bauxite processing methods

Abstract

A method for producing alumina from bauxite pre-processed by a process including calcination (2020) and leaching (2030), wherein the pre-processed bauxite is characterized by a loss on ignition of less than 2.5% by mass, the method comprising the steps of: (a) processing the pre-processed bauxite with an aqueous sodium hydroxide solution at a temperature of at least 100°C (2120), wherein the concentration of the aqueous sodium hydroxide solution is between 100 g Na2O / L and 220 g Na2O / L, preferably between 140 g Na2O / L and 200 g Na2O / L, more preferably between 155 g Na2O / L and 190 g Na2O / L, and even more preferably between 160 g Na2O / L and 180 g Na2O / L. (a) Na2O / L; (b) Separating the solid residue from the liquid phase (2130); (c) Crystallizing aluminum trihydrate by adding seed crystals (2170) (2160); (d) Separating the crystallized aluminum trihydrate from the liquid phase (2180); (e) Calcincing the aluminum trihydrate obtained in step (d) (2194) to obtain alumina (2196).

Description

Technical Field

[0001] This invention relates to the field of ore processing, and more particularly to the physical and chemical processing of bauxite. Specifically, this invention relates to a method for the thermal and chemical processing of bauxite with a low alumina / silica mass ratio. In this method, silicon is first removed from the ore by thermal pre-processing and subsequent leaching. The pre-processed ore is then used in the Bayer process to extract aluminum in the form of aluminum trihydrate, which can be converted into alumina. Background Technology

[0002] Aluminum is the third most abundant chemical element in the Earth's crust, after oxygen and aluminum. Aluminum, bound to oxygen, is found in numerous rocks. The main industrial ore for aluminum is bauxite, discovered in 1821 by geologist Pierre Berthier in the village of Les Baux (France). Bauxite is a complex mixture of aluminum oxides, iron oxides, and silicon oxides, and may contain various impurities such as titanium, calcium, and magnesium. More precisely, bauxite is a ore that primarily contains three minerals: gibbsite, boehmite, and diaspore, with small amounts of iron minerals, namely goethite and hematite (which give bauxite its characteristic color), as well as aluminosilicates (kaolinite, illite, etc.) and titanium minerals (anatase, rutile, ilmenite).

[0003] The main industrial method for extracting aluminum (aluminum in oxide form) from bauxite is the Bayer process, invented in the late 19th century. This method mainly comprises two steps: a first step in which the ore is leached under pressure with a sodium hydroxide solution (see patent DE 43 977, August 3, 1888); and a second step in which pure hydrated alumina is precipitated from the resulting sodium aluminate solution by inoculation with crystals of hydrated alumina (see patent DE65604, February 3, 1892). This precipitated hydrated alumina can then be thermally processed to dehydrate it; this thermal processing also determines the structure and morphology of the obtained alumina, taking into account its intended use (in the Hall-Héroult process, aluminum is prepared by electrochemical reduction of alumina in molten salt or technical alumina, particularly in the ceramics industry).

[0004] More specifically, the Bayer process primarily involves the selective erosion (dissolution) of alumina hydrates contained in bauxite using a recycled hot caustic soda solution (referred to as "caustic soda water"). After separation by decantation and washing of bauxite residues (referred to as "red mud"), a sodium hydroxide solution rich in sodium aluminate is cooled and subsequently decomposed (crystallization stage) to precipitate and extract alumina trihydrate (Al₂O₃-3H₂O); the latter is then washed and subsequently calcined at high temperature to obtain alumina (Al₂O₃). The caustic soda water, which has been depleted of sodium aluminate after the crystallization stage and diluted with feed water (primarily from washing bauxite residues), is evaporated and recycled to the erosion stage.

[0005] The composition of bauxite depends on its geographical origin. This variation in composition is related to the content of its main elements (Al, O, Si) and their impurities, as well as its mineral phase structure. For example, in some karst bauxite deposits (located on carbonate geological substrates) from mines in Iran, Kazakhstan, Azerbaijan, and Turkey, aluminum is primarily in the forms of boehmite and diaspore (two variations of monohydrates), while lateritic bauxite (located on aluminosilicate geological substrates) from mines in Guinea or Australia has a higher content of gibbsite (which is a trihydrate) and a lower content of silicon. Therefore, a simple parameter indicating the quality of bauxite is the ratio of alumina to silica, abbreviated as "A / S ratio". For example, bauxite from Guinea has an Al₂O₃ / SiO₂ ratio of approximately 20 or higher, while bauxite from Western Australia has an Al₂O₃ / SiO₂ ratio greater than 15. In bauxite deposits in North Queensland, aluminum exists primarily in the forms of boehmite and gibbsite.

[0006] However, aluminum content is not the only criterion: aluminum must also possess the chemical and crystalline forms that allow it to be extracted from bauxite via the Bayer process. It is known that the conventional Bayer process cannot dissolve aluminum contained in aluminosilicates: this portion of aluminum is lost in the red mud. Furthermore, it is known that aluminosilicates contained in the red mud carry away some sodium hydroxide, thus increasing the total sodium hydroxide consumption in the Bayer process; these are described in Xiaofeng Zhu et al.'s publication, "Basic Research on Calcification Transformation, Process of Low Grade Bauxite" (published in LightMetals 2013, pp. 239-244 (TMS)).

[0007] Like most ores, bauxite is traded on the world market and is often transported thousands of kilometers to its point of use. However, some countries tend to limit their purchases on the world market to support the use of their domestic mineral resources, even if those resources are of lower quality. This is particularly true of China. For example, as the world's largest aluminum producer, China's bauxite deposits and mines do not (or no longer) have the high A / S ratios of Australian bauxite. More precisely, in China, alumina deposits with low silica content (A / S > 8 or even > 6) are becoming increasingly scarce, while reserves of bauxite with high silica content are quite large. Furthermore, in many Chinese bauxite deposits with high silica content, most of the silica is present in the form of kaolinite, an aluminosilicate that also contains some of the aluminum present in the bauxite; these Chinese bauxite deposits also contain small amounts of quartz (an aluminosilicate) and muscovite (another aluminosilicate). However, as mentioned above, the conventional Bayer process cannot extract the aluminum present in the aluminosilicates.

[0008] For decades, primary aluminum production (and the corresponding consumption of bauxite) has increased regularly by a certain percentage each year. In recent years, the use of lower-quality bauxite has become a major economic and technical problem, particularly in Iran, Kazakhstan, and China. This problem has also emerged in other countries, such as Russia and Turkey, where researchers are particularly investigating the possibility of using bauxite with a low A / S ratio. Some of these lower-quality bauxite also have higher iron and / or sulfur content compared to aluminum-rich bauxite.

[0009] To alter the ore phase structure of bauxite, various preprocessing methods exist, including increasing the extractable proportion of aluminum in the Bayer process and facilitating silicon separation upstream of the Bayer process. Additionally, methods aim to reduce red mud formation and recover these elements in a usable form, thereby promoting iron separation upstream of the Bayer process.

[0010] For example, the article “Pre-beneficiation of low-grade Diasporic bauxite ore by reduction roasting” published in the 2015 review Int. J. Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering, Vol. 9(9), pp. 1023-1026 describes the calcination (referred to as “roasting”) of bauxite from Turkey with high Si and Fe content to convert iron into a phase that can be separated by magnetic methods. Other methods of calcination in reducing media have used CO (CN 103 614547 – Central South University; CN 104 163 445 – China Aluminum) or coal (CN 101 875 129 – Central South University).

[0011] For bauxite with a low A / S ratio, a method known as “calcination-carbonation” is described (e.g., see Ting'an Zhang et al., K. Yilmaz et al., “Calcification-carbonation method for alumina production by using low-grade bauxite”, Light Metals 2013, pp. 233-238 (TMS)). This method involves: processing the bauxite with an alkaline solution in the presence of lime (resulting in the formation of an insoluble calcium-aluminum-silicate phase); and processing this insoluble phase with pressurized CO2 (after decantation), thereby releasing some aluminum by forming two new insoluble phases, Ca3SiO4 and CaCO3.

[0012] Some other methods describe, for example, calcining a solid mixture of bauxite and Na₂CO₃ at 600°C to 1000°C, then dissolving the calcined mixture in NaOH at 75°C, followed by inoculation to precipitate a silicon-carrying phase (CN 102 180498 and CN 101 767 807, Aifang Pan); in an alternative to the method described in CN 205 603 238 (Hangzhou Jinjiang), the calcined mixture also contains lime; and Na₂CO₃ reacts with water to form an alkali, which is capable of leaching silica. Methods are also known in which bauxite is calcined without the addition of any product. CN 203 408047 describes a method for desulfurizing bauxite containing pyrite (FeS₂) (Xi'an University).

[0013] Smith and Xu-Parker (“Options for processing of high silica bauxites”, Travaux ICSOBA Vol. 35(39), 184-192 (2010)) described a thermochemical activation method for bauxite (called the “roasting-leaching method”). This method was originally developed for clay-type feedstocks (see US 2 939 764) and illite (see TJiang et al., “Desilication from illite by thermochemical activation”, Trans. Nonferrous Met. Soc. China, Vol 14(5), 1000-1005 (2004)), as well as bauxite containing diaspore monohydrate and without silicon (Q Zhou et al., “Temperature dependence of crystal structure and digestibility of roasted diaspore”, Trans. Nonferrous Met. Soc. China, Vol 14(1), 180-183 (2004)). In practice, the method comprises two distinct steps: the first step involves calcining bauxite at 980°C without additives, during which kaolin partially decomposes to form amorphous silica and alumina, referred to as transition alumina; and the second step, a leaching step, aims to selectively dissolve the amorphous silica under milder conditions than the Bayer process. The residue (calcined and leached bauxite) is then introduced into the conventional Bayer process. At first glance, the drawback of this method is the need for additional thermal energy to calcine the bauxite to achieve properties comparable to high-quality bauxite. This excessive energy consumption has hindered the development of the method. Furthermore, the method requires an additional feedstock, lime, to precipitate silicates derived from the dissolution of the amorphous silica formed during bauxite calcination.

