Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
  • D21C11/0078—Treatment of green or white liquors with other means or other compounds than gases, e.g. in order to separate solid compounds such as sodium chloride and carbonate from these liquors; Further treatment of these compounds
• • 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
  • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
  • C01F7/0646—Separation of the insoluble residue, e.g. of red mud
• • 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

Definitions

• the present invention relates to a method for the causticisation of alkaline solutions. More particularly, the method of the present invention relates to a method for the causticisation of alkaline solutions at elevated temperatures.
• 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.
• Aluminium hydroxide is added to the solution as seed to induce the precipitation of further aluminium hydroxide therefrom.
• the precipitated aluminium hydroxide is separated from the caustic aluminate solution, with a portion of the aluminium hydroxide 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.
• TA about 15 to 20 gL "1 to achieve high resultant TC/TA ratios and high lime efficiencies.
• the temperatures used by Cagnolatti are below 105°C, and no regard is had to the speed at which this reaction temperature is attained.
• Dry lime is used, and residence times approximate those of conventional outside causticisation, being 1 to 1 / hours.
• the efficacy of this process is substantially reduced as the TA of the feed liquor is increased.
• Cundiff spent liquors are used because 'the equilibrium . . . is shifted by the presence of sodium aluminate so as to further lower the proportion of soda ash causticised' (column 2, lines 19 to 22). Additionally, Cundiff notes that 'the reaction time does not materially effect the causticising efficiencies in terms of lime utilisation and degree of conversion to soda and therefore is not critical' (column 6, lines 43 to 46).
• Whittington Whittington, B I (1996), The Chemistry of CaO and Ca(OH) 2 relating to the Bayer Process', Hydrometallurgy, 43, 13-35
• Whittington notes the relationship between elevated temperatures and lime efficiencies, stating that "predictions that... increasing reaction temperatures thermodynamically favour CaC0 3 formation have been experimentally verified”.
• Whittington refers to several studies investigating the relationship between temperature and lime efficiency.
• Solymar and Zoldi (Solymar, K and Zoldi, J (1993), Light Metals, 185-194) report causticisation experiments at temperatures ranging from 120 to 240°C, and demonstrate that TC/TA increases with reaction temperature.
• the reaction temperatures studied exceed those used in conventional causticisation (about 100°C)
• the quoted TC/TA ratios at 160°C do not exceed those obtained under conventional conditions.
• Xu (Xu B A (1991 ), "Lime Chemistry in the Bayer Process", PhD Thesis, Murdoch University, and also XU, B A, Giles D E and Ritchie I M (1998), Hydrometallurgy, 48, 205-224) conducted experiments ranging from 25 to 75°C. TC/TA ratios calculated from these results do not exceed those obtainable by conventional caustisation.
• the higher temperatures are selected for the second stage simply to increase the rate of the reaction of the hydrocalumite to calcium carbonate (page 15, lines 1 to 5), and thus to reduce capital costs.
• the ratio of total caustic concentration to total alkali concentration of the alkaline solution when expressed in grams of sodium carbonate per litre of solution, exceeds approximately 0.82, after addition of the lime.
• both the quantity of lime and the alkaline solution are maintained about the reaction temperature before the quantity of lime is added to the alkaline solution.
• the quantity of lime is maintained at a temperature below the reaction temperature before the quantity of lime is added to the alkaline solution, the reaction mixture so produced being rapidly heated to the reaction temperature.
• the reaction mixture reaches the reaction temperature in less than 10 minutes.
• the reaction mixture reaches the reaction temperature in less than 5 minutes.
• the quantity of lime is preferably provided in the form of slaked lime.
• the slaked lime is provided in a highly reactive form, such as slaked lime putty.
• the slaked lime putty has a low particle size and a high solids density.
• the slaked lime has particle size d 50 » 8 - 10 ⁇ m, solids density > 400 gL "1 .
