Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/62—Carbon oxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
  • B01D53/80—Semi-solid phase processes, i.e. by using slurries
• • 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
  • C01F11/00—Compounds of calcium, strontium, or barium
  • C01F11/18—Carbonates
• • 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
  • C01F7/141—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by neutralisation with an acidic agent
  • C01F7/142—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by neutralisation with an acidic agent with carbon dioxide
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2251/00—Reactants
  • B01D2251/30—Alkali metal compounds
  • B01D2251/304—Alkali metal compounds of sodium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2251/00—Reactants
  • B01D2251/60—Inorganic bases or salts
  • B01D2251/604—Hydroxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2257/00—Components to be removed
  • B01D2257/50—Carbon oxides
  • B01D2257/504—Carbon dioxide
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2258/00—Sources of waste gases
  • B01D2258/02—Other waste gases
  • B01D2258/0283—Flue gases
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
  • Y02C20/00—Capture or disposal of greenhouse gases
  • Y02C20/40—Capture or disposal of greenhouse gases of CO2
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/141—Feedstock
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/151—Reduction of greenhouse gas [GHG] emissions, e.g. CO2

Definitions

• the present invention relates a carbon dioxide sequestration process.
• the present invention relates particularly though not exclusively to a carbon dioxide sequestration process to reduce greenhouse gas emissions from an alumina refinery.
• the Bayer process has been used to recover alumina values from bauxitic ores for over a century.
• the process centres on the following reversible equations, for gibbsitic and boehmitic or diasporic ores, respectively (1):
• FIG. 1 A schematic flowsheet showing a basic implementation of a traditional Bayer Process is illustrated in Figure 1.
• Blended bauxite ore is first mixed with a portion of the recycled spent liquor and subjected to grinding to reduce particle size.
• the resultant slurry is then treated via a process known as "desilication” or “slurry holding” to remove soluble silica minerals present in the bauxite, typically in the form of insoluble sodium aluminosilicates.
• the desilicated slurry is then mixed with the remainder of the spent liquor and the alumina values of the bauxite extracted via a process referred to as "digestion".
• digestion the conditions are manipulated so as to drive equation (1) or (2) towards the right hand side.
• the free caustic dissolves the aluminous mineral from the bauxite to form a concentrated sodium aluminate solution leaving behind a mud residue of undissolved minerals and impurities, principally inert iron oxides, hydroxides, (oxy)hydroxides, titanium oxides and silicious compounds.
• the mud residue is often red in appearance due to the presence of the iron minerals and is thus commonly referred to as "red mud".
• filter aid acts to prevent cloth blinding by the continuous formation of a bed of solids which trap the mud particles whilst still allowing the free flow of liquor through the interstices of the bed.
• An ideal filter aid will be cheap, chemically inert, and of such a size that it can trap the mud particles, and not restrict the flow of liquid, or contribute to blinding of the filter cloth themselves. In most alumina refineries, this role is performed by tricalcium aluminate (also referred to as TCA, C3A or C3AH6).
• the mud washing circuit relies on a counter-current decantation process to recover as much sodium aluminate as possible for re-use to minimise loss of alumina and caustic values and to cleanse the mud residue so that it can be disposed of in an environmentally acceptable manner.
• the washer overflow that subsequently exits the first stage mud washing tank is either directed to the settling tanks, or mixed with the settler overflow liquor to form clarified pregnant liquor.
• the washed mud residue from the final stage in the mud washing circuit is typically pumped to a mud disposal lake.
• the counter-current mud washing circuit is fed with wash water, typically fresh water, condensate (condensed steam) or recycled water from the mud disposal lake
• the precipitated gibbsite is separated via hydrocyclones, thickeners or filters.
• the remaining liquor, after evaporation to remove excess water that has entered the process with the bauxite and various washing steps is referred to as "spent liquor" and will have aluminate ions and hydroxyl ions present in an amount that depends on the temperature, seed surface area and residence time of the precipitation stage.
• Precipitation is favored by conditions that increase the supersaturation of the liquor, such as reducing the temperature, addition of gibbsite seed, increasing the concentration of aluminate ions, or diluting the solution.
• the spent liquor is recycled to digestion.
• the spent liquor that is recycled to digestion has dissolved alumina present in it.
• gibbsite crystals formed during precipitation are classified according to size with product grade material being calcined in a rotary kiln or fluidized bed calciner furnace whilst undersized particles are used as "seeds" which aid in the precipitation of gibbsite crystals during the precipitation stage.
