Method for extracting valuable substances from a bauxite residue

Abstract

The present invention relates to a method for extracting valuable substances from a bauxite residue, the method comprising the following steps: a) providing an aqueous suspension of the bauxite residue; b) adjusting the pH of the aqueous suspension to a value between 7.2 and 12.2; c) at least partially deagglomerating suspended mineral agglomerates of the bauxite residue; and d) separating the resulting mixture into an iron-rich fraction and at least one other, preferably silicate-rich, fraction, wherein at least one flocculant is added to the aqueous suspension, and the at least one flocculant comprises starch.

Description

[0001] Process for recovering valuable materials from a bauxite residue

[0002] The present invention relates to processes for recovering valuable materials from a bauxite residue.

[0003] Aluminum is extracted industrially from bauxite using the Bayer process. Bauxite typically contains only 30 to 60% aluminum oxide; the remainder is a mixture containing iron oxides and silicates, among other substances. The Bayer process separates sufficiently pure aluminum oxide from bauxite.

[0004] In a typical Bayer process, the bauxite is finely ground and mixed with caustic soda. This mixture is stirred in autoclaves under pressure and high temperatures, causing the caustic soda to dissolve the aluminum oxide from the bauxite by combining with it to form sodium aluminate. The undissolved bauxite components form what is known as bauxite residue. The bauxite residue is separated, and the aluminum oxide can be further processed by stirring, calcining, and other steps to obtain pure aluminum.

[0005] The bauxite residue (BR) produced during the Bayer process is also known as "red mud." This name derives from the red color caused by the iron compounds. Bauxite residue poses an enormous disposal problem. According to some sources, around 150 million tons of bauxite residue are generated as a waste product worldwide each year. Bauxite residue is often stored in large landfills or "red mud ponds," but in some countries it is simply discharged into rivers. Bauxite residue represents an environmental problem of extraordinary proportions.

[0006] Efforts are therefore underway to convert bauxite residue, previously considered a waste product, into usable materials. The goal is, in particular, to separate the iron components, which can make up a large portion of the bauxite residue.

[0007] A current overview of the state-of-the-art red mud treatment is provided by Yancun Qi ("The neutralization and recycling of red mud-a review." Journal of physics: conference series. Vol. 1759. No. 1. IOP Publishing, 2021; DOI 10.1088 / 1742-6596 / 1759 / 1 / 012004).

[0008] Furthermore, US Pat. No. 3,127,239 A discloses a process for separating alkali-insoluble components from aluminous ores of the bauxite type from alkaline bauxite digestion liquor in which they are suspended. A flocculant comprising homogenized gelatinized starch is added. Patent DE 702 397 C describes a process for processing red muds produced during the extraction of alumina from bauxite. The red mud is treated at low temperatures with dilute acids, with or without the addition of a neutral salt.

[0009] CN 113 857 209 A also discloses a process for recycling red mud based on acid treatment.

[0010] Another process for recovering valuable materials from a bauxite residue, in particular for separating iron-containing components, is known from EP 2 836 462 B1. The wet-chemical separation of at least some of the iron-containing components of the bauxite residue is to be made possible by a process comprising the following steps: providing an aqueous suspension of the bauxite residue; adjusting the pH of the suspension to a value between 7.2 and 12.2 and adding at least one dispersant; at least partially deagglomerating suspended mineral agglomerates of the bauxite residue by generating cavitation; and separating the resulting mixture into an iron-rich fraction and at least one further, preferably silicate-rich fraction.

[0011] Despite these and other processes known in the state of the art, the problem of recovering valuable materials from bauxite residue has not yet been satisfactorily solved. There is still a need for improved processes that enable, for example, higher yields, greater efficiency, and / or higher throughput. In particular, there is also a need for more cost-effective and environmentally friendly processes that can be carried out on the largest possible scale.

[0012] It is an object of the present invention to provide such methods.