[0014] Numerous scientific papers have been published on the roasting-leaching process, focusing on the calcination steps and providing differing indications regarding the optimal calcination temperature. The maximum calcination temperature appears to be determined by the formation of mullite: according to Xu et al. (“Thermal behaviors of kaolinite-diasporic bauxite and desilication from itby roasting-alkali leaching process”, Light Metals TMS 2002), kaolinite begins to decompose into amorphous silica and γ-alumina at 990 °C, but these two phases begin to react and form mullite at approximately 1100 °C. Under conventional Bayer process conditions, mullite is insoluble in sodium hydroxide; the recommended temperature is between 1000 °C and 1050 °C. Li et al. (“Desilication of bauxite ores bearing multi-aluminosilicates by thermochemical activation process”, Light Metals TMS 2009, pp. 57-61) also observed that mullite formation begins at 1100℃ and noted a decrease in desilication between 1150℃ and 1200℃, but concluded that bauxite activation is optimal between 1100℃ and 1150℃. N. Eremin's 1981 article (“Desilication of bauxite ores bearing multi-aluminosilicates by thermochemical activation process”, ICSOBA 1981, pp. 135-142) indicated a similar mechanism, with the optimal temperature between 925℃ and 1000℃. The article published by Moazemi et Rezai (“Desilication studies of diasporic bauxite by thermochemical treatment”, Proc. XI Int. Seminar on Minerals Processing Technology (MPT-2010), pp. 832-838) shows that the yield of extraction by leaching as a function of calcination temperature has a very narrow maximum at 1000℃.

[0015] Despite these advances in understanding the roasting-leaching process, it is noteworthy that this method is rarely used in industry because it adds significant additional costs to the conventional Bayer process. Furthermore, these additional costs relate to two aspects of particular concern in environmental reporting: energy consumption and water consumption.

[0016] The problem to be solved by the present invention is to develop a method or economically feasible improved process for using bauxite with low alumina content (A / S < 5, preferably < 4, or even more preferably < 3) in Bayer-type processes. Summary of the Invention

[0017] According to the present invention, the problem is solved by a bauxite processing method including pre-processing of the bauxite and by employing the Bayer process on the pre-processed bauxite, wherein pre-processing of the bauxite improves the ability of the bauxite to be used as a raw material in the Bayer process (which is known in itself).

[0018] The preprocessing of bauxite includes a first preprocessing step, which is a physical preprocessing step, i.e., a thermal preprocessing step. This thermal preprocessing aims to induce chemical and crystallization modifications in the bauxite (or at least chemical and crystallization modifications of some of the mineralogical components of the bauxite). Advantageously, the first preprocessing step of bauxite is performed on milled bauxite. This yields modified bauxite, which can then be introduced into the Bayer process, or a second step, i.e., a chemical preprocessing step, can be performed.

[0019] More specifically, the thermal pretreatment is performed at such a temperature and time that at least a portion of the silicates present in the bauxite are transformed into amorphous silica. For gibbsite bauxite, the temperature is advantageously between 1000°C and 1050°C, preferably between 1015°C and 1030°C, and even more preferably between 1015°C and 1025°C. For boehmite bauxite, the temperature is 40°C lower, and between about 960°C and about 1000°C, more preferably between 970°C and 990°C.

[0020] This method can be implemented in part at an industrial site where bauxite is mined, or entirely at an industrial site where the Bayer process is installed. The method requires specialized equipment, namely a furnace. It is advantageous to implement this method on ground bauxite.

[0021] According to the present invention, the bauxite processing method includes a second pre-processing step, which is a chemical step. This includes leaching the modified bauxite with sodium hydroxide. During this step, and under appropriate temperature, residence time, sodium hydroxide concentration, and solid / liquid ratio conditions, the amorphous silica obtained during calcination is dissolved, while a very small amount of alumina enters the solution.

[0022] The leaching step must be performed on the milled bauxite; therefore, it is advantageous to perform the milling upstream of the thermal pre-processing step. The milling method and desired particle size can be similar to those used in the conventional Bayer process. The processed bauxite can also be milled again before being introduced into the Bayer process.

[0023] A method of preprocessing natural bauxite, involving sequential calcination and leaching, yields a product referred to herein as "preprocessed bauxite," which is chemically and mineralogically distinct from natural bauxite. A simple parameter representing this distinctiveness of preprocessed bauxite is its loss on ignition; this loss on ignition is significantly lower (typically 10 to 20 times lower) than that of natural (unprocessed) bauxite.

[0024] Therefore, one object of the present invention is a method for producing alumina trihydrate or alumina from bauxite pre-processed by methods including calcination and leaching, wherein the pre-processed bauxite is characterized by having a loss on ignition of less than 2.5% by mass, preferably less than 2.0% by mass, and even more preferably less than 1.5% by mass. Advantageously, the pre-processed bauxite is further characterized by the absence of gibbsite and the presence of amorphous silica.

[0025] The method includes the following steps: (a) Pre-processed bauxite is processed (“leaching”) with an aqueous sodium hydroxide solution at a temperature of at least 100°C (typically in an autoclave), wherein the concentration of the aqueous sodium hydroxide solution is between 100 g Na2O / L and 220 g Na2O / L, preferably between 140 g Na2O / L and 200 g Na2O / L, more preferably between 155 g Na2O / L and 190 g Na2O / L, and even more preferably between 160 g Na2O / L and 180 g Na2O / L; (b) Separating solid residues from the liquid phase; (c) Crystallization of aluminum trihydrate by adding seed crystals; (d) Separation of crystalline aluminum trihydrate from the liquid phase; (e) Calcining the aluminum trihydrate obtained in step (d) to obtain aluminum oxide.

[0026] This final step is optional; if the purpose of the method according to the invention is to obtain aluminum trihydrate as a commercial product, the aluminum trihydrate obtained in step (d) can be dried. If the purpose of the method is to obtain alumina, then step (e) is required.

[0027] Advantageously, the temperature in step (a) is between 150°C and 350°C, preferably between 200°C and 300°C, more preferably between 220°C and 280°C, and even more preferably between 250°C and 270°C.

[0028] Advantageously, the pre-processed bauxite has an Al2O3 / SiO2 mass ratio greater than 8, preferably greater than 9, and even more preferably greater than 10. Advantageously, its alumina mass content is greater than 60%, preferably greater than 65%, and even more preferably greater than 70%. Its silicon oxide mass content is less than 12%, preferably less than 10%, and even more preferably less than 8%.

[0029] In an advantageous embodiment of the method according to the invention, the liquid phase from step (d) is reintroduced into the aqueous sodium hydroxide solution used in step (a).

[0030] Advantageously, the pre-processed bauxite has been pre-processed by calcination at a temperature between about 920°C and about 1200°C. This temperature is preferably between about 950°C and about 1070°C, even more preferably between about 1000°C and about 1050°C, especially in the case of gibbsite-type bauxite; for boehmite-type bauxite, a lower calcination temperature is preferred, between about 950°C and about 1100°C, more particularly between about 960°C and about 1000°C, even more preferably between about 970°C and about 990°C.

[0031] This calcination triggers both chemical and crystallization transformations in bauxite. More specifically, most of the diaspore (diaspore being the form in which the majority (and often almost entirely) of the alumina exists in bauxite with a low A / S ratio) is transformed into α-alumina. This transformation is achieved by separating some volatiles present in the bauxite or formed during the aforementioned chemical and crystallization transformations. Loss on ignition is a parameter that can be readily determined and comprehensively represents these chemical and crystallization transformations during calcination.

[0032] In calcined bauxite, aluminum, especially that contained in aluminosilicates, has higher solubility under the typical conditions of the bauxite leaching step in the Bayer process, and silicon has higher solubility under milder conditions than the Bayer leaching step. Therefore, by leaching calcined bauxite using an aqueous sodium hydroxide solution under milder reaction conditions than the Bayer leaching step, silicon dioxide can be dissolved.

[0033] Due to the chemical and crystallization transformations that occur during calcination, the mass percentage of diaspore in pre-processed bauxite is much lower than that in the raw bauxite; after calcination, the mass percentage of diaspore is preferably less than 5%, more preferably less than 3%, even more preferably less than 2%, and most preferably less than 1%. For the same reason, the mass percentage of kaolinite in pre-processed bauxite is preferably less than 4%, more preferably less than 3%, even more preferably less than 2%, and most preferably less than 1%. The mass percentages of diaspore and kaolinite can be determined using the commonly used method of X-ray crystallography analysis on powder samples of pre-processed bauxite.

[0034] For example, preferably, the pre-processed bauxite used in the method of the present invention has a loss on ignition of less than 2%, a percentage of gibbsite monohydrate of less than 3%, and a percentage of kaolinite of less than 3%, these percentages being by mass percentage. More preferably, the pre-processed bauxite has a loss on ignition of less than 2%, a percentage of gibbsite monohydrate of less than 2%, and a percentage of kaolinite of less than 2%, and even more preferably, a loss on ignition of less than 1.5%, a percentage of gibbsite monohydrate of less than 1%, and a percentage of kaolinite of less than 2%.

[0035] Another object of the present invention is a method for producing alumina trihydrate or alumina from bauxite, the method comprising the following steps: (i) Preprocessing bauxite to obtain the preprocessed bauxite, wherein the preprocessing comprises, in sequence: - Calcination, - Leaching with an aqueous sodium hydroxide solution. - Separating solids from the leaching aqueous phase, the separated solids representing the pre-processed bauxite. (ii) The pre-processed bauxite is processed by the method according to the first objective of the invention.

[0036] After preprocessing by calcination and leaching with sodium hydroxide, the bauxite can be fed into the method according to the invention. Advantageously, the Al₂O₃ / SiO₂ ratio of the bauxite is between 1 and 8, preferably between 1 and 7, even more preferably between 1 and 4, or even between 1 and 3 or between 2 and 3. Preprocessing of the bauxite according to the invention can significantly increase the Al₂O₃ / SiO₂ ratio, typically by two to three times. If the Al₂O₃ / SiO₂ ratio of the bauxite processed according to the method is greater than 8, the method is economically unfeasible because the additional alumina yield obtained from using preprocessed bauxite instead of natural bauxite is not sufficiently attractive, and the reduction in sodium hydroxide consumption is limited.

[0037] Another object of the present invention is to enable the production of alumina by means of the method according to the present invention.

