AU2019300344A1 - Process for obtaining size-calibrated tri-calcium aluminate particles and use of such particles in a process of alumina refining - Google Patents
Process for obtaining size-calibrated tri-calcium aluminate particles and use of such particles in a process of alumina refining Download PDFInfo
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- 239000002245 particle Substances 0.000 title claims abstract description 248
- 238000000034 method Methods 0.000 title claims abstract description 100
- 230000008569 process Effects 0.000 title claims abstract description 97
- HOOWDPSAHIOHCC-UHFFFAOYSA-N dialuminum tricalcium oxygen(2-) Chemical compound [O--].[O--].[O--].[O--].[O--].[O--].[Al+3].[Al+3].[Ca++].[Ca++].[Ca++] HOOWDPSAHIOHCC-UHFFFAOYSA-N 0.000 title claims abstract description 87
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 title claims description 27
- 238000007670 refining Methods 0.000 title description 9
- 239000000920 calcium hydroxide Substances 0.000 claims abstract description 163
- AXCZMVOFGPJBDE-UHFFFAOYSA-L calcium dihydroxide Chemical compound [OH-].[OH-].[Ca+2] AXCZMVOFGPJBDE-UHFFFAOYSA-L 0.000 claims abstract description 136
- 229910001861 calcium hydroxide Inorganic materials 0.000 claims abstract description 136
- 239000000725 suspension Substances 0.000 claims abstract description 82
- ANBBXQWFNXMHLD-UHFFFAOYSA-N aluminum;sodium;oxygen(2-) Chemical compound [O-2].[O-2].[Na+].[Al+3] ANBBXQWFNXMHLD-UHFFFAOYSA-N 0.000 claims abstract description 9
- 229910001388 sodium aluminate Inorganic materials 0.000 claims abstract description 9
- 238000002156 mixing Methods 0.000 claims abstract description 6
- ODINCKMPIJJUCX-UHFFFAOYSA-N Calcium oxide Chemical compound [Ca]=O ODINCKMPIJJUCX-UHFFFAOYSA-N 0.000 claims description 186
- 235000011116 calcium hydroxide Nutrition 0.000 claims description 150
- 235000012255 calcium oxide Nutrition 0.000 claims description 97
- 239000000292 calcium oxide Substances 0.000 claims description 93
- 238000009826 distribution Methods 0.000 claims description 37
- 239000000654 additive Substances 0.000 claims description 29
- 230000000996 additive effect Effects 0.000 claims description 29
- 235000008733 Citrus aurantifolia Nutrition 0.000 claims description 19
- 235000011941 Tilia x europaea Nutrition 0.000 claims description 19
- 239000004571 lime Substances 0.000 claims description 19
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 17
- 239000007787 solid Substances 0.000 claims description 13
- 238000010790 dilution Methods 0.000 claims description 9
- 239000012895 dilution Substances 0.000 claims description 9
- 239000000243 solution Substances 0.000 claims description 9
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 claims description 7
- 229910052938 sodium sulfate Inorganic materials 0.000 claims description 7
- 235000011152 sodium sulphate Nutrition 0.000 claims description 7
- 238000004090 dissolution Methods 0.000 claims description 6
- 150000003467 sulfuric acid derivatives Chemical class 0.000 claims description 6
- 239000007864 aqueous solution Substances 0.000 claims description 5
- 238000004519 manufacturing process Methods 0.000 claims description 5
- 229910052782 aluminium Inorganic materials 0.000 claims description 4
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 4
- 230000000694 effects Effects 0.000 claims description 4
- 238000011088 calibration curve Methods 0.000 claims description 3
- 229910052500 inorganic mineral Inorganic materials 0.000 claims description 3
- 235000010755 mineral Nutrition 0.000 claims description 3
- 239000011707 mineral Substances 0.000 claims description 3
- 238000005507 spraying Methods 0.000 claims description 3
- 238000001914 filtration Methods 0.000 description 15
- 239000003153 chemical reaction reagent Substances 0.000 description 12
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 description 9
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 9
- 241000196324 Embryophyta Species 0.000 description 8
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 8
- 239000012535 impurity Substances 0.000 description 8
- 239000011593 sulfur Substances 0.000 description 8
- 229910052717 sulfur Inorganic materials 0.000 description 8
- 238000004131 Bayer process Methods 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 5
- 239000011575 calcium Substances 0.000 description 5
- 239000000047 product Substances 0.000 description 5
- 239000002002 slurry Substances 0.000 description 5
- 239000000203 mixture Substances 0.000 description 4
- 238000013019 agitation Methods 0.000 description 3
- 238000005520 cutting process Methods 0.000 description 3
- 230000001419 dependent effect Effects 0.000 description 3
- 230000029087 digestion Effects 0.000 description 3
- 239000004744 fabric Substances 0.000 description 3
- 238000001033 granulometry Methods 0.000 description 3
- 210000004080 milk Anatomy 0.000 description 3
- 235000013336 milk Nutrition 0.000 description 3
- 238000000527 sonication Methods 0.000 description 3
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 2
- CDBYLPFSWZWCQE-UHFFFAOYSA-L Sodium Carbonate Chemical compound [Na+].[Na+].[O-]C([O-])=O CDBYLPFSWZWCQE-UHFFFAOYSA-L 0.000 description 2
- 229910052791 calcium Inorganic materials 0.000 description 2
- OSGAYBCDTDRGGQ-UHFFFAOYSA-L calcium sulfate Chemical compound [Ca+2].[O-]S([O-])(=O)=O OSGAYBCDTDRGGQ-UHFFFAOYSA-L 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 230000036571 hydration Effects 0.000 description 2
- 238000006703 hydration reaction Methods 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 239000012528 membrane Substances 0.000 description 2
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 239000002006 petroleum coke Substances 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000002360 preparation method Methods 0.000 description 2