[0013] The invention therefore relates to a process for recovering valuable materials from a bauxite residue, the process comprising the following steps: a) providing an aqueous suspension of the bauxite residue; b) adjusting the pH of the aqueous suspension to a value between 7.2 and 12.2; c) at least partially deagglomerating suspended mineral agglomerates of the bauxite residue; and d) separating the resulting mixture into an iron-rich fraction and at least one further, preferably silicate-rich fraction, wherein at least one flocculant is preferably added to the aqueous suspension, wherein the at least one flocculant comprises starch. The mineral agglomerates contained in the bauxite residue are agglomerated particles, which preferably comprise, among other substances, iron oxides and silicates. The particles are usually bonded together by electrostatic and steric bonding forces.In the process disclosed in EP 2 836 462 B1, the binding forces between the agglomerated mineral components are to be at least partially eliminated in order to enable deagglomeration. To this end, the surface charges of the mineral particles are to be balanced as much as possible by adjusting the pH value in the range of the isoelectric point or the point of zero charge (PZC). According to EP 2 836 462 B1, it is further provided that a dispersant is added to the suspension, which is intended to stabilize or maintain the particle isolation achieved by deagglomeration. This is intended to prevent released particles from reagglomerating with other undesirable particles and to promote deagglomeration. Deagglomeration is then to occur by generating cavitation, i.e., by the formation and dissolution of vapor-filled cavities (vapor bubbles) in the suspending medium of the bauxite residue, e.g.using a dissolver stirrer or by ultrasonication. An iron-rich fraction and a silicate-rich fraction are then separated.

[0014] EP 2 836 462 B1 points out that no additional additives such as flocculants or the like are required. This is fundamentally advantageous, since flocculants are often environmentally problematic and can sometimes be expensive.

[0015] Within the scope of the present invention, however, it has now surprisingly been found that starch – which is neither expensive nor environmentally harmful – can be used as a flocculant, thereby significantly improving the process. Without being bound by any theory, the inventors suspect that the flocculant enables the iron particles in the suspension to be captured and agglomerated more quickly and / or more purely. The agglomerated iron particles can then be separated as the iron-rich fraction. Starch is not only inexpensive and available in large quantities, but also does not pollute the environment. Furthermore, it has been shown, quite surprisingly, that starch works surprisingly well as a flocculant in the process according to the invention, even leading to better results than conventional chemical flocculants, such as the much more environmentally problematic polycarboxylate ester (PCE).PCE is widely used, but due to its harmfulness to aquatic organisms, it must not be discharged into sewers or surface waters. The invention has now shown that starch is not only an equivalent replacement for PCE as a flocculant, but even delivers better results. For example, potato starch achieved an approximately 3 percentage points higher iron content in the iron-rich fraction than PCE – this corresponds to a relative increase of approximately 7 percent (see Example 2 and Fig. 1). Considering the enormous quantities of bauxite residue to be treated and the large scale at which such a process is to be carried out, such an increase represents a veritable leap, with a significant impact on the economic viability of the process.The fact that this leap can be achieved using a means that is environmentally safe opens up new perspectives for the processing of bauxite residue and could be a significant step towards solving this major environmental problem.

[0016] Independently of the use of starch as a flocculant, a further surprising improvement of the process according to EP 2 836 462 B1 has emerged within the scope of the invention. It has unexpectedly been shown that, even without using starch as a flocculant, it is advantageous to carry out the deagglomeration of the suspended mineral particles of the bauxite residue without cavitation. Comparative tests have shown that deagglomeration with an inline disperser, which does not cause cavitation, leads to a higher iron content in the iron-rich fraction than when deagglomeration is carried out with cavitation using a toothed disk system (see Example 4). In addition, and independently of this, an inline disperser also makes it possible to shorten the dispersion time, which—also unexpectedly—allows for an even higher iron content to be achieved.At the same time, this makes it possible to significantly increase the overall process time and throughput.

[0017] Finally, it has been found that in the process according to the invention—i.e., using starch as a flocculant and / or deagglomeration with an inline disperser—the dispersant proposed in EP 2 836 462 B1, which is to be added to the bauxite residue suspension, can be completely dispensed with. This, in turn, increases the economic efficiency of the process and simultaneously reduces the environmental impact that can be associated with environmentally problematic dispersants.