[0038] Another object of the present invention is a facility for implementing the method according to the invention, comprising: - A unit used for preprocessing bauxite through calcination and leaching, thereby transforming bauxite into preprocessed bauxite; and - A unit for implementing the method according to the invention to produce alumina from the pre-processed bauxite. Its characteristics are: - The pre-processing unit includes: -- At least one calcining furnace for calcining bauxite, -- At least one leaching unit for leaching calcined bauxite using an aqueous sodium hydroxide solution (referred to as the "leaching solution"), and -- At least one solid-liquid separation unit for separating calcined and leached bauxite from the leachate; - The unit for producing alumina from the pre-processed bauxite includes -- At least one chamber (such as an autoclave or tubular device) for processing pre-processed bauxite using an aqueous sodium hydroxide solution (referred to as "Bayer solution") at a temperature of at least 100°C. -- At least one solid-liquid separation unit for separating solid residues (referred to as "red mud") from the Bayer liquid; -- At least one crystallization unit of aluminum trihydrate crystallized from the Bayer solution by adding a seed crystal of aluminum trihydrate; -- At least one solid-liquid separation unit for separating crystalline aluminum trihydrate from the Bayer liquid; -- Optionally, at least one calcination unit may be used to convert the aluminum trihydrate into alumina.

[0039] In this facility, the Bayer solution from the solid-liquid separation unit used to separate crystalline aluminum trihydrate from the liquid phase is recycled to the dissolution step. Attached Figure Description

[0040] exist Figure 1 and 2 In this system, three-digit designations represent physical objects (apparatus, components, or products), while four-digit designations represent method steps. After a phase separation step, the letter "L" indicates the liquid phase, and the letter "S" indicates the solid phase.

[0041] Figure 1 A simplified scheme of the Bayer process based on existing technology is shown.

[0042] Figure 2 A simplified embodiment of the method according to the present invention is shown.

[0043] Figure 3 and 4 Referring to Example 3, thermogravimetric differential thermal analysis (TGA-DTA) curves of the bauxite sample are shown; the increase in temperature corresponds to the calcination used to obtain calcined bauxite. In both figures, the curves referring to the left axis represent mass loss. The curves referring to the right axis represent mass loss per minute (…). Figure 3 ) and heat flow ( Figure 4 ).

[0044] Figure 5 The loss on ignition of monohydrate gibbsite-type bauxite with high silica content after calcination at different temperatures is shown (measured after calcination at 1060 °C). Detailed Implementation

[0045] 1. Conventional Bayer method

[0046] Comparison Figure 1 The invention is illustrated in detail below using the Bayer process according to the prior art. Bauxite from a bauxite mine is ground in the presence of a liquid phase (step 1100), which is sodium aluminate, as will be described in more detail below. The purpose of grinding is to facilitate the dissolution of bauxite and increase the specific surface area of ​​the bauxite accessible to the liquid phase during erosion. Typically, the target particle size is several hundred micrometers. Grinding is usually carried out by adding lime in the form of lime milk or solid form (step 1102). Lime serves three purposes: (i) during the dissolution of bauxite, lime reduces the consumption of sodium hydroxide because lime facilitates the precipitation of soluble silicates as calcium aluminosilicate rather than sodium aluminosilicate (which would otherwise carry away some of the sodium in sodium hydroxide, which is more expensive than lime); (ii) lime promotes the dissolution of aluminum and increases the yield of alumina extracted during dissolution; and (iii) lime improves the decantation of the slurry after erosion because lime facilitates the transformation of goethite, which is difficult to decant and filter, into hematite, which is more easily crystallized.

[0047] Then, the ground bauxite is eroded under pressure and high pressure in an autoclave or tubular exchanger by passing an aqueous sodium hydroxide solution (step 1110). The erosion (referred to as "dissolution") causes partial dissolution of the bauxite (step 1120), more precisely, the soluble portions of the bauxite (especially alumina, whether present as a monohydrate or trihydrate) form aluminate ions. In many cases, dissolution takes place in a closed autoclave or tubular exchanger at temperatures between 250°C and 270°C. In practice, the aqueous sodium hydroxide solution is an aqueous sodium aluminate solution. Commonly used sodium hydroxide concentrations are between 235 g Na₂O / L and 245 g Na₂O / L. Those skilled in the art know how to properly adapt the parameters of this step to the composition of the bauxite, particularly the temperature, residence time, and sodium concentration; the same observation applies to the amount of lime added in step 1102. For example, it is known that bauxite with a high content of alumina monohydrate (boehmite, and especially diaspore) requires a higher dissolution temperature compared to bauxite with a high content of trihydrate (gibbsite).

[0048] Temperatures between 250°C and 270°C ensure that all soluble alumina contained in the bauxite (including the gibbsite portion, which is the most difficult to dissolve from aluminum oxides, and whose content varies greatly) is dissolved. This temperature range is particularly important for karst bauxite (which is the primary area of ​​application for this invention). Some plants using karst bauxite are even designed to operate at temperatures up to 280°C to accommodate the use of bauxite with very unique compositions as needed.

[0049] In this leaching step 1120, bauxite can be placed in contact with a preheated liquid (a method known as the two-flow method), or a suspension of bauxite can be formed in Bayer liquor before heating (a method known as the single-flow method). In some plants, leaching step 1120 is performed in two steps, each at a different temperature, to first dissolve the readily soluble components and then dissolve the solid residue from the first step at a higher temperature. This alternative with two-flow leaching saves energy, but is considered to require a higher investment and complicate the process.

[0050] When the suspension is expanded through a continuous expansion step (step 1124), a portion of the water is evaporated (self-evaporation).

[0051] During decantation under ambient pressure (step 1130), the residue (referred to as "red mud") is separated from the liquid phase (liquid); a flocculant is added to increase the rate of phase separation and improve the clarity of the liquid (i.e., the amount of dry matter remaining in the liquid). The solid residue is referred to as "red mud"; it contains all the crystalline phases from the unreacted bauxite in leaching 1120, as well as the crystalline phases formed in the Bayer cycle. The recovered residue (referred to as "red mud") (step 1140) is washed with water (step 1142) to recover as much liquid as possible; this washing is typically performed using raw water and countercurrent (to minimize water usage); this is followed by decantation and / or filtration steps (not shown in the figure). Red mud is a powdery residue that is not easily reused and often ends up in special storage.

[0052] The liquid phase (“L”) from phase separation step 1130 is a sodium aluminate solution. After dilution (step 1150), aluminum trihydrate is crystallized by cooling the aluminate and adding seed crystals of aluminum trihydrate (step 1160) (step 1170). This crystallization step is usually referred to as “decomposition”; it lasts for about 40 hours. Cold water dilution (step 1150) reuses the water used to wash the red mud.

[0053] To ensure that the numerous parameters of this method (saturation of the liquid at the decomposition input, concentrations of Na₂O and impurities with different properties, decomposition initiation and final temperatures, seed surface area, crystallization technique, and particle size classification) are best suited to the desired properties of the alumina product, step 1170 requires certain expertise known to those skilled in the art. The physicochemical phenomena involved are particularly related to nucleation (spontaneous formation of fine particles in suspension), agglomeration of fine particles, and particle size classification by swirling and / or decantation.

[0054] The precipitated trihydrate is separated by decantation and filtration using various known techniques (step 1180); the trihydrate is then recovered (step 1190). The majority of the trihydrate must be recycled to decomposition step 1160, and the remaining trihydrate is dried (step 1192) and calcined (step 1194) to alumina. The alumina is then stored (step 1196) to transport it to the customer's location. The drying step (step 1192) is typically performed as the first step in calcination (step 1194), which is carried out in several stages. During this heating, the impregnation water is first removed starting at approximately 100°C, followed by the removal of the water constituting the trihydrate (starting at approximately 1000°C); the heating continues to obtain the desired crystalline structure. Two techniques are primarily employed: calcination in a furnace with a circulating fluidized bed (CFB); and gas suspension calcination (GSC). Older plants that still have rotary kilns are also suitable for implementing this step.

[0055] The liquid phase from phase separation step 1180 is an aqueous sodium hydroxide solution, which is more diluted than the aqueous sodium hydroxide solution used in step 1110 because water has been added to the liquid stream multiple times (water for washing the red mud (step 1142) and the trihydrate, and dilution water (step 1150)). Therefore, this aqueous sodium hydroxide solution must be concentrated by evaporating water (step 1210) for recycling (step 1220) to the sodium hydroxide solution used in the leaching step (step 1120). This aqueous sodium hydroxide solution can also be reused (step 1222) in the wet grinding step of bauxite (step 1100).

[0056] The trihydrate obtained in step 1190 can be washed and then dried; the wash water can be reused in the washing of red mud in step 1142 (not shown in the figure).

[0057] In the Bayer process according to the prior art, sodium hydroxide is consumed during the processing of bauxite to produce alumina. More specifically, this consumption relates to three mechanisms: (i) the formation of an insoluble sodium aluminosilicate phase during erosion (dissolution step 1120); (ii) residual sodium hydroxide being carried away by the slurry despite washing of the red mud (step 1142) (1140); and (iii) co-precipitation with alumina during the crystallization stage 1170. These losses must be compensated by adding fresh sodium hydroxide (1110). All washing liquid phases, including sodium hydroxide (including those during the chemical cleaning of tanks and pipelines), are recycled to the Bayer solution as much as possible.

[0058] 2. Preprocessing of bauxite according to the present invention

[0059] 2.1 Overview

[0060] Figure 2 The illustration shows an embodiment of the method according to the invention, which includes the preprocessing of bauxite. The preprocessed bauxite is then introduced into the Bayer process. Figure 1 The Bayer process steps shown in 11xx and 12xx are in Figure 2 The labels are 21xx and 22xx, while the labels for the pre-processing steps are 20xx.

[0061] According to a highly advantageous embodiment of the invention, some operating conditions of the Bayer process are adapted to the chemical and mineral composition of pre-processed bauxite; this will be described in detail below. Pre-processed bauxite is not a product that exists naturally; it must be obtained by industrial methods, i.e., by pre-processing methods. The chemical composition of pre-processed bauxite differs from that of natural bauxite (from which the pre-processed bauxite is obtained) in two key characteristics: pre-processed bauxite has a larger A / S ratio (because it contains fewer silicates), and it now contains very little water of crystallization. Furthermore, as will be explained in more detail below, after the transformations experienced during the various steps of pre-processing, pre-processed bauxite has a different mineral composition. For gibbsite-type bauxite containing mostly kaolinite silica, the key differences are as follows: gibbsite silica is dehydrated and transformed into alumina, which is mostly α-alumina; and kaolinite is dehydrated and transformed into metakaolinite (as described in section 2.3 below), thus enabling the silica to be dissolved by sodium hydroxide.

