PROCESS FOR OBTAINING TRICALCIUM ALUMINATE PARTICLES WITH CALIBRATED SIZE

Abstract

The present invention relates to a process for obtaining tricalcium aluminate (TCA) particles of calibrated size comprising the steps of providing a process liquor containing sodium aluminate; providing a suspension of calibrated-size calcium hydroxide particles; mixing said selected suspension of calibrated-size calcium hydroxide particles with said process liquor to react them in order to obtain said calibrated-size tricalcium aluminate particles.

Description

1 / 20 “PROCESS FOR OBTAINING TRICALCIUM ALUMINATE PARTICLES WITH CALIBRATED SIZE” TECHNICAL FIELD

[001] In a first aspect, the present invention relates to a process for obtaining tricalcium aluminate (TCA) particles of calibrated size 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 react them to obtain said tricalcium aluminate particles. In a second aspect, the present invention relates to the use of such coarse tricalcium aluminate particles in a process for purifying alumina process liquors. STATE OF THE ART

[002] Bayer process alumina refineries typically employ 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 primarily sodium aluminate is precipitated as the alumina product Al2O3. The filtration step to remove suspended impurities (typically fine solids containing Fe and / or Si), which have not yet been removed from the solution during clarification, is critical to ensure that such impurities do not contaminate the precipitated Al2O3 product. This impurity removal step is typically referred to as “safety 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 Southeast Asia).These filtration systems make use of a "filter aid" bed (or cake) of particles supported by a sieve, cloth, or filter membrane. The filter aid is... Petition 870250092346, dated 09 / 10 / 2025, page 12 / 40 2 / 20 typically tricalcium aluminate (Ca3[Al(OH)6]2), hereinafter abbreviated as TCA, is produced by the reaction of a portion of the process liquor, containing sodium aluminate NaAl(OH)4, with Ca(OH)2, in a molar ratio suitable for plant operation, generally from 0.5:1 to 3:1, but specific to plant conditions. The reagent Ca(OH)2 is typically added to the reactor as a slaked lime paste, 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.

[003] The feed liquor temperature for the process is typically in the range of 95-105°C. The slaked lime paste is preferably at a similar temperature or as high as practically possible, generally in the range of 70-95°C. The filter aid for the TCA 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 + 2NaAl(OH)4 > Ca3[Al(OH)6]2 + 2NaOH

[004] It is important to note that it is the TCA filter aid, rather than the filter cloth, that performs the filtering function. The properties of the TCA filter aid are therefore critical to the filtration performance. If the TCA particles are too fine, the voids in the filter bed may be too small, thus slowing down the filtration process and increasing backpressure filtration. Operationally, this requires frequent backwashing to prevent clogging, thus causing congestion in the plant flow. If the TCA particles are too coarse, the voids between the particles may be too large and suspended impurities are not sufficiently removed during the filtration process. The ideal TCA filter aid particle size and distribution are different for each alumina refinery and are dependent on several factors. Petition 870250092346, dated 09 / 10 / 2025, page 13 / 40 3 / 20 of the process and equipment, such as: the required flow rate, the load and particle size of suspended impurities, the type of filter, filtration conditions and filter cloth, type of membrane or sieve used.

[005] The problem for alumina refineries is that the optimum particle size distribution of the TCA filter aid, necessary to achieve filtration efficiency, cannot be controlled as a process parameter independent of the lime source. For example, US document 2004 / 0101470 A1, paragraph

[0024] describes the technical bias that the physical characteristics of the TCA filter aid are very highly dependent on the nature of the lime particles from which it is formed. Similarly, 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 particle size distribution of TCA.Thus, the particle size distribution of TCA is experienced operationally as a fixed consequence of the properties of the quicklime reagent used, as well as a combination of other plant conditions that are known to influence the particle size of TCA.

[006] However, this approach limits the ability to use alternative sources of quicklime reagent, even if using such alternatives may have other benefits, such as greater utilization efficiency. Frequently, the quicklime that provides satisfactory TCA particle size distribution is of low quality and low available CaO content. The use of such quicklime increases lime consumption due to low utilization efficiency, not only for the production of TCA filter aid, but also in other parts of the Bayer process alumina refinery where lime is used.

