OA17943A - Process and apparatus for obtaining material of value from a bauxite residue - Google Patents

Abstract

The invention relates to a process for obtaining material of value from a bauxite residue which is obtainable or has been obtained by the Bayer process. This process comprises the steps of a) providing an aqueous suspension of the bauxite residue, b) setting a pH of the suspension to a value between 7.2 and 12.2, c) at least partly deagglomerating suspended mineral agglomerates of the bauxite residue, and d) separating the resulting mixture into an iron-rich fraction and into at least one further, preferably silicate-rich fraction. The invention further relates to an apparatus (10) for performance of the process.

Description

The invention relates to a process for obtaining material of value from a bauxite residue obtainable by the Bayer process. Furthermore, the invention relates to an apparatus for performing the process.

In the alumina production (precursor for the aluminum melting plant) with the aid of the Bayer process, aluminum is dissolved out of finely milled bauxite as sodium aluminate with the aid of caustic soda lye. After seeding with crystallization nuclei, pure AI(OH₃) (gibbsite) is precipitated from the sodium aluminate solution separated from the bauxite residue, subsequently calcinated to aluminum oxide, and finally metallic aluminum is obtained by electrolysis. The remaining bauxite residue, which is also referred to as bauxite residue (BR), is chemically considered mainly composed of iron oxides and hydroxides, respectively, titanium oxides, aluminum oxide residues, silicates, calcium oxide, sodium oxide and residual caustic soda lye. Due to its red color caused by iron (III) oxide, this bauxite residue is also referred to as red mud.

According to the quality of the used bauxite, 1 to 3 tons of bauxite residue arise to each produced ton of aluminum as a non-avoidable attendant. Therefore, many millions of tons of bauxite residue arise each year, which présent a serious environmental and disposai problem together with the already présent amounts. The main problem is the high alkalinity of the bauxite residue with pH values of 13 to 14 due to its content of caustic soda lye. Moreover, toxically acting aluminum ions together with iron compounds présent a great danger to the ground water and additionally impede environmentally compatible disposai.

Therefore, the disposai of the bauxite residue is substantially effected by storage in sealed disposai sites. The caustic soda lye exiting on the floor of the disposai site is collected and returned into the Bayer process in some disposai sites. Usually, however, the caustic soda lye is simply sucked off and disposed of as wastewater or even leaks in uncontrolled manner in the worst cases. However, this form of storage r τ is costly and expensive since large disposai site areas and plants are required, and high costs arise for the transport of the bauxite residue. Additionally, the long-term costs arising by the déposition can only hardly be calculated and présent an additional economical problem because at least in Europe accruals for later disposai hâve to be constituted. At présent, disposai site stocks with about 2.5 billions of tons of bauxite residue exist. To this, about 80-120 millions of tons of bauxite residue are added per year.

The disposai costs could be reduced if the bauxite residue considered as waste product heretofore could be converted to usable materials of value or be used for obtaining materials of value. In particular, the séparation of the iron components is of great interest. The aim of each process should be that the obtained materials of value can be further used or marketed without expensive post-processing.

Since the beginning of the industrial employment of the Bayer process, attempts to extract the valuable ingrédients such as iron, titanium, vanadium or rare earths from the bauxite residue and provide them to a new use were not lacking. However, the bauxite residue is still mainly deposited in large settling pools as a mud or piled up in chamber filter presses in a kind of pile (heap) after partial drainage, which is known as so-called dry stacking. .

However, a process for separating the high-class iron ore with a simple wet-chemical process is not known heretofore. However, since iron oxides and hydroxides can constitute more than 50 % of the minerais présent in the bauxite residue, a wetchemical extraction of the iron containing compounds is of great interest.

The invention is based on the object to providé a process, which allows the wetchemical séparation of at least a part of the iron containing components in the bauxite residue as materials of value. A further object of the invention is to provide an apparatus for performing such a process.

According to the invention, the objects are solved by a process according to claim 1 for obtaining materials of value from a bauxite residue as well as by an apparatus according to claim 23 for performing this process. Advantageous configurations with r t convenient developments of the invention are specified in the dépendent daims, wherein advantageous configurations of the process are to be considered as advantageous configurations of the apparatus.

A first aspect of the invention relates to a process for obtaining materials of value from a bauxite residue from the Bayer process. According to the invention, therein, the wet-chemical séparation of at least a part of the iron containing components of the bauxite residue is allowed in that the process includes at least the steps of a) providing an aqueous suspension of the bauxite residue, b) adjusting a pH value of the suspension to a value between 7.2 and 12.2, c) at least partially disagglomerating suspended minerai agglomérâtes of the bauxite residue, wherein the minerai agglomérâtes in step c) are disagglomerated by génération of cavitation, and d) separating the resulting mixture into an iron-rich fraction and into at least one further, preferably silicate-rich fraction. Alternative^, it can be provided that the process is exclusively composed of these steps. Bauxite residue (or red mud) has a high portion of very small particles with diameters between about 20 nm and 1000 nm and therefore exhibits characteristics of a colloid. Colloids are complex Systems, in which various agglomerated particles are suspended/dispersed in a liquid, namely aqueous caustic soda lye, as in the case of the bauxite residue. Electrostatic and steric bonding forces act between the particles, which normally prevent simple séparation of the individual minerai particles or minerai fractions from each other and moreover also influence the chemical reactivity of the particles. In addition, bauxite residue contains zeolites in the higher percent range, which function as ion exchangers and for example prevent the simple elution of the residual caustic soda lye from the Bayer extraction. Characteristically, a bauxite residue suspension behaves like a non-Newton liquid and exhibits thixotropic behavior. The agglomération of various minerai particles such as for example of silicate components and iron minerais prevents simple séparation by gravity or with the aid of magnetic fields, as is known, because the nanoscale iron particles are fixedly connected to the other minerai components via mechanic, ionic and electrostatic forces. Ideally, a simple séparation would require spherical particles to be able to realize corresponding répulsive forces. However, there are virtually no spherical particles in the bauxite residue since silicates are usually platelet-shaped formed and the other minerai components hâve chaotic shapes without regular geometries.

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Thereby, the access of surface-changing chemicals is additionally impeded. Additionally, the electrostatic forces can be differently strong according to particle geometry and composition. Clay particles are negatively charged in almost ail cases.

Therefore, a séparation of minerai components from such a suspension requires first cancellation of the bonding forces between the agglomerated minerai components, that is disagglomeration. Furthermore, it has to be ensured that the disagglomerated colloïdal particles of a certain minerai do not again reagglomerate with different particles in undesired manner, whereby the initial state would finally again occur. On the other hand, chemically identically or similarly composed minerai particles are to conglomerate in orderthat larger, non-colloidal agglomérâtes form, which can then be separated in simple manner as an enriched fraction containing material of value.

The process according to the invention is based on the realization that for technically simple, scalable and wet-chemical séparation of at least a part of the iron containing minerai phases in the bauxite residue, these spécifie colloïdal characteristics of the bauxite residue hâve to be taken into account. Surface charges of particles can inherently be positive or négative. The interactions of the particles additionally dépend on the ionic strength of the suspension. The invention advantageously exploits the fact that the surface charges of the minerai particles of a BR suspension can be varied depending on pH.