[0062] The difference between the chemical composition of pre-processed bauxite and that of natural bauxite with a similar A / S ratio lies in its extremely low water of crystallization content. The loss of water of crystallization is a key parameter factored into the loss on ignition (by mass). For example, the loss on ignition of typical monohydrate gibbsite-type natural bauxite is greater than about 10%, while the loss on ignition of the pre-processed bauxite according to this invention is less than 2.5%, preferably less than 2.0%, and even more preferably less than 1.5%. Loss on ignition is a parameter known to those skilled in the art; further explanation will be given in Section 2.3 below.

[0063] According to the present invention, bauxite from a bauxite mine is ground (step 2000) after the addition of water (step 2002), filtered (step 2004), and the solid residue after filtration (step 2004) is calcined (step 2010). Here, this intermediate product is referred to as "calcined bauxite". An aqueous solution of sodium hydroxide is added (step 2020) and leaching of the calcined bauxite is performed (step 2030). After phase separation (step 2040), the solid phase, referred to herein as "leached calcined bauxite" or "pre-processed bauxite", is recovered and introduced into the Bayer process; depending on the particle size obtained during grinding in step 2000, it may need to be ground again (step 2100). Figure 2 (Not shown in the diagram). The liquid phase from the phase separation in step 2040 is processed with lime to precipitate silicates (step 2050). After another phase separation (step 2060), the residual white mud is recovered (step 2070). The liquid phase from the phase separation in step 2060 is an aqueous sodium hydroxide solution; it is recovered (step 2080) and partially recycled to the Bayer process etching solution. Advantageously, the lime 2052 introduced is in the form of lime milk.

[0064] According to the present invention, the pre-processing method may include a variety of alternative forms. For example, an additional liquid-phase filtration step may be performed after the phase separation step 2060. Figure 2 This step is not shown in the diagram; it is referred to here as 2062. Following phase separation step 2060, a washing step of the white mud may be included. Figure 2 This step is not shown in the diagram; it is labeled 2064 here. The wash water is recycled to step 1150. These two alternatives can be combined. Alternatively, the bauxite can be dry-milled, in which case the calcination step can be performed directly thereafter (2010).

[0065] As will be explained in detail below, the Bayer process for pre-processed bauxite can be carried out in the same plant as the Bayer process for unprocessed bauxite, i.e., using the same equipment and following the same process flow diagram (the only difference being the evaporation step 2210, which may be omitted in some alternative forms of the method according to the invention). However, if the same operating parameters (e.g., duration, temperature and / or concentration of sodium hydroxide) are used for the method implemented on pre-processed bauxite, the results obtained will differ from those obtained with unprocessed bauxite. Therefore, in some advantageous embodiments of the invention, the operating parameters for certain steps of the Bayer process implemented on pre-processed bauxite according to the invention are modified for commonly used Bayer process operations.

[0066] The inventors have discovered that the calcination temperature of bauxite (step 2010) significantly affects the yield of alumina extracted from leached bauxite. According to the invention, for gibbsite-type bauxite, this temperature must be greater than 980°C. When the calcination temperature is below 980°C, kaolinite is activated but not completely transformed; it reacts during leaching (step 2030) to form zeolite-type insoluble compounds. Therefore, a calcination temperature greater than 990°C is preferred. According to an advantageous embodiment, the calcination temperature is greater than 1000°C. When the temperature is between 1010°C and 1035°C, the transformation of kaolinite is complete; the preferred temperature is between 1020°C and 1030°C. The leaching process following the calcination method will be described below.

[0067] Pre-processed modified bauxite obtained by the calcination-leaching (roasting-leaching) method can be directly introduced into the Bayer process.

[0068] According to an advantageous embodiment of the invention, the method has been modified. More precisely, certain operating parameters have been changed, which in particular significantly reduces energy consumption.

[0069] 2.2 Specific Implementation Plan

[0070] To illustrate embodiments of the present invention, certain steps of the pre-processing method are described in detail herein.

[0071] Grinding (Step 2000)

[0072] In step 2000, grinding can be carried out in a cylindrical mill containing small balls or steel rods. The water volume (step 2002) can be approximately 0.7 m³ per metric ton of bauxite. 3 The bauxite loading rate is approximately 1000 kg / m³. 3 This yields a suspension of water and bauxite that can be separated by filtration on a filter press (step 2004). A residual impregnation water content of approximately 10% by mass is acceptable. The target particle size for grinding can be the same as that in the conventional Bayer process, i.e., approximately several hundred micrometers.

[0073] Calcination (Step 2010)

[0074] The calcination in step 2010 can be carried out in a rotary kiln or a static furnace. Progressive heating removes the impregnation water from the ore at the temperatures shown above, then removes the constituent water of the crystalline phases present in the bauxite, and then allows for the transformation of these phases. Under these conditions, the following was observed: - Most of the silicate form of silicon dioxide is transformed into amorphous silicon dioxide; - The monohydrate diaspore phase and the monohydrate boehmite phase are transformed into "α" type alumina; - Iron existing in the form of goethite (FeO(OH)) is transformed into hematite (Fe2O3) after calcination. - The phase containing carbon, carbonates and sulfur mainly thermally dissociates into CO2 and SO2, which are the volatile components.

[0075] Leaching (Step 2030)

[0076] After calcination, the calcined bauxite is leached in a sodium hydroxide solution (step 2020). This leaching step (step 2030) dissolves the transformed silica and certain impurities. The sodium hydroxide content in the liquid phase can be between about 70 g NaOH / L and about 160 g NaOH / L, preferably between about 90 g NaOH / L and about 150 g NaOH / L, and even more preferably between about 110 g NaOH / L and about 140 g NaOH / L. For example, a content of 129 g NaOH / L has been successfully used. This solution can be obtained from a mixture of recovered sodium hydroxide and 50% alkali solution, and the amounts of sodium hydroxide and alkali solution are adjusted to obtain the concentration required for leaching. When the content is below 70 g / L, the proportion of dissolved silica that can be leached is too low, the required residence time (i.e., the contact time between the solid and liquid phases) is too long, and the reserve sodium hydroxide solution circulating in the apparatus of this method is excessively diluted. When the concentration exceeds 150 g / L, the risk of aluminum loss due to alumina dissolution becomes significant. The temperature of sodium hydroxide aqueous solution is typically between 80°C and 120°C; if the temperature is too low, silica dissolves poorly, and if the temperature is too high, alumina tends to dissolve.

[0077] For example, calcined bauxite and sodium hydroxide solution (2010) can be introduced into a stirred reaction vessel in such a manner to obtain a solution containing approximately 80 kg / m³. 3 The initial suspension of solids. A reaction temperature of about 100°C is suitable; the residence time at this reaction temperature can be about 45 minutes.

[0078] Phase separation (step 2040)

[0079] The suspension from the reactor at leaching 2030 can be fed to a filter press for filtration, thereby performing phase separation in step 2040. The solid residue is "leached calcined bauxite" or "pre-processed bauxite"; a residual leaching liquor of about 10% by mass is acceptable. The liquid phase is a liquid loaded with dissolved silica from leaching 2030; it is purified by adding lime 2052. Advantageously, the feedstock consists of quicklime (CaO) or lime milk (Ca(OH)2). Quicklime with a fine particle size distribution is preferred, containing at least 85% CaO; typically it contains 85% to 95% CaO. The lime can be slaked with hot water in a stirred reactor (about 100 kg CaO / m³). 3 (To produce lime milk)

[0080] Precipitation of silicate (step 2050) and phase flow

[0081] The silicate precipitation step (step 2050) results in the formation of insoluble calcium silicate. In a reaction vessel, silica precipitation is carried out at 100°C in the presence of quicklime or lime milk (100 g CaO / L) for 2 hours. Advantageously, the CaO:SiO2 stoichiometric ratio is between 1.1 and 1.5. At the end of this operation, a silica content of approximately 35 kg / m³ is typically obtained. 3 Up to 50kg / m 3 Solid, preferably 39 kg / m 3 Up to 46kg / m 3 A suspension of solids.

[0082] Advantageously, phase separation is achieved by decantation of the suspension (step 2060). According to the invention, it is advantageous to recycle the clarified liquid phase (“overflow”) into the sodium hydroxide loop of the method, preferably partially recycled into leaching 2030, and partially recycled upstream of the dissolution step 2120 of the Bayer process.

[0083] A thickened suspension (“underflow”) known as white mud (2070) is extracted from the decanter; it consists of calcium silicate (typically 600 kg solids / m³). 3 Up to 700 kg solids / m 3 It can be supplied to a belt filter type, where it undergoes systematic washing with water (step 2072) to reduce the concentration of the impregnating agent. The residual diluted impregnating agent in the washed white mud can be approximately 10%; the concentration of sodium hydroxide in this impregnating agent is typically approximately 6 g NaOH / L to 10 g NaOH / L. The white mud is composed of calcium silicate, which is similar to the calcium silicate of calcareous quartz (approximately Ca). 4.31 Si 5.51 Al0.5 O 16 (OH)2 x 4 H2O). It can be transferred to an intermediate storage device and waited for reuse.

[0084] The water recovered after step 2072 for washing the silicate mud (white mud 2070) can enter the loop of the liquid phase 2080 obtained in step 2060, ready for use in the leaching step (step 2030) and the Bayer process (step 2120). In the latter case, its sodium hydroxide content needs to be readjusted (step 2110) to be lower than the initial content in step 2030 (e.g., 129 g NaOH / L). This readjustment is achieved by adding an equal part of 50% alkali solution.

[0085] Because sodium is absorbed into the precipitated silicates, and due to the impregnation with white mud, a certain amount of sodium hydroxide is lost during the preprocessing according to the invention. This loss must be compensated by adding an alkali solution, typically 50% alkali solution (step 2110). However, as explained below, the modified Bayer process according to the invention consumes less sodium hydroxide per metric tonne of alumina produced than the conventional Bayer process.

[0086] The Bayer liquid is recycled (step 2200); it consists of a mixture of liquid from the filtration step of the trihydrate (step 2180) and added alkali (step 2110), wherein the liquid from the filtration step of the trihydrate (step 2180) has been or has not been concentrated by water evaporation (step 2210).

[0087] 2.3 Meaning of Loss on Ignition

[0088] Loss on ignition (LOI) is a common and major characteristic of bauxite; the LOI value, expressed as a percentage by mass, is found on analytical certificates intended for delivery of bauxite for the Bayer process. LOI is typically determined by pre-drying at 105°C followed by calcination at 1060°C for 2 hours. Calcination of bauxite always results in a net loss of mass due to the release of volatiles, even if the oxidation reaction itself could lead to an increase in mass. This release of volatiles is caused by physical (especially sublimation) and chemical (especially thermal decomposition, such as dehydration, dehydroxylation, and thermal dissociation, as well as reduction) phenomena. More precisely, LOI primarily corresponds to the removal of water (i.e., water molecules bound in the crystal structure), carbon dioxide from organic matter and carbonate minerals, and certain other volatile compounds, particularly oxides of sulfur.