[007] A common experience in alumina refineries is that high-quality (i.e., higher % of available CaO) and highly reactive quicklime, generally Petition 870250092346, dated 09 / 10 / 2025, p. 14 / 40 4 / 20 changes the TCA towards a finer particle size distribution, without the reasons for this phenomenon being understood or articulated in the scientific or patent literature. Without an adequate control mechanism, alumina refineries are limited in their use of a lime reagent to that which provides acceptable TCA filter aid properties, even if the other properties of said lime reagent are suboptimal. This is particularly problematic for alumina refineries that require coarse TCA filter aid, i.e., with a d50 greater than 20 μm. Depending on the plant facilities and / or ore compositions, some refineries will require a specific TCA particle size distribution used as a filter aid.

[008] Thus, there is a need to provide an improved process for obtaining tricalcium aluminate with calibrated size without resorting to the use of lime reagent having low available CaO content. In addition, there is also a need for a process for obtaining tricalcium aluminate with calibrated size that can be used for the manufacture of coarse TCA filter aid. SUMMARY OF THE INVENTION

[009] In order to solve the problems and disadvantages of the previous technique mentioned above, the present invention relates to a process for obtaining tricalcium aluminate (TCA) particles with calibrated size comprising the steps of:

[010] a) provision of a process liquor containing sodium aluminate;

[011] b) provision of a suspension of calcium hydroxide particles with calibrated size;

[012] c) mixing said selected suspension of calibrated-size calcium hydroxide particles with said process liquor to allow them to react in order to obtain said calibrated-size tricalcium aluminate particles. Petition 870250092346, dated 09 / 10 / 2025, page 15 / 40 5 / 20

[013] Tricalcium aluminate (Caa[Al(OH)6]2) is hereinafter referred to as TCA.

[014] Sodium aluminate is hereinafter used to describe 2NaAl(OH)4 .

[015] The term “tricalcium aluminate particles with calibrated size” means, according to the invention, tricalcium aluminate particles with an optimum particle size distribution for use as a TCA filter aid in an alumina refining process. As mentioned above, the optimum TCA particle size is a mean between very fine and very coarse, to provide efficient filtration of suspended impurities while minimizing filter clogging, backpressure and flow problems. The particle size distribution is expressed as d50 measured by laser diffraction granulometry in methanol after sonication.

[016] The d50 of calibrated-size tricalcium aluminate particles may vary according to plant requirements, for example, calibrated-size tricalcium aluminate particles may have a d50 greater than 5 μm, preferably greater than 10 μm, more preferably greater than 15 μm, most preferably greater than 20 μm. In another embodiment of the invention, the d50 of calibrated-size tricalcium aluminate particles may be less than 200 μm, preferably less than 100 μm, even more preferably less than 50 μm.

[017] The term “calibrated size calcium hydroxide particles” means, according to the invention, calcium hydroxide particles having an optimum particle size distribution that is a mean between very fine and very coarse for obtaining optimal calibrated size TCA particles.

[018] It was surprisingly discovered 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 calcium hydroxide particles by a generally linear relationship. Thus, the technical bias Petition 870250092346, dated 09 / 10 / 2025, page 16 / 40 The long-held assumption that highly reactive quicklime generally shifts the TCA toward a finer particle size distribution has been surprisingly overcome. The slope of the linear relationship is specific to particular conditions, including: the solution composition, the molar ratio of Ca(OH)2 to NaAl(OH)4, the temperature of both the Ca(OH)2 reagent and the alumina refinery liquor, the maximum stirring speed (i.e., shear force), the hydraulic retention time, the particle retention time, the extent of precipitate seeding, and seed recycling. The relationship is also dependent on the properties of the quicklime used to produce Ca(OH)2, including its reactivity (as measured by its reaction rate with water) and its chemical composition. For all these reasons, the precise relationship between the particle size distribution of Ca(OH)2 and the TCA can be determined for each particular plant scenario.The determination of the relationship is based on the generation of a range of Ca(OH)2 reagents with increasing particle size distribution (meaning d25, d50 and d90 are increased), and the observation of the impact of using these reagents on the particle size distribution of the resulting TCA.