Therein, each minerai species has a balanced surface charge for a certain pH value, that is that the positive and négative charges compensate each other and the particle overall is electrically neutral. The corresponding pH value can therefore also be referred to as an isoelectric point or as a point of zéro charge (PZC). In order to separate the iron compounds of the BR from the remaining minerai components as quantitatively as possible, their présent surface charge has therefore first to be neutralized or even inverted to the opposite. Therefore, in step b) of the process according to the invention, adjustment of the pH value of the bauxite residue suspension to a value between 7.2 and 11.4 is first effected. By a pH value between

7.2 and 12.2, within the scope of the invention, pH values of 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8. 10.9, 11.0, 11.1,

11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12.0, 12.1 and 12.2 as well as corresponding intermediate values such as for example 9.40, 9.41,9.42, 9.43, 9.44, 9.45, 9.46, 9.47, 9.48, 9.49, 9.50 etc. are to be understood. Hereby, the surface charges of iron containing particles in the provided bauxite residue suspension can be adjusted such that a disagglomeration of the minerai agglomérâtes becomes possible at ail. With pH values above 12.2 or below 7.2, the particles cannot be disagglomerated anymore, but remain in suspension or agglutinate to even larger aggregates. After adjusting the pH value to a value in the range of the point of zéro charge, that is after changing the surface charge in particular of the iron-rich minerai components, in the following step c), an at least partial disagglomeration of the suspended minerai agglomérâtes of the bauxite residue is accordingly performed and the resulting mixture is finally separated into an iron-rich fraction and into at least one further fraction in step d). The further fraction is preferably a silicate-rich fraction. The minerai agglomérâtes in step c) are disagglomerated by génération of cavitation. Within the scope of the invention, the formation and dissolution of steam-filled cavities (steam bubbles) in the suspending agent of the bauxite residue is to be understood by cavitation. In the cavitation, one basically differentiates two limit cases, between which there are fluent passage forms. In the steam cavitation (hard or transient) cavitation, the formed cavities contain steam of the surrounding water. Such cavities collapse under effect of the external pressure by bubble implosion (microscopie steam impact). In the soft (stabile) cavitation, gases dissolved in the liquid enter the formed cavities and attenuate or preventthe collapse thereof. In coopération with the adjusted pH value or the modified surface charge of the individual particles, thus, particles adhering to each other can be disagglomerated by virtually shooting water, water steam or other gases between the particles by effect of the cavitation forces. Therein, the step of disagglomerating basically is not restricted to a certain method. However, the performance employing spécial fast rotating stirrers, also called dissolvers, ultrasonic generators or other suitable cavitation generating means is advantageous. In ail cases, the disagglomeration is based on the génération of cavitation in the suspension, which effects séparation of the particles by applying mechanical forces to the particles. Furthermore, the invention advantageously exploits the comparatively great density différence between the iron-rich and the other minerai components of the BR. Iron oxides and hydroxides for example hâve densities > 5 g/cm³, while silicates and titanium compounds hâve densities of 2.6 g/cm³ or less. This results in the disagglomerated iron containing particles being able to be separated at least predominantly from the not iron containing particles and being able to reagglomerate with other iron . containing particles. The iron-rich fraction formed hereby therefore sinks to the bottom and séparâtes by gravity alone from at least one further fraction, which accordingly is poor in iron and silicate-rich, respectively, and remains suspended or dispersed in the aqueous medium. Therefore, the iron-rich fraction can be separated from the further fraction in particularly simple manner as a material of value. By the séparation, thus, iron ore capable of smelting with an iron content of up to 55 % or more is obtained as a first material of value. This is particularly advantageous since the spécification ofthe ironworks industry for accepting iron containing starting products is at about 50 % to 55 % iron content. The actual iron ore yield inherently fluctuâtes within certain limits depending on the spécifie composition ofthe bauxite residue, but is regularly at least 45 % or more ofthe overall dry matter ofthe employed bauxite residue even with bauxite residues from old landfills. Within the scope ofthe invention, percent spécifications are basically to be understood as mass percent unless otherwise stated. As an additional material of value or additional mixture of materials of value, the further fraction is obtained, which includes a silicate material (that is mixture of various clays), which can for example be directly employed as a fertilizer or soil conditioner or be further processed. Thus, at least two different materials of value arise from reconditioning the bauxite residue. The . process according to the invention is technically particularly simply practicable and additionally simply scalable. Therefore, the process according to the invention can for example be performed immediately subséquent to the Bayer process by passing or introducing the arising bauxite residue into a corresponding apparatus for performing the process according to the invention.

In an advantageous development ofthe invention, it is provided that in step a), a ratio of solid to liquid between 1:2 and 1:5, in particular a ratio of 1:2.5 is adjusted in the suspension and/or that in step a) a bauxite residue with a water content between 20 % and 40 % is used, wherein the bauxite residue is preferably one or multiple times washed. By a ratio of solid to liquid between 1:2 and 1:5, in particular ratios of 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9, 1:3.0, 1:3.1, 1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9, 1:4.0, 1:4.1, 1:4.2, 1:4.3, 1:4.4, 1:4.5,

1:4.6, 1:4.7, 1:4.8, 1:4.9 and 1:5.0 as well as corresponding intermediate values are to be understood. The mentioned ratio range allows good manageability of the bauxite residue on the one hand and prevents unnecessarily large amounts of liquid having to be handled in step d) on the other hand. Therein, a ratio of 1:2.5 has proven particularly advantageous. By the bauxite residue having a water content between 20 % and 40 %, that is in particular water contents of 20 %, 21 %, 22 %, 23 %, 24 %, 25 %, 26 %, 27 %, 28 %, 29 %, 30 %, 31 %, 32 %, 33 %, 34 %, 35 %, 36 %, 37 %, 38 %, 39 % or 40 %, the amount of water to be added in step a) can be advantageously reduced. Moreover, the bauxite residue obtained by the Bayer process usually already has water contents of 28-35 % and thus can be directly used. By the bauxite residue being one or multiple times washed, in particular caustic soda lye can be recovered for the Bayer process and the pH value of the bauxite residue can be adjusted as required. Alternatively or additionally, it can be provided that a density of the suspension is adjusted to a value between 1.05 g/cm³ and 1.35 g/cm³, in particular between 1.07 g/cm³ and 1.30 g/cm³. By a value between 1.05 g/cm³ and 1.35 g/cm³, within the scope of the invention, in particular density values of 1.05 g/cm³, 1.06 g/cm³, 1.07 g/cm³, 1.08 g/cm³, 1.09 g/cm³, 1.10 g/cm³, 1.11 g/cm³, 1.12 g/cm³, 1.13 g/cm³, 1.14 g/cm³, 1.15 g/cm³,1.16 g/cm³, 1.17 g/cm³, 1.18 g/cm³, 1.19 g/cm³, 1.20 g/cm³, 1.21 g/cm³, 1.22 g/cm³, 1.23 g/cm³, 1.24 g/cm³, 1.25 g/cm³, 1.26 g/cm³, 1.27 g/cm³, 1.28 g/cm³, 1.29 g/cm³, 1.30 g/cm³, 1.31 g/cm³, 1.32 g/cm³, 1.33 g/cm³, 1.34 g/cm³ or 1.35 g/cm³ as well as corresponding intermediate values are to be understood. By the density of the suspension being adjusted to a value in the mentioned range before and/or during step c), a particularly fast and complété disagglomeration is allowed.

Further advantages arise by the température of the suspension being adjusted to a value between 30 °C and 70 °C before step c), in particular in step a). Hereby, the reaction times for the subséquent disagglomeration can be advantageously adjusted. By a température between 30 °C and 70 °C, in particular températures of 30 °C, 31 °C, 32 °C, 33 °C, 34 °C, 35 °C, 36 °C, 37 °C, 38 °C, 39 °C, 40 °C, 41 °C, 42 °C, 43 °C, 44 °C, 45 °C, 46 °C, 47 °C, 48 °C, 49 °C, 50 °C, 51 °C, 52 °C, 53 °C, 54 °C, 55 °C, 56 °C, 57 °C, 58 °C, 59 °C, 60 °C, 61 °C, 62 °C, 63 °C, 64 °C, 65 °C, 66 °C, 67 °C, 68 °C, 69 °C or 70 °C are to be understood. For example, the time required for step c) is usually about 20 minutes at a température of 40 °C, while it can be decreased to 15 minutes or less at températures above 50 °C. Moreover, a bauxite residue directly obtained from the Bayer process can already hâve a température of about 70 °C and therefore can be directly further processed within the scope of the présent process. At températures above 70 °C, the compositions of several hydrate compounds among the minerais of the bauxite residue change, whereby the disagglomeration is severely impeded or even made impossible. Therein, it can basically be provided that the température is one or multiple fîmes varied in the specified range.