• the reaction temperature is above 110°C.
• the reaction temperature is between about 110°C and 300°C.
• the reaction temperature is between about 110°C and 170°C.
• the preferred residence time of the quantity of lime in the alkaline solution is dependent on the reaction temperature, with higher temperatures corresponding to shorter residence times.
• the residence time of the quantity of lime in the alkaline solution is less than 15 minutes.
• the residence time is less than 10 minutes.
• the residence time is less than 5 minutes.
• the residence time is about 30 seconds.
• the alkaline solution has an initial total alkali concentration, expressed as grams of sodium carbonate per litre of solution, of between about 60 and 350.
• the alkaline solution has an initial total alkali concentration, expressed as grams of sodium carbonate per litre of solution, of between about 100 and 180.
• the alkaline solution is provided in the form of a Bayer process solution.
• the alkaline solution is preferably still provided in the form of a process side-stream as opposed to a digestion solution.
• the alkaline solution is provided in the form of a washer overflow liquor.
• the alkaline solution is provided in the form of a washer feed liquor.
• the method of the present invention has the added advantage of dissolving remaining aluminium containing solids.
• the magnitude of the quantity of lime added to the alkaline solution is dependent upon the level of carbonate in solution and the total alkali concentration of the alkali solution. Typically however, the magnitude of the quantity of lime added to the alkaline solution corresponds to between 5 and 12 grams of calcium oxide per litre of reaction mixture.
• the Bayer process solution is obtained from the Bayer circuit from a point upstream of precipitation.
• the reaction temperature is between about 110°C and 300°C. Preferably still, the reaction temperature is between about 110°C and 170°C.
• the residence time does not exceed 10 minutes. Preferably still, the residence time is less than 5 minutes.
• the Bayer process solution has a total alkali concentration of between 60 and 350 gL "1 .
• the Bayer process solution may be heated in one or more heating stages.
• the or each heating stage involves steam heating.
• the or each heating stage may be performed in a shell and tube type heat exchanger, a plate type heat exchanger, a spiral type heat exchanger. Where the heating stage involves steam, the heating stage may be performed by direct steam injection.
• the reaction vessel is agitated.
• the reaction vessel is provided in the form of a pipe-reactor.
• the method of the present invention may, before the reaction mixture is passed into the reaction vessel, comprise the additional step of:
• the causticised Bayer process solution is returned to the Bayer circuit prior to a solid separation stage to allow the removal of lime residue solids.
• the causticised Bayer process solution is returned to the Bayer circuit prior to a settling stage.
• the causticised Bayer process solution is returned to the Bayer circuit prior to a filtration stage.
• the causticised Bayer process solution is directed to a dedicated settling stage. In an alternate more specific from of the present invention, the causticised Bayer process solution is directed to a dedicated filtration stage.
• the method for causticisation of a Bayer process solution of the present invention includes the step of directing the causticised Bayer process solution to a dedicated solid separation stage to remove lime residue solids, the method may further comprise the step of:
• the method for causticisation of a Bayer process solution of the present invention may, before returning the causticised Bayer process solution to the Bayer circuit, comprise the additional step of:
• the or each cooling stage may be performed in a shell and tube type heat exchanger, a plate type heat exchanger, a spiral type heat exchanger.
• the method for causticisation of a Bayer process solution of the present invention comprises the step of cooling the causticised Bayer process solution in one or more cooling stages
• the method may further comprise the step of: donating heat generated by the or each cooling stage to the or each heating stage.
• At least a portion of the or each cooling stage is performed by flash cooling. Where at least a portion of the or each cooling stage is performed by flash cooling, vapour emitted during flash cooling may be used in the or each heating stage.