[0018] Apart from the differences mentioned above, the process according to the invention can be carried out as described in EP 2 836 462 B1. The content of EP 2 836 462 B1 is therefore hereby incorporated into the present application. In particular, the steps a) providing an aqueous suspension of the bauxite residue; b) adjusting a pH of the suspension to a value between 7.2 and 12.2; c) at least partially deagglomerating suspended mineral agglomerates of the bauxite residue; and d) separating the resulting mixture into an iron-rich fraction and at least one further, preferably silicate-rich fraction; can be carried out as described in EP 2 836 462 B1.

[0019] In the context of the invention, the bauxite residue is preferably obtainable or obtained by the Bayer process. This can be bauxite residue originating from a landfill. Existing landfills can thus be mined and recycled. However, the bauxite residue can also originate directly from a Bayer process. The process according to the invention can therefore be carried out immediately after the Bayer process, wherein, in step a) of the process, the bauxite residue resulting from the Bayer process is mixed with water to provide the aqueous suspension of the bauxite residue.

[0020] The process according to the invention comprises steps a) to d) as described above. The steps of the process are preferably carried out in the specified order. In a preferred embodiment, the process consists of steps a) to d).

[0021] In step a) of the process according to the invention, the bauxite residue can be mixed with water to obtain the aqueous suspension of the bauxite residue. Preferably, the bauxite residue is homogenized with water.

[0022] Homogenization can take place in a homogenization vessel. The homogenization vessel is preferably heatable, for example, via a double jacket. A stirrer can be used for homogenization. Homogenization can also be performed using an inline disperser. The suspension can circulate between the inline disperser and the homogenization vessel for homogenization, with the inline disperser having the option of switching over and pumping the suspension into the next process step.

[0023] The homogenizer vessel is preferably an open vessel. Dilution water can be added to the homogenizer vessel, preferably including process water separated from the silicate-rich fraction. Bauxite residue can be added from a dispersing vessel while stirring (or, in the case of an inline disperser, while continuously pumping in a circular flow).

[0024] In step b), the aqueous suspension of the bauxite residue is adjusted to a pH value between 7.2 and 12.2.

[0025] The pH value in step b) is preferably adjusted by adding an acid. Citric acid is particularly preferred. Citric acid has the advantage, among other things, of being environmentally safe.

[0026] In a preferred embodiment, the acid, especially citric acid, is added in an amount ranging from 0.25 to 2 wt.% based on the dry matter content of the aqueous suspension. The acid can advantageously be added in liquid form from a storage container via a metering pump. The amount of added acid can be determined by measuring the pH of the suspension.

[0027] It has proven advantageous if, in step b), the temperature of the suspension is also adjusted to a specific range. It is particularly advantageous if the temperature is set to a value between 20 °C and 90 °C, preferably between 30 °C and 70 °C.

[0028] The residence time of the bauxite residue in the homogenization vessel can preferably be 10 to 120 minutes, preferably 15 to 90 minutes, more preferably 20 to 70 minutes. The suspension is preferably kept in the pH range and / or temperature range specified above for the specified period of time, in particular 20 to 70 minutes, prior to deagglomeration in step c). This residence time particularly promotes the hydration of the individual mineral constituents, so that the individual particles are surrounded by a regular hydration shell.

[0029] From the homogenization tank, the suspension can be transferred to at least one dispersing tank, where deagglomeration can take place. Typically, deagglomeration occurs through the application of mechanical energy to separate the agglomerated particles into microscopically small individual particles. The mutual attractive forces can advantageously be minimized by adjusting the pH and then overcome by mechanical energy. After deagglomeration, the individual minerals in the suspension can be largely separated from one another. Simply put, the goal of deagglomeration is to ensure that each individual mineral particle present in the mixture floats independently in the aqueous phase.

[0030] The pH value can be adjusted again in the dispersing container (in addition to or alternatively to the adjustment already made in the homogenizing container).

[0031] As mentioned above, it is preferred in the context of the process according to the invention if the deagglomeration in step c) takes place without prior addition of a dispersant. This increases, among other things, the economic efficiency of the process and simultaneously reduces the environmental impact that environmentally problematic dispersants can otherwise cause.