- 230000009257 reactivity Effects 0.000 description 2
- 238000004626 scanning electron microscopy Methods 0.000 description 2
- 239000011734 sodium Substances 0.000 description 2
- 229910000029 sodium carbonate Inorganic materials 0.000 description 2
- 238000003756 stirring Methods 0.000 description 2
- 235000019738 Limestone Nutrition 0.000 description 1
- 229910018487 Ni—Cr Inorganic materials 0.000 description 1
- MXRIRQGCELJRSN-UHFFFAOYSA-N O.O.O.[Al] Chemical compound O.O.O.[Al] MXRIRQGCELJRSN-UHFFFAOYSA-N 0.000 description 1
- 241000638935 Senecio crassissimus Species 0.000 description 1
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 1
- 229910001570 bauxite Inorganic materials 0.000 description 1
- 238000001354 calcination Methods 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- VNNRSPGTAMTISX-UHFFFAOYSA-N chromium nickel Chemical compound [Cr].[Ni] VNNRSPGTAMTISX-UHFFFAOYSA-N 0.000 description 1
- 238000005352 clarification Methods 0.000 description 1
- 239000011335 coal coke Substances 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 238000010908 decantation Methods 0.000 description 1
- 230000000668 effect on calcium Effects 0.000 description 1
- 239000003480 eluent Substances 0.000 description 1
- 238000011010 flushing procedure Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 230000006870 function Effects 0.000 description 1
- 238000000227 grinding Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000009854 hydrometallurgy Methods 0.000 description 1
- 229910001026 inconel Inorganic materials 0.000 description 1
- 238000009434 installation Methods 0.000 description 1
- 239000006028 limestone Substances 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 230000008447 perception Effects 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 238000011085 pressure filtration Methods 0.000 description 1
- 238000004064 recycling Methods 0.000 description 1
- 238000004062 sedimentation Methods 0.000 description 1
- 238000007873 sieving Methods 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- -1 tricalcium aluminate hexahydrate Chemical class 0.000 description 1
- 238000005406 washing Methods 0.000 description 1
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
- C01F7/16—Preparation of alkaline-earth metal aluminates or magnesium aluminates; Aluminium oxide or hydroxide therefrom
- C01F7/164—Calcium aluminates
-
- 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
- C01F11/02—Oxides or hydroxides
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2/00—Lime, magnesia or dolomite
- C04B2/02—Lime
- C04B2/04—Slaking
- C04B2/06—Slaking with addition of substances, e.g. hydrophobic agents ; Slaking in the presence of other compounds
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/60—Particles characterised by their size
- C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
Abstract
The present invention describes a process for obtaining size-calibrated tri-calcium aluminate (TCA) particles comprising the steps of providing a process liquor containing sodium aluminate; providing a suspension of size-calibrated calcium hydroxide particles; mixing said selected suspension of size-calibrated calcium hydroxide particles with said process liquor to allow them to react to obtain the said size-calibrated tri-calcium aluminate particles.
Description
Process for obtaining size-calibrated tri-calcium aluminate particles and use of such particles in a process of alumina refining
Technical field
In a first aspect, the present invention is related to a process for obtaining size-calibrated tri calcium aluminate (TCA) particles comprising the steps of providing a process liquor containing sodium aluminate, then providing a suspension of coarse calcium hydroxide particles and mixing said selected suspension of calcium hydroxide particles with said process liquor to allow them to react to obtain the said tri-calcium aluminate particles. In a second aspect, the present invention is related to the use of such coarse tri calcium aluminate particles in a process of purifying alumina process liquors.
State of the art
Bayer process alumina refineries typically make use of a filtration process step to remove fine suspended particulate impurities prior to the alumina precipitation step, where the so-called "green liquor" or "process liquor" containing mainly sodium aluminate is precipitated as alumina Al203 product. The filtration step to remove suspended impurities (typically fine Fe and/or Si containing solids), that have not already been removed from solution during clarification, is critical to ensure that such impurities do not contaminate the precipitated Al203 product. This impurity removal step is typically referred to as "security filtration", "impurity filtration" or "solution polishing" and is conducted in well-known pressure filtration systems (e.g. Kelly leaf filters commonly used in Australia or Diastar filters commonly used in South East Asia). These filtration systems make use of a "filter aid" bed (or cake) of particles supported by a screen, filter cloth or membrane. The filter aid is typically tri-calcium aluminate (Ca3[AI(OH)6]2), hereinafter abbreviated as TCA, which is produced by reacting a portion of the process liquor, containing sodium aluminate NaAI(OH)4 with Ca(OH)2, in a molar ratio suited for the plant operation, generally from 0.5:1 to 3:1 but is specific to plant conditions. The Ca(OH)2 reagent is typically added to the reactor as a slaked lime slurry, also called milk of lime, or suspension, with a solids content ranging from 12-25 % w/w. Slaking occurs when quicklime, CaO, is reacted with excess water to form a suspension of Ca(OH)2.