[019] Preferably, said suspension of calibrated-size calcium hydroxide particles is obtained from quicklime slaking, 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 in accordance with standard EN4592:2010. Quicklimes having an available lime content greater than 90% are also generally referred to as “high-quality quicklime”.

[020] 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, that is, by using calcium hydroxide. Petition 870250092346, dated 09 / 10 / 2025, page 17 / 40 7 / 20 with calibrated size.

[021] Calibrated-size calcium hydroxide can be produced from high-quality quicklime having an available lime content greater than 85% as measured according to the EN459-2 standard, and generally exhibits 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 the generation of TCA filter aid, by reducing the consumption of lime reagent.

[022] In one embodiment of the invention, said suspension of calcium hydroxide particles with calibrated size is obtained from a quicklime slaking process in the presence of a thickening additive causing a thickening effect on the particle size distribution of the calcium hydroxide particles obtained by said slaking process.

[023] In one embodiment of the invention, said suspension of calcium hydroxide particles with calibrated size is obtained from a slaking process of formulated quicklime, prepared from quicklime to which a thickening additive has been added by spraying a solution containing a thickening additive onto quicklime.

[024] Preferably, said suspension of calibrated-size calcium hydroxide particles is obtained from a quicklime slaking process in the presence of 2% by weight or less, preferably 0.1 to 1.5% by weight, preferably 0.15 to 0.8% by weight and more preferably 0.2 to 0.65% by weight of a thickening additive relative to the weight of the quicklime, wherein said thickening additive is selected from among alkaline sulfates or alkaline earth sulfates.

[025] In a particular embodiment of the invention, said thickening additive is sodium sulfate. Petition 870250092346, dated 09 / 10 / 2025, page 18 / 40 8 / 20

[026] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a process in which:

[027] - a concentrated aqueous solution of thickening additive, in particular sodium sulfate, is provided by dissolving an amount of thickening additive of at least 10% by weight, preferably at least 30% by weight in water at a dissolution temperature adapted to dissolve such an amount of thickening additive;

[028] - said concentrated aqueous solution of thickening additive at said dissolution temperature is sprayed onto quicklime in an amount 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;

[029] - slaking of said quicklime formulated with water to provide said suspension of calcium hydroxide particles with calibrated size.

[030] Preferably, said suspension of calibrated-size calcium hydroxide particles is obtained from a process of slaking quicklime with a quantity of water such that the solid content in the milk of lime comprises between 10 and 45% by 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% by weight of the total weight of the milk of lime.

[031] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a process of partial slaking of prehydrated quicklime particles having at least a partially superficial layer of hydrated lime and a core of quicklime.

[032] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a process of Petition 870250092346, dated 09 / 10 / 2025, page 19 / 40 9 / 20 slaking of quicklime in a reactor cooled to a temperature less than or equal to 60 °C. The hydrated lime particles obtained under such conditions are generally coarser than the hydrated lime particles obtained under temperature conditions between 60 °C and 95 °C.

[033] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a quicklime slaking process in a reactor at a temperature greater than or equal to 95 °C. The hydrated lime particles obtained under such conditions are generally coarser than the hydrated lime particles obtained under temperature conditions between 60 °C and 95 °C.

[034] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a quicklime slaking process to obtain a first calcium hydroxide suspension followed by a particle size selection step of said first suspension to remove calcium hydroxide particles that have a particle size below a predetermined particle size and to retain calcium hydroxide particles larger than a predetermined particle size to obtain, possibly after dilution, said suspension of calibrated-size calcium hydroxide particles.

[035] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first calcium hydroxide suspension followed by:

[036] - a first step of selecting the particle size of said first suspension to remove calcium hydroxide particles having a particle size smaller than a predetermined first particle size and to retain calcium hydroxide particles larger than a Petition 870250092346, dated 09 / 10 / 2025, page 20 / 40 10 / 20 first predetermined particle size for obtaining, possibly after dilution, a second suspension of calcium hydroxide particles;

[037] - a second stage of particle size selection of said second calcium hydroxide particle suspension to remove calcium hydroxide particles having a particle size larger than a second predetermined particle size larger than said first predetermined particle size and to retain the remaining calcium hydroxide particles having a particle size between said first predetermined particle size and said second predetermined particle size to obtain, possibly after dilution, said calcium hydroxide particle suspension with calibrated size.