In further development of the invention, a particularly high iron yield is achieved in that the pH value is adjusted in step b) to a value between 7.4 and 11.4, in particular between 8.4 and 11.4.ln the range of values between 7.2 and 11.4, the surface charges of particularly many particles from different iron compounds can be advantageously influenced. In further development of the invention it is provided that the pH value adjusted in step b) is continuously and/or gradually varied in the range between 7.2 and12.2 during step c). By the pH value being continuously and/or gradually varied, alternatively or additionally, the isoelectric points of various iron compounds can be specifically approached or passed, whereby the yield of iron containing compounds can also advantageously be increased. Alternatively or additionally, the steps b) and c) can also be repeated two or multiple times.

Further advantages arise by the pH value being adjusted by addition of an acid, in particular a minerai acid, an organic acid, an acidic wastewater, an acidic condensate and/or FeCI₂. In further development of the invention it is provided that the pH value is adjusted by addition of a base, in particular of caustic soda lye and/or an alkaline wastewater. In further development of the invention it is provided that the pH value is adjusted by addition of a hydrolyzable compound, in particular an oil and/or a fat. Besides the adjustment of the pH value, which is a requirement for the disagglomeration, hereby, various further advantages can be achieved. For example, acidic or alkaline wastewaters, condensâtes and the like can be advantageously productively used for pH value adjustment as well as optionally for adjusting the solid/water ratio. Moreover, by the choice of the corresponding acid or base, influence can be exerted to the disagglomeration and to the reagglomeration of the particles and thereby to the yield of iron-rich fraction. For example, the inexpensively available compounds acetic acid and citric acid hâve proven to particularly increase the yield. By the use of a hydrolyzable compound, for example a plant oil or fat, besides a pH value adjustment, dispersants can additionally be formed in situ (by saponification of fatty acids), which can contribute to the miscibility and stabilization ofthe disagglomerated particles.

In a further advantageous development ofthe invention, it is provided that at least one calcium compound, in particular calcium oxide and/or calcium hydroxide and/or calcium sulfate, and/or at least one dispersant, in particular a surfactant, is added to the suspension before step c). By addition of a calcium compound, the buffer effect ofthe ion exchanging zeolite minerais ofthe bauxite residue can be advantageously repressed and the processability ofthe suspension as well as the yield of iron-rich compounds can be correspondingly improved. The Ca ions introduced into the suspension by the addition ofthe calcium compound(s) are incorporated in zeolites or zeolite containing compounds ofthe bauxite residue. These zeolites or zeolite containing compounds are predominantly sodium aluminum silicates, which hâve been formed during the bauxite extraction. By bonding the Ca ions in the zeolites, the ion exchange capabilities thereof are reduced, which in turn facilitâtes the adjustment of an optimum pH value in step b). Furthermore, the dispersion ofthe disagglomerated clay particles is improved, whereby a simplified séparation ofthe iron-rich fraction in step d), for example by gravity in the sedimenter, is achieved. By additionally to a dissolver stirrer or the like, the cavitation forces can also be e generated with the aid of ultrasound. To this, the stirring container is for example less acid has to be added than it would be the case without gypsum addition. Although gypsum (CaSO4x2H₂O) is substantially pH neutral, gypsum can partially dissolve and form calcium and sulfate ions. The sulfate ions bond to corresponding surface areas ofthe clay particles similarly as hydroxide ions and thus vary the electric surface charge thereof. Hereby, improved dispersion ofthe clay particles présent in the bauxite residue is allowed such that they are retained in the suspension colloidally dissolved. For example, gypsum from flue gas desulfurization plants (REA gypsum) and/or natural gypsum can be used as the gypsum. With the aid of a dispersant, as already mentioned, the particle isolation achieved in step c) can be stabilized or maintained. Thus, once released particles can be prevented from the reagglomeration with undesired other particles and the disagglomeration can be assisted. Therein, basically, sterically and/or electrostatically acting dispersants can be provided. In the steric stabilization, the particle affinic areas of the dispersant are on the minerai particle, while the residuals of the dispersant protrude into the dispersing medium. If two particles encounter each other, they cannot agglomerate because they are kept spaced by the dispersants. In the electrostatic stabilization, the dispersant carries electrical charge. The charge can basically be provided on the particle affinic and/or on the particle-remote end of the dispersant. Hereby, the charged parts of the dispersant form some kind of protective shell around the concerned particles. The electrosteric stabilization combines the mechanisms of the steric and the electrostatic stabilization.

In a further advantageous development of the invention, it is provided that between 0.1 % and 10 %, in particular between 2 % and 6 % of calcium compound and/or between 2 and 9 per mille of dispersant are added to the suspension related to the dry matter of the bauxite residue. Hereby, the processability of the bauxite residue, the disagglomeration degree and the yield of iron-rich fraction are advantageously increased. By a mass portion between 0.1 % and 10 %, in particular mass portions of 0.1 %, 0.2 %, 0.5 %, 1.0 %, 1.5 %, 2 %, 3 %, 4 %, 5 %, 6 %, 7 %, 8 %, 9 %, 10 % as well as corresponding intermediate values are to be understood. By the addition of the calcium compound, various Ca silicates and Ca aluminates are formed, which allow simpler division or séparation of the iron-rich fraction and better filterability of the silicate-rich fraction. While sodium silicates form only at températures above 110 °C, Ca silicates already form at room température and additionally provide for bonding of Na ions in the form of complex Na-Ca silicates. By the addition of one or more calcium compounds, due to the occurring minerai régénération, the viscosity of the BR suspension can additionally be changed. If gypsum is added as the calcium compound, mass portions between 0.5 % and 2 % hâve proven particularly advantageous. If too high amounts of calcium compound are added, undesired adhering effects between the clay particles can occur. By portions between 2 %₀ and 9 %o of dispersant, in particular portions of 2.0 %o, 2.5 %o, 3.0 %o, 3.5 %o, 4.0 %o, 4.5 %o, 5.0 %o, 5.5 %o, 6.0 %o, 6.5 %o, 7.0 %o, 7.5 %o, 8.0 %o, 8.5 %o or 9.0 %o as well as corresponding intermediate values are to be understood. With regard to the dispersant, portions between 3 per mille and 7 per mille usually hâve proven to be sufficient.

In a further advantageous development of the invention, it is provided that a separating agent, in particular a fluxing agent and/or particles, in particular iron oxide particles, is added to the suspension preferably before and/or during step c). Within the scope of the présent invention, compounds are understood by separating agents, which assistthe disagglomeration of the minerai agglomérâtes. Fluxing agents, which can also be referred to as liquefiers, plasticizing agents, super plasticizing agents or super liquefiers, are basically known from the field of concrète production and there serve for improvement of the flowability. The one-time or repeated addition of at least one such fluxing agent to the provided suspension advantageously results in decrease of the surface tension of the suspended minerai particles within the scope of the présent invention. Moreover, fluxing agents impede a reagglomeration in particular of the silicate-rich clay platelets in that their numerous side chains such as for example in PCE (polycarboxylate ester) construct steric hindrances. Thereby, fluxing agents assistthe disagglomeration and the following séparation into an ironrich and at least one further fraction by maintenance of the particle séparation. This allows significant increase of the iron yield. For the industrial employment, it is of particular interest that already low amounts of the fluxing agent resuit in considérable improvements of the disagglomeration, whereby the economy of the process is advantageously improved. For example, melamine sulfonate and/or melamine sulfonate dérivatives can be used as the fluxing agent. By these compounds, in addition, the surface tension of the présent water is decreased and a lubricating effect is effected, whereby the disagglomeration is also facilitated. Alternatively or additionally, basically, lignin sulfonates, naphthalene formaldéhyde sulfonates, polycarboxylates, polycarboxylate esters (PCE) and/or hydroxy carboxylic acids and the salts thereof can also be used as the fluxing agents. In PCE, the numerous side chains are in particular of importance since they constitute a particularly high steric hindrance for the undesired reagglomeration of already separated particles in the suspension.