The process feed liquor temperature is typically in the range of 95-105 °C. The slaked lime slurry is preferably at a similar temperature, or as high as practically possible, generally in the range of 70-95 °C. The TCA filter aid generation reaction is typically conducted at 95-98 °C. Although some plants use quicklime, CaO, directly in this process step, the CaO still undergoes hydration to Ca(OH)2, before reacting with sodium aluminate to form TCA:
3Ca(OH)2 + 2NaAI(OH)4 ® Ca3[AI(OH)6]2 + 2NaOH
It is important to note that it is the TCA filter aid, rather than the filter cloth that performs the filtration duty. The properties of the TCA filter aid are, therefore, critical to filtration performance. If the TCA particles are too fine, the filtration bed voids might be too small, thus slowing down the filtration process and increasing filtration backpressure. Operationally, this requires frequent back-flushing to prevent clogging, therefore causing a plant throughput bottleneck. If the TCA particles are too coarse, the voids between the particles may be too large and the
suspended impurities are not sufficiently removed during the filtration process. The ideal TCA filter aid particle size and distribution is different for each alumina refinery and is dependent upon a number of process and equipment factors, such as: the throughput rate required, the suspended impurity particle load and size, the type of filter, filtration conditions and filter cloth, membrane or screen type used.
The problem for alumina refineries is that optimal TCA filter aid particle size distribution, required to achieve filtration efficiency, cannot be controlled as a process parameter independent of the lime source. For example, US 2004/0101470 Al, paragraph [0024] describes the technical bias that the physical characteristics of the TCA filter aid are very highly dependent upon the nature of the lime particles from which it is formed. Also, Franca et al. "Some aspects of tricalcium aluminate hexahydrate formation on the Bayer process", Light Metals 2010, TMS (The Minerals, Metals & Materials Society) 2010 p.63-66., confirm the common perception that it is primarily the Calcium source that impacts the TCA particle size distribution. Thus, the TCA particle size distribution is operationally experienced as a fixed consequence of the quicklime reagent properties used, as well as a combination of other plant conditions which are known to influence particle size of the TCA..
However, this approach limits the ability to use alternative quicklime reagent sources, even if using such alternatives may have other benefits, such as higher utilization efficiency. Often the quicklime that provides satisfactory TCA particle size distribution is of a low quality and low CaO available content. The use of such quicklime increases the lime consumption due to the low utilization efficiency, not only in for TCA filter aid production but also in other parts of the Bayer process alumina refinery where lime is used.
A common experience in alumina refineries is that higher quality (i.e. higher CaO available %) and highly reactive quicklimes, generally shift the TCA towards a finer particle size distribution, without the reasons for this phenomenon being understood or articulated in scientific or patent literature. Without a suitable control mechanism, alumina refineries are restricted in their use of a lime reagent to that which provides acceptable TCA filter aid properties, even if said lime reagent other properties are sub-optimal. This is particularly problematic for alumina refineries that require coarse TCA filter aid, i.e. with a d50 larger than 20 pm. Depending on the plant installations and/or on the ore compositions, some refineries will require specific particle size distribution of TCA used as a filter aid.
There is thus a need to provide an improved process for obtaining size-calibrated tri-calcium aluminate without resorting to the use of lime reagent possessing low available CaO content. Moreover there is also a need for a process for obtaining size-calibrated tri-calcium aluminate which can be used for manufacturing coarse TCA filter aid.
Summary of the invention
In order to solve the abovementioned problems and drawbacks of the prior art, the present invention is related to a process for obtaining size-calibrated tri-calcium aluminate (TCA) particles comprising the steps of:
a) providing a process liquor containing sodium aluminate;
b) providing a suspension of size-calibrated calcium hydroxide particles
c) mixing said selected suspension of size-calibrated calcium hydroxide particles with said process liquor to allow them to react to obtain the said size-calibrated tri-calcium aluminate particles.
Tri-calcium aluminate (Ca3[AI(OH)6]2) is hereinhafter referred to as TCA.
Sodium aluminate is hereinafter used to describe 2NaAI(OH)4 .
By the term "size-calibrated tri-calcium aluminate particles" is meant according to the invention tri-calcium aluminate particles with an optimal particle size distribution for use as a TCA filter aid in a process of alumina refining. As mentioned above, the optimal TCA particles size is a compromise between too fine and too coarse, to provide an efficient filtration of suspended impurities while minimizing filter clogging, backpressure and throughput issues. The particle size distribution is expressed as the d50 measured by granulometry laser diffraction in methanol after sonication.