• gibbsite is dehydroxylated to form alumina.
• carbon dioxide is generated as a byproduct of the combustion of fossil fuels used to run the calciners.
• Carbon dioxide is also emitted in the stack gases produced from the power house which is operated to provide electricity to the alumina refinery and may also, in some alumina refineries, be generated by the lime kilns.
• the primary goal of the Bayer process is to economically extract the maximum amount of alumina values (Al) from the bauxite fed to digestion into solution and then completely recover this dissolved alumina from the solution in the form of gibbsite during precipitation.
• the upper limit of the refinery's precipitation yield is set by the difference between the solubility of alumina in a particular liquor at the digestion temperature and the solubility of alumina in that
• One of the main avenues for alumina loss in an alumina refinery is in the liquor that is pumped with the mud residue from the settling tanks into the mud washing circuit.
• This liquor is supersaturated pregnant liquor having effectively the same concentration of aluminate ions as the pregnant liquor sent to precipitation.
• the liquor that overflows each stage in the counter- current mud washing circuit becomes progressively more diluted and cooler with wash water. This effectively increases the supersaturation of the liquor, encouraging precipitation of gibbsite in accordance with equation (1 ).
• the mud particles in the residue have a high surface area that further encourages such precipitation of gibbsite in the mud washing circuit.
• any alumina that precipitates in the mud washing circuit in this manner is lost, as is any dissolved alumina in the liquor reporting to the mud disposal lake.
• Another avenue for alumina loss is alumina incorporation into TCA filter aid which is utilized during clarification to remove suspended fine solids. Over time, the TCA filter aid becomes contaminated with impurities and becomes "spent". TCA is relatively cheap and simple to produce. Under appropriate conditions, caustic aluminate solutions will react with calcium from a suitable source such as slaked lime to form thermodynamically stable and sparingly soluble TCA. This reaction is utilised most commonly in the alumina industry to produce TCA crystals of a controlled particle size for use as a filter aid.
• spent TCA filter aid is pumped to the red mud disposal area and can represent a loss of alumina values in the order of 40 to 90,000 tonnes per annum for a typical refinery. In this way, spent filter aid typically represents a significant portion of the "solid alkalinity" that is stored or disposed of by alumina refineries.
• a carbon dioxide sequestration process comprising the steps of:
• RO/AU a) introducing a source of carbon dioxide to a caustic aluminate solution to form a first treated stream comprising carbonate ions in solution and aluminium hydroxide in solid form;
• step b) subjecting the first treated stream of step a) to solid/liquid separation to recover alumina values in the form of aluminium hydroxide and produce a first clarified treated stream;
• step b) mixing the first clarified treated stream of step b) with tricalcium aluminate to form a second treated stream comprising calcium carbonate in solid form, aluminate ions in solution, and hydroxyl ions in solution; and,
• step d) subjecting the second treated stream of step c) to solid/liquid separation to remove calcium carbonate within which carbon dioxide has been sequestered, and produce a second clarified treated liquor stream.
• step c) is conducted at a temperature not less than 50°C. In another form, step c) is conducted at a temperature not less than 50°C and not greater than the atmospheric boiling point of the first treated stream. In one form, the first clarified treated stream of step b) is heated such that step c) is conducted at a temperature in the range of 50°C and not greater than the atmospheric boiling point.
• the caustic aluminate solution is a Bayer liquor.
• the Bayer liquor is one or more of; a spent Bayer liquor, an overflow stream from a mud washing stage, or a stream of lake water.
• the process further comprises the step of returning the second clarified treated stream of step d) to an alumina refinery at a location downstream of a digester and upstream of a precipitator.
• the caustic aluminate solution has an 'S' concentration of between 20 and 100 g/L.
• the tricalcium aluminate used in step c) is TCA filter aid or spent TCA filter aid.