[038] In one embodiment of the invention, said suspension of calcium hydroxide particles with calibrated size is obtained from a fluidization process of hydrated lime where the hydrated lime was prepared by dry hydration and its particle size was controlled by grinding and sieving to obtain the target size.

[039] Preferably, said calibrated size calcium hydroxide particles have a predetermined particle size distribution for obtaining TCA particles with a desired particle size distribution based on a calibration curve obtained by performing said steps a) ac) with at least three calcium hydroxide suspensions of different predetermined particle size distribution.

[040] All the modalities described here can be used alone or in a combination of them.

[041] In a second aspect, the present invention relates to the use of coarse tricalcium aluminate particles obtained by the process described above in an alumina refining process from an ore comprising Petition 870250092346, dated 09 / 10 / 2025, page 21 / 40 11 / 20 aluminum minerals.

[042] In another aspect, the present invention relates to the use of coarse tricalcium aluminate particles obtained by the process as described above in a process for manufacturing TCA filter aid. BRIEF DESCRIPTION OF THE DRAWINGS

[043] Figure 1 shows an image obtained by scanning electron microscopy of the TCA particles obtained according to the process of the invention. DESCRIPTION OF THE INVENTION

[044] According to a first aspect, the present invention relates to a process for obtaining tricalcium aluminate (TCA) particles with calibrated size comprising the steps of:

[045] a) provision of a process liquor containing sodium aluminate;

[046] b) provision of a suspension of calcium hydroxide particles with calibrated size;

[047] c) mixing said selected suspension of calibrated-size calcium hydroxide particles with said process liquor to allow them to react in order to obtain said calibrated-size tricalcium aluminate particles.

[048] Preferably, said calibrated-size calcium hydroxide suspension is obtained from quicklime slaking, 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 in accordance with standard EN459-2:2010. Quicklime having an available lime content greater than 90% is also generally referred to as “high-quality quicklime”.

[049] A typical alumina refining process is described in the document “The chemistry of CaO and Ca(OH)2”, BI Whittington, Hydrometallurgy 43 (1996) 13-35. An ore containing high amounts of aluminum such as bauxite is Petition 870250092346, dated 09 / 10 / 2025, p. 22 / 40 12 / 20 crushed and ground. The resulting product is transferred into a first tank for a pre-desilication stage where the crushed ore is brought into contact with milk of lime under agitation at temperatures of approximately 100 °C before being transferred to a digestion tank where a highly concentrated solution of NaOH and milk of lime are injected. This digestion stage occurs under high pressure and high temperature. The resulting slurry obtained after digestion is cooled and sent to a sedimentation tank to settle the solid fraction from the liquid fraction, here called process liquor. The solid fraction is washed with water in a series of countercurrent settling scrubbers. One of the overflow eluents from the scrubber is sent to a tank for causticizing where milk of lime is added to increase alkalinity by converting calcium carbonate to sodium hydroxide.In a separate process step, a small portion of the process liquor is reacted with milk of lime, also referred to as a suspension of calcium hydroxide particles, to obtain tricalcium aluminate particles that 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 alumina product, Al2O3.

[050] Since the alumina refining process comprises different stages involving the use of lime milk, the use of a lime milk derived from high-quality quicklime is advantageous for use in other components of the alumina refining process, another generation of TCA filter aid, by reducing the consumption of lime reagent.

[051] It has been found that it is possible to control the particle size distribution of tricalcium aluminate particles by controlling the particle size distribution of calcium hydroxide particles. According to the Petition 870250092346, dated 09 / 10 / 2025, page 23 / 40 13 / 20 invention, there are several ways to control the particle size distribution of calcium hydroxide particles to be used for the generation of tricalcium aluminate particles with calibrated size.