The d50 of Size-calibrated tri-calcium aluminate particles may vary according to the requirements of the plant, for example Size-calibrated tri-calcium aluminate particles may have a d50 of larger than 5 pm, preferably larger than 10 pm, more preferably larger than 15 pm, more preferably larger than 20 pm. In another embodiment of the invention the d50 of the size-calibrated tri-calcium aluminate particles may be lower than 200 pm, preferably lower than 100 pm, even more preferably less then 50 pm.
By the term "size-calibrated calcium hydroxide particles" is meant according to the invention calcium hydroxide particles having an optimal particle size distribution being a compromise between too fine and too coarse for obtaining optimal size-calibrated TCA particles.
It has been surprisingly found by the applicant that the particle size distribution of TCA particles obtained by the process according to the invention is linked to the particle size distribution of the calcium hydroxide particles by a generally linear relationship. Thus the long standing technical prejudice that highly reactive quicklimes generally shift the TCA towards a finer particle size distribution has been surprisingly overcome. The slope of the linear relationship is specific to the particular conditions, including: the solution composition, the molar ratio of Ca(OH)2 to NaAI(OH)4, the temperature of both the Ca(OH)2 reagent and the alumina refinery liquor, the tip speed of agitation (i.e. shear force), the hydraulic residence time, the particle residence time, the extent of precipitate seeding and seed recycling. The relationship is also dependent on the quicklime properties used to produce the Ca(OH)2, including its reactivity (as measured by its reaction rate with water) and its chemical composition. For all of these reasons, the precise relationship between particle size distribution of the Ca(OH)2 and the TCA may be determined for each particular plant scenario. The determination of the relationship is based on generating a range of Ca(OH)2 reagents with increasing particle size distributions (meaning the d25, d50 and d90 are increased), and observing the impact of using these reagents on the particle size distribution of the resulting TCA.
Preferably, the said suspension of size-calibrated calcium hydroxide particles is obtained from slaking quicklime, said quicklime having an available lime content greater than 82 %, preferably greater than 85 %, preferably greater than 90 %, more preferably greater than 92 %, more preferably
greater than 95 %, measured according to EN459-2:2010 standard. Quicklimes having an available lime content greater than 90 % are also generally referred as "high quality quicklime".
According to the invention, it is possible to control the particle size distribution of TCA particles by controlling the particle size distribution of calcium hydroxide particles, i.e. using size- calibrated calcium hydroxide.
Size-calibrated calcium hydroxide can be produced from high quality quicklime having a content of available lime greater than 85% as measured according to EN459-2 standard, and generally presents a high reactivity with water. In turn, the use of such high quality quicklime and milk of lime derived from high quality quicklime is also advantageous for use in other components of the alumina refining process, other than TCA filter aid generation, by reducing lime reagent consumption.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in the presence of a coarsening additive causing a coarsening effect on the particle size distribution of the calcium hydroxide particles obtained by said process of slaking.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking formulated quicklime, prepared from quicklime to which a coarsening additive has been added by spraying a solution containing a coarsening additive onto quicklime.
Preferably, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in the presence of 2 w% or less, preferably, 0.1 to 1.5 w%, preferably, 0.15 to 0.8 w% and more preferably from 0.2 to 0.65 w% of a coarsening additive relative to the weight of quicklime, wherein said coarsening additive is selected among alkaline sulfates or alkaline earth sulfates.
In a particular embodiment of the invention, the said coarsening additive is sodium sulfate.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process wherein
a concentrated aqueous solution of coarsening additive, in particular sodium sulfate, is provided by dissolving an amount of a coarsening additive of at least 10 w%, preferably at least 30 w% in water at a temperature of dissolution adapted to dissolve such an amount of coarsening additive;
said concentrated aqueous solution of coarsening additive at the said temperature of dissolution is sprayed onto quicklime with an amount comprised of between 5 dm3 and 30 dm3, preferably between 10 dm3 and 20 dm3 per dry metric ton of said quicklime to obtain a formulated quicklime;
slaking said formulated quicklime with water to provide said suspension of size- calibrated calcium hydroxide particles.
Preferably, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime with an amount of water such that the solid content in the milk of lime
solids content comprises between 10 and 45 % in weight of the total weight of the milk of lime preferably with a solid content in the milk of lime up to 40 %, preferably up to 35 %, more preferably up to 30 %, more preferably up to 25 % in weight of the total weight of the milk of lime.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking partially pre-hydrated quicklime particles having a superficial layer, at least partially superficial, of hydrated lime and a core of quicklime.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in a reactor cooled at a temperature lower than or equal to 60 °C. The particles of hydrated lime obtained in such conditions are generally coarser than particles of hydrated lime obtained in conditions of temperatures comprised between 60 °C and 95 °C.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in a reactor at a temperature greater or equal to 95 °C. The particles of hydrated lime obtained in such conditions are generally coarser than particles of hydrated lime obtained in conditions of temperatures comprised between 60 °C and 95 °C.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first suspension of calcium hydroxide followed by a step of particle size selection of said first suspension to remove particles of calcium hydroxide that have a particle size under a predetermined particle size and to retain the calcium hydroxide particles larger than a predetermined particle size for obtaining, possibly after dilution, said suspension of size-calibrated calcium hydroxide particles.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first suspension of calcium hydroxide followed by:
a first step of particle size selection of said first suspension to remove particles of calcium hydroxide having a particle size smaller than a first predetermined particle size and to retain calcium hydroxide particles larger than a first predetermined particle size for obtaining, possibly after dilution, a second suspension of calcium hydroxide particles
a second step of particle size selection of said second suspension of calcium hydroxide particles to remove particles of calcium hydroxide having a particle size larger than a second predetermined particle size greater than the said first predetermined particle size and to retain the remaining calcium hydroxide particles having a particle size comprised between said first predetermined particle size and said second predetermined particle size for obtaining, possibly after dilution said suspension of size-calibrated calcium hydroxide particles.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slurrying hydrated lime where the hydrated lime was prepared by dry hydration and its particle size was controlled by grinding and sieving to obtain the targeted size.