[0089] The loss on ignition (LOI) of bauxite depends on its chemical composition and mineral composition. For bauxite intended for the Bayer process, it is typically on the order of 10% to 30%. The LIO can be determined by calcining under the conditions described above and then weighing the ore before and after calcination. Differential thermogravimetric analysis (DGA) can also be used, which can characterize the minerals present in the bauxite.

[0090] The loss on ignition of monohydrate gibbsite bauxite with high silica content is typically between 10% and 18%, more typically between 12% and 15%, but these empirical values ​​do not limit the scope of the invention, wherein monohydrate gibbsite bauxite with high silica content forms a raw material that can be used in the present invention to prepare what is called pre-processed bauxite. Figure 5 The loss on ignition (LOI) of gibbsite-type bauxite calcined at different temperatures ranging from 980°C to 1030°C according to the present invention (measured after calcination at 1060°C for 2 hours) is shown. It can be clearly seen that the LIO is extremely low (0.10%) after calcination at 1030°C; in other words, the amount of volatiles released is extremely low when the material is heated above 1030°C and even as high as 1060°C.

[0091] For example, regarding the kaolinite phase contained in bauxite, the preprocessing according to the invention via calcination schematically produces the following reaction (the temperatures shown are approximate values): - Endothermic dehydration between 40℃ and 200℃: Al2Si2O5(OH)4 xn H2O decomposes into Al2Si2O5(OH)4 +n H2O; - Endothermic dehydroxylation between 530℃ and 590℃: Al2Si2O5(OH)4 decomposes into Al2O3x 2 SiO2 (altered kaolinite) + H2O; - Exothermic dissociation between 900℃ and 1000℃: Al2O3x 2 SiO2 (morphokaolinite) decomposes into 2 Al2O3x3 SiO2 (mullite-like) + SiO2 (amorphous) + γ Al2O3.

[0092] 3. The modified Bayer process according to the invention using pre-processed bauxite.

[0093] Phase separation (step 2040) preferably separates the solid from the liquid after filtration, thereby enabling the pre-processed bauxite to be used in the leaching step (step 2120) of the Bayer process. The inventors have made several observations, resulting in improvements to certain steps of the Bayer process; these improvements constitute an important feature of the present invention.

[0094] Dissolution (Step 2120)

[0095] Dissolution involves dissolving the aluminum phase contained in the pre-processed bauxite in a sodium hydroxide solution. This is achieved by simultaneously dissolving the α-alumina (produced during calcination) contained in the pre-processed bauxite with the previously present soluble aluminum using a Bayer solution at high temperatures. This step can be carried out under temperature and pressure conditions similar to the conventional Bayer process, i.e., in a closed autoclave or a pressurized tubular system (approximately 50 to 60 bar), with temperatures typically between 250°C and 270°C. Advantageously, heating is performed by gradually increasing the temperature to the reaction temperature. Advantageously, the residence time at the reaction temperature is between 30 and 60 minutes, preferably between 30 and 50 minutes, and even more preferably between 35 and 45 minutes.

[0096] A key feature of the method according to the invention is that the concentration of sodium hydroxide used for leaching pre-processed bauxite can be significantly lower than the concentration of sodium hydroxide used for ordinary bauxite in the conventional Bayer process. More particularly, this concentration is between 140 g Na₂O / L and 200 g Na₂O / L, preferably between 155 g Na₂O / L and 190 g Na₂O / L, and even more preferably between 160 g Na₂O / L and 180 g Na₂O / L. Advantageously, this concentration can be continuously monitored by measuring the conductivity of the liquid; this concentration can also be used for chemical analysis in the laboratory.

[0097] In one embodiment, although the concentration of caustic soda in the dissolving agent is low (162 g / L vs. 240 g / L), resulting in a higher circulation rate of the etching agent (12.11 m³ / s), the circulation rate of the etching agent is still relatively high. 3 / t vs.8.52m 3 / t), but the extraction yield of alumina is still very high; by modifying the plant settings, such as the saturation of the liquid (alumina concentration) and the WR of added lime (8% to 10.4%) and temperature (260°C), the extraction yield of alumina (without silica) is greater than 96%.

[0098] Evaporation (Step 2210)

[0099] At the dissolution outlet, the suspension expands, that is, the suspension reaches atmospheric pressure through continuous expansion; this operation can evaporate a large amount of water (self-evaporation).

[0100] Because the sodium hydroxide concentration in the Bayer solution used in the method according to the invention is significantly lower than that in the conventional Bayer process, much less water is evaporated. Furthermore, the pre-processed bauxite produces less red mud (2140) compared to most natural bauxite, which reduces the amount of water required for mud washing (2150), where the sodium hydroxide-laden wash water is introduced into the Bayer solution loop. In some cases, evaporation to dissolution 2120 during the expansion of the autoclave (step 2124) is sufficient to maintain the concentration of the recycled aluminate solution (2220); thus, evaporation step 2210 can be omitted. Evaporation step 2210 consumes thermal energy and requires significant investment in the evaporator; minimizing or even omitting this step is of significant economic benefit.

[0101] It should be noted that although some plants use the conventional Bayer process without evaporation step 1210, i.e., the plant uses only bauxite that produces very little red mud, it is completely impossible to omit this step when leaching bauxite with a low A / S ratio using the conventional Bayer process.

[0102] Deposition of red clay (step 2130)

[0103] At the dissolution outlet, the suspension is diluted with an aluminate solution derived from the first stage of red mud washing in step 2140. This dilution is adjusted according to the input wash water to achieve a solution concentration compatible with solid-liquid separation and crystallization in step 2160.

[0104] Decanting takes place in a device called a "decanter," which is a large-diameter vessel (typically with a flat or conical bottom) equipped with slow-speed stirring to enable separation. To improve the settling velocity of solid particles, decanting is optimized by using additives called flocculants.

[0105] In the method according to the invention, the absence of certain phases such as goethite allows for a reduction in the amount of flocculant used, wherein the goethite undergoes a transformation during the bauxite calcination in step 2010, and it is known that goethite can hinder flocculation.

[0106] Typically, the thickened suspension (underflow) is sent to the first washing stage. The clarified liquid (overflow) is then sent to a "safe" filter, the purpose of which is to remove extremely fine mud particles to ensure that a pure liquid is obtained for crystallization.

[0107] Washing the red clay (step 2140)

[0108] The washing of red mud in step 2140 is preferably carried out countercurrently; the wash water is then introduced into the final stage of the scrubber chain. The scrubber chain can be terminated by filtering the slurry from the previous scrubber using a filter press. The use of flocculants improves settling, thereby ensuring better slurry washing.

[0109] Furthermore, under the same conditions, a smaller amount of red clay and a higher tendency for red clay to decant can significantly improve washing because the amount of water available for the operation remains constant for a smaller amount of solids. While maintaining the same washing efficiency (both raw and pre-processed bauxite), the amount of water can be reduced, as explained below, resulting in a significant reduction in evaporation and thus saving considerable energy.

[0110] More precisely, the amount of water required for washing red mud produced by leaching pre-processed bauxite (typically reduced by about 60%) is much less than that required for washing red mud produced by leaching unprocessed bauxite (typically reduced by about 50%).

[0111] For example, the washing water volume and the tonnage of mud are as follows: - In the case of bauxite processed according to the method of the present invention, the water volume is 3.04 m³. 3 / t, the washed mud is 1.063t / t, - In the case of the original bauxite deposit, the water volume is 5.70 m³. 3 / t, the washed mud is 3.040t / t.

[0112] This invention discovers that when using bauxite pre-processed using the method of this invention, by rationally adjusting the main parameters of the Bayer process, especially those in the erosion process (residence time, solvent saturation, lime addition, etc.), it is possible to maintain excellent alumina dissolution yield (over 96% of alumina, excluding silica) using an erosion solvent with a low concentration of caustic soda (approximately 160 g / L to 170 g / L, compared to 238 g / L to 240 g / L in conventional erosion). These results, along with improved slurry washing efficiency, enable the recycling of Bayer liquor with a low concentration of caustic soda, thereby significantly saving energy and maintenance costs of the evaporation plant. Furthermore, by omitting the evaporation step 2210, savings are achieved in the construction of new production lines, particularly in the case of new production lines.

[0113] Deposition of white clay (step 2040)

[0114] Even taking into account the fact that the desilication step 2050, by precipitating silicates, produces a specific type of slurry (“white mud”) not found in the conventional Bayer process using unprocessed bauxite, the method according to the invention shows a significantly greater reduction in red mud than white mud. Furthermore, the subsequent experiences of these types of slurry (and their potential applications in the available products) differ. The method according to the invention typically reduces the amount of decanted slurry (steps 2040, 2060, and 2130) by 10% to 40%, preferably by 20% to 40%. For red mud, this reduction reaches 65% (step 2130). Moreover, the decantation tendency is higher because goethite transforms into hematite during the calcination stage of the ore. These two advantages result in several other superior effects: ease of plant operation (related to the reduction in the amount of slurry to be treated), reduced energy consumption, reduced flocculant consumption, and optimized plant maintenance costs.

[0115] Sodium hydroxide consumption and energy consumption according to the method of the present invention

[0116] In the Bayer process, sodium hydroxide consumption is related to both the formation of solid sodium compounds (sodium aluminosilicate crystallizing with alumina) and the loss of liquid sodium hydroxide (impregnating agents for red mud and trihydrate). Pre-processing methods involving calcination and leaching also result in sodium absorption (into silicates) and the loss of liquid sodium hydroxide (impregnation of white mud). All these losses are compensated by adding 50% alkali solution.

[0117] Using gibbsite-type bauxite with a low Al / Si ratio, the Bayer process consumes approximately 8 GJ of energy per metric tonne of alumina. This energy consumption does not take into account the calcination of the trihydrate (steps 1194, 2194); since the energy consumption of trihydrate calcination is not dependent on the source of the bauxite, the comparison here only extends to the trihydrate (1190, 2190). For the pre-processed bauxite according to the invention, by using natural karst bauxite with a low Al / Si ratio and omitting the evaporation step 2210, this energy consumption is reduced by 1.9 GJ / t. Considering that the pre-processing method, including the calcination step 2010 and the leaching step 2030, increases the energy consumption by approximately 2.2 GJ per metric tonne of alumina, it is noted that the excessive energy consumption for obtaining alumina per metric tonne according to the method of the invention is 0.3 GJ. This excessive energy consumption corresponds to approximately 4% of the energy consumption of the method according to the invention.