Alternatively or additionally, particles, in particular iron oxide particles, can specifically be added to the suspension. The added particles act as small projectiles due to their weight and their shape, which promote the disintegration of the agglomérâtes by collisions with agglomérâtes in the suspension in the disagglomeration. In particular in using iron oxide particles, the added particles additionally also function as seed crystals or crystal nuclei, which collect and bind the disagglomerated iron particles, whereby the iron yield is also advantageously increased. Therein, it can be provided that iron particles are used as the particles, which were already separated with the aid of the process according to the invention and are recycled into the process. Hereby, the process can be particularly economically performed with particularly high iron yields.

In a further advantageous development of the invention, it is provided that at least one fluxing agent is added to the suspension with a weight portion between 0.01 % and 1.0 %, in particular between 0.4 % and 0.6 % related to the dry matter of the bauxite residue. By a weight portion of the fluxing agent between 0.01 % and 1.0 %, in particular weight portions of 0.01 %, 0.10 %, 0.15 %, 0.20 %, 0.25 %, 0.30 %, 0.35 %, 0.40 %, 0.45 %, 0.50 %, 0.55 %, 0.60 %, 0.65 %, 0.70 %, 0.75 %, 0.80 %, 0.85 %, 0,90 %, 0.95 % and 1.0 % as well as corresponding intermediate values are to be understood. Hereby, the process can be particularly economically performed also within the industrial scope since the specified, relatively low fluxing agent amounts already resuit in considérable improvements of the disagglomeration and thereby reduce the time required for performing the process and allow improved yield.

In further development of the invention it is provided that particles are added, which hâve at least predominantly an average diameter between 0.3 pm and 25 pm, in particular between 0.4 pm and 20 pm. By particles having at least predominantly an average diameter between 0.3 pm and 25 pm, within the scope of the invention, particles are understood, of which at least 51 % hâve an average diameter of 0.3 pm, 0.4 pm, 0.5 pm, 0.6 pm, 0.7 pm, 0.8 pm, 0.9 pm, 1.0 pm, 1.0 pm, 1.5 pm, 2.0 pm, 2.5 pm, 3.0 pm, 3.5 pm, 4.0 pm, 4.5 pm, 5.0 pm, 5.5 pm, 6.0 pm, 6.5 pm, 7.0 pm, 7.5 pm, 8.0 pm, 8.5 pm, 9.0 pm, 9.5 pm, 10.0 pm, 10.5 pm, 11.0 pm, 11.5 pm,

12.0 pm, 12.5 pm, 13.0 pm, 13.5 pm, 14.0 pm, 14.5 pm, 15.0 pm, 15.5 pm, 16.0 pm, 16.5 pm, 17.0 pm, 17.5 pm, 18.0 pm, 18.5 pm, 19.0 pm, 19.5 pm, 20.0 pm, 20.5 pm, 21.0 pm, 21.5 pm, 22.0 pm, 22.5 pm, 23.0 pm, 23.5 pm, 24.0 pm, 24.5 pm, 25.0 pm or corresponding intermediate values. By the added particles being at least predominantly in the specified grain size range, they act particularly effectively as small projectiles due to their weight and their basically rather round shapes, which are brought to a high speed for example by a dissolver stirrer and assist the disagglomeration by collisions with agglomérâtes. At the same time, they serve as seed crystals/nuclei, collect and bind iron particles présent in the suspension. This is a time-dependent procedure. Similar effects can also be achieved by the employment of ultrasound and/or other disagglomeration means instead of a dissolver stirrer.

In further advantageous development of the invention, the cavitation or the cavitation forces required for disagglomeration are generated by at least one moved object, in particular by a dissolver stirrer, a shear stirrer, an impeller of a rotary pump, an impeller of a turbine, a shaker and/or a propeller. In further development of the invention the cavitation is generated by application of ultrasound to the suspension. Therein, the technically simplest possibility for cavitation is in the use of an object fast moved in the suspension. According to the Bernoullis's law, the static pressure of a liquid is the lower, the higher the speed is. If the static pressure drops below the évaporation pressure of the liquid, steam bubbles form. They are subsequently entrained into areas of higher pressure with the flowing liquid. With the new increase of the static pressure above the évaporation pressure, the steam in the cavities abruptly condenses. Therein, extreme pressure and température peaks occur. Local pressure changes can therefore be particularly simply generated with the aid of rotor blades, impellers, dissolver stirrers, shear stirrers, pumps, shakers and the like. Alternatively or additionally, cavitation can also be generated by application of ultrasound to the suspension. Therein, cavitation occurs in the pressure minimums of the oscillation. A further advantage of the use of ultrasound is in the comparatively high température input into the suspension such that température adjustment can be performed at the same time.

By the object for generating the cavitation being moved with a rotary frequency of at least 1000 min'¹, in particular of at least 2000 min'¹, through the suspension, a significant increase of the shear forces in the suspension can be advantageously achieved. For example, to this, particularly powerful dissolver stirrers can be used, which are able to achieve révolution speeds up to 3000 rmin or more. The use of fast rotating stirrers results in increased cavitation and as a resuit in particularly fast and complété crushing and dispersing of the minerai particle complexes.

In further development of the invention, a technically particularly simple, fast and inexpensive possibility of separating the iron-rich fraction is achieved in that the further fraction (clay fraction) is separated from the iron-rich fraction by sucking and/or decanting and/or filtering, in particular by vacuum filtering. Due to the good séparation of the two fractions, therein, additional adjuvants such as flocculants or the like are basically not required.

Further advantages arise in that the iron-rich fraction is washed and/or dried after séparation. This allows simplified further processing such as for example smelting and crude iron extraction of the iron-rich fraction.

In a further advantageous development of the invention, it is provided that at least one calcium compound, in particular calcium oxide and/or calcium hydroxide and/or calcium sulfate, is added to the further fraction after séparation. Hereby, the filterability and thereby the separability of the silicate-rich fraction is improved on the one hand, on the other hand, one obtains hereby a kind of clay, which is particularly well suitable as a soil conditioner. In particular by the addition of calcium sulfate, a product is obtained, which allows greening of BR disposai sites and the like due to the bioavailability of the sulfate ions. Moreover, Na ions présent in the further fraction are bonded in the form of Na-Ca silicates such that environmental hazard by exiting or eluted caustic soda lye does no longer exist in contrast to the bauxite residue.

In a further advantageous development of the invention, it is provided that between 2 % and 15 %, in particular between 5 % and 10 % of calcium compound is added to the further fraction related to its dry matter. By mass portions between 2 % and 15 %, within the scope of the invention, mass portions of 2 %, 3 %, 4 %, 5 %, 6 %, 7 %, 8 %, 9 %, 10 %, 11 %, 12 %, 13 %, 14 % or 15 % as well as corresponding intermediate values are to be understood. Hereby, the characteristics of the further fraction can be optimally adapted to its respective purpose of employment, for example as an additive to sand, acidic earth, lime, gypsum, fertilizer or as a filter medium or soil conditioner.

In a further advantageous development ofthe invention, it is provided thatthe température ofthe further fraction is adjusted to a value between 30 °C and 70 °C after séparation. By a température between 30 °C and 70 °C, within the scope of the invention, températures of 30 °C, 31 °C, 32 °C, 33 °C, 34 °C, 35 °C, 36 °C, 37 °C, °C, 39 °C, 40 °C, 41 °C, 42 °C, 43 °C, 44 °C, 45 °C, 46 °C, 47 °C, 48 °C, 49 °C, °C, 51 °C, 52 °C, 53 °C, 54 °C, 55 °C, 56 °C, 57 °C, 58 °C, 59 °C, 60 °C, 61 °C, °C, 63 °C, 64 °C, 65 °C, 66 °C, 67 °C, 68 °C, 69 °C or 70 °C as well as corresponding intermediate températures are to be understood. Hereby, the minerai régénération can be accelerated and specifically controlled. By the formation of different hydrate compounds, in addition, the présent residual water is at least partially bound, whereby a readily breakable material is obtained. Above about 70 °C, severely different hydrate compounds form, which rather impede the further processing.