Preferably, said size-calibrated calcium hydroxide particles have a particle size distribution predetermined for obtaining TCA particles with a desired particle size distribution on basis of a calibration curve obtained by realizing the said steps a) to c) with at least three suspensions of calcium hydroxide of different predetermined particle size distribution.
All the embodiments disclosed herein can be used alone or in a combination thereof.
In a second aspect, the present invention is related to the use of coarse tri-calcium aluminate particles obtained by the process as disclosed herein above in a process of refining alumina from an ore comprising aluminum minerals.
In another aspect, the present invention is related to the use of coarse tri-calcium aluminate particles obtained by the process as disclosed above in a process of manufacturing TCA filter aid.
Brief of the drawings
The figure 1 presents an image obtained by Scanning Electron Microscopy of the TCA particles obtained according to the process of invention.
Description of the invention
According to a first aspect, the present invention is related to a process for obtaining size- calibrated tri-calcium aluminate (TCA) particles comprising the steps of:
a) providing a process liquor containing sodium aluminate;
b) providing a suspension of size-calibrated calcium hydroxide particles
c) mixing said selected suspension of size-calibrated calcium hydroxide particles with said process liquor to allow them to react to obtain the said size-calibrated tri-calcium aluminate particles.
Preferably, the said suspension of size-calibrated calcium hydroxide is obtained from slaking quicklime, said quicklime having an available lime content greater than 82 %, preferably greater than 85 %, preferably greater than 90 %, more preferably greater than 92 %, more preferably greater than 95 %, measured according to EN459-2:2010 standard. Quicklimes having an available lime content than 90 % is generally also referred as "high quality quicklime".
A typical alumina refinery process is described in the document "The chemistry of CaO and Ca(OH)2", B.l. Whittington, Hydrometallurgy 43 (1996) 13-35. An ore containing high amounts of aluminum such as bauxite is crushed and milled. The resulting product is transferred into a first tank for a pre-desilication step wherein the crushed ore is put in contact with milk of lime under agitation at temperatures about 100 °C before being transferred to a digestion tank wherein highly concentrated solution of NaOH and milk of lime is injected. This digestion step occurs under high pressure and high temperature. The resulting mud obtained after digestion is cooled and sent to a sedimentation tank to settle out the solid fraction from the liquid fraction hereby called process liquor. The solid fraction is washed with water, in a series of counter current decantation washers. One of the washer overflow eluents is sent to a tank for causticization wherein milk of lime is added to increase alkalinity by converting sodium carbonate to sodium hydroxide. In a separate process step, a small part of the process liquor is used to be reacted with milk of lime, also referred to as a
suspension of calcium hydroxide particles, to obtain tri-calcium aluminate particles which are used as a filter aid to filter the remaining process liquor. The filtered process liquor is then cooled and settled to precipitate aluminum trihydroxide in a precipitation tank. Finally, the precipitate is calcined after washing to obtain a purified Al203 Alumina product.
Since the process of refining alumina comprises different steps involving the use of milk of lime, the use of a milk of lime derived from high quality quicklime is advantageous for use in other components of the alumina refining process, other TCA filter aid generation, by reducing lime reagent consumption.
It has been found that it is possible to control the particle size distribution of the tri-calcium aluminate particles by controlling the particle size distribution of the calcium hydroxide particles. According to the invention, there are various ways to control the particle size distribution of the calcium hydroxide particles to be used for generating size-calibrated particles of tri-calcium aluminate.
In an embodiment of the invention, quicklime can be slaked in the presence of a coarsening additive causing a coarsening effect on the particle size distribution of the calcium hydroxide particles obtained by said process of slaking.
Preferably, the said coarsening additive is selected among alkaline sulfate such as sodium sulfate as a non-limitative example or alkaline earth sulfates such as calcium sulfate as a non- limitative example.
The coarsening additive can be added to quicklime before the step of slaking for example by spaying the said coarsening additive onto quicklime and then the step of slaking is performed with sufficient amount of water to obtain a suspension of calcium hydroxide particles. Alternatively or in combination with the step of spraying, the coarsening additive can be added during the step of slaking, for example in the slaking water.