[052] In one embodiment of the invention, quicklime can be slaked in the presence of a thickening additive causing a thickening effect on the particle size distribution of the calcium hydroxide particles obtained by said slaking process.

[053] Preferably, said thickening additive is selected from alkaline sulfate such as sodium sulfate as a non-limiting example or alkaline earth sulfates such as calcium sulfate as a non-limiting example.

[054] The thickening additive can be added to the quicklime before the slaking stage, for example, by spraying said thickening additive onto quicklime and then the slaking stage is carried out with a sufficient quantity of water to obtain a suspension of calcium hydroxide particles. Alternatively or in combination with the spraying stage, the thickening additive can be added during the slaking stage, for example, in the slaking water.

[055] Preferably, the quicklime slaking step is carried out in the presence of less than 2% by weight, preferably 0.1 to 1.5% by weight, preferably 0.15 to 0.8% by weight and more preferably 0.2 to 0.65% by weight of a thickening additive relative to the weight of the quicklime. A minimum amount of thickening additive is required to provide the thickening effect on the calcium hydroxide particles and a maximum amount of thickening additive is adjusted to maintain the suspension of calcium hydroxide particles above a certain degree of purity and to prevent the formation of very coarse calcium hydroxide particles.

[056] In one embodiment of the invention, a suspension of particles of Petition 870250092346, dated 09 / 10 / 2025, page 24 / 40 14 / 20 calibrated size calcium hydroxide is obtained from a process in which:

[057] - a concentrated aqueous solution of sodium sulfate is provided by dissolving an amount of sodium sulfate of at least 10% by weight, preferably at least 35% by weight in water at a dissolution temperature adapted to dissolve such an amount;

[058] - said concentrated sodium sulfate solution at said dissolution temperature is sprayed onto quicklime in a quantity 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;

[059] - the said formulated quicklime is slaked with water to provide the said suspension of calcium hydroxide particles with calibrated size.

[060] Preferably, in the quicklime slaking stage, the amount of water is adapted to provide a calcium hydroxide suspension (or milk of lime) in which the solids content in the milk of lime is between 10 and 45% by weight of the total weight of the milk of lime, preferably with a solids content in the milk of lime of up to 40%, preferably up to 35%, more preferably up to 30%, most preferably up to 25% by weight of the total weight of the milk of lime. A minimum amount of solids content is required to provide a sufficient quantity of calcium hydroxide in a minimum volume of suspension and a maximum amount of solids content is preferably adjusted to minimize the volume of the suspension while maintaining good flowability and pumpability of the suspension.

[061] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a process of partial slaking of prehydrated quicklime particles having at least a partially superficial layer of hydrated lime and a core of quicklime.

[062] In one embodiment of the invention, said suspension of particles of Petition 870250092346, dated 09 / 10 / 2025, page 25 / 40 15 / 20 Calcium hydroxide with calibrated size is obtained from a quicklime slaking process in a reactor cooled to a temperature of 60 °C or less. The hydrated lime particles obtained under such conditions are generally coarser than the hydrated lime particles obtained under temperature conditions between 60 °C and 95 °C.

[063] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a quicklime slaking process in a reactor at a temperature greater than or equal to 95 °C. The hydrated lime particles obtained under such conditions are generally coarser than the hydrated lime particles obtained under temperature conditions between 60 °C and 95 °C.

[064] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a quicklime slaking process to obtain a first calcium hydroxide suspension followed by a particle size selection step of said first suspension to remove calcium hydroxide particles that have a particle size below a predetermined particle size and to retain calcium hydroxide particles larger than a predetermined particle size to obtain, possibly after dilution, said suspension of calibrated-size calcium hydroxide particles.This particle size selection step can be done, for example, by cutting the said first suspension with a sieve selected from commercially available sieves having sieve openings smaller than 125 μm (120 mesh), preferably smaller than 105 μm (140 mesh), more preferably smaller than 88 μm (170 mesh), more preferably smaller than 74 μm (200 mesh), more preferably smaller than 63 μm (230 mesh).