• Any of the above described methods may also concurrently remove phosphate from the alkaline solution.
• Figure 1 is a schematic flow sheet showing how a method in accordance with the present invention may be utilised in a Bayer Process circuit
• Figure 2 is a plot of TC/TA ratio against reaction time for a reaction involving the addition of a quantity of slaked lime to a sample of green Bayer process liquor to form a reaction mixture at 145°C, in accordance with a first example of the present invention
• Figure 3 is an X-ray diffraction spectrum of a solid phase isolated from the reaction mixture of Figure 2;
• Figure 4 is a thermogravi metric analysis trace of the solid phase of Figure 3;
• Figure 5 is a composite plot of liquor total caustic to total alkali (TC/TA) ratios against reaction time for reactions involving the addition of a quantity of slaked lime corresponding to 5, 7 and 12 gL "1 , to a sample of green Bayer process liquor with an initial TA of 220 gL "1 , at 145°C in accordance with a second example of the present invention
• Figure 6 is a composite plot of liquor TC/TA ratios against reaction time for reactions involving the addition of a 5 gL "1 of slaked lime to samples of green Bayer process liquor with an initial TA of 220, 240 and 260 gL "1 at 145°C in accordance with a second example of the present invention
• Figure 7 is a composite plot of liquor TC/TA ratios for experiments involving causticisation of green liquor with initial TA 150gL "1 with different samples of slaked lime in accordance with a third example of the present invention.
• Figure 8 compares peak TC/TA ratios at 130°C for experiments involving causticisation of a green liquor with initial TA 150gL "1 with different samples of slaked lime in accordance with a fourth example of the present invention.
• the method of the present invention is described in the context of the causticisation and phosphate reduction of a Bayer process solution, although such should not be seen as limiting the generality of the foregoing description.
• FIG. 1 shows a liquor 10 from a digestion stage 12 being fed into a thickener 14. Underflow 16 from the thickener 14 is passed to a series of washers 18, the overflow 20 from which is obtained from a Bayer circuit as a side stream, and heated to a temperature of about 145°C by way of heaters 22.
• a quantity of slaked lime putty 24 is pre-heated to about 145°C, then added to the heated Bayer process solution to form a reaction mixture in such a manner that the temperature of the reaction mixture rapidly exceeds the atmospheric boiling point of the Bayer process solution.
• the magnitude of the quantity of lime added to the alkaline solution is dependent upon the level of carbonate in solution and the total alkali concentration of the alkali solution. Typically however, the magnitude of the quantity of lime added to the alkaline solution corresponds to between 5 and 12 grams of calcium oxide per litre of reaction mixture.
• the mixture so formed is then passed to a reaction vessel 26, for example a pipe reactor, where it is retained for a period not exceeding about five minutes, thereby generating a causticised Bayer process solution.
• the causticised Bayer process solution is then cooled by way of coolers 28 before being directed to a dedicated solid separation stage 30 to remove lime residue solids and returned to the Bayer circuit.
• the lime residue solids may be collected use as an aid to the filtration of the main liquor stream.
• the vapour 32 generated by the flash cooling of the causticised Bayer process solution is transferred to the heating stage for the pre-causticised Bayer process solution and/or the heating stage for the lime putty.
• a green liquor solution was prepared and maintained at a temperature of 145°C in a 3-litre autoclave. Slaked lime slurry was blown into the autoclave using a large, stainless steel sampling bomb, thereby approximating the operation of plant digesters, and the resulting mixture sampled after 0, 10 and 30 minutes.
• Each sampling involved filtering a liquor sample through a 0.45 ⁇ m ACRODISC filter, analysing the filtrate for total caustic and total alkali by conventional titration methods, then washing and drying the solids at 60°C before analysing such by X- ray diffraction techniques.
• Figure 2 plots TC/TA ratio against reaction time and shows a marked increase in liquor TC/TA ratio, from 0.824 to 0.868, within 10 minutes of the start of the reaction.