[0118] The excessive energy consumption is extremely low because the method according to the invention increases the yield of alumina extracted from bauxite: in the example above used to estimate energy consumption, the method according to the invention reduces the amount of karst bauxite with a low Al / Si ratio required to obtain 1 metric ton of alumina from 3.5 t to about 2.6 t.

[0119] Furthermore, as mentioned above, the method according to the invention consumes less water and less sodium hydroxide compared to the conventional Bayer process used for low-grade karst bauxite. In the above example, sodium hydroxide consumption is reduced from 490 kg NaOH to 104 kg NaOH per metric ton of alumina produced. Lime consumption does indeed double, increasing from approximately 360 kg / metric ton of alumina to 800 kg / metric ton of alumina, but the cost of lime is approximately 10% of the cost of NaOH.

[0120] These cost savings are very attractive when the investment required for the additional pre-processing steps is taken into account. This investment includes adding a pre-processing unit to an existing plant using the Bayer process: in which no equipment modifications are required (except for the integration of the Bayer fluid between the "pre-processing" unit and the "Bayer" unit, including piping): the present invention significantly improves the Bayer process, but the improvements only relate to the operating parameters of the Bayer process, not to the industrial equipment.

[0121] Therefore, it is clear that this invention is highly economically beneficial. Furthermore, the significant reduction in the amount of red clay (which tends to have negative economic value) also reduces the reprocessing and storage costs of red clay. Regarding white clay (essentially silicates), it contains fewer heavy metals and other potentially toxic substances (if it eventually enters solution) compared to red clay; it is economically valuable. In fact, the mineralogical analysis of calcium silicate in the form of tobermorite suggests its particular potential for use in the construction industry.

[0122] Undoubtedly, the greatest advantage of the method according to the present invention is the potential to improve the economic viability (microeconomic and macroeconomic) of bauxite deposits by using mineral resources that cannot be used under economically competitive conditions using existing technological methods. For some plants using bauxite from geographically proximate mines, this potential provides savings in transportation-related costs.

[0123] Of course, pre-processed bauxite that has not been pre-processed at the same location can be supplied to Bayer process plants that have been improved according to the invention, which is also within the scope of the invention: the pre-processed bauxite can come from a separate pre-processed bauxite plant (e.g., built near the bauxite mine to save on bauxite transportation costs), or from an integrated plant (pre-processing unit + Bayer unit) with excess capacity in pre-processed bauxite. However, the first embodiment with a separate pre-processing unit is not preferred because in this case, the sodium hydroxide-loaded liquid phase from leaching step 2040 and washing 2072 of white mud cannot be reused by recycling the Bayer solution.

[0124] 4. Other advantages of the method according to the invention

[0125] As shown above, the method according to the invention has several advantages. Its main advantage is that the Bayer process can be applied to bauxite with a low Al / Si ratio, which cannot be used according to existing technologies or has low yields, and whose production costs are significantly higher compared to bauxite with a higher Al / Si ratio.

[0126] Another advantage is that the preprocessing according to the invention removes not only silicon, but also almost all of the organic carbon and most of the sulfur naturally present in bauxite. Organic carbon is known to accumulate in the soda solution of aluminates, and some of it precipitates as oxalate on the trihydrate. It is known that in the advantageous case of a lower oxalate concentration in the Bayer solution, there is greater potential to adapt the parameters of decomposition step 2160 (especially temperature, residence time, and seed crystal circulation rate) to obtain a product with controlled particle size, particle size distribution, and crystallite morphology. Furthermore, a lower oxalate concentration in the Bayer solution results in a higher yield for the crystallization step.

[0127] Another advantage is the reduction in the amount of red mud (up to 60%). This has two advantages: advantages in phase separation and advantages in its final processing. More precisely, the method according to the invention reduces the amount of red mud; it does produce a new type of residue, white mud, but the total amount of white and red mud is reduced (up to 25%) compared to red mud produced according to the conventional Bayer process. In addition to the reduction in mud volume, a higher tendency for decantation of the red mud produced by the method according to the invention has been observed (because goethite transforms into hematite during calcination (step 2010)), which allows for a reduction in the amount of flocculant added to the suspension, thereby aiding in the management of the decantation plant, reducing maintenance costs, and reducing energy consumption.

[0128] Another advantage is that it may reduce hard deposits of aluminosilicates on the surfaces of various devices used downstream of the dissolution step (step 2120); these hard deposits tend to impede heat exchange and must be removed from time to time during certain maintenance operations.

[0129] Another advantage is that the alumina obtained by the method according to the present invention has extremely low content of residual iron and silicon dioxide.

[0130] As mentioned above, another advantage is that the amount of water evaporated in the water evaporation step (step 2210) is reduced, and in many cases, the water evaporation step can be omitted. This is very helpful in saving energy.

[0131] The method according to the invention includes an additional calcination step (step 2010), which consumes heat energy and sodium hydroxide. However, most of the sodium hydroxide can be recycled and used in the Bayer process, and the energy consumption in the calcination step can be compensated by the energy savings achieved in the Bayer process.

[0132] The method according to the invention advantageously utilizes pre-processed bauxite obtained from natural bauxite with an A / S ratio between 1 and 8, preferably between 1.5 and 7. When the ratio is 7 or 8, the additional alumina benefits gained by calcining and leaching the pre-processed bauxite are diminished, and the benefits of reduced sodium hydroxide consumption are more limited because less silica can remove sodium hydroxide. This upper threshold depends on certain techno-economic parameters that can vary based on economic data. When the A / S ratio is 1, the mineral resource is particularly kaolinite, which has other technical uses. The A / S ratio of natural bauxite is rarely less than about 2.5 or 2, and the preferred lower limit of the method according to the invention is the use of pre-processed bauxite obtained from natural bauxite with an A / S ratio equal to 2.

[0133] Example

[0134] Example 1: Bauxite from Jiaokou

[0135] Bauxite powder from Jiaokou (China) is provided, and the chemical analysis of the bauxite powder is described in detail in Table 1 below.

[0136]

[0137] The ore contains a relatively large amount of alumina, but it is also rich in silicon (low A / S ratio, about 4.6), and the silicon dioxide is mainly present in the form of kaolinite (88%), with the remainder (12%) in the form of muscovite.

[0138] 200 g of the bauxite sample was calcined at 980 °C or 1030 °C. Calcination was carried out in a muffle furnace preheated to 200-250 °C, with the temperature increased as rapidly as possible. Calcination was continued for 30 minutes at the target temperature (980 °C or 1030 °C). At the end of calcination, the bauxite was cooled in a desiccator. Its chemical composition was analyzed by X-ray fluorescence, its structure by X-ray diffraction, and its volatile matter content (“loss on ignition”, abbreviated as LOI) was analyzed by weighing before and after heating to 1060 °C.

[0139] A 500 mL suspension of calcined bauxite at a concentration of 90 g / L was leached using a pure sodium hydroxide solution containing 100 g / L Na₂O. The resulting suspension was incubated at 100°C for 1 hour and then filtered through a 5 μm microporous membrane filter. The filtrate was stored in a furnace at 90°C; aliquots were taken for analysis. The leached bauxite was washed and dried.

[0140] Another leaching was performed on bauxite calcined at 980°C using a sodium hydroxide solution containing 260 g / L Na₂O.

[0141] Since bauxite calcined at 980℃ did not achieve good desilication, leaching tests were only conducted on bauxite calcined at 1030℃. The calcined and leached bauxite was etched at 260℃ according to the following scheme, with a residence time of 40 or 60 minutes: (i) Increase the temperature at 4°C / min until a first plateau temperature of 175°C is reached, and the plateau temperature is maintained for 5 minutes; (ii) Increase the temperature at 1.5°C / min until a second plateau temperature of 255°C is reached, and the plateau temperature is maintained for 5 minutes; (iii) Increase the temperature at 1°C / min until a third plateau temperature of 260°C is reached, and the duration of the plateau temperature is 40 minutes or 60 minutes.

[0142] The corrosive aqueous phase is an industrial Bayer solution with the following composition (g / L): Na₂O ctq = 238.0; Al₂O₃ = 128.6; Wr = 0.540; Na₂O cbte = 17.2; SiO₂ = 1.5; CaO = 0.001; Fe₂O₃ = 0.008; V₂O₅ = 0.020; C org = 2.4; Density at 20℃ = 1.3777 (The name “Na2O ctq” refers to the useful “caustic sodium” portion of sodium hydroxide, while the name “Na2O cbte” refers to the Na2O corresponding to the carbonate (“cbte”) residue; they can be distinguished by pH titration using methods known to those skilled in the art.)

[0143] Silica is precipitated by adding lime slurry; the amount of lime added is between 2.5% and 4%. This is described in detail in Example 2 below.

[0144] The bauxite loading was adjusted to obtain a WR (alumina / sodium hydroxide ratio) that varied between 1.00 and 1.28 at the end of erosion.

[0145] Example 2: Desilication Test

[0146] Various desilication tests were conducted using the following two methods: In the first series of experiments, different lime raw materials capable of purifying the leachate (i.e., causing silica to precipitate) were tested, and in the second series of experiments, the purification efficiency was tested as the leachate concentration varied.

[0147] a) Experiments on lime raw materials (comparison of solid lime and lime slurry)

[0148] Silica precipitation was carried out at 100°C for 2 hours using lime slurry containing 100 g / L CaO and solid CaO, respectively. The lime slurry was prepared by stirring at 70°C for 90 minutes and then at 85°C for 60 minutes.

[0149] After precipitation and filtration, the filtrate (SiO2, CaO and Al2O3) was analyzed, and the filter cake was then characterized by chemical analysis (SiO2, CaO and Al2O3), X-ray diffraction and scanning electron microscopy (SEM).

[0150] Silica precipitation was performed on 300 ml of the first hot filtrate from the leaching test of Wanji bauxite calcined at 1020℃. Two experiments were conducted: one using lime slurry and the other using solid lime.

[0151] Two preliminary analyses of the leachate showed SiO2 concentrations of 5.79 g / L and 6.10 g / L, or an average content of 5.96 g / L ([Si] = 0.1 M / L). Therefore, for a Ca / Si stoichiometry of 1.2, 2.03 g of solid CaO or 20 ml of lime slurry must be introduced into 300 ml of leachate.