[065] In one embodiment of the invention, said suspension of calibrated-size calcium hydroxide particles is obtained from a process of Petition 870250092346, dated 09 / 10 / 2025, page 26 / 40 16 / 20 slaking of quicklime to obtain a first suspension of calcium hydroxide followed by:

[066] - a first step of selecting the particle size of said first suspension to remove calcium hydroxide particles having a particle size smaller than a predetermined first particle size and to retain calcium hydroxide particles larger than a predetermined first particle size in order to obtain, possibly after dilution, a second suspension of calcium hydroxide particles.

[067] - a second stage of particle size selection of said second calcium hydroxide particle suspension to remove calcium hydroxide particles having a particle size larger than a second predetermined particle size larger than said first predetermined particle size and to retain the remaining calcium hydroxide particles having a particle size between said first predetermined particle size and said second predetermined particle size to obtain, possibly after dilution, said calcium hydroxide particle suspension with calibrated size.

[068] The first stage of particle size selection can be done, for example, by cutting said first suspension with a sieve selected from commercially available sieves having sieve openings smaller than 125 μm (120 mesh), preferably smaller than 105 μm (140 mesh), more preferably smaller than 88 μm (170 mesh), more preferably smaller than 74 μm (200 mesh), more preferably smaller than 63 μm (230 mesh). Subsequently, the second stage of particle selection can be done by cutting said second suspension with a sieve selected from commercially available sieves having sieve openings larger than those of the sieve used in the first stage of particle selection, such as sieves having openings larger than Petition 870250092346, dated 09 / 10 / 2025, page 27 / 40 17 / 20 105 μm (140 mesh), preferably larger than 125 μm (120 mesh), more preferably larger than 149 μm (100 mesh), more preferably larger than 177 μm (80 mesh).

[069] Preferably, in the process for obtaining tricalcium aluminate with calibrated size, said calibrated calcium hydroxide particles have a predetermined particle size distribution for obtaining TCA particles with a desired particle size distribution based on a calibration curve obtained by carrying out said steps a) ac) with at least three calcium hydroxide suspensions of different predetermined particle size distribution.

[070] In one embodiment of the invention, the calcination of limestone into quicklime can also be carried out using fuels (such as high-sulfur 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) to the quicklime product. It may be advantageous to measure the sulfur content in the quicklime and adapt the amount of thickening additive to be added for the slaking step according to the amount of sulfur in the quicklime, such as avoiding the formation of very coarse particles of calcium hydroxide. It is possible to establish, according to the invention, a relationship between the additive concentration, the particle size of Ca(OH)2 and the particle size of TCA, and this relationship is normally different for a quicklime with a low sulfur content and a quicklime with a higher sulfur content.

[071] Advantageously, in an alumina refining process, calibrated-size calcium hydroxide particles, preferably obtained from quicklime having an available lime content greater than or even higher than 82%, can be used at different stages of the process, including the tricalcium aluminate formation step. Therefore, the TCA particle manufacturing method provides not only Petition 870250092346, dated 09 / 10 / 2025, page 28 / 40 18 / 20 is a control method for the particle size distribution of TCA particles, but it also provides the possibility of using high-quality quicklime, with an available CaO content equal to or greater than 82%, to generate high-quality lime milk, thus increasing the efficiency of using this reagent in all units of the alumina refining process where it is used. EXAMPLES

[072] The effect of different types of milk of lime on the formation of tricalcium aluminate was evaluated by the reaction of those milks of lime with a synthetic liquor of the Bayer process.

[073] Bayer synthetic liquor can be prepared in an autoclave by heating an aqueous mixture containing 6 mol / dm3 of NaOH, 2.2 mol / dm3 of Al(OH)3 and 0.4 mol / dm3 of Na2CO3 to 180 °C while stirring.