A further aspect of the invention relates to an apparatus for performing a process according to any one ofthe preceding embodiments. In orderto allow a wetchemical séparation of at least a part of the iron containing components in the bauxite residue as materials of value, the apparatus according to the invention includes at least one container for receiving the aqueous suspension ofthe bauxite residue, a device for adjusting the pH value ofthe aqueous suspension, a device for at least partially disagglomerating suspended minerai agglomérâtes ofthe bauxite residue and a device for separating the resulting mixture into an iron-rich fraction and at least one further fraction. The advantages arising herefrom can be taken from the preceding descriptions ofthe process according to the invention and correspondingly apply to the apparatus according to the invention. Advantageous developments of the process are additionally to be considered as advantageous developments ofthe apparatus and vice versa.

The iron-rich fraction obtained by means of process according to any one ofthe preceding embodiments and/or by means of an apparatus according to the preceding embodiment can be used for obtaining iron. Hereby, a simple, scalable and wet-chemically practicable extraction of iron from the bauxite residue considered as waste heretofore with corresponding environmental and cost advantages is constituted.

The further fraction obtained by means of a process according to any one of the preceding embodiments and/or by means of an apparatus according to the preceding embodiment can be used as a filter body, in particular for heavy metals, for desulfurization and/or removal of arsenic, for water purification and/or exhaust gas purification as a pyrolysis catalyst, in particular in a biomass reactor as a soil conditioner and/or as an admixture to sand, acidic earth, lime, gypsum, cernent, concrète and/or plant fertilizer. By these uses the further fraction can be advantageously used as a further material of value besides the iron-fraction, whereby further environmental and cost advantages are constituted.

Further features of the invention are apparent from the daims, the embodiments and the drawing. The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the embodiments are usable not only in the respectively specified combination, but also in other combinations without departing from the scope of the invention. Therein, Fig. 1 shows a schematic diagram of an apparatus according to the invention for performing the process according to the invention for obtaining a material of value from a bauxite residue.

Example 1

Fig. 1 shows a schematic diagram of an apparatus 10 according to the invention for performing the process according to the invention for obtaining material of value from a bauxite residue. Therein, the apparatus 10 shown in Fig. 1 is basically also suitable for performing ail of the following examples.

The apparatus 10 includes a basically optional transfer point 12, in which bauxite residue (BR), which is for example immediately passed from the Bayer process and/or originates from old landfills, is transferred with the aid of a transport device 13 for obtaining material of value. The BR can be freed from aluminate solution, excess water or the like in the transfer point 12 by means of an also optional vacuum filter drum 14 or another suitable separating device. Subsequently, the BR is transferred into an also basically optional stirring container 16, in which a bauxite residue suspension is produced with the aid of a stirrer 17. Herein, as needed, a corresponding amount of water can be added from the water container 18 to adjust a desired viscosity, a desired dry matter portion and/or a desired initial pH value. From the stirring container 16, the aqueous suspension is pumped into a disagglomerator 20, which is equipped with a dissolver stirrer 22 and/or an ultrasonic device (not shown) and/or another suitable device for generating cavitation. The disagglomerator 20, which serves as a device for at least partially disagglomerating suspended minerai agglomérâtes of the bauxite residue in the shown embodiment, can hâve a single-walled or multi-walled container. Multi-walled containers in particular offer the advantage of improved capability of tempering and improved thermal insulation. Furthermore, the stirring container 16 can basically also be omitted and the suspension to be disagglomerated can be directly produced in the disagglomerator 20.

From a container 24, separating chemicals can be added to the suspension. The separating chemicals can for example be acids for adjusting the pH value of the suspension and/or fluxing agents such as polycarboxylate ester (PCE), melamine sulfonate or similar. The use of separating agents results in decrease of the surface tension of the minerai particles and assiste the dissolution of the complex bond. The container 24 therefore also serves as a device for adjusting the pH value of the aqueous suspension in the présent embodiment. Particularly the numerous side chains are of importance in PCE since they constitute a steric hindrance for the congrégation and binding of particles in the suspension to each other. In other words, they maintain the spatial séparation of the particles. For the industrial, economical employment, it is interesting that already low amounts of the fluxing agent (e.g. 0.4 % to 0.6 % of the solid matter) resuit in considérable improvements of the disagglomeration because they prevent new binding in particular of clay platelets to each other.

Furthermore, the separating chemicals can include particles. For example, iron oxide particles already obtained with the aid of the process can be added to the suspension to increase the iron oxide yield. The iron oxide particles preferably hâve a grain size range between 0.4 pm and 20 pm and act as small projectiles due to their weight and their rounded shape (small platelets like the clay particles), which are accelerated to high speed by the dissolver stirrer 22 and additionally disintegrate the agglomérâtes by collisions with agglomérâtes in the suspension. At the same time, they serve as seed crystals or seed nuclei and collect and bind smaller iron particles located in the suspension. This is a time-dependent procedure. Similar effects can also be achieved by the employment of ultrasound or other disagglomeration devices instead of the dissolver stirrer 22.

As a further separating chemical, a calcium compound such as for example burnt lime, slaked lime or gypsum (CaSO₄ x 2 H₂O) can be added to the suspension. Therein, gypsum can in particular considerably decrease the required amount of acid addition, in particular of the very effective and environmentally friendly citric acid, whereby corresponding cost réductions are constituted. Although gypsum itself is substantially pH neutral, gypsum can partially dissolve with formation of Ca and sulfate ions. The sulfate ions bind to the same surface locations of the clay particles as for example the OH groups of the citric acid and thus change the electric surface charge thereof in similar manner. Thereby, the sulfate ions help in the dispersion of the clay particles in order that they are présent colloidally dissolved or suspended. At the same time, released Ca ions, by incorporation in zeolites, which hâve substantially been formed as sodium aluminum silicates during the bauxite extraction, reduce the ion exchange capabilities thereof and thereby improve the adjustability of the optimum pH value for the dispersion of the minerai components of the BR suspension. This improves the following séparation of the resulting mixture by gravity in the downstream sedimenter 26. The sedimenter 26 thus serves as a device for separating the resulting mixture into an iron-rich fraction and at least one further fraction lower in iron in the shown embodiment. A gypsum addition can for example be effected in the form of REA gypsum of natural gypsum. The amount is optimally at 0.5-2 % and usually should not exceed 4 % of the dry matter of the suspension since otherwise adhering effects between the clay particles can occur.

The separating chemicals can basically be added before and/or during and/or after the disagglomeration individually and/or in any combinations. Therein, the separating chemicals can basically be kept available in a common compartment of the container 24. However, the container 24 can basically also hâve multiple separate compartments or multiple individual containers, in which varietal separating chemicals are each kept available and are added in the desired amount and order.

The dissolver stirrer 22 is preferably equipped with a fast rotating stirrer (up to 3000 rmin or more) to allow a cavitation as large as possible and in the conséquence a particularly effective rupture and dispersion of the minerai particle complexes as a préparation to the sédimentation. This conversion procedure is additionally improved by the spécifie adjustment of the pH value by corresponding addition of separating chemicals (e.g. of acids such as citric acid, sulfuric acid etc.). The disagglomeration as well as the reagglomeration of the minerai particles constitute time-dependent procedures, which dépend on various factors. After the partial or at least approximately complété disagglomeration of the minerai agglomérâtes, the resulting mixture is pumped into the sedimenter 26. Here, the mixture séparâtes with time into a silicate-rich, orange-colored floating phase and an iron-rich fraction sinking to the bottom with brownish color. The bottom phase is washed in a basically optional washer 28 and optionally at least partially dried. By washing the separated iron ore 30, the clay portion thereof can again be decreased. However, a possibly left clay portion can also be used as a binder for the production of iron ore pellets on the other hand. The drying is preferably effected with the aid of waste heat of the process. One obtains high-grade iron ore 30 with an iron content of at least 40 %, normally of above 50 % as the product, which can be directly used without further treatment steps for iron or steel production.