[0152] However, a slightly larger amount was used, namely 2.53 g of solid lime and 25 ml of lime slurry, which is equivalent to a stoichiometry of about 1.5.

[0153] Table 2 records the analytical results of the solution before and after precipitation, as well as the SiO2, Al2O3 and CaO in the precipitate, and Table 3 shows the precipitation ratio of SiO2 and Al2O3.

[0154] Therefore, it can be inferred that lime slurry precipitates more silica (87%) than solid lime (74%), but lime slurry causes slightly more aluminum to precipitate (49%) compared to solid lime (37%). It is also noted that lime slurry has almost no effect on the calcium content of the solution.

[0155]

[0156] X-ray diffraction characterization of the precipitates revealed that they consisted of the same crystalline phases: tobermorite formed during Si precipitation via calcium hydroxide; and calcite already present in the lime used.

[0157] Observations and microscopic analyses using SEM (see Table 4 below) showed that tobermorite was substituted by Al, Na and sometimes Mg, and that it existed as granular aggregates with a flaky appearance, hence the name CS-Hs (hydrated calcium silicate).

[0158]

[0159] After leaching, using lime milk is more effective at precipitating silica from the solution compared to using solid lime. Note that sodium hydroxide is more insoluble when using solid lime.

[0160] a) Lime concentration test

[0161] To determine the optimal concentration for desilication of the leachate, two concentrations were tested. These tests were conducted using a leachate agent derived from Jiaokou bauxite calcined at 1030°C, as described in Example 1 above.

[0162] Silica precipitation was performed at 98°C for 2 hours using lime slurry containing 100 g / L CaO (lime from a Chinese factory). Table 5 below shows the chemical composition of this lime.

[0163]

[0164] The CaO loading used was equivalent to 110% of the stoichiometry of SiO2 + Al2O3, thus enabling this experiment. The analysis and methods used are as follows: - Mud: LOI, Al2O3, TiO2, Fe2O3, CaO, SiO2, Na2O, total carbon and XRD - Liquids: Metrohm™ Thermogravimetry, complete ICP, complete chromatographic analysis and Phénix™ (organic carbon, mineral carbon).

[0165] The experimental conditions are summarized in Table 6. Table 7 shows the chemical analysis results before desilication, and Table 8 shows the chemical analysis results of the liquid agent after desilication. Table 9 provides information on the precipitation yields of silica and alumina.

[0166]

[0167] It was observed that reducing the concentration of the leachate by half resulted in a 10% decrease in the precipitation rate of silica. It was also observed that lime slurry had almost no effect on the CaO content of the solution.

[0168] Table 10 below shows the analysis of the precipitated silicates (white mud).

[0169]

[0170] X-ray diffraction patterns show that the precipitated silicate phase is tobermorite [Ca5Si6O]. 16 [(OH)2 x 4H2O] (ML=702). Excess calcium hydroxide and calcite were also formed.

[0171] When lime slurry is used for precipitation, the precipitation rates are 87% and 90%, respectively; when solid lime is used, the precipitation rate is 74%. It is estimated that sodium hydroxide is 0.0017 points insoluble relative to the dissolved silica.

[0172] Example 3: Bauxite from Xiaoyi

[0173] Powdered bauxite from Xiaoyi (Shaanxi Province, China) is provided, and the chemical analysis of the bauxite is described in detail in Table 11 below.

[0174]

[0175] The composition of this ore is significantly different from that of bauxite from Jiaokou. Specifically, it has a lower aluminum content and a higher silicon content: the A / S ratio is extremely low, at approximately 2.7; silica exists primarily as kaolinite. This bauxite is of "low grade" quality and would not be incorporated into methods according to existing techniques. Therefore, this bauxite presents a significant challenge to the methods according to the present invention.

[0176] Calcination and Leaching: Operating Conditions

[0177] A 30 g ore sample was placed in a silica crucible; the crucible was then placed in a muffle furnace at 1020 °C for 30 minutes. After calcination, the sample was removed from the furnace and allowed to cool in ambient air; the sample was then weighed.

[0178] The calcined bauxite was then leached in a solution containing 100 g / L Na₂O (equivalent to 130 g / L NaOH), resulting in a solids concentration of 80 g / L. This leaching was carried out in a jacketed reactor using a constant-temperature oil bath.

[0179] After leaching, the suspension is filtered through slow-speed filter paper (Whatman 589 / 3) in a Buchner funnel mounted on a flask connected to a vacuum pump. The first filtration is performed while the suspension is hot, followed by two more filtrations after the filter cake has been re-slurried with softened water at ambient temperature. The filter cake from the second filtration is washed with ethanol on the filter.

[0180] The raw ore, calcined bauxite, and leaching residues were characterized by chemical analysis, X-ray diffraction (XRD), DRIFTS (radio-infrared spectroscopy), and scanning electron microscopy (SEM). Furthermore, for bauxite from Xiaoyi (Shanxi Province), thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on the raw ore.

[0181] Results after calcination and leaching

[0182] Characterization by X-ray diffraction revealed that the crystalline phases in the initial bauxite, arranged in descending order of importance, were: gibbsite (AlO(OH)), kaolinite (Al2Si2O5(OH)4), magnetite (Fe3O4), anatase (TiO2), rutile (TiO2), hematite (Fe2O3), goethite (FeO(OH)), quartz (SiO2), mica (illite, muscovite (Al2O3AlO4)). 10 (OH)₂K), calcite (CaCO₃), palygorskite (MgAl)₄Si₈O 20(OH)2-8H2O. Therefore, the high silica content in this ore mainly comes from kaolinite, followed by quartz and other silicates (mica, palygorskite).

[0183] The combined use of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) essentially revealed the dehydroxylation of gibbsite and kaolinite (see [link]). Figure 3 and 4 At approximately 900°C, the curves observed indicated the hypothesis of crystallization of a newly formed compound, possibly mullite. These results led the inventors to set the calcination temperature to 1020°C.

[0184] After calcining bauxite from Xiaoyi at 1020℃, XRD characterization now only revealed the presence of corundum (α-Al2O3), magnetite, hematite, anatase, rutile, and quartz. Infrared spectroscopy showed that all signals associated with kaolinite disappeared, thus the planetary tetrahedral structure also vanished: only its highly distributed signal corresponding to amorphous silica remained.

[0185] As shown in Table 12 below, the mass loss of bauxite from Xiaoyi during calcination at 1020℃ was measured to be 14.02% with very good reproducibility through multiple experiments.

[0186]

[0187] Leaching tests of calcined minerals in a soda solution were conducted under the operating conditions defined at the beginning of this chapter. In this experiment, the observed dissolved mass percentages ranged between 16.8% and 17.8%. The results are summarized in Table 13 below.

[0188]

[0189] The desiliconization rate was 74.2%.

[0190] Mineralogical characterization of the leached bauxite by XRD revealed the presence of the same crystalline phases as before leaching: corundum (α-Al₂O₃), magnetite, hematite, anatase, rutile, and quartz. However, IR spectroscopy showed that the product was generally highly hydrophilic, which facilitated its suspension, and exhibited a significant decrease in signal intensity associated with Si-O elongation, consistent with the dissolution of the silicate phase.

[0191] Table 14 below compares the chemical analyses of raw bauxite, calcined bauxite, and leached bauxite:

[0192] The transformation of the ore after processing was clearly observed: the alumina content increased from 51.59% to 71.51%, and the alumina / silica ratio increased from 2.7 to 10.1. This indicates that the method according to the invention can transform "low-grade" bauxite into high-quality bauxite.

[0193] Erosion (leaching) of bauxite from Xiaoyi under industrial operating conditions.

[0194] The leached bauxite (all sizes less than 300 μm) was ground and dried in a furnace at 110 °C. Etching was then performed in a 150 ml autoclave at 260 °C for 40 minutes. The chemical composition of the etching solution was very close to that of industrial etching solutions in China: Na₂O ctq = 238 g / L, WR = 0.540, and Na₂O cbte = 17.2 g / L.

[0195] The lime added to the erosion process came from an alumina plant in China; it contained 86.02% CaO. The lime was slaked and added in the form of lime milk. Different amounts of lime were experimented with.

[0196] At the end of the etching process, the suspension was filtered. The solids were washed and prepared for chemical and crystallographic analysis (loss on ignition, X-ray fluorescence, and X-ray diffraction). Methrom analytical reagents (Na₂O, Al₂O₃, carbonates) were used.

[0197] The main parameters are recorded in Table 15 below.

[0198]

[0199] Some analytical results differ slightly from those in the table above. These results were obtained by different laboratories to confirm the validity of the described method.

[0200] It was clearly observed that the alumina contained in bauxite calcined at 1020°C and subsequently leached in a soda medium was soluble under the high-temperature operating conditions of the Chinese plant, and the extraction yield was absolutely acceptable, confirming the benefits of the method according to the invention. This result clearly demonstrates that corundum, depending on its degree of crystallinity and its particle size distribution, is soluble in a high-temperature soda medium.

[0201] Undoubtedly, the difference in the amount of lime added during dispersion and erosion is noteworthy. This important parameter affects yield, yield curves based on the alumina / caustic soda weight ratio, and the amount of insoluble sodium hydroxide.

[0202] These results enable the estimation of the specific consumption amount of the bauxite according to the method of the present invention.

[0203]

[0204] This embodiment illustrates several advantages of the invention. It is clear that the method according to the invention allows for better utilization of bauxite with low aluminum and high silicon content. A reduction in the amount of red mud-type residues is also observed, and the total amount of residues is significantly reduced (note that silicate precipitation produces its own white residue, which is absent in the prior art Bayer process; however, in the method according to the invention, the sum of these two residues is far less than that of prior art red mud). Considering that residues represent a cost factor, the environmental aspects associated with the residues of the invention present a very attractive advantage for operators; however, since the commercial value of this advantage depends on a variety of factors related to the plant, its global commercial value is difficult to assess.

[0205] Example 4: Leaching test of bauxite from Xiaoyi using low-concentration sodium hydroxide, which had been pre-processed using the calcination-leaching method of the present invention.