• Figure 1 is a simplified conceptual flow diagram of a basic implementation of a traditional prior art Bayer Process
• Figure 3 illustrates graphically the results of Example 1 as a comparison of C/S over time for both the RWB and AC series of tests;
• Figure 4 graphically illustrates the results for the matrix test program of Example 2 showing the final C at a constant 'S' of 17.9 g/L at 30°C, 60°C and 95°C;
• Figure 5 graphically illustrates the results for the matrix test program of Example 2 showing the final 'C at a constant 'S' of 37.1 g/L at 30°C, 60°C and 95°C;
• Figure 6 graphically illustrates the results for the matrix test program of Example 2 showing the final C at a constant 'S' of 56.5 g L at 30°C, 60°C and 95°C;
• Figure 7 graphically illustrates the results for the matrix test program of Example 2 showing the final C at a constant 'S' of 81.4 g/L at 30°C, 60°C and 95°C;
• 'C refers to the caustic concentration of the liquor, this being the sum of the sodium aluminate and sodium hydroxide content of the liquor expressed as equivalent g L concentration of sodium carbonate.
• 'A/C is thus the ratio of alumina concentration to caustic concentration.
• Free caustic is C-A (the caustic concentration minus the alumina concentration) with C and A each being expressed as equivalent g/L concentration of sodium carbonate.
• Lake water is the clarified liquor stream that is returned to the refinery from the mud disposal lake (if used) mixed together with collected rainwater. Lake water typically has the lowest A of any liquor stream. The lake water typically has a high carbonate concentration due to reaction of the lake water with carbon dioxide from the atmosphere.
• S refers to the soda concentration or more specifically to the sum of “C” and the actual sodium carbonate concentration, this sum once again being expressed as the equivalent g/L concentration of sodium carbonate.
• S-C sala concentration minus caustic concentration gives the actual concentration of sodium carbonate (Na 2 C0 3 ) in the liquor, in g/L.
• a Bayer liquor ' s carbonate impurity level is expressed in terms of the caustic to soda ratio, or 'C/S'.
• a fully causticised (carbonate-free) Bayer process liquor will possess a C/S ratio of 1.00.
• aluminium hydroxide is used throughout this specification, to refer to crystalline or amorphous compounds consisting of aluminium ions and hydroxide ions.
• 'aluminium hydroxide' is 'gibbsite' which is aluminium trihydroxide having the general formula of Al(OH) 3 .
• Gibbsite is also sometimes referred to in the literature as "hydrate” or “alumina trihydrate” or “aluminium trihydroxide” and sometimes expressed using the chemically incorrect formula A1 2 0 3 3H 2 0.
• the term 'aluminium hydroxide' is also broad enough to cover 'boehmite' which is an aluminium monohydroxide
• seed crystals' refers to particles generally with a size smaller than a nominal product size.
• the function of seed crystals is two-fold. Firstly, the seed crystals promote/enhance the production of gibbsite and secondly, the seed crystals encourage the growth of larger crystals
• 'Calcite' is calcium carbonate (CaC0 3 ).
• 'TCA' is tricalcium aluminate Ca 3 [Al(OH) 6 ] 2 which is also commonly written using the formula 3CaO.AI 2 0 3 .6H 2 0, (TCA6) or C3AH6 in cement industry notation.
• TCA is available as a side-product of causticisation in many alumina refineries. When TCA is made using plant liquors, it may contain impurities by incorporation of anionic impurities present in the plant liquor into the lattice.
• Synthetic TCA is a material that is generated in pure sodium aluminate solutions rather than using plant liquors and is thus pure tricalcium aluminate.
• Spent filter aid refers to TCA that has been used as a filter aid in an alumina refinery and is the filter aid is dumped on a periodic basis when a cycle of operation of pressure filtration is completed.
• spent filter aid retains the chemical formula of TCA and is likely to be contaminated with fine red mud solids.
• S refers to the sum of all sodium salts in solution, expressed as the equivalent concentration in g/L of sodium carbonate.
• the term 'autoprecipitation' refers generally to the growth of aluminium hydroxide not contributing to the production of alumina.
• a carbon dioxide sequestration process (10) is now described with reference to Figure 2.
• a source of carbon dioxide ( 12) is introduced to a caustic aluminate solution (14) in a first reaction vessel (18) to form a first treated stream (20) comprising carbonate ions in solution and aluminium hydroxide in solid form.
• the source of carbon dioxide (12) may be introduced to the first reaction vessel (18) using any suitable means, for example a sparging system.
• the source of carbon dioxide may be of any purity. Suitable sources include carbon dioxide present in the flue or 'stack gas' produced by a power station or calciners or carbon dioxide present in exhaust or "stack gas" produced by a lime kiln.
• the carbon dioxide gas reacts with sodium hydroxide present in the dilute Bayer liquor stream to produce sodium carbonate according to the following reaction:
• the carbon dioxide gas may react with sodium hydroxide present in the dilute Bayer liquor stream to produce sodium bicarbonate.