Preferably, the step of slaking quicklime is realized in presence of less than 2 w%, preferably 0.1 to 1.5 w%, preferably, 0.15 to 0.8 w% and more preferably from 0.2 to 0.65 w% of a coarsening additive relative to the weight of quicklime. A minimum amount of coarsening additive is required to provide the coarsening effect on calcium hydroxide particles and a maximum amount of coarsening additive is set up to keep the suspension of calcium hydroxide particles over a certain degree of purity and to prevent the formation of too coarse calcium hydroxide particles.
In an embodiment of the invention, a suspension of size-calibrated calcium hydroxide particles is obtained from a process wherein:
a concentrated aqueous solution of sodium sulfate is provided by dissolving an amount of sodium sulfate of at least 10 w%, preferably at least 35 w% in water at a temperature of dissolution adapted to dissolve such an amount;
said concentrated solution of sodium sulfate at the said temperature of dissolution is sprayed onto quicklime with an amount comprised of between 5 dm3 and 30 dm3, preferably between 10 dm3 and 20 dm3 of said concentrated solution per ton of said quicklime to obtain a formulated quicklime;
said formulated quicklime is slaked with water to provide said suspension of size- calibrated calcium hydroxide particles.
Preferably, in the step of slaking quicklime, the amount of water is adapted to provide a suspension of calcium hydroxide (or milk of lime) wherein the solid content in the milk of lime is comprised between 10 and 45 % in weight of the total weight of the milk of lime, preferably with a solid content in the milk of lime up to 40 %, preferably up to 35 %, more preferably up to 30 %, more preferably up to 25 % in weight of the total weight of the milk of lime. A minimum amount of solid content is required to provide sufficient amount of calcium hydroxide in a minimum volume of suspension and a maximum amount of solid content is preferably set up to minimize the volume of the suspension while keeping a good flowability and pumpability of the suspension.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking partially pre-hydrated quicklime particles having a superficial layer, at least partially superficial, of hydrated lime and a core of quicklime.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in a reactor cooled at a temperature lower than or equal to 60 °C. The particles of hydrated lime obtained in such conditions are generally coarser than particles of hydrated lime obtained in conditions of temperatures comprised between 60 °C and 95 °C.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in a reactor at a temperature greater or equal to 95 °C. The particles of hydrated lime obtained in such conditions are generally coarser than particles of hydrated lime obtained in conditions of temperatures comprised between 60 °C and 95 °C.
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first suspension of calcium hydroxide followed by a step of particle size selection of said first suspension to remove particles of calcium hydroxide that have a particle size under a predetermined particle size and to retain the calcium hydroxide particles larger than a predetermined particle size for obtaining, possibly after dilution, said suspension of size-calibrated calcium hydroxide particles. This step of particle size selection can be done for example by cutting the said first suspension with a sieve selected from the commercially available sieves having sieve openings inferior to 125 pm (120 mesh), preferably inferior to 105 pm (140 mesh), more preferably inferior to 88 pm (170 mesh), more preferably inferior to 74 pm (200 mesh), more preferably inferior to 63 pm (230 mesh).
In an embodiment of the invention, said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first suspension of calcium hydroxide followed by:
a first step of particle size selection of said first suspension to remove particles of calcium hydroxide having a particle size smaller than a first predetermined particle size and to
retain calcium hydroxide particles larger than a first predetermined particle size for obtaining, possibly after dilution, a second suspension of calcium hydroxide particles a second step of particle size selection of said second suspension of calcium hydroxide particles to remove particles of calcium hydroxide having a particle size larger than a second predetermined particle size greater than the said first predetermined particle size and to retain the remaining calcium hydroxide particles having a particle size comprised between said first predetermined particle size and said second predetermined particle size for obtaining, possibly after dilution said suspension of size-calibrated calcium hydroxide particles.
The first step of particle size selection can be done for example by cutting the said first suspension with a sieve selected from the commercially available sieves having sieve openings inferior to 125 pm (120 mesh), preferably inferior to 105 pm (140 mesh), more preferably inferior to 88 pm (170 mesh), more preferably inferior to 74 pm (200 mesh), more preferably inferior to 63 pm (230 mesh). Then the second step of particle selection can be done by cutting the said second suspension with a sieve selected from the commercially available sieves having sieve openings greater than the ones of the sieve utilized in the first step of particle selection, such as sieves having openings greater than 105 pm (140 mesh), preferably greater than 125 pm (120 mesh), more preferably greater than 149 pm (100 mesh), more preferably greater than 177 pm (80 mesh).
Preferably, in the process for obtaining size-calibrated tri-calcium aluminate, the said size- calibrated calcium hydroxide particles have a particle size distribution predetermined for obtaining TCA particles with a desired particle size distribution on the basis of a calibration curve obtained by realizing the said steps a) to c) with at least three suspensions of calcium hydroxide of different predetermined particle size distribution.
In an embodiment of the invention, calcination of limestone into quicklime, can also be realized by using fuels (such as high sulfur content coal, or petroleum coke, also called petcoke) that would impart a high sulfur content (typically in the form of a sulfate or other oxidized form of sulfur) onto the quicklime product. It can be advantageous to measure the sulfur content in the quicklime and to adapt the amount of coarsening additive to be added for the step of slaking as a function of the amount of sulfur in the quicklime, such as to avoid formation of too coarse calcium hydroxide particles. It is possible to establish according to the invention a relationship between the additive concentration, the Ca(OH)2 particle size and TCA particle size and that this relationship is normally different for a quicklime with a low sulfur content and a quicklime with a higher sulfur content.