The silicate-rich floating phase can directly be used after séparation or optionally be transferred into a further stirring container 32 with a stirrer 33. Here, the floating phase can be mixed with a calcium compound, for example with burnt lime, white lime, gypsum and/or slaked lime, with stirring and optionally be heated to a température between 20 °C and 65 °C, thus for example to a température of 20 °C, 21 °C, 22 °C, 23 °C, 24 °C, 25 °C, 26 °C, 27 °C, 28 °C, 29 °C, 30 °C, 31 °C, 32 °C, °C, 34 °C, 35 °C, 36 °C, 37 °C, 38 °C, 39 °C, 40 °C, 41 °C, 42 °C, 43 °C, 44 °C, °C, 46 °C, 47 °C, 48 °C, 49 °C, 50 °C, 51 °C, 52 °C, 53 °C, 54 °C, 55 °C, 56 °C, °C, 58 °C, 59 °C, 60 °C, 61 °C, 62 °C, 63 °C, 64 °C or 65 °C. The calcium compound can for example be stored in the container 34 and added via a rotary valve.

Herein, clay formation occurs by the minerais contained in the floating phase reacting with calcium with minerai régénération to a swelling clay-like calcium aluminate clay mud (CATO, 38). As the ingrédients of the CATO 38, predominantly calcium and sodium aluminates formed from the aluminum compounds contained in the BR as well as optionally goethite formed from possible iron oxides and hydroxides left in the floating phase. Therein, the proceeding main reactions are the formation of tricalcium aluminate

Ca(OH)₂ + 2 AI₂O₃ + 3 H₂O -> Ca₃AI₂[(OH)4]₃ as well as optionally the conversion from hématite into goethite:

Fe₂O₃ + H₂O -> 2 FeO(OH).

The formed CATO 38 can be dehydrated via a chamber filter press 36 or another déhydration device. The separated filtrate 40 can be returned to the disintegrator or stirring container 16, whereby considérable amounts of fresh water are saved and the required liquid can be at least predominantly circulated.

The obtained product mixture, that is the CATO 38, possesses a particularly large reactive surface and is suitable for various applications. For example, the CATO 38 can be dried and/or used as a filter element, in particular forfiltering plant oil and/or contaminated water and/or as a soil conditioner and/or as a purification agent and/or as a cernent additive and/or as a building material and/or as a minerai fertiliser. With the aid of the CATO 38, for example, crude plant oil from pressing oil plants can be refined and freed from undesired organic components. Due to its high spécifie surface, the CATO 38 is also suitable forfiltering contaminated water, wherein in particular acidic waters can be neutralized at the same time via its residual alkali content. Altematively or additionally, the CATO 38 can variously be used as a soil conditioner, purification agent, cernent additive, building material and/or minerai fertiliser. Therein, it can be provided that the CATO 38 is mixed with charcoal dust, whereby a black earth-like (terra prêta) mixture is obtainable, which constitutes a very fertile soil matching good European soil. In this manner, with the aid of the bauxite residue considered as waste heretofore, sustainable agriculture in the rain tillage farming zone and in other climate zones can be promoted.

Furthermore, the CATO 38 can be mixed with biomass, in particular with wood, chips, bark, straw, bagasse, leaf mass, plant waste, grass, foliages, dung, plant oil, sewage sludge, liquid manure, organic domestic waste and/or sawdust, and subjected to biomass gasification, whereby further materials of value are obtainable. Therein, the biomass gasification, which is preferably performed under oxygen exclusion, proceeds already at low températures between 250 °C and 450 °C, in particular between 280 °C and 400 °C due to the characteristics of the CATO. Moreover, the biomass gasification proceeds free of tar and without appréciable carboxylic acid formation (in particular without acetic acid or formic acid formation) due to the catalytic characteristics of the CATO 38.

For mixing the CATO 38, the apparatus 10 has a basically optional mixer 42. The compounds to be admixed, for example sand, lime, gypsum, charcoal, biomass etc., can be kept available in the container 44 and correspondingly added. According to the admixed further compounds, one thus obtains different further products 46 besides the iron ore 30, which in turn constitute valuable materials of value.

Example 2

As the starting material, bauxite residue from the Bayer process is used, which was multiple times washed and separated from the aluminate solution via the vacuum filter drum 14. The bauxite residue has a water content of about 35 % H₂O. In the stirring container 16, by addition of corresponding amounts of water, 2 liters of a bauxite residue suspension are produced, wherein the suspension has 1 kg of dry matter and a pH value of 13. With continuous stirring with the aid of a shear stirrer, the suspension is brought to a température of about 52 °C. Therein, the use of a shear stirrer advantageously allows the génération of high shear forces by the formation of overlapping layers. Hereby, the viscosity of the suspension advantageously decreases since the platelet-shaped silicate particles of the bauxite residue align substantially parallel to the layers and form new collective properties. The thixotropic suspension therefore becomes increasingly lower in viscosity with increasing stirring period until reaching a viscosity minimum. After completion of the shear load, the viscosity again increases by the stochastic rearrangement of the silicate particles depending on time. 1 literof acidic, oil containing condensate water from a biomass reactor is added to the suspension. The suspension is homogenized for ca. 30 minutes with stirring. The biomass reactor is a reactor, in which chips are pyrolyzed employing the clay product (CATO 38) from the process according to the invention described in more detail in the following. The bio oil from the acidic condensate reacts with saponification, which is recognizable by foam formation. Therein, the pH value of the suspension decreases to about 8.4.

Thereafter, the suspension is transferred into a container optimized in size, for example the disagglomerator 22, which is provided with a 200 mm dissolver stirrer. With the aid of the dissolver stirrer, the suspended minerai agglomérâtes of the bauxite residue are at least partially disagglomerated. After ca. 20 minutes of stirring time, the disagglomeration is terminated. After a settling time of ca. 5-30 minutes, a heavy phase of iron oxides has settled, above which a simply decantable, silicaterich floating phase is located. After extraction of the floating phase in a vacuum filter unit, this further fraction is transferred to another stirring vessel for further reconditioning. The separated iron ore phase is one or more times washed with water and filtered off, whereby it has a residual water content of about 30 %. The iron ore yield is 0.382 kg corresponding to 38 % of the employed dry matter. The Fe content is at about 55 % according to X-ray fluorescence analysis (RFA measurement), while Ti is at about 5 % and Na is at about 0.5 %. It is to be emphasized that the sodium is not présent free and thereby elutable, but bound in silicates.

From the floating phase, one obtains a kind of clay after filtering off the water, since now the silicate minerais considerably predominate. Due to the réduction of iron minerais, the color has changed from red to yellow-brown to yellow-orange. Upon cooling, new hydrates form such that a partial bond of the residual water in the newly formed minerais occurs. This crystal water can only again be expelled at températures above 130 °C. The silicate-rich fraction can be easily broken to mix it with other materials such as for example sand, acidic earth, lime, gypsum or fertilizer and for example produce a soil conditioner. In contrast to the original bauxite residue, environmental hazard by elutable caustic soda lye does no longer exist. The silicate-rich fraction has a nature-compatible pH value and can also be used as an additive for concrète, ceramic and the like.