• sodium bicarbonate Below pH of 10.5 the formation of sodiumbicarbonate becomes significant and thus the mode of operation for precipitation of sodium carbonate in preference to sodium bicarbonate would be operating above a pH of 10.5.
• the carbon dioxide also reacts with sodium aluminate present in the dilute Bayer liquor to produce including sodium carbonate and gibbsite according to the following reaction:
• the first treated stream (20) is then subjected to solid/liquid separation in a suitable first solid/liquid separator (22) to recover alumina values in the form of aluminium hydroxide (24) and produce a first clarified treated stream (26).
• the first clarified treated stream (26) is then be mixed with a source of tricalcium aluminate (32) in a second reaction vessel (34) to form a second treated stream (36) comprising calcium carbonate in solid form, aluminate ions in solution, and hydroxyl ions in solution.
• TCA reacts with the sodium carbonate present in the first clarified treated stream of liquor to produce calcium carbonate (as a solid), sodium aluminate (in solution) and sodium hydroxide (in solution) in accordance with the following reaction:
• the second treated stream (36) is to remove calcium carbonate and produce a second clarified treated liquor stream.
• the second treated stream (36) is subjected to solid/liquid separation using a second solid/liquid separator (38) to produce a second clarified treated stream (40) and a second solids stream (38) which is predominately calcium carbonate within which the carbon dioxide has been sequestered, as well as any unreacted TCA.
• the second clarified treated stream (40) is enhanced with aluminate ions and hydroxide ions and can be returned to any suitable location within the Bayer process, for example, to the settlers or the liquor polishing filters.
• the second solids stream (38) is in the form of a stable solid that can be readily discarded.
• Carbon dioxide is in this way sequestered in a form that allows for a reduction in greenhouse gas emissions from an alumina refinery.
• TCA conversion to calcite is more favourable at high temperatures and lower 'S 1 streams.
• the total yield of 'C increases with increasing 'S ⁇ however the efficiency of conversion decreases (as shown by the C/S).
• the caustic aluminate solution (14) that is fed to the first reaction vessel (18) may be a spent or dilute Bayer liquor stream, for example, a spent Bayer liquor stream from a mud washing circuit or another dilute Bayer liquor, with best performance being obtained with more dilute liquors with an 'S' concentration of between 20 and 100 g/L.
• the second reaction vessel (34) is operated at a temperature not less than 50°C. Whilst it is possible to operate the second reaction vessel (34) at a temperature greater than the atmospheric boiling point of the first treated stream (36), it is preferable for the second reaction vessel to be operated at a temperature not less than 50°C and not greater than the atmospheric boiling point of the first treated stream (26). It is to be understood that heating of the second reaction vessel (34) may occur by way of direct heating of the second reaction vessel or indirectly by way of heating of one or both of the first treated stream (26) or the source of TCA (32).
• Agitation conditions within the first and second reaction vessels (18 and 34, respectively) are not critical, although the contents of each of the first and second reaction vessels should preferably be completely suspended.
• Retention times in the first and second reaction vessels (18 and 34, respectively) may vary depending on such relevant factors as the operating temperature, the relative concentrations of the various streams being added and the efficiency of conversions occurring in each vessel. Retention times may thus be in the range of not more than 4 hours to not more than 20 hours.
• the caustic aluminate solution (14) that is fed to the first reaction vessel (18) may be a spent Bayer liquor stream, for example, a spent Bayer liquor stream from a mud washing circuit or another dilute Bayer liquor, with best performance being obtained with more dilute liquors with an 'S' concentration of between 20 and 100 g/L.
• the second treated stream (36) is to remove calcium carbonate, and produce a second clarified treated liquor stream.
• the second treated stream (36) is subjected to solid/liquid separation using a second solid liquid separator (38) to produce a second clarified treated stream (40) and a second solids stream (38) which is predominately calcium carbonate as well as any unreacted TCA.
• the second clarified treated stream (40) is enhanced with aluminate ions and hydroxide ions and can be returned to any suitable location within the Bayer process, for example, to the settlers or the liquor polishing filters.
• Alumina values are in this way delivered to a pregnant liquor where they can be recovered during precipitation.
• the first and second solid/liquid separators can be any suitable solids/liquid separator including one or more gravity settlers, pressure filters, cyclones, or centrifuges, but best performance is obtained using simple filters.