• the X-ray diffraction spectrum of Figure 3 indicates that calcium carbonate and tricalcium aluminate hexahydrate are both present in the solid phase of the reaction mixture, and the thermogravimetric analysis shown in Figure 4 confirms the existence of calcium carbonate.
• Example 1 Whilst the experiments of Example 1 illustrate that the method of the present invention is effective in causticising a Bayer process solution beyond the levels achieved by the prior art, the incomplete transfer of lime slurry from the stainless steel bomb renders such unsuitable for quantitative experiments.
• a quantity of slaked lime slurry was sealed in glass ampoules and reacted with 600 mL samples of blow-off liquor at 145°C in a 1 L autoclave.
• the experiment was performed with slaked lime charges corresponding to 5, 7 and 12 gL "1 CaO, and with blow-off liquor with total alkali concentrations of 220, 240 and 260 gL "1 sodium carbonate.
• Samples were taken at 0, 0.5, 2, 5, 10, 20 and 30 minutes after contact between the slaked lime charge and the blow-off liquor was initiated.
• Liquor samples were filtered twice through 0.45 ⁇ m ACRODISC filters, with the filtrate being analysed for total caustic and total alkali by conventional titration methods.
• the solids washed, dried at 60°C then subjected to X-ray diffraction and thermogravimetric analyses.
• Figure 5 is a composite plot of liquor total caustic to total alkali (TC/TA) ratios against reaction time for selected reactions involving the addition of a quantity of slaked lime corresponding to 5, 7 and 12 gL "1 to liquor with a TA of 220 gL "1 .
• the results were selected on the basis of comparable starting TC/TA ratios. Once allowances for different starting TC/TA ratios are made, the TC/TA against time curves for repetition reactions agree within standard error.
• Table 4 summarises lime efficiency data for the experiments represented by Figure 5, the lime efficiency data being calculated from the results of liquor titration analyses for samples taken after 30 seconds, where the TC/TA ratio is about its maximum, and after 30 minutes.
• Figure 6 is a composite plot of liquor TC/TA ratios against reaction time for selected reactions involving the addition of a 5 gL "1 of slaked lime to samples of green Bayer process liquor with initial TA values of 220, 240 and 260 gL "1 . Again, the reactions were selected on the basis of comparability of initial TC/TA ratios.
• Table 5 summarises lime efficiency data for the experiments represented by Figure 6, the lime efficiency data being calculated from the results of liquor titration analyses for samples taken after 30 seconds, where the TC/TA ratio is about its maximum, and after 30 minutes.
• Table 5 shows calculated lime efficiencies decrease from 99% at 220 gL "1 TA to 11% at 260 gL "1 TA, dropping to zero in the latter case after 30 minutes, so alkaline solutions with lower total alkali concentrations afford opportunities for greater increases in TC/TA ratio, and higher lime efficiencies.
• Dilute green liquor solutions were prepared by digesting spent liquor with hydrate. Samples of slaked lime slurry from the Kwinana refinery of Alcoa of Australia Ltd were sealed in glass ampoules, which were then placed in a Parr 1 L autoclave with 600 mL of the filtered prepared dilute green liquor. The autoclave and its contents were then heated to the desired reaction temperature. Upon attainment of the reaction temperature the glass ampoule was broken, releasing its contents into the hot liquor, and the experiment commenced. The range of reaction conditions under which the experiments were undertaken is as follows:
• the liquor samples were filtered twice through 0.45 ⁇ m acrodiscs and analysed by ALIAN titration analysis and for calcia in liquor by ICP.
• the solid reaction products were washed, dried at 60 °C and analysed by XRD.
• Example 2 Tests on the slaked lime used in Examples 1 and 2 indicated that it did not react to completion. Hence the results of Examples 1 and 2 do not necessarily reflect the maximum TC7TA ratios that could be attained using the method of the present invention. Accordingly the experiments of Example 2 were then repeated using standard and de-sanded slaked lime slurries from the Pinjarra refinery of Alcoa of Australia Ltd, under the conditions listed in Table 6.
