal 8/diluent mixtures using 1:1 volume ratios of organic to aqueous phase.
• Organic solutions were prepared in bulk by contacting plant liquor with 0.6 M tert- octylphenol in 1 -octanol and by contacting plant liquor with 0.6 M para- nonylphenol in 1 -octanol.
• the total soda content of each organic phase was determined by stripping the phase with dilute nitric acid followed by ICP- OES analysis.
• Each of the organic mixtures was then contacted with refinery lake water for three hours at 70 °C or with de-ionised water under the same conditions.
• the volume ratio of organic to aqueous phase was 4:1 and the experiments were conducted in duplicate. The results are presented in Table 15.
• Table 15 The recovery of soda from te/f-octylphenol/1 -octanol and para-nonylphenol/1- octanol mixtures by stripping with refinery lake water and de-ionised water.

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