• Use of TCA in this way allows for the recovery of caustic and alumina values from the mud washing circuit that may otherwise have been lost due to precipitation or discarded in soluble form with the liquor accompanying the mud.
• the gibbsite solids could potentially be sent back to digestion and recovered into the process, while C0 2 gas is effectively sequestered in the waste calcite solids, reducing greenhouse gas emissions (provided that the calcium carbonate is not subjected to calcinations to regenerate lime).
• the waste disposal of spent TCA filter aid is also reduced in scale.
• the mud residue is pumped from the first washer (52) to a second washer (56) and so on to the n* washer (60) while fresh water or lake water (62) is introduced firstly to the last (n-*) washer (60) in the mud washing circuit (50) and overflows to the n-l* washer (64) and so on up to the first mud washer (52).
• fresh water or lake water (62) is introduced firstly to the last (n-*) washer (60) in the mud washing circuit (50) and overflows to the n-l* washer (64) and so on up to the first mud washer (52).
• alumina values are lost in the mud washing circuit due to precipitation, as well as in soluble form in the liquor accompanying the mud to the mud residue disposal areas.
• the washer overflow liquor in all stages of the mud washing circuit is, like most liquors, supersaturated with respect to gibbsite.
• the wash overflow liquor becomes progressively more dilute and cooler with each successive stage of washing resulting in alumina
• the caustic aluminate solution (14) fed to the first reaction vessel (18) is the washer overflow from the n-l ,h washer (64).
• the present invention is equally applicable to the treatment of a plurality of washer overflow streams each being treated in one or a corresponding plurality of first reaction vessels (18). It is to be understood that the washer overflow liquor from any of the other mud washers could equally be used, however recovery of alumina values is most efficient when the washer overflow liquor is taken from any
• RO/AU one or each of the 2 to n-1 washer(s) (56 and 64, respectively) and least favourable when the caustic aluminate stream (14) is the overflow from the first or last washers (52 and 60, respectively) in the mud washing circuit (50). It is pointless to use the overflow from the first washer (52), as the overflow liquor from the first washer is fed to the settling tank in any event.
• the alumina concentration of the overflow liquor to the final (n 1 * 1 ) washer (60) is generally similar to lake water and is therefore too low to be of practical benefit. In addition, removal of alumina at this stage does little to prevent precipitation of gibbsite further up the mud washing circuit (50).
• Example 1 Conversion of TCA to Calcite Synthetic TCA was produced in the laboratory and reacted with 60g/L sodium carbonate (Na 2 CO;i) at a temperature of 60°C in both a rolling water bath (RWB) and autoclave to compare the kinetics of the reaction and the extent of conversion of TCA to calcite.
• Na 2 CO;i sodium carbonate
• RWB rolling water bath
• samples were taken using a 20mL syringe at intervals of 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300 mins, 24, 30, 48, 54 hours. These samples were filtered, cooled and then analysed using titration to determine the ' ⁇ ', 'C, and 'S' concentrations. After 54 hours the remaining mixture was removed from the RWB and filtered. The filtrate was collected and the solids were washed in cold DI water before being analysed on the XRD as a wet sample.
• the matrix test program is set out in Table 1 below.
• a kinetics test was designed to evaluate the rate of TCA conversion to calcite at the midpoint conditions set out in Table [1]. As such, tests were performed using 40g/L Na 2 C0 3 and 60°C. Samples were taken at 5, 15, 30, 45, 60, 90, 120, 180 and 240min and analysed for the ⁇ ', 'C, and 'S' concentrations. TCA was charged to 80% of the carbonate consumed in the respective test (80% of the final 'C concentration). The results for the kinetics test are shown in Table 2
• TCA slurry 12L was collected from an alumina refinery. The TCA slurry was filtered and washed with hot water. The solids were stored wet in a sealable bag. A wet sub-sample was taken and X-ray diffraction (XRD) performed. Another sub-sample was dried and X-ray fluorescence analysis (XRF) performed. Sodium carbonate solutions were made up by additions of a known standard to 1L volumetric flasks. The standards were analysed to determine the 'C ⁇ and 'S' concentrations.
• Example 2 The type of test set out in Example 2 was repeated at 60°C and 40g/l [Na 2 C0 3 ] to determine the effect, if any, of agitation on reaction kinetics. It was established conducting the test using a vigorously agitated autoclave was no faster than the equivalent test using a rolling waterbath.

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