[074] Three samples of formulated quicklime are prepared by mixing quicklime having the properties listed in Table 1 with, respectively, 0.3% by weight, 0.5% by weight and 0.7% by weight of Na2SO4 in said quicklime. TABLE 1 Total CaO >94% by weight, MgO < 2% by weight, SiO2 < 0.5% by weight, S < 0.04% by weight, CO2 < 1.9% by weight, C < 0.5% by weight, P < 0.03% by weight. Quicklime size: 10-60 mm. Reactivity time: T60 < 2 min. Petition 870250092346, dated 09 / 10 / 2025, page 29 / 40 19 / 20

[075] The said three samples of formulated quicklime were ground to < 5 mm and then slaked in water at room temperature in a slaker for 20 minutes with an amount of water to obtain approximately 16% by weight of solids content in the lime milk.

[076] The d50 particle size distribution for those three lime milks obtained is measured by laser diffraction and methanol granulometry after sonication. The d50 of the calcium hydroxide particles of the three lime milks obtained increases substantially and linearly with the amount of Na2SO4 in the formulated quicklime. The measured d50 of the calcium hydroxide particles is provided in Table 2.

[077] Three samples of tricalcium aluminate are prepared from 200 ml of Bayer process synthetic liquor as prepared above and an appropriate amount of one of three samples of milk of lime as prepared above such that the molar ratio of calcium to aluminum expressed in CaO equivalent to Al2O3 is 1 to 9. For each sample, 200 cm3 of Bayer process synthetic liquor are heated to 80 °C in a vessel preferably made of a corrosion-resistant, high-strength nickel-chromium material, also known commercially as INCONEL®. Then, the appropriate amount of the respective milk of lime is added to the vessel under stirring with a stirrer at 250 RPM. When the fluid paste reaches 95 °C, the fluid paste is allowed to react for 90 minutes at the same stirring rate. The fluid paste is then cooled to 70 °C for filtration.

[078] Figure 1 shows a Scanning Electron Microscopy image of the TCA particles obtained according to the invention process. It can be seen that the TCA particles are formed of aggregated crystals with dimensions below 5 μm, and that those TCA particles larger than 15 μm, as is often required for the effective operation of static vertical pressure leaf filters.

[079] The d50 of each of the tricalcium aluminate samples is measured by Petition 870250092346, dated 09 / 10 / 2025, pages 30 / 40 20 / 20 particle size distribution by laser diffraction in methanol without sonication. The d50 of the tricalcium aluminate particles increases substantially and linearly with the d50 of the calcium hydroxide particles in the milk of lime used. The d50 values ​​for the tricalcium aluminate particles and the respective d50 values ​​of the calcium hydroxide particles used to prepare those TCA particles are presented in Table 2. The amount of milk of lime used in the process of preparing the TCA particles is also included in Table 2. TABLE 2 Concentration of Na2SO4 additive (% w / w) in relation to the dry mass of quicklime dso of Ca(OH)2 particles in lime milk (pm) Amount of lime milk (16% by weight of solids content) used for the preparation of TCA (g) dso of TCA (Mm) 0.3 92 15.7 19 0.5 110 16.6 24 0.7 144 16.7 28

[080] This example shows that, by the process according to the present invention, it is possible to control the particle size distribution of tricalcium aluminate particles by adding a thickening agent and then using calcium hydroxide particles with calibrated particle size distribution to obtain TCA with calibrated size.

[081] In the quicklime used for the examples mentioned above, the sulfur content was relatively low (less than 0.04% by weight of the quicklime). Petition 870250092346, dated 09 / 10 / 2025, page 31 / 40

Claims

1 / 4 CLAIMS 1. Process for obtaining tricalcium aluminate (TCA) particles with calibrated size CHARACTERIZED in that it comprises the steps of: a) providing a process liquor containing sodium aluminate; b) providing a suspension of calcium hydroxide particles with calibrated size; c) mixing said selected suspension of calcium hydroxide particles with calibrated size with said process liquor to react them to obtain said tricalcium aluminate particles with calibrated size;wherein said calibrated-size calcium hydroxide particles have a predetermined particle size distribution for obtaining TCA particles with a desired particle size distribution based on a calibration curve obtained by performing said steps a) ac) with at least three calcium hydroxide suspensions of different predetermined particle size distribution, wherein in the process liquor, the process refers to an alumina refining process, or Bayer® process, focused solely on alumina refineries.