Advantageously, in a process of refining alumina, size-calibrated particles of calcium hydroxide, preferably obtained from quicklime having an available lime content of more than 82 %, or even higher, can be used in different stages of the process including the step of formation of tri-calcium aluminate. Therefore, the method of manufacturing TCA particles provides not only a control method for the particle size distribution of TCA particles but also provides the possibility of using a high quality quicklime, with a CaO available content equal or greater than 82 %, to generate high quality milk of lime, thereby increasing the utilization efficiency of this reagent in all process units of the alumina refining process where it is used.
Examples
The effect of different types of milk of lime on the formation of tri-calcium aluminate has been evaluated by reacting those milks of lime with a synthetic Bayer process liquor.
Synthetic Bayer liquor can be prepared in an autoclave by heating at 180°C under stirring an aqueous mixture containing 6 mol/dm3 of NaOH, 2.2 mol/dm3 of AI(OH)3, and 0.4 mol/dm3 of Na2C03.
Three samples of formulated quicklime are prepared by mixing quicklime having the properties listed in table 1 with respectively 0.3 w%, 0.5 w% and 0.7 w% of Na2S04 in weight of said quicklime.
Table 1
The three said samples of formulated quicklime were crushed to <5 mm and then slaked in water at room temperature in a slaker for 20 minutes with an amount of water to obtain about 16 w% of solid content in the milk of lime.
The particle size distribution d50 for those three milks of lime obtained are measured by granulometry laser diffraction in methanol after sonication. The d50 of the calcium hydroxide particles of the three milks of lime obtained increases substantially linearly with the amount of Na2S04 in the formulated quicklime. The measured d50 of calcium hydroxide particles are provided in table 2.
Three samples of tri-calcium aluminate are prepared from 200 ml of the synthetic Bayer process liquor as prepared above and from an appropriate amount of one of the three samples of milk of lime as prepared above such that the molar ratio of calcium to aluminum expressed in equivalent CaO to Al203 is of 1 to 9. For each sample, 200 cm3 of synthetic Bayer process liquor is heated to 80°C in a vessel preferably made of a high-strength, corrosion-resistant nickel chromium material, also known commercially as INCONEL®. Then the appropriated amount of the respective milk of lime is added in the vessel under agitation with a stirrer at 250 RPM. When the slurry reaches 95 °C, the slurry is allowed to react for 90 minutes at the same stirring rate. The slurry is then cooled to 70 °C for filtration.
The figure 1 presents an image obtained by Scanning Electron Microscopy of the TCA particles obtained according to the process of invention. It can be seen that the TCA particles are formed of aggregated crystals of dimensions below 5 pm, and that those TCA particles are larger than 15 pm, as often required for effective operation of pressure static vertical leaf filters.
The d50 of each of the tri-calcium aluminate samples are measured by granulometry laser diffraction in methanol without sonication. The d50 of the tri-calcium aluminate particles increases substantially linearly with the d50 of the calcium hydroxide particles of the milk of lime utilized. The values of d50 for the tri-calcium aluminate particles and the respective values of d50 of the calcium hydroxide particles utilized for the preparation of those TCA particles are presented in table 2. The amount of milk of lime utilized in the process of preparation of TCA particles is further included in table 2.
Table 2
This example shows that by the process according to the present invention, it is possible to control the particle size distribution of the tri-calcium aluminate particles by adding a coarsening agent and then using calcium hydroxide particles of calibrated particle size distribution to obtain size-calibrated TCA
In the quicklime utilized for the examples mentioned herein above, the content of sulfur was relatively low (smaller than to 0.04 % in weight of the quicklime).
Claims (15)
1. Process for obtaining size-calibrated tri-calcium aluminate (TCA) particles comprising the steps of:
a) providing a process liquor containing sodium aluminate;
b) providing a suspension of size-calibrated calcium hydroxide particles
c) mixing said selected suspension of size-calibrated calcium hydroxide particles with said process liquor to allow them to react to obtain the said size-calibrated tri-calcium aluminate particles.
2. Process according to claim 1 wherein said suspension of size-calibrated calcium hydroxide particles is obtainable from slaking quicklime having an available CaO lime content greater than 82,5 %, measured according to EN459-2:2010 standard.
3. Process according to any one of the preceding claims wherein said suspension of size- calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in the presence of a coarsening additive causing a coarsening effect on the particle size distribution of the calcium hydroxide particles obtainable by said process of slaking.
4. Process according to claim 3 wherein said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking formulated quicklime, prepared from quicklime to which a coarsening additive has been added by spraying a solution containing a coarsening additive onto quicklime.
5. Process according to claim 3 wherein said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in the presence of 0.1 to 1.5 w%, of a coarsening additive relative to the weight of quicklime, wherein said coarsening additive is selected among alkaline sulfates or alkaline earth sulfates.