Example 3

As the starting material, bauxite residue from the Bayer process is again used, which was multiple times washed and separated from the aluminate solution via a vacuum filter drum (e.g. vacuum filter drum 14) for caustic soda lye recovery. The bauxite residue, which still has a température of about 70 °C after washing, again has a water content of about 35 % H₂O. In a stirring container (e.g. stirring container 16), by addition of corresponding amounts of water, 2.9 liters of a bauxite residue suspension with a pH value of 12-13 are produced, wherein the suspension has a solid content of 1 kg. With continuous stirring with the aid of a disintegrating shear stirrer, the température is brought to about 56 °C. 25 ml of plant oil (e.g. crude râpe oil) are added to the suspension. The suspension is homogenized for 30 minutes with stirring. By the hydrolysis ofthe plant oil, the pH value ofthe suspension decreases to ca. 12.0. Subsequently, 100 ml of acetic acid, which was obtained by 1:10 dilution from glacial acetic acid (96 % HOAc), are added, whereby the pH value ofthe suspension decreasesto 7.9.

Thereafter, the suspension is transferred into a container optimized in size (e.g. disagglomerator 20), which is equipped with a 100 mm dissolver stirrer, and disagglomerated by génération of cavitation. After ca. 20 minutes of stirring time, the disagglomeration is terminated. After a settling time of ca. 5-30 minutes, a heavy phase of iron oxides has settled, above which a properly decantable floating phase is located. After extraction ofthe floating phase, the iron ore phase is washed with rinse water and the water is filtered off. The iron-rich fraction has a residual water content of about 30 %. The iron ore yield is 0.279 kg corresponding to 28 % ofthe originally employed dry matter. The Fe content is at 55 % according to RFA measurement, while Ti is at about 5 % and Na is at about 0.5 %. Again, the fraction does not hâve free, elutable sodium ions since the overall sodium content is présent bound in silicates.

After filtering off the water, the floating phase constitutes a kind of clay since the silicate minerais are severely enriched with respect to the iron compounds compared to the original bauxite residue. Due to the enrichment of iron minerais, the color has changed from red to bright orange. The filterability ofthe silicate-rich fraction can advantageously be improved if 5-10 % by weight of burnt lime are added to the silicate-rich fraction and the arisen mixture is brought to températures between about 43 and 49 °C. Upon cooling the reaction mixture, new hydrates form such that minerai binding of the residual water occurs. This can only be again expelled at températures beyond 130 °C. The material can be easily broken to mix it with other materials, e.g. with sand, acidic earth, lime, gypsum or fertilizer to produce a soil conditioner. The silicate-rich fraction can also be used as an additive for concrète, ceramic and the like. In contrastto the original bauxite residue, here too, environmental hazard by elutable caustic soda lye does no longer exist.

Example 4

As the starting material, bauxite residue from the Bayer process is again used, which was multiple times washed and separated from the aluminate solution via a vacuum filter drum (e.g. vacuum filter drum 14) for caustic soda lye recovery. The bauxite residue again has a water content of about 35 % H₂O. In a stirring container (e.g. stirring container 16), 2.9 liters of bauxite residue suspension containing 1 kg of dry matter with a pH value of about 13 are brought to a température of 60 °C with continuous stirring (shear stirrer). 25 ml of plant oil (crude râpe oil) are added to the suspension. Subsequently, the suspension is homogenized for about 30 minutes, wherein the plant oil is hydrolyzed in alkaline manner. The pH value of the suspension decreases to ca. 12.0. Subsequently ca. 100 ml of 0.5 % sulfuric acid are added until a pH value of about 9.1 appears.

Thereafter, the suspension is transferred to a geometrically optimized container (e.g. disagglomerator 20), which is equipped with a 100 mm dissolver stirrer. With the aid of the dissolver stirrer, which is operated with rotating speeds between about 2500 rmin and 3000 rmin, cavitation forces are generated in the suspension such that water molécules are shot between the agglomerated particles of the bauxite residue. Hereby, the particles are disagglomerated in coopération with the adjusted pH value. After ca. 20 minutes of stirring time, the disagglomeration is terminated. After a settling time of ca. 5-30 minutes, an iron-rich fraction has settled, above which a properly decantable floating phase is located.

After decanting the floating phase into a new stirring container (e.g. stirring container 32), the iron-rich phase is washed with rinse water and the water is filtered off (residual water 30 %). The iron ore yield is 0.382 kg corresponding to 38 % of dry matter. The Fe content is at 55 % according to RFA measurement. The contents of titanium are at 5 %, while the contents of sodium are at ca. 0.5 %. Here too, free Na ions are not présent since the sodium portion is bound in silicates. The floating phase is brought to a température of 45-49 °C in the further stirring container. Subsequently, the silicate-rich fraction is mixed with 3-10 % of burnt lime related to 60 % of the dry matter and reacted for about 90 minutes with homogenization. The developed mixture is dehydrated via a vacuum filter unit, wherein the filtering is possible substantially faster than without CaO addition. A weak alkaline solution with a pH value between about 12.4 and 12.6 and a porous clay with pore sizes below 1 mm arises, which is suitable as a filtering or absorption medium, e.g. for heavy métal and arsenic binding in the drinking water treatment. Here too, danger by releasable caustic soda lye does not exist anymore. The pH value ofthe clay-like product changes fast by aging to an unproblematic value of about 9. The silicate-rich product is also suitable as an additive for concrète, ceramic and the like.

Example 5

As the starting material, bauxite residue from the Bayer process is used, which was once washed and separated from the aluminate solution via a vacuum filter drum (e.g. vacuum filter drum 14) for caustic soda lye recovery. In a stirring container (e.g. stirring container 16), 2.6 liters of a bauxite residue suspension with a content of 1 kg of dry matter and a pH value of 13 are produced and brought to a température of 63 °C with continuous stirring (dissolver stirrer with medium speed). 25 ml of plant oil (crude râpe oil) are added to the suspension. Subsequently, the mixture is reacted for 10 minutes with homogenization. The pH value decreases during this time to ca. 12.0. Subsequently, 10 g of citric acid dissolved in 200 ml of water, is added in steps of each 50 ml. Hereby, the pH value ofthe suspension decreases stepwise over ca.

9.2 to ca. 7.4. After each addition ofthe citric acid solution, the suspension is stirred for 10 minutes with highest stirring speed. In this manner, the yield of iron or iron compounds can be advantageously increased since hereby the isoelectric points of different iron compounds are stepwise passed, wherefrom an improved disagglomeration results. Moreover, the citric acid functions not only as an inexpensive and simply manageable acid for adjusting the pH value, but it also prevents the reagglomeration of iron-rich and silicate-rich particles as a kind of grain refîner. Instead, the disagglomerated particles are highly efficiently separated and distributed in the suspension. After ca. 40 minutes in total, a heavy phase of iron oxides settles, above which a simply decantable floating phase is located.

After extraction of the floating phase, the iron ore phase is washed with rinse water and the water is filtered off (residual water 30 %). The iron ore yield is 0.428 kg corresponding to 42 % of dry matter. The Fe content is at ca. 55 % according to RFA measurement, while Ti is at 5 % and Na is at 0.5 %. Again, free sodium is not found since it is présent bound in silicates.

After filtering off the water (foam formation lowerthan in example 1), the floating phase présents itself as a kind of clay since the silicate minerais now considerably predominate with respect to the iron oxides. Accordingly, the color has changed from red to orange. The filtering becomes much more favorable if 5-10 % by weight of burnt lime are added before and the mixture reacts at températures of 42-49 °C. Hereby, new hydrates form such that a minerai binding of the residual water occurs. This crystal water can only be again expelled at températures beyond 130 °C. The developed material can be easily broken to mix it with other materials, e.g. with sand, acidic earth, lime, gypsum or fertilizer to produce a soil conditioner. A use as an additive for concrète, ceramic and the like is also possible. Here too, environmental hazard by releasable caustic soda lye does no longer exist.