[0032] Step c) comprises the at least partial deagglomeration of suspended mineral agglomerates in the aqueous suspension. The deagglomeration preferably takes place by subjecting the mineral agglomerates to high shear forces. This can be achieved by cavitation, e.g., by means of agitators running at high speeds that are specially shaped to achieve cavitation. For example, a toothed disk system can be used. However, it is preferred if the deagglomeration in step c) takes place without generating cavitation. As mentioned above, it has surprisingly been shown that the iron content of the iron-rich fraction can be further increased if the deagglomeration takes place without cavitation (see Example 4).

[0033] The use of a rotor-stator system, in particular an inline disperser, has proven particularly suitable for the process according to the invention. An inline disperser has the advantage, on the one hand, that no cavitation is generated. On the other hand, it has the further advantage of enabling a faster and, if necessary, continuous process. This can shorten the dispersion time, which—also unexpectedly—allows an even higher iron content to be achieved in the iron-rich fraction (cf. Example 2 and Fig. 1). At the same time, the overall process time is shortened and the throughput increased. Within the scope of the invention, it is therefore preferred if the deagglomeration in step c) is carried out using an inline disperser.

[0034] It has surprisingly been found to be advantageous if the deagglomeration in step c) takes place for a total duration of less than 120 minutes, preferably less than 90 minutes, more preferably less than 60 minutes, more preferably less than 45 minutes, more preferably less than 30 minutes, more preferably less than 20 minutes. Within the scope of the invention, it has been shown that such a short total duration can lead to an even higher iron content in the iron-rich fraction obtained (cf. Example 2 and Fig. 1). It is therefore preferred if the deagglomeration in step c) takes place for a total duration in the range of 1 to 120 minutes, preferably 2 to 90 minutes, more preferably 3 to 60 minutes, more preferably 5 to 45 minutes, more preferably 7 to 30 minutes, more preferably 10 to 20 minutes.

[0035] Preferably, at least one flocculant is added to the aqueous suspension, preferably to assist the agglomeration of iron-containing particles. The flocculant can be added after the deagglomeration is complete, i.e., after completion of step c). Alternatively, the flocculant can also be added beforehand, e.g., during the deagglomeration or even before the deagglomeration, i.e., for example, between steps a) and b) or between steps b) and c). Preferably, the addition takes place before step d), preferably between steps c) and d).

[0036] The goal of this step is to cause the individual particles to adhere to similar particles as much as possible and to form an agglomerate that is capable of settling and can be separated from the suspension by gravity over time, for example, due to the difference in density between the particles. Simply put, the iron particles are to be captured in the suspension and agglomerated. The flocculant is preferably added in an amount ranging from 0.05 to 0.40 wt.% based on the dry matter content of the aqueous suspension.

[0037] As mentioned above, starch has proven to be a particularly advantageous flocculant. It has resulted in a significant improvement in the process and even a higher iron content in the iron-rich fraction than when using significantly more environmentally harmful flocculants such as PCE (see Example 2 and Fig. 1). Therefore, the at least one flocculant is preferably starch.

[0038] Any type of starch can be used within the scope of the invention. However, potato starch has been shown to have certain particular advantages. For example, an excellent iron content in the iron-rich fraction was achieved with both potato and corn starch. However, it was found that the iron yield—i.e., the ratio between the amount of iron in the iron-rich fraction and the total amount of iron in the starting product, bauxite residue—was even higher when using potato starch than with corn starch (see Example 3 and Fig. 2B). Potato starch is therefore particularly preferred within the scope of the invention.

[0039] Additionally, and regardless of the type of starch used, it has proven highly advantageous to use activated starch as a flocculant. This is preferably acid-activated starch—that is, the starch is activated by treatment with an acid. Activated starch has been shown to further improve iron content and yield. Without being bound by any theory, the inventors suspect that activation leads to better starch expansion, allowing it to adhere even more effectively to the particles and accelerate agglomeration.

[0040] The starch can be activated with any type of acid, for example, HCl. However, it has proven particularly advantageous to use citric acid for starch activation. This is particularly preferred if the same acid was already used in step b) to adjust the pH, as this way no additional substances are introduced into the process. For activation, the starch can, for example, be kept in citric acid (e.g., 5 wt.% in water) at a temperature of at least 50°C for at least 1 hour.

[0041] The flocculant, in particular the activated potato starch, can be added in liquid form from a storage container via a dosing pump.