6. Process according to claim 3 wherein said suspension of size-calibrated calcium hydroxide particles is obtained from a process wherein
a concentrated aqueous solution of coarsening additive, preferably sodium sulfate is provided by dissolving an amount of coarsening additive of at least 10 w%, in water at a temperature of dissolution adapted to dissolve such an amount of coarsening additive; said concentrated aqueous solution of coarsening additive at the said temperature of dissolution is sprayed onto quicklime with an amount comprised of between 5 dm3 and 30 dm3 of said concentrated solution per ton of said quicklime to obtain a formulated quicklime;
said formulated quicklime is slaked with water to provide said suspension of size- calibrated calcium hydroxide particles.
7. Process according to any one of the preceding claims wherein said suspension of size- calibrated calcium hydroxide particles is obtained from a process of slaking quicklime with an amount of water such that the solid content in the milk of lime is comprised between 10 and
45 %, in weight of the total weight of the milk of lime, in weight of the total weight of the milk of lime.
8. Process according to any one of the preceding claims wherein said suspension of size- calibrated calcium hydroxide particles is obtained from a process of slaking partially pre hydrated quicklime particles having a superficial layer, at least partially superficial, of hydrated lime and a core of quicklime.
9. Process according to any one of the preceding claims wherein said suspension of size- calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in a reactor cooled at a temperature lower than or equal to 60 °C.
10. Process according to any one of the claims 1 to 8 wherein said suspension of size-calibrated calcium hydroxide particles is obtained from a process of slaking quicklime in a reactor at a temperature greater or equal to 95 °C.
11. Process according to any one of the preceding claims wherein said suspension of size- calibrated calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first suspension of calcium hydroxide followed by a step of particle size selection of said first suspension to remove particles of calcium hydroxide that have a particle size under a predetermined particle size and to retain the calcium hydroxide particles larger than a predetermined particle size for obtaining, possibly after dilution, said suspension of size- calibrated calcium hydroxide particles.
12. Process according to any one of the preceding claims wherein said suspension of size- calibrated calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first suspension of calcium hydroxide followed by:
a first step of particle size selection of said first suspension to remove particles of calcium hydroxide having a particle size smaller than a first predetermined particle size and to retain calcium hydroxide particles larger than a first predetermined particle size for obtaining, possibly after dilution, a second suspension of calcium hydroxide particles a second step of particle size selection of said second suspension of calcium hydroxide particles to remove particles of calcium hydroxide having a particle size larger than a second predetermined particle size greater than the said first predetermined particle size and to retain the remaining calcium hydroxide particles having a particle size comprised between said first predetermined particle size and said second predetermined particle size for obtaining, possibly after dilution said suspension of size-calibrated calcium hydroxide particles.
13. Process according to any one of the preceding claims wherein said size-calibrated calcium hydroxide particles have a particle size distribution predetermined for obtaining TCA particles with a desired particle size distribution on basis of a calibration curve obtained by realizing the said steps a) to c) with at least three suspensions of calcium hydroxide of different predetermined particle size distribution.
the claims 1 to 13 in a process of purifying alumina process liquors from an ore comprising aluminum minerals.
15. Use of coarse tri-calcium aluminate particles obtained by the process according to any one of the claims 1 to 13 in a process of manufacturing TCA filter aid.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP18182967 | 2018-07-11 | ||
| EP18182967.2 | 2018-07-11 | ||
| PCT/EP2019/068757 WO2020011952A1 (en) | 2018-07-11 | 2019-07-11 | Process for obtaining size-calibrated tri-calcium aluminate particles and use of such particles in a process of alumina refining |
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| AU2019300344A1 true AU2019300344A1 (en) | 2021-01-21 |
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| AU2019300344A Pending AU2019300344A1 (en) | 2018-07-11 | 2019-07-11 | Process for obtaining size-calibrated tri-calcium aluminate particles and use of such particles in a process of alumina refining |
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| Country | Link |
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| EP (1) | EP3820820A1 (en) |
| AU (1) | AU2019300344A1 (en) |
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| WO (1) | WO2020011952A1 (en) |
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| AUPQ889100A0 (en) * | 2000-07-20 | 2000-08-10 | Worsley Alumina Pty Ltd | Improved process for filter aid production in alumina refineries |
| US7326400B2 (en) * | 2005-08-11 | 2008-02-05 | Chemical Lime Company | Treatment of high sulfate containing quicklime |
| BE1021522B1 (en) * | 2012-09-12 | 2015-12-07 | S.A. Lhoist Recherche Et Developpement | HIGH FINENSE LIME MILK COMPOSITION |
| WO2017152960A1 (en) * | 2016-03-08 | 2017-09-14 | S.A. Lhoist Recherche Et Developpement | Process for manufacturing a milk of slaked lime of great fineness and milk of lime of great fineness thereby obtained with process water |
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2019
- 2019-07-11 AU AU2019300344A patent/AU2019300344A1/en active Pending
- 2019-07-11 BR BR112020026329-0A patent/BR112020026329A2/en unknown
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| WO2020011952A1 (en) | 2020-01-16 |
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