• the reactivity of the slaked lime also affects lime efficiencies. Despite a slightly higher lime charge, the lime efficiencies obtained using Pinjarra desanded slaked lime are consistently higher than those attained with Kwinana slaked lime. This is due to both the higher TC/TA ratios reached in the Pinjarra desanded experiments as well as the presence of unreacted Ca(OH) 2 in the residue of the experiments with Kwinana slaked lime. Further, different results were generated by the sanded and de-sanded Pinjarra slaked lime samples, the de-sanded samples producing better results. The results generated by both the sanded and de-sanded Pinjarra samples being superior to the Kwinana samples.
• the putty is produced by slaking commercial dry lime in steam. This process produces a very fine (d 50 « 8 - 10 ⁇ m) slaked lime slurry with a high solids density (> 400 gL "1 ).
• Figure 8 compares the maximum TC/TA ratios obtained with each of the slaked lime slurries tested.
• Pinjarra desanded slaked lime is the most reactive of all of the slaked limes tested, producing maximum TC/TA ratios of 0.926 to 0.930.
• the lime putty is as reactive as Pinjarra standard slaked lime, but less reactive than the desanded lime.
• Point Comfort slaked lime is as reactive as Pinjarra standard slaked lime, but not as reactive as Pinjarra desanded slaked lime.
• the Kwinana and Wagerup refineries produced the least reactive slaked lime slurries.
• Point Comfort slaked lime is as reactive as Pinjarra slaked lime before it is de-sanded, but not as reactive as Pinjarra slaked lime after sand removal. In terms of reactivity, Point Comfort slaked lime ranks among the better performers in the suite of slaked limes tested.
• the reactivity of the slaked lime slurry will have a significant effect on the maximum TC/TA ratio at any reaction temperature. Slaked limes that do react completely, leaving unreacted Ca(OH) 2 in the causticiser residue, will produce lower TC/TA ratios than those obtained from highly reactive slaked lime. This difference in reactivity may be due to variations in the control of the slaking process.
• the method of the present invention is highly effective at causticising alkaline solutions, in the form of Bayer process solutions, beyond the levels achievable by the prior art. This is achieved with excellent lime efficiency, by producing calcium carbonate as the dominant product.
• the diminished production of tricalcium aluminate hexahydrate arising from the method of the present invention will result in decreased residual red mud alkalinity, reducing the environmental difficulties associated therewith.
• the calcium carbonate byproduct of the method of the present invention will be of a particle size suitable for use as a filter aid.
• the highly desirable short residence times of the present invention offer a considerable advantage over the residence times associated with conventional causticisation, being over an hour.
• the fine CaC0 3 produced by the method of the present invention may be utilised as a low-cost filter aid, replacing the tricalcium aluminate hexahydrate filter aid presently used and thereby further reducing the lime requirement.
• the method of the present invention may also concurrently remove phosphate from the alkaline solution.
• lime residue solids produced by the method of the present invention may be re-calcined and utilised as a source of lime in the method of the present invention.
• the lime residue solids produced by the present invention may be utilised in one ore more subsequent causticisation operations downstream from the point of application of the method of the present invention.

Landscapes

• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Life Sciences & Earth Sciences (AREA)
• Geology (AREA)
• Inorganic Chemistry (AREA)
• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• General Life Sciences & Earth Sciences (AREA)
• Geochemistry & Mineralogy (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
• Treating Waste Gases (AREA)

Applications Claiming Priority (3)

Publications (2)

Family

ID=3814183

Family Applications (1)

Country Status (5)

Families Citing this family (8)

Family Cites Families (5)

• 1999
  • 1999-04-23 AU AUPP9982A patent/AUPP998299A0/en not_active Abandoned
• 2000
  • 2000-04-20 WO PCT/AU2000/000346 patent/WO2000064812A1/en active Application Filing
  • 2000-04-20 BR BRPI0009920-1A patent/BR0009920B1/pt not_active IP Right Cessation
  • 2000-04-20 CA CA002370956A patent/CA2370956C/en not_active Expired - Fee Related
  • 2000-04-20 EP EP00918583A patent/EP1194376A4/de not_active Withdrawn

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events