[0206] Based on the excellent results of Example 3, the inventors sought to further improve the method. Considering that the pre-processing method for bauxite (roasting-leaching method) significantly reduces the quality of bauxite at the Bayer process input, and thus the quality of the red mud at the output (while the quality of alumina produced remains constant), it is conceivable that the washing of the mud would be more efficient in the case of the method of the present invention. If the same washing efficiency is maintained, the amount of clarified water used for washing the mud can be reduced, which reduces the total amount of water evaporated in the Bayer process. Therefore, the energy consumption in this stage of the method is greatly reduced. In fact, in the Bayer process, the reduction in evaporation necessarily accompanies a reduction in the concentration of caustic soda in the leaching agent. Therefore, the inventors conducted leaching tests on leached bauxite from Xiaoyi at a Na₂O ctq concentration of 171.5 g / L, which is much lower than the concentration of 238 g / L used in the first test (Example 3). A Na₂O ctq concentration of 171.5 g / L is equivalent to a Bayer unit (technically termed a "refinery") operating without an evaporator.

[0207] In this Example 4, the bauxite from Xiaoyi used in Example 3 was processed using the same method as described in Example 3, but the Bayer process leaching was performed with a lower sodium hydroxide concentration: the Na₂O ctq concentration was reduced from 238 g / L to 171.5 g / L. The results are summarized in Table 17 below.

[0208]

[0209] It was noted that although the concentration of caustic soda in the etching solution was significantly reduced, the solubilization yield of alumina remained very high.

[0210] The results of this study can significantly reduce energy consumption and operating costs in alumina refineries using the technology of this invention.

[0211] Example 5: Bauxite from Wanji (Henan Province)

[0212] Bauxite was supplied from Wanji (Henan Province).

[0213] Calcination and leaching tests, as well as physicochemical analyses, were conducted under the same conditions as those performed on bauxite from Xiaoyi (Example 3). A key difference between the two ores lies in the form of silica in the bauxite. For the ore from Wanji, characterization by X-ray diffraction revealed the following crystalline phases: Boehmite (AlO(OH)), muscovite (K) 1-x Na x (Al) 2-y Fe y (AlSi3)(O) 10 (OH)2), kaolinite (Al2Si2O5(OH)4), quartz (SiO2), siderite (FeCO3), goethite (FeO(OH)), hematite (Fe2O3), magnetite (Fe3O4), anatase and rutile (TiO2).

[0214] Therefore, the SiO2 content mainly comes from muscovite and quartz, and secondly from kaolinite.

[0215] Table 18 below records the analysis of raw bauxite, calcined (1020℃) bauxite, and leached bauxite:

[0216] After calcination, X-ray diffraction characterization showed that kaolinite disappeared, diaspore was transformed into corundum (α-Al₂O₃), and siderite and goethite were transformed into hematite. Other phases remained, such as muscovite (residual structure), quartz, magnetite, anatase, and rutile. The detection limit of this X-ray characterization was less than 1% by mass.

[0217] Leaching in the soda medium reduced the SiO2 content from 12.58% to 8.58%, or 4 percentage points, and increased the alumina / silica ratio from 4.0 to 7.3 (see Table Z2 above).

[0218] The observed difference between the ore from Xiaoyi (a 12 percentage point decrease in silica content) stems from the fact that the SiO2 content in the ore from Xiaoyi mainly comes from kaolinite, while the SiO2 content in the bauxite from Wanji comes from muscovite, which is difficult to leach even after calcination.

[0219] However, the leaching test showed that sodium hydroxide consumption was reduced by 60% relative to the amount of alumina consumed, and bauxite consumption was reduced by 25%.

[0220] Example 6: Bauxite from Shanxi

[0221] Another type of bauxite from Shanxi (China) is provided. Its alumina / silica ratio is 1.94. X-ray diffraction analysis shows it to be a gibbsite-type bauxite, with its silicates primarily composed of kaolinite and containing small proportions of muscovite and quartz. Iron is primarily present in the form of goethite.

[0222] The bauxite was calcined at 1030℃ for 30 minutes. Leaching of the calcined bauxite was then carried out at 100℃ using sodium hydroxide with a concentration of 100 g Na₂O / L.

[0223] Table 19 below records the analysis of raw bauxite, calcined (1030℃) bauxite, and leached bauxite:

[0224] X-ray diffraction characterization after calcination showed the disappearance of kaolinite and the appearance of a siliceous phase—mullite. A protrusion was observed in the 2θ region between 18° and 20° in the diffraction pattern, indicating the formation of a poorly crystalline silica phase. Boehmite monohydrate disappeared. Corundum was present. Goethite transformed into hematite.

[0225] Following leaching, X-ray diffraction characterization revealed the continued presence of mullite. The protrusions in the diffraction pattern disappeared. Most of the amorphous silica dissolved. Corundum, hematite, rutile, and anatase phases remained.

[0226] Leaching of calcined ore in a soda ash medium reduced the SiO2 content from 27.75% to 14.49%, or increased the alumina / silica ratio from 1.94 to 5.06. The desilication yield increased to 62.9%. The amount of sodium hydroxide introduced was 0.98%.

Claims

1. A method for producing alumina trihydrate or alumina from pre-processed bauxite, wherein the bauxite has an Al2O3 / SiO2 mass ratio of less than 5 and is pre-processed by a method including calcination (2020) and leaching (2030), wherein the pre-processed bauxite is characterized in that its loss on ignition is less than 2.5% by mass, preferably less than 2.0%, and even more preferably less than 1.5%. The method includes the following steps: (a) The pre-processed bauxite is processed (2120) (referred to as "dissolution") using an aqueous solution of sodium hydroxide in a closed autoclave or a pressurized tubular system at a temperature between 250°C and 270°C and a residence time between 30 min and 50 min, and then the aqueous solution is allowed to reach atmospheric pressure through continuous expansion, thereby causing the water to evaporate, wherein the concentration of the aqueous solution of sodium hydroxide is between 155 g Na2O / L and 190 g Na2O / L, preferably between 160 g Na2O / L and 180 g Na2O / L; (b) Separation of solid residues from the liquid phase (2130); (c) Aluminum trihydrate crystallization (2160) by adding seed crystals (2170); (d) Separate the crystallized aluminum trihydrate (2180) from the liquid phase. (e) Optionally, the aluminum trihydrate obtained in step (d) can be calcined (2194) to obtain alumina (2196). Furthermore, in the method, the water evaporation step of the liquid phase is omitted after step (d).

2. The method according to claim 1, characterized in that, The temperature in step (a) is between 250°C and 270°C.

3. The method according to claim 1 or 2, characterized in that, The pre-processed bauxite has an Al2O3 / SiO2 mass ratio greater than 8, preferably greater than 9, and even more preferably greater than 10.

4. The method according to any one of claims 1 to 3, characterized in that, The pre-processed bauxite contains more than 60% alumina by mass, preferably more than 65%, and even more preferably more than 70%.

5. The method according to any one of claims 1 to 4, characterized in that, The pre-processed bauxite contains less than 12% silica by mass, preferably less than 10%, and even more preferably less than 8%.

6. The method according to any one of claims 1 to 5, characterized in that, The pre-processed bauxite has a loss on ignition of less than 2%, a boehmite mass ratio of less than 3%, and a kaolinite mass ratio of less than 3%. Preferably, the loss on ignition is less than 2%, the boehmite mass ratio is less than 2%, and the kaolinite mass ratio is less than 2%. Even more preferably, the loss on ignition is less than 1.5%, the boehmite mass ratio is less than 1%, and the kaolinite mass ratio is less than 2%.

7. The method according to any one of claims 1 to 6, characterized in that, The liquid phase from step (d) is reintroduced (2220) into the aqueous sodium hydroxide solution used in step (a).

8. The method according to any one of claims 1 to 7, characterized in that, The pre-processed bauxite is pre-processed by leaching (2030) with an aqueous sodium hydroxide solution.

9. The method according to any one of claims 1 to 8, characterized in that, The pre-processed bauxite is pre-processed by calcination (2020) at a temperature between 920°C and 1120°C, preferably between 950°C and 1070°C, and even more preferably between 1000°C and 1050°C.

10. The method according to any one of claims 1 to 9, characterized in that, The pre-processed bauxite is pre-processed by calcining monohydrate gibbsite-type bauxite at a temperature between 1000°C and 1050°C, preferably between 1010°C and 1035°C.

11. The method according to any one of claims 1 to 10, comprising the following steps: (i) Preprocessing bauxite to obtain the preprocessed bauxite, wherein the preprocessing comprises, in sequence: - Calcination (2010) - Leaching using sodium hydroxide aqueous solution (2030). - Solids (2040) are separated from the leaching aqueous phase, the separated solids representing the pre-processed bauxite. (ii) The pre-processed bauxite is processed by the method according to any one of claims 1 to 10.

12. The method according to any one of claims 1 to 11, characterized in that, The Al2O3 / SiO2 ratio of the bauxite before preprocessing is between 1 and 4, with the optimal ratio being between 1 and 3.

13. An alumina that can be obtained by the method according to any one of claims 1 to 12.

14. An apparatus for carrying out the method according to any one of claims 11 or 12, comprising: - Bauxite is pre-processed through calcination and leaching to transform it into pre-processed bauxite units; as well as - A unit for implementing the method according to any one of claims 1 to 9 to produce alumina from the pre-processed bauxite. Its characteristics are: - The pre-processing unit includes -- At least one calcining furnace for calcining the bauxite, -- At least one leaching unit for leaching calcined bauxite using an aqueous sodium hydroxide solution (referred to as "leaching solution"), and -- At least one solid-liquid separation unit for separating calcined and leached bauxite from the leachate; - The unit for producing alumina from the pre-processed bauxite includes -- At least one chamber for processing the pre-processed bauxite using an aqueous sodium hydroxide solution (referred to as "Bayer solution") at a temperature of at least 100°C -- At least one solid-liquid separation unit for separating solid residues from the Bayer liquid; -- At least one crystallization unit of aluminum trihydrate crystallized from the Bayer solution by adding a seed crystal of aluminum trihydrate; -- At least one solid-liquid separation unit for separating crystalline aluminum trihydrate from the Bayer liquid; -- Optionally, at least one calcination unit may be used to convert the aluminum trihydrate into alumina.

15. The device according to claim 14, characterized in that, The Bayer solution from the solid-liquid separation unit used to separate crystalline aluminum trihydrate from the Bayer solution is recycled to the dissolution step (2120).

16. The device according to claim 15, characterized in that, The device does not include an evaporation unit, wherein the Bayer liquid from the solid-liquid separation unit used to separate crystalline aluminum trihydrate from the Bayer liquid is concentrated by evaporation after reaching atmospheric pressure before being recycled to the dissolution step (2120).

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