[0042] Finally, in step d) of the process, the mixture is separated into an iron-rich fraction and at least one further, preferably silicate-rich fraction. The separation into these two fractions can be based on different specific gravity, with the iron-rich fraction having a lower specific gravity than the silicate-rich fraction. Separation in step d) is preferably carried out by selective sedimentation, preferably in a sedimentation tank. The flocculant can also be added in the sedimentation tank. After deagglomeration in step c), the suspension can be passed from the dispersing tank into the sedimentation tank. In this tank, agglomeration of similar particles can then occur, forming, among other things, larger units of iron oxide agglomerates, which sink due to their higher specific gravity.At the bottom of the sedimentation tank, the settling iron oxide can be compacted and continuously discharged via rotary valves or screw conveyors. The remaining, preferably silicate-rich fraction can be discharged at the overflow of the sedimentation tank.

[0043] The iron-rich fraction can also be referred to as the "heavy fraction," "iron oxide concentrate," or "iron mineral concentrate." This fraction is typically the one with the highest density. It contains predominantly iron minerals, particularly iron oxide and iron hydroxide. It separates first from the suspension and can be removed from the separator, for example, after compaction by vibration. The iron-rich fraction can ultimately be reused in iron production, thus forming a valuable raw material.

[0044] The remaining, preferably silicate-rich fraction can also be referred to as the "light fraction." This may be an iron-depleted fraction containing clay minerals.

[0045] In a preferred embodiment, the further, preferably silicate-rich fraction obtained in step d) is subjected to a solid-liquid separation, preferably by filtration and / or centrifugation, in order to separate process water.

[0046] Calcium hydroxide can be added to the other, preferably silicate-rich fraction before solid-liquid separation. This can improve the separation or filtration properties.

[0047] In a preferred embodiment, the process water separated from the further, preferably silicate-rich fraction is recycled into the suspension in step a). It can be fed into a homogenization tank and used in the homogenization of fresh bauxite residue.

[0048] All percentages (%) herein refer to weight percentages (wt%) unless otherwise indicated.

[0049] The present invention is illustrated by the following examples and figures, to which it is of course not limited. Figure 1: Comparison between starch and PCE as flocculants. The results are shown for different deagglomeration times.

[0050] Figure 2: Comparison of potato starch and corn starch as flocculants. (A) Iron content in the sediment. (B) Iron yield in the sediment.

[0051] Example 1 - General process description

[0052] In the following, an exemplary embodiment of the method according to the invention is described, as it was also used in the comparative experiments in the following examples.

[0053] Materials used

[0054] - Bauxite residue (red mud)

[0055] - Citric acid (solid, as anhydrite)

[0056] - tap water

[0057] - Flocculants: polycarboxylate ethers (PCE), potato starch or corn starch

[0058] Process flow

[0059] The bauxite residue was slurried with water and stirred using a toothed disc system or an inline disperser. The suspension was heated to 50 °C and the pH was adjusted to 9.6 by adding citric acid.

[0060] The flocculant was prepared. In the case of PCE, it was simply mixed with water. Starch was activated with 5 wt% citric acid in water for at least 1 hour at a minimum temperature of 50°C. The flocculant was then added to the suspension, and the rotation speed was increased to break up the particles. When using the toothed disc system, this generated cavitation. When using the inline disperser, no cavitation occurred.

[0061] For sedimentation, the crushed particles were allowed to settle. Iron-containing particles settled more quickly due to their higher specific gravity. Decanting separated the heavy from the light fraction.

[0062] The samples were analyzed by SEM and examined for their elemental composition. In particular, the iron content in the heavy (iron-rich) fraction was determined. Example 2: Comparison of starch vs. PCE

[0063] Two series of experiments were conducted, one with PCE and the other with potato starch as flocculant. The deagglomeration time was varied. The experiments were carried out using an inline disperser as described in Example 1.

[0064] The results are shown in Fig. 1. A significant increase in iron content in the iron-rich fraction was observed when starch was used as a flocculant compared to PCE.

[0065] In addition, the results showed that the iron content achieved in the iron-rich fraction was higher the shorter the total duration of deagglomeration was kept.