Example 6

As the starting material, bauxite residue from the Bayer process is again used, which was once washed and separated from the aluminate solution via a vacuum filter drum (e.g. vacuum filter drum 14) for caustic soda lye recovery. In a stirring container (e.g. disagglomerator 20), the geometry of which is adapted to the diameter of a used dissolver stirrer, 2.6 liters of bauxite residue suspension containing 1 kg of dry matter and having a pH value of 13 are brought to a température of 63 °C with continuous stirring (dissolver stirrer with medium speed). An optimum solid/water ratio is regularly at about 1:2 to 1:5. 5 % of CaO related to the bauxite residue dry matter are added to the suspension and reacted for 40 minutes with homogenization. Therein, exchange of Na ions for Ca ions occurs in the zeolite-like silicate compounds of the bauxite residue such that the ion exchange capability and thus the buffer action of these compounds is severeiy reduced. The l

pH value of the suspension decreases to ca. 12.4 to 12.6. Subsequently, 10 g of citric acid dissolved in 200 ml water are added in steps of each 50 ml. Therein, the pH value of the suspension gradually decreases over ca. 9.2 to 7.4. After each step, it is stirred with highest stirring speed for 10 minutes to achieve disagglomeration of the iron-rich and the silicate-rich particles. Before, during and/or after the addition of the citric acid solution, one or more dispersants, surfactants and the like with a concentration in the range of 0.2 per mille can basically be added to achieve an additionally improved particle séparation. Alternatively or additionally to a dissolver stirrer or the like, the cavitation forces can also be generated with the aid of ultrasound. To this, the stirring container is for example provided with a sonotrode or another suitable device for generating ultrasound.

After about 40 minutes, a heavy phase of iron oxides has settled, above which a properly decantable floating phase is located. After extraction of the floating phase, the iron ore phase is washed with rinse water and the water is filtered off (residual water content 30 %). The iron ore yield is 0.457 kg corresponding to almost 46 % of the dry matter. The Fe content is at ca. 55 % according to RFA measurement, for Ti at 5 % and for Na at 0.5 %. Again, free sodium is not found since it is présent bound in silicates.

The floating phase présents itself as a kind of clay after filtering off the water (foam formation lower than in example 1), in which the silicate minerais now considerably predominate with respect to the iron compounds. The filtering becomes much more favorable if 5-10 % of burnt lime are added to the silicate-rich fraction and the silicate-rich fraction is heated to températures between 43 and 49 °C. Herein, new hydrates form, whereby a minerai binding of the residual water is effected. This residual water can only be again expelled at températures beyond 130 °C. The developed material can be easily broken to mix it with other materials, e.g. with sand, acidic earth, lime, gypsum orfertilizer to produce a soil conditioner. A use as an additive for concrète, ceramic and the like is also possible. Here too, environmental hazard by releasable caustic soda lye does no longer exist. The porous clay can also be employed as a filter mass for exhaust gas purification, biogas purification, desulfurization and the like. Furthermore, the silicate-rich fraction is suitable as a filter medium for immobilization of heavy metals and in particular of arsenic.

Moreover, the separated floating phase (clay) can be used as a catalyst mass in a biomass reactor, wherein it suppresses formation of tar and advantageously increases the hydrogen yield in the low-temperature pyrolysis in the température range between 230 °C and 550 °C.

The parameter values specified in the documents for defining process and measurement conditions for the characterization of spécifie properties of the inventive subject matter are to be considered as encompassed by the scope of the invention even within the scope of déviations - for example due to measurement errors, System errors, weighing errors, DIN tolérances and the like.

Claims (23)

1. Process for obtaining material of value from a bauxite residue, which is obtainable or obtained by the Bayer process, including the steps of:

a) providing an aqueous suspension of the bauxite residue;

b) adjusting a pH value of the suspension to a value between 7.2 and 12.2;

c) at least partially disagglomerating suspended minerai agglomérâtes of the bauxite residue, wherein the minerai agglomérâtes are disagglomerated by génération of cavitation in step c); and

d) separating the resulting mixture into an iron-rich fraction and into at least one further, preferably silicate-rich fraction.

2. Process according to claim 1, characterized in that in step a) a ratio of solid to liquid between 1:2 and 1:5, in particular a ratio of 1:2.5 is adjusted in the suspension and/or that in step a) a bauxite residue with a water content between 20 % and 40 % is used, wherein the bauxite residue preferably is one or multiple times washed.

3. Process according to claim 1 or 2, characterized in that a density of the suspension is adjusted to a value between 1.05 g/cm³ and 1.35 g/cm³, in particular between 1.07 g/cm³ and 1.30 g/cm³.

4. Process according to any one of claims 1 to 3, characterized in that the température of the suspension is adjusted to a value between 30 °C and 70 °C before step c), in particular in step a).

5. Process according to any one of claims 1 to 4, characterized in that the pH value is adjusted to a value between 7.4 and 11.8, in particular between 8.4 and 11.5 in step b).

6. Process according to any one of daims 1 to 5, characterized in that the pH value adjusted in step b) is continuously and/or gradually varied in the range between 7.2 and 12.2 during step c).

7. Process according to any one of daims 1 to 6, characterized in that the pH value is adjusted by addition of an acid, in particular a minerai acid, an organic acid, an acidic wastewater, an acidic condensate and/or FeCI₂.

8. Process according to any one of daims 1 to 7, characterized in that the pH value is adjusted by addition of a base, in particular of caustic soda lye and/or an alkaline wastewater.

9. Process acccording to any one of daims 1 to 8, characterized in that the pH value is adjusted by addition of a hydrolyzable compound, in particular an oil and/or a fat.

10. Process according to any one of daims 1 to 9, characterized in that at least one calcium compound, in particular calcium oxide and/or calcium hydroxide and/or calcium sulfate, and/or at least one dispersant, in particular a surfactant, is added to the suspension before step c).

11. Process according to daim 10, characterized in that between 0.1 % and 10 %, in particular between 2 % and 6 % of calcium compound and/or between 2 and 9 per mille of dispersant are added to the suspension related to the dry matter of the bauxite residue.

12. Process according to any one of daims 1 to 11, characterized in that at least one separating agent, in particular a fluxing agent and/or particles, in particular iron oxide particles, is added to the suspension preferably before and/or during step c).

13. Process according to claim 12, characterized in that the at least one fluxing agent is added to the suspension with a weight portion between 0.01 % and 1.0 %, in particular between 0.4 % and 0.6 % related to the dry matter of the bauxite residue.

14. Process according to claim 12, characterized in that particles are added, which at least predominantly hâve an average diameter between 0.3 pm and 25 pm, in particular between 0.4 pm and 20 pm.

15. Process according to any one of daims 1 to 14, characterized in that the cavitation is generated by at least one moved object, in particular by a dissolver stirrer, a shear stirrer, an impeller of a rotary pump, an impeller of a turbine, a shaker and/or a propeller.

16. Process according to any one of daims 1 to 15 characterized in that the cavitation is generated by applying ultrasound to the suspension.

17. Process according to claim 15, characterized in that the object for generating the cavitation is moved with a rotational frequency of at least 1000 min’¹, in particular of at least 2000 min’¹, through the suspension.

18. Process according to any one of daims 1 to 17, characterized in that the further fraction is separated from the iron-rich fraction by sucking and/or decanting and/or filtering, in particular by vacuum filtering.

19. Process according to any one of daims 1 to 18, characterized in that the iron-rich fraction is washed and/or dried after séparation.

20. Process according to any one of daims 1 to 19, characterized in that at least one calcium compound, in particular calcium oxide and/or calcium hydroxide and/or calcium sulfate, is added to the further fraction after séparation.

21. Process according to daim 20, characterized in that between 2 % and 15 %, in particular between 5 % and 10 %, of calcium compound is added to the further fraction related to its dry matter.

22. Process according to any one of daims 1 to 21, characterized in that the température of the further fraction is adjusted to a value between 30 °C and 70 °C after séparation.

23. Apparatus (10) for performing a process according to any one of daims 1 to 22, including:

- a container (16, 20) for receiving the aqueous suspension of the bauxite residue;

- a device (24) for adjusting the pH value of the aqueous suspension;

- a device (20) for at least partially disagglomerating suspended minerai agglomérâtes of the bauxite residue; and

- a device (26) for separating the resulting mixture into an iron-rich fraction and at least one further fraction.