2. Process, according to claim 1, CHARACTERIZED in that said suspension of calcium hydroxide particles with calibrated size is obtainable from the slaking of quicklime having an available lime content (CaO) greater than 82.5%, measured according to the EN459-2:2010 standard.

3. Process, according to claim 1 or 2, CHARACTERIZED in that said suspension of calcium hydroxide particles with calibrated size is obtained from a quicklime slaking process in the presence of a thickening additive causing a thickening effect on the particle size distribution of the calcium hydroxide particles obtainable by said slaking process, wherein the thickening additive is selected from among alkaline sulfates or alkaline earth sulfates.

4. Process, according to claim 3, CHARACTERIZED in that said suspension of calcium hydroxide particles with calibrated size is obtained from a slaking process of formulated quicklime, prepared from quicklime to which a thickening additive has been added by spraying a solution containing a thickening additive onto the quicklime.

5. Process, according to claim 3, CHARACTERIZED in that said suspension of calcium hydroxide particles with calibrated size is obtained from a slaking process of quicklime in the presence of 0.1 to 1.5% by weight of a thickening additive relative to the weight of the quicklime, wherein said thickening additive is selected from among alkaline sulfates or alkaline earth sulfates.

6. Process according to claim 3, CHARACTERIZED in that said suspension of calibrated-size calcium hydroxide particles is obtained from a process in which: - a concentrated aqueous solution of thickening additive, preferably sodium sulfate, is provided by dissolving an amount of thickening additive of at least 10% by weight in water at a dissolution temperature adapted to dissolve such amount of thickening additive; - said concentrated aqueous solution of thickening additive at said dissolution temperature is sprayed onto quicklime with an amount 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 calibrated-size calcium hydroxide particles.

7. Process, according to claim 3, CHARACTERIZED by the fact Petition 870260062174, dated 06 / 25 / 2026, page 6 / 14 3 / 4 that said suspension of calcium hydroxide particles with calibrated size is obtained from a process of slaking quicklime with a quantity of water such that the solid content in the milk of lime is between 10 and 45%, by weight of the total weight of the milk of lime.

8. Process, according to claim 3, CHARACTERIZED in that said suspension of calibrated-size calcium hydroxide particles is obtained from a process of partial slaking of pre-hydrated quicklime particles having at least a partially superficial layer of hydrated lime and a core of quicklime.

9. Process, according to claim 3, CHARACTERIZED in that said suspension of calcium hydroxide particles with calibrated size is obtained from a quicklime slaking process in a reactor cooled to a temperature less than or equal to 60 °C.

10. Process according to claim 3, CHARACTERIZED in that said suspension of calcium hydroxide particles with calibrated size is obtained from a quicklime slaking process in a reactor at a temperature greater than or equal to 95 °C.

11. Process, according to any one of claims 3 to 10, CHARACTERIZED in that said suspension of calibrated-size calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first suspension of calcium hydroxide followed by a particle size selection step of said first suspension to remove calcium hydroxide particles that have a particle size below a predetermined particle size and to retain calcium hydroxide particles larger than a predetermined particle size to obtain, possibly after dilution, said suspension of calibrated-size calcium hydroxide particles. Petition 870260062174, dated 06 / 25 / 2026, page 7 / 14 4 / 4 12. Process, according to any one of claims 3 to 11, CHARACTERIZED in that said suspension of calibrated-size calcium hydroxide particles is obtained from a process of slaking quicklime to obtain a first calcium hydroxide suspension followed by: - ​​a first particle size selection step of said first suspension to remove calcium hydroxide particles having a particle size smaller than a predetermined first particle size and to retain calcium hydroxide particles larger than a predetermined first particle size in order to obtain, possibly after dilution, a second suspension of calcium hydroxide particles;- a second stage of particle size selection of said second calcium hydroxide particle suspension to remove calcium hydroxide particles having a particle size larger than a second predetermined particle size larger than said first predetermined particle size and to retain the remaining calcium hydroxide particles having a particle size between said first predetermined particle size and said second predetermined particle size to obtain, possibly after dilution, said calcium hydroxide particle suspension with calibrated size. Petition 870260062174, dated 06 / 25 / 2026, page 8 / 14;

Images