[0066] Example 3: Comparison of potato starch vs. corn starch

[0067] To compare potato starch and corn starch as flocculants, tests were conducted using a toothed disk as described in Example 1. The starch was used in amounts of 0.05 wt.%, 0.10 wt.%, and 0.20 wt.%, each based on the dry matter of the bauxite residue. The iron content achieved in the iron-rich fraction and the total yield were determined. To determine the yield, the total amount of iron present in the bauxite residue used was calculated and compared with the amount of iron ultimately obtained in the iron-rich fraction.

[0068] The iron content is shown in Fig. 2A. Potato and corn starch produced comparably good results.

[0069] The achieved iron yield is shown in Fig. 2B. This demonstrates a clear advantage of potato starch over corn starch. At each of the three tested addition levels, a higher yield was achieved with potato starch than with corn starch. The difference was particularly pronounced at 0.05 wt% starch, where a (very good) yield of 53.8% was achieved with corn starch, while potato starch achieved an even significantly better yield of 65.8%.

[0070] Example 4: Comparison of deagglomeration with vs without cavitation

[0071] To investigate the influence of cavitation on deagglomeration, experiments were conducted once with a toothed disc system (generates cavitation) and once with an inline disperser (does not generate cavitation) as described in Example 1. PCE was used as the flocculant.

[0072] The key difference between the two methods is the method of energy input. While the toothed disc achieves dispersion through cavitation, the inline disperser uses a rotor-stator mixer that breaks and divides the grains through a narrow gap using strong crushing and transverse forces.

[0073] Two tests were conducted using each dispersion method. The following results were obtained for the iron content in the iron-rich fraction:

[0074] - With cavitation (tooth disc) an average iron content of 44.2 wt.% was achieved (individual values: 45.3 wt.% and 43.1 wt.%).

[0075] - Without cavitation (inline disperser), an average iron content of 46 wt.% was achieved (individual values: 45.0 wt.%, 47.0 wt.%).

Claims

Claims 1. A process for recovering valuable materials from a bauxite residue, the process comprising the following steps: a) providing an aqueous suspension of the bauxite residue; b) adjusting a pH of the aqueous suspension to a value between 7.2 and 12.2; c) at least partially deagglomerating suspended mineral agglomerates of the bauxite residue; and d) separating the resulting mixture into an iron-rich fraction and at least one further, preferably silicate-rich fraction, wherein at least one flocculant is added to the aqueous suspension, wherein the at least one flocculant comprises starch.

2. The process according to claim 1, wherein the pH in step b) is adjusted by adding citric acid.

3. The process according to any one of the preceding claims, wherein the acid is added in an amount ranging from 0.25 to 2% by weight based on the dry matter content of the aqueous suspension.

4. The process according to any one of the preceding claims, wherein in step b) the temperature of the suspension is adjusted to a value between 30 °C and 70 °C.

5. The process according to any one of the preceding claims, wherein the deagglomeration in step c) is carried out without prior addition of a dispersant.

6. The process according to any one of the preceding claims, wherein the deagglomeration in step c) is carried out without generating cavitation.

7. The process according to any one of the preceding claims, wherein the deagglomeration in step c) is carried out with an inline disperser.

8. The process according to any one of the preceding claims, wherein the deagglomeration in step c) is carried out for a total duration of less than 45 minutes.

9. The process according to any one of the preceding claims, wherein the flocculant is added in an amount ranging from 0.05 to 0.40 wt.% based on the dry matter content of the aqueous suspension.

10. The process according to any one of the preceding claims, wherein the starch is potato starch.

11. The method according to any one of the preceding claims, wherein the starch is an activated starch.

12. The process according to any one of the preceding claims, wherein the separation in step d) is carried out by selective sedimentation.

13. The process according to any one of the preceding claims, wherein the further, preferably silicate-rich fraction obtained in step d) is fed to a solid-liquid separation in order to separate process water.

14. The process according to any one of the preceding claims, wherein calcium hydroxide is added to the further, preferably silicate-rich fraction prior to the solid-liquid separation.

15. The process according to any one of the preceding claims, wherein the process water separated from the further, preferably silicate-rich fraction is recycled into the suspension in step a).

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